Relief of excited-state antiaromaticity enables the smallest red emitter

The design and synthesis of efficient and tunable light-emitting materials is a crucial area of research in organic electronics and bioimaging. Traditionally, the achievement of low-energy electronic transitions, necessary for long-wavelength emission (e.g., red, near-infrared), has been closely tied to the use of extensive π-conjugated systems. These systems provide a delocalized electron cloud that lowers the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), leading to lower energy emission wavelengths. However, this approach often leads to complex synthetic procedures and large, potentially less biocompatible molecules. This research aimed to challenge the conventional wisdom regarding the necessity of extensive π-conjugation for long-wavelength emission. The researchers hypothesized that a carefully designed molecule with a smaller core structure, strategically functionalized, could achieve comparable or even superior performance. The importance of this study lies in its potential to simplify the design and synthesis of light-emitting materials while offering the possibility of creating smaller, potentially more biocompatible molecules for applications such as bioimaging and light-emitting devices. A successful demonstration could drastically reduce the synthesis complexity and improve the accessibility of red emitters, impacting various fields.

The authors extensively review existing literature on the design and synthesis of small molecule fluorophores. They highlight existing approaches for achieving long-wavelength emission, particularly those relying on intramolecular charge transfer (ICT) or excited-state intramolecular proton transfer (ESIPT) mechanisms, and the typical use of large π-conjugated systems. The review touches upon the challenges of creating small molecule fluorophores with large Stokes shifts, essential for applications where separation of excitation and emission wavelengths is critical to minimizing background noise. This review forms a foundation for their innovative approach, which bypasses the traditional strategies based on ICT and ESIPT and instead focuses on the excited-state antiaromaticity.

The study employs a multi-pronged methodology combining synthetic chemistry, spectroscopic techniques, and high-level quantum chemical calculations. **Synthesis:** The researchers synthesized three regioisomers of diacetylphenylenediamine (DAPA) – *o*-, *m*-, and *p*-DAPA – using a straightforward synthetic approach detailed in the supplementary information. Further structural modifications were carried out using *p*-DAPA as a template, introducing various carbonyl electrophiles to generate a library of DAPA derivatives (compounds 4-10). Control compound 11, with a strongly electron-withdrawing trichloroacetyl group, was also synthesized. Crystal structures for several key compounds were confirmed using single-crystal X-ray diffraction. **Spectroscopic measurements:** UV-Vis absorption and fluorescence spectroscopy were employed to characterize the photophysical properties of the synthesized compounds in chloroform solution. Quantum yields and fluorescence lifetimes were measured using an integrating sphere and time-resolved photoluminescence techniques, respectively. Fluorescence imaging was used to visualize the emission colors of the synthesized fluorophores. **Computational studies:** Advanced quantum chemical calculations were performed to elucidate the electronic structure and excited-state dynamics of the DAPA isomers and derivatives. The mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT) method was used to calculate potential energy surfaces and locate conical intersections. Calculations were also conducted to determine nucleus-independent chemical shift (NICS) values and harmonic oscillator model of aromaticity (HOMA) indices, providing insight into the aromaticity/antiaromaticity of the benzene core in both ground and excited states. The effect of solvent polarity on emission energy was also assessed to investigate the contribution of ICT. Various computational details are provided in the methodology section of the paper. The MRSF-TDDFT and CASSCF calculations were performed using the GAMESS and Dalton packages, respectively.

The key findings of this research are multifaceted and significant: 1. **Discovery of the smallest red emitter:** *p*-DAPA (FW = 192) is identified as the lightest molecule capable of red emission, challenging the long-held assumption that large π-conjugated systems are required for such low-energy emission. 2. **Structure-property relationship:** A systematic study of DAPA isomers revealed that *o*- and *p*-DAPA are emissive, while *m*-DAPA is non-emissive. This difference is attributed to variations in their excited-state energy landscapes, directly linked to the position of the acetyl and amino groups. 3. **Full-color tunability:** Through facile modifications to *p*-DAPA, the researchers developed a library of fluorophores emitting across the entire visible spectrum. This tunability is achieved via controlled adjustments of the peripheral substituents, influencing the HOMO-LUMO energy gap. 4. **Large Stokes shifts:** All emissive DAPA derivatives exhibit large Stokes shifts (around 4500 cm⁻¹), which are exceptionally large for small molecules. This phenomenon is not attributed to ICT or ESIPT, as is commonly observed in similar systems. 5. **Relief of excited-state antiaromaticity:** The researchers propose a novel explanation for the large Stokes shifts based on the relief of excited-state antiaromaticity (ESAA). Theoretical calculations showed that the benzene core becomes antiaromatic in the excited state (at the Franck-Condon geometry) and that this antiaromaticity is relieved through geometric relaxation, leading to a significant reduction in the excited-state energy and the substantial Stokes shift. The role of intramolecular hydrogen bonding in facilitating this geometric relaxation is also highlighted. The strong dependence of the excited state antiaromaticity on the substitution pattern explains why only *o* and *p* isomers are fluorescent. 6. **No significant ICT or ESIPT:** Experimental and theoretical data rule out significant contributions from ICT and ESIPT to the large Stokes shift and fluorescence of the DAPA derivatives. The lack of significant solvatochromism, along with the modest dipole moments in the excited state, support this conclusion.

This study significantly advances the understanding of molecular design principles for efficient light-emitting materials. The discovery that the relief of excited-state antiaromaticity in a simple benzene core can lead to large Stokes shifts and red emission opens up new avenues for designing small molecule fluorophores. The results challenge the conventional wisdom that extended π-conjugation is necessary for long-wavelength emission. The straightforward synthetic approach and tunability of the DAPA platform offer advantages over existing methods, potentially leading to a wider range of applications. The mechanistic understanding gained through computational modeling provides a framework for the rational design of new fluorophores with specific spectral and photophysical properties. The lack of significant ICT or ESIPT contribution expands the design parameters for large Stokes shift materials.

This paper demonstrates that a simple benzene core, suitably functionalized, can produce efficient and tunable light emission, including the smallest known red emitter. The large Stokes shifts exhibited by these fluorophores are explained by the relief of excited-state antiaromaticity assisted by intramolecular hydrogen bonding, not ICT or ESIPT. The facile synthesis and tunability of the DAPA platform pave the way for the development of new small-molecule fluorophores for diverse applications such as bioimaging and optoelectronic devices. Future research could explore further structural modifications to enhance brightness and explore the potential of these fluorophores in different environments and applications.

The study primarily focuses on solution-phase characterization of the fluorophores. Further investigation into the solid-state properties and performance in device applications is needed. While the theoretical calculations provide compelling support for the proposed mechanism, other contributing factors may warrant exploration. The range of carbonyl substituents investigated could be expanded to further explore the structure-property relationship and to achieve fine-tuned color control. The current study primarily focuses on chloroform as the solvent; the influence of different solvent environments on emission properties needs further evaluation.