Optical properties and electroluminescence of poly(3-alkyl-methoxythiophene)s

Synthetic

Metals

102 (1999)

918

Optical propertiesandelectroluminescence of poly(3-alkyd-methoxythiophene)s A. Bolognesi, C. Botta, L. Cecchinato Istituto di Chimica delle Macromolecole,

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Abstract

We report the photohuninescence (PL) properties of a new class of highly regioregular methoxy substituted polythiophenes which display good electroluminescenceproperties. The thermal dependence of the photoluminescence.is studied in detail and compared to the differential scamring calorimetry analysis of polymers with different alkyl-side chain length. The thermochromism of the PL is discussedin term of the different stmcural properties of the polymers. Keywords: Polythiophenes, Photoluminescence,Thermochromism, Electroluminescence.

The poly(3-alkylmethoxythiophene)s studied in this work have been synthetized by a Ni catalyzed polymerization as reported in ref. 2; their structure is shown in Pig.1. With the particular synthetic procedure followed Y7-100% regioregularities were obtained; molecular weights were in the range of 10.000-l 5.000, as detected by SiSze Exclusion Chomatography referred to standard polystyrene. The PL measurements were performed under vacuum as reported elsewhere [ I].

have been obtained in a single layer light emitting diode (lTO/P5OML4l).

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I

II III Fig. 1-Structures of poly(3pentylmethoxythiophene) P50M (I), of poly(3-hexylmethoxythiophene) P6OM (II), of poly(3decylmethoxythiophene) PI OOM (RI). PlOOM shows PL properties similar to P6OM [ 11. The sharp change in the PL spectral shape and consequent blue shift occur at the temperature of a transition from an ordered to a less ordered phase (as detected by DSC), occurring at about 90°C and 110°C for PlOOM and POOM (see the inset in Fig. 2), respectively. These temperatures do no1 correspond to the polymer melting in agreement with previous obser-vations on poly(3-octyhhiophene) [3] where similar transitions were studied in details. P60M and P5OM PLs, as a function of temperature, are reported in Fig.2. On going from PGOM to P5OM thermochromic effects in the temperature range studied are strongly reduced and only an increase in the PL intensity of PSOM is observed by heating up to 110°C. This behaviour is coherently related to the thermal properties of P5OM w-hich does not show any phase transitions before its melting point ai 207°C. External electroluminescence efficiencies of l-.05% 0379-6779/99/S - see front PII: SO379-6779(98)00955-2

matter

0 1999 Elsevier

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: wavelength

( nm )

Fig.2. Photoluminescence at different temperatures for P6OM (upper spectra) and P5OM (lower spectra). Exciting wavelength 457.8 mn. In the inset the DSC scans. Progetto Finalizzato MSTAII is aknowledged for support

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References

[l] A. Bolognesi, C. Botta, G. Bajo, R. Gsterbacka, T. Gstergard, and H. Stubb, submitted. [2] A. Bolognesi, W. Porzio, G. Bajo, L. Fanning, to be submitted ]3] A. Bolognesi, W. Porzio, Z. Geng, T. Ezquerra, Eur. Polym. J. 32 (1996) 1097

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