Photophysical and Photochemical Processes of 9,10-Dihydro-9-silaphenanthrene Derivatives:  Photochemical Formation and Electronic Structure of 9-Silaphenanthrenes

Abstract

Photophysical and photochemical processes of 9-methyl- and 9-phenyl-9,10-dihydro-9-silaphenanthrene derivatives have been studied at room temperature and 77 K in comparison with the carbon analogue, 9,10-dihydrophenanthrene. These 9,10-dihydro-9-silaphenanthrene derivatives show smaller fluorescence quantum yield and remarkably larger Stokes shifts than those of the carbon analogue. In contrast, their phosphorescence quantum yields are two times larger than those of the carbon analogue, although the absolute value is not so large (approximately 0.1). Reaction products and intermediates produced by the 266 nm light photolysis have been studied, and it has been confirmed that 9-methyl- and 9-phenyl-9-silaphenanthrenes have been photochemically formed in methylcyclohexane at 77 K, in addition to the formation of radical cations of 9,10-dihydro-9-silaphenanthrene derivatives and the carbon-centered radical: 9-hydro-9-silaphenanthrenyl radical.

Performance of Quasi-Degenerate Scaled Opposite Spin Perturbation Corrections to Single Excitation Configuration Interaction for Excited State Structures and Excitation Energies with Application to the Stokes Shift of 9-Methyl-9,10-dihydro-9-silaphenanthrene

“…Quite understandably from the large Stokes shift, the molecule is highly distorted at S 1 equilibrium compared to the S 0 structure (See Figure and Table for the detailed depictions of the geometries), and the oscillator strength of the emission at S 1 equilibrium is as large as 0.200. This is consistent with the relatively high quantum yield (28%) for emission from this molecule at 77 K . Also from Figure , one can predict that an S 0 −S 1 conical intersection will eventually take place at large C−Si separation.…”

“…In addition, because S 1 and S 2 are so close in energy, with ∼0.1 eV gap for a wide range of values of the C−Si bond length, the two absorptions will not be separately resolved with the broad spectrum obtained from experiment . In fact, at the ground state equilibrium geometry, the oscillator strength of S 0 → S 2 transition is much larger than that of S 0 → S 1 transition (0.017 versus 0.001), and therefore, presuming vibronic mixing is small, it is likely that the measured spectrum actually corresponds to the S 0 → S 2 transition.…”

“…Accordingly, nonradiative deactivation from S 1 will be promoted significantly even at moderate temperature. In fact, the experimental quantum yield for emission was found to be only 3% at room temperature …”

Synthesis and Properties of a Kinetically Stabilized 9-Silaphenanthrene

“…The UV/vis spectrum of 1a in hexane [λ max = 230 (ε 1 × 10 5 ), 262 (6 × 10 4 ), 288 (4 × 10 4 ), 304 (2 × 10 4 ), 366 (2 × 10 4 ), 376 (2 × 10 4 ), 404 nm (1 × 10 4 )] is shown in Figure …”

Computational Investigation of Brook-Type Silabenzenes and Their Possible Formation through [1,3]-Si→O Silyl Shifts

“…In the late 1970s and early 1980s, the groups of Barton and West initiated research on the formation and trapping of the parent silabenzene as well as substituted silabenzenes and disilabenzenes. , A range of different thermal and photolytic methods to generate and isolate silabenzenes were subsequently developed. − For example, Märkl and Schlosser synthesized a silabenzene, 2,6-bis(trimethylsilyl)-1,4-di- tert -butylsilabenzene, which was stable up to −100 °C in a 4:1:1 THF/Et 2 O/petroleum ether solvent mixture…”