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.

Photochemistry and photophysics of cholesta-5,7,9(11)-trien-3ß-ol: a fluorescent analogue of cholesterol.

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. CTL is known to be photochemically active, but little consideration has been given to the formation efficiency and possible toxicity of its photoproducts. In degassed tetrahydrofuran (THF) solution, we isolated the photoproduct of CTL and of its 25-hydroxy derivative (HOCTL), and X-ray crystal structures were obtained for HOCTL and the photorearrangement product. The X-ray crystal structure and its 1H NMR spectrum confirm the product structure as a pentacyclic HOCTL isomer. In the presence of air in THF, endoperoxide formation via [2+4] addition of 1O2* across the B ring of CTL or HOCTL becomes the dominant photoreaction. The UV spectrum and decay kinetics of the triplet state of HOCTL, the precursor of 1O2*, are determined by transient absorption spectroscopy. We confirm the proposed structure of the endoperoxide by X-ray crystallography. Kinetics analysis of quantum yields provides rate constants for photophysical and photochemical events.

Generation of Long-Lived Photoinduced Charge Separation in a Supramolecular Toroidal Assembly.

Efficiencies of artificial photosynthetic and photocatalytic systems depend on their ability to generate long-lived charge-separated (CS) states in photoinduced electron transfer (PET) reactions. PET, in most cases, is followed by an ultrafast back electron transfer, which severely reduces lifetime and quantum yield of CS states. Generation of a long-lived CS state is an important goal in the study of PET reactions. Herein, we report that this goal is achieved using a hierarchically self-assembled anthracene-methyl viologen donor-acceptor system. Anthracene linked to two ß-cyclodextrin molecules (CD-AN-CD) and methyl viologen linked to two adamantane units (AD-MV2+-AD) form an inclusion complex in water, which further self-assembled into well-defined toroidal nanostructures. The fluorescence of anthracene is highly quenched in the self-assembled system because of PET from anthracene to methyl viologen. Irradiation of the aqueous toroidal solution led to formation of a long-lived CS state. Rational mechanisms for the formation of the toroidal nanostructures and long-lived photoinduced charge separation are presented in the paper.

Translational Diffusion and Unstable Conformer Trapping in Glassy Isopentane at 77 K.

Diffusional quenching in isopentane (IP) glass at 77 K is demonstrated by the reduction of triphenylene phosphorescence lifetimes in the presence of 1,3-pentadiene and/or molecular oxygen. Fluorescence spectra and lifetimes of cis- and trans-1,2-di(1-methyl-2-naphthyl)ethene in IP glass at 77 K reveal that the cis → trans photoisomerization leads to the trapping of unstable conformers of the trans isomer. The claim that IP at 77 K is not sufficiently viscous to trap unstable photoproduct conformers is invalidated.