A major source of inspiration for the design and synthesis of optical nano-structures comes from the light-harvesting antenna complexes of natural photosynthetic systems. The antenna complexes are comprised of a large number of pigments that are arranged in a rigid three-dimensional matrix. The natural antenna complexes absorb light and funnel the resultant energy to the reaction centers by means of excited-state energy transfer processes. The versatile optical (absorption and emission), redox, and photochemical properties of porphyrins make them ideally suited as components of artificial photosynthetic nanostructures. Continuing efforts to realize the mimicry of solar energy harvesting complexes have enabled the design and synthesis of various types of covalently linked porphyrin arrays with the goal of applying these arrays to molecular photonic/electronic devices and artificial photosynthesis systems. The recent success in elaborating various porphyrin architectures using several types of linkers via meso position attachment has brought up the issues on the electronic coupling, of which the extent is largely determined by the interconnection length and relative orientation between the adjacent porphyrin moieties.
Molecular electronics constitutes a multidisciplinary research area focusing on the potential utilization of molecular scale systems and molecular materials for electronic or optoelectronic applications. There are two main optical methods for characterizing molecular systems for the application as molecular photonic/electronic devices. One is ultrafast spectroscopic measurement, which characterizes the functionality of target systems. Although there have been several successful approaches in ultrashort optical pulse generation and characterization technologies, a full characterization of ultrashort optical pulses (intensity and phase) can lead to the realization of coherent coupling/dynamics experiments in condensed phase to reveal dynamic vibrational mode coupling in photo-excitation/relaxation processes. The other is space-resolved measurement, which examines the morphologies and single molecular level properties. Although the space-resolved spectroscopic measurement has already been established as a spot-lighted method, especially in biology and for specification of micro-structured semiconductor devices, it is still at a rudimentary stage to combine with time-resolved spectroscopic measurements for the characterization of molecular-scale photonic/electronic materials. We will devote our efforts to build up time-resolved confocal microscopy by improving the time resolution as well as extending the applicable spectral range. Our strong point over other groups in the characterization of molecular photonic/electronic devices is that we have a lot of experiences with molecular systems that can function as active elements in molecular-size electronics. If the project is carried out successfully, our group is able to contribute to the actualization of molecular photonic/electronic devices by time- and space-resolved spectroscopic characterization.
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.
The proposed novel ultrafast light-control and molecular photonic device technologies can be a promising application in the near future, which gives a reason for the continual development as a core technology in optical science. We can also build the foundation for a venture enterprise in which the key technology itself is invaluable. In all, this technology has a great potential to play a pioneering role to innovate the current industry by leading one step further towards the advanced technology for future application and establishing an overall scientific culture over the country.