Fischer et al.
108
The Role of the Triplet State in the Photocoloration of the Dianthrones. A Reinvestigation T. Bercovlcl, R. Korensteln, G. Flscher, and E. Flscher” Department of Structural Chemistry, The Weizmann Institute of Science, Rehovot, Israel (Received May 2 1, 1975)
Flash-photolytic investigations show conclusively that photocoloration in the dianthrones, to form the B isomer, takes place via the triplet state of the starting isomer A. Supporting evidence is provided by triplet sensitization experiments, in which B is formed exclusively, under conditions where isomer C is the sole product of direct excitation of A.
In an earlier paperla we provided detailed evidence for our conclusion that in dianthrone, I, and its derivatives the photocoloration to what is .by now commonly called the “B” form passes through the triplet state of the starting form A:
0
R’ hv --t
(D is an unstable isomeric precursor of B). This conclusion was elaborated in a recent note,lb but contradicts earlier results by Huber and coworkers3 who tried to correlate the triplet state and the formation of the photochromic isomer, in particular in triacetin solutions. According to Huber3 and to a more recent paper by Gschwind and Wild,2 most if not all the optical absorption in the visible region assigned to B, and arising from uv irradiation, appears virtually instantaneously, i.e., within the time resolution of the experimental setup (ca. 0.05 msec), while the triplet absorption decays a t a much slower rate. No D was observed by these authors. They therefore concluded that B is formed directly from the excited singlet of A, parallel to the triplet: 3A* hv / A ‘A*
-
\I3
No clear-cut distinction between the colored B and C isomers was established by the above authors, though a t sufficiently low temperatures little, if any, C is formed.la The nature of B and C was established r e ~ e n t l y . ~ In an effort to reconcile the results of both groups, and to find the possible reasons for the obvious discrepancies, we have now carried out additional flash experiments, using dilute solutions of the tetramethyl derivative I1 investigated by both groups, in three widely differing solvents: a mixture of aliphatic hydrocarbons (methylcyclohexane-2methylpentane, l:l), a mixture of alcohols (l-propanol-2propanol, 3:2), and the highly viscous triacetin (glycerol triacetate). In addition, earlierlb sensitization experiments with the dimethyl derivative 111, dissolved in methylene chloride, were extended, and carried out also with I and 11. As described earlier, the photoformation of D and B is controlled by the nature and the viscosity of the solvent medium in which A is dissolved. For each solvent a temperature T1 exists a t which practically no photocoloration is observed in flash photolysis, except that ascribed to the triplet 3A*. (No disagreements exist regarding this point.) At a somewhat higher temperature T2 the viscosity is sufficiently lower to allow formation of D, the precursor1 of B. At this temperature the kinetics of the formation of D, and B, can be folthe subsequent spontaneous conversion D
-
The Journal of Physical Chemistry, Vol. 80, No. 2, 1976
R‘ 0
A
3 1 8 0
0
R’
’ \
\
\
0 \
R’
0
C
I: R
= R’ =
Iv
H; 11: R = R’ = CH,; I11 R = CH,; R’ = H
lowed flash photolytically. The photocyclization product “C”, the formation of which is strongly temperature dependent,la is formed to some extent at T2 in triacetin, and not at all in the other two solvents. Since it is thermally stable at T2, it gives rise to a permanent increase in absorption a t its main peaks, in the range 410-500 nm, and a t its minor peak, in the range 550-750 nm. At Tz, isomer B, the absorption of which covers the range 500-750 nm (cf. Figure 1) is the only stable photoproduct in 1P-2P and in MCH2MP, and the major one in TA. Table I summarizes the values of T1 and T2 employed in the present study. The kinetics of decay of the triplet 3A* and of formation and decay of D were followed, as before,l at 490 and 720 nm, respectively. (On the time scale of the present experiments the formation of the triplet 3A* is instantaneous.) With each solvent flash-photolytic experiments were carried out at two temperatures T1 and T2. Each kinetic curve was taken concurrently on two identical oscilloscopes operating at different time scales. In Figure 2 we present the results obtained with a 5 X M solution of I1 in 1P-2P. The results in MCH-2MP closely parallel those in the propanol mixture a t the corresponding temperatures. Figure 3 shows the observations in a similar solution in TA. The results in the first two solvents can be summarized as follows (Figure 2). (a) At T1 transient absorptions a and b appear “instantaneously” a t both 490 and 720 nm, and
