An INTRODUCTION to SENSITIZED PHOTOCHEMICAL REACTIONS ROBERT LIVINGSTON University of Minnesota, Minneapolis, Minnesota
I.
P
HOTOCHEMISTRY in its practical aspects must have forced itself upon the consciousness of the earliest thinking men. Possibly the first photochemist was some Egyptian weaver, grateful to the sun for helping to bleach his crude fabric, disgruntled with the same sun for fading the colors of his carefully dyed materials. Strangely enough the first law of photochemistxy was not formulated until 1817, when Grotthus stated that only light which is absorbed can produce a photochemical change. The earliest exact photochemical experiments, such as Draper's work on the photochemical interaction of hydrogen and chlorine, were performed with reactants which absorb light directly. However, photochemical reactions may sometimes be induced by the addition of a light-absorb'mg substance to a potentially reactive but transparent system. Such substances, which induce photochemical reactions without undergoing permanent chemical change themselves, are called photosensitizers. Photosensitizers play the same r41e in photochemical reactions that catalysts ( I ) do in thermal reactions. The directness of this analogy becomes apparent when we realize that light, like matter, is made up of discrete particles, called photons or light quanta. It is entirely correct to write chemical equations involving photons as well as atoms and molecules, and to think of photons as constituting a special class of atoms.* A primary photochemical reaction is one which occurs between a photon and a molecule or atom. hv fA +X (Primary reaction), where hv stands for a photon of frequency v, A for a molecule of the reactant (or photosensitizer), and X for the direct product (or products) of this interaction. In most photochemical reactions, the primary step is followed by one or more secondary . stem. * X +A
+ B -+M + N (Secondaryreactians).
where A and B represent the reactants and M and N the stable products. As a simple example, let us consider the photochemical decomposition of hydrogen iodide, which may be summarized as follows:
-
hv + H I
H+HI-+Hz+I
GENERAL PRINCIPLES
+H + I
(Secondaryreactions)
21 +I*
The sum of these three equations represents, in a purely formal way, the total or stoichiometric reaction. hv
+ 2HI = H2 + L (Stoichiometric reaction)
The last equation gives the information that when one photon is absorbed two molecules of hydrogen iodide are decomposed into their elements. It gives no information as to how this happens; that information is summarized in the three preceding equations. The primary step in a photosensitized reaction may be written as: hv fS --+P (Primary reaction) where S represents the sensitizer and P the primary product (or products). The product P undergoes a reaction (or sometimes initiates a series of reactions) resulting in the formation of the stable reaction products and the regeneration of the sensitizer. This reaction may be represented by the following equation: P fA
+ B --+ M f N + S (Secondaryreactions)
The secondary reactions are thermal reactions; only the primary step is a true photochemical process. It should be remembered that, while a photosensitizer may be thought of as a catalyst for a photochemical reaction, hu fA B +M N (Stoichiometric reaction).
+
+
it is entirely incorrect to consider the photosensitizer, light, or the combination of the two, as a catalyst for the correspondmg thermal reaction, A
+ B +M f N (Stoichiometric reaction).
It should also be remembered that, although all photosensitizers absorb lixht, not all substances which absorb light are capable of acting as photosensitizers. The action of sensitizers is highly specific. The several types of photosensitization are discussed in the following Dazes. .
-
11.
PHOTOSENSITIZATION BY MONATOMIC
GASES*
Mercury vapor, a monatomic gas, is one of the sirn-
(Primaryreaction)
* This statement and its subsequent applications amount to a plest of photosensitizers. A normal atom of mercury,
tacit acceptanceof the Einstein Photochemical Equivalence L~~ as applied to primary photochemical processes. For a presentstion of the law in its customary explicit form the reader should consult Ref. (Z), PP. 2 1 4 ; Ref. (3). PP. 13-26; Ref. ( 4 ) , pp. 36W3; Ref. (5). pp. 1-10.
such as is present in merally Vapor a t ordinary temperatures, can absorb ultra-violet light of wave-length
-
* See Ref. ( 5 ) , pp. 200-3, and 259.
Also Ref. (Z), p. 37; Ref.
(3), pp. 1 2 W 4 ; and Ref. ( 4 ) , pp. 276-86 and 583-602.
