Journal of Photochemistry and Photobiology, A: Chemistry, 44 (1988) 143 - 152 143

ABILITY OF PLATINUM(H) COMPLEXES OF 2,2’-BIPYRIDINE AND l,lO-PHENANTHROLINE WITH HALIDES AND PSEUDO-HALIDES TO PHOTOSENSITIZE THE PRODUCTION OF ‘02

S. SHUKLA, S. S. KAMATH and T. S. SRIVASTAVA?

Department of Chemistry, Indian Institute of Technology, Powai, Bombay 400076 {India)

(Received September 17,1987; in revised form March 16,1988)

Summary

[Pt(NN)X,] complexes (where NN is l,lO-phenanthroline or 2,2’- bipyridine, and X is N3, CN, Cl, SCN, Br or I) have been found to be photo- sensitizers in the photo-oxidation of 2,2,6,6-tetramethyl-4-piperidinol to the nitroxide radical. This photo-oxidation reaction involves singlet molecular oxygen as an intermediate and its presence has been confirmed in quenching studies using bis-(di-n-butyldithiocarbamato)nickel(II) as an efficient ‘02 quencher with a quenching rate constant of 1.9 X 10” M-l s-l. The ability of l-Pt(NN)(X),] complexes to photosensitize the above photo-oxidation reaction follows the order [Pt(NN)(N,),] > [Pt(NN)(CN),] > [Pt(NN)C12] > [Pt(NN)(SCN)2J > [Pt(NN)Br,] > [Pt(NN)I,]. With the exception of the cyanide ion, this order parallels the positions of the ligand-field splitting of X in the spectrochemical series.

1. Introduction

Photochemical oxidative addition of chlorine derived from chloroform has been reported to occur on irradiation of [Pt(bipy)C12 ] at a wavelength of 280 nm [ 11. 2,2’-Bipyridine and related ligands form the luminescent ruthenium( II), osmium( II) and iridium( III) complexes which photosen- sitize the production of ‘02 [2 - 41. Complexes of palladium( II) and platinum(H) of type [M(bipy)X2] (where M - Pd or I%, bipy E 2,2’-bipyri- dine and XX - dianion of catechol, 4-tert-butylcatechol, thiosalicylic acid or 3,4dimercaptotoluene) give an absorption band above 400 nm correspon- ding to ligand-to-ligand charge transfer and they photosensitize the genera- tion of ‘02 [ 5, 61. In this paper we report that [Pt( bipy)X2], and [Pt(phen)- X2] (where phen E l,lO-phenanthroline and X s Cl, Br, I, SCN, N3 or CN)

+Author to whom correspondence should be addressed.

lOlO-6030/88/$3.50 0 Elsevier Sequoia/Printed in The Netherlands

144

photosensitize the generation of ‘OZ. The ability of these complexes to photosensitize the production of ‘02 depends on the nature of the ligands attached to platinum(I1).

2. Experimental details

2.1. Materials [ Pt( phen)X*] and [ Pt( bipy)X,] were prepared and characterized by the

literature methods [ 7 - lo]. Potassium tetrachloroplatinate( II) (Strem, U.S.A.), 1, lo-phenanthroline monohydrate and 2,2’-bipyridine (RDH, India) and hematoporphyrin IX (Sigma, U.S.A.) were of reagent grade and used as such. Other chemicals used were of analytical grade. The reagent grade solvents were purified before use by standard procedures [ 111.

2.2. Measurements The absorption spectra of [Pt(phen)X,] and [ Pt( bipy)X*] in dimethyl-

formamide (DMF) were recorded on a Shimadzu UV-260 UV-visible recording spectrophotometer. The electron paramagnetic resonance (EPR) spectra of nitroxide (NO) radicals formed by reaction of 2,2,6,6-tetra- methyl-4-piperidinol (NH) molecules with ‘02 molecules [ 123 were recorded on a Varian E-12 ESR spectrometer (X-band) using an aqueous EPR cell. The quantitative EPR measurements were made by comparing the spectrum of the solution with an unknown concentration of nitroxide radicals with the spectrum of a known concentration of nitroxide radicals under identical instrumental conditions.

