LIGHT-TRIGGERED MOLECULAR DEVICES - PHOTOCHEMICAL SWITCHING OF OPTICAL AND ELECTROCHEMICAL PROPERTIES IN MOLECULAR WIRE TYPE DIARYLETHENE SPECIES

Organic photochromic systems represent a starting point for the elabor ation of light-triggered molecular switching devices. The novel bispyr idinium and bispyridine compounds 1(2+) and 6 were synthesized as thei r uncyclized isomers from 3,5-dibromo-2-methylthiophene in overall yie lds of 43 and 44%, respectively. The diarylethene photochromes 2 and 1 0-13, substituted with electron donors and accepters, were prepared fr om 5-methylthiophene-2-carboxaldehyde in 21-32% overall yield. All of the compounds were found to exhibit pronounced photochromic properties . Irradiation with UV light resulted in essentially complete photocycl ization of the open forms to the intensely coloured closed isomers whi ch could, in turn, be reconverted back to the open state with visible light of lambda > 600 nm. The absorption maxima of the described compo unds in their closed forms are shifted far towards, and even into, the near-IR region. Whereas no thermochromic properties were observed for the open isomers, the rates of thermal decolouration of the cyclized forms was found to be highly dependent on the nature of the substituen ts on the thiophene rings. It was demonstrated that reversible photoch emical interconversion between the two photochromic states could be us ed to effectively switch a number of physical properties. Thus, the mo lecules 1(2+) and 12 represent two kinds of redox switches, the former in reduction and the latter in oxidation, in which electron conductio n is switched on in the closed state and off in the open state. Compou nd 12 may also be considered to be a photoswitchable analogue of tetra thiafulvalene type substances. On the other hand, compound 2 displays a marked increase in nonlinear optical activity on conversion from the open to the closed form. Such systems are prototypes of photoswitchab le molecular wires where electron conduction and push-pull interaction can be reversibly modulated by an external stimulus, namely, irradiat ion by light.