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Investigation of the radical-ion stages in sensitized trans-cis photoisomerization of fumaronitrile by 1H CIDNP
Volume
121. number
CHEMICAi
4.5
INVESTlGATION IN SENSITIZED OF FU~RO~~LE AX
KRUPPA,
LETTERS
15 November 1985
OF THE RADICAGION STAGES TRANS-CIS PHOTOISOMERKZATION BY ‘H CIDNP T.V. LESHINA
and R.Z.
Insrirure o/ ChemicaI Kinerics and Combunion. Received
PHYSICS
SAGDEEV
Novosibirsk
90. 630090,
USSR
29 July 1985
The thermodynamic criterion or reversible electron phototransfer correlates with CIDNP effects in sensitized trans-cis photoisome~tion ol fumaronitrile in aoetonirrile. It tallows from CIDNP analysis xhat rhc free radical anion OF FIJITIWOII~~~~~C does no1 undergo geometrial isomerization
1. Introduction The radicalion pathway for the sensitized transcis isomerization of substituted olefms is welI established [l--3]. However,in the literature the role of radical ions in the isomerization process is treated differently_ It is assumed that trans-cis isomerlzation is accomp~~ed during the lifetime of the triplet excited state of the olefm molecule due to the degradation of the electronic energy [4,5] _ In the majority of studies of radical-ion mechanism s of isomerization, it is claimed that the triplet excited olefm molecule is a product of back electron transfer in the triplet radicalion pair (RIP) [ 1,3] _However, there are examples in the literature when the energy of the RIP is not suffcient to form the triplet olefm, and yet both the isomertiation and CIDNP are observed. It has been thought that in this case the isomerization occurs in the free radical ion of the olefm [6]. There is also another point of view, that isomerization even in polar solvents is accomplished by energy transfer, and CIDNP effects reflect only the secondary way of formation of the radical ions of the initial and isomerized olefms [7]. Clarification of the role of radical ions in the process of sensitized trans-cis isomerization may be achieved in two ways _First, by considering the details .of the CIDNP fo~ation in the electron phototransfer reaction [8], and second, the joint analysis 386
of thermodynamic parameters of RIP and CIDNP. This paper presents a 1H CIDNP study of the trans-cis photoisomerization of fumaronitrile under the action of different sensitizers with different thermodyn~c parameters. Note that fumaronitrile is a more favourable example than the most widely studied stilbenes and substituted styrenes [l-3,6,7]. In the case of ~~onit~e it is easy to exclude the direct excitation of the olefm itself, which would have caused some difficulties in the CIDNP analysis, because of the formation of several types of RIP.
2. Expeximental Fumaronitrile (trans-1,2dicyanoethylene), naphthalene, 1 .%limethylnaphthaIene, diphenylacetylene, anthracene, 9 ,1 O~e~yl~~acene and pyrene were purified by sublimation. Acetonitrile and deuteroacetonitrile were boiled over C&l2 and then twice distilled. The samples vLere studied with Tesla BS 487C and Varian XL-200 NMR spectrometers provided with devices for photoClDNP observation. The samples contained 1O-2 M of sensitizer and 5 X 10e2 M of furnaronitrlle. The light source was a high-pressure mercury lamp (1000 W). The samples were irradiated with the full Ii&t of the la&p and with the use of glass ftiters (300 <&us ==-400 nm). 0 009-2614/85/% 03.30 0 Elsevier Science Publishers B.V. ~o~-Ho~~d Physics ~b~g Division)
Volume 121, number 4,s
CHEMICAL PHYSICS LETEZRS
15 November 19B.S
3. Results and discussion To describe the processes of the CIDNP formation in the high magnetic fields in the reactions of photoinduced electron transfer, let us consider scheme 1. The main difference of *the radical-ion reactions from reactions of the neutral radicals lies in the possibility of the recombination of triplet RIP *_ The possibility of photo~duced electron transfer and the recombination of the triplet RIP were estimated by Weller’s equation [9] I AGL =(E+ -E_ AG2 = 3E;
-(E+
- e2/eR) - lE6
,
- E_ - e2/&)
,
AGS = 3Eg - (E+ - E_ - e2feR) , where E+ and E_ are the half-wave redox potentials of donor and acceptor, e2f& is the Coulomb interaction energy between the pair of radical ions (R = OS1 nm), lz3fZ* is the energy of the excited states of donor and acceptor, and AGj is the variation of the free energy. If AG, < 0 then electron transfer from donor to acceptor is possible with the formation of RIP of a multiplicity corresponding to that of the precursor. The multiplicity of a pair may change during the lifetime of the RIP. The process of variation of mnltiplicity of RIP is the ma~etosensiti~ stage which is responsible for formation of the nuclear polarization. In reversible electron phototransfer reactions selection * Recombination is the back electron tra&fer in the RIP.
