research summaries for teachers
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JOHN H. RICHARDS California Insltub of Technology Parodeno, California 91 109
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physical organic chemistry
The Role of the Triplet The energy injected into a molecule when it absorbs a quantum of light can lead to a wide variety of chemical transformations, many of them of considerable synthetic importance (e.g., photosynthesis). I n some of these processes, light energy can be absorbed by one molecule and this energy transferred to a second mole cule which then undergoes chemical reaction; the overall process is said to involve "sensitization" of the second, reacting molecule by the first. This article will discuss some general aspects of photochemical reactions, in particular, (1) the role of triplet states, (2) an illustrative case of photosensitization, and (3) the mechanism of the photochemical rearrangement of santonin. Singlet and Triplet States
Photochemical reactions are strongly influenced by the multiplicity of the intermediates, that is whether the electrons have their spins paired, as they are in the ground state (So)or have their spins unpaired as they are in triplet states (T,). Consider two electrons in the highest filled orbital of a molecule. On absorption of a quantum of light one of these electrons can be promoted to a higher orbital which originally contained no electrons. Each orbital now contains one electron; the two electrons have opposite spins (as represented by the orientations of the arrows in Fig. 1) and the state is a singlet. If the excited electron has been promoted to the lowest unfilled orbital, the resulting state (recall that the electron spins are paired) is the first excited singlet (8,). If the excited electron has been promoted to the next lowest unfilled orbital and the spins of the electrons in the two singly occupied orbitals are paired, the state is termed the second excited singlet (8%). For each excited singlet state, there is a corresponding triplet state in which the two electrons (the one promoted to an unfilled orbital and the one remaining in the originally filled orbital) have the same or parallel spins. Hunt's rule states that the triplet will have an energy lower than the corresponding singlet. The triplet state, related to the first excited singlet state, is called the first triplet (TI). There can be no triplet state corresponding to the ground singlet because this would require two electrons in the same orbital to have
' Contribution No. 3666 from the Gates and Crellin Lsboratories of Chemistry. 398
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Journal of Chemical Education
Figure 1. Ground and flrrt three excited states of hvo electrons in the highest filled orbitol of o molecule.
the same spin, a clear violation of the Pauli exclusion principle. A common way of summarizing the various spectroscopic states of a molecule is a Jablonski diagram (Fig. 2). I n this diagram, the solid arrow represents the absorption of light by the ground state (So)to produce an Sl state, the wavy line indicates emission of this energy as light (fiuorescence), the dotted arrow the conversion of a singlet to the corresponding triplet state (in this case, SI -t TI), a process called intersystem crossing; and the broken, wavy arrow the emission of light as the molecular returns to the ground state from the first triplet state, a process called phosphorescence. Singlet and triplet states differ markedly in many ways. I n general, intersystem crossing, because it involves a change in multiplicity (the total number of electrons with each spin) is not an "allowed" process and therefore usually occurs relatively slowly. For example, the lifetime of most excited singlet states is of the order of sec or less. On the other hand, the lowest triplet state commonly has a lifetime of 10W4sec or longer. (Decay from higher triplet states to TIis rapid because the conversion of one triplet state into another triplet state does not involve a change in multiplicity.) For our purposes, the major differences between singlet and triplet states is the profoundly dif-
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state yields one singlet state plus one triplet state. TI' Sol SO' TI' Consider, now, the use of benzophenone as a photosensitizer for the isomerization of piperylene. Figure 3 shows the relative So, SI, and T I states of transpiperylene and benzophenone; the energy levels are expressed in kcal/mole which by virtue of the relationship E = hv are directly related to the frequency and therefore the wavelength of light necessary to excite the molecule.
+
+
BENZOPHENONE 12
I Figure 2.
