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ESR evidence of the electron-transfer processes in the solid state photochemical reactions between organotin compounds and quinones
volume 84, number 3
CHFMICAL PHISYCS LETTERS
15 December 1981
E!3REMDENCE OF TIiE ELECTRON-TRANSFER PROCESSESM THE SOLID STATE PHOTOCHEMICALREACTIONS BETWEEN ORGANOTIN COMFOUNDS AND QUINONES THE TRJPLET STATE OF THE PRIMARY RADICAL ION PAIRS S~UJI
EMORI, Douglas WERI and Jeffrey K S WAN
Departmwt af Chemntry, Queen’s Utnverslty, Km,qston Ontano, Canada K7L 3N6 Rccewcd 29 July 198 1
The ESR observatlons and charactcnzatlon of the trtplct raduxd ron paus formed dunng the trraduttlon of B sobd solunon of qumone m oganotm at 77 K estabhsh uncqutvocally the pnmary electron transfer proccss between the orgmotm compounds and qumones
The ablhty of qumones to form paramagnets radlcal comulexes unth a vanety of organometals has been recewmg mcreased attentton To estabhsh the nature of the pnmary photochemlcal promessesbetween the qumone exclted tnplets and organometals such as organotm compounds, we have recently apphed the tlme-resolved CIDEP (chemlcally mduced dynamlc electron polanzatlon) to the paramagnehc organometal adducts of qumones [ 11 Whde the CIDEP observatlons can be readdy accounted for by a charge-transfer mechamsm for the quenchmg of qumone tnplet, mvolvmg a pnmary radlcal Ion pair of qumone and the organotm, &rect spectroscoplc evtdence of the pnmary electrc” transfer radeal ion paw m thls Important model process 1shere-to-fore stil1 lackmg Prenous chemrcal studm have suggested organotm compounds as h$ly effectwe electron donors m a vanety of charge-transfer mteractlons [2] For example, the thermai charge transfer addltlon reaction of tetracyanoethylene to vanous tetraalkyltms was found to proceed also photochemlcally when bemg lrrddlated wlthm the charge-transfer ab. sorptlon band [3] Other studtes mcludmg the reactlon of hexaalkyldmn wlth phenanthroqumone [4], substltuted o-qumones [S1) and pqumones [6] also Jtpported thc charge-transfer concept wluch was Brst formulated by Kosower [7] who proposed that photochemlcal and thermal processes of electron transfermay share common pathways leadmg to the 512
formatlon of the same 1011pairs In thls report, we have unequrvocally estabhshed the prtmary electron trans fer process m the quenchmg of tnplet qumones by organotm by a direct ESR observatlon and charactenzatlon of the tnplet state of thc pnmary radlcal ion parrs formed m the photolysls of a sohd sofutlon of qumones m organotm at 77 K A small amount of qumone (1%) wat added to phenyltm compounds Ph.$nX (X = Ph, Cl, CH$!OO) Thc muLture was melted by gentle warrr mg and degaaicd under high vacuum The meltmg pomts are 227,105, and 123’C for Ph,Sn, Ph,SnCI, and Ph3SnCOOCH3, respecttvely The samplc was then sealed off under vacuum and Irradlated dt 77 K mthm the mrcrowave cavlty of a Vanan B3 X-band spectrometer The hght source was a super pressure Hg lamp Upon lrradmtlon of the sohd solutlon iit 77 K. a typtcal ESR spectrum shown m fig. 1 was observed The ESR spectrum exfnblts the characrenstic features of an &y symmettrc tnplet state speaes as well as a streng absorptlon due to the doublet state of some radlcals For all the systems studled the Mis = 2 tranntlon of the fnplet state ISalso observed, The ESR observahons a@ accounted for by the emtence of a radlcal IOII pan trapped m the polyc~staltie sample at 77 K. The det&d ana!yss of the different ZFS parameters dependmg upon the ccMñnatlons of mdmdual qumone w~th organotm compounds (table 1) confirms the mvolvement of both the quI-
x‘
”
-I
-
TJ^-
%.
