Violation of Hund's multiplicity rule in the electronically excited states of conjugated hydrocarbons

Abstract

This paper is dedicated to Professor Camille Sandor-on the occasiotz of his 65th birthday SHIRO KOSEKI, TAKESHI NAKAJIMA, and AZUMAO TOYOTA. Can. J. Chem. 63, 1572Chem. 63, (1985. Violation of Hund's multiplicity rule in the electronically excited states of conjugated hydrocarbons is studied by using the Pariser-Pam-Pople type SCF MO method and the ab initio MO method with STO-3G basis set, both methods being augmented by CI-type treatments. It is shown that for symmetrical structures (Dzl,) of the nonalternant hydrocarbons, propalene, pentalene, and heptalene, the lowest excited singlet state is energetically lower than the corresponding triplet state. This is mainly due to the spin polarization (SP) effects. For D2/, structures of pentalene and heptalene the open-shell excited singlet state is predicted to be lower in energy than the closed-shell state, with the result that the former is really the ground state. Further, calculations made by including electron correlation effects reveal that in linear polyenes and polyacenes, the lowest excited singlet "minus" state (using Pariser's classification of the alternancy symmetry species) is lower in energy than the corresponding triplet state. The energy lowering of the singlet "minus" state in linear polyenes is due mostly to the mixing with the doubly excited configurations (tnm + nn), while the considerable part of it in polyacenes is due to the SP effects. [Traduit par le journal]

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

“…For 2T‐4N, and similarly to the case of 2T‐N, we found values up to % ( %) for the S 1 ( T 1 ) excited‐states, and thus also slightly higher for the singlet than for the triplet wavefunction. Doubly‐excited configurations are known to lead to a stabilization few times larger for singlet than for triplet excited‐states, which supports the values obtained for 2T‐N and 2T‐4N essentially because Coulomb correlation primarily involves the interaction between anti‐parallel spins [73,74] . For 2T‐7N, those values are % ( %) for the S 1 ( T 1 ) excited‐states, and therefore similar to 2T‐N and 2T‐4N.…”

Violation of Hund's multiplicity rule in the lowest excited singlet–triplet pairs of cyclic bicalicene and its higher homologues

“…Cyclic Bisesquifulvalene (2).-The situation here is quite similar to the case of cyclic bicalicene. The energy gap for the lowest excited singlet-triplet pair is 0.111 eV before SP effects are included.…”

Organic Molecules with Inverted Gaps between First Excited Singlet and Triplet States and Appreciable Fluorescence Rates

“…We note that ωB2PLYP only reproduces an inverted singlet-triplet gap for 2, but not for 1. This is the result of a systematic and correctable offset compared to benchmark methods like ADC (2) or EOM-CCSD (vide infra). Scheme 1.…”

“…Very few, i.e. on the order of 10, organic INVEST molecules were predicted on the basis of computations alone 2,13,14 with little to no experimental evidence, 15,16 and, as far as we know, no inorganic INVEST molecule is known to date. Besides inherently inverted molecules, it has been shown in recent years that the influence of the environment can also make the gap negative, 17 for instance in exciplexes, 18 through strong light-matter coupling in microcavities, 19 and polarizable environments.…”

“…This prediction holds for all but a handful of all known compounds, both organic and inorganic. 2,3 Hence, it is the basis for Jablonski diagrams 4 in educational material about electronic spectra of molecules illustrating that it is almost considered a basic truth in chemistry. [5][6][7][8][9][10][11][12] Molecules violating Hund's first rule in their first excited singlet and triplet energies are said to possess an "inverted" singlet-triplet gap (herein termed the INVEST property).…”