Catalysis of Photochemical Reactions Opportunities and Pitfalls A. Alblnl The University. Pavia, Italy The recent literature offers as increasing number of reports about photochemical reactions accelerated by some added substance. The term catalytic effect has been used in connection with these ~henomena.and the conceot of catalyzed photoreaction has been recently more fully elahorated ( I ) . It is the purpose of this paper to present a classification of catalytic effects in photochemical reactions, both because this is a promising area of development for further research and because some characteristic properties of photochemical versus thermal reactions are elucidated in this wav. The increase of the reaction rate in the presence of a catalyst is due to the availability of a new reaction pathway involving some intermediate of which the catalyst is a part (2). Therefore, the concentration of the catalyst appears in the rate equation and a phenomenological description of catalysis can he achieved by reference to the rate equation, the question then remaining of identifying the mechanism of catalysis. Thus, a substance is said to act as a catalyst in a thermal reaction when i t amears in the rate eauation hut not in the stoichiometrical equation, or better, aiopting the definition by Bell which includes autocatalysis, when its concentration appears in the velocity expression to a higher Dower than it does in the eauilibrium expression (3). a definition of a catalyst Accordingly, Wubbels in a photochemical reaction as a substance that appears in the quantum yield expression for reaction from a particular excited state t o a power greater than its coefficient in the stoichiometric equation ( I ) . As the quantum yield is the ratio between the rate of reaction and the rate of absorption of the light (I.),this appears a legitimate extension of Bell's definition, which simply takes into account that the reagent of photochemical reactions, the excited state, is itself created at a certain rate by light absorption. However, in order to attribute a chanee in the observed auantum vield to a change in the reactivity of a state, the greater complexity of photochemical reactions must be taken into account. Photochemical reactions are inherently multistep Drocesses involving-. ohvsical (i.e.. without nuclear motion) as . well as chemical steps. Thus, a ground state substrate is ~ r o m o t e dt o some electronicallv excited state which then reacts to yield a ground state product. The latter may he a reactive species which undergoes further thermal reaction to yield the actually isolated Disregarding for the moment secondary thermal processes, the quantum yield for overall process is
' The discussion is limileo to "clean" photoreactions. In Me practical case competitive absorbance by the products, secondary photoreactions. etc.. eao to ado tional complicatnon an0 corrections are introduced in calculation of the quantum yield (see for example ref. 4a). The distinction between sensitization (energy transfer with d6excitation of the sensitizer and excitation of the substrate: analogously for electron transfer) and catalysis (chemical interaction with the substrate, leads toward the thermodynamic equilibrium) Is pivotal in modern molecular photochemistry (4b). However, some confusion exists and the term "photocataiysis" Is often used when in fact sensitization is meant. The present contribution attempts to offeran unambiguous, albeit not universally accepted, classification.
where 7 is the efficiency in reaching thereactive excitedstate A* from the one originally formed by absorption ( A * * )and k, and kd the rates of reaction and physical decay from that state.' In the presence of the catalyst C, in the simple hypothesis that the product is the same as in the uncatalyzed reaction, the process is
;*Y.
