Photoinduced electronic interactions between acridine derivatives and small gold nanoparticles: A spectroscopic insight

Abstract

The central tricyclic moiety of acridine derivatives has fascinating redox properties. We take advantage of the electronic properties of two such acridine derivatives, 9-amino acridine (9AA-HCl) and acridine yellow, recognized so far as potential antimicrobial agents, since these drugs can act both as potential electron donor and acceptor, depending upon the reduction potential of its adjacent chemical entity. Precise timescale of formation of contact ion pair (CIP) and solvent separated ion pair in solution involving donors and acceptors has been much encountered and debated in literature till date. In our present communication, we like to decipher the mechanism and timescale of individual steps of photoinduced electron transfer (PET) between these drugs and gold nanoparticles (AuNPs) with dimension of 3-5 nm, which is important and presently our subject of interest since AuNPs could be used for shuttling of electrons between different donor and acceptor moieties. Steady-state and time-resolved absorption and fluorescence studies evidently reveal the signature of PET both in singlet and triplet states. Using ultrafast time-resolved fluorescence techniques we have tried to quantify precisely the time required for formation of CIP which is within fs-ps time regime. Moreover, 9AA-HCl, being a planar molecule, dimerises at excited state in presence of high concentration of AuNPs, due to its immobilisation over their surface. The occurrence of excited state proton transfer in triplet state for 9AA-HCl-AuNPs conjugate is depicted by laser flash photolysis experiments. The presence of all such charge transfer channels, provokes us to delineate the intricacies of the mechanisms involved which would help further to evaluate the possibility of making devices for energy storage using these drugs.