Hydroxyl-directed differentiation of photocyclization pathways in triphenylamine monitored by transient absorption spectroscopy

Photoinduced [6π]-electrocyclization of triphenylamine provides an efficient route to the construction of carbazole frameworks. In this work, 4-(diphenylamino)phenol (TPA-OH) was selected as a model compound to examine how hydroxyl substitution breaks the symmetry of triphenylamine and differentiates its photocyclization pathways. Transient absorption spectroscopy revealed that the singlet excited-state absorption of 1TPA-OH at 627 nm decays within 1.6 ns, accompanied by the emergence of the triplet excited-state absorption of 3TPA-OH at 508 nm, which subsequently decays with a lifetime of 45.7 ns. Thereafter, a positive absorption signal appears at 441 nm, corresponding to the triplet cyclization intermediate 3DHC0-OH. The long-lived absorption band near 608 nm is attributed to overlapping contributions from 1DHC0-OH and subsequent product-related species. Intrinsic reaction coordinate and Mayer bond-order analyses confirm that ring closure is governed by intramolecular C–C bond formation. Hydroxyl substitution changes the equivalence of the ring-closing sites, allowing ring closure either between the two unsubstituted phenyl rings (EXO cyclization) or through involvement of the hydroxyl-substituted phenyl ring (ENDO cyclization). On both the potential-energy and free-energy surfaces, the EXO pathway is favored over the ENDO pathway, exhibiting a lower potential-energy barrier (4.5 vs 26.5 kcal mol−1), a substantially lower Gibbs free-energy barrier (28.4 vs 112.1 kJ mol−1), and a more stable cyclized product (−42.6 vs −4.5 kJ mol−1). Although hydroxyl substitution does not alter the dominance of the EXO channel, it provides mechanistic insights and suggests a potential avenue for the targeted synthesis of carbazoles via the ENDO pathway.