Correlation between Crystal Habit and Luminescence Properties of 4,4-Difluoro-1,3-dimethyl-4-bora-3a,4a-diaza-s-indacene, An Asymmetric BODIPY Dye

The asymmetric substituted BODIPY dye 4,4-difluoro-1,3-dimethyl-4-bora-3a,4a-diaza-s-indacene crystallizes in three different crystal habits, that is, as needles (I), leaves (II), or microcrystalline sublimed crystals (HI). All crystals share the same crystal structure but exhibit varying solid-state fluorescence from yellow-orange to deep red. The crystal structure mainly consists of one-dimensional chains of J-aggregates which leads to excitonic luminescence at lambda = 600 nm. The point-dipole approximation is used to calculate the excitonic splitting to a value of 3300 cm(-1), which is in good agreement with the experimental observations. The influence of the macroscopic appearance on the luminescence properties is discussed in terms of reflectivity of the surface and reabsorption within the material. It turns out that the long-range order modulates the solid-state luminescence due to the small Stokes shift of the dye.