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5 Radical—Radical Transformations at Reencounters

5 Radical—Radical Transformations at Reencounters

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RADICAL—RADICAL TRANSFORMATIONS AT REENCOUNTERS



V-a



197



V-b

e = 35.9

e = 17.5



e = 16.0



e = 4.7



FIGURE 9.5 Polarity-dependent polarization patterns in photosensitized hydrogen abstractions from triethylamine DH (sensitizer A, 9,10-anthraquinone). For the formulas, see Chart

9.3. Shown are the signals of the olefinic a and b protons of the product N, N-diethylvinylamine,

V-a (6.05 ppm) and V-b (3.45 ppm), as functions of the relative permittivity e (given at the

right). Top, pure acetonitrile-d3; bottom, pure chloroform-d3; other traces, mixtures of these two

solvents. All spectra were normalized with respect of the absolute amplitude of V-a. Further

explanation, see text.



The occurrence of the radical ion pattern in a polar solvent indicates that the

photoreaction involves an electron transfer step

A* ỵ DH ! A



.







ỵ DH



.







9:7ị



under these circumstances. However, a direct reaction of the radical cation DH. ỵ to

give the vinylamine V is chemically inconceivable; the direct precursor to V must be

the neutral radical D. . Hence, a deprotonation of DH. ỵ yielding the conjugated base

D. is a key step of the electron transfer-induced formation of V.

Extensive studies of the sensitizer dependence30 and the solvent dependence31 of

the polarization patterns led to the identification of two parallel pathways of that

deprotonation. One is a proton transfer within the spin-correlated radical pairs, with

the radical anion A. À acting as the base. The other is a deprotonation of free radicals,

in which case the proton is taken up by surplus starting amine DH. Furthermore,

evidence was obtained from these experiments that even in those situations where the

polarization pattern suggests a direct hydrogen abstraction according to Equation 9.6

these reactions proceed as two-step processes, electron transfer (Eq. 9.7) followed by

deprotonation of the radical cation by either of the described two routes. The whole

mechanism is summarized by Chart 9.3 for triethylamine as the substrate. Best suited

for an analysis is the product V.



198



CHEMICAL TRANSFORMATIONS WITHIN THE PARAMAGNETIC WORLD



DH:







•+



DH • + :



N



D• :



N



Pattern # (



)



V:



N



Pattern Đ (



N



)

O



A=

O

3A*



1



A



+ DH



DH DH+2



3





+



DH







A



+



#



DH



3



AH D



AH D



#



DH

+

A



kdep

1



+



Đ



D

+

AH



Đ



D



#



V



V



CHART 9.3



As in the preceding section, a pair substitution is central to the scheme, and there are

again two limiting cases depending on the rate of that step relative to the spincorrelated life: If in-cage deprotonation is negligibly slow on the CIDNP timescale, the

second radical pair is not formed, so V results from deprotonation of the radical cations

after escape from the first pair; the escaping radical cations, and hence the product V,

exclusively bear the polarization pattern of DH. ỵ . If, on the other hand, in-cage

deprotonation is extremely fast on the CIDNP timescale, the first radical pair has no

chance of developing polarizations before it is transformed into the second radical pair,

so CIDNP develops only in the latter pair; therefore, the escaping neutral radicals

exclusively bear the polarization pattern of D. , which is then carried over to the product

V. This example illustrates the power of CIDNP: Although the same product, V, can be

formed by two parallel routes, a clear distinction is possible because these routes yield

different polarization patterns.

In contrast to the PET sensitizations discussed in the preceding section, the

concentrations do not influence the rate constant of the in-cage proton transfer,

kdep, because that key step involves the radical pair as an entity, so is a first-order

process. However, kdep can be changed by varying the thermodynamic driving force.

With different sensitizers,30 this can be effected only in rather large steps, but a much

finer gradation can be realized by the relative permittivity of the reaction medium,31,32

which only influences the free energy of the radical ion pair but leaves that of the pair of

neutral radicals essentially unaffected.



