ALEKSANDARGOLUBOVIC
3.352
Photoconductive Properties of Aceanthraquinoxaline and Related Pyrazines by Aleksandar Golubovic Space Physics Laboratory, Energetics Branch, Air Force Cambridge Research Laboratories, Bedford, Massachusetts (Recefved September 1 8 , 1968)
01730
Many of the pyrazines obtained by reacting aceanthraquinone with 1,2-diaminesare highly photoconductive. Measurements in both surface and sandwich cell geometries show close correspondence between the photocurrent response and the absorption spectra of the organic materials. The photocurrent of surface cells tended to be 10-20% lower when measured under vacuum rather than in dry air. These observations as well as the nature of transient photocurrents and the effect of purification suggest that formation of holes at the surface is the predominant mechanism of charge carrier generation. Variations in phot'ocurrentto dark current ratios among the various pyrazines indicate that (1) methyl substitution or extension of the aromatic system by fusion of additional rings substantially enhances the photoconductive response; (2) replacement of carbon by nitrogen within a ring system lowers the response; (3) substitution with electron-withdrawing groups such 8s nitro or bromine essentially quenches the photoconductivity.
Introduction In recent years, semi- and photoconductivity have been reported in several aza aromatic compounds.'-s Generally, the moderate photoresponse of these aza compounds is much lower than that of their nitrogenfree aromatic counterpart^.^^^ On the other hand, the dark resistivities (usually in the insulator range) show lower values than the corresponding nitrogen free analogs.a Definite conclusions from the body of reported data would be premature, mainly because measured dark conductivities are especially sensitive to material purity, a factor not adequately characterized in many previous measurements. In the course of a program on the synthesis of new organic materials with useful photoconductive properties and in order to elucidate further the effect of nitrogen atom ring substitution as well as variation of ring substituents on the relative photoconductivity, we prepared a number of pyrazines. The fact that various symetrical and asymetrical pyrazines are fluorescent at room temperature, indicating a high singlet state population, offered a good possibility of photoconductivity.4 However, such a short-lived excitation state does not always contribute to a photocurrent.8 Indeed, preliminary electrical tests showed that photoconductivity was by no means a universal property of compounds with the basic pyrazine s t r u c t ~ r e . ~However, good photoconductive materials were prepared by condensation of aceanthraquinone with 1,a-aromatic diamines which gave various pyrazines based on aceanthraquinoxaline (I)
I The Journal of Physical Chemistry
We report here on a study of the photoconductive properties of these pyrazines and in particular on the effect of slight structural variations in the moiety B, moiety A remaining fixed.
Experimental Section The pyrazines were prepared by refluxing equimolecular quantities of aceanthraquinone and a 1,2diamine in glacial acetic acid for 0.5-1 hr. Extensive effort has been put into purification of starting materials and final products. Progressive purification slightly increased the photoconduction up to a maximum after which additional purification had no essential effect. Similar observations were made by Noddack, et aL1O The crude microcrystalline product was recrystallized two or three times from an appropriate solvent (dimethylformamide, acetic acid, chloroform, or acetonitrile). We accomplished additional purification of the recrystallized materials by vacuum sublimation and column chromatography (silica gel 0.02-0.5 mm, E. Merck, A. G. Darmstadt, Germany) with chloroform appIied as eluent. Purification by zone refining was attempted and not successful due to partial decomposition of material. All starting materials underwent similar purification. Most of the diamines came from commercial sources; others were (1) H. Inone, K. Noda, and E. Imoto, Bull. Chem. SOC.J a p . , 37, 332 (1964). (2) H. Akamatu and H. Inokuchi, J . Chem. Phys., 18, 810 (1950). (3) H.Inokuchi, Bull. Chem. S O C . J a p . , 2 5 , 28 (1952);27, 22 (1954). (4) M. Kleinerman, L. Aaarraga, and S. P. McGlynn, J . Chem. Phys., 37, 1825 (1962). (5) B. Rosenberg, (bid., 37, 1371 (1962). (6) A. Q. Kalle, Belgian Patent 640264. (7) J. J. Brophy and J. W. Buttrey, "Organic Semiconductors," The Macmillan C o . , New York, N. Y., 1962. (8) B. Rosenberg, J . Chem. Phys., 29, 1108 (1958). (9) A. Golubovic, unpublished results. (10) N. Noddack, H. Meier, and A. Haus, 2. Phys. Chem. (Leipaig), 212, 55 (1959).
