7484
J . Phys. Chem. 1991,95, 7484-7488
Evidence for Matrix Interaction in Doped Polymer Films: An EPR Analysis of o-ToiMinerTCNQ in Bisphend A Polycarbonate Y. C. Fann and S. A. Jansen* Department of Chemistry and Materials Science, Temple University, Philadelphia, Pennsylvania 191 22 (Received: July 23, 1990; In Final Form: January 17, 1991)
Polycarbonate (PC) films of chargetransfer complexes formed from 7,7',8,8'-tetracyanoquinodimethane (TCNQ) and +tolidine (oT) were studied by electron paramagnetic resonance (EPR) spectroscopy. The formation of dendritelike microcrystallites within the polymer network was found. The crystallization phenomena depends on the film thickness and substrate temperature as well as evaporation process. Magnetic susceptibility and power saturation studies show significant differencesas concentrations vary. Discontinuities in magnetic susceptibility were observed at about 200 K and at the glass transition temperatures of charge-transfer doped PC films. Multiredox steps were characterized from cyclic voltammetry (CV) on pristine charge-transfer complexes. A strong interaction between dopants and the polymer matrix was observed by gel permeation chromatography and differential scanning calorimeter.
TABLE I: Redox Potentiab of TCNQ, o-Tdidiae, and Tbdr Charge-Transfer Salts in 1:l THFCH,CN Solvent System Using Clur Carbam ladicator Electrode (Sappthg Ekcldyte, 0.1 M Tetrawtbvhmmoaium Tetrafl-te); Potentials in Vdts vs Pt Electrode i t Scan Rate 50 mV/s Et E A V) E,' E, - (10.005 . TCNQ -0.28 -0.88 0.18 0.43 o-tolidine TCNQ-oT = l:lb -0.71 -0.15 0.19 0.41 TCNQ-oT = 1:lc -0.79 -0.17 0.31d TCNQ-oT = 1:2 -0.80 -0.18 0.16 0.34 TCNQ-oT = 2:l -0.76 -0.16 0.17 0.36
Introduction
Many charge-transfer (CT) agents have been utilized in a variety of development and application technologies. For instance, organometallic CT agents are used as toners in color printing, and simple molecular dopants are employed as photoconductors in the "photocopying" process. In addition, metal-polymer compounds have been used as electrostatic discharge materials. To be used this way, metal or steel is doped into polymers to increase conductivities, thus increasing the density of the material and expense. Recently, Dirk and co-workers reported that dye-doped polymers show interesting nonlinear optical properties with potential as quadratic electrooptic materials.' For these applications, charge-transfer agents or molecular species are dispersed in high concentration (1-40 wt %) into a polymeric matrix with suitable electronic requisites to provide a material capable of withstanding the mechanical stress. There are several material problems associated with this type of system. One of these problems is the long period stability as the conducting species or the polymer of the matrix is frequently degraded by a photoexcitation or thermal process in the presence of oxygen or aging in oxidizing environment. In addition, solubility of the dopant is also a consideration. There is a miscibility problem as the C T agents and dyes are of low molecular weight and the polymers are high molecular weight, and as expected, the matrix modulates the electronic transport process. Several studies of the formation of conducting polymer films containing low weight percent of organic CT salts, the so-called reticulate doping method, have been reported.**' The reported preparation of such conducting films involved simply casting a solution of polymer and C T complex onto a glass substrate. Their results show that the conductivity increased significantly (ca. lOI5 orders of magnitude) with respect to that of pristine polymer films and the conductivity is critically dependent on the composition of casting solution. Our work has focused on the analysis of electronic and magnetic properties of CT salts of OTand TCNQ in a polymeric matrix, as this system shows several interesting features. Multiple stoichiometries of the oT-TCNQ salt can be prepared by simple means. Aniline functional groups in the oT provide two sites for interaction within the polymer matrix. Molecular orbital calculations4 show electronic structures similar to photoconductors such as (diethy1amino)benzaldehyde
' E , = (E, + E,)/2. bMix two equimolar solids in solvent. CDissolve complexes in solvent. dOxidation peak only.
diphenylhydrazone (DEH), which when doped into polycarbonate is an effective photoconductor. Experimental Section
390.
