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 (C2h-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 τrotamer ≈ 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 E1act toward perpendicular configuration P both from cis and trans configurations. In tN, the radiative relaxation with τR ≈ 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.