thermal and photochemical generation...

THERMAL AND PHOTOCHEMICAL GENERATIONOF ELECTRONICALLY EXCiTED ORGANIC

MOLECULES. TETRAMETHYL-1,2-DIOXETANEAND NAPHTHVALENE

NICHOLAS J. TURRO and PETER LECHTKEN

Department of Chemistry, Columbia University,New York, NY 10027, USA

ABSTRACTIn this paper some examples of reactions which yield electronically excitedproducts are presented. In particular, the 1 ,2-dioxetanes are discussed. Thesemolecules cleave cleanly into two carbonyl fragments when heated or irradiated.It will be shown that these simple, high energy, four atom arrays can efficientlygenerate electronically excited carbonyl fragments when they decompose.Surprisingly, tetramethyl- 1,2-dioxetane (V) yields acetone triplet selectivelyupon thermolysis or photolysis.

A study of the kinetics of thermal decomposition of V as a function ofsolvent, and the mechanistic implications of these observations will be dis-cussed. Based on summation of available evidence, we propose that thethermolyses of I ,2-dioxetanes require specific vibrational motions whichenhance spin—orbit coupling as the molecule fragments, and allow efficientdecomposition into triplet states.

In the photochemistry of V an exceptional 'anti-Stokes' sensitization isdemonstrated, which suggests the possibility of efficient execution of 'bluelight' photochemistry with 'red light', and a 'quantum chain' reaction.

It is demonstrated that, at 77°K, naphthvalene undergoes efficient photo-chemical conversion to naphthalene triplets.

Finally, the relationship of radiationless electronic relaxation and primaryphotochemical processes is discussed in the framework of our results.

INTRODUCTIONIn this paper we shall be concerned with organic reactions in solution that

generate electronically excited products by either thermal (equation 1) orphotochemical (equation 2) activation of reactants.

R 4p* (1)

R hv .R* p* (2)

Such reactions are of potential interest to photochemists, spectroscopists,kineticists and photobiologists for any number of reasons. Photochernistsmay see in the study of such reactions a unique method of selectively produc-

363

NICHOLAS J. TURRO AND PETER LECHTKEN

ing electronically excited molecules which are not easily accessible by normalmeans of photochemical excitation. Spectroscopists may see in such reactionsa connection between the ubiquitous and imperfectly understood radiation-less processes which interconvert the electronic states of a molecule and thechange of electronic states implicit in reactions 1 and 2. Kineticists may beintrigued by the selective discharge, into specific groups of atoms, of sufficientchemical energy to produce an electronically excited product. Photobiologists,and the public at large, have long been fascinated by nature's display ofchemielectronic pyrotechnics in fireflies and luminous plants, both of whichare presumably manifestations of reaction 1.

In spite of the potential interest of reactions such as 1 and 2, hardly anyinformation is available concerning the mechanisms of formation of electron-ically excited products. We hope this paper will stimulate interest in this areaby our discussion of several examples of reactions 1 and 2, which have beenstudied at Columbia during the last year or so. In particular, the thermaland photochemiçal reactions of tetramethyl-1,2-dioxetane have provided awell-suited system for detailed study of reactions 1 and 2. In addition, theemission spectroscopy of naphthvalene also provides a particularly strikingexample of reaction 2.

ELECTRONIC STRUCTURE AND SPECTROSCOPYOF PEROXIDES

The outstanding property of the peroxide linkage is surely the feeblebonding which connects the two oxygen atoms, the O—0bond strength oforganic peroxides generally being less than 39 kcal/mole. This property ofperoxides seems to originate in the repulsions of the non-bonded electronpairs on the adjacent oxygen atoms. A possible description of the highestenergy electrons of the ground state of the peroxide bond is (it0)2, i.e. themixing of two adjacent n0 orbitals causes splitting into a ir and it pair,each of which is doubly occupied in the ground state'. The lowest availableunfilled orbital is probably of o character. Thus, the lowest energy electronictransition in simple peroxides can be characterized by t0 —+ oo. Essentially,instantaneous cleavage of the 0—0 bond is clearly expected to followelectronic excitation.

Simple alkyl peroxides2 show only weak, nearly structureless end absorptionin the region below 400 nm. For example, di-tert-peroxide2 possesses aninflection ( 5) at 260 nm. We are unaware of any systematic studies orreports of emission from. alkyl peroxides3. Although sketchy, the literature isconsistent with the idea that the excitation — o results in dissociationor predissociation.

PHOTOCHEMICAL REACTIONS OF PEROXIDESUntil recently, very little was known about the mechanistic details of the

photochemistry of peroxides in solution. This situation probably results fromthe difficulty of direct excitation of peroxides, due to their inherently weak

• absorption above 250 nm. In spite of this, the recent work of Story4 andCriegee5 has shown that some clever and important synthetic photoreactions

364

ELECTRONICALLY EXCITED ORGANIC MOLECULES

of peroxides can be achieved. For example, elegant syntheses of macrocyclicrings4 (equation 3) and cyclobutadienes5 (equation 4) have been achieved.

—-— + 3CO2 (3)

CH3CI

Cl Cl 00

CIII" I1 [ ClII'"Cl ]+ CH3OCH3

CH3

Although no detailed mechanistic work on these reactions has beenreported, it seems reasonable to assume that the initial photochemical stepis cleavage of the 0—0 bond, followed by secondary thermal decomposition.This picture is consistent with the observation that di-tert-butyl peroxidemainly undergoes O—O bond cleavage when it is excited with ultra-violetlight2.

Sensitized decompositions of peroxides have been reported3'6 and itappears that triplet benzophenone can effect the decomposition of dibenzoyl-peroxide6. This experiment suggests, but does not require, that the peroxidelinkage may7 have triplets in the range of 70 keal/mole.

Perhaps the spectroscopy and photochemistry of peroxides can besummarized as follows: not very much is known, but a lot of interestingexperiments remain to be done.

1,2-DIOXETANES1,2-Dioxetanes, given by the general structure I, have been the subject of

intense recent study. Such structures have been implicated in many chemi-luminescent reactions8. ez-Peroxylactones such as II and Ill are implicatedin bioluminescent reactions9 and in a commercial method of producing coldlight'0 via a chemiluminescent reaction. The breakthrough, pioneeringsynthesis of trimethyl-1,2-dioxetane by Kopecky and Mumford11 and the1,2-addition of singlet oxygen to electron rich and hindered ethylenesachieved by Foote and Mazur'2, Bartlett and Schaap' ,and the recent workof Adam et al.14, and Richardson'5 et a!., represent a monumental stepforward for the study of intriguing, reactive but isolable intermediates. Therecent proposal that dioxetanes are intermediates in the ozonolysis of alkenesby Story16 may signal yet another breakthrough in the synthesis of thisfamily of remarkable cyclic peroxides.

