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A comparison of the solid state photochemistry and electron beam chemistry of p-diethylaminobenzaldehyde diphenylhydrazone
0146-5724/87 $3.00+ 0.00 Pergamon Journals Ltd
Radiat. Phys. Chem. Vol. 29, No. 3, pp. 219-225, 1987 Int. J. Radlat. Appl. lmtrum. Part C
Printed in Great Britain
A COMPARISON OF THE SOLID STATE PHOTOCHEMISTRY AND ELECTRON BEAM CHEMISTRY OF p-DIETHYLAMINOBENZALDEHYDE DIPHENYLHYDRAZONE J. PACANgKY,H. COUFAL, R. J. WALTMAN, R. Cox and HSmG CHEN' IBM Almaden ~ h Center, 650 Harry Road, San Jose, CA 95120-6099 and IIBM, Information Products Division, 5300 Diagonal Highway, Boulder, CO 80302, U.S.A. (Reechoed 18 June 1986; in revised form 25 July 1986)
Almraet--While p-diethylaminobenzaldehyde diphenylhydrazone, 1, efficiently photoconverts to l-phenyl-3-(4-diethylamino-l-pbenyi)-l,3-indazole, 2, with a quantum yield ~ ffi0.4 + 0.1, excitation by a high energy electron beam (175 kV) induoes very little chemistry. The G value for decomposition of 1 is <0.07; also, the products which are a result of the electron beam exposure differ from the photochemical products. A most likely cause of the extreme contrast between the u.v. and electron beam excitation is the ability of I to efficiently ionize and transport holes in the solid state.
INTRODUCTION Due to its excellent solid state electrical properties the hydrazone, p-diethylaminohenzaldehyde diphenylhydrazone, 1, hereafter called DEH, is used as a hole transporting agent in the charge transport layer of commercial photoconductor. °-s) In order to understand the role of DEH as a hole transporting agent, it is pertinent at this point to briefly describe the composition and photoconduction process used in copiers and printers. An organic layered photoconductor, such as the one in which DEH is used, consists of an aluminized-plastic substrate upon which a thin charge generation layer (.,,0.3 ~m) and a relatively thick charge transport layer (.,,20~m) are sequentially coated. In say a copier, the photoconductor is wrapped around a cylinder and copies are produced by transferring toner particles (formed into shapes by electrostatic images on the surface of the photoconductor) from the photoconductor to a blank sheet of paper. The process by which the electrostatic image is created consists of first placing negative charges on the surface of the charge transport layer; this creates a potential difference across the photoconductor due to the image charge formed in the aluminum suhstrate. An electrostatic image is thus formed on the surface of the photoconductor by producing via light absorption an electron-hole pair in the charge generation layer; the electrons move to the grounded aluminum substrate while the holes migrate to and neutralize the negative ions on the surface of the photoconductor. The copying process is complete when charged toner particles are spread on the surface of the imaged photoconductor and are finally electrostaticaily transferred to and fused onto a blank paper. All of this is discussed at length in the
references already cited but we should note here that the charge transport layer must be transparent to the fight used to form the electron-hole pairs in the charge generation layer. Specifically, when DEH is used to transport holes in the charge generation layer and chlorodiane blue is the material used for the charge generation layer, light with ,l > 500 nm is used to generate the electron-hole pairs. It is also pertinent to add that several thousand copying cycles occur during the lifetime of a photoconductor and hence the process of hole transport where a charge migrates on DEH molecules appears to be relatively free of chemistry which may be associated with charge transfer. The phenomenon of transport of electrons and holes in solids, and photoconduction has been studied by a large number of solid state physicists; a number of important texts ¢6-~°~and reviews°'-Is~ have been written. Upon absorption of u.v. light ~6) (~. <400nm), DEH rapidly converts to 1-phenyi-3-(4-diethylamino-l-phenyl)-l,3-indazole, 2, presumably via the photocyclization, and/or photo-oxidation mechanism outlined in Scheme I. The facile solid state photochemistry is not a technological problem because commercial photoconductors operate at wavelengths longer than 400 nm to take advantage of low cost light sources such as He/Ne or GaAs lasers. The photochemistry is however, interesting for our continuing investigation into systems of technoiogicaF '7~ and theoretical importance. °s) We therefore decided to study and contrast the solid state high energy electron beam chemistry of 1 with its solid state photochemistry. Comparisons of this nature may provide insight into the different pathways taken by the energy dissipated from an electron beam. In general when a system is exposed to a high 219
220
J. PACANSKY et aL
Et2N,~'~c/H (/"~(b) ~ hv ~-~.~-N
I
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02
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/N NEt2
2 Scheme 1 energy electron beam the primary step is ionization or excitation into nonionizing electronic states. The system I has a low ionization potential and as shown by its abifity to transport holes in the solid state, one would expect ionization to predominate as the major excitation process and concomitantly, a chemical product distribution which commences with ionization. Since the chemistry as a result of excitation into the lower valence excited states of 1 has been investigated, then this information will be used in this report to determine, albeit circuitously, whether the electron beam excitation eventually produces the same excited states.
