TAPAN K. MUKHERJEE
3006
this increase in observed lifetime seems to be due to properties of the buIk solvent and also eliminates the possibility of reaction with the chlorinated solvent. Anew, weak absorption at 5200-5500 was observed when a solution of 1-methylquinolinium iodide in 1propanol was flashed. This absorption may be due to the 1-methylquinoline free radical. The rate of decay of the transient was too fast for measurement and its identity is speculative. However, the 1-propanol should not give rise to species absorbing in this spectral region and the iodine atom-1-propanol chargetransfer complex should lie at much lower wavel e n g t h ~ ,The ~ ~ reversibIe ~~~ nature of the system suggests that the absorption was not due to a reaction intermediate. The possibility that this absorption was due to the R+I2- species shown in reactions 4 and 5 was eliminated on the basis that the apparent lifetime of this species was much less than that for the 1 2 -
with which it was in equilibrium [reaction 51. Since phosphorescence has been seen from excited chargetransfer complexes,’* the possibility also exists that a triplet-triplet transition was obswved. It should be noted that the rapid decay of the observed transient does not imply anything about the rate constant. What is measured is J c / E ; thus IC may be large or small depending on the value of the extinction coefficient.
AcknowledgmenC. This work was supported by a NASA Institution Grant to the University of Virginia. RFC wishes to thank NASA for a predoctoral feIlowship. (17) T. A. Gover and G. Porter, F. R. S., Proc. Roy. Soc., Ser. A, 262, 476 (1961). (18) Von J. Czekalla, G. Briegleb, W. Herre, and H. J. Vahlensieck, Z. Elektrochem., 63, 715 (1959).
Photoconductive and Photovoltaic Effects in Dibenzothiophene and Its Molecular Complexes’ by Tapan K. Mukherjee Energetics Branch, A i r Force Cambridge Research Laboratories, Bedford, Massachusetts
01730
(Receiued August 88, 1060)
The photoconductivity of dibenzothiophene crystals was measured in the sandwich and surface type cells. The photocurrent spectral response and the supression of photocurrent by small amount of fluorescence quencher tetracene demonstrate that optical excitation in the 340-230-mr region generates carriers by a mechanism involving singlet exciton states. Mobile holes are majority carriers. Hole trapping, which modifies the conductivity type, can be accomplished by doping with electron acceptors. The photo emf consists of two stages; the fast rising positive emf in the region of low optical absorption changes sign in the stationary state, From the rise and decay characteristics of the short-circuit current in the regions of high and low optical absorption it is concluded that part of the total photo emf is diffusion controlled. The magnitude of the photovoltage in dibenzothiophene can be sensitized by doping with 2,7-dinitr0fluoren-A~~-malononitrile (DDF), 1,3,54rinitrobenzene (TNB), and 2,4,5,7-tetranitrofluorenone(TJYF). A number of new 1:1 chargetransfer complexes have been synthesized. The surface photocurrent peaks of these complexes, although considerably red shifted from the corresponding CT absorption peaks, are within the absorption edges. Dibenzothiophene-T4NF complex, the best photoconductor of this series, was investigated in some detail. Electrons are found to be the majority carriers in this complex. A comparison of the photoconductivity and fluorescence of dibeneothiophene with anthracene did not yield any meaningful correlation between the two phenomena. I n the case of dibewothiophene crystal, the photoconductivity seems to be independent of the selfquenching process.
Introduction
that the long-wavelength photocurrents in anthracenea
The majority Of the Organic photoconductors are fluorescent materials. It has also been known that doping with small concentrations of fluorescent quenchers results in the supression of photoconductivity of the host rnole~ules.~Chaiken and Kearns have shown
(1) Part of this work was presented by T. K. Mukherjee, Abstracts, 156th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1968, No. PHYS 23. (2) D. 0. Northrop and 0. Simpson, PYOC. Roy. SOC.,Ser. A, 244, 377 (1958). (3) R.F.Chaiken and D. R. Kearns, J.Chem. Phys., 45,3966 (1966)).
The Journal of Physical Chemistry, Vol. 7 4 , No. 16, 1970
3007
PHOTOCONDUCTIVE AND PHOTOVOLTAIC EFFECTS IN DIBENZOTHIOPHENE Table I: Elemental Analyses of Dibenzothiophene Charge-Transfer Complexes (1:1) Crystallization solvent
Acceptor"
205 237 228 249 280
CHCla CHC13 CHaCN CHaCN CHCla CHaCN
TNF TaNF DDF DTF TFM a
De0, OC
+
I -
Calod %
-
C
H
N
8
60.11 55.14 66.92 61.42 56.75
2.62 2.22 2.80 2.39 2.04
8.41 10.29 11.15 12.79 14.18
6.41 5.88 6.38 5.85 5.41
,.----
Found -% -C
H
N
8
59.92 55 I43 66.72 61.17 56.59
2.75 2.28 2.65 2.19 1.81
8.29 10.17 10.95 12.80 14.09
6.28 5.93 6.36 6.01
5.57
See text for abbreviations.