PHOTOCONDUCTIVE PROPERTIES OF ACEANTHRAQUINOXALINE
1353
Table I: Surface Cells
Moiety Ba
Dark current (Id)? A X 10-19
Photocurrent (Ip),'A X 10-10
Ip/Id
Photoresponse threshold, mp
55 0
531
8.6
4.7
7.2
17.0
236
555
6.6
9.2
138
524
5.2
0.41
7.4
588
4.0
2.9
72.5
552
3.2
1.1
34.3
5 18
1.1
6.9
63.0
540
7.0
8.7
12.5
528
0.23
0.0083
3.95
0.29
0.007
2.4
2.5
1.1
0.25
0.12
a For complete formulas in question, see I in the text. Last three entries chromatic light peak of response (15 mW/cmz). Irradiated area 8.5 mma.
synthesized following published procedures.11J2 Aceanthraquinone was prepared by the method of Liebermann and Zsuffa.la We measured absorption spectra on a Cary spectrophotometer Model 14. Electrical measurements were performed on both sandwich- and surface-type cells. The latter were of comb design with 14 pairs of legs. Each leg is a 10-mm length of gold or nichrome deposited on a quartz plate (15 X 20 mm) with a gap of 0.25 mm. The selected photoconductive material formed a uniform layer of 1-2-p thickness upon slow sublimation onto this plate under controlled temper-
44.0
48.0
show complete formulas.
546
518
550
* Applied do voltage 200 V.
e
Mono-
ature and pressure (0.1 mm). The electrical characteristics of the surface cells were measured in a chamber described by Meier" and Kaufhold and Hauffe.lS The sandwich cells consisted of material similarly sublimed to a thickness of 0.05 mm onto a transparent quartz plate (18 X 24 mm) coated with (11) E. Knoevenagel, J . Prakt. Chem., 89, 1 (1914). (12) G . R. Lappin and F. B. Slesak, J . Amer. Chem. Soc., 7 2 , 2806 (1950). (13) 0.Liebermann and M. ZsufPa, Chem. Ber., 44, 202 (1911). (14) H.Meier, 2. Wiss. Phot., Photophystk Photochem., 5 3 , 1 (1958). (15) J. Kaufhold and K. Hauffe, 2.Elektrochem., 69, 168 (1965). Volume YSp Number 6 Mall 1969
ALEHSANDARGOLUBOVIC
1354 Table 11: Sandwich Cells Dark current Moiety B
Applied dc voltage 200 V.
ID/Id
5.9
1550.00
26,270
1.83
1.98
4.6
3300.00
72 000
1.57
2.30
1.65
7800.00
47,000
2.36
2.44
0.4
1.65
411
4.0
155.00
3,880
0.27
1.74
1.3
30.00
2,300
7.6
2.06
1.0
41.00
4,100
25.0
2.08
3.4
34.00
1,000
A X 10-18
(Id).'
b
SPECTRUM --(INOPTICAL CHCI,)
I,, ,
0.1
300
, ,
, , ,,, ,, ,
350
400
77-
1.55
2.10
1.54
voltage by means of a Keithley high-voltage supply Model 241 and recorded the photocurrents measured by a Keithley Electrometer Model 610B on an electronic associates X-Y variplotter Model 1100. The irradiated area of the surface cells was standardized a t 8.5 mm2. Sandwich cells were irradiated by a tungsten light (100 W) from a distance of 10 cm.
Results and Discussion The dark as well as the photocurrents showed linear (ohmic) behavior up to applied fields of 15,000 V/cm. At the higher fields, the voltage-current relationship shows the curvature expected from space charge limited current effects. A summary of results for surface type cells is presented in Table I and for sandwich type cells in Table 11. The dark current-temperature relationship for sandwich cells measured under reduced pressure (0.1 mm) in the range of 25-110" followed the usual expression, I = IOexp[-E/2kT], from which activation energies E were computed. Ratios of photocurrent (Ip)t o dark current ( I d ) best reveal the effect of structural changes in moiety B on the electrical properties of the pyrazines;16 the ratios for both cells differ in absolute values but show similar trends for the various structures. One notes that methyl group substitution in moiety B roughly doubles the photoconductivity and produces further enhancement if moiety B is a naphthalene ring. On the other hand, ring substitution by electron-withdrawing groups such
x;;*, L.-\
j
t
%
450
500 mu
Figure 1. Optical spectrum and photocurrent action spectrum of aceanthraquinoxaline (surface cell). The Journal of Physical Chemistry
12.1
Activation energy, eV
100-W tungsten light.