EXPERIMENTAL DEH was prepared using standard chemical procedures 09) by reaction of p-diethylaminobenzaldehyde and 1-1, diphenylhydrazin¢ hydrochloride in ethanol. Thin films of DEH for optical spectroscopic measmements were prepared by spin coating onto CsI, quartz disks, or silicon wafers using tetrahydrofuran as the solvent, or solvents with a higher boiling point. The tetrahydrofuran was refluxed over LiAIH+ and distilled from the same vessel as required. All solutions were used immediately and handled in an inert atmosphere to prevent peroxida formation. For neat DEH films nominally thicker than I prn, spin coating routinely produced glassy films of excellent quality for infrared spectroscopy. For films thinner than l/~m, usually required for u.v. absorption spectroscopy, polymers like polymethacrylate (PMA) or polymethylmethacrylate (PMMA), were used as an inert matrix. The short wavelength absorption of PMA and P M M A commences at 250 van and, in comparison to DEH, has a very low extinction coefficient. The films used for u.v. absorption spectroscopy and photochemical studies analyzed by u.v. absorp-
tion spectroscopy contained 40% DEH in PMMA by weight. The films were spin coated onto quartz disks using tetrahydrofuran as a solvent and had a thickhess ~-0.1 ~m. The u.v. absorption spectra were recorded using a Cary 14 spectrometer. The photochemical equipment consisted of an Eimac VIX-150 watt high pressure xenon lamp fitted with a 10 cm water filter and standard Coming glass filters to isolate the spectral region of interest, e.g. exclusively into the first excited state of DEH (band maximum at 365 nm), or into higher excited states. A Coming filter No. 7-60 was particularly useful because it only transmits light between 290 and 400 nm where DEH has its longest wavelength absorption. Experiments conducted with or without filters in the photochemical equipment, however, gave the same photochemistry. All of the photochemical exi~riments were performed in the presence of air. The quantum yield for the decomposition of DEH was determined by irradiation of thin films of DEH in PMMA (5% by weight). The films were 1.25/Am thick, and were exposed to light emitted from the Eimac VIX-150 watt lamp. A Schott UV-PIL interference filter with a maximum transmittance at 378 nm and a 9.5 nm half bandwidth was used to isolate the wavelength used for the analysis. The fight intensity was measured using a Scientech 36-001 calorimeter fitted with a Isoperibol enclosure 364)203 and a 36-2002 power indicator. The wavelength 378 nm was chosen so that a minimum amount of light was absorbed by the photoproduct of DEH, 2. Nominally, the initial absorbance of the film was 0.42, the light intensity was 0.6 mW/cm 2, and the reaction was almost complete when an incident energy density equal to 60 m J/era 2 was used. After correcting the absorbance at 378 nm for the absorption of the indazole, (+) 2, the quantum yield thus determined was ~ = 0.4 __ 0. I. Electron beam exposure of the samples was achieved using a CB 150 Electron Processor (Energy
Electron beam chemistry of DEH Sciences, Inc., Woburn, Mass.) which allows exposures of the samples in an atmosphere of nitrogen. Basically, the instrument consists of a shielded conveyor that transports the sample under an electron beam. Electrons emanating from a rod-like filament are accelerated and subsequently exit through a Ti/AI alloy window to form a "planar" shaped electron beam perpendicular to the direction in which the sample moves. The electron beam gun operates at accelerating voltages between 150 and 175 kV. The dose delivered to the sample is controlled by adjusting either the beam current or the conveyor speed. The course of the electron beam induced decomposition of DEH was followed using i.r. spectroscopy. Infrared spectra of the solid thin film samples coated on I in. diameter sificon substrates were recorded using a Perkin Elmer 621 IR Spectrometer. The silicon substrates, obtained from Diode Corp., Framington, Mass., are polished on both sides and are tapered 200-300 p m (8-12 rail) to eliminate interference fringes. The samples were irradiated in the sample holder shown in Fig. 1. A 7 p m thick aluminum foil protected the sample from the plasma under the electron beam. A bare silicon substrate was exposed to a 175 kV beam for doses up ~o 10000 kGy (1000 Mrad) without the appearance by any absorption bands. Dosimetry was performed using the aminophenyl methane dye doped films produced by Far West Technology, Goleta, Calif. The thickness of the dosimetric films was 50/tm (2 rail) and the absorbed dose was measured by recording the optical density at 510 ran before and after exposure in the sample holder shown in Fig. 1. A silicon wafer was placed beneath the dosimetric film to include the fraction of the absorbed dose from backscattering of the electron beam. Since the density of DEH, p ffi 1.12 g/cm 3, is almost identical to that for the dmimetric film, p = 1. ! 4 g/cm 3, no further corrections were made to obtain the absorbed dose. Samples of irradiated DEH in sufficient quantities for product analysis were prepared using the tray shown in Fig. 2. A well, 75 p m (3 mil) deep and of area equal to 160cm 2 was machined into a stainless steel block. A viscous solution of DEH in tetrahydrofuran was placed in the tray to flood the well and subsequently, the sample thickness was brought to 7 5 p m (3 mii) by pulling a straightedge across the sides of the well and the remaining solvent allowed to Wing
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~ A l u m i ~ m
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Fig. 1. The sample holder used to exposethin films of DEH coated on a silicon substrate to a 175 keV electron beam.
221 5x5 in 2 well, 3 mil deep
~
StainlessSteelPlate
Fig. 2. The sample holder used to expose DEH to a 175 kV beam in quantities large enough for product analysis. dry. After the sample was exposed to the 175 kV electron beam it was collected and stored in capped bottles for analysis. The sample thickness, 75 pm (3 mii), was chosen to irradiate as large a quantity of sample as possible, without exceeding the Grfid 2°) range Rc, for energy deposition. These parameters and the usual geometry employed in the exposure of a thin film are illustrated in Fig. 3. A beam of electrons with energy E0, travelling in the z direction, is incident normally on a film with thickness z = t. The maximum depth to which energy is dissipated by the electron beam is given by the G ~ n range: 1~ = (O.049 / p )E ~,s
(1)
where E0 is the incident electron beam energy in keV, the density p = 1.12 g/era=, Ro is the C_r~n range for the electron beam in microns; for DEH, Ro = 368/~m when F.e = 175 keV. The rate of energy dissipation is obtained from: dE
--~ = ( - Eo/RG)A ( f )
(2)
where A ( f ) is the depth dose function
A(f)=O.6+6.21f-
12.4fe -F 5.69f 3
(3)
and f = z/R G. The energy of the beam at a depth z, in the film is readily found by integrating equation (2) as given below:
J0 \ d z /
"
(4)
The DEH film thickness chosen in this study does not exceed Ro, but the rate of energy dissipation varies from " 0 . 2 to 0.75 keV/pm across the film. The radical studies were all performed using a Varian E-9 EPR spectrometer. The electrochemical experiments (cyclic voltammetry) were performed under anbient conditions in acetonitrile using 20 g/! TMAFB (tetramethyl ammonium tetrafluoroborate) as the supporting electrolyte and the experimental apparatus already described. °~)
RESULTSANDDISCUSSION Thin films of DEH (thickness -- 10 pm) were spin coated onto Si wafers and exposed to electrons accelerated by 175 kV. The exposures were administered by allowing DEH to receive a total dose of 100, 200, or 500kGy (10, 20 or 50Mrad) in a single pass; thus, DEH was irradiated, for example, at
222
J. PACANSKY
et aL
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3000
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I 2000
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1600
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i 600
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I 1000
I 800
I 600
t 400
200
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I
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Fig. 3. The i.r. spectrum of DEH r e ~ r d ~ (a) befoc¢, and (b) after exposure to D = 3000 kGy (300 Mrad)
from a 175 keV electron beam.
