KOTES
July, 1963 lowed in detail the temperature and pressure behavior of only the peak intensity. Series 11.-With 10-cm. fused-quartz cells in a Cary 14 spectrophotometer a t room temperature, spectra of NZcontaining VC1, vapor a t its equilibrium pressure over the liquid were obtained. To calculate concentrations we employed the equilibrium vapor pressure data found in series I and that of Tchukarev, et 121.~ Series 111.-Pure liquid VCla (obtained from the Vanadium Corporation of America) was put into thin cells constructed from optically flat Pyrex plates, gold foil spacers, and epoxy resin. The cell thickness was measured interferometrically. The spectra were determined a t room temperature. All manipulations were carried out in the dry X2atmosphere. Series 1V.-Utilizing the cells of series [I1 or fused silica cells, we measured spectra 6f 0.05 to 507, solutions of VCla in well dried CC14. For all series, extreme precautions against contamination and decomposition were observed. All results of series 11,111, and I V were taken from complete frequency sweeps.
Calculations and Results The data from runs at the elevated temperatures were processed with the equation E = aRT/PI, where a ( = log(lo/l)) is the decadic optical density, P the partial pressure of absorbing species, I the cell length, R the ideal gas constant, and T the absolute temperature. The experimental intensity is expressed as oscillator strength, f f
=
(s) J’E dc
=
lais the incident intensity of monochromatic light of wave number ii, I is the intensity after passage through the absorbing system, m the electronic mass, c the velocity of light, N Avogadro’s number, and e the electronic charge. The integration is extended over the complete absorption band. The pressures, P, of VC1, a t the recorded temperatures were taken directly from the sickle gage of the apparatus after corrections were applied for a small pressure of Clt (originating from slight decomposition of VCL). Results are summarized in Table I. INTENSITIES OF THE
TABLE I 9 kK. VCla ABSORPTION BAND Series
Series
Series I vapor 25O 54O 69O 200 200 200
I1
I11
vapor 100
liquid 0 019
150 2 2
147 216
Series I V CCln s o h . 0 0264 10 0
.rnax/(l./
molecm.)
looof
142 4 2 2
indicate an upper limit of dimer: monomer < 1:4000. The vapor pressure was observed to be 16.85 f 0.10 torr a t 32OOK. h good linear plot of log P v a p 11s. 1/T (XI to 150’) shows the enthalpy of vaporization to be 10.0 kcal./mole, which supports the latest’ of the two previous measurement^.^,^ I t is iiistructive to compare these data for TrC& and TiC14: TiCI,
vc1,
B.p., “ K .
OS(vap) a t b.p., e.u.
AH(vap), kcal./mole
411.5 ‘$22
23.8 23.7
9.8 10.0
The similarity of these properties suggests that VCL and Tic14 are very similar, particularly in the strength of their intermolecular cohesive forces. Presumably, TiC14 is not appreciably associated in liquid or vapor; the above similarity would suggest that VC1, also is not dimerized in either state. The Trouton constant (AX(vap) a t the normal boiling point) has almost the normal value; it is nearly the same for the two molecules Tic14 and VCL. This is further evidence against strong molecular association. The studies of Whittaker and Yost3 led them to speculate that VC14 is a “normal” liquid, not associated; our studies support their view.
4.33 X 10-6J~ dv/kK 1. mole-’ cm.-l
Path (mm.)
1563
141 3 2 2
123 2.16
122 209
128 222
All runs in series I and I1 (vapor) give emax --146 f 4 l./mole cm. The oscillator strength is independent of temperature and pressure over the 25-70” range, and is independent of physical state. (The condensed phases show slightly lower, broader bands with the same integrated intensity as the vapor.) These observations add weight to other observationss-ln that the molecular species present in all three states (vnpor, liquid, solution) is in all probability the same--mom omer-as one would expect the ligand field band intensity to depend drastically upon molecular symmetry16 which should change markedly with polymerization. Mass spectrometric and vapor density measurements9 (7) S. A. Tchukarev, hl. A. Oranskaya, T. A. Tolmetcheva, and A. K. Yakhkind, Zh. Neorgan. Xhzm., 1, 30 (1956). (8) H. E. Roscoe, Ann. Chem. Lzebzgs, 7 , 70 (1869). (9) J. R. Marquart, unpublished. (10) W. N. Lipscomb and A . G. Whittaker, J. Am. Chem. Soc., 67, 2019 (1945).
SEBIICONDUCTIVITY AND PHOTOCONDUCTIVITY OF PURINES AKD PYRIXIDINES BY SAD HA^ BASUAND WALTERJ. MOORE Chemical Laboratory, Indiana Unaverszty, Bloomzngton, Indzana
Received January 86, 1969
Since the original suggestion of Szent-Gyorgyi’ there has been a continuous interest in the possibility that electronic semiconductivity and photoconductivity might play a part in biochemical mechanisms.2 For example, some unusual effects of radiation on the nucleic acids have been observed. A quantum yield = l was found for the inactivation of a transforming principle (DNA) by soft X-rays3 and = 0.1 to 1 was reported for various phage preparations.4 Such high quantum yields in macromolecules suggested that an exciton might travel freely through the molecule until it is trapped a t a site a t which the elementary reaction of deactivation cart occur. Such exciton mechanisms are closely related to the phenomenon of photoconductivity. If, during the course of its motion, the exciton encounters a strong donor or acceptor entity (i.e., a trapping center), then either the positive hole or the negative electron will be trapped, leaving its counterpart mobile. The trapping effect, therefore, leads to the generation of charge carriers which can carry electric current through a conduction band.
