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

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 ( \({K}_{SV}=45.2 {M}^{-1}\) $K_{\mathit{SV}}=45.2M^{-1}$ ) and bimolecular quenching rate constant ( \({k}_{q}=0.77\times {10}^{10} {M}^{-1}.{s}^{-1}\) $k_q=0.77\times {10}^{10}{M}^{-1}.s^{-1}$ ), 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, \({\Delta G}^{o}\) ${\mathrm{\Delta }G}^o$ ~ -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.