TAPAN K. MUKHERJEE
3848
Photoconductivity of Electron Acceptors.
I.
Nitro Derivatives
of Fluoren-~~~-malononitrile
by Tapan K. Mukherjee Energetics Brunch, Air Force Cambridge Reaeurch Laboratories, Bedford, Massachusetts (Receioed June 3, 1966)
The phenomenon of photoconduction in four dinitro isomers of fluoren-AQa-malononitrile, 2,4,7-t,rinitr0fluoren-A-~~-malononitrile, and 2,4,5,7-tetranitro fluoren-AQa-malononitrilehas been investigated. The bulk and surface photoelectrical characteristics of the 2,7dinitro isomer have been studied in some detail. The large photocurrent in this compound is associated with the excitation energy corresponding to 515 mp. A strong fluorescence emission peak at 520 mp is observed.
Introduction The photoconductivity of molecular solids possessing polynuclear aromatic structures, vix., anthracene,' perylene,2 and heterocyclic amine^,^ has been studied extensively. These materials are typical electron donors. On the basis of the fact that the photoconductive dyest4 like malachite green, rhodamine B, pinacyanol etc., form donor-acceptor complexes5 and anion radical saltsGwith electron acceptors, they can also be classified as donors. Recently some studies on the photoconduction in several donor-acceptor complexes have been p u b l i ~ h e d . ~In * ~contrast, very little information is available about the light-induced conductivity of electron acceptor molecules. In connection with the work on sensitization of electrostatic imaging processes, Hoegig lists a number of electron acceptors as photoconductive materials. The photoconductivity observed in the partialIy nitrated polyacenaphthylene'O probably arises from the donoracceptor interaction. Reucroft and co-workers thoroughly investigated the nature" and origin12 of the bulk photoconductivity of p-chloranil, along with sever a1 other halogenated benzoquinones. As a part of a program on organic p-n junctions, the photoelectric characteristics of several electron acceptors have been examined in this laboratory. In this paper we wish to report the preparation and photoconductive properties of a number of nitro derivatives of fluoren-AQa-malononitrile. The Journal of Physical Chemistry
Experimental Section Materials. Malononitrile was condensed with 2,4dinitrofluorenone (Ia),Ia 2,bdinitrofluorenone (Ib),14 2,B-dinitrofluorenone(Ic),'~2,7-dinitrofluorenone (Id),14 2,4,7-trinitrofluorenone (IC),and 2,4,5,7-tetranitrofluorenone (If), respectively, by a previously published procedureI6to give the corresponding dicyanomethylene (1) J. Kommandeur, J. Phys. Chem. Solids, 22, 339 (1961). (2) B. J. Mulder, Rec. Trao. Chim., 713 (1965). (3) H. Tnoue, K. Noda, and E. Imoto, BUZZ. Chem. Soc. Japan, 37, 332 (1964). (4) H. Meier, Angew. Chem., 77, 633 (1965). (5) J . E. LuValle, A. Leifer, M. Koral, and M. Collins, J . Phys. Chem., 67, 2635 (1963). (6) T. K. Mukherjee, to be published. (7) H. Akamatu and H. Kuroda, J . Chem. Phys., 39, 3364 (1963). (8) ,M.C. Tobin and D. P. Spitzer, ibid., 42, 3652 (1965). (9) H. Hoegl, J . Phys. Chem., 69, 755 (1965). (10) A. Inami, K. Morimoto, and Y. Hayashi, Bull. Chem. SOC. Japan, 37,842 (1964). (11) P. 3. Reucroft, 0. N. Rudyi, R. E. Solomon, and ,M.M .Labes, J . Phys. Chem., 69,779 (1965). (12) P. J. Reucroft, 0. N. Rudyi, R. E. Solomon, and M. M. Labes, J. Chem. Phys., 43, 767 (1965). (13) F. Ullmann and J. Broido, Ber., 39, 360 (1906). We are thank-
ful to Professor Allan K. Colter for a sample of this compound. (14) C. Courtot, Ann. Chim., 14, 5 (1930). (15) This compound as obtained from Aldrich Chemical Co. was found t o be quite impure. It was purified by repeated chromatography over a silica gel column using chloroform as the eluent, followed by crystallization from acetonitrile, mp 235-236O. (16) T. K. Mukherjee and L. A. Lavasseur, J . Org. Chem., 30, 644 (1965).
