Transient electronic and vibrational absorption studies of the photo-Claisen and photo-Fries rearrangements.

The liquid-phase photo-Claisen and photo-Fries rearrangement dynamics of allyl phenyl ether and phenyl acetate in cyclohexane solution have been interrogated via ultrafast transient absorption spectroscopy. Following excitation at 267 nm, the reaction progress is monitored on a picosecond time-scale by electronic and vibrational absorption spectra obtained from broadband UV/Visible and mid-infrared probe pulses. The evolution of the ground and excited electronic states of the parent molecule, the radicals produced by photo-induced homolytic bond ﬁ ssion, and intermediate cyclohexadienones formed via recombination of the produced radical pair are followed, providing new insight and detail on the reaction mechanisms. Subsequent kinetic analysis allows determination of rate coe ﬃ cients as well as quantum yields for the processes involved. These examples serve to highlight the utility of employing broadband UV-Visible and infrared probe spectroscopies, in conjunction, to unravel the mechanisms of photochemical reactions in solution. The underlying photo-physics that initiates bond ﬁ ssion in this class of molecules is also addressed in the context of the role of dissociative (n/ p ) s * excited states.


Introduction
The rearrangements of substituted phenols from either aryl phenyl ethers or phenyl esters into 2-or 4-substituted phenol products are a well-studied class of reactions in synthetic chemistry. First identied by Claisen 1 and Fries 2,3 in the early 1900's, the thermal Claisen rearrangement results in a 2substituted phenol produced via a concerted intramolecular reaction whereas a Lewis acid catalyzes the Fries rearrangement and, depending on the experimental conditions, can result in either a 2-or a 4-substituted phenol. Several decades aer these initial discoveries, photochemical versions of both rearrangements were uncovered, known as the photo-Claisen 4 and the photo-Fries rearrangements. 5,6 Subsequent mechanistic studies suggested that UV excitation of the reactant causes homolytic bond ssion producing a radical pair which can then recombine to form the original parent molecule. Alternatively, the recombination can occur at either the 2-or 4-positions on the aromatic ring to form a cyclohexadienone intermediate which subsequently undergoes hydrogen atom transfer to form the corresponding substituted phenol as depicted in Scheme 1. [7][8][9][10][11][12] A previous ultrafast study of a 4-substituted phenyl acetate using a UV/Visible probe at several discrete wavelengths by Lochbrunner et al. provided time-resolved support of this mechanism. 13 The thermal versions of these rearrangements have received extensive attention and evaluation as a key tool in a wide range of syntheses. [14][15][16] With improved understanding and further investigation, the photochemical versions of these rearrangements may offer the same synthetic utility as their thermal counterparts but with the added benet of being a 'greener' synthetic route, 17 negating the need for additional reagents (in the case of the Fries rearrangement) and, potentially, requiring reduced energy input. The photo-Fries reaction has previously been used synthetically for the production of compounds including diazoamide macrocycles, 18 hydroxyphenones 19 and chroman-4-one derivatives. 20 A photo-Fries rearrangement has also been identied as the main degradation process of paracetamol following UV excitation. 21 The photo-Claisen rearrangement has recently been used in micelle formation. 21 This study uses both transient electronic absorption (TEA) and transient vibrational absorption (TVA) spectroscopy to follow the evolution of the species produced by the UV excitation of allyl phenyl ether (PhOC 3 H 5 ) and phenyl acetate (PhOC(O)CH 3 ) in cyclohexane solutionwell-known examples of the photo-Claisen and photo-Fries reactions. As we show below, the combination of transient interrogation by broadband UV/Visible and broadband IR probe pulses reveals a much fuller mechanistic picture of these photo-initiated rearrangements than either technique alone, allowing determination of production and loss rates for various key species and intermediates and estimates of quantum yieldsinformation which, in turn, affords greater understanding of the underlying photophysics driving these processes.

