Experimental and theoretical studies on the intramolecular charge-transferemission of phenyldisilanes

Fluorescence and its polarization spectra of phenylpentamethyldisilane (1) and its para-substituted derivatives (para-substituent: -C(CH3)(3) (2), -OC H3 (3), and -N(CH3)(2) (4)) were measured in a poly(vinyl alcohol) film at 77 K. Intramolecular charge transfer (ICT) fluorescence together with local ly excited (LE) emission was found even for the compounds having an electro n-donating substituent at the para-position (2 and 3) as well as that of 1. The ICT state of these compounds showed an in-plane long axis polarization . By the measurements of fluorescence lifetimes, it was found that there wa s no dynamic equilibrium between the ICT and LE states, indicating that the ICT state originates from the nonrelaxed excited singlet (S-n(7)) state. A b initio MO calculations (CASSCF and MRMP methods) were performed for the g round and excited states of phenyldisilane with planar and perpendicular co nformations. The following results were obtained: (I) upon excitation, the ICT of phenyldisilane occurs from the phenyl moiety to the disilanyl group regardless of planar or perpendicular conformation, (2) in the course of th e ICT formation, conformational changes in the Si-dimethyl group adjacent t o the phenyl group play an important role rather than a twisting motion of the disilanyl group, and (3) the ICT state consists of the pi,pi'* state pr oduced by ICT from the phenyl moiety to the disilanyl group, where pi'* den otes the excited pseudo-pi orbital.