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Quantum-chemical and experimental investigations of photochromic transformations in quinone compounds
Journal of Molecular Structure (Theochem), 181 (1988) 285-296 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
285
QUANTUM-CHEMICAL AND EXPERIMENTAL INVESTIGATIONS OF PHOTOCHROMIC TRANSFORMATIONS IN QUINONE COMPOUNDS
N.P. GRITSAN Institute of Chemical Kinetics and Combustion, Novosibirsk 630090 (U.S.S.R.) (Received
13 August 1987)
ABSTRACT Analysis of quantitative information on photochemical migration of hydrogen atom, proton, acylic and arylic groups in anthraquinone derivatives obtained by experimental and quantumchemical methods has been performed. The migration of acyl is found to occur in the xx* state and to be thermally reversible. The concepts of conservation of orbital symmetry in the photochemical process are applied to account for the relation of the compound reactivity investigated to the chemical structure. Data from quantum-chemical calculations and experimental spectroscopy are used to construct energy correlation diagrams. The “nn* state is found to be reactive in hydrogen photochemical migration. Aryl migration proceeds non-adiabatically without conserving the excitation. The dependence of rate constants of thermal migration of hydrogen atom and acylic group on substituent nature, medium and temperature is studied. A quantitative theoretical interpretation of the results based on calculations by the PPP method is proposed.
INTRODUCTION
Intramolecular photochemical migrations of proton, hydrogen and more complex atomic groups refer to one of the simplest types of photochemical reactions of complex organic compounds. Studying such processes is of great interest as it allows the understanding of qualitative and quantitative peculiarities of these reactions to be applied to the basic statements of theoretical chemistry (the principle of orbital symmetry and spin multiplicity conservation, theoretical models treating the photochemical reactions as the processes of radiationless deactivation of excitation, etc.) [l-5]. The present paper is concerned mainly with the reactions of photochemical and dark migrations of hydrogen, phenylic and acylic groups in anthraquinone derivatives; these have been the object of several of our experimental studies (see, for example, refs. 6-10).
0166-1280/88/$03.50
0 1988 Elsevier Science Publishers
B.V.
286 EXPERIMENTAL
The basic tools used are flash photolysis with microsecond resolution, the low-temperature technique permitting the registration of spectral characteristics of intermediates stabilised at 77 K. The experiments have enabled us to determine the nature of intermediates, the rate constants for their transformation, the quantum yields of formation and, besides, to follow the dependence of all the quantitative characteristics of the processes on chemical structure. The absorption spectra of stable compounds and conjectural intermediates, the distribution of electron density and the enthalpy of reactions are calculated quantum-chemically using the Parise-Parr-Pople method with conventional Nishimoto-Forster or Dewar parameterizations [ 11,121. If necessary, the calculations are performed by AM1 and MNDO techniques [ 13,141. RESULTS AND DISCUSSION
Photochemical migration of hydrogen and proton has been at the center of numerous theoretical and experimental studies. Thus photoenolization of orthe-alkylketones and proton transfer in salicylates has been studied fairly extensively [15-171. Photochemical migration of hydrogen in orthomethylquinones was discovered during the last decade [6,18]. Migration of heavy atomic groups is still not clearly understood. Recently we [8] recorded the photochemical, thermally reversible, migration of acylic group for a series of methoxy and amino derivatives of l-acetoxy9,10-anthraquinone and non-substituted 9-acetoxy-1,4-anthraquinone. The migration of an acylic group fails with irradiation of the simplest compounds cofa given type, namely 1-acetoxy-9,10-anthraquinone and some of its derivatives. An analysis of photochemical behavior of all compounds investigated provides the following picture for the dependence of photoactivity on quinone structure: (i) 1-Acetoxyanthraquinone and a series of its derivatives of slightly different optical absorption spectra than that of non-substituted anthraquinone do not transform under irradiation into corresponding derivatives of 9-acetoxyl,lO-anthraquinone. The triplet and singlet states of type nti are the lowest excited states of these compounds and of anthraquinone. In alcohols the compounds with high quantum yield are reduced to appropriate hydroquinones at room temperature. (ii) 2-Methoxy-2-amino- and 4-methoxyderivatives of 1-acetoxy-9,10-anthraquinone and 9-acetoxy-1,4-anthraquinone with high quantum yields are isomerized under irradiation into the corresponding l,lO-anthraquinone derivatives due to photochemical migration of acylic group. The spectra of these compounds exhibit a longwave band of the xx* type. Calculations show the triplet and singlet of this type to be the lowest excited states.
