Langmuir 1986, 2, 788-791
788
201
\
E
H \
10-
++ W
I
I C
01 '
- 50
0
50
I IO0
Temperature ( " C )
Figure 6. Temperature dependence of ZJZ,, in PDAIbarium stearate (112) LB film (20 layers). (a-c) See text. creasing temperature. The change of I e / I mvalue was reproducible and reversible within region a. On reaching a critical temperature (region b), however, the I e / I mvalue suddenly decreased. After this thermal transition (region b), the Ie/Imvalue is lower (region c) compared with region a and was not restored to region a on cooling to the initial temperature. This shows that an irreversible transition occurred. The I J I m values shown in region c are comparable to those of PDA in cast films, where the pyrene moieties are oriented randomly. Therefore, the sudden decrease in I e / I mvalue is attributable to the destruction of layer structures in LB films. A similar tendency was found with PDA/barium stearate = 1/50 LB film. X-ray diffraction measurements were carried out both before and after the transition. Heat treatment was carried out to induce the transition a t 70 "C for 5 min, and the X-ray diffraction measurements were done at room temperature. Before the heat treatment, all peaks in the X-ray diffraction pattern can be assigned to one doolspacing which corresponds to Ba-Ba spacing with twice the mon-
olayer thickness (50.5 A, (a) in Figure 5 ) , suggesting that this LB film is a Y-type film, and the phase separation into stearic acid and PDA phases in the monolayer has not occurred. After the heat treatment which caused the sudden decrease in I e / I mvalues, both X-ray diffraction intensity and the doolspacing decreased ((b) in Figure 5 ) . The decrease of the diffraction intensity indicates the destruction of the layer structure. The decrease of Ba-Ba spacing is due to the conformational change of hydrocarbon chain from the all-trans state. The destruction of the oriented structure may cause the cleavage of excimer formation sites and decrease the efficiency of energy migration to the excimer formation sites. The evidence for the energy migration process was obtained by measuring the degree of the polarization of monomer emission of PDA; e.g., a low degree of polarization (2.6 X lo-*) was obtained with a PDA/barium stearate = LB film. In LB films with large mole fractions of PDA (PDA/ barium stearate = 1/2 and LB films), a similar thermal transition occurs, resulting in a sudden decrease in I e / I m (Figure 6). Here, however, the dependence of the Ie/I,,, value on temperature in region a is different from that in Figure 4. For these samples, the Ie/Imvalue decreases with increasing temperature. The reason the dependence of the I e / I mvalue on temperature differs with the mole fraction of PDA in the LB film is not clear at the present stage. We do believe that Ie/Imis sensitive to important features of monolayer structure and dynamics which future studies will help us to understand. In summary, the change of the I e / I mvalue with temperature gives information on the chromophore orientations in LB films and provides a sensitive measure of the destruction of layer structure in LB films by thermal treatment. The transition of the layer structure of LB films has been generally studied by means of X-ray diffraction measurements. However, there are several cases in which the X-ray method cannot be used, e.g., when the diffraction patterns are very weak. In such cases, the fluorescence measurement as described above is a convenient method to study the thermal transition. Registry No. PDA, 69168-45-2;barium stearate, 6865-35-6.
Spectroscopic Detection of Charge Exchange in Phthalocyanine Dispersions John R. Harbour* and Mary Jane Walzak Xerox Research Centre of Canada, Mississauga, Ontario, Canada L5K2L1 Received May 28, 1986. In Final Form: August 1, 1986 The use of ESR for the spectroscopicdetection of redox reactions in metal-free phthalocyanine particles
in dispersion has been demonstrated. The ESR signals obtained upon electroreduction are readily differentiated from those achieved by electrooxidation by both g factor and line width. Extended electroreduction reveals that one out of four molecules within the particle is reduced. The additional fact that these electrons are localized (i.e., the intensity of the ESR signal follows the Curie law) suggests that interactions between the molecules are limited and that only local interactions or clusters ( n 5 4) may exist.
