A Case-Study on the Photophysics of Chalcogen-Substituted Zinc(II) Phthalocyanines

Abstract

Singlet dioxygen has been widely applied in different disciplines such as medicine (photodynamic therapy or blood sterilization), remediation (wastewater treatment) or industrial processes (fine chemicals synthesis). Particularly, it can be conveniently generated by energy transfer between a photosensitizer's triplet state and triplet dioxygen upon irradiation with visible light. Among the best photosensitizers, substituted zinc(II) phthalocyanines are prominent due to their excellent photophysical properties, which can be tuned by structural modifications, such as halogen- and chalcogen-atom substitution. These patterns allow for the enhancement of spin-orbit coupling, commonly attributed to the heavy atom effect, which correlates with the atomic number ( $Z$ ) and the spin-orbit coupling constant ( $\zeta$ ) of the introduced heteroatom. Herein, a fully systematic analysis of the effect exerted by chalcogen atoms on the photophysical characteristics (absorption and fluorescence properties, lifetimes and singlet dioxygen photogeneration), involving 30 custom-made β-tetrasubstituted chalcogen-bearing zinc(II) phthalocyanines is described and evaluated regarding the heavy atom effect. Besides, the intersystem crossing rate constants are estimated by several independent methods and a quantitative profile of the heavy atom is provided by using linear correlations between relative intersystem crossing rates and relative atomic numbers. Good linear trends for both intersystem crossing rates (S₁-T₁ and T₁-S₀) were obtained, with a dependency on the atomic number and the spin-orbit coupling constant scaling as $Z^{0.4}$ and ${\zeta }^{0.2}$ , respectively The trend shows to be independent of the solvent and temperature.

Keywords: heavy atom effect; metal phthalocyaninates; photophysics; phthalocyanines; spin-orbit coupling; synthesis and characterization; time-resolved and steady-state photoluminescence spectroscopy.