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Self-sensitized photo-oxidation of platinum(II) complexes of 1,10-phenanthroline with 3,4-dimercaptotoluene and thiosalicylic acid by 1O2
Journal
of Photochemistry
and Photobiology,
50 (1989)
A: Chemistry,
199 - 20‘7
199
SELF-SENSITIZED PHOTO-OXIDATION OF PLATINUM(H) COMPLEXES OF l,lO-PHENANTHROLINE WITH 3,4-DIMERCAPTOTOLUENE AND THIOSALICYLIC ACID BY ‘0, S. SHUKLA, S. S. KAMATH and T. S. SRIVASTAVAT Department (Zndia)
of Chemistry,
Indian
Institute
of Technology,
Powai,
Bombay
400 076
(Received March 16, 1989; in revised form July 4, 1989)
Summary [Pt(phen)(DMT)] and [Pt(phen)(TSA)] have been prepared and characterized. They show five absorption bands between 250 and 600 nm in dimethylformamide (DMF). A low energy band at 570 nm in [Pt(phen)(DMT)] and at 490 nm in [Pt(phen)(TSA)] shows a bathochromic shift with a decrease in the polarity of solvent and it is assigned to ligand-to-ligand charge transfer transitions. [Pt(phen)(DMT)] and [Pt(phen)(TSA)] undergo self-sensitized photo-oxidation involving the ‘02 species as an intermediate, as indicated by kinetic studies. This result is further supported by quenching studies of the photo-oxidation of these platinum(I1) complexes using NaNs as ‘0, quencher. The rate constant of NaN3 quenching of ‘02 in the selfsensitized photo-oxidation of [Pt(phen)(DMT)] has also been determined and it is calculated as 1.2 X lo8 M-l s-i in DMF.
1. Introduction The photo-oxidation of [ Pt( bipy)( DMT)] (where bipy is 2,2’-bipyridine and DMT is the dianion of 3,4-dimercaptotoluene) in chloroform has been reported on irradiation of the ligand-to-ligand charge transfer band at 577 nm [I]. The primary step of this reaction involves an electron transfer from the 3,4dimercaptotoluene ligand of the platinum(I1) complex to chloroform. Recently [Pt(bipy)(DMT)] and [Pt(bipy)(TSA)] (where TSA is the dianion of thiosalicylic acid) complexes have been shown to generate the singlet molecular oxygen ( ‘02) by energy transfer from their triplet excited state to triplet molecular oxygen, and these complexes are further oxidized by self-sensitized photo-oxidation [ 2, 31. The self-sensitized photo-oxidation of organic sensitizers is already known and they have also been used to TAuthor to whom correspondence 1010/6030/89/$3.50
should be addressed. @ Elsevier Sequoia/Printed
in The Netherlands
200
determine the quenching rate constants [4, 51. In this paper we report the self-sensitized photo-oxidation of [Pt(phen)(DMT)] and [Pt(phen)(TSA)] in the dimethylformamide (DMF) and measure the quenching rate constant of this photo-oxidation reaction of [Pt(phen)(DMT)] using NaN, as quencher.
2. Experimental details 2.1. Materials [Pt(phen)(DMT)] was prepared by the following method. [Pt(phen)Cl,] (1 mmol) was dissolved in 25 ml of dimethyl sulphoxide. 3,CDimercaptotoluene (1 mmol) was dissolved in 2 ml of 1 M NaOH (2 mmol) and this was added to the above solution with stirring. The reaction mixture was stirred at 28 “C for 30 min. The resulting solution was diluted with 100 ml of chloroform. The chloroform solution of this complex was repeatedly washed with water. The chloroform layer was filtered and concentrated to obtain purple crystals. The complex was recrystallized from a chloroform-aceto’ne mixture. The crystals were filtered, washed with a small quantity of cold chloroform, acetone and finally with petroleum ether. The purple crystals were dried in a vacuum desiccator over anhydrous calcium chloride. Analysis. Calculated for C,.$,,N,S,Pt: C, 43.10%; H, 2.65%; N, 5.29%. Found: C, 43.07%; H, 3.06%; N, 5.04%. [Pt(phen)(TSA)] as an orange complex was prepared by the method given for [Pt(phen)(DMT)] except that thiosalicylic acid was used in place Calculated for C,&l,,N,O$Pt: C, of 3,4-dimercaptotoluene. Analysis. 43.26%; H, 2.28%; N, 5.31%. Found: C, 43.36%; H, 2.72%; N, 5.02%. Potassium tetrachloroplatinate(I1) (Strem, U.S.A.), l,lO-phenanthroline monohydrate (Glaxo, India), thiosalicylic acid, sodium azide (SISCO, India) and 3,4-dimercaptotoluene (Fluka, Switzerland) were bought and used as such. The reagent grade solvents were purified before use by standard methods [ 61. 2.2. Measurements
The physical methods used have been described elsewhere [ 7 - 81. 2.3.
