Small is Beautiful: Electronic Origin and Synthetic Evolution of Single-Benzene Fluorophores

ConspectusSingle-benzene fluorophores (SBFs) are small molecules that produce visible light by using only one benzene ring as the sole aromatic core. This Account centers around the chemistry of a new class of SBF that we accidentally discovered but rationally developed and refined afterward. In a failed experiment that took an unintended reaction pathway, we encountered the bright green fluorescence of ortho-diacetylphenylenediamine (o-DAPA). Despite its uninspiring look, reminiscent of textbook examples of simple benzene derivatives, this molecule had neither been synthesized nor isolated before. This discovery led to our studies on the larger DAPA family, including isomeric m-DAPA and p-DAPA. Remarkably, p-DAPA is the lightest red fluorophore, with a molecular weight of only 192. While o- and p-DAPA are emissive, m-DAPA rapidly undergoes internal conversion, facilitated by sequential proton transfer reactions in the excited state.Leveraging the synthetic utility of the amine group, we carried out straightforward single-step modifications to create a full-color SBF library from p-DAPA as the common precursor. During the course of the investigation, we made another fortuitous discovery. With increasing acidity of the N-H group, the excited-state intramolecular proton transfer reaction is promoted, opening up additional pathways for emission to occur at even longer wavelengths. Tipping the balance between the two excited-state tautomers enabled the first example of a single-benzene white-light emitter. We demonstrated the practical utility of these molecules in white light-emitting devices and live cell imaging.According to the particle-in-a-box model, it is difficult to expect a molecule with only one small aromatic ring to produce long-wavelength emission. SBFs rise to this challenge by exploiting electron donor-acceptor pairs around the benzene core, which lowers the energy of light absorption. However, this answers only half of the question. Where do the exceptionally large spectral shifts in the light emission of SBFs originate from? Chemists have long been curious about the molecular mechanisms underlying the dramatic spectral shifts observed in SBFs. Prevailing paradigms invoke the charge transfer (CT) between electron donor and acceptor groups in the excited state. However, without a large π-skeleton for effective charge separation, how could benzene support a CT-type excited state? Our experimental and theoretical studies have revealed that large excited-state antiaromaticity (ESAA) of the benzene core itself is responsible for this remarkable phenomenon. The core matters, not the periphery. With appropriate molecular design, large and extended π-conjugation is no longer a prerequisite for long-wavelength light emission.