Electrochemical cell with stilbazolium merocyanines oriented in a nematic liquid crystal

Abstract

The photovoltaic effect and current—voltage ( I(V)) characteristics of an electrochemical cell, consisting of a layer of merocyanine (1-(6′-hydroxyhexyl)-4-/(3,5-di- tert-butyl-4-oxocyclohexa-2,5-dienylidene)ethylidene/-1,4- dihydropyridine and 1-(11′-hydroxyundecyl)-4-/(3,5-di- tert-butyl-4-oxocyclohexa-2,5-dienylidene)ethylidene/-1,4-dihydropyridine) in a nematic liquid crystal ( p-methoxybenzylidene p′-butylaniline (MBBA) plus p-ethoxybenzylidene p′-butylaniline (EBBA)) sandwiched between two semiconducting (In 2O 3) electrodes and covered with polyimide orienting layers, were investigated. Both dyes are uniaxially oriented as a result of the guest—host effect, but the dye with the longer chain to a lower degree. Both dyes occur in free base and protonated forms. The maximum of the photopotential action spectrum is located in the absorption region of the protonated form of the dye. The first illumination of the cell generates “formation” potentials of opposite sign for the two merocyanines investigated. The difference in sign can be related to several processes, such as the different perturbations of the liquid crystal arrangement (which can produce an additional polarized charge of the matrix) and the different concentrations of free base and protonated forms. The dye located at the illuminated semiconducting electrode (which is responsible for the formation of the surface charge) and the dye molecules dissolved in the bulk volume of the liquid crystal are both responsible for this effect. A sequence of light and dark periods applied to a “formed” cell generates an additional negative potential related to electron tunnelling from the excited dye molecules to the semiconducting electrode. The cells are sensitive to the polarization of the light. All the effects observed can be related to the interaction of the dye molecules with the liquid crystal matrix and to the electric field formed at the junction between the illuminated semiconducting electrode and the pigmented liquid crystal layer.