J. MATER. CHEM., 1991, 1(6), 939-941 Dielectric Relaxation Peaks in the Low-frequency/High-temperature Range of the Loss Permittivity Curve studied by the Thermostimulated Depolariration Current (TSDC) Technique in 2,3,7,8-Tetramethoxychalcogenanthrenes-TCNQ Charge-transfer Complexes Ricardo Diaz Calleja,” Enrique Sanchez Martinez” and Gunter Klarb ” E. T.S.I.I.,Universidad Politecnica de Valencia, Camino de Vera sln, E-46071 Valencia, Spain lnstitut fur Anorganische und Ange wandte Chemie der Universitat Hamburg, Martin-Luther King Platz 6, D2000 Hamburg 13, Germany The dielectric behaviour of the 1: 1 charge-transfer complexes of the 2,3,7,8-tetramethoxychalcogenanthrenes (5,10-dichalcogena-cycl~diveratrylenes) Vn,EE’ (E=E’ =S, Se; E =S, E’= Se) with 7,7,8,8-tetracyano-pquinodi-methane (TCNQ) is studied by the thermostimulated depolarization current (TSDC) technique as an alternative experimental method to the more conventional a.c.measurements. In the low-frequency side of the dielectric loss curves (E” vs. frequency plots) the relaxation peaks of the compounds are hidden by a continuous increase in dielectric loss. By a transformation of complex permittivity E* =E’-id’ into complex polarizability a* =a’-id’ loss peaks can be observed in the a” vs. frequency plots (E. Sanchez Martinez, R. Diaz Calleja, P. Berges, J. Kudning and G. Klar, Synth. Met., 1989, 30, 67). The existence of these relaxations is now proven by the TSDC met hod. Keywords: Dielectric relaxation; Charge transfer; Polarizability; Thermostimulated depolarization current Dielectric loss curves contain one or several relaxation peaks, many of which are dipolar in origin.These peaks are related to small motions in the molecule concerning these groups. Free charges can also produce peaks, for example peaks appearing at higher temperatures than the peak associated with the glass-rubber transition in amorphous polymers. In heterogeneous samples consisting of different phases and having different dielectric constants and conductivities, charges accumulated near the interface when the sample is heated and subjected to an electric field can be neutralized in a TSDC experiment resulting in peaks. This phenomenon, called the Maxwell-Wagner effect can be expected in semicrys- talline polymers, the amorphous part of which has higher conductivity than the crystalline part, in systems in which an electret metallized on only one side is shorted together with an air gap, or in a two-layer system.In order to prevent these effects we have used two-sided silver metallized samples and direct-contact electrodes. In our powdered samples, on the other hand Maxwell-Wagner-Sillars (M WS) relaxation is usually described by a single relaxation time, i.e. a Debye-like relaxation. However, an analysis of the dielectric and loss curves carried out previously1T2 indicates that this relaxation follows a Cole-Cole equation and is governed mainly by a distribution of relaxation times. Owing to the specific charac- ter of our samples a contribution of an MWS phenomenon cannot be totally excluded, but we think that the observed peak is of primarily dipolar origin.In many cases a continuous increase of the loss permittivity has been observed in the low-frequency side of the spectrum, commonly attributed to d.c. conductivity behaviour. For example, this effect has been found in the spectra of the 1 :1 charge transfer (CT) complexes of the 2,3,7,8-tetramethoxy- chalcogenanthrenes (5,1O-dichalcogena-cyclo-diveratrylenes), Vn,EE’ (E =E’=S, Se; E =S, E’ =Se), with 7,7,8,8-tetracyano- quinodimethane (TCNQ), in which the relaxation peak is partially (Vn2S2 .TCNQ) or totally (Vn,SSe*TCNQ and Vn,SE2 -TCNQ) hidden by this increase of conductivity.’ I I H3C CH3 VnZEE’ TCNQ A transformation of the complex permittivity E* =E’ -id’ into the complex polarizability a* =a’ -ia” according to a*--(E * -1)/(E* +2) (1) can be used;3 loss peaks have been observed in the a” us.frequency curves, previously not seen in the a’ us. frequency curve^.^,^ Eqn. (1) acts as a normalization procedure for the polarizability a*. E* and a* representations are equivalent, and from a macroscopic point of view the use of E* or a* is only relevant to demonstrate the presence of a relaxation peak. On the other hand, thermostimulated depolarization cur- rents (TSDCS)~.