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Transient behaviour and luminescence quenching of Rub(bpy)32+ bound to an insoluble polymeric phase
Journal ofPhotochemistry
and Photobiology,
A: Chemistry, 44 (1988)
51-
55
TRANSIENT BEHAVIOUR AND LUMINESCENCE QUENCHING Ru(bpy),‘+ BOUND TO AN INSOLUBLE POLYMERIC PHASE J. L. BOURDELANDE,
C. CAMPA, J. CAMPS, J. FONT+and
51
OF
P. DE MARCH
Repartament de Quimica (Quimica Organica), Facultat de Ciencies, Universitat Autonoma de BarceIona, Bellaterra, 08193 Barcelona (Spain) F. WILKINSON? Department (U.K.)
and C. J. WILLSHER
of Chemistry,
University of Technology,
(Received November 6, 1987;
Loughborough,
Leics. LE 11 3TlJ
in revised form February 15, 1988)
Summary The spectroscopic behaviour of Ru(bpy),2* bound to an insoluble polystyrene support is similar to that reported in homogeneous solution and in other heterogeneous media. Following pulsed excitation at 532 nm, emission from the MLCT complex is observed, the decay of which contains both unimolecular and bimolecular components. Weak transient absorption within the sample is also observed and *Ru(bpy),2+ could be identified by its absorption maximum at 375 nm and depletion minimum at 430 nm. Experiments using methylviologen show that luminescence quenching is about forty times slower than in aqueous solution, indicating that the hydrophobic nature of the polymer impedes the approach of the quencher to the ruthenium complex. The inclusion of a hydrophilic sequence in the sample has little effect on improving the quenching efficiency.
Tris( 2,2 -bipyridine)ruthenium( ‘ II), Ru( bpy), 2+, is a well-studied material and it has been widely used, either in homogeneous solution or immobilized on a suitable substrate, for the dissociation of water [l] . We have already described the synthesis and use i of polymeric ruthenium complex 1 in which one of the rings of a bipyridyl ligand is covalently bound to insoluble cross-linked polystyrene [2]. (In addition to the washing procedure outlined in ref. 2, polymer 1 was washed with fluorescence grade CH2C12 to be certain that no impurities, especially unbound complex, remained in the polymer. It was confirmed th t‘ no unbound ruthenium complex was present, since the last washing showe no luminescence.) Photoinduced electron transfer from this complex was f:% und to be very inefficient and the reasons for this are unclear. We Tow wish to report on TAuthors to whom correspondence should be addresseld. I lOlO-6030/88/$3.50
0 Elsevier Sequ&/Printed
in The Netherlands
52
the laser-induced emission and transient absorption as studied by diffuse reflectance laser flash photolysis [ 31. The characterization of surfaces by excited state techniques and the reactions that occur thereon have recently been reviewed by Thomas [4]. Three polymers were used in the present study: (i) a “dilute” sample la containing 0.03 mmol of Ru(bpy),*+ units per gram of polymer, corresponding to functionalization of 1 in 250 pendant phenyl groups; (ii) a “concentrated” sample lb bearing 0.43 mmol of complex per gram of polymer, which corresponds to 1 in 15 functionalization; (iii) polymer 2 which possesses a hydrophilic sequence between the polystyrene backbone and the ruthenium complex. Polymer 2 contains 0.24 mmol of Ru(bpy)32+ units per gram of polymer, i.e. 1 in 30 phenyl groups are functionalized 151.
.Pulsed nanosecond excitation at 532 nm of la, lb or 2 suspended in 2:l dioxane:water generates a transient emission which has the same spectrum as the metal-to-ligand charge transfer of the complex in solution. Pure Rutbpy), *+ shows the same emission both in the form of microcrystals or when suspended in chloroform, but no emission is detected from the plain polymer. Exclusion of oxygen causes no observable difference in the decay kinetics of the emission. There is little variation in spectral and kinetic behaviour between the different polymeric complexes, and their decays are only slightly slower than in microcrystalline Ru(bpy)32+. A typical decay is given in Fig. 1, and from inset A it can be seen that the initial and final slopes of the first-order analysis differ by a factor of at least 5, which implies that the decay is multiexponential. However, inset B shows that a second.order rate law can be applied to the first 80% of the decay; second-order processes have been reported for Ru(bpy),*’ in various heterogeneous environments [ 61, and in our case it would suggest that preferential fuction,alization at the polymer surface has occurred. Transient absorption within the polymeric ruthenium complexes is not so easy to detect. For example, in a deoxygenated 2:l dioxane:water suspension of la, a small absorption was observed after excitation at 532 nm and the time-resolved transient difference spectrum is given in Fig. 2. The spectral features, which comprise a maximum at 370 nm and a depletion minimum at 430 nm, may be assignable to *Ru(bpy)32+ [7]. Upon excitation at 354 nm a very weak transient absorption spanning the 450 - 700 nm region was observed for microcrystalline Ru(bpy),*+, and in lb a transient
Fig. 1. Decay of laser-induced emission from la suspended in N2-purged 2:l dioxane: water. A,,, = 532 nm; h,,, = 650 nm; excitation intensity, 150 mJ pulse-‘; pulse width, 15 ns. Inset A shows first-order kinetic analysis and inset I3 second-order analysis.
