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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Synthetic bacteriochlorins enable systematic tailoring of substituents about the bacteriochlorin chromophore and thereby provide insights concerning the native bacteriochlorophylls of bacterial photosynthesis. Nine free-base bacteriochlorins (eight prepared previously and one prepared here) have been examined that bear diverse substituents at the 13- or 3,13-positions. The substituents include chalcone (3-phenylprop-2-en-1-onyl) derivatives with groups attached to the phenyl moiety, a “reverse chalcone” (3-phenyl-3-oxo-1-enyl), and extended chalcones (5-phenylpenta-2,4-dien-1-onyl, retinylidenonyl). The spectral and photophysical properties (τs, Φf, Φic, Φisc, τT, kf, kic, kisc) of the bacteriochlorins have been characterized. The bacteriochlorins absorb strongly in the 780–800 nm region and have fluorescence quantum yields (Φf) in the range 0.05–0.11 in toluene and dimethylsulfoxide. Light-induced electron promotions between orbitals with predominantly substituent or macrocycle character or both may give rise to some net macrocycle [LEFT RIGHT ARROW] substituent charge-transfer character in the lowest and higher singlet excited states as indicated by density functional theory (DFT) and time-dependent DFT calculations. Such calculations indicated significant participation of molecular orbitals beyond those (HOMO − 1 to LUMO + 1) in the Gouterman four-orbital model. Taken together, the studies provide insight into the fundamental properties of bacteriochlorins and illustrate designs for tuning the spectral and photophysical features of these near-infrared-absorbing tetrapyrrole chromophores.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The development of chromophores with tunable absorption across the near-infrared (NIR) spectral region is essential for diverse applications. Light in the NIR region affords the deepest penetration in soft tissue and hence is ideal for use in photomedicine [1-4]. Absorption that can be stepped across the NIR region would complement absorption of fluorophores in the near-UV and visible regions and thereby enhance multicolor protocols [5, 6]. The ability to capture sunlight in the photon-rich NIR region is essential for highly efficient bioinspired and biohybrid photosynthetic solar-conversion systems [7-9].

Bacteriochlorophylls are nature's chromophores for absorbing sunlight in the NIR region. The native chromophores and analogs derived therefrom have limited synthetic malleability due to a nearly full complement of substituents about the perimeter of the macrocycle and susceptibility to adventitious dehydrogenation. De novo syntheses have recently begun to provide access to bacteriochlorins that can be tailored in a variety of ways and that are stable toward such macrocycle oxidation [10-13]. Our own work has focused on creation of the bacteriochlorin skeleton, wherein a geminal dimethyl group is placed in each reduced, pyrroline ring, thereby blocking adventitious pathways leading to dehydrogenated products (i.e. chlorins and porphyrins) [14, 15]. The incorporation of auxochromes [16] such as phenyl, vinyl and acetyl groups at selected positions allows wavelength tuning of the strong NIR absorption band [17-20], known as the Qy band [21]. Chart 1 shows representative free-base (metal-free) bacteriochlorins B1B11 [14, 17-20] that bear such auxochromes at the 3-position, 13-position or both positions. Other synthetic approaches to impart a bathochromic shift of the long-wavelength band of bacteriochlorins include attachment of auxochromes [22] and modification in the macrocycle skeleton [23].

The NIR absorption spectra of representative members of B1B11 are provided in Fig. 1. In each case, a prominent fluorescence feature is found ca 5 nm to longer wavelength than the Qy absorption maximum (data not shown). The free-base bacteriochlorins and zinc or magnesium chelates (where available) have modest fluorescence yields (0.1–0.2), relatively long singlet excited-state lifetimes (3–5 ns), high yields of the triplet excited-state (0.5–0.7) and relatively long triplet excited-state lifetimes (50–150 μs) [17, 18, 20, 24].

image

Figure 1. Absorption spectra of representative bacteriochlorins, bacteriooxophorbine and bacteriochlorin imide studied previously, normalized in the Qy band. The compounds (Chart 1) are B1 (a, black), B3 (b, orange), B5 (c, purple), B7 (d, blue), B9 (e, red), B10 (f, green) and B11 (g, magenta).

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The parent synthetic bacteriochlorin (B3) that bears no substituents (other than the geminal dimethyl group in each pyrroline ring) has the Qy absorption band at 713 nm (and fluorescence at 716 nm). A substantial (58 nm) bathochromic shift to 771 nm is attained in the 3,13-diformylbacteriochlorin (B9). Other auxochromes (e.g. phenyl, vinyl, ethynyl, ester, acetyl) give shifts of intermediate magnitude. For a given set of 3,13-substituents, a 5-methoxy group gives a hypsochromic shift in the Qy band of up to ca 20 nm (e.g. B6 versus B8, Chart 1). Combinations of these substituents afford a palette of stable synthetic bacteriochlorins with a strong, relatively sharp 15–20 nm full width at half-maximum (FWHM) NIR absorption that can be stepped in ca 10 nm increments from 690 to 770 nm (Fig. 1). Absorption farther into the NIR region has been obtained by incorporation of a 5-membered imide ring that spans the 12–15 positions of the macrocycle [18]. Two representative bacteriochlorin–imides (B10 and B11), with or without a 5-methoxy group, are shown in Chart 1 and have the Qy band at 793 or 818 nm respectively (Fig. 1).

For fundamental studies as well as applications, it would be desirable to complement the existing set of 3,13-substituted bacteriochlorins that absorb in the 690–770 nm region with analogs that incorporate alternate auxochromes at the same two substituent sites to give even more bathochromic absorption shifts (similar to or exceeding those of an imide ring). One approach has been to utilize condensation reactions involving acetyl-substituted bacteriochlorins and a variety of aldehydes to prepare bacteriochlorin–chalcones [25]. The structures are shown in Chart 2. The name “chalcone” (Greek chalkos, copper ore) was given by Kostanecki and Tambor to the red-yellow condensation product of benzaldehyde and acetophenone (i.e. benzylideneacetophenone) in keeping with the names of other colored aryl ketones such as flavone and xanthone [26]. The enone benzylideneacetophenone is the parent member of the family of chalcones. The literature concerning chalcones is now vast; a Web of Science search on “chalcone” elicits >5000 citations owing to the importance of this motif in plant biochemistry (polyketides, flavonoids, anthocyanines) [27-30] and in medicinal chemistry [31-35]. We chose chalcones as a bacteriochlorin substituent that could both be readily formed [35, 36] and might impart a bathochromic shift in the bacteriochlorin absorption spectrum [25].

Here, we have performed a condensation of a formyl-substituted bacteriochlorin with acetophenone to prepare a reverse chalcone. The photophysical properties (in both polar and nonpolar media) and molecular-orbital (MO) characteristics of the reverse chalcone as well as the previously synthesized bacteriochlorin–chalcones are described. Collectively, these studies afford 3,13-substituted bacteriochlorins with extended NIR absorption and photophysical properties suitable for use in solar-light-harvesting systems and photomedical research.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

3,13-Bis[(E)-3-phenylprop-1-en-2-onyl]-8,8,18,18-tetramethylbacteriochlorin (B-REV-chPh)