These isolable molecules are remarkable in that they are kinetically stableat 300°K, in spite of the fact that they release about 100 kcalJmole in heat and,

365


in addition, about 6—10 kcal/mole of positive entropy when they dissociateinto two carbonyl fragments (equation 5), a reaction common to all 1,2-dioxetanes. Thus, there is effective energy storage in a structurally simplefour atom array which superficially looks capable of 'busting up' by arelatively uncomplicated chemical reorganization.

0—0 0—0 0—0I I

R—C —C —R R—C—C C—CII // \\R R R 0 0 0

I II III

0—0 0+ II (5)x—Y Y

AIf < 100 keal/moleIV AS < +20—30 eu

Reactions conforming to equation 5 are generally chemiluminescent, i.e.enough energy is funnelled into a fragment so as to produce electronicexcitation and eventual emission from that moiety. An extraordinaryproperty has been proposed8 for molecules of type IV in this regard: thattheir decomposition can be catalysed by electronic energy acceptors!

Chemists who delight in the study of molecules possessing high energycontent can not only probe into the reasons underlying the fantastic kineticstability of 1,2-dioxetanes, but they can also design experiments to answersuch questions as: (1) can a reaction be slow because it is too exothermic?(2) is there a chemical equivalent to the Franck—Condon principle whichprevents massive release of chemical energy into vibrational energy? (3) areorbital symmetry factors operating? (4) what are the yields, multiplicitiesand mechanisms of formation of the electronically excited states responsiblefor the chemiluminescence in reactions of type 5? (5) can energy acceptorsreally 'catalyse' the decomposition of dioxetanes and if so, what is themechanism of such an astonishing reaction?

SPECTROSCOPY AND PHOTOCHEMISTRY OF1,2-DIOXETANES

Table I lists a number of 1,2-dioxetanes that have been reported in theliterature. We shall discuss the kinetic parameters there listed later in thispaper. The molecule, tetramethyl-1,2-dioxetane (V), is of special interest tous here, since it represents the only alkyl dioxetane which is expected to yieldidentical carbonyl fragments upon decomposition. Furthermore, our priorinterests in the solution photochemistry of acetone allow us a splendidopportunity to search rationally for the electronically excited states ofacetone when V is fragmented1 .

The exceptional electronic characteristics to be expected of 1,2-dioxetanesare apparent by simple visual observation of tetramethyl-1,2-dioxetane,which is a yellow, low melting solid. The ultra-violet absorption spectrum of

366


Table I The kinetic parameters of some isolable 1,2-dioxetanes and related systems

AH iS k(x104)Molecule Solvent (kcal/mole) (eu.) (550, sec') Ref.

0—0_____ Cd4 22 —5 6 15

0—0_______ CCL 23 —4 3 11

C6H6 25 — 1 0.6 This work

0—0_____ CCL 22 —5 6 15

Ph

0—0

C2H50.L-LOC2H5 C6H6 30 13

0—0______ C6H6 30 13

C2H50J "0C2F15

0—0

CH3O OCH3 C6H6 1 12

CH3O OCH3

(CH3)3COOC(CH3)3 CH2÷2 36 +11 _.10_h1 21

ri vapour 60 +11 ,..l0_26 21

vapour 51 +6 l0_20 21

EJ vapour 32 +1 21

vapour 37 +4 ,,l0_0 21

367


I 1 - I-

— u.v. in C6 H,2 Qt r.t.

30-— — — excitation in C6 H,2 at rt.

excitation in glass, 77°K

/ \20 ! s

I '-..../ \w / \/

I

200 240 280 320 360 400nm

Figure 1. Absorption and excitation spectra of tetramethyl-1,2-dioxetane.

V is shown in Figure 1. The emission spectroscopy of V is even more amazingin that at room temperature or at 77°K, photochemical excitation of V resultsin the characteristic luminescence of acetone. The excitation spectrum of theacetone luminescence is consistent with the absorption spectrum of V (seeFigure 1). Indeed, acetone luminescence (pure fluorescence) can be producedfrom V which is excited with light of wavelength as long as 450 nm.

0—0

H3C —C—C—CH3 —n---- 2 (CH3)2 C0 (6)

H3C CH3

V

Product analysis demonstrated that the photolysis of V in benzene atroom temperature leads quantitatively to two moles of acetone (equation 6).Furthermore, the quantum yield for disappearance of V is 1.0. A questionwhich immediately arises is: when singlet V. formed by direct absorptiondecomposes, what are the percentages of acetone singlet (A1) and triplet (A3)formed? In order to answer this question we need an experiment! probewhich can quantitatively distinguish A, and A3 and which also allowsextrapolation to complete trapping of A1 and A3. Such a method is availablesince A, and A3 can be selectively 'titrated' with trans-dicyanoetbylene (VI),as shown in equations 7 and 8.

The key to this method of analysis is that VI yields completely differeniproducts with A, and A3, namely the oxetane (VI!) and cis-dicyanoethylen(VIII), respectively. The photoreactions given in equations 7 and 8 arbelieved to be well understood' . Knowledge of limiting quantum yields fotformation of VII and VIII allows measurement of the limiting yields of A1and A3 by simple extrapolation of a plot of the yields of VII and VIII (formec

368


CR3H CN k H3C-4—-O

(CH3)2C0' + >< (7)

k NC HNC CN

(CH3)2C0 VI VII

H CN k H H(CH3)2 CO3 + >==-< (8)

NC H NC CN

VI VIII

in the presence of decomposing V) to infinite concentration of VI, i.e. atinfinite concentration k0[VI] k + ksT and k[VI] >> k-i-, where kF andksT are the rate constants for fluorescence and intersystem crossing, theimportant processes involved in deactivation of A1, and kT is the rate constantfor unimolecular decay of A3 (probably determined by a mixture of inter-system crossing and ct-cleavage). The results of our study with 366 nmexcitation are given in Table 2. Unexpectedly, photochemical excitation of Vresults in preferential formation of A3. It should be emphasized that theapproximate 4 to 1 preference for formation of A3 over A1 is not due tointersystem crossing of A1, since the extrapolation to infinite concentrationof VI means we have trapped every A1 before intersystem crossing can occur.

Table 2. Production of electronically excited acetone in the direct photochemical, and tripletphotosensitied decomposition of tetramethyl- 1 ,2dioxetanea

Method Excited State yield" Excitation wavelength

A1 A3Direct photochemical 10

253035

43 366 nmc320 nmd297 nmd240 nm'

Triplet photosensitizedef - -50 436 nm

One mole of excited acetone formed per mole of dioxetane decomposed is defined as 100 per cent, i.e. only one excited acetoneis assumed to form per molecule of V decomposed.Percentage of excited acetone formed per mole of photons absorbed.At 6C in benzene solution by titration with rans-dicyanoethylene. The 366 nm light is solely absorbed by the dioxetaneunder the applied conditions. = 1.0.