General
irradiation procedures
The photochemical irradiation was carried out by the modified method of Diamond et al. [9] as described below. A stabilized beam of light from a 200 W tungsten-halogen lamp operating at 18 V was first passed through a Thomas Scientific F-56 filter, which cut out light below 455 nm, placed about 28 cm from the light source. The filtered light was then directed through a solid clear-cut acrylic cylinder (16 X 5 cm) placed 1 cm away from the filter. The molecular-oxygen-saturated DMF solution to be photolysed was placed in a stoppered quartz cuvette of the path length 1 cm and was irradiated by placing it 2 cm on the other side of the cylinder for different irradiation times. The course of photolysis was followed by the difference
201
absorption spectroscopy spectrophotometer.
in the visible region using a Shimadzu UV-260
3. Results 3.1. Characterization of [Pt(phen)(DMT)] and [Pt(phen)(TSA)] The molar conductance values of [Pt(phen)(DMT)] and [Pt(phen)(TSA)] in DMF (1 X 10e3 M) are 2.0 and 4.2 cm* ohm-’ mol-’ respectively. This suggests that these complexes are non-electrolytes [lo]. The IR bands at 1515 and 1530 cm-’ respectively in [Pt(phen)(DMT)] and [Pt(phen)(TSA)] have been assigned to ring stretching frequencies of coordinated l,lO-phenanthroline [ll]. The v(S-H) vibrations present at 2540 cm-’ in the 3,4-dimercaptotoluene ligand and at 2520 cm-’ in the thiosalicylic acid ligand are not present in [Pt(phen)(DMT)] and [Pt(phen)(TSA)] complexes respectively. The v(CO,-) band is observed in [Pt(phen)(TSA)] at 1600 cm-i which is due to the coordinated oxygen of the -CO,- group. The band at 490 cm-’ in [Pt(phen)(DMT)] and the band at 492 cm-i in [Pt(phen)(TSA)] are assigned to v(Pt-S) [12]. The ‘H-NMR signals of [Pt(phen)DMT)] and [ Pt(phen)(TSA)] were assigned following the assignments of [Pt(bipy)(DMT)] and [Pt(bipy)(TSA)] [13]. The integrated areas obtained for protons of l,lO-phenanthroline and protons of the dianion of 3,4-dimercaptotoluene or thiosalicylic acid are in the desired ratio in [Pt(phen)(DMT)] or [Pt(phen(TSA)]. The electronic absorption band maxima and their extinction coefficients (in parentheses) of [Pt(phen)(DMT)] in DMF are 570 (4.6 X lo3 1 mol-’ cm-‘), 410 (2.3 X 103), 360 (shoulder, sh), 325 (sh) and 275 nm (2.0 X 104) and those of [Pt(phen)(TSA)] in DMF are 490 (1.8 X 103), 390 (sh), 374 (3.4 X 103), 329 (3.8 X 103) and 278 nm (1.9 X 104). The band at 570 nm in [Pt(phen)(DMT)] and the band at 490 nm in [Pt(phen)(TSA)] are highly solvent dependent and they experience a bathochromic shift with a decrease in the polarity of solvents from DMF to CHC13 (38 nm shift in [Pt(phen)(DMT)] and 35 nm shift in [Pt(phen)(TSA)]). This is attributed to charge transfer from the highest occupied molecular orbital of DMT or TSA to the lowest unoccupied molecular orbital of l,lO-phenanthroline ligand via a metal [l, 131. Other bands in the region 275 - 410 nm may be assigned on the basis of the assignments for [Pt(phen)Cl,] [ 141. 3.2. Self-sensitized
[Wphen)(TWl
photo-oxidation
of [Pt(phen)(DMT)]
and
[Pt(phen)(DMT)] (2 X 10h4 M) in a molecular-oxygen-saturated solution of DMF was irradiated with light of wavelength above 455 nm. The course of photolysis was monitored by recording the difference absorption spectra of the visible region after different irradiation times. The decrease in absorbance at 570 nm is accounted for by the decay of [Pt(phen)(DMT)] as a result of photolysis. These difference spectra reveal new positive and
202
negative peaks as shown in Fig. 1. Figure 2(a) shows a plot of log(concentration of [ Pt(phen)( DMT)] ) us. different irradiation times. This plot is linear, which indicates first-order kinetics for the above reaction. The 570 nm absorption band of [Pt(phen)(DMT)] in DMF disappears after irradiation (4 - 5 h) until all the complex is converted into new product(s) and a new 486 nm band with about 50% reduced intensity as compared to the original 570 nm band intensity appears. Similar behaviour is observed when [Pt(bipy)(DMT)] in DMF is irradiated until all the complex is converted into the product(s) [ 21. [Pt(phen)(TSA)] (2 X 10e4 M) in molecular-oxygen-saturated DMF was irradiated with light of wavelength above 455 nm. The difference absorption spectra of [Pt(phen)(TSA)] were recorded after different times. The decay of this complex is indicated by a decrease in absorbance at 490 nm as shown in Pig. 3. A linear plot of log(concentration of [Pt(phen)(TSA)]) us. irradiation time of photolysis is obtained, which is shown in Fig. 2(b). This linear plot indicates the first-order kinetics for the photo-oxidation of [Pt(phen)(TSA)]. The 490 nm absorption band of [Pt(phen)(TSA)] in DMF disappears on irradiation for long enough (4 - 5 h) to convert all the complex into new product(s). This product does not absorb above 400 nm. Similar behaviour is observed when [Pt(bipy)(TSA)] in DMF is irradiated until the complex is converted into product(s) [ 21. Molecular-oxygen-saturated solutions of [Pt(phen)(DMT)] and [Pt(phen)(TSA)] in DMF were quite stable in the dark and were not affected
+T------
, 300
Fig. 1. Electronic of [Pt(phen)(DMT)]
6min
500 WAVELENGTH
I 700
900
-
absorption difference spectra of molecular-oxygen-saturated (2 X lop4 M) in DMF at different irradiation times.
