Photoinduced Charge Transfer Between Tetracyano-Anthraquino-Dimethane Derivatives and Conjugated Polymers
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Physikalische Chemie, Johannes Kepler Universität, 4040 Linz, Austria Departamento de Química Orgánica, Universidad Complutense, 28040 Madrid, Spain
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ABSTRACT The photo-induced charge transfer between tetracyano-anthraquino-dimethane (TCAQ) derivatives and poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene) MDMO-PPV was studied by means of photoinduced absorption (PIA) spectroscopy in the VIS and IR spectral region and lightinduced electron spin resonance (LESR) spectroscopy. Three different TCAQ derivatives with alkyl-side chains for increased solubility compared to TCAQ and one of them attached to a C60 molecule serve as strong electron acceptors. The photoinduced absorption in the VIS near-IR range shows a broad plateau around 1.8 eV followed by two peaks at 1.35 and 1.24eV for all three acceptors. All PIA features have a power law excitation intensity dependence with an exponent close to 0.5 as expected for bimolecular kinetics. The modulation frequency dependent excited state absorption decreases with a power law. LESR studies of all donor-acceptor combinations show one radical line at a g = 2.0028 with ∆H = 3.5 Gauss, originating from TCAQ anion and the polymer cation.
INTRODUCTION Light induced charge transfer from conjugated polymers and their oligomers to different tetracyano-p-quinodimethane (TCNQ) derivatives and fullerenes has been studied extensively with different methods [1-4]. It is well known that the charge transfer process from a conjugated polymer to C60 takes place on an ultra-fast timescale [2].The back transfer is remarkably slow; lifetimes of the order of milliseconds can be observed for the charge separated states. Various PIA studies on thin films of pristine conjugated polymers have shown the formation of the triplet state after photo-excitation. In mixed films of polymers with electron acceptors the triplet state absorption is quenched due to charge transfer. Absorption detected magnetic resonance (ADMR) experiments on films of pristine poly[2-methoxy-5-2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) have shown, that the origin of the photoinduced absorption in the range between 1.1 -1.6 eV is due to excited triplet states (spin 1) whereas mixed films of MEH-PPV and C60 show additional photoinduced absorption features but being due to charged separated states of spin ½ [5]. LESR studies on mixed films of MDMO-PPV and a C 60 derivative 1-(3-methoxycarbonyl)-propyl-1-1phenyl-(6,6)C61 (PCBM) showed two light induced ESR lines, one with a g-factor of 2.0025 being due to the polymer and the other one with a g-factor 1.9995 stemming from the fullerene anion [6]. In an other more comparative study
Figure 1: Chemical structure of the three TCAQ derivative electron acceptors used in the experiments. different TCNQ derivative acceptors were tested with respect to their charge transfer efficiency to conjugated polymer [4]. Both the amount of luminescence quenching and the PIA signal of the charged states were investigated for MEH-PPV and various TCNQ derivatives as electron acc
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