PHOTOCONDUCTIVITY OF ELECTRON ACCEPTORS
3849
Table I : Melting Points, Elemental Analyses, and Fluorescence Emission of Nitro Derivatives of Fluoren-AoP-malononitrile Mp, OC
Compound
2,4Dinitro (IIa) 2,5-Dinitro (IIb) 2,6Dinitro (IIc:) 2,7-Dinitro (IId) 2,4,7-Trinitro (IIe) 2,4,5,7-Tetranitro(IIf)
259-162 250-252 225-226 298-299 266-268" >40Ob
-Calcd C
60.38 60.38 60.38 60.38 52.89 47.07
Found
H
N
C
H
N
Fluorescence
1.90 1.90 1.90 1.90 1.38 0.98
17.60 17.60 17.60 17.60 19.25 20.58
60.61 60.35 60.42 60.47 52.49 47.25
2.06 2.08 1.98 2.04 1.56 1.05
17.45 17.57 17.48 17.80 19.33 20.57
Yellow (+++) Orange(++) Yellow-orange ( -I-) Yellow-green (++-I-+) Yellow ( f ) Yellow-green ( f )
+
See ref 16. I, T. K. Mukherjee and A. Golubovic, Abstracts, 149th National Meeting of the American Chemical Society, Detroit, Mich., April 1965, p 53P.
derivatives (IIa-IIf) shown below.
ACN
0
CN
I
I1
(a) RI = NOz, Rz = RO = R4 = H (b) R2 = NO2, R1 = RB = R4 = H (c) Ra = N02, R1 = R2 = Rq = H (d) Ra = NO2, R1 = Rz = Rt = H (e) RI = R4 = NO2, Rz = R3 = H (f) RI = R2 = Rg = NOz, R3 = H Table I records their melting points and elemental analyses. The last column shows a comparison of fluorescence emission of the solids, as observed visually under ultraviolet excitation. Purification of these compounds by zone-refining technique was not successful due to partial carbonization of the melt. The compounds (I and 11) were extensively purified by a combination of column chromatography, crystallization, and vacuum sublimation. The purity was checked by melting points, thin layer chromatography, and molecular extinction coefficients. It is important to note that the compounds IIa-IIe are fairly strong electron acceptors and react readily with alkali. Consequently, all operations were performed in acid-washed glassware. Photoconductivity Measurements. The surface conductivity experiments were performed on a "comb"type gold grid deposited on glass plate. The interelectrode spacing was 0.25 mm. Thin layers of the substances were deposited either by solvent evaporation or by vacuum sublimation techniques. The latter method was extensively used in the case of the 2,7dinitro compound (IId). The surface cell w&s held
by electrical leads in an evacuable glass chamber fitted with an optically flat quartz window. The cell was attached to a thermocouple, and heat was supplied by preheated nitrogen. The assembly was essentially of the same design described by Meier." For bulk conductivity measurement, the substances were solution-evaporated on the conductive sides of NESSA quartz plates. The other electrode consisted of a spring-activated aluminum disk. For the evaluation of the bulk photoconductivity efficiency &/Id), the"sandwich" cell was placed at a distance of 7 cm from a 200-w incandescent lamp (100 pw/cm2, intensity of incident radiation), and the system was protected from stray light. For spectral dependence of photoconduction, a 900w xenon light source in conjunction with a Bausch and Lomb grating monochromator was used. The output from the monochromator was focused on the cell with two quartz lenses; the spectral distribution was determined by a bismuth-silver thermopile. The photocurrent was corrected to an incident illumination intensity corresponding to 38 pv developed by the light source at 470 mp. The regions between 313 and 366 mp were scanned by using a G.E. 100-w mercury arc. For photoconduction activation energy, the 1-mm exit slit of the monochromator was used, while for spectral dependence studies, the exit slit was placed at the 0.25 mm position. Light intensity was varied by the use of Kodak neutral density filters, followed by calibration with a thermopile. The field applied to the specimen was taken from a Keithley Model 241 regulated high voltage power supply, and the current measurement was carried out with the aid of a Keithley Model 610A micromicroammeter and an x-y recorder (EA1 Variplotter Model 1110). Results and Discussion Room temperature bulk resistivities of the four (17) H . Meier, Z . Phyaik. Chem., 208, 340 (1958).