solutions
203
0
4 lRRADlATIC+4
6
TIME
12 16 IN MINUTES
-
20
Fig. 2. Plots of log( concentration) us. irradiation time of molecular-oxygen-saturated tions of (a) [Pt(phen)(DMT)] (2 X 10e4 M) and (b) [Pt(phen)(TSA)] (2 X lop4 M).
-0.25
r 300
Fig. 3. Electronic of [Pt(phen)(TSA)]
500 WAVELENGTH
absorption (2 x lop4
700 (nm)
solu-
9
-
difference spectra of molecular-oxygen-saturated M) in DMT at different irradiation times.
solution
204
when irradiated in the presence of molecular nitrogen. However, more than 90% of the photo-oxidation of the above complexes (2 X 1O-4 M) is quenched when irradiated in the presence of NaNs (4 X 10e4 M). 3.3. Determination of [Pt(phen)(DMT)]
of the quenching rate constant by sodium azide
k,
for photo-oxidation
Five different concentrations (1 X 10-l to 2 X 10e3 M) of [Pt(phen)(DMT)] in saturated-molecular-oxygen DMF were irradiated by light of wavelengths between 455 and 800 nm at different time intervals. The course of photolysis was followed by a decrease in absorption at 577 nm using difference absorption spectroscopy. The above experiment was repeated in the presence of a 1.5 X lop4 M solution of azide as physical quenchers when the photolysis reaction was retarded. The plot of
(--dWl/dt), (---d[Sl/dth -
’
vs. [S]
gives a straight line (see Fig. 4) [15]. Knowing the values of the slope and the intercept from this plot and kd in DMF from the literature [ 161, k, can be calculated and it is found to be 1.2 X 10’ M-’ s-l. The physical quenching
Fig. 4. Self-sensitized photo-oxidation (1.5 x low4 M) as quencher.
of [Pt(phen)(DMT)]
in the presence of NaN3
205
ho can thus be calculated from the slope as well as from the intercept. Both have the same ho value of 1.2 X lo8 M-l s-l. 4. Discussion The above results suggest the following mechanism of self-sensitized photo-oxidation of [Pt(phen)(DMT)] and [Pt(phen(TSA)] . The last step in parentheses involving NaN, as physical quencher of the self-sensitized photooxidation of platinum complexes is also included. hv
s-‘s
3s + 30,
=
s + ‘o*
k,
lo2
+
S __f
‘0, + S -%
30, + $3 -
30, + S (physical quenching)
SO, (chemical quenching)
kQ
‘02+Q--30,+Q where S is the ground state of the sensitizer complex; its first excited singlet state is ‘S and its first excited triplet state is 3S, hv is the energy of a photon, ISC is the intersystem crossing, ET is the energy transfer, hd is the rate constant of the quenching of ‘02 by a DMF molecule, k, is the rate constant of the physical quenching of ‘02 by S, k, is the rate constant of the chemical quenching of ‘0, by S, and k, is the rate constant of the physical quenching of the self-sensitized photo-oxidation of S by NaN3 as Q. On the basis of the above mechanism the rate of the self-sensitized photo-oxidation of S under steady state conditions assuming kd 9 (k, + k,) [S] is given by
and
log[Sl = -kt
+ c [2]
(1)
where k = Ia,,#~o, X (k,/kd) (labs is the intensity of irradiation and 410, is the quantum yield of ‘02). If the decay of S is followed by visible absorption
206
spectroscopy, a plot of log[S] or log A (A is the absorbance of S) us. different irradiation times gives a striaght line with the slope of k (see Figs. 2(a) and 2(b)). Such linear plots will be observed only in the initial stages of the disappearance of S (less than 15% of the sensitizer is oxidized in 30 min) during which time the average sensitizer concentration is nearly constant. This supports the involvement of ‘0, in the self-sensitized photo-oxidation of s. If one assumes kd + k,[S] % k,[S], the rate of the self-sensitized photooxidation of S under steady state conditions is given by
= Iab&O,
x
kSS1 iI51 k[Sl + k,
(2)
If the last step of the above mechanism is included, the self-sensitized oxidation of [Pt(phen)(DMT)] in the presence of NaN, (Q) can be written as
=
Iabs@O,
x
krtsl
WY + kd + kat&l
(3)
From eqns. (2) and (3), the following relationship can be obtained: t--d[SlW)o-'