Determined by comparing dioxetane luminescence (pure acetone fluorescence) with that of acetone solutions of identicalabsorption at those wavelengths. Triplet yields not determined.Biacetyl sensitized at 6C in benzene. See text.Measured by sensitization of the Type TI cleavage reaction of valerophenone. See text.

Table 2 also shows the effect of excitation wavelength on the yield of A1.A steady increase in the measured efficiency of formation is observed as theexciting wavelength decreases.

It should be noted that the net efficiencies for formation of A1 and A3 at366 nm apparently are less than unity. It is not absolutely clear whether thisis due to a large experimental error or whether there is some real inefficiencyin formation of electronically excited acetone.

369


Dr Richard Hautala suggested an interesting method to test for an inter-mediate preceding the acetone fluorescence derived from direct excitationof V. He reasoned that the sensitive and precise method of single photoncounting was eminently suited to detect such an intermediate, since the risetime of directly excited acetone fluorescence could be determined and com-pared to the rise time of acetone fluorescence derived from photoexcitationof V. indeed, the two rise times were found to be identical with an experi-mental precision of the order of 0.1 x iO sec! We interpret this result torequire that no intermediate of lifetime greater than i0 10sec precedes singletacetone formation from singlet V, since such an intermediate would havecaused a delay in the rise time of acetone fluorescence. This result, of course.is consistent with the notion, mentioned earlier, that the electronically excitedO—O bond is probably cleaved within a few vibrational periods.

PHOTOSENSITIZED DECOMPOSITION OFTETRAMETHYL-1,2-DIOXETANE: AN 'ANTI-STOKES'

TRIPLET EXCITATION TRANSFERWe have investigated the biacetyl photosensitized decomposition of V.

It was immediately obvious that the phosphorescence, but not the fluorescence,of biacetyl is strongly quenched by V (k i07 M sec 1). Areelectronicallyexcited states of V and/or acetone produced by this quenching? The answeris resoundingly positive, since sensitization of the decomposition of V bybiacetyl triplets in the presence of valerophenone results in efficient sensitizedType II cleavage of the latter molecule. We calculate the efficiency of forma-tion of valerophenone triplets to be 50 per cent, based on an analysis ofthe data in terms of known kinetic schemes and an observed 30 per centchemical yield of acetophenone!

Since the triplet energy of biacetyl is about 56 kcal/mole while that ofvalerophenone is 72 kcal/mole, sensitization of formation of valerophenonetriplets by biacetyl triplets constitutes a formal example of highly endothermic'anti-Stokes' triplet excitation transfer.

It thus seems that equation 9 is appropriate to describe the result of theinteraction of biacetyl and V. The exothermicity of cleavage of V to two

[CH3COCOCH3]3 + + CH3COCOCH3(9)

+ +

Ix0

+ _(11)

370

0

Ix


acetones (AH — 100 kcal/mole) plus the triplet energy of biacetyl (E3 56kcal/mole) makes the formation of two acetone triplets, equation 12, a formalpossibility. However, triplet triplet annihilation and 'cage quenching' bybiacetyl might reduce the efficiency of formation of 'free' acetone triplet. Inthis regard, it is interesting to note that the biacetyl which has sensitized the

[CH3COCOCH3]3 + V + 2 >=O + CH3COCOCH3

deccmposition of a molecule of V does not efficiently quench the tripletacetone molecule which is formed. Perhaps this indicates a long rangequenching or a 'backside' displacement of the triplet acetone leaving group!

The efficient quenching of biacetyl triplets by V reminds us of the quenchingof singlet states by strained molecules such as quadricyclane'8 and hexa-methyl dewar benzene'9. However, our studies have not progressed sufficientlyfor us to decide upon the details of the quenching mechanism.

We should point out, however, that Wilson and Schaap2° have proposedthat triplet anthracenes sensitize the decomposition of cis-diethoxy-1,2-dioxetane, in order to explain the observation that oxygen 'protects' thedioxetane from an apparent induced decomposition in the presence ofanthracenes.

The results reported here are quite preliminary and many studies remain tobe done. Two very interesting types of experiments suggest themselves:(a) generation of 'blue light' (or electronic excitation corresponding toabsorption of blue light) by use of red light excitation (equations 13 and 14)and (b) a quantum chain mechanism (with equation 15 as a possible propaga-tion step).

sens + 'red light' —÷ sens*

sens*+ V+ )=O+ )==O+ V - 2 >=O* +

THERMAL DECOMPOSITION OFTETRAMETHYL-1,2-DIOXETANE

As expected, heating of V in a variety of solvents results in quantitativeformation of two moles of acetone (equation 16). Kopecky and Mumford1'had reported earlier that trimethyl-1,2-dioxetane is chemiluminescent. Other

O—O

H3C— C—C—-CH35O-7O'

2 (CH3)2C0

H3C CH3V

examples of chemiluminescence concomitant with dioxetane decompositionhave been observed'2' 14—16, 20, White demonstrated that trimethyl-1,2-

371

PAC—33—-2—H


dioxetane could sensitize the triplet reactions of a number of acceptors22,with net efficiencies of up to about five per cent. Wilson and Schaap2° sug-gested that the dioxetane X yields ethyl formate singlets in high, possiblyquantitative yield. GUsten and Ullman23 proposed that III is the activespecies in an oxalate—peroxide sensitized reaction, with net reaction effic-iencies approaching five per cent in favourable cases. Chemiluminescencefrom XI and XII have been observed during their thermolysis.

0—0 0—0 0—0I I

C2H50—C —C—0C2H CH3O—C —C —OCH3 (CH3)3C—C

I1II CH300CH3 H

x XI XII

These results leave little doubt that electronically excited states areproduced upon decomposition of 1,2-dioxetanes. However, the precisenature of the electronically excited species and their mechanism(s) of forma-tion have not been defined. Let us now consider how the mechanism ofdecomposition of V was elucidated.

