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-
R. HEILIGMAN-RIM, Y. HIRSHBERG, AND E. FISCHER
2466
2
..
'
"*"E
e.(
h
'
a
i
mr
Fig. 1 .-Solvent dependence of spectra of merocyanines: full curves, in non-polar solvents; broken curves, in alcohol. (a) Compound IB, 0.9 x 10-6 mole/l. a t -100': 1, in methylcyclohexane-petrol ether 1:1; 2, in ethanol-methanol 4:1, (b) compound I11 a t room temperature: 1, in decalin, saturated solution; 3, in benzene; 2, in propanol, 1.6 X 10- mole/l.: (c) compound 11, at room tem erature: 1, in decalin, 2.3 X 10-4 mole/l.; 2, in ethanox 0.9 X 10" mole/l.; (d) compound IVB: 1, in methylcyclohexane-isohexane 1:1 a t -150°, 2.2 X 10-6 mole/l.; 2, in propanol a t - l l O o , 1.7 X mole/.
Me IY
IA
I
Me IH
I
I
Me
C2HB
I1
111
Me
I VB
(1) irradiation with ultraviolet light, which is absorbed by both modifications, results in a photoequilibrium mixture containing between about 70 and 100% (B), whereas irradiation wit,hvisible light which is absorbed only by (B) converts all of it into iA). Results will be reDorted for the influence of On the absorption 'peeSOfvent ltnd Of trum of modification (B).
Vol. 66
Results The form of the spectra of the merocyanine-like compounds obtained by irradiation of photochromic spiropyrans, such as I or IV, as well as that of the spectra of "merocyanines proper," such as I1 and 111, depends to a great extent on the nature of the solvent used (cf. a similar observation regarding the thermochromic colored modification of the spiropyrans).s In non-polar solvents the spectra are, furthermore, strongly affected by the temperature. The effect of solvents and of the temperature on these spectra manifests itself both in frequency shifts and in variation of the ratio of the intensities of some absorption bands, these variations leading in extreme cases to complete disappearance of one or more bands. Solvent Effects.-These are illustrated in Fig. 1. Figures l a and Id compare the spectra of compounds I B and IVB in a polar and in a non-polar solvent at low temperature. Figures l b and IC show a similar solvent dependence, at room temperature, in the spectra of the two merocyanines I1 and 111, which resemble in structure the colored modification of I. The polar solvent is seen to suppress and even to obliterate some of the absorption bands. Temperature Effects.-These are even more striking, as illustrated in Fig. 2. Figure 2a shows the effect of cooling on the spectrum of a solution of IB in a non-polar solvent. At - 100" the intensities of the two main absorption bands in the visible, at about 530 and 560 mp, are approximately equal; as the solution is cooled the ratio of intensities changes in favor of the 530 mp band until the value At the same of this ratio reaches 2.3 at -160'. time the peaks of both bands undergo a bathochromic shift from 525 to 532 mp and from 559 to 577 mp, and a new absorption band appears at 610 mp. However, further cooling from -160 to -183' causes no further change in the shape of the absorption spectrum. The process to which the temperature-linked spectral changes are due thus appears to be "frozen" at about - 160°. Figure 2b shows the effect of cooling on a solution of IVB in a non-polar solvent. It is seen that the intensity of the 585 mp absorption band diminishes while new absorption bands appear at 735 and 670 mp. In Fig. 2d a similar effect of cooling on a solution of the same compound in another non-polar solvent, of higher viscosity, is seen. In this solvent, a mixture of methylcyclohexane and decalin, the change of the shape of the spectrum with cooling ceases already at about - 145O. In a polar solvent cooling from - 100 to - 130' does not affect the form of the spectrum in any way. On the other hand, the emission spectrum of these compounds in a polar solvent changes with cooling, at least in some cases; thus the fluorescence of IR, which is orange-red at -100O, becomes yellow at -140', whereas in a non-polar solvent it remains orange-red down to - 180'. Thermally-Irreversible Spectral Changes.-The spectral changes described hitherto are thermally reversible, ie., gradual cooling from -100 to - 160" transforms the spectrum of IB as showii ill (3) (a) Part I, Y.Hirshberg and E. Fischer, J. Chcm Soc., 29i 1954); (b) Part 11, ibid., 3129 (1954).