Wavelength inml Fig. 2. Transient difference spectrum of la in Na-purged 2:l dioxane:water 532 nm excitation: (i) 100 ns, (ii) 200 ns and (iii) 400 na after laser flash.
following
absorption whose spectrum showed depletion at 450 nm and a maximum at 520 nm was observed. The latter transient could be due to Ru(bpy)3+ [8], but owing to the poor quality of the signals we cannot draw any firm conclusions, and kinetic data can only be extracted with confidence from laserinduced emission. Figure 3 shows steady state Stern-Volmer plots for luminescence quenching by methyl viologen (MV2’) of the excited complexes lb and 2 suspended in 2:l dioxane:water, as well as the quenching of *Ru(bpy)s2+
0.2
0.4 IMV*+i
0.6
0.8 x
lo*
1.0
1.2
(Ml
Fig. 3. Stern-Volmer plot for luminescence quenching of excited Ru(bpy)s2+ 2 (curve b) and Ib (curve c) by MV2+ in Nz-purged 2:l dioxane:water.
(curve a),
dissolved in the same solvent. The lifetimes of the excited states of lb and 2 can be deduced from the final slope of the first-order analysis of the emission decay for these polymers and are in the region 900 - 950 ns. These lifetimes are similar to those obtained for pure Ru(bpy)s2+ in solution [l]. From the slopes of the Stern-Volmer plots it is observed that k, drops from (1.1 + 0.1) X 10’ mol-’ dm3 s-l for Ru(bpy)32+ in water to (2.8 + 0.1) X 10’ mol-’ dm3 s-l f or lb suspended in dioxane :water. The poor quenching efficiency is considered to result from the hydrophobic nature of the polystyrene support which would be solvated preferentially by dioxane (a similar effect has been described for Ru(bpy)32’ contained in a cation exchange resin in a water-organic solvent mixture, for which a two-phase microheterogeneous solvent domain was proposed [9] ), while the water (containing MV2*) is repelled, thus inhibiting contact between *Ru(bpy)32+ and the quencher. This effect could be reduced in polymer 2 where the hydrophilic polyether sequence should facilitate the approach of MV2+ to the complex. However, k, for this sample is very similar to that for lb, which demonstrates that the polyether sequence does not compensate for the hydrophobic nature of the polystyrene. These results show that the spectroscopic and photophysical properties of Ru( bpy),*+ are unaffected
55
by attachment to this polymer support, but that the photochemistry is substantially altered, and it appears that other polymeric supports should be investigated. A&no wledgments Support from F’undacibn Ram6n Areces of Spain is gratefully acknowledged. We would like to thank the British C6uncil and Ministerio de Educaci6n y Ciencia for financial assistance under the “Acciones Integradas” scheme. C.J.W. wishes to thank the European Office of the U.S. Army for support under contract number DAJA 45-85-COOZO.
References 1 K. Kalyanasundaram, Coord. Rev., 46 (1982) 159. M. Griitzel, K. Kalyanasundaram and J. Kiwi, Solar Energy Materials, Springer, Berlin, 1982, pp. 37 - 126. 2 P. Bosch, C. Campa, J. Camps, J. Font, P. de March and A. Viigili, An. Quim. Ser. C, 81 (1985) 162. 3 C. J. Willsher, J. Photochem., 28 (1985) 229. 4 J. K. Thomas, J. Phys. Chem., 91 (1987) 267. 5 J. L. Bourdelande, C. Campa, J. Font and P. de March, Eur. J. Polym., submitted for publication. 6 B. H. Milosavljevic and J. K. Thomas, J. Phys. Chem., 87 (1983) 616. S. Kelder and J. Rabani, 3. Phys. Chem., 85 (1981) 1637. J. W. Perry, A. J. McQuillan, F. C. Anson and A. H. Zewall, J. Phys. Chem., 87 (1983) 1480. 7 R. Bensasson, C. Salet and V. Balzani, J. Am. Chem. SOC., 98 (1976) 3722. 8 D. Meisel, M. S. Matheson, W. A. Mulac and J. Rabani, J. Phys. Chem., 81 (1977) 1449. 9 S. L. Buell and J. N. Demas, J. Phys. Chem., 87 (1983) 4675.
