Crystallographic, Photophysical, NMR Spectroscopic and Reactivity Manifestations of the "8-Heteroaryl Effect" in 4,4-Difluoro-8-(C ₄H₃ X)-4-bora-3 a,4 a-diaza-s-indacene (X = O, S, Se) (BODIPY) Systems

We have synthesized and fully characterized three novel, yet closely related, heterocyclically meso-substituted (BODIPY) fluorophores 4,4-difluoro-8-(C₄H₃X)-4-bora-3a,4a-diaza-s-indacene (X = O, 2-/3-furyl (7/10); Se, 2-selenenyl (9)) through the use of 2-D NMR (COSY, HSQC, and HMBC), single crystal X-ray diffraction, mass spectrometry, elemental analysis, UV-vis spectroscopy, and fluorescent decay lifetimes, for comparison to the previously reported thienyl species (X = S, 2-/3-thienyl (8/11)). Specifically, 7-11 differ formally by chalcogen (O, S, or Se) or chalcogen placement. Solid state comparisons reveal major effects stemming from subtle structural differences which allows for insights into fluorescent crystal engineering. For the 2-heteroatom substitution, an increase in molecular weight (7 < 8 < 9) correlates with an increasing unit cell-volume, a greater orthogonality for the C₄H₃X group, and a lower value for φ_F. Solution and density functional theory (DFT) results reveal interesting platforms for potential in fluorescent studies for neurology. 2-Heterocyclic species show larger λ_{abs,max/em,max} values relative to 3-heterocyclic ones, based on electron withdrawing effects. 10 has the greatest φ_F value herein (0.25, toluene). Fluorescence lifetimes were found to be 2.60 (7), 0.74 (8), 0.27 (9), 4.26 (10), and 1.86 ns (11); λ_em,max decay was studied for 8. Heterocyclic differences give rise to somewhat different pyrrolic NMR spectroscopic shifts as well. These compounds resist decomposition as seen from titrations with H ₂O₂, and uniformly undergo λ_abs,max red-shifting and lowered φ_F values as they become brominated with Br₂.