Scheme 1.
Scheme 2. of nuclear spin states occurs due to the difference in the formation rates of A and D from the singlet and triplet RlPs and in the rates of the diffusion in the bulk. Scheme 2 shows the situation when the product is formed as a result of isomerization of the triplet molecule. The arrows shows the channels of polarization passing through to the molecules of the initial and isomerized acceptor olefm. It follows from scheme 2 that there are three ways that polarization can arise in the initial olefm (if both AC2 ( 0, and AC3 < 0): recombination of the singlet and triplet RIPS with the formation of olefm in the ground state and recombination of the triplet RIP with formation of the tripIet excited olefm; there are also two ways (if only AG, < 0, or AG, < 0). Within scheme 2 the isomerized olefm will be polarized only if AG, < 0. The relaxation times of the triplet molecules are 10-6-10-7 s [lo] and their lifetimes in the triplet state are in the nanosecond range. Thus, the nuclear polarization of the initial olefm has two contributions with opposite signs, from S and T pairs. The magnitudes of these CoRt~butions depend on the recombination constants K, and Kt, on the relation between the isomerization rate constants of triplet olefin to the cis and tram structure and on the relaxation time of the olefm in the T state. Therefore, the use of the observed coefficients of CiDNP of the cis- and transolefm in the discussion of the mechanism of the isomerization (relation between the X, and K,, the contribution of the radical-ion pathway) as done in ref. [6] is incorrect: Only in the case of identical signs of 387
CHEMICAL PHYSICS LEl-TJZRS
Volume 121, number 4.5
15 November I985
Table 1 Experimental results, thermodynamic parametas of se.usitizersand of radical-ion reactions acr
SerlSiliZer compound
IE* a) (ev)
3E’ a)
=I
.AG1 u)
AGa
CIDNP sign sensitiel
E+b’cv)
FN
-Mri
naphthalene
3.94
2.64
154
-
+
1 .Sdknethylnaphthalene
3.88
259
1.35
CO
-
+
-
CO
>o
<0
no
+
-no
1.82
1.09
9,lo-dimcthylanttuacene
-3.2
1.75
0.87
CO
>O
no
no
no
pyrene diphenylacetylene
3.34 -4.0
2.11 2.71
1.06 al-5
70
no
no
co
-
-
no +
antluacene
3.27
b) The data are mainly from ref. [ 121 (versus SCE). a) The data are mainly Tom ref. [ll]. C) E_(FrQ = -1.36 V(versus SCE) [13]. d) 3~*(~ = 255 eV 1141.
the CIDNP of the cis- and transolefrns may one claim thatK, 0, AG, < 0). The line broadening of the fumaronitrile signal in the NMR spectrum during the irradiation is assumed to indicate the formation of the radical ions of the furnaronitrile in the system where AGJ < 0, AG, > 0 (anthracene, 9,lOdimethylanthracene, pyrene). Measurements of the dependence of line broadening on the concentration of the diamagnetic fumaronitrile show that this broadening arises from electron exchange between the firrnaronitrile radical anion and its diamagnetic precursor. Thus, the radical-ion mechanian of the sensitized photoisomerization is indeed effective only in systems 388
where formation
of the triplet olefm is possible (AG2 of maleonizrile observed. This fact allows one to exclude the possibility of the isomerization of fumaronitrile free radical ion, which-is discussed for the radical ions of stilbene [ 161 and Bmethylstyrene [6], because of the pola&&ion absence in the case AG2 > 0. Note, however, that during prolonged irradiation with the full light of the lamp in the samples with such sensitizers as anthracene, 9jlOdimethyl~thracene and pyrene, a small (3-S% of the initial furnaronitrile concentration) amount of maleonitrile is formed; we expect this to be a result of non-radical mechanisns. < 0). Only in this case is the polarization
References
-
[l] T.V. Lcshirra,S.G. Belyaeva, VJ. Mu-yasolva, RZ. Sa&eev and YuN. Molin, Chem. Pbys. LMters 75 (1980) 438. [2] W. Hub,tJ. KlUta. S. Schneida. F.Darr. J_D. Oand FD. Jxwk. J. Phys. C&em. 88 (1984) 2308. [3] W. Hub. S. Schneidkr, F. Dan. JD. Oxman aud FD. Lewis.J. Am. Chem. Sot. 106 (1984) 708. [4] G.S. Hammond, J. Saltiel,AA. Lamok, NJ.Turro,
J.S. Bradshaw. D.O. Cowan, R.C. Couruell, V. Vogt and C. Dalton. J. Am. Cbem. Sot. 86 (1964) 3197.