400
2536.719, which is known as the resonance radiation of mercury. When an atom absorbs a photon, it is raised to an excited state, that is, its energy content is increased by an amount hv. If the excited atom does not undergo a collision during the life of the excited state, about seconds, it will emit light (i. e., a photon) of the wave-length absorbed. If it undergoes a collision the light energy may be degraded to beat or converted into chemical energy. This may be illustrated by the following experiment. If a beam of ultra-violet radiation of 2536.7A is focused upon an evacuated quartz flask, all of the light (except that which is absorbed or reflected by the walls) will pass through the flask. If mercury vapor at a low pressure (for example, 0.1 millimeter) is introduced into the flask, a large number of the photons, which make up the light beam, will be absorbed and then be re-radiated in all directions. However, as long as the pressure is low the number of photons appearing in the scattered light will be equal to the number absorbed from the focused beam. If in addition to the mercury vapor the flask contains a chemically inert gas a t a high pressure (for example, nitrogen a t one atmosphere), a large percentage of the excited atoms will undergo collisions and some of these will distribute their energy with the colliding molecules; i. e., be deactivated. As a result, the intensity of the scattered light will be greatly diminished and an equivalent amount of energy will appear as heat, raising the temperature of the gas. If the deactivating gas instead of being nitrogen consists of atoms which have a lower excitation energy than mercury (for example, thallium vapor), part of the energy lost by the excited mercury atoms will appear as radiation characteristic of the deactivating atom. This phenomenon is h o w n as sensitized fluorescence. This effect, the extinction of atomic fluorescence, is of particular importance to photochemistry when the deactivating gas is a chemically active diatomic or polyatomic substance. For example, if an excited mercury atom collides with a hydrogen molecule, part of the energy of excitation can be used to dissociate the molecule and the remainder of the energy will then appear as kinetic energy of the three atoms. H g * f Hn+H g + 2H (Primary reaction)
product of the action will be water. If ethylene is substituted for oxygen in the system, it will be partly reduced to ethane and partly polymerized. A number of other mercury-sensitized reactions, including the decomposition of ammonia, of methyl alcohol, or of other organic vapors, have been reported. There is no a prwri reason why other monatomic gases should not be as efficient sensitizers as mercury. However, for practical reasons the list of available substances is very limited. For convenience of experimentation the wave-length of the resonance radiation should be long enough so that the radiation is not too strongly absorbed by quartz or air. This restriction rules out most of the non-metals, though by the use of a special technic the dissociation of hydrogen has been photosensitized by xenon to radiation of 146919 (5). The use of metallic vapors, other than mercury, is greatly limited by the inconveniently high temperatures which would be necessary to produce an appreciable vapor pressure. However, cadmium vapor has been used to sensitize the reduction and polymerization of ethylene to radiation of 322619. 111.
PHOTOSENSITIZATION BY DI- AND POLYATOMIC GASES*
The absorption spectra of di- and polyatomic gases consist of broad bands or even regions of continuous absorption, some of which are frequently located in the near ultra-violet or even the visible. A s a result, such gases are relatively very efficient in their utilization of the radiant energy from such convenient sources as sunlight, carbon arcs, incandescent filaments in quartz or glass, etc. The primary process for a molecular photosensitizer may involve either the formation of an excited molecule, hu
+ AB --+ AB* (Primary reaction),
or the dissociation of the molecule into atoms or radicals, hv
+ AB + A
+ B (Primary reaction).
Action of the latter type is of particular importance to photochemistry. Chlorine is not only a typical diatomic photosensitizer but is by far the most important member of this group. Its absorption spectrum shows two distinct regions. For wave-lengths longer than 4785A, there is a fine-structured band with a convergence l i t at 478519; for shorter wave-lenghs in the visible and near ultra-violet, the absorption is continuous. This continuous, or non-quantized absorption corresponds to the instantaneous dissociation of the molecule into a normal ind an excited atom. This is the primary step for chlorine-sensitized photochemical reactions,
If the total pressure of the gases present is reasonably high, the hydrogen atoms will make many collisions with the molecules present before they have an opportunity to recombine (at the wall or in the presence of a thud body) (6). If one of the gases present can react with hydrogen atoms, a photochemical reaction results. A mixture of hydrogen and oxygen does not react when it is illuminated with radiation of ~ 5 3 6 . 7 1 9 hv C12+C1 Cl* (Primary process). since it is transparent to such radiation. However, if Two chlorine-sensitized reactions are the oxidation of a trace of mercury vapor is added to the mixture, a monoxide and the formation of water from its photosensitized reaction will take place. The hydro- carbon gen atoms which will be formed under these conditions, *Ref. (4), pp. 620-1, 6 0 0 3 ; Ref. (2), p. 39; Ref. (3), pp. will react with oxygen molecules, and the stable 13440; Ref. (5).pp. 256 and 262.
