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J. Phys. Chem. B 1997, 101, 9224-9232
Charge Transport in the Mesomorphic Free-Radical Compound Bis(octakis(dodecyloxy)phthalocyaninato)lutetium(III) Anick M. van de Craats,† John M. Warman,*,† Hiroshi Hasebe,‡ Rie Naito,‡ and Kazuchika Ohta‡ IRI, Delft UniVersity of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands, and Department of Functional Polymer Science, Shinshu UniVersity, Ueda 386, Japan ReceiVed: July 8, 1997X
We have used the pulse-radiolysis time-resolved microwave conductivity technique (PR-TRMC) to study the charge transport properties of the free-radical sandwich complex bis(octakis(dodecyloxy)phthalocyaninato)lutetium(III), [(C12O)8Pc]2Lu. This compound displays multimesomorphic thermotropic behavior with two columnar liquid crystalline phases, Dx and Dh, between the freshly precipitated crystalline solid at room temperature, Kv, and the isotropic liquid, I, at 196 °C. In contrast with all other discotic materials previously studied using the PR-TRMC technique, the mobility of charge carriers is found to increase markedly at the transition from the Kv phase to the first columnar mesophase, Dx. The mobility displays a further substantial increase on entering the higher temperature hexagonal columnar phase, Dh. The intracolumnar mobility determined just above the Dx f Dh transition temperature of 90 °C is 0.17 × 10-4 m2/(V s), which is the highest yet determined for a columnar liquid crystalline material. The decay kinetics of the mobile charge carriers show a complex dependence on temperature with a first half-life that increases by orders of magnitude at the K f Dx and Dx f Dh transitions. The results are discussed in terms of both intramolecular and intermolecular structural changes.
Introduction Octakis(alkoxy)-substituted phthalocyanines (CnO)8PcM (M ) 2H or metal(II)) are disklike molecules consisting of a flat aromatic core and peripheral hydrocarbon side chains. Such discotic phthalocyanines self-organize into columnar stacks1 and provide a one-dimensional pathway for charge transport. The possibility of rapid, channeled conduction in such materials makes them candidates for applications as charge transport layers in xerography, electrophotography, and electronic devices. An important advance in this direction was made with the synthesis of mesomorphic, octakis(alkoxy) derivatives of the intrinsic semiconducting compound Pc2Lu(III).2 In the crystalline solid phase of (CnO)8PcM compounds the phthalocyanine macrocycles are usually tilted with respect to the stacking axis of the columns. When the hydrocarbon chains melt to give a liquid crystalline mesophase, however, horizontal stacking of the macrocycles usually occurs.3,4 Since it has been proposed that the electronic coupling between Pc units should be proportional to the area of π-π overlap between the neighboring macrocycles, such a fully eclipsed, horizontal arrangement has been predicted to be optimal for rapid intracolumnar charge migration.5,6 However, clear-cut experimental evidence for more rapid charge migration within horizontally stacked columns has not as yet been found. In the studies of Schouten et al.3,7-10 a large decrease in charge carrier mobility was in fact observed at the solid to mesophase transition of (CnO)8PcM compounds despite the change from a tilted to a horizontal columnar configuration. The decrease was, however, equally pronounced for a branchedchain derivative for which horizontal stacking was characteristic * To whom correspondence should be addressed. † Delft University of Technology. ‡ Shinshu University. X Abstract published in AdVance ACS Abstracts, October 1, 1997.
S1089-5647(97)02201-3 CCC: $14.00
of both the solid and the liquid crystalline phases. The observed decrease was therefore attributed mainly to an increase in columnar disorder and structural fluctuations that accompany the solid to mesophase transition. While the magnitude of the effect can vary, all of the discotic materials studied so far using the pulse-radiolysis time-resolved microwave conductivity technique (PR-TRMC), which includes porphyrins, perylenes, triphenylenes, and a variety of phthalocyanines, have shown a decrease in charge carrier mobility on entering a higher temperature phase. The vast majority of pure, metal-free and metal-substituted phthalocyanine derivatives are good insulators with HOMOLUMO gaps of close to 1.8 eV and dark conductivities less than 10-11 S/m.11 Rare exceptions are the free-radical complexes of lithium, PcLi, and lutetium, Pc2Lu, both of which have much lower bandgaps and relatively large dark conductivities. Values as high as 6 × 10-3 and 1 × 10-3 S/m have been reported for single-crystal and thin-film samples of the lutetium compound.12-14 A lower limit to the mobility of charge carriers in the film sample was estimated to be 1.3 × 10-4 m2/(V s), which is comparable to mobilities found in time-of-flight studies of pure, single-crystal phthalocyanine.15,16 These and other properties have led to compounds such as Pc2Lu being called “intrinsic semiconductors”.17 Several mesomorphic derivatives of Pc2Lu have been prepared by substitution of alkyl, alkoxy, alkoxymethyl, and alkoxyphenyl groups at the periphery of the Pc macrocycles.2,18-20 These compounds display a variety of thermotropic phase behavior, including often the formation of a horizontally stacked, hexagonally packed columnar liquid crystalline phase, Dh, over a limited temperature range prior to melting or decomposition. Electrical measurements on thin layers of [(C18OCH2)8Pc]2Lu and [(C12O)8Pc]2Lu gave limiting low-frequency conductivities many orders of magnitude lower than for unsubstituted Pc2Lu.12-14,18,19 This is most probably due to barriers to charge © 1997 American Chemical Society
