Abstract

Understanding how the photophysical properties of conjugated polymer films depend on film morphology is essential in designing and optimizing devices such as organic solar cells, light emitting diodes, and field-effect transistors. It was previously shown that absorption and photoluminescence in poly(3-hexylthiophene) or P3HT spin-cast films could be understood by treating the polymer pi-stacks as weakly-coupled H-aggregates, consistent with Kasha’s designation for aggregates of “side-by-side” oriented chromophores.[1] By contrast, the photophysical response of isolated chains of conjugated polymers, such as the red form of polydiacetylene, can be understood in terms of linear J-aggregates [2,3] – i.e. as a linear array of “head-to-tail” coupled repeat unit “chromophores”. In this talk we present the HJ-aggregate model, an extension of the Kasha model to include exciton motion along the polymer chain as well as across chains. The model shows how competition between interchain and intrachain electronic interactions impacts the photophysical response. Surprisingly, a given polymer can display both J-like and H-like photophysics – i.e. “Jekyll-Hyde” behavior – depending on the morphology.[4] Strong disorder and the associated small conjugation lengths weaken the intrachain interactions while strengthening the interchain interactions leading to H-aggregate behavior. Conversely, intrachain interactions are stronger and interchain interactions are weaker in well-ordered long polymer segments favoring J-aggregate behavior. The theory accounts for recent observations of J-aggregate-like photophysical behavior in P3HT nanofibers, in marked contrast to the H-aggregate behavior displayed by spin-cast films.[5] The HJ-aggregate model is also successful in understanding the photophysics of the red-phase MEH -PPV aggregates⁶ and MEH -PPV single molecules.[7]

1. M. Kasha, Radiation Research 20 (1), 55 (1963).

2. M. Schott, in Photophysics of molecular materials: from single molecules to single crystals, edited by G. Lanzani (Wiley-VCH, Weinheim, 2006), pp. 49.

3. H. Yamagata and F. C. Spano, J. Chem. Phys. 135, 054906 (2011).

4. H. Yamagata and F. C. Spano, J. Chem. Phys. 136 (18), 184901 (2012).

5. E. T. Niles, J. D. Roehling, H. Yamagata, A. J. Wise, F. C. Spano, A. J. Moule, and J. K. Grey, J. Phys. Chem. Lett. 3 (2), 259 (2012).

6. A. Kohler, S. T. Hoffmann, and H. Bassler, J. Am. Chem. Soc. 134 (28), 11594 (2012).

7. P. F. Barbara, A. J. Gesquiere, S.-J. Park, and Y. J. Lee, Acc. Chem. Res. 38 (7), 602 (2005).