Symmetry‐Breaking Charge‐Transfer Chromophore Interactions Supported by Carbon Nanodots

Abstract Carbon dots (CDs) and their derivatives are useful platforms for studying electron‐donor/acceptor interactions and dynamics therein. Herein, we couple amorphous CDs with phthalocyanines (Pcs) that act as electron donors with a large extended π‐surface and intense absorption across the visible range of the solar spectrum. Investigations of the intercomponent interactions by means of steady‐state and pump‐probe transient absorption spectroscopy reveal symmetry‐breaking charge transfer/separation and recombination dynamics within pairs of phthalocyanines. The CDs facilitate the electronic interactions between the phthalocyanines. Thus, our findings suggest that CDs could be used to support electronic couplings in multichromophoric systems and further increase their applicability in organic electronics, photonics, and artificial photosynthesis.

Abstract: Carbon dots (CDs) and their derivatives are useful platforms for studying electron-donor/acceptor interactions and dynamics therein. Herein, we couple amorphous CDs with phthalocyanines (Pcs) that act as electron donors with al arge extended p-surface and intense absorption across the visible range of the solar spectrum. Investigations of the intercomponent interactions by means of steady-state and pump-probe transient absorption spectroscopyr eveal symmetry-breaking charge transfer/separation and recombination dynamics within pairs of phthalocyanines.T he CDs facilitate the electronic interactions between the phthalocyanines.T hus,o ur findings suggest that CDs could be used to support electronic couplings in multichromophoric systems and further increase their applicability in organic electronics,p hotonics,a nd artificial photosynthesis.
Multichromophoric systems play an important role in both organic electronics and photosynthetic light harvesting,w ith the spatial arrangement of the chromophores governing their electronic communication. [1] Fore xample,i nt he photosynthetic reaction center of purple bacteria, electron transfer from abacteriochlorophyll "special pair", surrounded by two branches of protein-bound cofactors,i sp receded by as ym-metry-breaking charge-transfer (SB-CT) state;t his is generally defined as arising from ap hotoexcited process,w here ap air of identical chromophores produces ad esymmetrized charge-transfer (CT) state.T his ultrafast-formed state governs charge separation in photosystem II. [2] Gathering acomprehensive understanding is vital for designing and preparing efficient organic electronic devices.M olecular materials which are capable of undergoing SB-CT will very likely find applications in fields such as organic photovoltaics, [3] photonics, [4] and artificial photosynthesis. [5] In recent years,t here have been many efforts to study interactions between chromophores in nonbiological molecular assemblies.M ost notable are pairs of arenes, [6] perylenes, [7] perylene diimides, [8] boron dipyrromethenes/dipyridylmethenes, [9] and metallodipyrrins, [3,10] to name just af ew. In terms of effective interchromophore communication, the spacer between them is one of the most critical variables.F or example,z inc porphyrins have been linked through phenylene(s), [11] naphthalene, [11,12] phenanthrene, [12] hexaphenylbenzene, [13] hexa-peri-hexabenzocoronene bridges, [14] as well as well-defined nanographene spacers. [15] Recently, porphyrins,which were linked through hexa-peri-hexabenzocoronene (HBC) spacers,were shown to interact through the p-surface of the HBC.T he most compelling evidence for this is ad istinct split of the porphyrin Soret-band absorption. [14a] In the context of photoactive chromophore assemblies, carbon dots (CDs) have been attracting attention. [16] These small (< 10 nm) carbon-based nanoparticles have been employed, both as electron donor (D) and acceptor (A), in electron-donor/acceptor systems based on covalent or noncovalent approaches. [17] Notably our groups have previously investigated amorphous nitrogen-doped CDs (referred to as a-N-CDs or nitrogen-doped carbon nanodots,N CNDs), and their electron-transfer interactions with a meso-tetraarylporphyrin (H 2 P). [17a] It was found that the covalent NCND-H 2 P conjugate,i mmediately after photoexcitation with visible light, undergoes charge separation to form the one-electronoxidized form of H 2 Pa nd the one-electron-reduced form of NCND.W enow turn our attention to phthalocyanines (Pcs), well-known electron donors,which possess unique properties such as good thermal and (photo)chemical stabilities,aswell as intense absorptions in the visible range. [18] Our interest stems also from some previous reports of Pc blends with different CDs,namely,graphitized carbon dots (discoidal and quasi-spherical in shape). [19] These blends were used to improve the absorption of visible light, [19a] to enhance electron-transfer/transportc haracteristics in inorganic quantum dots solar cells, [19b] and to enhance the nonlinear optical response of the nanomaterials. [19d] It is important to note that no symmetry-breaking charge transfer (SB-CT)/separation (SB-CS) has been observed in any of the aforementioned CDbased D-A materials.
