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The Effect of Structural Conformational Changes on Charge Transfer States in a Light-Harvesting Carotenoid-diaryl-Porphyrin-C60 Molecular Triad

We present a detailed study of charge transfer (CT) excited states for a large number of structural conformations in a light-harvesting Carotenoid-diaryl-Porphyrin-C60 (CPC60) molecular triad. The molecular triad undergoes a photoinduced charge transfer process exhibiting a large excited state dipole moment, making it suitable for application to molecular-scale opto-electronic devices. An important consideration is that the conformational flexibility of the CPC 60 triad impacts its dynamics in solvents. Since experimentally measured dipole moments for the triad of ~110 Debye (D) and ~160 Debye strongly indicate a range in conformational variability for the triad in the excited state, studying the effect of conformational changes on the CT excited state energetics furthers the understanding of its charge transfer states. We have calculated the lowest CT excited state energies for a series of 14 triad conformers, where the structural conformations were generated by incrementally scanning a 360 degree torsional (dihedral) twist at the C60-porhyrin linkage and the porphyrin-carotenoid linkage. Additionally, five different CPC60 conformations were studied to determine the effect of pi-conjugation and particle-hole Coulombic attraction on the CT excitation energies. Our calculations show that structural conformational changes in the triad show a variation of ~0.4 eV in CT excited state energies in the gas-phase. The corresponding calculated excited state dipoles show a range of 88 D - 188 D.