CHEMISTRY OF ORGANOSILICON COMPOUNDS .266. DUAL FLUORESCENCE OF ARYLDISILANES AND RELATED-COMPOUNDS - EVIDENCE FOR THE FORMATION OF 1(SIGMA-PI-STAR) ORTHOGONAL INTRAMOLECULAR CHARGE-TRANSFER STATES

Fluorescence and absorption spectra of 14 arylpentamethyldisilanes and related compounds are recorded. Several compounds reveal dual fluorescence corresponding to two singlet excited states. The high polarity of the lowest excited singlet states with charge-transfer nature is evidenced by the solvent shifts. Structural effects clearly demonstrate that Si-Si (and Si-Ge) bonds and aryl groups function as electron donors and acceptors, respectively. The excited-state dipole moment of PhSiMe2SiMe3 is found to be 4.3 D in good agreement with a calculated value of 3.8 D derived by an assumption of planar arrangement of the molecular frame, in which σ and π orbitals are orthogonal to each other, with full charge separation. The orthogonality of σ and π orbitals in the excited states is proved by the stereoelectronic effects on the fluorescence. The circumstance found for the present σπ system is similar to twisted intramolecular charge-transfer (TICT) states of nπ and sudden polarization in the excited singlet of ππ systems, and orthogonal intramolecular charge transfer (OICT) is proposed as a general term. © 1990 American Chemical Society.