3934
F. Bolletta, M.Gleria, and V, Balzani
R
and Quenching of the Cis
+
Trans Isomerization
~t~?~latinum~l~~ abrizio Bolletta, Mario Gleria, and Vincenro Balzani* istiiuto Chimico "6.Ciamician" de//' Universita, Laboratorio di Fotochimica e Radiazioni d'Aita Energia de/ C N R , 40126 Boiogna, ltaiy (Received June 27, 1972)
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The possibility of sensitizing and quenching the cis trans intramolecular isomerization of Pt(g1y)z has been investigated using various triplet donors and acceptors. Pyrazine (26.2 kK) and xanthone (26 kK) trans isomerization (@qlm 0.03 and 0.1, respectively), whereas no were found to sensitize' the cis sensitization was found when thioxanthone (22.9 kK), quinoline (22 kK), naphthalene (21.2 kK), and biacetyl (19.2 kK) were used. Niaq2+ ions (8.9 kK) were found to quench the photoisomerization, whereas no quenching effect was obtained with Mnaqz+ ions (18.9 kK). The lack of sensitization by the donors in Lhe 22.9-19.2 kK range and the lack of quenching by Mnaq2+ (18.9 kK) are interpreted on the basis of unfavorable Franck-Condon factors due to the distortion of the excited state that is responsible for the cis -trans isomerization.
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Introduction Sensitization and quenching techniques have profitably been used in organic photochemistry to individualize the state(s) responsible for the observed photoreactivity.1 In the last few years, the use of these techniques has also become popular in the photochemistry of coordination compounds and some interesting results have already been obtained concerning the role played by the various excited states in the photoaquation of Cr(II1) complexes2 and in the redox decomposition of Co(II1) complexes.3 However, to our knowledge no example of sensitized or quenched isomerization of coordination compounds has so far been reported. In the field of organic photochemistry, on the contrary, sensitization and quenching of isomerization reactions have extensively been i n v e ~ t i g a t e d , land ?~ the results so obtained have given important information on the involvement of distorted excited states and have also generated stimulating discussions on nonvertical energy transfer and related prob1ems.l It seems obvious that the extension of such investigations to the intramolecular isomerization of coordination compounds may also offer interesting information as far as the excited state distortions and the energy transfer processes are concerned. We wish to report here a study concerning the sensitizatrans isomerization of tion and quenching of the cis Pt(g1y)z (gly = glycinato anion). The choice of this complex was based om the fact that, a t present, it i s the sole coordination compound which has been shown t o undergo a geometrical photoisomerization uia an intramolecular m e ~ h a n i s m . ~The a participation of distorted excited states in such an intramolecular isomerization reaction has already been discussed in some detail.536
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Materials. cis-Bis(glycinato)platinum(TI), cis-Pt(gly)z, was prepared following the indications given in ref 5a. trans-Pt(gly)z was obtained by the methods of Pinkard, el al 7 Xanthone (xanthen-9-one), thioxanthone (thioxanthen-%one), biacetyl (2,3-butanedione), and naphthalene "suitable for sensitizer use" were obtained from the Baker Chemical Co. and ysed without further purifications. ReThe Journai of Physicai Chemistry, VoI. 76, No. 26, 7972
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agent grade pyrazine was purified by vacuum sublimation. All of the other chemicals were of reagent grade. Apparatus. Radiations of 254 nm were obtained as previously described.8 Those of 313, 334, and 365 nm were obtained from a Hanau Q 400 mercury vapor lamp by using Schott and Genossin interference filters.9 Radiations of 401 nm were obtained as described in ref 10. The intensity of the incident light was measured by means of the ferric.oxalate actinometer,ll and was of the order of Nhulmin for all of the wavelengths used. The fraction of the absorbed light was calculated on the basis of the transmittance of the solutions. Spectrophotometric measurements were performed with an Qptica CF4 NI spectrophotometer and the emission spectra were recorded with a Turner Spectro 210 spectrofluorimeter. Procedures. The general procedure for the study of the sensitization and quenching of the cis-Pt(gly)z isomerization was as follows. Weighed amounts of the complex and of the potential sensitizer or quencher were dissolved in the selected medium (water or water-ethanol 50% v/v). A sample ( 3 ml) of the solution was put into a spectrophotoA. A. Lamola, "Energy Transfer and Organic Photochemistry," A. A . Lamola and N. J, Turro, Ed., Interscience. New York, N. Y., 1969, p17. (a) G . E. Porter, S. N. Chen, H . L. Sch!afer, and H. Gausmann, Theor. Chim. Acta, 20, 81 ( l 9 7 1 ) ; ( b ) C. H. Langford and I-. Tipping, Can. J , Chem., 50, 887 (1971); ( c ) N. Sabbatini and V. Baizani, J. Amer. Chem. SOC.,in press. M. A. Scandola and F. Scandola, J . Amer. Chem. SOC.. 92, 7278 119701. > - -, (a) N. J. Turro, Photochem. Photobioi., 9, 555 (1969); (b) J. Saitiel and E. D.Maaarit. J. Amer. Chem. Soc.. 94. 2742 (1972) , , and references citediherein. (a) F. Scandola, 0. Traverso, V. Balzani. G . L. Zucchini, and V. Carassiti, Inorg. Chim. Acta, 1, 76 (1967); (b) V, Balzani and V. Carassiti, J. Phys. Chem., 72, 383 (1968); (c) F. S. Richardson, D. D. Shillady, and A. Waldrop, Inorg. Chim. Acta, 5, 279 (1971). V. Balzani and V. Carassiti, "Photochemistry of Coordination Compounds," Academic Press, London. 1970. F. W. Pinkard, E. Sharrat, and W. Wardlaw, J. Chem. SOC., 1012 (1934). V. Balzani, V. Carassiti, L. Moggi, and F. Scandola, inorq. Chem., 4,1243 ( 1 965). V . Balzani, R . Ballardini, N. Sabbatini, and L. Moggi, Inorg. Chem., 7.1398 11968). V. Balzani, R. Ballardini, M. T. Gandolfi, and L Moggi, J. Amer. Chem. Soc.. 93,339 (1971). C . 6 . Hatchard and C. A. Parker, Proc. Royal SOC.. Ser. A, 235, 518 (1956).
