2486
J . Phys. Chem. 1985,89, 2486-2492
and (0 volatile o ~ i d e . ~ In ~ , addition, ~' the presence of nonnoble metal impurities can lead to highly stable mixed oxide phas-
Coenen and the many clarifying discussions with my colleagues H. P. Bowel, G. Pirug, D. Wesner, and C. Freiburg are gratefully acknowledged.
es.6,28.32,34,35,38
Acknowledgment. The skillful technical assistance of F. P.
Registry No. Pd, 7440-05-3.
Energy- and Electron-Transfer Processes of the Lowest Triplet Excited State of Tetrakis(diphosphito)diplatinate(I I ) John R. Peterson and K. Kalyanasundaram* Institut de Chimie Physique, Ecole Polytechnique FZdCrale de Lausanne, CH- 101 5 Lausanne, Switzerland (Received: November 9, 1984; In Final Form: February 6, 1985)
In addition to the characterization of the luminescence in aqueous and nonaqueous solvents (HzO, DzO,CH3CN, CH30H, and DMF), energy- and electron-transfer processes involving the lowest triplet excited state of the title compound have been investigated. Efficient transfer of excitation energy from 3Pt2(P0P)44-'occurs to various acceptors including naphthalene and Ru(bpy)32f. Direct spectral evidence for the efficient formation of redox products is also presented both in reducive quenching (with N,N-dimethylaniline, tetramethylbenzidine,and TMPD) and in oxidativequenching (propylviologensulfonate, PVS). With methylviologen and other cationic quenchers, ion-pair formation drastically reduces the yields of photoredox products.
Introduction
Bimolecular electron and energy-transfer reactions of electronically excited transition-metal complexes have been the subject of intense study in recent years.l Thanks to numerous studies mainly in the past decade, Ru(bpy);+ and related complexes have clearly emerged as a superior class of photosensitizers for various electron- and energy-transfer processes in homogeneous solutions at room temperature.2 The myriad of photoredox reactions-both oxidative and reductive-of R ~ ( b p y ) ~ are ~ + already * finding applications in systems for the photochemical conversion of solar energy. In order to extend this type of novel photochemistry to other systems, it is of interest to examine other potential candidates. One such new class of compounds that deserve careful scrutiny are the polynuclear metal complexes. Gray et al., in particular, have identified several polynuclear metal complexes which appear to possess photophysical and photoredox properties ideally suited for exploitation as energy and redox sensitizers.) One intriguing possibility with these systems is that they may be able to undergo net multielectron photoredox reactions, though this is yet to be demonstrated. One particular binuclear complex which is currently receiving some attention is a diplatinum(I1) complex, tetrakis(diphosphito)diplatinate(II), often abbreviated as Pt2(POP);-, POP = (H205P22-).4-6 It is a very interesting complex possessing several (1) (a) T. J. Meyer, Progr. Inorg. Chem., 30,389 (1983); (b) N. Sutin and C. Creutz, Pure Appl. Chem., 52, 2717 (1980); (c) V. Balzani, F. Boletta, F. Scandola, and R. Ballardini, Pure App. Chem., 51, 299 (1979); (d) D. G. Whitten, Acc. Chem. Res., 13, 83 (1980). (2) K. Kalyanasundaram, Coord. Chem. Reu., 46, 159 (1982). (3) (a) D. G. Nocera, A. W. Maverick, J. R. Winkler, C.-M. Che, and H. B. Gray, ACS Symp. Ser. No., 211, 21 (1983); (b) P. K. Eidem, A. W. Maverick, and H. B. Gray, Inorg. Chim. Acta, 50, 59 (1981); (c) W. C. Trogler and H. B. Gray, Acc. Chem. Res., 11, 232 (1978). (4) (a) R. P. Sperline, M. K. Dickson, and D. M. Roundhill, J . Chem. Soc., Chem. Commun., 62 (1977); (b) M. A. Filomena Dos Remedios Pinto, P. J. Sadler, S. Neidle, M. R. Sanderson, A. Subbaiah, and R. Kuroda, J . Chem. Soc., Chem. Commun., 13 (1980); (c) C.-M. Che, W. P. Schafer, H. B. Gray, M. K. Dickson, P. B. Stein, and D. M. Roundhill, J . Am. Chem. Soc., 104, 4253 (1982); (d) M. K. Dickson, W. A. Fordyce, D. M. Appel, K. Alexander, P. Stein, and D. M. Roundhill, Inorg. Chem., 21, 3857 (1982); (e) P. Stein, M. K. Dickson, and D. M. Roundhill, J . Am. Chem. Soc., 105, 3489 (1983).
