Long-Lived Defects in Polycarbazolyldiacetylenes. Photoinduced Vis and Infrared Spectra
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ABSTRACT Photoinduced absorption spectra of different polycarbazolyldiacetylenes in the visible and infrared regions are reported. Long-lived bipolaronic defects, photogenerated by excitation above the band-gap (2.54 eV), are found responsible for the observed signals. Associated with these defects, photogenerated infrared active vibrations (IRAV) are observed as absorption bands or apparent bleachings arising from Fano-type interference. This different behaviour originates from tihe different degree of overlap of the IR.AV iiodes with dhe associated low-energy bipolaroric absorption.
INTRODUCTION Various conjugated polymers have been studied extensively due to their novel and potentially useful physical properties. Among these materials, the polydiacetylenes are of particular interest for their large optical non-linearities and short excited-state relaxation times I l1. These polymers, which can be obtained as almost defect-free single crystals via solid-state polymerization, ideally behave as wide band-gap monodimensional semiconductors. By varying the substituent groups attached to thebconjugated backbone, arepro uce tte .... o-I 1 pr"--- materials with great differences in optical absorption spectra are produced but hIgf ofUpratica. tion i ueviL'ces .car would require detailedI kn•owledge of' their electronic structures. Photoinduced absorption studies have often provided information about photogenerated defects and their recombination dynamics. In this way. photoexcited triplet excitons or,
alternatively, bipolaronic defects have been identified in different polydiacetylenes 12- 101.
In this paper we will discuss the photoinduced spectrum of poly(l-(N-carbazolyl)penta-l,3diyn-5-ol) (polyCPDO) and poly( I-(N-carbazolvl)penta- 1.3-diyn-5-acetoxy) (polyCPDA). Among conjugated polymers, polycarbazolyldiacetylenes show very interesting structural,
mechanical, thermal and optical properties 19-131. Substantial red shifts relative to other polydiacetylenes are in fact observed either of the lowest energy optical transition and of the photoconductivity. They have been attributed to the increase in the polarizability of the chain environment determined by the carbazolyl substituents. RESULTS AND DISCUSSION Fig. I shows the room-temperature electronic absorption spectra of polyCPDO and polyCPDA. These are measured on polycrystalline samples obtained by isothermal polymerization of the micronized monomers in KBr pellets. The spectrum of polyCPDO (full line), which is redshifted with respect to spectrum of polyCPDA (dashed line), shows a non resolved structure at the position of the singlet excitonic absorption typical of highly ordered polydiacetylene chains. In the same figure the photoluminescence spectra of these polymers, obtained upon excitation at 488 nm (2.54 eV), are reported. Here featureless emission bands are observed with a peak displacement comparable to that occurring in the absorption edges. The shift between absorption and photoluminescence maxima may be attributed to the relaxation of the thermalized s
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