Conjugated Donor Acceptor Research Articles

(Invited) Axially Assembled Photosynthetic Antenna-Reaction Center Mimics

A series of porphyrin based donor-acceptor conjugates in which the arrangement of various components are in perpendicular orientation to the porphyrin plane have been constructed and used to study the pathways of electron and energy transfer processes. The employed aluminum(III) porphyrin in these studies is found to be synthetically more advantageous as it provides reaction site on both faces of the porphyrin plane. The axial hydroxyl group on one side of the aluminum(III) porphyrin plane is able to form a covalent ester or ether bond with an energy harvester or electron acceptor while Lewis bases such as pyridine or an imidazole group form a metal-ligand axial coordination on other side of the plane hosting an electron donor or acceptor. The advantage of this configuration of the conjugates is that the donor and acceptor sides of the complex are located on opposite faces of the porphyrin which minimizes the rate of charge recombination. Moreover, the excited state \U0001d70b* orbital of porphyrin and axially coordinated ligand lie in the same space which causes an increase in the electronic coupling thus facilitating the electron/energy transfer processes. The distance dependence of sequential electron transfer in vertical, linear supramolecular triads constructed using tetrathiafulvalene (TTF), aluminum(III) porphyrin (AlPor) and fullerene (C60) entities will be discussed. Further, results of photoinduced energy transfer followed by electron transfer in antenna-reaction center mimicking BODIPY-AlPor-C60 supramolecular triads will be highlighted. Figure 1

Porphyrin Donor and Tunable Push-Pull Acceptor Conjugates-Experimental Investigation of Marcus Theory.

We report on a series of electron donor-acceptor conjugates incorporating a ZnII -porphyrin-based electron donor and a variety of non-conjugated rigid linkers connecting to push-pull chromophores as electron acceptors. The electron acceptors comprize multicyanobutadienes or extended tetracyanoquinodimethane analogues with first reduction potentials ranging from -1.67 to -0.23 V vs. Fc+ /Fc in CH2 Cl2 , which are accessible through a final-step cycloaddition-retroelectrocyclization (CA-RE) reaction. Characterization of the conjugates includes electrochemistry, spectroelectrochemistry, DFT calculations, and photophysical measurements in a range of solvents. The collected data allows for the construction of multiple Marcus curves that consider electron-acceptor strength, linker length, and solvent, with data points extending well into the inverted region. The enhancement of electron-vibration couplings, resulting from the rigid spacers and, in particular, multicyano-groups in the conformationally highly fixed push-pull acceptor chromophores affects the charge-recombination kinetics in the inverted region drastically.

Indacenodithiophene-based conjugated polymers incorporating alkylthiophene side chains: Improvement of organic solar cell performance

Varying the structures of conjugated side chains can be an efficient approach toward tailoring the optoelectronic properties of conjugated polymers. In this study, we synthesized four indacenodithiophene (IDT)-based side chain conjugated donor–acceptor alternating polymers. We introduced four different acceptor units (difluorobenzothiadizole: PIDTHT-FBT; difluoroquinoxaline: PIDTHT-QF; dicyanoquinoxaline: PIDTHT-QCN; diketopyrrolopyrrole: PIDTHT-DPP) to study the effects of the conjugated side chains on the optoelectronic properties (absorption spectra, energy levels, hole mobility) of the polymers and their resulting organic photovoltaic (OPV) devices. The energy levels of the highest occupied molecular orbitals for the new polymers were deeper when they featured alkylthiophene side chains. Films of these materials blended with PC71BM exhibited sufficiently high space-charge-limited current (SCLC) hole mobilities (>1.2 × 10−5 cm2 V−1 s−1) for efficient charge carrier extraction. The greatest OPV performance was that of the device incorporating PIDTHT-QF, with a power conversion efficiency of 5.15% under illumination with AM 1.5G solar light (1000 W m−2). Thus, side chain conjugated (two-dimensional) IDT polymers incorporating optimized acceptor groups are prospective materials for OPV applications.