0022-3654/85/2089-2486$01 .50/0
of the useful attributes that one finds in R ~ ( b p y ) ~ These ~ + . include (a) high solubility in aqueous and nonaqueous solvents, (b) strong absorption in the near-UV-visible region ( 6 = 34000 at 368 nm) so that quite dilute solutions can be used, (c) the donor excited state is a low-lying triplet having the convenient energy of 2.51 eV (57.9 kcal/mol) and, most importantly, (d) the lowest excited state shows a very intense (4 > 0.5) green phosphorescence in room temperature solutions with a lifetime of nearly 10 ps. The intense luminescence as well as the long lifetime facilitate monitoring of the excited-state reactions. In the past couple of years, several publications have appeared reporting on the photochemistry and redox chemistry of this Pt c o m p l e ~ . ~Closer . ~ examination of the data reveals that, in addition to most of these reports being of a preliminary in nature, they are contradictory in terms of both the excited state lifetimes and the ability to undergo photoredox chemistry (cf. discussion on this later). Herein we report our detailed studies on the characterization and energy- and electron-transfer processes of the lowest triplet excited state of this Pt complex. Steady-state as well as laser flash photolysis data are presented for the occurrence of both triplet-triplet energy transfer as well as oxidative, reductive quenching of the 3Pt2(POP)4"*.
Experimental Section Materials. K4[Pt2(POP)4]was synthesized from phosphorus acid (H3PO3) and K2PtC14by the procedure of Che, Butler, and (5) (a) C-M. Che, L. G. Butler, and H. B. Gray, J . Am. Chem. Soc., 103, 7796 (1981); (b) W. B. Hener, M. D. Totten, G. S. Rodman, E. J. Hebert, H. J. Tracy, and J. K. Nagle, J. Am. Chem. Soc., 106, 1163 (1984); (c) A. Vogler and H. Kunkely, Angew. Chem., In?. Ed. Engl., 23, 316 (1984); (d) S. A. Bryan, M. K. Dickson, and D. M. Roundhill, J. Am. Chem. SOC.106, 1882 (1984); (e) K. A. Alexander, P. Stein, D. B. Hedden, and D. M. Roundhill, Polyhedron, 2, 1389 (1983); (f) C-M. Che, S. J. Atherton, L. G. Butler, and H. B. Gray, J . Am. Chem. SOC.,106, 5143 (1984). (6) (a) W. A. Fordyce, J. G. Brummer, and G. A. Crosby, J . Am. Chem. Soc., 103, 7061 (1981); (b) S . F. Rice and H. B. Gray, J . Am. Chem. Soc., 105, 4571 (1983); (c) C.-M. Che, L. G. Butler, H. B. Gray, R. M. Crooks, and W. H. Woodruff, J . Am. Chem. Soc., 105, 5492 (1983); (d) J. T. Markert, D. P. clements, M. R. Corson,and J. K. Nagle, Chem. Phys. L e t f . ,97, 175 (1983); ( e ) M. K. Dickson, S. K. Pettee, and D. M. Roundhill, Anal. Chem., 53,2159 (1981); (f) A. Cox, T. J. Kemp, W. J. Reed, and 0. Traverso, results reported in T. J. Kemp, Progr. React. Kinet., 10, 301 (1980); (g) Y. Shimizu, Y. Tanaka, and T. Azumi, J . Phys. Chem., 88, 2423 (1984).
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985 2487
Reactions of Triplet Excited State of Pt2(POP)44-
T -1" A b s o r p t i o n Spectrum
TABLE I: Excited-State Luminescence Characteristics of Pt2(POP)rc in Homogeneous Solutions at Room Temperature
09 P t Z (POP)
phosphorescence solvent HZO' DZ0 CHjOH DMF CH3CN
fluorescence A,,*=, nm A,., 399 400 400 399 398
nm
k,(O,), M-l s-'
.07
ps
2.4 X lo9 2.0 x 109 1.4 X lo9 1.5 X lo9
.os
9.4 9.4
1.25 1.44 0.32 0.38
?
?
Tphoad=eratd, ps
513 512 51 1 51 1 51 1
9.5
10.0
"In HzO degassed solutions, @(phos)= 0.55
Tphol=ra'd,
'
-
in ~ o ~ c - 0 1
340
**
a t *
p . h
-'
*
I
*
*
a
* 0.03.
Gray.5a After recrystallization, pale greenish yellow crystals were obtained and the measured absorption maxima and molar extinction coefficients in water (E = 34000 M-' cm-I at 368 nm) are in good agreement with those of Crosby et a1.6a For studies in nonaqueous solvents, the triphenylarsonium salt was prepared from the potassium salt via a metathesis reaction as reported by Gray et al.Sa The K salt was found to be very soluble in water. Freshly prepared solutions from doubly recrystallized salts are practically colorless and are stable for several days, though a slight dark brown/black color develops upon storage over several weeks (probably due to some residual phosphorus acid still present, as solutions of nonrecrystallized salts in water turn brownish rather quickly, in less than 1-2 h). All quencher salts are commercial p.a grade samples from Fluka or were available from previous work in this laboratory.' Methods. Steady-state luminescence measurements were carried out on a Perkin-Elmer Hitachi MPF 4F spectrofluorimeter equipped with a Hamamatsu R928 PM tube and a correction unit. Phosphorescence quantum yields were determined by using quinine sulfate in 1 N H2S04as a standard. Time-resolved measurements were carried out with a fast flash kinetic spectroscopy unit, comprised of a JK Lasers System 2000 series Q-switched ND: YAG (oscillator)/glass (amplifier) laser. Quenching rate constants were determined from Stern-Volmer plots of emission intensity or lifetime data unless stated otherwise; the phosphorescence emission of Pt2(pop)44-was monitored at 515 nm. Except for a few experiments that involved direct excitation of naphthalene, all laser photolysis experiments employed 353-nm triplet Nd laser pulses (1 5 ns, 20-50 mJ/pulse). Absorption spectra of transients were constructed from measured transient absorbances recorded every 5 nm point by point and drawing a smooth line through them. All solutions were thoroughly degassed prior to use by bubbling ultrapure Ar or N2 for at least 15 min. To avoid loss of the solvent (CH,OH) during degassing and consequent changes in the concentration of the sensitizer and the quenchers, N2 or Ar was first passed through a scrubber (CH,OH) to saturate the gas with the solvent vapor, prior to passing it through the photolysis solution. This is absolutely essential, especially for the steady-state quenching experiments.