r---l
Tlg 1 ESR spectraof a sobd solutron cuntauung 1% 2,5-dlphenyl-p~enxqumone UItetrapbenyltm at 77 K BeforePrad&on breken bne, Mter rnadnt~~n sohdhm
none and the organotm as the two radrcal components formmg the tnplet state of the rad& ton pan Thus, the photochemlcal reaction must proceed na an electron transfer mechamsm
77K (Q- +Ph3Snx’)P, (Q- + Ph3SnX+)p, -t Q- + Ph$n+ + X Q- + ph$n+ + Ph3SozQ
(3)
Photolysts at 77 Kof a pure Ph3SnCl or Ph3SnCQQCH3 polycrystalhne sample did not yleId an) tnplet ESR stgnals, as the presence of qumone was necessary to xmtite the photochenncal electron transfer teactlon The observed doublet radzal agnak TabIe 1 ZFS parameters ( 10e3 cm-“) for the radxal loonpaus Ph$nX+/QQmuone
15 December i981
CHEMICAL PHISYCS LJZIERS
Volume 84, numbcr 3
acceptor
1,4-benzoqumone
2.S-d~pheuyl-pbenzuqumonc 1,4-xtaphthoquumne 9,l~thraqumone nlunl”tlltyfn’.“~ r -__ >olthu-naphthcquuotte 3,Sd+butykAenzuqmno~e camphoroqumone
Ph&
_
x
5.59 5 47
-
*
m the~~,sofidSoI&on *systemsboth before and^durmg the $kM&&r&&iëthé folimatson of radlcal spec~&e~~~~~~ and (3) ETa thermal process and~tlZ@e “primarvii%chonTl) can be mttlated thermally but oií@af &íii?r teinperatu- Smce the sohd solutlon was prepared from the ml 1‘at hg& temperatures, reactrons wdl proked and douolct radlcals wdl be formed However, when the solutlon was n+ medntely cooled to 77 K wtthout madtitlon, no tnpief s~@Were obsaved, akhough the doublet rad& mgnals obnously remamed Tfus fact suggests that the tnplet specíes formcd durmg the photolfis at 77 K ISnot due to the randomly trapped of ;le radral spehes from reacttons (2) and (3) Therefore, the tnpfet state denves from the trappme of the pnmary radrcal mn pur, produced dnectly from the pnmary electron tranfir process m reacbon (1) While the electron-transfer process in the quenchmg of an exated triplet qumone by organometal wlth low iomzatmn potenttals has generally been assumed, we must emphasze that only now have we concluave iy Identlfìed the pnmary radical ion pan by tnplet ESR and thus uneqmvocally prowded the physrcal endence of the electron transfer process ‘TheIdentlty of the doublet radlcal speacs m tha crystal after photolysls can be obtamed m many cases by slmpIy dtssolvmg the sohd m tobtene and the solutron ESR spectrum of the radtcal would be v el1 resolved For example, for the orthoqumones such as phenanthroqumone and 3,5-dl-r-built-benzoqua’anc the solution ESR spectra Identlfy the tnphenyltm mdlcal adduct to qumone Ph3Sn-Q as *he stable radical species For 1&taphthoquïnone the doublet ra&cal 1ssunply the qumone rad~calamon as the tm adduet to a pam-qumone m reactton (3) ISprobabIy less effiment at 77 K F-h3sncl
5
34 5 60 558 548 563
*
970
10 90 9.52
Ph3sncoocH3
558 540 5 34 17 24 23 10 2958 ~ 513
The ZFS parameters obtamed for different sold solution systems are used to estunate the mean d~stances reff between the two radical 1011s WI~~UI the pnmary
1011 paw.
15 December 1981
CHEMICAL PHISYCS LETTERS
Volume 84, number 3
The m aximum split tinp of the chpoleIS gtven [8] by the famlhar relation-
&pole mteractlon
we the ESR results provide duect evidence For the electron transfer process m the quenchmg of excited triplet qutnone by organottn, these systems affor a great opportunity to study the CIDEP phenomena of pnmary radical pau-s III sohd state.
&P
D = -; ~.&*/47&, where the symbols have their usual meanmg For the Ph,Snr/Qradical Ion pairs m tetraphenyltm, the estunated reFt IS LYIthe range of 7 76-7.89 A, dependmg upon mdlvldual qumones Thus result IS considered to be very reasonable, smce the nearest-neghbour dism tetraphenyltln crystal IS 6 65 8, (P42 1 c, II = 11.85, c = 6.65 a, z = 2) [9,10] In the trlphenyltm chiorlde crystal (P2t/a, a = 18 65,b=9_59,c= 19.02 &B= 105_5”,Z=8) [II],
tance
the estimated reff for the Ph3SnCl*/Q-
radical ion Parr 1s III the range of 6 3-2-6 5 1 A. Here, the nearestneighbour distance in tnphenyltm chloride crystal is
5.74 ,k It IS interestmg to note that this distance may &crease tion to
at low temperature,
because the phase transl-
a polymeric coo&nation structure ~th tngonal b~pyranudal coordmation of the tm atom has been suggested by NQR study [ 121 Finally, for the trlphenyltin acetate-qumone system the estmated reff for the Ph3SnCOOC@3/Qradical ion pair vaned substantially dependmg upon the types of qumones. Thus, reff = 4 56-5 34 a for ortho-qumones and reK = 7 78-7.90 i$ for paraqumones. The crystal data for tnphenyltm acetate
are
not available m hterature, but the relatively shorter rcff for the ortho-qumone tnphenyltm acetate rarhcal ion pau can be ranonnhzed by takmg mto account the acetate bndgmg between tin atoms wluch was suggested by the IR spectrum of solid rnphenyltln acetate
t131.