Simultaneous Substitution of Silicon and Fluorine Atom on Donor-Acceptor Copolymers for Photovoltaic Applications

To investigate the effect of silole-containing and fluorination on the photovoltaic performance of polymer solar cells, two conjugated donor–acceptor copolymers have been synthesized via Stille coupling reaction. Both donor–acceptor alternating copolymers consisted of 4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene as the donor unit and nonfluorinated or fluorinated 2,1,3-benzooxadiazole (BO) as the acceptor unit, along with a thiophene group as the π-bridge between the donor and acceptor unit. For comparison, the copolymer composed of the 4,4-diethylhexyl-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) donor unit and the BO acceptor unit has also been prepared. UV–vis spectra of the three copolymers display a panchromatic absorption ranging from 300 to 1100 nm. The optical band gap obtained from the Tauc relation for the CPDT-containing polymer is 1.54 eV. With regard to the silole-containing copolymers, the optical band gaps for nonfluorinated copolymer and fluorinated copolymer are reduced to 1.46 and 1.42 eV, respectively. In comparison with both nonfluorinated polymers, the photovoltaic performance of fluorinated polymer is significantly improved due to the increased J sc and enhanced V oc. Based on the indium tin oxide (ITO) coated glass/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)/polymer:phenyl-C61-butyric acid methyl ester/LiF/Al device structure, the optimal device efficiency obtained from the fluorinated copolymer at room temperature under the illumination of AM 1.5 is 3.74%, with a high V oc up to 0.87 V.

Geometry‐Controlled Photoinduced Charge Separation and Recombination in a Trans‐A2B2‐Functionalized Donor–Acceptor Conjugate Composed of a Multimodular Zinc Porphyrin and Fullerene

AbstractThe front cover artwork is provided by the D′Souza group at the Department of Chemistry, University of North Texas. The image shows the optimized structure of a trans‐A2B2‐functionalized multimodular zinc porphyrin/fullerene supramolecular donor–acceptor conjugate and the geometry‐governed photochemical events which occur upon photoexcitation of the porphyrin. Read the full text of the article at 10.1002/cptc.201600017.

High-efficiency perovskite solar cells employing a conjugated donor–acceptor co-polymer as a hole-transporting material

In this work, we have successfully demonstrated that molecularly p-doping of donor–acceptor co-polymer PCPDTBT as an efficient hole-transporting material in perovskite solar cells with a decent power conversion efficiency of 15.1%.

Geometry‐Controlled Photoinduced Charge Separation and Recombination in a Trans‐A2B2‐Functionalized Donor–Acceptor Conjugate Composed of a Multimodular Zinc Porphyrin and Fullerene

AbstractA novel trans‐A2B2‐functionalized multimodular zinc porphyrin (TPA‐BT)2ZnP has been synthesized through a multistep sitedirected synthesis as an electron donor for light‐energy‐harvesting applications. The Zn porphyrin was functionalized by appending two triphenylamine–bithiophene moieties (TPA‐BT) at the trans‐located meso‐positions of the porphyrin ring to improve the electron‐donor capability. Facile oxidation of the Zn porphyrin due to electron‐rich substituents was evidenced by cyclic voltammetry, and the spectrum of oxidized (TPA‐BT)2ZnP was obtained using spectroelectrochemical methods. The donor–acceptor conjugate was subsequently built by coordinating an electron acceptor, C60Im, through metal–ligand axial coordination. The conjugate was characterized by spectral, electrochemical, and computational techniques. The geometry and electronic structure of the (TPA‐BT)2ZnP:ImC60 conjugate, deduced from the B3LYP/6‐31G* method, revealed the frontier HOMO to be delocalized over the (TPA‐BT)2ZnP macrocycle whereas the LUMO was localized on the C60 entity. Free‐energy calculations performed according to Rehm–Weller's approach suggested the possibility of the occurrence of photoinduced electron transfer processes from the singlet excited Zn porphyrin to C60. Efficient photoinduced charge separation in the conjugate was demonstrated from studies involving femtosecond transient absorption techniques. The measured rate of charge separation was slightly higher than previously reported donor–acceptor systems formed from simple Zn porphyrin and C60Im due to facile oxidation of (TPA‐BT)2ZnP. The charge recombination rate was also found to be faster and this has been attributed to the closely located C60 and triphenylamine entities of (TPA‐BT)2ZnP, facilitating through‐space charge recombination process. The present study highlights the importance of the 3D organization of the self‐assembled donor–acceptor conjugate in governing the speed of electron transfer.