n
A*
Products
and the quantum yield is
In the inverse from
and in the region when k,c[C]
>> k,
After the excited state oarticioatine in the catalvzed reaction has been recognized,-it mu& he srhown to he trke catalysis by excluding the possibility that the effect is due t o an increased efficiency of formation rather than of reaction of that state. Thus a sensitizer S, which would absorb light concurrently with A and transfer energy to it, would cause, at least within certain limits of concentration, a linear increase of with IS]-' similar to that predicted by eq 4, but energy transfer (eqs 5,6)is certainly not to be confused with ~atalysis.~
+-'
h"
S-S*
A similar effect on +would he also caused by an additive which modifies by acting on the phorophysical parameters of the suhstrate A. This is the case with additives increasing the intersystem crossing from initially formed A'* to reactive A3* (e.g., Hg(CNh facilitates the ISC to the triplet state of sinelet excited N-vinvlcarhazole. and thus increases the photokmerization quantum yield ( 5 ) )or increasing the absorbivit\f of forbidden bands (ex.. . - . I'dCI, enhances the A A3* absorption of some olefins and thus their triplet reaction (.6.. )). With these limitations an increase in the quantum yield is indicative of an increase in the chemical reaction of the excited state versus its physical decay and, considering the linear part of the plot +-I versus [C]-1, the ratio intercept1 slope corresponds to k,c/kd. When the lifetime of the excited state ( r = llkd) is known or can be guessed at, i t is useful to calculate k,Cfrom this ratio, both as a consistency test, since an unreasonably high value for k,C would indicate that a
-
Volume 63 Number 5 May 1986
383
longer-lived species rather than the excited state is being catalyzed (see below), and as this is the quantity of interest for mechanistic conclusions. As an example, when comparing the effect of a catalyst on the photochemical reactions of aseries of substrates, hrCandnot k,clhdis plotted against the chosen parameter (e.g., electron density a t the reaching center, etc.). Of course, in favorahle cases the direct determination of h. and k.C. is nossihle with nulsed ex~eriments. . Sotneexamples ofcatalyzed reactionsofexciwd statescan be discussed. Thus aromatic alkenes are significantly polarized in their singlet excited stare and these undergo protonation by acidsand hydration under general acid catalysis (7). 1'
C=C
Ar
, '
/
\
-
H
I/ C-C
\+
H+
/ Ar
-H20
-Ht
OH H
\I
,C-C Ar
I/
Nap1'
Act-
0
Ace-
x- NapX. Nap? --t
Ac.
u
Notice that in both cases the interaction between A* and the catalyst is the only part of the process occurring on the excited state surface, after which the system is hack in the realm of thermal chemistry, where the actual reaction takes place. In other terms, i t has taken advantage of the much more favorahle position of the protonation and, respectively, oxidation equilibria to transform excited states, which would otherwise decay to the ground state, into the corresponding cation or, respectively, radical cation, and these species react before the equilibrium is reversed in the ground state. Thus light absorption and quenching of the excited state by the catalyst trigger a ground state reaction that would require much more drastic conditions without the detour through the excited state. More strictly adherent to the mechanism of thermal catalysis are some photoreactions in which the catalyst opens a new reaction pathway remaining on the excited state surface. Examples are to be found among reactions of excited complexes C'exci~lexes") (AB)* (9a). Interaction with a third molecule gives a termolecular excited complex (.'exterleu") from which decay to the product in comparison with decay to the reagents is more efficient. ~ l t h o u g hsome of the evidence is controversial (10) such a reaction scheme is demonstrated both through the temperature effect on quantum yield (96)and the formation of new products, including several examples of autocatalysis (C = A or B) (9c-f). A* + B
-
/
(AB)'
3
A+B
(ABC)'
J A-B
+C
On the whole, unambigous examples of catalyzed reactions of excited state are limited in number, as would be expected on the basis of theextremely short lifetimeof these species (typically r = 10-6-10-12 s). Thus, chemically reactive excited states bv definition have available an easv reaction pathway (withnegligible activation energy, heice the usual near independence from temperature of photochemi384
Journal of Chemical Education
X
t = Intermediate
The efficiencyof the overall photochemical reaction can be
increased a) by enhancina- the oo~ulation of the reactive excitedstate: b\ bvactuallv . . ~ catalvzino , " the excited state reaction: c) by comp ex ng me ground rtate: tne complex then absorbs light and gives the deslreo reactoon d) oy inflbencing me tnermal panltlon ng al a gra~ndstate intermed ate towards me desired prod~ct.