+
+
Experimental Procedures In order to obtain further information on such transfer processcs in nucleic acids, we decided to measure the conductivity and the photoconductivity of their constituent purines and pyrimidines. (1) A. Szent-Gydrgyi, Nature, 148, 157 (1941). (2) D. D. Eley, “Biological Semiconduotivity,” in ”Horizons in Biochemistry,” ed. by M. Kasha and B. Pullman, Academic Press, New York, N. Y . . 1962. Drovides a comolete review. (3) R. Latarjet, H. Ephrussi-Taylor, and N. Rebeyrotte, Rad Res., Suppl. 1, 417 (1959). (4) H. Ephrussi-Taylor and R. Latarjet, Biochim. Biophgs. Acta, 16, 183 (1955); G. Stent and €1. Fuerst, J. Gen. Physiol., 58. 441 (1955).
XOTES
1564
Vol. 67
TABLE I PHOTOCONDUCTIVITY AND SEMICONDUCTIVITY OF PURINES AND PYRIRZIDINES Photocurrent (Ai) X 1011
Compound
Dark current X 1011,amp.
air
vac.
air
vac.
air
vac.
Adenine Uracil Thymine Cytosine Guanine Hypoxanthine
0.26 .50 .55 .50 .22 .25
2.88 5.75 4.45 4.25 2.74 5.75
1.24 4.10 3.20 2.80 1.09 3.50
0.71 .10 .70 .70 .60 2.24
0 0 0 0 0 0
0.56 .55 .55 .60 .47 .55
0.90 1.38 1.09 1.40 1.10 1.10
290 ma
, 2.6
I
WAVELENGTH,
mp
Fig. 1.-Comparison of absorption spectrum of 0.02 M solution of adenine in water (solid line) and photoconductive action spectrum of adenine powder in air (dashed line). Ideally these measurements should be made on single crystals but we have not been able to grow suitable crystals of these quite insoluble compounds. Therefore we used thin films of finely powdered samples. The bases were chromatographically purified preparations from the Pabst Laboratories; repeated recrystallizations did not alter their spectral characteristics. The electrodes were quartz plates (Corning Glass Works) rendered conductive by an adherent stannic oxide layer. Blank measurements on sucrose showed essentially zero conductance and photoconductance, indicating that the preparation of the films did not abrade conductive material from the plates. The plates showed a transmission of only 15% a t 260 mp rising sharply t o IO070 from 320 mp. I n drawing the action curve, the photocurrent was corrected for the transmission characteristics of the quartz and the spectral intensity of the light. The electrical measurements were made by the standard d.c. technique6 for such “sandwich cells” with the use of a vibrating-reed electronieter. Monochromatic light was isolated from a hydrogen or mercury arc lamp by means of a Bausch and Lomb 250 nmi. grating monochromator. Intensity of light was measured with an IP28 phototube calibrated against an oxalic acid dosimeter. Absolute intensity data have little significance since much of the incident light is lost by scattering from the powdered sample, and from 270 to 400 mp no light passes completely through the conductivity cell. The sample thickness was set by Teflon spacers a t 0.0086 cm. The incident light intensity at, 290 mp was 120 uw./cm.2; a t 360 mw, 265 wRr./cm.a. Measurements of dark conduction were made in the temperature range 30 to 70”. The dark conduction was the same in air and in vacuo of torr, but the photocurrent was markedly dependent on the ambient atmosphere. The electric field across the electrodes was 2500 volts/cm. Ohm’s law was followed over the range 600 to 9000 volts/cm.
Results and Comments The experimental results are summarized in Table I. The reproducibility of current measurements on duplicate samples was about 10%. The photoconduc(6) B. Rosenberg, J . Chem. Phye., 81, 238 (1969).
E, e.v.
360 ma
tive action spectrum followed the absorption spectrum of a concentrated water solution of the base. An example is shown in Fig. 1. This correlation is evidence that the measured photoconductivity is a molecular property of the crystals and is not simply a surface effect in the powdered samples. On the other hand, the importance of surface effects is emphasized by the markedly higher photocurrents in the samples exposed to air. In air, appreciable photocurrent was detected in the range 320 to 380 mb, but this was practically absent in vacuo. I n air, the photocurrent was higher when the sample was illuminated through the positive electrode than when it was illuminated through the negative electrode. In vacuo, the photocurrent was independent of the direction of illumination. The last two columns in Table I give the activation energy E for the semiconduction, computed from c = UO exp(-E/kT). The activation energies were markedly lower in air, although the dark currents at 30” were about the same. Since the band gap for the onset of the photoconduction process is about 3.5 e.v., it is likely that it is not related to the semiconductivity observed from 30 to 70’. The higher photocurrent in air when the sample is illuminated through the positive electrode is in accord with a mechanism in which the predominant current carriers are positive holes. It is possible that adsorbed oxygen molecules at the crystallite surfaces are then acting as trapping centers for electrons. However, the question of the detailed mechanism of conduction in these materials is likely to be solved only by ail exhaustive study of monocrystalline specimens of ultrahigh purity, in which surface effects and trapping by impurity centers can be more readily isolated. The present results are a t least consistent with exciton mechanisms in irradiated nucleic acids and suggest that adsorption of electron acceptors may influence the electronic properties. Acknowledgment.-This work is part of a program supported by the Office of Naval Research. TYe are indebted to Henry Mahler for helpful discussions. ~
A LBNGMUIR MEASUREMENT OF T H E SUBLIMATION PRESSURE OF MANGAKESE (11) FLUORIDE BY REKATO G. BAUTlSTA .4ND JOHN L. %TARGRAVE* Department of Chemistry, Uninersfty of Wzsconszn, lMadzson, Wisconsin Received Fehruarg 16. 1966
The sublimation pressure of a single crystal of MnF2 has been measured by the Langmuir free-evaporation technique. Until this work was carried out, no other
*
Rice Univereity, Houst.on, Texas.
