Research Contents

 

  Another important objective in our proposal is to investigate the pi-pi stacking phenomena in various molecular architectures such as liquid crystals, supramolecues, molecular aggregates, nanofibril structures (nanofibers), and so on. These molecular systems have recently attracted much attention due to their versatile structures and potential applications in many areas such as display, OLED, FET, biomimicries, drug delivery, solar cell devices. In the formation of such molecular structures, the pi-pi stacking plays a crucial role by providing intermolecular interaction forces between constituent molecular elements. In this sense, it is easily conceivable that the functionalities of these materials stem from the pi-pi stacking phenomena, but a deep understanding of this feature is still in the rudimentary stage. Depending on the distance and orientation between the adjacent constituent molecular systems as well as pi-conjugation (number of pi-electrons) associated with molecular topologies, the pi-pi stacking interactions would be different, leading to versatile molecular architectures. Thus the determination as well as control of the extent of pi-electron delocalization in these systems is important for further application as molecular electronic materials. The pi-electron delocalization in organic materials, which possess a lower density of electronic states that are more strongly coupled to vibrational modes, is more complicated and mobilities are, in general, lower than in inorganic crystals. Higher carrier mobility leads to larger current densities and faster switching times and is one of the keys to achieving practical FETs, brighter LEDs, and even organic solid state lasers. Ideally, the transport should be as “coherent” as possible in these organic materials. Scattering due to vibrations or environmental interactions causes wavefunctions to lose coherence and become localized, and these same processes also cause the optical dephasing which occurs after the carrier absorbs a photon. To investigate coherent transport, we will study various molecular assemblies interconnected through pi-pi stacking, which controls the electron delocalization among the pigments in molecular assemblies. Femtosecond pump/probe polarization anisotropy experiments on the carrier absorption would directly measure the depolarization time and provide an indirect measure of the scattering time and thus of how far electron delocalization propagates. By doing these experiments, we can correlate the fast scattering (from the dephasing) with the spatial domain of pi-electron delocalization and quantitatively determine the scale of pi-electron delocalization. The problem of exciton delocalization is similar to that of charge transport, and thus ultrafast optical experiments can be used to monitor the diffusion of these electron-hole pairs to gain further insight into the functionalities of various molecular assemblies, which provides prospects of the application as molecular photonic/electronic materials.

   Optical spectroscopy is a valuable tool to investigate these materials, and the ability to spatially resolve these dynamics would be a great aid in unraveling the meso-macroscopic connection. The most flexible and experimentally tractable technique is that of confocal microscopy and micro spectroscopy. Spatial resolution of a few hundred nanometers would enable us to address problems like the localization of exciton trapping and fluorescence quenching sites, and the influence of molecular assemblies on charge transport. For example, we will be able to identify spatial regions of very low (or high) carrier mobility in molecular assemblies and gain insight into how morphology influences transport. For example, we can determine the exciton dynamics as a function of aggregate size and composition in a search for the “best” type of aggregates. By correlating static structure with dynamical properties, we can quantitatively answer the question of how structure from the nanometer to micron scale affects the functioning of these materials, and how they might be improved by, for example, making more or less ordered samples. Unlike the vast majority of previous experiments, the combination of spatial resolution and ultrafast nonlinear optical spectroscopy will allow us to gain a quantitative understanding of how structure on the micro- and mesoscopic levels determines the chemical, morphological, and electronic properties of these materials.