Unraveling energy transfer and fluorescence quenching dynamics in biomolecular complexes: a comprehensive study of imiquimod-rifampicin interaction

In nature, numerous biomolecules are implicated in charge transfer (CT) and energy transfer (ET) mechanisms crucial for fundamental processes such as photosynthesis. Unveiling these mechanisms is pertinent to multiple disciplines including chemistry, engineering and biochemistry. This article presents a detailed study involving two molecules forming a model system with efficient ET properties. Specifically, their complex exhibits dark quenching phenomena arising from fluorescence resonance energy transfer (FRET) from the donor (imiquimod) to the acceptor (rifampicin). In addition, the energy transfer properties were also elucidated by considering the two forms of rifampicin (RIF), non-ionic and zwitter-ionic in the solution. Supplemented by spectroscopic findings, molecular dynamics simulations and time dependent density functional theory (TD-DFT) calculations conclusively validate the ET properties from imiquimod (IMQ) to RIF forms. Interestingly, these ET processes were found to be associated with pi-pi and hydrogen bond interactions. Their contribution was observed to depend upon the non-ionic and zwitter-ionic form of RIF. A comprehensive exploration of various excitation and deactivation pathways, with particular emphasis on the energy transfer process.