Modulation of excimer formation and photostability of avobenzone inside the nanocavities of cyclodextrins

Abstract

Avobenzone (AVZ), a UVA filter, is extensively used in sunscreen lotion and protects the skin from harmful UVA radiation. Despite their importance in sunscreen formulation, less attention has been given to its light induced excited state properties. Herein, we have explored the photophysical properties of AVZ in aqueous solutions and cyclodextrins (CDs) complexes by employing steady-state, time-resolved fluorescence, and molecular modelling tools followed by photostability analysis. AVZ exhibits dual emission from monomer and excimer forms in aqueous solutions. Spectroscopic studies revealed that AVZ forms multiple inclusion complexes with β-CD and γ-CD depending upon the host’s cavity size. These versatile host–guest complexes resulted in the modulation of the excimer formation of AVZ. The presence of α-CD and β-CD complexes with AVZ inhibited the excimer forms. In contrast, γ-CD facilitated excimer formation because its larger cavity could encapsulate the AVZ dimer. Stoichiometry and association constants for multiple host–guest complexes were determined using Benesi-Hildebrand (BH) plot. Job’s plot was used to confirm the stoichiometry of complexes. Time correlated single photon counting (TCSPC) technique was used to depict the excited state lifetime values of AVZ:CD inclusion complexes. Furthermore, these inclusion complexes were characterized by Fourier transform-infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Molecular docking followed by semiempirical calculations confirm the experimental results on stoichiometries and binding constants of host–guest complexes. UV light-induced absorption kinetics revealed that the photostability of AVZ inside the β-CD cavity is much better than other inclusion complexes of CDs. Overall, the present study helps us in further understanding of excited state behavior of AVZ inside hydrophobic nanocavities.