Following an established procedure [25], the reaction was carried out using a CEM Discover Synthesis Unit (CEM Corp., Matthews, NC), which was equipped with an infrared sensor for temperature monitoring. The reactions were carried out in an open vessel. Thus, a mixture of B9 (15 mg, 0.035 mmol), acetophenone (16 μL, 0.14 mmol) and NaOH (28 mg, 0.70 mmol) in absolute ethanol (17 mL) was reacted in an open vessel (long-neck 125 mL round-bottom flask) equipped with magnetic stirring and a reflux condenser and subjected to microwave irradiation at 300 W. The protocol was as follows: [1] heat from room temperature to reflux, [2] continue heating to hold at reflux for 30 min, [3] allow to cool to room temperature (ca 2 min), [4] check the reaction mixture by TLC analysis and [5] repeat steps 1–4 until most of the starting material has disappeared. In so doing, the reaction mixture was heated for 2 h. The crude mixture was transferred to a round-bottom flask and concentrated. The resulting crude product was dissolved in CH2Cl2 and washed with a saturated aqueous solution of NH4Cl. The organic layer was separated, dried over Na2SO4, concentrated under reduced pressure and chromatographed [silica, hexanes/CH2Cl2 (1:1) [RIGHTWARDS ARROW] (1:9)] to afford an orange solid (<1 mg, ca 4%): 1H NMR (CDCl3, 300 MHz) δ – 1.29 (brs, 2H), 1.97 (s, 12H), 4.44 (s, 4H), 7.58–7.75 (m, 6H), 8.19–8.35 (m, 6H), 8.63 (s, 2H), 8.99 (s, 2H), 9.07 (s, 2H), 9.25 (d, = 15.0 Hz, 2H); laser-desorption mass spectrometry in the absence of a matrix [37] obsd 630.5; ESI-MS obsd 631.3064, calcd 631.3068 (C42H38N4O2); λabs (CH2Cl2) 333, 351, 395, 543, 800 nm.

Photophysical measurements

Measurement of the fluorescence (Φf) and triplet excited-state (Φisc) quantum yields, singlet (τS) and triplet (τT) lifetimes and transient-absorption studies unless noted otherwise, utilized dilute (μm) Ar-purged solutions at room temperature. Samples for Φf measurements had an absorbance <0.1 at the excitation wavelength. The Φf values ( ± 0.01) were generally determined as described previously [17] with respect to two standards and the results averaged. The standards were [1] free-base meso-tetraphenylporphyrin (FbTPP) in toluene, for which Φf = 0.070 was established with respect to the zinc chelate ZnTPP in nondegassed toluene (Φf = 0.030) [38], consistent with prior results on FbTPP [39], and [2] 8,8,18,18-tetramethylbacteriochlorin in Ar-purged toluene, for which Φf = 0.14 was established with respect to FbTPP and chlorophyll a (Chl a) in deoxygenated benzene [40] or toluene (Φf = 0.325).

The τs value ( ± 0.1 ns) for most of the bacteriochlorins (τs ca 1 ns or longer) was first probed using a time-correlated single photon counting instrument that employed Soret excitation flashes derived from a nitrogen-pumped dye laser (PTI LaserStrobe) and a Gaussian instrument response function of 0.6 ns. The τs values measured by transient absorption are consistent with those determined via fluorescence. The τT values were determined using a conventional flash photolysis setup. The apparatus utilized excitation flashes (ca 5 ns, ca 10 mJ, 490–550 nm) from a dye laser pumped by a Q-switched Nd:YAG laser (Cobra-INDI, Spectra-Physics), continuous probe light filtered by monochromators and a photomultiplier tube detector followed by an amplifier and digital oscilloscope with an overall instrument response function of ca 0.1 ns.

The Φisc (triplet yield) values ( ± 0.07) were obtained using a transient-absorption technique in which the extent of bleaching of the ground-state Qx and Qy bands due to the lowest singlet excited state was measured immediately following a ca 100 fs flash (in the Qx or Qy bands) and compared with that due to the lowest triplet excited state at the asymptote of the singlet excited-state decay. For the Qy region, the contribution of stimulated emission was taken into account. For both states and spectral regions, the extent of bleaching in the presence of excited-state absorption in the transient difference spectra was determined by various methods (to encompass a reasonable range of spectral shapes) including Gaussian fitting, integrations and linear interpolation of the excited-state absorption across the ground-state bleaching region. An average value of the triplet yields obtained by these methods is reported for each bacteriochlorin.

Molecular-orbital calculations

DFT calculations were performed with Spartan '10 for Windows version 1.2.0 [41] in parallel mode on a PC equipped with an Intel i7–975 cpu, 24 GB ram and three 300 GB, 10k rpm hard drives. The calculations employed the hybrid B3LYP functional and basis set 6–31G*. The equilibrium geometries were fully optimized using the default parameters of the Spartan program. Molecular-orbital (MO) images were plotted from Spartan using an isovalue of 0.016.

TDDFT calculations were performed in parallel mode [42] with Gaussian '09 version B.01 64-bit for linux using OpenSUSE version 11.4 or 12.1. One of two PC systems was used for Gaussian runs. The hardware for the first PC system used for Gaussian runs is the same as that used for the Spartan calculations and the second is a PC equipped with an intel i7–980 cpu, 24 GB ram and two 600 GB 10k rpm hard drives. Geometries used for the TDDFT calculations were from optimizations at the B3LYP/6-31G* level. TDDFT single-point calculations were performed at the B3LYP/6-31G* level.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Synthesis

We previously synthesized a 3,13-dibromobacteriochlorin [20], which provided a valuable substrate for conversion into the corresponding 3,13-diacetylbacteriochlorin B8 [20] and 3,13-diformylbacteriochlorin B9 [19]. We found that treatment of the 3,13-diacetylbacteriochlorin with an aldehyde under microwave irradiation resulted in facile formation of the condensation product, namely the bacteriochlorin–chalcone B-chPh as shown in Scheme 1 [25].

The scope of the reaction encompassed a wide variety of aldehydes. The aldehydes and resulting products include the following: (1) 1,3-bis(methoxymethoxy)benzaldehyde afforded the MOM-substituted bacteriochlorin–chalcones B-chM1 and B-chM2 (MOM = methoxymethyl); cleavage of the MOM group of B-chM2 gave resorcinol-substituted bacteriochlorin–chalcone B-chRsc. (2) p-N, N-dimethylaminobenzaldehyde afforded B-chDma. (3) Cinnamaldehyde afforded B-chCin, the vinylog of B-chPh. (4) Retinal afforded the retinyl-substituted bacteriochlorin–chalcones B-chRet1 and B-chRet2. The bacteriochlorin–chalcones (Chart 2) exhibited a bathochromic shift of the long-wavelength absorption band (Qy) of up to 24 nm from that of the parent 3,13-diacetylbacteriochlorin (B8) [25].

The availability of diformylbacteriochlorin B9 suggested analogous elaboration via condensation with a methyl ketone. Thus, the condensation of B9 and acetophenone was carried out to give the bacteriochlorin–chalcone, wherein the vinyl group is attached to the bacteriochlorin macrocycle and the carbonyl group is attached to the phenyl unit (Scheme 2). Such arrangement is the reverse of that for the bacteriochlorin–chalcone B-chPh in Scheme 1. Hence, the product shown in Scheme 2 is termed a reverse bacteriochlorin–chalcone, termed B-REV-chPh. The yields of the bacteriochlorin–chalcones shown in Scheme 1 and Chart 2 were quite reasonable, reaching as high as 58%. However, the reaction to form the reverse chalcone proceeded in low yield, with both starting material and monosubstituted bacteriochlorin (observed by laser-desorption mass spectrometry) remaining in the crude reaction mixture even after prolonged reaction time. The isolated yield of the desired B-REV-chPh was low (ca 4%), but sufficient material was obtained for spectroscopic studies (vide infra). The photophysical and molecular-orbital characteristics of the entire set of molecules are now described.

Absorption spectra

The absorption spectrum of a typical bacteriochlorin [21] such as the 3,13-substituted free-base bacteriochlorins studied previously (Chart 1) is normally comprised of four main features (Fig. 1). Progressing from longer to shorter wavelength these bands (and typical spectral ranges) are Qy (690–770 nm), Qx (520–580 nm), Bx (360–400 nm) and By (340–360 nm). The Bx and By features are also known as the Soret bands. Each of these main features is the origin transition [Qy (0,0), Qx (0,0), Bx (0,0), By (0,0)], for which hereafter the (0,0) designator will not be indicated for ease of presentation. A much weaker (1,0) vibronic overtone feature can be found 1000–1500 cm−1 to high energy than the Qy and Qx origin transitions. Features in the Soret region that represent the (1,0) vibronic overtones of Bx and By are often partially overlapped with the origin transitions. Additional features may also contribute to the Soret-region absorption. The Qy absorption band is intense, with an extinction coefficient on the order of 100 000 m−1 cm−1 [15].

