Dec., 1962
OPTICAL PROPERTIES
OF ADSORBED 2-(2’,4’-DINITROBENZYL)-PYRIDINE
2430
OPTICAL PROPERTIES OF 2-(%’,4’-DINITROBENZYL)-PYRIDINE IN THE ADSORBED STATE BY G. KORTUM,~ M. KORTUM-SEILER,~ AND S. D. BAILEY Con2nhtiun from the Pioneering Research Division, Quartemtaster Research and Engineering Center, Natick, Mass. Received May 26, 1962
Diffuse reflectance spectroscop haa been used in an investigation of the reversible photochemical conversion of 2-(2’,4’dinitrobenzy1)-pyridine in the adJlsorbed state. The tautomeric shift associated with this conversion was studied in the phase boundary of those adsorbents which serve aa electron acceptors. Two first order reactions were found which probably are due to the fading of a positive ion and of the neutral molecule, respectively. Activation energies have been estimated from the kinetic data.
The Kubelka-Munk theory,z describing the process of “diffuse reflectance” of finely powdered absorbing material, has not been extensively used because the invariably superimposed “regular reflectance” levels and broadens the derived spectra. To get the true absorption spectrum from reflectance measurements one therefore has to eliminate the regular part of the reflection. This can be done easily by triturating the sample together with a large excess of a non-absorbing standard of the same particle ~ i z e . ~Since , ~ in this process all organic and many inorganic compounds are adsorbed on the surface of the standard, this method is especially suitable for the investigation of the optical behavior of adsorbed molecules. Recent work., for instance on the reversible adsorption of different dyes on surfaces of appropriate ads or bent^,^.^ has shown that frequently a chemisorption is taking place which is accompanied by a pronounced change of color. This can be attributed to an electron-donor-acceptor-process between adsorbed material and the adsorbent. Equally, irreversible photochemical reactions of adsorbed molecules have been investigated by this method.’ The method, therefore, appeared promising for the investigation of the reversible photochemical reaction of 2-(2’,4’-dinitrobenzy1)-pyridine, which turns blue by illumination and fades again in the dark to its original colorless form. This reaction takes place in the crystalline form as well as in solution,8 but the rate of fading in solution is very rapid and can only be observed at low temperaturesg or by a special flash technique a t room temperature. lo The reaction has been interpreted8as a tautomeric shift which involves either the pyridine or the neighboring nitro group. (1) Visiting scientists from the Institute for Physical Chemistry, University of Tubingen, Tubingen, Germany. (2) P. Kubelka and F. Munk, Z . Techn. Phya., 14, 513 (1931); P. Kubelka, J. Opt. Soc. Am., 38, 448, 1067 (1948). (3) G. Kortum and G. Schreyer, Anoew. Chem.. 67, 694 (1955). (4) G. Kortum and J. Vogel, 2. Phuaik. Chcm. (Frankfurt). 18, 110, 230 (1958). (5) G. Kortum, J. Vogel, and W. Braun, Aneew. Chem.. 70, 651 (1958). (6) G. Kortum and J. Vogel, Chem. Ber., BS, 706 (1960). (7) G . Kortum and W. Braun, Ann. Chem., 634, 104 (1960). (8) R. Hardwick, H.S. Mosher, and P. Passailaigue. Trans. Faraday SOC.,66, 44 (1960); H. S. Mosher, C. Sauers, and R. Hardwick, J. Chem. Phus.. 82, 1888 (1960). (9) J. Sousa and J. Weinstein, J . Org. Chem., 41,3155 (1962). (10) G. Wettermark, J. Am. Chem. Soe.. 84, 3658 (1962).
