New Intramolecular Fluorescence Probes That Monitor Photoinduced Radical and Cationic Cross-Linking

The fluorescence probes trans,trans-1,4-bis[2-(2‘,5‘-dimethoxy)phenylethenyl)-2,3,5,6-tetrafluorobenzene (1) and trans,trans-1,4-bis[2-(3‘,4‘,5‘-trimethoxy)phenylethenyl)-2,3,5,6-tetrafluorobenzene (2) show solvatochromic behavior yielding slopes in the Lippert−Mataga equation of −13 300 and −13 700 cm-1, respectively. 1 sensitively probes progress in both photoinduced radical and cationic cross-linking. Both 1 and 2 show minor tendencies for trans−cis photoisomerization. Substituted stilbenes such as trans-2-(2‘,5‘-dimethoxyphenyl)ethenyl-2,3,4,5,6-pentafluorobenzene (1r) and trans-2-(3‘,4‘,5‘-trimethoxyphenyl)ethenyl-2,3,4,5,6-pentafluorobenzene (2r) are less solvatochromic in fluorescence but evidence enhanced tendency toward photoisomerization.