The absorption spectra of the 3,13-substituted bacteriochlorin–chalcones studied here have similar overall absorption characteristics as the simpler 3,13-substituted analogs, with notable differences in detail. Spectra obtained in toluene are shown in Figs. 1-3. The spectra are similar to those measured in dimethylsulfoxide (DMSO). Spectral characteristics are summarized in Table 1. The Qy absorption band of each bacteriochlorin–chalcone bearing a single vinyl group shown in Chart 2 (i.e. except B-chCin) lies in the range 777–800 nm, which represents a significant bathochromic shift from the positions (690–771 nm) of the 3,13-bacteriochlorins studied previously. The latter positions include 713 nm for the unsubstituted parent B3 and 768 nm for the diacetylbacteriochlorin B8 (Figs. 1 and 2A). The Qy absorption bands of the bacteriochlorin–chalcones have a FWHM in the range 22–34 nm (26 nm average) in toluene and 26–38 nm (32 nm average) in DMSO. These Qy FWHM are greater than those of ca 20 previously studied 3,13-substituted bacteriochlorins, which have widths in the range 12–25 nm (19 nm average) [17]. The greater FWHM in DMSO versus toluene is paralleled by a corresponding decrease in Qy peak intensity (relative to the Soret maximum) in DMSO versus toluene. The compensating effects on bandwidth and peak height indicate that the integrated intensity (oscillator strength) of the Qy band generally does not change appreciably with solvent. The Qx bands lie in the range 540–550 nm (Fig. 2B and C), which represents a bathochromic shift from the range 489–536 nm found [17] for the simpler 3,13-disubstituted analogs.

Table 1. Spectral properties of bacteriochlorins.a
CompoundSolventB1 abs (nm)B2 abs (nm)B3 abs (nm)Qx abs (nm)Qy (nm)Qy abs FWHM (nm)IQy/IBmaxQy em (nm)Qy em FWHM (nm)
  1. a

    Data acquired at room temperature. “sh” = shoulder.

  2. b

    From ref. [17].

  3. c

    From ref. [76].

B3 b Toluene340365 489713120.84971616
B8 b Toluene360389 533768191.1977120
B-chRsc c MeOH346388325 sh544781201.6479527
B-chM1 Toluene350391330 sh550786251.6779226
DSMO350391330 sh546787311.3079833
B-chM2 Toluene350363380 sh542777221.9078123
DMSO350365380 sh538778281.4378729
B-chDma Toluene355391 547782291.3078728
B-chPh Toluene350388330 sh549785261.5979125
DMSO349389330 sh546785311.3879833
B-chCin Toluene354399336 sh550787261.4879426
DMSO353400336 sh548787321.1579834
B-REV-chPh Toluene350394332542800262.1880828
DMSO353396333542800331.5681334
B-chRet1 Toluene360381420545780231.7578624
DMSO361378419543780291.5079032
B-chRet2 Toluene352378420555792340.5179630
DMSO351379421551790380.4080238
image

Figure 2. Absorption spectra in toluene at room temperature of reference bacteriochlorins (A) and bacteriochlorin–chalcones (B, C), normalized at the Qy band. The insets show an expanded view of the Qy region.

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image

Figure 3. Qy absorption (A) and fluorescence (B) spectra of the set of bacteriochlorin–chalcones in toluene at room temperature.

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The near-UV Soret absorption characteristics of the bacteriochlorin–chalcones also have notable differences compared with the simpler 3,13-substituted counterparts. For example, the Soret-region absorption both of B-chDma and B-chRet1 show overlapping features in the 330–400 region to which the By and Bx origin transitions contribute, plus a substantial tail that contains several partially resolved features that extends into the Qx region (Fig. 2B). The general similarity of the near-UV spectra of B-chDma and B-chRet1, and the fact that the spectrum for B-chRet1 in this region is more complex than might be expected by the simple sum of the absorptions of a retinyl moiety plus an acetyl-containing reference bacteriochlorin (Fig. 2B) has implications regarding the nature of the excited states and transitions. In particular, the macrocycle and the retinyl moiety (like the other chalcone substituents) are likely not independent units, but instead possess (higher energy) excited states of mixed parentage that give rise to the complex near-UV absorption spectrum. This point will be explored further below in conjunction with the MO calculations.

The Soret absorption of B-chM1, B-chPh and B-REV-chPh do not show such a tailing absorption between the normal B and Qx regions, but show three features (near 330, 350 and 400 nm). Clearly, one or more transitions in addition to the nominal By and Bx contribute to the near-UV Soret absorption of the bacteriochlorin–chalcones, and the relative positions of all these bands depend on the complex. Such new contributions likely involve excited states that result from electron promotions between orbitals that have electron density on the chalcone substituents (and macrocycle), perhaps with some net charge-transfer (CT) character. Such possibilities (discussed below) make tenuous the assignment of the standard By and Bx tetrapyrrole transitions. For this reason, the prominent wavelength maxima (or distinct shoulders) in the Soret region are listed in Table 1 as B1, B2 and B3.

In the way of a specific comparison, Figs. 2B and 3A and Table 1 reveal that the Qy band of the reverse chalcone B-REV-chPh is shifted 15 nm to longer wavelength than that of the normal counterpart B-chPh (800 versus 785 nm in both toluene and DMSO). The bathochromic shift of the Qy band in the reverse versus normal bacteriochlorin–chalcone is accompanied by (1) an increase in intensity of the Qy band relative to the Soret absorption; and (2) a small (ca 5 nm) hypsochromic shift in the Qx band (Fig. 2B). Another specific comparison indicates that the incorporation of one additional double bond in the 3,13-chalcone substituents in B-chCin versus B-chPh results in only a 2 nm bathochromic shift in the Qy absorption band (787 versus 785 nm; Table 1).

Fluorescence spectra

The fluorescence spectrum of each bacteriochlorin–chalcone is dominated by the Qy origin band. For most compounds, the Qy fluorescence maximum is 4–7 nm to longer wavelength in DMSO than in toluene, although the Qy absorption maximum for each complex is the same to within 1 nm in the two media. Relative to the Qy absorption maximum (Fig. 3A), the Qy emission feature (Fig. 3B) lies on the average 6 nm to longer wavelength (90 cm−1 to lower energy) in toluene and 10 nm to longer wavelength (160 cm−1 to lower energy) in DMSO. The Qy fluorescence bands have a FWHM in the range 23–30 nm (26 nm average) in toluene and 27–38 nm (33 nm average) in DMSO, similar to the Qy absorption band (Table 1). The FWHM of fluorescence band of the bacteriochlorin–chalcones is modestly greater than the value of 21 nm for ca 20 simpler (nonchalcone) 3,13-substituted bacteriochlorins studied previously [17], which have the Qy emission feature at shorter wavelength.

Fluorescence quantum yields

The fluorescence quantum yields (Φf) of several bacteriochlorin–chalcones (B-chM1, B-chM2, B-chDma, B-chPh and B-chCin) and reverse chalcone B-REV-chPh are in the range 0.07–0.11 in toluene and DMSO, with an average value of 0.09 (Table 2). These values are smaller than the average Φf of 0.15 reported for the set of ca 20 bacteriochlorins (in toluene) bearing simple 3,13 substituents (e.g. acetyl, formyl, ester, phenyl) [17]. The Φf values for bacteriochlorin–chalcones B-chM1, B-chM2 and B-chDma are effectively the same in DMSO versus toluene. Bacteriochlorin B-chPh indicates a ca 30% reduction in Φf in DMSO versus toluene (0.08 versus 0.11). Similarly, cinnamyl-derivative B-chCin (one additional double bond in the chalcone substituents) has an apparent ca 20% reduction in Φf in DMSO versus toluene (0.09 versus 0.11). A more prominent (about two-fold) reduction in Φf in DMSO versus toluene (0.05 versus 0.11) is observed for 13-retinyl-containing B-chRet1.