The synthesis and purification of amphiphilic conjugated donor–acceptor block copolymers

Nanoscale domains of donor and acceptor materials are crucial for charge generation in organic photovoltaic devices. These domains are difficult to achieve reproducibly in blended materials, but could be engineered into single-material active layers composed of donor–acceptor block copolymers (BCPs). In this work, we report the synthesis and purification of two novel fully conjugated BCPs, P3HT-b-PFTEGTBT and P3HT-b-PFTEGT6BT. To enhance phase separation and self-assembly, these two polymers incorporate tetraethylene glycol side chains into the PFTBT acceptor block, generating an amphiphilic system. The chemical disparity of the donor and acceptor blocks allowed the development of a purification strategy capable of isolating the desired BCP from homopolymer contaminants. This is a significant improvement over previous systems in which homopolymer contaminants can dominate the reaction mixture, affecting performance and morphology. We show that preliminary morphological analysis indicates spontaneous phase separation in thin films, demonstrating the efficacy of this design strategy and the potential application of these materials in organic photovoltaic devices. Hydrophilic tetraethylene glycol moieties are incorporated into the electron-accepting block of a donor–acceptor block copolymer (BCP) to afford two novel amphiphilic materials based on the P3HT-b-PFTBT scaffold. These amphiphilic BCPs are designed to provide enhanced morphological control as organic photovoltaic active layer materials. The chemical disparity introduced by the hydrophilic side chains enabled the design of a purification system capable of isolating the BCP product, an advantage over previously reported systems.

Synthesis and Photoinduced Electron-Transfer Reactions in a La2 @Ih -C80 -Phenoxazine Conjugate.

A newly designed electron donor-acceptor conjugate consisting of an endohedral dimetallofullerene (La2 @Ih -C80 ) and phenoxazine (POZ) was successfully synthesized under Prato conditions. Our results document that the 1,3-dipolar cycloaddition took place across the [5,6] junction to afford exclusively the corresponding [5,6] cycloadduct. The structure of the conjugate was characterized by means of NMR spectroscopy, absorption spectroscopy, and electrochemical studies. Computational calculations suggest that the electron density of the highest occupied molecular orbital (HOMO) is distributed on the POZ moiety, whereas that of the lowest unoccupied molecular orbital (LUMO) is located at the endohedral La atoms, leading to efficient separation of the HOMO and LUMO in the conjugate. Time-resolved absorption spectroscopic investigations and spectroelectrochemical measurements corroborate the formation of the energetically low-lying (La2 @Ih -C80 ).- -(POZ).+ radical-ion-pair state by means of ultrafast through-space electron transfer.

Phthalocyanine-Perylenediimide Cart Wheels.

The electronic features of Zn(II) and Ru(II) phthalocyanines (Pcs) have been modulated by direct peripheral attachment of up to eight ferrocenes. The presence of peripheral ferrocenes noticeably impacts the electronic properties of the corresponding ZnPc and RuPc complexes 7, 12 and 9, 15, respectively-a notion that is supported by optical spectroscopy with bathochromic shifts of up to 8-10 nm per ferrocene unit. Cyclic voltammetry and optical spectroscopy reveal long-distance (10-11 bonds) electronic interaction between ferrocene units. The ZnPc and RuPc complexes have been integrated into a series of orthogonal, supramolecular bis(phthalocyanine)-perylenediimide electron donor-acceptor conjugates, 2a,b and 3a,b. In these cart-wheel-shaped arrays, coordination of ditopic perylenediimide 16, containing two pyridyl substituents at its imido positions, enabled selective interactions with the metal centers of phthalocyanines 7, 12, 9, and 15. The presence of ferrocenes in, for example, Zn complexes 2a and 3a triggers a fast energy transfer from the excited-state PDI to ZnPc. In the RuPc-PDI conjugates, substitution with ferrocenes produces a slight acceleration of the charge separation upon photoexcitation of the PDI chromophore. However, charge recombination is accelerated by 2 orders of magnitude in ferrocene-containing conjugates when compared to that in the analogous tert-butyl-substituted array 1b.

Controlling the Surface Organization of Conjugated Donor-Acceptor Polymers by their Aggregation in Solution.

The aggregation of conjugated polymers is found to have a significant influence on the surface organization of deposited films. Difluorobenzothiadiazole-based polymers show a strong pre-aggregation in solution, but the addition of 1,2,4-trichlorobenzene efficiently reduces such aggregates, leading to the transition of the surface organization from edge- to face-on orientation in deposited films.

