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

doi:10.1002/cphc.202000926

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

University of Namur

Research output: Contribution to journal › Article › peer-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 language	English
Pages (from-to)	553-560
Number of pages	8
Journal	ChemPhysChem
Volume	22
Issue number	6
Early online date	16 Dec 2020
DOIs	https://doi.org/10.1002/cphc.202000926
Publication status	Published - 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.

Funders	Funder 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 Innovación	PID2019‐106114GB−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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10.1002/cphc.202000926

Cite this

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

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.",

keywords = "ab initio calculations, electron correlation effects, exchange interaction, heptazine derivatives, singlet-triplet energy gap",

author = "G. Ricci and E. San-Fabi{\'a}n and Y. Olivier and Sancho-Garc{\'i}a, {J. C.}",

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doi = "10.1002/cphc.202000926",

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AU - Olivier, Y.

AU - Sancho-García, J. C.

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N2 - 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.

AB - 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.

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

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

Abstract

Funding

Keywords

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High Performance Computing Technology Platform

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