My research is focused on the understanding of intermolecular effects in ordered media. In particular we are investigating the effect different media impart upon chemical reactivity with the goal of designing organic systems with specific functional control. This broad research goal encompasses a number of different fields including semiconducting polymers for molecular electronics, biodegradable polymers for drug delivery and the synthesis of carbon nanotubes of one type through topochemical reactions. A brief description of each research area is included below.
Semiconducting polymers have potential to be used in a wide variety of applications including flexible electronics and photovoltaics. For any of these applications, a good energy overlap of the corresponding frontier orbitals is required for efficient electron transfer processes. Traditionally, the frontier orbital levels are adjusted by the proper choice of either electron donating or withdrawing substituents. With regioregular polythiophene semiconducting polymers, however, there is only one remaining aromatic position on the polythiophene core available for substitution and any substitution at this position would limit the conjugation length of the polymer due to steric interactions with the regioregular alkyl substituents required for processability. In order to overcome these limitations, a bithiophene core has been prepared with a fused benzene ring. The central benzene ring can be substituted in order to control the orbital levels of the polymer and the symmetric nature of the fused bithiophene core prevents any concerns about regioregularity in the polymerization. To lower the LUMO energy level, an electron withdrawing phenyl ethynyl substituent was attached to the benzene ring with para alkyl substituents to increase solubility of the formed polymer. Our group designs and synthesizes monomer cores of the fused benzodithiophene unit with extended conjugation which are then subsequently polymerized in the group of Prof. Mihaela Stefan (Iovu).
Block copolymers that contain a hydophobic and hydrophilic block can form micelles in aqueous environment. These micelles have been studied as possible drug delivery agents where the hydrophobic drug is included in the block copolymer. Using polylactones for the two block components, the ester linkages can slowly degrade in the aqueous environment which evenutally releases the drug from the inner, hydrophobic part of the micelle. By controlling the substituents on both the hydrophilic and hydrophobic part of the block-polylactone, many factors to improve the ability of using this micellar approach to drug delivery can be affected including the loading capacity of the micelle, the degradation rate of the micelle and the ability to target the micelle to various tumor sites in the body. This project is a collaboration with the polymer group of Prof. Mihaela Stefan (Iovu).
In collaboration with the NanoTech Institute at UTD we are attempting to prepare single walled carbon nanotubes through an organic synthesis approach. Instead of trying to purify a mixture of carbon nanotubes produced by current methods, we are attempting to prepare one type of carbon nanotube. Our approach is to prepare carbon rings with diacetylene groups at specified positions. In the solid-state these rings will pack to allow a topochemical polymerization of the diactylene units. This reaction will produce a tubular carbon framework of a certain dimension. Thermal annealing will then be applied with appropriate catalysts to remove remaining hydrogen and form the single walled carbon nanotube.
Another research area is designing monolayers with specific intermolecular interactions within the monolayer structure. We are studying monolayers grown on transparent glass surfaces because we are initially probing the intermolecular interactions on the surface through optical studies of a photochromic spiropyran that is attached. Upon ultraviolet irradiation this spiropyran converts into a photomerocyanine structure with an absorbance in the visible region. This chromophore can be detected and the decay rate of the photomerocyanine to the spiropyran form in the dark is directly related to the specific intermolecular interactions within the monolayer.
Two component SAM is synthesized and attached to transparent glass surface.
Upon photolysis of slide the photomerocyanine absorbance is observed, and the decay rate of the photomerocyanine in dark is dependent upon the interactions within the monolayer
The photochromic SAM slide is placed in light path of photodiode array UV-Vis machine. The slide can be photolyzed in air as shown or immersed in appropriate solvent.
Another research area is studying the effect of "hard" and "soft" cages upon the stability of included organic compounds. Once again organic photochromic materials are used to probe these effects by studying the photogeneration and lifetime of the photogenerated species within the caged environment compared to fluid studies. The type of cages employed are host/guest single crystals like cyclodextrins (hard cages) which form channel inclusion voids where appropriately sized guests may reside and organogels (soft cages) where guests can be included within the optically clear viscoelastic environment where fluid flow has been prevented. Each system offers different types of intermolecular stabilization interactions which can increase the lifetime of the photogenerated photomerocyanines.
Bromo Substituted Spiropyran Included in gamma-CD Host