Photochemistry and Photophysics of Cholesta-5,7,9(11)-trien-3ß-ol in Ethanol.

Cholesta-5,7,9(11)-trien-3ß-ol (9,11-dehydroprovitamin D3, CTL) is used as a fluorescent probe to track the presence and migration of cholesterol in vivo. We recently described the photochemistry and photophysics of CTL in degassed and air-saturated tetrahydrofuran (THF) solution, an aprotic solvent. The zwitterionic nature of the singlet excited state, 1CTL* is revealed in ethanol, a protic solvent. In ethanol, the products observed in THF are accompanied by ether photoadducts and by photoreduction of the triene moiety to four dienes, including provitamin D3. The major diene retains the conjugated s-trans-diene chromophore and the minor is unconjugated, involving 1,4-addition of H at the 7 and 11 positions. In the presence of air, peroxide formation is a major reaction channel as in THF. X-ray crystallography confirmed the identification of two of the new diene products as well as of a peroxide rearrangement product.

α-Methylstilbene Isomers: Relationship of Structure to Photophysics and Photochemistry.

Significant differences in the photochemical and photophysical behavior of trans-α-methylstilbene and trans-stilbene have been attributed to structural changes caused by the steric requirements of the methyl group. We present here the X-ray structures of cis- and trans-α-methylstilbene (c- and t-MeSt). This is the first X-ray structure of a cis-stilbene. Despite the pronounced departure from phenyl group coplanarity, the solid-state packing of t-MeSt resembles that of trans-stilbene in that both exhibit disorder with a bicycle pedal structural relationship, dynamic in t-St but static in t-MeSt. We compare the X-ray structures with calculated structures. We also compare our steady state and transient photochemical and spectroscopic results with predictions in a recent theoretical paper that anticipated some of our experiments. Deviations from planarity imposed by the methyl substitution account for the shorter lifetimes of the trans excited states. The rapid torsional relaxation of 1t-MeSt* to the twisted intermediate 1p*, ktp = 2.9 × 1012 s-1, observed using fs transient absorption spectroscopy, explains the sharp decrease in the fluorescence quantum yield of t-MeSt. We correct misconceptions that have appeared in the literature concerning the shape of the stilbene potential energy surface in S1. The nonplanarity due to methyl substitution leads to chirality issues that are relevant in biological molecules such as the protonated Schiff bases of retinal in the opsins.

Ultrafast study of 9-diazofluorene: direct observation of the first two singlet states of fluorenylidene.

Ultrafast photolysis of 9-diazofluorene (DAF) produces a broadly absorbing transient within the instrument time resolution (300 fs), which is assigned to an excited state of the diazo compound. The diazo excited state fragments to form fluorenylidene (Fl) in both its lowest energy singlet state (1Fl, 405-430 nm, depending on the solvent) and a higher energy singlet state (370 nm, 1Fl*). The excited singlet carbene has a lifetime of 20.9 ps in acetonitrile and decays to the lower energy singlet state (1Fl), which relaxes to the triplet ground state (3Fl) in acetonitrile, cyclohexane, benzene, and hexafluorobenzene. The equilibrium mixture of singlet and triplet fluorenylidene reacts with these solvents. Singlet fluorenylidene reacts with methanol and cyclohexene in competition with relaxation to 3Fl. One of the reaction products in methanol is the 9-fluorenyl cation. The rate of intersystem crossing (ISC) in hexafluorobenzene and other halogenated solvents is remarkably slow given that carbene ISC rates are generally fastest in nonpolar solvents. An explanation of this effect is advanced.