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

The excited state properties of a series of $\mathrm{\alpha ,\omega -}\mathrm{dimethyl-oligo}\text{{2,5-}\mathrm{bis}\text{[2-(S)-}\mathrm{methylbutoxy}\mathrm{]-p-}\mathrm{phenylene\; vinylene}\mathrm{\}s}$ $(\mathrm{OPV}n\mathrm{s},$ with $n$ the number phenyl rings) are investigated for $\text{n=2–7}$ 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 ${\mathrm{(S}}_1{\mathrm{\leftarrow S}}_0)$ and triplet ${\mathrm{(T}}_{\mathrm{n}}{\mathrm{\leftarrow T}}_1)$ transition energies decrease with conjugation length. For the $S_1$ state of $\mathrm{OPV}n$ 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 $\text{(⩽0.04\hspace{0.05em}}\mathrm{eV}\mathrm{).}$ The Huang–Rhys parameter and relaxation energies in the excited state associated with $S_1{\mathrm{\leftarrow S}}_0$ and $T_{\mathrm{n}}{\mathrm{\leftarrow T}}_1$ 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 $S_1$ with respect to $S_0$ $\text{(0.165±0.02\hspace{0.05em}}\mathrm{eV})$ is considerably larger than for $T_{\mathrm{n}}$ with respect to $T_1$ (0.03 eV), but the latter increases for the shorter oligomers, in contrast to the relaxation energy of $S_1.$ The $\mathrm{OPV}n\mathrm{s}$ $\text{(n⩾5)}$ 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.