Relief of excited-state antiaromaticity enables the smallest red emitter

The study challenges the conventional belief that long-wavelength emission requires extended π-conjugation. It investigates whether a single benzene ring scaffold can yield long-wavelength fluorescence via relief of excited-state antiaromaticity. The focal questions are: (i) how o- and p-DAPA exhibit long-wavelength visible emission from a small benzene core; (ii) what structural features render m-DAPA non-emissive; and (iii) whether structural elaboration of the minimalist DAPA motif can generate a family of fluorophores spanning the entire visible spectrum. The work aims to establish photophysical design rules based on excited-state (anti)aromaticity and intramolecular hydrogen bonding, rather than intramolecular charge transfer (ICT) or excited-state intramolecular proton transfer (ESIPT).

Large Stokes shifts in organic fluorophores are often attributed to ICT or ESIPT processes (e.g., Refs. 5, 6, 17–19). Recent reports showed single-benzene fluorophores (SBFs) with unusual long-wavelength emission and solvent-independent behavior (Refs. 29–33), frequently rationalized by ICT from HOMO–LUMO asymmetry. However, for a benzene-sized core, effective charge separation is questionable. Baird’s rule predicts antiaromatic character for the lowest excited states (S1/T1) of annulenes, including benzene (Refs. 36–46), suggesting that relief of excited-state antiaromaticity (ESAA) via geometric relaxation may drive large Stokes shifts. Prior analyses of aromaticity metrics such as NICS and HOMA provide tools to assess (anti)aromaticity changes upon excitation (Refs. 47–48).

Synthesis: o-, m-, and p-DAPA were accessed and p-DAPA was used as a diversification hub due to synthetic compatibility. Single-step reactions of p-DAPA with various carbonyl electrophiles yielded mono- and di-functionalized derivatives (compounds 4–10), and control compound 11. Several structures (4–8, 10) were confirmed by single-crystal X-ray diffraction, revealing intramolecular N–H···O and C–H···O hydrogen bonds that restrict torsional degrees of freedom. Photophysics: UV–vis absorption and steady-state fluorescence spectra were measured in CHCl3 (typical concentration 50 μM); absolute quantum yields were determined by integrating sphere; lifetimes by time-resolved photoluminescence (450 nm diode excitation). Solvatochromism was examined across solvents; Lippert–Mataga analysis estimated excited-state dipole moments. Computation: Ground and excited states were explored using mixed-reference spin-flip TDDFT (MRSF-TDDFT) at the MRSF/BH&HLYP/6-31G* level with PCM solvent model. Minimum-energy paths on S0/S1/S2 PESs and minimum-energy conical intersections (MECIs) were located (branching plane updating; geodesic interpolation). Aromaticity metrics were computed via NICS(1)zz using CASSCF(2,2)/6-31G* (GIAO) at MRSF-optimized geometries; geometric aromaticity assessed by HOMA indices at S0,min and S0@S1,min. Energies, geometries, and oscillator strengths were extracted to rationalize radiative vs non-radiative pathways, proton transfer coordinates, and accessibility of conical intersections.

