Circularly Polarized Luminescence and Circular Dichroism of Bichromophoric Difluoroboron-ß-diketonates: Inversion and Enhanced Chirality Based on Spatial Arrangements and Self-Assembly.

We synthesized two bichromophoric difluoroboron-ß-diketonates (DFB) connected in para and meta positions by using cyclohexane diamine as a chiral bridge (para and meta (R/S)-CyDFB). TD-DFT calculations revealed that the variation in connectivity of the DFB units leads to different spatial arrangements and a chirality inversion of the bichromophoric DFB. Higher gabs values were obtained in (R/S)-CyDFB connected in para as compared to meta position. Aggregation of para (R/S)-CyDFB in mixture of solvents increase the glum values as compared to its monomeric form. Ultrasonication and heating induced the formation of highly ordered nano-helical wires of para (R/S)-CyDFB that increased the glum values to 0.015. On the other hand, meta (R/S)-CyDFB failed to form highly ordered self-assembled wires due to hindered H-binding sites. These observations indicate that the chiroptical properties of DFB bi-chromophore system can be modulated with self-assembly and spatial arrangement of the chromophores.

Mechano- and Photoresponsive Behavior of a Bis(cyanostyryl)benzene Fluorophore.

The mechanoresponsive behavior and photochemical response of a new bis(cyanostyryl)benzene fluorophore (CSB-5) were investigated. Green fluorescence with λem,max of 507 nm was found for CSB-5 in chloroform solution, mirroring the behavior of a previously reported similar dye (CSB-6). Alternatively, crystalline samples of CSB-5 exhibited orange fluorescence with λem,max of 620 nm, attributable to excimer emission. Although the emission color change was not clearly noticeable by naked eye, CSB-5 exhibited mechanochromic luminescence, due to transformation into the amorphous state upon grinding the crystalline powder. Interestingly, rubbed films of CSB-5 prepared on glass substrates exhibited a pronounced emission color change from orange to green when exposed to UV light. This response is the result of a photochemical reaction that occurs in the amorphous state and which causes a decrease of the excimer emission sites so that the emission color changes from excimer to monomer. The crystalline material did not display such a photoinduced emission color change and the difference in photochemical reactivity between crystalline and amorphous states was exploited to pattern the emission color of rubbed films.

Mechano-responsive circularly polarized luminescence of organic solid-state chiral emitters.

Two difluoro-boron ß-diketonate complexes bearing chiral amido groups have been synthesized. Their mechano-responsive luminescence and chiroptical properties have been investigated in the solid state. Both compounds display a bright blue-green emission and a significant circularly polarized luminescence (CPL) signal in the crystalline state, with |g lum| values as high as 2.2-2.4 × 10-2. A bathochromic shift in emission, together with a decrease of |g lum| values to c.a. 3 × 10-3, is induced upon application of a shearing stress. For the DFB-Hex-amide compound, interestingly, sign inversion of the anisotropy factor g lum is observed under mechanical stimulation ("mechano-CPL effect"), which can be rationalized by a switching between monomer and excimer emission.