Photophysics and photochemistry of trans-4-nitrostilbenes and trans-2,4-dinitrostilbenes: Effect of intramolecular charge transfer

The photophysical and photochemical properties of trans-R-2′,4′-dinitrostilbene (II) and a series of trans-R-2′,4′-dinitrostilbenes (II-R, R: 2-NO₂, 3-NO₂, 4-Br, 4-F, 4-Me, 4-C₃H₇, 4-OCH₂C₆H₅, 3,4,5-(OMe)₃, 4-OEt, 4-NMe₂, 4-NEt₂) were studied in solution as a function of solvent polarity and temperature. The quantum yield of fluorescence (Φ_f) is very small for all II-R at 25°C. At −196°C Φ_f is moderate for several derivatives in 2-methyltetrahydrofuran (MTHF) but strongly enhanced for those bearing electron donating substituents. For the latter compounds the quantum yield of trans → cis photoisomerization is low, but for the other compounds Φ_{t → c} is substantial (0.2-0.5 in toluene or MTHF at room temperature). The triplet state absorbs typically in a broad spectral range; its lifetime (τ_T) lies in the 20-200 ns range and is longer for R = 4-NEt₂; at −196°C τ_T of all II-R approaches milliseconds. The results are compared with those of trans-4-R-4′-nitrostilbenes (I-R, R: NO₂, H, OMe, NH₂, NMe₂, NEt₂). Phosphorescence of singlet molecular oxygen was observed for several mono- and dinitrostilbenes at room temperature. Generally, the quantum yield of singlet oxygen formation is much smaller than that of intersystem crossing into the triplet state. The triplet mechanism accounts for trans→cis photoisomerization and the contribution of this pathway is lowered by intramolecular electron transfer to the nitro group(s). The similarities and differences between I-R and II-R type compounds and the effects of intramolecular charge transfer are discussed.