Photoexcited and ground-state diradical(oid) character in a triquino[3]radialene

Abstract

The unusual [3]radialene motif at the centre of triquino[3]radialene permits exceptional electronic conjugation between the three quinone arms of the molecule, giving rise to a remarkably low-energy and intense absorption spectrum. Since triquino[3]radialene's initial synthesis in the 1970s, questions have been posed around the molecule's potential diradical(oid) character. In this work we present new synthetic approaches to triquino[3]radialene and a thorough spectroscopic and computational evaluation of its properties. Despite broad UV-visible absorption, the triquino[3]radialene is non-emissive. An investigation by transient absorption spectroscopy reveals that the initially-formed excited state of triquino[3]radialene decays to a triplet state with a lifetime of 0.82 μs. Using computational chemistry, we compare triquino[3]radialene to Yang's biradical and galvionxyl, with which it is structurally similar. DFT reveals insights into the role of aromaticity in the photoexcited triplet state, where quinoidal rings become more aromatic. MCSCF and CIPT2 calculations reproduce the experimental spectra and reveal a diradical character of y = 0.05 in triquino[3]radialene.

Fig. 1. [3]Radialene and its selected analogues. In the lower box, cycles with [4n + 2] π-electrons are highlighted yellow.

Fig. 2. Examples of diradical and diradicaloid molecules. Aromatic sextets are shaded yellow. R = Ph.

Scheme 1. Revised synthesis of triquino[3]radialene, which substitutes benzene and PbO₂ in the original for CH₂Cl₂ and DDQ, respectively.

Fig. 3. (a) Transient absorption of 2 after excitation at 800 nm; (b) ground-state spectrum of 2 (black) and decay-associated spectra of the short-lived species S₁ (orange) and the longer-lived species T₁ (green).

Fig. 4. (a) Difference in bond lengths between triplet and singlet geometries (triplet–singlet); (b) spin density (CAM-B3LYP/6-31G*), isosurface plotted at 0.002; (c) magnetically-induced current densities of a truncated model of 2 (tBu replaced with H), calculated 1.7 Å above the plane using SYSMOIC.