Most organic fluorophores in low-concentration solutions exhibit a photoluminescence quantum yield (PL QY) close to 1, whereas at higher molecular concentrations, PL QY significantly decreases [1]. This reduction in fluorescence QY with increasing concentration is known as concentration quenching. Since concentration quenching can negatively impact system or instrument performance, understanding its underlying physical mechanisms is crucial [2]. In organometallic compounds like chlorophyll (Chl), this phenomenon is particularly intriguing due to its relevance in photosynthetic complexes. However, a definitive model explaining concentration quenching has yet to be established.
This study aimed to investigate the effects of concentration on Chl-a molecules in solvents of different polarity. To achieve this, absorption (Abs) and fluorescence (FL) spectra, along with fluorescence decay kinetics and PL QY were measured. Chl-a solutions were prepared by dissolving the molecules in toluene (Tol), dimethyl sulfoxide (DMSO), ethanol (EtOH).
Spectral properties of Chl-a solutions strongly depend on solvent properties. In nonpolar Tol, Chl-a spectral changes are the most significant: with increasing concentration Qy (665 nm) Abs band gets wider. In FL, formation of a new 705 nm FL band is observed, which relative intensity increases with concentration. In aprotic polar DMSO, Chl-a Abs spectra changes are observed only for the highest 25 mM concentration, where Qy band shifts to shorter wavelengths - this may indicate H type aggregate formation. In FL spectra, with increasing concentration, 705 nm band formation is observed, however its relative intensity is 3 times lower than in Tol solutions. In protic polar EtOH, Chl-a spectra does not depend on concentration. For Chl-a molecules in all investigated solvents, the decrease of FL intensity with increasing concentration is observed. For concentrations higher than 1 mM, the average FL lifetime decreases, indicating that concentration quenching is present in solutions.