Excited State Dynamics of Zinc–Phthalocyanine Nanoaggregates in Strong Hydrogen Bonding Solvents

{Zinc 2,9,16,23-tetra- tert -butyl-29 H,31 H -phthalocyanine} (ZnPC) exists as monomeric species in DMSO and is reasonably strong fluorescent. But ZnPC forms H-aggregates in water and hexafluoroisopropanol, which are strong hydrogen bonding solvents. Nanoaggregates are nearly nonemissive. Transient absorption spectroscopic technique has been used to investigate the excited state relaxation processes in both monomeric and aggregated forms of ZnPC. The lifetime of the S 1 state of the monomoric form in DMSO is long (τ = 3.4 ns) but the excited states of ZnPC nanoaggregates show much faster ground state recovery (within 100 ps). The longest lifetime component, τ 3, which is independent of excitation density, has been assigned to the unimolecular decay of the S 1 -exciton in the absence of annihilation reaction, while τ 1 and τ 2 are the lifetimes obtained by the two-component fit of the nonexponential decay arising due to the time-dependent decay rates of the S 1 -excitons because of diffusive migration controlled exciton - exciton annihilation reaction. Rates of the annihilation reaction (2.0 × 10 –6 cm 3 s –1 ) and exciton migration (4.3 × 10 –5 m 2 /s) as well as diffusion length (about 85 nm) of the S 1 -exciton created in the ZnPC nanoaggregates in HFIP have been determined.