Experimental
The TEA and TVA spectra were recorded using the ULTRA laser facility at the STFC Rutherford Appleton Laboratory. 22 An amplied titanium sapphire laser system generated 800 nm (band center) pulses with 50 fs pulse duration at a 10 kHz repetition rate. The broadband UV/Visible probe was generated by focusing a portion of the 800 nm radiation into a CaF 2 disc which was rastered in the two planes orthogonal to beam propagation in order to reduce photodamage. A further portion of the 800 nm light was used to pump an optical parametric amplier to produce mid-IR pulses with a bandwidth of $500 cm À1 . In each experiment the pump and probe pulses were overlapped in the sample with their respective linear polarization vectors aligned at the magic angle, before the transmitted radiation was dispersed by a grating onto a 512 element silicon array (for UV/Visible detection) or a 128 element mercury cadmium telluride array (for IR detection). The samples were owed continuously through a Harrick cell with a 100 mm PTFE spacer between CaF 2 windows at solution concentrations chosen to ensure an absorbance of 0.5 at 267 nm (30 mM for PhOC 3 H 5 and 180 mM for PhOC(O)CH 3 ). The $1 ps experimental response function is limited by background noise induced by the cell windows and solvent (cyclohexane), thus masking the true instrumental response time of $100 fs. PhOC 3 H 5 (99%), PhOC(O)CH 3 (99%), and cyclohexane were obtained from Sigma-Aldrich and used without further purication.

Transient absorption spectroscopy
We begin by considering the TEA spectra of PhOC 3 H 5 and PhOC(O)CH 3 in cyclohexane shown in Fig. 1(a) and 2(a) respectively. At the earliest pump-probe delays studied, $2 ps, both systems exhibit a broad increase in absorption extending across the full probe region (340-670 nm). In accord with previous aromatic molecules studied via TEA these features can be assigned to absorption from an excited electronic state of the molecule under study. [23][24][25] The temporal evolution of these features are depicted in Fig. 1(c) and 2(d) and are t well by a single exponential decay which provides a measure of the lifetime of the excited state molecule. A value of 96(4) ps (where the number in parentheses represents the one standard deviation uncertainty in the last signicant digit) is obtained for PhOC 3 H 5 , while the data for PhOC(O)CH 3 reveal an excited state lifetime of 30(3) ps.
The TEA spectra of PhOC 3 H 5 and PhOC(O)CH 3 exhibit another similarity, a structured feature at 398 nm. This feature has been observed several times previously and is assigned to the phenoxyl radical produced following O-R bond ssion. [24][25][26] Due to the substantial overlap of this absorption with that of the excited state parent, the detailed kinetics of the phenoxyl radical cannot be obtained. Homolytic bond ssion following UV excitation of PhOC 3 H 5 /PhOC(O)CH 3 will also produce an allyl/acetyl radical. No absorption of the allyl radical has been reported in the visible probe region studied here, but absorption due to the acetyl radical has previously been recorded in the 490-660 nm range (l max ¼ 532 nm), with a measured absorption cross section, s ¼ (1.1 AE 0.2) Â 10 À19 cm 2 per molecule (a molar extinction coefficient, 3, of 29 M À1 cm À1 ). 27 No feature attributable to the acetyl radical is evident at these wavelengths in Fig. 2(a), but this is not unexpected given the weakness of this absorption (cf. for phenoxyl at l max ¼ 398 nm, s ¼ 1.2 Â 10 À17 cm 2 per molecule, 3 ¼ 3155 M À1 cm À1 ). 26 The TVA spectra recorded following 267 nm photoexcitation of PhOC 3 H 5 and PhOC(O)CH 3 are shown in Fig. 1(b), 2(b) and (c), respectively. The spectra of PhOC 3 H 5 show six features within the probe region studied (1400-1700 cm À1 ), three bleach features reecting depletion of the ground state population induced by the UV pump pulse at 1495, 1586 and 1598 cm À1 (with strong overlap been the latter two features) and three transient absorptions at 1415, 1516 and 1672 cm À1 .
