Introduction

The intermolecular interaction between electron donor D and electron acceptor A leads to the formation of a new molecular assembly, DA, called charge transfer (CT) complex, first introduced by Mulliken [1, 2]. Organic CT complexes and salts have extensively been investigated in various areas of chemistry and material sciences both from the theoretical and experimental aspects as they exhibit diverse kind of interesting physical properties and functionalities related to optical, magnetic, electrical (super) conductivity and dielectric properties [3, 4]. Since the discovery of the first metallic type CT complex TTF-TCNQ in 1973 [5], attention to organic CT complexes has been focussed for several decades, aimed mainly at the discovery of materials with good electrical conductivity or even room temperature superconductivity [6]. Later on, attention has been turned to technologically relevant properties of CT complexes due to their potential in the improvement of electronic and optoelectronic devices [7].

The solid organic CT complexes are one of the most important subclasses of organic semiconductors [8, 9] which often exhibit similar conduction mechanism to inorganic semiconductors where hole and electron conduction layers are typical carriers separated by a band gap. CT complexes exhibiting mixed ionic-electronic conduction have also been reported [10–13].

Although, the electrical conductivity of charge transfer complexes of pyridine analogue with halogens and interhalogens has been reported in literatures [14–16]; however, the detailed studies of frequency dependence a.c. conductivity and complex impedance spectroscopy of these complexes are lacking. In this paper, the electrical properties of the CT complexes obtained by the interactions of three n-donors namely pyridine, 4-methylpyridine and 3,5-dimethylpyridine with iodine monochloride (ICl) as σ-acceptor are reported.

Despite the fact that the electrical conduction property of a donor-acceptor complex depends upon many factors, to manifest high conductivity, the mixed valence (or partial CT state) electronic structure and specific molecular packing architecture (segregated stacking and mixed stacking) within the molecular assembly are the most essential criteria [17]. The degree of charge transfer, δ from donor (D) to acceptor (A), in the complex (D+δA-δ) is primarily governed by the ionization potential (I p) of D and electron affinity (E a) of A. A neutral CT complex with δ = 0 is obtained if E a > I p, whereas the completely ionic state corresponds to δ = 1, 2..... when E a < I p. Both neutral and completely ionic CT complexes are insulators with few exceptions [18, 19]. It is reported [7, 20] that mixed valence CT complexes within the window 0.50 < δ <0.74 with segregated stacks of the D and A components generally exhibit metallic electrical conduction; otherwise, alternating stack CT complexes are either semiconductors or insulators.

For the study of microstructural and electrical properties of many electronic materials, the AC impedance spectroscopy has long been used as a non-destructive experimental technique [21]. In this technique, a sinusoidal perturbation is applied on the test system and the AC response is analysed. The ratio of the voltage response to the current perturbation is the impedance which is calculated as a function of the frequency of the perturbation. The interpretations of the impedance spectra (−Z// im vs Z/ real) are aided by analogy to physically plausible equivalent circuits involving simple components such as capacitors and resistors connected in many modes. From this analysis, a meaningful insight into the material behaviour can be obtained. The analysed values of the electrical parameters (conductivity, dielectric constant, loss, capacitance, etc.) give explicit results when compared with those measured at arbitrarily selected fixed frequencies. This powerful technique is useful for the measurement of real and imaginary part of impedances for a wide range of frequency, determination of relaxation frequency and separation of grain, grain boundary and grain-electrode effects [22, 23].

Experimental

Materials

Pyridine (from Ranbaxy Lab. Ltd.), 4-methylpyridine (from Sigma Aldrich), 3,5-dimethylpyridine (from Sigma Aldrich) and ICl (from Ranbaxy fine chemical Ltd.) were used without further purification. Hexane (Merck) was used after distillation as a solvent.

Synthesis and characterization of complexes

The solid CT complexes of the pyridine analogue with ICl were prepared by stirring equimolar quantities of the donor with ICl in hexane, which resulted in precipitation of the complex as light yellow solid. The separated solids were filtered and washed several times with minimum amount of hexane and these are dried under vacuum over anhydrous calcium chloride. The resultant complexes were characterised by routine techniques like elemental analysis, UV-Vis, FTIR, 1H-NMR spectroscopy and powder XRD.

The elemental analysis of the carbon, hydrogen and nitrogen content was performed by using Perkin Elmer CHN 2400 Series II analyzer. The total halogen contents were estimated by a simple gravimetric method with AgNO3 solution.