514
Thts research 1s supported by the Natural Sciences and Engmeermg Research Council of Canada. DW kvlshes to acknowledge the award of a Queen’s Graduate Fellowship.
References K Koch], K S Chen and J K S Wan.Chem Phys Letters73 (1980) 5.57. J Phys Chem. 85 (1981), to be pubhshed 121 S K Kochl, Organomctrdhc mcchamsms and catalysis (Acadermc Press New York, 1978) I31 H C Gardner and J K Koclu, J Am Chcm Sot 98 (1976) 2460. J Y Chen, H C. Gardner and J K Koch. J Am Chem. Sot 98 (1976) 6150 I41 K hlochlda, J K_ Kochl, K S Chen and J K S Wan, J Am. Chem Sot. 100 (1978) 2927. 151 G A Razuvaev, V A TsarJapkm, L V Gorbunova, V K Cherkasov. G A. Abakumov and E S Khmov. J Organomet Chcm 174 (1979) 47 161 A Albert1 and A Hudson, J Chem Sot Perkm 11 (1979) 1098, I; S Chen,T. roster and J K S Wtn. J Chem Sot Perkm II (1979) 1288 t71 E M Kosower, Pros Phys Ore Chcm. 3 (1965) 81.
111 J
ISI I91 1101 [ItI II’-1 1131
W Cordy and R Morehouse,
Phys
Rev 151 (1966)
207 C Gmcomello. Gazz Chrn ItaL 68 (1938) 422. N A Alchmod and G G Aleksandrov, J. Struct Chem. I I (1970) 824 N G Boku, C N Zakharova and Yu T Stiuchkov, J Struct Chem 11 (1970) 828. T Srwnstnva, J. Organomet Chem LO (1967) 373 N-W. iUcodc and RE Tunes. J Chem Sot. A (1968) 1873
CHFMICAL PHISYCS LETTERS
15 December 1981
E!3REMDENCE OF TIiE ELECTRON-TRANSFER PROCESSESM THE SOLID STATE PHOTOCHEMICALREACTIONS BETWEEN ORGANOTIN COMFOUNDS AND QUINONES THE TRJPLET STATE OF THE PRIMARY RADICAL ION PAIRS S~UJI
EMORI, Douglas WERI and Jeffrey K S WAN
Departmwt af Chemntry, Queen’s Utnverslty, Km,qston Ontano, Canada K7L 3N6 Rccewcd 29 July 198 1
The ESR observatlons and charactcnzatlon of the trtplct raduxd ron paus formed dunng the trraduttlon of B sobd solunon of qumone m oganotm at 77 K estabhsh uncqutvocally the pnmary electron transfer proccss between the orgmotm compounds and qumones
The ablhty of qumones to form paramagnets radlcal comulexes unth a vanety of organometals has been recewmg mcreased attentton To estabhsh the nature of the pnmary photochemlcal promessesbetween the qumone exclted tnplets and organometals such as organotm compounds, we have recently apphed the tlme-resolved CIDEP (chemlcally mduced dynamlc electron polanzatlon) to the paramagnehc organometal adducts of qumones [ 11 Whde the CIDEP observatlons can be readdy accounted for by a charge-transfer mechamsm for the quenchmg of qumone tnplet, mvolvmg a pnmary radlcal Ion pair of qumone and the organotm, &rect spectroscoplc evtdence of the pnmary electrc” transfer radeal ion paw m thls Important model process 1shere-to-fore stil1 lackmg Prenous chemrcal studm have suggested organotm compounds as h$ly effectwe electron donors m a vanety of charge-transfer mteractlons [2] For example, the thermai charge transfer addltlon reaction of tetracyanoethylene to vanous tetraalkyltms was found to proceed also photochemlcally when bemg lrrddlated wlthm the charge-transfer ab. sorptlon band [3] Other studtes mcludmg the reactlon of hexaalkyldmn wlth phenanthroqumone [4], substltuted o-qumones [S1) and pqumones [6] also Jtpported thc charge-transfer concept wluch was Brst formulated by Kosower [7] who proposed that photochemlcal and thermal processes of electron transfermay share common pathways leadmg to the 512