INTERCONVERSIONS OF BIRADICALS



199



FIGURE 9.6 Photosensitized hydrogen abstraction from triethylamine DH by 9,10-anthraquinone A (for the formulas, see Chart 9.3). The rate constant of in-cage deprotonation kdep, as

obtained from the polarity pattern, is shown as a function of the relativity permittivity e of the

reaction medium (mixtures of acetonitrile and chloroform). The timescale of the CIDNP effect

provides a kinetic window, within which such a quantitative treatment is applicable. Further

explanation, see text.



An interesting threshold behavior is found. Within a transition range from about

À90 kJ/mol to about À130 kJ/mol, the polarization pattern completely changes from

that of the radical ion pair to that of the pair of neutral radicals. In-cage deprotonation is

thus negligibly slow compared to the “natural” life of the radical ion pair on the more

positive side of this range and too fast to be observable on the more negative side. Two

spectra within that range have also been included in Fig. 9.5 and serve to illustrate the

gradual transmutation of the patterns.

Within the kinetic window provided by the transition regime, the pair substitution

process is amenable to a quantitative analysis. Again, the equations can be converted to

a form that contains the parameters of the two radical pairs implicitly and expresses

them by the polarization patterns in the two limiting cases.32 The use of the patterns,

that is, polarization ratios, has the advantage that all factors influencing the absolute

size of the polarizations cancel. Figure 9.6 shows the result for the sensitizer 9,10anthraquinone. It is seen that the kinetic window spans a chemically very important

range, namely, from about the diffusion-controlled limit to about two orders of

magnitude below that limit.



9.6 INTERCONVERSIONS OF BIRADICALS

As far as CIDNP is concerned, the most important difference between radical pairs and

biradicals is that the interdiffusion of the paramagnetic centres is restricted in the latter

case. This has two implications.



200



CHEMICAL TRANSFORMATIONS WITHIN THE PARAMAGNETIC WORLD



First, the exchange interaction often does not fall off to zero. On the one hand, this

reduces the efficiency of intersystem crossing between |Si and |T0i. On the other hand,

it opens up another intersystem crossing pathway, namely, between |Si and |T1i (or,

rarely, |T ỵ 1i) because the potential energy curves of these states intersect at some

point, and the system spends more time in that region if diffusion is not free.

Second, there is usually no escape channel because the two radical termini cannot

separate completely. This usually causes a fundamental difference between CIDNP

from radical pairs and CIDNP from biradicals: Being nuclear spin sorting as described

above, intersystem crossing between |Si and |T0i crucially relies on both exit channels

leading to different products, whereas intersystem crossing between |Si and |TỈ1i

occurs by simultaneous electron–nuclear spin flips, and so creates net nuclear

polarizations without the need of different exit channels.

Hence, in most cases, biradicals do not exhibit radical-pair type CIDNP but S–TÀ1type CIDNP. The two variants are easily distinguishable because the former gives rise

to both absorptive and emissive polarizations with, ideally, a grand total of zero

(compare Fig. 9.3), while the latter manifests itself by the same phase, usually

emission, of all CIDNP signals.

The 1,5-biradical BR1 formed during the Paterno–B€uchi reaction of excited benzoquinone B with quadricyclane Q (for the formulas, see Chart 9.4) provides one of the

extremely rare examples of a short-chain biradical that produces CIDNP of the radical

pair type.33 Extracts of the CIDNP spectra are displayed in Fig. 9.7. The occurrence of

both absorption and emission in the same product is clear evidence for this mechanism

of polarization generation.



CHART 9.4



201



INTERCONVERSIONS OF BIRADICALS



H1

P1



H7



H7¢

P2

H7¢



H7



H1



3



2

ppm



1



FIGURE 9.7 CIDNP effects in the Paterno–B€

uchi reaction of benzoquinone B with quadricyclane Q to give the oxolane P1 and the oxetane P2. Shown are the most strongly polarized

signals, of H1, H7, and H70 , in the two products. The formulas and the assignment are given in

Chart 9.4. For clarity, the spectra of P1 and P2 have been separated, and all other signals have

been blanked. Further explanation, see text.