Table 2. Photophysical properties of bacteriochlorins.a
CompoundSolventτS (ns)τT (µs) Φ f Φ isc Φ ic kf−1 (ns)kisc−1 (ns)kic−1 (ns)
  1. a

    Data acquired at room temperature in Ar-purged solutions. The experimental error is  ± 0.01 for the Φf values,  ± 0.05 for Φisc values and  ± 0.1 ns for τS values. Triplet lifetimes determined in tetrahydrofuran or 2-methyltetrahydrofuran.

  2. b

    From ref. [17].

  3. c

    From ref. [76].

B3 b Toluene4.01690.140.620.24296.517
B8 b Toluene2.9550.110.490.40265.97.3
B-chRsc c MeOH3.4780.020.660.321705.210.6
B-chM1 Toluene2.4540.080.440.48305.55.0
DSMO2.2 0.070.420.51315.24.3
B-chM2 Toluene2.6720.080.600.32334.38.1
DMSO2.6 0.07  38  
B-chDma Toluene2.2640.100.500.40224.45.5
B-chPh Toluene2.5580.110.490.40235.16.3
DMSO2.3 0.080.450.47295.14.9
B-chCin Toluene2.4 0.100.410.49245.94.9
DMSO2.2 0.090.370.54245.94.1
B-REV-chPh Toluene2.4510.080.440.48305.55.0
DMSO2.2 0.08     
B-chRet1 Toluene2.6650.110.480.41245.46.3
DMSO1.5 0.050.360.59304.22.5

The fluorescence spectra shown in Fig. 3B were generally obtained using at least three different excitation wavelengths across the Soret region (345–435 nm) to encompass the main spectral features and ensure that the same emission shape was obtained for each bacteriochlorins as was the case. The fluorescence yields obtained using the different excitation wavelengths were generally the same to within the experimental uncertainty. Because the different absorption features may arise from excited states that have variations in macrocycle-substituent character, these observation suggest that, independent of parentage, the energy flows in high yield to the lowest singlet excited state of the bacteriochlorin macrocycle from which fluorescence occurs. This finding includes the retinyl-containing bacteriochlorin B-chRet1, which has the most extended chalcone substituent. For the latter complex, the Φf values in both toluene and DMSO are the same using Qx and Soret excitation.

Singlet excited-state decay characteristics

The singlet excited-state lifetimes (τs) were measured by fluorescence decay (and are consistent with transient-absorption time profiles). The values for several bacteriochlorin–chalcones (B-chM1, B-chM2, B-chDma and B-chPh) and reverse chalcone B-REV-chPh are in the range 2.2–2.6 ns in toluene and DMSO, with an average value of 2.5 ns (Table 2). These values are shorter than the average τs of 3.9 ns reported for the set of ca 20 simpler (nonchalcone) 3,13-substituted bacteriochlorins [17]. For several bacteriochlorin–chalcones (B-chM1, B-chM2, B-chDma, B-chPh and B-chCin) and reverse chalcone B-REV-chPh, the τs value is ca 10% smaller in DMSO versus toluene (e.g. 2.3 ns in DMSO versus 2.5 ns in toluene for B-chPh). The reduction in excited-state lifetime in a polar versus nonpolar medium is even more pronounced for the mono-retinyl–bacteriochlorin B-chRet1 (1.5 ns in DMSO versus 2.6 ns in toluene); the ca two-fold reduction in τs parallels the reduction in Φf noted above.

The enhanced excited-state decay of B-chRet1 in DMSO versus toluene was explored further using the transient-absorption studies using a ca 100 fs excitation pulse (780 nm) and probing to 7.5 ns after the flash. Figure 4 shows that in both toluene and DMSO there is a smooth decay of the excited singlet state (1 ps spectrum) to form the excited triplet state (7 ns spectrum) along with ground-state recovery. There is no clear indication of any intermediates such as CT excited states along competitive decay of the singlet excited state to the ground state in DMSO. Nonetheless such states could contribute to the shorter τs in the polar solvent without being sufficiently populated to easily observe or could contribute directly to the singlet excited state by mixing with the normal (π,π*) electronic configurations of the bacteriochlorin macrocycle.

image

Figure 4. Transient-absorption difference spectra obtained following excitation of bacteriochlorin B-chRet1 in toluene (A) and DMSO (B) with a 120 fs excitation flash at 780 nm.

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Triplet excited-state quantum yields and lifetimes

The yield of intersystem crossing from the lowest singlet to triplet excited state (Φisc), also commonly called the triplet yield (ΦT), of each of several bacteriochlorin–chalcones (B-chM1, B-chM2, B-chDma, B-chPh and B-chCin) and reverse chalcone B-REV-chPh in toluene is in the range 0.41–0.60 with average value of 0.48. This value is slightly lower than the average value of 0.53 for ca 20 analogs bearing simple 3,13-substituents studied in toluene [17]. A subgroup of the same bacteriochlorin–chalcones (B-chM1, B-chPh and B-chCin) in DMSO have Φisc values in the range 0.37–0.42 with an average value of 0.41. In particular, although within experimental uncertainty, there is consistently a ca 10% lower intersystem-crossing yield for the same bacteriochlorin–chalcones in DMSO versus toluene (e.g. 0.45 versus 0.49 for B-chPh and 0.37 versus 0.41 for B-chCin). The effect is greater for retinyl–bacteriochlorin B-chRet1, wherein Φisc drops from 0.48 in toluene to 0.36 in DMSO. These solvent effects on Φisc generally parallel reductions in τs values and therefore derive from the solvent dependence not of the intersystem-crossing rate constant but of that for internal conversion (vide infra).

The lifetime of the lowest triplet excited state of select bacteriochlorin–chalcones (in deoxygenated THF or 2-MeTHF) was measured at room temperature using transient-absorption spectroscopy. The τT values are in the range 51–78 μs with an average of 61 μs. These values are somewhat lower than those results found previously for a large set of bacteriochlorins bearing simple 3,13-substituents, in which the τT values were found to be in the range 46–190 μs with an average of 90 μs. Shorter triplet lifetimes are expected as the energy of the lowest triplet excited state moves to lower energy (in parallel with the energy of the lowest singlet excited state; Table 1). This expectation derives from the energy gap law for nonradiative decay [43].

Singlet excited-state decay rate constants

The observables τs, Φf and Φisc (Table 2) for decay of the lowest-energy singlet excited state (S1) are connected to the rate constants for S1[RIGHTWARDS ARROW]S0 spontaneous fluorescence (kf), S1[RIGHTWARDS ARROW]S0 internal conversion (kic) and S1[RIGHTWARDS ARROW]T1 intersystem crossing (kisc) via Eqs. (1)(3).

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The internal conversion yield can be calculated from Eq. (4).

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The kf, kisc and kic values can be calculated from the τS and the yields via Eq. (5), where i = f, isc or ic.

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The Φic, kf, kisc and kic values obtained using Eqs. (3)-(5), along with the measured values of τs, Φf, Φisc for the bacteriochlorins are collected in Table 2.

Molecular-orbital characteristics

The energies and electron-density distributions of the frontier MOs of the bacteriochlorin–chalcones (standard, reverse, extended) and reference bacteriochlorins [unsubstituted parent (B3) and 3,13-diacetyl complex (B8)] were obtained from density functional theory (DFT) calculations. Figures 5-8 show the electron-density maps and energies of the MOs spanning HOMO-5 to LUMO+4. The MO energies for all the bacteriochlorins are collected in Table 3 to facilitate comparisons.