Conjugated Donor–Acceptor Polymers Entailing Pechmann Dye-Derived Acceptor with Siloxane-Terminated Side Chains Exhibiting Balanced Ambipolar Semiconducting Behavior

We present four conjugated donor–acceptor (D–A) polymers PBPDT-Si, PBPDSe-Si, PBPDT, and PBPDSe entailing a new electron acceptor Pechmann dye framework, i.e., bipyrrolylidene-2,2′(1H,1′H)-dione (BPD), and thiophene and selenophene as the respective electron donors. PBPDT-Si and PBPDSe-Si contain siloxane-terminated side chains, while PBPDT and PBPDSe bear branching alkyl chains. The respective HOMO energies of PBPDT-Si and PBPDSe-Si are slightly higher than those of PBPDT and PBPDSe, whereas the respective LUMO energies of PBPDT-Si and PBPDSe-Si are slightly lower than those of PBPDT and PBPDSe. The results reveal that (i) thin films of all four polymers show ambipolar semiconducting performances under a nitrogen atmosphere, (ii) hole and electron mobilities are more balanced for PBPDT-Si and PBPDSe-Si, and (iii) the employment of siloxane-terminated side chains is beneficial for improving charge transporting compared to branching alkyl side chains. Thin film hole and electron mobilities of PBPDT-Si can ...

Influence of donor-acceptor conjugation between thiophene-phthalazinone structures containing Sp3 CN bond on the frontier orbital levels and optic-electronic properties

Herein, an electron-deficient phthalazinone unit containing a unique Sp3 hybrid nitrogen atom as an acceptor to cut off the ether bond was presented and three Donor–Acceptor (D–A) conjugated fluoropolymers via Classical Aromatic Nucleophilic Displacement Polymerization (CANDP) method were successfully synthesized. These polymers exhibit excellent thermostability. The 5% weight loss temperature of PDT2TF under nitrogen atmosphere is high, up to 470 °C. This is due to that the D–A conjugation between thiophene-phthalazinone units affects its resonance energy and stabilizes the thiophene-phthalazinone structures. These three fluoropolymers show strong absorptions and fluorescence in visible light region both in solution and thin film states. The band gaps (Eg) of these polymers are narrower than that of their corresponding di-NH capped monomers, owing to the good electronic communication properties of the Sp3 nitrogen atom in the phthalazinone unit. The Eg value of PDT3TF is decreased to 2.03 eV. These results indicate that phthalazinone unit is an efficient acceptor and could exhibit strong D–A effect with thiophene unit. Also, the Sp3 nitrogen atom in the phthalazinone shows good electronic wave function and charge transport properties. This facile CANDP synthetic method combined with Sp3 CN bond linked electron-deficient phthalazinone unit affords polymers with controllable photoelectric properties. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 3470–3483

Synthesis of a Conjugated D-A Polymer with Bi(disilanobithiophene) as a New Donor Component

A new conjugated donor-acceptor (D-A) polymer pDSBT2-BT containing bi(disilano-bisthiophene) and benzothiadiazole as donor and acceptor units, respectively, was prepared. The polymer showed a broad UV-vis absorption band at λmax = 599 nm in chlorobenzene. The absorption band was shifted to λmax = 629 nm when the polymer was measured as a film, indicating enhanced interchain interactions of the polymer. Bulk hetero-junction polymer solar cells (BHJ-PSCs) were fabricated using pDSBT2-BT and PC71BM as host and guest materials, respectively. Optimization of cell fabrication conditions provided a maximal power conversion efficiency of 3.3% and the following cell parameters: Voc = 0.86 V, Jsc = 7.56 mA/cm2, and FF = 0.51. Although the efficiency still leaves much to be desired, these data underscore the potential of pDSBT2-BT as a high-voltage polymer solar cell material.

New bulky side chain substituted benzodithiophene based 2D-conjugated polymers for optoelectronic applications

In order to explore the effect of bulky side chain substituted benzodithiophene (BDT) based polymers on optoelectronic properties, here we have designed and synthesized two new 2D conjugated donor–acceptor (D–A) copolymers P1 and P2 via Stille coupling of 2,3-bis(4-(2-ethylhexyloxy)phenyl)thiophene (BAPT) substituted BDT as (D) unit and 1,3-di(2-bromothien-5-yl)-5-(2-ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione (TPD) or 2,5-ethylhexyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione (DPP) as (A) units. The new polymers P1 and P2 showed broad absorption windows with vibronic shoulders indicate π–π stacking of polymer backbones. In addition P1 and P2 showed deep highest occupied molecular orbital (HOMO) energy levels of −5.50, −5.35 eV, respectively which allow delivering high open-circuit voltages (Voc) in bulk heterojunction polymer solar cells (BHJ PSCs). The donor photon energy loss (Eg–eVoc) of P1 and P2 are 0.87 and 0.57 which is comparable to the previous reports. BHJ PSCs were fabricated with P1 and P2, and they displayed high Voc of 0.99 and 0.78 V, respectively, with maximum power conversion efficiency of 2.05 and 0.96 % in additive free BHJ PSCs. The polymer field effect transistor mobilities of P1 and P2 are 8.0 × 10−3, 9.2 × 10−5 cm2/V s, respectively.