(-d[Sl/dt),
USI =-+_ kid&l
kd ka[Ql
The values of (-d[S]/dt),and (-d[S]/dt), are used to obtain a linear plot of
(--Wlldt),
k--d[Sl/dt), -
’
(4) for different concentrations
of S
vs. [S]
(see Fig. 4) with a slope of k,/k,[Q] and an intercept of kd/kQ[Q]. The ratio’ of slope/intercept gives k,/kd. The value of k, calculated from this relationship knowing the value of k, in DMF to be 1.4 X 105sC1 [16] is 1.2 X lo5 M-l s-i. Using these values of k, and k,, the rate constant of the physical quenching of ‘0, by sodium azide k,, can be calculated from the slope of k,/k,[Q] and the intercept of kd/kQ[Q] respectively. Both the slope and the intercept give the same kQ value of 1.2 X lo8 M-’ s-i. This k, value in DMF is close to the reported value of k, as 3.5 X 10’ M-’ s-l in methanol obtained by using methylene blue as the photosensitizer [ 161. The products of the photolysis of [Pt(phen)(DMT)] and [Pt(phen)(TSA)] have not been isolated but they are suggested on the basis of their characteristic absorption spectra. The disappearance of the 570 nm band (characteristic of two Pt-S bonds) and the appearance of the 485 nm band in [Pt(phen)(DMT)] in DMF for long irradiation times suggest that one of the sulphur atoms in the complex is oxidized because the band at 485 nm and other high energy bands in the oxidized product are similar to bands
207
present in the complex [Pt(phen)(TSA)] [2]. Similarly the disappearance of the 490 nm band (characteristic of one Pt-S bond) in [Pt(phen)(TSA)] in DMF for long irradiation times suggest that one of the sulphur atoms of the dianion of thiosalicylic acid is oxidized [2]. The oxidized product shows only absorption bands below 400 nm and they can be assigned to ‘II-~* and d-r* transitions as assigned earlier for [Pt(phen)Cl,] [14]. The oxidation of a sulphur atom of [Pt(phen)( DMT)] and [Pt(phen)(TSA)] to sulphonate with the formation of a Pt-0 bond from the coordination of the sulphonate group replacing a Pt-S bond, is suggested as the oxidation product [2,17,18]. 5. Conclusion [Pt(phen)(DMT)] and [Pt(phen)(TSA)] undergo self-sensitized photooxidation with the involvement of ‘0, as an intermediate. Acknowledgment We thank the DST (Government of India) for financial support. S. S. Kamath (SRF) is grateful to CSIR, New Delhi, for financial assistance. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
A. Vogler and H. Kunkely, J. Am. Chem. Sot., 103 (1981) 1559. K. H. Puthraya and T. S. Srivastava, Polyhedron, 4 (1985) 1579. K. H. Puthraya and T. S. Srivastava, J. Indian Chem. Sot., 62 (1985) 843. C. S. Foote and T. Y. Ching, J. Am. Chem. Sot., 97 (1975) 6209. B. M. Monroe and J. J. Mrowca, J. Phys. Chem., 83 (1979) 591. B. S. Furniss, A. J. Hannaford, V. Rogers, P. W. G. Smith and A. R. Tuchell, Vogel’s Textbook of Practical Organic Chemistry, Longman, London, 1978, pp. 264 - 279. S. Shukla, S. S. Kamath and T. S. Srivastava, J. Photochem. Photobiol., A: Chem., 44 (1988) 143. S. Shukla, S. S. Kamath and T. S. Srivastava, J. Photochem. Photobiol., A: Chem., 47 (1989) 287. I. Diamond, S. G. Granelli and A. F. McDonagh, Biochem. Med., 17 (1977) 121. W. J. Geary, Coord. Chem. Rev., 7 (1971) 81. A. H. Schitt and R. C. Taylor, J. Znorg. Nucl. Chem., 9 (1959) 211. K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1963, pp. 189 - 220. L. Kumar, K. H. Puthraya and T. S. Srivastava, Znorg. Chim. Acta, 86 (1984) 173. L. N. Minochkina, V. I. Fadeeva and N. B. Zorob, Anal. Left., 12 (1979) 451. D. J. Carlsson, T. Suprunchuk and D. M. Wiles, Can. J. Chem., 52 (1974) 3728. B. Bellus, in B. Ranby and J. F. Rabeck (eds.), Singlet Oxygen, Wiley, New York, 1978, pp. 61 - 110. C. S. Foote, in W. A. Pryer (ed.), Free Radicals in Biology, Academic Press, New York, 1976, pp. 85 - 133. C. S. Foote, in M. A. J. Rogers and E. L. Powers (eds.), Oxygen and Oxy Radicals in Chemistry and Biology, Academic Press, New York, 1981, p. 431.
