Nondestructive monitoring of the photochromic state of dithienylethene monolayers by surface plasmon resonance

The use of surface plasmon resonance (SPR) as a nondestructive, nonerasing readout of the isomerization state of a photochromic dithienylethene covalently linked to a chemically modified gold surface was investigated. Four different binding layers were examined: 11-mercaptoundecanol (MUO), an amine-modified 11-mercaptoundecanol (MUO-NH2), dextran, and an amine-modified dextran. The binding of dithienylethene to the modified gold surface and photoisomerization of the photochrome in the bound state were established by FTIR. Solvent effects were measured for every layer tested using ethanol and hexanes. In general, large, easily measurable SPR signal changes could be detected under conditions where photoisomerization of the dithienylethene photochrome was not quenched by the gold plasmon, establishing SPR as a viable form of readout for potential dithienylethene-based optical data storage or processing devices. Dextran-bound photochrome in ethanol exhibited the largest SPR response upon photoisomerization, but is more prone to time-dependent fluctuations resulting from swelling of the dextran layer (caused by slow diffusion of the solvent) than the other layers. Large responses are also provided by MUO-NH2 and MUO, and the signal is much more stable than that for dextran.