Light induced intramolecular electron and energy transfer events in rigidly linked borondipyrromethene: Corrole Dyad

We have designed and synthesized a photo-induced energy/electron donor–acceptor conjugate comprising of corrole linked to BODIPY at the 5-position via ester linkage. The dyad was characterized by elemental analysis, MALDI-MS, UV-Visible, 1H NMR fluorescence spectroscopy (steady-state and time-resolved) as well as electrochemical methods. A comparison of the UV–visible and 1H NMR spectra of the dyad with those of the corresponding individual model compounds (i.e., BODIPY-CO2H and BPFC-OH) reveal that there exist minimum π–π interactions between BODIPY and corrole π-planes. Quenched emission of BODIPY and corrole part of the dyad has been observed in five different solvents. Excitation spectral data provided evidence for an intramolecular excitation energy transfer (EET) from the singlet BODIPY to the corrole and an intramolecular photoinduced electron transfer (PET) from singlet state of corrole to ground state of BODIPY. Detailed analysis of the data suggests that Forster׳s dipole–dipole mechanism does not adequately explain this energy transfer but, an electron exchange mediated mechanism can, in principle, contribute to the intramolecular EET.

Effect of Spacer Connecting the Secondary Electron Donor Phenothiazine in Subphthalocyanine-Fullerene Conjugates in Promoting Electron Transfer Followed by Hole Shift Process.

Sequential electron/hole transfer between energetically well-positioned entities of photosynthetic reaction center models is one of the commonly employed mechanisms to generate long-lived charge-separated states. A wealth of information, applicable towards light energy harvesting and building optoelectronic devices, has been acquired from such studies. In the present study, we report on the effect of spacer (direct or via phenoxy linkage) connecting the hole shifting agent, phenothiazine (PTZ), on photoinduced charge stabilization in subphthalocyanine-fullerene donor-acceptor conjugates. In these conjugates, the subphthalocyanine (SubPc) and fullerene (C60 ) served as primary electron donor and acceptor, respectively, while the phenothiazine entities act as hole shifting agents. The newly synthesized compounds were characterized by optical absorption and emission, computational, and electrochemical methods. The redox potentials measured using differential pulse voltammetry were used to estimate free-energy changes for charge separation, hole migration, and charge recombination processes. Using femto- and nanosecond transient absorption techniques, evidence for charge separation, and kinetics of charge separation and recombination were obtained in polar benzonitrile and nonpolar toluene solvents. In the conjugate where the phenothiazine entities are directly linked to SubPc, evidence for sequential electron transfer followed by hole shift leading to long-lived charge separated state was weak, primarily due to the delocalization of HOMO on both SubPc and PTZ entities. However, in case of the conjugate where the PTZ and SubPc are linked via phenoxy spacers, sequential electron transfer/hole shift was observed leading to the formation of long-lived charge-separated states. The present study brings out the importance of the spacer group connecting the hole shifting agent in model donor-acceptor conjugates to generate long-lived charge-separated states.

(Invited) Photoinduced Charge Transfer Reactions and Excited State Properties in Triphenylamine C60 Donor-Acceptor Conjugates