(iii) There exists a third group of compounds, namely the 3-amino and 4amino derivatives of 1-acetoxy-9,10-anthraquinone. Under irradiation they do not transform photochemically. The triplet and singlet of the type XX* are the lowest excited states of these compounds. The properties enumerated may be readily understood and accounted for by correlation diagrams assuming that the phototransfer of an acylic group proceeds adiabatically with excitation and symmetry of wavefunctions conserved. The energy diagrams for the compounds studied have been constructed as follows: (i) The position of the ‘XX* levels was estimated from the absorption spectra with the O-O band 2800 cm-l lower than the maximum one [ 191. (ii) The energy of the %R* state was deduced from the E (m*) experimental value of 23800 cm-’ for anthraquinone and a linear correlation between E (m* ) and the energy of the lower free x*-orbital, calculated quantum chemically [20]. The corrections do not exceed 1600 cm-l. (iii) The single-triplet splitting between the m* type states was assumed to be 2000 cm-l as for anthraquinone. (iv) The singlet-triplet splitting between the xx* type states is taken to be equal to 6000 cm-‘. The value is typical for the quinone derivatives and is deduced from ref. 21. (v) While constructing the energy diagrams for hypothetical products we have used the experimental spectra of the corresponding 9-phenoxy-l,lO-anthraquinone derivatives as the spectra of 9-phenoxy- and 9-acetoxy-l,lO-anthraquinones are similar. (vi) The difference in the energy of ground states of the initial compounds and products has been calculated using the quantum-chemical procedures. With the energy diagrams constructed by the above technique, we have correlated the levels of the initial compounds and photomigrated products by the symmetry of wavefunctions. The only symmetry element in the system under study is a plane of aromatic rings. The wavefunctions of 0 and x electrons display a different symmetry relative to this plane. Thus the excited r-m*states of the initial compounds correlate with those of the m* type. The xx* type states of the initial compounds correlate with the product states of the type zx*. Adiabatic transfer of the acylic group is feasible if the lowest excited states correlate with the state of the product lower in energy, and vice versa. The adiabatic reaction is impossible providing the correlating product lies higher in energy. Figures l-3 demonstrate three types of correlating diagrams for photochemical migration of acylic group in 9-acetoxy-1,4_anthraquinone, 1-acetoxy-9,‘10anthraquinone and 1-acetoxy-4-hexilamino-9,10-anthraquinone. These compounds belong to the above mentioned three types. A favorable position for the correlating levels of acylic adiabatic migration is attained only in the case of 9-acetoxy-1,4-anthraquinone (Fig. 1). For two other compounds the adi-
288
40
J. Ii” cm
20
‘nQ*_ -3fi*
-lnT* ?-W-!l&
~_‘~L-
3!m*-
-
20 -%!lt”
SO 0
SC.
s
SO
Fig. 1. State correlation diagram for the allowed photochemical acetoxy-1,4_anthraquinone. Fig. 2. State correlation diagram for the forbidden 1-acetoxy-9,10-anthraquinone.
migration
of acetoxy group in 9-
photochemical
migration
of acetoxy group in
photochemical
migration
of acetoxy
40 t
J-1 o-,3
-1 cm
20
-
‘!Rst”---k-CR* *
-3P7z*
W-
Fig. 3. State correlation diagram for the forbidden 1-acetoxy-4-hexylamino-9,10-anthraquinone.
group in
289
abatic migration does not hold because of energy or symmetry considerations (Figs. 2, 3). In all compounds investigated, isomerization under light occurs only in the cases of the first type correlation diagrams. The non-photoactive anthraquinone derivatives possess diagrams of the third type. In the case of the second type diagrams, migration of the acylic groups fails but reduction to the corresponding hydroquinones in alcohols is feasible at room temperature. The process of photochemical migration of the acylic group has much in common with photochemical reaction of proton transfer via intramolecular hydrogen bond. Such reactions are the best studied and best known adiabatic photochemical processes. The adiabatic reaction of proton transfer was first observed in 1955 by Weller [ 221. Such processes are also known for anthraquinones and manifest themselves in a large Stokes shift of luminescence spectrum or in the presence of dual luminescence which is more characteristic for anthraquinones with intramolecular hydrogen bond [ 231. Existence of a strong intramolecular hydrogen bond, however, does not guarantee proton transfer at excitation. For example, in the case of 1-oxy-, l-amino- and 1,4-dioxyanthraquinones this reaction is not observed [ 23-261. Such a result is probably caused by the fact that for these anthraquinones the excited singlet state of the ketoform lies higher in energy and the adiabatic process proves impossible. Our energy diagram for these compounds verify this hypothesis qualitatively. Figure 4 exemplifies a diagram for 1-oxyanthraquinone. Proton transfer is detected in l+dioxyanthraquinone [ 231 and in this case the rate constant is very high. Only its lower estimate 2 1012 s-l is presented. Unfortunately, in the case of the acylic group transfer studied the adiabatic
0
0
Fig. 4. State correlation diagram for proton transfer in 1-oxyanthraquinone.
