Four-Component Fluorescence of trans-1,2-Di(1-methyl-2-naphthyl)ethene at 77 K in Glassy Media. Conformational Subtleties Revealed

Transient Rotamerism and Photoisomerization Dynamics of trans- and cis-Naphthylstilbene

The rotamerism and photoisomerization of trans- and cis-1,2-di-(2-naphthyl)ethylene (tN and cN) are studied with stationary and transient absorption spectroscopies assisted by quantum chemical calculations. Absorption and emission spectra of rotamers (rotational isomers) tN-S (C_2h-symmetric), tN-A (C₁), and tN-S' (C₂) are derived with a 53:47 ratio of tN-S to tN-A. Upon photoexcitation, the equilibration of the rotamers in S₀ (rotamerization) is observed in the bleach region with characteristic time τ_rotamer ≈ 0.5 ns. With excitation at 364 nm, the S₀ equilibrium shifts because, mainly, tN-A is bleached and the rotamerization becomes traceable, whereas with excitation at 345 nm, the equilibrium is preserved and the bleach spectrum remains unchanged. It is just long-lived (∼2 ns) S₁ that allows for monitoring the rotamer dynamics in S₀. Replacement of the stilbene phenyl rings with larger naphthyls increases the S₁ → P torsional barrier E_1act toward perpendicular configuration P both from cis and trans configurations. In tN, the radiative relaxation with τ_R ≈ 3.7 ns becomes the main deactivation channel, and accordingly, the measured decays show nearly no dependence on the solvent viscosity. The cis-to-trans photoisomerization occurs via two paths: adiabatic cS₁ → P → tS₁ (20%) and more common nonadiabatic cS₁ → P → S₀ (80%). The barrier cS₁ → P in the cis-isomer is reduced in polar solvents because of a zwitterionic character of P. The P-state is directly detected with the cN isomer in acetonitrile by an excited-state absorption band at 400 nm developing with 0.7 ps and decaying with 1.6 ps. Two dihydrophenanthrene (DHP)-like products result from photoexcited cN. The metastable DHP-A builds up transiently from cN-A, and its spectrum at about 550 nm matches the published DHP absorption. The stable DHP-S' accumulates under stationary illumination and is formed either from excited cN-S' or metastable DHP-A.

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.

Photochemistry and Photophysics of the 3-Styrylidenebenz[e]indanes

The photochemical and photophysical properties of the extended conformers of trans- and cis-1-(2-naphthyl)-2-phenylethenes (t- and c-NPE_B ) are strikingly different than those of their rigid analogues, trans- and cis-3-styrylidenebenz[e]indane (t- and c-BPE). The fluorescence quantum yield and lifetime at 25°C in methylcyclohexane drop from 0.76 and 22.2 ns in t-NPE_B to 0.0051 and 0.25 ns in t-BPE, and there are complementary changes in the photoisomerization quantum yields. In both cases, photoisomerization occurs in S₂ , a stilbenic excited state. The differences in behavior are traced to the diminished S₁ /S₂ energy gap on the alkyl substitution afforded by the five-membered ring in BPE. The effect of viscosity on the torsional relaxation of t-BPE, evaluated in glycerol/methanol mixtures at 25°C and in 95/5 and 99.9/0.1 glycerol/methanol (% v/v) as a function of T, is well accounted for by the medium-enhanced barrier model. Absorption and fluorescence spectra of t-BPE in pure glycerol reveal aggregate formation. Quenching of t-BPE fluorescence in methylcyclohexane by tri-n-butylamine (TBA) leads to exciplex fluorescence. Comparison with analogous results for t-NPE_B suggests that charge transfer involves primarily the naphthalenic S₁ state in t-NPE_B and the stilbenic S₂ state in t-BPE.