Herein, we report the preparation of acovalent electrondonor/acceptor conjugate consisting of NCNDs and az inc phthalocyanine (ZnPc). Notably,weshow that, upon absorption of light at l = 387 nm, the NCNDs carrying the ZnPcs undergo aS B-CT (ZnPc dÀ -ZnPc d+ ), which is followed by aS B-CS to afford (ZnPc À -ZnPc + ). Similarly,a fter photoexcitation at l = 675 nm, aS B-CS is also observed, which, in this particular case,e volved from ah ot-S 1 state.T aken in concert, the results show that NCNDs provide as caffold to spatially arrange ZnPcs and facilitate the electronic communication between two or more of them.
TheN CND-ZnPc conjugate ( Figure 1) was prepared by ac arbodiimide (EDC) mediated coupling of the amino groups of the NCNDs [20] and the carboxyphthalocyanine (ZnPc); [21] the characterizations of the conjugate were in accordance with the literature (see the Materials Section in the Supporting Information). Following the reaction and the removal of DMF,t he reaction mixture was purified by sizeexclusion chromatography (SEC,S ephadex LH-20 resin in methanol). Thef irst dark blue band, corresponding to the NCND-ZnPc conjugate,w as collected, the organic solvent was removed under reduced pressure,and the final blue solid was obtained after freeze-drying an aqueous solution. The successful formation of the NCND-ZnPc conjugate was apparent as soon as it was dissolved in methanol, as it had an oticeable difference in solubility in polar organic solvents compared to the starting carboxyphthalocyanine.F urthermore,from aKaiser test, which was performed with NCNDs and NCND-ZnPc, we estimated the presence of al ower amount of free amino groups (100 mmol g À1 )o nt he nanoparticle surface,c ompared to pristine NCNDs (1350 mmol g À1 ). Additional characterization by FTIR spectrophotometry (KBr) confirmed the formation of the amide bond ( Figure S1). Specifically,inthe conjugate we observe the absence of the ZnPc carboxylic band at ñ = 1716 cm À1 ,t he presence of the ZnPc signature bands in the form of CÀH stretching at ñ = 2961, 2928, and 2850 cm À1 as well as CÀH bending at ñ = 1380 cm À1 ,and the appearance of C=Oand NÀ Hbending signals of amide bands at ñ = 1637 and 1565 cm À1 , respectively.T he composition was further probed by X-ray photoelectron spectroscopy (XPS;F igure S2). Thes urvey spectrum of the hybrid material shows the three peaks corresponding to the C1s, N1s, and O1ss pecies (the atomic percentages for C, N, Oare 84.3, 8.0 and 7.7) and the Zn 2p 3/2 species is visible in the high-resolution spectrum. Theh igher C/N ratio observed for the hybrid (C/N = 10.5) compared to the NCNDs (C/N = 4.25) [20a] is consistent with the presence of the organic macrocycle (deconvoluted spectra are shown in Figure S3). TheN 1s spectrum also provides evidence of functionalization ( Figure S3a,b). It can be deconvoluted into surface components observed for the two single entities, [20a] but the percentage of primary amine is lower compared to the NCNDs alone,i na greement with the decreased value of the Kaiser test.
Thermogravimetric analyses were used to compare the weight residues (or loss) of the hybrid and reference materials ( Figure S4). Ther esidual weight at 450 8 8Cf or the NCND-ZnPc and the NCND alone are 50 %a nd 14 %, respectively. Taking into account that zinc phthalocyanines have good thermal stability (> 90 %r esidual weight at 450 8 8C), [22] we conclude that the lower residual loss from the conjugate is due to the presence of the ZnPc,and it can be roughly calculated that the latter constitutes about half of the material. Finally, morphological characterization by atomic force microscopy (AFM) showed the presence of larger nanoparticles (4.19 AE 1.11 nm), when compared to pristine NCNDs [20a] (Figure S5).
