Halogen-Bonded BODIPY Frameworks with Tunable Optical Features*.

Area of Science:

• Materials Science
• Supramolecular Chemistry
• Organic Electronics

Background:

• Tuning optical properties of BODIPY materials in solid-state is vital for photorelated applications.
• Controlling crystal packing is key to achieving desired optical features.
• Halogen bonding (XB) offers a precise method for molecular assembly.

Purpose of the Study:

• To achieve controlled crystal packing in BODIPY supramolecular assemblies using halogen bonding.
• To investigate the impact of XB interactions on the optical properties of BODIPY materials.
• To explore the relationship between crystal structure and photophysical behavior.

Main Methods:

• Design and synthesis of BODIPY molecules with halogen-bond donor (I, Br) and acceptor (-NO2) groups.
• Characterization of crystal structures, including mono-coordinate motif B3, B4 (1D tubular) and bifurcated motif B4-II (1D zigzag).
• Spectroscopic analysis to study photophysical properties, including fluorescence and intersystem crossing rates.

Main Results:

• Formation of isostructural motifs (B3, B4) and symmetric bifurcated motifs (B4-II) via N-O⋅⋅⋅I, Br XB interactions.
• XB interactions promoted singlet-to-triplet and triplet-to-singlet intersystem crossing.
• Unexpected fluorescence enhancement observed in B4-II due to partial electron delocalization.
• Tunable indirect optical band gaps (1.85–2.50 eV) achieved through XB-driven crystal packing.

Conclusions:

• Deliberate design of BODIPY molecules with XB functionalities enables precise control over solid-state supramolecular assembly.
• XB interactions significantly influence photophysical properties, including fluorescence and intersystem crossing.
• The study demonstrates a strategy for tuning optical band gaps in BODIPY materials for advanced photorelated applications.