Fluorescence concentration quenching in phthalocyanine solutions: Experimental observation and theoretical insights

Concentration quenching (CQ) of fluorescence (FL) is a phenomenon in which an increase in the molecular concentration of a fluorophore leads to a reduction of its FL quantum yield. Although it has been recognized for decades, its microscopic origin remains not fully understood. CQ is commonly attributed to excitation migration from fluorescent species to non-emissive traps, although the physical nature of these traps is often unspecified. In this work, we study CQ in solutions of free-base and zinc-containing phthalocyanines (Pcs) by combining steady-state and time-resolved FL spectroscopy as well as quantum-chemical calculations. While steady-state FL spectra remain essentially unchanged across the 0.1-10 mM concentration range for both Pc molecules, the absorption spectra exhibit concentration-dependent changes, which are especially pronounced for free-base Pc. The FL decay kinetics for both Pcs with increasing concentration evolve monotonically from mono-exponential to distinctly non-exponential behavior, altogether indicating the formation of ground-state aggregates that are non-emissive. Density functional theory calculations show that both Pc molecules can form optically dark, thermodynamically stable H-dimers, suggesting that such dimers may serve as the statistical pair traps responsible for CQ. Together, these results provide a microscopic picture of CQ in phthalocyanine solutions and contribute to the understanding of the underlying photophysics and its application in the light-to-energy conversion molecular system, where efficient energy transfer is crucial.