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 condi-
tions.
MECHANISM OF PHOTOTRAPSFORMATION ISSPIROPYRASS
Dec., 1962
247 1
TABLE I HALF-LIFE TIMES, (MIN.) OF COJARERADICATION WITH "VISIBLE" LIGHT Temp.,
IB
(MCH-IP) (MCH-IH)
IB IB (MCH-D) IIB (MCH-MCP) IIB (MCH-D)
*C. -100 6 28 13
.. ..
-110
100 03
25
...
- I20
- 130
200 150
m 0)
...
80
+ +
--.)
- 150
W
W
W
Q)
100
...
..
...
43 43 Data in parentheses are explained in the following paragraph.
tempt t o gain insight into the mechanism of these processes. Results 1. Temperature Dependence of the Phototransformation A 4 B.-The rate of color formation by irradiation with ultraviolet light a t either 365, 313, or 303 mp was found to decrease slightly on cooling both in non-polar solvent mixtures (methylcyclohexane with either isopentane, isohexane, isooctane, methylcyolopentane, or decalin) and in alcoholic solvents (ethanol with methanol, 1-propanol with 2-propanol). Thus for compound IA, with the same light source used throughout most of these experiments, half-life times of color formation were found to increase gradually from about 1.5 min. at - 100' to 9 min. at - 183' in methylcyclohexane -isohexa,ne, and from 1.2 min. at -100" to 2.2 min. a t - 150" in methylcyclohexane-decalin. Similar results were obtained also in alcoholic solutions of this compound in the range - 100 to - 150". With compound IIAretardation of colorformation by cooling is more pronounced in both types of solvents. Thus, in methylcyclohexane-decalin the half-life times of color formation brought about by irradiation with the light of 365 313 303 mp lines increase gradually from 2 sec. a t -100" to 28 sec. a t - 150". All the above solvent mixtures form rigid glasses at sufJiciently low temperatures within the above-mentioned temperature range. It appears that this fact has no pronounced effect on the rate of merocyanine formation. 2. Temperature Dependence of the Phototransformation B -+ A.-As mentioned in the Introduction, the phototransformation B + A is of special interest, and the results therefore will be described a t some length. (a) Temperature Dependence of the Process B A w i t h Visible Light,-In all non-polar solvents the rate of this process is strongly reduced by lowering of the temperature. Table I summarizes the results obtained with compounds IB and IIB, in the form of half-life times of spiropyran formation. The concentration of the latter was followed a t the characteristic sharp absorption peak in the ultraviolet .6 Irradiation was carried out with the light emitted by a mercury arc above 410 mp, Le., in practice the three lines a t 436, 546, and 578 mp. (It was found that the ratio between the quantum yields of light a t 546 and 436 mp in non-polar solvents was not affected by change of temperature.) In all cases B was formed by ultraviolet irradiation a t -100" for 113, a t -110" for IIB and the solution then was cooled to the required temperature. Solvent mixtures used were methylcyclohexane (MCH) with eiither methylcyclopentane (MCP) , isohesane (IH), or isopentane (IP) (all these mix-
- 140
68
53
- 183 0) Q)
(132)"
(24)"
w
160 260 (3)"
(SIQ
-
--r b
-1zo*c
WAVELENGTH
_1
my
Fig. 1.-Com ound I, 0.75 X 10-6 mole/l. in MCH-IH (2: l), irradiatefwith visible light a t different temperatures; full lines, IB; broken lines, consecutive stages of irradiation; dash-dot lines, IA. (a), (b), and (d), irradiated with 578 546 436 mp; (c), irradiated with 578 mp (curve 3), heated t o -110" (curve 4), and recooled to -140' (curve 5, identical with curve 2).
+
+
tures form rigid glasses a t the temperature of liquid air), and methylcyclohexane with decalin (D) (this mixture forms a glass which tends to crack at temperatures below - 160'). Actual absorption spectra in the course of irradiation are shown in Fig. la, Id, and 3d, where the increase in the intensity of the ultraviolet and the decreases in the intensity of the visible bands are shown to run parallel. Deviations from this behavior are dealt with later on. The data in Table I show that in all non-polar solvent mixtures the rate of B 3 A decreases on cooling, and approaches zero somewhere between -120" and -145". The first two mixtures are fluid down to -135O, whereas the MCH-D mixture is highly viscous already at -100". In solutions in alcohols the rate of formation of spiropyran from the corresponding merocyanine is little affected by variation of temperature when the process is brought about by irradiation with light at 546 and 578 mp. Typical results for the process IB + IA in ethanol-methanol, effected by irradiation with light a t 546 mp, are given in Table I1 in the form of half-life times of color eradication a t various temperatures. TABLE I1 HALF-LIFETIMES, MIN. Temp., O C . Time, min.
