nee., 1962
TRaNSFER O F
TRIPLET STATE ESERGY
Fig. 2. Equation m-7 also predicts a linear plot of (l/Rr) 2’s. (1/A!I) a t constant thionine concentration. This plot is shown in Fig. 9b using the steady-state rate data in Fig. 5 . We note that it is necessary to use the steady rates of fading in (m-7) to obtain satisfactory plots, but the data are not strong enough to draw conclusions from this. ,4n estimate of the dependence of the rate of polymerization R, can be made from the mechanism. The rate of appearance of the radical M,. is given by d?tl,./dt = ka(ST)M ka(OH)lll 2Rf Using the usual procedure in polymer kinetics, one assumes that the rate of removal of M,. is bimolecular in lLilx.z with rate constant kt and that the rate of polymerization is given by eq. m-8.
2569
added to the thionine-acrylamide-EDTA system a t concentrations less than those of LT, then T will he regenerated
LT
+ O2 -+
+
T Hz02 (m-10) and an H202-suppressed photopolymerization will occur. If an excess of O2 over LT is added to the system, then TW believe that after reactions m-9 and m-10 have occurred, an 02-inhibition photopolymerization will follow. The possibility that the mechanism of formation of ST is by hydrogen abstraction from monomer9 appears improbable from the energetic point of view. Available bond energy data2’ for C-H bonds indicate that they are in the range of 70 to 100 kcal./mole, which is beyond the 46-kcal., mole range of light-excited thionine. Data are not available on the strength of the N-H bond in the amide R, =-- k,M(ILl,~) = k,M(2Rr/kt)l/2 (m-8) group of acrylamide, but it is possible that in alkaline From eq. m-8, R, should follow a lam, should solution it might be in the 46-kcal. range. To c2M8/2),and should check this we have photopolymerized acrylic acid depend on b1 as b!Ia/a/(cl depend on T as l/(cl C ~ T ) ’ /Figure ~. 4 shows in oxygen-free aqueous solution a t pH 8.4 with 3 M thionine. In this case thionine fading that the (Io)’/zis approximately followed, and a X plot of data from Fig. 4 shows R, to be roughly was not observed, but copious polymerization relinear with Ma/p. The thionine-rate data of Fig. sulted. Hydrogen abstraction from acrylic acid 2 , however, do not show a linear dependence of T would seem to be impossible because hydrogen is us. 1/R,2. We find, therefore, that the functional bound only as C-H bonds in this molecule. It dependence of R, given by eq. m-8 is not as well must be pointed out. however, that if one assumes verified as the functional dependence of Ri given semi-dye formation by hydrogen abstraction, one can write a mechanism very similar to that written by eq. m-7. Assumption of this mechanism allows one to use above. The dependence of Ri and R, from such a the well known explanations of the effects of added mechanism is virtually the same as that derived electron donor RH2 in the absence of oxygen: the in eq. m-7 and m-8. Consequently, further successive two-electron reduction of T* to LT which experimentation with different dyes, monomers, and solvents is needed to resolve further the nature of suppresses reactions m-1 and m-2 the polymerization-initiator species. T* RH2 -+ LT Ro (m-9) ( 2 5 ) E. W. R. Stacie, “Atomic and Free Radical Reactions,” when Ro is the oxidized form of RH2. If oxygen is Second Ed., Reinhold Pub1 Corp., New York, N. Y . , 1954,p. 97.
+
+ +
+
+
TRANSFER OF TRIPLET STATE ENERGY AND THE CHEMISTRY OF EXCITED STATES BY F. WILKISSON~ Physical Chemistry Laboratory, South Parks Road, Oxford, England Received M a y 85, 1968
The general principles of the way in which triplet energy transfer can be used to obtain inforniation concerning photoreactive states in solution is outlined. Aromatic hydrocarbons are recommended as good inert acceptors while aromatic ketones and aldehydes often make good donors. Results on t v o systems where these principles have been applied are presented. The first example concerns the photoreactions of a number of substituted anthraquinones with isopropyl alcohol. Some derivatives, “good sensitizers,” give acetone n ith a quantum yield of unity in the presence or absence of oxygen. In the absence of oxygen they also produce anthraquinol with a quantum yield of unity. Other derivatives, “poor sensitizers,” give these same reactions with quantum yields < 0.1. The photoreactive state of anthraquinone, itself a good sensitizer, is shonn to be its triplet state. Thus the presence of compounds with lower triplet states inhibits drastically its photoreactions while those with higher triplet levels have no effect. The rate constant for the reaction of triplet anthraquinone with isopropyl alcohol is estimated to be 3 X 106 1. mole-’ see.-’. The lifetime of triplet anthraquinone in a 4: 1 mixture of benzene:isoproppl alcohol therefore is 1.3 x 1O-isec. The second example concerns the photodecomposition of 1-iodonaphthalene to give iodine (quantum yield a t 3130 A . = 0.09). Many compounds with higher trip!et levels than I-iodonaphthalene sensitize its decomposition (quantum yield of sensitized production of iodine with 3660 A . irradiation = 0.04-0.05). Triplet l-iodonaphthalene therefore undergoes some but not complete decomposition. Although 2-acetonaphthone does not photoreact with isopropyl alcohol, it does photosensitize the decomposition of 1-iodonaphthalene. This is interpreted aa showing that the triplet state of 2-acetonaphthone is present in solution but it does not react as it is F A * and not n--Ai’ in character.