2.3. Irradiations (a) The irradiation of solutions was carried out on a merry-go-round

apparatus. The stabilized light source of a 150 W tungsten-halogen lamp operating at 18 V was housed at the centre of a double-jacketed Pyrex vessel thermostatted at 28 ? 1 “c by circulating water through the jacket. The solu- tions to be irradiated were contained in Pyrex tubes (20 cm X 1 cm) and DMF-saturated molecular oxygen was bubbled into them for 15 min. They were kept on a merry-go-gound apparatus about 5 cm from the light source, revolving at a speed of 33 rev min-i.

(b) Typically, molecular-oxygen-saturated DMF solutions of [ Pt( NN)- X2 ] (200 pm) were irradiated in the presence of 2,2,6,6-tetramethyl-4-piperi- dinol (10 mM) with light of wavelengths from 300 to 800 nm for different irradiation times in the merry-go-round apparatus. The ‘02 molecules formed combine with 2,2,6,6-tetramethyl-4-piperidinol molecules to produce nitroxide radicals. The amount of nitroxide radicals was measured by the EPR method (see Section 2.2). Furthermore, the formation of nitroxide radicals was not observed when the solution of the platinum complex and 2,2,6,6-tetramethyl-4-piperidinol was kept in the dark or irradiated in the presence of molecular nitrogen.

145

(c) Molecular-oxygen-saturated solutions containing [Pt( phen)(NJ)J (5 X lo-’ M), bis(di-n-butyldithiocarbamato)nickeI(II) (lo-’ or low6 M) and five different concentrations of 2,2,6,6-tetramethyL4piperidinol (ranging from 1 X 10-l to 2 X 10y3 M) were irradiated using light of wavelengths ranging from 300 to 800 nm for 1 h. The amount of nitroxide radicals produced after irradiation for 1 h was measured by EPR (see Section 2;2).

3. Results

3.1. Electronic absorption spectra of f Pt(NN)X,] The electronic absorption maxima of [Pt(phen)XJ and [Pt(bipy)X,]

and their extinction coefficients in DMF are given in Table 1. These spectra

TABLE 1

Electronic absorption spectral data for [Pt(NN)Xz] in DMF

Complex Band maxima A,, (nm)

Bund 1 Band 2 Band 3 Band 4

[Pt(phenY% 1

WWen)Brz 1

IPtWen)I2 1

[WPhen)(SCNh 1

[WPhen)(CNh 1

lWPhen)(N3)2 1

lWbiPy)C12 1

WWPy)Br2 1

IF’WiPvV2 1

EPWiPvl(sCW 1

lWbiPv)(CNh 1

IWbiPyW3)2 1

392 (0.35)a

397 (0.29)

390 (0.44)

381 (0.13)

376 (0.47)

402 (0.40)

390 (0.36)

393 376 (0.44) (sh)

394 378 (0.26) (sh)

398 364

(sh) (0.22)

370 (0.46)

398 360 (0.39) Csh)

370 (0.29)

370

(shJb

363 (0.46)

361

(sh)

355 (0.40)

350 (0.48)

326 314 (0.99) (0.86)

327 313 (1.31) (1.04)

327 (1.05)

326 312 (0.80) (0.76)

325 313 (1.32) (1:04)

325 310 (0.89) (sh)

327 (0.51)

330 (0.42)

300

(sh)

320

(sh)

323 (0.67)

295

(sh)

280 (2.11)

280

(sh)

290

(sh)

282

tsh)

278 (1.57)

287 (1.67)

278

(2.05)

268 (2.38)

268 (2.00)

270 (1.71)

270 (2.24)

270 (2.36)

260

(sh)

268 (1.52)