Table 3. Molecular-orbital energies of bacteriochlorins.a
CompoundHOMO–5HOMO–4HOMO–3HOMO–2HOMO–1HOMOLUMOLUMO+1LUMO+2LUMO+3LUMO+4H−L
  1. a

    DFT calculations utilized the structures shown in Charts 1 and 2 except for the following: For B-chM1 and B-chM2, –OCH3 was used rather than –OCH2OCH3 (OMOM) as the terminal group on the chalcone substituent. The orbitals with energies indicated in bold italics have the closest electron-density distributions to the normal macrocycle frontier MOs, which in the four-orbital model are HOMO-1, HOMO, LUMO and LUMO+1 (see B3 and B8). The last column (H−L) lists the energy gap between the HOMO and LUMO.

B3 −7.04−6.87−6.85−6.67 4.99 4.46 2.20 0.93 +0.65+0.96+1.572.26
B8 −7.21−6.89−6.64−6.64 5.26 4.83 2.78 1.31 −1.06−0.40+0.632.05
B-chRsc −6.17−6.16−6.02−6.02 5.15 4.71 2.74−1.89−1.731.03−0.251.97
B-chM1 −6.57−6.57−6.09−5.88 5.18 4.74 2.73−1.781.20−0.68+0.672.01
B-chM2 −6.05−6.04−5.84−5.84 5.10 4.67 2.68−1.80−1.650.98−0.181.99
B-chDma −6.21−6.20−5.13−5.10 4.89 4.46 2.44−1.47−1.350.77+0.012.02
B-chPh −6.58−6.58−6.25−6.25 5.16 4.73 2.75−1.90−1.741.05−0.321.98
B-chCin −6.50−6.50−5.82−5.81 5.11 4.67 2.71−2.08−1.911.08−0.601.96
B-REV-chPh −6.58−6.58−6.43−6.33 5.33 4.83 2.90 −1.89−1.651.20−0.411.93
B-chRet1 −6.51−6.44−6.06 5.13 −4.92 4.68 2.69 −2.211.19−1.05−0.571.99
B-chRet2 −6.26−6.25−5.21 5.19 −5.01 4.59 2.64 −2.14−1.961.05−0.781.95
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Figure 5. Characteristics of the five lowest unoccupied molecular orbitals of bacteriochlorins. The DFT calculations for B-chM1 and B-chM2 used –OCH3 rather than –OCH2OCH3 (OMOM). The orbitals with energies indicated in red have the closest electron-density distributions to the normal macrocycle LUMO and LUMO+1 (see B3 and B8).

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Figure 6. Characteristics of the five highest occupied molecular orbitals of bacteriochlorins. The DFT calculations for B-chM1 and B-chM2 used –OCH3 rather than –OCH2OCH3 (OMOM). The orbitals with energies indicated in red have the closest electron-density distributions to the normal macrocycle HOMO-1 and HOMO (see B3 and B8).

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Figure 7. Characteristics of the five lowest unoccupied molecular orbitals of bacteriochlorins. The orbitals with energies indicated in red have the closest electron-density distributions to the normal macrocycle LUMO and LUMO+1 (see B3 and B8 of Fig. 5).

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Figure 8. Characteristics of the five highest occupied molecular orbitals of bacteriochlorins. The orbitals with energies indicated in red have the closest electron-density distributions to the normal macrocycle HOMO-1 and HOMO (see B3 and B8 of Fig. 6).

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Time-dependent DFT (TDDFT) calculations were also performed. These calculations (at the B3LYP/6-31G* level) were not used as predictors of the exact energies/wavelengths or oscillator strengths of the features spanning the near-UV to NIR absorption spectrum of the bacteriochlorins. In particular, the calculations underestimate the energies of the electronic states responsible for the main transitions by amounts typically in the range 0.2–0.6 eV. In addition, although the relative oscillator strengths of the Soret (B), Qx and Qy features are predicted reasonably well for some of the bacteriochlorins, this is not true in all cases including the structurally least complex parent bacteriochlorin (B3). For this molecule, the predicted Qy/B intensity ratio is off ca 10-fold. Such TDDFT calculations are generally thought to become progressively less reliable with an increasing extent of excited-state CT character.

Owing to the above considerations, the TDDFT calculations were utilized qualitatively for aiding in the understanding of the likely one-electron promotions that comprise the Qy excited state, the Qx excited state and the higher-energy excited states that give rise to the more complex Soret-region absorption of the bacteriochlorin–chalcones compared with the simpler bacteriochlorins. The contributors to the excited states that are likely responsible for near-UV (Soret, B) and NIR (Qy) absorption of the bacteriochlorin–chalcones are generally apparent via inspection of the MOs that are in energy proximity to the standard frontier MOs of simple bacteriochlorins (the MO set that underlies the four-orbital model described below). The TDDFT calculations generally identify the same contributors to the excited-state compositions. Together the MO characteristics and TDDFT predictions afford a self-consistent assessment of the physical basis for key differences and similarities in the optical properties and some photophysical characteristics of the bacteriochlorin–chalcones versus analogs that bear simple substituents at the same 3,13-positions of the macrocycle.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

In the following sections, we first describe the context and rationale for the studies described herein. We then discuss the singlet excited-state properties of the diverse bacteriochlorins. The properties are then interpreted with insights gained from molecular-orbital calculations.

Overview

Our objectives in bacteriochlorin chemistry have been multifold. One goal is to prepare diverse substituted bacteriochlorins and learn how the nature and pattern of substituents alters the spectral and photophysical properties of these NIR-absorbing tetrapyrrole chromophores [14, 15, 17-20, 24, 25]. The knowledge gained in so doing should provide insight into the properties of native photosynthetic chromophores, but cannot readily be achieved with the native bacteriochlorophylls given their nearly full complement of substituents. A second and somewhat related goal is to learn how to tune the long-wavelength absorption band across the NIR region. The availability of a palette of such wavelength-tunable bacteriochlorins could be used in diverse studies and applications in solar-energy conversion and in photomedicine. A third goal is to develop a building block toolkit for rapid assembly of multipigment architectures that contain diverse bacteriochlorins. In contrast to the almost bewildering assortment of arrays that contain porphyrins [44, 45], relatively few (∼20) bacteriochlorin-containing arrays have been prepared [46].

The family of bacteriochlorins examined herein, which contain chalcones, extended chalcones and reverse chalcones, figures prominently in each of the aforementioned objectives. The possible roles in tuning spectral properties are apparent; the possible role in a modular building block strategy resides in the fact that all such compounds can be formed via chemistry that (1) employs basic conditions (to which the bacteriochlorins are stable); and (2) complements the most prevalent methods employed at present, which typically rely on palladium-mediated coupling reactions. Although the precursor bacteriochlorins B8 and B9 here were prepared using palladium-mediated reactions, chalcone chemistry may provide general advantages in various approaches to the preparation of tandem conjugates or multipigment arrays. The synthesis of normal chalcones proved substantially more efficient than that of the single reverse chalcone prepared herein. The condensation approach may be most convenient for use with polyenals such as retinal, which is available in multigram quantities yet can be sensitive to handle [47]. Other approaches to synthetic polyenes of the retinoid or carotenoid family can entail elaborate synthesis [48].

To our knowledge, only two other types of tetrapyrrole–chalcones have been prepared. One type consists of monochalcone analogs of B-chPh, B-chCin and B-chRet2, wherein a synthetic chlorin is employed in lieu of the bacteriochlorin [25]. A second type consists of meso-tetraarylporphyrins bearing one or four chalcone moieties appended via alkoxy or ester linkages to the para-positions of the meso-aryl groups [49]. The porphyrin–chalcones have been examined for cellular uptake, cytotoxicity and phototoxicity. A family of coumarin–chalcones also has been prepared and characterized spectroscopically [50], and also studied by computation [51].

