J. Phys. Chem. 1996, 100, 5485-5491
5485
Polaron Pair Formation, Migration, and Decay on Photoexcited Poly(phenylenevinylene) Chains Gerwin H. Gelinck,† John M. Warman,*,† and Emiel G. J. Staring‡ Radiation Chemistry Department, IRI Delft UniVersity of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands, and Philips Research Laboratories, Professor Holstlaan 4, 5656 AA EindhoVen, The Netherlands ReceiVed: October 30, 1995; In Final Form: January 10, 1996X
Poly(2-(3,7-dimethyloctoxy)-5-methoxy-1,4-phenylenevinylene), (dMOM-PPV), undergoes a thermochromic transition on heating in benzene at approximately 35 °C from a red gel to a yellow solution. The latter is metastable on cooling to room temperature and eventually reverts to the gel form after several hours. The absorption and emission spectrum of the gel can be resolved into two components, one which is identical with that for the solution and which is therefore associated with single-strand segments, and a second of similar spectral shape but lower energy which is ascribed to aggregated segments. Photoexcitation of dMOMPPV in both the gel and solution results in a transient change in the microwave conductivity as monitored using the time-resolved microwave conductivity (TRMC) technique. The change in microwave conductivity for the gel is approximately an order of magnitude larger than for the solution. No TRMC transient is found on flash photolysis of a benzene solution of a dMOM-PPV derivative with 13% nonconjugated units in the polymer backbone. The effects are attributed to formation of mobile charge carriers (polaron pairs) on a single polymer chain in the case of the solution, whereas in the gel interchain charge transfer results in the predominant formation of charge carriers located on separated chains. The product of the quantum efficiency for polaron pair formation, φp, and the sum of the mobilities, Σµ, is φpΣµ ) (2.0 ( 0.3) × 10-8 m2/Vs and (2.0 ( 0.5) × 10-9 m2/Vs for the gel and solution respectively. The decay in microwave conductivity is nonexponential and extends to hundreds of nanoseconds.
Introduction 19901
that polySince the report by Burroughes et al. in (phenylenevinylene), PPV, and its derivatives can serve as the emissive layer in light-emitting diodes, there has been renewed interest in the processes of charge generation, migration, and recombination in such π-bond-conjugated polymeric materials. Commonly used methods of investigation involve photoexcitation of the polymeric material with concomitant monitoring of changes in the electrical or optical properties. An important finding in such studies was that the photoconductivity of PPV and its derivatives has a threshold very close to the onset of the first optical absorption band.2-8 On the basis of this, the binding energy of the lowest excited singlet state with respect to dissociation into mobile, polaronic charge carriers is concluded to be only a few tenths of an electronvolt at most. The detailed mechanism of photoinduced charge separation remains a matter of debate, however. According to the SSH model proposed by Su et al.,9 excitation across the π-π* “bandgap” in PPV results directly in the formation of an electron-hole pair as in inorganic semiconductors. This is followed by (sub)picosecond self-localization of the electronic charges to form negative and positive polarons, as a result of the strong coupling between the π-orbitals and the local chain geometry. The polaron pair may diffusively separate or combine to form an exciton, which can decay by radiative or nonradiative channels. In the SSH model therefore charges are allowed to separate on a single PPV chain and may remain separated for times considerably longer than the natural lifetime of the exciton state of a few hundred picoseconds. In an alternative model the initial product of photoexcitation is a neutral exciton (or “strongly correlated electron-hole pair”). †
IRI Delft University of Technology. Philips Research Laboratories. X Abstract published in AdVance ACS Abstracts, March 1, 1996. ‡
0022-3654/96/20100-5485$12.00/0
On a single polymer chain rapid radiative or radiationless decay back to the ground state is predicted. Charge separation can occur only via electron transfer to an adjacent chain resulting in a positive and negative polaron on separate chains.10-13 Charge separation in this model requires therefore a degree of polymer aggregation. It should not occur on photoexcitation of isolated PPV chains. The work presented here was initiated in an attempt to provide information which might shed light on charge carrier generation by monitoring the change in the microwave conductivity (dielectric loss) occurring on flash photolysis of dilute solutions of a PPV derivative. The time-resolved microwave conductivity (TRMC) technique which was used can detect the formation of micromolar concentrations of mobile charges within a dielectric medium with nanosecond time resolution. It has previously been extensively applied to the study of ions and dipolar species formed on flash photolysis of small molecules.14 This work represents the first application to photoinduced charge separation in polymeric systems. Experimental Section In Figure 1 is shown the primary molecular structure of the polymeric materials studied. The parameter n is the average fraction of conjugated segments as determined by 1H NMR with an estimated error of (0.02. The synthesis and characterization of the compounds have been described earlier.15 Molecular weights were determined by gel-permeation chromatography, which was calibrated using polystyrene standards in THF. The weight-average (number-average) molecular weights, MW (MN), of dMOM-PPV and dMOM-PPV(0.87) were 180 000 (70 000) and 220 000 (82 000), respectively. Solutions were prepared by dissolving the polymers in UV spectroscopic grade benzene at ca. 60 °C. Absorption spectra were recorded on a Uvikon 940 UV/Vis spectrophotometer. The © 1996 American Chemical Society
5486 J. Phys. Chem., Vol. 100, No. 13, 1996
Gelinck et al.
Figure 1. Molecular structures of the polymeric materials studied in the present work.
extinction coefficients of dMOM-PPV and dMOM-PPV(0.87) were determined to be 15,600 and 15,800 mol-1 dm3 cm-1 per phenylenevinylene unit at the absorption maxima of 503 and 481 nm respectively. The temperature of the cuvette containing the solution of interest could be varied using a RL6 Lauda cryostat. Time-resolved fluorescence measurements were carried out using a 0.8 ns fwhm, 1.5 mJ pulse of 337 nm light from a PRA LN1000 N2 laser. The solutions were contained in 1 cm quartz cuvettes with an optical density of ca. 0.15 at 337 nm. They were deaerated by bubbling with Ar for ca. 10 min. The light emitted at 90° to the laser beam was detected using an ITL TF 1850 photodiode (