109
Photocoloration of Dianthrones i
[
1.4-
r
"\
(090) (0.60)i
A
m
5
0ae
Ki
0
(0.30)
ae (0.12)
100
/ r
I
02 r
:-
--
+ -------
g
-I
18-
C
\
16-
i
(0.90) (0.60)
L
I IO r
11,
1-
(0.00)100
O
0 6t
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, , ,
,
, , ,
I " "
,j -
0 4-
L
AI
300
L
400
I
A
500
--1-.-1 600
700
800 A,nm
Figure 1. Absorption spectra of the various isomeric forms of compound II and 111. The curves are named as detailed in text. (A) Cornpound 111 in CH2C12, at -70'. Curve C was obtained by 405-nm irradiation, and is completely reconverted into A by irradiation at 546 -k 578 nm. Curve B was obtained by 436-nm irradiation of the same solution, to which biacetyl had been added to give a 0.1 M solution. (B) Compound II in triacetine. The absorption curves of D and of triplet A were obtained at -78' (3A* by means of flash photolysis), B and C at -70'. Curve B resulted from 366-nm irradiation, followed by 546 -k 578-nm irradiation to erase C, and describes a mixture of ca. 80% B and 20% A. Curve C was obtained by and describes the absorption calculated for the pure C isomer.
TABLE I : Values of T,and T,Employed with Various Solvents Solventa MCH-2MP 1P-2P TA -186 -1 69 -7 8 TI, :c -173 -1 54 -60 T,, c a MCH = methylcyclohexane, 2MP = 2-methylpentane, 1 P = 1-propanol, 2P = 2-propanol, TA = glycerol triacetate, commonly known as triacetin. decay to zero at identical rates a t both wavelengths. (b) At Tz,an "instantaneous" increase b' in absorption a t 720 nm is followed by a further slow increase up to c' and then a decrease to a stable value d'. b' is identical with b. At 490 nm, a transient absorption a' decays to a low final value r at a rate similar to that of the slow increase a t 720 nm. The
Figure 2. Oscilloscope traces obtained in flash-photolytic measurements of solutions of II in 1P-2P at -169 and -154', at 490 and at 720 nm. The initial rise at 490 nm is higher at -169', because in the experiment at -154' the solution had already been flashed several times, and a considerable part of A thereby converted into B. Wavelengths and time scales as indicated in each figure; upper part at -169', lower part at -154', at otherwise identical conditions. Optical density values are given in parentheses at the left-side vertical scale. The meaning of a, b, and a'-d'is explained in the text.
values for a , b, a', b', e', and d' in Figure 2, expressed as optical densities, were as follows: a = 0.60, b = b' = 0.09, a' = 0.36, c' = 0.32, d' = 0.18. As usual in such measurements, the oscilloscope trace measures percent absorption (100%percent transmission) vs. time. It is therefore greatly distorted a t low transmissions, as compared with the corresponding curve of optical density vs. time, in particular if the full-scale vertical deflection of the scope corresponds to the complete transmission range 0-100%. (Cf. left side scale of Figure 2, where optical density values are given in parentheses.) We explain these results as follows. At T I the only transient is triplet 3A*, which returns practically completely to ground-state A, because of the viscosity controlled energy barrier' separating it from the primary photoproduct D: A 3A* ?-* D. At Tz this barrier is passed a t a rate much beyond that of the returr. to the ground state, so that we ob+
The Journal of Physical Chemistry, Vol. 80, No. 2, 1976
110
Fischer et al.
720
c
4
z
A
’iIO0
20
ms
Flgure 3. Oscilloscope traces obtained in flash-photolytic measurements of solutions of compound II in TA at -78’ (upper half)and at -60’ (lower half), at 490 and 720 nm, and with the timescales as indicated in each case.
