Association function of conjugated polymer charge-transfer complex

Abstract

The donor–acceptor ground-state charge-transfer complex (CTC) formed in solution between a conjugated polymer, poly[methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV), and a low-molecular-weight organic acceptor, 2,4,7-trinitrofluorenone (TNF), is studied by optical absorption and Raman spectroscopy. The CTC absorption as a function of TNF content shows a threshold increase that is in conflict with the model commonly used for optical characterization of low-molecular-weight CTCs. The shift of MEH-PPV characteristic Raman band at 1585 cm⁻¹ also exhibits a threshold dependence upon TNF addition. We assign the threshold in both the absorption and Raman data to the CTC concentration. To describe the threshold in the terms of the common model, we extend it by introducing an association function instead of a constant. The association function of acceptor concentration has been calculated to be K_a ∼ 1.5–3 M⁻¹ below the threshold, to increase steeply up to K_a ∼ 6–7.5 M⁻¹ just after the threshold, and then to grow gradually up to K_a ∼ 40 M⁻¹. The CTC molar absorption coefficient has been found to be ε_CTC = (12.7 ± 0.6) × 10³ M⁻¹ cm⁻¹ at 635 nm. We explain the threshold as a result of the positive feedback: the CTC formation induces planarizaton of conjugated polymer segments that in turn facilitates further CTC formation.