A practicable
strategy to a reversible mechanochromic material
featuring interconversion of classical/frustrated Brönsted
pairs has been established. We report the mechanochromic property
of 2,6-bis(4-biphenyl)isonicotinic acid (1), which features
a frustrated Brönsted pair in the crystalline form and a classical
Brönsted pair after grinding. A large mechanochromic shift
was found from 428 to 505 nm. In addition, compound 1 also exhibits acidochromic behavior, which further proves that the
formation of an acid-base interaction is responsible for the mechanochromic
phenomenon.
Five m-phenylene-embedded cycloparaphenylenes m 3 [9]CPP 1−5 were synthesized by the platinum-mediated cyclooligomerization strategy with high overall yields. The structures of m 3 [9]CPP 1−3 were determined by X-ray diffraction analysis. Compared to [9]CPP, m 3 [9]CPP 1 caused a significant blueshift in the UV−vis absorption and fluorescence spectra. This result shows that the radial π-conjugation is distorted and partially interrupted. The photophysical properties of m 3 [9]CPP 1 were further tuned by the introduction of various substituents for m 3 [9]CPP 2−5. Methoxy group substitution at m-phenylene did not change the photophysical properties significantly. Replacement of m-phenylene by tetrafluoro-m-phenylene achieved a significant blueshift. When the carboxyl group was embedded at m-phenylene or the methoxy group was embedded at p-phenylene, significant redshifts were observed with blue color emission. Theoretical calculations revealed that the decrease in the HOMO−LUMO gap in m 3 [9]CPP 4 and 5 is favorable for the redshift of the fluorescence spectrum.
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