Crossover to an Even-Parity Lowest Excited Singlet in Large Oligorylenes: A Theoretical Study

The series of oligorylenes from perylene to pentarylene are theoretically studied with the purpose to predict the dipole character of the lowest excited singlet state, S 1, for the larger oligomers and the polymer. Like polyenes and other extended conjugated systems, oligorylenes have two low-lying excited singlet states of odd (B 3u ) and even (A g ) parity whose relative ordering determines the nature of the basic photoexcitation processes. The equilibrium geometries of oligorylenes in the ground and excited electronic states are obtained within the PM3 Hamiltonian with configuration interaction. Structural relaxations are found to reverse the order of 1 1 B 3u and 2 1 A g with increasing number of peri -linked naphthylene repeat units, N. The adiabatic 1 1 B 3u state is below 2 1 A g for perylene, in line with its strong fluorescence. For terrylene, S 1 = 2A g and S 2 = 1B 3u are nearly degenerate. For N = 4 and 5, S 1 = 2A g is predicted lower than 1B 3u by 0.17 and 0.24 eV, respectively. These results explain the decrease of fluorescence quantum yield in tert -butyl-substituted oligorylenes, especially on going from terrylene (74%) to quaterylene (5%).