Synthesis, Photophysical and Electronic Properties of Mono‐, Di‐, and Tri‐Amino‐Substituted Ortho‐Perylenes, and Comparison to the Tetra‐Substituted Derivative

Abstract We synthesized a series of new mono‐, di‐, tri‐ and tetra‐substituted perylene derivatives with strong bis(para‐methoxyphenyl)amine (DPA) donors at the uncommon 2,5,8,11‐positions. The properties of our new donor‐substituted perylenes were studied in detail to establish a structure‐property relationship. Interesting trends and unusual properties are observed for this series of new perylene derivatives, such as a decreasing charge transfer (CT) character with increasing number of DPA moieties and individual reversible oxidations for each DPA moiety. Thus, (DPA)‐Per possesses one reversible oxidation while (DPA)4‐Per has four. The mono‐ and di‐substituted derivatives display unusually large Stokes shifts not previously reported for perylenes. Furthermore, transient absorption measurements of the new derivatives reveal an excited state with lifetimes of several hundred microseconds, which sensitizes singlet oxygen with quantum yields of up to 0.83.

startingp oint for the synthesis of the new derivatives as for our previouslyr eported [30] tetra-substituted ortho perylene (DPA) 4 -Per,w hich is the high yielding, regioselective Ir-catalyzed CÀHb orylation of perylene.I no rder to obtain the mono-, di-, and tri-borylated derivatives, the number of equiva-lents of B 2 pin 2 was reduced from 5t o1 .T hus, am ixture of Bpin-Per, (Bpin) 2 -Per, (Bpin) 3 -Per ands ome (Bpin) 4 -Per was obtained. The mono-a nd di-borylated products were successfully separated by column chromatography.T wo furtherr egioisomerso f(Bpin) 2 -Per are possible, namely 2,5-bis(Bpin)-perylenea nd 2,11-bis(Bin)-perylene.H owever,t hese two isomers could not be isolated, whereas 2,8-bis(Bpin)-perylene ((Bpin) 2 -Per)w as isolated successfully.F or the follow-up reactions, the tri-borylated perylene, in contrast, was used as amixture containing some residual di-borylatedp erylene. THF is the solventofc hoice for the borylationreactions, as borylations attempted in MTBE or cyclohexane gave either mostly the higher borylated derivatives and no mono-o rd i-borylated product or no conversion was observed at all. The borylated compounds Bpin-Per, (Bpin) 2 -Per and (Bpin) 3 -Per were further converted to their corresponding bromo derivatives by halodeboronation, [33,34] analogously to our previously reported (Br) 4 -Per derivative. [30] Thus,t he respective borylated derivates were suspended in THF and MeOH (1:1) at 50 8C, and CuBr 2 dissolved in H 2 Ow as added to the reaction mixture dropwise. The mixtures were stirred at 95 8Cf or 48 h, until monitoring of the reactions by TLC confirmed full consumptiono ft he starting material. As the brominated derivatives are poorly soluble, only Br-Per was characterized by solution 1 HNMR spectroscopy,w hile (Br) 2 -Per and (Br) 3 -Per were used as am ixture for the respective follow-up reactions. TheB uchwald-Hartwig amination reactionw as accomplished using Pd 2 (dba) 3 ·CHCl 3 as the catalystp recursor andS phos as the ligand;K O tBu was used as ab ase and the reactionm ixture was stirred at 120 8Cf or 48 h. The aminated perylene products exhibit good solubility in commono rganic solvents, and thus purification by column chromatography was possible. All three derivatives (DPA)-Per, (DPA) 2 -Per,a nd (DPA) 3 -Per,t he latter obtainedf rom am ixture of di-and tri-brominated perylene,w ere isolated in good yields and fully characterized.