290
mechanism of migration cannot be confirmed as easily as in the case of proton transfer. The compounds in question and their photolysis products do not luminesce. We have failed to identify the reaction state multiplicity. However, the migration is likely to occur in a triplet state. There is some indirect evidence for this assumption. Photochemical reaction of phenylic group migration in peri-aryloxy-p-quinones was first reported in 1971 [ 271. The few available studies on this mech,anism [ 28,291 indicate a triplet nature of the reaction state. In contrast to the above migration processes of proton and acylic group, the phenylic group transfer is photochemically reversible. The quantum yields of direct and reverse photoreactions for a series of 1-aryloxy-9,10-anthraquinone derivatives investigated are closely related and amount to 0.03-0.09. The diagrams for the energy levels constructed using the above technique show that the two kinds of position for the lowest excited zx* type states are realized for the compounds studied (Figs. 5,6). A different position of the levels does not, however, affect the magnitude of photolysis quantum yields of these compounds. The forma-
‘nfi?
3
nllt*
‘rfR*
‘JS” 3nR* 20 -
‘f%.”
3E3r* d* 1g3 cm
33&
Fig. 5. Diagram of energy levels for 1-phenoxy-2-acylamino-9,10-anthraquinone of its photolysis.
and the product
291
10
I
SO
0
A0 0
0
a0
‘p= 0.09
[II>
0
zIIzIz OCH3
(9=0*02
Fig. 6. Diagram of energy levels of 1-phenoxy-4-methoxy-9,10-anthraquinone and its photolysis product.
tion of an intermediate spiroform in a triplet state has been assumed recently [ 281. This assumption is, however, poorly argumented and needs further experimental study of the mechanism of peri-aryloxyanthraquinone photolysis. It is only clear that this process is nonadiabatic, qualitatively different in mechanism from those for photochemical migration of proton and acylic group. Now we consider briefly the fourth type of migration photochemical processes, the hydrogen transfer in ortho-alkylquinones [ 6,7,18]. The latter is close in mechanism to photoenolization of ortho-alkylketones which exhibits the following characteristics [ 16,171: (i) the photoenolization of ortho-alkylketones occurs in both singlet and triplet states of the r-m*type; (ii) the constants for hydrogen atom transfer in the r-m* triplet states are KHN log+lo’o s-l; (iii) the formation of the triplet state of the enol form is registered, the enol triplet lifetime depends on medium and may vary from N 100 ns to some ,LLS.
The triplet states of enol form convert into a ground enol state; oxygen accelerates the rate of conversion. Our estimates of the constant for hydrogen transfer in l-methylanthraquinone indicate that the constant is high, K? 10’ s-l. The formation of an intermediate, 9-oxy-l,lO-anthraquinone-1-methide, in a triplet state has been registered recently [ 301 in the case of 1-methylanthraquinone. Hence the mechanism of o&ho-alkylquinone photolysis is analogous to that for orthoalkylketone photolysis. The situation in the case of photoenolization of quinones and ketones is quite different compared with the above processes. In all three cases the quantity of 0 and rc electrons in the initial compound and in its photoinduced form was the same. In the case of photoenolization, a keto form (p-quinone) has two o electrons more than the enol form (ana-quinone) . Hence two a electrons convert into two x electrons in the course of the photochemical process. As for the symmetry relative to a molecule plane of the full wavefunction, the ground states of both p-quinone (ketone) and ana-quinone (enol) are symmetrical. However, the symmetry of wavefunctions of separate electrons changes. In the case of hydrogen transfer (photoenolization) the nn* type state appears to be reactive by contrast to the n?r* type states in the first three cases. As shown, the photoenolization in a triplet state proceeds via enol formation (or anu-quinone) in a triplet state. The process may be represented by the scheme
Q
hv
na + mn
e
Tn-I_)0
(m+l)rr
+
3Q*
(n_l)a
(m+l)n
I--trLLer
3AQ* (n-2)0 (m+2)7c
+AQ (n-2)0 (m+2)n
The symmetry of the full wavefunction is seen to be broken during the process. Nevertheless, the process is highly efficient, K- 10’ s-l and a feasible reason for this is that the primary process defining the rate is the formation of (a, x) biradical in a triplet state. HO H\C/H
3“R *
3m
*
In this case the principle of wavefunction symmetry conservation is not violated in hydrogen transfer. An intermediate (a, n) biradical relaxes into a triplet enol state whose absorption is registered experimentally. It is noteworthy that in the literature there exists some obscurity in referring the intermediate absorption to the triplet state of either enol or biradical [ 30,311. A correlation diagram for such a type is presented in Fig. 7. Such a diagram has been suggested earlier [ 171 for the process of ortho-alkylketone photoenolization.
293
SO
0
Fig. 7. State correlation methylanthraquinone.