+
+
elements. The kinetics of both of these reactions are complicated by the presence of side reactions; the formation of phosgene in the first case, and of hydrogen chloride in the second. They have been the subject of many careful investigations, and their mechanisms, while too complex to be discussed here, are reasonably well established. A number of other chlorine-sensitized reactions are known, among which may be mentioned the decomposition of ozone and of chlorine monoxide. An example of the remarkable specificity of photosensitization is the complete inability of bromine to sensitize the oxidation of carbon monoxide. Nitrogen dioxide is an interesting molecular photosensitizer of a slightly different type. Its absorption spectrum resembles that of chlorine in that the absorption extends throughout the visible and into the ultra-violet, and is divided into distinct regions, but unlike chlorine it does not show a region of continuous absorption. For wave-lengths longer than 3700A the absorption spectrum consists of bands exhibiting the usual fine structure, and the chemical equation corresponding to absorption in this region is: hv
+ NOS +NO$* (Primary reaction)
For wave-lengths shorter than 3700A the bands are "diiuse"; that is, they exhibit no fine structure. This absence of (rotational) fine structure has been interpreted as evidence that when a photon of wave-length less than 3700A is absorbed an excited molecule is formed, in which the energy is distributed between various degrees of freedom including electronic excitation. Within a very short time (about 10-la seconds) the electron drops to a lower state, thereby increasing the energy of vibration of the molecule above its limit of stability, and the molecule dissociates. This phenomenon is known as predissociation. The process may be represented by the equation: hu
+ NOz --+N O + 0 (Primary reaction)
O+NOs+NO+O1 2NO
+ NO2+ N O + 0 (Primaryreaction) N O + N.Os +3 N 0 1 0 + NO2+N O + OP (Secondary reactions) 2NO + 0, +2NOz hu
1
If we add the last equation to twice the sum of the first three and divide the resultant by two, the equation obtained, hv
+ NIOs = 2NOn +
+
01
+2 N 0 1
}
(Secondary reactions)
The concentration of the oxygen atoms at the photochemical steady state is always small because of the
(Stoichiometricreaction),
represents the stoichiometric reaction. It should be remembered that this (stoichiometric) reaction does not take place in the absence of the sensitizer, nitrogen peroxide. Since the photosensitizer is one of the stable products of the reaction, this process is analogous to the autocatalysis of a thermal reaction; it might be called autosensitization. The mechanism of this reaction also illustrates the fact that in a true case of photosensitization the sensitizer is not used up, but is continuously reformed, in analogy with the regeneration of the catalyst in a homogeneous catalytic reaction. Nitrogen dioxide also acts as a photosensitizer for the formation of water from its elements (7), but in this case oxygen atoms are the active agents. For this reaction, the action of the photosensitizer is relatively inefficient, since the reaction of oxygen atoms with hydrogen molecules,
+ NOa+N O + 0 (Primary reaction).
This is the primary step for all reactions photosensitized by NO2. When a flask containing nitrogen dioxide (which is of course in equilibrium with nitrogen tetroxide) is irradiated by a focused beam of near ultra-violet light, the pressure of the gas increases because of the formation of nitric oxide and oxygen. However, this increase in pressure soon reaches a constant (steady state) value, which is much less than the increase that would result if all the nitrogen dioxide were converted into a mixture of nitric oxide and oxygen. The change in pressure increases as the intensity of the absorbed light increases. These fads are fully explained by the following sequence of reaction steps. hv
specific reaction rate of their interaction with nitrogen dioxide molecules. If a suitable reactant is present the steady state is not attained, and a new series of secondary reactions is induced. Either nitric oxide or oxygen atoms may induce thermal reactions. In the decomposition of nitrogen pentoxide photosensitized by nitrogen dioxide, nitric oxide is the active agent. The reaction steps involved in the process are :
0
+ Hn+HsO (Secondary reaction),
occurs much less readily than the corresponding reaction with nitrogen dioxide, 0
+ NO1+N O + 01(Secondary reaction).