and Photobiology,
A: Chemistry, 44 (1988)
51-
55
TRANSIENT BEHAVIOUR AND LUMINESCENCE QUENCHING Ru(bpy),‘+ BOUND TO AN INSOLUBLE POLYMERIC PHASE J. L. BOURDELANDE,
C. CAMPA, J. CAMPS, J. FONT+and
51
OF
P. DE MARCH
Repartament de Quimica (Quimica Organica), Facultat de Ciencies, Universitat Autonoma de BarceIona, Bellaterra, 08193 Barcelona (Spain) F. WILKINSON? Department (U.K.)
and C. J. WILLSHER
of Chemistry,
University of Technology,
(Received November 6, 1987;
Loughborough,
Leics. LE 11 3TlJ
in revised form February 15, 1988)
Summary The spectroscopic behaviour of Ru(bpy),2* bound to an insoluble polystyrene support is similar to that reported in homogeneous solution and in other heterogeneous media. Following pulsed excitation at 532 nm, emission from the MLCT complex is observed, the decay of which contains both unimolecular and bimolecular components. Weak transient absorption within the sample is also observed and *Ru(bpy),2+ could be identified by its absorption maximum at 375 nm and depletion minimum at 430 nm. Experiments using methylviologen show that luminescence quenching is about forty times slower than in aqueous solution, indicating that the hydrophobic nature of the polymer impedes the approach of the quencher to the ruthenium complex. The inclusion of a hydrophilic sequence in the sample has little effect on improving the quenching efficiency.
Tris( 2,2 -bipyridine)ruthenium( ‘ II), Ru( bpy), 2+, is a well-studied material and it has been widely used, either in homogeneous solution or immobilized on a suitable substrate, for the dissociation of water [l] . We have already described the synthesis and use i of polymeric ruthenium complex 1 in which one of the rings of a bipyridyl ligand is covalently bound to insoluble cross-linked polystyrene [2]. (In addition to the washing procedure outlined in ref. 2, polymer 1 was washed with fluorescence grade CH2C12 to be certain that no impurities, especially unbound complex, remained in the polymer. It was confirmed th t‘ no unbound ruthenium complex was present, since the last washing showe no luminescence.) Photoinduced electron transfer from this complex was f:% und to be very inefficient and the reasons for this are unclear. We Tow wish to report on TAuthors to whom correspondence should be addresseld. I lOlO-6030/88/$3.50
0 Elsevier Sequ&/Printed
in The Netherlands
52
the laser-induced emission and transient absorption as studied by diffuse reflectance laser flash photolysis [ 31. The characterization of surfaces by excited state techniques and the reactions that occur thereon have recently been reviewed by Thomas [4]. Three polymers were used in the present study: (i) a “dilute” sample la containing 0.03 mmol of Ru(bpy),*+ units per gram of polymer, corresponding to functionalization of 1 in 250 pendant phenyl groups; (ii) a “concentrated” sample lb bearing 0.43 mmol of complex per gram of polymer, which corresponds to 1 in 15 functionalization; (iii) polymer 2 which possesses a hydrophilic sequence between the polystyrene backbone and the ruthenium complex. Polymer 2 contains 0.24 mmol of Ru(bpy)32+ units per gram of polymer, i.e. 1 in 30 phenyl groups are functionalized 151.
.Pulsed nanosecond excitation at 532 nm of la, lb or 2 suspended in 2:l dioxane:water generates a transient emission which has the same spectrum as the metal-to-ligand charge transfer of the complex in solution. Pure Rutbpy), *+ shows the same emission both in the form of microcrystals or when suspended in chloroform, but no emission is detected from the plain polymer. Exclusion of oxygen causes no observable difference in the decay kinetics of the emission. There is little variation in spectral and kinetic behaviour between the different polymeric complexes, and their decays are only slightly slower than in microcrystalline Ru(bpy)32+. A typical decay is given in Fig. 1, and from inset A it can be seen that the initial and final slopes of the first-order analysis differ by a factor of at least 5, which implies that the decay is multiexponential. However, inset B shows that a second.order rate law can be applied to the first 80% of the decay; second-order processes have been reported for Ru(bpy),*’ in various heterogeneous environments [ 61, and in our case it would suggest that preferential fuction,alization at the polymer surface has occurred. Transient absorption within the polymeric ruthenium complexes is not so easy to detect. For example, in a deoxygenated 2:l dioxane:water suspension of la, a small absorption was observed after excitation at 532 nm and the time-resolved transient difference spectrum is given in Fig. 2. The spectral features, which comprise a maximum at 370 nm and a depletion minimum at 430 nm, may be assignable to *Ru(bpy)32+ [7]. Upon excitation at 354 nm a very weak transient absorption spanning the 450 - 700 nm region was observed for microcrystalline Ru(bpy),*+, and in lb a transient
Fig. 1. Decay of laser-induced emission from la suspended in N2-purged 2:l dioxane: water. A,,, = 532 nm; h,,, = 650 nm; excitation intensity, 150 mJ pulse-‘; pulse width, 15 ns. Inset A shows first-order kinetic analysis and inset I3 second-order analysis.