The inherent color yet relatively small size of chalcones has prompted a large number of synthetic [35, 36], photophysical [52-58] and computational [33, 59-61] studies, predominantly of “push-pull” chalcones. As one relatively recent example, Rurack et al. described the synthesis and photophysical characterization of a family of push–pull chalcones of the general form shown in Chart 3 [53]. Typical electron-donating “push” substituents included p-N,N-dimethylaniline, whereas electron-withdrawing “pull” units included benzothiazole. Unlike the parent chalcone (benzylideneacetophenone, A = D = phenyl), which absorbs broadly in the UV (λmax = 312 nm, ε = 26 700 m−1 cm−1) in 95% ethanol [62], the push–pull chalcones exhibit absorption in the visible region (ε ca 20 000 m−1 cm−1) and, in polar solvents, a strong Stokes'-shifted emission in the NIR region [53]. The s-cis configuration of the enone moiety (as displayed in each diagram herein) of the chalcones is believed to be more stable than other configurations [33]. The bacteriochlorin–chalcones examined herein include those with the bacteriochlorin attached directly to the carbonyl moiety (A site, Chart 3) and those attached to the alkene unit (D site) of the chalcone framework.

A second variation in the chalcone unit concerns extended conjugation of the alkenyl system. Thus, the extended chalcone in B-chCin is a conjugated dienone. A benchmark for this auxochrome—(E,E)-cinnamylideneacetophenone (derived from condensation of cinnamaldehyde and acetophenone)—is yellow and absorbs broadly with λmax = 342 nm (ε = 39 000 m−1 cm−1) in methanol [63, 64]. The extended chalcones B-chRet1 and B-chRet2 are derived by reaction of the visual chromophore retinal with the bacteriochlorin containing an acetyl group. A nonbacteriochlorin analog (Ret-Ind), derived from retinal and indan-1,3-dione, is shown in Chart 4 [65]. Ret-Ind absorbs strongly in the region near 500 nm and exhibits rapid (multistep depending on excitation energy/wavelength) excited-state decay to the ground state in <50 ps with an overall time constant that depends on solvent polarity [66].

Pathways and rate constants for decay of the singlet excited state

The measured quantities τs, Φf and Φisc for decay of the lowest-energy singlet excited state were used to obtain values for Φic and the rate constants for the three decay pathways of the lowest singlet excited state (kf, kisc and kic), as described in the Results section. The values for all of these fundamental photophysical properties are collected in Table 2. Here, we discuss the excited-state decay rate constants for the bacteriochlorins bearing different types of substituents.

The S1 [RIGHTWARDS ARROW] S0 radiative rate constants (kf) for several bacteriochlorin–chalcones (B-chM1, B-chM2, B-chDma, B-chPh, B-chCin, B-chRet1) and reverse chalcone B-REV-chPh are in the range (24 ns)−1 to (38 ns)−1. The average value of (26 ns)−1 for these compounds in toluene is effectively the same as the value of (28 ns)−1 obtained in DMSO. These values are comparable to the average value of (27 ns)−1 found previously for bacteriochlorins bearing simple 3,13-substituents [17].

The S1 [RIGHTWARDS ARROW] T1 intersystem-crossing rate constants for several bacteriochlorin–chalcones (B-chM1, B-chM2, B-chDma, B-chPh, B-chCin and B-chRet1) and reverse chalcone B-REV-chPh are in the range (4.2 ns)−1 to (5.9 ns)−1. There is no systematic difference in the values for a given bacteriochlorin in toluene versus DMSO, except for retinyl–bacteriochlorin B-chRet1 (Table 2) for which the kisc value appears to modestly increase from (5.4 ns)−1 in toluene to (4.2 ns)−1 in DMSO. The latter effect can be understood if the lowest singlet excited state develops (more) macrocycle [LEFT RIGHT ARROW] substituent CT character in the polar versus nonpolar solvent as a result of stabilization of contributing CT excited-state electronic configurations. A modest shift of electron density from the substituent to the macrocycle in the excited state could result in a modest increase in spin-orbit coupling, which underlies the intersystem-crossing process. In this regard, the average kisc value for the 3,13-chalcone substituted and retinyl–bacteriochlorins of (5 ns)−1 is modestly greater than the value of (8 ns)−1 found previously for the 3,13-substituted bacteriochlorins bearing simple substituents (e.g. acetyl, formyl, phenyl) [17]. The possible contribution of such CT character to the excited states is described below in conjunction with the molecular-orbital characteristics.

The third pathway for decay of the lowest singlet excited state is nonradiative internal conversion into the ground state. The rate constants for S1 [RIGHTWARDS ARROW] S0 internal conversion (kic) for several bacteriochlorin–chalcones (B-chM1, B-chM2, B-chDma, B-chPh, B-chCin and B-chRet1) and reverse chalcone B-REV-chPh are in the range (2.4 ns)−1 to (6.3 ns)−1. The average value for these compounds in toluene is (5.9 ns)−1 and that in DMSO is (3.6 ns)−1. The difference in average values reflects a systematic reduction in kic for the compounds in DMSO versus toluene. These differences can be seen in the following comparisons (Table 2): (4.3 ns)−1 versus (5.0 ns)−1 for B-chM1, (4.9 ns)−1 versus (6.3 ns)−1 for B-chPh, (4.1 ns)−1 versus (4.9 ns)−1 for B-chCin, (2.4 ns)−1 versus (5.0 ns)−1 for B-REV-chPh and (2.5 ns)−1 versus (6.3 ns)−1 for B-chRet1. The solvent effect is greatest for the latter compound, which is the retinyl-substituted bacteriochlorin. Such effects also may be connected with an increase in excited-state CT character in the polar versus nonpolar solvent associated with stabilization of CT configurations and greater mixing with macrocycle (π,π*) configurations in giving rise to the wavefunction for the lowest singlet excited state. Such CT configurations may enhance nonradiative deactivation by a number of mechanisms. Such mechanisms include solvent-induced displacement of the excited- versus ground-state potential-energy surfaces and enhanced Franck-Condon factors involving the internal molecular coordinates [43]. Such involvement of the chalcone or retinyl moiety in the lowest singlet excited state could additionally enhance nonradiative decay via isomerization or other motions of the substituent that alter interactions with the bacteriochlorin macrocycle and thus the excited-state electron-density distribution.

Molecular-orbital characteristics and electronic structure

Gouterman's four-orbital model [67-69] is a useful framework for understanding the effects of substituents on the optical properties of tetrapyrrole chromophores. We have applied this model to synthetic chlorins [16, 70, 71] and bacteriochlorins [17, 24, 72] bearing simple substituents (e.g. acetyl, formyl, vinyl, ester, ethynyl, phenyl) in various patterns about the periphery of the macrocycles. All that is required to implement this model are the energies of the four-frontier MOs (HOMO-1, HOMO, LUMO, LUMO+1), which can be obtained from DFT calculations. The four-orbital model does not give absolute energies or oscillator strengths of the absorption features, but does well in assessing trends in the relative positions and relative intensities of the main optical features (Qy, Qx, Bx, By) as a function of substituent types and macrocycle sites.

Here, we use the four-orbital framework to provide a physical basis for understanding the manner in which the chalcone (normal, reverse, extended) substituents affect the absorption spectra and photophysical properties of the bacteriochlorins. As will be seen, these substituents require the consideration of additional MOs beyond the normal four-orbital set. These orbitals may place considerable electron density on the substituents, in addition to the macrocycle, and the excited states resulting from electron promotion may involve shifting of electron density between the macrocycle and the substituents.