-95 6.5
-105 6
-120 5
-140
7
-150 13.5
-1% 24
The rate of color eradication passes through a shallow maximum between -105 and -120' and then decreases slowly on further cooling. The same trend was observed with light at 578 mp. However, when the process is brought about by
2472
R. HEILTGMAN-RIM, Y. HIRSHRERG, AND E. FTSCHER
irradiation with light a t 436 mp the temperature dependence of its rate is different: after passing through a similar maximum a t - 115 to - 120' its rate is sharply slowed down by further cooling and approaches zero already at -140". In alcoholic solutions the formation of spiropyran runs strictly parallel to the disappearance of color during irradiation of B with visible light, throughout the temperature range investigated, - 95 to - 150". It should be recalled here that in solutions in nonpolar solvents the shape of the spectrum of the merocyanine is strongly temperature dependent while in alcoholic solutions it is altogether unaffected by variations of temperature. (b) Phototransformation B -+ A Effected by Visible Light in Rigid Media at Low Temperature. -In the experiments described in the preceding paragraph, B always was formed from A by ultraviolet irradiation at about -loo", a t which temperature all solvent mixtures are fluid. However, as shown in the preceding publication, if the process A + B is carried out in a rigid medium a t - 183", B created under these conditions is spectroscopically different from the B formed a t -100" and then cooled to -183". In solutions in a MCH-D mixture a similar behavior is observed already at about -1.55" (cf. Fig. 3 in the preceding paper). These colored modifications, denoted by X, proved to be photoconvertible into the corresponding spiropyrans by risible light. Figure Id describes a typical experiment a t - 183" in which spiropyran formation and dye eradication are seen to run parallel, exactly as a t -100' (Fig. la). Figure 3d shows a similar behavior in compound IIB irradiated with light at 546 mp a t -1183'. Moreover, from the kinetics apparent from Fig. 2d, it seems that under these conditions at least a large part, if not all, of B is reconvertible into A. It is particularly striking to compare the comparatively rapid photoconversion I B + IA at -183", a t which temperature the medium forms a rigid glass, with the virtual absence of this reaction a t - 130°, where the medium is still fluid. IIB -+ IIA at, - 183 is still more rapid (half-life time 8 min. under irradiation with light a t 546 mp). Some relevant results are given in Table I, in parentheses, and show that in compound I B the half-life time of color eradication under these conditions is about five times longer than a t -lOOo, whereas in IIB it is even shorter than a t -110". I n solutions in a MCH-D mixture this phenomenon occurs already a t -150". It should be recalled here that if the X form is heated to about -165" (in MCH-IH), it is transformed spontaneously into the B modification, and cannot be re-formed by cooling. (c) Phototransformation B -+ A under the is no direct Action of Ultraviolet Light.-There proof for the occurrence of the transformation B 4 A under the action of ultraviolet light on compounds I B and IIB as there is no way of isolating these compounds or of achieving complete transformation of IA and IIA into IB and IIB. I n this case the existence of this process can only be deduced indirectly from the fact that the process U.V. A + B does not go to completion. In reversible
Yol. 66
photoisomerizations photoequilibrium is established when the rates of the two opposing photoreactions A B under the action of the particular photoactive light used, are equal. Hence, in the ab-
*
U.V.