258 (1.31)

268 (1.52)

260

(sh)

270

(sh)

aExtinction coefficients in 1 mol-l cm-l X lo4 are given in parentheses. bsh is shoulder_

146

reveal four bands, tentatively assigned as in ref. 13 and 14. In [Pt(phen)XJ, bands 1 and 3 are due to the charge transfer transitions from the platinum d orbitals to fl antibonding orbitals of l,lO-phenanthroline. Bands 2 and 4 of [Pt(phen)Xa] are assigned to intraligand x-n* transitions of l,lO-phenan- throline. Similarly, bands 1 and 3 of [Pt(bipy)X,] are due to charge transfer transitions from the platinum d orbital to 7r antibonding orbitals of 2,2’- bipyridine and bands 2 and 4 are assigned to intraligand x+r* transitions of 2,2’-bipyridine.

3.2. Photo-oxidation of 2,2,6,6-tetramethyl-4-piperidinol sensitized by

/ Pt(phen)W The photo-oxidation of 2,2,6,6-tetramethyl-4-piperidinol sensitized by

[Pt(phen)X,] gives nitroxide radicals (see paragraph (b) of Section 2.3). There is a linear relationship between the amounts of nitroxide radicals formed and the irradiation times for the different [Pt(phen)X,] complexes, as shown in Fig. 1. [Pt(phen)(N,),] shows the steepest slope, suggesting that

4-Q- pt(Phen) ( WJI~]

- [Ptcphen) (cN)~]

- [Pt(phen) Cl2 3

-o-o- [Pt( phen) (SCN)d

@ [Pt(phen) ~~~ ]

= [Pt (phen) I2 1

o

0.0 0 20 LO 60 80

IRRADlATlON TIME IN MINUTES -

Fig. 1. Plots of amount of nitroxide radicals produced on irradiation of [Pt(phen)X2] (200 pm) in the presence of 2,2,6,6-tetramethyl-4-piperidinol (10 mM) in DMF us. irradiation time.

147

TABLE 2

Relative yields and efficiencies of [Pt( NN)Xz] for sensitizing the generation of IO2

Complex Relative yield of IO2 Relative efficiency’ of production referred ‘02 production referred to to F’t(NWN3)zl CWNWW&l

1.00 1.00 0.33 0.41 0.20 0.19 0.10 0.15 0.06 0.07 0.03 0.04

1.00 1.00 0.27 0.31 0.20 0.19 0.12 0.16 0.04 0.05 0.02 0.02

aRelative integrated areas under absorption curves between 300 and 800 nm of [Pt-

(phen)(N3)21, WW-)(CVd, IWphen)CM, [PW=n)(SCNM, fPt(phen)Br21 and [Pt- (phen)12] for 1 X 10e4 M are in the ratio 1.00:0.81:1.03:0.69:0.81:0.73 respectively; similarly, relative integrated areas of [Pt(bipy)(N3)2], [Pt(bipy)(CN)z], [Pt(bipy)Clz],

[PtWpy)(SCN)21, [Wbipy)Br21 and [PtWw)I21 f or 1 X 10e4-are in the ratio 1.00:0.88: 1.07:0.75:0.73:1.07 respectively.

this complex gives the highest IO2 yield. On the other hand, [Pt(phen)I,J shows the smallest slope and thus this complex gives the lowest ‘02 yield. The yields of IO2 production by the different complexes referred to [Pt- (phen)(N,),] have been compared and the values are given in Table 2. The efficiences of IO 2 production by the different complexes referred to [Pt(phen)(N&] after correcting for the relative amounts of light absorbed have also been compared and the values are also given in Table 2. The relative efficiences or relative yields of different complexes for sensitizing the production of ‘02 follow the order [Pt(phen)(N,),] > [Pt(phen)(CN)J > [Pt(phen)Cl,] > [Pt(phen)(SCN)J > [Pt(phen)BrJ > [Pt(phen)IJ.