The electronic absorption spectra of the donors, acceptor and the resulting complexes in hexane were recorded over a wavelength range of 200–900 nm using Shimadzu U-3900 spectrophotometer. The diffuse reflectance spectra of the solids were recorded with a JASCO V-750 UV Visible spectrophotometer in the region 200–900 nm.

The FT-IR spectra were recorded on a Shimadzu FT-IR spectrophotometer (IRPrestige-21) within the range of 4000–250 cm−1.

The 1H-NMR spectra were recorded in CDCl3 solvent. Powder XRD analysis was carried out at room temperature by using Cu radiation (λ = 1.5405 Å) in the range 2° ≤ 2θ ≤ 70° at a scanning rate of 1°/min and step height of 0.05°.

Electrical properties

The solid complexes were pelletized into discs of 13 mm diameter by using a hydraulic press (Spectralab) at a pressure of 9 Kbar. The thicknesses (0.5 to 1.5 mm) were measured with a screw gauge. Uniform graphite layer was applied on the opposite faces of the pellets and was sandwiched between two thin sheets of copper electrodes with even surface. Before recording the data, each sample was sintered at 50 °C for 30 min to ascertain the interface contact between the electrodes and the sample. The electrical properties were measured by complex impedance method using LCR HiTester (HIOKI 3522) frequency response analyser in the frequency range from 10 Hz to 100 kHz. AC conductance (G), impedance (Z) and phase angle (θ) were measured as a function of frequency at different temperatures. The sample cell was kept for 30 min at each measuring temperature in order to reach its thermal equilibrium. The impedance spectra were modelled as an ‘equivalent circuit’ by using complex non-linear least squares fitting of both real and imaginary part of impedance. LEVM 3.0 software [24] has been used for fitting purpose. The best fitted equivalent circuit has been determined by observing the coherence between the experimental and the simulated data in the complex impedance plots. D.C. conductance of the pellets was measured at various temperatures by using Keithley 2400 source meter. Electrical conductivity was calculated using the pellet dimensions and the measured resistance.

Determination of ionic transport number

The total ionic transport numbers, t ion, were evaluated by the standard Wagner polarization technique [25]. The cell ‘SS| CT complex |SS’ was polarized by a step potential of about 1 V and the resulting current was monitored as a function of time [26, 27], where SS stands for stainless steel. The SS acted as blocking electrodes for the above cell. The t ion values were evaluated from the polarization current curve by using the following equation:

$$ {t}_{\mathrm{i}\mathrm{on}}=\frac{\left({i}_{\mathrm{i}}-{i}_{\mathrm{f}}\right)}{i_{\mathrm{i}}} $$
(1)

where i i and i f are the initial and final steady state currents, respectively.

Results and discussion

Elemental analysis

Elemental analyses data (C, H, N and total halogen) of the complexes were given in Table 1. It is observed that the experimental values agree quite well with the calculated values for 1:1 complexes. The same stoichiometries in the solution state are confirmed by the linearity of the Benesi-Hildebrand plots (S1).

Table 1 Elemental analysis data of the CT complexes
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UV-Vis spectroscopy

In the electronic absorption spectra (S2) of the resulting complexes (in hexane), an additional absorption band at λ max = 333.4, 316.8 and 322.4 nm for pyridine-ICl, 4-methylpyridine-ICl and 3,5-dimethylpyridine-ICl complexes, respectively, indicates the formation of CT complexes.

In the diffuse reflectance spectra of the solid complexes (Fig. 1), the bands at 256, 322 and 371 nm for pyridine-ICl, at 239, 262, 301 and 361 nm for 3,5-dimethylpyridine-ICl and at 253, 312 and 427 nm for 4-methylpyridine-ICl complexes indicate the presence of ICl2 − ion in the solid state [28]. The formation of ICl2 − ion is attributed to the solid state transformation of the CT complexes (e.g. 4-methylpyridine-ICl) as follows:

$$ 2\ \upgamma -\mathrm{Pic}-ICl\to {\left(\upgamma -{\mathrm{Pic}}_2-\mathrm{I}\right)}^{+}+ICl{{}_2}^{-} $$
Fig. 1
figure 1

Diffuse reflectance spectra of pyridine-ICl (blue), 3,5-dimethylpyridine-ICl (red) and 4-methylpyridine-ICl (purple) complexes