The charge-transfer agents, o-tolidine (Eastman Kodak Co.) and TCNQ (Strem Chemicals, Inc.), were used without further purification, and their chemical structures are shown in Figure 1. The pristine CT complexes, (oT)(TCNQ), ( n = 0.5, 1, 2). were prepared by a literature methodS and recrystallized with acetonitrile. The results described here are based on 1:l salt only. Bisphenol A polycarbonate (PC,Makrolon 5303) films, 1 and 10 wt %, containing C T complexes were prepared a t room temperature. Hot tetrahydrofuran (THF) solutions (-50 "C) of PC and CT complex mixtures were cast onto a glass substrate, and the solvent was evaporated at ambient temperature and pressure. CV experiments were carried out by IBM cyclic analyzer EC-225. A 0.1 M solution of tetramethylammonium tetrafluoroborate in a 1:l solvent mixture of THF and acetonitrile in volume was used as the electrolyte. A Bruker ER-200D EPR spectrometer equipped with ER035M N M R gaussmeter and ER-4111 VT temperature control unit was used for all the EPR measurements. The precision of the gaussmeter is 0.001 G and temperature was maintained during varied temperature experiments within 0.1 O C . EPR experiments were performed on pristine and polymer films of varying concentration and complex stoichiometry. DPPH was used as a spin concentration reference. A Waters gel permeation chromatography (GPC) and a Perkin-Elmer DSC-7 differential scanning calorimeter (DSC) were used for GPC and glass transition temperature (T) measurements, respectively. The micrographs were taken under &kon microscopy
(3) Quill, K.; Underhill, A. E.; Kathirgamanathan, P. Synrh. Met. 1989, -32. - -329. -- . (4)Hoffmann, R. J . Chem. Phys. 1963,39, 1397. Hoffmann, R.;Lipscomb, W. M. Ibld. 1962, 36, 3179:Ibld. 1962,37,2872.
(5) Ashwell, G. J.; Chyla, A,; Heimer, N. E.; Metzger, R. M.; Allen, J. G.,Mol. Cryst. Liq. Cryst. 1985, 120, 137.
( I ) Dirk, C. W.: Kuzyk, M. G.Chem. Mater, 1990, 2,4. (2)Jeszka, J. K.;Ulanski, J.: Kryszewski, M. Nature (Lodon) 1981.289,
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0022-3654191 12095-7484$02.50/0
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1991 American Chemical Society
Matrix Interaction in Doped Polymer Films
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7485
N*
c.c+cfN @ NHzC&&NH
o
4c
- Tolidine
-
c.
TmQ
TABLE 11: Line Widths and Average g Values of the Pristine TCNQ-oT CT Salts Varied as a Function of Stoichiometry and Temperature (8) line width (*0.00005) (st0.005),G CT salts temp, K I:l
Figure 1. Chemical structures of o-tolidine and TCNQ.
150 200 250 295
a) 2: 1
1 :2
b
-1.15v
100
100 150 200 250 298 100 150 200 250 295
2.002 55 2.002 73 2.002 50 2.002 68 2.002 89 2.002 74 2.002 66 2.002 69 2.002 6 1 2.002 57 2.002 69 2.002 66 2.002 63 2.002 59 2.002 64
1.47 1.44 1.24 1.03 0.93 1.65 1.63 1.28 0.98 0.87 I .43 1.39 1.29 1.07 0.93
I
+l.w
Figure 2. CV traces of 1:l TCNQ-oT complexes: (a) freshly prepared; (b) CT complex in solvent.
equipped with a polarizing apparatus. Results and Discussion Pristine. TCNQ and oT form a dark green complex in both
THF and acetonitrile solution. The CV studies have shown the CT complexes can undergo four quasi-reversible redox steps based on the difference of cathodic and anodic potentials. Figure 2a shows the CV trace of the fresh prepared pristine 1:l mole ratio CT complex in a 1:l solvent mixture of THF and acetonitrile. The redox potentials of CT complexes, (oT)(TCNQ), (n = 0.5, 1, 2), tend toward an average position relative to the redox potentials of oT and TCNQ. Dissolving the solid complex yields a CV trace of which two peaks coalesce into a irreversible peak at 0.3 V compared to the freshly prepared mixture of individual solutions. This is shown in Figure 2b. After about 1 h a similar CV trace as shown in Figure 2b was observed for the fresh prepared solution, which indicates a strong complex formation as the solution reaches an equilibrium state. Table I gives the redox potentials of the CT complexes, which are similar to those previously reported for simple complexes of TCNQ.6 EPR studies were performed on pristine CT salts as references for films of varying stoichiometries. These studies have shown the pristine CT complexes exhibit a single exchange narrowed line. The g values, which varied between 2.0025 and 2.0030, are strongly dependent on stoichiometries and temperature. The line widths and average g values varied as a function of temperature and stoichiometry are shown in Table 11. No discontinuity in magnetic susceptibility was observed between 100 and 300 K. Their line widths at room temperature for the pristine complexes are ca. 0.9 G and increased to a maximum of 1.5 G at 100 K. ( 6 ) Jones, M.T.; Chestnut, D. B. J. Chem. Phys. 1963.38, 131 1; Jones, M.T.; Jansen, S.A.; Roble, J.; Ashwell, G. J. Mol. Crysr. Liq. Crysr. 1W5,
120, I 1 1.
