J. Phys. Chem. 1993,97, 9863-9870
9863
Effect of Accumulated Radiation Dose on Pulse Radiolysis Conductivity Transients in a Mesomorphic Octa-&alkoxy-Substituted Phthalocyanine Pieter G. Schouten,’ John M. Warman, and Matthijs P. de Haas Radiation Chemistry Department, IRI, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands Received: May 27, 1993”
The transient conductivity induced by nanosecond pulse radiolysis of octa-n-nonyloxyphthalocyaninehas been studied as a function of accumulated radiation dose, ED, from 10 Gy to 250 kGy. The end-of-pulse conductivity, AamplD,is unaffected by ED up to 150 kGy. Above this, AaWp/Dbegins to decrease and at 250 kGy is 65% of the low-dose value. An increase in the rate of decay of the transient conductivity in the solid state can be observed at values of ED as low as 10 kGy. On heating to the liquid crystalline phase and resolidifying, the conductivity decay returns to that found for the virgin material. However, Aacpp/Dremains low. The solid to mesophase transition temperature, as monitored by differential scanning calorimetry, was found to be a few degrees lower after a total accumulated dose of 260 kGy.
Introduction Of the numerous different interests in phthalocyanine (Pc) derivatives,’J much research activity has been focused on their semiconducting properties. The stacked aromatic macrocycles provide a one-dimensional path for charge transport via the overlapping ?r-orbitals. Since 1982 it has been known that when long aliphatic chains are substituted at the periphery of the Pc molecule, thermotropic liquid crystalline materials are obtained.”O X-ray diffraction measurements have shown that the Pc molecules are columnarly stacked in both the crystalline and the liquid crystallinephase even though for the latter the hydrocarbon chains have melted. In pure Pc materials only an extremely small intrinsic concentration of charge carriers will exist because of the 2-eV gap between the highest occupied and lowest unoccupied molecular orbitals.ll Therefore, charge carriers are generally produced by chemical or electrochemical doping.12 However, high dopant concentrations,on the order of 10 mol %, are usually used, which may cause significant changes in the phase transition characteristics of the materials and even more importantly may affect changes in themolecular ordering.13-17 Materials containing high concentrationsof doping agents may therefore have very different physical properties, in particular with respect to the dynamics of charge carriers, compared to the unperturbed material. Use can be made of the ionizing power of high-energy radiation to produce charge carriers in pure materials at concentrationsof approximately 10 pM or less by means of pulse radiolysis.lS In our group we monitor changes in the microwave conductivity of a material when irradiated by a nanosecond pulse of 3-MeV electrons using the time-resolved microwave conductivity (TRMC) technique.l&ZO The radiation-induced conductivity in alkylsubstituted phthalocyanines and porphyrins has been attributed to the high one-dimensional mobility of electrons and/or holes along the stacking axis of the macrocyclic columns.21-23 In former pulse-radiolysis TRMC experimentsno effects were observed during the course of the measurements, which could be ascribed to degradation of the sample due to the accumulated irradiation dose. In this paper we report a study in which we have deliberately increased the total dose given to an octaalkoxy phthalocyanine material far in excess of that normally applied in the pulse experiments to see at what dose effects on the conductive properties are found. There is to date little information on radiation damage to phthalocyanine derivatives to be found 9
Abstract published in Aduance ACS Abstracts, August 15, 1993.
0022-3654/93/2097-9863$04.00/0
in the literaturez4and none on the recently synthesized mesomorphic compounds as studied in the present work.