Within the four-orbital model, the By and Qy excited states are comprised of linear combinations of the (y-polarized) electronic configurations that result from HOMO [RIGHTWARDS ARROW] LUMO and HOMO-1 [RIGHTWARDS ARROW] LUMO+1 one-electron promotions. The By state reflects the symmetric combination and the Qy state the antisymmetric combination of the configurations. For porphyrins, the By and Bx states have roughly 50/50 contributions of the HOMO [RIGHTWARDS ARROW] LUMO and HOMO-1 [RIGHTWARDS ARROW] LUMO+1 configurations. The respective constructive versus destructive interference of the two associated transition dipole moments (which have comparable strength) results in a strong By band in the near-UV (390–430 nm) region and a very weak Qy band in the visible (500–600 nm) region. The reduction in one pyrrole ring in chlorins and a second pyrrole ring in bacteriochlorins progressively and predominantly increases the energy of one of the two filled orbitals (the original porphyrin HOMO or HOMO-1) and the energy of one of the two unfilled orbitals (the original porphyrin LUMO or LUMO+1). Consequently, for bacteriochlorins, the Qy state has primarily (70–90% based on TDDFT calculations) the character of the HOMO [RIGHTWARDS ARROW] LUMO configuration (and 10–30% HOMO-1 [RIGHTWARDS ARROW] LUMO+1) and vice versa for By. In parallel, the Qy band moves to lower energy and gains intensity at the expense of the By band.

The excited-states Bx and Qx are similarly derived (within the four-orbital model) from configurations resulting from the (x-polarized) one-electron promotions HOMO [RIGHTWARDS ARROW] LUMO+1 and HOMO-1 [RIGHTWARDS ARROW] LUMO. Again, for porphyrins the Bx and Qx states have roughly equal contributions of the two electronic configurations, resulting in strong Bx and weak Qx bands that are in the same spectral region as (and may substantially overlap) the Qy and Bx bands depending on the porphyrin and its metalation state. However, unlike the y-polarized states, upon progressing to chlorin and then to bacteriochlorin, the energies (spectral positions) and intensities of the Bx and Qx bands are expected to be far less affected (by pyrrole-ring reduction). Consequently, in bacteriochlorins the large spectral splitting of the By and Qy (compared with chlorins or porphyrins) generally causes these two bands to spectrally sandwich the less perturbed Bx and Qx pair. Thus, the spectrum of a typical bacteriochlorin has four main features that in progressing from higher to lower energy are By, Bx, Qx and Qy, with the Qy band in the NIR spectral region (Fig. 1).

Description of the effects of the incorporation of simple 3,13-acetyl groups within the four-orbital approach will serve as a backdrop for examining the effects of 3,13-chalcone groups. The HOMO-1, HOMO, LUMO and LUMO+1 orbitals of the unsubstituted parent B3 are shifted by 0.27–0.58 eV to more negative values upon incorporation of 3,13-acetyl groups in B8 (Figs. 5 and 6 and Table 3). Electron density resides on the acetyl groups to a different degree depending on the orbital for B8, but the distribution on the macrocycle in each orbital is quite similar to that in the parent B3. For B8 the least electron density resides on the acetyl groups in HOMO-1 and the most in LUMO. The LUMO has stabilized the most of the four-frontier MOs by addition of the 3,13-acetyl groups, resulting in a diminution of the HOMO–LUMO energy gap (Table 3) and a bathochromic shift in the Qy band (Table 1 and Fig. 9). The Qy spectral shift occurs because within the four-orbital model the HOMO [RIGHTWARDS ARROW] LUMO is the major contributor to the Qy excited state (with a lesser contribution from HOMO-1 [RIGHTWARDS ARROW] LUMO+1). The TDDFT calculations support this picture, in which the Qy state is calculated to be comprised primarily (≥75%) HOMO [RIGHTWARDS ARROW] LUMO and most of the remaining being HOMO-1 [RIGHTWARDS ARROW] LUMO+1 for both B3 and B8. Similarly, TDDFT indicates that the By excited state has more HOMO-1 [RIGHTWARDS ARROW] LUMO+1 than HOMO [RIGHTWARDS ARROW] LUMO character (consistent with the four-orbital model) along with other configurations that involve lower energy filled orbitals and/or higher energy unoccupied orbitals. These latter orbitals for B8 have electron density on the acetyl groups (and the macrocycle). For B3 and B8, the TDDFT calculations also indicate (consistent with the four-orbital model) that the Qx state has >90% total contribution from HOMO-1 [RIGHTWARDS ARROW] LUMO and HOMO [RIGHTWARDS ARROW] LUMO+1. The Bx state also has substantial contribution of the same two contributions and for B8 additional contributions of one-electron promotions between orbitals that have electron density on both the macrocycle and the acetyl groups. The result is that these two reference compounds have prototypical bacteriochlorin absorption spectra (Fig. 1 and Table 1).

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Figure 9. HOMO–LUMO energy gap versus the absorption energy of the Qy origin band. Data are shown for compounds both in toluene and DMSO where spectra were measured in the two solvents (Table 1). The solid line is a fit to all the data.

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Scheme 1. Synthesis of a bacteriochlorin–chalcone.

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Scheme 2. Synthesis of a reverse bacteriochlorin–chalcone.

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Chart 1. Representative bacteriochlorins studied previously. The position of the Qy absorption band is indicated [14, 17-20].

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Chart 2. Bacteriochlorin–chalcones prepared previously [25].

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Chart 3. Push–pull chalcone architecture.

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Chart 4. Benchmark extended chalcones.

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The electron-density maps (Figs. 5-8) for the normal bacteriochlorin–chalcones (B-chPh, B-chM1, B-chM2, B-chRsc, B-chDMA), reverse chalcone (B-REV-chPh) and modestly extended chalcone (B-chCin) generally show similar characteristics to each other. Similarities and significant differences exist for the greatly extended retinyl-containing bacteriochlorin–chalcones (B-chRet1 and B-chRet2). The key points are as follows:

