Solvent and Substitution Effects on the Excited-State Dynamics of Triphenylamines [6π] Electrocyclization: A Laser...

The excited-state dynamics and photochemical behavior of mono-, di-, and trisubstituted triphenylamines were investigated in different media by nanosecond laser flash photolysis. Photoexcitation produces the triplet excited state of the triarylamines, which undergoes competitive radiationless decay and intramolecular [6π]-electrocyclization to form the corresponding triplet N-aryl-4a,4b-dihydrocarbazoles. These intermediates subsequently yield singlet dihydrocarbazoles that undergo back-electrocyclization, disproportionation, or oxygen trapping. The transient species were characterized by their absorption spectra, lifetimes, quantum yields, and kinetic parameters. Both the efficiency of electrocyclization and the rate of radiationless deactivation depend strongly on solvent polarity and substituent electronic effects. Linear correlations between rate constants and Reichardt's solvent polarity parameter point out that cyclization is favored in polar solvents, whereas radiationless decay predominates in nonpolar media. Hammett analyses further demonstrate systematic substituent effects on the competing excited-state pathways, including triarylamines bearing perfluoroalkyl substituents. These results provide a comprehensive kinetic and mechanistic description of the photoinduced electrocyclization of substituted triphenylamines and establish quantitative relationships between molecular structure, solvent environment, and excited-state reactivity.