of Photochemistry
and Photobiology,
50 (1989)
A: Chemistry,
199 - 20‘7
199
SELF-SENSITIZED PHOTO-OXIDATION OF PLATINUM(H) COMPLEXES OF l,lO-PHENANTHROLINE WITH 3,4-DIMERCAPTOTOLUENE AND THIOSALICYLIC ACID BY ‘0, S. SHUKLA, S. S. KAMATH and T. S. SRIVASTAVAT Department (Zndia)
of Chemistry,
Indian
Institute
of Technology,
Powai,
Bombay
400 076
(Received March 16, 1989; in revised form July 4, 1989)
Summary [Pt(phen)(DMT)] and [Pt(phen)(TSA)] have been prepared and characterized. They show five absorption bands between 250 and 600 nm in dimethylformamide (DMF). A low energy band at 570 nm in [Pt(phen)(DMT)] and at 490 nm in [Pt(phen)(TSA)] shows a bathochromic shift with a decrease in the polarity of solvent and it is assigned to ligand-to-ligand charge transfer transitions. [Pt(phen)(DMT)] and [Pt(phen)(TSA)] undergo self-sensitized photo-oxidation involving the ‘02 species as an intermediate, as indicated by kinetic studies. This result is further supported by quenching studies of the photo-oxidation of these platinum(I1) complexes using NaNs as ‘0, quencher. The rate constant of NaN3 quenching of ‘02 in the selfsensitized photo-oxidation of [Pt(phen)(DMT)] has also been determined and it is calculated as 1.2 X lo8 M-l s-i in DMF.
1. Introduction The photo-oxidation of [ Pt( bipy)( DMT)] (where bipy is 2,2’-bipyridine and DMT is the dianion of 3,4-dimercaptotoluene) in chloroform has been reported on irradiation of the ligand-to-ligand charge transfer band at 577 nm [I]. The primary step of this reaction involves an electron transfer from the 3,4dimercaptotoluene ligand of the platinum(I1) complex to chloroform. Recently [Pt(bipy)(DMT)] and [Pt(bipy)(TSA)] (where TSA is the dianion of thiosalicylic acid) complexes have been shown to generate the singlet molecular oxygen ( ‘02) by energy transfer from their triplet excited state to triplet molecular oxygen, and these complexes are further oxidized by self-sensitized photo-oxidation [ 2, 31. The self-sensitized photo-oxidation of organic sensitizers is already known and they have also been used to TAuthor to whom correspondence 1010/6030/89/$3.50
should be addressed. @ Elsevier Sequoia/Printed
in The Netherlands
200
determine the quenching rate constants [4, 51. In this paper we report the self-sensitized photo-oxidation of [Pt(phen)(DMT)] and [Pt(phen)(TSA)] in the dimethylformamide (DMF) and measure the quenching rate constant of this photo-oxidation reaction of [Pt(phen)(DMT)] using NaN, as quencher.
2. Experimental details 2.1. Materials [Pt(phen)(DMT)] was prepared by the following method. [Pt(phen)Cl,] (1 mmol) was dissolved in 25 ml of dimethyl sulphoxide. 3,CDimercaptotoluene (1 mmol) was dissolved in 2 ml of 1 M NaOH (2 mmol) and this was added to the above solution with stirring. The reaction mixture was stirred at 28 “C for 30 min. The resulting solution was diluted with 100 ml of chloroform. The chloroform solution of this complex was repeatedly washed with water. The chloroform layer was filtered and concentrated to obtain purple crystals. The complex was recrystallized from a chloroform-aceto’ne mixture. The crystals were filtered, washed with a small quantity of cold chloroform, acetone and finally with petroleum ether. The purple crystals were dried in a vacuum desiccator over anhydrous calcium chloride. Analysis. Calculated for C,.$,,N,S,Pt: C, 43.10%; H, 2.65%; N, 5.29%. Found: C, 43.07%; H, 3.06%; N, 5.04%. [Pt(phen)(TSA)] as an orange complex was prepared by the method given for [Pt(phen)(DMT)] except that thiosalicylic acid was used in place Calculated for C,&l,,N,O$Pt: C, of 3,4-dimercaptotoluene. Analysis. 43.26%; H, 2.28%; N, 5.31%. Found: C, 43.36%; H, 2.72%; N, 5.02%. Potassium tetrachloroplatinate(I1) (Strem, U.S.A.), l,lO-phenanthroline monohydrate (Glaxo, India), thiosalicylic acid, sodium azide (SISCO, India) and 3,4-dimercaptotoluene (Fluka, Switzerland) were bought and used as such. The reagent grade solvents were purified before use by standard methods [ 61. 2.2. Measurements
The physical methods used have been described elsewhere [ 7 - 81. 2.3.