It has already been noted that the reactions of singlet and triplet acetonewith trans-dicyanoethylene yield the oxetane VII and cis-dicyanoethyleneVIII, respectively, as the exclusive products24. It is possible to titrate simul-taneously A1 and A3 with trans-dicyanoethylene as they are produced fromthe thermolysis of V. The limiting yields25 of A1 and A3 from thermolysis ofV are reported in Table 3. We are immediately confronted with the ratherstartling result that, based on titration with VI, the thermolysis of V yieldsA1 in less than one per cent while the yield of A3 is about 50 per cent! Anunexpected result of this type requires independent and convincing confirma-tion before it can be considered as valid. However (Table 3), the same yieldsof A1 and A3 were measured, within experimental error, (1) by measurementof the chemilurninescence (pure acetone fluorescence) of V; (2) by sensitiza-tion of biacetyl fluorescence and phosphorescence; (3) by chemical titrationwith cis-diethoxyethylene. Thus, the evidence is quite compelling that Vhas a preference for decomposing into A3 rather than A1 (Figure 2).

We were able to show beyond any reasonable doubt that acetone singlets,rather than some other reactive species (like, say, the dioxetane itself), attack

> v*

Figure 2. Thermolysis of tetramethyl-1,2-dioxetane.

372

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t-DCE. This was done by demonstrating that Stern—Volmer quenching ofthe chemiluminescence of V by t-DCE (benzene solution) yields the sameslope (kqr) as that obtained from quenching of acetone fluorescence byt-DCE25.

Incidental to this study we noticed that high concentration of V decreasesthe net yield of A3, indicating that V is a quencher of A3. Consistent with thispossibility is the observation that degassed solutions show less chemilumines-cence than aerated solutions. We interpret this result as follows: since oxygen isknown to be an efficient quencher of A3 but not A1, oxygen protects V againstsensitized decomposition by A3 (which apparently does not generate A1),thereby allowing more molecules of V to form A1 (the state responsible forchemiluminescence). This result is the basis of our earlier conclusion thatquenching of biacetyl triplets by V does not generate A1.

In addition to the remarkably efficient and selective formation of A3relative to A1, we wish to point out that the blue (acetone fluorescence)chemiluminescence is a 'red herring' with respect to the major method forexcited state production from V. This shows the hazards of making mechanis-tic conclusions solely on the basis of low efficiency chemiluminescence.Finally, V is a 'self quencher' of chemielectronic production of A3, whiledissolved oxygen is a 'promoter' of chemiluminescence. We conclude that lowconcentrations of V in the absence of oxygen (i.e. under conditions thatchemiluminescence is decreased) are most favourable for efficient productionof chemically useful excited states (triplets) from V.

ACTIVATION PARAMETERS FOR THE DECOMPOSiTION OFTETRAMETHYL-1 ,2-DIOXETANE. SOLVENT EFFECTS

In order to gain further insight concerning the mechanism of thermolysisof V, the activation parameters for fragmentation were measured. Since therate of decomposition of V is directly proportional to the rate of formationof acetone singlets, which is in turn directly proportional to the chemilumines-cence intensity, therefore the kinetics for decomposition of V are easilyfollowed by the sensitive and precise method of fluorimetry. The activationparameters for decomposition of V in several solvents are listed in Table 4. There

Table 4. Solvent dependence of the activation parameters for thermal decomposition of tetra-methyl-1,2-dioxetane

SolventLH

(kcal/mole)AS(eu.)

k(x104)(55°, sec 1)

Acetonitrile 30 + 11 0.41,3-Cyclooctadiene 27 + 5 0.5trans-2,4-Hexadiene 27 + 5 0.52,5-Dimethylbexadiene 26 + 2 0.64-Bromotoluene 26 + 2 0.6Cyciohexane 25 — I 0.7Benzene 25 —1 0.7Cyclohexene 23 —8 1.0

Isopropanol 20 — 15 1.1

Methanol 13 —34 8.0

374

ELECTRONICALLY EXCJTED ORGANIC MOLECULES

CH3CN

Figure 3. Correlation of AH and /S for the thermolysis of tetramethyl-1,2-dioxetane invarious solvents. The two ah'àhols studied appear to fall on a separate line.

is a rather large spread in th values of and AS in different solvents.However, a rather good straight line is achieved when H* is plotted versusAS (Figure 3). This plot is thus evidence for an isokinetic relationship26, i.e.

= f3AS*, where /3 isthe isokinetic temperature, equal to 86° in thiscase. The occurrence of an'isokinetic relationship is consistent with one majorinteraction mechanism in the thermolysis of V26. Alcohols (Figure 3) do notadhere strictly to the relationship, however.

The integrated chemiluminescence intensity cL is proportional tothe yield of A1 in the thermolysis of V. Significantly a plot of In CL versusthe activation energy for decomposition of V in different solvents yields astraight line (Table 5).. This result suggests that the mechanism involvingformation of A1 is reli'ted to the major decomposition path.

The work of Ricl'ardson and O'NeaP5 has shown that the activationparameters for the decomposition of the dioxetanes XIII and X1V in carbontetrachioride are :experimentally indistinguishable (Ea = 23.0 kcal/mole,

Table 5. Solvent dependence of chemiluinescence fromtetramethyl- 1 ,2-dioxetane

Solvent AH(kcal/mole)

CL0/

Acetonitrile 30 0.28Benzene 25 0.20CyclohexeneEthanol

2316b

0.130.07

Cheniiluninescence yield determined by integration of the corrected acetonefluorescence observed during thermolysis of V. T equals 69.

Estimated from the values for methanol and isopropanol. See Table 4.

375

CH3.-O.-Br

0 C

0

I


22.4 kcal/mole; log A = 12.2, EtS —5 e.u.). They interpretedthese data as evidence for a two-step mechanism, in which the key energeticand structural feature required to reach the transition state is breakingof the 0—0 bond. In a concerted mechanism the phenyl group wouldbe expected to interact differently from the methyl with respect to the develop-ing carbonyl bond (as has been suggested to be the case in fragmentationsof alkoxy radicals). In agreement with their premise, these authors showedthat calculated activation parameters, based on a two-step mechanismwith rate-determining formation of a biradical intermediate, were in goodagreement with their experimental data.

0—0

H3C—C—C —H

H5C6 H

XIV

REACTIONS WHICH PRODUCE PRODUCTS JN ANELECTRONICALLY EXCITED STATE OR WHICH UNDERGO

A CHANGE IN ELECTRONIC STATESome fundamental and thought-provoking ideas should be considered

before the possible detailed mechanisms for photochemical and thermaldecomposition of V are explored; namely, the concepts of chemical reactionswhich undergo a change of electronic state along the reaction profile and/orchemical reactions which produce products in electronically excited states.These ideas are shown schematically in Figure 4. In this diagram the con-version R —* P and R* _+ are changes in state which produce an electro-nically excited product (electronic excitation denoted by * and vibrationalexcitation by ). The conversion R* - P* is a radiationless process in whicha change of electronic state must occur somewhere along the reaction coordi-

R (or

Figure 4. Schematic description of processes involving formation of products in electronicallyexcited states.

