Does bridging geometry influence interfacial electron transfer dynamics? Case of the enediol-TiO₂ system

We have employed femtosecond transient absorption spectroscopy in enediol-TiO2 systems (catechol, resorcinol, and quinol) to understand localized vs delocalized interfacial electron transfer dynamics in dye-nanoparticle systems. Optical absorption studies confirmed the formation of a charge transfer (CT) complex between the enediols and TiO2 nanoparticles. CT interaction between enediols and TiO2 was found to be decreased from catechol to resorcinol to quinol. The decrease in interaction strength from catechol to quinol was explained on the basis of a reduced overlap between the HOMO localized on the enediol and the conduction band of TiO2. Femtosecond transient absorption studies confirmed an ultrafast electron injection (<50 fs) into the conduction band of TiO2 in all enediol-TiO2 systems. Interestingly back electron transfer (BET), which follows multiexponential dynamics, is faster in the catechol-TiO2 system as compared to the other two enediol-TiO2 systems. As we increase the distance between the bridging ligand from catechol to quinol, we find that decay time increases proving the influence of bridging distance between enediols and TiO2. Our theoretical studies indicate an increase in the delocalization of the injected electron over several Ti atoms as the distance between the bridge linkers increases. Analysis of BET results in the framework of the Marcus theory indicated a significant influence of electronic coupling on BET in the enediol-TiO2 systems