Thermal vibrational disorder of a conjugated polymer in charge-transfer complex

Temperature dependences of optical absorption and Raman spectra of ground-state charge-transfer complex (CTC) formed in blends of a conjugated polymer, poly[2-methoxy-5-(2(')-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV), and low-molecular-weight acceptor, 2,4,7-trinitrofluorenone, were studied. Upon cooling from 320 to 120 K, the polymer strongest Raman band shows a 0.5 cm(-1) low-frequency shift while it demonstrates a 0.5 cm(-1) high-frequency shift in the CTC. This behavior is explained by the temperature dependence of polymer-acceptor charge transfer in the CTC: The amount of transferred charge decreases by 25% upon cooling. At the same time, both the pristine polymer and CTC demonstrate a 0.05-0.1 eV redshift of the absorption edge. To account for these temperature shifts, we propose a model that relates temperature variations in the effective conjugation length with thermal torsion vibrations of the conjugated chains. Comparison of the model and experimental data reveals that the torsion rigidity of conjugated chains involved in the CTC is 30% stronger than that of the pristine ones. This enhanced rigidity increases the conjugation length of MEH-PPV chains in the CTC by 20%. However, the major contribution to the MEH-PPV absorption edge shift in the CTC is assigned to a local built-in electric field effect induced by the ground-state charge transfer.