The Solid-State Multi-Color Fluorescence Switching from a [2.2]Paracyclophane-Based Triarylborane

The fluorophores, the fluorescence of which can be switched between multi bright colors in the solid state, show promising applications not only in the sophisticated multicolor display but also in the advanced encryption and anti-counterfeiting systems. However, it is very challenging to obtain such fluorophores. Herein, we disclose such an example, g-BPhANMe₂-Cp, which contains an electron-donating dimethylamino (NMe₂) and an electron-accepting [(2-dimesitylboryl)phenyl]acetyl at the pseudo-gem position of [2.2]paracyclophane skeleton. This molecule can display tricolor mechanochromic luminescence (MCL) due to the different responses of the mechanically ground amorphous state to heating and solvent-fuming. Owing to the absence of intermolecular π-π interactions in the solid state, the fluorescence efficiency is very high irrespective of its morphological state (Φ_F=0.60-0.87). Moreover, this molecule also displays reversible acidochromic luminescence (ACL) by protonation and deprotonation of NMe₂ with trifluoroacetic acid (TFA) and triethylamine (TEA), respectively. The protonated sample fluoresces (Φ_F=0.31) at much shorter wavelength due to the interruption of intramolecular charge transfer process. Therefore, with the combination of tricolor MCL and ACL properties, the solid-state emission of g-BPhANMe₂-Cp can be switched among four bright fluorescence colors of yellow, green, cyan and blue via treatment with appropriate stimulus.