376

H3C CCHH3C H

XIII

// P*

-iiv I

R —- Rt____. )(R p*P


nate. In effect, all photoreactions involve at least one radiationless processbefore a final, ground state product is formed.

Radiationless electronic excitation and de-excitation are generally classi-fied as 'physical' processes ii these processes occur within the frameworkof the electronic states of a single molecule. Hammond27 has convincinglypointed out that such a classification is not fundamentally required. He hasindicated that such a classification may violate our most deep-seated intuitivenotions in certain cases such as collapse of a perpendicular ethylene to cis andtrans-isomers: the physical or chemical nature of the process would dependon the structure of the starting olefin! Indeed, one can perhaps bettersupport a radically different point of view, namely that all electronicallyexcited states are electronic isomers of the ground state, thus all electronicstate interconversions involving at least one electronically excited state arechemical in nature. This view of things would logically include radiativeprocesses where, in absorption, the photon is a reagent and, in emission,the photon is a product. An advantage of this attitude is that organic photo-chemistry then broadens, in a natural way, its purview to include all reactionswhich interconvert electronic states, whether or not light is involved inany of the excitation and/or de-excitation processes. The 'photo' part simplymeans that light happens to be the most convenient general way of producingor detecting an electronically excited state. Now let us return to Figure 4.The process R* P* might be (a) an intramolecular excited state inter-conversion such asS2 —+ S1,S1 - T1, etc. or(b)an excited state interconversionsuch as —÷ A1 + A0 or V - A3 + A0, in which electronically excitedstates of a molecule different from R are generated. The process Rleads to an electronically excited state from a ground state starting material.The electronically excited product is identical in all cases to the species producedby light excitation, i.e. hi' + P — P*. Furthermore, it seems sensible to tryto treat analysis of the mechanism of the process R —-*P*lna manner analogousto the treatment of the P —+ P process. This frame of reference is easilyextended to the cases in which P undergoes a proreaction itself, to yield aproduct P'.

Now we also can see that chemiluminescence, the process R - P*_ P + hi',is simply a specific corollary of the more general theory of generation ofproducts in electronically excited states.

The theory of radiationless transitions and chemical reactions can betreated under the general concept of electronic relaxation. The availabletheories of radiationless transitions may, therefore, provide a source ofuseful and refreshing ideas and language to discuss rate processes. Some of theconnections between the usual manners in which an organic chemist and aspectroscopist might discuss electronic relaxation processes can be gatheredfrom a comparison of the familiar28'29 equations 17 and 18. Instead ofthe usual attempts to evaluate AH* and AS*, the key conceptual quantitiesof transition state theory (the model of kineticists), we shift to dynamic

rate x exp (—iH/RT) x exp(AS#/R)

rate cc (tfr,JHI/i1)2 <Hfr/f>2 <XiIXi>AE1f

377


perturbation theory (equation 18) for consideration of conversion ofelectronic energy into nuclear motion (vibrational energy) as we pass fromthe 'transition state' (i/i) to upper vibrational states of the product (/4).This theory is thus essentially vibronic in nature. The important, usableconcepts in equation 18 are the electronic matrix element <4j1HI4f>, thevibrational overlap integral <XIIXf> and the energy gap between the inter-acting states. With the exception of the vibrational overlap integral, equation18 is the basis for the orbital symmetry30' 31 rules in which specific nuclearmotions .and electronic interactions are expected to be particularly favouredin concerted reactions, i.e. the 'best' electronic interactions between and i/Jrare vibrationally induced. Certainly, many organic photochemists will notfind equation 18 to be a comfortable and/or comprehensible theoreticalframework in which to discuss rate processes. Yet we are all probably familiarenough with orbital language to commonly write down pretty pictures ofoverlapping lobes in order to rationalize our observations. Perhaps theimportant feature of equation 18 of general impact is that, in addition to theexpected electronic overlap term and symmetry considerations, new possi-bilities for H come to mind, and the initial vibrational motions which couple&/i and &/i are emphasized. We shall see in the discussion of the mechanism ofdecomposition of V, that we can profitably make good use of the ideassuggested by equation 18 to infer some fascinating detail about bond-breaking processes; and gain some insight into how V is able effectively tostore so much energy, in spite of the apparently simple electronic reorganiza-tion which could release an enormous amount of free energy.

MECHANISMS FOR THE PHOTOCHEMICAL AND THERMALDECOMPOSiTION OF TETRAMETHYL-1,2-DIOXETANE

Let us summarize the prominent features of the photochemical and thermaldecomposition of V which must be handled by any mechanism: (a) twomolecules of acetone are produced per molecule of V decomposed under allconditions which were studied; (b) the formation of triplet acetone is favouredover that of singlet acetone under both photochemical and thermal excitationof V; (c) the solvent dependence of the activation parameters for thermolysisof V reveals an isokinetic relationship; (d) hydroxylic solvents do not adherestrictly to the isokinetic relationship; (e) the formation of electronicallyexcited products from V is not completely efficient in either its thermolysisor photolysis; (I) the activation parameters for decomposition of other 1,2-dioxetanes do not depend much on structure.

First let us attack the mechanism of the thermal reaction in non-polarsolvents, e.g. benzene, cyclohexane or carbon tetrachioride. The activationparameters in such solvents are iH 23 to 25 kcal/mole and LS —1to —5 e.u. Contrast these values with those for decomposition of di-tert-butylperoxide (equation 19) and fragmentation of a cyclobutane (equation 20).The lower enthalpy of activation of V relative to acyclic alkyl peroxidescan be taken care of in any mechanism by invoking ring strain, which isestimated as about 26 kcal/mole'5. However, the low, slightly negative,value of tS for V and other dioxetanes (Table 1) suggests that either the

378


(CH3)3C0—OC(CH3)3 —--- 2(CH3)3CO

LH 35 kcal/mole (19)AS* +lOe.u.

LIIA

+

60 kcal/mole (20)

±lOe.u.

starting material has not gained a substantial increase in flexibility, or has alow probability in reaching to the transition state, or that some ordering ofsolvent is required. The latter seems unlikely to be important in cyclohexane.Several specific proposals to explain the AS values are (a) the 0—0 bondbreaks but reforms, thereby achieving the transition state, but not proceedingon to product; (b) achievement of the transition state requires a change inelectronic state30, i.e. a singlet to triplet conversion, or a conversion of amassive amount of chemical energy into vibration energy, such as formationof two vibrationally excited acetone molecules; () specific restrictivemolecular motions are required to achieve the transition state.