General
irradiation procedures
The photochemical irradiation was carried out by the modified method of Diamond et al. [9] as described below. A stabilized beam of light from a 200 W tungsten-halogen lamp operating at 18 V was first passed through a Thomas Scientific F-56 filter, which cut out light below 455 nm, placed about 28 cm from the light source. The filtered light was then directed through a solid clear-cut acrylic cylinder (16 X 5 cm) placed 1 cm away from the filter. The molecular-oxygen-saturated DMF solution to be photolysed was placed in a stoppered quartz cuvette of the path length 1 cm and was irradiated by placing it 2 cm on the other side of the cylinder for different irradiation times. The course of photolysis was followed by the difference
201
absorption spectroscopy spectrophotometer.
in the visible region using a Shimadzu UV-260
3. Results 3.1. Characterization of [Pt(phen)(DMT)] and [Pt(phen)(TSA)] The molar conductance values of [Pt(phen)(DMT)] and [Pt(phen)(TSA)] in DMF (1 X 10e3 M) are 2.0 and 4.2 cm* ohm-’ mol-’ respectively. This suggests that these complexes are non-electrolytes [lo]. The IR bands at 1515 and 1530 cm-’ respectively in [Pt(phen)(DMT)] and [Pt(phen)(TSA)] have been assigned to ring stretching frequencies of coordinated l,lO-phenanthroline [ll]. The v(S-H) vibrations present at 2540 cm-’ in the 3,4-dimercaptotoluene ligand and at 2520 cm-’ in the thiosalicylic acid ligand are not present in [Pt(phen)(DMT)] and [Pt(phen)(TSA)] complexes respectively. The v(CO,-) band is observed in [Pt(phen)(TSA)] at 1600 cm-i which is due to the coordinated oxygen of the -CO,- group. The band at 490 cm-’ in [Pt(phen)(DMT)] and the band at 492 cm-i in [Pt(phen)(TSA)] are assigned to v(Pt-S) [12]. The ‘H-NMR signals of [Pt(phen)DMT)] and [ Pt(phen)(TSA)] were assigned following the assignments of [Pt(bipy)(DMT)] and [Pt(bipy)(TSA)] [13]. The integrated areas obtained for protons of l,lO-phenanthroline and protons of the dianion of 3,4-dimercaptotoluene or thiosalicylic acid are in the desired ratio in [Pt(phen)(DMT)] or [Pt(phen(TSA)]. The electronic absorption band maxima and their extinction coefficients (in parentheses) of [Pt(phen)(DMT)] in DMF are 570 (4.6 X lo3 1 mol-’ cm-‘), 410 (2.3 X 103), 360 (shoulder, sh), 325 (sh) and 275 nm (2.0 X 104) and those of [Pt(phen)(TSA)] in DMF are 490 (1.8 X 103), 390 (sh), 374 (3.4 X 103), 329 (3.8 X 103) and 278 nm (1.9 X 104). The band at 570 nm in [Pt(phen)(DMT)] and the band at 490 nm in [Pt(phen)(TSA)] are highly solvent dependent and they experience a bathochromic shift with a decrease in the polarity of solvents from DMF to CHC13 (38 nm shift in [Pt(phen)(DMT)] and 35 nm shift in [Pt(phen)(TSA)]). This is attributed to charge transfer from the highest occupied molecular orbital of DMT or TSA to the lowest unoccupied molecular orbital of l,lO-phenanthroline ligand via a metal [l, 131. Other bands in the region 275 - 410 nm may be assigned on the basis of the assignments for [Pt(phen)Cl,] [ 141. 3.2. Self-sensitized
[Wphen)(TWl
photo-oxidation
of [Pt(phen)(DMT)]
and
[Pt(phen)(DMT)] (2 X 10h4 M) in a molecular-oxygen-saturated solution of DMF was irradiated with light of wavelength above 455 nm. The course of photolysis was monitored by recording the difference absorption spectra of the visible region after different irradiation times. The decrease in absorbance at 570 nm is accounted for by the decay of [Pt(phen)(DMT)] as a result of photolysis. These difference spectra reveal new positive and
202
negative peaks as shown in Fig. 1. Figure 2(a) shows a plot of log(concentration of [ Pt(phen)( DMT)] ) us. different irradiation times. This plot is linear, which indicates first-order kinetics for the above reaction. The 570 nm absorption band of [Pt(phen)(DMT)] in DMF disappears after irradiation (4 - 5 h) until all the complex is converted into new product(s) and a new 486 nm band with about 50% reduced intensity as compared to the original 570 nm band intensity appears. Similar behaviour is observed when [Pt(bipy)(DMT)] in DMF is irradiated until all the complex is converted into the product(s) [ 21. [Pt(phen)(TSA)] (2 X 10e4 M) in molecular-oxygen-saturated DMF was irradiated with light of wavelength above 455 nm. The difference absorption spectra of [Pt(phen)(TSA)] were recorded after different times. The decay of this complex is indicated by a decrease in absorbance at 490 nm as shown in Pig. 3. A linear plot of log(concentration of [Pt(phen)(TSA)]) us. irradiation time of photolysis is obtained, which is shown in Fig. 2(b). This linear plot indicates the first-order kinetics for the photo-oxidation of [Pt(phen)(TSA)]. The 490 nm absorption band of [Pt(phen)(TSA)] in DMF disappears on irradiation for long enough (4 - 5 h) to convert all the complex into new product(s). This product does not absorb above 400 nm. Similar behaviour is observed when [Pt(bipy)(TSA)] in DMF is irradiated until the complex is converted into product(s) [ 21. Molecular-oxygen-saturated solutions of [Pt(phen)(DMT)] and [Pt(phen)(TSA)] in DMF were quite stable in the dark and were not affected
+T------
, 300
Fig. 1. Electronic of [Pt(phen)(DMT)]
6min
500 WAVELENGTH
I 700
900
-
absorption difference spectra of molecular-oxygen-saturated (2 X lop4 M) in DMF at different irradiation times.
