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

Funding

The work in Alicante is supported by “Ministerio de Ciencia e Innovación” of Spain (Grant No. PID2019‐106114GB−I00). Computational resources in Namur are provided by the “Consortium des Équipements de Calcul Intensif” (CÉCI), funded by the “Fonds de la Recherche Scientifiques de Belgique” (F.R.S.‐F.N.R.S.) under Grant No. 2.5020.11. G.R. acknowledges a grant from the “Fonds pour la formation a la Recherche dans l'Industrie et dans l'Agriculture” (F.R.I.A.) of the F.R.S.‐F.N.R.S.

FundersFunder number
F.R.I.A.
F.R.S.‐F.N.R.S.2.5020.11
Fonds de la Recherche Scientifiques de Belgique
Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture
Ministerio de Ciencia e InnovaciónPID2019‐106114GB−I00
Ministerio de Ciencia e Innovación

    Keywords

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

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