As shown by the kinetic traces in Fig. 1(c) and (d), the transient absorption features at 1415 and 1516 cm À1 show very different kinetic behavior from that exhibited by the feature centered at 1672 cm À1 . Fig. 1(c) shows the transient behavior of the feature at 1415 cm À1 . It exhibits an exponential decay with a time constant of 108 (8) ps. This transient population change suggests that the feature can be assigned to absorption of the excited state of PhOC 3 H 5 as its lifetime is comparable to that of the excited state absorption (ESA) recorded in the TEA spectra; when these two kinetic traces are t simultaneously to an exponential decay a lifetime of 98(4) ps is obtained. The other transient absorption at 1672 cm À1 shows different kinetics; it is rst visible at pump-probe delays of $10 ps and grows with an exponential rise time of 150(10) ps. Given this kinetic evolution, it might initially appear tempting to assign this feature to absorption of the phenoxyl radical previously identied in the TEA spectra. This assignment would be incorrect, however, as both previous measurements and calculations do not nd or predict any phenoxyl radical absorption in the 1670 cm À1 region. 28 Instead, absorptions in this mid-IR wavenumber region are usually associated with a carbonyl group. Two of the proposed intermediates contain a carbonyl group, viz. the substituted 2,4-and 2,5-cyclohexadienones. These are formed when the produced radical pair geminately recombines with, in this case, the aliphatic group rejoining the phenoxyl at either the 2 or the 4 position. However, the predicted carbonyl stretching wavenumbers for these two species are very similar and cannot be distinguished in this experiment (Tables 1 and 2 in the ESI † detail the calculations of the normal modes of PhOC 3 H 5 , PhOC(O)CH 3 and the substituted 2,4-and 2,5-cyclohexadienone intermediates that have aided these assignments). The feature at 1866 cm À1 exhibits distinctly different kinetic behaviour from the other transient features and is shown in Fig. 2(e). The feature grows for $40 ps following excitation, and then begins to decay with some fraction remaining at long time delays. This kinetic behaviour, and the wavenumber at which the band appears, suggests that the feature is due to the acetyl radical produced via O-R bond ssion. Previous measurements of the IR spectrum of the acetyl radical in hexane solutions, which found a C]O stretching wavenumber of 1864 cm À1 , serve to reinforce this assignment. 29 Its appearance reects production by bond ssion from the initially excited electronic state of the parent molecule, whereas its subsequent decay is attributable to geminate recombination to form the aforementioned substituted cyclohexadienones or the S 0 state parent molecule. The long-time absorption indicates that some fraction of the radical products escapes recombination and persists beyond 2.5 ns in solution.
The two features at 1697 and 1765 cm À1 evolve concurrently and, analogous to PhOC 3 H 5 , can be ascribed to formation of either or both the substituted 2,5-or 2,4-cyclohexadienone intermediates. The intermediates in this case show two C]O stretch absorptions, as two distinct carbonyl groups are present: the enone, which is part of the six-membered ring (at 1697 cm À1 ) and the carbonyl of the acetyl group (at 1765 cm À1 ). These features exhibit an exponential rise of 48(2) ps. As was the case for PhOC 3 H 5 , the predicted wavenumbers of the carbonyl stretching modes in the substituted cyclohexadienones are too close to allow them to be differentiated in the present TVA spectra. However, the predicted IR transition strengths (detailed in the ESI †) offer another possible route to discriminating between these two intermediates. The calculated ratio of the enone to acetyl C]O stretch band intensities is 0.98 in the case of the 2-substituted cyclohexadienone, while in the 4-substituted cyclohexadienone it is 2.3. The experimentally observed ratio is 1.9. If the calculated IR intensities are reliable, this would suggest that the ratio of substituted 2,5-cyclohexadienone to 2,4-cyclohexadienone is $2 : 1. The previous time resolved study of the photo-Fries rearrangement used a 4-substituted phenyl acetate specically to exclude possible competition between the two sites of cyclohexadienone formation. 13 The TVA spectra also allow any repopulation of the ground electronic state of these two molecules to be monitored via the intensity of the ground state bleach features. Fig. 1(d) and 2(f) show that, over the timescale probed, 2.5 ns, $30% of the initially excited population of PhOC 3 H 5 returns to the S 0 state while, for PhOC(O)CH 3 , this recovery is $54%. When the extracted kinetic traces are t to an exponential function, good agreement with the rise time for formation of the intermediate cyclohexadienones (170 (35) ps and 60(10) ps for PhOC 3 H 5 and PhOC(O)CH 3 , respectively) is found. We recognize that internal conversion (IC) from the photoexcited state could contribute to the parent bleach recovery at early pump-probe delays, but any such contribution is hard to discern in the present data as the excited state lifetimes in both systems are similar to that observed for geminate recombination. Any contribution from IC is thus neglected in the following kinetic analysis.