U.V.
sence of B 4 A the reaction A + B should go to completion and the conversion of A into €3 should he U
v.
complete. Since this is not the case, B -+ A must be inferred to occur. Moreover, if the ratio heil \
tween the yields of the two processes A
B and B A would vary with temperature, this should affect the state of photoequilibrium S o pronounced temperature effect of this sort has been observed in I and I1 with light at 365 or 313 mp in either solvent and it thus may be deduced that, +
u v. +
u.v.
the rate of B + A does not depend on temperature to any appreciable extent.* Thus, in this respect the behavior of ultraviolet light, in a nonpolar solvent, seems to differ from that of visible light described in section 2a. (d) Phototransformation of the Merocyanine Modification B Not Involving Formation of Spiropyrans ("Ring Closure").-As described in section 2a, the phototransformation of B into A by visible light in non-polar solvents is slowed down on cooling and virtually stops around - 140". However, a t such temperature another effect is observed as a result of irradiation with visible light, as shown in Fig. ICfor compound IB. Irradiation a t 578 mp causes pronounced changes throughout the visible and near-ultraviolet region, but has no effect on the absorption in the region of the characteristic spiropyran band around 245 mp. It thus appears that a sort of "internal" photoisomerization within the Vis.
merocyanine, B -+ B', i.e., photoisomerization between different stereosiomers oi the colored form, takes place, proceeding with a much higher efVIS.
ficiency than B + A and without retardation by cooling. Thus, the half-life of the change depicted in Fig. l c is about 3 min. at -140" and practically also a t -130 and -150". This may be compared with a half-life of about 45 min. observed for v1s.
B 4 A a t -100" under otherwise similar conditions. If a solution thus changed by visible irradiation a t - 140' is gradually warmed up, the above change reverts, the rate of this reversion increasing with temperature. For the case described in Fig. I C this reversion is complete within a few minutes a t -110". As a result the original merocyanine spectrum is re-formed, and recooling to -140" results only in the usual temperature effect on the spectrum (cf. Part IV). A complete cycle is thus performed as follows: "color formation" by ultraviolet irradiation a t - 140" (curve 2) , "color change" by visible irradiation a t - 140" (curve 3), heating to -110" (curve 4), and recooling to -140' (curve 5, identical with 2). All steps except the first one involve only the meroryanine. "I
(8) The occurrence both of the process B --f A and of the temperature dependence of the position of photoequilibrium has been proved in certain nitro derivatives of henzosplropyrans. wh1rh will be ~ e ~ ~ o r t e d in the follouing part in this series
MECHANISM OF PHOTOTRAYSFORMATIOS IN SPIROPYRANS
Dec., 1962
2473
Visible irradiation a t temperatures above - 140" causes two concurrent photoreactions, namely, B A and B -+ B', as illustrated in Fig. l b for -120". An initial rapid change in t,he visible, without any change around 245 mp, is followed by a slower change in both regions. Exact and rapid measurements irtdicat,e a similar behavior even a t -loo", though here B + B' is of course over-f
vis.
shadowed by both B A and B' -t B (thermal reaction). Kinetic curves for all photoreact'ions described are shown in Fig. 2. Figures 3b and 3c show similar interconversions of the stereoisomers of -f
C
-14n.c
via.
IIB running coiicurrent,ly with processes IIB IIA and I I A Y IIB. 3. Quantum Yields.
+
(a) Relative Quantum U.Y.
U.V.
Yields of A --f 13 and B + A.-In case of interconversion of two isomers A and B under the action of light, the photoequilibrium established by irradiation with light a t any particular wave length, X, depends only on the molar extinction coefficients E of the two isomers a t the particular A, and on the quanmm yields (PA-B and (PB-A of t'he photoconversion in both direction^.^ Since a t equilibrium the rates of reaction in both directions are equal
o
40
BO
120
160
o
40
ao
120
160
T I M E , MlNlJTES
Fig. 2.-Temperature dependence of the kinetics of phototransformation of I B brought about by irradiation with visible light (cf. Fig. 1); full lines, variation of I B concentration, followed by optical density at 565 mp; broken lines, formation of spiropyran IA followed by the changes in optical density a t 245 mp. (a), (b), and (d), 546 436 mp; (c), irradiated with irradiated with 578 578 mp.