Theo ptoelectronic properties of the NCND-ZnPc conjugate were first analyzed by comparison with the two references,n amely NCNDs and ZnPc,u sing steady-state absorption and fluorescence spectroscopy (Figure 2, methanol at room temperature).
Thea bsorption spectrum of ZnPc shows the typical Qband absorption at l = 674 nm and aw eaker Soret-band absorption at l = 346 nm. TheN CNDs,o nt he other hand, show the typical absorption spectrum with abroad signal that tails into the visible region. TheN CND-ZnPc conjugate presents ad ifferent absorption spectrum than the two references.I np articular,t he Q-band absorptions split into two maxima of similar intensity at l = 678 and 670 nm (Figure 2l eft, inset). This phenomenon, known as Davydov splitting, [23] is associated with electronic communication between two or more ZnPc units that are in proximity to each other. [14a, 24] It is noteworthy that Davydov splitting is not observed in molecular aggregates of ZnPc,since in the latter case an ew band appears at l = 640 nm ( Figure S6). The NCND-ZnPcs were additionally studied in the excited state and, again, compared to the references (Figure 2, right). To excite ZnPc, an excitation wavelength of l = 387 nm (3.2 eV) was used and the samples were adjusted to obtain the same optical density.Z nPc shows as trong fluorescence at l = 686 nm, with aq uantum yield of 0.15. In the NCND-ZnPc conjugate,af luorescence quenching of 22 %i so bserved, together with a4nm red-shift to l = 690 nm. An analogous red shift is seen, with alower fluorescence quenching of 9%, on using an excitation wavelength of l = 675 nm (1.84 eV; Figure S7). Thed ifference in the fluorescence quenching efficiency is due to the type of electronic transition which is excited in each particular case (see below). These findings suggest that symmetry-breaking interactions supported by the NCNDs promote additional decay pathways upon excitation.
This aspect was further investigated by electrochemical means (Figure 3). Forthe NCND reference,rather weak and broad signals,w hich are hard to dissect, were recorded. The other reference,Z nPc,s hows oxidations at + 0.59 and + 0.82 Vv ersus Ag/AgCl and reductions at À0.73 and À0.85 Vv ersus Ag/AgCl. NCND-ZnPc,i nc ontrast, showed three (rather than two) oxidations and reductions at + 0.55, + 0.72, + 0.84 Vand À0.78, À0.90, À1.01 V, respectively.T his result strongly suggests that, when coupled, the ZnPccentered orbitals are split because of ground-state electronic interactions with another ZnPc,w hich is facilitated by the NCNDs.F or this reason, aSB-CS state is expected at around 1.33 eV above the ground state.
To better understand the excited-state dynamics responsible for the ZnPc fluorescence quenching, femtosecond timeresolved transient absorption spectroscopy (fsTAS) experiments were performed. These were done for the conjugate and the ZnPc reference,w ith excitation at l = 387 nm in methanol and at room temperature.T he ZnPc reference ( Figure S8) showed positive transients at l = 485, 590, 630, and 808 nm, next to an egative signal at l = 674 nm. These features persist to the nanosecond timescale,a fter which apositive transient at l = 480 nm and negative ones at l = 609 and 674 nm appear. Theg lobal analysis suggests at hreespecies kinetic model with two exponentially decaying species with time constants of 7psa nd 2.2 ns,t ogether with an infinitely lived species.T hese two decays are ascribed to an internal conversion S 1 ! S 2 and an intersystem crossing T 1 ! S 1 .T he triplet (T 1 )e xcited states were much longerlived with respect to the resolution of the experiments,which in turn helps to rationalize their infinitively lived nature. [25] Similar results were obtained when the ZnPc was excited at l = 675 nm ( Figure S9). Thee xcitation leads to ah ot-S 1 population that decays in 1.6 ps to afford S 1 .T hen, the intersystem crossing T 1 ! S 1 takes 2.9 ns,preceding the decay to the ground state that occurs on the microsecond timescale.