3.3. Photo-oxidation of 2,2,6,6-tetramethyl-4-piperidinol sensitized by

/ Pt(bipy)&I The linear plots of the amount of nitroxide radicals produced for

[Pt(bipy)X2] us. irradiation time are given in Fig. 2. [Pt(bipy)(N,),] gives the highest slope and [Pt(bipy)I,] the smallest slope. The values of the relative yield of IO2 production by the different complexes referred to [Pt(bipy)- (N&l are given in Table 2. The values of the relative .efficiency of lo2 production by the different complexes after absorption correction are also given in Table 2. The relative yields or relative efficiencies of different com- plexes for sensitizing the production of ‘02 follow the order [Pt(bipy)(N,),] > [Pt(bipy)(CN), 3 > [Pt(bipy)ClJ > [Pt( bipy)(SCN)J > [Pt( bipy)Br,] >. [Pt( bipy)I,] . This order is the same as that obtained for [Pt(phen)XJ . :

148

N [Pt(biPY) ( ~3121

- [Pt( biPY) (CN)2]

- [@(bipy) C12 ]

-D-O- [Pt(biPY) (SCN)2]

-n-n- [Pt(biPY) er2]

- [Pt(biPY) 12 J

I-

0.0 Y T 7 I

0 20 40 60 80 100

IRRADIATION TIME IN MINUTES -

Fig. 2. Plots of amount of nitroxide (NO) radicals produced on irradiation of [Pt(bipy)- X2 J (200 pm) in the presence of 2,2,6,6-tetramethyl-4-piperidinol (10 mM) in DMF us. irradiation time.

3.4. Quenching of photu-oxidation of 2,2,6,6-tetramethyl-4-piperidinol sensitized by [Pt(phen)(N&j in the presence of bis(di-n-butyldithio- carbama to)nickel(II)

Two concentrations of bis(di-n-butyldithiocarbamato)nickel(II) were used in the quenching of the photo-oxidation of Z&2,6,6-tetramethyl-4- piperidinol sensitized by [ Pt(phen)( Ns)J ( see paragraph (c) of Section 2.3). l/[NO] was plotted against l/[NH] for the two concentrations of the nickel- (II) complex [ 151, and two linear plots with their intercepts on the y axis meeting at the same point were obtained. They are shown in Fig. 3. The k, values obtained for the nickel(H) complex are 1.30 X 1O1* M-l s-i and 2.50 X lOlo M-l s-’ respectively for concentrations of 10m5 M and 10e6 M. The average k, is 1.9 X 10” M-l s-l.

149

[NH]-’ (M-‘1 -

Fig. 3. Plots of reciprocal of nitroxide (NO) radical concentration us. reciprocal of 2,2,6,6-tetramethyld-piperidinol (NH) concentration in the presence of 1 X 10Y5 M (curve a) and 1 x 10e6 M (curve b) bis(di-n-butyldithiocarbamato)nickel(II).

4. Discussion

The above results suggest that the following mechanism of ‘02 genera- tion is involved in the photo-oxidation of 2,2,6,6-tetramethyl-4-piperidinol sensitized by [E%(NN)X,] . The last step in parentheses is also included for nickel(I1) complex as a physical quencher.

hv s - ‘s

ISC ‘S----t 33

ET 3s+30*- s+‘oz

k, lo2 +NH- NHOz -NO

(‘0,+Q++30Z+Q)

150

S is [Pt(NN)X,] photosensitizer, its first excited singlet state is ‘S, its first excited triplet state is %, hv is the energy of a photon, ISC is inter- system crossing, ET is energy transfer, kd is the rate constant of quenching of IO2 by DMF solvent molecules, 12, is the rate constant of chemical quenching of ‘02 by 2,2,6,6-tetramethyl-4-piperidinol (NH) to produce nitroxide radicals (NO) and k, is the rate constant of physical quenching by the bis( di-n-butyldithiocarbamato)nickel( II) as the ‘02 quencher (Q).