There can be no doubt that a large number of oxidation reactions could be photosensitized by nitrogen peroxide, and it is to be anticipated that when the substance to be oxidized (the acceptor) is a complex molecule that the process would be much more efficient.* Unfortunately, photosensitized reactions of this type have not received the attention they merit. Any other molecule which splits off an oxygen atom in a photochemical primary process should be capable of acting as a photosensitizer for oxidation reactions. At temperatures above 300°C. ammonia acts as photosensitizer for the formation of water from its elements. As in the case of nitrogen dioxide, the primary act is a predissociation process, which in this case yields a hydrogen atom.
-
* Compare
Ref. (6), pp. 3 8 4 7 and Ref. (5), pp. 216-9.
hu
+ NHa--+
NHz*--+ NH2
+ H (Primary reaction)
the intensity of the fluorescent light is greatly diminished. If, instead of these salts, oxalic acid is added, the fluorescence is diminished and some of the oxalic acid is decomposed. If the concentration of the oxalic acid is much greater than that of the uranyl ion, the fluorescence is completely quenched and the ratio of the number of molecules of oxalic acid decomposed to the number of photons absorbed becomes unity. This process may be represented by the following stoichiometric equation:
Light of wave-length between ZOOA and 1865A is required. This reaction is unlike the cases we have discussed in that no steady state is set up. On prolonged irradiation the ammonia is completely decomposed into nitrogen and hydrogen. Both nitrogen and hydrogen are transparent to the light used in these experiments. If only one molecule of water was formed for each photon absorbed, ammonia would scarcely be considered as a photosensitizer for this reaction, since this kv (COOH), = COa CO 4- H 2 0 would result in the decomnosition of an ammonia molecule for each water molecule formed. However, under The action of potassium bromide and of oxalic acid the experimental conditions the formation of water on the advated uranyl ion may be to the is a chain reaction.* That is, the reaction of an oxygen of nitrogen and hydrogen, respectively, on the molecule and a hydrogen atom sets up a series or chain resonated mercury atom. of secondary reactions resulting in the formation of a Under certain conditions ferric salts are also capable The number of of acting as sensitizing agents. However, the mecblarge number of water water molecules formed in a chain depends upon the ,i, their action does not follow the simple pressure and temperature of the system. Under proper exchange process, which seems sufficient to explain conditions i t is even possible to explode a mixture of reactions sensitized by uranyl ion, but appears to inand Oxygen a little ammonia volve alternate reduction of ferric and oxidation of exposing it to an intense source of ultra-violet light. ferrous ions. The reactions sensitized by ferric ion Several other reactions of this type have been studied which have been studied appear to be complicated by and i t has been pointed out that other substances, such complex-ion formation, hydrolysis, and general salt as hydrogen sulfide, should be expected to act like effects. ammonia. It should be noted that reactions photoThe sensitizing action of the halogens for reactions sensitized by ammonia are not typical cases of photo- ocming in solution seems to be analogous to their sensitization and are not in direct analogy to catalyzed action in the gas phase, Their primary behavior may homogeneous thermal reactions. They bear about the be rep,sented as: same relation to a simple case of photosensitization kv XI -+2 X (Primary reaction) that an induced reaction (with a lame induction factor) does to a simple case of homogeneous catalysis. It An example of such a process is the oxidation of hywould be quite consistent to call them photo-induced driodic acid to iodine by dissolved oxygen. This reacreactions, or to say that the oxidation of hydrogen is tion occurs slowly in the dark, but after some iodine has induced by the photochemical decomposition of am- been formed it is greatly accelerated by visible light. monia. The light is absorbed by the iodine which is dissociated into atoms. The iodine atoms set up a cycle of secondary reactions which result in the oxidation of hydriodic The phenomenon of photosensitization is by no acid and the regeneration of iodine molecules. The means confined to gaseous systems. A number of exact nature of these thermal steps is still somewhat in photosensitized reactions occurring in aqueous or non- doubt. This reaction, like the photosensitized decomaqueous solutions have been studied. The decom- position of nitrogen pentoxide, is an example of autoposition of oxalic acid sensitized by uranyl ion, UOz++, sensitization. An interesting example of a bromo-sensitized reaction is a typical example of a sensitized reaction occurring in aqueous solution. The action of this ion in a dilute occurring in an organic medium (carbon tetrachloride aqueous solution resembles in many ways the action of solution) is the isomerization of the diethyl ester of mercury vapor. This analogy may be illustrated by maleic acid. The primary step is : considering a series of experiments corresponding to hv Brr -+2Br (Primary reaction) those described in Section 11. If a narrow beam of light is focused upon a dilute solution of uranyl sulfate a The bromine atoms add to the ester molecules breaking number of the photons making up the light beam are the double bonds, and forming molecules of an unstable absorbed by the uranyl ions. Some of these photons intermediate. I are degraded to heat, but many are re-radiated as H-C-COOEt H-C-COOEt fluorescent light (though not necessarily a t the same 11 + B r 4 (Secondaryreaction) H-&cooEt wave-length). If certain salts, such as potassium H-c-cooEt iodide, sodium bromide, etc., are added to the solution
+
-
+
+
+
* See Ref. (2), pp. 40-5; Ref. ( 6 ) , pp. 118-20.