-
serve in effect A t/- 3A* D. The instantaneous increase in absorption a t 720 nm at T2 is solely due to 3A* which, on the time scale of our experimental set-up, is formed “instantaneously” from IA*. Since D absorbs at 720 nm much more strongly than 3A*, the gradual formation of D from 3A* is accompanied by an increase in absorption. At 490 nm 3A* absorbs much more than D and therefore a decrease in absorption is observed, parallel to the increase at 720 nm. The formation of D is followed by its spontaneous conversion into B, which again is a viscosity controlled reacti0n.l Since a t 720 nm B absorbs less than D, this conversion results in a decrease in absorption, down to that of B, which is stable a t Tz. This is why the absorption a t 720 nm passes through a peak value with time, depending on the D and D B. rates of the spontaneous processes The absorption of 3A* extends all the way from the peak a t 490 nm to that at 890 n p , first described by Wild;2 a t 720 nm it is about one seventh that at 490 nm (0.09 and 0:60, respectively). In TA solutions, Figure 3, the results a t first sight do not appear as straightforward. In particular, not all of the initial absorption at 720 nm and -60’ is accounted for by the absorption due to 3A*, as observed a t -78’. Expressing the results as optical densities, we have a = 0.27, a’ = 0.26, b = 0.040, b’ = 0.061, c’ = 0.19, and d’ = 0.087. Thus, out of an initial rise a t 720 nm of b’ = 0.06, only b = 0.04 can be attributed to the triplet 3A*, while 0.02 is not accounted for. However, this is still only 2/19, i.e., about lo%, of the total absorbance c’ assigned to D formed under these conditions. We note two experimental complications encountered with solutions in TA. First, TA is not a stable solvent, and we succeeded to obtain reproducible results only with freshly
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The Journal of Physical Chemistry, Vol. 80, No. 2, 1976
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distilled solvent, using an efficient fractionating column a t reduced pressure. Second, the cyclization product C which is formed to some extent a t -60°, directly from the singlet excited lA*, could account for some of the initial absorption at 720 nm. (Cf. Figure 1,curve C.) We therefore conclude that in the first two solvents D, and therefore indirectly B, is formed solely via 3A*, while in TA this is the major pathway, and possibly the sole one. Of course, these conclusions hold strictly only for the temperatures T2 at which our measurements were made. In the absence of conflicting evidence we may assume a similar mechanism a t higher temperatures too, though in principle a singlet mechanism may open up a t high temperatures. I t remains now to explain the discrepancy between the above results and those of the Zurich group, who found no evidence for the existence of the D isomer in triacetin, and no correlation between the photocoloration to B and the decay of triplet 3A*. Two technical reasons might be responsible: the quality of the solvent TA, and the intensity of the flash. The difficulties with the solvent TA have been mentioned above, and indeed experiments carried out in Zurich with our solution proved the existence of D, by obtaining decay curves a t 720 nm which pass through a maximum, basically similar to our Figure 3 (lower part).5 As far as the flash intensity is concerned, we have recently described6* a case in which the course of secondary thermal reactions is affected by it. However, we believe that in the present case the major reason for what looks like a large discrepancy even with the same solution is actually the simple one elaborated above, i.e., the low sensitivity of the measurements a t low optical transmissions. The flash intensities in Zurich were probably about five times higher than ours, even using our short cells, and therefore a’-d’ there were mostly in the range below 40% transmission. I t seems advisable that one should always operate a t the lowest flash intensities which are still compatible with the required experimental accuracy. In view of the ease with which transmission changes in the range between 100% and, e.g., 60%, can be measured accurately, there is normally no reason to employ flash intensities causing changes in transmission beyond this range. To summarize, it seems that the reports of instantaneous photocoloration in TMD solutions are based on the two technical points mentioned above, and on the fact that the optical absorption of triplet 3A* and of C extend all through the visible range. The “instantaneous” formation of C via a singlet mechanism, regarding which no disagreement exists may thus be responsible for observed differences between b and b’ in Figures 2 and 3.6b We