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The optical band gaps of the CT complexes were estimated from the optical absorption edge of the spectrum by using the Tauc relation [29]

$$ Ah\nu ={\left(h\nu -{E}_{\mathrm{g}}\right)}^n $$
(2)

where A is the absorbance, E g is the optical band gap corresponding to a particular absorption of photon of energy hν and n is ½ for allowed direct, 3/2 for forbidden direct, 2 for allowed indirect and 3 for forbidden indirect transitions in the material. It is observed that (Ahν)1/2 versus hν plots are linear (Fig. 2) and the intercept on the energy axis on extrapolating; the linear portion of the curves to A = 0 gives the indirect band gap. The values of E g are given in Table 2.

Fig. 2
figure 2

(Ahν)1/2 vs hν (photon energy) plot of A 4-methylpyridine-ICl (red), B  3,5-dimethylpyridine-ICl (black) and C pyridine-ICl (green) complexes

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Table 2 Physical properties, optical band gap and ionic transport no. of the CT complexes
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FTIR spectroscopy

Few selected IR frequencies of the donor on complex formation are affected in both intensities and wave number values (S3a–c). This could be interpreted on the basis of expected electronic environment change upon complexation. The detailed vibrational assignments were reported in Table 3. These assignments were in accordance with earlier works [30, 31].

Table 3 FTIR frequencies (cm−1) of the donors and their CT complexes
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It is observed that the spectra of the CT complexes show all the principal features for free donors. Many modes on complex formation have been shifted to higher frequency in comparison to the free donor. Similar upward shifts were observed for metal coordinated pyridine [32] and metal coordinated pyridine analogues [33, 34]. It has been reported [35] that iodine monochloride (ICl) has an allowed fundamental absorption at 382 cm−1 in the gas phase and the I-Cl stretching frequency decreases on complex formation with strong donor like pyridine. The bands at 290 cm−1 for pyridine-ICl, 265 cm−1 for 4-methylpyridine-ICl and 273 cm−1 for 3,5-dimethylpyridine-ICl complexes have been assigned to I-Cl stretching in the complexes [36].

1H-NMR spectra

The 1H-NMR spectra of CT complexes (S4a-c) showed the δ values for ring and methyl group protons in the donor molecules are shifted towards lower field (Table 4). The smallest change was observed for α protons on comparing the chemical shifts of β, γ and methyl protons; it could be explained by the paramagnetic effect observed previously [37] in free donors. The observed down-field shift for all protons is attributed to the decrease of electron density on each ring carbon as a consequence of partial positive charge at the nitrogen atom on complex formation.

Table 4 1H Chemical shifts (δ ppm) of the CT complexes and the parent donors
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Powder XRD analysis

The room temperature X-ray diffraction pattern of pyridine-ICl complex (S5a) showed a good agreement with the single crystal data [38]. The good agreement between the observed (d obs ) and calculated (d cal ) inter-planar spacing (Table 5) confirmed that the prepared complex has a monoclinic structure, as reported earlier, with unit cell lattice parameters a = 4.274 Å, b = 12.319 Å, c = 14.094 Å, β = 94.99° and unit cell volume 739.3 Å3.

Table 5 Comparison of dobs and dcal (Å) values of some reflections for the CT crystals at room temperature
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For the lack of crystal structure data about 4-methylpyridine-ICl and 3,5-dimethylpyridine-ICl complexes, Powder-X software has been applied in order to index the diffraction pattern. It is found that the diffraction pattern of both the complexes are best fitted to a monoclinic unit cell with the estimated lattice parameters a = 8.335 Å, b = 12.830 Å, c = 18.313 Å, β = 90.03° and V = 1958.3 Å3 for 4-methylpyridine-ICl and for 3,5-dimethylpyridine-ICl complex are a = 6.636 Å, b = 18.935 Å, c = 8.789 Å, β = 97.06° and V = 1095.99 Å3. The inter-planar spacing (d hkl ) values were calculated by using Eq. (3) for monoclinic system and a quite good agreement between the observed (d obs ) and calculated (d cal ) values (Table 5) confirmed the estimated unit cell parameters.

$$ \frac{1}{d_{hkl}^2}=\frac{h^2}{a^2{\mathrm{Sin}}^2\beta }+\frac{k^2}{b^2}+\frac{{\mathrm{l}}^2}{c^2{\mathrm{Sin}}^2\beta }-\frac{2 hlCos\beta }{ac{\mathrm{Sin}}^2\beta } $$
(3)