\
Analogously, naphthalene (Nap) undergoes photon-addition in the presence of r electron acceptor (Acc) via the corresponding radical cation (8). Ace
Excited State
cal reactions). otherwise the reaction bas no chance to compete with phisical decay to the ground state. If this is the case, there is not much room for catalysis, as the uncatalyzed process is very fast (commonly observed rate constants are several orders of magnitude larger than those observed in thermal processes). fn the othe; instance, the reaction between the excited stateand thecatalyst is required to bevery fast for the latter to have a significant effect, particularly if we stick to the idea that catalysts are active a t a low concentration, and this in practice limits available mechanisms to electron transfer and proton transfer. In contrast. a mechanism different from catalvsis of an excited state ;eaction has a hetter chance t o enhance the efficiency of the overall photochemical process (see the figure). A first possibility is that the catalyst complexes the ground state substrate. Thus, for example, Lewis acid complexes of unsaturated esters undergo efficient 2+2 cycloaddition, whereas the uncomplexed substrates do not (11). In this case, the catalyst does not modify the reaction of an excited state of the original substrate, but rather a different chemical substance, with a different absorption spectrum, is irradiated. Therefore, while i t is possible that in a limited concentration range V1denends on IC1-I in a wav similar to that ~ r e d i c t e dbv eqs 3 a i d 4, the bbserved q u k t u m yield will vary with thk wavelength of irradiation, and parameters relative t o the reaction of the complexed excited state are obtained taking into account the around state eauilihrium constant and the absorptivity of thk complex andof the free substrate a t the wavelength(s) of interest. (
A*
A .B)*
--r
Products
This class of reactions includes a large number of synthetically useful photoreactions. Thus, for example, irradiation of metal (mostly Cu (1))-olefin complexes often lead to a chemistry (e.g., cycloaddition) not encountered with uncomplexed olefins (12).The cis isomer is predominant in the mixture obtained by steady state irradiation of complexed conjugated alkenes whereas uncomplexed alkenes give a -1:l trans-cis mixture (13). Another possibility of accelerating the overall reaction is catalysis of the reaction of some primary photoproduct rather than of an excited state. A*
~
~
u-0
Stable products
Tvnical examnles are found amona medium ring (five- to seven-member)~cycloalkenes.~ h e s ecompounds undergo sensitized nhotochemical isomerization to the strained trans isomers. he latter are obviously ground state, albeit highenergy, compounds and a t room temperature spontaneously revert to the stable cis isomer, so that no net chemistry results. However, in the presence of an acid the trans derivative is trapped and ionic addition or carbocationic rearrangement take place (14) leading to irreversible reaction. Metal complexation likewise stabilizes the trans isomer (15) and makes new reactions possible (16).. These reactions clearly differ from the previously mentioned acid-catalyzed addition of aromatic alkenes, in which the excited state, not a ground state product, is protonated. A common occurrence is that the primary product of a photochemical reaction is highly reactive particles, such as radicals, ions, zwitterions, carhenes. I t is often possible that a small amount of a catalyst diverts the reaction of these species towards a product in preference t o another, or towards irreversible reaction in preference to recombination (or cleavage) to the starting material. Thus, for example, the irradiation of benzophenone leads to bimolecular reduction to benzopinacol in neutral (or weakly acidic) alcohols and to complete reduction to benzhydro1 in the presence of sodium alkoxide. The base interacts with the intermediate ketyl radicals, not with the excited state. (17).
7
~ ~ ~ t rPhzy d -
F
Ph.
+ RzCO
OH OH
Similarly, the quantum yield for reduction of benzophenone bv the mixture of diisooronvlamine (DIA)and t-butvlamine (TBA) is increased by'ad&& a sm& amount of a thiol, so that the aminyl radicals from TBA are transformed into aaminoalkyl radicals (from DIA) thus avoiding back hydrogen transfer between P ~ & O Hand t-BuNH., which would lead hack to starting materials (18).
A large group within this category is formed by electron transfer photoreactious in that the easier pathway for the radical ions of opposite sign is usually back electron transfer regenerating the starting material. The chance of chemical reaction can be improved, for example, by secondary electron transfer, which removes the charged species and lengthens their lifetime, or proton transfer from the cation radical to vield a neutral radical. ~ h u sfor , example, general base catalysis has been demonstrated for the Photo-Smiles rearraneement of some aminoalkoxybenzenes, the key step beingdeprotonation of the
zwitterions resultine from attack of the amino eroun on the benzene ring (19)-Likewise, excited dicyanonaphtalene (DCN) is reduced hv toluene formine DCN-' and PhCH7+.. A d back electron transfer to the starting compounds is prevented in the presence of water, which mediates proton transfer yielding DCNH. and PhCH2. (20). The reaction of a radical ion can also be made more efficient by prolonging its lifetime through quenching of the radical ion of opposite sign (thus, for example, protonation of A-' allows BH+. more time to react (21)). The following scheme accounts for the last mentioned examples
BH+.F C
CHt
4
A-.+
n-.
reaction of B.
BB.
AH- + B.
--t
reaction of AH. andlor B.
