Surface plasmon excitation of porphyrin self-assembly monolayers on an Au surface

摘要

Surface plasmon (SP) enhanced fluorescencespectroscopy has been employed to elucidate the molecularorientation, molecular exchange process, andphotoelectrochemical properties of photofunctional self-assembly monolayers (SAMs) for the first time. A 48 nm Au filmon a BK-7 right-angle prism was treated with a porphyrindisulfide ((-S(CH2)10CONH-Por)2, Por=p-(tris(p-tolyl)porphyrinyl) phenyl) (1)giving a porphyrin SAM (2). Spexcitationof 2 using p-polarized 425 nm light at an incident angle of 55°gave a strongly p-polarized fluorescence of the porphyrin. Thisindicates selective excitation of the porphyrins having transitionmoments effectively overlap with the SP field vectors and thesuppressed molecular motions in 2. The Qbands in thefluorescence excitation spectrum were considerably enhancedattributable to the field enhancement of SP. The molecularexchange process of a decanethiol SAM with 1 has beenelucidated based on changes in the fluorescence properties. Thephotoelectrochemical measurement using 632.8 nm light gave amuch more intense photocurrent (18.6 μA cm-2) than thatprovided by conventional direct photoexcitation (2.7μA cm-2).Moreover, the photocurrent was observed using near-IR lightwhile the energy is considerably lower that the lowest singletexcited state of the porphyrin.