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

Structural and electronic features of ground and excited states of the bis(difluoroboron)-1,2-bis-(pyrrol-2-yl)methylene-hydrazine (BOPHY) fluorophore, a seemingly extended version of the popular 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) fluorophore, are presented. Geometries of S₀ and S₁ electronic states are highly puckered, as confirmed by a combination of density functional theory (DFT), time-dependent (TD)-DFT, CASSCF-PT2, EOM-CCSD calculations and density functional theory-based molecular dynamics (DFT-MD). Packing effects are responsible for planarization in the solid state. Without the network of a solid matrix, planar conformation of BOPHY is an easily accessible transition state of inversion between two puckered conformations and hence solvated BOPHY is suggested to sample the conformational space between the two puckered geometries. The peculiar features of puckering as well as inversion via a planar TS are unaltered with a large range of lateral substitutions. Concentration-dependent electronic absorbance measurements were carried, which showed that the transformation of the low activation energy between the puckered and planar conformations is responsible for the broadening of the absorption spectrum. BOPHY, a four-ring system, is not an electronic extension of the three-ring BODIPY system since the excitation characteristics suggest BOPHY to behave as two electronically unlinked fragments despite the fact that the two subunits are covalently bonded.