(e.g., soluble photolysis products that react rapidly when exposed to oxygen in the air). The ketone-alcohol solutions (for photopotential, spectra, or electron spin resonance work) were photolyzed using either an Osram HBQ-200 or PEK-109 high pressure mercury arc lamp. The radiation was rendered approximately monochromatic in the 3130 or 3660 A. region by appropriate was used for Corning glass filters. Ferrioxdate actinometry. Phosphorescence measuremetits were made on vacuum degassed, sealed-tu be samples in rigid ethanol (unless otherwise stated) a t liquid Nz temperatures. Irradiation from the PEK-109 lamp was filtered to allow transmission a t 3150-4000 A. which then passed through a rapid closing shutter system to the sample. At right angles to the excitation light, the phouphorescence was monitored by a photomultiplier tube (filtered t o allow transmission only above which fed an input signal to an Bmerican Instru4400 i.) ment Go. photomultiplier microphotometer whose output si nal was recorded on a Sanborn Model 154-100 recorder (&art speed: 100 mm. per second). Matheson, Coleman and Bell reagent grade alcohols were purified by the following procedure: to a 2-1. round-bottom flask containing ca. 200 ml. of alcohol were added 5 g. of granular magnesium metal and 0.5 g. of iodine crystals. The solution was refluxed for 1 hr., the remainder of the alcohol added (1 to 1.5 I.), refluxed for an additional 3-4 hr., and then distilled from a 60-cm. glass helices packed column. Only the middle fractions were collected and used. (44) C. A. Parker and C. G. Harchard, Proc. Roy. Soc. (London), A220, 101 (1953); i b z d , 8236, 518 (1956); "Photochemistry in the Liquid and Solid States," John Wiley a n d Sons, Inc., New Yrrk, N. I-.,1960,p. 41.
The ketones used in this study were known compounds prepared by reported methods.",* They were purified by crystallization, sublimation, or column chromatography. The samples of the chloro- and hydroxy-substituted anthraquinones were kindly supplied by Dr. E. .J. Rowen and Dr. D. Seaman, Oxford University.
Acknowledgment. -The authors wish to acI;nowledqc the c.oiitrihutions of Mr. ,J. H. Sharp, N r . T. C. Li, A h . 0. Paez, and Drs. 1'. West and G. Black to this paper. lye are indebted to Drs. S. I. Chan and L. Piette for their helpful suggestions concerning the electron spin resonance aspects of the work, and to Dr. F. Wilkinson of the Physical Chemistry Laboratories, Oxford University, for interesting discussions of the problem. This research was supported in part by a grant from the Petroleum Research Fund administered by the American Chemical Society. Grateful acknowledgment is hereby made to the donors of this fund. The authors also gratefully acknowledge partial support of this work by grant No. 4P-109 from the Public Health Service, Air Force Contract, AF 19(604)-8096, from the Geophysics Research Directorate of the Air Force Cambridge Research Laboratories, and a grant from the Yational Science Foundation. (4.7) P. Graminaticakis, Cornpt. rend., 236, 546 (19.52). (46) J. P. Cordner and K. H. Parisacker, J . Chem. Soc., 102 (1953).
R.WD STRUCTURE AND TRANSPORT OF HOLES AND ELECTRONS IN HOMOLOGS OF ANTHRACENE1 BY G. D. THAXTON, R. c. JARNAGIN, Physics and Chemistry Departments, The University of Narth Carolina, Chapel Hill, North Carolina AND
M. SILVER
A r m y Research Ofice (Durham), Durham, North Carolina, and Physics Department, University of North Carolina, Chapel Hi!l, North Carolina Received M a y 85. 196.9
Calculations of t'he band structure and of the mobility of excess holes and of excess electrons in homologs of anthracene have been coniplcted. Following LeBlanc, the tight binding approximation w m used and applied to naphthalene, tetracene, and pentacme. Calculated mobility tensors and band widths indicate the mobility properties of excess charge carriers in all four molecu1:ir crystals to be much alike, Experimental results for the crystals other than anthracene are not yet available.
