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

• Published in: The journal of physical chemistry. B Vol. 124; no. 6; pp. 1049 - 1064
• Main Authors: Quick, M. T, Quick, M, Ioffe, I. N, Richter, C, Mahrwald, R, Druzhinin, S, Kovalenko, S. A
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 13-02-2020
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/acs.jpcb.9b10710

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 S 0 (rotamerization) is observed in the bleach region with characteristic time τrotamer ≈ 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 (∼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 1act 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 c S 1 → P → t S 1 (20%) and more common nonadiabatic c S 1 → P → S 0 (80%). The barrier c S 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.