On the structure of meso-substituted F-BODIPYs and their assembly in molecular crystals: An experimental-theoretical approach

Molecular crystals of meso-(5-((4-formylphenyl)ethynyl)thiophen-2-yl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene 4a and meso-(4-((4-formylphenyl)ethynyl)phenyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene 4b were synthesized using palladium(0)-catalyzed Sonogashira cross coupling reactions and characterized using an experimental-theoretical approach. The crystal structures of F-BODIPYs 4a-b are structurally analogous, yet their crystal packing motifs displayed remarkable differences due to contrasting hydrogen-bonding interactions. A CASTEP study of the unit cell for each compound provided the theoretical band structure and density-of-states (DOS) diagrams for each molecular crystal, suggesting a semiconductor character for both solids. Analysis of the computed electron density with Bader's QTAIM describes the intermolecular-contact network within each periodic system, while Natural Bonding Orbitals (NBO) display in-phase overlap between stacked units, denoting intermolecular delocalization of the electron density via stacking. TDDFT analysis of solution UV-Vis spectra gives insight into the nature of the observed excitations along with the effects derived from the relative rotation of π-subsystems in solution.