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

The photoelectrochemical investigation of 1,3,3-trimethylspiro(2H-1-benzopyran-2,2'-indoline) (SP-0) and 1,3,3-trimethyl-6-nitrospiro(2H-1-benzopyran-2,2'-indoline) (SP-1) has been performed in a dimethylformamide (DMF) solution at room temperature and at a low temperature (-42 o C). Nitro-substituted SP-1 is electroactive in the experimental potential range of -1.8 V 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 o 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