Charge Transport in [(C12O)8Pc]2Lu carrier motion formed by domain and/or grain boundaries within the sample. In agreement with this, the conductivity did increase significantly at higher frequencies, reaching values of 0.82 × 10-7 and 1.8 × 10-7 S/m at 104 Hz for the solid, K, and Dh phases, respectively.19 For both compounds there was an indication of a slight increase in the dark conductivity at the K to Dh transition. However, this could be due to a change in the characteristics of the domain boundaries rather than to a change in the intracolumnar transport properties of charge carriers within monodomains. In an attempt to learn more about charge carrier dynamics in this interesting class of self-assembling, “semiconducting” molecular materials, we have carried out a study of the radiationinduced conductivity of [(C12O)8Pc]2Lu using the pulseradiolysis time-resolved microwave conductivity technique (PRTRMC). The results indicate an increase in charge carrier mobility to occur at the crystalline solid to mesophase transition and a further abrupt increase to occur at a transition to a higher temperature mesophase. The intracolumnar mobility attained at this transition is 0.17 × 10-4 m2/(V s), which is the highest value ever measured for a liquid crystalline material. Possible reasons for the “anomalous” behavior of the Lu derivative are discussed in terms of structural changes occurring within the material. Experimental Section To perform radiation-induced conductivity measurements, approximately 25 mg of the lutetium compound [(C12O)8Pc]2Lu was compressed into a rectangular shaped cavity of dimension 2 × 6 × 3 mm3 in a polyimide block. The block was placed in the microwave cell.3 Polyimide was used because of its high melting point and the absence of significant radiation-induced conductivity transients in this material, making PR-TRMC measurements on small samples up to at least 200 °C possible. The pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) technique is described in greater detail elsewhere.3,10,21 Briefly, the sample was ionized by pulsed irradiation with 3 MeV electrons from a Van de Graaff accelerator using pulse widths of 20 and 50 ns. The integrated beam charge per pulse, Q (in nC), was monitored routinely. The energy deposited in the sample per pulse, D, was close to uniform throughout the sample9 and equal to 550Q J/m3 measured using thin-film radiochromic dosimeters (Far West Technology Nr92). Taking an average of 25 eV for the formation of one electronhole pair, a maximum concentration of ca. 50 µM electronhole pairs is initially generated during a 50 ns, 4 A pulse (material density 1 g/cm3). This concentration corresponds to approximately 2 × 10-4 ionization events per [(C12O)8Pc]2Lu molecule. Effects due to accumulated dose (i.e.; radiation damage) on the magnitude and decay kinetics of the transient conductivity in a similar discotic phthalocyanine compound have been found to occur only for total doses orders of magnitude higher than used in the present series of pulse-radiolysis measurements.9 Changes in the conductivity on pulsed irradiation were monitored as changes in the microwave power reflected by the sample over the frequency range 26.5-37 GHz. For small changes the change in reflected microwave power is directly proportional to the radiation-induced conductivity, ∆σ, and the proportionality factor can be calculated.22 If mobile charge carriers are formed in the sample, the conductivity will increase to ∆σeop at the end of the pulse. This end-of-pulse conductivity can be normalized to the energy deposited in the sample, D, resulting in the experimental parameter ∆σeop/D.
J. Phys. Chem. B, Vol. 101, No. 45, 1997 9225
Figure 1. Primary molecular structure of the sandwichlike phthalocyanine compound, bis(octakis(dodecyloxy)phthalocyaninato)lutetium(III), [(C12O)8Pc]2Lu.
Charge carrier mobilities can be derived from dose normalized end-of-pulse conductivity values, ∆σeop/D, using the relationship7
∑µ ) (∆σeop/D)Ep/Weop
(1)
In (1), Ep is the average energy required to form one electronhole pair and Weop is the fraction of the charge carriers formed within the pulse that still survive at the end, i.e., do not rapidly decay via geminate recombination or fast localization processes. We have used the same procedure as was used previously for discotic phthalocyanines and triphenylenes3,10,23,24 to estimate values of Ep ) 25 eV and Weop ) 0.36 for [(C12O)8Pc]2Lu. As pointed out previously, the mobility values obtained using (1) are only approximate10 with the main uncertainty being associated with the value of Weop. We have estimated the error in the absolute value of ∑µ obtained using the present method to be (20%. Comparison with time-of-flight results for a discotic triphenylene derivative has shown this to be a reasonable estimate.24 For the bulk samples used in the present work no attempt was made to orient individual monodomains. The mobility value determined using (1) is therefore an effective isotropic value corresponding to random orientation of the columnar axes within microdomains. Charge transport in the present type of material is in fact expected to be highly anisotropic and to occur almost exclusively along the axis of the macrocyclic stacks.3,25 The one-dimensional, intracolumnar mobility is then related to the isotropic value ∑µ by
∑µ1D ) 3∑µ
(2)
Recently, very good agreement has been found between the mobility values determined using the present PR-TRMC approach and a time-of-flight (TOF) method for the compound hexakis(hexylthio)triphenylene, HHTT.24 By using fast time resolution and microwave probing, the PR-TRMC method is to a large extent free from effects due to domain or grain boundaries. This allows measurements to be made equally well on polycrystalline as well as liquid crystalline materials as was shown for HHTT. Results and Discussion Before presenting the conductivity measurements, the investigations carried out in order to characterize the structure of the compound studied will be discussed. Material Characterization. Bis(octakis(dodecyloxy)phthalocyaninato)lutetium(III), denoted [(C12O)8Pc]2Lu, was synthe-
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Figure 2. Absorption spectra at various temperatures of a drop-cast film of [(C12O)8Pc]2Lu for the UV/vis region and the NIR region depicted in (A) and (B), respectively.
