RAMAN-SPECTROSCOPIC DETERMINATION OF THE STRUCTURE AND ORIENTATION OFORGANIC MONOLAYERS CHEMISORBED ON CARBON ELECTRODE SURFACES

Azobenzene (AB) and 4-nitroazobenzene (NAB) were covalently bonded to carbon surfaces by electrochemical reduction of their diazonium deriva tives, The N-1s features of XPS spectra of modified surfaces had inten sities expected for monolayer coverage, However, the Raman spectra wer e significantly more intense than expected, implying an increase in sc attering cross section upon chemisorption, A likely explanation is res onance enhancement of the carbon/adsorbate chromophore analogous to th at reported earlier for dinitrophenylhydrazine (DNPH) chemisorption, V ibrational assignments indicate that the C-C vibration between azobenz ene and the carbon surface is in the 1240-1280 cm(-1) region, and this conclusion is supported by spectra obtained from [C-13]graphite. Obse rvation of depolarization ratios for 4-nitroazobenzene and DNPH on gra phite edge plane indicate that NAB is able to rotate about the NAB/car bon C-C bond, while chemisorbed DNPH is not. The partial multiple bond character of the DNPH linkage to graphite is consistent with the obse rvation that the DNPH pi system remains parallel to the graphitic plan es.