Relaxation processes in the excited state of phenylsiloxanes
The relaxation processes in the excited state of phenylsiloxanes and related compounds have been studied by measurements of fluorescence decay, quantum yields, time-resolved fluorescence spectra and intersystem-crossing yields. The absorption and emission spectra of the phenylsiloxanes used correspond to the π,π* transition (1B2u–1A1g) in benzene. The local (1Lb) emissions for the silicon compounds used are very weak: the quantum yields are less than 0.033. For the compounds 2–4 with phenyl groups, very weak intramolecular excimer emissions (ΦfE < 0.017) are observed together with weak local (1Lb) emissions (ΦfM < 0.025) at 298 K. The intersystem-crossing quantum yields of the siloxane compounds obtained by the thermal lensing method are very small (ΦISC < 0.1) at 298 K. The fluorescence decay kinetics of the compounds 1–4, and 6 and 7 with an Si—O bond give double-exponential decay functions. The decay rates of the siloxane compounds are significantly faster than those of the carbon analogues. The siloxane compounds are photochemically stable (the reaction quantum yields at 254 nm are less than 1 × 10–3). On the basis of these results, it can be said that the main channel of the deactivation processes in the excited singlet states (local and excimer states) at room temperature is internal conversion to the ground state. The fast decay rate (i.e. the rate of internal conversion) of the siloxane compounds may be due to the perturbation of a pseudo-π system of the Si—O bond to the S1(1Lb) state.