Enhancing Molecular Alignment and Photostability in Organic EO Materials using Single-Molecule Microscopy

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Single-molecule spectroscopic techniques will be employed to enhance the efficiency and photostability of organic electro-optic (EO) materials. Recent advances in organic photonic materials and their inclusion into novel device architectures suggests that these materials will play an important role in the next generation of EO switching devices for telecommunications and other applications. Although the promise of organics to provide enhanced EO activity, faster switching speeds, and at a lower cost relative to current materials has been recognized for some time, there are two issues that limit the wide-spread use of these materials: chromophore alignment and photostability. The studies are designed to provide insight into the molecular details underlying these issues, and with this insight strategies for enhanced material performance can be identified and pursued.Alignment in organic EO materials is studied by investigating the rotational dynamics of single-molecules as a function of external perturbation (in particular, electric field and temperature). EO activity arises at the ÷(2) level of material susceptibility which requires that the material non-centrosymmetric. Acentric order in these materials is introduced by poling, a process in which an external electric field interacts with the permanent dipole moment of the chromophore to (theoretically) restrict chromophore reorientation thus providing for material alignment. The molecular-level details of poling are poorly understood, and our recent studies have established that only modest alignment is achieved in this process. Furthermore, poling is generally performed 5 to 10° C below the glass transition temperature of the polymer host, but what do chromophore reorientational dynamics look like at these temperatures? What is the interplay between temperature, polymer relaxation, and poling-induced order? The single molecule studies will provide molecular-level insight into the poling process, and subsequently refinement of this process. Optical poling in binary chromophore organic glasses is also studied. Optical poling provides for a two-fold enhancement in EO activity relative to electric-field poling alone. Theory suggests that this enhancement arises from the optical field reducing the spatial dimensionality of the host. The team will measure the rotational dynamics of single molecule in the presence and absence of the optical poling field to directly test this hypothesis. The photostability of organic EO materials is investigated by measuring the time-dependent emission (blinking), spectral diffusion, and excited-state lifetimes of single molecules. Time-tagged, time-correlated single photon counting techniques are used to directly correlate blinking behavior to the underlying photophysics that result in population and depopulation of the non-emissive or dark state. The experiments are combined with Monte-Carlo simulations to identify these states which serve as a gateway to material photodecomposition. A unique aspect of this work is that the team will employ single molecule crystal isolation techniques to test vexing questions concerning molecular photophysics in complex environments, with the crystal providing a host where solvation is well-defined and controlled.The advancement of fundamental knowledge will have impact on the fields of quantum information and of photonics. The graduate students directly involved in these studies will receive a multidisciplinary education in basic physics, materials science, and nanofabrication. The research, while fundamental in nature, is readily accessible to undergraduates and will benefit from the involvement of undergraduate students in the program. The fundamentals of optical absorption and emission provide unique opportunities for illustrating nanoscience to pre-Kindergarten through high school students. For example, the emission from visible quantum dots provides excellent visual demonstrations that will be used in the outreach activities of the PIs in high needs Buffalo Public Schools. This outreach will be enhanced significantly by the incorporation of a middle school science teacher in the PIs research activities during the summer. Simultaneously, this summer program will also enable the PIs to benefit from the experience of the teacher in the development of educational tools for use at the middle and high school levels. Finally, the PIs will organize a summer workshop for high school students to provide an introduction to the exciting research in nanoscience.