A conjugated photoresponsive dithienylethene–ferrocene system: applications in secret writing and decoding information

Abstract

Although photoswitchable or photoactivatable fluorescent diarylethenes are extensively applied in various fields such as optical memory, optical transistors, bioimaging, and super-resolution imaging, they encounter drawbacks of fluorescence quenching in the aggregated/solid state. Therefore, the design and synthesis of solid state “turn-on” fluorescent photochromic materials remain a great challenge. Here, in this work, a ferrocene appended dithienylethene (DTE-Fc) bridged π-conjugated photochromic material, 4o/4c, with high photoreaction quantum yields (Φ_o→c = 0.83 and Φ_c→o = 0.021) and fatigue resistance properties, was rationally designed. The conversion of pale violet, 4o, to green color, 4c, is associated with a slightly higher rate constant (k_solution = 0.007 s⁻¹ and k_solid = 0.005 s⁻¹) than the conversion of 4c to 4o (k_solution = 0.004 s⁻¹ and k_solid = 0.003 s⁻¹) both in solution and in the solid state indicating that photocyclization occurs at a higher rate than cycloreversion. It is a unique example of a “turn-on” mode fluorescent (Φ_F = 0.299) DAE without a traditionally known fluorophore or a S,S-dioxide moiety, in solution and in the solid state. The “turn-on” mode fluorescence behaviour was further validated by the time-resolved fluorescence spectroscopic measurement and quantum chemical calculations (DFT). Furthermore, this DTE-Fc derivative, 4o/4c, is found to be highly efficient in a lithographic technique (erasable ink) and deciphering secret codes (QR/bar code), opening real opportunities for practical applications in anti-counterfeiting.