Introduction Over the past several years, we have been involved in a study of electron transfer both to and from small particle~.'-~ It was our goal not only to demonstrate this (1) Harbour, J. R.; Walzak, M. J. Carbon 1984, 22, 191. (2) Harbour, J. R.; Walzak, M. J.; Julien, P. Carbon 1985, 23, 185.
0743-7463/86/2402-0788$01.50/0
exchange but also to gain some insight into the way in which particles accept or donate electrons. To accomplish this, we attempted to spectroscopically detect these ex(3) Harbour, J. R.; Walzak, M. J. Carbon 1985, 23, 687. (4) Harbour, J. R.; Walzak, M. J. Carbon, in press. (5) Harbour, J. R.; Walzak, M. J. Photogr. Sci. Eng. 1984, 28, 224.
0 1986 American Chemical Society
Langmuir, Vol. 2, No. 6, 1986 789
Charge Exchange i n Phthalocyanine Dispersions
changes using ESR. The actual redox reactions were achieved electrochemically with platinum electrodes inserted within the particle dispersions. Our initial efforts focused on carbon black parti~les.l-~ It was demonstrated that carbon blacks in dispersion can accept electrons and that these electrons are in fact observable by ESR. From this study it was demonstrated that chemical oxidation of the carbon blacks creates the surface sites which act as electron-accepting sites. Upon electroreduction, these surface sites accept an electron reversibly and give rise to a narrow ESR signal (g N 2.0027) whose intensity follows the Curie law. This latter observation implies that the sites are localized. There is also an inherent conduction ESR (CESR) signal due to charge carriers within the conductive black particle^.^^' It turns out that an equilibrium exists between these bulk charge carriers and the localized surface states. These surface states can be populated by either lowering the temperature or by electrochemical reduction. Electrooxidation of the initial dispersion did not result in any changes in the ESR spectrum. ZnO particles were also found to manifest an ESR signal upon electrored~ction.~ This ESR signal was centered a t g = 1.96 and is due to trapped conduction electrons. In addition to the ESR changes, it was also determined that the dielectric loss increases upon electroreduction. This is presumably due to the added electrons within the conduction band which interact with the electric component of the microwave radiation (dielectric loss), leading to a decrease in the cavity Q. Both observations were reversible. This present paper reveals our efforts on the redox reactions of metal-free phthalocyanines (H2Pc). It has previously been demonstrated by Loutfy and Sharp* that H2Pc can be reduced on a hanging mercury drop electrode or oxidized on a Pt disk electrode. Our results confirm that these particles can accept and donate electrons and that they also manifest different ESR signals upon electroreduction and oxidation.
Experimental Section A Varian E109-E EPR spectrometer was used to obtain the data. The electrochemical cell has been previously describede2 Briefly, the cell is composed of a capillary with a glass wool plug a t the bottom. The H2Pc is then placed on top of this plug and a thick platinum wire inserted from the top into the H2Pc layer. This entire electrode is then inserted into an ESR tube containing a thin platinum counter electrode and the electrolyte solution. Hence there is no mechanical stirring of the dispersion. The ESR signal occurs at an applied potential of 2.6 V for electroreduction and 2.4 V for electrooxidation. Since this system contains no reference electrode, the real potential is unknown. The ESR signals do not change as the potential is increased to 10 V. However, the rate of the redox reaction is increased and we, therefore, chose 10 V as the applied potential for the saturation experiments. Saturation of the redox reaction was taken as the point when the ESR signal reached its maximum value. As discussed in the text, this is actually a minimum value for the extent of reduction. Methylene chloride was used as the solvent with 0.05 M tetrabutylammonium perchlorate as electrolyte. The dispersion was purged of O2by bubbling thoroughly with nitrogen gas. The 1 / T data for the signals were obtained simultaneously with that of a standard (Mn2+in SrO) to correct for any changes in Q. DPPH was used as the standard for the determination of the number of spins. The line shapes for the electrochemically generated signals were Lorentzian and did not change with time of electrolysis. (6) Singer, L. S. Proc. Carbon Conf., 5th 1963, 11, 37. (7) Mrozowski, S. Carbon 1965, 3, 305. (8) Loutfy, R. 0.; Sharp, J. H.J . Appl. Electrochem. 1977, 7, 315.