The average grain size (D) of the samples was calculated from the most intense x-ray diffraction peak by using the classical Scherer [39] formula

$$ D=\kern0.5em \frac{K\ \uplambda}{\beta\ Cos\ \theta } $$
(4)

where λ is the x-ray wavelength (for Cu-Kα radiation λ = 1.5405 Å), K is a constant (0.90 for Cu grid), θ is the Bragg diffraction angle (half the scattering angle) and β is the full width at half maximum (FWHM) in radians of the main peak. The average particle size for pyridine-ICl, 4-methylpyridine-ICl and 3,5-dimethylpyridine-ICl complexes was found to be 55, 45 and 61 nm, respectively.

Impedance studies

The Nyquist diagrams (−Zʺ vs Z′) are shown in Fig. 3a–c for pyridine-ICl at 303, 313, 318, 323, 333, 343 and 353 K; for 4-methylpyridine-ICl at 303, 313, 318, 323, 328, 333 and 338 K and for 3,5-dimethylpyridine-ICl at 303, 318, 323, 333, 338 and 343 K. The spectra 3a and 3c relative to pyridine-ICl and 3,5-dimethylpyridine-ICl consist of a single semicircle whose radii decrease with the rise in temperature. The spectra 3b related to 4-methylpyridine-ICl are characterised by the appearance of depressed semicircles at higher frequencies with an inclined spike on the low frequency side. The inclined spike on the low frequency side of the semicircles in the spectrum of 4-methylpyridine-ICl is a characteristic of polarization phenomena at the electrode-material interface. Hence, conduction in 4-methylpyridine-ICl is ascribed mainly due to ions. The absence of inclined spike in the impedance spectrum of pyridine-ICl and tiny spike like portion in the low frequency side of the impedance spectrum of 3,5-dimethylpyridine-ICl attribute to either electronic or mixed ionic-electronic conduction [40].

Fig. 3
figure 3

Nyquist plots of a pyridine-ICl, b 4-methylpyridine-ICl and c 3, 5-dimethylpyridine-ICl complex

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Equivalent circuit fitting

The electrical behaviour of a system can be interpreted in terms of an equivalent circuit containing real electrical elements in various combinations. It should be worth mentioning that a particular electrical circuit can be customized in a variety of combinations of simple electrical elements but at a halt results the same overall a.c. response. Though, there is no unique equivalent circuit for a particular system; however, an equivalent circuit can be selected on the basis of the following criteria:

  1. (i)

    Keeping in mind as to what kind of impedances is probable to be present in the system in question and how they connected.

  2. (ii)

    Whether the response of the proposed circuit is consistent to the experimental data.

  3. (iii)

    Whether the simulated values of the circuit elements (R, C, etc.) are realistic and their variation with temperature is logical.

In general, an equivalent circuit used to describe the impedances of polycrystalline solid materials consists of three parallel RC elements connected in series corresponds to grain interior, grain boundary and electrode [41]. A single parallel RC element gives one semicircular arc passing through the origin in the Nyquist −Zʺ vs Z′ plot and the low frequency intercept on the real Z′ axis corresponds to the resistance R of the element. We expect three semicircular arcs corresponds to the grain (bulk), grain boundary and electrode contributions. In practice, all the semicircles may not be noticed. They may overlap, depress or distort due to the presence of (i) distribution in relaxation times and (ii) very small differences of the time constants of various relaxation processes, if any. The non-ideal nature of the experimental impedance spectrum enforced to fit the experimental data to evaluate the contributions of various circuit elements which otherwise cannot be worked out from the experimental spectrum. In this paper, LEVM 3.0 software [24] has been used for fitting purpose.

Pyridine-ICl complex

Although the impedance spectrum (Fig. 3a) seems like a single semicircle at the first sight, the experimental data did not fit well to an equivalent circuit containing a single parallel RC element. The best fit is achieved by using an equivalent circuit comprising of three RC elements connected in series (Fig. 4a). Hence, in this complex grain, grain boundary and electrode contributions are responsible for total hindrance in the material. The values of all parameters of the equivalent circuit, after fitting at different temperatures, are given in Table 6. The validity of the proposed equivalent circuit is confirmed by the excellent coherence between the experimental and the simulated data in the complex impedance plots. The temperature variation of the R i parameters (in terms of σ i) obeys the Arrhenius law (Fig. 5).