Figure 3. Microphotographs of 10 wt o/o CT complex in PC (a) 40X and (b) 400X on one of the aggregates.
The uncertainties in the "g" are 3 X lod and 1 X IO4 G for line widths. Multiple stoichiometries were used as references as Jeszka et al.' have reported changes in D+A- association upon doping into PC. Cr-Doped Films. (i) Characterization. Both 1 and 10 wt 3' 6 of CT-doped PC films are yellow and translucent. Optical microscopy was used to ascertain the level of mixing and miscibility of the oT-TCNQ complexes in the PC matrix. At low concentrations, ca. 1 wt %, a solid solution of the CT complex and PC is formed without phase boundaries or any particulates. At higher concentrations,ca. 10 wt 96,which are visible at 40 magnification, two distinct types of crystallites, one needlelike and the other dendritelike, are formed within the polymer network. It is observed in all the stoichiometries of 10 wt '% films. In addition, these (7) Jeszka, J. K.; Tracz, A.; Shafee, E. E.;Ulanski, J.; Kryszewski, M.J. Phys. D: Appl. Phys. 1986, 19, 1047.
Fann and Jansen
7486 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991
b) 198 mW
Minutes
/I/--
Figure 4. GPC curves of the pristine PC powder, film, and 1 wt % CT-doped film. TABLE III: Glass Transition Temperatures of the histine PC Film, Powder. and the 1:l 1 wt % CT-Doped Film T,,OC samples samples T,,OC 1 wt 4% doped film 133.3 149.0 PC powder 10 wt 4% doped film 112.8 PC film 141.4 ~~~~~
20 G
~
TABLE IV: Total Spin Density of CT-Doped Films Relative to the Pristine 1:l CT Agent in Percentage film thickness, y m wt % relative % 50 1 13 f 1.0 50 5 23 f 1.2 50 IO 40 f 2.0 80 IO 70 i 3.6
microcrystallites are arranged in nearly regularly spaced aggregates with the spacing distance being on the order of 160 pm. Figure 3 shows the aggregate arrangement in the 1:l CT complex doped IO wt % film at different magnifications. Whether this phenomenon is related to the nucleation sites within the polymer network or the interaction between polymer and CT complexes is yet to be determined. Infrared (IR) vibrations at 730 cm-' suggest no perturbation or modification of the end groups of PC during the film preparation? Thus, it is doubtful that aggregation or nucleation is occurring at the end groups. By increasing the amount of solvent in the casting solution (i.e., lowing the viscosity), larger microcrystallites formed within the PC matrix was observed. It was also noted that the dendritelike microcrystallites within the polymer matrix become larger and extend through the dimension of the films as the thickness of films increases. The polymeric films appear to be more homogeneous with less tendency toward crystallization being observed when cast onto a hot glass substrate (ca. 100 "C). X-ray powder diffraction on 1 and 10 wt % show amorphous characters for all films. In comparison with films cast from neutral TCNQ in PC, the same microcrystalline structures were found with at least two species present-one due to T C N Q and one due to TCNQ in both 5 and 10 wt %-but no aggregates were identifiable. In addition to dendritelike microcrystallites, diamondlike crystallites within the TCNQ-PC polymer matrix were found, which are believed due to neutral TCNQ. GPC measurements show a pronounced shift of the molecular weight distribution in 1 wt % film compared with pristine PC powder, and the film indicates a significant interaction between CT agent and the polymer matrix. GPC curves of the pristine PC powder, film, and 1 wt % doped film are shown in Figure 4. In addition, a significant decrease of the glass transition temperature ( TB!in CT-doped films compared with T of the pristine PC, shown in Table Ill, also suggest a strong eT-PC matrix interaction. (ii) EPR Spectroscopy. EPR studies have shown that a severe reduction of magnetic susceptibility is noted upon doping oTTCNQ into PC. The values relative to pristine oT-TCNQ are given in Table 1V. For the 10 wt % 80-pm film, greater crys(8) Fann, Y. C.; Jansen, S.A., to be published.
