DIPOLAR TRIPLET-STATES OF P-NITROANILINE AND N-ALKYL DERIVATIVES WITHONE-FOLD, 2-FOLD, AND 3-FOLD SYMMETRY

Photoexcitation of p-nitroaniline (PNA) and its N-alkyl derivatives re sults in the formation of triplet states with dipole moments considera bly larger than that of the ground state, as monitored by time-resolve d microwave conductivity (TRMC). The triplet state lifetime, tau(T), i s only 54 ns for PNA in benzene but increases to 1400 ns on N,N-dimeth yl substitution and to 275 ns in the more polar solvent p-dioxane. Thi s sensitivity of tau(T) is attributed to the close proximity of all of the lowest lying singlet and triplet n pi and pi pi* states of PNA, which results in substantial S-1 <-> T-1 mixing, The dipole moment of the ''pure'' (3) pi pi State is estimated to be 11 D. Methylene-bridg ed arrays of PNA moieties with two- and three-fold D-n symmetry, 2-PNA and 3-PNA, are also found to have highly dipolar triplet states, indi cating that symmetry breaking and exciton localization occur in T-1. F lip-flop interchange between the resonant dipolar structures is estima ted to take place on a time scale of tens of picoseconds, Collective i nteractions between the PNA moieties in the multichromphoric arrays re sults 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.