and pyrene4 are substantially reduced by tetracene and acridine. These authors suggested that this behavior can be used as a test for the extrinsic process of charge carrier generation. In order to find a possible relationship between the photoconduction efficiency and the radiative properties associated with the optically excited states of photoconductive molecules, a number of structurally related fluorescent compounds were selected for investigation. I n the case of the annellated derivatives of some fivemembered ring compounds (I),the fluorescence quantum yield decreases as fluorene (0.50) > carbazole (0.35) >
I -R(a) (b)
-CHr
(C)
-0-
(d)
-S-
-NH-
Compound Fluorene Carbazole Dibenzofuran Dibenzothiophene
dibenzofuran (0.29) > dibenzothiophene (0.03).5An evaluation of the photoconduction properties revealed that, among these compounds, only dibenzothiophene is photoconductive. Since very little information about the photoconductivity of sulfur-containing heterocyclic compounds is available, a detailed investigation of this compound was undertaken. A number of new stoichiometric complexes were synthesized and their photoelectrical properties studied.6
Experimental Section 1. Materials. Dibenzothiophene (Eastman Kodak, White Label) was crystallized first from glacial acetic acid followed by two crystallizations from acetonitrile. The electron acceptors were chosen so that the resulting complexes were fairly insoluble and they could be recrystallized without appreciable dissociation to the components. 2,4,7-Trinitrofluorenone (TNF) and Z14,5,7-tetranitrofluorenone(T4NF) were crystallized from glacial acetic acid. 2,7-Dinitro-(DDF) , 2,4,7trinitro- (DTF) , and 2,4 ,5 ,7-t e tranitro- (TFM) fluorenAga-malononitrile were obtained from previous work.'
Spectrograde methylene chloride was used as solvent. The complexes were prepared by mixing hot solutions of equimolar quantities of dibenzothiophene and the acceptor. The precipitates were filtered, washed with the solvent, and recrystallized. In Table I, the elemental analyses and the final decomposition temperatures of the complexes are recorded. 2. Absorption Spectya. All spectra were measured on a Cary Model 14 spectrophotometer. The solid spectrum of dibenzothiophene was taken on a vacuum sublimed layer, as well as on a cluster of single crystals obtained by cooling the melt between two tin oxide coated (NESSA) plates. The charge-transfer complex was ground up with one drop of Cargille's nondrying ~ and smeared on a quartz immersion oil ( n 2 6 1.5150) plate. The spectrum of the smear was taken against a blank quartz plate placed in the reference beam. Cargille's oil was found to be superior to a Nujol mull and this procedure avoids the troublesome KBr disk technique. 5. Photocells. Crystal sections, cut from zonerefined* water-white ingots, were polished successively with fine emory paper, ethyl acetate-soaked lens paper, washed with benzene, and finally dried at 50" under vacuum. Single crystals were oriented with the long axis parallel to the cleavage plane. For surface cells, epoxy-based silver electrodes were painted across the long axis on a single-crystal region which was selected by examining the section under a polarizing microscope. Alternatively, few crystals of zone-refined dibenzothiophene were melted on an Incone1 grid deposited on quartz. A small piece of thin quartz plate placed on the solid helped the formation of an even layer of the melt. As the melt cooled, a cluster of randomly oriented needles was formed. (4) R. E'. Chaiken and D. R. Kearns, J . Chem. Phys., 49,2846 (1968). (5) D. W. Ellis and B. S. Solomon, ibid., 46, 3497 (1967). (6) The solution spectroscopic properties of the charge-transfer complexes of dibenzothiophene and the related donors (I) have been reported: T. K. Mukherjee, J. Phys. Chem., 73, 3442 (1969). (7) T. K. Mukherjee, ibid., 70, 3848 (1966). (8) The zone refinement was carried out at the Franklin Institute Research Laboratories, Philadelphia, Pa., under Contract F1962869-C-0129.
The Journal of Physical Chemistry. Vol. 7 4 , No. 16,1070
3008 The single crystal of dibenzothiophene invariably cracked when it was pressed between two semitransparent KESSA electrodes. The low melting point of this compound (98”) precluded vacuum deposition of metallic electrodes on the surface of a pressed pellet. The photoconductivity action spectrum of a cracked crystal was identical with the spectrum obtained from the cell prepared by cooling a melt between two NESSA plates. All bulk conductivity experiments were performed on melt-cooled crystals. The deeply colored CT complexes were deposited on the grid by slowly evaporating a hexane suspension of the powdered solid. Before the electrical measurements, the chemical homogeneity of each cell was visually examined under the microscope. Only those cells which did not show the presence of crystals of separate components were used. Evaporation from ethyl acetate and other polar solvents resulted in visible dissociation of the complexes. 4. Measurements. The apparatus for measuring the de conductivity consisted of a Keithley Model 610A electrometer, a Keithley Model 241 voltage power supply, Bausch and Lomb Model 33-86-45 grating monochromator, and a 1000-W xenon light source equipped with “Schoeffel” power supply. The light intensity was measured by a calibrated t h e r m ~ p i l e . ~ For the normalization of the bulk current data, the absorption and reflection loss due to the NESSA plates was compensated by using a blank plate as the thermopile window. For recording, an EA1 Variplotter Model 1100 X - Y recorder was used. In the case of the molecular complexes, the harmonics from the lower wavelengths were removed by appropriate filters. Surface conductivity measurements were performed under vacuum.
TAPAN I