Information concerning photoreactive states is not easily obtained and although the triplet state (1) Albright and Wilson Research Fellow, Pembroke College, Ox-
ford.
often has been proposed as the photoreactive state in a variety of systems due to its longer lifetime and greater biradical character than excited sing1et states, its role in such reactions has been estab-
F. WILIUNSON
2570
lished for only a few cases. It is important to have criteria for the participation of triplet states in reactions other than the effect of added oxygen, as this effect may be difficult to interpret due to the extremely reactive nature of oxygen. Oxygen not only quenches both singlet and triplet states, but it also reacts with any easily oxidized state, radical, intermediate, or product. For many systems, a number of chemically inert compounds can be found which are non-absorbing in a region where the reactive molecule absorbs and which are selective in quenching only triplet states. Such compounds quench triplet states by a process of electronic energy transfer which may be represented by the equation
Vol. 66
and there seems no reason why these should not be greatly extended. The general principles of thc applications of this process, many of which are illustrated later in this paper, are outlined briefly here. Consider a photoreactive molecule M. It may be used as either a donor RI*(triplet)
+ A(sing1et) +M(sing1et) 3. A* (triplet)
(1)
or as an acceptor D*(triplet)
+ hl(sing1et) +D(sing1et) + M* (triplet)
(11)
kt
+ A(sing1et) +D(sing1et) +
In each case partners should be chosen which allow the relative energy levels of each pair to be as A*(triplet) shown in Fig. 1. A filter can be used which allows only the donor to absorb light. A very concenwhere D denotes the donor, A, the acceptor, and trated solution of the acceptor often makes an exthe asterisk, an electronically excited state. This cellent filter for this purpose. Under such circumprocess, which leads to the sensitized phosphores- stances energy transfer from the excited singlet cence of acceptor compounds in rigid media,2 re- state of the donor to that of the acceptor is encently has been found to occur with high efficiency ergetically impossible. Transfer from excited singin normal ~ o l u t i o n . ~For ~ ~many ~ donor and ac- let donors to the triplet state of the acceptors, alceptor pairs values of -1O'O 1. mole-1 set.-' in though energetically possible, is unlikely due to hexane and benzene and -3 X 108 1. mole-' set.-' the lack of spin conservation. Furthermore, this in ethylene glycol have been found for kt. These process has been shown to be absent for quite a values are very similar to those obtained for oxygen number of systems under conditions where efquenching of triplet states* and are what would be ficient triplet transfer was 0ccurring.'*~~*6 expected for a process which depends only on the If process (I) above occurs with high efficiency, rate of diffusion of the reactants together, Le., it should be possible to add a high enough concenone which occurs a t every collision. In rigid media tration of the acceptor so that the decay of the the rate constants are very low, -lo2 1. mole-' triplet M is all due to energy transfer. Under set.-' or less, but this corresponds to transfer these conditions any remaining reactions would be distances equivalent to normal collision diameter^.^ due to the singlet state and these reactions could S o evidence for long range transfer and no de- therefore be studied in the absence of reactions due pendence of the transfer efficiency upon the to triplet states. On the other hand, reactions strength of the S, T1transition probability in which have been inhibited by the presence of the either the donor or the acceptor has been acceptor may reasonably be attributed to triplet Porter and Wilkinson3bhave made the following states. The presence, lifetime, and heights of tripgeneral observations from flash photolysis studies of let states which decay too rapidly to be detected by 20 donor and acceptor pairs. When the energy flash photolysis or other techniques or which do not of the donor triplet was considerably higher than phosphoresce in rigid media may be established that of the acceptor, Le., greater than ca. 1000 using process (I), provided always that it can be em.-', the transfer was diffusion controlled. As shown that the role of the quencher is simply to the triplet energies became comparable, the ef- accept energy by the physical process of energy ficiency decreased and no quenching by molecules transfer. Other types of quenching often are poswith triplet levels higher than that of the donor was sible and therefore it is important to take care in observed. choosing quenchers for this kind of study. Use of Triplet Energy Transfer in Distinguishing Aromatic hydrocarbons constitute a good class between Reactions of Excited Singlet and Triplet of quenchers because (a) much is known about States.--Since triplet energy transfer occurs with their energy states, (b) they are generally chemisuch high efficiency, it should be possible to make cally inert, and (c) the sensitized production of their use of it to learn more about triplet states. A triplet states can be illustrated by detecting their number of applications already have been made6-* triplet-triplet absorption spectra, which have been well characterized, using flash photolysis tech(2) A. Terenin a n d V. Ermolaev, Trans. Faraday Soc., 82, 1042 (lYj6). niquese3b (3) (a) H. L. J. Backstrdm and K. Sandros, Acta Chem. Scand.. l e , Process (11) leads to the photosensitized produc823 (1958); (b) G . Porter and F. Wilkinson, Proc. Rot/. SOC.(London), tion of the triplet state d M by-passing-excited A264, l(1961). (4) G. Jackson, R. Livingston, and A. C. Pugh, Trans. Faraday Soc., singlet states. This allows a study to be made of 56, 1635 (1960). the chemistry of the triplet state of M. By choos(5) V. Ermolaev and A. Terenin, J . c h i n . phys., 55, 698 (1958). ing donors which give high quantum yields of trip(6) H.I,. 3. Biokstrdm a n d K. Sandroa, Acta Chem. Seand., 14,48 let production (many aromatic aldehydes and (1960). D*(triplet)
(7) Q. S . Hammond, P.A. Leermakers, and N. J. Turro, J . A m . Chem. Soc., 83, 2395. 2396 (1961).
