Ž . Synthetic Metals 113 2000 129–133 www.elsevier.comrlocatersynmet Influence of inter-ring bridge on the optical properties of thiophene-based oligomers Mariacecilia Pasini, Silvia Destri, Chiara Botta, William Porzio ) Istituto di Chimica delle Macromolecole, C.N.R., Õia E. Bassini 15, 20133 Milan, Italy Received 5 October 1999; accepted 16 December 1999 Abstract Four tetramers, based on thiophene and phenylene residues in which the torsion angles between thiophene rings are blocked by a chemical bridge, are characterized with X-ray diffraction analysis and optical spectroscopy by comparison with quaterthiophene. Structural determinations show different packing arrangements for substituted and unsubstituted oligomers, resulting in different p-overlap among adjacent molecules. While in solution, the locked molecules display red-shifted absorptions and decreased Stokes-shifts, in the solid state, their optical properties strongly depend on the packing arrangements. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Blocked-oligomers; Structure; Optical properties 1. Introduction Ž . Thiophenes oligomers Tn have been widely studied because of their extensive p-electron conjugation which makes them interesting for a large number of optoelec- tronic applications as active layers in field effect transis- w x wx tors 1–3 and light emitting diodes 4 . Moreover, using molecular design, they can be easily modified and used as model compounds in the study of the spectroscopical properties related to structural features of different parent polythiophenes. It is well known that molecular conformations of olig- othiophenes largely influence the degree of conjugation and therefore the energy position of the absorption and wx emission spectra 5 . In fact, the absorption spectra of unsubstituted Tn in solution do not show any vibronic structure and are blue-shifted with respect to the data obtained from solid state samples. This is related to the coexistence, in solution, of a variety of nonplanar confor- mations due to the torsional mobility around the inter-ring ` C C bond of the molecules. Differently, in the solid state, a planar conformation is attained for unsubstituted wx molecules 6 . However, through proper chemical substitu- tions, it is possible to control both the molecular conforma- tion in the solutions and the chain packing arrangement in ) Corresponding author. the solid state. This allows to obtain a class of good model compounds for the study of the relation between the structural and optical properties of thiophene-based sys- tems. With the aim of controlling the molecular conformation in order to increase the planarity, particularly in solution, of thiophene-based oligomers, we had synthesized and studied three molecules constituted by four rings. In the first molecule T4bb, see Scheme 1, only thiophene residues are present and rotations between the first and second ring, and between the third and the fourth ring, are prevented by two methylene bridges on the b position of the rings. In the second compound PhT2bPh of Scheme 1, the thio- phene residues, which are bridge-blocked, constitute the molecule core and two phenyl rings having conformational freedom end the molecule. The third molecule T4bbR4 has been synthesized in order to increase the solubility of T4bb by substituting the hydrogens on the bridge with two butyl chains. In previous works, T4b, also reported in Scheme 1, wx wx was synthesized 7 and was optically characterized 8 . In the present paper, the optical properties of T4, T4b, T4bb, T4bbR4 and PhT2bPh, by using absorption and Ž . photoluminescence PL spectroscopies in solution and in the solid state, are reported. In particular, the comparison among the above molecules and T4, also shown in Scheme 1, allows us to obtain information about the effect of the conformational mobility on the luminescence properties. Such molecules were synthesized aiming to reduce the conformational freedom hence to increase the PL quantum 0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. Ž . PII: S0379-6779 00 00189-2