REVERSIBLE PHOTOBLEACHING OF DYES
527
molecule enters into some chemical reaction with the quencher, and the net effect is reversed by subsequent dark reactions. Examples are presented of all three types of action occurring in gaseous systems. Some preliminary data, however, indicate that the specific energy transfer may be of major importance in the quenching of dyes by various ions. It is pointed out that if this view is supported by additional data we have in these studies a means for obtaining data concerning metastable states which cannot now be obtained in any other way. REFEREXCES (1) FRANK, I. Jl.,ASD VAVILOV,S. I.: Z.Physik 69, 100 (1931). M . : J. Am. Chem. Soc. 66, 2100 (1944). (2) LEWIS,G. S . ,A X D KASHA, (3) ROLLEFSOS, G . K., A N D STOUGHTOS, R . R.: J. Am. Chem. Soc. 63, 1517 (1941). (4) STERN, O . , A S D T’OLVER, 31.:z. wiss. P h o t . 19, 275 (1920). ( 5 ) STOUGHTON, R . W., ASD ROLLEFSOS, G . K.: J. Am. Chem. Soc. 61,2634 (1939). (6) SVESHXIKOFF, W . X.:Acta Physicochim. U.R.S.S. 4, 453 (1936). (7) CMBERGER, J., ASI) La MER,V. K.: J. Am. Chem. Soc. 67, 1099 (1945); also three papers presented a t the 111th Jleeting of the .American Chemical Society, which was held at .-\tlantic City, S e w Jersey, April: 1947.
T H E REVERSIBLE PHOTOBLEACHING OF DYES AKD PIGMEKTS ROBERT LIVINGSTOS
School of C h e m s t i ut Instatute of Technology, [*navejsity of Mznnesota, Mznneapolzs, Xznnesota Received Octobei 23, 1947
The primary process of a photochemical reaction may be defined as including (12) “the initial act of absorption and those immediately following processes which are determined by the properties of the initially excited electronic state.” The photochemist who studies the reactions of simple molecules in the gas phase has the distinct advantage that more or less complete information in regard to the primary process may be obtained directly from spectroscopic data. This advantage does not exist for the photochemistry of complex molecules. Indeed, the study of the primary process in such complicated reactions as dye-sensitized photooxidations in solution is often the most difficult part of the investigation of the reaction. It is doubtful if the detailed nature of the primary process, or as it is sometimes called (15) the “inner or hidden” mechanism, of any photochemical reaction involving dyes or pigments is known with any certainty. I‘ntil fairly recently it was maintained very tenaciously by a group of photochemists that fluorescence and photochemistry were complementary processes. Piesented at t h e S> mposiu~ii0 1 1 Radiation C h e n n s t r j and Photochemistir n h i c h n as hc~ldtit t h c I - n i l t , i s i t v of S o t i c D a n i r . S o t i e Dame. Indiana, ,June 21-2i. 1947
528
ROBERT LIVINGSTON
That is, all of the photochemical action was produced by a direct interaction between a substrate molecule and the sensitizing molecule in the relatively shortlived excited state which leads to fluorescence. This hypothesis is scarcely consistent with the mll-known fact (4) that there is no parallelism between the efficiency of dyes as photosensitizers and their maximum Rucrescent yields obtained in the absence of the substrate. Indeed, dyes are known which are capable of sensitizing photooxidation with yields approaching unity, but whose maximum fluorescence yields in the absence of the reactants nrc less than 0.01, Rlore definite evidence against this simple theory n-as obtained by Gnffron (2) in the study of chlorophyll-sensitized photooxidations and in the photochemical autooxidation of rubrene. Particularly in the latter case, it was demonstrated that the minimum life of the activated state which leads to the chemical reaction must be more than a thousandfold greater than the life of the fluorescent state. Thus, the photochemical evidence seems to demand the existence of a long-lived activated form of the sensitized molecule, which is reached indirectly through the first excited state (1). Further evidence for the existence of long-lived excited states of complex molecules comes from the studies of T’avilov (18, 20), Kautsky (6), Pringsheim (14), Lewis (9), and others on dyes adsorbed or dissolved in extremely viscous solvents. Under these ccnditions a phosphorescence is observed which has the same n-ave-length distribution as the ordinary fluorescence but whose life is temperature dependent. At very Ion- temperatures, when the life of this radiation is relatively long, a nen- long-lived emission becomes detectable. This long-lived