Photoexcitations of Oligothiophenes with Reduced Conformational Mobility
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ABSTRACT We present a comprehensive study of the optical and electronic properties of a series of oligothiophenes in which one or two inter-ring torsional angles have been blocked by chemical bridging. These give us the possibility to investigate the role of the conformational mobility and of the coupling with the inter-ring torsion on the deactivation process of the singlet excited states. We find that both the radiative and the non-radiative deactivation channels are affected by the inter-ring bridging. INTRODUCTION The class of thiophene oligomers (nT) have attracted considerable attention as model compounds for a better understanding of the electronic and optical properties of the parent polymer polythiophene [tI and because thin films of nT have found applications as active layers in field effect transistors [2-4] and light emitting diodes [5]. The optical properties of nT can be easily tuned by changing the conjugation length of the chain or via the chemical substitution of hydrogen atoms by many different groups. With the intent of controlling the molecular conformation and enhance the solubility, we have synthesized and studied a series of quaterthiophenes in which one or two inter-ring torsional angles have been blocked by chemical bridging. Two types of adjacent ring bridgings have been realized one rigid, which employs a CH, group, corresponding to a planar inter ring configuration and the other flexible, with a CH.OCH2 group, in which a torsional angle up to 200 is expected. Both cw and time resolved spectroscopical techniques have been applied on bridged nT in solutions. The comparison with the parent unsubstituted oligomer T4 provides information on the role of the reduced conformational mobility. EXPERIMENT The chemical structures of the four studied quaterthiophenes are shown in Fig. 1. The chemical bridging have been realized via a CH 2 unit in T4B and via one or two CH 2OCH2 127 Mat. Res. Soc. Symp. Proc. Vol. 488 0 1998 Materials Research Society
groups in T4BO and T4BBO, respectively. All these compounds have been synthesized via the chemical route reported in Ref. 6. The absorption spectra of oligomeric solutions were recorded with a Cary 2400 spectrophotometer at room temperature. Photoluminescence (PL) spectra were obtained using a double monochromator equipped with a photomultiplier and exciting with the 350-363 rim lines of an Ar+ laser. PL quantum yields (QY) of oligomers solutions in tetrahydrofuran (THF) were determined at concentration of about 9.1 x 10.6 mol/L (T4), 10-. mol/L (T4B), 4.3 x 10' mol/L (T4BO) and 4.6 x 10'5 mol/L (T4BBO) by using quinine sulfate solutions as a reference.
T4
T4B 13
T4BO 1
T4BBO 11
Fig. 1 Chemical structures of quaterthiophenes. PL time resolved measurements on solutions were performed by exciting with the second harmonic of a pulse compressed Nd:YAG laser. Temporal dispersion of the PL signal was achieved through a Hamamatsu optical sampling oscilloscope, which combines a high time resolution (;20 ps) with a high dynamical range (&10). All the oligo
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