Effect of solvent on photophysical properties of tetranitro zinc phthalocyanine

Abstract

This study investigates the influence of solvent on the photophysical properties of a tetranitro zinc phthalocyanine derivative synthesized through a cyclotetramerization reaction using 4-nitrophthalonitrile and zinc acetate in N, N-dimethylaminoethanol. Its photophysical behavior was systematically analyzed in DMSO, DMSO:H₂O (1:1), NMP, and NMP:H₂O (1:1) media using UV-Vis absorption, fluorescence, and phosphorescence spectroscopy. The UV-Vis spectra revealed the presence of a single, intense Q-band around 700 nm, whose position and intensity depend on the solvent polarity and the degree of hydrogen bonding, indicating solvent-dependent aggregation behavior. Fluorescence measurements showed well-defined emission bands only in pure solvents (DMSO and NMP), while in aqueous mixtures the fluorescence was strongly quenched, suggesting non-radiative processes and possible hydrogen-bond-assisted aggregation. Room-temperature phosphorescence, recorded under pulsed excitation with a xenon lamp, confirmed the population of triplet states, with emission bands around 850 nm and lifetimes on the order of tens of microseconds, depending on the solvent environment. The phosphorescence intensity was notably higher in DMSO compared to NMP, indicating differences in triplet-state stabilization. These results demonstrate that solvent polarity, hydrogen bonding, and the presence of water significantly influence both, singlet and triplet excited-state dynamics, of the tetranitro zinc phthalocyanine derivative. The findings provide valuable insights for optimizing the photophysical response of phthalocyanine-based systems in light-harvesting, photodynamic therapy, and optoelectronic applications, where solvent tuning can be employed to control emission properties and excited-state lifetimes.