Wavelength inml Fig. 2. Transient difference spectrum of la in Na-purged 2:l dioxane:water 532 nm excitation: (i) 100 ns, (ii) 200 ns and (iii) 400 na after laser flash.
following
absorption whose spectrum showed depletion at 450 nm and a maximum at 520 nm was observed. The latter transient could be due to Ru(bpy)3+ [8], but owing to the poor quality of the signals we cannot draw any firm conclusions, and kinetic data can only be extracted with confidence from laserinduced emission. Figure 3 shows steady state Stern-Volmer plots for luminescence quenching by methyl viologen (MV2’) of the excited complexes lb and 2 suspended in 2:l dioxane:water, as well as the quenching of *Ru(bpy)s2+
0.2
0.4 IMV*+i
0.6
0.8 x
lo*
1.0
1.2
(Ml
Fig. 3. Stern-Volmer plot for luminescence quenching of excited Ru(bpy)s2+ 2 (curve b) and Ib (curve c) by MV2+ in Nz-purged 2:l dioxane:water.
(curve a),
dissolved in the same solvent. The lifetimes of the excited states of lb and 2 can be deduced from the final slope of the first-order analysis of the emission decay for these polymers and are in the region 900 - 950 ns. These lifetimes are similar to those obtained for pure Ru(bpy)s2+ in solution [l]. From the slopes of the Stern-Volmer plots it is observed that k, drops from (1.1 + 0.1) X 10’ mol-’ dm3 s-l for Ru(bpy)32+ in water to (2.8 + 0.1) X 10’ mol-’ dm3 s-l f or lb suspended in dioxane :water. The poor quenching efficiency is considered to result from the hydrophobic nature of the polystyrene support which would be solvated preferentially by dioxane (a similar effect has been described for Ru(bpy)32’ contained in a cation exchange resin in a water-organic solvent mixture, for which a two-phase microheterogeneous solvent domain was proposed [9] ), while the water (containing MV2*) is repelled, thus inhibiting contact between *Ru(bpy)32+ and the quencher. This effect could be reduced in polymer 2 where the hydrophilic polyether sequence should facilitate the approach of MV2+ to the complex. However, k, for this sample is very similar to that for lb, which demonstrates that the polyether sequence does not compensate for the hydrophobic nature of the polystyrene. These results show that the spectroscopic and photophysical properties of Ru( bpy),*+ are unaffected
55
by attachment to this polymer support, but that the photochemistry is substantially altered, and it appears that other polymeric supports should be investigated. A&no wledgments Support from F’undacibn Ram6n Areces of Spain is gratefully acknowledged. We would like to thank the British C6uncil and Ministerio de Educaci6n y Ciencia for financial assistance under the “Acciones Integradas” scheme. C.J.W. wishes to thank the European Office of the U.S. Army for support under contract number DAJA 45-85-COOZO.
References 1 K. Kalyanasundaram, Coord. Rev., 46 (1982) 159. M. Griitzel, K. Kalyanasundaram and J. Kiwi, Solar Energy Materials, Springer, Berlin, 1982, pp. 37 - 126. 2 P. Bosch, C. Campa, J. Camps, J. Font, P. de March and A. Viigili, An. Quim. Ser. C, 81 (1985) 162. 3 C. J. Willsher, J. Photochem., 28 (1985) 229. 4 J. K. Thomas, J. Phys. Chem., 91 (1987) 267. 5 J. L. Bourdelande, C. Campa, J. Font and P. de March, Eur. J. Polym., submitted for publication. 6 B. H. Milosavljevic and J. K. Thomas, J. Phys. Chem., 87 (1983) 616. S. Kelder and J. Rabani, 3. Phys. Chem., 85 (1981) 1637. J. W. Perry, A. J. McQuillan, F. C. Anson and A. H. Zewall, J. Phys. Chem., 87 (1983) 1480. 7 R. Bensasson, C. Salet and V. Balzani, J. Am. Chem. SOC., 98 (1976) 3722. 8 D. Meisel, M. S. Matheson, W. A. Mulac and J. Rabani, J. Phys. Chem., 81 (1977) 1449. 9 S. L. Buell and J. N. Demas, J. Phys. Chem., 87 (1983) 4675.