sized according to the method of Belarbi et al.18 After purification by column chromatography, the solid was recrystallized several times from ethyl acetate. This produced dark green, polycrystalline material which is referred to subsequently as the “virgin” sample and denoted the “Kv” phase. In what follows we will present data relevant to the primary molecular structure and the higher order structures of the different phases of the material. Primary Molecular Structure. The primary molecular structure of the central Pc2Lu unit in the Kv phase of [(C12O)8Pc]2Lu is expected to be similar to that found for unsubstituted Pc2Lu by single-crystal X-ray diffraction, as illustrated in Figure 1.26 The lutetium atom is sandwiched between two Pc macrocycles to which it is 8-fold-coordinated via the (deprotonated) isoindole nitrogens. As a result, the Pc units, which are 45° staggered with respect to each other, are distorted from their usual planarity and are concave with a central interdisk distance of 2.7 Å compared with the normal cofacial distance between Pc units of 3.3 ( 0.1 Å.3 Theoretical treatments suggest that individual Pc2Lu units can be considered nominally as Lu3+Pc2•3-.27,28 The presence of one unpaired electron per molecular unit has been proven by ESR measurements.2,13,14,29 Those measurements also showed that the free electron is associated with the extensive π-systems of the Pc macrocycles. There remains some discussion as to whether the unpaired electron is localized on a single Pc macrocycle at any given time, i.e., as Pc2-Lu3+Pc•-,27 or can be considered to be completely delocalized between the two macrocycles of the Pc2 sandwich pair.28 It has been suggested in fact that one Pc macrocycle is slightly more distorted than the other.26 The UV/vis and NIR absorption spectra of a thin layer of the present compound were very similar to those obtained for Pc2Lu,27,28 including the bands at 460 nm and ca. 1400 nm which are characteristic of the free-radical species as shown in Figure 2. Phase Transitions and Secondary Structures. The mesomorphism of [(C12O)8Pc]2Lu and other peripherally substituted Pc2Lu derivatives has been well demonstrated previously.18-20 In the present work we have used differential scanning calorimetry (DSC), X-ray diffraction, polarization microscopy (PM), and absorption spectrophotometry in an attempt to characterize the structures of the various phases of [(C12O)8Pc]2Lu. For this purpose samples from the same batch and with an as similar as possible thermal history to that used in the PR-TRMC experiments were studied.
Figure 3. DSC (differential scanning calorimetry) results for the first heating, cooling, and second heating runs of the freshly precipitated sample [(C12O)8Pc]2Lu.
Figure 4. Phase transition temperatures and corresponding enthalpy changes for the [(C12O)8Pc]2Lu complex as determined by DSC.
DSC measurements were performed on a 2 mg sample of the virgin material with a scanning rate of 10 °C/min over the temperature range from 0 to 210 °C. Results for the first heating, first cooling, and second heating runs are shown in Figure 3. The phase transition temperatures observed and the corresponding enthalpy changes are given in the scheme in Figure 4. Previous DSC measurements for a second heating reported18 transitions at 42, 85, and 189 °C only slightly lower than the present findings on a second heating of 45, 90, and 196 °C.
Charge Transport in [(C12O)8Pc]2Lu
J. Phys. Chem. B, Vol. 101, No. 45, 1997 9227 TABLE 1: Intercolumnar and Intracolumnar Distances, D and c, and the Corresponding Density G Derived from X-ray Results for the Dh Mesophase of [(C12O)8Pc]2Lu at Various Temperatures
a
Figure 5. X-ray diffraction patterns of [(C12O)8Pc]2Lu at various temperatures recorded during the first heating of the virgin sample from RT. (A) The pattern of the K2v phase at 28 °C, (B) the pattern of the mesophase denoted Dx at 67 °C, and (C) the patterns of the Dh phase for increasing temperature. Lower traces are offset for clarity.
On heating the virgin sample from 0 °C, a small transition is observed at 18 °C. This is, however, associated with only a small enthalpy change (∆H ) 2.8 kJ/mol) and is assigned to a slight structural change within the crystalline solid, denoted K1v f K2v. A second transition occurs at 61 °C with a very large ∆H of 202 kJ/mol. This value is close to twice the value of 120 kJ/mol found for the solid to liquid crystal transition of metal-free octakis(dodecyloxy)phthalocyanine ((C12O)8PcH2).3 In the latter case the large ∆H value is known from NMR and X-ray diffraction studies to be due to complete melting of the alkyl chains. There is little doubt therefore that melting of the peripheral alkyl chains is also the underlying cause of the 61 °C transition for the [(C12O)8Pc]2Lu compound, indicating that the transition is to a liquid crystalline phase. This is supported by the X-ray and polarization microscopy results which are discussed in more detail below. A third peak in the heating curve is observed to occur at 90 °C. While the associated ∆H value of 70 kJ/mol is substantially smaller than that found at 61 °C, it is still quite large and indicates a significant structural change and possibly a second melting stage. No further DSC peaks are observed up to the eventual transition to the isotropic liquid at 196 °C for which ∆H ) 5.9 kJ/mol. The X-ray diffraction patterns taken at several temperatures during a first heating trajectory are shown in Figure 5. We begin with the results above 90 °C for the second mesophase since these are the most readily interpreted. Thus, all of the
temp (°C)
D (Å)
c (Å)
F (g/cm3)
94 100 115 130 151 170 180 190
35.0 35.0 35.1 34.9 33.4 32.8 32.6 32.4
3.26 3.27 3.30 3.33 (3.58)a (3.71)a (3.76)a (3.80)a
1.00 0.99 0.98 0.98 (1.00) (1.00) (1.00) (1.00)
Based on the measured D and a constant density of 1.00 g/cm3.
