The molecular orientation dependence of excitation energy transfer from adsorbed molecules to a metal surface

1997 Fiscal Year Final Research Report Summary

• Project/Area Number: 08640654
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: Physical chemistry
• Research Institution: Toho University
• Principal Investigator: OHSHIMA Shigeru Toho University, Faculty of Science, Assistant Professor, 理学部, 助教授 (60168911)
• Co-Investigator(Kenkyū-buntansha): UCHIDA Akira Toho University, Faculty of Science, Assistant Professor, 理学部, 助教授 (30176680) ; OONOSHI Isao Toho University, Faculty of Science, Professor, 理学部, 教授 (50057634)
• Project Period (FY): 1996 – 1997
• Keywords: silver / surface / fluorescence / fluoranthene / angular distribution / thin film / excitation energy transfer / molecular orientation

Research Abstract

A light-guide was newly mounted in the existing vacuum chamber ; the light-guide was rotatable around a temperature variable sample-holder. This improvement of enabled us to investigate energy transfer from adsorbed molecules to a metal substrate and the dependence of energy transfer on molecular orientations only by measuring fluorescence of the molecules.
Fluorescence measurements were performed for fluoranthene adsorbed on silver and glass substrates. Fluorescence decay curves were represented by the sum of two exponential functions, A_1exp(-t/tau_1)+A_2exp(-t/tau_2), for both the substrates. The relative quantum yield, F,was defined as (A_1tau_1+A_2tau_2)/tau_0, where tau_0 was the fluorescence life-time of fluoranthene in solution. F was 0.75 for the glass substrate and independent of the thickness of fluoranthene films. On the other hand, PHI for the silver substate was 0.21 for a fluoranthene film with a thickness of 100 * ; the decrease in PHI is ascribed to an energy transfer f rom the molecules to the surface.
The angular distribution of the fluorescence from fluoranthene on a silver surface was also measured. At T=180 K,just after evaporation, no maxima appeared in the "horizontal" situation, where the polarization direction of a polarizer was parallel to the plane of observation of fluorescence. This means that the transition moment for the fluorescence was parallel to the surface. With increasing T,the distributions for the horizontal situation were getting to close to those for the "vertical" one, indicating that crystallization was occurring.The direction of the transition moment of fluorescence was calculated by the INDO method.On the basis of the direction, the adsorption state was deduced. Molecules were adsorbed at first randomly and with increasing T they were rearranged to a crystalline sate, where the molecules were oriented their transition moment parallel to the the surface.
The adsorption states of fluoranthene and energy transfer from its excited states to the silver surface were investigated on the basis of fluorescence measurements. However, a simultaneous measurement could no be performed. The reasons were as follows : was because the fluorescnece intensity was not as strong as we had expected and the film thickness could not be controlled precisely.