Design of phthalocyanine metal complexes for efficient far-red to near-IR light-initiated photopolymerizations

Abstract

Efficient near-infrared (NIR) photopolymerization is promising for applications such as hydrogel bioprinting, composite manufacturing, and other technologies that benefit from deep light penetration and low-energy activation. Yet, design principles for optimizing NIR photoinitiators, particularly those based on earth-abundant or metal-free elements, remain limited. Here, a library of phthalocyanine (Pc) and naphthalocyanine (Nc) derivatives was synthesized, characterized, and evaluated as photoredox catalysts for NIR-induced radical polymerizations. Variations in metal center (Zn, Si, Pd), α-substitution (pentyl or butoxy), and π-extension (Pc vs. Nc) enabled tuning of light absorption, excited-state energies and lifetimes, and triplet excited state quantum yields. Polymerization kinetics were quantified using real-time FTIR spectroscopy under LED irradiation (740–940 nm), with photon absorption normalized to allow direct comparison of internal quantum yields. This work provides a framework for quantitative benchmarking of NIR photoinitiators under controlled conditions. Among the catalysts studied, a Pd–Pc complex showed the highest internal efficiency, while a Si–Nc catalyst outperformed a leading commercial cyanine initiator, highlighting the potential of silicon as a sustainable alternative to precious metals. These results establish clear structure–property relationships and offer guiding principles for the design of next-generation NIR photoinitiators suited for biomedical and advanced manufacturing technologies.