The reaction affords two products, an oxolane P1 and an oxetane P2, which exhibit a

mirror-image relationship of their CIDNP patterns. The three most strongly polarized

signals, of H1, H7, and H70 , with intensity ratios of about 2 to ỵ 3 to ỵ 3.5, have been

shown in the gure; all the other protons are also polarized, but more weakly. The

observed pattern is found to be in excellent agreement with the relative proton

hyperfine coupling constants of the neutral benzosemiquinone radical and of the

tert-butoxybicyclo[2.2.1]heptenyl radical, which were tested as model compounds

for the two radical moieties.33 The biradical BR1 is thus the source of the polarizations.

It is formed in a triplet state, its singlet exit channel produces the oxolane P1, and its

triplet exit channel the oxetane P2.

Evidently, P1 is obtained from BR1 by a straightforward combination of the two

radical centres, but an extensive skeleton rearrangement must occur on the route to the

product P2. Because it is natural to assume that no carbon–hydrogen bonds are broken

in that process, the polarization of each proton serves as a label of the carbon atom it is

attached to. The mechanism displayed in Chart 9.4 sums up the result, and identifies

the structural changes as a cyclopropylmethyl–homoallyl rearrangement of the

quadricylcane-derived moiety.

Both the biradical with the cyclopropylmethyl structure, BR2, and that with the

other homoallyl structure, BR3, are too short lived for CIDNP generation, the former

because of steric strain and the latter because it is a 1,4-biradical of the Paterno–B€uchi

type, that is, a structure that is known to undergo fast and efficient intersystem crossing

without the participation of the nuclear spins. The latter property explains not only the

absence of any polarizations from BR3 but also the fact that S–T0-type polarizations



202



CHEMICAL TRANSFORMATIONS WITHIN THE PARAMAGNETIC WORLD



–S (P2) / S (P1)

1.0



0.5



0

240



260



280

T (K)



300



320



FIGURE 9.8 Ratio ÀS(P2)/S(P1) of the CIDNP signals of H7 in the two products P1 and P2

formed by the Paterno–B€uchi reaction of benzoquinone with quadricyclane as function of the

experimental temperature T. Further explanation, see text.



from BR1 are observable at all in that system: because BR3 acts as a chemical sink, the

rearrangement of BR1 to give BR3 provides the analog to an escape reaction; hence,

those nuclear spin states that decrease the rate of intersystem crossing in BR1 are

enriched in BR3, and thus in P2, while those that increase that rate end up preferentially

in P1.

As opposed to the previous examples, the rate of the “pair substitution” BR1 Ð

BR2 Ð BR3 can be varied by neither the reactant concentrations nor the solvent

polarity because it is intramolecular and only involves neutral species. However, the

ratio of polarizations of corresponding protons in P1 and P2 exhibits a pronounced

temperature dependence,34 which is shown in Fig. 9.8 and can be explained in the

following way. Ideally, these opposite polarizations should have exactly equal

magnitudes, but their ratio deviates from À1 if nuclear spin relaxation in the

paramagnetic intermediates is taken into account. Biradicals with nuclear spin states

that slow down intersystem crossing of BR1 live longer, so their nuclear spins suffer a

stronger relaxation loss.

The rate-limiting step of the complicated biradical interconversions is the

formation of the three-membered ring, that is, the transformation of BR1 into

BR2, which follows an Arrhenius law. The known temperature dependence of

dipolar relaxation provides a gauge against which the activation energy of the

rearrangement can be measured. Again, a polarization ratio is analyzed, so all other

influences on the absolute CIDNP intensities—in particular, the temperature dependence of the CIDNP effect itself—cancel. By setting up an appropriate kinetic

model, the relative rates of intersystem crossing of BR3 and BR1 (about 5 : 1), the

rate of formation of BR2 relative to intersystem crossing of BR1 (about 4 : 1), and the

activation energy of the transformation BR1 ! BR2 (17 kJ/mol) are obtained from

these experiments.34



REFERENCES



203



9.7 CONCLUSIONS

These case studies illustrate the power of CIDNP spectroscopy. Short-lived paramagnetic intermediates can be identified because their EPR spectrum remains frozen in as a

polarization pattern of the nuclear spins in much longer-lived secondary species, and

the pathways of their subsequent reactions can be traced out because these polarizations behave as nearly ideal labels. As the examples have shown, transformations

of radical pairs into other radical pairs, with or without the participation of a third

molecule as a scavenger, and transformations of biradicals can all be investigated

by this method, which yields information that is often inaccessible by other

techniques.