narrow-angle reflections observed can be explained by a twodimensional hexagonal lattice with one molecule per unit cell and an intermolecular distance, D, of 35.0 Å. The broad feature centered at 4.6 Å is typical of molten alkyl chains, and the 3.3 Å reflection, which is clearly visible for temperatures just above the 90 °C transition, is the characteristic cofacial contact distance between aromatic moieties. There is little doubt, therefore, that this phase consists of a two-dimensional hexagonal array of columns in which the planes of the macrocyclic Pc cores are orthogonal to the columnar axes. We denote it accordingly Dh in agreement with a previous assignment.18 Of particular interest in this Dh phase is the apparent equivalence of all Pc macrocycles and the lack of a reflection corresponding to a distance of ca. 6.6 Å, which would have been expected for orthogonal stacking of [(C12O)8Pc]2Lu molecular units. It would appear that the Pc macrocycles have adopted their usual planar geometry and that the distance between neighboring Pc units is constant and uninfluenced by the presence or absence of a lutetium ion between alternate pairs. This lack of correlation of the spacing of the Pc macrocycles with the position of the lutetium ion was pointed out previously by other workers.18 The change in the cofacial structure from the concave sandwich arrangement found for crystalline Pc2Lu26 and thought to prevail in the Kv phase of the present compound could explain the much broader optical absorption in the 1400 nm region in the Dh phase shown in Figure 2. This absorption band is assigned to a z-axis-polarized, inter-Pc charge-transfer transition which would be expected to be particularly sensitive to the geometrical relationship between the two macrocycles. As the temperature is raised within the Dh phase, the X-ray diffraction pattern becomes less well-defined, as shown in Figure 5C. For temperatures of ca. 150 °C and higher the feature at approximately 3.4 Å is in fact no longer discernible above the noise level. Evidence that an orthogonally stacked, hexagonally packed columnar structure is retained is, however, still to be found in the narrow-angle reflections. The temperature dependence of the intercolumnar and intracolumnar distances, D and c, respectively, are listed in Table 1. For the highest temperatures the value of c was derived from the measured D values, assuming a constant density of 1.0 g/cm3. The measurements indicate a substantial decrease in the intercolumnar spacing and a corresponding increase in the intracolumnar distance from 3.26 to 3.80 Å just prior to complete melting. Clearly, there is a gradual change from a well-ordered columnar structure at close to 90 °C to an increasingly disordered columnar structure as the temperature is raised. This increase in disorder is found to have dramatic effects on both the charge carrier mobility and the decay kinetics of the PR-TRMC transients. We turn now to the lower temperature mesophase below 90 °C which we denote Dx. The X-ray diffraction pattern associated with this phase is shown in Figure 5B, and the main reflections are listed in Table 2. The diffraction pattern depicted
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TABLE 2: X-ray Distances for the Most Prominent Reflections Found for the Dx Mesophase of [(C12O)8Pc]2Lu at 67 °C with the Calculated Values and Miller Indices (hk) for a Two-Dimensional Hexagonal Lattice Structure with Z ) 1 distance (Å) observed (int)a
calculatedb
Miller indices (hk)
29.1 (s) 18.5 (s) 16.8 (w) 14.8 (w) 11.0 (m) 9.35 (w) 8.96 (w) 8.09 (w)
29.1
10, 11
16.8 14.5 11.0 9.7 8.4 8.1 7.3 6.7 6.3 5.8
21 22, 20 31, 32 30, 33 42 41, 43 40, 44 52, 53 51, 54 50, 55
6.14 (w) 5.89 (w) 4.74 (w) 4.6 (b) 4.07 (m) 3.46 (w)
a s ) strong, m ) medium, w ) weak, b ) broad. b Based on a Z ) 1 hexagonal lattice with D ) 33.5 Å.