100kGy (I0 Mrad) per pass until a total dose of 3000-4000kGy (300-400 Mrad) was accumulated, and after each exposure an i.r. spectrum was recorded. These experiments were conducted in atmospheres of N2, 02, and air, respectively.In fact, exposures were also conducted without using the protective. Al foil shown in Fig. I. The i.r.spectra recorded, however, were identical regmrdless of the atmosphere in which the exposure was conducted or the dose used for the irradiation.The i.r. spectra recorded at doses D == 0 and 3000kGy (0 and 300 Mrad), for example, for the N 2 atmosphere exposure, 200 kGy (20 Mrad)/pass is shown in Fig. 3. The i.r. spectrum recorded after exposure to 3000kGy (300 Mrad) is for all practical purposes identicalto unirradiatedD E H , thus indicatingthat at least to the dose administered D E H is resistant to chemical damage by the high energy electron beam. A n estimate may be obtained for the extent of the damage induced by the electron beam by recognizing that the sensitivitylimits of dispersive, i.r.analysis is "0.5% T. Since this corresponds to a relative change in concentration of -~ I-2%, the 3000 kGy (300 Mrad) dose could have damaged lessthan 1 % of the original D E H molecules. In order to obtain corroborative information for the i.r. analysis analytical high performance liquid chromatography (HPLC) methods were used. Chromatograms gathered under identical conditions for samples of unirradiated and irradiated D E H , D = 2750 kGy, (275 Mrad) were compared; a vari-
able wavelength u.v.-visible detector was used to interrogate the eluent as it exited the HPLC column. When the detector was set at 365 nm only one peak was observed in the chromatogram of the irradiated DEH. This was unequivocally assigned to DEH by comparison with the chromatogram of unirradiated DEH. Furthermore, the integrated area under the irradiated DEH peak was only 0.7% less than the area for virgin DEH. Since this is within the nominal reproducibility of HPLC experiments (~- 1%) under these conditions, then it supports the very small damage level found using i.r. spectroscopic analysis. When the detector was set at 210 nm, in addition to DEH, two other very small peaks were observed in the irradiated material; however, the integrated area of each was too small to be reliably obtained. In order to at least obtain a u.v. absorption spectrum of the trace products we followed analytical procedures developed to detec impurities in DEH. A large sample (2 g) of exposed DEH, D = 3000 kGy (300 Mrad), was dissolved in acetone and subsequently, the DEH was precipitated from the solution by the addition of water, and centrifuged to remove the precipitate. Since it is difficult to remove all of the DEH from the sample (because the original sample is for all practical purposes DEH), in addition to the two peaks for the products, the peak for DEH was detected. We were able to obtain a u.v. scan of the radiation products by using a diode array detector to interrogate the eluent as it emerged from the HPLC apparatus. The u.v. scan of the products, along with the scan for
Electron beam chemistry of DEH
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Fig. 4. A u.v. scan (absorbance vs wavelength in nm) of the two products, (1) and (2), formed when DEH is exposed to a 175 kV electron beam. The u.v. absorption spectrum for DEH also obtained from the same eluent as it emerged from the HPLC apparatus is included for reference. DEH is shown in Fig. 4; each u.v. absorption spectrum for a product was recorded as it sequentially separated from the HPLC column and passed the detector (the absorbance shown in Fig. 5 cannot be put on an absolute scale because we do not know the concentrations o r extinction coefficients of the products). The trace of the product labelled (1) has a maximum at 271 nm while the product (2) has maxima at 283 and 340 nm (the solvent, 90% acetonitrile plus 10% water, is the mobile phase used for the HPLC). By comparison of the u.v. absorption spectra of photo and electron beam induced products we can unequivocally demonstrate that the product distributions are different, respectively. Figure 5 contains u.v. absorption spectra of a thin film of 40% D E H by weight in P M M A recorded before (Fig. 5, curve a) and after (Fig. 5, curve b) exposure to light 0, > 340 urn). The spectrum recorded after u.v. exposure is attributed to 2 which is clearly seen to be the case by examination o f the solution u.v. absorption spectra for authentic samples o f I and 2 shown in Fig. 6. The band maximum for 2 observed in the solution spectrum and the spectrum after irradiation
1.0 0.9 0.8 0.7
~ O5 ~
04 0.3 0.2 0.1 0.0
Fig. 5. The u.v. absorption spectrum of a thin film of DEH in PMMA (40% by weight) (a) before and (b) after expmure to u.v. light (~ > 340 rim) in air. Note that after irradiation the long wavelength absorption of DEH shifts to shorter wavelengths and that in comparison with the u.v. absorption spectrum of indazole 2, only 2 appears to be formed. R.P.C. 29/3---D
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,-
0.3 0.2 0.1 0.o
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Fig. 6. The u.v. absorption spectrum of DEH in tetrahydrofuran: (--) concentration = 2 x 10-5 M, ~ = 365, 335, 300 nm, ~ s 3 x 104, 2.5 × 104, 1.7 x 104 (l/mole cm). The u.v. absorption spectrum of indazole 2 ( - - - - ) in acetonitrile: concentratioti = 2 x 10-s M, ~ = 345, 285.5, 257.5, ~ = 2.98 x 104, 2.3 x 104, 2.8 x 104 I/(mole cm). in the solid state are identical. Of greater significance, however, is that the u.v. absorption spectrum of 2 does not match with the electron beam induced products shown in Fig. 4. Consequently, we cannot find any major radiation induced products common to both u.v. and electron beam excitation. The efficiency for a u.v. and an electron beam induced reaction is usually expressed in the form of a quantum yield, ¢~, and a G value, respectively. As discussed in the Experimental Section ~ - 0.4 for the u.v. conversion o f I to 2. An upper limit for the G value for decomposition of 1 was determined to be <0.07 by using the following expression: C =
IOOA/E
(5)
where A and E are the number of DEH molecules decomposed when an energy E, in eV, is absorbed per gram of DEH. G thus was determined from the HPLC analysis where 0.7% of the material was decomposed by a d o s e - - 2 7 5 0 kGy (275 Mrad). We must add that this G value is an upper limit and probably is a reflection of our ability to measure small concentrations of radiation induced products. Thus, in addition to the different photo and electron beam induced product distribution, the effgiencies for each process differ significantly; the rate o f the photochemical reaction is very rapid and goes to completion in a very short time period, the electron beam induced reaction is extremely slow and barely detectable. Qualitative results of EPR measurements are presented in this report primarily to obtain information on the role of radiation induced products with open shells. The EPR spectrum of the radical cation o f DEH was investigated because o f its ability to efficiently transport charge in the solid state. Its EPR spectrum, electrochemically generated in a Varlan electrochemical cell within the EPR spectrometer cavity, comisted of a single broad line 1.98 -1-0.05 mT (19.8 4- 0.5 G peak-to-peak) which appeared the same
