Morphology-dependent optical properties of substituted poly(p-phenylene–ethynylene) (PPE) films

Two different batches of substituted poly(p-phenylene–ethynylene) [PPEs] were synthesized, where the substituents: di-2-butyl(a) and di-2-ethylhexyl(d), were placed on the 2 and 5 positions of the backbone phenyl rings. The photoinduced absorption (PA) spectrum of PPE(a) shows a polaron band at about 850 cm , whereas PPE(d) does not show polaron bands at all. At T 1⁄4 10 K the PA spectrum of PPE(a) in the visible spectral range has two main bands around 1.25 and 1.9 eV, respectively, and a broad shoulder at 2.1 eV. Using the photoinduced absorption detected magnetic resonance (PADMR) spectroscopy, we found that in PPE(a) the 1.9 eV PA band is associated with spin 12; thus it is due to long-lived photogenerated polarons, whereas both the 1.25 eV PA band and 2.1 eV band do not have any spin 1 2 ; resonance. In PPE(d) there is a very sharp peak at 1.45 eV, which is due to triplets excitons, and a broad band around 2.0 eV similar to PPE(a), which is due to aggregates. r 2003 Elsevier B.V. All rights reserved.