0
diagram
for
the
photochemical
hydrogen
transfer
in
l-
We have verified experimentally [ 71 the fact that in hydrogen transfer both the nr* type states and those of the type xx* may be reactive; the value of quantum yield, however decreases substantially. Particularly drastic differences are observed in the photolysis at 77 K [ 71. In this very case a detailed mechanism has not yet been ascertained. All these processes of photochemical migration of atoms and atomic groups are reversible. A phenyl group migration is photochemically reversible. The reverse migration of hydrogen atom and acylic group in anthraquinones proceeds only in a thermal fashion. The mechanism of dark migrations of hydrogen and acyl displays both common and different features. The dependences of rate constants of these processes on medium, substituents and temperature are different. The kinetic curves of absorption decay of the short-lived photolysis products of l-methyl- and 1-acyloxy-9,10-anthraquinone derivatives are described using the exponential time dependence. The characteristic time of absorption decay in all cases is independent of the initial reagent concentration and of the dissolved oxygen. In the case of 9-acyloxy-l,lO-anthraquinone derivatives the rate constant of dark transformations is actually independent of solvent nature. In contrast, the rate constant for the dark migration of hydrogen atom in quinones largely depends on solvent nature and drops by - 3 orders of magnitude from hexane to alcohols. As for quinonemethides, the rate of the reverse dark migration is high even at 77 K in non-polar solvents so that the hydrogen transfer has not been registered in methylhexane at 77 K by stationary meth-
294
ods. Such a difference in the dependences of process rates is satisfactorily interpreted on the basis of calculations of dipole moments of the initial compounds and the products of dark transformations. In the case of l-acetoxyanthraquinones, the dipole moments of the initial and final states are similar, that of the intermediate state is, probably, also close to them. In the case of dark hydrogen migration the initial state is quinonemethide, a polar compound (d N 4 D), and the product, l-methylanthraquinone, exhibits a very small dipole moment. Hence the dipole moment of the intermediate state is less than that of the initial one and the process is strongly inhibited in polar solvents. The rough estimates for solvent action according to the Onsanger model are in satisfactory agreement with experiment. The rate constant for the acylic group dark migration is practically independent of the substituent nature in the 9,10-anthraquinone nucleus but changes by two orders of magnitude in passing from 9,10-anthraquinone to 1,4-anthraquinone. Such a difference is accounted for by the change in the reaction enthalpy. However scanty the experimental data, one may plot a correlation log K with the enthalpy calculated quantum-chemically (Fig. 8). The tangent of the dependence is close to an analogous Polyani-Semenov dependence for the reactions of hydrogen abstraction by radicals. The temperature-dependence of the rate constant of acyl migration in 9-phenoxy-4-acetoxy-l,lO-anthraquinone is measured. A preexponential factor is small (2 x lOlo s-’ ) and testifies to a higher rigidity of the intermediate state. Such values are characteristic for
5.0 -
3.0 1
I 25
I 50 dti
colt 9
I 75
kJ
Fig. 8. Acyl migration rate constant dependence on enthalpy of reaction.
Fig. 9. Acyl migration rate constant dependence on the nature of migrant substituent.
the reactions with a six-member transfer in quinone-methides, the preexponential The 0-acyl group nature 0
state. For those of hydrogen transfer factor equals 7 x 1011+ 5 x 10” s-l.
(4-R) affects substantially the rate of the acylic dark migration [lo]. Increasing electron-donor ability of the R substituent, decreases the rate of rearrangement from 2.5~10~ s-’ (R=CH3) to 3.1~10~ s-’ (R=OC2H5) and 3.3 s-l (R = N ( CHB ) 2). An arylic nucleus involved in migration, levels the electron influence of the substituents considerably. Figure 9 demonstrates a linear correlation of the rate constant logarithm with the electrophilic constants of substituents. Constant p proved to be equal to 0.60; its sign is positive and agrees with the fact that the higher the positive charge on a migration carbon atom, the higher the process rate. ACKNOWLEDGEMENT
The author expresses her gratitude experimental investigations.
to Dr. L.S. Klimenko
for aid in all the
296
REFERENCES
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
W.G. Dauben, L. Salem and N.J. Turro, Act. Chem. Res., 8 (1975) 41. L. Salem, J. Am. Chem. Sot., 96 (1974) 3486. J. Jortner, S.A. Rice and R.M. Hochstrasser, Adv. Photochem., 7 (1969) 149. A. Heher, Mol. Photochem., 1 (1969) 257. S.J. Formosinho, J. Chem. Sot., Faraday Trans. 2,72 (1976) 1332. N.P. Gritsan, V.A. Rogov, N.M. Bazhin, V.V. Russkikh and E.P. Fokin, Teor. Eksp. Khim., 15 (1979) 290. N.P. Gritsan and N.M. Bazhin, Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk, 3 (1979) 138. S.A. Russkikh, L.S. Klimenko, N.P. Gritsan and E.P. Fokin, Zh. Org. Khim., 18 ( 1982) 2224. N.P. Gritean, S.A. Russkikh, L.S. Klimenko and V.F. Plyusnin, Teor. Eksp. Khim., 19 (1983) 455. L.S. Klimenko, N.P. Gritsan, A.V. Konstantinova and E.P. Fokin, Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk, 6 (1984) 84. K. Nishimoto and L.S. For&r, Theor. Chim. Acta, 14 (1969) 80. M.J.S. Dewar and N.L. Sabehi, J. Phys. Chem., 661 (1962) 2310. M.J.S. Dewar and W. Thiel, J. Am. Chem. Sot., 99 (1977) 4899,4907. M.J.S. Dewar,E.G. Zoebisch,E.F. HeaIyand J.J.P. Stewart, J. Am. Chem. Sot., 107 (1985) 3902. W. Klijpffer, Adv. Photochem., 10 (1977) 311. C.V. Cumar, S.K. Chattopadhyay and K.P. Das, J. Am. Chem. Sot., 105 (1983) 5143. R. Haag, J. Werz and P.J. Wagner, Helv. Chim. Acta, 60 (1977) 2595. E. Rommel and J. Wirz, Helv. Chim. Acta, 60 (1977) 38. R. Nurmukhametov, Pogloschenie i Lyuminestzentziya Molekul, Khimiya, Moscow, 1971. C. Leibovice and J. Deschamps, Theor. Chim. Acta, 4 (1966) 321. D.N. Shigorin and N.A. Scheglova, Zh. Fiz. Khim., 52 (1978) 1126. A. Weller, Naturwissenschaften, 42 (1955) 175. S.R. Flom and P.F. Barbara, J. Phys. Chem., 89 (1985) 4489. F. Dorr, Z. Elektrochem., 64 (1960) 580. T.P. Carter, H.H. Van Benthen and G.D. Gillispie, J. Phys. Chem., 87 (1983) 1891. G. Smulevich, A. Amirav, A Even and J. Jorter, Chem. Phys., 73 (1982) 1. Yu.A. Gerasimenko and N.T. Poteleschenko, Zh. Org. Khim., 7 (1971) 2413. Yu.P. Strokach, V.A. Barachevskii, N.T. Sokolyuk and Yu.E. Gerasimenko, Khim. Fiz., 6 (1987) 320. A.V. El’tsov (Ed.), Organicheskie Fotokhromy, Khimiya, Leningrad, 1982, p. 224. N.