Photochemistry_of_Organic

Scheme 5.16

Most interesting results were obtained by measuring the lifetimes of these biradicals in magnetic fields B ranging from 50 mT (Earth s magnetic field) to 0.1 T. The ratio of the ISC rate constants in the presence and absence of a magnetic field, kISCB =kISC0 , increased from 1 (B ¼ 50 mT) to a maximum of 1.13 and then decreased to an asymptotic value of 0.8–0.9 for B ¼ 0.1 T. The maximum was reached at 3 mT for n ¼ 12, at 12 mT for n ¼ 11 and at 60 mT for n ¼ 10. The analysis of these and other data40 proved that ISC is dominated by SOC (76% for n ¼ 12 and 88% for n ¼ 11), but that hyperfine coupling (HFC) gains importance for the longest chain lengths. It was also shown that for 1,n-dibenzyl biradicals HFC becomes dominant for n > 6, because SOC is smaller in the absence the acyl group (recall that SOC increases with the fourth power of the atomic number Z, Section 4.8).

Case Study 5.3: Mechanistic photochemistry – the photo-Favorskii reaction of p-hydroxyphenacyl compounds

p-Hydroxyphenacyl (pHP) is an effective photoremovable protecting group for anionic substrates X (see also Special Topic 6.18). The release of, for example, diethyl phosphate from compound 25 [X ¼ OPO(OEt)2, Scheme 5.17] is accompanied by a photo-Favorskii rearrangement of pHP to p-hydroxyphenylacetic acid (26), which absorbs only below 300 nm. Following fast and efficient ISC, the reaction is initiated

Scheme 5.17 Mechanism of the photocleavage of p-hydroxyphenacyl diethyl phosphate

from the lowest the triplet state T1 of 25 (lmax 390 nm). It proceeds only in aqueous solvents and the lifetime of T1 decreases in a quadratic fashion with increasing water concentration in wet acetonitrile;383 with diethyl phosphate as the leaving group X it amounts to several microseconds in degassed, dry CH3CN, but is reduced to 60 ps in wholly aqueous solution.462 Thus water plays an essential role in the elimination of the leaving group.

Time-resolved resonance Raman spectroscopy of 25 in 50% aqueous CH3CN proved that the final product 26 appears with a rate constant of 2.1 109 s 1 following pulsed excitation of 25.207 The appearance of 26 was slightly delayed with respect to the decay of T1(25), k ¼ 3.0 109 s 1, that was determined independently by optical pump– probe spectroscopy in the same solvent. The intermediate that is responsible for the delayed appearance of 26, t 0.5 ns, is attributed to the triplet biradical 327.462 It shows weak, but characteristic, absorption bands at 445 and 420 nm, similar to those of the phenoxy radical. ISC is presumably rate limiting for the decay of 327, which cyclizes to the spiro-dienone 28. The intermediate 28 is not detectable; its decay must be faster than its rate of formation under the reaction conditions. Decarbonylation of 28 to form p- quinone methide (29) competes with hydrolysis to 26 at low water concentrations. Hydrolysis of 29 then yields p-hydroxybenzyl alcohol (30) as the final product.

Detailed knowledge of this mechanism was needed to establish the release rate of the leaving group. With its fast and efficient release of diethyl phosphate, t ¼ 60 ps, F 0.4, the p-hydroxyphenacyl protecting holds promise for use in studies of fast reactions such as the primary steps in protein folding (random coil to a-helix formation) (Figure 5.18).

H2O

Figure 5.18

5.4.5Carbocations and Carbanions

Recommended reviews.391, 463

Carbocations have held an important place as reactive intermediates in physical organic chemistry since the 1920s following pioneering studies of H. Meerwein, C. Ingold, S. Winstein and others. G. Olah showed that many carbocations are persistent in superacid solvents. Vinyl cations were long considered to be extremely short-lived or even nonexistent as reactive intermediates, but they are now readily accessible for study by light-induced heterolytic fission of halides, acetates and alcohols in polar solvents.464 The decay of carbocation intermediates is usually first order due to solvent addition, the rate of which increases with increasing nucleophilicity of the medium. 2,2,2-Trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) are especially useful for the investigation of carbocations by laser flash photolysis, because these solvents favour the efficient formation of carbocations, yet they are slow to trap them. The lifetime of carbocations is insensitive to the presence of dioxygen in solution.