Dee., 1962
SEVERAL FORMS OF COLORED MODIPICATION OF SPIROPYRANS
Fig. 2a, while gradual heating from - 160 to - 100' causes the same change in the reverse direction. This oycle can be repeated again and again, the reversibility indicating that a thermal equilibrium is established at each temperature. The spectrum corresponding to this thermal equilibrium also is obtained when a colorless solution of IA is irradiated with ultraviolet light a t any temperature in the above range. If, however, the colored form of I is formed at - B O o by ultraviolet irradiation of IA at this temperature, its spectrum is different from that of the dye obtained by ultraviolet irradiation a t -lOOo and subsequently cooled to -BO', as shown in Fig. 3a. In curve 1, the ratio between the intensities of the peaks at 530 and 560 mp is even smaller than a t - loo', and corresponds to the hypothetical ratio a t some higher temperature. (This cannot be proved experimentally because above - 100' thermal transformation into the colorless IA sets in.) When the dye produced by irradiation a t - 183' (Fig. 3a, curve 1) is gradually heated, it is transformed spontaneously, between about -160' and -150°, into the dye formed by irradiation a t this temperature. Recooling to - 183' causes only minor changes in the spectrum. Spectra 1 and 2 of Fig. 3a are seen to differ in their absorption not only in the visible but also in the ultraviolet. Ultraviolet irradiation of IA in a methylcyclohexane-isopentane rigid glass a t - 183' thus is seen to result in a spectrum corresponding to a non-equilibrium state which is frozen-in under these conditions At temperatures above - 180' this spectrum changes spontaneously into that corresponding to an equilibrium state. Cooling of this latter form to -183' results in its being frozen-in at the equilibrium corresponding to -160'. In a mixture of methylcyclohexane and decalin, which is more viscous and reaches rigidity already at somewhat higher temperatures, the non-equilibrium rstate seems to be frozen-in already a t -150', as t;hown in Fig. 3c. Similar phenomena were obeerved with solutions of compound IV, with the results shown in Fig. 3b and 3d. In all these cases the non-equilibrium form of the dye can be created only by ultraviolet irradiation of the solutions in rigid glasses a t sufficiently low temperatures. As will be described in detail in Part V of this series, those colored forms of both I and IV which are obtained by irradiation a t - 183' (full curves in Fig. 3), and only they, can be made to revert into the corresponding colorless forms by irradiation with visible light a t this temperature. This is another manifestation of the profound difference between these dyes and those corresponding to the broken curves of Fig. 3, which can be "erased" by visible light only very slowly and to a slight extent.
2467
I
..