224
J. PACANSKY et al.
at two concentrations of DEH(10 -4 and 10 -6 M in acetonitrile) and at both high and low conversion, thus ruling out exchange-broadening. Therefore, the many inequivalent magnetic nuclei in the D E H cation radical are most likely causing the broad line with lack of resolved hyperfine structure. A typical EPR spectrum is shown in Fig. 7a. The radical was reversibly formed and was stable when the electrodes were disconnected from the cell. It was also found that the unfiltered light from a xenon high pressure u.v. lamp produced no detectable radical cation of DEH. When polycrystalline DEH (or D E H in a glassy state) was irradiated an EPR spectrum was not observed at temperatures higher than 170K even after prolonged exposure to u.v. light. Below 170 K a low intensity EPR signal is observed whose spectrum consists of a single line of 1.1 _+0.05roT (11 + 0.5 G) width with no resolvable hyperfme structure as shown in Fig. 7b. The EPR signal is symmetrical, relatively resistant to microwave power saturation and has broad wings. The magnitude of the signal weakens slowly as the temperature is increased until it reaches the region of 150-170K whereupon it rapidly becomes too weak to observe. Interestingly, DEH single crystais from slow evaporation of methanol-ether solutions will not give this EPR signal under the same conditions, and even large doses of 50 keV X-rays at 77 K failed to produce an EPR signal of sufficient size to observe. The EPR studies presented thus far reveal that free radical formation via u.v. or X-ray excitation o f glassy or polycrystalline D E H is a very inefficient pr ~oo~__~
Et
H
/ ~
ff~
~N-~
(a) ).55V
I .OV
d~
Voltage
(b)
EL.NsEt
075V
1,30V
Voltage
Fig. 8. (a) The fundamental a.c. voltammogram for DEll (10 -6 M in TMAFB/acetonitrile). The voltammogram indicates that the two electron stepwise oxidation and reduction at 0.55 and 1.0V is a reversible process. (b) The f u n d a n ~ l voltammogram for indazole 2 (10-6M in TMAFB/ acetonitrile). The voltammogram indicates that the one electron oxidation-reduction at 0.75 V is reversible while the one at 1.30V is not reversible.
The close resemblance of the glassy or polycrystalline EPR spectrum of irradiated DEH with the solution radical cation spectrum suggests that a hole, i.e. the radical cation of DEH is formed and trapped in the solid state at low temperatures. At higher temperatures (above 170 K), or in single crystals the hole is mobile and may move to a gas--solid L. 2 0 1 ~ u l I-I interface where either reaction or charge recombination occurs. This, of course, requires that the radical cation of DEH must be both physically and chemically stable and that the electron-hole recombination proceues reforming neutral DEH must not lead to Fig. 7. (a) The EPR spectrum of DEH pt'od~_~__ by excited states that have a high probability for chemelectrolysis of 10-4M DEH in It~toaitrile with 2011/1 istry. We were able to at least demonstrate using TMAFB (tetramethyi =mmoaium ~M~°oorate) added as cyclic voltametry that DEH is reversibly oxidized and supporting eJectrolyte. (b) The EPR spectrum recited after DEll was expmed for 20 rain with the full reduced in solution and hence, the radical cation etc. is stable. Figure 8a contains the a.c. voltammogram output of a 200 W Hg lamp at 10 K.
-h
Electron beam chemistry of DEH for DEH using acetonitrile as a solvent. The curve in the direction to the left of the figure represents the oxidation process; the reduction process is represented by the curve in the right direction. Large deviations between the two curves are taken as evidence that an irreversible oxidation-reduction occurs. As shown in Fig. 8a for DEH, the first a n d second oxidation potentials occur at 0.55 and 1.0 V, respectively and the oxidation-reduction process is reversible. In contrast to D E H the first and second oxidation potential of indazole 2 (see Fig. 8b) are at higher potentials, 0.75 and 1.30 V respectively; furthermore, the first oxidation-reduction process is reversible while the second is without doubt irreversible.
225
hole is very stable and, most of all, when neutralized ~ f 0 ~ ' ~ ~ E H and not another chemical species.
1. 2. 3. 4. 5. 6.
7. 8.
CONCLUDING REMARIf~ AND SUMMARY
9.
When DEH is excited by u.v. light in the solid phase, a very efficient photochemical reaction commences. One major primary product is quantitatively formed via a unimolecular rearrangement by u.v. excitation into the first excited electronic state centered at 365 nm, or into higher electronic states. Since the shortest wavelength for u.v. exposure was 290 nm (below the enerSY required for photoionization), the photochemistry only involves neutral molecules. Excitation o f D E H by a 175 kV electron beam induces a chemical change which is barely detectable and in comparison to u.v. excitation has an entirely different product distribution. As a result o f these observations we conclude that electron beam impact and the plethora of events produced by it, does not leave DEH on the same part o f the excited valence electronic states responsible for the photochemistry. A plausible explanation for this and the low G value for decomposition o f DEH, is the ease with which DEH ionizes and transports holes. Once formed the
10. 11. 12. 13. 14. 15. 16. 17. 18.
19. 20. 21.
IUgFEIIENClgS P. J. Melz, R. B. Champ, L. S. Chang, C. Chiou, G. S. Keller, L. C. Liclican, R. R. Neiman, M. D. Shattuck and W. J. Weiche, Photogr. Sci. Eng. 1977, 21, 73. R. O. Loutfy, C. K. Hsiao and P. M. Kazmaier, Phologr. Sci. Eng. 1983, 27, 5. R. B. Champ and M. D. Shattuck, U.S. Patent 3,824,099 (1974). H. W. Anderson and M. T. Moore, U.S. Patent 4,150,987 (1979). D. M. Pm and J. Yanus, Photogr. Sci. Eng. 1983, 27, 14. R. M. Schaffert, Electrophowgraphy. Focal, London, 1980. J. H. Deuauer and H. E. Clark, Xerography. Focal, London, 1965. H. Meir, Organic Semiconductors, VoL 2, Monographs in Modem Chemistry. Verla8 Chemic, Weinheim, 1974. R. H. Bube, Photoconductivity in Solids. Wiley, New YoA, 1960. P. R. Gtrlich, Phatocanductioity in Soikls. Routledge and Kegan Paul, London, 1967. H. Seki, J. Amorphous and LRI. Semiconductors 1974, 1015. J. Mort, Photoelectronic Properties of Phatoconducting Polymers. In Electronic Properties of Polymers (Edited by H. Mort) John Wiley, New York, 1982. H. Biuler, Phys. Status Solidi B 1981, 107, 9. G. Phister and H. Scher, Adv. Phys. 1978, 27, 747. H. Bislder, Philos. Mag. b 1984, ~ , 347, J. pacansky, D. W. Brown and H. Coufal, J. Photochemistry, accepted for publication. J. Pacansky and J. Lyerla, IBM J. Res. Develop. 1979, 23, 42; J. Pacansky and H. Coufal, J. Am. Chem. Soc. 1980, 107, 410. J. pacansky, D. W. Brown and J. S. Chang, J. Phys. Chem. 1981, 8$, 2562; J. Pacansky and B. Schrade, J. Chem. Phys. 1983, 78, 1033; N. Honjou, J. pacansky and M. Yoshimine, J. Am. Chem. Soc. 1985, 107, 5332. F. Fieser and D. Fieser, Advanced Organic Chemistry, Reinhold, New York, 1961. A. E. Grfin, Z. Naturforsch 1957, 12a, 89. R. K. Galwey and K. K. Kanazawa, IBM RJ 1977, 1926.
