Planarized emitters exhibiting thermally activated delayed fluorescence (TADF) have attracted attention due to their narrow emission spectra, improved photostability, and high quantum yields, but with large singlet–triplet energy gaps (ΔE ST ) and no heavy atoms, the origin of their TADF remains a subject of debate. Here we prepare two isomeric, coplanar donor–acceptor compounds, with HMAT-2PYM performing dual TADF and room-temperature phosphorescence but with HMAT-4PYM exhibiting only prompt fluorescence. Although conventional TADF design principles suggest that neither isomer should exhibit TADF, we reveal differences in the excited state potential energy surfaces that enable spin-flip processes in only one isomer. We also find that hydrogen bonding is absent between the planar units of these emitters, despite earlier claims of intramolecular hydrogen bonding in similar compounds. Overall, this work demonstrates that potential energy surface analysis is a practical strategy for designing coplanar TADF materials that might otherwise be overlooked by conventional TADF design metrics.

The rotational spectra of 2-fluoroanisole (2-FA) and 3-fluoroanisole (3-FA) were investigated using Fourier transform microwave (FTMW) spectroscopy in the 4-26 GHz range. Assigned transitions correspond to the lowest energy rotamer for 2-FA which has the O-CH3 group directed away (anti) from the fluorine substituent whereas for 3-FA, the spectrum is consistent with the presence of two rotamers arising from syn and anti orientations of the methoxy moiety relative to fluorine. Ab initio calculations at the MP2/cc-pVTZ level were used to estimate the equilibrium (re) geometries of the three observed rotamers. Their assignments were confirmed through the observation of the rotational transitions of eight minor isotopologues ( 13 C and 18 O) in natural abundance for each species. The mass dependence (rm (1) ) structures derived using the experimentally determined rotational constants compare favourably with the ab initio estimates.The resulting sets of geometric parameters suggest that the aromatic ring backbone is distorted by the introduction of the angular methoxy substituent, with a tendency to induce bond length alternation around the ring, and by the electron withdrawing effects of fluorine.

In this study, the photosensitive nature of reactive enamine and polyenamine intermediates is investigated to improve our understanding of light-mediated aminocatalytic reactions. Experimental optical absorption data and TD-DFT calculations reveal that these intermediates are excited directly from the HOMO on the enamine moiety to low-lying unoccupied orbitals localized on the catalyst scaffold. This indicates that the photophysical properties of enamine intermediates can be tuned for visible light-mediated reactions by modifications to the aminocatalyst.

Donor–acceptor materials containing tris(triazolo)triazine (TTT) acceptors have recently gained attention as green to deep-blue fluorophores exhibiting thermally activated delayed fluorescence (TADF).