Compact Near‐Infrared‐Absorbing Triarylmethane Dyes Based on Antiaromatic Fluorenyl Cations

Electronic‐structure control using antiaromaticity offers an effective molecular design strategy for developing compact organic near‐infrared (NIR)‐absorbing dyes. However, current NIR‐absorbing dyes are costly, toxic, or difficult to synthesize. In this study, we designed and synthesized donor–π–acceptor‐type triarylmethane dyes combining an antiaromatic fluorenyl cation framework with an electron‐donating julolidine unit. DFT, TD–DFT, NICS, and structural analyses indicated the formation of the fluorenyl cation framework enhances antiaromatic character and planarity and narrows the HOMO–LUMO gap, while electron‐donating substituents at the 3,6‐positions further promote long‐wavelength absorption. The NMe 2 ‐substituted dye 1b‐ClO 4 exhibited an absorption maximum at 696 nm in CH 2 Cl 2 , with an absorption edge extending beyond 900 nm. Counteranion exchange had little effect on the absorption wavelength, whereas repeated measurements revealed appreciable variability in absorption intensity. In addition, 1b‐ClO 4 showed weak photoluminescence (PL) at 916 nm upon excitation at 706 nm, with a PL quantum yield below 0.01. Photostability under the LED irradiation conditions employed in this study showed some dependence on the counteranion. The dye also showed little spectral change under acidic conditions but pronounced absorption changes upon base addition. These results demonstrate that control of the fluorenyl cation framework and donor strength provides a promising strategy for compact NIR‐absorbing dyes.