fluorescence*ha3 a life which is independent of temperature and a mean wave length which is distinctly longer than that of the ordinary fluorescence. The heat of activation which is associated with the life of the phosphorescence is equal to or greater than the difference in energy betwen the phosphorescent quanta and the long-lived fluorescent quanta. Jabloliski ( 5 ) was the first to interpret these facts in terms of a simple energy diagram involving three electronic levels. The absorption and fluorescence of the molecule are due to transitions between the ground state, A, and the first excited state, R. Radiative transitions between these states and the third state, C, which has an energy level betxeen A and K , are forbidden. Hon-ever, a radiationless transition (internal conversion) (19) betkeen states B and C‘ can occur. Since the molecules are in solution, the (generalized) oscillational energy in excesb of the thermal amount is quickly lost by collision with solvent molecules. Phosphorescent emission occurs when a molecule in state C‘ acquires sufficient energy of activation to revert to state 11. Since phosphorescence is not commonly observed in solution a t ordinary temperatures, it must be possible for state c‘ to go over to a high oscillational lelrel of state il and so degrade its energy as heat. This radiationless transition determines the life of state (‘ under ordinary conditions. In The oldci t l c f i i i i t i o i i b of i:uoi ncl phc y’llol ('ice l l L C I13 C i l C usctl I l l t I l l , l’apel I l e l n ~ed eniission t h e hnlf-!itc of inch 16 stionply teiiipt~i:rluredependent 1 5 called phosphorescence, n hile dela) ed e m i m o i i TI hose half-life is tempci atui c indcpendcnt is rcferrcd t o as I o n p - l i ~r d fluoi cwcxiice I,cnis :ind his con 0 1 heiq (9 :idopted t h e opposite coriveiition.
REVERSIBLE PHOTOBLEACHING O F DYES
5213
extremely viscous solvents or in the absorbed state, this radiationless transition becomes relatively unimportant and phosphorescence is observed. At Ion- temperatures the life of the phosphorescence exceeds the natural life of the temperature-independent long-lived fluorescence, which then becomes the dominant proce~s.~ Additional information regarding the inner mechanism of photosensitization by chlorophyll can be obtained from a study of the reversible photobleaching of chlorophyll solutions. This effect v a s discovered by Porret and Rabinovitch (16) in 1937; its existence \vas confirmed and it was further studied by Livingston in 1941 (10). Recently the investigation of this phenomenon has been renewed by JIcBrady and Livingston (11). Unlike the reversible photobleaching of thiazine dyes (3, 21), the photobleaching of chlorophyll solutions occurs in the absence of added reducing agents. The studies of Holst (3) show that the bleaching of methylene blue in the presence of phenylhydrazine is a typical case of a photostationary state, analogous to the photochemical stationary state involving iodine and ferrous ion (17). In cases of this type there appears t o be little probability of obtaining information regarding the nature of the primary process by a study of the reversible bleaching. However, since chlorophyll is photobleached in solutions containing no added reactants, it appears much more probable that the course of the bleaching may be directly related to the primary process of photosensitization. Figures 1, 2, and 3 illustrate typical cases of photobleaching of chlorophyll solutions. The duration of each experiment in seconds is plotted as abscissa and the decrease in molarity of chlorophyll as ordinate. The interval betn-een measurements is 5 sec. The point a t the end of each light interval is indicated by a circle; that folloiring a dark interval, by a solid dot. In the experiments illustrated by the figures the source of actinic light Tvas a 1000-watt projection lamp, equipped n-ith suitable lenses and with filters which transmit the red end of the spectrum and absorb most of the infrared as well as the blue-violet end of the spectrum. The solutions were made up in purified methanol, and were JI in respect to chlorophyll a. The details of the experimental meas2 X urements have been discussed elsewhere (11). In computing the decrease in the molarity of chlorophyll it x-as assumed (16) that the bleached form of chlorophyll does not absorb a t all in the red end of the spectrum. Figure 1 is typical of the bleaching in air-free methanol solutions. I-nder these conditions the steady-state bleaching ranged from 0.2 to 0.6 per cent, depending upon the sample of solvent and chlorophyll used. The steady-state bleaching was attained in less than 3 sec. (n-hich was the period of the galnmomcSo diecusqion of (lit hcr t h e cause of t h e long life of s h t e C ( 1 1 t l i L iiiechsiiiqiii 111 n h i c h t h e iiiolrcult gors f i o i n state B i o state C i s included in tiic pirccding statenlent l'hosr contioic,rsi,tl point3 h a ~ cbeen t i c a t r d cx.tensivcly clcrnlic~re(1. 