-W. Grummt and M. Friedrich, Z. Chem., (1985) 434. J.S. Scaiano, Act. Chem. Res., 15 (1982) 252.
285
QUANTUM-CHEMICAL AND EXPERIMENTAL INVESTIGATIONS OF PHOTOCHROMIC TRANSFORMATIONS IN QUINONE COMPOUNDS
N.P. GRITSAN Institute of Chemical Kinetics and Combustion, Novosibirsk 630090 (U.S.S.R.) (Received
13 August 1987)
ABSTRACT Analysis of quantitative information on photochemical migration of hydrogen atom, proton, acylic and arylic groups in anthraquinone derivatives obtained by experimental and quantumchemical methods has been performed. The migration of acyl is found to occur in the xx* state and to be thermally reversible. The concepts of conservation of orbital symmetry in the photochemical process are applied to account for the relation of the compound reactivity investigated to the chemical structure. Data from quantum-chemical calculations and experimental spectroscopy are used to construct energy correlation diagrams. The “nn* state is found to be reactive in hydrogen photochemical migration. Aryl migration proceeds non-adiabatically without conserving the excitation. The dependence of rate constants of thermal migration of hydrogen atom and acylic group on substituent nature, medium and temperature is studied. A quantitative theoretical interpretation of the results based on calculations by the PPP method is proposed.
INTRODUCTION
Intramolecular photochemical migrations of proton, hydrogen and more complex atomic groups refer to one of the simplest types of photochemical reactions of complex organic compounds. Studying such processes is of great interest as it allows the understanding of qualitative and quantitative peculiarities of these reactions to be applied to the basic statements of theoretical chemistry (the principle of orbital symmetry and spin multiplicity conservation, theoretical models treating the photochemical reactions as the processes of radiationless deactivation of excitation, etc.) [l-5]. The present paper is concerned mainly with the reactions of photochemical and dark migrations of hydrogen, phenylic and acylic groups in anthraquinone derivatives; these have been the object of several of our experimental studies (see, for example, refs. 6-10).
0166-1280/88/$03.50
0 1988 Elsevier Science Publishers
B.V.
286 EXPERIMENTAL
The basic tools used are flash photolysis with microsecond resolution, the low-temperature technique permitting the registration of spectral characteristics of intermediates stabilised at 77 K. The experiments have enabled us to determine the nature of intermediates, the rate constants for their transformation, the quantum yields of formation and, besides, to follow the dependence of all the quantitative characteristics of the processes on chemical structure. The absorption spectra of stable compounds and conjectural intermediates, the distribution of electron density and the enthalpy of reactions are calculated quantum-chemically using the Parise-Parr-Pople method with conventional Nishimoto-Forster or Dewar parameterizations [ 11,121. If necessary, the calculations are performed by AM1 and MNDO techniques [ 13,141. RESULTS AND DISCUSSION
Photochemical migration of hydrogen and proton has been at the center of numerous theoretical and experimental studies. Thus photoenolization of orthe-alkylketones and proton transfer in salicylates has been studied fairly extensively [15-171. Photochemical migration of hydrogen in orthomethylquinones was discovered during the last decade [6,18]. Migration of heavy atomic groups is still not clearly understood. Recently we [8] recorded the photochemical, thermally reversible, migration of acylic group for a series of methoxy and amino derivatives of l-acetoxy9,10-anthraquinone and non-substituted 9-acetoxy-1,4-anthraquinone. The migration of an acylic group fails with irradiation of the simplest compounds cofa given type, namely 1-acetoxy-9,10-anthraquinone and some of its derivatives. An analysis of photochemical behavior of all compounds investigated provides the following picture for the dependence of photoactivity on quinone structure: (i) 1-Acetoxyanthraquinone and a series of its derivatives of slightly different optical absorption spectra than that of non-substituted anthraquinone do not transform under irradiation into corresponding derivatives of 9-acetoxyl,lO-anthraquinone. The triplet and singlet states of type nti are the lowest excited states of these compounds and of anthraquinone. In alcohols the compounds with high quantum yield are reduced to appropriate hydroquinones at room temperature. (ii) 2-Methoxy-2-amino- and 4-methoxyderivatives of 1-acetoxy-9,10-anthraquinone and 9-acetoxy-1,4-anthraquinone with high quantum yields are isomerized under irradiation into the corresponding l,lO-anthraquinone derivatives due to photochemical migration of acylic group. The spectra of these compounds exhibit a longwave band of the xx* type. Calculations show the triplet and singlet of this type to be the lowest excited states.