(8) G. Porter s n d F. Wilkinaon. Trans. Faradag Soc.. 87, 1686 (1961).
Dec., 1962
TRANSFER OF TRIPLET STATEENERGY
ketones are ideal for this purpose) and by increasing the concentration of M until virtually all the triplet donor is disappearing by process (I), the triplet state of M can be produced in high yields with the possibility of higher quantum yields for certain reactions of M than are normally found when M itself is irradiated. This is because photochemical and photophysical reactions of the singlet state of 31 which normally compete for each quantum absorbed are not present. There also is the possibility of populating by process (11) triplet states of compounds which upon self irradiation do not normally undergo internal conversion to form triplets. Another point of interest is that photochemical reactions due to triplet states often can be effected with longor wave lengths, due to the sensitized production of triplet states, than those necessary using self absorption, with obvious advantages for some systems. Some of these possibilities have been raised and illustrated by Hammond and cow o r k e r ~and , ~ some other examplesfare givendater in this paper.
Results and D'scussion This work represents an attempt by the author to apply the principles outlined in the previous section to some photochemical reactions of general interest. Some of these results are of a preliminary nature while others are a t present being prepared for publication elsewhere in a more detailed form. The discussion here will be confined, wherever possible, to the application of triplet energy transfer. Photochemical Reactions of Quinones.-Much previous work has been done on the photochemistry of quinones. A selection of references is given b e l o ~ . l O - ~The ~ pbotosensitized oxidation of various alcohols by quinones especially has attracted attention and much is known about the reaction mechanism (see ref. 10 and 11). However, until now the photoreactive state has been determined for only one system. Using flash photolysis techniques, Bridge and Porter16 have shown that the poor sensitizer duroquinone reacts via its singlet and not its triplet state to give durosemiquinone. The question arises-do other quinones, especially those which react 1Tith high quantum yields, also react via their singlet states? It was decided to attempt to answer this question by making use of triplet energy transfer. Anthraquinone and a number of its derivatives were selected for this purpose and isopropyl alcohol was chosen as substrate. Solutions of various anthraquinones were irradiated with 3660-A. irradiation in the presence and absence of oxygen. The results obtained are (9) G. S. Hammond, pi. J. Tnrro, and P. A. Leerrnakers, J. Phys. Chem., 66, 1144 (1962). (10) J. L. Bolland and H. R. Cooper, Proe. Roy. SOC.(London), 8226, 405 (1954). (11) C. F. Wells. Trans. Faraday Soc., 67, 1703 (1961). (12) B. Atkinson and M. Di, %bid., 64, 1331 (1958). (13) A. Bertlioud and D. Porret, Helv. chtm. Acta, 17, 694 (1934). (14) G. 0. Schenk and G. Koltsenberg, Nalurwrss., 41, 4.52 (1954). (15) P. A. Leighton and G. 8. Forbes, J . Am. Chem. Soc., 61, 3549 (1929). (16) N. K. Bridge and G. Porter, Proc. Roy. Soc. (London), 8 2 4 4 , 259, 276 (1958).
2571 -1
sg donor
acceptor
s,
Fig. 1.-Energy levels of donor and acceptor. summarized in Table I. I n neutral degassed solutions, the disappearance of the anthraquinones and the production of the anthraquinols were followed by absorption measurements. Unless otherwise stated, each anthraquinone (AQ) gave quantitative conversion to its anthraquinol (AQH2)and complete recovery of the AQ was effected by admitting oxygen. We shall note in passing that those systems w-bich show reversible quantitative conversion in neutral and acid solutions to give AQH2 give reversible and quantitative production of the semiquinone radical ion AQ.- in alkaline solution. The absolute values of the quantum yields given in Table I would be expected to be subject to errors not greater than 10%. W e r e quantum yield measurements are not available a t present, the words high and low are used to indicate the efficiency of the reaction as judged by comparison with those systems where the yields have been measured and which have been studied under almost identical conditions. The estimates of quantum yields are high about unity, and low