Volume 70, Number 18 December 1866
TAPAN K.MUKHEFUEE
3850
Table I1 : Volume Resistivities of Nitro Derivatives of Fluoren-1100-malononitrile Compound
Thickness mm
2,4Dinitro (IIa) 2,5--Dinitro (IIb) ?,G-Dinitro (IIc) 2,7-Dinitro (IId) 2,4,7-Trinitro (IIe) 2,4,5,7-Tetranitro (IIf)
0.14 0.20 0.24 0.18 0.29 0.23
dinitro isomers (IIa-IId), the trinitro (IIe), and the tetranitro (IIf) derivatives measured in the dark and under steady-state polychromatic illumination are listed in Table 11. The photoconduction efficiencies are shown in the last column. I n view of the polycrystalline nature of the materials and the variable thicknesses of the cells, the photoconductive efficiencies are relative to each other. The fact that on illumination the bulk conductivities of these compounds increase by 2-4 orders of magnitude qualifies them as good photoconductive materials. I n contrast, the photocurrents in the starting ketones (Ia-Ie) were higher than the corresponding dark currents by 0-1 order of magnitude. T h u s , replacement of the carbonyl group by the dicyanomethylene function results in the enhancement of photoconductivity in this
I
I
I
350
400
A( mp) Figure 1. Absorption spectrum of 2,7-dinitrofluoren-A~-malononitrile in Nujol.
The Journal of Physic& Chemistry
Dark, ohm-om
8x 2 x 9.0 x 2.0 x 1.0 x 2.0 x
Efficiency =
Light, ohm-om
9x 5 x 1.0 x 1x 7.0 X 8.0 X
10'6 1014 1014 10'6 10'8 10'8
Ip/Id
8,800 4,000 6,000 20,000 142 250
1011 10'0 10" 10" 1Olo 1O1O
series of compounds. 2,7-Dinitrofluoren-A'"-malononitrile (IId), the best photoconductor in this series, was selected for further investigation. 2,7-Dinitro$uoren-AQa-malononitrile:A bsoyption Spectra and Specfral Dependence of Photoconductivity. The structure of 2,7-dinitrofluoren-Aga-malononitrile (IId) was established by (a) analogy,16(b) elemental analyses of itself and that of the 1 : l molecular complex with pyrene, and (c) infrared and ultraviolet spectra. The electronic absorption spectrum, taken in mineral oil, shows peaks at 343,360, and 410 mp (shoulder) (Figure 1). A solution in acetonitrile showed peaks at 360 mp (log e 4.30), 342 mp (log e 4.40), 312 mp (log E 4.74), and 300 mp (log e 4.62). In the fluorescence spectrum the mirror image relationships of the absorption peaks are nearly retained, except that the long wavelength emission is considerably shifted to 520 rt 5 mp (Figure
I
I
I
I
I
300 Figure 2. Fluorescence spectrum of 2,7-dinitrofluoren-Aw-malononitdein dioxane. Excitation a t 350 mu.
1
I
600
PHOTOCONDUCTIVITY OF ELECTRON ACCEPTORS
80 70
'
I
3851
I
-
-
-DARK
PHOTOCURRENT AT 515mp
+
E a a
CURRENT ALONE
PHOTOCURRENT AT 360mp
60-
3
:6:
9-
I
40-
w320
380
440
500
560
620
L
I
Y
c
1. -10
680
0
5
WAVELENGTH ( m p )
Figure 3. Spectral response of bulk photoconductivity of 2,7-dinitrofl~oren-A@~-malononitrile: full curve, photocurrent at 400 v (amp X 1010); broken curve, photocurrent a t 10 v (amp X 1012); thickness of the layer, 0.07 mm.
2). The emission spectrum of a thin layer of the compound was complicated by scattering, although the strongest peak at 520 mp was clearly observed. The phosphorescence emission from compound IId could not be detected in single crystals, glycerine, or EPA (ether-pentane-alcohol) glass. The photocurrent action spectrum of IId was characterized by a strong response at the 515-mp region. Comparatively, only a small response was noticed in the expected region, 360 mp, of maximum absorption. The possibility of photoconduction at 515 mp due to second-order excitation from the grating was avoided by the use of an ultraviolet cutout filter. Burshteinls and Avdeenko and co-w~rkers'~ have shown that the anticorrelation of the photoconductivity and absorption spectra is more pronounced a t weak fields and is due to weak charge separation masking the spectral dependence of the quantum yield of carrier generation. Conversely, at higher fields the carriers are effectively ionized. By a careful study of the effect of the external field on the photoconductivity a t different wavelengths, Reucroft and co-workers12 were able to prove that pure p-chloranil showed normal dependence of the photocurrent on the wavelength of the exciting light. The bulk photoconductivity action spectra (positive electrode illuminated) for solution-evaporated layers of IId were obtained at several field strengths. In Figure 3 the spectra at 10 and 400 v are shown. The maxima of photocurrents a t intermediate fields showed regular variation of intensity without any shift
lo3
10' FI ELD STRENGTH ( V / c m )
io5
Figure 4. The surface current as a function of applied field strength of 2,7-dinitrofluoren-A@*-malononitrilephotocell.