S
PIPERYLENE 1 120 kcal
W
SINGLET STATES
TRIPLET STATES
A Jablonski diogrom of the singlet and triple states.
ferent chemical reactions which characterize these two types of excited molecules. Because of their longer lifetimes, triplet states generally play a more important role in chemical processes than do singlet states. An illustrative example of reactions that involve triplet intermediates is provided by cis-trans isomerization such as that of piperylene or stilbene.
trans-piperylene
trans-stilbene
cis-piperylene
cis.stilbene
Because intersystem crossing is so inefficient, irradiation of cis-piperylene at 221 mp (where it absorbs strongly to convert the So to the Sl state) does not result in rapid cis-trans isomerization; virtually all the molecules in excited singlet states decay to the ground state before undergoing intersystem crossing to the triplet state which would lead to isomerization (1). Photosensitization
Though intersystem crossing is, indeed, generally an inefficient process, some molecules such as those containing carbonyl groups undergo intersystem crossing with efficiencies as high as 100%. Benzophenone is an example, in which, after absorption of light to give an excited singlet state, virtually every molecule then undergoes intersystem crossing to the T I state. Thus, triplet states of benzophenone can be generated with a quantum yield of very nearly one (2). The essence of photosensitization is that an excited molecule can transfer its energy (and, if it is a triplet, also its triplet character) to another molecule of a different kind (5). For this transfer of energy to be a rapid and efficient process, the energy of the triplet donor should be greater than that required to excite the acceptor molecule from its ground state to first excited triplet state. Note that the overall conversion conserves multiplicity; one triplet state plus one singlet
51-
m+
82.8kcal 1, -69.6
kcal 11
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58.8 kcal
Figum 3. The relative 50, SI,and TI dotes of tmnrpiperyione end banrophenone.
The Sl state of benzophenone lies 82.8 kcal/mole higher than the ground state (So) which corresponds to the absorption by benzophenone a t 341 mp and the SI state of trans-piperylene is more energetic than the ground state by 120 kcal which corresponds to the absorption at 221 mp. Especially important is the relationship between the triplet energies of the two substances; the T I state of benzophenone a t 69.6 kcal is more energetic than the T I state of piperylene a t 58.8 kcal (the opposite of the energy relations of the Sl states). As a result, transfer of energy and triplet character from the T I state of benzophenone to the So state of trans-piperylene is energetically favorable and, furthermore, is allowed because the multiplicity of the reactants (one singlet state and one triplet state) is the same as the multiplicity of the products (one triplet state and one singlet state). For the reasons cited above, irradiation of a mixture of benzophenone and trans-piperylene a t 345 mp effects the isomerization of trans-piperylene by way of the following steps: (1) benzophenone absorbs light and is excited to its S1 state, (2) the Sl state undergoes inter-system crossing with nearly 100% efficiency to the T 1 state, (3) this T1 state of benzophenone transfers its energy and triplet character to the Sostate of transpiperylene which is raised to its T I state, the benzophenone returns to its So state, (4) the T 1 state of piperylene can decay to the Sostate of either cis or trans stereochemistry thus effecting the isomeriaation. The various steps are outlined (P stands for piperylene): Volume 45, Number 6, June 196-8
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&OSa dzCOsl
+
+ 2 ~ ~ r pso l
-h"
PTl-
Absorption
QICOSl +,COT,
+
+ , c o ~ ~ PI,
cis PSa
+ trans Pso
Intersystem crossing
LS0
5LS~
Absorption
Photosensitization Isomerization
Rearrangement of Santonin 4 ~~1
The photochemistry of santonin has a long and illustrious history. I n aprotic media, the rearrangement follom a course from santonin, I, to lumisantonin, 11, (4) thence to mazdasantonin, 111, and another substance, IV (in addition small amounts of other snbstances are also formed) (5, 5). Irradiation of mazdasantonin (5) in protic media leads to photosantonic acid (7, 8) (V) derivatives.
