When Does Fusing Two Rings Not Yield a Larger Ring? The Curious Case of BOPHY

Area of Science:

• Photophysics and photochemistry
• Organic electronics
• Computational chemistry

Background:

• The bis(difluoroboron)-1,2-bis-(pyrrol-2-yl)methylene-hydrazine (BOPHY) is a fluorophore structurally related to the widely used 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) system.
• Understanding the structural and electronic properties of novel fluorophores is crucial for developing advanced optical and electronic materials.

Purpose of the Study:

• To investigate the ground (S0) and excited (S1) state geometries and electronic characteristics of the BOPHY fluorophore.
• To elucidate the conformational dynamics, including puckering and inversion, and their influence on spectral properties.
• To compare the electronic behavior of BOPHY with that of BODIPY.

Main Methods:

• Utilized a combination of computational methods including density functional theory (DFT), time-dependent (TD)-DFT, CASSCF-PT2, and EOM-CCSD.
• Performed density functional theory-based molecular dynamics (DFT-MD) simulations to study conformational flexibility.
• Conducted concentration-dependent electronic absorbance measurements.

Main Results:

• Both S0 and S1 states of BOPHY exhibit highly puckered geometries, which planarize in the solid state due to packing effects.
• In solution, BOPHY exists as a mixture of puckered conformations, with a planar transition state accessible via low activation energy.
• Excitation characteristics reveal that BOPHY behaves as two electronically independent fragments, unlike the BODIPY system.

Conclusions:

• The puckered nature and inversion dynamics significantly influence the absorption spectrum broadening of BOPHY.
• BOPHY does not represent a simple electronic extension of BODIPY; its electronic structure comprises two uncoupled subunits.
• The conformational flexibility and electronic decoupling offer unique possibilities for designing new functional materials based on the BOPHY scaffold.