(iii) There exists a third group of compounds, namely the 3-amino and 4amino derivatives of 1-acetoxy-9,10-anthraquinone. Under irradiation they do not transform photochemically. The triplet and singlet of the type XX* are the lowest excited states of these compounds. The properties enumerated may be readily understood and accounted for by correlation diagrams assuming that the phototransfer of an acylic group proceeds adiabatically with excitation and symmetry of wavefunctions conserved. The energy diagrams for the compounds studied have been constructed as follows: (i) The position of the ‘XX* levels was estimated from the absorption spectra with the O-O band 2800 cm-l lower than the maximum one [ 191. (ii) The energy of the %R* state was deduced from the E (m*) experimental value of 23800 cm-’ for anthraquinone and a linear correlation between E (m* ) and the energy of the lower free x*-orbital, calculated quantum chemically [20]. The corrections do not exceed 1600 cm-l. (iii) The single-triplet splitting between the m* type states was assumed to be 2000 cm-l as for anthraquinone. (iv) The singlet-triplet splitting between the xx* type states is taken to be equal to 6000 cm-‘. The value is typical for the quinone derivatives and is deduced from ref. 21. (v) While constructing the energy diagrams for hypothetical products we have used the experimental spectra of the corresponding 9-phenoxy-l,lO-anthraquinone derivatives as the spectra of 9-phenoxy- and 9-acetoxy-l,lO-anthraquinones are similar. (vi) The difference in the energy of ground states of the initial compounds and products has been calculated using the quantum-chemical procedures. With the energy diagrams constructed by the above technique, we have correlated the levels of the initial compounds and photomigrated products by the symmetry of wavefunctions. The only symmetry element in the system under study is a plane of aromatic rings. The wavefunctions of 0 and x electrons display a different symmetry relative to this plane. Thus the excited r-m*states of the initial compounds correlate with those of the m* type. The xx* type states of the initial compounds correlate with the product states of the type zx*. Adiabatic transfer of the acylic group is feasible if the lowest excited states correlate with the state of the product lower in energy, and vice versa. The adiabatic reaction is impossible providing the correlating product lies higher in energy. Figures l-3 demonstrate three types of correlating diagrams for photochemical migration of acylic group in 9-acetoxy-1,4_anthraquinone, 1-acetoxy-9,‘10anthraquinone and 1-acetoxy-4-hexilamino-9,10-anthraquinone. These compounds belong to the above mentioned three types. A favorable position for the correlating levels of acylic adiabatic migration is attained only in the case of 9-acetoxy-1,4-anthraquinone (Fig. 1). For two other compounds the adi-
288
40
J. Ii” cm
20
‘nQ*_ -3fi*
-lnT* ?-W-!l&
~_‘~L-
3!m*-
-
20 -%!lt”
SO 0
SC.
s
SO
Fig. 1. State correlation diagram for the allowed photochemical acetoxy-1,4_anthraquinone. Fig. 2. State correlation diagram for the forbidden 1-acetoxy-9,10-anthraquinone.
migration
of acetoxy group in 9-
photochemical
migration
of acetoxy group in
photochemical
migration
of acetoxy
40 t
J-1 o-,3
-1 cm
20
-
‘!Rst”---k-CR* *
-3P7z*
W-
Fig. 3. State correlation diagram for the forbidden 1-acetoxy-4-hexylamino-9,10-anthraquinone.
