Relaxation in excited states of porphycene in low-temperature argon and nitrogen matrices

Picosecond transient spectroscopy was used to study excited state deactivat ion processes of porphycene embedded in solid argon and nitrogen matrices a t low temperatures. Comparison of time profiles of the signals due to groun d state bleaching and stimulated fluorescence reveals that the latter is de layed by about 100 ps with respect to the former when the molecule is excit ed into the S-3 or S-4 electronic states, corresponding to Soret bands. No delay between the two signals is observed for excitations into Q bands, cor responding to S-1 and S-2 transitions. This is evidence of slow vibrational relaxation from S-3 and S-4 into S-1 and S-2, due to the considerable S-1- S-3 energy gap and similarity of the potential energy profiles in different electronic states. On the contrary, for nonrigid dibenzo-derivatives of po rphycene which have an 'intruder' state located between the Q and Soret ban ds, the relaxation from excited electronic states is faster than the experi mental time resolution of about 30 ps. (C) 2000 Elsevier Science B.V. All r ights reserved.