Theoretical Characterization of Aromatic Exciplex Fluorescence

The negative effects of soot on the environment and human health are well known, but efforts to decrease soot production in combustion processes are hampered by the absence of accurate, transferable models for soot formation. Uncertainties about the soot nucleation mechanism, including the size and properties of the molecules involved and the relative importance of chemical and physical stabilization, have made model development difficult. Electronic spectroscopy methods such as laser-induced fluorescence (LIF) have the potential to characterize transient soot nuclei, but interpreting spectra requires a comprehensive understanding of the photoresponse of likely soot precursors, namely polycyclic aromatic hydrocarbon (PAH) dimers and clusters. To build up a picture of this photoresponse using theory, it is necessary to evaluate which methods are capable of treating the relevant molecules at reasonable cost while capturing the excited-state and noncovalent interactions involved in excimer and exciplex formation, a key excited-state process for aromatic clusters. In this work, we describe extensive benchmarking of basis set error in highly-accurate perturbatively-corrected multireference calculations of exciplex interaction strength and use the best possible multireference approach to evaluate the performance of less-expensive time-dependent density functional theory (TDDFT) results. Using the most accurate TDDFT methods, we explore how the geometric and electronic properties of the monomers influence excited-state interactions in complexes, considering a large database of complexes. A predictive model for exciplex fluorescence emissions of complexes containing six-membered ring PAHs based on monomer HOMO-LUMO gaps is proposed. We describe the contrasting photoresponse of PAHs containing five-membered rings, where nonaromatic groups produce conformational flexibility that has a strong impact on absorption and emission behavior.