Electronic Properties of a Novel Class of Conjugated Systems: Transition Metal Substituted Oligothiophenes
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In consideration of Eq. 1 chemists have syntetized a variety of strongly absorbing conjugated structures with attached electron-donors and acceptors in order to produce asymmetric charge distributions and this approach has been successful in engineering molecules with enhanced second order nonlinearities. In this context investigation on organometallic systems have been greatly intensified [3]. Incorporation of metal in a conjugated structure introduces new flexibility in the molecular design because transition metals can exhibit different oxidation states and a variety of ligand environments. Moreover the high polarizability of the d electrons, ensures nonlinear activity for conjugated molecules to which transition metal compounds are attached. In this paper we report on the spectroscopical manifestation arising from the interaction between the electrons of a conjugated system and the d electrons of a transition metal, as well as an estimate of the second order molecular hyperpolarizability using the solvatochromic effect. In particular, among the stable and isolable metal complexes, our attention has been focussed on the Fischer-type carbene complexes [4], which are organometallic compounds containing a VI group transition metal (Cr, Mo, W) of general structure depicted in Scheme 1. Because of the six dative bonds the metal atom in the ground state is negatively charged so there is a large dipole moment from the transition metal to the carbenic bond. This situation is expected to be reversed in the first excited state which usually corresponds to the transition from an occupied d orbital of the metal to a pz empty carbene orbital followed by the charge delocalization throughout the oligothiophenic system. 211 Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
The nature of this low-lying Charge Transfer (CT) transition and the modification of the it electronic structure in the oligothiophene system induced by the metal substitution have been studied by using optical and Raman spectroscopy. Scheme 1
CH3-( 7H 2-0
M=W, Cr
(CO) 5M[ n
n=2, 3 R= H, C=C(CN)2
EXPERIMENTAL The synthesis of the metal substitued oligothiophenes studied in the present work (see Scheme 1) has been described in detail in previous publications [5]. The compounds have been dispersed in Kbr pellets for the IR absorption measurements while UV-Vis absorption, Raman measurements have been carried out in solution. Dichloromethane has been used as a solvent for the Uv-Vis absorption spectra and carbondisulfide for Raman spectra. The UV-Vis absorption spectra have been measured with a Cary 2300 spectrophotometer. A Bruker FTIR IFS66 has been used for IR absorption and for the Raman spectra obtained with a 1064 nm Nd-Yag laser excitation.The Raman spectra with excitation in the visible region have been obtained with the laser lines of an Argon-ion laser, using a flat field triple monochromator with an intensified diode array detector (Jasco TRS300 monochromator and 1420 EG&G detector). RESULTS Electroni
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