Probing the role of pre-aggregation in rhodamine-based organic photocatalysis: a combined computational and single-molecule approach

• Applicants: Professor Dr. David Egger; Professor Dr. John Lupton
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 439215932

In photocatalysis visible light is used in the activation of chemical reactions, which bears important potential applications in tackling environmental challenges of the modern age. In this proposal, we suggest the microscopic investigation of rhodamine-based organic photocatalysis via strongly interlinked theoretical, spectroscopic, and synthetic efforts. Rhodamine-6G (Rh-6G) has been studied extensively but has only recently been found to activate reactions in aryl bromides, the substrate molecules, via absorption of two photons in a consecutive photoelectron transfer (conPET) reaction. As the Rh-6G dye is commercially available in great abundances, rhodamine-based photocatalysis holds the promise of being suitable for industrial processing. Our preliminary findings indicate that an important pre-aggregation between Rh-6G and the organic substrate occurs, which could be crucial for the yield of the photocatalytic reaction. However, the microscopic role of pre-aggregation effects in the conPET reaction of Rh-6G is currently unknown.The overarching goal of this project is developing a microscopic understanding of the dynamics in organic photocatalysis on the level of the single Rh-6G molecule. Specifically, we will focus on the impact of geometrical and dynamical effects on the photocatalytic reaction, such as a pre-aggregation between Rh-6G and the substrate molecule. Concerted theoretical and spectroscopic techniques will be employed in order to address our first objective, namely to quantify the interaction strengths of the Rh-6G–substrate molecular complex by examining a vast parameter space that includes the use of different substrates and solutions. Our second objective concerns unprecedented insight into the energy- and timescales involved in the conPET reaction of Rh-6G by non-adiabatic molecular dynamics and by an independent tuning of the wavelengths in the two-photon spectroscopy experiment. The proposed combination of computation and single-molecule spectroscopy will allow for detecting how the excess photoenergy is deposited in the molecular compound as well as for probing the role of pre-aggregation effects in the excited-state of the Rh-6G–substrate complex. We expect that the results of our studies will lead us to suggest optimized conditions for enhancing the photocatalytic efficiency of rhodamine-based organic photocatalysis. Furthermore, this project also addresses important methodological challenges in both theory and experiment. On the theoretical side, we propose to implement and test a scheme that allows for more accurate calculations of excited-state nuclear gradients and non-adiabatic couplings via a non-empirical, but system-dependent tuning of the range-separation parameter in hybrid density functional theory. Experimentally, we plan to broaden the pool of potential reactants by developing synthetic organic linker compounds, so that we can apply our techniques in organic solvents rather than in water.

• DFG Programme: Research Grants