Fastest molecular photochromic switches based on nanosecond isomerizing benzothiazolium azophenolic salts

Abstract

In search of strategies to improve the switching speeds and fatigue resistances of photochromic molecular switches, we have analysed the kinetic behaviour of a series of photoactive azoderivatives which combine the strong electron withdrawing character of both thiazolium and benzothiazolium salts with the ability of azophenols to establish keto–enolic equilibrium. The excitation of these molecules with a green laser at 532 nm, a wavelength that is compatible with biological applications, induces trans-to-cis photoisomerisation in less than 5 ns. The photogenerated cis isomer reverts to the thermodynamically stable trans form with lifetimes as short as 55 ns in ethanol at room temperature. Thus, a full switching cycle can be completed on a nanosecond timescale. To the best of our knowledge, these are the fastest molecular photochromic switches found heretofore. Furthermore, all of the switches reported herein are able to tolerate thousands of switching cycles with no sign of decomposition, even in the presence of molecular oxygen, which is convenient for further technological applications. In addition, these molecules can be adsorbed onto microcrystalline cellulose thereby exhibiting a thermal isomerisation rate as fast as that observed in alcoholic solutions. This feature affords novel solid switchable materials which operate under similar conditions with nanosecond switching speeds.