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.