+
+
[A]EA(PA= [BIEB'PB and therefore the equilibrium constant K is given by
K = - [AI =[BI
(PB~B (PAEA
and the ratio between the quantum yields in both directions is 'PA
'PB
-
[BIEB [AlEA
Since both the merocyanine and the spiropyran absorb in the ultr:tviolet, the irradiation of a spiropyran with ultraviolet light results, a t photoequilibrium, in incomplete conversion into the merocyanine.6 The ratio of the quantum yields of the reactions in the two opposite directions (PA/(PB can be dculat8edfrom the above equation. The values of TABLE I11 CA/PB at
C nm p o u n d
(I) a t 313 m
Solvent
p
(I) a t 365 mp
MCH-IH alcohol MCH-IH alcohol MCH-D alcohol
-looo 5 10 2.5 6.5
PA/CB a t
-180'
5
... 2.5
Fig. 3.-(a), (b), (d), compound IIB irradiated with visible light a t different temperatures; full lines, before irradiation; broken lines, consecutive stages of irradiation. (a), (b), 1.1 x 10-6 mole/l. in MCH-D, irradiated with 578 546 436 mp; (d), 2.2 X 10-6 mole/l. in MCH-IH, irradiated with 546 mp; (c), IIA, 1.1 X 10-5 mole/l. in MCH-MCP, irradiated with 365 mp; curves, 5, 6, and 7 show interconversion of stereoisomers of IIB.
+
+
ring opening is seen to be higher than in ring closure. The values given above are subject to the uncertainty in the equilibrium constant [A]/[B] and in E B as described before.6 In compounds I and I1 the ratio ( P A j ( P B practically does not change with temperature. vis.
(b) Relative Quantum Yields of B -t A at Different Wave Lengths.-The spiropyran A does not (11) at 365 mp 10 10 absorb in the visible region, EA = 0, and FA ob3.5 ... viously has no meaning. It is however of interest, to compare the relative ~ B ' S a t various wave (PA/(PB thus calculated from results obhined for I photoequilibrated by light a t 365 and a t 313 mp, lengths in the visible region. Qualitative investiand for I1 photoequilibrated by light of 313 mp, gations reported earlier5from this Laboratory were a t -100" and -180", are given in Table 111. now repeated and analyzed quantitatively under In all cases the efficiency of ultraviolet light in well defined conditions, as described in the following section. The results may be summarized as (9) (a) G. Zimmerman, et d.,J . A n . Chem. Soc., 80, 3528 (1958); follows: in non-polar solvents the yields a t the (b) E. Fiacher, ibid., 82, 3249 (1960); ( 0 ) D. Schulte-Frohlinde. Ann., 616. 114 (1958). four wave lengths investigated (the mercury emis-
...
R. HEILIGMAN-RIM, Y. HIRSHBERG, AND E. FISCHER
2474
sion lines at 405,436, 546, and 578 mp) were identical within a factor of two, all of them dccrcasing on cooling; in alcoholic solut,ions in the presence of triethylamine (added as a weak base) the situation with respect to the ratio between the quantum yields at different wave lengths is similar; however, in the absence of a base, even in alcohol freshly distilled from potassium hydroxide, the efficiency of light at 436 mp was 30-80 times higher than that of light at 546 or 578 mp, corroborating our earlier results.6 The range 30-80 is given here because values depend strongly on the alcohol used. Moreover, in such solutions the yield with light at 436 mp falls sharply on cooling, and below about - 130“ approaches zero, whereas with light a t 546 or 578 mp the yield is substantially independent of temperature (cf. section 2a above). (c) Calculation of the Relative Quantum Yields. -The intensity IXof the incident monochromatic light of wave length X (expressed as the number of light quanta per minute), the transmission TXof the solution at the same wave length, and the observed half-lives ~11%of the process via.
B + A were used as follows: under otherwise identical conditions the yields at various X’s are inversely proportional to the numbers of light quanta QX absorbed during T I / , ( x ) . During any infinitesimal time interval dt the number of quanta absorbed is I x ( ~ TX)dt. During finite periods of irradiation the change of TXwith time has to be taken into account by replacing (1 - 2‘) by 1 -
However, an exact evaluation shows that the error introduced by using the arithmetic mean of TXinstead of the last expression is small in comparison with the experimental errors of the method. The final approximation for the number of quanta absorbed during the interval TI/^ a t any wave length X is therefore Q X = Ix(1 pA)~l/l, where TXis the average transmission of light at wave length X during n/*. For the present purpose only relative values for IX a t various wave lengths are needed. These were measured by Bowen’s “fluorescent screen” method, as adapted to a somewhat similar purpose by Weber and Teale.Io This method is based on the observation that the fluorescence yield of rhodamine B is virtually independent of the wave length of the exciting light. The intensity of fluorescence, as measured by a photomultiplier, is therefore proportional to the rate a t which quanta at wave length X hit the solution, ie., Ix!and the photocurrent thus can serve as a measure of Ix. Relative values of IXthus measured agree reasonably well with those calculated from the spectral energy distribution data of the mercury arc, given by the manufacturer, and the transmission of the filter combinations used.