+lOG--
/Figure 1. ESR spectrum of x-H2Pc (polymorph 1) after electroreduction. Spectrometer settings: microwave power, 10 mW; modulation amplitude, 1.6 G; receiver gain, 2 x 10; T = 20 O C .
inherent sample 1 2 3
g
2.0030 2.0029
AHpp,G 6.1 5.0
Table I reduced g 2.0030 2.0031 2.0030
m,,,G 1.9 1.9 1.8
oxidized g 2.0024 2.0024 2.0024
AH,,,G 0.8 0.7 0.7
The three samples of two polymorphs of metal-free phthalocyanine were (1)x-H2Pc, (2) x-HzPc, and (3) P-H2Pc. The (1) x-H2Pc was obtained by ball milling a-HzPc9and the (2) x-HzPc was synthesized by using the Brach method8 which involved reductive condensation of phthalonitrile in 2-(dimethylamino)ethanol. Sample 2 differed from sample 1in having a much larger inherent ESR signal. These phthalocyanines are insoluble in most solvents except a-chloronaphthalene. Therefore the redox reactions are heterogeneous rather than homogeneous. The absorption spectra of these two polymorphs in dispersion have been reported.'O
Results and Discussion Reduction. Electroreduction of any of the three H,Pc samples resulted in the generation of an intense, narrow, Lorentzian ESR signal at g = 2.0030 (Figure 1). These results are summarized in Table I. In all three samples the g factor and AHppvalues are essentially equivalent. (AHppnarrows initially to a final value of 1.9 G and then remains constant with time.) The signals generated have intensity orders of magnitude greater than the inherent signalsll observed for these pigments. (The g factors and AHppvalues for these inherent signals are also included in Table I.) Since all the polymorphs behaved similarly under electroreduction, the following results focus on polymorph (l),x - H ~ P cwhich , has the smallest inherent signal (3 X 1015spins/g) and the largest surface area (-70 m2/g). The electroreduced signal was stable with time (in the absence of 0,) and could be removed by electrooxidation. In fact, further oxidation led to a new signal as described below. After -75 min of electroreduction the intensity of the ESR signal corresponded to a value of -3 X 1020spins/g. This can be compared to the value of 1.1 X 1021molecules/g of (9) Byrne, J. F.; Kury, P. F. US. Patent 3357989, 1967. (10)Loutfy, R. 0. Can. J . Chem. 1981, 59, 549. (11) Harbour, J. R.; Loutfy, R. 0. J . Phys. Chem. Solids 1982,43, 513.
