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.

Zinc phthalocyanine tetrasulfonate (ZnPcS4), a potential photosensitizer for photodynamic therapy (PDT), has been studied using femtosecond laser spectroscopy. The excited-state dynamics in water have been found to be fast (< 80 ps) and dominated by intermolecular aggregation. Since the proposed mechanism for PDT is energy transfer from the triplet excited state of the photosensitizer to triplet O2 creating singlet O2, the short lifetime is expected to be unfavorable for producing singlet O2. This leads to the suggestion that the presence of biological substrates may have an effect on the excited-state dynamics. To test this hypothesis, the lifetimes of the excited states of ZnPcS4 have been directly measured in the presence of a model membrane, n-hexadecyltrimethylammonium bromide (CTAB). The excited-state dynamics of ZnPcS4 in buffer solutions and with human serum albumin (HSA) have also been measured. The presence of HSA and CTAB increases the excited-state lifetime significantly relative to that observed in water. The longer lifetime of ZnPcS4 in CTAB (> 1 ns) indicates that the micellar surface favors monomer formation. By increasing the excited-state lifetime, the surface substantially increases the photosensitizing potential of ZnPcS4.

Zinc phthalocyanine tetrasulfonate (ZnPcS4), a potential photosensitizer for photodynamic therapy (PDT), has been studied using femtosecond laser spectroscopy. The excited-state dynamics in water have been found to be fast (< 80 ps) and dominated by intermolecular aggregation. Since the proposed mechanism for PDT is energy transfer from the triplet excited state of the photosensitizer to triplet O2 creating singlet O2, the short lifetime is expected to be unfavorable for producing singlet O2. This leads to the suggestion that the presence of biological substrates may have an effect on the excited-state dynamics. To test this hypothesis, the lifetimes of the excited states of ZnPcS4 have been directly measured in the presence of a model membrane, n-hexadecyltrimethylammonium bromide (CTAB). The excited-state dynamics of ZnPcS4 in buffer solutions and with human serum albumin (HSA) have also been measured. The presence of HSA and CTAB increases the excited-state lifetime significantly relative to that observed in water. The longer lifetime of ZnPcS4 in CTAB (> 1 ns) indicates that the micellar surface favors monomer formation. By increasing the excited-state lifetime, the surface substantially increases the photosensitizing potential of ZnPcS4.

Meso-tetra(hydroxyphenyl)chlorin (m-THPC) is a new photosensitizer developed for potential use in photodynamic therapy (PDT) for cancer treatment. In PDT, the accepted mechanism of tumor destruction involves the formation of excited singlet oxygen via intermolecular energy transfer from the excited triplet-state dye to the ground triplet-state oxygen. Femtosecond transient absorption measurements are reported here for the excited singlet state dynamics of m-THPC in solution. The observed early time kinetics were best fit using a triple exponential function with time constants of 350 fs, 80 ps and > or = 3.3 ns. The fastest decay (350 fs) was attributed to either internal conversion from S2 to S1 or vibrational relaxation in S2. Multichannel time-resolved absorption and emission spectroscopies were also used to characterize the excited singlet and triplet states of the dye on nanosecond to microsecond time scales at varying concentrations of oxygen. The nanosecond time-resolved absorption data were fit with a double exponential with time constants of 14 ns and 250 ns in ambient air, corresponding to lifetimes of the S1 and T1 states, respectively. The decay of the T1 state varied linearly with oxygen concentration, from which the intrinsic decay rate constant, ki, of 1.5 x 10(6) s-1 and the biomolecular collisional quenching constant, kc, of 1.7 x 10(9) M-1 s-1 were determined. The lifetime of the S1 state of 10 ns was confirmed by fluorescence measurements. It was found to be independent of oxygen concentration and longer than lifetimes of other photosensitizers.

Meso-tetra(hydroxyphenyl)chlorin (m-THPC) is a new photosensitizer developed for potential use in photodynamic therapy (PDT) for cancer treatment. In PDT, the accepted mechanism of tumor destruction involves the formation of excited singlet oxygen via intermolecular energy transfer from the excited triplet-state dye to the ground triplet-state oxygen. Femtosecond transient absorption measurements are reported here for the excited singlet state dynamics of m-THPC in solution. The observed early time kinetics were best fit using a triple exponential function with time constants of 350 fs, 80 ps and > or = 3.3 ns. The fastest decay (350 fs) was attributed to either internal conversion from S2 to S1 or vibrational relaxation in S2. Multichannel time-resolved absorption and emission spectroscopies were also used to characterize the excited singlet and triplet states of the dye on nanosecond to microsecond time scales at varying concentrations of oxygen. The nanosecond time-resolved absorption data were fit with a double exponential with time constants of 14 ns and 250 ns in ambient air, corresponding to lifetimes of the S1 and T1 states, respectively. The decay of the T1 state varied linearly with oxygen concentration, from which the intrinsic decay rate constant, ki, of 1.5 x 10(6) s-1 and the biomolecular collisional quenching constant, kc, of 1.7 x 10(9) M-1 s-1 were determined. The lifetime of the S1 state of 10 ns was confirmed by fluorescence measurements. It was found to be independent of oxygen concentration and longer than lifetimes of other photosensitizers.