Intersystem crossing
Electron demotion to dipolar state
VII
HOOC
v Recent work (5,5,9) has shown that the mechanisms of these photochemical rearrangements possess many of the characteristics discussed previously. Thus, the first two steps in the photoconversion of santonin to lumisantonin are (1) absorption of light by santonin to a singlet state and (2) intersystem crossing with nearly 100yc efficiency from this excited singlet state to a triplet state which then isomerizes. (In this case the substrate of the isomerization, santonin, itself undergoes intersystem crossing to generate a triplet. In the case of piperylene, this was not true.) Photosensitization of santonin rearrangement by benzophenone is also possible, in which case three steps lead to santonin triplets (1) absorption of light by benzophenone which is excited to its SIstate (2) intersystem of benzophenone to its T1state (3) transfer of energy and triplet character from benzophenone to santonin which is then promoted to its TI state. Further details of the mechanism which account for the skeletal rearrangements that occur have been proposed by Zimmerman ( 0 ) and seem to he confirmed in detail for the rearrangement of lumisantonin (10). In this mechanism, as applied to the rearrangement of lnmisantonin, attainment of the triplet state is followed by bond breaking (to VI) and electron demotion to a dipolar state (VII) which then undergoes skeletal rearrangements of a kind which have ample analogy in the known chemistry of carbonium ions. 400
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Journal of Chemical Education
I11 Skeletal rearrangement
An interesting aspect of the work on the mechanism of the photochemical rearrangement of lumisantonin is the observation (10) of a blue species when lnmisantonin is irradiated in ethanol a t 77°K. The blue species -is not a solvated electron and shows no epr signal and, in consequence, has been proposed to he the dipolar intermediate VII. At 77'R the microscopic rigidity of the medium may prevent the skeletal motions which are necessary to convert VII into mazdasantonin (111). This blue species in stable without change for at least 36 hr in the ethanol glass a t 77'K but on warming to 10O0K, the blue species lasts only a few seconds and gives rise to the usual photoproducts observed on irradiation of lumisantonin. Conclusion
In addition to its importance in the cis-trans isomerization of piperylene and rearrangement of santonin and lnmisantonin and its use in photosensitization, the triplet state has come to have wide and varied application in a number of other photochemical conversions, many of which are of great synthetic utility (11). Literature Cited
(1) HAMMOND, G. S., SALTIEL, J., LAMOLA, A. A,, TURRO, N. J., BRADSHAW, J. S., COWEN,D. O., COUNSELL, C., VOGT,V., AND DALTON, C., J . Am. Chem. Sac., 86, 3197 (1964). (2) MOORE, W. M., HAMMOND, G. S., AND FOSS,R. P., J . Am. Chem. Soe., 83, 2789 (1961). H. L. J.. AND S A N T .~ K., S , . Ada Chem. Scand., (3) BACKSTROM, 14, 48 (1960). D., BOSSHARD, H., BRUDERER, H., BUCAI,G., (4) ARIGONI, KREBAUM, L. J., Helu. Chim. Ada, 40, 1732 (1957).
,
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(7) (8) (9)
FISCH,M. H., AND RICHARDS, J. H., J. Am. Chem. Soc., 85, 3029 (1963). FISCA. M. H.. AND RICHARDS. J. H., J. Am. Chem. Soc.. in press. BARTON, D. H. R., DE MAYO,P., AND SHAFIQ,M., J. Chem. SOC.,3314 (19.58). v m TAMELEN, E. E., LEVIN,S. H., BRENNER, G., WOLMSKY, J., AND ALDRICH, P., J. Am. Chem. Soc., 81, 1666 (1959). Z M M B R ~ ~ H. A NE., , AND SCHUSTER, D. I., J. Am. Chem. Soe., 83, 4486 (1961); and 84, 4527 (1962). ZIMMERMAN, H. E., Adv. in Pholochem., 1, 83 (1963); Tetrahedra, 19, Suppl. 2, 393 (1963).
(10) Ftscn, M. H., AND RICHARDS, J. H., J. Am. Chem. Soc., in press. (11) For further general discussion see: Tumo, N. J., "Molecolm Photochemistry," W. A. Benjamin, Inc., New York, 1965. KIN, R. O., "Organic Photochemistry," McGraw-Hill Book Co., New York, 1966. CALVERT, 3. G., AND PITTS, J. N., JR., "Photochemie try," John Wiley & Sons, he., New York, 1966. Advances i n Photochemistry, an approximately Annual Series beginning in 1963, Interscience Publishers (a division of John Wiley & Sons, Inc.) New York.
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