Experimental Section Figure 1 shows the molecular structure of the nonyloxysubstituted Pc studied in this work. In the solid phase the molecules self-organizeintocolumnar stackswith the macrocycles tilted with respect to the stacking axis.s When heated, a phase transition occurs at 107 OC as seen by differential scanning calorimetry (DSC); see Figure 2. This is due to a transition from the solid to the liquid crystalline phase (also termed mesophase) in which the hydrocarbon chains have completely melted.* In this state the Pc macrocycles are still stacked, but now horizontal with respect to the stacking axis. On recooling, the mesophase to solid phase transition is found to occur at 67 OC. When the material is heated to 300 O C it decomposes. The sample was contained in a microwave cell which consisted of a 14-mm length of rectangular, 7.1 X 3.55 mmz, gold-plated, copper waveguide closed at one end with a metal plate and flanged at the other, as shown in Figure 3. A 25-mg sample of the freshly prepared Pc compound was compressed into a rectangular shaped cavity of 2 X 6 X 3 mm3 (length X width X depth) dimension in a perspex block. When this perspex block is put in the waveguide cell, the sample is situated at the center as is also shown in Figure 3. In the pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) measurements the cell was irradiated with single 10-ns pulses of 3-MeV electrons from a Van de Graaff accelerator. The copper top wall of the cell was reduced to 0.39-mm thickness to minimize the attenuation of the electronbeam. The integrated beam charge in the pulse was routinely measured. Any change in conductivity of the sample after the pulse was measured as a reduction in the microwave power reflected by the sample using TRMC. For small changes this absorption is directly proportional to the conductivity change, Au. The quantitativerelation between the microwave loss and Au was determined using computational and data-fitting procedures described previously.l8-z0 The effect of accumulated dose on the radiation-induced conductivity signal was investigated in the following way: a TRMC transient was monitored for the fresh sample with no prior irradiation history using a small number of single-shot, low-dose (ca. 10 Gy) pulses. The sample was then heated to above the solid-to-mesophase transition temperature at which a few TRMC transients were monitored. The sample was then recooled to room temperature and monitored again. The dose 0 1993 American Chemical Society
9864 The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
Schouten et al.
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Figure 1. Primary, secondary, and tertiary structure of the peripherally octa-n-nonyloxy-substitutedphthalocyanine, Pc9. 2000
TABLE I: History of the Irradiation Procedure T ether with the Associated Accumulated Irradiation Dose, ?D temp (“C) comments CD (kGY)
1500
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1000 A
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140
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Figure 2. Differential scanning calorimetry cycles for the freshly precipitated sample (solid line) and after an accumulated dose of 260 kGy (dashed line).
Electron beam
W = perspex block 20x7.1x3.55 mm
fl -y, L::iG3
= sample cavity 2 x 6 ~ 3mm
Figure 3. Schematic representation of the TRMC detection cell.
in the sample was hereafter increased in larger steps of approximately 12 kGy by irradiating the cell with typically 1000 pulses of 5-11s duration. After each such pulse train a TRMC transient was taken. The total beam charge from both the TRMC measurement pulses and intervening, high-dose pulse trains was continuously summed. This was then used to derive the total accumulated dose, ED, corresponding to the point just prior to a TRMC transient measurement. The temperatureof the sample
~~~
fresh sample in the mesophase solid after cooling increasing dose in the mesophase solid after cooling after 16 h
~
0 2.3 3.8 3.8-241 25 1 254 254
was also changed at certain periods in the radiation cycle. The history of these changes together with the associated value of ED is presented in Table I. The electron beam profile at the position of the microwave cell was measured by directing the beam onto a coaxial microtarget which consisted of an insulated 0.4-mm-diameter copper wire. The beam was swept across the target using magnetic deflection coils, and the current density on the target was measured using a Tektronix 7623 oscilloscope with a 7S11 sampling unit with a S4 plug-in. The profile is shown in Figure 4 together with the cell dimensions and the position of the Pc sample for comparison. To measure the energy deposition of the 3-MeV electrons, use was made of Far West Technology-92 radiochromic film dosimeters. The dosimetersconsisted of 50-pm-thick foils of nylon impregnated with the radiation-sensi t ive dye hexahydroxyethylpararosaniline nitrile. On exposure to ionizing radiation the dye changes from colorless to blue.25 After irradiation the change in the optical density of the foil was measured at 510 nm, and this was converted to the amount of energy deposited per unit mass, the dose in Gy = J/kg, using a calibration curve supplied by Far West Technology. The calibration curve was verified within the range 10-100 kGy by comparing the dose determined using the radiochromicfoils with Fricke dosimetry26using a cobalt source with a dose rate of 0.46 Gy/s. The dose determined with the foils proved to be consistent within 1%. From the literature it is known that these foils have an equivalent response to yrays and high-energy electrons and that the dose measured is independent of dose rate over the range 10-1-1013 Gy/s.27 To measure the dose deposited by the 3-MeV electrons at different depths within the microwave cell, foils were alternately placed between five 0.53-mm-thick and one 0.28-mm-thick polyethylene (PE) plate of 0.947 g/cm3 density. This medium was pulseirradiated with 3-MeV electrons with a dose rate of ca. lo9Gy/s. The dose measured at various positions in the path of the electron beam is shown in Figure 5. After these series of measurements, the material was studied by differentialscanningcalorimetryon a Mettler 7 DSC apparatus to investigate whether changes in phase transition temperatures and/or accompanying enthalpy values had been induced upon irradiation.
