Electronic absorption spectra and geometry of (E)-stilbene and ‘stiff’ stilbenes

Abstract

Examination of the electronic absorption spectra of (E)-stilbene (1), indeno[2,1-a]indene (2), 5,6,11,12-tetrahydrochrysene (3), (E)-1,1′-bi-indanylidene (4), and (E)-1,1′-bitetralinylidene (5) in fluid solution at room temperature and in glassy solution at 77 K led to the following conclusions. (a) The torsion angles of the C–Ph bonds in (1) deviate from 0° to some extent in the fluid solution and are nearly 0° in the glassy solution. (b) The (E)-stilbene skeleton in (2)—(4) is planar or nearly planar in the fluid as well as in the glassy solution. (c) The torsion angles of the C–Ph bonds in (5) substantially deviate from 0° in the fluid solution and become smaller in the glassy solution. However, they still deviate from 0° to a considerable extent. The observed spectral change is explained by the use of the Frank–Condon principle.