Light-Harvesting Crystals Formed from BODIPY-Proline Biohybrid Conjugates: Antenna Effects and Excitonic Coupling

Area of Science:

• Materials Science
• Photochemistry
• Crystallography

Background:

• Boron dipyrromethene (BODIPY) dyes are known for their strong fluorescence.
• The solid-state optical properties of BODIPY derivatives can differ significantly from their solution behavior.
• Understanding solid-state photophysics is crucial for developing advanced optical materials.

Purpose of the Study:

• To investigate the solid-state photophysical properties of a specific BODIPY derivative.
• To elucidate the role of crystal packing and intermolecular interactions on optical behavior.
• To correlate structural features with observed absorption and emission spectra.

Main Methods:

• Single-crystal X-ray diffraction to determine molecular and crystal structure.
• UV-Vis absorption and fluorescence spectroscopy in solid-state and solution.
• Time-resolved fluorescence measurements.
• Theoretical modeling using the extended dipole approach.

Main Results:

• The BODIPY derivative crystallizes in platelets with strong red fluorescence (Φ_F = 0.34).
• Solid-state spectra show marked differences from solution spectra, indicating strong intermolecular interactions.
• A pseudo-dimer structure with hydrogen-bonded proline groups was identified as a key building block.
• Accretion into narrow channels leads to significant excitonic splitting in absorption, explained by coupling of ~30 BODIPY units.
• Fluorescence originates from a surface dimer, populated by energy transfer from interior molecules.

Conclusions:

• Crystal packing and excitonic coupling in densely packed BODIPY channels significantly alter optical properties.
• The observed spectral changes are driven by intermolecular electronic interactions rather than intramolecular effects.
• The study provides insights into controlling solid-state fluorescence through supramolecular assembly.