Radiat. Phys. Chem. Vol. 29, No. 3, pp. 219-225, 1987 Int. J. Radlat. Appl. lmtrum. Part C
Printed in Great Britain
A COMPARISON OF THE SOLID STATE PHOTOCHEMISTRY AND ELECTRON BEAM CHEMISTRY OF p-DIETHYLAMINOBENZALDEHYDE DIPHENYLHYDRAZONE J. PACANgKY,H. COUFAL, R. J. WALTMAN, R. Cox and HSmG CHEN' IBM Almaden ~ h Center, 650 Harry Road, San Jose, CA 95120-6099 and IIBM, Information Products Division, 5300 Diagonal Highway, Boulder, CO 80302, U.S.A. (Reechoed 18 June 1986; in revised form 25 July 1986)
Almraet--While p-diethylaminobenzaldehyde diphenylhydrazone, 1, efficiently photoconverts to l-phenyl-3-(4-diethylamino-l-pbenyi)-l,3-indazole, 2, with a quantum yield ~ ffi0.4 + 0.1, excitation by a high energy electron beam (175 kV) induoes very little chemistry. The G value for decomposition of 1 is <0.07; also, the products which are a result of the electron beam exposure differ from the photochemical products. A most likely cause of the extreme contrast between the u.v. and electron beam excitation is the ability of I to efficiently ionize and transport holes in the solid state.
INTRODUCTION Due to its excellent solid state electrical properties the hydrazone, p-diethylaminohenzaldehyde diphenylhydrazone, 1, hereafter called DEH, is used as a hole transporting agent in the charge transport layer of commercial photoconductor. °-s) In order to understand the role of DEH as a hole transporting agent, it is pertinent at this point to briefly describe the composition and photoconduction process used in copiers and printers. An organic layered photoconductor, such as the one in which DEH is used, consists of an aluminized-plastic substrate upon which a thin charge generation layer (.,,0.3 ~m) and a relatively thick charge transport layer (.,,20~m) are sequentially coated. In say a copier, the photoconductor is wrapped around a cylinder and copies are produced by transferring toner particles (formed into shapes by electrostatic images on the surface of the photoconductor) from the photoconductor to a blank sheet of paper. The process by which the electrostatic image is created consists of first placing negative charges on the surface of the charge transport layer; this creates a potential difference across the photoconductor due to the image charge formed in the aluminum suhstrate. An electrostatic image is thus formed on the surface of the photoconductor by producing via light absorption an electron-hole pair in the charge generation layer; the electrons move to the grounded aluminum substrate while the holes migrate to and neutralize the negative ions on the surface of the photoconductor. The copying process is complete when charged toner particles are spread on the surface of the imaged photoconductor and are finally electrostaticaily transferred to and fused onto a blank paper. All of this is discussed at length in the
references already cited but we should note here that the charge transport layer must be transparent to the fight used to form the electron-hole pairs in the charge generation layer. Specifically, when DEH is used to transport holes in the charge generation layer and chlorodiane blue is the material used for the charge generation layer, light with ,l > 500 nm is used to generate the electron-hole pairs. It is also pertinent to add that several thousand copying cycles occur during the lifetime of a photoconductor and hence the process of hole transport where a charge migrates on DEH molecules appears to be relatively free of chemistry which may be associated with charge transfer. The phenomenon of transport of electrons and holes in solids, and photoconduction has been studied by a large number of solid state physicists; a number of important texts ¢6-~°~and reviews°'-Is~ have been written. Upon absorption of u.v. light ~6) (~. <400nm), DEH rapidly converts to 1-phenyi-3-(4-diethylamino-l-phenyl)-l,3-indazole, 2, presumably via the photocyclization, and/or photo-oxidation mechanism outlined in Scheme I. The facile solid state photochemistry is not a technological problem because commercial photoconductors operate at wavelengths longer than 400 nm to take advantage of low cost light sources such as He/Ne or GaAs lasers. The photochemistry is however, interesting for our continuing investigation into systems of technoiogicaF '7~ and theoretical importance. °s) We therefore decided to study and contrast the solid state high energy electron beam chemistry of 1 with its solid state photochemistry. Comparisons of this nature may provide insight into the different pathways taken by the energy dissipated from an electron beam. In general when a system is exposed to a high 219
220
J. PACANSKY et aL
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I
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/N NEt2
2 Scheme 1 energy electron beam the primary step is ionization or excitation into nonionizing electronic states. The system I has a low ionization potential and as shown by its abifity to transport holes in the solid state, one would expect ionization to predominate as the major excitation process and concomitantly, a chemical product distribution which commences with ionization. Since the chemistry as a result of excitation into the lower valence excited states of 1 has been investigated, then this information will be used in this report to determine, albeit circuitously, whether the electron beam excitation eventually produces the same excited states.