group in
289
abatic migration does not hold because of energy or symmetry considerations (Figs. 2, 3). In all compounds investigated, isomerization under light occurs only in the cases of the first type correlation diagrams. The non-photoactive anthraquinone derivatives possess diagrams of the third type. In the case of the second type diagrams, migration of the acylic groups fails but reduction to the corresponding hydroquinones in alcohols is feasible at room temperature. The process of photochemical migration of the acylic group has much in common with photochemical reaction of proton transfer via intramolecular hydrogen bond. Such reactions are the best studied and best known adiabatic photochemical processes. The adiabatic reaction of proton transfer was first observed in 1955 by Weller [ 221. Such processes are also known for anthraquinones and manifest themselves in a large Stokes shift of luminescence spectrum or in the presence of dual luminescence which is more characteristic for anthraquinones with intramolecular hydrogen bond [ 231. Existence of a strong intramolecular hydrogen bond, however, does not guarantee proton transfer at excitation. For example, in the case of 1-oxy-, l-amino- and 1,4-dioxyanthraquinones this reaction is not observed [ 23-261. Such a result is probably caused by the fact that for these anthraquinones the excited singlet state of the ketoform lies higher in energy and the adiabatic process proves impossible. Our energy diagram for these compounds verify this hypothesis qualitatively. Figure 4 exemplifies a diagram for 1-oxyanthraquinone. Proton transfer is detected in l+dioxyanthraquinone [ 231 and in this case the rate constant is very high. Only its lower estimate 2 1012 s-l is presented. Unfortunately, in the case of the acylic group transfer studied the adiabatic
0
0
Fig. 4. State correlation diagram for proton transfer in 1-oxyanthraquinone.
290
mechanism of migration cannot be confirmed as easily as in the case of proton transfer. The compounds in question and their photolysis products do not luminesce. We have failed to identify the reaction state multiplicity. However, the migration is likely to occur in a triplet state. There is some indirect evidence for this assumption. Photochemical reaction of phenylic group migration in peri-aryloxy-p-quinones was first reported in 1971 [ 271. The few available studies on this mech,anism [ 28,291 indicate a triplet nature of the reaction state. In contrast to the above migration processes of proton and acylic group, the phenylic group transfer is photochemically reversible. The quantum yields of direct and reverse photoreactions for a series of 1-aryloxy-9,10-anthraquinone derivatives investigated are closely related and amount to 0.03-0.09. The diagrams for the energy levels constructed using the above technique show that the two kinds of position for the lowest excited zx* type states are realized for the compounds studied (Figs. 5,6). A different position of the levels does not, however, affect the magnitude of photolysis quantum yields of these compounds. The forma-
‘nfi?
3
nllt*
‘rfR*
‘JS” 3nR* 20 -
‘f%.”
3E3r* d* 1g3 cm
33&
Fig. 5. Diagram of energy levels for 1-phenoxy-2-acylamino-9,10-anthraquinone of its photolysis.
and the product
291
10
I
SO
0
A0 0
0
a0
‘p= 0.09
[II>
0
zIIzIz OCH3
(9=0*02
Fig. 6. Diagram of energy levels of 1-phenoxy-4-methoxy-9,10-anthraquinone and its photolysis product.
tion of an intermediate spiroform in a triplet state has been assumed recently [ 281. This assumption is, however, poorly argumented and needs further experimental study of the mechanism of peri-aryloxyanthraquinone photolysis. It is only clear that this process is nonadiabatic, qualitatively different in mechanism from those for photochemical migration of proton and acylic group. Now we consider briefly the fourth type of migration photochemical processes, the hydrogen transfer in ortho-alkylquinones [ 6,7,18]. The latter is close in mechanism to photoenolization of ortho-alkylketones which exhibits the following characteristics [ 16,171: (i) the photoenolization of ortho-alkylketones occurs in both singlet and triplet states of the r-m*type; (ii) the constants for hydrogen atom transfer in the r-m* triplet states are KHN log+lo’o s-l; (iii) the formation of the triplet state of the enol form is registered, the enol triplet lifetime depends on medium and may vary from N 100 ns to some ,LLS.
The triplet states of enol form convert into a ground enol state; oxygen accelerates the rate of conversion. Our estimates of the constant for hydrogen transfer in l-methylanthraquinone indicate that the constant is high, K? 10’ s-l. The formation of an intermediate, 9-oxy-l,lO-anthraquinone-1-methide, in a triplet state has been registered recently [ 301 in the case of 1-methylanthraquinone. Hence the mechanism of o&ho-alkylquinone photolysis is analogous to that for orthoalkylketone photolysis. The situation in the case of photoenolization of quinones and ketones is quite different compared with the above processes. In all three cases the quantity of 0 and rc electrons in the initial compound and in its photoinduced form was the same. In the case of photoenolization, a keto form (p-quinone) has two o electrons more than the enol form (ana-quinone) . Hence two a electrons convert into two x electrons in the course of the photochemical process. As for the symmetry relative to a molecule plane of the full wavefunction, the ground states of both p-quinone (ketone) and ana-quinone (enol) are symmetrical. However, the symmetry of wavefunctions of separate electrons changes. In the case of hydrogen transfer (photoenolization) the nn* type state appears to be reactive by contrast to the n?r* type states in the first three cases. As shown, the photoenolization in a triplet state proceeds via enol formation (or anu-quinone) in a triplet state. The process may be represented by the scheme
Q
hv
na + mn
e
Tn-I_)0
(m+l)rr
+
3Q*
(n_l)a
(m+l)n
I--trLLer
3AQ* (n-2)0 (m+2)7c
+AQ (n-2)0 (m+2)n
The symmetry of the full wavefunction is seen to be broken during the process. Nevertheless, the process is highly efficient, K- 10’ s-l and a feasible reason for this is that the primary process defining the rate is the formation of (a, x) biradical in a triplet state. HO H\C/H
3“R *
3m
*
In this case the principle of wavefunction symmetry conservation is not violated in hydrogen transfer. An intermediate (a, n) biradical relaxes into a triplet enol state whose absorption is registered experimentally. It is noteworthy that in the literature there exists some obscurity in referring the intermediate absorption to the triplet state of either enol or biradical [ 30,311. A correlation diagram for such a type is presented in Fig. 7. Such a diagram has been suggested earlier [ 171 for the process of ortho-alkylketone photoenolization.
