Rare-Gas Excimers and Exciplexes: Surprising Challenges for Time-Dependent Density Functional Theory Approaches

Abstract

Excimers (excited dimers of identical monomers) and exciplexes (excited complexes of distinct monomers) are bound by interactions including electrostatics, charge transfer, London dispersion, and excitonic splitting (excimers only). As such, these systems are challenging to describe theoretically. Time-dependent density functional theory (TD-DFT) is the mostly widely used approach for excited-state computational chemistry; however, it is not yet clear whether such methods are able to adequately describe excimer and exciplex binding. In this work, we present a systematic TD-DFT benchmarking study of rare-gas excimers and exciplexes. Interaction energy curves are computed for these systems with CC3/aug-cc-pV5Z as an accurate reference. 79 variations of dispersioncorrected and uncorrected TD-DFT methods are benchmarked that span Jacob’s ladder and include recent spin-scaled double-hybrids. It is shown that TD-DFT tends to overbind the rare-gas excimers and exciplexes. This is surprising given that TD-DFT has known limitations in describing London dispersion forces which typically results in underbound complexes. The overbinding of rare-gas excimers is attributed to red-shifted excitation energies of their Rydberg states. The Helium excimer has particularly strong binding arising from large excitonic splitting. For this excimer, TD-DFT is generally underbinding, which is due to greater long-range character for the dissociated monomers compared to the bound excimer. On average, the double-hybrid TD-DFT methods are the most robust and in particular SCS-PBE-QIDH has the lowest mean absolute error (1.18 kcal mol−1). There remains a need for additional benchmarking and method development to address the various interactions involved in excimer and exciplex binding.

Keywords: TD-DFT, excimer, exciplex, benchmarking, rare-gas dimer