Compared to ZnPc, NCND-ZnPc shows very different excited state dynamics.Excitation at l = 387 nm leads to two minima at l = 633 and 670 nm (Figure 4). These signals are  accompanied by positive transients at l = 547 and 750 nm, which, importantly,w ere not seen in the ZnPc reference. Moreover,t hese findings suggest the typical fingerprint absorption of the one-electron-oxidized ZnPc. [26] In contrast, on the nanosecond timescale,the transient absorption spectra resemble those observed for the ZnPc reference.T he kinetic model which is derived from these data involves three exponentially decaying species and af ourth one,w hich is persistent on the timescale of these experiments.
Taking also into account the results of fluorescence spectroscopy,aparallel target model was necessary to fit the transient absorption measurements.This model considers two parallel cascades.One of the two cascades,responsible for the fluorescence,i ncludes the same singlet excited state. [27] This intersystem crossing occurs in 2.2 ns to afford the corresponding microsecond-lived triplet excited state that was seen in the ZnPc reference ( Figure 5). Thes econd cascade is related to the fluorescence quenching and it is characterized by two excited states with distinct transients at l = 547, 750, and 925 nm. Notably,the maxima at l = 547 and 750 nm match the ones responsible for the observed formation of one-electron-oxidized ZnPc. [26] TheN CNDs in contrast are characterized by arather broad and featureless band, which extends all across the visible range of the solar spectrum. Moreover,t he one-electron-reduced ZnPc shows amaximum at l = 1000 nm [28] that resembles the one recorded for the conjugate.T his global information suggests as ystem formed from aN CND nanoparticle that binds at least two ZnPcs.H ere,aSB-CT/SB-CS occurs,b yo xidation of one ZnPc moiety,o no ne hand, and ar eduction of another ZnPc moiety,o nt he other hand. Thus,t hese two species that are involved in the fluorescence quenching could be assigned in one case to as ymmetry-breaking charge-transfer ZnPc dÀ -ZnPc d+ state (SB-CT), which is populated directly upon the absorption of light. In the other case,t he cascade could involve the symmetry-breaking charge-separated ZnPc À -ZnPc + state (SB-CS), which is formed in 37 ps and reinstates the ground state through charge recombination in 108 ps ( Figure 5). When excited at l = 675 nm, the excited-state dynamics of NCND-ZnPc are quite similar ( Figure S10). More specifically,a lthough the SB-CS state evolves from ahot-S 1 state in 11 ps,the charge recombination takes 101 ps, and matches the results obtained on exciting at l = 387 nm ( Figure 5). To further compare these states, Figure S11 shows the differential spectra of the NCND-ZnPc generated species upon excitation at l = 387 and 675 nm.
In summary,w eh ave prepared ac ovalent conjugate between N-doped carbon nanodots (NCNDs) and zinc phthalocyanines (ZnPcs). Symmetry-breaking (SB) interactions within aZnPc pair were evident in the absorption profile of the NCND-ZnPc conjugate,w hich shows the Q-band absorptions presenting Davydov splitting into two maxima of similar intensity.N otably,s teady-state fluorescence experiments show quenching in the conjugate,a ccompanied by ared-shift of the fluorescence maximum, on excitation at both l = 387 and 687 nm. This electronic communication, between at least two ZnPc species close to one another on the NCNDs, was confirmed also by electrochemical experiments.F urther investigations,b ym eans of steady-state and pump-probe transient absorption spectroscopy,revealed the SB-CT/SB-CS and recombination dynamics in the hybrid material. The present system, therefore,d iffers from the CND-H 2 Pc onjugate,w hich lacked any evidence of close contacts between the H 2 Ps on the periphery of the NCNDs.
We have thus provided further evidence of the beneficial integration of CDs into electron-donor/acceptor assemblies, this time by performing an ew role as as caffold. Although precise control of the number and position of the chromophores coupled on the CD surface remains beyond reach, the results presented herein should stimulate further research in CD-based D-A systems and applications in solar-energy conversion. Fore xample,t he use of CDs as scaffolds for chromophores of adifferent nature,aswell as using CDs with different structures is envisaged.