In every case, C is regenerated by reaction with some product and thus, a t least in this sense, acts as a catalyst. For the sake of classification. catalvsis of the reaction of primary products may be sensu latu considered a catalysis of the reaction of the excited state from which that product is formed, although this is not directly involved in the catalvzed reaction. In some, but not all. cases the quantum vield for the formation of the end product will depend on the catalyst concentration .IC1. according to ea 3 or similar expressions when now q is the effirienry for the formation of the primary product from the initially formed (not the reactive) excited-state and kd, k,, k,C are t h e parameters for the reaction of this product, the low and thus kinetic relevant step of the overall reaction. Obtaining these parameters can give useful information about the type of product which is being catalyzed (e.g., a strained compound, such as transcyclohexene, or a radical, etc.), although further support is reouired before a mechanism is recoenized.3 " In conclusion, an increase of the quantum yield of aphotochemical reaction by addition of some substance is usually not due to acceleration of the reaction of the excited state involved, but either to com~lexationof the around state substrate before excitation o; to catalyzed reactions of some primary photoproduct. In some cases a simple relation beand [C]-' obtains but the numerical values availtween able from this plot can be given a physical significance only after definition of a mechanism. Apart from mechanistic implications, the main object lies of course in finding out new chemistry. In this we are encouraged by nature, which actually resorts t o complexation (or orderine of the molecules in a structure) before excitation or to fast secondary interaction with primary photoproducts in order to reach maximum efficiency in biological photochemical reactions, from photosynthesis to vision. Indeed, it is likely that we know only afraction of possible photochemical process, or, in other terms, that only in a limited number of cases (the easier ones), conditions have been found which make irreversible reaction of excited states and/or primary products competitive with return to the starting materal. Toward this aim an advanced knowledge of the events immediately following excitation will be more helpful than the effect observed on the quantum yield of the overall reaction. Indeed, for example, the evidence provided by pi-
+-'
Wubbels includes in his discussion all catalytic effects, independently from the step of the photochemical reaction which is influenced ( I ) . A s it has been shown above, however, the obtained kinetic parameters mostly do not refer to the reaction of an excited state. Volume 63
Number 5
May 1986
385
cosecond spectroscopy i s introducing a different portrait of the dynamic interactions between states and surrounding molecules and of the evolution of encounter complexes and primary photoproducts, and t h i s w i l l probably foster the use of the kinetic approach different from the one traditionally .. used (22). From t h e didactic point of view, emphasis should be on the different species involved, rather than on a general kinetic approach if the mechanistic significance of the equations proposed and the limit of their application are n o t appreciated. I t has been said 20 years ago that "a student who sees a difficultv in definine catalvsis i s ~rohahlvin a better position thanone who has learned a standard i e f i n i tion" (2).This i s s t i l l correct about catalvsis and the more so about the inherently more complex cataiysis of photochemical reaction^.^,^
