Tuning the near-infrared absorption of meso-C≡N-substituted BODIPY via group modification at various sites

Abstract Boron dipyrromethene (BODIPY) derivatives exhibiting near-infrared absorption demonstrate significant potential in photodynamic therapy and bioimaging. This study investigates how substitution positions and electron-donating groups affect the absorption of both the unsubstituted meso-CN BODIPY core (compound 1) and its derivatives (compounds 2–14). Compound 1 exhibits an absorption maximum at 545 nm. This peak undergoes a blueshift to 528 nm upon the introduction of –OH groups at the 1,7-positions in its derivative, compound 4, whereas methyl substitution (3) leads to a slight blueshift in absorption to 536 nm. In contrast, the presence of –NH2 groups at the 2,6-positions (9) leads to a significant shift to 798 nm, placing it within the near-infrared I range (700–950 nm). Similarly, the introduction of –OH groups at the 2 and 6 positions (7 and 8) results in smaller shifts to 578 nm and 669 nm, respectively. Substitutions at the 3,5-positions exhibit similar but less significant effects. The observed redshift trend correlates with the electron-donating strength of the substituents: –NH 2 > –OH > Me > H. It is demonstrated that placing electron-donating groups at the 2,6-positions effectively narrows the HOMO–LUMO gap: 9 (2.210 eV) < 8 (2.573 eV) < 6 (2.895 eV) < 10 (2.981 eV) < 1 (3.014 eV) and enhances BODIPY core electron density: 9 (+12.46%) > 8 (+1.15%) > 6 (–7.22%) > 10 (–9.84%) > 1 (–10.93%). It is concluded that a donor–acceptor (D-A) structure enhances intramolecular charge transfer (ICT), thus causing a redshift in the absorption spectrum. These results offer important guidance for the development of near-infrared BODIPY compounds.