I
It
XOn
N
/I
HO
0
I11
I
We have studied this reaction in the phase boundary of several adsorbents by measuring the reflectance spectra of mixtures, where the molar fraction of the nitro compound was to against the pure adsorbent as standard. Adsorption was accomplished by grinding for several hours in a ball mill in the dark in a COzatmosphere. The sample then was exposed to diffuse daylight for 30 minutes and measured again. The spectra on NaCl as adsorbent are shown in Fig. 1. The logarithm of the Kubelka-Munk function
F(R,)
‘c = (1 - R J 2 2R
S
is plotted against the wave number. R, 3 I(sample)/I(standard) is the measured relative diffuse reflectivity, E the molar extinction coefficient, c the molarity, and s the scattering coefficient, which is essentially independent of the wave length. The spectra are therefore identical with the true absorption spectra, except for a parallel shift in the values of log E . Quite analogous spectra were obtained on silica and lithium fluoride as adsorbents. Prolonged irradiation changes the compound irreversibly both in the case of the pure substance and in the solutions.* The adsorbed nitro compound when illuminated turns blue, like the pure crystalline compound (A max E 600 mp), with the color fading slowly over a period of about 10 hours in the dark. The spectrum can be measured easily therefore a t room temperature. Since earlier studies had shown that the interaction between adsorbed molecules and the adsorbent can in some cases be prevented by adsorbed
G. KORTUM, M. KORTUM-QIETLER, A U D S.D. BAILEY
2440
0
-I
-2
3
!3'
k -3
Fig. 1.-Reflectance spectra of 2-( 2',4'-dinitrobenzy1)pyridine adsorbed on NaCl before exposure to light (a) and after exposure to 30 minutes of diffuse sunlight (b).
VoI. 66
Quite unexpectedly the conversion to the blue form by illumination failed when the compound was adsorbed on MgO. Instead, an irreversible reaction in the dark took place, indicated by two maxima in the visible region a t about 670 and 480 mp, respectively, which did not change upon irradiation in the ultraviolet. This is shown in Fig. 2. This reaction, in contrast to the tautomeric shift, seems to be accelerated by water and can best be investigated on adsorbents which are only air-dried. An indication of the reaction mechanism was given by earlier experiments5 on the reflectance spectra of s-trinitrobenzene adsorbed on RIgO. Whereas s-trinitrobenzene adsorbed on SiOz or NaC1 remains colorless, the adsorption on XfgO produces a bright red compound, the spectrum of which is analogous to that of an alkaline solution of the same compound in water. This reaction has been explained according to polarographic investigationsll as an addition of OH- ions on the benzene nucleus: NO2
SO2
-A We therefore triturated 2,4-dinitrotoluene with an excess of MgO and SiOz, respectively, and found that it reacts with MgO in the same way whereas on silica it remains unchanged. We may therefore conclude that an analogous reaction takes place between the dinitro compound and MgO a t the phase boundary, the MgO acting as an electron donor
-I
-2
-3, 0
24
2%
32
v
3'6
4b X/03C??l.-'
Fig. 2.-Reflectance spectrum of 2-(2',4'-dinitrobenzy1)pyridine adsorbed on magnesium oxide.
/I
N /( 0
IV
water,js6we a t first carried out all experiments in the absence of moisture by heating the adsorbents The quinoid structure of this compound explains to 500" for several hours and sealing the sample the absorption in the visible and prevents the cells with quartz plates in a glove-box in a dry conversion I -t 111, but would not prevent the carbon dioxide atmosphere. It was found, how- tautomeric reaction I -+ 11. From the fact that ever, that the exposure of the adsorbed and con- the dinitro compound does not turn blue on MgO verted compound to moist air did not change the by illumination we conclude that the mechanism intensity of absorption, so that later experiments of this conversion consists more likely in an intrawere made with adsorbents which were air-dried molecular shift I % I11 than in a shift I @ 11. only. The strong partial dipole moments of the This can be confirmed further by the optical benitro compound appear to be capable of displacing havior of the 2-(4'-nitrobenzy1)-pyridine, in which the water molecules from the surface of the adsorb(11) L. Holleck and G. Perret, Z. Eiektrochrml, 59, 114 (1953); 6 0 , ent. 463 (1956).
Tlcc., 1962
2441
OPTICAL PROPERTIES O F L4DSORBE1) ~-(2’,4’-T)rSITT fi, which might be due to different interaction of the cation or the neutral molecule, respectively, with the surface.
