Energy relaxation paths in matrix-isolated excited molecules: Comparison of porphycene with dibenzoporphycenes

When excited with large excess of energy over the lowest excited singlet state, two alkyl derivatives of dibenzo[cde, mno]porphycene undergo rapid relaxation to S1. The S1 depopulation occurs in 10-20 ps, leading directly to S0. The relaxation rates are only weakly temperature or medium-dependent. This behavior contrasts with properties of parent porphycene, where the relaxation is much slower in argon matrices: after excitation into S3/S4, it takes about 100 ps to reach S1, which then decays in 15 ns. The differences are explained by the planarity and rigidity of parent porphycene, as opposed to nonplanar, flexible structure of dibenzo derivatives.