Benzoyl‐Xanthenoxanthenes: Versatile Chromophores for Light‐Engaging Applications

In this work, we present a modular donor-acceptor strategy that produces oxidatively stable, benzoyl-fused peri-xanthenoxanthene (PXX) ribbons with near-infrared (NIR) emission and light-harvesting properties. Benzoyl fusion at the pseudo-peri positions, achieved through intramolecular Friedel-Crafts planarization, creates the first benzoyl-functionalized PXX monomers, dimers, and trimers that exhibit deep red-NIR absorption and emission without significantly raising the HOMO level. The ribbons display strong, tunable absorption and fluorescence bands, with up to 63% of emission in the NIR and an NIR fluorescence quantum yield reaching 0.36 in solution. Lewis adducts formed between boron-based Lewis acids and carbonyl acceptor sites further enhance the electron-accepting nature of the ketones, producing pure NIR emission ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>Φ</mml:mi> <mml:mi>F</mml:mi> <mml:mi>NIR</mml:mi></mml:msubsup> <mml:annotation>$\mathit{\Phi}_{\mathrm{F}}^{\mathrm{NIR}}$</mml:annotation></mml:semantics> </mml:math> ≈ 0.15-0.16). Spectroelectrochemical investigations uncover reversible electrochromism and electrofluorochromism, allowing redox-gated switching and NIR absorption signatures, useful for sensing and display technologies. When embedded in a nematic liquid crystal phase, the ribbons function as cascaded Förster resonance energy transfer (FRET) antennas, achieving near-quantitative single-step energy transfer ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>Φ</mml:mi> <mml:annotation>$\mathit{\Phi}$</mml:annotation></mml:semantics> </mml:math> _FRET = 0.97) and highly efficient two-step transfer ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mi>Φ</mml:mi> <mml:annotation>$\mathit{\Phi}$</mml:annotation></mml:semantics> </mml:math> _FRET = 0.70), enabling directional energy funneling. Their complementary absorption broadens the excitation window to cover the entire visible spectrum, making them highly efficient panchromatic light-harvesting materials.