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

The excited state properties of a series of a,v-dimethyl-oligo$2,5-bis@2-(S)-methylbutoxy#- p-phenylene vinylene%s (OPVns, with n the number phenyl rings! are investigated for n52–7 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 (S1ÃS0) and triplet (TnÃT1) 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 (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