in Figure 5B remains the same for the whole temperature region of 61-90 °C for the Dx phase. Also listed in Table 2 are the first 11 reflections expected for a Z ) 1, two-dimensional hexagonal lattice with D ) 33.5 Å. As can be seen, many of the reflections observed are reproduced quite well by the calculated values with the only major exception being the strong reflection at 18.5 Å. Additional, strong support for a 2-D hexagonal lattice in the Dx phase is provided by the polarization micrograph of this material in Figure 6A which shows abundant evidence of 60° angular intersections. The reflections below 5 Å in the Dx phase contain a broad feature centered at ca. 4.6 Å which can be readily assigned to the molten saturated hydrocarbon chains. The sharp 3.46 and 4.07 Å reflections are both potential candidates for the average intracolumnar distance between Pc macrocycles, i.e., a 002 reflection from columnarly stacked [(C12O)8Pc]2Lu units. The more prominent 4.07 Å reflection would have been the logical choice for the intracolumnar Pc-Pc distance were it not for the fact that this results in a density of only 0.87 g/cm3 when it is combined with the value of D ) 33.5 Å given above. This is much lower than the density derived for the Dh phase for which the structural characterization is clear-cut. An intracolumnar Pc-Pc distance of 3.46 Å leads on the other hand to a more reasonable density of 1.02 g/cm3, which is close to that determined for the Dh phase and similar to that found for monomeric alkoxy-substituted phthalocyanines.3 On the basis of this and the substantial evidence for a hexagonal columnar packing, we tentatively conclude that the [(C12O)8Pc]2Lu units are also horizontally stacked in the Dx phase but with a slightly larger average distance between the Pc macrocycles than in the Dh phase. We have no definitive explanation of the prominent 4.07 and 18.5 Å reflections. The former may result from alkyl chain regions that are not fully molten in Dx as has been previously suggested by others.2 As mentioned above, partial chain melting in Dx could also explain the relatively large enthalpy change associated with the Dx f Dh transition. The strong 18.5 Å reflection is thought to possibly be due to a periodic, helical superlattice structure within the columnar stacks. A further insight into the structure of the Dx phase is given by the change in the optical absorption spectrum found on heating a drop-cast film which is shown in Figure 2. At the Kv to Dx transition the intensity of the absorption decreases only
Figure 6. (A) Photomicrographs of the Dx mesophase (upper photo) and (B and C) of the Dh mesophase of [(C12O)8Pc]2Lu (two lower photos).
slightly, and the main spectral features remain almost unchanged. Particularly important is the lack of a pronounced change in the NIR band at 1400 nm which is assigned to a z-polarized interligand electron-transfer transition.28 The concave sandwich structure of the [(C12O)8Pc]2Lu units would therefore appear to remain intact in Dx. At the Dx f Dh transition large changes in the intensity of the absorption in the UV/vis region occur, and the NIR band absorption, while not significantly decreased in magnitude, becomes much broader and slightly red-shifted. We attribute the effect on the NIR band to a change from the sandwich structure with concave Pc macrocycles, characteristic of Pc2Lu and the lower temperature phases of [(C12O)8Pc]2Lu, to a structure in which the Pc macrocycles have adopted their more usual planar configuration. This would be in accord with the equivalence of the Pc units which is apparent from the X-ray analysis of the Dh phase. The considerable overall decrease in intensity in the UV/vis region can be explained by a change from isotropically oriented domains in the Dx phase to a preferred orientation of the columnar axes orthogonal to the glass substrate in Dh. Since
Charge Transport in [(C12O)8Pc]2Lu all of the UV/vis transition moments are x,y-polarized, i.e., in the plane of the Pc macrocycles,28 this concerted orientation would considerably decrease their absorption coefficients. The z-polarized NIR band would be much less affected by this preferred orientation. This explanation is substantiated by the photomicrographs of the Dh phase in Figure 6B,C. These show mainly homeotropically aligned domains interspersed with “coffee-bean” features characteristic of a hexagonally ordered structure. The fully crystalline nature of the Kv phase below 61 °C is illustrated in Figure 5A by the very sharp, multiline X-ray diffraction pattern found at 28 °C. A full analysis of this pattern has not been attempted. The absence of a pronounced reflection corresponding to ca. 3.3 Å is, however, taken as an indication that the Pc2Lu units in the Kv phase are tilted with respect to their stacking axis as is found for unsubstituted, crystalline Pc2Lu and for monomeric octakis(alkoxy)phthalocyanines in their crystalline phase.3,10,26 The pronounced X-ray reflections at 7.58 and 3.81 Å are thought to correspond to 001 and 002 intracolumnar reflections. For Pc2Lu the intracolumnar distance has been found to be 8.05 Å, i.e., somewhat larger than the 7.58 Å reflection found for the present compound.26 DSC measurements made during cooling from the melt are shown in Figure 3. After the relatively small enthalpy change at the I f Dh transition at 196 °C the next phase change is found at 80 °C. This is only 10 °C lower than the temperature assigned to the Dx f Dh transition found on the first heating, and the absolute magnitude of the enthalpy change is similar as is shown in the scheme of Figure 4. This is therefore assigned to the reverse, Dh f Dx, phase change. The hysteresis is considerably smaller than the ca. 25 °C commonly found for the Dh T K transition of monomeric octakis(alkoxy)phthalocyanines, in agreement with the less profound structural rearrangement expected at the mesophase to mesophase transition of the present compound. On further cooling a transition is observed at 35 °C with an enthalpy change which is relatively large at 108 kJ/mol but still only approximately half of the enthalpy of the K2v f Dx transition observed at 61 °C on the first heating. We assign this to a transition to a solid phase with a somewhat less wellorganized crystalline structure than that formed on precipitation from solution. Since this second solid phase is obtained on cooling from the isotropic liquid, we denote it Ki. On subsequent heating and cooling the Ki f Dx transition is found to be reversible with a hysteresis of approximately 10 °C as is also found for the Dx f Dh phase change. The above structural considerations will be discussed further in a subsequent section in light of the charge carrier mobility results obtained. Conductivity Measurements. On pulsed irradiation readily measurable changes in conductivity were observed for all phases of the sandwichlike phthalocyanine complex [(C12O)8Pc]2Lu. In Figure 7 typical conductivity transients are shown as measured at temperatures corresponding to the K2v, Dx, Dh, and I phases during the first heating run of the virgin sample. The one-dimensional charge carrier mobility sum, ∑µ1D, was calculated from the end-of-pulse conductivity per unit dose, ∆σeop/D, for such transients. At longer times the conductivity decays back to zero due to charge carrier recombination and localization. This aspect of the results will be considered separately after first discussing the relationship between ∑µ1D and the structural measurements. Charge Carrier Mobility. The mobility is plotted as a function of temperature for the first heating run of the virgin sample in Figure 8A. At the K1v f K2v transition only a very
J. Phys. Chem. B, Vol. 101, No. 45, 1997 9229
Figure 7. Conductivity transients (ns to ms) of the four different phases K2v, Dx, Dh, and I found on heating the virgin sample of [(C12O)8Pc]2Lu.