7 , b (I!, and it 1 oultl br heyonti t l i r scope of t h e piesiiit pctpci t o p ~ e w n tthcni .idequatrl\ In the o1)iiiioii o f t h e author. t h c e\peiiiiiciitnl facts obtainrd for diffeient t ~ p e of s compounds cannot bc fiiretl by any one simple formulation v h i c h attcnipts to spccify t h c e\act nntuic nf thc i I c c t i o n i c states and of t h e 1ad;ntionlrss tiansitio~is
530
ROBERT LIVINGSTON
-
Time
-
50
0 Scc
~
-_
100 Tme-Sec
_t - I50
$
0
FIG.1 FIG.2 FIG.1. Reversible bleaching of chlorophyll in air-free methanol solutions FIG 2. Irreversible bleaching of chlorophyll in the presence of air
I ~
0
50
100
1
I 150 h m a See
-
200
250
3(
F I G :3 Reversible bleaching of chloiophyll (prepared with carbon tetrachloride) methanol solutions.
111 ail
-free
ter used) and the half-life of the bleached material was less than 0.5 sec. Experiments performed with different intensities of the actinic light demonstrated that the steady-state bleaching is directly proportional to the square root of
REVERSIBLE PHOTOBLEACHIKG O F DYES
53 1
the intensity of the absorbed light. The initial concentration of chlorophyll appears to have no effect upon the bleaching escept indirectly, on-ing to the change in the intensity of the absorbed light. Both the steady-state bleaching and the rate of the reverse dark reaction appear to be independent of temperature in the rage from 5" to 25°C. The reversible bleaching is always accompanied by a relatively slon- irreversible bleaching, its rate usually being less than 10 per cent of the reversible effect. Oxygen completely inhibits the reversible bleaching and somewhat increases the irreversible action. However, even in solutions saturated Tvith air, the rate of the irreversible reaction is never more than 20 per cent of the rate of the reversible process which vould be observed in the same solution in the absence of air. Figure 2 illustrates the bleaching of a solution saturated with air. The reverse reaction, which occurs in the dark, appears to be sensitive to small traces of impurities. For esample, the bleaching of solutions of chlorophyll prepared by a slightly modified procedure follon-s the course illustrated by figure 3 rather than figure 1. The only differencein the two methods of preparation n-as that in the latter case (figure 3 ) carbon tetrachloride n-as substituted for the ether which was used as a solvent in the standard method of purifying chlorophyll. Further experiments showed that this difference was due to impurities present in the carbon tetrachloride used in the experiments. Table 1 summarizes the effects produced by a number of different added substances. It is noteworthy that reactive reducing agents (such as hydroquinone) are practically without effect, while oxalic acid and osidizing agents (such as methyl red and iodine) have a marked effect. The striking change produced by dilute solutions of iodine is paralleled by the efficiency with which iodine quenches the fluorescence of chlorophyll and changes its absorption spectrum. Iodine apparently forms a compound with chlorophyll and so modifies its photochemical proper tie^.^ Although the data presently available are not sufficient t o permit a definite decision as t o the mechanism of the reversible bleaching, it is possible to set up a series of reaction steps which are consistent with the known facts: (1) (2) (3 (4)
(5) (6)
GH
+ hv
+ GH*
-+ GH*
+ GH GH* HG HG -+ GH HG BH + GH2 GH2 B -+ GH
+ hvf
---f
+
+
+ 13 + BH
The symbols used have the following significance : GH is normal chlorophyll; GH* is electronically excited (singlet state) chlorophyll; HG is long-lived activated chlorophyll (probably in a tautomeric, triplet state); GH? is the partly reduced, bleached form of chlorophyll; and BH is a reactive, reducing impurity present in the solvent. This mechanism leads to the following expression for the change in chlorophyll concentrations, AC, at the steady state. In agreement
* The evidence for these effects of iodine will be presented else\\herc LTith Dr W
Watson.