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~I
3935
in the thermostated (25") cell tquipment. All of the experiut at a pH of 6-1 since it has been showrr that inr ilxro p3-1 nmge cis-Pt(gly)~.only undergoes ~ i LIS ~I1 a m piicitunsomeriial ~ ~ ion.*2~After a e bli~ ~rradmk~anijierrod, thc irradiated solution was metric cell that
'~"JBS$aced
RLKXI
~
4
~
Pyparlt?e Xanthorse Thicxanihane Qu in01 I nc
Naphthalone
sild biawtyi were found to mterfere a j h c beparatiun, they w ~ r cpxtracted
Uiacslyl 11aq2 +
P\liaq*
+
~ 26.2 28.0
'i'S.9 22.0 21.2 19 2 18.9C
~
~
~
Yes Yes N0 o li!0
hiQb No Yes
0.5
'Energy of the lowest triplet excited state unless otherwise noted. 'The biacetyl phosphorescence is quenched (see text). 'This excited state, which is the lowest one for Mnaq2+, is a quartet.
rescent t o fhoresceai cinrrsion was
'I%e mar:imum soiubilky of czs-Pt(gly)z m water at 25" ~ b i i u t2 X 10-3 M and the trans isomer was about fava t:nres less solu5ie Bn all of the other solvents suitable for ~ ~ ~ ~ ~ o estudies ~ i e m both i ~the a ~complexes were much !sss d u b l t . . For this reasoc, and because of the small extinictionl coefficients of cis-Pt(g l y ) ~above 230 nm, only wtic3:i r)r water-cthuro: 50% v/v mixtures could bp used as a fiiihenit. 'Thas, L I COI ~ rse, somewhat limited the possrioilicy of using standard donors since many of them are water urganie mol~cules.l~13 314 Moreover, some of the mors that wore soluble in water or water-ethanol (specifically, ~ i p ~ ~ e n ~ 7 ~ a fi-naphthylamine, mine, one, p-toliaenesulfonic acid, and naphthalene-lacid) wcrc found to undergo photodecomposition and o4~herctoraors, like acetone and biphenyl, nd to give s e m a p photoreaction with czs-Pt(gly)z. be e o r n p ~ m d hnd ~ i Ihais to be discarded As fox 8 s the qrnenclzing of the isomerization reaction ~ 7 2 1 5 li c~uaccrnd, the smd1 absorptlon coefficlent and the 108%oclnbility of t 8s Pt(gly)s did not allow us to use any of the standard mganrc querachers. However, the hydrated 10k'as of s3mF" ?,*cinsitl metal6 proved to be suited for aransitionwas determined to be f = 0.11. The in7s* tersystem crossing quantum yield was measured using excitation radiation corresponding to both T excitation (240-300 nm) and n r* excitation (310-350 nm). The value of the intersystem crossing quantum yield, $ ~ s c= 0.99 & 0.07, was found to be independent of the excitation region within the experiment uncertainty. The lifetime of the phosphorescent state was also measured and was found to be indepmdent of the excitation region. The phosphorescent lifetime was measured to be T~ = 0.42 f 0.04 see,
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ti^^
Phthalazine (2,3-diiizanaphthalene) has recently been ~ - ~ of the possibility of the subject of much 5 t ~ d y because strong interactiun between the nonbonding electron pairs. It is now generdly accepted that the lowest excited singlets is of type I & ( n , n * ) which in the pure crystal is located 23,577 ern-1 abcive the ground state. The next higher excited singlet is of type lAl(?r,a*), 32,250 cm-1 above the ground state The lowest triplet state132 is of type 3Bz (r7n-*)? with energy 22,780 rm-1 above the ground state. Little information 15 available concerning the dependence (if any) of the iumiriescent and radiationless processes
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such as fluorescence, phosphorescence, and intersystem crossing on the wavelength of absorption. Li and Lim4 report that the phosphorescence quantum yield of phtbalazine changes by a factor of 3 upom change in excitation wavelength. They reported that the phosphorescent quantum yield for excitation wavelengths less than 310 nm was three .times greater than the quantum yield for excitation (1) E. C . Lim and J. Stanislaus, J. Chem. Phys., 53, 2096 (:970). (2) H. Baba, I . Yamazaki, and T. Takemura, Specrochim. Acta, Part A , 27, 1271 (1971). (3) R . M. Hochstrasser and D. A. Wiersma, J . Cham. Phys.. 56, 528 (1972). (4) Y. H. l_iand E. 6.Lim, J . Chem. Phys., 56,1004 (1972) The Journal ot Physical Chemistry Voi 76 No. 26, !972
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