Intriguing multichannel photoinduced electron transfer in lanthanide(III)-diphenylamine systems

Photoinduced electron transfer (PET) and charge transfer (CT) interactions of trivalent lanthanide ions (Ln(3+)) with a fluorogenic diphenylamine (DPA) donor in aqueous solution have been investigated. Present donor-acceptor systems have a relevance to the leaching out and mitigation of lanthanide ions in the geoenvironment where DPA finds its presence as industrial and agricultural waste. The formation of weak ground state CT complexes in the present systems is confirmed from ground state absorption studies. There is no emissive exciplex formation as evidenced from steady-state (SS) fluorescence measurements. SS fluorescence quenching results, however, indicate substantial static quenching, attributed to ground state complex formation. Time-resolved (TR) fluorescence quenching results show a quite efficient dynamic quenching process. It is established that the TR fluorescence quenching of DPA by lanthanide ions is not due to energy transfer but due to PET from excited DPA to Ln3+. Direct evidence for the PET mechanism is obtained from laser flash photolysis measurements where a transient absorption band at around 670 nm for the DPA cation is clearly evident. The time constant for DPA cation growth confirms that PET occurs from the excited S-1 state of DPA to lanthanide ions. No correlation is, however, observed for the estimated quenching constants with the free energy changes of the PET reactions, due to the participation of multiple PET channels involving vacant electronic states of lanthanide ions. As realized, lanthanide ions are complex electron acceptors in the PET reactions and many extensive follow up studies are expected to understand the details of the multichannel PET in these geologically important redox systems.