Resolving the radical cation formation from the lowest-excited singlet (S1) state of terthiophene in a TiO2–SiO2 hybrid polymer matrix

Photoexcitation dynamics of terthiophene (3T) molecules incorporated in an organic–inorganic matrix containing titanium in the network are investigated by polarized transient absorption with femtosecond time resolution. We observed the decay of excited-state absorption of neutral 3T and simultaneous rise of 3T+⋅ radical cation absorption. The observed kinetics of electron transfer are independent of excess vibrational energy in the S1 state, and can be described by a biexponential function with time constants of ∼1 ps (for ∼62% of the excited 3T molecules) and ∼8 ps (for ∼33%). The angle between the S1←S0 absorption dipole moment of neutral 3T and the absorption dipole moment of the 3T+⋅ radical cation is estimated to be ∼30°. About one-half of the generated charge pairs do not recombine within 0.5 ns, which is an attractive feature for application in photovoltaic devices.