~ have been widely used as a complementary technique of the more conventional dielectric a.c. measure- ments in the audiofrequency range in order to detect and analyse dipolar relaxation.TSDC gives a better resolution of the relaxation peaks, owing to the fact that the equivalent frequency is lower than in a.c. dielectric measurements. We therefore applied the TSDC technique to the CT complexes Vn,EE’-TCNQ in order to interpret the results’ of the a.c. measurements in terms of this method. According to TSDC a neat peak should appear in the low-temperature range of the TSDC spectrum of each compound, which corresponds to that produced by the transformation in the a” us. frequency curve. Experimental The dielectric a.c. measurements are described elsewhere. ’ The thermostimulated depolarization currents were deter- mined by use of a SOLOMAT TSC/RMA spectrometer. The technique employed was as f01lows.~ The sample in disc form was metallized with colloidal silver and a d.c.electric field was applied at a high temperature. The sample was then cooled to low temperature with the field maintained. Next it was short-circuited and reheated at a linear rate, measuring the discharge current with an electrometer as a function of temperature. Measurements were made between 153 and 343 K after polarization under the conditions given in Table 1. Depolarizations were done at a rate of 7 K min-’. Results The TSDC thermograms of the CT complexes are given in Fig. 1. In each case a clean depolarization peak is present. The parts of the spectra at higher temperatures, showing a continuous increase of the conductivity, are not reproduced.Activation energies E, are usually calculated from the initial slope of the intensity of current5 according to d(ln j,)/d(l/T)= -rnE,/R (2) where j, is the reduced current (intensity of current/area), R is the gas constant, and m is a parameter related to the broadness of the peak. Thus, for rn= 1 we have a Debye single peak. But, in general, the values of rn are temperature depend- ent and can be obtained from dielectric loss measurements 20.0 10.0 0.0 I I 1 I I 160 180 200 220 N 160 180 200 220 T/K J. MATER. CHEM., 1991, VOL. 1 or alternatively from polarization loss. We adopt for E” (or a”) an equation E” sech(rnx) (3) where is the loss at the maximum, x is ln(o,,,/o)= (E,/R)(l/T-l/Tmax),and E, is the activation energy of dielec- tric a.c.data. As proposed by Fuoss and Kirkwood7 the parameter rn can be estimated from a cosh-’ (E~,~/E’’)us. x plot. The parameter rn is approximately equivalent to the param- eter 1 -h in the equation proposed by Cole and Cole for the permittivity E*=E~+(E~-E,)/[~+(ioz)’-”] (4) where E~ and E, are the unrelaxed and relaxed permittivities, respectively. It has been demonstrated’ that the 1-h parameter in eqn. (4)is the same as the corresponding one in a*=a, +(ao-am)/[I+(ioz)’-”’] (5) where a* is calculated from E* by means of eqn. (1). The corresponding values of m and 1 -h for the three compounds are given in Tables 1 and 2, respectively. For Vn2S2*TCNQrn was calculated from the E” curve. In the two other cases, where no maximum was present in the E” curves, m had to be evaluated from the a” curves.The values of j, were directly taken from the thermograms in Fig. 1. The activation energies E, were then calculated using eqn. (2). The values of E, obtained (Table 1) are close to those calculated from a.c. measurements. ’ In fact, small discrepancies between the two sets of values are normally found and may be due to the different techniques employed. Discussion Activation energies are valuable for estimating the tempera- ture at which the TSDC peak corresponding to the a.c. peak must appear. For this purpose a time (frequency) temperature transformation has to be made. In fact, TSDCs are obtained as a time response and a.c.audiofrequency measurements in the frequency dominion. Frequency-time conversion requires Fourier transformation. However, exact formulae are not obtainable; approximate ones were proposed several years ago by Schwarzl and Struik’ and Van Turnhout6 according to 1.475RT2j,(T) &’I( r)= EOEE, where R is the gas constant, T is the temperature, j, is the current density, c0 is the vacuum permittivity, E is the electric field, and E, the activation energy. Also, f=-0.113E, SRT~ (7) Fig. 1 Thermostimulated depolarization currents for the CT com- where E,, R and T have been formerly defined, s is the inverse plexes Vn,EE.TCNQ. (a) E=E’=S; (b) E=S, E=Se; (c) E=E= of the heating rate, and fis the frequency. Eqn. (7) gives the equivalent frequency at the maximum and E acts as a Se compound ~~ ~~ Vn,S, .TCNQ Vn,SSe *TCNQ Vn ,Se, -TCNQ go Table 1 TSDC data for the CT complexes Vn,EE.TCNQ“ T*/K E/V mm-’ m EJkJ mol-’ j710-3 HZ 323 500 0.53 57.4 2.52 313 100 0.57 57.0 2.32 353 50 0.43 55.9 3.06 Tp=polarization temperature; E =electric field; E, =activation energy; m and fcalculated by means of eqn.