.--
(d) Absolute Quantum Yields of A 2 B and Vis. B + A.-These were calculated from the basic equation given by Z k ~ m e r m a n . ~The ~ intensity of the incident monochromatic light was measured by 10) G. Weber and F. J. Teale, Trans. Faraday Soc., SS, 646 (1957).
Vol. 66
ferrioxalate actinometryll for light of wave lengths up to 436 mp. The absolute IX values for light a t longer wave lengths were calculated from t,hose at a shorter wave length and the relative IX values measured as described in section 3c above. The quantum yields thus calculated again depend on the accuracy with which EB and the photoequilibrium constants can be determined. These reservations should be kept in mind when evaluating the results summarized in Table IV for compound I a t - 100”. TABLE IV ABSOLUTEQUANTUMYIELDS AT Photoaoiive lighi, mw
‘PA-B
x 1 x
5
.... ....
546
Solvent
9B-A
5 x 10-2 6 X lo-*
313
- 100”
10-8
5 X 10-3
5 x 10-4
Ethanol-methanol Non-polar Ethanol-methanol Non-polar
Values for (PB-A a t higher temperatures cannot be measured because then thermal isomerization B + A sets in. In view of the sharp temperature via.
dependence of (PB + A in non-polar solvents, described in section 2a, t,he value of (PB-A at 546 mp, given above, must be regarded as valid for -100“ only. Higher values probably would be obtained if measurement at higher temperatures were possible. Discussion General Conclusions.-The radiative conversion of a spiropyran into a merocyanine involves two aspects : (a) redistribut,ion of charge following electronic excitation and (b) change of molecular configuration. It can be inferred from the absorption spectrum of spiropyrans that the primary electronic excitation leading to A + B photoisomerization takes place in the aromatic, naphthopyran, part of the spiropyran molecule. The net result of the change of the electronic structure is the increase of the number of T elect,ronsby two. This result may be represented by scheme I, whereby two Q electrons are transferred to the a electron system of the molecule and the C-0 bond is broken
.& eo. .?,
r ., ( ’
(’
‘ ( 5
.? ..
’
(j.
&
. . -----+
.s
.
(.
(’
{j;;(’’
I (cr and n electrons are denoted by points, ?r electrons by circles). The “ring opening” results in the formation of a high-energy isomer of the merocyanine in which the configuration of the spiropyran is still partly retained. In this isomer, designated “X,” the distance between the oxygen and the “spiranic” carbon atoms is probably relatively small. This configuration could be related to the cis isomer with respect to the central bond in the methine chain, in which the strong steric repulsion (11) C. G . Hetchard and C. A. Parker, Proc. Roy. Soe. (London), A586, 518 (1956).
MECHAKISM OF PHOTOTRANSFORMATION IN SPIROPYRANS
Dee., 1962
2475
would be balanced by the electrostatic attraction between the negatively charged oxygen and the positively charged nitrogen or carbon atoms
I;
hv
GJ
The B 4A, or “ring closure,” process has hitherto been observed only in nitrogen-containing spiropyrans.12 The net result of the charge redistribution in the course of this process might be represented either by scheme I! in the reverse direction, or alternatively, by scheme 11, where the n electrons of the nitrogen atom are involved.
