Synthesis and red emission properties of indolo[3,2-b]carbazole derivatives modified by β-diketone boron difluoride

Abstract

Organic fluorescent materials are extensively utilized in organic light-emitting diodes, biological imaging, fluorescent probes and other fields due to their easily controlled photophysical properties and high photostability. However, due to the constraints of energy band theory, compared to blue and green fluorescent materials, most organic red fluorescent materials possess large conjugated structures and introduce push-pull groups to regulate the distribution of electron clouds, which typically exhibit low fluorescent quantum efficiency. To investigate the regulatory mechanism of molecular spatial structure on the photophysical properties of red fluorescent materials, three derivatives of indolo[3,2-b]carbazole modified with β-diketone boron difluoride groups (named ICZ-H, ICZ-F and ICZ-CH₃) were designed and synthesized. A systematic study was conducted on the photophysical properties in organic solvents and aggregated states, and the relationship between their spatial structure and properties was explored in detail. The UV–Vis absorption and fluorescent emission spectra in various organic solvents revealed that all three compounds exhibited strong absorption peaks near 490 nm, with maximum molar absorption coefficient exceeded 6.0 × 10⁴ L/(mol cm)⁻¹. The fluorescent intensities decreased with increasing solvent polarity, accompanied by a red-shifted in the fluorescent peak position. In the meantime, density functional theory calculations were utilized to visualize the process of intramolecular charge transfer. The aggregation fluorescent behaviors in a mixed solvents of tetrahydrofuran and water showed that the fluorescent intensity initially decreased and then increased, suggesting that all three indolo[3,2-b]carbazole derivatives retained the fluorescent emission property during the aggregation process. The fluorescent emission spectra in the solid state showed that the fluorescent peaks of all three compounds were all over 630 nm, which belonged to typical red fluorescent materials. The study on the structure-properties relationship of this material provides an experimental basis and theoretical guidance for the design of red-emitting fluorescent materials with excellent photophysical properties.