532
ROBERT LIVINGSTON
with the experiments, this equation indicates that the steady-state bleaching is proportional to the square root of the intensity of absorbed light and that it __ SOLVXKT
TABLE 1 T h e eJect of added si~bstnnceupon the reuerszble bleaching of chlorophyll -~
1
ADDED SUBSTANCE
i
I
I
1
1
~
I-
1
REVERSE REACTION
1
Order
k(g)
i
-1
41
, -
C11 ,OH
CH,O€I CH O H
,
1 Formic acid(")l Formic acid 1 Formic scid
1
0.01
0.01 0.01
I
i
';
30 3
1-1
I 1
10-'5
2
1
x
lo-' , 22
d
I
lo-' : 150 d
1
I 1
I
~
2
x
10-1
1
(11) (11)
10--5
The d a t a are consistent with reference 16. Reference 10 indicates k N 6 X lo6. Qualitatively similar t o methanol solutions except t h a t the irreversible reaction is
f :LSt e 1 C ' These direigent iesults suggest t h a t the effect is due t o a n impurity in the formic acid I d ' The addition of allylthiourea neutralizes the effect of this impurity. ( e ) A compound between iodine and chlorophyll is apparently formed. ( f ) The iireversible reaction accounts for about half of the yield. An after-bleaching occuis for about 0.5 sec. after t h e light is cut off. k' Sccond-order k ' s are expressed i n liters/mole X sec.; t h e first-order k's i n reciprocal seconds
varies with an uncontrolled factor, the concentration and the chemical nature of an unknown impurity.
REVERSIBLE PHOTOBLESCHIKG O F DYES
533
I indicates the intensity of the absorbed light (in appropriate units) and k , is the rate constant for the ithreaction step. T17hile the assumption that the partly reduced radical, GH:, is the bleached form of chlorophyll is plausible, it is completely arbitrary. I n fact, the evidence presented in table l suggests that it is more likely that the bleached form is the partly oxidized radical, G. The former alternative was chosen in the present mechanism since it is consistent with the following (1) simple explanation for the inhibitory action of oxygen:
+ +
+ +
0 2 GHz -+ GH HOz. (7) (8) HOP B -+ BH 0 2 These reactions fit in with the HOg mechanism (21) of sensitized autooxidations. In the presence of a relatively high concentration of a reducing agent, as allylthiourea ( 2 ) , the HO? would be reduced and the radical B mould react, presumably, with some of the radicals formed in the reaction chain. ,Another may in which the inhibition by oxygen could be explained would be to assume that the paramagnetic oxygen molecule catalyzes steps 3 and 4 (increasing their absolute rates a t least one hundredfold). Since these steps are radiationless transitions which (presumably) violate the intercombination rule, this assumption appears quite plausible. This explanation has the advantage that it does not require that GH, be the bleached form of chlorophyll. Furthermore, it is consistent with the reported (6) quenching of chlorophyll phosphorescence by oxygen. It is questionable, however, whether it can be reconciled with the experimental data on sensitized autooxidations ( 2 ) . It is obviously incon,iistcnt I\ it11 the H 0 2 mechanism (el) tor photooxidations. Gaffron’s mechanism ( 2 ) for these reactions, which postulates that the reaction is initiated by a direct interaction between molecules of the reductant and of activated chlorophyll, may possibly be compatible with this assumption, but would lead to the (untested) prediction that high concentrations of oxygen should retard the autooxidations. Professor Franck5 has suggested that the action of iodine in enhancing the bleaching and changing the reverse reaction to first order is due to the addition of an iodine molecule to a double bond of an excited chlorophyll molecule, thereby interrupting the resonating system. This photochemical reaction is preceded by a dark reaction between chlorophyll and iodine.4 The reverse reaction will be the unimolecular dissociation (or rearrangement) of the bleached compound, “SI2, as follows: HGI2 -+GH Iz This mechanism leads to the prediction that iodine should increase the quantum yield of bleaching as well as reduce the rate of the reverse reaction. A similar explanation for the effects of methyl red and of oxalic acid seems probable.