Scheme 5.18 Making photochemically generated phenyl cations visible by trapping with TMB

The highly reactive phenyl cation has long escaped detection because it does not absorb in the near-UV region; it has been identified by titration with 1,3,5-trimethylbenzene (TMB) (Scheme 5.18).465

Fluorenyl cation (31), an antiaromatic carbocation, is formed by irradiation of 9- fluorenol in protic solvents (Scheme 5.19).466–469 The cation 31 exhibits a narrow

absorption band at 515 nm. The lifetime of the precursor singlet state of 9-fluorenol (lmax,abs ¼ 630 nm, lmax,em ¼ 370 nm) is strongly reduced in polar solvents, ranging from 1.7 ns in cyclohexane to <10 ps in water. The acceleration of heterolytic cleavage of the C OH bond in protic solvents must be attributed to fast proton transfer to the incipient

OH leaving group, because fission of the C OH bond occurs on a time scale that is faster (<10 ps) than that required for solvation of the ions (25 ps).468 The lifetime of the

cation 31 varies from 20 ps in water and TFE,468 to hundreds of nanoseconds in alkali

difference of DrG ¼ 114 kJ mol 1.

Scheme 5.19

5.4.6Enols

Recommended reviews.471–473

Flash photolysis has contributed much to our understanding of enol chemistry in aqueous solution. The equilibrium constant for the enolization of simple ketones, KE ¼ cenol/cketone, is usually too small permit a direct determination of enol concentrations cenol at equilibrium. For example, the enolization constant of acetone is pKE ¼ 8.3; the relative amount of 2-hydroxypropene is thus only 5 ppb in aqueous solution. Nevertheless, most reactions of carbonyl compounds proceed via enol intermediates, which, once

222 Photochemical Reaction Mechanisms and Reaction Intermediates

correspond to the HOMO–LUMO transition, but the forbidden, doubly excited state of Ag symmetry (see end of Section 4.7) lies only 1000 cm 1 above S1(t).490

ISC to the triplet state is inefficient in media of low viscosity at ambient temperature; the triplet state must be populated by sensitization. Pulsed laser irradiation of a sensitizer in the presence of high concentrations of either c or t produces the same transient intermediate exhibiting a broad absorption band rising in intensity from 400 to 360 nm.491 The intermediate decays with a lifetime of 60 ns to give nearly equal amounts of the c and t

isomers and is attributed to the twisted triplet state 3p*. The energy of 3p* was determined by photoacoustic calorimetry to lie 195 4 kJ mol 1 above the ground state of t.251 The

The comparable yields of c and t formed by direct excitation of either isomer have long been taken as evidence that the E–Z photoisomerization proceeds via a common intermediate 1p*, which acts as the funnel for IC to the ground state. However, femtosecond laser studies do not appear to be consistent with such an intermediate possessing a lifetime of more than 150 fs because there is no measurable delay between the disappearance of 1c* and the appearance of ground-state product absorption.482

Theoretical calculations show that the reaction cannot be described solely by the torsional angle f.493–495 A conical intersection (Special Topic 2.5) has been located along a cyclization reaction coordinate leading to DHP.493,496 DHP opens up very efficiently on

irradiation, presumably through the same conical intersection. Contrary to an early suggestion,497 the minimum at 1p*, probably arising by admixture of the doubly excited 1 Ag wavefunction (see end of Section 4.7), is shallow and the energy gap to the ground state is fairly large at this geometry. It is therefore unlikely that 1p* represents the dominant funnel for IC to the ground state. However, in spite of the extensive work done, this issue is still under discussion.

The groundand triplet excited-state PES of the homologues 1,4-diphenylbuta-1,3- diene (DPB)498 and 1,6-diphenylhexa-1,3,5-triene (DPH)499 have also been elucidated in detail by both experiment and calculation. Stilbene and DPH represent two extreme cases because the extended conjugation in DPH lowers the triplet energies of planar conformers more than those of twisted ones. All three have rather flat triplet PESs with respect to torsional motion about the formal double bonds. The perpendicular geometry 3p* represents an energy minimum in stilbene. In contrast, the perpendicular geometry corresponds to an energy maximum in 3DPH. Decay from the 3p* state of stilbene and from roughly planar triplets in DPH accounts for the 1000-fold difference in triplet lifetimes (60 ns for stilbene, 60 ms for DPH). The roughly equal amount of (E)- and (Z)- stilbene formed by sensitization of either isomer is consistent with decay from a perpendicular triplet, whereas the partitioning from 3DPH reflects an equilibrium distribution of the planar conformers that strongly favours the all-trans triplet. Conformational equilibration is also fast in 3DPB, which decays mostly from a perpendicular minimum that lies slightly above the global all-trans minimum. Quantum chain processes of triplet–triplet energy transfer may lead to quantum yields of photoisomerization quantum yields far in excess of unity.500

