Cyclic trans-stilbenes: Synthesis, structural and spectroscopic characterization, photophysical and photochemical properties

Trans-Stilbene and several cyclic derivatives with hindered free rotation around the C(vinyl)–C(phenyl) single bond were studied by various spectroscopic techniques. Those derivatives which contain 6- or 7-membered aliphatic rings do not exhibit any measurable S₁ → S₀ fluorescence. The introduction of two methoxy groups into the 6-membered aliphatic ring derivative accelerates its photoreactivity to such an extent that fluorescence or resonance Raman spectroscopy investigations become impossible. The introduction of aliphatic rings has only a little effect on the frequency of the phenyl-ring stretching vibrations, but pronounced ones on the IR and Raman intensities. The frequency of the vinylic stretching mode is downshifted between 5 and 100 cm⁻¹ except in the case of the derivative with the 4-membered ring, which experiences a hypsochromic shift of 80 cm⁻¹. Only trans-stilbene and its 4-membered cyclic analogues were amenable to fluorescence lifetime measurements. Analysis of VT-NMR spectra of stilbene 7 reveals a barrier of 15.9 kcal mol⁻¹ for equilibration of aliphatic equatorial and axial H-atoms.