  1. The HOMO and LUMO orbitals of the various bacteriochlorin–chalcones retain overall similar electron-density distributions on the macrocycle as in the reference compounds (B3 and B8), but with electron density now residing on the substituents, much more so for the LUMO than the HOMO. The HOMO-1 orbitals of the bacteriochlorin–chalcones place little electron density on the substituents, just like reference bacteriochlorins B3 and B8. Such considerations are relevant to potential shifting of electron density between macrocycle and substituent as a result of one-electron promotions from HOMO or HOMO-1 to unoccupied MOs that have substantial chalcone-substituent character.
  2. The LUMO is generally stabilized by the substituents more than the HOMO, resulting in a decrease in the HOMO–LUMO gap to an extent that depends on the bacteriochlorin (Table 3). The HOMO–LUMO gap for bacteriochlorins bearing 3,13-chalcone groups (like simple substituents) tracks the position of the Qy absorption band (Fig. 9). Again, this correlation follows because the Qy state is expected to contain substantial HOMO [RIGHTWARDS ARROW] LUMO character (within the four-orbital model). Indeed, the TDDFT calculations indicate that the Qy state for most of the bacteriochlorin–chalcones has predominantly (m75%) HOMO [RIGHTWARDS ARROW] LUMO character.
  3. The unoccupied orbitals LUMO+1 to LUMO+4 of the bacteriochlorin–chalcones are basically combinations of the normal macrocycle LUMO+1 (for B3 or B8) and orbitals that have electron density spread across the chalcone substituent. Of this set (LUMO+1 to LUMO+4), the MO that has the most electron density on the macrocycle and the least on the substituents (and thus is closest in characteristics to the normal macrocycle LUMO+1) is now LUMO+3. The exception is B-chM1, for which this orbital is LUMO+2. Accordingly, the Soret-region absorption spectrum for the bacteriochlorin–chalcones is expected to be more complex than normal, given that for simple bacteriochlorins the By transition (typically, the highest energy of the main Soret-region features) in the four-orbital model has primarily HOMO-1 [RIGHTWARDS ARROW] LUMO+1 character, and the Bx transition (typically, the lower energy of the main Soret-region features) has substantial HOMO [RIGHTWARDS ARROW] LUMO+1 character (along with substantial HOMO-1 to LUMO character). The near-UV spectrum is expected to have contributions from excited states derived in part from electron promotions from HOMO-1 and HOMO (and/or lower filled orbitals) to one or more of the filled orbitals in the LUMO+1 to LUMO+4 set. The contribution closest to HOMO-1 [RIGHTWARDS ARROW] LUMO+1 of the simple bacteriochlorins will be HOMO-1 [RIGHTWARDS ARROW] LUMO+3 for most bacteriochlorin–chalcones. The TDDFT calculations indicate that some of the contributions derive from electron promotions involving filled orbitals down in energy to at least HOMO-5 and unoccupied orbitals up in energy to LUMO+4. The resulting mixed-parentage transitions often have mixed x and y polarization. Some states/transitions may have net CT character in shifting electron density between macrocycle and chalcone substituent. This analysis gives a physical basis for the more complex, multifeatured near-UV Soret-region absorption spectra of the bacteriochlorin–chalcones (Fig. 2) than for bacteriochlorins that bear simple substituents (Fig. 1). For these new NIR-absorbing compounds, the additional absorption transitions incorporated into the near-UV Soret region can be viewed as beneficial to the overall light-harvesting capacity.
  4. The TDDFT calculations on the normal, reverse and modestly extended (cinnamyl) bacteriochlorin–chalcones indicate small (generally <10%) contributions to the Qy excited state of one-electron promotions beyond those expected based on the four-orbital model. Again, the normal four-orbital configurations are expected to be mainly HOMO [RIGHTWARDS ARROW] LUMO and HOMO-1 [RIGHTWARDS ARROW] LUMO+X, where LUMO+X denotes the orbital most closely analogous to the LUMO+1 of the reference bacteriochlorins. The minor non–four-orbital contributions could potentially lend some small net substituent [LEFT RIGHT ARROW] macrocycle CT character to the Qy excited state depending on bacteriochlorin and potentially a mild solvent-polarity dependence of the photophysical properties. However, as noted in the Results section (Table 2), a medium-polarity dependence is not apparent for most normal, reverse and modestly extended (cinnamyl) bacteriochlorin–chalcones, and when seen is only a <15% effect. Thus, in general the bathrochromically shifted (and strong) NIR absorption attained in the synthetic bacteriochlorin–chalcones is not accompanied by significant deleterious or solvent-dependent effects on the lowest singlet excited state (Qy) and thus the key photophysical properties.
  5. For the more extended bacteriochlorin–chalcones (B-chRet1 and B-chRet2), the key characteristics concerning the HOMO and LUMO are noted above for the shorter chain chalcones. This parallelism includes the correlation of HOMO–LUMO energy gap with Qy spectral energy/wavelength (Fig. 9). B-chRet1 and B-chRet2 also share the characteristic noted for the shorter chain chalcones that the LUMO+1 of reference bacteriochlorins (B3, B8) is effectively incorporated into four orbitals (LUMO+1 to LUMO+4) that have variations in electron density on the macrocycle (with the reference LUMO+1 density distribution) versus being spread across the chalcone (and retinyl) groups. The unoccupied orbital that is the closest to the normal macrocycle LUMO+1 (with the smallest electron density on the substituents) is LUMO+2 for B-chRet1 and LUMO+3 for B-chRet2. Thus, as in the case of the shorter chain and reverse bacteriochlorin–chalcones, the Soret region of B-chRet1 and B-chRet2 is more complex than for the reference bacteriochlorins (Figs. 1 and 2). The differences again can be understood in terms of one-electron promotions involving molecular orbitals that have various degrees of macrocycle and retinyl character that replace and/or supplement characteristic configurations of the original four-orbital set.
  6. For B-chRet1, the macrocycle HOMO-1 of reference bacteriochlorins (and the normal, reverse and slightly extended bacteriochlorin–chalcones) becomes the HOMO-2 for B-chRet1 (which contains one acetyl and one retinyl group), which also has a small amount of electron density across the retinyl group. The HOMO-1 of B-chRet1 in contrast has very little electron density on the macrocycle and is dominated by electron density on the retinyl moiety. Thus, electron promotions from HOMO-1 of B-chRet1 to filled orbitals that have substantial electron density on the macrocycle have the potential to contribute CT character to the resulting excited states.
  7. B-chRet2 follows a similar picture to that for B-chRet1 except that the HOMO-1 of B-chRet2 remains closest to the normal macrocycle HOMO-1, but acquires a modest amount of electron density along the two retinyl groups. The HOMO-2 of B-chRet2 in contrast has little electron density on the macrocycle and a large amount of electron density on the two retinyl groups, analogous to HOMO-1 of the mono-retinyl analog B-chRet1.
  8. For the extended chalcone analogs (B-chRet1 and B-chRet2), TDDFT calculations indicate that the primary HOMO [RIGHTWARDS ARROW] LUMO contribution to the Qy excited state is supplemented by configurations that do not have simple four-orbital model flavor. Such one-electron promotions (e.g. HOMO-1 [RIGHTWARDS ARROW] LUMO and HOMO [RIGHTWARDS ARROW] LUMO+1 for B-chRet1) could impart some macrocycle [LEFT RIGHT ARROW] retinyl CT character on the basis of the electron-density distributions noted above (Figs. 7 and 8). In this regard, the retinyl-bacteriochlorins are distinct from the numerous, less strongly coupled carotenoid–spacer–tetrapyrrole multads pioneered by the groups of Gust, Moore and Moore, where the tetrapyrrole consists of a porphyrin [73], purpurin [74] or phthalocyanine [75]. Inspection of Table 2 indicates that B-chRet1 has a 2–3-fold shorter rate constant for nonradiative internal conversion (kic) in toluene compared with the other bacteriochlorin–chalcones, and that this rate constant drops by about 3-fold again in the more polar DMSO. These findings are consistent with enhanced CT character for the Qy state of B-chRet1 versus the other bacteriochlorin–chalcones in toluene, with further enhancement in DMSO due to solvent stabilization of the CT configurations and thereby a greater contribution to the Qy wave function. Collectively, these considerations tie the observed photophysical behavior of B-chRet1 to the effects of the retinyl substituents on MO characteristics and electronic structure.

In summary, the bacteriochlorin–chalcones described herein are readily synthesized, absorb strongly in the 780–800 nm region, fluoresce with small Stokes shift and quantum yields (Φf) in the range 0.05–0.11 and have singlet excited-state lifetimes (τs) in the range 1.5–2.6 ns in toluene and DMSO; the spectral features exhibit modest or no sensitivity to solvent polarity. An adequate description of the near-UV (Soret) region absorption requires significant participation of electron promotions between occupied and unoccupied molecular orbitals beyond those in the Gouterman four-orbital model, with some extent of macrocycle [LEFT RIGHT ARROW] substituent charge-transfer character in the lowest and higher singlet excited depending on the nature of the chalcone substituent. Accordingly, the bacteriochlorin–chalcones are of fundamental interest for electronic and spectroscopic studies and represent viable architectures for light-harvesting applications, particularly where absorption across the visible and NIR regions is desirable.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This work was supported by grants from the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy to D.F.B. (DE-FG02-05ER15660), D.H. (DE-FG02-05ER15661) and J.S.L. (DE-FG02-96ER14632). Mass spectra were obtained at the Mass Spectrometry Laboratory for Biotechnology at North Carolina State University. Partial funding for the facility was obtained from the North Carolina Biotechnology Center and the National Science Foundation. Transient-absorption studies were performed in the Ultrafast Laser Facility of the Photosynthetic Antenna Research Center (PARC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award no. DE-SC0001035.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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