REFERENCES

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2.

3.

4.

5.

6.

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Goez, M. Annu. Rep. NMR Spectrosc. 2009, 66, 77–147.

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Ward, H. R.; Lawler, R. G. J. Am. Chem. Soc. 1967, 89, 5518–5519.

Kaptein, R.; Oosterhoff, L. J. Chem. Phys. Lett. 1969, 4, 195–197.

Closs, G. L. J. Am. Chem. Soc. 1969, 91, 4552–4554.

Eckert, G.; Goez, M.; Maiwald, B.; Mueller, U. Ber. Bunsen-Ges. 1996, 100, 1191–1198.

Goez, M.; Rozwadowski, J.; Marciniak, B. J. Am. Chem. Soc. 1996, 118, 2882–2891.

Roth, H. D. Z. Phys. Chem. 1993, 180, 135–158.

Goez, M.; Rozwadowski, J.; Marciniak, B. Ang. Chem., Int. Ed., 1998, 37, 628–630.

Eckert, G.; Goez, M. J. Am. Chem. Soc. 1994, 116, 11999–12009.

Eckert, G.; Goez, M. J. Am. Chem. Soc. 1999, 121, 2274–2280.

Ward, H. R. Acc. Chem. Res. 1972, 5, 18–24.

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Pedersen, J. B. J. Chem. Phys. 1977, 67, 4097–4102.

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CHEMICAL TRANSFORMATIONS WITHIN THE PARAMAGNETIC WORLD



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10

SPIN RELAXATION IN

Ru-CHROMOPHORE-LINKED

AZINE/DIQUAT RADICAL PAIRS

MATTHEW T. RAWLS1, ILYA KUPROV2, C. MICHAEL ELLIOTT3,

4

AND ULRICH E. STEINER

1



National Renewable Energy Laboratory, Golden, CO, USA

Chemistry Department, University of Durham, Durham, UK



2

3



Department of Chemistry, Colorado State University, Fort Collins, CO, USA



4



Fachbereich Chemie, Universit€at Konstanz, Konstanz, Germany



10.1 INTRODUCTION

Photosynthesis is the inspiration for all efforts to harvest solar energy ranging from

solar cells to light-induced water splitting. Historically, synthetic molecular systems

that undergo light-induced electron transfer reactions have featured prominently in

efforts to functionally mimic photosynthesis.1–9 Like the natural system, many of the

intermediates and transient products of these light-induced electron transfer reactions

are radicals; thus, they are subject to spin-chemical effects.

While artificial photosynthetic mimics come in many manifestations, our efforts

have focused predominantly on the class of molecules represented by the structure in

Fig. 10.1. These molecules consist of a visible-light-absorbing chromophore in the

form of a trisbipyridineruthenium(II) complex (C) linked by flexible polymethylene

chains to one or more electron donors (D) and an electron acceptor (A). The electron

acceptor is an N,N0 -dialkylated-2,20 -bipyridine (a so-called “diquat”); and the electron

donors are N-alkylated phenothiazines. The diquat type acceptor was chosen because



Carbon-Centered Free Radicals and Radical Cations, Edited by Malcolm D. E. Forbes

Copyright Ó 2010 John Wiley & Sons, Inc.