- Emission behavior of DAPA isomers: o- and p-DAPA are fluorescent in CHCl3, while m-DAPA is non-emissive. p-DAPA (FW 192.22) displays red emission (λmax,em = 618 nm) and is the lightest red emitter identified by the authors. o-DAPA: λmax,abs = 432 nm (ε = 5160 M−1 cm−1), λmax,em = 531 nm, Stokes shift Δν ≈ 4320 cm−1, ΦF = 0.26, τ = 6.44 ns, kr = 4.0×10^7 s−1, knr = 1.2×10^8 s−1. p-DAPA: λmax,abs = 482 nm (ε = 2890), λmax,em = 618 nm, Δν ≈ 4570 cm−1, ΦF = 0.06, τ = 1.84 ns, kr = 3.3×10^7 s−1, knr = 5.1×10^8 s−1. m-DAPA: λmax,abs = 350 nm (ε = 7050), non-emissive under these conditions. - Tunable library covers visible spectrum: Carbonyl substitutions on p-DAPA yielded 4–10 with blue-shifted absorption/emission relative to p-DAPA and large Stokes shifts (Δν = 4490–5800 cm−1). Representative data: 5: λabs = 405 nm (ε = 5810), λem = 502 nm, Δν = 4770 cm−1, ΦF = 0.73, τ = 8.93 ns, kr = 8.1×10^7 s−1, knr = 3.1×10^7 s−1; 6: λabs = 416 nm, λem = 518 nm, ΦF = 0.61; 7: λabs = 435 nm, λem = 543 nm, ΦF = 0.29; 8–10 (mono-functionalized): λabs = 445–456 nm, λem = 556–581 nm, ΦF = 0.23–0.30. Emission and excitation energies correlate linearly with Hammett parameters. - Hydrogen-bonding enhances fluorescence: Increased carbonyl substitution strengthens intramolecular H-bonds, restricting internal rotations and reducing non-radiative decay. Di-functionalized 5–7 show smaller knr than mono-functionalized 8–10 and parent p-DAPA. - Mechanistic origin: Large Stokes shifts arise from relief of excited-state antiaromaticity (ESAA) of the benzene core via bond-length redistribution during S1 relaxation, rather than ICT or ESIPT. MRSF-TDDFT shows for o- and p-DAPA bright S0→S1 transitions (o: 3.73 eV, f = 0.3133; p: 3.25 eV, f = 0.2921), barrierless relaxation to emissive S1,min, and conical intersections (CI10, CI10,1pt/2pt) located above FC, suppressing non-radiative decay. For m-DAPA, the bright S2 (4.70 eV, f = 1.0503) lies near a dark S1 (4.66 eV); internal conversion via CI21 and access to low-oscillator-strength S1,1pt (4.07 eV, f = 0.0818) and S1,2pt (3.73 eV, f = 0.0000) minima and CI10,2pt enable efficient non-radiative decay, explaining its non-emissive nature. - Aromaticity metrics: NICS(1)zz for p-DAPA switches from aromatic at S0,min (−24.0 ppm) to antiaromatic at S1 FC (+31.2 ppm), then partially relieved at S1,min (+8.8 ppm). Similar trends hold for o-DAPA and 5–10 (Table 2). HOMA decreases at S0@S1,min vs S0,min, indicating reduced ground-state aromaticity at the S1-relaxed geometry, which together with ESAA relief narrows the S1–S0 emission gap. - Control and solvent effects: Control 11 (strongly electron-withdrawing acyl) exhibits dual emission with an ESIPT band (λem = 579 nm, Δν = 8290 cm−1) and a local band (λem = 482 nm, Δν = 5030 cm−1), confirming ESIPT can occur when N–H acidity is high. DAPA fluorophores show minimal solvatochromism; Stokes shifts remain ≈4500 cm−1 across solvents except DMSO/EtOH that disrupt H-bonds. Lippert–Mataga analysis yields modest excited-state dipole moments for p-DAPA (5.1 D) and 5 (6.0 D), much smaller than typical ICT dyes (~20 D). - Generality: Computational analysis of related SBFs (amino-sulfonyl and amino-ester) shows ESAA upon vertical excitation and its relief upon relaxation to S1,min, supporting the general mechanism.

The findings address the core questions by demonstrating that a single benzene ring can support long-wavelength emission when ESAA is relieved via geometric relaxation stabilized by intramolecular hydrogen bonds. The emissive o- and p-isomers possess accessible S1 minima protected from non-radiative decay by high-lying conical intersections, whereas m-DAPA’s geometry positions donor and acceptor groups to enhance N–H acidity and facilitate ESIPT-like proton motion in the excited state, opening low-lying dark states and conical intersections that quench fluorescence. The consistent large Stokes shifts across the DAPA library, their weak solvent dependence, small excited-state dipoles, and lack of ESIPT signatures (except in the control 11) collectively support a mechanism dominated by ESAA relief. This reframes design principles for small-molecule fluorophores: rather than extending π-conjugation or enforcing ICT/ESIPT, controlling excited-state (anti)aromaticity and hydrogen-bond networks can yield efficient, color-tunable emission, including red emission from the lightest reported emitter. The results are relevant to the design of compact emitters for bioimaging and optoelectronics, where large Stokes shifts minimize self-absorption and spectral crosstalk.

This work introduces a minimalist fluorophore platform based on a single benzene core (DAPA) that achieves long-wavelength emission, including the lightest red emitter (p-DAPA, FW 192). Through late-stage carbonyl functionalization, a full-color library (4–10) with large, solvent-insensitive Stokes shifts (~4,500–5,800 cm−1) is realized. Combined experimental and MRSF-TDDFT/CASSCF analyses reveal that large Stokes shifts originate from relief of excited-state antiaromaticity via bond-length redistribution, assisted by intramolecular hydrogen bonding, not ICT or ESIPT. The positioning of H-bond donor/acceptor pairs dictates access to emissive vs non-emissive pathways, explaining the isomer-dependent behavior. These mechanistic insights establish design rules for compact, color-tunable fluorophores and suggest applications in bioimaging and light-emitting devices. Future work may expand substituent space, explore solid-state and device performance, and leverage ESAA control in other small-ring scaffolds.

The study focuses on solution-phase photophysics (primarily in CHCl3). Solvent disruption of intramolecular hydrogen bonds (e.g., in DMSO or EtOH) alters Stokes shifts, indicating sensitivity to hydrogen-bonding environments. No device-level or in situ bioimaging data are reported here.