Infrared and Raman Spectroscopy of Organic Electron Conductors
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INTRODUCTION The transport properties of organic conductors obtained from molecular complexes of tetrathiafulvalene (TTF) depend on the degree of charge transfer p between the donor (TTF) and the acceptor molecules. The changes in the electronic distribution due to the charge transfer affect the intramolecular forces and this, in turn, produces a shift in the IR and Raman bands. In particular, the coupling between conduction electrons and intramolecular phonons in totally symmetric modes, is proportional to the occupancy of the conduction band, to the phonon amplitude u along the normal coordinate and to (aE/8u)u=0 , where E is the energy of the molecular orbital upon which the conduction band is based [l]. In this work we present results regarding the determination of p in a series of these compounds using IR and Raman spectroscopy. The structures of the complexes can be divided in two groups: fluorenone derivatives and dicyanofluorene derivatives (Figure 1). The studied complexes were the following: DNF(I): 2,7-dinitro-9-fluorenone. R1 =0, R2 =R 3 =H TNF(II): 2,4,7-trinitro-9-fluorenone. R1 = 0, R2 = N0 2 , R3 = H T 4 NF(IlI): 2,4,5,7-tetranitro-9-fluorenone. R1 =0, R2 = R3= NO 2 DDF(IV): 9-dicyanomethylene-2,7-dinitrofluorene. R1=
CN 'R2 = R =H
= CNN R2=N2R3
DTF(V): 9-dicyanomethylene-2,4,7-trinitrofluorene. R1
CN' R =N2 R =H /CN
DT 4 NF(VI): 9-dicyanomethylene-2,4,5,7-tetranitrofluorene. R1 =$
, R2 =R 3 = NO 2 CN '
295 Mat. Res. Soc. Symp. Proc. Vol. 328. t1994 Materials Research Society
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