[S] qD;;kch and J. Bkks, Chem. Phys. Lettcm 38 (1976) [6] ND: Roth and MLM Sm. J. Am; Chem. sot. lOl(1979) 1898; 102 (1980) 4303. [7] C.Agostini,A. Gambaro,G. Ge.nnui. G. Giacometi and L. padme& Chem. Phys. Letters 78 (1981) 5%.
Volume 121. ntiber
4.5
CHEMICAL
[8] A.L Kruppa, T.V. Leshba, R-2. Sagdeev, KM Salikhov and F.S. Sarvarov. Chem. Phys. 67 (1982) 27. [9] I-L Beens and A. Weller. Chem. Phys. Letters 2 (1968) 140. [lo] K.M. SaJikhov, Yu.N. Molin. R.Z. Sagdeev and AL. Buch&henkb, Polarization and magnetic effects in radical reactions (Akademiai Kiado. Budapest, 1984) chs. 4. 7. [ll] A.J. Gordon and RA. Ford, The chemist’s companion (Wiley, New York, 1972) ch. 5.
PHYSICS LETTERS
15 November
1985
[ 121 C.K. Mann and K.K. Barnes, Electxochemical reactions in nonaqueous systems (Dekker, New York, 1970) ch. 3. [13 ] DR. Arnold and P.C. Wong, J. Am. Chem. Sot. 101 (1979) 1894. [14] PC. Wang, Can. J. Chem. 60 (1982) 339. [15] G.L. Closs and MS. Czeropski, J. Am. Chem. Sot. 99 (1977) 6127. [16] FD. Lewis, J.R. Petisce, JD. Oxman and MJ. Nepras. J. Am. Chem. Sot. 107 (1985) 203.
389
121. number
CHEMICAi
4.5
INVESTlGATION IN SENSITIZED OF FU~RO~~LE AX
KRUPPA,
LETTERS
15 November 1985
OF THE RADICAGION STAGES TRANS-CIS PHOTOISOMERKZATION BY ‘H CIDNP T.V. LESHINA
and R.Z.
Insrirure o/ ChemicaI Kinerics and Combunion. Received
PHYSICS
SAGDEEV
Novosibirsk
90. 630090,
USSR
29 July 1985
The thermodynamic criterion or reversible electron phototransfer correlates with CIDNP effects in sensitized trans-cis photoisome~tion ol fumaronitrile in aoetonirrile. It tallows from CIDNP analysis xhat rhc free radical anion OF FIJITIWOII~~~~~C does no1 undergo geometrial isomerization
1. Introduction The radicalion pathway for the sensitized transcis isomerization of substituted olefms is welI established [l--3]. However,in the literature the role of radical ions in the isomerization process is treated differently_ It is assumed that trans-cis isomerlzation is accomp~~ed during the lifetime of the triplet excited state of the olefm molecule due to the degradation of the electronic energy [4,5] _ In the majority of studies of radical-ion mechanism s of isomerization, it is claimed that the triplet excited olefm molecule is a product of back electron transfer in the triplet radicalion pair (RIP) [ 1,3] _However, there are examples in the literature when the energy of the RIP is not suffcient to form the triplet olefm, and yet both the isomertiation and CIDNP are observed. It has been thought that in this case the isomerization occurs in the free radical ion of the olefm [6]. There is also another point of view, that isomerization even in polar solvents is accomplished by energy transfer, and CIDNP effects reflect only the secondary way of formation of the radical ions of the initial and isomerized olefms [7]. Clarification of the role of radical ions in the process of sensitized trans-cis isomerization may be achieved in two ways _First, by considering the details .of the CIDNP fo~ation in the electron phototransfer reaction [8], and second, the joint analysis 386
of thermodynamic parameters of RIP and CIDNP. This paper presents a 1H CIDNP study of the trans-cis photoisomerization of fumaronitrile under the action of different sensitizers with different thermodyn~c parameters. Note that fumaronitrile is a more favourable example than the most widely studied stilbenes and substituted styrenes [l-3,6,7]. In the case of ~~onit~e it is easy to exclude the direct excitation of the olefm itself, which would have caused some difficulties in the CIDNP analysis, because of the formation of several types of RIP.