Harbour and Walzak
790 Langmuir, Vol. 2, No. 6, 1986 120
t
0
X-HzPc Polymorph # 1
0.02
0.04
0.06
(Temperature)-'
008
0.10
/
011
in Kelvin
Figure 2. Intensity of the ESR signal obtained upon electroreduction of x-H2Pc (polymorph 1)vs. 1/T. HzPc which reveals that approximately one out of every four molecules has been reduced. It is interesting to note that after 15 min of electroreduction, when the signal intensity had saturated, the ESR signal actually increased by 35% when the voltage was returned to zero. It was this latter intensity which was used to determine the number of spins. A temperature study of this ESR signal demonstrated that the intensity is following the Curie law (Figure 2). This implies that the spins are localized. Oxidation. When any of three samples were electrochemically oxidized, an intense ESR signal resulted which was different from that obtained by reduction. These Lorentzian signals had g factors of 2.0024 with AHpp-0.8 G (see Table I). (AHppnarrows to 0.8 G rather quickly during oxidation and then remains constant during the remaining oxidation.) This signal could be removed by electroreduction, demonstrating the reversibility of the redox reactions. In fact, it was observed that one can reversibly take the system back and forth between the oxidized and reduced states. A temperature study of the intensity of this oxidized signal revealed a dependence which was very close to that expected for the Curie law. For the Curie law the ratio of intensities at 97 and 147 K should be 1.5. For reduction, a value of 1.6 (using Mn2+in SrO as a standard) was obtained with very little scatter in the results. Although there was more scatter in the results for oxidation, a leastsquares fit of the points also gave a ratio of 1.6. These results are therefore consistent with localized sites which can donate or accept electrons. It is interesting to note that color changes were also evident during the electrochemical process. This was observed in a flat EPR cell (with Pt electrodes) outside the cavity in order to better visualize the color changes. During oxidation, an emerald green color developed at similar potentials to those applied in the ESR cell while a violet to brown color appeared at higher oxidation potentials. It was difficult to observe any color change initially upon electroreduction (the electroreduction process evidently increased the degree of dispersion), but an orange color developed under more negative potentials. These color changes appeared to be reversible. Similar color changes have been observed by Walton et a1.12 These results are consistent with the generation within the particles of radical anions (Ered= -1.2 V vs. SCE6) (12) Walton, D.; Ely, B.; Elliott, G. J . Electrochen. SOC.1981, 128, 2479.
under electroreduction and radical cations (Eox= 1.0 V vs. WE8) under electrooxidation. Prolonged reduction produces dianions which spontaneously release electrons to generate radical anions when the voltage is zeroed and the circuit remains closed. The g factor of 2.0030 for the reduced state is higher than that found in the oxidized state 2.0024). This is consistent with reported values of radical anions and cations for both soluble phthalocyanines &(anion radical) = 2.0031,13g(cation radical) = 2.002014) and chlorophylls &(Chl+) = 2.0025,15 g(Ch1-) = 2.0028l')). The AHppvalues are significantly narrower than those values reported in the literature for single radical anions or cations. Although it is well-known that a reduction in the line width can be associated with a delocalization over several H,Pc molecules (AHppnarrows by a factor of r ~ ' ' ~ ) , ' ~ it can more readily be accounted for by exchange narrowing in this case. The electrochemistry in this experiment is thought to occur on a particle-to-particle basis. Hence, when a particle comes into contact with the Pt electrode it is significantly reduced before leaving the electrode surface. One would therefore not expect a narrowing in line width with time which is consistent with observation after the initial narrowing occurred. This initial narrowing (which occurs up to 10% of the total reduction) could be accounted for by an initial particle-to-particle sharing of charge which is precluded once the magnitude of the double layer and resultant repulsive force increases. The very high number of sites involved in this process is remarkable. Even so, the value of one electron accepted for every four molecules is a minimum value since we have no way of knowing if all the particles have been reduced. In any event, these particles are evidently not acting as true semiconductors since a temperature study of the intensity of the ESR signal revealed that the electrons are localized (electrons in a band give rise to a conduction ESR signal with intensity independent of temperature). Rather, the molecules either singly or in a small cluster (n 5 4) accept or donate electrons. Although semiconductors such as ZnO can accept electrons in localized conduction band traps, the very high number of electron-accepting sites in H2Pc relative to ZnO favors the electrons being localized on a molecule or small cluster of molecules ( n 5 4) (Le., no extended interaction). Finally, no change in Q was observed under these electrochemical conditions with H,Pc. This can be contrasted to the ZnO case where electrons dumped into the conduction band cause a increase in dielectric loss. The reduction of one of every four molecules results in a large number of spins in each particle, and as a result the particles are highly charged (balanced in solution by the counterions of the electrolyte which are present at 0.05 M). The repulsion of particles from each other was observed in these cells as the space occupied by the particles actually increased within the capillary as the electroreduction continued. This charge did not effect quantitative ESR measurements as all the particles (
