I  U  P  A  C






News & Notices
.. Prize for Young Chemists
.... 2002 winners

Organizations & People

Standing Committees

Divisions

Projects

Reports

Publications

Symposia

AMP

Links of Interest

Search the Site

Home Page

 

 

Winner of the IUPAC Prize
for Young Chemists - 2002

 

Jinsang Kim wins one of the first 4 IUPAC Prize for Young Chemists, for his Ph.D. thesis work entitled "Supramolecular Assemblies of Conjugated Sensory Polymers and the Optimization of Transport Properties."

Current address (at the time of application)

California Institute of Technology
Division of Chemistry and Chemical Engineering
Mail code 210-41
Pasadena, CA 91125, USA

E-mail: jinsang@cheme.caltech.edu or jinsang@alum.mit.edu

Academic degrees

  • Ph.D. MIT, Department of Materials Science and Engineering, June 2001
  • M.S. Seoul National University, Fiber and Polymer Science, Feb. 1993
  • B.S. Seoul National University, Fiber and Polymer Science, Feb. 1991, Magna Cum Laude

Ph.D. Thesis

Title Supramolecular Assemblies of Conjugated Sensory Polymers and the Optimization of Transport Properties
Adviser Prof. Timothy M. Swager, Department of Chemistry, Massachusetts Institute of Technology
Thesis Committee Prof. C.A. Ross, Prof. M.F. Rubner (co-adviser), Prof. E.L. Thomas, Department of Materials Science and Engineering, Massachusetts Institute of Technology.

Essay

Conjugated polymers (CPs) have become emerging materials for electronic applications due to the tunability of their properties through variation of chemical structure. Their applications, which currently include LEDs, FETs, plastic lasers, batteries, and sensors, are expanding to many novel areas. The two critical parameters that determine the function of CP-based devices are chemical structure and nano-structure of a polymer in the solid-state. While the physical properties of isolated polymers are decided by their chemical structure, these properties are altered in the solid-state due to electronic coupling between polymer chains determined by their interpolymer packing. However, in more than 20 years of CPs research, the development of effective precise methods for controlling the nano-structure of polymers in the solid-state has been limited because polymers often refuse to assembly into organized structures due to their amorphous character and large molecular weight.

My Ph.D. research has focused on understanding and optimizing energy transport properties of CP thin films by achieving supramolecular assemblies, with the ultimate goal to fabricate high performance solid-state fluorescence chemosensors. A good sensor must satisfy two key properties - selectivity and sensitivity. While selectivity can be optimized by rational chemical design of receptor groups, sensitivity is strongly influenced by transport properties of sensory materials in the solid-state. For fluorescence chemosensors, fluorescence energy transport between polymers as well as through a polymer backbone must be optimized to realize high sensitivity.

We have utilized the unique behavior of surfactant materials at the air-water interface to control precisely and directly the conformation and interpolymer orientations of CPs. We designed surfactant monomers that have preferred orientations at the air-water interface and the initial equilibrium orientations can be converted to different orientations by applying surface pressure. Unique combinations of these building blocks created polymers in three inter-convertible structures with different conformation and/or interpolymer orientation at the air-water interface (Figure 1). Applying mechanical force to the monolayers controls the conformation of the isolated polymer chain and/or interpolymer interactions. Critical to the analysis is the ability to monitor optical spectra of the monolayers while switching between different structures with our in situ UV-Vis and fluorescence spectroscopy setup. As the effective conjugation length of a polymer was altered by applying surface pressure, the absorption lmax of its monolayer showed a corresponding shift either in the blue or red direction, and as the polymer's p-planes co-facially oriented themselves towards each other, the fluorescence of the monolayer was gradually quenched. This was the first direct and clear picture of conformational and interpolymer interaction effects on the intrinsic optical properties of CPs achieved by delicate control of nano-structures.

Figure 1 - Jinsang Kim and Timothy M. Swager Nature 2001, 411, 1030.