Photophysical properties
Perylene's S 1 ! S 0 transition has aw ell-defined vibronic fine structure with an intervalo f1 400 cm À1 and is allowed with molar absorption coefficients of 34 000 m À1 cm À1 at l max (abs) = 400 nm in toluene. The absorption spectra of our derivatives (DPA)-Per, (DPA) 2 -Per, (DPA) 3 -Per and also, for comparison, our previously reported [30] (DPA) 4 -Per,a re depicted in Figure2.The lowest energy transition l max (abs) of all four compounds is strongly bathochromically shifted in the order (DPA)-Per < (DPA) 2 -Per < (DPA) 3 -Per < (DPA) 4 -Per comparedt ot he parent perylenea nd is located between 472-499 nm. The intensity of this band follows the same order,w ith the mono-aminated derivativeh aving the least allowed S 1 ! S 0 transition with am olar absorption coefficient of 7000 m À1 cm À1 at l max (abs) = 472 nm while the tetra-aminated peryleneh as the most strongly allowed S 1 ! S 0 transition with am olar absorption coefficient of 20 000 m À1 cm À1 at l max (abs) = 499 nm. In contrast to the absorptiono fp erylene, the S 1 ! S 0 transitiono ft he aminated derivatives is rather broad and does not showavibrational progression. In addition, the absorption spectra indicate that the derivativew ith the least number of DPAu nits has the highest charge transfer (CT) characteri nt his series, whichd ecreases with increasing number of donor moieties.
All four derivatives possess rather moderate fluorescence quantum yields (F = 0.15-0.26), emitting in the orange-red region of the electromagnetic spectrum.I nterestingly,t he intensity of the fluorescenced ecreases in the order (DPA) 4 -Per > (DPA) 3 -Per > (DPA) 2 -Per > (DPA)-Per,w ith the mono-aminated derivativee xhibiting the weakestf luorescencew ith F = 0.15, while the tetra-aminated perylene exhibits the strongest fluorescence in this series with F = 0.26 (Table1). The emission shifts bathochromically with increasing number of DPAu nits in toluene, which correlates with the expected increaseo ft he HOMO energy with each DPAu nit. [35] Interestingly, (DPA)-Per possesses the strongest solvatochromic effect so that, in THF, the emission shifts bathochromically in the order (DPA) 4 -Per < (DPA) 3 -Per < (DPA) 2 -Per < (DPA)-Per.T his implies that the excited state of (DPA)-Per is more polar than that of (DPA) 4 -Per. Hence, the CT (charge transfer) character of these compounds decreases in the order (DPA)-Per > (DPA) 2 -Per > (DPA) 3 -Per > (DPA) 4 -Per,w hich might be ar esult of the increased symmetry of the more substituted derivatives. [36,37] The emission maxima and radiative decay rates of the derivatives (DPA)-Per (l max (em) = 557 nm, k r = 1.5 10 7 s À1 )a nd (DPA) 2 -Per (l max (em) = 558 nm, k r = 1.2 10 7 s À1 )a re very similar to each other,w hile there are larger differences between (DPA) 2 -Per and (DPA) 3 -Per,a nd the emission maxima of (DPA) 3 -Per (l max (em) = 563 nm, k r = 2.0 10 7 s À1 )a nd (DPA) 4 -Per (l max (em) = 569 nm, k r = 2.2 10 7 s À1 )a re also very similar to each other.A pparently,t he more highly substituted derivatives are stabilized by the polar solventl ess efficientlyt han the two less-substituted derivativesa st hey possess less CT character. The apparent Stokess hifts of (DPA)-Per and (DPA) 2 -Per,w hich are larger than those of (DPA) 3 -Per and (DPA) 4 -Per,a lso show that the geometry of the former two derivatives undergoes a larger changeu pon excitation than those of the latter two derivatives, and this is more pronouncedi nT HF than in toluene. Such large Stokes shifts are very unusual for perylened erivatives and have not been reported thus far,t ot he best of our knowledge. [38,39] However,this is anecessary propertyf or applications such as imaging and luminescent solar concentrators, as the reabsorption of the emittedl ight is reduced. [38,39] Yamato and co-workers observed asimilar trend of the solvatochromism in the series of 1-(diphenylamino)pyrene, 1,3-(diphenylamino)pyrene, 1,3,6-(diphenylamino)pyrene and 1,3,6,8-(diphenylamino)pyrene. [37] The intrinsic lifetimes of our four derivatives are quite long compared to that of perylene (t 0 = 4ns), while the tetra-aminated derivative has the shortest lifetimei n this series (t 0 = 46 ns), in accordance with the Strickler-Berg relation, as this derivativeh as the largest molar absorption coefficient. [40] These are the longest intrinsic lifetimes of perylenes reportedt od ate.