Results and Discussion Characterization of Luminescence in Homogeneous Solutions. The luminescence of the Pt2(POP)44-excited states in aqueous solution has been the subject of numerous earlier investigations.6 In homogeneous solutions (aqueous or nonaqueous) Pt2(POP)4e exhibits a weak fluorescence (A, 407 nm) and a very strong 5 15 nm). Spectro(room temperature) phosphorescence (A,, scopic studies on the nature of the luminescence indicate that these arise from the transitions ]AZu 'Al, and 3A2u 'A respectively. We have examined the luminescence of Pt2POg4- in several neat solvents in addition to water and the observed emission maxima, lifetimes, and quantum yields are summarized in Table I. As can be seen from the data, the influence of solvent as well as replacement of hydrogen by deuterium in water (D20) has only a very small effect on the luminescence properties of both the singlet and triplet excited states. In all cases, oxygen quenching
-
.03
-
:-
-
(7) V. H. Houlding, T. Geiger, U. KMe, and M. Gratzel, J . Chem. SOC., Chem. Commun., 681 (1982).
-. 01300
360
420
480
540
600
W n v o l o n g t h (nm)
Triplet Extinction Coefficient (465 n m ) + 5500 ].mole-lcm-l
064
0
0
80
160
240
320
400
L o s a r Intensity (orb. u n i t s )
Figure 1. Top: Transient absorption (difference) spectrum recorded at the end of laser pulse, following excitation of a degassed solution of Ptz(POP)44-(2.0 X M) in methanol. Bottom: Determination of triplat-state extinction coefficient of Pt2(POP)t- in CHJOH via totdl saturation method using high-intensity laser pulses, X = 465 nm, I = 5
mm. is rather efficient with measured kqvalues all in the range of (1-2) X lo9 M-' s-l. As monitored by emission lifetimes, the intense phosphorescence does not show any concentration quenching upto lo4 M (both in aqueous and nonaqueous solvents) though Sadler et al.4bhave reported concentration quenching at concentrations as low as 15 pM. The measured phosphorescence lifetimes in various solvents are all in the neighborhood of 10 ps, in excellent agreement with the values of Gray et al.5a in water, but are significantly different from those cluster of values around 6-7 ps (6.2,6f 5.5,6a and ?.lSbks). Since these lower values are closer to the observed half-lifes in water, it is possible that there may have been an error in earlier work of reporting emission half-lifes as lifetimes! Our measured quantum yield values in water (4 = 0.55) are in good agreement with values recently reported by Nagle et a1 (4 = 0.52).5b Triplet-State Absorption Spectrum. Figure 1 presents the transient difference spectrum of Pt,(POP),"- in degassed aqeuous solutions recorded following excitation with 15-ns, 353-nm light pulses from a Q-switched tripled Nd laser. The spectrum is characterized by two absorption maxima, one around 460 nm and the other around 330 nm, and it is qualitatively similar to that reported earlier by Gray et al.5a The assignment of the transient absorption as due to the triplet state has been confirmed by (a) matching the decay of the transient absorption with that of the phosphorescence emission around 510 nm and (b) the extreme sensitivity of the transient absorption to the presence of oxygen in solution. The molar extinction coefficient for the triplet absorption around 460 nm has been determined by "saturation methods", viz, measuring the maximum absorbance with a saturating intense laser pulse. As can be seen in Figure 1, saturation is readily achieved and the estimated E value at 465 nm is 5500 f 500 M-' cm-l. Though not in this wavelength region, at wavelengths above 500 nm the intense phosphorescence emission
2488 The Journal of Physical Chemistry, Vol. 89, No. 12, 1985
Peterson and Kalyanasundaram
TABLE II: Quenching of Pt2(WP),& Phosphorescence by Various Organic and Inorganic Molecules in Homogeneous Solutions auenchers" concn range. M solvent knr.M-' s-I k"'. M-I s-' remarks ~ _ _ oxygen azulene anthracene
Energy Transfer HZO 2.4 x CH3OH 3.5 x CH3OH H20 1.8 x CH30H 1.2 x CHSOH