tin oxide. The sandwich assembly was then completed by spring attachment of an identical conductive quartz plate. The monochromatic light source for surface cells was a Bausch and Lomb grating monochromator No. 33-86-45 with a Hanovia 800-W xenon light pressure arc lamp. Entrance and exit slits were 5 and 2 mm, respectively. An Eppley bismuth-silver thermopile provided a calibration of light intensity. The final data were normalized to equal intensity throughout the 300-600-mp range after the light intensity at 470 mp (highest intensity of xenon lamp) was standardized a t 15 mW/cm2. For both types of cells, we regulated dc
0.6
Speciflc dark ohm-cm resistance, X 1016
Photocurrent (ID),bA X 10-10
(16) It is of interest that aceanthraqulnone itself showed fair photoconductivity. For 8 sandwich cell ID/&= 2380.
PHOTOCONDUCTIVE PROPERTIES OF ACEANTHRAQUINOXALINE
-----
OPTICAL SPECTRUM
-6
PHOTOCURRENT
I
t n 0.1
-
300
350
400
500
450
550 rnp
Figure 2. Optical spectrum and photocurrent action spectrum of aceanthranaphthopyrazine (surface cell).
as nitro or bromine largely quenches the photoconductive response. Similar trends were observed by Noddack, et aZ.,10 in structures resulting from methylation of amino groups in triphenyl dyestuffs. The order of photoconductive response with structural variation followed there was CHs >> CSHB> H > CeH6. The order in the present work may be summarized schematically as follows
fi
As a qualitative generalization, the relative photo0.71
I
I
I
I
,
,
, ,,
,
I
I
I
I
, ,
I
I I
OPTICAL SPECTRUM (IN CHCI,) PHOTOCURRENT
17
$:
-
0.1
I 300
350
3
4
I
t
i
1
400
I
1 . 9
I
450
500 m p
Figure 3. Optical spectrum and photocurrent action spectrum of aoeanthrapyridopyrazine (surface cell).
Ij 0
1355
No N
0.4
-
0.8
1.6
m sec
Figure 4. Transient photocurrent in aceanthraquinoxaline pellet (0.2 om). Applied field is 250 V.
conductivities appear to correlate with the number as well as the degree of delocalization of the ?r electrons. Hence an increase in the number of aromatic rings or methyl substitution (hyperconjugation effects) enhances photoconductivity while substitution of electron-withdrawing groups diminishes it. Previous work in which Carlton, et aZ,,l7 observed the effect on the electrical conductivity of structural variations in the polyazophenylenes leads to a similar generalization. The most important factor was the extent of ?r-orbital overlap along a conjugated chain. In apparent contradiction to the present results, Inami, et a1.,18 found that the nitration of the polyacenaphthylenes substantially increased their photoconductivity. It is probable as suggested by these authors that some charge-transfer mechanism is operative with the polyacenaphthylenes which is not occurring in our compounds. The decrease in photocurrent upon replacement of carbon with nitrogen atoms within a ring system, thereby lowering the ?r-electron density, is also consistent with this picture. Surprisingly, the dark resistivities of our azo compounds do not differ significantly from their aromatic counterparts in contradistinction to the trend observed by InokuchilD in various phenazines when compared to their fully aromatic counterparts. The action spectrum of the photocurrent for thinlayer surface cells conforms in all cases to the absorption spectra of the organic material. Some typical examples are given in Figures 1, 2, and 3. For the thicker surface cells and for the sandwich cells, the (17) D. M. Carlton, D. K. McCarthy, and R . H. Genz, J . Phys. Chem., 6 8 , 2661 (1964). (18) A. Inami, K . Morimoto, and Y. Hayashi, Bull. Chem. SOC. Jap., 37, 842 (1964). (19) H. Inokuchi, i b i d . , 25, 28 (1952).
