A multi-state fluorescent switch with multifunction of AIE, methanol-responsiveness, photochromism and mechanochromism

Abstract

In this article, we report a newly designed molecule, RhTPE, synthesized by fusing rhodamine with TPE, which realizes both emission color change and on/off switching. Compared to most of the reported fluorescent switches, such a unique structure endowed RhTPE with the multifunction of AIE, methanol-responsiveness, photochromism and mechanochromism, owing to the dynamic ring opening–closing isomerization. RhTPE is acid-sensitive and can recover to the ring-closed form (OF) by base treatment. Both RhTPE and trifluoroacetic acid (TFA)-treated RhTPE display AIE properties. It is noteworthy that during the aggregation process of TFA-treated RhTPE, the cyan emission strengthens while the red emission disappears, due to the aggregation-induced ring-closing reaction. RhTPE shows selective responsiveness to methanol out of all the twelve screened solvents, probably owing to the low pK_a value and high polarity of methanol. More interestingly, RhTPE demonstrates excitation wavelength-dependent photochromism in several different organic solvents, which is very rarely reported in rhodamine systems. In the solid state, RhTPE has three well-distinguished high contrast states of cyan, red and dark. The reversible switch between any two states can be achieved with acid/base or mechanical force/chemical vapor.