Alkyl chain-dependent ESIPT luminescent switches of phenothazine derivatives in response to force

Abstract

The response of organic materials to mechanical force has attracted significant attention because of its potential applications. Organic molecules with excited state intramolecular proton transfer (ESIPT) luminescence and responsive properties to force are rarely explored. Herein, three phenothiazine derivatives with ESIPT luminescence were synthesized and the influence of alkyl chain on their mechanochromic behaviors was investigated. The results indicate that their fluorescence spectra exhibited large bathochromic shift in polar solvents, meaning intramolecular charge transfer inherent characteristic. Moreover, the compounds with shorter alkyl chain possessed longer emission wavelength in pristine solids. All three compounds exhibited reversible mechanochromism and shifted their fluorescence band to lower energy region. XRD investigations suggested that mechanochromic process was accompanied with the phase transition between crystalline and amorphous states. Furthermore, fluorescence spectral shift values are dependent on alkyl chain length. The compound with ethyl group exhibited the smallest spectral shift (20 nm), whereas the compound with octyl group had the largest shift (31 nm). Solvent fuming and thermal annealing could promote fluorescence recovery. C2PAHN required high temperatures, and C8PAHN recovered its fluorescence at low temperatures, indicating that an alkyl chain length may regulate thermal recovery.