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 k_ct 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 k_ct; that is the reorganization energy λ, the electronic coupling V_ij, 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 k_ct. 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 I_h-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.