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Singlet‐Triplet Excited‐State Inversion in Heptazine and Related Molecules: Assessment of TD‐DFT and ab initio Methods

2020; Wiley; Volume: 22; Issue: 6 Linguagem: Inglês

10.1002/cphc.202000926

1439-7641

Gaetano Ricci, Emilio San‐Fabián, Yoann Olivier, J. C. Sancho‐García,

Electron Spin Resonance Studies

We have investigated the origin of the S1 -T1 energy levels inversion for heptazine, and other N-doped π-conjugated hydrocarbons, leading thus to an unusually negative singlet-triplet energy gap ( ΔEST<0 ). Since this inversion might rely on substantial doubly-excited configurations to the S1 and/or T1 wavefunctions, we have systematically applied multi-configurational SA-CASSCF and SC-NEVPT2 methods, SCS-corrected CC2 and ADC(2) approaches, and linear-response TD-DFT, to analyze if the latter method could also face this challenging issue. We have also extended the study to B-doped π-conjugated systems, to see the effect of chemical composition on the results. For all the systems studied, an intricate interplay between the singlet-triplet exchange interaction, the influence of doubly-excited configurations, and the impact of dynamic correlation effects, serves to explain the ΔEST<0 values found for most of the compounds, which is not predicted by TD-DFT.