Relief of excited-state antiaromaticity enables the smallest red emitter

Abstract

It is commonly accepted that a large π-conjugated system is necessary to realize low-energy electronic transitions. Contrary to this prevailing notion, we present a new class of light-emitters utilizing a simple benzene core. Among different isomeric forms of diacetylphenylenediamine (DAPA), o- and p-DAPA are fluorescent, whereas m-DAPA is not. Remarkably, p-DAPA is the lightest (FW = 192) molecule displaying red emission. A systematic modification of the DAPA system allows the construction of a library of emitters covering the entire visible color spectrum. Theoretical analysis shows that their large Stokes shifts originate from the relief of excited-state antiaromaticity, rather than the typically assumed intramolecular charge transfer or proton transfer. A delicate interplay of the excited-state antiaromaticity and hydrogen bonding defines the photophysics of this new class of single benzene fluorophores. The formulated molecular design rules suggest that an extended π-conjugation is no longer a prerequisite for a long-wavelength light emission.

Fig. 1. Single benzene fluorophores.

a HOMO–LUMO energy level diagrams of benzene derivatives calculated at the MRSF/BH&HLYP/6-31 G* level of theory. b Chemical structures of DAPA isomers along with photographic images of CHCl₃ solution samples taken under 365 nm UV lamp. LUMO lowest unoccupied molecular orbital, HOMO highest occupied molecular orbital.

Fig. 2. Construction of isomeric single benzene fluorophores.

Synthetic routes for o-, m-, and p-DAPA. p-TsOH para-toluenesulfonic acid, EtOH ethanol.

Fig. 3. Structure-dependent light absorption and emission.

Absorption (thin lines) and normalized emission (thick lines) spectra of o-DAPA (green), m-DAPA (black), and p-DAPA (red) in CHCl₃ (sample concentrations = 50 μM). The inset compares the absorption (thin lines) and emission (thick lines) spectra of o- and p-DAPA; the absorption spectra are normalized to the absorbance at the longest maximum absorption wavelengths (λ_max,abs), whereas the emission spectra are normalized to the maximum fluorescence intensity. Fl., fluorescence.

Fig. 4. Full-color fluorophore library of DAPA.

a Chemical structures of p-DAPA derivatives 4–11, along with capped-stick representation of 5 generated with crystallographically determined atomic coordinates. Hydrogen bonds are denoted by dotted lines. b Fluorescence images of 4–10 and p-DAPA in CHCl₃ under irradiation of 365 nm UV light (top), and normalized emission spectra (bottom). c Chromaticity coordinates (CIE) of 4–10 and p-DAPA in CHCl₃.

Fig. 5. Potential energy surfaces.

Calculated S₀ (black), S₁ (red), and S₂ (blue) potential energy surfaces of o-DAPA (a), m-DAPA (b), and p-DAPA (c). The geometries were optimized at the MRSF/BH&HLYP/6-31 G* level of theory. Using computed structures, minimum energy paths (MEPs) were constructed, and optimized using the geodesic interpolation method_. For each transition, calculated wavelengths and oscillator strengths are shown in bold and italic, respectively. FC Franck–Condon region, CI conical intersection, IC internal conversion, PT proton transfer.

Fig. 6. Relief of excited-state antiaromaticity assisted by intramolecular hydrogen bonds.

a Schematic energy diagram with calculated NICS(1)_zz values (in bold) at the optimized geometries of p-DAPA. b Calculated NICS(1)_zz grids parallel to the molecular plane of p-DAPA. c, d Bond lengths (Å) and HOMA value of p-DAPA at the S_0,min (c), and S₀@S_1,min (d) geometry. FC Franck–Condon, min minimum, NICS nucleus-independent chemical shift, HOMA harmonic oscillator model of aromaticity.