of the peak at 515 mp. Similar behavior was also observed in vacuum-deposited surface cells.2o Dependence on the Potential. The dark and light (at 360 and 515 mp) current-voltage characteristics of the sublimed surface cells show an ohmic dependence up to an approximate field strength of 8 X loa v/cmz (Figure 4). Above this, the dark current varies as the square of the field strength until the region of very high current is reached. This is the typical behavior of space-charge-limited currents. In the case of the photocurrent a t 515 mp, the region of square law dependence is very short, and the current rises steeply, indicating that traps are rapidly filled.21 Intensity Dependence. The intensity dependence of the photocurrent is shown in Figure 5. A good linear plot is obtained for light intensities at each of the spectral regions a t 360 and 515 mp. The photocurrent-intensity follows an I , = kI" relationship, (18) A. I. Burshtein, Soviet PhysAoZid State, 5 , 922 (1963). (19) A. Avdeenko, Y. V. Naboikin, and S. P. Asina, ibid., 6 , 2779 (1965). (20) In several experiments on sublimed surface cells, the maximum of the photocurrent varied between 520 and 525 mp. By illumination from the back side of the cell [G. Tollin, D. R. Kearns, and M. Calvin, J . Chem. Phys., 32, 1013 (1960)l this discrepancy was eliminated. It is important t o sublime a very thin layer of the s u b stance; otherwise a high dark current, giving erratic electrometer reading, will result. (21) S. Matumoto, Bull. Chem. SOC.Japan, 38, 997 (1965).
Volume 70. Number 19 December 1966
TAPAN K. MUKHERJEE
3852
~ 0 0 p ” l r
I
,
E f e c t of Oxygen. Both the dark and the photoconductivity of the material IId decreased when oxygen was adsorbed by the surface, indicating that electrons23 may be the majority carriers. Dependence on Temperature. The temperature dependence of the dark and photo- (at 515 mH) conductivity were measured on the surface cells. The activation energies ( M )were calculated from the relationship i = io exp(-MIKT), where i is the current, io is a constant, K is the Boltzmann constant, and T is the absolute temperature. The slopes obtained at an ascending temperature were identical with those at the descending temperature. These experiments provided the values of dark activation energy (A&) as 1.14 ev, and photoactivation energy (A&) as 0.5 ev, r e spectively .
Conclusion
I
IlIllII 100
I
I
IO INTENSITY -
(.% I.
Figure 5 . Surface photocurrent us. light intensity of 2,7-dinitrofl~oren-A*~-malononitrile photocell : X-X-X, 360 m p ; A-A-A, 515 mp;
.-.-. , white light.
where I, is photocurrent and I is intensity. A value of x = 0.60 was obtained at 360 and 515 mp. By following the argument developed by Almelch and Harrison,22 one can infer that the charge carriers are produced at these wavelengths by a single excitation mechanism. The current-intensity plot for intense polychromatic light shows a slope of 1.09. Rise and Decay. Since the rate of rise of photocurrent in 2,7-dinitro compound (IId) is very fast, it has not been possible to measure the “rise characteristic.” The rate of decay of conductance in darkness from steady state in light (at 515 mp) followed secondwith a rate ‘Onstant Order ‘010 ohm’sec’ Approximately 98% of the conductivity is lost within 5 sec of cessation of illumination.
The Journal of Physical Chemistrv
The outstanding feature in this work is represented by the high photoconductivity of 2,7-dinitrofluorenA9”-malononitrile (IId), especially at the spectral region of weak absorption. This behavior is persistent even at higher fields. Although numerous experiments on different batches of highly purified samples confirm the findings, the possibility of the presence of traces of sensitizing impurities cannot be completely eliminated. This caution is necessary in photoconductivity studies of most organic materials due to the fact that available analytical tools are not sensitive enough to detect and characterize impurities below a certain concentration level. In the absence of any phosphorescence in IId, the participation of the triplet state in the conduction phenomenon cannot be considered. Alternatively, it can be speculated that the excited singlet state (A*) on reaction with the ground state (A) generate the “excimer” (AA*),which decays by emitting at longer wavelength. Further studies are required to examine the nature of this excited level and its possible impact on the photoconduction process.
Acknowledgment. The author wishes to express his thanks to Dr. A. Golubovic for his help in the instrumentation. Thanks are also due to Mr. D. Bogan and Mr. R. Andersson for technical assistance. ( 2 2 ) N. Almelch and S. E. Harrison, J. Phys. Chem. Solids, 26, 1571 (1965).
(23) H. Meier, 2. Wiss. Phot., 53, 1 (1958).