Results Dose Distribution within the Irradiated Sample. In Figure 4 the cross-sectional geometry of the electron beam is displayed as
Radiolysis of Octa-n-alkoxy-SubstitutedPhthalocyanine
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9865
t
5
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Figure 4. Intensity profile of a 3-MeV electron beam. Also shown for comparison are the length and width of the sample in the irradiated volume of the TRMC detection cell.
radiochromic foil positioned at the interface between two media isdependenton both absorbers. At the CulPE interfacethe higher dose is probably caused by a higher concentrationof short-range secondaryelectrons ejected from the much higher electron density copper, while at the PElCu interface electron energy is backscattered into the polyethyleneby the copper. The distance over which these nonequilibrium effects are operative is assumed to bevery small compared with the depth of the cell. When therefore the values measured at the metal interfaces are omitted, the straight line in Figure 5 can be drawn yielding
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& 0.0
0.00
0.05
0.10
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Depth (mollcm2)
Figure 5. Dose/depth distribution of a 3-MeV electron beam in polyethylene placed in a TRMC detection cell.
measured with the copper microtarget. The beam is Gaussian shaped with a full width at half-maximum of 18.8 mm at the position of the sample. The beam geometry is compared with the dimensions of the irradiated volume within the cell in Figure 4. At the extremities of width and length the intensityis still 90%and 83%, respectively, of the value at the beam center. The phthalocyanine sample studied in the present work was in fact contained in an even smaller cavity within the perspex sampleholder as shown in Figure 3. The maximum variation in intensity across this sample area is found to be less than 10%. The depth dependence of the dose deposition within the cell measured using the radiochromic foils is shown in Figure 5. The dose per unit beam charge, D/Q, is given in gray per nanocoulomb. The depth is expressed in mol of electrons per cm2, which is obtained via depth (mol of electrons/cm2) = dp&M/MM
(1) where d is the distance in cm, p~ the density of the material in g/cm3, ZMthe total number of electrons per molecule, and M M the molecular weight of the material. For the polyethyleneused these values are 0.947 for p ~ZM , = 8,and M M = 14. The dose/ depth distribution is seen to be rather uniform except for the foils placed against the copper walls which show a somewhat higher value. Eisen et a1.28 found that the dose deposition in a
D/Q(Gy/nC) = 0.66 - 0.79depth (2) Since D/Qis linear in the depth, the average value of D/Qin the cell is now equal to the value of D/Q at the depth at half the maximum distance (3) Then the average D / Q is found combining (l), (2), and (3) to be
b/Q (Gy/nC) = 0.66 - 0.79 O.Sd-pMZM/MM (4) Using the values for p ~ &, , and M M given above and dmax= 0.355 cm,we find D / Q = 0.59 Gy/nC in polyethylene. Effects of Accumulated Dose. In Figure 6 two microwave conductivity transients are shown for Pc9, both taken at room temperature: one for the freshly precipitated sample and one after heating the sample to 110 OC in the mesophase and subsequent cooling. The latter had received an accumulated dose of 3.8 kGy as a result of intervening PR-TRMC measurements. Both the end-of-pulse conductivity per unit dose, Aaq/D, and thedecay kinetics are seen to beunaffected either by going through the phase transition and back or by imparting the first 3.8 kGy. The dose was increased in the solid phase at room temperature, and conductivity transients were measured at intervening total doses, some of which are displayed in Figure 7. It is observed that Au,,/D remains unchanged up to an accumulated dose of approximately 150 kGy as shown in Figure 8. Above 150 kGy, however, Aaq/D is found to decrease gradually until, at 250 kGy, it is only 65% of the low-dose value. Changes in decay kinetics are found to appear at a much lower accumulated dose. Because the kinetics are disperse, both the
9866
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The Journal of Physical Chemistry, Vol. 97, No. 38, 199'3
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Time (s) Figure 6. Conductivity transients in the solid phase of Pc9 at room temperature: freshly precipitated sample (open circles) and after heating to 110 OC in the mesophase and subsequent cooling having received an accumulated dose of 3.8 kGy (closed circles).
