Spectroscopy of a terthiophene–vinylbenzoate Vladimir Chukharev,* a Nikolai Tkachenko, a Alexander Efimov, a Pirjo Vainiotalo b and Helge Lemmetyinen a a Institute of Materials Chemistry, Tampere University of Technology, PO Box 541, FIN-33101 Tampere, Finland. E-mail: Vladimir.Chukharev@tut.fi b Department of Chemistry, University of Joensuu, PO Box 111, FIN-80101 Joensuu, Finland Received 8th May 2003, Accepted 21st May 2003 First published as an Advance Article on the web 12th June 2003 A new terthiophene–vinylbenzoate compound has been synthesized for applications in molecular optoelectronic devices. The photophysical properties of the compound have been studied in a series of solvents. The compound is characterized by a high emission yield (43% in cyclohexane) and a large solvent-dependent Stokes shift (90–120 nm). The shift is attributed to a considerable change in the dipole moment in the excited state as compared to that in the ground state. The emission spectra have been analyzed in the frame of semi-classic charge-transfer theory. This gave estimates for the emitting state free energy, the solvent and internal reorganization energies, and the vibrational frequency. Fast dynamics of the emitting state have been studied by using femtosecond pump–probe and up-conversion methods. In polar solvents, the intramolecular vibrational energy redistribution in the excited state takes place in a sub-picosecond time domain and may result in a molecular configuration different from the all-trans conformer in the ground state. The conformational difference between the excited and ground states makes it possible to use the compound for light amplification. The amplification coefficient can be greater than 2 cm -1 , as demonstrated by preliminary experiments. Introduction Thiophene and its derivatives have attracted a great deal of attention owing to their potential applications in molecular and optoelectronics. In particular, conjugated oligomeric and polymeric thiophenes have been demonstrated to func- tion as molecular wires, molecular light-emitting devices and electron-donating components of molecular photo- diodes. 1–5 This wide application area became possible due to the extensive investigations of the photophysical properties of thiophene derivatives undertaken during the last few decades. 6–10 The present study is devoted to detailed spectroscopic investigation of a terthiophene–vinylbenzoate derivative (TSe) (3 in Scheme 1) in various solvents. The primary goal of the work was to obtain a compound which could be used in Langmuir–Blodgett (LB) films as a secondary electron donor. This application of oligothiophenes is justified by results obtained with polythiophene layers in combination with a phytochlorin–fullerene primary donor–acceptor layer 5 and in mixed oligothiophene–fullerene films. 11,12 The synthesized compound, TSe, is characterized by an electronic conjugation involving three thiophene units, one vinyl, and one phenyl moiety, and contains a polar ester group to facilitate form- ation of LB films. Studies of TSe LB films are in progress and will be published elsewhere. However, the compound itself displays interesting photophysical properties in solu- tion, and this paper reports on the ultrafast dynamics of the excited state and on the steady-state emission properties of the molecule. Methods and materials Compounds Methyl 4-[(E )-2-(2,2':5',2-terthien-5-yl)vinyl]benzoate (3). The title compound was prepared in two steps (Scheme 1) by Diels–Alder formylation of terthiophene (1) and subsequent condensation of the resulting formylterthiophene (2) with (4-methoxycarbonylbenzyl)triphenylphosphonium bromide by a method similar to that described in the literature. 13 A typical synthetic procedure is as follows. 2,2':5',2-Terthiophene (200 mg, 0.8 mmol) and dimethyl- formamide (5 ml) were dissolved in 1,2-dichloroethane (5 ml), and the solution cooled down to 0 °C. POCl 3 (0.08 ml, 136 mg, 0.88 mmol) was added, the reaction mixture stirred at 0 °C for 15 min, then at room temperature for 30 min, and finally refluxed for 2 h. The orange–brown solution was poured into saturated solution of sodium acetate and stirred for 1 h. The water phase was extracted with diethyl ether (40 ml) and then twice with dichloromethane (40 ml). The combined organic extract was washed with water (100 ml), dried over anhydrous sodium sulfate, and the solvent evaporated. The dry yellow residue was chromatographed on silica gel (dichloromethane) to give 2,2':5',2-terthiophene-5-carbaldehyde (2; 134 mg, 59% yield). 1 H-NMR (300 MHz, DMSO-d 6 ) δ/ppm: 7.1–7.14 (m, 1H, 4-H), 7.35–7.61 (m, 5H, 3, 3', 3, 4, 4'-H), 7.99 (d, 1H, J = 4.0 Hz, 5-H), 9.87 (s, 1H, CHO). MS (MALDI-DHB) m/z: 276.40 [M + ]. 2 (134 mg, 0.49 mmol), (4-methoxycarbonylbenzyl)- triphenylphosphonium bromide (280 mg, 0.58 mmol), and Scheme 1 Steps in the synthesis of TSe (3). DOI: 10.1039/b305144g Photochem. Photobiol. Sci., 2003, 2, 1044–1049 1044 This journal is © The Royal Society of Chemistry and Owner Societies 2003