CP monolayers with the above mentioned controlled specific structures were transferable to a substrate by the Langmuir-Blodgett method. Therefore, a wide variety of structurally well-defined CP films can be fabricated as demonstrated by the construction of well-aligned poly(p-phenyleneethynylene)s (PPEs) films. By applying mechanical compression and flow field while polymers were transferred from the air-water interface to a sold substrate we induced alignment of polymers, and were able to make monolayers composed of nano-scale PPE fibrils. Bilayer grid films of these fibrils were then obtained by changing the dipping direction of a substrate (Figure 2).

Figure 2 - reproduced from Jinsang Kim, Sean K. McHugh and Timothy M. Swager Macromolecules 1999, 32, 1500.

The energy transport properties of these well-defined CP films were then systematically studied. Two model systems were investigated to understand the photophysical and energy transport properties of PPEs. Multilayer PPE films were either surface modified with immobile luminescent traps or bulk modified with diffusible traps. From the experimental and simulation results, we found that well-aligned luminescent rigid-rod PPEs provide fast fluorescent energy transfer within the sensory films, but that fluorescent energy transfer has an inherent 16-layer limit in the z-direction. In our fluorescence sensory film design, receptors are located at the surface of a film, while the underlying layers provide signal amplification through energy transfer to the surface layer. Therefore, a 16-layer CP-based film showed maximum sensitivity obtainable for a single component film.

Our next goal was to devise a film structure that could overcome the inherent 16-layer limit. In a single component multilayer film, energy travels randomly in the z-direction because the energy levels of the polymer layers are essentially degenerate. To lift this degeneracy, a multicomponent film composed of three different polymers with tailored optical properties was fabricated. The polymers were designed so that their absorption and emission spectra have good overlap to facilitate efficient Förster energy transfer. In the multicomponent film, polymer bandgap decreases toward the top layer, allowing unidirectional energy flow from bottom to top (Figure 3). This vectorial energy transfer design, which is analogous to the way nature optimizes its energy harvesting system, overcame the 16-layer limit, amplifying the sensory signal from the top layer even further.

Figure 3 - reproduced from Jinsang Kim, D. Tyler McQuade, Aimee Rose, Zhengguo Zhu and Timothy M. Swager J. Am. Chem. Soc. 2001, 123, 11488.

Besides the sensitivity studies, the effect of receptor design on the selectivity of a potassium chemosensor was investigated. Physiologic potassium level is very important but potassium is not selectively detected in the presence of sodium. We utilized the fact that co-facially aggregated CPs lose their fluorescence to develop a potassium chemosensor. Alternating PPE copolymers containing 15-crown-5 were synthesized. By analyzing the effect of the side group bulk on every other repeating unit to the sensitivity, we have shown that the detection mechanism is co-facial interpolymer aggregation induced by potassium binding. The copolymers selectively detect bigger K+ over Na+ and Li+ by intermolecular aggregation induced by 2:1 complexes among 15-crown-5s and K+ (Figure 4). A highly selective and sensitive K+ chemosensor has been developed.

Figure 4 - reproduced from Jinsang Kim, D. Tyler McQuade, Sean K. McHugh and Timothy M. Swager Angew. Chem. Int. Ed. 2000, 39, 3868.

In summary, we have developed an effective method to control the conformation and/or spatial arrangements of polymers in the solid-state. This unique method allows us to address an unsolved challenging question. For the first time we deconvoluted the roles of individual polymer chains from the contributions of interpolymer interactions in the optical properties of CP films. Well characterized energy transport property of CP films in controlled structures led us to adapt nature's effective energy harvesting system to a novel sensory film design for ultimate signal amplification. This method and the film design are readily applicable for general CP research and development of diagnostic fluorescence sensors.


Page last modified 7 May 2002.
Copyright ©2002 International Union of Pure and Applied Chemistry.
Questions or comments about IUPAC, please contact, the Secretariat.
Questions regarding the website, please contact Web Help.