Kinetic modelling
Scheme 2 details the kinetic scheme used to model the processes observed in the photo-Claisen and photo-Fries rearrangements of PhOC 3 H 5 and PhOC(O)CH 3 , respectively. k d is the rate coefficient for dissociation of the photo-excited molecule to form a radical pair, PhO_ + R_. This radical pair can then be lost in two ways; recombination (described by a rate coefficient k r ) or escape into the bulk solvent, yielding PhO_ e + R_ e (described by the rate coefficient k e ). Recombination can lead to several observable products: S 0 parent molecules and the substituted 2,4-and 2,5-cyclohexadienones. An analogous scheme has been used previously to model the kinetics of electron photodetachment from halide anions. 30,31 Treating the (PhO_ + R_) radical pair as a single entity allows analytical solutions for the time dependent concentrations of the four species of interest to be obtained. This treatment has been favoured over one where the geminate recombination process is modelled diffusionally, 31 with the assumption that the radical pair is formed instantaneously aer UV excitation. Since the lifetimes of the electronically excited parent molecules are in the tens of ps range, such an assumption would be inappropriate in this case. In the case of PhOC(O)CH 3 , all the transient populations necessary for complete kinetic modelling (viz. the excited and ground electronic states of the parent molecule, the substituted cyclohexadiene, and the C(O)CH 3 radical) are accessible. It is important to note that the "in cage" and "bulk" acetyl radicals have the same spectral signature, and the observed signal is consequently the sum of both. Fitting all of the available data, simultaneously, to the kinetic equations detailed in the ESI † yields the rate coefficients and quantum yields shown in Table 1.
In deriving the latter, we assume an initial quantum yield for dissociation, f d ¼ 1. f r and f e are the respective quantum yields for recombination and escape which can be obtained from the relative magnitudes of k e and k r . The quantum yield for recombination is itself the sum of that for reforming S 0 parent molecules, f S0 , and that for forming the cyclohexadienone adducts, f CHD . The percentage ground state bleach recovery provides a measure of f S0 , with f CHD subsequently being obtained from the relation f r ¼ f S0 + f CHD .
Equivalent kinetic analysis for PhOC 3 H 5 is hampered by the lack of transient signal for the C 3 H 5 radical. k d has been acquired from analysis of the transient excited state population, but reliable values for k r and k e are unobtainable. The TEA experiments do, however, provide a relative measure of the long-time phenoxyl radical product yield for both molecules. The above analysis for PhOC(O)CH 3 shows that the long-time phenoxyl radical yield (PhO_ e ) represents 26% of the originally excited population. Direct comparison of the relative intensities of the long-time phenoxyl absorption at 398 nm thus provides the estimate f e ¼ 0.14 in the case of PhOC 3 H 5 . Such a comparison is valid as the initial sample concentrations were chosen to ensure the same optical density, 0.5, at the excitation wavelength of 267 nm. Recalling Scheme 2, f e ¼ 0.14 implies that the total recombination quantum yield following 267 nm excitation of PhOC 3 H 5 is f r ¼ 0.86. As for PhOC(O)CH 3 , f S0 can be obtained from analysis of the ground state bleach recovery, yielding f S0 ¼ 0.30 and thus, by difference, f CHD ¼ 0.56.