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Accumulated radiation dose (kGy) Figure 9. First (open circles) and second (closed circles) half-life times at room temperature versus accumulated dose.
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Accumulated radiation dose (kGy) Figure((. End-of-pulseconductivityin thesolid phaseat room temperature versus accumulated dose (open circles). The dark circle is obtained after the 247 kGy irradiated sample has been heated to 110 O C in the mesophase and subsequently cooled.
first and the second half-lives are plotted versus dose in Figure 9. Within the first 50 kGy the second half-life, Le., the longliving component of the decay, becomes considerably shorter, decreasing from 35 to 6 ps. The first half-life decreases more gradually with dose as does the second half-life above an initial dose of 50 kGy. After an accumulateddoseof 250 kGy, the material was heated again to 110 O C in the mesophase. Figure 10 shows Aa,/D to be about half the value it was in the mesophase at a low
I l.l&J
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F i p e 10. Conductivity transients in the mesophase at 110 OC at an accumulated dose of 2.3 kGy (open circles) and 251 kGy (closed circles).
accumulated dose of 2.3 kGy. The decay, however, resembles that of the low-dose sample almost completely. On cooling back to room temperature, Auoop/Dreturned to the same low value it had at 250 kGy before heating as indicated by the dark circle in Figure 8. The decay, however, was slower and closer to that for the fresh material as shown by the end-of-pulse normalized transients in Figure 11. After standing an additional 16 h at room temperature, the decay was found to have returned even more closely to that of the fresh sample. Differential calorimetry scans of a freshly precipitated sample and a sample subjected to an accumulated dose of 260 kGy are shown in Figure 2. Upon heating, the phase transition to the liquid crystalline phase is found to have decreased from 106.6 to 105.1 O C . On cooling, the transition from the liquid crystalline to the crystalline phase is found to be lowered by 3.7 OC. The accompanyingenthalpy values are also found to have decreased somewhat after irradiation, from 112 to 101 kJ/mol at the K M transition and 106 to 95 kJ/mol at the M K transition. The DSC data are summarized in Table 11. A small-angle X-ray diffraction analysis of a sample of the closely related Pc12 compound in the mesophase before and after an accumulated dose of ca. 26.3 kGy due to an extensive series of PR-TRMC measurements showed no observable change to have occurred in the hexagonal, columnar structure.
-
-
Discussion Energy Deposition. The experimental results for the dosedepth relationship, as represented by (2), show a linear rela-
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9867
Radiolysis of Octa-n-alkoxy-SubstitutedPhthalocyanine
2alkoxy tails, F(D)oR, will be given a first approximation by -
3
0
254 kGy,
after heatinglcooling
Time (s)
Figure 11. End-of-pulse normalized conductivity transients at room temperature for the fresh sample (plus sign), after an accumulated dose of 247 kGy (squares), after subsequent heating and cooling to the mesophase (filled circles), and after 16 h (open circles).
TABLE II: Onset Transition Temperatures and Accompanying Enthalpy Changes for the Freshly Precipitated Sample of Pc and after Having Received an Accumulated Dose of 258 kGya TK-M("C) AH (kJ/mol) 106.6 112 fresh sample 105.1 101 260 kGy TM-K("C) fresh sample 61.2 63.5 260 kGy K = crystalline phase; M = mesophase.