The molecular heterojunction triphenylamine/C60 is a potential material to construct high-efficient dye-sensitized solar cells. We have carried out a combined molecular dynamics and time-dependent density functional theory study to analyze charge separation and exciton delocalization in excited states of two constitutional isomers of this interface.[1] Our results reveal the presence of exciton states characterized by partial charge-transfer character (hybrid states); which are suggested to promote the direct charge separation process by excitation. These states may have a significant impact on the efficiency of the light-harvesting ensembles. That is to say, a greater amount of such hybrid states is found at short distances between the triphenylamine fragment and the C60 cage; as a result, charge transfer reactions can occur faster. This trend resembles the experimental evidence since rate constants for photoinduced charge transfer reactions kct are bigger for the constitutional isomer with the shortest distance between the triphenylamine fragment and the C60 cage.[2] We have also evaluated the performance of U-CAM-B3LYP/6-31G* and TD-CAM-B3LYP/6-31G* level of theories to compute the different parameters affecting kct ; that is the reorganization energy λ, the electronic coupling Vij , and free energies ΔG.[3] Our study is done under the theory of nonadiabatic electron transfer. By using the standard Marcus formulation, our results show that the outer-sphere contribution to λ and ΔG for the recombination reaction are the parameters that more uncertainty introduces to the estimation of kct . We implemented a low-cost methodology based on U-DFT for the evaluation of the outer-sphere part of λ and ΔG; wherein Franck-Condon and relaxed charge-transfer states are conveniently modeled, including properly the solvent reorganization energy, by U-CAM-B3LYP/6-31G* coupled to continuum solvation models. Finally, it is worth to mention that the replacement of the C60 cage by Ih -Sc3N@C80 generates longer-lived charge-transfer states; which is attributed to the lower ΔG of the charge recombination reaction.[2] Our group is currently analyzing this trend for the whole family of endohedral metallofullerenes; for which dynamics is expected to be in the normal region of the Marcus parabola. [1] J. P. Martínez, S. Osuna, M. Solà, A. Voityuk, Theor. Chem. Acc. 2015, 134, 12. [2] J. R. Pinzón, D. C. Gasca, S. G. Sankaranarayanan, G. Bottari, T. Torres, D. M. Guldi, L. Echegoyen, J. Am. Chem. Soc. 2009, 131, 7727–7734. [3] J. P. Martínez, M. Solà, A. Voityuk, 2015, in preparation. Figure 1

(Invited) Long-Lived Charge Separated States Exclusively from the Triplet Excited States in Palladium Porphyrin-Fullerene Conjugates

Molecular engineering of donor-acceptor conjugates to control their photoinduced processes to yield the much desired long-lived charge separated states has been one of the major goals in artificial photosynthesis. The charge separated states of sufficient lifetime and stored energy are key in ultimately driving the photocatalytic processes of either light-to-electricity conversion or light-to-fuel production. In simple donor-acceptor dyads, charge separation from the singlet excited donor or acceptor due to rapid charge recombination process results in a short-lived charge separated state of singlet spin character. Utilization of high energy triplet sensitizers instead of the traditionally employed singlet sensitizers could be an elegant approach to generate long-lived charge separated states. The radical ion-pairs of triplet spin character charge recombine to the ground state slowly due to spin forbidden nature of the process. However, witnessing such a process purely from the triplet excited state without the interference of the singlet excited state and other competing photochemical events such as excitation energy transfer has been challenging to establish. Here, we report our strategy to observe electron transfer exclusively from the triplet excited state of the donor in PdP-C60 dyads (see figure below). Using transient absorption spectral studies operating at different timescales and time-resolved electron paramagnetic resonance (TR-EPR) studies, photoinduced electron transfer from 1PdP* to C60 leading to the formation of PdP•+-C60 •- charge separated state of singlet spin character has been witnessed in 2 (instead of triplet-triplet energy transfer observed in 1 1) due to the close proximity of the donor and acceptor entities. In contrast, by lowering the energy of the [(TPA)3PdP]•+-C60 •- state owing to the presence of electron rich TPA entities (facile oxidation) compared to that of 3C60*, electron transfer from 3(TPA)3PdP* to C60 leading to [(TPA)3PdP]•+-C60 •- charge separated state of triplet spin character with lifetime in the microsecond range has been observed in 3 in polar solvents. Results of these findings and its significance will be highlighted. C. Obondi, G. N. Lim, F. D’Souza, Triplet-Triplet Excitation Transfer in Palladium Porphyrin – Fullerene and Platinum Porphyrin – Fullerene Dyads’ J. Phys. Chem. C 2015, 119, 176-185. Figure 1

Theoretical estimation of the rate of photoinduced charge transfer reactions in triphenylamine C60 donor-acceptor conjugate.

Fullerene-based molecular heterojunctions such as the [6,6]-pyrrolidine-C60 donor-acceptor conjugate containing triphenylamine (TPA) are potential materials for high-efficient dye-sensitized solar cells. In this work, we estimate the rate constants for the photoinduced charge separation and charge recombination processes in TPA-C60 using the unrestricted and time-dependent DFT methods. Different schemes are applied to evaluate excited state properties and electron transfer parameters (reorganization energies, electronic couplings, and Gibbs energies). The use of open-shell singlet or triplet states, several density functionals, and continuum solvation models is discussed. Strengths and limitations of the computational approaches are highlighted. The present benchmark study provides an overview of the expected performance of DFT-based methodologies in the description of photoinduced charge transfer reactions in fullerene heterojunctions. © 2016 Wiley Periodicals, Inc.