Effect of substitution on the solid-state fluorescence properties of styrylbenzoxazole derivatives with terminal dicyanomethylene group

Abstract

Three styrylbenzoxazole derivatives with terminal dicyanomethylene group were studied in order to understand how minor modifications brought to the benzoxazole ring influence the photoluminescence properties. The three compounds were weakly fluorescent in organic solution because of molecular motions around the styryl single bonds. The unsubstituted (1) and methoxy (2) derivatives showed clear solid-state luminescence enhancement (SLE) properties. Distinct emission characteristics were attributed to the formation of amorphous and crystalline particles owing to scanning electron microscopy. These two compounds were also strongly luminescent as microcrystalline powders. Compound 1 that crystallized as slip stacks was a better emitter than the methoxy derivative 2, in which intermolecular interactions are more numerous, according to X-ray diffraction analysis. In contrast, compound 3 was virtually not emissive, confirming that in this series of dyes the presence of the hydroxy group is detrimental to PL emission, as rarely observed among SLE-active fluorophores.

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“…It can be noticed that total PL extinction was observed for samples 1c and 1e, where deformation is the strongest. Such a deactivation pathway would be particularly original because H-bonds are generally considered to rigidify the molecular structures and activate the SLE processes of numerous fluorophores [64−67], although the opposite behaviour was recently reported by us for hydroxy-substituted PBO derivatives [68]. Clearly, dimeric structures like the ones studied in the present work would deserve more attention regarding their solid-state photophysical properties.…”

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“…This compound is indeed very weakly fluorescent in fluid organic solvents, but fluorescence intensity increases with increasing viscosity, and the PLQY reaches 0.28 in the solid state (Fig. 4a) [45]. When increasing the proportion of water into a THF solution of 2, the emission intensity at the PL maximum wavelength (584 nm) is strongly enhanced up to 45 folds (Fig.…”

“…The presence of a methoxy group on the BO ring (compound 3) does not enhance the SLE behavior, while the substitution by a hydroxy group is strongly detrimental to PL emission in the solid state. No clear MFC effect was detected for molecules 2 and 3, which arrange in densely packed crystal networks [45]. Yadav et al [44] have also reported compounds 4 and 5 with only one terminal electronwithdrawing cyano group.…”

“…Recently, our group has reported an improved procedure to access to formyl-BPO precursor by condensation of 2-aminophenol with terephtalaldehyde and subsequent treatment with 2,3dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in 45% overall yield [45] (Scheme 2). The Schiff bases 10-12 were prepared in good yields (60 to 84%) from the condensation of p-(benzoxalzolyl)anilines (obtained from 2-aminophenol with p-aminobenzoic acid in PPA) with the aromatic aldehydes in refluxing ethanol [54] (Scheme 3).…”

“…Figure 2. Molecular structure of compounds 1[43], 2 and 3[45], 4-7[44], 8[49] and 9[50,52] with solid-state PL maxima (λ em ) and photoluminescence quantum yields (Φ PL ).…”