9” 3
Figure 4. I1
At this stage no attempt will be made to draw conclusions regarding the mechanism of these charge redistributions. The observations reported here pertain largely to the second aspect of the process of photoisomerization-the rearrangement of “X” into the %ormal” equilibrium mixture of several stereoisomers of B, all of which are in the trans configuration with respect to the central bond of the methine cliain. In Fig. 4, A denotes the spiropyran, and B the equilibrium mixture of the stereoisomers B’, B”, B”’, and B’”’ Reactions are numbered 1 to 20, and can be either thermal ones, denoted by italic numbers, or photochemical ones, denoted by starred numbers. The following tentative postulates are forwarded to explain the results described above for solutions in non-polar solvents. (1) At sufficiently low temperatures the rates of 1 and 2 are practically zero and do not have to be taken into account. These are the conditions of “thermal stability of the colored modifications.” (2) There is rio direct conversion A 4 (B’, B”, B”’, B””) or (B’, B”, B”’, B’”’) 4A, either thermal or radiative. Steps “11” and “12” are actually 3 or 4 2*, respectively. superpositions of 1* In other words, a spiropyran can be formed only from that isomer of the corresponding merocyanine which has the right configuration and is therefore “closable.” ( 3 ) Steps 4, 6, 8, 10 are slowed down sharply on cooling, to an extent depending somewhat on the nature of the non-polar solvent mixture used. The observed temperature dependence of “12” actually is due to 4 , which becomes the rate determining step when 4 < 2*. ‘This effect does not depend on the viscosity of the solutions, since it is observed both in fluid and in highly viscous media. (4) Steps S , 6 7, 5 are slowed down by the combined effect of low temperature and high viscosity
+
(12)
Cf.,lioaeirr, footnote 7.
+
and practically stop under conditions existing in a MCH-D mixture a t around -150°, and in a MCH-IH mixture a t around -180”. Ultraviolet irradiation of the spiropyran under such conditions results in the formation of “X” only, which can be reconverted into the spiropyran by visible light. ( 5 ) Step 4* (and the analogous 6*, 8*, lo*) does not occur, as otherwise “12”’ should proceed even with merocyanines formed a t about - 100” and then cooled to - 180”. Step 3* probably also does not take place as shown by the fact that the colored modification formed a t - 180” is completely convertible into the spiropyran by visible irradiation. (6) If, as stated in ( 3 ) , step 4 virtually stops a t around - 140”, further cooling should freeze-in the equilibrium established at this temperature. On the other hand such a solution, when irradiat,ed with visible light at - B O ” , changes very little. It therefore must be concluded that the fraction of X in the mixture is very low a t around -140”, since any X present is photoconvertible into the spiropyran. (7) The thermal equilibrations among the B’s depend on the temperature and the viscosity to various extents. Some take place down to the temperature of high viscosity (- 160” in MCH-IH, - 140” in MCH-D) as evidenced by the continuous change of the absorption spectrum on cooling. Others already stop around - 125”,as shown by the fact that the spectral changes caused in the merocyanine modification by visible irradiation a t about -140” persist at this temperature are reversed by heating to above - 125”. At least some, and perhaps all, of the reactions l5*--2O* (and other photointerconversions between B’s not shown in the scheme for lack of space) are possible (Fig. IC, 3c), and may be compared to the photoconversion of geometrical isomers. Since all isomers B‘-B”’’ absorb in the visible, irradiation should lead to a photoequilibrium mixture. Some of these photoisomerizations 15*-20* may be over-shadowed by “12”’ (at temperatures where it also takes place), and also by thermal steps 15-20.
R. HEILIGMAN-RIM, Y. HIRSHBERG, AND E. FISCHER
2476
(8) It is evident from point (2) that the quantum yield measured for “12*” is meaningless. When 4 is slow in comparison with 2*, the over-all rate of “12*” will be given by that of 4, any Xformed from B being immediately photoconverted into A; if 4 is fast as compared with 2*, the over-all rate will equal that of 2*. It is by no means certain that at - 100 and -110”, the highest temperatures a t which photochromism of I and 11, respectively, can be investigated, 4 >> 2*. The quantum yield of “12*” under these conditions may therefore serve only as a lower limit for that of 2*. (9) The above postulates explain the observed dependence of the rate of “12*” on temperature, when brought about by visible light. With ultraviolet light the situation is less certain. I n the compounds whose behavior has been described here p~ for ultraviolet light seems to decrease only slightly on cooling. However, this is based on photoequilibria in which [B]/[A] 3 10, and any small shift in these photoequilibria strongly affects the ratio c p ~ l p . 4 calculated from it. In the absence of additional evidence it is impossible to decide whether “12*” with ultraviolet light also decreases on cooling, or if ultraviolet light, as distinct from visible light, is capable of causing 4* or even directly “12*” (10) In this context it should be recalled that certain spiropyrans are converted into merocyanines by ultraviolet light, but visible light does not cause the reverse p r o ~ e s s . ~ Nevertheless 1* does not go to completion in these cases,6 indicating that a photoequilibrium is reached in which the rates of “ll*” and ‘(12*” are equal. A possible explanation would be that in these cases 4 is too slow within the whole accessible temperature range, and therefore “12*” = 4 2* cannot take place with visible light. Ultraviolet light is capable of acting as suggested a t the end of point 9. (11) The above results do not rule out the existence of a temperature dependence of the really photochemical part of process “12*”, ie., 2*. In fact, the observed temperature dependence of the process was originally13 regarded to be a consequence of a potential barrier between the excited states of X and A. (A similar explanation was forwarded for the observed temperature dependence of rates of photoisomerization in azocompounds and in ~tilbene.~~~l~) (12) The mechanism of photoconversion in alcoholic solution is apparently quite different and probably involves the complex solute-solvent, rather than the solute alone. With this complex “ll*” and “12*” seem to take place directly and therefore depend only slightly on the temperature. In addition, light a t 436 mp seems to act not on the merocyanine but on the cation formed from it with protons donated by the soli-ent alcohol a R H+ [BH]+; b vis. [BH]++ ([AH]+) -+-A Hf
.
