In the present work, we have studied electronic excitation and photocurrent generation of a porphyrin self assembly monolayer (SAM) on a gold surface by Surface Plasmon (SP).
SPs were generated on a gold surface with an attenuated total reflection (ATR) method. A 50 nm gold film was evaporated on the hypotenuse face of a BK-7 right-angle prism. The ATR minimum of bare film was measured with p-polarized 632.8 nm light and the SPR angles (theta-Rs) were at 43.6° and 71.9° in air and water, respectively (Fig. 1). A porphyrin disulfide  was used as a modification reagent to introduce porphyrin chromophores (-S(CH2)10CONH-Por, Por = p-(tris(p-tolyl)porphyrinyl)phenyl) on the gold surface (Chart 1). After the modification, the theta-R shifted to 43.9° and 72.2° in air and water, respectively, which is attributable to the porphyrin SAM formation.
Assuming the refractive index of the SAM as 1.5, the thickness was estimated to be 1.7 nm. This suggests that the -S(CH2)10CONH-Por moiety takes linear conformation perpendicular to the gold surface. Since the SP field has the maximum on the surface and decays exponentially into the space along the axis perpendicular to the surface, the porphyrin SAM can obtain effectively the energy of SP field. The emission from the SAM was measured using p-polarized excitation light at the incident angle (theta-f) of 45°. The characteristic fluorescence of tetraarylporphyrin was observed, while the excitation spectrum (Fig. 2) was quite different from the transmittance absorption spectrum of the -S(CH2)10CONH-Por film.
The intensity ratio between the QX(1,0)-band and Soret band, IQ/IS = 4.0 was significantly larger than those of the absorption spectrum (0.06) and the direct photoexcitation (0.44). Since the amplitude of the electromagnetic field of SP exceeds that of the incident field, the evanescent field of SP is expected to enhance inelastic emission such as Raman scattering from molecules adjacent to the metal film. The electric field of SP on the gold surface created by 650 nm light is approximately 50 times larger than that of the incident light. This is consistent with the preferable excitation at the longer wavelength light in the SP excitation. The decay profile of the fluorescence was analyzed by two components with 200 ps (82%) and 4.5 ns (18%).
Electrochemical measurements were carried out in an oxygen-saturated 0.1 mol dm-3 aqueous solution of Na2SO4 at 26°C using the modified gold film as a working electrode, an Ag/AgCl reference electrode, a platinum counter electrode, and a potentiostat (chart 2). When SP excitation of the modified gold film was started with p-polarized 632.8 nm light from a 13 mW He-Ne laser at theta-f = 73°C without application of the bias voltage, a cathodic current of 1.12 micro A was observed and it decreased exponentially to reach a constant current of 700 nA at approximately 30 s later (Fig. 3). It is noteworthy that only the small photocurrent of 160 nA was observed upon direct photoirradiation with the same light. Thus, the photocurrent generation proceeds effectively through the SP excitation of the SAM attached on the gold surface. The photocurrent decreased by argon bubbling and the initial photocurrent was recovered by successive oxygen bubbling. This suggests that 1Por* is quenched by molecular oxygen via electron transfer to give a radical cation of the porphyrin (Por· +) and superoxide (O2· -) or reduction products of molecular oxygen. In order to quench 1Por* effectively, methyl viologen (MV2+) was added as an electron acceptor. The increase of the initial and constant currents were observed with an increase of the concentration of MV2+. The initial peak current of 1.66 micro A (27 micro A cm-2) was obtained in the presence of 40 mmol dm-3 of MV2+. The results indicate that the electron transfer quenching occurred to give MV· + and Por· +.
The present work is the first example showing that the SP excitation can be used as a unique and effective excitation source to cause the bimolecular reaction of 1Por* covalently linked to the gold surface via electron transfer to generate photocurrent. We are currently studying the photocurrent generation by the SP excitation using other gold film electrodes modified by -S(CH2)nCONH-Por and a mixture of -S(CH2)10CONH-Por and -S(CH2)4CONH-Por to improve the photocurrent behavior.
Ishida, A.; Sakata, Y.; Majima, T. (1998) Chem. Comm., 267-268. Surface plasmon excitation of a porphyrin covalently linked to a gold surface
Ishida, A.; Sakata, Y.; Majima, T. (1998) Chem. Comm., 57-58. Photocurrent Generation by Surface Plasmon Excitation via Electron Transfer Quenching of Excited Porphyrins Linked Covalently to a Gold Film Electrode