Scheme 5.22 The peroxyoxalate chemiluminescence reaction

known as the peroxyoxalate chemiluminescence reaction. The authors noted: The vapors produced by the reaction have the unusual property of inducing the fluorescence of suitable indicators, e.g., a filter paper impregnated with anthracene. These observations are suggestive of a metastable excited electronic state or some other highly energetic species. The luminescence was strongly enhanced by the addition of fluorescent chromophores such as 9,10-diphenylanthracene (DPA) and the emission corresponded to the fluorescence of these substances. It took over 40 years to provide evidence that the energetic species formed in this reaction is indeed the long-suspected511 1,2-dioxetanedione (32, a dimer of carbon dioxide!) (Scheme 5.22),512 and it remains to be shown that the rise and fall of this intermediate are kinetically related to the observed rise and fall of the chemiluminescence.509, 513 High-quality ab initio calculations have shown that structure 32 represents a minimum on the PES514 but, surprisingly, a reliable calculation of the barrier to its decomposition is not available at present.

This reaction is the active principle of the cold light sticks originally marketed by American Cyanamid, now OmniGlow. The last step in Scheme 5.22 cannot be due to a simple cleavage of 32 yielding an excited CO2 molecule, the excitation energy of which is far too high. In Section 5.2, we saw that the encounter of an excited molecule with an electron donor or acceptor may lead to the formation of radical ions when the free energy of electron transfer is negative (Equation 5.1). Naturally, the reverse process may occur when the recombination of radical ions is sufficiently exergonic to form an excited state of one of the neutralized reactants. Thus, the reaction of DPA and other activators with 32 is thought to produce a radical ion pair 1(DPA. þ CO2. þ CO2), which forms 1DPA* by return electron transfer.515 Such processes were named chemically initiated electron exchange luminescence (CIEEL);516 they operate in many other chemiluminescent systems. The radical ions may also be produced by electrochemical oxidation and reduction processes, resulting in electrochemiluminescence (ECL).508 Essentially the same process is exploited in LEDs and OLEDs (Special Topic 3.2) and excimer lasers (Special Topic 3.1). Four important isolable chemiluminescent species are shown in Scheme 5.23.

The cleavage of tetramethyldioxetane (33) to two acetone molecules represents a [2 þ 2] cycloreversion reaction that classifies as ground-state forbidden by the Woodward–Hoffmann rules (Section 4.9). Hence one may expect a diabatic crossing of zero-order PESs in this reaction (Figure 4.35). Sophisticated calculations indicate that the weak O O bond breaks initially and that ISC to the triplet state occurs in the resulting biradical intermediate.517 Exothermic triplet–singlet energy transfer from the chemiexcited triplet acetone to a heavy-atom substituted chromophore (9,10-dibromoanthracene) then promotes efficient fluorescence emission.507

The majority of bioluminescent organisms live in the ocean, but there are many terrestrial forms, notably beetles, that exploit bioluminescence. The bioluminescence of fireflies

Scheme 5.23 Isolable chemiluminescent reagents

proceeds by oxidation of D-luciferin (34) to oxyluciferin (35) that is assumed to involve a highly energetic dioxetanone intermediate (Scheme 5.24).504 The emission wavelength depends on the medium, pH and temperature. The origin of the pH dependence is far from understood.518 An enol tautomer of 35 may be involved.

Scheme 5.24 Luminescent reaction of fireflies

5.7 Problems

1.The pKa of phenalene ionizing to its anion 4 (Case Study 4.1) is about 20. The first absorption band of phenalene lies at n~0 0 ¼ 2.9 mm 1. Predict the acidity constant of phenalene in the lowest excited singlet state. [pKa ¼ 1]