Dependence of Interfacial Charge Transfer on Bifunctional Aromatic Molecular Linkers in CdSe Quantum Dot Sensitized TiO₂ Photoelectrodes

Quantum dot (QD) sensitization of TiO₂ is a powerful method to improve its performance as a photoanode material in solar energy conversion. The efficiency of sensitization depends strongly on the rate of interfacial electron transfer (ET) from the QDs to TiO₂. To understand the key factors affecting the ET, arene-substituted (ortho, meta, and para) bifunctional linkers with single or double aromatic rings were employed to link CdSe QDs to TiO₂ and control the strength of their interaction as well as the rate of interfacial ET. Interestingly, the para-substituted aromatic linker, 4-mercaptobenzoic acid (4MBA) with the longest distance between the carboxyl and thiol groups, shows the best photoelectrochemical (PEC) performance, when compared to those of ortho-subtituted (2-mercaptobenzoic acid, 2MBA) and meta-substituted (3-mercaptobenzoic acid, 3MBA) aromatic linkers. Two other bifunctional linkers with double aromatic rings, 4′-mercapto-[1,1′-biphenyl]-4-carboxylic acid (4M1B4A) and 6-mercapto-2-naphthioc acid (6M2NA), were also studied for comparison. Ultrafast transient absorption (TA) spectroscopy was used to study the exciton dynamics in CdSe QDs and determine the interfacial ET rate constant (<i>k</i>_ET). The <i>k</i>_ET results are consistent with the trend of PEC measurements in that 4MBA shows the highest <i>k</i>_ET. To gain further insight into the ET mechanism, we performed density functional theory (DFT) calculations to examine the intrinsic properties of the linkers. The results revealed that the favorable wave function distribution of the molecular orbitals of 4MBA and 4M1B4A are responsible for the higher interfacial ET rate and PEC performance due to better interfacial coupling, a factor that dominates over distance. The present study provides important new insight into the mechanism of interfacial ET using aromatic bifunctional linkers, which is useful in designing QD sensitized semiconductor metal oxide nanostructures for applications including photovoltaics and solar fuel generation

Experimental and Theoretical Investigations of the Bromination of Phenols with β and γ Aliphatic Substituents, including Rings

Bromination reactions of substituted and ring fused phenols were studied by both experiment (<i>t</i>-BuNH-Br) and computation (density functional theory). The outcomes support each other, indicating a clear and predictable regioselective preference among 3,4-bis-alkylated and 3,4-ring-fused phenols