t See Ref. ( 4 ) , pp. 605-20;
also Ref. (Z),pp. 3&9; Ref. (3),
pp. 140-52; Ref. (5). pp. 20E-9.
The breaking of the double bond permits free rotation of the groups about the central carbon-to-carbon bond.
~ i c ~ajr-certam ~ c probability that the bromine atom will be eliminated (or removed) when the groups are in their trans position, permitting the double bond to reform and stabilize the fumaric ester.
nitely understood. It is by no means impossible that the reduced form of the dye plays a part in the process, by some mechanism roughly analogous to the nitrogen dioxide-sensitized oxidation reactions. V.
PHOTOSENSITIZED REACTIONS IN HETEROGENEOUS
SYSTEMS* Br
Br
HC-COOEt
11
EtOOC-CH
+ Br
(Secondary reaction)
It should be noted that these steps result in the elimination of the bromine atom which is then free to add to another ethyl maleate molecule and so induce further racemization. This theoretical conclusion is supported by the experimental observation that about 300 molecules are racemized for each photon absorbed. If it were not for the known occurrence of side reactions and the direct recombination of Br atoms it would be expected that the number of molecules reacting per photon would approach infinity. In addition to these simpler inorganic substances certain complex organic compounds, notably dyes, are capable of photosensitizing chemical reactions. It is commonly stated that only those dyes which are fluorescent can act as photosensitizers. However, as in the case of uranyl ion, photosensitization does not depend upon the occurrence of fluorescence, although the two phenomena seem to be complementary functions. When a molecule of non-fluorescent dye absorbs a photon it degrades the energy to heat. When a molecule of a fluorescent dye absorbs a proton, the molecule is raised to an activated state. If, in the brief time while it retains this energy, it collides with a chemically reactive molecule, it may give up all or part of its excess energy to the second molecule, and so induce a chemical reaction. If no such collision occurs it may either transmit the energy to the solvent as heat, or radiate it as a photon of fluorescent light. In other words, photosensitization and fluorescence may be thought of as two independent phenomena competing for the same quantum. A sensitized reaction of this type, which is a t least superficially simple, is the oxidation, in acetone, of allyl thiocarbamide by dissolved oxygen, the sensitizer being ethylchloroaphyllid. When the concentration of the acceptor (allyl thiocarbamide) is very much greater than the concentration of the dye, one molecule of oxygen disappears for each photon absorbed; but when the relative concentration of the acceptor is small the number of photons absorbed is much greater than the number of oxygen molecules reduced. This was to be expected, since under the latter conditions there is a much greater opportunity for the activated dye molecule to lose its energy before it collides with a molecule of the acceptor. I t must be admitted that the intimate or detailed mechanism of photosensitization by dyes is not defi-
A number of photosensitized reactions occurring in heterogeneous systems have been studied. The kinetics of these reactions are very complex, since the usual series of photochemical and thermal reactions can be accompanied by adsorption, contact catalysis, and diffusion phenomena. One of the best known examples of this type is the sensitization of photographic emulsions (8). While undoubtedly a vast amount of study has been devoted to this subject, the greater part of it has been made from a purely practical viewpoint, and no satisfactory general theory of the action has been suggested. Further work on the sensitization of homogeneous reactions by dyestuffs may be valuable in interpreting the mechanism of photographic sensitization. Solid zinc oxide has been reported as acting as a