On the basis of the above mechanism, the rate of formation of nitroxide radicals by the oxidation of NH with IO2 under steady state con- ditions is given by

d[INOl = &absho

krWH1 dt * k,[NH] + kd

(1)

I abs is the intensity of irradiation and Glo2 is the quantum yield of ‘02 production. If Iabs is not constant for the different complexes and the k,, k, and [NH] are kept constant, the slope of the plot of the amount of nitroxide radicals produced vs. irradiation time is related to labs+fo (yield of ‘02 pro- duction). The slopes of the linear plots of amount 08 nitroxide radicals produced vs. irradiation time for [Pt(NN)X,] are shown in Figs. 1 and 2. The relative yields of IO2 production for the different complexes referred to IWNWN,),l were obtained from the slopes of Figs. 1 and 2 and are given in Table 2. On correcting for the relative amounts of light absorbed (or relative integrated areas under the absorption curves) by the different com- plexes using eqn. (2)

@102)2 = tslope)2 x (labsh

o?h0,h (slope)l x kbs)2 (2)

the relative efficiencies of ‘02 production for the different complexes referred to [Pt(NN)(N,),] were obtained. These values are also given in Table 2. The relative efficiencies of ‘02 production follow the order [Pt(NN)(N,),] > [Pt(NN)(CN),] > [Pt(NN)Cl,] > [Pt(NN)(SCN),] > [Pt(NN)Br,] > [Pt- (NN)I,]. The order seems to depend on the positions of ligand-field splitting of X in the spectrochemical series, with the exception of the cyanide ion.

If the last step in the above mechanism is included, the 12, value can be obtained from the relation [ 151

k, =

i

slope k, kd -- intercept [Q ] L&l

(3)

The values of the intercept and slope in eqn. (3) can be obtained from the linear plot of l/[NO] vs. l/[NH] (see Fig. 3). k, = 5.04 X 10’ M-’ s-l [12, 15, 163, k, = 1.40 X lo5 s-l (DMF) 1171 and [Q] = lo-’ or 10Y6 M are also known for eqn. (3). On substituting these values into eqn. (3), the unknown kq can be calculated (see Section 3.4). The average value of k, is 1.9 X 1O’O M-l s-‘, which is close to the value reported in ref. 17. From the plot in Fig. 3, it is clear that the intercept on the y axis does not change with a change in the concentration of the nickel complex. However, the slope

151

increases with increasing concentration of nickel(U) complex. These results suggest that the deactivation of ‘02 physical quenching is mainly involved in the above reaction [ 171.

F’YNWM complexes show four absorption bands. The triplet state (d,n*) of the longest-wavelength band is photochemically important [IS]. The ability of [Pt(NN)X,] to photosensitize the generation of ‘0, depends mainly on the positions of the ligand field splitting of the X anions in the spectrochemical series [ 191. [Pt(NN)X,] complexes have CZV symmetry and their ligand-field triplet state, 3A2, corresponds to the ligand-field 3B1s state of platinum(I1) porphyrin in D a symmetry [20, 211. The bromo and par- ticularly the iodo complexes are expected to have a ligand-field 3A2 state energy just below the lowest triplet state (d,n*). They show the lowest ef- ficiency of ‘02 production because the 3A2 state may provide a pathway for radiationless decay. In the sulphur-bonded thiocyanato complexes the ligand-field 3A2 state energy is at a position such that it competes ap- preciably with the lowest triplet state (d,r*) and thus IO2 is generated less efficiently. In the chloro complexes, the 3A2 state has a somewhat higher energy than the lowest triplet state (d,n*) of the thiocyanato complexes and thus the former complexes are better photosensitizers for the generation of lO2. In the azido complexes the crystal-field 3A2 state has a much higher energy than the lowest triplet state (d,n*) because of the stronger ligand field of azide ion. The @and-field 3A2 state of the azido complexes cannot provide a pathway for radiationless decay. Thus, the azido complexes behave as good photosensitizers. In the cyan0 complexes, the ligand-field 3A2 state is expected to have even higher energy with respect to the lowest triplet state (d,m*) than in the azido complexes, because the cyanide ion has a very strong ligand field. However, the relatively poor photosensitizing ability of cyan0 complexes compared with the azido complexes suggests that some photodissociation of the Pt-CN bond may be involved. Thus, the photo- sensitizing ability of [Pt(NN)X,] depends on the energy difference between the lowest triplet state (d,n*) and the ligand-field 3A2 state. The larger is this difference the better is its ability to photosensitize ‘02 production.

