Elucidating the Mechanism of Copper-Induced Photoluminescence Quenching in 2-Phenylbenzimidazole-5-Sulfonic Acid

Abstract

To explore the possible impact of 2-Phenylbenzimidazole-5-sulfonic acid (PBSA) on the function of a sunscreen, in this work we investigate the binding of copper metal ions (Cu²⁺) to PBSA. Due to the existence of an intrinsic interaction phenomenon between Cu²⁺ ions and PBSA molecules, the photoluminescence (PL) quenching arises owing to the charge transfer from PBSA to Cu²⁺ ions. The mechanism of fluorescence quenching is probed experimentally following excitation at 306 nm by evaluating various quenching parameters with the help of the Stern-Volmer plot. Through the assessment of the values of the Stern-Volmer constant ( $K_{\mathrm{SV}}=45.2M^{-1}$ ) and bimolecular quenching rate constant ( $k_q=0.77\times {10}^{10}{M}^{-1}.s^{-1}$ ), it is deduced that the dynamic mode of PL quenching is operative between PBSA and Cu²⁺ ions. We evaluate the number of binding sites (n = 1) that advocate the presence of a single binding site in PBSA for Cu²⁺ ions. The numerical value of standard Gibbs free energy change, ${\Delta G}^o$ ~ -27.485 kJ.mol^-1 implies the spontaneous binding between Cu²⁺ ions and PBSA molecules. The results obtained give an insight into the mechanism of metal-induced PL quenching of water soluble PBSA sunscreen.

Keywords: 2-Phenylbenzimidazole-5-sulfonic acid; Dynamic quenching; PBSA; Photoluminescence quenching; Stern–Volmer plot.