i
ereoisomersprobably is due also to the sharp increase in viscosity in these temperatiire regions, where both solvent mixtures form rigid glasses. The high viscosity may be expected to impede the motion of large groups within the molecule, which must occur during interconversion of isomers. At the moment of formation of the dye by ultraviolet irradiation of the spiropyran, and the resulting rupture of the carbon-oxygen bond, the dye molecule is pro‘bably in an unstable form X, in which the oxygen atom and the original spiranic carbon atom are still close to each other. This sterically unstable isomer then undergoes a series of rearrangements involving the methine bridge, and leading eventually to an equilibrium mixture of some or all of the isomers (a)-(d), as well as X. As pointed out above, these rearrangements involve comparatively large movements of one-half of the molecule in relation to the other one. The combined effect of low temperature and very high viscosity of the medium therefore may be expected to slow down or even prevent this rearrangement, and thus freeze-in the unstable isomer X immediately following its formation. These expectations are borne out by the experiments illustrated in Fig. 3, which shorn the result of ultraviolet irradiation of the spiropyrans in rigid media, mxh a$ mixtures of methylcyclohexane and isohcvnne at - 180”, or methylcyclohexane and t 1 w : i l i i i :it - 1 ,?(lo, Siivli irrndi:~tioiii w i i l t s in the
2469
formation of a colored modification whose absorption spectrum is quite different from that of the dye formed from the above after it had been heated to about -150’ (or -120’ in the m.c.h.-decalin mixture) and then recooled to the original low temperature. Such heating results in establishment of thermal equilibrium between the different stereoisomers, the equilibrium then being frozen-in by recooling. Additional evidence in favor of the above conclusions is furnished by the observation (to be described in detail in Part T’ of this series) that the dye produced in the rigid glass is reconverted into the spiropyran by irradiation with visible light under the same conditions. This is not the caw with the dye formed from the previous one on heating and recooling. Another feature of the spectrum of the dye formed on ultraviolet irradiation in the rigid glass is its similarity to the spectrum observed at the highest temperature a t which the dyes are not yet converted spontaneously into the spiropyrans (about -100’). This is seen from comparison of the “high temperature” curves in Fig. 2 with the “rigid glass” full curves in Fig. 3. Hence it can be assumed that the species formed by irradiation in a rigid medium is a high-energy isomer. One should, however, bear in mind that all the postulated stereoisomers probably have rather similar spectra, and the similarity of the above spectra does not necessarily prove the identity of the corresponding isomers. The over-all picture emerging from the above discussion thus may be summarized as follows: Ultraviolet irradiation of the spiropyrans results in formation of an unstable isomer, X, of the colored modification. In media whose viscosity is below a certain limit most of this isomer is immediately converted into other, more stable isomers, the equilibrium between the various isomers being established as long as the viscosity of the medium does not exceed that limit. However, in a rigid medium a t low temperature the high viscosity of the medium and the low energy content of the dye molecule combine to prevent the spontaneous conversion of X into the other isomers. In fluid media a tiny fraction of the dye exists in the form of X, this fraction increasing with temperature bccause of the higher energy of X. Only this isomer is photoconvertible into the spiropyran by visible light. Photoconversion of all the dye thus will proceed as long as the thermal equilibration between X and the various other isomers of thc colored modification takes place a t a sufficiently high rate. When this equilibrium mixture is frozen by cooling to rigidity, visible light can transform into the spiropyran only that small fraction of the dye which is in the form of X. On the other hand, if the dye is formed from the spiropyran by ultraviolet irradiation in a rigid medium, most of it remains in the form of X, and therefore is photoconvertible by visible light. In an alcoholic solution the contribution of the dipolar mesomeric structure is higher than in nonpolar solvents, owing to interaction with the polar solvent. In addition one of the stereoiromcr. may hc prcfcrcntislly stnhilizccl 1)y solvntioir, thc
2470
R. HEILIGUN-RIM,Y. HIRSHBERG, AND E. FISCHER
resulting adduct being sensitive to visible light. This accountsfor the absence of pronounced effects of c o o k on the absorption spectra of the dyes and on the rate of their photoconversion into spiropyrans by visible light in alcoholic solution. Experimental Irradiations and spectrophotometric measurements were carried out in special dewar-type quartz cells, in a Cary
T’ol. 66
.
Model 14 recording s ectrophotometer Complete details are given in part 111ofthis series and in earlier publications. Ex eriments at the temperature of liquid air were carried out the absorption cell placed in a specially comtructed copper block cooled by liquid air. The copper block with the cell was placed in a silica dewar flask with optical windows.
wig
Acknowledgments.-The authors wish to thank Mr. M. Kaganowitch for synthesizing the compoullds investigated, and Mrs. N*Caste1 for technical assistance.