Reactivity with oxygen
In our previousr eport, we showed that (DPA) 4 -Per can sensitize singlet oxygen with aq uantum yield of 0.60, which was detected by its luminescencea t1 272 nm upon excitation of an O 2 -saturated toluenes olution. [30] Transienta bsorption measurements confirmed the presence of along-lived excited triplet state upon photoexcitation, from which energy transfer to the ground state oxygen takes place.H ence, we were motivated to investigate whether the quantum yield of singlet oxygen generation can be fine-tuned by the number of DPAu nits. The fluorescenceq uantumy ield F f increases with the number of DPAu nits. Thus, (DPA) 4 -Per exhibits the highest fluorescence quantum yield in this series ( Table 2). This indicates that the other derivatives could have an even higher rate of ISC (intersystem crossing) than (DPA) 4 -Per and, consequently,t he other derivatives could be more efficient singlet oxygen sensitizers. Compared to the standard, perinaphthenone, for which the quantum yield for generation of singlet oxygen is nearu nity, (DPA) 3 -Per, (DPA) 2 -Per and (DPA)-Per sensitize singleto xygen with quantum yields F D of 0.72, 0.78 and 0.83, respectively ( Table 2). To confirm the formationo fapossible triplet state upon photoexcitation, we performed transient absorption measurements, which revealed ab road excited state absorption in the range of 400-700nmw ith comparable lifetimes (t 1 = 150 ms, t 2 = 650 ms) to our previously reported (DPA) 4 -Per ( Figure 3, Figure S2). Hence, our experiments indeed show that ISC becomes less likelyw ith increasing number of DPAu nits and, therefore, the quantum yield of singlet oxygen production decreases whereas the fluorescencequantum yield increases.

Electrochemistry
As the tetra-substituted derivative (DPA) 4 -Per revealed four reversible oxidation potentials, which is uniquef or perylenes, [30] we conducted cyclic and square wave voltammetry studies on our new compounds in CH 2 Cl 2 with 0.1 m [nBu 4 N][PF 6 ]a nd detected one reversible oxidationp er DPAu nit ( Figure 4, Table 3). The first oxidation potential shifts to lower values with increas- The band maxima of the absorptiona nd emission were used to determine the Stokes shift.
[c] The intrinsic lifetime was calculated as t 0 = t/F.[ d] The non-radiative rate constantsw ere calculated as k nr = (1ÀF)/t.[ e] The radiative rate constantsw ere calculated as k r = F/t.  Vw ith respect to Fc/Fc + .A sp reviously described, [30] the third and fourth oxidations of (DPA) 4 -Per are quite close to each other; therefore, we previously conducted af urther measurement using the weakly coordinating anion (WCA)-containing electro- 4 ]t hat is known to separatec harged speciesbetter in electrochemical studies. Hence, four reversible oxidations at 0.04, 0.24, 0.41 and 0.51 Vw ith respect to Fc/Fc + were detected. [30] Thus, we now have as eries of perylene derivatives which allows one to tune the electrochemical properties dependingo nt he requirements of ap articular application.

DFT and TD-DFT calculations
DFT and TD-DFTcalculations were performed to rationalize the observedt rends and properties of these perylened erivatives.
The ground state structures were first optimized in the gasphase at the B3LYP/6-31G + (d, p) level of theory.P revious studies [23] have shown that range-separated hybrid functionals are necessary to obtain ar eliable pictureo ft he nature and relative energetic ordering of the excited states. We have thus used the CAM-B3LYP functional for the subsequent TD-DFTcalculations of the perylenederivatives. Unsubstituted peryleneh as al arge HOMO-LUMO gap (3.05 eV) ( Figure 5) while the addition of aD PA unit leads to a destabilization of the occupied orbitals as the nitrogen p z orbitals and the p-orbitals of the methoxy phenylr ings mix very well with the perylene core, and the unoccupied orbitals are less influenced by these substitutions. Therefore, the HOMO-LUMO gap decreases with the addition of DPAu nits so that (DPA) 4 -Per possesses the smallest gap in the series. However, it is important to note that the destabilization of the HOMO is large upon addition of one DPAu nit to perylene, being 0.32 eV,b ut does not change much with the addition of further DPAu nits. The lower energy occupied orbitals (HOMOÀ3 to HOMOÀ1) are strongly influenced by the DPAu nits at the ortho positions of perylene. Thus, addingo ne DPAu nit destabilizes the HOMO-1 by 1.57 eV and by 2.21 eV for the tetrasubstituted derivative in comparison to perylene. With the ad- dition of each DPAu nit, these lower occupied orbitals are more and more destabilized so that in (DPA) 2 -Per,H OMOa nd HOMOÀ1a re almostd egenerate, in (DPA) 3 -Per,t hree orbitals are almostd egenerate (HOMO, HOMOÀ1a nd HOMOÀ2) and, in (DPA) 4 -Per,f our orbitals (HOMO to HOMOÀ3) are nearly degenerate. [30] Hence, the observede lectrochemical behavior is consistentw ith our calculations, as each DPAu nit adds one orbital close to the HOMO of the respective derivative, so that the ease of removing an additional electron from the system increases with the number of substituents.
