A Multi‐Functional Electro‐Optical Molecular Device. The Photoelectrochemical Behavior of Spirobenzopyrans in Dimethylformamide

The photoelectrochemical investigation of l,3,3‐trimethylspiro(2H‐l‐benzopyran‐2,2′‐indoline) (SP‐O) and l,3,3‐trimethyl‐6‐nitrospiro(2H‐l‐benzopyran‐2,2′‐indoline) (SP‐1) has been performed in a dimethylformamide (DMF) solution at room temperature and at a low temperature (‐42°C). Nitro‐substituted SP‐1 is electroactive in the experimental potential range of ‐1.8V to +0.8 V (vs. Ag/Ag, while no nitro‐substituted parent spirobenzopyran SP‐0 is lacking in electroactivity. In the case of SP‐1, nearly identical electrochemical behavior was found for the closed‐ring (SP) and open‐ring (MC) isomer at room temperature or at ‐42°C, suggesting that the ring form, i.e. open or closed, has no influence on the redox reaction of SP‐1. Moreover, reversible color changes were observed in the DMF solution of SP‐1 depending on reduction and oxidation potentials, namely, a change to yellow in the reduction and to deep violet in the oxidation. Both voltammetric and spectroelectrochemical evidences suggest the production of a nitrobenzopyran radical anion of closed‐ring (yellow) in a reduction and an open‐ring isomer MC (deep violet) in an oxidation either from SP and from MC of SP‐1 in the DMF solution. This behavior leads to the discovery of a novel photochromism and electrochromism “three state” conjugated system which may potentially be applicable to the construction of multi‐functional electro‐optical molecular devices.