BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Talks.cam//talks.cam.ac.uk//
X-WR-CALNAME:Talks.cam
BEGIN:VEVENT
SUMMARY:Interplay Between Intrachain and Interchain Interactions in Semico
 nducting Polymers: “Jekyll-Hyde” Behavior in Common Emissive Polymers 
 - Prof. Frank Spano\, Temple University\, USA
DTSTART:20130712T133000Z
DTEND:20130712T143000Z
UID:TALK45594@talks.cam.ac.uk
CONTACT:Dr. Girish Lakhwani
DESCRIPTION:Understanding how the photophysical properties of conjugated p
 olymer films depend on film morphology is essential in designing and optim
 izing devices such as organic solar cells\, light emitting diodes\, and fi
 eld-effect transistors. It was previously shown that absorption and photol
 uminescence in poly(3-hexylthiophene) -or P3HT - spin-cast films could be 
 understood by treating the polymer pi-stacks as weakly-coupled H-aggregate
 s\, consistent with Kasha’s designation for aggregates of “side-by-sid
 e” oriented chromophores.[1] By contrast\, the photophysical response of
  isolated chains of conjugated polymers\, such as the red form of polydiac
 etylene\, can be understood in terms of linear J-aggregates [2\,3] – i.e
 . as a linear array of “head-to-tail” coupled repeat unit “chromopho
 res”.   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. Sur
 prisingly\, a given polymer can display both J-like and H-like photophysic
 s – i.e. “Jekyll-Hyde” behavior - depending on the morphology.[4] St
 rong disorder and the associated small conjugation lengths weaken the intr
 achain interactions while strengthening the interchain interactions leadin
 g to H-aggregate behavior. Conversely\, intrachain interactions are strong
 er and interchain interactions are weaker in well-ordered long polymer seg
 ments favoring J-aggregate behavior. The theory accounts for recent observ
 ations 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 photophy
 sics of the red-phase MEH-PPV aggregates[6] and MEH-PPV single molecules.[
 7]\n\n\n1. M. Kasha\, Radiation Research 20 (1)\, 55 (1963).\n\n2. M. Scho
 tt\, in Photophysics of molecular materials: from single molecules to sing
 le crystals\, edited by G. Lanzani (Wiley-VCH\, Weinheim\, 2006)\, pp. 49.
 \n\n3. H. Yamagata and F. C. Spano\, J. Chem.  Phys. 135\, 054906 (2011).\
 n\n4. H. Yamagata and F. C. Spano\, J. Chem.  Phys. 136 (18)\, 184901 (201
 2).\n\n5. 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 (2
 012).\n\n6. A. Kohler\, S. T. Hoffmann\, and H. Bassler\, J. Am. Chem. Soc
 . 134 (28)\, 11594 (2012).\n\n7. P. F. Barbara\, A. J. Gesquiere\, S.-J. P
 ark\, and Y. J. Lee\, Acc. Chem. Res. 38 (7)\, 602 (2005).\n
LOCATION:Kapitza Building Seminar Room\, Cavendish Laboratory\, Department
  of Physics
END:VEVENT
END:VCALENDAR