Excited-State Relaxation Process of Free-Base and Oxovanadium Naphthalocyanine in Near-Infrared Region

Excited-state relaxation process of free-base and oxovanadium naphthalocyanine (H(2)Nc and VONc) in solutions and in polymer films was studied by transient absorption measurements. In polymer films only H-aggregate was observed with H2Nc with increasing its weight fraction, whereas VONc formed both H- and J-type aggregates. The transient absorption of singlet excited-state of H(2)Nc and VONc in toluene solution decayed with time constant of 25 +/- 30 and 12 +/- 2 ps, respectively. The relaxation from singlet excited state of H(2)Nc and VONc in toluene solution is governed by the IC and ISC, respectively. The central metal ion, VO2+, acceralated the ISC by the spin-orbital coupling due to unpaired electron. The excited-state relaxation in polymer films differed from that in toluene solution, which originates from the exciton-exciton annihilation process. The excitation power dependence of the excited-state dynamics and weight ratio dependence of absorption spectrum suggests that the aggregation of VONcs contributes to faster decay from the singlet excited-state. The excition-exciton annihilation occurs more efficiently in VONc system compared with H(2)Nc system. The dipole-dipole interaction depending on the aggregated structure controls excited-state relaxation processes in polymer films. This is mainly due to the differences in the transition dipole moment between Hand J-type aggregates. The ultrafast deactivation of VONc in polymer films can be applied to all-optical ultrafast modulation in the optical telecommunication wavelength region.