Relief of excited-state antiaromaticity enables the smallest red emitter

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

doi:10.1038/s41467-021-25677-2

Heechan Kim, Woojin Park + Show 4 more

Open Access

https://doi.org/10.1038/s41467-021-25677-2

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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.

Highlights

It is commonly accepted that a large π-conjugated system is necessary to realize low-energy electronic transitions
We found that the excited-state antiaromaticity of the benzene core itself[36,37], rather than the typically assumed intramolecular charge transfer (ICT) or excited-state intramolecular proton transfer (ESIPT), is responsible for their peculiar photophysical properties
Visual observation of its green emission under UV lamp prompted our interest in its improved synthesis and comparative studies with regioisomeric m-DAPA and p-DAPA

Summary

Introduction

It is commonly accepted that a large π-conjugated system is necessary to realize low-energy electronic transitions. Existing strategies to realize large Stokes shift rely on (i) intramolecular charge transfer (ICT)[5,6,16], (ii) excited-state intramolecular proton transfer (ESIPT)[17,18,19], (iii) fluorescence resonance energy transfer (FRET)[20,21], (iv) desymmetrization of the molecular structure[6,22,23,24], (v) excimer/exciplex emission[25], or (vi) excited-state planarization of the benzannelated 8π system[26,27,28] These design principles are applicable to known fluorogenic motifs, but often compromise other photophysical properties such as emission quantum yield. While the electronic origin of the large Stokes shifts of SBFs has often been ascribed to the HOMO–LUMO asymmetry implying ICTtype transitions[29,30,31,32], it is less clear how the small benzene core can promote significant charge separation

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