Temperature Gating of the Ring‐Opening Process in Diarylethene Molecular Switches

Abstract

have been demonstrated. Photoswitching effects are based on the reorganization of the p-conjugated backbone of the molecule that consequently leads to different physical properties. Knowledge of the vital factors governing photochromic processes is essential for future utilization of diarylethenes as smart materials as well as for a fundamental understanding of the photoprocesses involved. Hence, temperature-dependent studies are necessary to reveal new aspects and limits of the switching process, as previously shown by measurements at elevated temperatures. [19] Additionally, determining possible switching restrictions at low temperatures is particularly important in view of carrying out conductance measurements at the single-molecular level, which are restricted at room temperature because of the high mobility of the gold atoms constituting the electrodes in this type of experiment. In order to achieve stability of the metal/molecule/metal system it is often necessary to perform measurements at low temperatures. [20] Herein, we explore the temperature dependence of the photochemical behavior of dithienylcyclopentenes. We report the suppression of the ring-opening process with decreasing temperature, leading to the complete absence of the photoreaction below a cutoff temperature. By contrast, we demonstrate that the reverse ring-closure process shows no significant temperature dependence above 115 K. These observations of the temperature dependence for the ring-opening process are essential to understand the behavior of these molecular switches before they can be used in advanced technological applications. The theoretical description of the ringopening process is not completely understood. Furthermore, the observed suppression of the ring opening might have important implications for gating of photochromic processes. Like for other photoreactions that have a photostationary state with less than 100% product, the ring-closure process for diarylethenes is accompanied by ring opening, because both forms have absorption peaks in the UV region, hence decreasing total conversions. [21] On the other hand, if ring opening can be blocked, total conversion increases. [22]