+
RLE 1;REKCES (11 I ’ H 4 \ C h . J . 4 ~ 1 L ) i\IhG+roh, R J C‘hem Phys. 9, 184 (1941). (2) G ~ ~ F R OHK I3ei 60B, ‘is5 (1927); Uiochem Z 264, 251 (1933) j
P r i v a t e communication
534
F. E. BLACET
(3) HOIST.G . : %. physik. C'heni. A179, 172 (1937); A182,321 (1938). (4) HPRI).F.. .ISD LIVISGSTOS,R . : ,J. Phys. Chcni. 44, 865 (1910). ( 5 ) J ~ s t o f i s ~ tAr , : d c t a I'hys. l'olon. 4, 311 (1935). (6) KAUTGKY, H., HIRXTI.A , . ASD FLESCH. W .: Her. 66,401 (1932 ; 68, 132 (1936). ( 7 ) LEWIS,G., ASD C A L V I K11.: , J . .!ai. C'lieni. SOC.67, 1232 (1945). (8) LEWIS,G., A N D KASHA, 11.:J . Cliern. SOC.66, 2100 (l!U (9) LEWR,G . , LIPKIS,D . . A S D ~ L I G E I T , . . : J . .\in. Chrm. Roc. (10) LIVISGSTOS,R . : *J. P h (11) ~ ~ B R A D J . ,Y. 4 S. D LTVI d Chrni. 62, S o . 4 (1948:, (12) SOYES, IV. A , , A K D LE citristry o,f Gases. 1). 133. Reinhold Publistling Corporation, Sex\- l o r k (1941) , (13) PRIKGSHEIJI! 1'. : Fluorescerlz a n d Phosphoresceiii, 11. 160. J . Springer. I3erlin (1!)2S). (14) P R I N G S H E I J I , p . . AND VOC:EI,R. H . : J. Chem. Phys. 33, 31q5 (1936). (15) RABIKOWITCH, F:,: Photosynthesis, Vol. I. p. 494. Interscience Press. I n ? . . Sen. York (1945). (16) RABINOWITCH, E.. A K D PORRET, P . : S a t u r e 140,321 (1937). ( l i ) RIDEAL,E., A K D WILLIAMS, 2:. G . : J . Chem. Sac. 127, 258 (1925). (18) SCHISCHI,O~-SKY, A .I> \-A\-ILOV, S.: Z.Physik (F,S.S,R.I6, 379 (1934). (19) TELLER, E . : J. Phys. Chem. 41, 109 (1937). (20)VAVILOT,S., AKD LEVSHIK, W.:Z . Physik 36,920 (19261. (21) WEISS. J . : Trans. Faraday Soc. 36,48 (1939).
T H E PHOTOCHEMISTRT O F T H E ALDEHYDES' F. E. BLBCET Department of Chenrzsiril, l - r n t e r s z f u of C a l z f o m z a , /,os .-ltigeles, Cal7jornza R e c e z i c d Octobei 2 3 . 1947
The absorption spectra and vapor pressures of the aldehydes are such as to invite photochemical studies of them in the gaseous phase. They all have a first region of absorption in the near ultraviolet with a span of approximately 1000 A. On the long-wave-length side this region is always discontinuous in character, showing some fine structure in the case of the more simple molecules. The fine structure blends into a diffuse or what has been designated a predissociation spectrum, and this in turn changes over to a continuum on the short-wavelength fringe of the region. second absorption region soon starts in below this and continues perhaps another thousand L%ngstromunits well into the Schumann region. A few investigators have ventured into this second region, but for the most part it remains unexplored, waiting for the development of better equipment and better technique for quantitative studies. We shall return in this discussion, therefore, to the first absorption region, where much work has been done. It is believed that here at least a beginning has been made toward an appreciation of the chemical processes which follow the absorption of radiant energy. 1 Presented at the Syniposium on Radiation ('henustiy and l'hotochemistij held a t the L-niveisity o f S o t r c Ilanic, S o t r c Dame. Indi:ma, .June 24-27,1947.
I\
hich was
