Valence state engineering in multi-heteroatom-doped PAHs: a strategy for tunable photophysical properties and phototheranostic potentials

Abstract

Precise modulation of the electronic structures in heteroatom-doped polycyclic aromatic hydrocarbons (hetero-PAHs) is essential for advancing organic optoelectronic materials. Herein, we report a facile synthetic strategy for hetero-PAHs co-doped with N, O, and S/Se, achieved via acid-catalyzed Pictet–Spengler reactions and thermally induced ipso-substitution. Systematic π-extension results in a large redshift in optical absorption/emission and enhances fluorescence quantum yield (Φ_F) from 4.0% to 30.5%. Furthermore, a post-synthetic valence state engineering approach enables precise tuning of photophysical properties: (i) oxidation of thiophene units to thiophene S,S-dioxides increases Φ_F to 42.3% and enhances singlet oxygen generation, and (ii) pyridinium-functionalized hetero-PAHs exhibit strong near-infrared absorption, leading to high photothermal conversion efficiency (up to 61%) at the molecular level. Notably, Se-doped derivatives outperform their S-doped counterparts, underscoring the heavy-atom effect in triplet-state modulation. This work provides a versatile platform for tailoring hetero-PAH electronic structures via valence state manipulation, offering potential applications for organic electronics and phototheranostics.