PHOTOCHROMISM IN SPIROPYRANS. PART V.’ ON THE MECHANISRI: OF PHOTOTRANSFORMATION BY RAHELHEILIGMAN-RIM, YEHUDA HIRSHBERG, AND ERNSTFISCHER Laboratory of Photochemistry and Spectroscopy, The Weizmann Institute of Science, Rehovoth, Israel Received Mazl 26. 1962
Phototransforma.tions spiropyran merocpanine and merocyanine + spiropyran have been studied in some photochromic spiropyrans under varying environmental conditions. The dependence of the rates of these processes on the temperature and the chemical nature and viscosity of the solvent waa observed. The relative and absolute quantum yields of both processes at different frequencies of the photoactive light were determined. The results indicate the occurrence of consecutive and concurrent phototransformations and thermal interconversions between stereoisomers of the colored modification, besides the “main” phototransformation spiropyran merocyanine. -.f
Introduction two parts of the spiropyran molecule are approxiThe general ideas about thermochromism and mately perpendicular to each other. The photophotochromism have been summarized in previous isomerization spiropyran + merocyanine thus publications,2-6 and in the introduction to the pre- involves two aspects: (1) delocalization of two u ceding paper (Part IV). The photochromic spiro- electrons, following excitation, and resulting in pyrans to be dealt with here are exemplified by breaking of the C-0 bond; (2) rearrangement of compounds IA (1,3,3-trimethylindolinonaphtho- the methine chain and rotation of the two parts spiropyran), and IIA (N-methylacridinonaphtho- of the molecule in relation to each other so as to spiropyran), which are transformed by the action approach coplanarity. Such a process appears of ultraviolet light into strongly colored modifica- fairly plausible as soon as the energy required for tions usually represented by merocyanine-like delocalization of the C-0 bond electrons is supplied either in the form of thermal energy or as formulas, such as I B and I I B radiation energy. The transformation in the reverse direction, B 4 A (“ring closure”), involves the same two aspects, i.e., two electrons have to be Ik &BI demoted and the molecule has to be induced to undergo the rearrangements in the methine chain and the relative rotation of the two parts of the s o s 0 molecule before a spiropyran molecule can be reI I Me 3lt. formed. Processes A + B and B + A thus seem to be basically different, with B 4 A being a priori less probable. This also is confirmed by the fact that radiative ring opening has been observed with all thermochromic and many non-thermochromic spiropyrans, whereas radiative ring closure by visible light was found hitherto only in spiropyrans derived from indoline, such as I, or from acridine, such as II.’ However, radiative ring closure by The B modifications can be converted into the A ultraviolet light does seem to take place, as shown B under the ones by irradiation with visible light. The number by the incomplete conversion A of T electrons in a merocyanine molecule exceeds action of ultraviolet light, which probably is due to by two their number in a spiropyran. The mero- simultaneous occurrence of both A B and B A. The experiments reported here on the temperacyanine molecule is planar, while the planes of the ture dependence of the rates of photoisomerization (1) Part IV. J . Phys. Chem.. 66, 2465 (1962). reactions in both directions and the determination (2) E. Fischer and Y. Hirshberg, J. Chem. Soc., 4522 (1952). (3) (a) Part I , Y. Hirshberg and E. Fischer, ibid., 297 (1954); of their quantum yields were undertaken in an at-
fi-w
.--)
(b) Part 11, ibid., 3129 (19.54). (4) Y.Hirshberg, E. H. Frei, and E. Fischer, ibid., 2184 (1953). ( 5 ) Y. Hirshberg, J . A m . Chem. Soc.. 7 8 , 2304 (1956). (6) P a r t 111, R. Heiligman-Rim, Y. Hirshberg, and E. Fischer, J . Chem. Soe., 156 (1961).
(7) However, recent experiments in this Laboratory showed that auch ring closure with visible light does take place with certain bipyrospirans, oontaining only oxygen a8 hetero-atoms, under specific conditions.
