| Abstract
| - Femtosecond measurements of transient absorption, bleach, andstimulated emission are used to study theexcited-state dynamics of phthalocyanine tetrasulfonate(PcS4) and zinc phthalocyanine tetrasulfonate(ZnPcS4)in solution. In water the excited-state decay process is fast anddominated by energy relaxation due tointermolecular aggregation. In dimethyl sulfoxide (DMSO) bothPcS4 and ZnPcS4 exist predominantlyinthe monomeric form and exhibit very different dynamics from that of theaggregates. The decays are muchslower and the observed processes are strongly dependent on the probewavelength. For PcS4 in DMSO,when probed at 790 nm, the dynamics are dominated by stimulatedemission which is observed for the firsttime in solution. At other wavelengths either transient absorptionor bleach dominates the signal. All theobserved dynamics can be well fit using a double-exponential functionwith a fast and slow component. Thefast decay has a time constant of 10 ± 4 ps for both phthalocyanineswhile the slow decay has a time constantof 370 ps for PcS4 and 460 ps for ZnPcS4,respectively. The overall excited-state decay dynamicscorrelatewell with the recovery of the ground electronic state, indicating thatthe recovery is the predominant processon this time scale. On the basis of a simple three-state kineticmodel, the fast decay (10 ps) is attributedprimarily to a conversion from the second to the first excited singletstate, possibly involving vibrationalrelaxation in S1. There might also be a smallcontribution from aggregates. The first excited-stateS1subsequently decays with a time constant of 130 ps for PcS4and 160 ps for ZnPcS4, respectively. Thisdecay is due to a combination of radiative and nonradiative relaxationfrom S1 to S0 and intersystemcrossingfrom S1 to the triplet state.
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