photosensitizer for a number of widely different reactions occurring in both gaseous and liquid systems. The mechanism of this effect is still entirely mysterious. There is some possibility that this action may be due to the presence of an impurity, such as zinc nitrate or some partial decomposition product of it (9). Photosynthesis is the process by which green plants synthesize carbohydrates from carbon dioxide and water; it is sensitized by the dyestuff chlorophyll (10). The complete reaction apparently involves an elaborate series of reaction steps, including heterogeneous reactions and diiusion processes. It is an endothermic reaction, and results in the storing of solar energy in the form of chemical energy. This in turn supplies the energy for practically all vital processes. It is unique among photosensitized reactions in that several (probably four) photons are absorbed (and utilized) in the reduction of each molecule of reactant (carbon dioxide). I t is commonly believed that the first product of the reduction of carbon dioxide is formaldehyde, which is then elaborated to starch or some other carbohydrate. This stoichiometric process may be represented by the equation: COZ
+ HIO + 4(?)hv = HCHO + O1 (Stoichiometric reaction)
In spite of the large amount of information about photosynthetic processes which has been obtained, its intimate mechanism is quite unknown. This is not snrprising when the enormous complexity of the system and the difficulty of obtaining reproducible measurements on a vital system are remembered. VI. SUMMARY
In the foregoing discussion no attempt has been made to present either the historical development, or a ' S e e Ref. (3), pp. 1 5 2 4 0 ; also Ref. (2). p. 39; Ref. (4). PP. 62W5; Ref. ( 5 ) , pp. 27&80.
complete outline of the subject. Furthermore, such closelv related subiects as chemiluminescence* and the sensitization of ionic reactions by chemically inert gases (11)have not been included. Weigert's "sensitization of the second kind" has not been mentioued (12). Several of the older theories of photosensitization have been intentionally omitted since they are not T e e Ref. (P), pp. 31&54;
Ref. (3). pp. 161-5.
in agreement with our present knowledge of physics. Reactions which seemed best adapted to illustrate the general principles have been described in some detail, while others, perhaps equally important, have been either briefly mentioued or else omitted entirely. However, if this introduction has served to acquaint the reader with the scope and fascination of the subject, it has accomplished the only purpose for which i t was intended.
BIBLIOGRAPHY
(Asfar as possible reference has been made only to swnmarizing articles, monographs, and texts. References to the original work mav he formd in the 0 1 1. ~ ~ 4 ) .. -~--, -. .. .cited (1) R. LNINGST~N, J. CHEM.EDUC., 7,2887 (1930). (2) D. W. G. STYLE,"PhotochemistTYII' E. P. Dutton & Co., New York City, 1930. (3) G. B. KISTIAKOTKSKY, "Photochemical processes," A. C. S. Monograph, Chemical Catalog Co., New York City, 1928. (4) R. 0. GRIP PIT^ AND A. MCKEOWN,"Photo-processes in gaseous and liquid systems." Longmans, Green & Co., New York City, 1929. (5) K. F. BONKOEFPER AND P. HARTECK, "Grundlagen der Photochemie," Steinkopf, Leipzig, 1933.
(6) L. S. KASSEL, "The kinetics of homogeneous gas reactions," A. C. S. Monograph, Chemical Catalog Co.. New York Citv 1032. . ~ ~ > -.-, (7) H. J. SCFIUMACHER, I. Am. Ckem. Soc., 52, 2584 (1930); NonrusnamGruamns, Proc. Roy.Soc.. A139,107(1933). S,E,SEEPPARD, Ind. Eng, Chem,, 22,555(1930). (9) J. M c M o m s AND R. G. DIWNSON, I. Am. C h m . Soc., 54,4248 (1932). (10) H. A. S P O E ~"Photosynthesis," , A. C. S. Monograph, Chemical Catalog Co., New York City, 1926. (11) S. C. LIND,"The chemical effects of alpha particles and electrons," A. C. S. Monograph, Chemical Catalog Co., New York City, 1928. PTOC. Farnday Soc., 1931,542. (12) F.WEIGERT, ~