Figure 5. Power variation of 1:l TCNQ-oT 1 wt % film.
2.00806 2.00286
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0
20
40
80
80
100
120
140
160
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Figure 6. g values of 1:l TCNQ-oT salt, 1 and 10 wt 9% PC films, as a function of microwave power.
tallization is noted. Thus, the levels of interaction with polymer m d the tendency toward crystallization increase the spin density of the matrix. Further studies on the TCNQ-aT films have shown the presence of two distinct magnetic species. These were resolved by a saturation/population study. Both species are observed in the 1 and 10 wt % films for all stoichiometries in the temperature range 100-300 K. Figure 5 shows the EPR spectra of (oT)(TCNQ) complex dispersed in PC at 1 wt %. Clearly, one species saturates easily and is not identifiable at microwave power levels, above 30 mW. The other species seems characteristic of magnetic exchange as observed in many charge-transfer complexes based on TCNQ. The g values measured from the 1 and 10 wt % films are strongly dependent on power, composition, and temperature. It was noted both pristine and 1 wt 9% films show a weak dependence on microwave power, whereas the 10 wt 9% films showed a strong dependence continuing through out high levels of microwave power. This dependence is believed due to the presence of multiple magnetic species with different relaxation properties. Figure 6 shows the g values in a variety of microwave power for 1:l pristine complex, 1 and 10 wt % films. Though we cannot absolutely describe the effect of the matrix on the 'g", a pronounced effect is observed when comparing the average g value of the 1 and 10 wt % films with standards such as free TCNQ, free oT, and the pristine solid. Relative magnetic susceptibility measurements provide consistent information and further substantiate the conclusion that multiple magnetic species are present. For 1 wt % film, at low microwave powers, 2-10 mW, the magnetic susceptibilityappears dominated by fixed sites and follows a Curie-type susceptibility. At high power, 198 mW, and low temperatures, 100-200 K, an activated species appears present. No saturation effects were observed at this power for the narrow line. Such activation, typically derived from singlet-triplet processes in TCNQ chains
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7487
Matrix Interaction in Doped Polymer Films
2*0030r
2.0029
Ts
2.0028 2t
I I I
I 1 I
Temp. (K)
2.0028
Figure 7. Relative magnetic susceptibility as a function of temperature from 100 to 300 K at microwave power 198 mW; a discontinuity at ca. 200 K is noted.
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Figure 8. (a) Line widths and (b) g values for both 1 and 15 wt 96 PC film at temperature region 1W300 K, significant changes of line widths and g values corresponding to the magnetic transition temperature are noted. or conduction type electrons, have been identified in several TCNQ complexe~.~ The activation fit to a Boltzmann model associated with this region is ca. 0.20 eV. At roughly 200 K, a discontinuity in the magnetic susceptibility is observed. The effect is shown in Figure 7. The discontinuity is not observable at low microwave power through the temperature region 100-300 K and is not ( 9 ) Chestnut, D. B.;Phillip, W. D. J . Chem. Phys. 1961, 35* 1002.
observed under any microwave condition in the pristine CT salts. The origin of this discontinuity is not yet well understood but is suspected to be the formation of TCNQ dimer or crystallization of traces of solvent trapped within the PC matrix. Wegner and co-workers reported that a discontinuity of magnetic states observed from poly( 11-vinylfluoranthene) a t 195 K due to the formation of the "Wegner" type dimer, [F2]'+X-, where F = fluororanthene and X = C104-, BF4-, or PF,-.*O A specific discontinuity of magnetic states in TCNQ salts caused by crystallization of solvent trapped during the preparation of CT salts has been noted." Though similar phenomena have been observed, the magnetic states of our PC-doped films are activated below the critical temperature and are Curie type above the critical temperature and thus do not appear consistent with this rationale. Line widths and g's show significant changes at the critical temperature, which is shown in Figure 8. The trend of the g above the transition is not well understood a t the present time but is a consideration for further investigation. The higher 'g" (10 wt 5%) is similar to "g"'s from TCNQ dimers in Figure 6. Another example of the discontinuity of magnetic states was reported in polymeric charge-transfer complexes by Sathyanarayana.I2 In the case of polymeric charge-transfer complexes, polymers act as electron donors/acceptors and the CT complexes transform from initial paramagnetic complexes to final diamagnetic products with a discontinuity at critical temperature. For the 10 wt 95 film with aggregates, a similar discontinuity effect was observed at about the same temperature region. From 100 to 200 K, an activated species was observed with a lower activation energy, ca. 0.07 eV compared to that of the 1 wt % film. These observations are consistent with those of 1 wt %. In addition to this low-temperature magnetic transition, another discontinuity appeared at the glass transition temperature (T) for both 1 and 10 wt % films at low microwave power, ca. 2 m d . At high power, ca. 198 mW, the discontinuity is not identifiable (IO) Eichele, H.; Schwoer, M.;Krohnkc, C.; Wegner, G.Chem. Phys. Lcrr.
