The main theme in our work on conjugated polymers has been understanding how the interactions between polymer chains affect the electronic properties of bulk conjugated polymer films.  A quick perusal of the literature in this field reveals an enormous amount of controversy:  different groups studying the same or related polymers have obtained very different results in a wide variety of experiments; without agreement on the basic electronic properties, it is difficult to know how to proceed to improve the quality of optoelectronic devices (LEDs or solar cells) based on these materials.  What we have found is that the  pi-electrons on neighboring conjugated polymer chains can interact with each other, so that an interchain species (sometimes referred to as an "aggregate", "excimer" or "interchain polaron pair") can be formed in the electronic excited state.  Moreover, the presence of these interchain excited states depends sensitively on how the chains are packed.  For example, the figure at left shows the fluorescence spectra of 5 films of the same polymer (MEH-PPV; see inset for chemical structure) cast from solutions with different polymer concentrations and using different solvents (CB = chlorobenzene; THF = tetrahydrofuran).  Clearly the fluorescence spectrum depends sensitively on the film preparation conditions!  What we have found is that the interactions between polymer chains in the film, which are characterized by a red-shift and lowered quantum yield of the fluorescence, can be increased by 1) increasing the polymer concentration in the solution from which the film is cast; 2) choosing a solvent like CB that causes the polymer to unfold in solution, promoting interactions between chains, and 3) thermally annealing the film by heating above the polymer's glass transition temperature.   Thus, different groups who make their films in different ways (by casting from different solvents, using different spin speeds, heating their substrates differently, etc.) are preparing samples with a different degree of interchain contact and thus differing amounts of interchain electronic excited states.  No wonder there was so much controversy in the literature!  With our new understanding of the interchain electronic species and how to control the morphology of conjugated polymer films to enhance or eliminate them, we have been able to understand the photophysics of these materials and to significantly improve the efficiency of  polymer-based LEDs.  Our current efforts are focused on using charged conjugated polymers (conjugated ionomers and polyelectrolytes) to take advantage of electrostatic forces in controlling conformation for applications in LED's and photovoltaic cells.  See, e.g.,

I. B. Martini, A. D. Smith and B. J. Schwartz, "Exciton-exciton Annihilation and the Production of Interchain Species in Conjugated Polymer Films:  Comparing the Ultrafast Stimulated Emission and Photoluminescence Dynamics of MEH-PPV Films," Phys. Rev. B 69(3) 035204, 1-12 (2004). 

B. J. Schwartz, "Conjugated Polymers as Molecular Materials:  How Chain Conformation and Morphology Influence Energy Transfer and Interchain Interactions," Annu. Rev. Phys. Chem. 54, 141 (2003).  

R. D. Schaller, P. T. Snee, J. C. Johnson, L. F. Lee, K. R. Wilson, L. H. Haber, R. J. Saykally, T.-Q. Nguyen and B. J. Schwartz, "Nanoscopic interchain aggregate domain formation in conjugated polymer films studied by third harmonic generation (THG) near-field scanning optical microscopy (NSOM)," J. Chem. Phys. 117(14), 6688-98 (2002).

R. D. Schaller, L. F. Lee, J. C. Johnson, L. H. Haber, R. J. Saykally, J. Vieceli, I. Benjamin, T.-Q. Nguyen and B. J. Schwartz, "The Nature of Interchain Excitations in Conjugated Polymers:  Near-Field Optical Studies of Spatially-Varying Solvatochromism in Annealed MEH-PPV Films," J. Phys. Chem. B 106(37), 9496-506 (2002).

T.-Q. Nguyen and B. J. Schwartz, "Ionomeric Control of Interchain Interactions, Morphology and the Electronic Properties of Conjugated Polymer Solutions and Films," J. Chem. Phys. 116(18) 8198-208 (2002).

T.-Q. Nguyen, B. J. Schwartz, R. D. Schaller, J. C. Johnson, L. F. Lee, L. H. Haber and R. J. Saykally, "Near-Field Scanning Optical Microscopy (NSOM) Studies of the Relationship between Interchain Interactions, Morphology, Photodamage, and Energy Transport in Conjugated Polymer Films," J. Phys. Chem. B 105(22), 5153-60 (2001).

T.-Q. Nguyen, R. Y. Yee and B. J. Schwartz, "Solution Processing of Conjugated Polymers: The Effects of Polymer Solubility on the  Morphology and Electronic Properties of Semiconducting Polymer Films," J. Photochem. Photobio. A 144, 21-30 (2001).

T.-Q. Nguyen, R. C. Kwong, M. E. Thompson and B. J. Schwartz, "Higher Efficiency Conjugated Polymer-Based LEDs by Control of Polymer Film Morphology and Interchain Interactions," Synth. Met. 119, 523-4 (2001).

T.-Q. Nguyen, R. C. Kwong, M. E. Thompson and B. J. Schwartz,  "Improving the Performance of Conjugated Polymer-Based Devices by Control of InterchainInteractions and Polymer Morphology," Appl. Phys. Lett. 76(17), 2454-6 (2000).

T.-Q. Nguyen, I. Martini, J. Liu and B. J. Schwartz, "Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphologyon Exciton-Exciton Annihilation and Aggregation in MEH-PPV Films," J. Phys. Chem. B 104(2), 237-55 (2000).

T.-Q. Nguyen, V. Doan and B. J. Schwartz, "Conjugated Polymer Aggregates in Solution: Control of Interchain Interactions,"J. Chem. Phys. 110(8), 4068-78 (1999).

V. Doan, V. Tran and B. J. Schwartz, "Intensity-Dependent Stimulated Emission in Conjugated Polymers: The Mechanism for Line Narrowing," Chem. Phys. Lett. 288, 576-84 (1998).
 

Also related to the above is our work on conjugated polymers embedded in the channels of mesoporous silica glass.
 

This work was supported in part by the National Science Foundation under award DMR-9971842