AH (kJ/mol) 106 95
tionship. For materials with a much larger electron density, the dose-depth relationship may be somewhat different (not linear, different slope). The linear energy transfer (LET) for electrons for a low-Z material may be assumed to be proportional to the electron density, and the weak dependence on the average excitation potential in the expression for LET under the logarithm29 may be neglected. Therefore, for low-2 materials with electron densities somewhat smaller or larger than the polyethylene used above, we may assume (2) to hold for the determination of the dose-depth relation, with the depth defined as (1). The average value for D/Qin the cell is now simply given by (4) for an arbitrary material. For the present Pc9 compound with a density8 of 0.94 g/m3 a total number of electrons, ZM= 906, and M M = 1652.5 it follows using (4) that the average dose per unit beam charge is 0.59 Gy/nC as for polyethylene. Since the value of 0.59 is the dose per unit beam charge in J/kg, the corresponding energy deposition per unit volume in Pc9, with density 940 kg/m3, is determined to be 555 J/m3 per nC. The total energy deposition for the largest accumulated dose of 260 kGy in the present experiments corresponds to a volume dose of 2.4 X lo8 J/m3. This is equal to an average energy deposition per Pc9 molecule of 4.4 eV. Since the phthalocyanine cores and peripheral alkane mantle regions of Pc9 would be expected to react differently to high-energy radiation as far as chemical changes are concerned, it is of importance to know what fraction of the energy is absorbed initially in each component. For the general case of an octaalkoxyphthalocyanine with an alkoxy group OC.H2,,+1, the fraction of energy absorbed in the
For Pc9, F(D)oR is found to be 0.71. Considerably more than half of the energy will therefore be deposited initially in the alkoxy regions filling the space between the phthalocyanine columns. Charge andEnergy Migration. The energy of a 3-MeV electron is transferred to the molecules of the medium it traverses in discrete ionization and excitation events. The overall range of a 3-MeV electron is approximately 1.5 cm in a medium of density 1.O g / ~ m ~ . From ~ O the average energy transferred per energy loss event of 40 eV,3l the average distance between events is found to be approximately 2000 A. Phthalocyanine has a much smaller ionization potential (5.2 eV) and larger electron affinity (3.2 eV) than saturated hydrocarbons in the condensed phase.32 Holes and electrons formed within the saturated hydrocarbon regions of the sample which diffuse close to the phthalocyanine cores would therefore be expected to become localized on the aromatic macrocycles. This scavenging of charge carriers from the hydrocarbon regions will be in competition with geminate recombination of electron-hole pairs which in saturated hydrocarbon liquids has been determined to take place on a time scale of a few picose~onds.~3 We can make a rough estimate of the probability that geminate ions formed in the hydrocarbon regions will be "scavenged" in this way, W,, using the WAS equation which has been found to be a good empirical description of electron and hole scavenging by solutes in saturated hydrocarbon liq~ids.3~
6)
/ ( 1+ (6) Using a characteristic value35 for the scavenging efficiency parameter a of 10 M-* and an effective Pc concentration of 0.57 M in Pc9 gives W,= 0.7. This estimate is crude, assuming as it does that Pc9 can be considered to be a homogeneous solution of Pc moieties in a liquid hydrocarbon solvent. If anything, the scavenging efficiency of the Pc units would be expected to be less because of the columnar stacking. It may be concluded with reasonablecertainty therefore that while a large fraction of charge carriers may be scavenged from the hydrocarbon regions, a reasonably high fraction will still undergo geminate recombination there, leading to excited-state formation. Excited states can be formed via geminate electron-hole recombination or by direct excitation. The energy level of the SIstate of saturated hydrocarbons is approximately 6 eV above the ground ~ t a t e . ~This 6 is to be compared with the first excited state of phthalocyanines at only approximately 2 eV." There is a high probability therefore that, in addition to a large fraction of the initially formed charge carriers in the hydrocarbon regions becoming localized on the Pc moieties, excitation energy will also be transferred efficiently to the aromatic cores. This migration of energy will be in competition with emissive and dissociative decay modes of the excited saturated hydrocarbon states which are discussed in a later section. Phthalocyanine derivatives have been found to be extremely photostable, resulting in their application as ink pigments and in xerographic reproduction.2 Pc excited states must therefore dissipate their energy almost exclusively as fluorescence or as heat, resulting in negligible chemical change. The very low quantum efficiency of charge carrier generation of only 10-8 on irradiation of metal-free phthalocyanine single crystals with light (694 nm)3* indicates that ionization events must be followed by extremely rapid recombination. Radiation-InducedConductivity. On the basis of the previous section, one can conclude that charge carriers formed on irradiation of an octaalkoxy-substituted phthalocyanine will invariably either undergo geminate recombination or become localized on the Pc cores on a time scale much shorter than the
w,= 6
9868
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The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
nanosecond time resolution of the present experiments. The observation of large, long-lived conductivity transients in these materials has led us to conclude that the charge carriers while localized on the Pc cores must still be capable of rapid, onedimensional movement along the core a x i ~ . ~This 3 conclusion has been supported by observations on analogous porphyrin derivatives for which the radiation-induced conductivity is found to disappear on complete melting to the isotropic liquid in which the columnar order is destroyed.22 The close to exponential dependence of the lifetime of the conductivity transients on the lengthof thealkyl chains has further substantiated this conclusion and indicates that the decay of the conductivity is determined mainly by intercolumnar charge recombination via long-distance electron tunneling.21 The complete absence of a conductivity transient for the octamethoxyphthalocyanine derivative indicates the importance of a substantial insulating hydrocarbon layer between the Pc stacks in order to result in charge separation and an extended lifetime of the charge carriers. The absolute magnitude of the conductivity is proportional to the concentration of mobile charge carriers, N , and their mobility, 1.1, i.e.
