P-030

Vilius Čirgelis

vilius.cirgelis@ftmc.lt

Simona Streckaitė, Jevgenij Chmeliov, Leonas Valkūnas, Andrius Gelžinis

Center for Physical Sciences and Technology and Vilnius University

Concentration quenching in solutions of chlorophyll a molecules

Most organic fluorophores in dilute solutions exhibit a photoluminescence quantum yield (PL QY) close to 1. However, as molecular concentration increases, PL QY decreases significantly. This decline in fluorescence efficiency, known as concentration quenching, can adversely affect the performance of various systems and instruments, making it essential to understand the underlying physical mechanisms. In organometallic compounds such as chlorophyll (Chl), this phenomenon is particularly intriguing due to its relevance in photosynthetic complexes. Despite extensive research, a definitive model explaining concentration quenching remains elusive.

This study aimed to investigate the effects of concentration on Chl-a molecules dissolved in solvents of varying polarity. To achieve this, absorption (Abs) and fluorescence (FL) spectra were measured, along with fluorescence decay kinetics and PL QY. Chl-a solutions were prepared in toluene (Tol), dimethyl sulfoxide (DMSO), and ethanol (EtOH).

The spectral properties of Chl-a solutions are highly dependent on solvent polarity. In nonpolar Tol, Chl-a exhibits the most pronounced spectral changes: as concentration increases, the Qy (665 nm) Abs band broadens, and a new fluorescence band at 705 nm emerges, with its relative intensity increasing at higher concentrations. In aprotic polar DMSO, changes in the Abs spectrum are observed only at the highest concentration (25 mM), where the Qy band shifts to shorter wavelengths, suggesting the possible formation of H-type aggregates. Similarly, the 705 nm fluorescence band appears with increasing concentration, but its relative intensity is three times lower than in Tol solutions. In protic polar EtOH, the Chl-a spectra remain largely unaffected by concentration.

Across all solvents, fluorescence intensity decreases as concentration increases. Additionally, for concentrations exceeding 1 mM, the average fluorescence lifetime shortens, confirming the presence of concentration quenching in Chl-a solutions.

This work was supported by the Research Council of Lithuania (LMT Grant No. S-MIP-23-31).