Biradicaloid and Polyenic Character of Quinoidal Oligothiophenes Revealed by the Presence of a Low-Lying Double-Exciton State

Abstract

Evidence of the biradicaloid and polyenic character of quinoidal oligothiophenes is reported by proving at the CASSCF//CASPT2 computational level the presence of a low-lying double exciton state responsible for the weak features observed in the NIR absorption region of the longest members of this class of molecules. The energy lowering of this state, accompanying the length increase in the oligomers, causes a displacement of the ground-state equilibrium geometry toward more biradicaloid structures because of the more efficient S0-S1 state mixing. Furthermore, it is shown that the doubly excited state is strongly coupled to the ground electronic state, and the coupling is mediated by a collective mode dominated by the out-of-phase stretching of adjacent CC bonds, recently shown to govern the Raman activity. All together, this evidence offers a unified view of the low-lying electronic states for quinoidal oligothiophenes and polyenes.

Open-shell polycyclic aromatic hydrocarbons

“…The most salient experimental features of a PAH with biradical character include the line broadening of 1 H NMR arising from the existence of thermally excited triplet species, and a relatively small HOMO-LUMO gap to facilitate doubly excited electronic configuration into the ground state configuration. 1,40 However, in contrast to the theoretical predictions, all functionalized stable hexacene and heptacene derivatives display 1 H NMR signals with sharp splitting and narrow line widths, indicating a closed-shell ground state. [41][42][43][44] Alternatively, investigations on nonacene derivatives provide promising observations which may arise from their singlet open-shell ground state (Fig.…”

Modern zethrene chemistry

“…One signature of the diradicaloid character is the presence of a low-lying excited singlet state dominated by the doubly excited (H,H¡L,L) electronic configuration. 18 The S 0 ¡S 1 transition is one-photon forbidden but two-photon allowed, and as a consequent, a weak absorption shoulder is observed at low energy edge in open-shell singlet diradicaloids, and these molecules usually show large two-photon absorption (TPA) crosssections. Experimentally, the diradical character index can be evaluated if both one-photon and two-photon absorption spectra can be completely measured and the singlet-triplet energy gap can be determined:…”

Recent Progress in Quinoidal Singlet Biradical Molecules

“…Negri et al reported that a weak low-energy band was observed for thienoquinoid compounds with large biradical character. 59 The reason why bisanthene has negligible biradical character while that of teranthene is appreciable is as follows. Upon transforming from the Kekulé to the biradical form for anthenes, one CC π-bond is cleaved, as shown in Figure 14.…”