2. Expeximental Fumaronitrile (trans-1,2dicyanoethylene), naphthalene, 1 .%limethylnaphthaIene, diphenylacetylene, anthracene, 9 ,1 O~e~yl~~acene and pyrene were purified by sublimation. Acetonitrile and deuteroacetonitrile were boiled over C&l2 and then twice distilled. The samples vLere studied with Tesla BS 487C and Varian XL-200 NMR spectrometers provided with devices for photoClDNP observation. The samples contained 1O-2 M of sensitizer and 5 X 10e2 M of furnaronitrlle. The light source was a high-pressure mercury lamp (1000 W). The samples were irradiated with the full Ii&t of the la&p and with the use of glass ftiters (300 <&us ==-400 nm). 0 009-2614/85/% 03.30 0 Elsevier Science Publishers B.V. ~o~-Ho~~d Physics ~b~g Division)
Volume 121, number 4,s
CHEMICAL PHYSICS LETEZRS
15 November 19B.S
3. Results and discussion To describe the processes of the CIDNP formation in the high magnetic fields in the reactions of photoinduced electron transfer, let us consider scheme 1. The main difference of *the radical-ion reactions from reactions of the neutral radicals lies in the possibility of the recombination of triplet RIP *_ The possibility of photo~duced electron transfer and the recombination of the triplet RIP were estimated by Weller’s equation [9] I AGL =(E+ -E_ AG2 = 3E;
-(E+
- e2/eR) - lE6
,
- E_ - e2/&)
,
AGS = 3Eg - (E+ - E_ - e2feR) , where E+ and E_ are the half-wave redox potentials of donor and acceptor, e2f& is the Coulomb interaction energy between the pair of radical ions (R = OS1 nm), lz3fZ* is the energy of the excited states of donor and acceptor, and AGj is the variation of the free energy. If AG, < 0 then electron transfer from donor to acceptor is possible with the formation of RIP of a multiplicity corresponding to that of the precursor. The multiplicity of a pair may change during the lifetime of the RIP. The process of variation of mnltiplicity of RIP is the ma~etosensiti~ stage which is responsible for formation of the nuclear polarization. In reversible electron phototransfer reactions selection * Recombination is the back electron tra&fer in the RIP.
Scheme 1.
Scheme 2. of nuclear spin states occurs due to the difference in the formation rates of A and D from the singlet and triplet RlPs and in the rates of the diffusion in the bulk. Scheme 2 shows the situation when the product is formed as a result of isomerization of the triplet molecule. The arrows shows the channels of polarization passing through to the molecules of the initial and isomerized acceptor olefm. It follows from scheme 2 that there are three ways that polarization can arise in the initial olefm (if both AC2 ( 0, and AC3 < 0): recombination of the singlet and triplet RIPS with the formation of olefm in the ground state and recombination of the triplet RIP with formation of the tripIet excited olefm; there are also two ways (if only AG, < 0, or AG, < 0). Within scheme 2 the isomerized olefm will be polarized only if AG, < 0. The relaxation times of the triplet molecules are 10-6-10-7 s [lo] and their lifetimes in the triplet state are in the nanosecond range. Thus, the nuclear polarization of the initial olefm has two contributions with opposite signs, from S and T pairs. The magnitudes of these CoRt~butions depend on the recombination constants K, and Kt, on the relation between the isomerization rate constants of triplet olefin to the cis and tram structure and on the relaxation time of the olefm in the T state. Therefore, the use of the observed coefficients of CiDNP of the cis- and transolefm in the discussion of the mechanism of the isomerization (relation between the X, and K,, the contribution of the radical-ion pathway) as done in ref. [6] is incorrect: Only in the case of identical signs of 387
CHEMICAL PHYSICS LEl-TJZRS
Volume 121, number 4.5
15 November I985
Table 1 Experimental results, thermodynamic parametas of se.usitizersand of radical-ion reactions acr
SerlSiliZer compound
IE* a) (ev)
3E’ a)
=I
.AG1 u)
AGa
CIDNP sign sensitiel
E+b’cv)
FN
-Mri
naphthalene
3.94
2.64
154
-
+
1 .Sdknethylnaphthalene
3.88
259
1.35
CO
-
+
-
CO
>o
<0
no
+
-no
1.82
1.09
9,lo-dimcthylanttuacene
-3.2
1.75
0.87
CO
>O
no
no
no
pyrene diphenylacetylene
3.34 -4.0
2.11 2.71
1.06 al-5
70
no
no
co
-
-
no +
antluacene
3.27
b) The data are mainly from ref. [ 121 (versus SCE). a) The data are mainly Tom ref. [ll]. C) E_(FrQ = -1.36 V(versus SCE) [13]. d) 3~*(~ = 255 eV 1141.