Literature Clted (1) Wubbels. G. G.Acc. Chem. Res. 1983.16, 285. (2) Leiofen. J. C. J. Chem.Edue. 1964.41.23. (31 Bd, R. P. '"Acid-BaseCatslmis"; Oxford UniversityPress, 1941: p 3. (4) (a) Buna. N. J. J. Pholorhem. l981.15,1: (b) Hammond. G. S.: Ssltiel.J. Lamola, A. A.;Turro,N. J.; Bradshew,J. 5.; Cowan.0. 0.; Counsell. R.C.; vogt, V.;Ddton.C. J . Amel Cham. Soc. 1964.86.3197. (51 Bmifenbaeh, J.: Sommer, F.; Unger, G. Momfsh.Chem. L910,lOI. 32. (6) Wan6 T.M.. PhD. Thesis, UniueraitydArimna. 1973:Dirs. Absf. l974,34B, 5920: Chsm. Abs. I974,81,9080Su; eikd in Sslomon, R G. Tetrahedron. 1983.39,485. (71 Wan, P.; Yaks, K. J. Org. Chem 1983,48,869. 181 Yasuda, M.: Pac, C.: Sakwai, H. J. O m Chem. 1981.46.788; Yaruda, M.; Pac, C.; Sakursi. H.J. Chrm. Sor., Perkin Tram. 1,1981,746. (91 (a) Csldwell, R. A,; Creed. D. Acc. Chem. Res. 1980. 13.45: (b) GrcUmann. K. H.; Suckow, U. Chem Phys Ldt. 1973. 32, 250: ( 4 Saltlel, J.: Townsend, D. E.; Watson, B. D.: Shannon, P.; Finson, S. C. J. Amer. Chem. Soc. 1917,99, W (dl Yang, M. C.: Shold, D. M.; Kim, B. J. Amar. Chom. Soc. 1976, 98, 6587: (e) Campbell, R. 0.: Liu. R. S. H. Molec. Photorhom. 1914. 6, 207: (fl Sslfiel, J.; Townaand.D.E. J.Amer. Chem.Soc. 1973.95.6140. (101 Ksupp. G.;Teufel,E. Chom.Bor. I980,113,3669. (IllLewis,F.D.;Oxman,J.D. J.Amer. Chem.Sor. 1984. Iffi, 466. (121 Salomon, R. G.;Kahi. J. K. J. Amrr. Chrm. Soc. 1973. 9, 1889; Even. J. T. M.; Macker,A.Rec.J.RoyNeLh. Chem Soc 1979.98.423. (13) Lewis,F.O.:Oxman,J.D. J.Amer. Chsm.Sor. 1981,103,7345. (141 Kcopp,P. J. J. Amar. Cham.Soc 1969.91.5783; Marshall,J. A.;Hwhstetler,A. R.J . Amsr Chem. Soc. 1969.91.648. (15) Deyrup,J. A,; Betlrowaki,M. J. Drg. Cham. 1912,37,3561. (16) Salomon, R. G.; Folting, K.; Stmib, W. E.; Kahi. K. J. Amer. Cham. Sor. 1914,95, >,A%
Cohen. S. G.: Sherman. W. V. J. Amw. Chsm. Soe. 1963.85.1642. Albini, A ; Faasni,E.: Sulpizio, A. J. Amar. Chem. Soe. 1984,106,3562. Matte8,S.L.: Farid.S. J . Cham.Soc. Chem.Commun. 1986.457. Simon,J.D.,Peten.K.S. Acc. Chem.Res. 1984,17,277.
986
Journal of Chemical Education
(231 Child.. R. F.. Duffw. B.;~iks-Gibds.A. J.Om Chem. 1984.49,4352. (24) For.M.A.Acc. Chsm RPJ.1983.16.314.
'The present discussion has been impiicity limited to homogeneous catalysis. When the catalyst is heterogeneous, a clear distinction of the soecies interactiw with it is even more imooftant. A nice
df
~.~
discussion the cis-trans oh&oisomerization of coniubated olefins , ~ " ~- in the presence of solidac ds has been recently given by Chi ds. Ratner than by any effects on the photochemicalreaction, the isomeric ratio is determined in that case by preferential absorption of one of the isomers (23).On the contrary the so-called "heterogeneous photocatalysis," which involves light absorption by a powdered semiconductor and sensitization of a reaction of the (adsorbed) substrate is outside the scope of the present discussion. Finally, it can be noted that inhibition, or quenching, of a photochemical reaction is more familiar ohenomenon than 7~~ oositive catalv~, Sis, as excited states or other n ~ g h energy particles involved in these ~
~~
~~~
~
~
~
~~~
~~~~
(24,
~
~
~~~
reactions are easly quenched or deactivated by smal amount
of
i m p d i e s . Negat.ve catalysis can oe *metically treated as before, the inverse of the quantum yield now depending on the catalyst concentration according to the following equation
kc
where is the rate for the catalyzed decay process. Photochemists genera ly plot the ratno of the quantum yield for unquenched ( P I versus quenched (+) process against [C], viz.
This is the familiar Stern-Volmer eauation. which directlv , affords the ratio between the rate of quenchingikdc)and the sum of all decay rate in the absence of the ouencher - - ik. ~~~" * ~. ~I., , . The same cgeatdiscussed above for positive catalysis holds here, ~
7~~
~
~~
~~~
+
and one must distinguish whether negative catalysis isdue toreaction of C with a given excited state (e.g.. energy or electron transfer) or with some primary photoproduct, or if it is due to pre-complexation of the ground state. It would be bener to reserve the term quenching to direct interaction of C with an excited state, in analogy with the original application of this termto quenching of excited state lumines-
cence.
