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

Surface plasmon (SP) enhanced fluorescence spectroscopy has been employed to elucidate the molecular orientation, molecular exchange process, and photoelectrochemical properties of photofunctional self-assembly monolayers (SAMs) for the first time. A 48 nm Au film on a BK-7 right-angle prism was treated with a porphyrin disulfide ((-S(CH 2 ) 10 CONH-Por) 2 , Por = p -(tris( p -tolyl)porphyrinyl)phenyl) ( 1 ) giving a porphyrin SAM ( 2 ). SP excitation of 2 using p-polarized 425 nm light at an incident angle of 55° gave a strongly p-polarized fluorescence of the porphyrin. This indicates selective excitation of the porphyrins having transition moments effectively overlap with the SP field vectors and the suppressed molecular motions in 2 . The Q bands in the fluorescence excitation spectrum were considerably enhanced attributable to the field enhancement of SP. The molecular exchange process of a decanethiol SAM with 1 has been elucidated based on changes in the fluorescence properties. The photoelectrochemical measurement using 632.8 nm light gave a much more intense photocurrent (18.6 µA cm -2 ) than that provided by conventional direct photoexcitation (2.7 µA cm -2 ). Moreover, the photocurrent was observed using near-IR light while the energy is considerably lower than the lowest singlet excited state of the porphyrin.