EXPERIMENTAL DEH was prepared using standard chemical procedures 09) by reaction of p-diethylaminobenzaldehyde and 1-1, diphenylhydrazin¢ hydrochloride in ethanol. Thin films of DEH for optical spectroscopic measmements were prepared by spin coating onto CsI, quartz disks, or silicon wafers using tetrahydrofuran as the solvent, or solvents with a higher boiling point. The tetrahydrofuran was refluxed over LiAIH+ and distilled from the same vessel as required. All solutions were used immediately and handled in an inert atmosphere to prevent peroxida formation. For neat DEH films nominally thicker than I prn, spin coating routinely produced glassy films of excellent quality for infrared spectroscopy. For films thinner than l/~m, usually required for u.v. absorption spectroscopy, polymers like polymethacrylate (PMA) or polymethylmethacrylate (PMMA), were used as an inert matrix. The short wavelength absorption of PMA and P M M A commences at 250 van and, in comparison to DEH, has a very low extinction coefficient. The films used for u.v. absorption spectroscopy and photochemical studies analyzed by u.v. absorp-
tion spectroscopy contained 40% DEH in PMMA by weight. The films were spin coated onto quartz disks using tetrahydrofuran as a solvent and had a thickhess ~-0.1 ~m. The u.v. absorption spectra were recorded using a Cary 14 spectrometer. The photochemical equipment consisted of an Eimac VIX-150 watt high pressure xenon lamp fitted with a 10 cm water filter and standard Coming glass filters to isolate the spectral region of interest, e.g. exclusively into the first excited state of DEH (band maximum at 365 nm), or into higher excited states. A Coming filter No. 7-60 was particularly useful because it only transmits light between 290 and 400 nm where DEH has its longest wavelength absorption. Experiments conducted with or without filters in the photochemical equipment, however, gave the same photochemistry. All of the photochemical exi~riments were performed in the presence of air. The quantum yield for the decomposition of DEH was determined by irradiation of thin films of DEH in PMMA (5% by weight). The films were 1.25/Am thick, and were exposed to light emitted from the Eimac VIX-150 watt lamp. A Schott UV-PIL interference filter with a maximum transmittance at 378 nm and a 9.5 nm half bandwidth was used to isolate the wavelength used for the analysis. The fight intensity was measured using a Scientech 36-001 calorimeter fitted with a Isoperibol enclosure 364)203 and a 36-2002 power indicator. The wavelength 378 nm was chosen so that a minimum amount of light was absorbed by the photoproduct of DEH, 2. Nominally, the initial absorbance of the film was 0.42, the light intensity was 0.6 mW/cm 2, and the reaction was almost complete when an incident energy density equal to 60 m J/era 2 was used. After correcting the absorbance at 378 nm for the absorption of the indazole, (+) 2, the quantum yield thus determined was ~ = 0.4 __ 0. I. Electron beam exposure of the samples was achieved using a CB 150 Electron Processor (Energy
Electron beam chemistry of DEH Sciences, Inc., Woburn, Mass.) which allows exposures of the samples in an atmosphere of nitrogen. Basically, the instrument consists of a shielded conveyor that transports the sample under an electron beam. Electrons emanating from a rod-like filament are accelerated and subsequently exit through a Ti/AI alloy window to form a "planar" shaped electron beam perpendicular to the direction in which the sample moves. The electron beam gun operates at accelerating voltages between 150 and 175 kV. The dose delivered to the sample is controlled by adjusting either the beam current or the conveyor speed. The course of the electron beam induced decomposition of DEH was followed using i.r. spectroscopy. Infrared spectra of the solid thin film samples coated on I in. diameter sificon substrates were recorded using a Perkin Elmer 621 IR Spectrometer. The silicon substrates, obtained from Diode Corp., Framington, Mass., are polished on both sides and are tapered 200-300 p m (8-12 rail) to eliminate interference fringes. The samples were irradiated in the sample holder shown in Fig. 1. A 7 p m thick aluminum foil protected the sample from the plasma under the electron beam. A bare silicon substrate was exposed to a 175 kV beam for doses up ~o 10000 kGy (1000 Mrad) without the appearance by any absorption bands. Dosimetry was performed using the aminophenyl methane dye doped films produced by Far West Technology, Goleta, Calif. The thickness of the dosimetric films was 50/tm (2 rail) and the absorbed dose was measured by recording the optical density at 510 ran before and after exposure in the sample holder shown in Fig. 1. A silicon wafer was placed beneath the dosimetric film to include the fraction of the absorbed dose from backscattering of the electron beam. Since the density of DEH, p ffi 1.12 g/cm 3, is almost identical to that for the dmimetric film, p = 1. ! 4 g/cm 3, no further corrections were made to obtain the absorbed dose. Samples of irradiated DEH in sufficient quantities for product analysis were prepared using the tray shown in Fig. 2. A well, 75 p m (3 mil) deep and of area equal to 160cm 2 was machined into a stainless steel block. A viscous solution of DEH in tetrahydrofuran was placed in the tray to flood the well and subsequently, the sample thickness was brought to 7 5 p m (3 mii) by pulling a straightedge across the sides of the well and the remaining solvent allowed to Wing
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W.f.r ÷ Co., .g
~ A l u m i ~ m
SampleHol~r
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Fig. 1. The sample holder used to exposethin films of DEH coated on a silicon substrate to a 175 keV electron beam.
221 5x5 in 2 well, 3 mil deep
~
StainlessSteelPlate
Fig. 2. The sample holder used to expose DEH to a 175 kV beam in quantities large enough for product analysis. dry. After the sample was exposed to the 175 kV electron beam it was collected and stored in capped bottles for analysis. The sample thickness, 75 pm (3 mii), was chosen to irradiate as large a quantity of sample as possible, without exceeding the Grfid 2°) range Rc, for energy deposition. These parameters and the usual geometry employed in the exposure of a thin film are illustrated in Fig. 3. A beam of electrons with energy E0, travelling in the z direction, is incident normally on a film with thickness z = t. The maximum depth to which energy is dissipated by the electron beam is given by the G ~ n range: 1~ = (O.049 / p )E ~,s
(1)
where E0 is the incident electron beam energy in keV, the density p = 1.12 g/era=, Ro is the C_r~n range for the electron beam in microns; for DEH, Ro = 368/~m when F.e = 175 keV. The rate of energy dissipation is obtained from: dE
--~ = ( - Eo/RG)A ( f )
(2)
where A ( f ) is the depth dose function
A(f)=O.6+6.21f-
12.4fe -F 5.69f 3
(3)
and f = z/R G. The energy of the beam at a depth z, in the film is readily found by integrating equation (2) as given below:
J0 \ d z /
"
(4)
The DEH film thickness chosen in this study does not exceed Ro, but the rate of energy dissipation varies from " 0 . 2 to 0.75 keV/pm across the film. The radical studies were all performed using a Varian E-9 EPR spectrometer. The electrochemical experiments (cyclic voltammetry) were performed under anbient conditions in acetonitrile using 20 g/! TMAFB (tetramethyl ammonium tetrafluoroborate) as the supporting electrolyte and the experimental apparatus already described. °~)
RESULTSANDDISCUSSION Thin films of DEH (thickness -- 10 pm) were spin coated onto Si wafers and exposed to electrons accelerated by 175 kV. The exposures were administered by allowing DEH to receive a total dose of 100, 200, or 500kGy (10, 20 or 50Mrad) in a single pass; thus, DEH was irradiated, for example, at
222
J. PACANSKY
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3000
2500
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3000
2500
I 2000
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1600
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I 1000
I 800
I 600
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Fig. 3. The i.r. spectrum of DEH r e ~ r d ~ (a) befoc¢, and (b) after exposure to D = 3000 kGy (300 Mrad)
from a 175 keV electron beam.
