An Order-Disorder Transition in the Conjugated Polymer MEH-PPV Anna Kö hler,* Sebastian T. Hoffmann, and Heinz Ba ̈ ssler Organic Semiconductors, Experimental Physics II, Department of Physics and Bayreuth Institute of Macromolecular Science(BIMF), University of Bayreuth, Bayreuth 95440, Germany ABSTRACT: The poly(p-phenylene vinylene) derivative MEH-PPV is known to exist as two morphologically distinct species, referred to as red phase and blue phase. We show here that the transition from the blue phase to the red phase is a critical phenomenon that can be quantitatively described as a second order phase transition with a critical temperature T c of 204 K. The criticality is associated with the trade-off between the gain in the electronic stabilization energy when the π- system of a planarized chain can delocalize and the concomitant loss of entropy. We studied this transition by measuring the absorption and fluorescence in methyltetrahydrofuran (MeTHF) in two different concentrations as a function of temperature. The spectra were analyzed based upon the Kuhn exciton model to extract effective conjugation lengths. At room temperature, the chains have effective conjugation lengths of about five repeat units in the ground state (the blue phase), consistent with a disordered defect cylinder conformation. Upon cooling below the critical temperature T c , the red phase with increased effective conjugation lengths of about 10 repeat units forms, implying a more extended and better ordered conformation. Whereas aggregation is required for the creation of the red phase, its electronic states have a predominant intrachain character. 1. INTRODUCTION Morphologically induced polychromism is a frequently encountered phenomenon in conjugated polymers. It was first recognized when studying the fluorescence from polydiacetylene in solution. 1-4 Depending on the kind of solvent, concentration and temperature, photoluminescence is emitted either from a high energy state (the “blue phase”) or from a lower energy state (the “red phase”) that differ in the extent of delocalization of the π-bond electrons . 5 There has been a lively debate whether or not this color change is a single chain phenomenon 2,6 or is rather a signature of aggregation. 7,8 While polychromism is of scientific interest as it indicates changes in the underlying electronic structure, it has also potential for commercial applications. For example, the temperature-dependence of absorption and emission in derivatives of polydiacetylene is being considered for sensor applications. 9,10 A more recent, and for opto-electronic applications also more relevant example is the chromism that been observed in conjugated polymers of the poly(p-phenylene vinylene) family, 11-21 the poly(p-phenylene) family 22-26 and the polythiophene family. 27-30 In these materials, the polychrom- ism indicates changes in electronic coupling between the molecular subunits. This has a strong impact on the performance of devices such as transistors, solar cells or light- emitting diodes. 24,25,29-33 Understanding the relationship between the conformation of polymer chains and their electronic structure in the excited state is thus a key issue in the field. We need to comprehend (i) which physical phenomenon causes the polychromism and its dependence on experimental parameters such as temperature, concentration and molecular weight, and (ii) what is the role of intrachain and interchain interactions in this process. In the current manuscript, we address the origin of the polychromism for the prototypical polymer MEH-PPV (poly- (2-methoxy-5-(2‘-ethylhexyl)oxy 1,4-phenylene vinylene)). MEH-PPV is well-known to show a bimodal distribution of emission maxima. 11-14,17,18,34-38 The two associated chain conformations are referred to as “blue phase” and “red phase”. We measured the absorption and fluorescence spectra of MEH- PPV with molecular weight of about M w = 60 ± 10 kD, equivalent to about 230 ± 40 repeat units per chain, in methyl- tetrahydrofuran (MeTHF) solution between 300 K and 80 K. The concentrations were 5 × 10 -6 mole of repeat unit/liter of solvent and 10 -7 mole of repeat unit/liter of solvent. The aim of this study is to gain insight into the condition and mechanism for the formation of a bimodal distribution of excited states in MEH-PPV. Our results suggest that, (i) the transition from the blue phase to the red phase can be described as a cooperative phenomenon that leads to a second order (continuous) phase transition. It is associated with a disorder-to-order transition, corresponding to a change in the polymer structure. (ii) We find that both, intrachain and interchain interactions facilitate the establishment of the more ordered, that is, a red phase. Thus, in MEH-PPV the formation of the red phase is assisted by concentration dependent aggregation, yet the electronic nature of the excited state has a predominant intrachain character. Received: March 12, 2012 Published: June 19, 2012 Article pubs.acs.org/JACS © 2012 American Chemical Society 11594 dx.doi.org/10.1021/ja302408a | J. Am. Chem. Soc. 2012, 134, 11594-11601