Singlet and triplet excitations of chiral dialkoxy-p-phenylene vinylene oligomers

Abstract

The excited state properties of a series of α,ω-dimethyl-oligo{2,5-bis[2-(S)-methylbutoxy]-p-phenylene vinylene}s (OPVns, with n the number phenyl rings) are investigated for n=2–7 in solution at ambient temperature, under matrix-isolated conditions at low temperature, and as nanoaggregates using absorption (time-resolved), photoluminescence, photoinduced absorption, circular dichroism, and circular polarized luminescence spectroscopies. The singlet (S1←S0) and triplet (Tn←T1) transition energies decrease with conjugation length. For the S1 state of OPVn the lifetime strongly decreases with chain length due to enhanced nonradiative decay and radiative decay. The increase in the nonradiative decay rate constant is much more pronounced, and as a result the photoluminescence quantum yield is less for longer oligomers. Studies at low temperature afforded spectra with well-resolved vibronic fine structure. Under these conditions the Stokes’ shift is very small (⩽0.04 eV). The Huang–Rhys parameter and relaxation energies in the excited state associated with S1←S0 and Tn←T1 transitions and their evolution with conjugation length have been determined via a Franck–Condon analysis of the vibronic fine structure. For OPV7, the relaxation energy of S1 with respect to S0 (0.165±0.02 eV) is considerably larger than for Tn with respect to T1 (0.03 eV), but the latter increases for the shorter oligomers, in contrast to the relaxation energy of S1. The OPVns (n⩾5) form nanoaggregates in polar solvents such as ethanol. The changes in absorption and emission together with a strong circular dichroism and circularly polarized emission indicate the formation of chiral H-type aggregates under these conditions.