205



206



SPIN RELAXATION IN Ru-CHROMOPHORE-LINKED AZINE/DIQUAT RADICAL PAIRS



X

N







4[PF6 ]

N

N



II

Ru



N



N



N

N



X



N

N

+

+

N



FIGURE 10.1 Donor–Chromophore–acceptor triad. X ¼ a chalcogenide atom: oxygen,

sulfur, or selenium.



it is possible to tune its redox potential by almost a half a volt by simply altering the

length of the alkyl chain connecting to two quaternary nitrogens. Three different but

closely related donors are considered in the following discussions: phenothiazine

(PTZ), phenoxazine (POZ), and phenoselenazine (PSZ). Finally, the fact that the

structure represented in Fig. 10.1 contains two rather than a single donor moiety is a

matter of synthetic expediency and is of no functional consequence. Subsequently, we

will refer to structures analogous to the one in Fig. 10.1 as DCA triads.

In the ground state, these DCA triads are singlets. Upon irradiation with visible

light, a metal-to-ligand charge transfer (MLCT) transition occurs in which an electron

from a metal-based d-orbital is transferred to a pà orbital on one of the bipyridine rings.

In less than a picosecond, this state undergoes intersystem crossing to yield a triplet,

3

MLCT.10 It is from this 3 MLCT state that a series of intramolecular electron transfer

steps ensues leading ultimately to the charge separated state (CSS). The steps in this

electron transfer cascade are first the oxidative quenching of the 3 MLCT state by

the diquat acceptor followed by transfer of an electron from one of the donors to the

oxidized chromophore; thus, in the resulting CSS the acceptor is reduced, one of the

donors is oxidized and the chromophore is returned to the ground state.11 This specific

class of DCA triads is quite unusual in that the CSS state is formed with essentially

unity quantum efficiency irrespective of the identity of the specific azine donor (i.e.,

PTZ, POZ, or PSZ) or diquat acceptor.11 Detailed models of the energetics and kinetics

of the CSS formation and decay back to the ground state in these specific DCA triad

systems have been provided earlier.11–21



207



INTRODUCTION



Of particular relevance to the present discussion is the observation that the CSS,

which is a biradical cation, is formed with essentially pure triplet spin correlation.12

For energetic reasons, this triplet radical pair cannot recombine to form the 3 MLCT

state and can only form the singlet ground state. Therefore, direct recombination is

spin forbidden. Moreover, because the radical pair which constitute the CSS

product can separate only to a limited distance, essentially every CSS recombination event is between the same geminate radical pair—in other words, every reduced

acceptor is ultimately oxidized by the donor radical cation that was formed from the

same initial photochemical event. The spin behavior of the DCA triad CSS can be

effectively explained by application of the relaxation mechanism of Hayashi and

Nagakura.22

Scheme 10.1 shows how this model is applied to the DCA triad CSS under

consideration.12,23 The reduced diquat acceptor and oxidized azine donor are



“normal” organic radicals that can physically separate within CSS to at least 20 A.

3

1

Therefore, at zero applied magnetic field, the triplet CSS ( CS) and singlet CSS ( CS)

are nearly degenerate and isotropic hyperfine coupling provides a path to mix these

states allowing for CSS decay. In the zero field case, this mixing and transition occur

rapidly relative to the singlet decay (ks). Upon application of a magnetic field, Zeeman

splitting of the 3 CS occurs yielding three energy states: a magnetic field independent

T0 state and eld dependent T and T ỵ states. The path to the ground state for the T0

state remains unchanged while at small fields the splitting of the field dependent states

exceeds the hyperfine coupling energy, thus diminishing singlet–triplet mixing from

those states and retarding their rate of decay to the ground state.12

The model in Scheme 10.1 suggests that application of a magnetic field to the CSS

should result in biexponential decay kinetics consisting of a field independent (T0)

portion along with a field dependent portion (T and T ỵ ). Within the CSS of these

DCA triads the oxidized donor and the reduced acceptor each exhibit intense, isolated



3CS(T +)



k’r,1

1CS



kS,T0



3CS



1CS



kS,T0



kr,1



0



kr,1

k ’r,1



kS



Zero field



kT



kS



Ground state (S 0)



3CS(T



3CS(T _)



Nonzero field



Ground state (S 0)



SCHEME 10.1



)



kT



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