1981, 77, 31 1.
(11) Waller, A. M.;Compton, R. G. Synrh. Mer. 1990, 35, 371. (12) Palaniappan, S.;Sathyanarayana, D. N. Magn. Reson. Chem. 1990, 28, 233.
7488 The Journal of Physical Chemistry, Vol. 9 5 No. 19, 1991 a)
Fann and Jansen a) 30OK
Thouundo 6r
n
X
I
200
300 320 340 380 980 400 420 440 400 400
600
Temp. (K)
4 7 c) 450K
If
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d) 400K
1
I
200
300 320 340 360 380 400 420 440 400 400 600 Temp. (K)
Figure 10. Relative magnetic susceptibility as a function of temperature from 300-450 K at microwave power: (a) 2 mW and (b) 198 mW; a discontinuity is noted at T8at 2 mW but not at 198 mW. from magnetic susceptibility but is apparent in the g values measured from the 10 wt % film. Figure 9 shows g values changes as a function of temperature from 300 to 450 K at microwave powers of 2 and 198 mW. Line widths decrease from a maximum 2.38 G with a sharp change at T, and toward a minimum 1.83 G as temperature increases. Figure 10 shows magnetic susceptibilities change as a function of temperatures from 300 to 450 K for the 10 wt % film at microwave powers of 2 and 198 mW. The reason of the discontinuity is believed due to be that polymer matrix changes above the T,. EPR signals also show significant changes with respect to temperature. When the temperature is above 450 K,the cooling process becomes irreversible for both 1 and 10 wt 9% films with "g" remaining at the minimum. Figure 1 1 shows EPR spectra of the IO wt 9% films at 300 K, T,, and 450 K from the heating process and 400 K from the cooling process respectively at low microwave power. The irreversible process is believed due to the decomposition of films above 450 K. Conclusion
This work has demonstrated that simple charge-transfer salts can be doped significantly at high concentrations in a polymeric medium without disrupting all correlated phenomena. In addition, microcrystallites of these complexes can be introduced into the polymer matrix by controlling the amount of solvent, temperature,
. 1OG
Figure 11. EPR spectra of 10 wt W PC film at 300, T,,and 450 K from the heating process and 400 K from the cooling process.
and evaporation processes. The magnetic exchange is preserved in both the low-concentration solid and high-concentration aggregate films. The average g value of 1 wt % film is similar to that of a free TCNQ radical anion, suggesting weakening of the strong donor-acceptor interaction through oT-PC interaction. There appears to be three critical temperature regions, 100-200, 200-Ts, and T,-450 K. These effects, which differ with composition, are believed due to strong oT-polymer and TCNQTCNQ interactions,which seem parallel with crystallization within the matrix. Above T,,the g values suggest a minimum of interaction with the polymer matrix and approach that of 1:l oTTCNQ. The strong difference of "g"on temperature and power is believed due to changes in the effective stoichiometry of the oT(x)-TCNQ@) complex, initially x = y = I , in the matrix. We believe this is driven by oT-polymer interaction. After "annealing" heating to T,,the ( g ) remains constant at 2.00265, which approaches the ( g ) of the pristine 1:l oT-TCNQ salt. Future work will focus on further characterization of the optical and magnetic properties, explicit measurements of the resistivity within the films, and elucidation of structural features on the molecular level with emphasis on structural analysis of the crystallites within the aggregates.
Acknowledgment. We thank Mr. David Grasso for assistance with photography, Dr. M. M.Labes for providing the microscopy accessory, and Dr. G. Myers for assistance with the X-ray powder diffraction measurements.
