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

G. Ricci, E. San-Fabián, Y. Olivier, J. C. Sancho-García

Research output: Contribution to journalArticlepeer-review

Abstract

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 ((Formula presented.)). 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 (Formula presented.) values found for most of the compounds, which is not predicted by TD-DFT.

Original languageEnglish
Pages (from-to)553-560
Number of pages8
JournalChemPhysChem
Volume22
Issue number6
Early online date16 Dec 2020
DOIs
Publication statusPublished - 17 Mar 2021

Keywords

  • ab initio calculations
  • electron correlation effects
  • exchange interaction
  • heptazine derivatives
  • singlet-triplet energy gap

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