A d d = e[N+(t)r+ + N-(01.1-1
(7) in which e is the elementary charge. For a pulse length much shorter than thedecay timeof the carriers by either recombination or trapping, the end-of-pulse conductivity will be proportional to the product of the sum of the mobilities of the charge carriers and the concentration of those charge carrier pairs formed during the pulse which contribute to the conductivity, Np Auwp = eNp(cL++ CL-) (8) If Ep is the average energy in electronvolts required to form an electron-hole pair in the material, then a minimum value of the sum of the charge carrier mobilities, &,,,in, can be derived from the end-of-pulse conductivity per unit dose:
(9) Using an estimate for organic materials of 25 eV for Ep,39+40 a value of Zpmin= 4 X lo" m2/(V s) is determined for Pc9 at room temperature in a fresh sample. Chemical Effects of Radiation. An estimate of radiation damage to unsubstituted copper phthalocyanine of 1 molecule per 2.5 x lo4eV absorbed energy has been made on the basis of structural changes observed in an electron microscope This indicates this large aromatic molecule to be one of the most stable organic structures with respect to high-energy radiation.41 Taking 29% of the 260-kGy total accumulated dose deposited in the present experiments to be directly absorbed by the Pc fraction as calculated via eq 5, one would predict that only 1 in 19 000 of the Pc moieties would be damaged. Even if all of the energy were to be concentrated in the Pc units, only approximately 1 in 6000 would suffer damage based on the above literature estimate of the average energy required. It is possible however that more subtle chemical changes in the Pc moieties, which do not lead to structural perturbations, would be missed in the electron microscopy study. A higher yield of "chemical", as opposed to structural, damage may therefore occur. This will be discussed further in a subsequent section. When compared with the phthalocyanine value given above, the average energy required to chemically change a saturated hydrocarbon molecule by high-energy radiation is extremely small, on the order of only approximately 20 eV.31 When this value is used tocalculate thedegreeof chemical changein the hydrocarbon tails of Pc9, we find that 260 kGy would result in close to 1 in 50 of the tails being damaged, assuming 7 1% of the energy to be initially absorbed and fully effective in those regions as given by eq 5 . This estimate is an upper limit since energy and charge
migration from the saturated hydrocarbon regions to the Pc core will almost certainly occur to a large extent as mentioned above. The aromatic macrocycle will in other words act to protect the alkoxy tails from radiation damage. This "protective effect" of an aromatic moiety has been shown for alkylbenzenes withvariable length alkyl groups.42 For these compounds the Hz and low molecular weight product yields were found to be much lower than would have been expected for the saturated hydrocarbon segment alone. One of the major products of saturated hydrocarbon radiolysis4' is usually H2. This is formed either by secondary abstraction by initially formed H atoms, processes A and B, or by molecular elimination, process C.
RH2* H'
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(B) (C)
The freeradical products of (A) and (B)can form higher molecular weight compounds by radical-radical combination (D) or unsaturated compounds via disproportionation, reaction E.
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Carbon