formatlon of the same 1011pairs In thls report, we have unequrvocally estabhshed the prtmary electron trans fer process m the quenchmg of tnplet qumones by organotm by a direct ESR observatlon and charactenzatlon of the tnplet state of thc pnmary radlcal ion parrs formed m the photolysls of a sohd sofutlon of qumones m organotm at 77 K A small amount of qumone (1%) wat added to phenyltm compounds Ph.$nX (X = Ph, Cl, CH$!OO) Thc muLture was melted by gentle warrr mg and degaaicd under high vacuum The meltmg pomts are 227,105, and 123’C for Ph,Sn, Ph,SnCI, and Ph3SnCOOCH3, respecttvely The samplc was then sealed off under vacuum and Irradlated dt 77 K mthm the mrcrowave cavlty of a Vanan B3 X-band spectrometer The hght source was a super pressure Hg lamp Upon lrradmtlon of the sohd solutlon iit 77 K. a typtcal ESR spectrum shown m fig. 1 was observed The ESR spectrum exfnblts the characrenstic features of an &y symmettrc tnplet state speaes as well as a streng absorptlon due to the doublet state of some radlcals For all the systems studled the Mis = 2 tranntlon of the fnplet state ISalso observed, The ESR observahons a@ accounted for by the emtence of a radlcal IOII pan trapped m the polyc~staltie sample at 77 K. The det&d ana!yss of the different ZFS parameters dependmg upon the ccMñnatlons of mdmdual qumone w~th organotm compounds (table 1) confirms the mvolvement of both the quI-
x‘
”
-I
-
TJ^-
%.
r---l
Tlg 1 ESR spectraof a sobd solutron cuntauung 1% 2,5-dlphenyl-p~enxqumone UItetrapbenyltm at 77 K BeforePrad&on breken bne, Mter rnadnt~~n sohdhm
none and the organotm as the two radrcal components formmg the tnplet state of the rad& ton pan Thus, the photochemlcal reaction must proceed na an electron transfer mechamsm
77K (Q- +Ph3Snx’)P, (Q- + Ph3SnX+)p, -t Q- + Ph$n+ + X Q- + ph$n+ + Ph3SozQ
(3)
Photolysts at 77 Kof a pure Ph3SnCl or Ph3SnCQQCH3 polycrystalhne sample did not yleId an) tnplet ESR stgnals, as the presence of qumone was necessary to xmtite the photochenncal electron transfer teactlon The observed doublet radzal agnak TabIe 1 ZFS parameters ( 10e3 cm-“) for the radxal loonpaus Ph$nX+/QQmuone
15 December i981
CHEMICAL PHISYCS LJZIERS
Volume 84, numbcr 3
acceptor
1,4-benzoqumone
2.S-d~pheuyl-pbenzuqumonc 1,4-xtaphthoquumne 9,l~thraqumone nlunl”tlltyfn’.“~ r -__ >olthu-naphthcquuotte 3,Sd+butykAenzuqmno~e camphoroqumone
Ph&
_
x
5.59 5 47
-
*
m the~~,sofidSoI&on *systemsboth before and^durmg the $kM&&r&&iëthé folimatson of radlcal spec~&e~~~~~~ and (3) ETa thermal process and~tlZ@e “primarvii%chonTl) can be mttlated thermally but oií@af &íii?r teinperatu- Smce the sohd solutlon was prepared from the ml 1‘at hg& temperatures, reactrons wdl proked and douolct radlcals wdl be formed However, when the solutlon was n+ medntely cooled to 77 K wtthout madtitlon, no tnpief s~@Were obsaved, akhough the doublet rad& mgnals obnously remamed Tfus fact suggests that the tnplet specíes formcd durmg the photolfis at 77 K ISnot due to the randomly trapped of ;le radral spehes from reacttons (2) and (3) Therefore, the tnpfet state denves from the trappme of the pnmary radrcal mn pur, produced dnectly from the pnmary electron tranfir process m reacbon (1) While the electron-transfer process in the quenchmg of an exated triplet qumone by organometal wlth low iomzatmn potenttals has generally been assumed, we must emphasze that only now have we concluave iy Identlfìed the pnmary radical ion pan by tnplet ESR and thus uneqmvocally prowded the physrcal endence of the electron transfer process ‘TheIdentlty of the doublet radlcal speacs m tha crystal after photolysls can be obtamed m many cases by slmpIy dtssolvmg the sohd m tobtene and the solutron ESR spectrum of the radtcal would be v el1 resolved For example, for the orthoqumones such as phenanthroqumone and 3,5-dl-r-built-benzoqua’anc the solution ESR spectra Identlfy the tnphenyltm mdlcal adduct to qumone Ph3Sn-Q as *he stable radical species For 1&taphthoquïnone the doublet ra&cal 1ssunply the qumone rad~calamon as the tm adduet to a pam-qumone m reactton (3) ISprobabIy less effiment at 77 K F-h3sncl
5
34 5 60 558 548 563
*
970
10 90 9.52
Ph3sncoocH3
558 540 5 34 17 24 23 10 2958 ~ 513
The ZFS parameters obtamed for different sold solution systems are used to estunate the mean d~stances reff between the two radical 1011s WI~~UI the pnmary
1011 paw.
