Modeling and Characterization of Exciplexes in Photoredox CO₂Reduction: Insights from Quantum Chemistry and Fluorescence Spectroscopy

Abstract

Interactions between excited-state arenes and amines can lead to the formation of structures with a distinct emission behavior. These excited-state complexes or exciplexes can reduce the ability of the arene to participate in other reactions, such as CO 2 reduction, or increase the likelihood of degradation via Birch reduction. Exciplex geometries are necessary to understand photophysical behavior and probe degradation pathways but are challenging to calculate. We establish a detailed computational protocol for calculation, verification, and characterization of exciplexes. Using fluorescence spectroscopy, we first demonstrate the formation of exciplexes between excited-state oligo-(p-phenylene) (OPP), shown to successfully carry out CO 2 reduction, and triethylamine. Timedependent density functional theory is employed to optimize the geometries of these exciplexes, which are validated by comparing both emission energies and their solvatochromism with the experiment. Excited-state energy decomposition analysis confirms the predominant role played by charge transfer interactions in the red shift of emissions relative to the isolated excited-state OPP*. We find that although the exciplex emission frequency depends strongly on solvent dielectric, the extent of charge separation in an exciplex does not. Our results also suggest that the formation of solvent-separated ionic radical states upon complete electron transfer competes with exciplex formation in higher-dielectric solvents, thereby leading to reduced exciplex emission intensities in fluorescence experiments.Article pubs.acs.org/JPCA

Photoredox Chemistry with Organic Catalysts: Role of Computational Methods

“…These peaks have been observed for, among others, interacting arene and amine systems including the CO 2 cycle described in Figure 4. 78,79 Despite challenges associated with the identification of exciplex geometries on largely shallow, excited-state potential energy surfaces, only a handful of prior studies establish procedures for TDDFT-driven geometry optimization and characterization. 80,81 In a more recent study of exciplexes formed between excited-state OPP*and TEA, authors of this perspective outlined the detailed procedure for TDDFT optimization of exciplexes.…”

“…80,81 In a more recent study of exciplexes formed between excited-state OPP*and TEA, authors of this perspective outlined the detailed procedure for TDDFT optimization of exciplexes. 79 As the two fragments exhibit fractional charges in the exciplex state that cannot be determined a priori or specified as constraints, TDDFT and not CDFT was employed in this study. To minimize errors arising from known limitations of TDDFT in describing excited and charge-transfer states, the study calculated exciplex geometries using both hybrid as well as long range-corrected hybrid density functional approximations.…”

“…In addition, very small step sizes and tight tolerances for convergence of gradients are recommended for the TDDFT optimization of exciplexes. 79 The same study also outlined several methods for verifying whether TDDFT optimization converges correctly to an exciplex state in addition to validation with experimental fluorescence spectra and solvatochromic shifts. 79 A simple test of convergence to an exciplex is to replace the donor atom containing the lone pair with a group that does not contain a lone pair (e.g., N in TEA with a CH group).…”

“…79 The same study also outlined several methods for verifying whether TDDFT optimization converges correctly to an exciplex state in addition to validation with experimental fluorescence spectra and solvatochromic shifts. 79 A simple test of convergence to an exciplex is to replace the donor atom containing the lone pair with a group that does not contain a lone pair (e.g., N in TEA with a CH group). If the exciplex geometry is correctly identified, this replacement will result in the absence of the red-shifted emission peak.…”

“…By applying this technique to the TDDFT-optimized exciplex structure between OPP* and TEA, the authors of this perspective successfully demonstrated that exciplex stabilization is dominated by interfragment relaxations arising from incomplete charge transfer from TEA to OPP*. 79 EDA therefore offers a suite of powerful characterization methods to not only deconvolute interactions between chromophores, substrates, and electron donors/ acceptors but also generate quantitative insights into the role of solvent in condensed-phase photoredox chemistry.…”