(2) and (7). J. MATER. CHEM., 1991, VOL. 1 94 1 Table 2 Data evaluated by comparison of the a.c. and TSDC measurements for the CT complexes Vn,EE'.TCNQ compound a0 a, EO Vn,S, -TCNQ 0.836 0.384 7.77 Vn,SSe * TCNQ 0.952 0.400 60.5 Vn,Se, -TCNQ 0.890 0.390 25.3 I I T/K Fig. 2 Dielectric loss permittivities for the CT complexes Vn,EE'*TCNQ, calculated by means of eqn.(6). 0,E = E'= S; A, E=S, E'=Se; 0,E=E'=Se normalization factor in order to reduce the current to a common basis in terms of E"(T). The equivalent frequencies f are given in the last column of Table 1 and the calculated E"(T)curves are shown in Fig. 2. Values of meet in an acceptable range and are of the same order of magnitude as the value directly measured for Vn2S2 -TCNQ by means of a dielectric bridge. A conclusive test of the equivalence of the two sets of relaxations is to apply an Arrhenius equation in order to check the position along the temperature axis in connection with the respective frequencies. This is easy in the case of Vn2S2 *TCNQ owing to the previously mentioned fact that for this compound a relaxation peak at ca.200 Hz at low temperatures (275 K) is seen in the E" us. frequency curve. Accordingly, gives a temperature of 184 K for the estimated frequency of 2.52 x Hz close to the experimental value of 190 K for the peak temperature in the TSDC spectrum. For the other two compounds, as there are no peaks visible in the &"-f diagrams, frequencies have to be translated from E* to a* according to &o+2 (l-h)-lL=f,0a (9) E, I-h LlHz flHz 2.87 0.63 3 x lo3 I xi03 3.00 0.68 4 x 10, 9.7 2.92 0.59 140 7.7 0.5 0.6 0.7 0.8 Uf Fig. 3 Cole-Cole plots in terms of the polarizability for the CT complexes Vn,EE'-TCNQ. (a) E=E'=S; (b) E=S, E'=Se; (c) E= E' = Se frequencies of the maximum loss in the permittivity and in the polarizability representations, respectively, and 1 -h is the Cole-Cole exponent in eqn.(4) and (5). f, can be calculated according to eqn. (9) from f, taken from the al-f curves. The values of ao, a,, cO,E,, 1 -h, andf, are given in Table 2. Again, by application of eqn. (8) for Vn,SSe*TCNQ and Vn2Se2 .TCNQ temperatures of 204 and 20 1 K, respectively, are obtained, in good agreement with the observed ones of 197 and 195 K (Fig. 1). Occasionally, discrepancies between the two sets of values can arise due to the long succession of calculation steps. Although it is not the purpose of this paper to interpret the dielectric relaxation behaviour in terms of some specific molecular motion in these CT complexes (we lack information about conformational energies of these materials) we ascribe tentatively and partially the relaxation in these molecules (Vn2EE') to the butterfly flapping motion, as observed in folded molecule^,^ in which the dipolar methoxy groups -OCH3 in addition to the E and E' chalcogenides are dielectrically active.However, an MWS contribution to this peak is not totally excluded. We thank the Volkswagenstiftung and the Fonds der Chem- ischen Industrie for financial support. References 1 E. Sanchez Martinez, R. Diaz Calleja, W. Gunsser, P. Berges and G. Klar, Synth. Met., 1989, 67, 30. 2 W. Hinrichs, P. Berges, G. Klar, E. Sanchez Martinez and W. Gunsser, Synth. Met., 1987, 20,357. 3 G. Schufmann and W. Gunsser, Z. Naturforsch., Teil A, 1977, 32,1059. 4 E. Sanchez Martinez, R. Diaz Calleja, P. Berges, J. Kudning and G. Klar, Synth. Met., 1989, 32, 79. 5 C. Bucci, R. Fieschi and G. Ghidi, Phys. Rev., 1966, 148, 816. 6 J. Van Turnhout, Thermally Stimulated Discharge of Polymer Electrets, Elsevier, Amsterdam, 1975. 7 R. M. Fuoss and J. G. Kirkwood, J. Am. Chem. SOC., 1941, 63, c0 and E, can be calculated from a. and a,: 385. 8 F. R. Schwarzl and L. C. E. Struik, Adu. Molec. Relax. Proc.,2ao+1. 2a,+l 1967, 1, 201.&O=-, 1-ao &,== 1 -a, 9 Y. Koga, H. Takahashi and K. Higasi, Bull. Chem. SOC. Jpn., 1973,46, 3359. a. and a, can be easily estimated from the a"-@' plots (Cole- Cole diagrams; Fig. 3). Moreover,f, andf, in eqn. (9) are the Paper 1/01600H; Received 5th April, 1991