Relief of excited-state antiaromaticity enables the smallest red emitter

This paper challenges the common belief that large π-conjugated systems are necessary for low-energy electronic transitions. The researchers present a new class of light-emitters based on a simple benzene core, specifically diacetylphenylenediamine (DAPA) isomers. They found that *o*- and *p*-DAPA are fluorescent, while *m*-DAPA is not, with *p*-DAPA being the lightest known molecule exhibiting red emission (FW = 192). Modifying the DAPA system creates a library of emitters across the visible spectrum. Theoretical analysis reveals that the large Stokes shifts result from the relief of excited-state antiaromaticity, not intramolecular charge transfer or proton transfer. The study establishes new molecular design rules, suggesting that extended π-conjugation is not essential for long-wavelength light emission.

Heechan Kim, Woojin Park, Younghun Kim, Michael Filatov, Cheol Ho Choi, Dongwhan Lee