Discussion
The combined use of TEA and TVA techniques has allowed direct conrmation of the mechanism for the rst few steps of the photo-Claisen and photo-Fries reactions shown in Scheme 1, while kinetic modeling has provided additional information about the timescales on which these processes occur and their respective quantum yields. Both techniques reveal absorption attributable to the respective excited state molecules and give dissociation lifetimes (in cyclohexane) of 96(4) ps for PhOC 3 H 5 and 28(1) ps for PhOC(O)CH 3 . Homolytic O-R bond ssion is identied as the main population loss channel following photoexcitation. IC to the S 0 state cannot be completely ruled out, but is considered a minor process on the basis that the ground state bleach recovery exhibits the same early time kinetics to that of a process (adduct formation) that is unquestionably due to geminate recombination. The tens of picoseconds excited state lifetimes identied in the present study provide a measure of the rate at which population transfers from the initial excited state to one of a dissociative nature. The excited state potential energy landscape relevant to O-R bond ssion in a range of aromatic compounds, including PhOC 3 H 5 and PhOC(O)CH 3 , was explored in an early theoretical paper. 32 This study found the S 1 state of PhOC 3 H 5 to be a 1 pp* state, the PES of which is intersected by a dissociative 1 ps* state (where the s* orbital is located along the O-R bond) correlating to PhO + C 3 H 5 products. The acetyl group in PhOC(O)CH 3 introduces additional non-bonding and weakly antibonding orbitals. The S 1 state in this case was shown to have substantial carbonyl-centered 1 np* character (i.e. to be formed by removing an electron from an n-orbital on the carbonyl O atom to the p* orbital of the acetyl group), S 2 is the ring-centered 1 pp* state and S 3 is the dissociative 1 ps* state that forms conical intersections (CIs) with both the 1 np* and 1 pp* states at short R O-R bond lengths and with the S 0 state at longer R O-R . The 1 np* and 1 pp* states were predicted to lie close in energy, however, and it is not clear which excited state contributes most of the oscillator strength when exciting at 267 nm. These early calculations 32 also imply the existence of a barrier to O-R bond dissociation from the S 1 potential minimum (PhOC 3 H 5 ) or from that of the S 2 state (PhOC(O)CH 3 ).
Given that UV excitation of both PhOC 3 H 5 and PhOC(O)CH 3 results in homolytic O-R bond ssion and formation of a radical pair that includes a phenoxyl radical, the well-studied case of phenol (PhOH) is an appropriate point from which to attempt to rationalise the photophysics of PhOC 3 H 5 and PhOC(O)CH 3 . The potential energy landscape of phenol is qualitatively similar to those exhibited by PhOC 3 H 5 and PhOC(O)CH 3 . O-H bond ssion is observed even when exciting to S 1 ( 1 pp*) levels lying at energies below the S 1 ( 1 pp*)/S 2 ( 1 ps*) CI, i.e. where there is a signicant energy barrier to dissociation. 33 Time-resolved measurements show the H atom yield rising on a nanosecond timescale, 34 which can be understood in terms of O-H bond dissociation by tunneling through the barrier under the S 1 /S 2 CI. At shorter excitation wavelengths (i.e. at energies above the S 1 /S 2 CI), O-H bond ssion occurs on a much faster timescaleeither as a result of directly populating the dissociative S 2 state or following fast radiationless transfer to the S 2 PES following initial population of the higher lying S 3 ( 1 pp*) state. 35 One key difference between the PESs of PhOC 3 H 5 , PhOC(O)CH 3 and phenol is the difference in bond dissociation energies (i.e. the asymptotic energy associated with radical production). The PhO-H bond strength in phenol is 3.72 eV whereas the PhO-R bond strengths in PhOC 3 H 5 and PhOC(O)CH 3 are only 2.16 eV and 3.35 eV, respectively. 