electron. Application of periodic boundary conditions enables one to obtain band widths and transport, of excess holes and of excess electrons mobilities for both holes and electrons. LeBlanc made computations only for anthracene. in anthracene has recently been reported by LeBlanc.2 LeBlanc applied the tight binding ap- It is desirable to extend these type calculations to proximation to construct crystal wave functions other members of the naphthalene-anthracene in order to describe the motion of excess charge series (members which will probably be accessible carriers. The crystal wave functions are formed to experiment) and see what major differences, from linear combinations of molecular orbitals con- if any, may be expected in the mobility properties. In addition to calculations for naphthalene, structed within the Huckel approximations from Slater 2p, atomic orbitals.s Linear combinations tetracene, and pentacene, the computations for of molecular orbitals for the highest bonding state anthracene were repeated. LeBlanc4 has advised describe the band for the excess hole, and linear us of an error in his original work. The repeated combinations of the molecular orbitals for the lowest computations were done to ensure computational anti-bonding stake describe the band for the excess consistency with LeBlanc's corrected results. Molecular and Crystal Structure.-Naphthalenes (1) Partially supported b y the Army Research Office (Durham) Introduction
A theoretical treatment of the band structure and
and the National Science Foundation. (2) 0. H. LeBlsnc, Crust., 2, 238 (1949).
G. D. THAXTON, R. C. JARNAGIN, ASD M. SILVER
2462
constant mean free time T $ ) mobility in the Z crystallographic direction should be viewed with some skepticism. It would be = T~ and (b) constant mean free path T $ ) X !;(IC) I = A,,. The first case permits the calculation interesting to have experimental data with which of the mobility in closed form. For that reason to compare this result. Since a rigid lattice has been assumed in these (a) is assumed and the mobility components are calculations and is an obvious over-simplification, given hy then one may argue that no significance can bc attached to the large anisotropy of velocity components in naphthalene nor to the 180-fold asymmetry in electron and hole velocity components in which v,&) is the it" component of the carrier along the a' direction. However, these calculations clearly indicate the desirability of looking for these velocity Ti and is given by effects and suggest crystalline naphthalene to be the more promising material to examine, parhv,(k) = dB(kj'/dk (18) ticularly a t low temperatures where the rigid latThe quantity (u, u,) is a statistical average over the tice assiimption may be more closely attained. appropriate band. By treating the density of ?Tith the exception of increased anisotropy in the states and the Boltzmann factor as constant over ab plane for tetracene and pentacme, there does the extremely narrow bands, the integration is not appear to be any startling differences hetiyeen easily performed. The results are given in Table the predicted mobilities in the substances investi111. gated. I n tetracene and pentacene the off-diagonal Finally the mobility tensors (tensors with com- components of the mobility tensor are no longer ponents (v, vJ) have been diagonalized and the negligible and the principal axes are near I; and components along the principal axes are given in Table IV. In all four substances, for both holes rather than along a' and b a s in naphthalene and and electrons, it is found that c' is a principal anthracene. Also, if T~ iq comparable for holes and electrons, then there is a reversal of their relative axis of the mobility teiisor. 111 addition li and mobility in the bdirection for tetracene and pentaare principal axes for both holes and electrons in naphthalene and anthracene. In tetracene and cene compared to their relative mobility in the pentacene the principal axes are obtained by rota- same direction for naphthalene and anthracene. On the basis of these calculations one would tions of the orthogonal coordiiislte system about e'. In tetracene the rotation angle is 59.5' for predict that the magnitude of the mobilities in all holes and 51.5' for elevtrons and in pmtaccnc the of these materials should lie wmparable but with rotation angle is 63.7' for holes and 65.5' for clcc- significant differences in thcir anisotropy.
The conditions for maxima and minima in E(;) are found to be G.:.C = 0 and ;.a! = %% for both naphthalene and anthracene. These conditions are used to obtain the band widths for these substances. The extremum conditions are not so simple in tetracene and pentacene and for simplicity only estimates are made for the band widths in these two substances. The band widths are given in Table 11.
