The 2pπ*–3dπ interaction in aromatic silanes. Fluorescence from the 1(2pπ, 3dπ) intramolecular charge-transfer state

Abstract

The 2pπ*–3dπ interaction in the excited state of aromatic silanes has been studied by means of absorption and emission spectroscopy. Broad and structureless fluorescence spectra of phenyldisilanes and naphthyldisilanes with large Stokes shifts have been observed and they have been attributed to the emissions from the intramolecular charge-transfer (c.t.) states with large dipole moments. Evidence that the c.t. emission originates from the 1(2pπ, 3dπ) state produced by the 2pπ*(aromatic ring)→ 3dπ(Si—Si bond) intramolecular charge transfer is given by the effect of steric twisting on the emission. It is shown that the fast formation (⩽ 1 ns) of the c.t. state from the locally excited state 1(π, π*)1B2(or 1Lb) of phenyldisilanes takes place, followed by rapid decay (⩽ 1 ns) of the intersystem crossing 1(2pπ, 3dπ)→3(π, π*). However, no c.t. emission has been observed for aromatic monosilanes and polycyclic aromatic disilanes (aromatic rings 3) even in fluid polar solvents. The 2pπ*→3dπ intramolecular c.t. mechanism is discussed in comparison with that of the twisted intramolecular charge-transfer (t.i.c.t.) state.