The ability of [Pt(phen)(N,),J and [Pt(bipy)(N,),] to photosensitize the generation of ‘02 was compared with that of hematoporphyrm IX. W(phen)(N&l and [Pt(bipy)(N,),] show 71% and 66% efficiency respec- tively in the production of ‘02 compared with hematoporphyrin IX, after correcting for the relative amounts of light absorbed.

5. Conclusion

The results obtained in this study show that the different platinum(I1) diimine complexes can mediate the net photo-oxidation reaction involving singlet molecular oxygen as an intermediate. The anionic ligands in these complexes have pronounced effects on their ability to photosensitize the above reaction.

152

Acknowledgment

We thank the Department of Science India) for financial support of this research.

References

and Technology (Government of

1 2 3

4 5 6 7 8 9

10 11

12 13 14

15 16 17

18 19

20 21

A. Vogler and H. Kunkely, Angew. Chem. Int. Ed. En&, 21 (1982) 209. J. N. Demas, D. Diemente and E. W. Harris, J. Am. Chem. Sot., 95 (1973) 6864. J. N. Demas, E. W. Harris, C. M. Flynn, Jr., and D. Diemente, J. Am. Chem. Sot., 97 (1975) 3838. J. N. Demas, E. W. Harris and R. P. McBride, J. Am. Chem. SOC., 99 (1977) 3547. L. Kumar, K. H. Puthraya and T. S. Srivastava, Inorg. Chim. Acta, 86 (1984) 173. K. H. Puthraya and T. S. Srivastava, Polyhedron, 4 (1985) 1579. F. A. Palocsay and J. V. Rund, Inorg. Chem., 8 (1969) 5214. K. D. Hodgesand J. V. Rund, Inorg. Chem., 14 (1975) 525. J. L. Burmeister and F. BasoIo, Inorg. Chem., 3 (1964) 1587. K. Bowman and Z. Dori, Inorg. Chem., 9 (1970) 395. A. 1. Vogel, A Textbook of Practical Organic Chemistry, Longman, London, 1956, 3rd edn., pp. 163 - 179. Y. Lion, E. Gandin and A. Van de Vorst, Photochem. PhotobioZ., 31 (1980) 305. L. A. Rossiello, Gazz. Chim. Ital., 97 (1968) 1177. P. M. Gidney, R. D. Gillard and B. T. Heaton, J. Chem. Sot. DaZton Trans., (1973) 132. A. Zweig and W. A. Henderson, Jr., J. Polym. Sci., Polym. Chem. Ed., 13 (1975) 717. S. S. Kamath and T. S. Srivastava, unpublished results, 1987. D. Bellus, in B. Ranby and J. F. Rabek (eds.), Singlet Oxygen, Wiley, New York, 1978, pp. 60 - 110. D. L. Webb and L. A. Rossiello, Inorg. Chem., 10 (1971) 2213. W. L. Jolly, Modern Inorganic Chemistry, McGraw-Hill, New York, 1984, pp. 409 - 410. D. Eastwood and M. Gouterman, J. Mol. Spectrosc., 35 (1970) 359. F. A. Cotton, Chemical Applications of Group Theory, Wiley, New York, 1971, 2nd edn., p. 364.