solutions
203
0
4 lRRADlATIC+4
6
TIME
12 16 IN MINUTES
-
20
Fig. 2. Plots of log( concentration) us. irradiation time of molecular-oxygen-saturated tions of (a) [Pt(phen)(DMT)] (2 X 10e4 M) and (b) [Pt(phen)(TSA)] (2 X lop4 M).
-0.25
r 300
Fig. 3. Electronic of [Pt(phen)(TSA)]
500 WAVELENGTH
absorption (2 x lop4
700 (nm)
solu-
9
-
difference spectra of molecular-oxygen-saturated M) in DMT at different irradiation times.
solution
204
when irradiated in the presence of molecular nitrogen. However, more than 90% of the photo-oxidation of the above complexes (2 X 1O-4 M) is quenched when irradiated in the presence of NaNs (4 X 10e4 M). 3.3. Determination of [Pt(phen)(DMT)]
of the quenching rate constant by sodium azide
k,
for photo-oxidation
Five different concentrations (1 X 10-l to 2 X 10e3 M) of [Pt(phen)(DMT)] in saturated-molecular-oxygen DMF were irradiated by light of wavelengths between 455 and 800 nm at different time intervals. The course of photolysis was followed by a decrease in absorption at 577 nm using difference absorption spectroscopy. The above experiment was repeated in the presence of a 1.5 X lop4 M solution of azide as physical quenchers when the photolysis reaction was retarded. The plot of
(--dWl/dt), (---d[Sl/dth -
’
vs. [S]
gives a straight line (see Fig. 4) [15]. Knowing the values of the slope and the intercept from this plot and kd in DMF from the literature [ 161, k, can be calculated and it is found to be 1.2 X 10’ M-’ s-l. The physical quenching
Fig. 4. Self-sensitized photo-oxidation (1.5 x low4 M) as quencher.
of [Pt(phen)(DMT)]
in the presence of NaN3
205
ho can thus be calculated from the slope as well as from the intercept. Both have the same ho value of 1.2 X lo8 M-l s-l. 4. Discussion The above results suggest the following mechanism of self-sensitized photo-oxidation of [Pt(phen)(DMT)] and [Pt(phen(TSA)] . The last step in parentheses involving NaN, as physical quencher of the self-sensitized photooxidation of platinum complexes is also included. hv
s-‘s
3s + 30,
=
s + ‘o*
k,
lo2
+
S __f
‘0, + S -%
30, + $3 -
30, + S (physical quenching)
SO, (chemical quenching)
kQ
‘02+Q--30,+Q where S is the ground state of the sensitizer complex; its first excited singlet state is ‘S and its first excited triplet state is 3S, hv is the energy of a photon, ISC is the intersystem crossing, ET is the energy transfer, hd is the rate constant of the quenching of ‘02 by a DMF molecule, k, is the rate constant of the physical quenching of ‘02 by S, k, is the rate constant of the chemical quenching of ‘0, by S, and k, is the rate constant of the physical quenching of the self-sensitized photo-oxidation of S by NaN3 as Q. On the basis of the above mechanism the rate of the self-sensitized photo-oxidation of S under steady state conditions assuming kd 9 (k, + k,) [S] is given by
and
log[Sl = -kt
+ c [2]
(1)
where k = Ia,,#~o, X (k,/kd) (labs is the intensity of irradiation and 410, is the quantum yield of ‘02). If the decay of S is followed by visible absorption
206
spectroscopy, a plot of log[S] or log A (A is the absorbance of S) us. different irradiation times gives a striaght line with the slope of k (see Figs. 2(a) and 2(b)). Such linear plots will be observed only in the initial stages of the disappearance of S (less than 15% of the sensitizer is oxidized in 30 min) during which time the average sensitizer concentration is nearly constant. This supports the involvement of ‘0, in the self-sensitized photo-oxidation of s. If one assumes kd + k,[S] % k,[S], the rate of the self-sensitized photooxidation of S under steady state conditions is given by
= Iab&O,
x
kSS1 iI51 k[Sl + k,
(2)
If the last step of the above mechanism is included, the self-sensitized oxidation of [Pt(phen)(DMT)] in the presence of NaN, (Q) can be written as
=
Iabs@O,
x
krtsl
WY + kd + kat&l
(3)
From eqns. (2) and (3), the following relationship can be obtained: t--d[SlW)o-'