+
+
+
(13) R. Heiligman-Rim, Y . Hirshberg, and E. Fisoher, Bull. Res Counc Israel, 8 8 , No 3 (1959). (14) 5 hlalkin arid I‘. Fisoher, J . Phya. Chem., 66, 2482 (1862).
T‘ol. 66
[BH]+ of compound I has an absorption maximum a t about 480 mp and absorbs appreciably a t 436 mp (€436 18,000a t -loo”), whereas the absorption of I B a t this wave length is very low (€436 450 a t - 100”). As long as the thermal step a takes place, the whole of B eventually will be converted into A, even if the concentration of (BH)+ is too low to bc detected spectroscopically. As long as this is so, the absolute quantum yield a t 436 mp should be calculated using the €436 of (BH) +, whirh is 40 times as high as that of B. The true yields a t 436 and 546 mp thus calculated are of the same order of magnitude. At sufficiently low temperatures, process a is suppressed, and light a t 436 mp, like that at 546 or 578 mp, acts only on B, the absorption of which at this wave length is negligible. Viscosity Eff ects.-The above postulates involve the idea of a combined effect of low temperature and high viscosity to “clamp-in” a dye molecule in a certain configuration, assumed by the molecule immediately following its radiative formation. Effects of the viscosity or rigidity of the medium therefore should be discussed in a more general nay. In doing so, it is necessary to differentiate between molecular rearrangements taking place by virtue of thermal equilibria being established (i.e., where the energy stored in the molecule is the only source of energy), and between those brought about by irradiation (Le., where energy is added to the system from outside in the form of light). With regard to the first case, the well known selfdiffusion in crystals shows that even in the solid state molecular motion is not completely inhibited. Pimentells observed that 502, HJO, and KH3 diffuse rapidly in solid argon a t temperatures above 35’K., while a t 20°K. the diffusion of even diatomic species in solid xenon, argon, and nitrogen is negligible.16 These experiments deal with small molecules trapped in lattices of atoms or small molecules. The present observations refer to large molecules or molecular groups in glasses also consisting of large molecules. It appears that under these ronditions either low temperature or high viscosity alone do not suffice to prevent the thermal rearrangement of the highenergy form of the merocyanine, and also some of the internal rearrangements of the various “stable” merocyanines. Apparently the thermal energy of the molecules in viscous media suffices to overcome the resistance to motion offered by the medium, down to rather low temperatures. Other experimental results also show that high viscosity alone does not inhibit thermal isomerization at room temperature. Thus, the merocyanine modification of certain nitrospiropyrans dissolved in parafin wax a t room temperature reverted into spiropyran approximately at the same rate as in fluid solvents l7 On the other hand, the rate of this thermal isomerization was appreciably slowed down by cooling the paraffin wax solution to 0”. T h i k view is confirmed by the experiments of Hardwick. Mosher, and Passailaigue,18who reported that the ( olored (15) Q C Pimentel. J A m . Chem. S o c , 80, G L (1958) (16) E. D Becker and C C Pimentrl J Cirni I ’ / v Y ( 1956)
(17) R TI( ~ I ~ g i ~ ~ n - R I ~l nI iI ~ ~ I ~ iIw~l l l~t ?~ I ~ I ’ ~
28, 024
Dee., 1962
MECHANISM OF PHOTOTRAKSFORMATION IN SPIROPYRAXS
