Time-Resolved Spectral Measurement of Dendrimer Molecular Films with Rhodamine B Core
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Time-Resolved Spectral Measurement of Dendrimer Molecular Films with Rhodamine B Core M. Zhou, A. Otomo, S. Yokoyama and S. Mashiko KansaiAdvanced Research Center, Communications Research Laboratory 588-2 Iwaoka, Iwaoka-cho, Nishi-ku, Kobe 651-2492, Japan ABSTRACT We investigated molecular thin films fabricated using novel organic materials, such as dendrimers with a rhodamine B (Rh-B) core, by a pump-probe measurement for ultrafast time-resolved spectroscopy with a time resolution of approximately 160 fs. The photon energy transfer caused by the interaction of the dendrimer molecules was measured in terms of absorbance. The wavelength of the pump light was 550 nm and a white continuum light spanning from 450 nm to 750 nm was used as a probe light. A spin-coated molecular film made of dendrimer molecules with Rh-B core has a relatively high molecular density. We measured a strong stimulated emission at a wavelength of 612 nm, whose fast decay was assumed to be caused by intermolecular interactions between the dendrimer molecules with Rh-B core. The lifetime of the molecules in the excited state was measured to be as short as 10 ps. Several types of molecular films with different molecular densities were compared, and cases with different pump energies were also discussed. INTRODUCTION Organic molecules have been attracting increasing attention in recent years because they are expected to open the door to the development of advanced optical functional devices, by using their remarkable characteristics of having a fast optical response and strong nonlinear optical properties [1-2]. However, it is known that when organic materials have a high molecular density, the interaction between molecules often prevents an accumulated optical nonlinear effect. Therefore, measurements of microscopic and macroscopic nonlinearities using LB film, polymers, and other nonlinear optical systems have revealed new relationships between molecular structure and nonlinear optical properties [2-3]. The size of a molecule is also a factor in improving hyperpolarizability. We have focused our attention on some dye molecules, especially on rhodamine molecules because they have good fluorescence efficiency, to develop novel organic optical functional devices. A new step-wise synthesis method that can control structures and sizes of molecules has been developed [4]. Thus, we used a structure with a Rh-B core to prevent extinction by a novel structure called dendrimer molecule [5-6]. Due to the structure of dendritic macromolecules, molecular size will grow each successive generation. Their core-shell structure can keep the cored dye molecule in the center of dendrimer and can be used to control the distance between the dye cores of organic molecules. As a result, an organic molecular film has a relatively high molecular density that can be fabricated with little self-quenching energy transfer, so that it performs with high fluorescence efficiency. Figure 1 shows a schematic of the step-wise synthesis method and the structure of a dendrimer genera
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