Figure 8. (A) One-dimensional mobilities and (B) first half-lives of the charge carriers formed in [(C12O)8Pc]2Lu on pulsed irradiation during the first heating trajectory of a virgin sample. The vertical dashed lines correspond to the temperatures at which transitions are observed by DSC on the first heating trajectory.
small increase is detected. Above this transition ∑µ1D increases gradually up to approximately 60 °C, where it undergoes an abrupt enhancement associated with the solid to mesophase transition, K2v f Dx. A further sudden increase by a factor of 3 is observed at close to 90 °C, the temperature corresponding to the Dx f Dh transition. Above 90 °C ∑µ1D gradually decreases until at the clearing point of 196 °C an abrupt decrease by a factor of approximately 2 occurs. The most remarkable aspect of the temperature dependence as presented in Figure 8A is the considerable increase in the mobility observed on entering the higher temperature Dx and Dh phases. This is particularly unusual for a transition from a rigid crystalline solid to a more disordered liquid crystalline
9230 J. Phys. Chem. B, Vol. 101, No. 45, 1997 phase. In all other discotic materials studied with the PR-TRMC technique, including triphenylenes,23,24 porphyrins,8 and other alkoxy-substituted phthalocyanines,10 the conductivity has been found to decrease, often substantially, at K f D transitions even when these involve a change from a tilted to horizontal stacking of the macrocycles. This has been attributed to the net negative effect on the intracolumnar charge carrier mobility of the thermally induced columnar fluctuations which become possible when the alkyl side chains melt.3, 10 The present, anomalous results are undoubtedly in part due to an unusually low charge carrier mobility in the crystalline solid phase of [(C12O)8Pc]2Lu. The value of ∑µ1D ≈ 2 × 10-6 m2/(V s) is in fact more than 1 order of magnitude lower than for the K phase of other alkoxyphthalocyanines.3,10 We attribute this low mobility to the sandwich structure of the individual [(C12O)8Pc]2Lu units in the crystalline phase. This results in a relatively large distance of ca. 8 Å between identical sites in the tilted molecular stacks. The concavity of the Pc macrocycles will also result in a reduction in the π-π overlap between adjacent [(C12O)8Pc]2Lu units. Both effects would tend to decrease the electronic coupling within the columns, reduce the rate of charge migration, and induce localization at individual [(C12O)8Pc]2Lu units. The gradual increase in the mobility with temperature in the crystalline phases is in agreement with shallowly localized charges whose site-to-site transport is thermally activated. The abrupt increase in mobility at the transition to the first mesophase can be attributed in part to a change from tilted to horizontal stacking of the molecular cores. This results in a decrease in the site-to-site jump distance and an increase in the overlap between adjacent Pc macrocycles. Both changes would be expected to have a positive influence on charge transport. In monomeric Pc derivatives the positive effects of such changes at the K f Dh transition are found to be outweighed by the increased disorder in the columnar structure which results in a net decrease in charge mobility.3,10 This negative effect of columnar disorder may, however, be considerably less for the Dx phase of [(C12O)8Pc]2Lu which appears to be more rigid than a normal Dh phase possibly due to incomplete melting of the peripheral hydrocarbon chains as mentioned in the previous section. The further abrupt increase in mobility at the Dx f Dh transition is particularly interesting since this is, to our knowledge, the first observation of such an effect for a mesophase to (higher temperature)mesophase transition. In the case of HHTT, for example, the transition from the more rigid helical discotic mesophase, termed H, to the higher temperature Dh phase is accompanied by a large decrease in mobility.24,30 As pointed out above, such an effect would be expected in view of the increased columnar disorder. In addition to the anomalous increase at the Dx to Dh phase found for the present compound, the absolute magnitude of the mobility just above the Dx f Dh transition, ∑µ1D, is surprising since it is more than a factor of 3 larger than that determined for the Dh phase of the monomeric compound (C12O)8PcH2 at the same temperature.3,10 We consider that these anomalous effects are related to the free-radical nature of [(C12O)8Pc]2Lu and the structural change which accompanies the Dx f Dh transition. In particular, the apparent change from a concave sandwich structure to a structure in which the Pc macrocycles are planar and equidistant within the columnar stacks could result in delocalization of the radical site not just between the adjacent Pc macrocycles within a given [(C12O)8Pc]2Lu complex but also between the Pc macrocycles of neighboring complexes. The resulting increase in intracolumnar binding over and above that due to electrostatic
van de Craats et al.