the CIDNP of the cis- and transolefrns may one claim thatK,
where formation
of the triplet olefm is possible (AG2 of maleonizrile observed. This fact allows one to exclude the possibility of the isomerization of fumaronitrile free radical ion, which-is discussed for the radical ions of stilbene [ 161 and Bmethylstyrene [6], because of the pola&&ion absence in the case AG2 > 0. Note, however, that during prolonged irradiation with the full light of the lamp in the samples with such sensitizers as anthracene, 9jlOdimethyl~thracene and pyrene, a small (3-S% of the initial furnaronitrile concentration) amount of maleonitrile is formed; we expect this to be a result of non-radical mechanisns. < 0). Only in this case is the polarization
References
-
[l] T.V. Lcshirra,S.G. Belyaeva, VJ. Mu-yasolva, RZ. Sa&eev and YuN. Molin, Chem. Pbys. LMters 75 (1980) 438. [2] W. Hub,tJ. KlUta. S. Schneida. F.Darr. J_D. Oand FD. Jxwk. J. Phys. C&em. 88 (1984) 2308. [3] W. Hub. S. Schneidkr, F. Dan. JD. Oxman aud FD. Lewis.J. Am. Chem. Sot. 106 (1984) 708. [4] G.S. Hammond, J. Saltiel,AA. Lamok, NJ.Turro,
J.S. Bradshaw. D.O. Cowan, R.C. Couruell, V. Vogt and C. Dalton. J. Am. Cbem. Sot. 86 (1964) 3197.
[S] qD;;kch and J. Bkks, Chem. Phys. Lettcm 38 (1976) [6] ND: Roth and MLM Sm. J. Am; Chem. sot. lOl(1979) 1898; 102 (1980) 4303. [7] C.Agostini,A. Gambaro,G. Ge.nnui. G. Giacometi and L. padme& Chem. Phys. Letters 78 (1981) 5%.
Volume 121. ntiber
4.5
CHEMICAL
[8] A.L Kruppa, T.V. Leshba, R-2. Sagdeev, KM Salikhov and F.S. Sarvarov. Chem. Phys. 67 (1982) 27. [9] I-L Beens and A. Weller. Chem. Phys. Letters 2 (1968) 140. [lo] K.M. SaJikhov, Yu.N. Molin. R.Z. Sagdeev and AL. Buch&henkb, Polarization and magnetic effects in radical reactions (Akademiai Kiado. Budapest, 1984) chs. 4. 7. [ll] A.J. Gordon and RA. Ford, The chemist’s companion (Wiley, New York, 1972) ch. 5.
PHYSICS LETTERS
15 November
1985
[ 121 C.K. Mann and K.K. Barnes, Electxochemical reactions in nonaqueous systems (Dekker, New York, 1970) ch. 3. [13 ] DR. Arnold and P.C. Wong, J. Am. Chem. Sot. 101 (1979) 1894. [14] PC. Wang, Can. J. Chem. 60 (1982) 339. [15] G.L. Closs and MS. Czeropski, J. Am. Chem. Sot. 99 (1977) 6127. [16] FD. Lewis, J.R. Petisce, JD. Oxman and MJ. Nepras. J. Am. Chem. Sot. 107 (1985) 203.
389