36 These reduced bond strengths reect the additional stability of the allyl and acetyl radicals relative to the hydrogen atom produced by phenol photolysis. Recent pump-probe TEA experiments conrm that the same photoexcitation and dissociation dynamics persist for phenol in solution in weakly interacting solvents like cyclohexane though, unsurprisingly, additional post-excitation processes such as geminate recombination of the PhO + H products are also observed. 24,25 How then can we reconcile excited state dissociation on a ps timescale following excitation of PhOC 3 H 5 and PhOC(O)CH 3 ? Several points merit note. The topologies of the excited state PESs for PhOC 3 H 5 and PhOC(O)CH 3 may be qualitatively similar to that of phenol, but the fragments formed via homolytic O-R bond ssion are very different; allyl and acetyl are much larger than an H atom, and tunneling is not a credible fragmentation mechanism. A second consideration is the nature of the level or levels within the excited state manifold that will be populated by absorption of a 267 nm photon. The S 1 -S 0 origin of PhOC 3 H 5 in the gas phase has been identied at 4.50 eV, so absorption of a 267 nm photon (with energy 4.67 eV) will not form PhOC 3 H 5 (S 1 ) molecules with substantial internal (vibrational) excitation. Indeed, given that the systems under study are solvated, any such vibrational energy is likely to be quenched by vibrational energy transfer to the surrounding solvent bath. Vibrational cooling in similar systems has been seen to occur on timescales of $10 ps (ref. 37) and should thus be competitive for systems such as PhOC 3 H 5 and PhOC(O)CH 3 with deduced S 1 lifetimes of tens of picoseconds.
Nonetheless, the present data conrm O-R bond ssion to be the dominant decay process following 267 nm photoexcitation of both PhOC 3 H 5 and PhOC(O)CH 3 as did the previous study of a 4-substituted phenyl acetate, 13 and it is pertinent to consider possible limitations of the previous theoretical treatment. The cuts along R O-R in these early PES calculations 32 were constrained to C s geometries (i.e. all the heavy atoms were held in a plane), but a later combined experimental (laser induced uorescence) and theoretical study of isolated PhOC 3 H 5 iden-tied no fewer than four conformers lying within 500 cm À1 of the planar ground state minimum. 38 Recent studies of S-CH 3 bond ssion in thioanisoles may also be relevant in this regard. As in the phenols, the CI between the bound S 1 ( 1 pp*) and dissociative S 2 ( 1 ps*) states of thioanisole is calculated to lie at energies above that of the S 1 (v ¼ 0) level at planar geometries, 39 but the barrier to dissociation is seen to decrease upon relaxing the excited state geometry and by changing the C-S-CH 3 torsion angle. 40 For completeness, we note that the early theoretical study of PhOC(O)CH 3 did explore the effect of varying the torsion angle, but found no decrease in the height of the barrier to O-R bond ssion. 32 New multi-dimensional ab initio calculations on these systems would surely be useful in helping to reconcile the present (and related) 13 experimental data. Of course, solvation could also contribute to this apparent mismatch between experiment and theory. Inspection of the available absorption data, presented in the ESI, † shows some red-shi in the peak of the long wavelength UV absorption spectrum of PhOC 3 H 5 in cyclohexane (cf. the gas phase), but the real quantity of interest would be any differential shi in the relative energies of the bound 1 pp*( 1 np*) states and the dissociative 1 ps* PES (since this would affect the height of any barrier under this CI). The low oscillator strength and the diffuseness of the s* ) p transition precludes its direct observation in absorption measurements.
Post-dissociation, the next step in Scheme 1 determines the fate of the radical pair. This involves competition between two processes: geminate recombination and solvent cage escape.