As we have seen, there is convincing evidence that the 0—0 bond isnearly fully broken in the transition state, i.e. the transition state is 'biradicallike' in character, since the activation parameters for thermolysis are structure-insensitive and because a biradical model for the transition state allowsaccurate calculation of the activation parameters' .Whatabout the possibilitythen, that the biradical forms but then recollapses to dioxetane? We cannotthink of any rigorous way to rule this possibility out; however, the reversereaction should have an activation energy of about 8—10 kcal/mole1 . Thecleavage of the carbon—carbon bond should be quite facile from the biradical,since Ea for fragmentation of a methyl group from the tert-butoxy radical isonly 15 kcal/mole21. Since the activation energy for 3-cleavage of butoxyradicals decreases as the stabilization of the leaving group increases3 1, thebiradical XVII may well have an activation energy for cleavage much less than8 kcal/mole. Furthermore, fragmentation should be increased in rate in asolvent like cyclohexene, if we use tert-butoxy radicals as a model, sinceWalling and Wagner32 found that cyclohexene bad a profound effect ofenhancing cleavage of alkoxy radicals over other modes of decomposition.

+ (21)

XVII

The second explanation, i.e. that a singlet—triplet conversion is requiredin going to the transition state, is particularly intriguing. The reaction N20N2 + 0 is believed to be an example of such a non-adiabatic process33.

379

PAC—33---2—I


However, no well documented examples of organic reactions which followsuch a reaction surface are known, although cis—trans isomerization of certainethylenes was once thought to conform to this model33. In any case, thelack of a heavy atom effect (4-bromotoluene falls on the isokinetic line) and therather large positive value of LS in acetonitrile suggests that the spin fliprequired to obtain the major product, acetone triplet, occurs after thetransition state is achieved, for all intents and purposes.

0-0

[oIIIO]+ (22)

>=O

The possibility of a conversion of a massive amount of electronic energyinto vibrational energy seems intuitively unreasonable, i.e. a violation of thechemical equivalent of the Franck—Condon principle (the nuclear factor inequation 18 is very small). However, we must admit that our results suggestthat some fraction of V may, in fact, follow such a path.

We shall now show that the third mechanism, in which specific molecularmotions are required to achieve the transition state, is consistent withthe observed AS values, and furthermore, has the ability to correlate theother observations quite satisfactorily.

The most obvious types of nuclear motion, which might be more restrictivein V# than in the ground state, are those in which pendant groups are causedto interfere with one another in achieving V, thereby reducing vibrationaland rotational freedom. Arguments based on this attitude have severedifficulties when confronted with the data on activation parameters givenin Table 1. Certainly, if eclipsing effects were operating to cause the lowactivation entropies, we would expect a wide variation in which shoulddecrease with increasing encumbrances due to pendant groups on the carbonatoms. In fact, the most hindered dioxetane for which a value of AS isavailable, namely V, has the highest value fLtS. Note, however, the increasein zXH as one proceeds from the 3,3-dimethyl (XIII) to trimethyl (XIX) totetramethyl dioxetanes. If this effect is ascribed to destabilizing interactionsbetween pendant groups in the transition state, there cannot be much entropyloss associated with the group interactions. Note, moreover, that in a 'classic'case of a biradical fragmentation, namely, fragmentation of 1,2-dimethyl-cyclobutane [into two molecules of propylene (Table 1)], the values of ASare highly positive (+ 11 e.uj! Furthermore, cis-1,2,3,4-tetramethylcyclo-butene (XX) provides a good model for what to expect when two methylgroups are compelled to rotate as a transition state is approached. Noticethat for XX relative to cyclobutene, itH increases (by 5 kcal/mole) and ASincreases (by 3 e.u.) while for XIII relative to V, AH increases (by 4 kcal/mole)and AS increases (by 4 e.u.)! Since the thermal ring opening of XX is conrota-tory, the methyl groups have a very specific motion which achieves the transi-tion state. The disrotatory motions are not productive in forming the lowestenergy transition state. The above discussion suggests to us that a close rela-tionship may exist between the 'classical' concerted ring opening of cyclo-

380


butenes and the ring opening of dioxetanes. Indeed, in the one example ofsolvent effects on the activation parameters for ring opening of cyclobutenes,we have been able to locate in the literature21, an apparent isokinetic relation-ship holds! Let us now follow up this line of thinking a bit more closel,i.

Consider the cleavage of the 0—0 bond of V as being nearly complete inthe transition state V and then consider the final product, triplet acetone(equation 23). Notice that a 900 rotation of the bonding lobe on the left hand

(23)

A3

oxygen generates the final n, m state. Furthermore, this electronic motionis formally analogous to that required for maxima! spin orbital coupling in amolecule so that the normal prohibition to spin interconversion is expected tobe substantially lowered34!

That this should be the case can be easily demonstrated to anyone whobelieves that 1n, it± * spin interconversions are allowed relative to'n. it * and 'it, it ± 3m, interconversions. Equation 23 showspictorially how the formal mathematical operation of the spin orbitalcoupling operator34 corresponds to the motion of a p-like orbital through90°, and is thereby theoretically connected to the type of motion which con-verts V* into A3 and A0! We note that it is the initial nuclear motions whichprobably are important here, in the same way that one must suppose thatinitial nuclear motions dominate the rates of formation of transition states forphotoreactions which adhere to orbital symmetry30' 31 rules, e.g. electro-cyclic ring openings and closures. Thus, an idea which has long been partand parcel of the theory of state interconversions used by molecular electronicspectroscopists34 may now be applied in a very intriguing way to help under-stand the mechanism of decomposition of an organic molecule, i.e. the con-version of V to A3 + A1 is a vibrationally induced intersystem crossing.

Carrying this idea a bit further, we can understand the wide range of AHand AS* values on the basis of a mechanism which involves a transitionstate which is biradical-like in character, but which requires a 'twist' motionof the fragmenting sigma orbital of the 0—0 bond. Let us consider the valuesof and in cyclohexane, and assuming the solvent effect to be negli-gible here, the AH# value (25 kcal/mole) represents a value for breaking the0—0 bond and the AS value (— 1 e.u.) represents a loosening of molecularmotions as the 0—U bond breaks (increase in entropy) but also a specfIc kindof motion as shown in equation 23 (decrease in entropy) thereby leading to anearly zero value for AS*. The breaking of the 0—0 bond can be assisted bycharge transfer stabilization of the transition state by the solvent (cyclohexene,AH = 23 kcal/mole). This is quite analogous to the effect observed byWalling and Wagner32 on the fragmentation of alkoxy radicals. These authors

381

A0

NICHOLAS i. TURRO AND PETER LECHTKEN

found that cyclohexene solvent has a marked effect which strongly favoured 13-scission (equation 24) over hydrogen abstraction. There is an entropy price tobe paid, in this mechanism, and we note that indeed IXS* becomes morenegative (—8 e.u.) than the value in cyclohexane. Hydrogen bonding mightalso be expected to assist cleavage of the O—O bond in analogy to the resultsof Walling and Wagner32. Indeed, LIH* is much lower in isopropanol(20 kcal/mole) and drops further (to 12 kcal/mole) when the more acidic, lesshindered molecule, methanol is solvent. The entropy price paid here is mainlydue to tying up the solvent in specific orientations and is more costly(S — 15 e.u. and —34 e.u., for isopropanol and methanol, respectively).