100kGy (I0 Mrad) per pass until a total dose of 3000-4000kGy (300-400 Mrad) was accumulated, and after each exposure an i.r. spectrum was recorded. These experiments were conducted in atmospheres of N2, 02, and air, respectively.In fact, exposures were also conducted without using the protective. Al foil shown in Fig. I. The i.r.spectra recorded, however, were identical regmrdless of the atmosphere in which the exposure was conducted or the dose used for the irradiation.The i.r. spectra recorded at doses D == 0 and 3000kGy (0 and 300 Mrad), for example, for the N 2 atmosphere exposure, 200 kGy (20 Mrad)/pass is shown in Fig. 3. The i.r. spectrum recorded after exposure to 3000kGy (300 Mrad) is for all practical purposes identicalto unirradiatedD E H , thus indicatingthat at least to the dose administered D E H is resistant to chemical damage by the high energy electron beam. A n estimate may be obtained for the extent of the damage induced by the electron beam by recognizing that the sensitivitylimits of dispersive, i.r.analysis is "0.5% T. Since this corresponds to a relative change in concentration of -~ I-2%, the 3000 kGy (300 Mrad) dose could have damaged lessthan 1 % of the original D E H molecules. In order to obtain corroborative information for the i.r. analysis analytical high performance liquid chromatography (HPLC) methods were used. Chromatograms gathered under identical conditions for samples of unirradiated and irradiated D E H , D = 2750 kGy, (275 Mrad) were compared; a vari-
able wavelength u.v.-visible detector was used to interrogate the eluent as it exited the HPLC column. When the detector was set at 365 nm only one peak was observed in the chromatogram of the irradiated DEH. This was unequivocally assigned to DEH by comparison with the chromatogram of unirradiated DEH. Furthermore, the integrated area under the irradiated DEH peak was only 0.7% less than the area for virgin DEH. Since this is within the nominal reproducibility of HPLC experiments (~- 1%) under these conditions, then it supports the very small damage level found using i.r. spectroscopic analysis. When the detector was set at 210 nm, in addition to DEH, two other very small peaks were observed in the irradiated material; however, the integrated area of each was too small to be reliably obtained. In order to at least obtain a u.v. absorption spectrum of the trace products we followed analytical procedures developed to detec impurities in DEH. A large sample (2 g) of exposed DEH, D = 3000 kGy (300 Mrad), was dissolved in acetone and subsequently, the DEH was precipitated from the solution by the addition of water, and centrifuged to remove the precipitate. Since it is difficult to remove all of the DEH from the sample (because the original sample is for all practical purposes DEH), in addition to the two peaks for the products, the peak for DEH was detected. We were able to obtain a u.v. scan of the radiation products by using a diode array detector to interrogate the eluent as it emerged from the HPLC apparatus. The u.v. scan of the products, along with the scan for
Electron beam chemistry of DEH
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Fig. 4. A u.v. scan (absorbance vs wavelength in nm) of the two products, (1) and (2), formed when DEH is exposed to a 175 kV electron beam. The u.v. absorption spectrum for DEH also obtained from the same eluent as it emerged from the HPLC apparatus is included for reference. DEH is shown in Fig. 4; each u.v. absorption spectrum for a product was recorded as it sequentially separated from the HPLC column and passed the detector (the absorbance shown in Fig. 5 cannot be put on an absolute scale because we do not know the concentrations o r extinction coefficients of the products). The trace of the product labelled (1) has a maximum at 271 nm while the product (2) has maxima at 283 and 340 nm (the solvent, 90% acetonitrile plus 10% water, is the mobile phase used for the HPLC). By comparison of the u.v. absorption spectra of photo and electron beam induced products we can unequivocally demonstrate that the product distributions are different, respectively. Figure 5 contains u.v. absorption spectra of a thin film of 40% D E H by weight in P M M A recorded before (Fig. 5, curve a) and after (Fig. 5, curve b) exposure to light 0, > 340 urn). The spectrum recorded after u.v. exposure is attributed to 2 which is clearly seen to be the case by examination o f the solution u.v. absorption spectra for authentic samples o f I and 2 shown in Fig. 6. The band maximum for 2 observed in the solution spectrum and the spectrum after irradiation
1.0 0.9 0.8 0.7
~ O5 ~
04 0.3 0.2 0.1 0.0
Fig. 5. The u.v. absorption spectrum of a thin film of DEH in PMMA (40% by weight) (a) before and (b) after expmure to u.v. light (~ > 340 rim) in air. Note that after irradiation the long wavelength absorption of DEH shifts to shorter wavelengths and that in comparison with the u.v. absorption spectrum of indazole 2, only 2 appears to be formed. R.P.C. 29/3---D
/ \
,-
0.3 0.2 0.1 0.o
I . . . . ~'~0A
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,
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Fig. 6. The u.v. absorption spectrum of DEH in tetrahydrofuran: (--) concentration = 2 x 10-5 M, ~ = 365, 335, 300 nm, ~ s 3 x 104, 2.5 × 104, 1.7 x 104 (l/mole cm). The u.v. absorption spectrum of indazole 2 ( - - - - ) in acetonitrile: concentratioti = 2 x 10-s M, ~ = 345, 285.5, 257.5, ~ = 2.98 x 104, 2.3 x 104, 2.8 x 104 I/(mole cm). in the solid state are identical. Of greater significance, however, is that the u.v. absorption spectrum of 2 does not match with the electron beam induced products shown in Fig. 4. Consequently, we cannot find any major radiation induced products common to both u.v. and electron beam excitation. The efficiency for a u.v. and an electron beam induced reaction is usually expressed in the form of a quantum yield, ¢~, and a G value, respectively. As discussed in the Experimental Section ~ - 0.4 for the u.v. conversion o f I to 2. An upper limit for the G value for decomposition of 1 was determined to be <0.07 by using the following expression: C =
IOOA/E
(5)
where A and E are the number of DEH molecules decomposed when an energy E, in eV, is absorbed per gram of DEH. G thus was determined from the HPLC analysis where 0.7% of the material was decomposed by a d o s e - - 2 7 5 0 kGy (275 Mrad). We must add that this G value is an upper limit and probably is a reflection of our ability to measure small concentrations of radiation induced products. Thus, in addition to the different photo and electron beam induced product distribution, the effgiencies for each process differ significantly; the rate o f the photochemical reaction is very rapid and goes to completion in a very short time period, the electron beam induced reaction is extremely slow and barely detectable. Qualitative results of EPR measurements are presented in this report primarily to obtain information on the role of radiation induced products with open shells. The EPR spectrum of the radical cation o f DEH was investigated because o f its ability to efficiently transport charge in the solid state. Its EPR spectrum, electrochemically generated in a Varlan electrochemical cell within the EPR spectrometer cavity, comisted of a single broad line 1.98 -1-0.05 mT (19.8 4- 0.5 G peak-to-peak) which appeared the same
