Molecular design of NIR-II organic fluorescent probe based on adjusting the shielding unit

Organic fluorophores are extensively utilized in the second near-infrared window (NIR-II) for bioimaging applications, owing to their superior biocompatibility and favorable metabolic properties. This study focused on the shielding unit-donor-acceptor-donor-shielding unit (S-D-A-D-S) architecture of organic it-conjugated molecules, which mitigates aggregation-induced quenching by the modification of terminal shielding units. The introduction of thiophene groups as donor units serves to augment conjugation length, thereby inducing a redshift in the fluorescence wavelength with an increasing number of donor units. Density functional theory (DFT) and timedependent DFT (TD-DFT) methods were employed to investigate the frontier molecular orbitals, electronic absorption, and fluorescence spectra of the designed compounds. An in-depth analysis elucidates the fluorescence mechanisms and the impact of shielding units on the fluorescence properties of the compounds. The results indicate that the phenoxazine group (POZ) linked at position 'a' provides the most pronounced shielding effect. Extending the conjugation in the POZ-a compound results in an extend fluorescence wavelength within the NIRII spectrum, with the POZ-5 compound achieving a fluorescence wavelength of up to 1308 nm in aqueous environment. Furthermore, modulating the it-conjugated bridge's length enables precise control over the compound's fluorescence wavelength, luminescence intensity and color purity, with negligible impact from the shielding units on the compound's fluorescence performance.