R®R—C-—O• R---CO (24)

R® R®

R—C—O• ® R—C0 ®I (25)

R— C —OS R—CO

One may now ask the question, is there any 'linear-type' of cleavage of theO—O bond? We have no good arguments for its specific occurrence, butperhaps the fragmentation of V into singlet states (A1 + A0 or 2A) involvessuch a cleavage. It is interesting to note, in this regard, that as the activationenergy rises, the relative yield of A1 + A0 increases (Table 5).

With respect to the photochemical decomposition of V, we do not havesufficient data for a detailed mechanistic picture. It seems reasonable,however, to suppose that the O—O bond would be cleaved easily in the excitedstates of V. The factors determining ratio of singlet and triplet yields are a bitmore difficult to assess now, since they may reflect the rate of intersystemcrossing of V(S1) rather than the rates of direct partitioning of V(S1) intoA1 and A3. The wavelength dependence is consistent with either view, if wehypothesize that higher excitation energy favours V(S1) —p A1 + A0, withrespect to intersystem crossing which eventuates in V(T1) —+A3 + A0.

The faster rate of cleavage ofT1 relative to S1 has a possible analogy in theobservation that n, irtriplets of alkanones undergo a-cleavage at a muchfaster rate than n, t*singlets17.

A SEARCH FOR CATALYSIS OF FRAGMENTATION OF VBY ENERGY ACCEPTORS

One of the possible reasons for the remarkable stability of dioxetanes hasbeen put forth in the previous section. Let us now consider the fascinating

382


question: can an electronic or vibrational energy acceptor 'catalyse' thefragmentation of dioxetanes by (a) diminishing the problem of conversion ofsubstantial amounts of chemical energy into vibrational energy, i.e. equation26 or (b) by providing a path for a 'non-classical' electronic energy from a

+ Q 2 + Q* (26)

ground state donor to produce an electronically excited acceptor, i.e. equation27.

+ Q 2 )=o + Q* (27)

Both these mechanisms, if they are elementary reactions, will require someelectronic coupling or complexing for reaction to occur efficiently. Thus, therate of fragmentation of V may suffer from an increase in L\G due to a morenegative AS. We should thus search for catalysis by analysing AH.

Before doing so, let us point out that Rauhut8'37 suggested some time agothat the decomposition of III might be catalysed by certain aromatic com-pounds (e.g. anthracenes) in order to explain enhanced decomposition ratesin some chemiluminescent reactions. He8 also suggested that the acceptoris behaving as an electron donor in these catalyses so that the ionizationpotential of the acceptor might be important in determining the effectivenessof a catalyst. Mazur and Foote'2 have reported the apparent catalysis ontetramethoxy dioxetane by zinc tetraphenylporphine. Kopecky and Mum-ford'1 found that 0.2 M biacetyl increases the rate of decomposition of tn-methyl-1,2-dioxetane by a factor of nearly two. However, Wilson and Schaap2°have shown that enhanced rates of decomposition may be due to energyacceptors, relaying their newly acquired electronic excitation energy to aground state dioxetane, thereby causing its 'sensitized' decomposition. Thus,the role of any real 'catalysis' by added energy acceptors is not fully established.

We have measured the activation parameters for thermal decompositionof benzene solutions V in the presence of energy acceptors (Thble 6). In eachcase, the fluorescence of the acceptor20'38 was excited by decomposing V.

Table 6. Activation parameters for thermolysis of benzene solutions of tetramethyl-1,2-dioxetane/ in the presence of energy acceptors

Acceptora H(kca1jmo1e 1) (eu.)

k( x 104J(60°C, sec 1)

ePcj. ST

None 25 —1 1.3Rubrene 26 +0.5 1.0 0.4 0.019,10-Dimethyl-A 31 + 16 0.5 1.0 0.259,1O-Dibromo-A 28 +8 0.6 120 0.99,10-Dichloro-A 27 +4 1.1 4.3 0.459,1O-Diphenyl-A 20 — 16 2.2 0.6 0.16

A denotes anthracene; 0.01 M in beneene, aerated solutions. Ratio acceptor/dioxetane = 1:2.Relative chemiluminescence yields, corrected for the relative fluorescence quantum yields under the applied conditions (69°in benzene),

383


Triplet to singlet energy transfer (equation 28) has been proposed as themechanism for excitation of acceptor fluorescence. The data in Table 6 arenot easily correlated by such a thesis. Thus, 9,10-dibromoanthracene is ex-

>O3 + Q —>=01

+ Q' (28)

- Q + hv (29)

pected to be the best quencher in any triplet to singlet mechanism39, butit is not in terms of It should be noted, however, that 9,10-dibromo-anthracene is the most efficient acceptor with respect to the observed quantumyield of (corrected) sensitized fluorescence.

Our data are too preliminary to allow us to generate any firm conclusionsconcerning the mechanism by which tH* and AS* are caused to vary tosuch extremes. However, it does seem quite apparent that the electronicstructure of the acceptor is playing a major role in determining the electronicinteractions which occur in this remarkable 'enthalpy catalysis' of aromaticenergy acceptors.

THE THERMOLYSIS AND PHOTOLYSIS OF NAPHTHVALENEKatz, Wang and Acton4° have recently reported a brilliant synthetic route

to benzvalene (XXI) and naphthvalene (XXII). In a collaborative effort, wehave examined the thermolysis and photolysis of XXI and XXII in a spectro-photofluorimeter, to determine ilelectronically excited products are produced.Although results on the thermolyses have been inconclusive to date, the dataachieved on photolysis of XXII at 77°K allow some definitive and very strikingconclusions to be made about napbthvalene's photochemistry.

Excitation of XXII in a methyl cyclohexane glass at 77°K revealed theinitial occurrence of two emissions, one short-lived emission maximizingnear 320 nm which is assigned to naphthvalene fluorescence on the basis of itsspectral similarity to the known fluorescence of XXII. Of greater interestis the second, long-lived emission maximizing at about 520 nm which is

hv

iIi'IIII // -÷ ZIXII(S1) (S1)

+hvT1)

Figure 5. Photochemical conversion of naphthvalene into naphtbalene.