PHOTOCHROMY AND THERMOCHROMY OF ANILS BY M. D. COHENAKD G. M. J. SCHMIDT Department of X-Ray Crystullography, Weizmann Institute of Science, Rehouoth, Israel Received M a y 66, 1988
In crystalline salicylidene anilines, photochromy and thermochromy are mutually exclusive properties. Thermochromy involves an intramolecular proton shift to the quinonoid tautomer; this shift occurs both in the ground state and excited state. In the photochromic process there is also an intramolecular proton shift in the excited state, but the nature Of the colored product is not yet unambiguously defined. Photochromy is a property of the isolated anil molecule in an arbitrary rigid matrix; thermochromy, on the other hand, necessitates certain specific structural types.
Introduction Some crystalline anils of salicylaldehyde (I) are
photochromic ; on irradiation with near ultraviolet light t,heir color deepens with the formation of an absorption band in the visible. This color-deepening can be reversed (eradicated) by irradiating with light absorbed in the latter band, or thermally in the dark. Photochromy in this series is a topochemically determined phenomenon, Le., there seems to be no correlation of activity with substituents X’ and Y , but the packing arrangement in the crystal is of importance. Thus, for example, of the chemically very similar 4-chloro- and 4-bromo-anils, the latter is photochromic but the former not. Further, several anils have two polymorphic modifications of which only one is photochromic. Summary of Experimental Observations We have reported previously that in rigid glassy solutions all anils derived from salicylaldehyde and substituted salicylaldehydes are photochromic, whether or not they are so in the crystaL2 By varying the chemical structure of the ani1 molecule used in these experiments we were able, then, to determine what features of the molecule are essential for photocolorability. We found that the anils of orthomethoxy- and of para-hydroxy-benzaldehydes, and
of benzaldehyde itself, are not colorable; thus the ortho-hydroxyl group is essential to the process. KOcases of photochromy in the crystal have been found among the anils of hydroxy-naphthaldehydes. However, the anils of 2-hydroxyl-1-naphthaldehyde (11) and of 1-hydroxy-2-naphthaldehyde (111) are photochromic in “high temperature” glassy solutions. Anils of I1 and I11 also have absorption bands in the visible when dissolved in hydroxylic solvents a t room temperature, or in non-polar This absorption solvents a t low temperat~res.2~V~v~ is not observed in the absence of the hydroxyl group, nor is it found in solutions of anils of 2hydroxy-3-naphthaldehyde. In the crystal, the dark-fading of the photo-color is a typical rate process with activation energy of the order of 20 to 25 kcal. mole. - l a Above a certain temperature this process is so rapid that photochromy cannot be observed by conventional methods. This “upper temperature limit” is characteristic of the particular anil and crystal modification studied. On the other hand, for many anils it has been observed that for irradiation carried out a t successively lower temperatures the yield of photo-color drops off, and there is a “lower temperature limit” below which photochromy is not observed. Because photochromy is, thus, found only between certain temperature limits it was necessary to study each compound over a range of temperatures. These experiments showed that many of the crystalline anils are thermochromic and that, in fact, thermochromy and photochromy are mutually exclusive properties; thus, each particular anil crystal modification is either photochromic or thermochromic, but not both. Keither photo-
(1) Further reports on these and other topochemical systems are to be submitted shortly to J . Chsm. Soc. (2) (a) M. D. Cohen, Y. Hirshberg, and G. M. J. Schmidt in “Hydrogen Bonding.” D. Hadzi. Ed. Pergamon Preen, London, 1959, p. 293; (b) M. D. Cohen and G. M. J. Schmidt in “Reactivity of Solids,” J. H. de Boer, Ed., Elsevier, Amsterdam, 1961,p. 558.
(3) M. D. Cohen, Y.Hirshberg, and G. M. J. Schmidt, abstracts in Bull. Res. Council l ~ d6A,, 167 (1957); XIVth IUPAC Congress, Paris, 1957,Vol. 11, p. 84. (4) W. Vosz, “Elektronenapektren aromatischer Azomethin- und Azaverbindungen,” Thesis, Technische Hochschule, Stuttgart, 1960.
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