+
-
-
- -
2
SEVERAL FORMS OF COLORED MODIFICATION OF SPIROPYRANS
Dec., 1962
2465
TABLE I11
-
VELOCITY COMPONENTS (1010 cm.z/sec.*) ---.4nthracene Electron
-NaphthaleneElectron Hole
1.79 1.26 0.031 0.000 0.000 0.000
6.57 5.67 0.034 -0.007 0.000 0.000
0.009 2.38 0.046 0.000 0.000 0.000
Hole
----Tetracene---Electron
Hole
-PentaccneElectron
Hole
3.77 6.35 0.742 -0.168 0.000 0.000
8.73 9.90 0.036 -0.018 2.63 -0.008
5.31 6.01 0.034 -0.004 -0.648 -0.003
8.22 8.83 0.014 -0.005 0.360 -0.003
5.02 6.13 0.027 -0.004 -0.734 -0.003
TABLE IV VELOCITY COMPONENTS IN PRINCIPAL AXESCOORDINATE SYSTEM. DENOTEDBY a”, b”, AND C” (1010 cm.2/sec.2) --Naphthalene--Electron Hole
V,”2 Vb“’ VC”2
1.79 1.26 0.031
-
0.009 2.38 0.046
Acknowledgments.-The
--. -
Anthracene Electron Hole
6.57 5.67 0.034
3.77 6.35 0.742
authors wish to t,haiik
Dr. 0. R. LeBlanc, Jr., of the General Electric Company for several very helpful letters regarding
-
Tetracene Electron Hole
12.01 6.62 0.036
4.93 (5.40
0.034
--
--
Pentacene Electron Hole
9.00 8.05 0,014
4.66 6.50 0.027
these calculations, and also Dr. J. Trot’ter for a let,ter containing revised crystal data for tetracene and pent,acene.
PHOTOCHROMISRI I K SPIROPYRAXS. PART 1V.l EVIDEK CE FOR THE EXISTEKCE OF SEVERAL FORIJlS OF THE COLORED MODIFICATIOS B Y RAHEL
HEILIGMAN-RIM, YEHUD.4 HIRSHBERG, ,4ND ERNST FISCHER
Lubor(Ltory of Photochemistry and Spectroscopy, The JPeirmann Institute of Science, Rehouoth, Israel RecezLed M a u 26, 1062
Both the wave length and the relative intensity of bands in the absorption spectra of the colored modifications of photochromic spiropyrans strongly depend on the character of the solvent used. Moreover, cooling causes pronounced changes in the spectra of solutions in non-polar solvents, while the spectra of alcoholic solutions are not affected by variation of temperature. The effect of cooling ceases when a certain very low temperature ( - 150°, -160’) is reached. At this temperature all solvent mixtures used become highly viscous. When the colored modification is produced by ultraviolet irradiation at low temperature in such highly viscous glassy media, its spectrum is different from that of the colored modification produced by irradiation a t a higher temperature and then cooled; the spectrum assumes the latter shape when the solution is warmed and then cooled again. The hypothesis of the existence of several stereoisomers of the dye molecule is forwarded, to account for these observations. Some of these stereoisomers are interconvertible thermally as long as the thermal energy of the molecules does not drop below a certain value and the viscosity of the medium does not exceed a certain limit. The various isomers difler both in their spectra and in their convertibility into the spiropyran by visible light.
Introduction The gciieral ideas about thermochromism and photochromism of spiropyrans, developed in previous publications, may be summarized as follows. Thermochromic “spiropyrans” exist in solution as an equilibrium mixture of two isomers-a colorless spiropyranic modification (A) and a colored merocyanine-like modification (B). The position of this equilibrium varies widely with the nature of the compound and the solvent, and with the temperature ; high temperatures arid polar iolvents favoring the colored modification (B). The rate a t which thermal equilibration takes place (starting from a non-equilibrium mixture) also depends on the same factors and is reduced to practically Aero at sufficiently low temperatures. At buch tem-
peratures any phototransformations which might take place can be investigated without the complication of thermal transformations between (A) and (B). The spiropyrans hitherto investigated can be classified as follows with regard to phototransformation: (1) Those in which the two modifications can be interconverted by the action of ultraviolet light, and (B) isconverted into (A) by irradiation with visible light; (2) those in which ultraviolet light converts (A) mto (13) and vice i’ersa but visible light has no effect; (3) t8hosein which the two modifications are not interconvertible by light, although both do exist. Compounds of classes (1) and (2) are called The present report deals with compounds of class ( I ) , as exemplified by I and 1V; and of class (3), ns exemplified by I1 and 111. In compounds of class
(1) Part 111 le IIedigtnaii-lhn E I C i o n soc 1% r l l l b l )
( 2 ) ( a ) Y I I i n h h ~ r g ,Compt rend , 231, QOJ ( l D W ) , (h) berg ,Ind E. l’iuclicr, J Chem S a c , C,PY (1953).
lIirslibeir
nil L
1 1 5 1lirr
Y . Hirsh-