Fig. 4
figure 4

Equivalent circuit a –(RgCg)–(RgbCgb)–(RelCel)– and b –(Rcpe)–(cpe)–

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Table 6 Fitted equivalent circuit parameters for the CT complexes
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Fig. 5
figure 5

Arrhenius plots of σg, σgb and σel for pyridine-ICl and σdc of the three complexes

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4-methylpyridine-ICl complex

The existence of low frequency straight line indicates the presence of double layer capacitance of electrode-material interface [42] and the semicircular portion corresponds to the parallel combination of resistance and capacitance. But the angle of inclination of the straight line and depressed semicircle indicate the presence of distributed microscopic properties of the material, which is called constant phase element (cpe). Hence, cpe has been introduced in place of ideal capacitive circuit element. The experimental complex impedance spectrum can be best fitted to the equivalent circuit given in Fig. 4b. The depressed semicircle corresponds to the parallel combination of bulk resistance and a constant phase element (cpe) in series with another cpe corresponds to the inclined spike due to interfacial polarization. The inclined spike (as observed in case of a real solid electrolyte) in the complex impedance plot indicates that 4-methylpyridine-ICl complex is mainly ionic. However, for an ideal solid electrolyte, one would expect a vertical straight line (instead of inclined one) on the low frequency side followed by a perfect semicircle (instead of depressed one) in the complex impedance plot.

3,5-dimethylpyridine-ICl complex

The impedance spectra at lower temperatures of this complex have been best fitted to an equivalent circuit comprising of three RC elements connected in series corresponding to grain, grain boundary and electrode interface. However, at 333 K and above, a spike at lower frequency region of the impedance spectra become prominent indicating different conduction mechanism at higher temperatures. Hence, similar equivalent circuit as that of 4-methylpyridine-ICl complex (i.e. 4b) is appropriate for the spectra at 333, 338 and 343 K. At higher temperatures, ionization of the complex transforms the material from mixed conductor to predominantly ionic conductor.

The total conductivity (σT) of the sample has been calculated using the total resistance and the dimensions of the pellet with the following equation:

$$ {\sigma}_{\mathrm{T}}=\kern0.5em \frac{1}{R_{\mathrm{T}}\ }\kern0.5em \mathrm{x}\kern0.75em \frac{t}{A} $$
(5)

where t is the thickness, A is the area of the pellet and R T is the total resistance estimated from the experimental spectrum at low frequency intercept of the semicircle with the real axis. Similarly, the bulk conductivity (σ g) has been calculated using bulk resistance (R g) value obtained from fit data. It is also seen that the value of Σ Ri from fit data (Table 6) equals to the value of R Total estimated from experimental spectrum (Table 7) which once again confirms the validity of the proposed equivalent circuits.

Table 7 Total conductivity and d.c. conductivity of the complexes at various temperatures
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AC conductivity study

The frequency dependence of a.c. conductivity i.e. logσ vs logω plots for 4-methylpyridine-ICl complex is characterised by three distinct regimes (i) low frequency polarisation region (ii) the medium frequency plateau region and (iii) high frequency dispersion region (Fig. 6). But for pyridine-ICl and 3,5-dimethylpyridine-ICl complex, only the later two regions are observed. In case of pyridine-ICl and 3,5-dimethylpyridine-ICl complex, the absence of the first low frequency regime attributed that there is no/negligible interfacial polarisation. In the low frequency region, conductivity increases with the increase in frequency which is attributed to the polarization at the electrode-sample interface. Here, flow of the charges accumulated at the interface is responsible for increase in conductivity. In the intermediate plateau region, conductivity is almost frequency independent and is equal to the bulk or d.c. conductivity of the sample. The higher frequency dispersion region can be explained by Jonscher’s universal power law [43]

$$ {\sigma}_{ac}={\sigma}_{\mathrm{dc}}+A{\omega}^s $$
(6)

where σ ac and σ dc are the a.c. and d.c. conductivity, respectively, ω is the angular frequency which equals to ω = 2πf, A is a constant and S is the frequency exponent having values between 0 and 1. From the slope of the plot between logσ versus logω, the frequency exponent S can be obtained and the intercept of the horizontal portion on the vertical axis equals to logσdc.

