Ultrafast Studies of Excited-State Dynamics of Phthalocyanine and Zinc Phthalocyanine Tetrasulfonate in Solution

Abstract

Femtosecond measurements of transient absorption, bleach, and stimulated emission are used to study the excited-state dynamics of phthalocyanine tetrasulfonate (PcS4) and zinc phthalocyanine tetrasulfonate (ZnPcS4) in solution. In water the excited-state decay process is fast and dominated by energy relaxation due to intermolecular aggregation. In dimethyl sulfoxide (DMSO) both PcS4 and ZnPcS4 exist predominantly in the monomeric form and exhibit very different dynamics from that of the aggregates. The decays are much slower and the observed processes are strongly dependent on the probe wavelength. For PcS4 in DMSO, when probed at 790 nm, the dynamics are dominated by stimulated emission which is observed for the first time in solution. At other wavelengths either transient absorption or bleach dominates the signal. All the observed dynamics can be well fit using a double-exponential function with a fast and slow component. The fast decay has a time constant of 10 ± 4 ps for both phthalocyanines while the slow decay has a time constant of 370 ps for PcS4 and 460 ps for ZnPcS4, respectively. The overall excited-state decay dynamics correlate well with the recovery of the ground electronic state, indicating that the recovery is the predominant process on this time scale. On the basis of a simple three-state kinetic model, the fast decay (10 ps) is attributed primarily to a conversion from the second to the first excited singlet state, possibly involving vibrational relaxation in S1. There might also be a small contribution from aggregates. The first excited-state S1 subsequently decays with a time constant of 130 ps for PcS4 and 160 ps for ZnPcS4, respectively. This decay is due to a combination of radiative and nonradiative relaxation from S1 to S0 and intersystem crossing from S1 to the triplet state.