Figure 9. (A) One-dimensional mobilities and (B) first half-lives of the charge carriers formed in [(C12O)8Pc]2Lu on pulsed irradiation during the cooling and second heating trajectory. The vertical dashed lines correspond to the temperatures at which transitions are observed by DSC on the second heating trajectory.
and π-π interactions could result in an increase in the rigidity and intracolumnar order within the cores of the columnar stacks compared with that prevailing in the lower temperature phases or the nonradical monomeric analogues. Evidence for the delocalization of the radical site is provided by the considerable broadening and red-shift of the absorption spectrum in the Dh phase. It should be mentioned, however, that the ESR freeradical line width which should be affected by spin delocalization has been found to be unchanged on entering the Dh phase.2 The absolute value of 0.17 × 10-4 m2/(V s) for ∑µ1D in the Dh phase of [(C12O)8Pc]2Lu, just above the Dx f Dh transition temperature, is the largest mobility value yet determined for a liquid crystalline material, a factor of 2 larger than the largest value previously reported of 0.08 × 10-4 m2/(V s) for the helical columnar H phase of hexakis(hexylthio)triphenylene, HHTT.24 The subsequent decrease in ∑µ1D within the Dh mesophase on further heating can be attributed to increasing intracolumnar disorder as evidenced in the X-ray data shown in Figure 5C, which has a negative influence on the charge carrier mobility. In the isotropic liquid phase, I, the conductivity signal is small but still comparable to that of the organized phases. This indicates that in the I phase some ordering still exists with small aggregates providing short pathways for rapid charge carrier migration. On cooling from the melt, ∑µ1D increases abruptly at the I f Dh transition temperature (see Figure 9A). As was found with DSC, the I f Dh transition shows no hysteresis. The increase is, however, to a ∑µ1D value smaller than that found immediately prior to melting on the first heating trajectory. A gradual increase in ∑µ1D occurs on further cooling, mirroring the changes of the first heating. The absolute value of ∑µ1D, however, remains a factor of 2-3 lower than that found initially. We conclude that the material reverts only partially to an extensive hexagonal lattice when cooled from the melt. A substantial fraction of the sample would appear to remain disordered. A sudden decrease in ∑µ1D on further cooling is
Charge Transport in [(C12O)8Pc]2Lu observed at a temperature ca. 10 °C lower than that found for the Dx f Dh transition in the first heating trajectory. We associate this therefore with the reverse Dx f Dh mesophase to mesophase transition. Again in the Dx phase the value of ∑µ1D is approximately a factor of 2 less than found in Dx in the first heating trajectory, indicating either greater columnar disorder or a large fraction of disorganized domains. On further cooling from 75 to -50 °C no significant change in ∑µ1D is observed. The actual ∑µ1D values in the lowtemperature region found on cooling are larger than the values found for the virgin polycrystalline material. We conclude that there is no reversion to a phase with characteristics similar to those of the Kv phase. The fact that no indication of an abrupt change in the mobility is found at 35 °C despite the observation of a relatively large DSC peak is not completely understood. It would appear either that the structure and rigidity of the columnar cores remain unchanged when the alkyl chains “freeze” to form the Ki phase or that the Dx phase supercools under the conditions of the conductivity measurement. Decay Kinetics. The after-pulse decay kinetics of the mobile charge carriers formed display an even more dramatic dependence on temperature than the mobility values as determined from the end-of-pulse magnitudes of the transients. As can be seen from the transients in Figure 7, both the form and the time scale of the decays change markedly from one phase to the other. We have not carried out a full analysis of the decay kinetics and discuss here only the changes occurring in the first halflife of the conductivity transients, t1/2. This parameter is plotted together with the mobility data in Figures 8 and 9. As can be seen, during the first heating, the half-life increases from a value of only ca. 100 ns in the Kv phase to microseconds in Dx and as long as 0.3 ms initially in the Dh phase. The lifetime in the Dh phase decreases by almost 2 orders of magnitude on further heating before undergoing a sudden decrease by an order of magnitude at the Dh f I transition. Interestingly, the t1/2 cooling trajectory follows closely that of the first heating down to 60 °C whereas the mobility values for the cooling trajectory were a factor of 2-3 lower. Since the decay kinetics may be considered to be an intrinsic property of the types of domains being probed within the sample, we conclude that these are probably the same type of organized domains as in the first heating trajectory. The lower effective mobility values must therefore result from the fact that on cooling from the isotropic liquid only part of the sample reverts to well-organized domains with the remaining fraction being relatively amorphous and not supportive of high charge carrier mobilities. As for the mobility values, t1/2 does not return to the value for the freshly precipitated Kv phase on cooling to temperatures below 60 °C. The half-life in fact remains close to the value of 1 µs found for the Dx phase on the first heating, even down to -50 °C. The fact that no abrupt change in the lifetime is found at approximately 35 °C where a pronounced DSC peak is observed is taken to indicate that the Dx phase does in fact supercool under the conditions of the TRMC measurements. This could be due to either the different rates of cooling in the two measurements or the considerable difference in the size of the samples. While the sudden increases in mobility on entering the higher temperature mesophases of the present compound are anomalous, the increase in lifetime has been observed before for monomeric phthalocyanine derivatives. It is attributed to a substantial decrease in the rate of charge carrier recombination via tunneling between columns when the intervening saturated hydrocarbon mantle becomes liquidlike.31 The magnitude of