The TD-DFT calculations show that the nature of the S 1 ! S 0 transition in all of our derivatives remains HOMO!LUMO in character as in perylene and, as noted previously,f or our tetrasubstituted ortho perylenes (Table 4). [30] However, the HOMO! LUMO transition in (DPA)-Per has more CT character,w hile in (DPA) 4 -Per it has more LE (locally excited) character.I ndeed, the proportion of CT characterd ecreases with increasing number of DPAu nits. This trend is in accordance with the increasing molar absorption coefficients with increasing number of DPAu nits (DPA-Per e = 7000 m À1 cm À1 , (DPA) 2 -Per e = 11 000 m À1 cm À1 , (DPA) 3 -Per e = 12 000 m À1 cm À1 , (DPA) 4 -Per e = 20 000 m À1 cm À1 )a nd with stronger solvatochromism of the less substituted derivatives, resulting from the CT nature of the loweste xcited singlet state. The larger CT character for the loweste nergy absorption band for DPA-Per is also reflected in the relativelys mall oscillator strength (0.300) found for this transition.

Conclusions
We have synthesized as eries of ortho perylene derivatives (DPA) n -Per (n = 1-3) substituted with one, two andt hree bis(para-methoxyphenyl)amine (DPA) moieties and compared their properties to those of our previously reported four-fold substituted species (DPA) 4 -Per.T he key step in the synthesis of the new perylene derivatives was the partial Ir-catalyzed CÀH borylation of perylene. We showed that these donor-substituted perylenes possesst unable photophysicala nd electrochemical properties. Hence, with increasing number of donor moieties at the ortho positions, the CT character of our derivatives decreases while the LE character increases. All of our compoundsp ossessv ery long intrinsic singlet lifetimes (t 0 = 46-126 ns), and transient absorption measurements confirm af urther excited state with al ifetimeo fseveral hundred microseconds which effectively sensitizes singlet oxygen. Interestingly, the quantum yield for the sensitizationo fs inglet oxygen de-  creasesw hen more DPAu nits are added to the ortho positions of perylene. Thus, our mono-substituted derivatives ensitizes singlet oxygen with aq uantum yield of 0.83 whereas the tetrasubstituted derivative has a F D value of 0.60. Furthermore, cyclic voltammetry and square wave studies reveal one reversible oxidation per DPAu nit, which,t ot he best of our knowledge has not been observed previously for perylenes. These unique trends are rationalized by theoretical investigations that show that the DPAm oieties mix very wellw ith the occupied peryleneo rbitals, which leads to their strong destabilization. With each DPAu nit, the number of occupiedo rbitals lying energetically close to the HOMO increases, making it easier to removea ni ncreasing number of electrons.

Experimental Section
General considerations:T he catalyst precursors [Ir(COD)(OMe)] 2 [42] and Pd 2 (dba) 3 ·CHCl 3 [43] were prepared according to literature procedures, B 2 pin 2 was ag ift from AllyChem Co. Ltd. while other starting materials were purchased from commercial sources and used as received. Solvents used for synthesis were HPLC grade, further treated to remove trace water using ac ommercial solvent purification system from Innovative Te chnology Inc. and deoxygenated using the freeze-pump-thaw method. 1 H, 13 11 BNMR signals are quoted relative to BF 3 ·OEt 2 .C hemical shifts are listed in parts per million (ppm) and coupling constants in Hertz (Hz).
