Intramolecular Photoinduced Charge Transfer and Recombination Dynamics in Vinylarene Terminated Organosilanes

Abstract

We report on charge-transfer dynamics of newly designed acceptor-donor-acceptor organosilanes, with a specific focus on how donor-acceptor combination and local chemical environment can be used to control the lifetime for intramolecular charge-separation between silane electron donors and organic acceptors. In this work linear oligosilanes were capped with arene-vinyl end groups of variable electron-accepting strength: weak (diester vinyl), intermediate (ester,cyano vinyl), and strong (dicyanovinyl). Ultrafast transient absorption spectroscopy was used to characterize their structure-dependent charge-transfer and recombination behaviors. All structures exhibit similar photoinduced ultrafast spectral dynamics that we ascribe to relaxation of the nascent charge-separated excited state followed by a return to the ground state via charge recombination. We find that relaxation of the nascent "hot" charge-separated excited state scales with the strength of dipole-dipole interactions between solvent molecules and the polar arene-vinyl acceptor. Furthermore, electron-accepting strength governs whether electronic coupling dictates charge recombination rate: weak acceptors produce charge-separated states that exhibit relatively large electronic coupling for back-electron transfer (approaching the adiabatic limit) that result in fast recombination, whereas the strong and moderate-strength acceptors support more stable charge-separated states with weaker coupling and longer lifetimes. We find that recombination rates increase substantially for structures with weak and moderate-strength acceptors in cyclohexane (i.e., negligible solvent reorganization energy), which we attribute to an increased electronic coupling in a nonpolar solvent environment where charge pairs are weakly screened. In contrast, for structures with strong electron acceptors, the very low reorganization energy of cyclohexane places back-electron transfer even further into the Marcus inverted regime, with a resultant increase in charge-separation lifetime. Together these results provide critical insights on how to tune photoinduced charge-transfer behavior in organic-inorganic hybrids that have potential material applications in molecular electronics and optoelectronics.