293
SO
0
Fig. 7. State correlation methylanthraquinone.
0
diagram
for
the
photochemical
hydrogen
transfer
in
l-
We have verified experimentally [ 71 the fact that in hydrogen transfer both the nr* type states and those of the type xx* may be reactive; the value of quantum yield, however decreases substantially. Particularly drastic differences are observed in the photolysis at 77 K [ 71. In this very case a detailed mechanism has not yet been ascertained. All these processes of photochemical migration of atoms and atomic groups are reversible. A phenyl group migration is photochemically reversible. The reverse migration of hydrogen atom and acylic group in anthraquinones proceeds only in a thermal fashion. The mechanism of dark migrations of hydrogen and acyl displays both common and different features. The dependences of rate constants of these processes on medium, substituents and temperature are different. The kinetic curves of absorption decay of the short-lived photolysis products of l-methyl- and 1-acyloxy-9,10-anthraquinone derivatives are described using the exponential time dependence. The characteristic time of absorption decay in all cases is independent of the initial reagent concentration and of the dissolved oxygen. In the case of 9-acyloxy-l,lO-anthraquinone derivatives the rate constant of dark transformations is actually independent of solvent nature. In contrast, the rate constant for the dark migration of hydrogen atom in quinones largely depends on solvent nature and drops by - 3 orders of magnitude from hexane to alcohols. As for quinonemethides, the rate of the reverse dark migration is high even at 77 K in non-polar solvents so that the hydrogen transfer has not been registered in methylhexane at 77 K by stationary meth-
294
ods. Such a difference in the dependences of process rates is satisfactorily interpreted on the basis of calculations of dipole moments of the initial compounds and the products of dark transformations. In the case of l-acetoxyanthraquinones, the dipole moments of the initial and final states are similar, that of the intermediate state is, probably, also close to them. In the case of dark hydrogen migration the initial state is quinonemethide, a polar compound (d N 4 D), and the product, l-methylanthraquinone, exhibits a very small dipole moment. Hence the dipole moment of the intermediate state is less than that of the initial one and the process is strongly inhibited in polar solvents. The rough estimates for solvent action according to the Onsanger model are in satisfactory agreement with experiment. The rate constant for the acylic group dark migration is practically independent of the substituent nature in the 9,10-anthraquinone nucleus but changes by two orders of magnitude in passing from 9,10-anthraquinone to 1,4-anthraquinone. Such a difference is accounted for by the change in the reaction enthalpy. However scanty the experimental data, one may plot a correlation log K with the enthalpy calculated quantum-chemically (Fig. 8). The tangent of the dependence is close to an analogous Polyani-Semenov dependence for the reactions of hydrogen abstraction by radicals. The temperature-dependence of the rate constant of acyl migration in 9-phenoxy-4-acetoxy-l,lO-anthraquinone is measured. A preexponential factor is small (2 x lOlo s-’ ) and testifies to a higher rigidity of the intermediate state. Such values are characteristic for
5.0 -
3.0 1
I 25
I 50 dti
colt 9
I 75
kJ
Fig. 8. Acyl migration rate constant dependence on enthalpy of reaction.
Fig. 9. Acyl migration rate constant dependence on the nature of migrant substituent.
the reactions with a six-member transfer in quinone-methides, the preexponential The 0-acyl group nature 0
state. For those of hydrogen transfer factor equals 7 x 1011+ 5 x 10” s-l.
(4-R) affects substantially the rate of the acylic dark migration [lo]. Increasing electron-donor ability of the R substituent, decreases the rate of rearrangement from 2.5~10~ s-’ (R=CH3) to 3.1~10~ s-’ (R=OC2H5) and 3.3 s-l (R = N ( CHB ) 2). An arylic nucleus involved in migration, levels the electron influence of the substituents considerably. Figure 9 demonstrates a linear correlation of the rate constant logarithm with the electrophilic constants of substituents. Constant p proved to be equal to 0.60; its sign is positive and agrees with the fact that the higher the positive charge on a migration carbon atom, the higher the process rate. ACKNOWLEDGEMENT
The author expresses her gratitude experimental investigations.
to Dr. L.S. Klimenko
for aid in all the
296
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