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 (C-1), and tN-S' (C-2) are derived with a 53:47 ratio of tN-S to tN-A. Upon photoexcitation, the equilibration of the rotamers in So (rotamerization) is observed in the bleach region with characteristic time tau(rotamer) approximate to 0.5 ns. With excitation at 364 nm, the S-0 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 (similar to 2 ns) S-1 that allows for monitoring the rotamer dynamics in S-0. Replacement of the stilbene phenyl rings with larger naphthyls increases the S-1 -> P torsional barrier E-lact toward perpendicular configuration P both from cis and trans configurations. In tN, the radiative relaxation with tau(R) approximate to 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 cS(1) -> P -> S-1 (20%) and more common nonadiabatic cS(1) -> P -> S-0 (80%). The barrier cS(1) -> 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.