The former leads either to reforming the parent precursor or formation of a substituted cyclohexadienone. As noted above, this substitution may occur at either the 2-or 4-position on the ring (dened relative to the C-O bond) but neither of the present probe techniques allows us to distinguish between these two intermediates. Cage escape leads to a persistent radical signal. In the case of PhOC(O)CH 3 , the present kinetic modeling suggests that the recombination rate in cyclohexane is $3-times faster than the rate of radical cage escape; for PhOC 3 H 5 , the relative probability of radical pair recombination is yet greater ($86%). A more striking difference between the two systems is the relative branching among the recombination products. For PhOC 3 H 5 , about two thirds of the recombination events result in substituted cyclohexadienones whereas, for PhOC(O)CH 3 , the majority recombination product is the S 0 parent molecule. A plausible explanation for this difference is that once on the 1 ps* PES, the separating PhO/C 3 H 5 products are well posed to recombine in a manner analogous to the concerted reaction observed in the thermal Claisen rearrangement. Such a mechanism should result in preferential production of the substituted 2,4-cyclohexadienone. No such pathway exists for the Fries rearrangement of PhOC(O)CH 3 , consistent with the deduced lower fractional branching into the adduct in this case.
The nal step in Scheme 1 is intramolecular hydrogen transfer within the cyclohexadienone intermediate to form the nal substituted phenol product. The present TVA experiments are also capable of probing in the 3000 cm À1 spectral region, where the O-H stretch vibration associated with creation of a phenol product should appear, but no such signal was observed even at the longest pump-probe delays (2.5 ns)consistent with previous experimental estimates of the timescale on which hydrogen atom transfer occurs in these systems. 7,10,13

Conclusions
By using a combination of transient electronic and vibrational absorption spectroscopies, the temporal evolution of the photo-Claisen and photo-Fries rearrangements of PhOC 3 H 5 and PhOC(O)CH 3 in cyclohexane solution have been followed for several nanoseconds following photoexcitation at 267 nm. During this time window, the ability to follow both electronic and vibrational absorption of the samples has allowed us to monitor the evolving population of the photoexcited parent molecule and the ensuing O-R bond ssion (revealed by the appearance of phenoxyl radical absorption in the TEA experiment and, in the case of PhOC(O)CH 3 , absorption due to the acetyl radical in the TVA experiment). Subsequent geminate recombination of the resulting radical paireither to yield a substituted cyclohexadienone intermediate or to reform the parent moleculeis observed also, via growth of new absorption features and ground state bleach recovery in the TVA experiment. The nal step in both the photo-Claisen and photo-Fries rearrangements, involving intramolecular hydrogen transfer to transform the cyclohexadienone intermediate into a substituted phenol, is not observed within the time delays probed.
Kinetic modelling of the time evolving signals of the various observed species has allowed determination of rate coefficients for each of the above processes, and their relative quantum yields. The fraction of radical pairs formed following photoexcitation that are deduced to escape the initial solvent cage is greater for PhOC(O)CH 3 ($26%) than for PhOC 3 H 5 (14%). Thus most of the radical pairs formed in both cases undergo geminate recombinationbut with different outcomes. In the case of PhOC(O)CH 3 , reforming parent S 0 population is favoured over formation of the substituted cyclohexadienone intermediate by a factor of $2 : 1, whereas PhOC 3 H 5 shows the opposite behavior. This, we suggest, reects the fact that the nascent allyl radical arising upon O-R bond ssion in the latter case is particularly well posed to add at the 2-position of the ring (as in the thermal Claisen rearrangement).
At a more general level, the present study provides further illustrations of (i) the similarities between the primary UV photochemistry that drives O-R bond ssion in these molecules and that responsible for O-H bond ssion in simpler systems like phenol, (ii) the ubiquity of ps* excited states in driving excited state bond ssions in such molecules, and (iii) the utility of detailed gas phase studies of simpler prototypes in guiding our understanding of the, usually more complex, photochemistry of condensed-phase systems for which little gas-phase spectroscopy is available and/or for which high-level theoretical calculations are prohibitive (due to the size of the molecules involved). The present study also highlights the value of broadband transient absorption methods (over the widest possible range of probe wavelengths) 41 for elucidating mechanistic aspects of photo-initiated organic reactions in solutionallowing identication of key species, the timescales on which they are created or destroyed and the respective product yields.