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" chargeseparated 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.

Highly selective addition of cyclosilanes to alkynes enabling new conjugated materials

“…This lack of synthetic control limits the discovery of structure-property relationships in main group materials. For example, while linear oligosilanes (e.g., Ar(SiMe) n Ar) capped with aromatic end groups have been extensively studied for molecular electronics, 1,2 metal-organic framework (MOF) secondary building units, 3 charge transport, 4 photoinduced charge transfer, [5][6][7] and nonlinear optical properties, [8][9][10] there has been very limited investigation of cyclic s,p-conjugation, 11 as cyclosilanes are typically synthesized by methods [12][13][14] that restrict functional group diversity on an organic fragment. Nonetheless, the wellestablished conformation-dependence of s-conjugation 15 strongly suggests that the restricted degrees of freedom in cyclosilanes should give rise to properties distinct from linear oligosilanes.…”

Organic Donor‐Acceptor Complexes As Potential Semiconducting Materials

“…N,N′‐Diphenyl‐N,N′‐di‐ p ‐tolylbenzene‐1,4‐diamine (MPDA), an electron donor and hole transport material, was complexed with F 4 ‐TCNQ in different solvents to investigate the impact of chemical environment on their equilibrium constants [134] . ( 66 , Figure 15) Steady‐state and temperature‐dependent spectral studies revealed that the complex formation is enthalpically and entropically favoured in higher permittivity solvents like 1,2‐dichloroethane.…”

Disilane-bridged architectures: an emerging class of molecular materials

“…Recently, a new series of ADA organosilanes 18–19 has been developed. 67 In these linear oligosilanes, disilane and tetrasilane chains were used to control the electron-donating ability; meanwhile, terminal groups with variable electron-accepting strength were employed. All the studied compounds have similar photoinduced ultrafast spectral dynamics, which involve the relaxation of the excited state with CS, and subsequent return to the ground state through charge recombination (CR).…”