HRMS were recorded using aT hermo Scientific Exactive Plus Orbitrap MS system with either an Atmospheric Sample Analysis Probe (ASAP) or by Electrospray Ionization (ESI). Elemental analyses were performed on an Elementar vario MICRO cube elemental analyzer.
Cyclic voltammetry experiments were performed using aG amry Instruments Reference 600 potentiostat. As tandard three-electrode cell configuration was employed using ap latinum disk working electrode, aplatinum wire counter electrode, and asilver wire, separated by a Vycor tip, serving as the reference electrode.  state measurements were performed in standard quartz cuvettes (1 cm x 1cmc ross section). UV/Vis absorption spectra were recorded using an Agilent 1100 diode array UV/Vis spectrophotometer. Excitation, emission, lifetime and quantum yield measurements were recorded using an Edinburgh Instruments FLSP920 spectrometer equipped with a450 WXenon arc lamp, double monochromators for the excitation and emission pathways, and ar ed-sensitive photomultiplier (PMT-R928P) and an ear-IR PMT as detectors. The measurements were made in right-angle geometry mode and all spectra were fully corrected for the spectral response of the instrument. All solutions used in photophysical measurements had a concentration lower than 10 À5 M. .
Fluorescence quantum yield measurements:F luorescence quantum yields of the samples were measured using ac alibrated integrating sphere (150 mm inner diameter) from Edinburgh Instruments combined with the FLSP920 spectrometer described above. For solution-state measurements, the longest wavelength absorption maximum of the compound in the respective solvent was chosen for the excitation. In order to exclude self-absorption, the emission spectra were measured with dilute samples (ca. 0.1 OD at the excitation wavelength).
Fluorescence lifetime measurements:L ifetime measurements were conducted using the time-correlated single-photon counting method (TCSPC) on the FLSP920 spectrometer equipped with a high-speed photomultiplier tube positioned after as ingle emission monochromator.M easurements were made in right-angle geometry mode, and the emission was collected through ap olarizer set to the magic angle. Solutions were excited with either a3 15 (pulse width 932.5 ps), 376 (pulse width 72.6 ps) or a472 nm (pulse width 90.6 ps) pulsed diode laser at repetition rates of 1-5 MHz and were recorded at emission maxima. Decays were recorded to 10 000 counts in the peak channel with ar ecord length of at least 40 00 channels. The band-pass of the monochromator was adjusted to give as ignal count rate of < 20 kHz. Iterative reconvolution of the IRF with one decay function and nonlinear least-squares analysis were used to analyze the data. The quality of all decay fits was judged to be satisfactory,b ased on the calculated values of the reduced c 2 and Durbin-Watson parameters and visual inspection of the weighted and autocorrelated residuals.
Transient absorption measurements:T ransient absorption spectra were measured with an Edinburgh LP920 laser flash spectrometer equipped with an EKSPLA NT340 Nd:YAG laser with integrated optical parametric oscillator,a4 50 WX ef lash lamp, aH amamatsu R955 photomultiplier and aT ektronix TD3012B oscilloscope for detection of the spectra. The transient maps were obtained by measuring decay profiles in 4nms teps between ca. 25 000 cm À1 (400 nm) and 14 085 cm À1 (710 nm). The instrument response (ca. 8ns) of the set-up was determined by measuring the scattered light using aL UDOX AS-30 colloidal silica suspension in water. Decay curves were fitted with the tail-fit function of the spectrometer software. The quality of all decay fits was judged to be satisfactory,b ased on the calculated values of the reduced c 2 and Durbin-Watson parameters and visual inspection of the weighted and autocorrelated residuals. All solvents were spectroscopic grade and were used without further purification. The sample solutions in the quartz cuvettes were carefully degassed by bubbling argon through the solutions. The samples were excited with ca. 3-6 ns laser pulses at 10 Hz repetition rate. Measurements were performed at pulse energies of 1.2 mJ (excitation at 460 and 550 nm). The stability of the samples was verified by recording the steadystate absorption spectra before and after the time-resolved measurements.
