The fluorescence of cyclotetrasilanes with an extremely large Stokes shift (up to 13700 cm-l) has been observed in rigid matrices at 77 K. The large Stokes shift is interpreted in terms of variation in potential energy for the S, and So states as a function of the ring Si-Si bond distance. Fluorescence properties are dependent on the molecular structures (planar and bent forms) of the compounds ; the fluorescence quantum yield, lifetime, and Stokes shift for the former are greater than those for the latter.

Photolysis of cyclotetrasilanes has been carried out by laser flash photolysis in methylcyclohexane (MCH) at 293 K and steady-state photolysis in methylcyclohexane-isopentane (MP; 3: 1, v/v) glass at 77 K. The photolysis patterns are remarkably dependent on the molecular structures of cyclotetrasilanes. The planar persilylcyclotetrasilanes 1 and 2 give rise to the corresponding disilenes (D1 and D2) with absorption peaks at 410 nm at 293 K and at 415 nm at 77 K. For the peralkylcyclotetrasilanes 3 and 4 having bent structures, photolysis occurs to give the corresponding silylene and cyclotrisilane at 293 K, but not in MP glass at 77 K. These photolyses originate from the excited singlet state (Sl), judging from a rapid build-up of the transient absorption after pulsing. Transient peaks for diisopropylsilylene (S3) and t-butylmethylsilylene (S4) are observed at 530 and 515 nm, respectively, with a lifetime of 38 or 50 ns. The photochemical reactions of cyclotetrasilanes are discussed from the viewpoint of their molecular structures Little attention has been paid to the photochemistry and photophysics of organosilicon compounds until recent1y.l-lo However, the excited-state chemistry of aromatic disilanes has been extensively studied. '-' It has been found that the intramolecular charge-transfer