15 December 1981
CHEMICAL PHISYCS LETTERS
Volume 84, number 3
The m aximum split tinp of the chpoleIS gtven [8] by the famlhar relation-
&pole mteractlon
we the ESR results provide duect evidence For the electron transfer process m the quenchmg of excited triplet qutnone by organottn, these systems affor a great opportunity to study the CIDEP phenomena of pnmary radical pau-s III sohd state.
&P
D = -; ~.&*/47&, where the symbols have their usual meanmg For the Ph,Snr/Qradical Ion pairs m tetraphenyltm, the estunated reFt IS LYIthe range of 7 76-7.89 A, dependmg upon mdlvldual qumones Thus result IS considered to be very reasonable, smce the nearest-neghbour dism tetraphenyltln crystal IS 6 65 8, (P42 1 c, II = 11.85, c = 6.65 a, z = 2) [9,10] In the trlphenyltm chiorlde crystal (P2t/a, a = 18 65,b=9_59,c= 19.02 &B= 105_5”,Z=8) [II],
tance
the estimated reff for the Ph3SnCl*/Q-
radical ion Parr 1s III the range of 6 3-2-6 5 1 A. Here, the nearestneighbour distance in tnphenyltm chloride crystal is
5.74 ,k It IS interestmg to note that this distance may &crease tion to
at low temperature,
because the phase transl-
a polymeric coo&nation structure ~th tngonal b~pyranudal coordmation of the tm atom has been suggested by NQR study [ 121 Finally, for the trlphenyltin acetate-qumone system the estmated reff for the Ph3SnCOOC@3/Qradical ion pair vaned substantially dependmg upon the types of qumones. Thus, reff = 4 56-5 34 a for ortho-qumones and reK = 7 78-7.90 i$ for paraqumones. The crystal data for tnphenyltm acetate
are
not available m hterature, but the relatively shorter rcff for the ortho-qumone tnphenyltm acetate rarhcal ion pau can be ranonnhzed by takmg mto account the acetate bndgmg between tin atoms wluch was suggested by the IR spectrum of solid rnphenyltln acetate
t131.
514
Thts research 1s supported by the Natural Sciences and Engmeermg Research Council of Canada. DW kvlshes to acknowledge the award of a Queen’s Graduate Fellowship.
References K Koch], K S Chen and J K S Wan.Chem Phys Letters73 (1980) 5.57. J Phys Chem. 85 (1981), to be pubhshed 121 S K Kochl, Organomctrdhc mcchamsms and catalysis (Acadermc Press New York, 1978) I31 H C Gardner and J K Koclu, J Am Chcm Sot 98 (1976) 2460. J Y Chen, H C. Gardner and J K Koch. J Am Chem. Sot 98 (1976) 6150 I41 K hlochlda, J K_ Kochl, K S Chen and J K S Wan, J Am. Chem Sot. 100 (1978) 2927. 151 G A Razuvaev, V A TsarJapkm, L V Gorbunova, V K Cherkasov. G A. Abakumov and E S Khmov. J Organomet Chcm 174 (1979) 47 161 A Albert1 and A Hudson, J Chem Sot Perkm 11 (1979) 1098, I; S Chen,T. roster and J K S Wtn. J Chem Sot Perkm II (1979) 1288 t71 E M Kosower, Pros Phys Ore Chcm. 3 (1965) 81.
111 J
ISI I91 1101 [ItI II’-1 1131
W Cordy and R Morehouse,
Phys
Rev 151 (1966)
207 C Gmcomello. Gazz Chrn ItaL 68 (1938) 422. N A Alchmod and G G Aleksandrov, J. Struct Chem. I I (1970) 824 N G Boku, C N Zakharova and Yu T Stiuchkov, J Struct Chem 11 (1970) 828. T Srwnstnva, J. Organomet Chem LO (1967) 373 N-W. iUcodc and RE Tunes. J Chem Sot. A (1968) 1873