224
J. PACANSKY et al.
at two concentrations of DEH(10 -4 and 10 -6 M in acetonitrile) and at both high and low conversion, thus ruling out exchange-broadening. Therefore, the many inequivalent magnetic nuclei in the D E H cation radical are most likely causing the broad line with lack of resolved hyperfine structure. A typical EPR spectrum is shown in Fig. 7a. The radical was reversibly formed and was stable when the electrodes were disconnected from the cell. It was also found that the unfiltered light from a xenon high pressure u.v. lamp produced no detectable radical cation of DEH. When polycrystalline DEH (or D E H in a glassy state) was irradiated an EPR spectrum was not observed at temperatures higher than 170K even after prolonged exposure to u.v. light. Below 170 K a low intensity EPR signal is observed whose spectrum consists of a single line of 1.1 _+0.05roT (11 + 0.5 G) width with no resolvable hyperfme structure as shown in Fig. 7b. The EPR signal is symmetrical, relatively resistant to microwave power saturation and has broad wings. The magnitude of the signal weakens slowly as the temperature is increased until it reaches the region of 150-170K whereupon it rapidly becomes too weak to observe. Interestingly, DEH single crystais from slow evaporation of methanol-ether solutions will not give this EPR signal under the same conditions, and even large doses of 50 keV X-rays at 77 K failed to produce an EPR signal of sufficient size to observe. The EPR studies presented thus far reveal that free radical formation via u.v. or X-ray excitation o f glassy or polycrystalline D E H is a very inefficient pr ~oo~__~
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H
/ ~
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(a) ).55V
I .OV
d~
Voltage
(b)
EL.NsEt
075V
1,30V
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Fig. 8. (a) The fundamental a.c. voltammogram for DEll (10 -6 M in TMAFB/acetonitrile). The voltammogram indicates that the two electron stepwise oxidation and reduction at 0.55 and 1.0V is a reversible process. (b) The f u n d a n ~ l voltammogram for indazole 2 (10-6M in TMAFB/ acetonitrile). The voltammogram indicates that the one electron oxidation-reduction at 0.75 V is reversible while the one at 1.30V is not reversible.
The close resemblance of the glassy or polycrystalline EPR spectrum of irradiated DEH with the solution radical cation spectrum suggests that a hole, i.e. the radical cation of DEH is formed and trapped in the solid state at low temperatures. At higher temperatures (above 170 K), or in single crystals the hole is mobile and may move to a gas--solid L. 2 0 1 ~ u l I-I interface where either reaction or charge recombination occurs. This, of course, requires that the radical cation of DEH must be both physically and chemically stable and that the electron-hole recombination proceues reforming neutral DEH must not lead to Fig. 7. (a) The EPR spectrum of DEH pt'od~_~__ by excited states that have a high probability for chemelectrolysis of 10-4M DEH in It~toaitrile with 2011/1 istry. We were able to at least demonstrate using TMAFB (tetramethyi =mmoaium ~M~°oorate) added as cyclic voltametry that DEH is reversibly oxidized and supporting eJectrolyte. (b) The EPR spectrum recited after DEll was expmed for 20 rain with the full reduced in solution and hence, the radical cation etc. is stable. Figure 8a contains the a.c. voltammogram output of a 200 W Hg lamp at 10 K.
-h
Electron beam chemistry of DEH for DEH using acetonitrile as a solvent. The curve in the direction to the left of the figure represents the oxidation process; the reduction process is represented by the curve in the right direction. Large deviations between the two curves are taken as evidence that an irreversible oxidation-reduction occurs. As shown in Fig. 8a for DEH, the first a n d second oxidation potentials occur at 0.55 and 1.0 V, respectively and the oxidation-reduction process is reversible. In contrast to D E H the first and second oxidation potential of indazole 2 (see Fig. 8b) are at higher potentials, 0.75 and 1.30 V respectively; furthermore, the first oxidation-reduction process is reversible while the second is without doubt irreversible.
225
hole is very stable and, most of all, when neutralized ~ f 0 ~ ' ~ ~ E H and not another chemical species.
1. 2. 3. 4. 5. 6.
7. 8.
CONCLUDING REMARIf~ AND SUMMARY
9.
When DEH is excited by u.v. light in the solid phase, a very efficient photochemical reaction commences. One major primary product is quantitatively formed via a unimolecular rearrangement by u.v. excitation into the first excited electronic state centered at 365 nm, or into higher electronic states. Since the shortest wavelength for u.v. exposure was 290 nm (below the enerSY required for photoionization), the photochemistry only involves neutral molecules. Excitation o f D E H by a 175 kV electron beam induces a chemical change which is barely detectable and in comparison to u.v. excitation has an entirely different product distribution. As a result o f these observations we conclude that electron beam impact and the plethora of events produced by it, does not leave DEH on the same part o f the excited valence electronic states responsible for the photochemistry. A plausible explanation for this and the low G value for decomposition o f DEH, is the ease with which DEH ionizes and transports holes. Once formed the
10. 11. 12. 13. 14. 15. 16. 17. 18.
19. 20. 21.
IUgFEIIENClgS P. J. Melz, R. B. Champ, L. S. Chang, C. Chiou, G. S. Keller, L. C. Liclican, R. R. Neiman, M. D. Shattuck and W. J. Weiche, Photogr. Sci. Eng. 1977, 21, 73. R. O. Loutfy, C. K. Hsiao and P. M. Kazmaier, Phologr. Sci. Eng. 1983, 27, 5. R. B. Champ and M. D. Shattuck, U.S. Patent 3,824,099 (1974). H. W. Anderson and M. T. Moore, U.S. Patent 4,150,987 (1979). D. M. Pm and J. Yanus, Photogr. Sci. Eng. 1983, 27, 14. R. M. Schaffert, Electrophowgraphy. Focal, London, 1980. J. H. Deuauer and H. E. Clark, Xerography. Focal, London, 1965. H. Meir, Organic Semiconductors, VoL 2, Monographs in Modem Chemistry. Verla8 Chemic, Weinheim, 1974. R. H. Bube, Photoconductivity in Solids. Wiley, New YoA, 1960. P. R. Gtrlich, Phatocanductioity in Soikls. Routledge and Kegan Paul, London, 1967. H. Seki, J. Amorphous and LRI. Semiconductors 1974, 1015. J. Mort, Photoelectronic Properties of Phatoconducting Polymers. In Electronic Properties of Polymers (Edited by H. Mort) John Wiley, New York, 1982. H. Biuler, Phys. Status Solidi B 1981, 107, 9. G. Phister and H. Scher, Adv. Phys. 1978, 27, 747. H. Bislder, Philos. Mag. b 1984, ~ , 347, J. pacansky, D. W. Brown and H. Coufal, J. Photochemistry, accepted for publication. J. Pacansky and J. Lyerla, IBM J. Res. Develop. 1979, 23, 42; J. Pacansky and H. Coufal, J. Am. Chem. Soc. 1980, 107, 410. J. pacansky, D. W. Brown and J. S. Chang, J. Phys. Chem. 1981, 8$, 2562; J. Pacansky and B. Schrade, J. Chem. Phys. 1983, 78, 1033; N. Honjou, J. pacansky and M. Yoshimine, J. Am. Chem. Soc. 1985, 107, 5332. F. Fieser and D. Fieser, Advanced Organic Chemistry, Reinhold, New York, 1961. A. E. Grfin, Z. Naturforsch 1957, 12a, 89. R. K. Galwey and K. K. Kanazawa, IBM RJ 1977, 1926.