Volume 7% Number 6 May 1060
1356
IRWIN
photocurrent response curve maxima are red shifted with respect to the optical spectra. These results as well as the linear dependence of photocurrent on light intensity observed in the 400-600-mp region for all the pyrazines suggest that carriers are generated by an extrinsic process involving interaction of singlet excitons with surface impurity sites.*O The increase in photoconduction with initial purification (see Experimental Section) also points to an extrinsic process. The entries in Tables I and I1 are the values obtained in dry air. The photocurrent of surface cells tended to be 10-2075 lower when measured under vacuum, an indication that holes are the mobile carriers.la Measurements of transient photocurrents in a 2-mm thick
COHENAND K. DOUGLAS CARLSON
pellet of aceanthraquinoxaline gives further evidence that holes are the majority carriers. Figure 4 shows oscilloscope response traces to a 100-psec flash. The extreme trapping of charge prevented calculation of mobilities. However, the stronger trapping of electrons, especially in air, is clearly seen by the long decay of electrons when the negative side is illuminated.
Acknowledgments. The author wishes to acknowledge the assistance of Mr. R. Sidor and Miss Noreen A. Dimond and valuable suggestions and helpful discussions with Dr. J. Silverman. (20) R. F. Chaiken and D. R. Kearns, J . Chem. Phys., 45, 3966 (1966).
Density Distributions and Chemical Bonding in Diatomic Molecules of the Transition Metalsla by Irwin Cohenlb Department of Chemistry, Youngstown Stale University, Youngstown, Ohio
and K. Douglas Carlson Department of Chemistry, Case Western Reserve University, Cleeeland, Ohio
44106
(Received September 1 8 , 1 9 6 8 )
Electronic charge density distributions calculated from truncated-matrix Hartree-Fock wave functions for isoelectronic T i 0 and ScF are analyzed in terms of the interpretive concepts of chemical bonding. The analyses are based on an examination of the spatial characteristics of the orbital charge density contour maps, the features of density difference diagrams which compare the molecular orbitals with covalent and lone-pair charge distributions, and on a study of charge-transfer populations and charge moments. An important aspect is the transformation of the canonical orbital basis to the intrinsic localized basis defined by Edmiston and Ruedenberg. With this transformation, the complicated shell structures are simplified to well-defined hybrid lone-pair functions and to bonding orbitals which involve hybrid combinations of the usual valence orbitals of the atoms. It is shown that both T i 0 and ScF may be considered to be molecules with triple bonds ( ~ 2 ~ 4of ) strong ionic character. The oxide, however, is about equally intermediate to a covalent or ionic molecule, whereas the fluoride is predominantly ionic.
Introduction In recent years, a number of ab initio calculations based on the matrix Hartree-Fock method of Roothaan have been carried out for certain diatomic molecules of the transition metals. These calculations have dealt with the nitride molecule TiN,2 the oxides ScO,* Ti0,4 and V0,6 the fluoride S C F ,and ~ others currently under investigation.6 The principal emphasis of these studies has been the classification of electronic ground states in the absence of reliable or complete experimental information. This article makes further use' of these calculations to describe the electronic structures of these molecules in terms of chemical bonding concepts drawn The Journal of Physical ChemistTy
from analyses of their electronic charge density distributions. The electronic structures of these heteronuclear (1) (a) Research sponsored by the Air Force OWce of Scientiflc Research, Oface of Aerospace Research, U. S. Air Force, under AFOSR Grant No. 68-1438. (b) National Science Foundation College Teacher Research Participant, Case Western Reserve University. 1967-1968. (2) K. D. Carlson, C. R. Claydon, and C. Moser, J . Chem. Phys., 46, 4963 (1967). (3) K. D. Carlson, E. Ludeiia, and C. Moser, i b i d . , 43, 2408 (1965). (4) (a) K. D. Carlson and R. K. Nesbet, ibid., 41, 1051 (1964); (b) K.D.Carlson and 0. Moser, ibid., 46, 35 (1967). (5) K . D.Carlson and C. Moser, ibid., 44, 3259 (1966). (6) A. 0.Wahl, 0. Moser, and K. D. Carlson, unpublished calcula-
tions.