Theoretical studies:A ll DFT and TD-DFT calculations were carried out with the Gaussian 09 (Rev.E .01) program package [44] and were performed on ap arallel cluster system. GaussView 5.0.9 was used to visualize the results, to measure calculated structural parameters, and to plot orbital surfaces (isovalue: AE 0.02 [e a 0 À3 ] 1/2 ). The ground-state geometries were optimized using the B3LYP functional [45][46][47] in combination with the 6-31G + (d, p) basis set. [48,49] The optimized geometries were confirmed to be local minima by performing frequency calculations and obtaining only positive (real) frequencies. Based on these optimized structures, the lowestenergy gas-phase vertical transitions were calculated (singlets, 5 states) by TD-DFT,u sing the Coulomb-attenuated functional CAM-B3LYP [50] in combination with the 6-31G + (d, p) basis set.
(Bpin)-Per, (Bpin) 2 -Per and (Bpin) 3 -Per:Inan argon-filled glovebox, aY oung'st ube was filled with perylene (1.00 g, 3.96 mmol, 1.0 equiv), B 2 pin 2 (1.0 g, 4.0 mmol, 1.0 equiv), [Ir(OMe)COD] 2 (39 mg, 60 mmol, 1.5 mol %) and dtbpy (32 mg, 0.1 mmol, 3.0 mol %). The tube was removed from the glovebox, 40 mL of THF was added under argon, and the reaction was stirred at 70 8C for 16.5 hu ntil reaction monitoring by TLC (hexane/CH 2 Cl 2 ,2 :1) showed that the starting material was consumed. After removing the solvent, the two products were separated by automated flash column chromatography (Biotage SNAP cartridge KP-Sil 50 g; 10-15 %C H 2 Cl 2 in hexane).  (Br)-Per:T oaround bottom flask fitted with ar eflux condenser, (Bpin)-Per (260 mg, 0.68 mmol, 1.0 equiv) was dissolved in 10 mL of THF and heated to 50 8C. Then, 10 mL of MeOH was added to this solution and CuBr 2 (460 mg, 2.1 mmol, 3.0 equiv) dissolved in 10 mL of H 2 Ow as added and the mixture was stirred at 95 8Cf or 2 du ntil reaction monitoring by TLC (hexane/ CH 2 Cl 2 ,2 :1) showed that the starting material was consumed. The solution was diluted with water and extracted with CH 2 Cl 2 .The organic phases were collected, and the solvent removed under reduced pressure. The crude product was purified by automated flash chromatography (Biotage SNAP cartridge KP-Sil 10 g; 10 %C H 2 Cl 2 in hexane) and obtained as ay ellow solid (115 mg, 51 %). The crude solid product was not completely pure due to its low solubility,w hich did not allow further purification. Therefore, it was not possible to obtain a reasonable 13  (Br) x -Per:T oar ound bottom flask fitted with ar eflux condenser,a mixture of (Bpin) 2 -Per and (Bpin) 3 -Per (730 mg) was dissolved in 35 mL of THF and heated to 50 8C. Then, 35 mL of MeOH was added to this solution and CuBr 2 (2.31 g) dissolved in 35 mL of H 2 O was added and the mixture was stirred at 95 8Cf or 2duntil reaction monitoring by TLC (hexane/ CH 2 Cl 2 ,2:1) showed that the starting material was consumed. The solution was diluted with water and extracted with CH 2 Cl 2 .T he organic phases were collected, and the solvent was removed under reduced pressure. The crude products of (Br) 2 -Per and (Br) 3 -Per were not completely pure due to their low solubility which did not allow further purification. HRMS (DPA)-Per:I na na rgon-filled glovebox, (Br)-Per (140 mg, 0.34 mmol, 1.0 equiv), bis(4-methoxyphenyl)amine (310 mg, 1.4 mmol, 4equiv), Pd 2 (dba) 3 ·CHCl 3 (9 mg, 8.7 10 À6 mol, 3mol %), SPhos (8 mg, 2 10 À5 mol, 6mol %) and KOtBu (150 mg, 1.4 mmol, 4equiv) were added to aY oung'st ube. The tube was removed from the glovebox, 8mLt oluene was added under argon, and the reaction was stirred at 120 8Cf or 3d .T he solvent was removed in vacuo and the crude product was purified by automated flash chromatography (Biotage SNAP cartridge KP-Sil 10 g; 10 %E tOAc in hexane). The desired product was obtained as ar ed-orange solid (120 mg, 74 %). 1