Dual-channel deep-NIR-emissive N-embedded PAHs with hybridized local and charge-transfer excited-state

Abstract

Polycyclic aromatic hydrocarbon (PAH) molecules have been extensively investigated, and they showcase excellent optoelectronic properties, which are promising for optical applications, including deep-penetration bioimaging and NIR lasers. However, constructing PAHs with deep-NIR (800–1700 nm) photoluminescence is a long-standing challenge, owing to the limitation of the energy gap law. Herein, three N-atom-doped PAHs APAH-a–c with electronic acceptor–donor–acceptor (A–D–A) configuration were produced via a facile sandwich-like peri-fusion pathway. On the basis of these three model molecules, their electronic characters and physicochemical properties were comprehensively studied by X-ray crystallographic analyses, various spectroscopic analyses and theoretical calculations. Our outcomes revealed that core-twisted APAH-b and APAH-c displayed unique dual-emission with reasonable photoluminescence quantum yield (PLQY) in low-polarity solvents, which were further red-shifted to ∼975 nm with increasing solvent-polarity due to their hybridized local and charge-transfer (HLCT) character. In sharp contrast, compound APAH-a with a planar π-skeleton did not show any charge-transfer (CT) character or fluorescent solvatochromism effect, indicating a synergistic effect of skeletal nonplanarity and electron-withdrawing N-doping for formation of an HLCT excited-state. Additionally, varying the π-configuration also gives rise to other differentials, such as aromaticity, band gap and open-shell characteristics. In conclusion, our findings offer a valid strategy for the development of deep-NIR-emissive PAH-derivatives via fine-tuning the geometrical structure of the π-framework.