384


assigned to naphthalene phosphorescence, based on comparison of the life-time and spectral distribution of this emission with that of an authenticsample of naphthalene. From the known phosphorescence yield of naphtha-lene, we can calculate that the quantum yield of the reaction is about 0.6 to 0.7!Thus we have uncovered another example of an efficient formation of anexcited state product. The results of this research are summarized in Figure 5.

CONCLUSION

After consideration of the dearth of examples of photoreactions whichproduce electronically excited products, Hammond27 concluded that 'Itseems unlikely that there will be any large number of systems in which a givenchemical change is vastly more rapid in excited states than in correspondingground states, unless the chemical change is driven by internal conversion ofelectronic energy.' He meant by this that reactions of the type R* —+P will, ingeneral, dominate R* * followed by P* —* P. Förster35 has suggestedthat the potential surfaces interconnecting R* and P* must be unfavourablefor interconversion, and that a wide separation be maintained between thepotential surfaces of p*and P. This point of view is similar to that expressed byDougherty36. The same types of ideas seem to be transferable to the descrip-tion of thermal reactions which produce electronically excited products. Theproblem with these attitudes is that, while they are theoretically valid andprovide us with a certain amount of insight, they do not immediately suggestthe types of structures which will meet the requirement of producing electroni-cally excited products. To begin with, it is non-trivial to surmount the problemof exothermicity required for the step R P* or R* P*.

The two examples discussed here do have some features in common whichmight be of assistance in deciding what kinds of reactions might have thebest chance of forming electronically excited products. Both involve: (a)a four-electron pericyclic reaction and (b) release of an enormous amountof energy if the path R —*P or R* — P were to be followed without the inter-vention of P*. Intuitively, we expect that a chemical analogue to the spectro-scopists' Franck—Condon principle should exist. In other words, reactionsinvolving conversion of large amounts of electronic excitation energyinto vibrational energy should be slow, relative to available paths which avoidsuch a situation, i.e. step-wise processes such as R — P'k — P and R*- P might be favoured when the exothermicity of the overall reaction isvery large. Perhaps then the problem is that, in general, molecular motionswhich allow the proper intersection of the energy surfaces of R and P*or R* and P* are not favourable. In this regard, it is intriguing to note thatboth of the reactions leading to excited state products considered in this paper,are formally pericyclic in nature. We do not wish to imply that they are requiredto be concerted, however, but merely that a 2 —÷ 2it interconversion isinvolved in both cases.

*0 0— JJ (30)

385


* '1

(31)

Thus, we can speculate that a felicitous blend of high exothermicity andproper molecular motions contrives to allow efficient formation of electroni-cally excited products from V and XXII.

--'180

///

\\\'*_:

Figure 6. Possible state correlation diagram depicting the fragmentation of tetramethyl-1,2-dioxetane. Energies in kcal/mole. Not drawn to scale.

In conclusion, it is revealing to consider an analysis of the state correlationof decomposition of 1,2-dioxetanes. Kearns' has pointed out that the groundelectronic state of 1,2-dioxetanes might correlate with an electronicallyexcited state of the carbonyl fragment. Thermochemical considerationsallow only one carbonyl fragment to be formed in an electronically excitedstate, which, in turn is required to be n, ,t (Figure 6). In an incisive anticipa-tion of our results, Kearns noted that 'there may be a crossing of the potentialenergy curves for excited states (of products) and the ground state (of dioxetane).While small distortions from C2,, symmetry could prevent crossing of theground state with an excited singlet curve, crossing of an excited triplet statecould occur. . . one of the carbonyl ketones could be left in an excitedtriplet state as a result of the expected curve crossing. . . anddioxetanes shouldbe photochemically unstable with respect to cleavage.' Even though thisanalysis applies strictly to a thoroughly concerted, one-step mechanism fordecomposition, we feel that the effective mechanistic difference will be slightbetween a two-step rate-determining biradical formation followed by rapid

386

\ \\ \\\ /\/K\/ \

/\\ \\£H*

1000—0 d

—il

00


cleavage of the C—C bond and a one-step process with advanced cleavageof the O—O bond relative to the C—C bond.

ACKNOWLEDGEMENTS

The authors are indebted to many people for helping them to perform theexperiments and to shape the ideas presented in this paper. Professor KarlKopecky generously provided them with a detailed synthesis of V. Mr MarkNiemczyk skillfully synthesized V and executed some of the preliminaryexperiments with V. To Professors Emil White, David Kearns and WilliamRichardson they are grateful for stimulating and concept-shaping discussionsof the mechanism of dioxetane decomposition. Dr Richard Hautala suggestedand executed the rise time search for an intermediate. Professor ThomasKatz, Mrs Eileen Carnahan and Mr Arthur Lyons collaborated with them onthe experiments and interpretation of naphthvalene photochemistry.

The generous financial support of this work in the form of a NATOFellowship to P. Lechtken and grants from the Air Force Office of ScientificResearch (Grant AFOSR-1848D) and The National Science Foundation(Grant NSF-GP-26602x) is gratefully acknowledged.

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2 J• Calvert and J. N. Pitts Jr, Photochemistry, p 443. Wiley: New York (1965).1. E. Leffler and J. W. Miley, J. Amer. Chem. Soc. 92, 7005 (1971).P. R. Story and P. Busch, Advanc. Organic Chem. 8, 67 (1972).R. Criegee and R. Huber, Chem. Ber. 103, 1855, 1862 (1970).

6 S. R. Fahrenholtz and A. M. Trozzolo, J. Amer. (7'iem. Soc. 93, 251 (1971) and referencestherein.C. Walling and M. J. Gibian, J. Amer. Chem. Soc. 87, 3413 (1965); Photosensitized decomposi-tion of peroxides may be complicated by non-vertical energy transfer and 'chemical sensitiza-tion': P. S. Engel and B. M. Monroe, Advanc. Photochem. 8, 245 1971).

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10 A product called 'Coolite' is available commercially as a flexible plastic rod which, when bent,cracks an interior glass tube whose ingredients initiate a chemiluminescent reaction.K. R. Kopecky and C. Mumford, Canad. J. Chem. 47, 709 (1969);C. Mumford, PhD Dissertation, University of Alberta (1969).

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18 S. Murov and 0. S. Hammond, J. Phys. Chem. 72, 3797 (1968).19 G. Taylor, Chem. Phys. Letters, 10, 355 (1971).20 T. Wilson and A. P. Schaap, J. Amer. Chem. Soc. 93, 4126 (1971).

387

NICHOLAS J. TURRO AND PETER LECHTKEN21 s W.Benson and H. E. O'Neal, Kinetic Data on Gas Phase Unimolecular Reactions, NSRDS-

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40 T. J. Katz, E. J. Wang and N. Acton, J. Amer. Chem. Soc. 93, 3782 (1971).

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