note that this is one of the few examples in which the triplet photoproduct can be sequence excited singlet followed by conventional flash photolysis, and the variation with viscosity of the rate of the second step can be measured.la So much for results with TMD. As mentioned before,l low-temperature flash experiments with dianthrone itself, I, and with a variety of its derivatives available to us, have shown that in all cases the triplet mechanism of photocoloration is either the sole or a t least the predominant one at the temperatures of investigation. (In most cases B is formed directly from 3A*, without evidence of D.) Indirect support for the triplet mechanism is provided by sensitization experiments,lbv7 with biacetyl serving as a triplet sensitizer. We have now used a similar system to obtain more direct support. As reported,1a,8 the ratio between -+
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Photocoloration of Dianthrones
111
the two colored photoproducts B and C of I1 and of I11 is a function of the solvent and the temperature. In solutions of I11 in methylene chloride, CH2C12, C is virtually the sole photoproduct.l* We could now show that nevertheless irradiation a t 436 nm (where only biacetyl absorbs) of a solution of I11 and biacetyl in CHzC12 forms exclusively the B isomer. Since it is commonly accepted that the photocyclization product C is formed directly from the excited singlet lA*, we conclude that in this solvent intersystem croming 3A* is inefficient and lA* disappears mainly by lA* photocyclization and by fluorescence:
-
A
- F
'A*
C
-
However, once 3A* is formed via energy transfer, it is transformed into B in this solvent just as in others: 3A* B. Relevant experiments were carried out both flash photolytically a t and below room temperature, and a t -70' in the Cary 14 spectrophotometer. C was identified b y its double peak a t 460 and 480 nm and by its photoconversion back into A with light a t 546 and 578 nm. Typical concentrations employed were: biacetyl, 10-1 M and 111, 5 X M. Figure l a describes an experiment a t -70'. Very similar results were obtained with CH2C12 solutions of I1 and of I. In the presence of biacetyl, irradiation produces B in both cases. In the absence of biacetyl the major photoproduct of I1 is its C isomer, while I is photooxidized to helianthrone, IV, via a C-like precursor observable only by flash metho d ~ . ~
Experimental Section The low-temperature techniques have been described before.1° The copper block technique was employed throughout. In the flash-photolytic experiments we used cells made of rectangular cross-section Pyrex tubing (4 x
1 2 mm inside) with a light path of 20 mm along the direction of the measuring light beam.l' The peak optical density of the solutions a t the 390-nm peak was about 0.4 in the direction of the flash (4 mm), so that homogeneous irradiation was assured. In the regular flash experiments the light from the flash tubes was passed through Corning 9863 colored glass filters. In view of the Pyrex Dewar and cell this meant an effective wavelength range of irradiation between 330 and 400 nm. In the sensitization experiments the wavelength range 410-450 nm was transmitted by means of a suitable filter combination. The flash apparatus12 consisted of two oxygen-filled 10-cm flash tubes operating in series at up to 20 kV with either one or two 1 p F capacitors. A 100W, 12-V tungsten-iodine monitoring lamp was used with suitable light filters, a 500-mm Bausch and Lomb monochromator, a 9558 EM1 photomultiplier, and Tektronix 564 storage oscilloscopes.
References and Notes (1) (a) T. Bercovici, R . Korenstein, K. A Muszkat, and E. Fischer, Pure Appl. Chem., 24, 531 (1970), where earlier papers are cited extensively; (b) T. Bercovici and E. Fischer, Helv. Chim. Acta, 56, 1114 (1973). (2) K. H. Gschwind and U. P. Wild, Helv. Cbim. Acta, 56, 809 (1973). (3) J. R. Huber, U. Wild, and Hs. H. Gunthard, Helv. Chim. Acta, 50, 841 (1967). (4) R. Korenstein, K. A. Muszkat, and Sh. Sharafi-Ozeri, J. Am. Chem. Soc., 95, 6177 (1973). (5) U. Wild, private communication. ( 6 ) (a) G. Fischer, E. Fischer, K. H. Grellmann, H. Linschitz, and A. Temizer, J. Am. Chem. Soc., 96, 6267 (1974): (b) Professor Wild has meanwhile agreed5 that under the circumstances this summary reflects the feelings of both groups. (7) H. H. Richtoi, R. L. Strong, and L. J. Dombrowski, Israel J. Chem., 12, 791 (1974). (8) G. Kortum, Ber. Bunsenges. Phys. Chem., 78, 391 (1974). (9) R. Korenstein, unpublished results. (10) E Fischer, Mol. Photochem., 2, 99 (1970). (1 1) E. Fischer, Mol. Photochem., 6, 111 (1974). (12) T. Bercovici, R. Heiligman-Rim, and E. Fischer, Mol. Photochem., I,23 (1969).
The Journal of Physical Chemistry, Vol. 80, No. 2, 1976
