Dipolar Triplet States of p-Nitroaniline and N-Alkyl Derivatives with One-, Two-, and Three-Fold Symmetry

Photoexcitation of p-nitroaniline (PNA) and its N-alkyl derivatives results in the formation of triplet states with dipole moments considerably larger than that of the ground state, as monitored by time-resolved microwave conductivity (TRMC). The triplet state lifetime, τT, is only 54 ns for PNA in benzene but increases to 1400 ns on N,N-dimethyl substitution and to 275 ns in the more polar solvent p-dioxane. This sensitivity of τT is attributed to the close proximity of all of the lowest lying singlet and triplet nπ* and ππ* states of PNA, which results in substantial S1 ↔ T1 mixing. The dipole moment of the “pure” 3ππ* state is estimated to be 11 D. Methylene-bridged arrays of PNA moieties with two- and three-fold Dn symmetry, 2-PNA and 3-PNA, are also found to have highly dipolar triplet states, indicating that symmetry breaking and exciton localization occur in T1. Flip-flop interchange between the resonant dipolar structures is estimated to take place on a time scale of tens of picoseconds. Collective interactions between the PNA moieties in the multichromphoric arrays results in a marked blue shift in the first absorption maximum from 384 nm for 1-PNA to 375 nm for 2-PNA and to 354 nm for 3-PNA.