Fig. 6
figure 6

Variation of a.c. conductivity with frequency for a 4-methylpyridine-ICl, b 3,5-dimethylpyridine-ICl and c pyridine-ICl complexes at various temperatures

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It is seen that as temperature increases, conductivity increases at all frequencies which attributes to thermal activation. It is also observed that the characteristic frequency, (the frequency at which dispersion region started to deviate from the d.c. conductivity plateau) at which relaxation effect begins to appear, shifted to the higher frequency with increase in temperature and arrived at a position beyond the measured frequency window limit. Another interesting observation is that the dispersion is more prominent at lower temperature, which is reflected in the value of S which decreases with the rise of temperature as shown in Fig. 7. All these observations indicate that the tested materials exhibit semiconducting behaviour and the a.c. conduction mechanism is due to barrier hopping [44, 45].

Fig. 7
figure 7

Variation of frequency exponent(s) with temperature

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Measurement of ionic transport number

From the current vs time plots (Fig. 8) at room temperature, the total ionic transport number (t ion) of the complexes were calculated using the Eq. 1. The values of t ion (Table 2) imply that the conduction mechanism in pyridine-ICl complex (t ion = 0.11) is predominantly electronic, in 3,5-dimethyl-ICl complex (t ion = 0.58) is mixed ionic-electronic and in 4-methylpyridine-ICl complex (t ion = 0.95) is mainly ionic as came across from impedance spectra.

Fig. 8
figure 8

Current vs time plots of a pyridine-ICl, b 3,5-dimethylpyridine-ICl and c 4-methylpyridine-ICl complex at 303 K

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DC conductivity measurement

The current-voltage characteristics of the samples in the pellet form recorded at various temperatures by using Keithley 2400 source meter are shown in Fig. 9 (S6a, b). It is observed that the I/V curves are linear indicating the ohmic behaviour. From the slope of the I/V curve d.c. conductivity can be calculated by using the following relation

$$ {\sigma}_{\mathrm{dc}}=\mathrm{slope}\times \frac{t}{A} $$
(7)
Fig. 9
figure 9

Current-voltage plots of 3,5-dimethylpyridine-ICl complex at different temperatures

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where t is the thickness and A is the surface area of the sample. The d.c. conductivities measured from the I/V curves (using source meter) are well agreement to those calculated from the intercept of logσ versus logω plot (Table 7). Further, total conductivity, σ Total, measured from the experimental Nyquist plots nearly equals to the σ dc values and follows the Arrhenius law (Fig. 5). The activation energy for d.c. conductivity, calculated from the Arrhenius plots are 0.51, 0.71 and 0.86 eV for 4-methylpyridine-ICl, pyridine-ICl and 3,5-dimethylpyridine-ICl, respetively.

The increasing order of conductivity at any particular temperature is pyridine-ICl < 3,5-dimethylpyridine-ICl << 4-methylpyridine-ICl. Among the three, the highest conductivity of 4-methylpyridine-ICl complex is due to the higher ionisation (t ion = 0.95) and lower band gap (E g = 2.21 eV); on the contrary, pyridine-ICl complex with lower ionisation (t ion = 0.11) and higher band gap (E g = 2.85 eV) reveals lowest conductivity.

Conclusion

The investigations of the three charge transfer complexes by complex impedance spectra and ionic transport number measurements confirm that 4-methylpyridine-ICl can be considered purely ionic which originates from (γ-Pic2-I)+ and ICl2 − ions, while 3,5-dimethylpyridine-ICl is a mixed ionic-electronic conductor and pyridine-ICl is predominantly electronic in nature. The electrical properties of pyridine-ICl and 4-methylpyridine-ICl can be described by equivalent circuits –(RC)–(RC)–(RC)– and –(Rcpe)–(cpe)– respectively. The former circuit corresponds to grain, grain boundary and electrode contributions whereas the latter circuit corresponds to bulk resistance and double layer capacitance of electrode-material interface. Interestingly, electrical properties of 3,5-dimethylpyridine-ICl complex fits in both the equivalent circuits. At low temperature, it corresponds to that of –(RC)–(RC)–(RC)– and at high temperature it corresponds to –(Rcpe)–(cpe)–. The nature of the variation of d.c. conductivity with temperature suggests semiconducting behaviour with activation energy 0.71, 0.51 and 0.86 eV for pyridine-ICl, 4-methylpyridine-ICl and 3,5-dimethylpyridine-ICl complexes, respectively. The frequency dependence of a.c. conductivity obeys the Jonscher’s power law.