J. Phys. Chem. B, Vol. 101, No. 45, 1997 9231 the effect for [(C12O)8Pc]2Lu is, however, much larger than has previously been found. In particular, the dramatic change in both the form and the time scale of the decay at the Dx to Dh transition is difficult to understand in terms only of the change in the intercolumnar distance from 33.5 to 35.0 Å. This could be taken as further evidence that the aliphatic chains remain partially frozen in the Dx phase. The dramatic decrease in the lifetime of the conductivity transients that occurs in the Dh phase on heating is considered to result from an increasing rate of molecular exchange between columns. Charge recombination can then occur via intercolumnar diffusion of molecular ions more rapidly than by intercolumnar long-distance electron tunneling which is required if intercolumnar exchange cannot occur. The even more rapid rate of charge recombination in the isotropic liquid can also be ascribed to the increase in the rate of molecular diffusion. Conclusions The free-radical compound [(C12O)8Pc]2Lu displays complex thermotropic mesogenic behavior with two discotic liquid crystalline phases Dx and Dh. In both phases the Pc macrocycles are horizontally stacked in columns with average intracolumnar Pc-Pc distances of 3.46 and 3.26 Å, respectively. In the lower temperature Dx phase the [(C12O)8Pc]2Lu units retain a sandwich structure in which the Pc units are distorted from their normal planar geometry as found for Pc2Lu. There is evidence that the transition from the freshly precipitated crystalline solid Kv phase to the Dx mesophase involves only partial “melting” of the alkyl chains with “melting” being completed at the Dx to Dh transition. In the Dh phase the Pc macrocycles adopt a planar geometry and are equidistant within the columnar stacks. The one-dimensional charge carrier mobility increases anomalously at the Kv f Dx and Dx f Dh transitions, reaching a maximum value of 0.17 × 10-4 m2/(V s) in the Dh phase, which is the highest value yet found for a discotic liquid crystalline material using PR-TRMC. The first increase is attributed to a change from tilted to orthogonal stacking while retaining the sandwich structure of the individual [(C12O)8Pc]2Lu units. The second increase is attributed to the equivalence of the Pc macrocycles within the columns which leads to delocalization of the radical site between neighboring [(C12O)8Pc]2Lu units and results in additional intracore binding over and above the electrostatic and π-π interactions normally responsible for columnar self-aggregation. The dramatic changes in the lifetime of the radiation-induced conductivity transients found are attributed mainly to the negative effects of alkyl chain melting on the rate of intercolumnar electron tunneling and, at higher temperatures, to the eventual occurrence of charge recombination preferably via intercolumnar diffusion of molecular ions. Acknowledgment. K.O. thanks the Japanese Ministry of Education, Science and Culture for financing a stay of five months in The Netherlands. Dr. ir. G. Hakvoort (Delft University of Technology) is thanked for his assistance with the DSC measurements. References and Notes (1) Chandrasekhar, S. Liq. Cryst. 1993, 14, 3. (2) Piechocki, C.; Simon, J.; Andre´, J.-J.; Guillon, D.; Petit, P.; Skoulios, A.; Weber, P. Chem. Phys. Lett. 1985, 122, 124. (3) Schouten, P. G. Charge Carrier Dynamics in Pulse-Irradiated Columnar Aggregates of Mesomorphic Porphyrins and Phthalocyanines. Ph.D. Thesis, Delft University of Technology, 1994. (4) Ohta, K.; Jacquemin, L.; Sirlin, C.; Bosio, L.; Simon, J. New J. Chem. 1988, 12, 751.
9232 J. Phys. Chem. B, Vol. 101, No. 45, 1997 (5) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525. (6) Schultz, H.; Lehmann, H.; Rein, M.; Hanack, M. Struct. Bonding 1991, 74, 41. (7) Schouten, P. G.; Warman, J. M.; De Haas, M. P.; Van der Pol, J. F.; Zwikker, J. W. J. Am. Chem. Soc. 1992, 114, 9028. (8) Schouten, P. G.; Warman, J. M.; De Haas, M. P.; Fox, M.-A.; Pan, H.-L. Nature 1991, 353, 736. (9) Schouten, P. G.; Warman, J. M.; De Haas, M. P. J. Phys. Chem. 1993, 97, 9863. (10) Schouten, P. G.; Warman, J. M.; De Haas, M. P.; Van Nostrum, C. F.; Gelinck, G. H.; Nolte, R. J. M.; Copyn, M. J.; Zwikker, J. W.; Engel, M. K.; Hanack, M.; Chang, Y. H.; Ford, W. T. J. Am. Chem. Soc. 1994, 116, 6880. (11) Simon, J.; Andre´, J.-J. Molecular Semiconductors; Springer: Berlin, 1985. (12) Andre´, J.-J.; Holczer, K.; Petit, P.; Riou, M.-T.; Clarisse, C.; Even, R.; Fourmigue, M. Chem. Phys. Lett. 1985, 115, 463. (13) Turek, P.; Petit, P.; Andre´, J.-J.; Simon, J.; Even, R.; Boudjema, B.; Guillaud, G.; Maitrot, M. J. Am. Chem. Soc. 1987, 109, 5119. (14) Petit, P.; Holczer, K.; Andre´, J.-J. J. Phys. (Paris) 1987, 48, 1363. (15) Cox, G. A.; Knight, P. C. J. Phys. C: Solid State Phys. 1974, 7, 146. (16) Usov, N. N.; Benderskii, V. A. Phys. Status Solidi 1970, B37, 535. (17) Maitrot, M.; Guillaud, G.; Boudjema, B.; Andre´, J.-J.; Strzelecka, H.; Simon, J.; Even, R. Chem. Phys. Lett. 1987, 133, 59.
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