(-d[Sl/dt),
USI =-+_ kid&l
kd ka[Ql
The values of (-d[S]/dt),and (-d[S]/dt), are used to obtain a linear plot of
(--Wlldt),
k--d[Sl/dt), -
’
(4) for different concentrations
of S
vs. [S]
(see Fig. 4) with a slope of k,/k,[Q] and an intercept of kd/kQ[Q]. The ratio’ of slope/intercept gives k,/kd. The value of k, calculated from this relationship knowing the value of k, in DMF to be 1.4 X 105sC1 [16] is 1.2 X lo5 M-l s-i. Using these values of k, and k,, the rate constant of the physical quenching of ‘0, by sodium azide k,, can be calculated from the slope of k,/k,[Q] and the intercept of kd/kQ[Q] respectively. Both the slope and the intercept give the same kQ value of 1.2 X lo8 M-’ s-i. This k, value in DMF is close to the reported value of k, as 3.5 X 10’ M-’ s-l in methanol obtained by using methylene blue as the photosensitizer [ 161. The products of the photolysis of [Pt(phen)(DMT)] and [Pt(phen)(TSA)] have not been isolated but they are suggested on the basis of their characteristic absorption spectra. The disappearance of the 570 nm band (characteristic of two Pt-S bonds) and the appearance of the 485 nm band in [Pt(phen)(DMT)] in DMF for long irradiation times suggest that one of the sulphur atoms in the complex is oxidized because the band at 485 nm and other high energy bands in the oxidized product are similar to bands
207
present in the complex [Pt(phen)(TSA)] [2]. Similarly the disappearance of the 490 nm band (characteristic of one Pt-S bond) in [Pt(phen)(TSA)] in DMF for long irradiation times suggest that one of the sulphur atoms of the dianion of thiosalicylic acid is oxidized [2]. The oxidized product shows only absorption bands below 400 nm and they can be assigned to ‘II-~* and d-r* transitions as assigned earlier for [Pt(phen)Cl,] [14]. The oxidation of a sulphur atom of [Pt(phen)( DMT)] and [Pt(phen)(TSA)] to sulphonate with the formation of a Pt-0 bond from the coordination of the sulphonate group replacing a Pt-S bond, is suggested as the oxidation product [2,17,18]. 5. Conclusion [Pt(phen)(DMT)] and [Pt(phen)(TSA)] undergo self-sensitized photooxidation with the involvement of ‘0, as an intermediate. Acknowledgment We thank the DST (Government of India) for financial support. S. S. Kamath (SRF) is grateful to CSIR, New Delhi, for financial assistance. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
A. Vogler and H. Kunkely, J. Am. Chem. Sot., 103 (1981) 1559. K. H. Puthraya and T. S. Srivastava, Polyhedron, 4 (1985) 1579. K. H. Puthraya and T. S. Srivastava, J. Indian Chem. Sot., 62 (1985) 843. C. S. Foote and T. Y. Ching, J. Am. Chem. Sot., 97 (1975) 6209. B. M. Monroe and J. J. Mrowca, J. Phys. Chem., 83 (1979) 591. B. S. Furniss, A. J. Hannaford, V. Rogers, P. W. G. Smith and A. R. Tuchell, Vogel’s Textbook of Practical Organic Chemistry, Longman, London, 1978, pp. 264 - 279. S. Shukla, S. S. Kamath and T. S. Srivastava, J. Photochem. Photobiol., A: Chem., 44 (1988) 143. S. Shukla, S. S. Kamath and T. S. Srivastava, J. Photochem. Photobiol., A: Chem., 47 (1989) 287. I. Diamond, S. G. Granelli and A. F. McDonagh, Biochem. Med., 17 (1977) 121. W. J. Geary, Coord. Chem. Rev., 7 (1971) 81. A. H. Schitt and R. C. Taylor, J. Znorg. Nucl. Chem., 9 (1959) 211. K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1963, pp. 189 - 220. L. Kumar, K. H. Puthraya and T. S. Srivastava, Znorg. Chim. Acta, 86 (1984) 173. L. N. Minochkina, V. I. Fadeeva and N. B. Zorob, Anal. Left., 12 (1979) 451. D. J. Carlsson, T. Suprunchuk and D. M. Wiles, Can. J. Chem., 52 (1974) 3728. B. Bellus, in B. Ranby and J. F. Rabeck (eds.), Singlet Oxygen, Wiley, New York, 1978, pp. 61 - 110. C. S. Foote, in W. A. Pryer (ed.), Free Radicals in Biology, Academic Press, New York, 1976, pp. 85 - 133. C. S. Foote, in M. A. J. Rogers and E. L. Powers (eds.), Oxygen and Oxy Radicals in Chemistry and Biology, Academic Press, New York, 1981, p. 431.