modificat,ion of :2-(2',4'-dinitrobenzyl)-pyridine is not stable a t room temperature even when the compound is dissolved in a rigid glass of sorbitol and fructose. On t,he other hand, this colored modification is rendered stable by cooling, even in a fluid medium. It thus may be concluded that in thermal transf0r:mations the mechanical effect of viscosity becomes a factor of importance only a t low t'emperatureir, when the energy content of the reactant molecules is very low. Still less important is the part that the viscosity of t.he medium plays in photochlemical reactions. There the degradation of the light energy absorbed by molecules of the reactants may result in local heating and melting of the rigid glass or of the crystal lattice of t,he solvent in the vicinity of the molecules of the absorbing species. Thus DeMore and Davidsoni9 report that bimolecular photochemical reactions take place during photolysis of ozone in nitrogen matrices at 20'K. (or 4°K.). Lewis, Magel, and Lipkin20also consider that it hardly would be reasonable t o expect that the rigidity of a solvent alone could hold an excited molecule in a fixed position, although it might inhibit thermal molecular motions (turning or diffusion) a t low temperatures.2l From t,he widely differing results reported here for different compounds studied under similar conditions it may be concluded that the individual characteristics and energy relations of the molecules undergoing transformat'ion play a far more important part in the determinat)ionof the rates and quantum yields of photochemical reactions than the mechanical properties of the medium. In closing, it is appropriate t'o give some very crude data for the macroscopic viscosity-rigidity, a t various t'emperatures, of the glass-forming solvent mixt'ures used. It' is hoped to make more ac(18) R. Hardwick. H.S. Mosher, and P. Passailaigoe, Trans. Farad a y Soc., 66, 44 (1960). (19) W.B. DehIore m d N. Davidson, J. Am. Chem. Suc., 81, 5869 (1959). (20) G. K.Lewis, T. T. Magel, and D. Lipkin, ibid., 62, 2973 (1940). (21) G. K.Lewis and J. Bigeleisen, ibid., 66, 520 (1943).
2477
curate low-temperature measurements of macroscopic viscosity and of viscosity in molecular dimensions in the near future. TABLE V TEMPERATURE DEPENDENCE OF VISCOSITYO F SOLVENT MIXTURES
Solvent mixture RICH-IH ( 2 : l ) MCH-MCP (1 : 1) MCH-decalin ( 1 : l ) MCH-decalin ( 2 : l ) Ethanol-methanol (4:1) 1-Propanol-2-propanol (3:2)
Onset of viscosity -132O -123O
90" -100' - 125'
-140'
Lower limit of useful temp. rangeQ -185' -185' -160' -180' 170'
-
- 90'
-160'
Very viscous -135O
-130°
- 80' - 95' - 100' - 65'
-
80'
Rigid -138" -133' -100'
-110'
-
" Y o experiments were carried out a t temperatures below that of liquid air.
Experimental Spectrophotometry and Irradiation Techniques.-These were essentially as described in the preceding paper.' Relative and Absolute Light Intensities, Actinometry.Relative intensities of the monochromatic light isolated from a mercury arc by suitable filters were measured with Bowen's method of "proportional quantum counting" as described in section 3c. With a Philips spectral lamp and either Corning glass filter combinations (365, 436, 546, 578 mp), interference filters (405 mp), or glass filter-solution combinations (313 mM), the following relative numbers ZA of quanta hitting the solutions were found Wavelength,mfi 313 28 IX
385 100
40.5 57
436 52
546 318
578 128
>410
810
ilbsolute actinometric measurements were made with Hatchard and Parker's micromethod under conditions identical with those under which the spiropyrans were irradiated. The absolute intensity at 365 mp was about 4 X 10'' quanta per minute.
Acknowledgments.-The authors wish to thank Prof. G. Stein and Prof. H. Linschitz for most helpful discussions, Mr. M.Kaganowitch for synthesizing the compounds investigated, and Mrs. N. Caste1 for technical assistance. The efficient help of Mr. T. Rercovici is gratefully acknowledged.
