A Computational Investigation on the Characteristic Electron Delocalization of Symmetrical Squaraine Dyes

Abstract A series of 14 highly conjugated, NIR‐absorbing, charge‐transfer‐free biradicaloid symmetrical squaraine (SQ) dyes were investigated to elucidate the degree of delocalization and nature of aromaticity within their constituent rings. This comprehensive study utilizes a multidimensional approach, incorporating various aromaticity indices, including the Harmonic oscillator model of electron delocalization (HOMED), Aromatic fluctuation index (FLU), Nucleus‐independent chemical shift (NICS), Electron density of delocalized bonds (EDDB), and magnetically induced current densities (MAGIC). For clarity, each SQ dye is divided into three distinct regions: the central core (Ring A), the side substituents (Ring B), and the terminal fused phenyl rings (Ring C, applicable to SQ11‐SQ14). The results reveal significantly diversified aromaticity among the three rings: the Ring A exhibits poor aromaticity/delocalization (balanced paratropic and diatropicity) irrespective of the side substituents, while Ring B and Ring C exhibit good to excellent aromaticity (diatropicity). Beyond the traditional charge‐transfer‐based Donor–Acceptor–Donor (D–A–D) framework, we propose that the electronic structure of SQ dyes may alternatively be described as an Aromatic–Nonaromatic–Aromatic system, with the central ring exhibiting biradicaloid character.