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

Abstract

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 (III). 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 λ = 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.

Intermolecular states in organic dye dispersions: excimers vs. aggregates

“…In class A (aggregate), the decay of singlet excitons is again dominated by transfer to excimer-forming sites, but there is parallel transfer to distinct, strongly redshifted aggregates. Similar species are often attributed to BODIPY J-aggregates, 6,[12][13][14][15][16][17] but the observed time-resolved behaviour and temperature dependence reveal this to be an over-simplification. The states must be described specifically as J-coupled dimers, exhibiting exceptional near-unity quantum efficiency.…”

“…In the case of BODIPY dyes, it has been well documented in a wide range of derivatives that dispersion within a polymer matrix or deposition of pure films results in significant PL quenching accompanied by spectral shifts. 9,10,[12][13][14][15][16][17][18][19][20][21] Analogous effects have also been described in a wide range of covalent dimers, 11,15,[21][22][23][24][25][26] though the geometries are typically a poor match for the structures obtained in the solid state. Quenching in these systems is often attributed to the formation of non-emissive H-aggregates, identified by a broadening of the absorption spectrum at higher energies.…”

“…10,[15][16][17]26,27 Red-shifted PL spectral features have been variously attributed to excimers and J-aggregates, typically on the basis of their spectral position alone. 6,9,[12][13][14][19][20][21][22][23][24] The properties of these emissive states have not been analyzed in detail, nor the mechanism of their formation. We have recently demonstrated that the excimer state formed in a high-concentration BODIPY dispersion enables exciton-polariton formation through a novel radiative pumping mechanism, 28 highlighting the potential of these low-energy states for applications if their properties can be better understood.…”

Contrasting Solid-State Fluorescence of Diynes with Small and Large Aryl Substituents: Crystal Packing Dependence and Stimuli-Responsive Fluorescence Switching

“…Effort has been put forward to tune the molecular packing through structural modification and thereby influence the solid state fluorescence output. [23][24][25][26][27][28][29][30][31][32][33][34][35] In this regard, small organic derivatives with olefin, acetylene, phenylenevinylene and phenyleneethynylene spacers have found current interest. 19,24,[36][37][38][39][40] Davis et al reported the solid state fluorescence of alkoxy-cyano substituted diphenylbutadiene derivatives.…”

Small BODIPY Probes for Combined Dual 19F MRI and Fluorescence Imaging

“…[9,10] The red shift of the observed fluorescencep resumably arises from reabsorption or aggregation. [21] For future medical or biological experiments, it is also crucial to test the cytotoxicity of the synthesized molecules. HUVEC isolated from human vessels served as am odel with utmost relevance for the human in vivo situation.…”