Effects of Polymer Matrices on the Time-Resolved Luminescence of a

3162

J. Phys. Chem. 1995, 99, 3162-3167

Effects of Polymer Matrices on the Time-Resolved Luminescence of a Ruthenium Complex Quenched by Oxygen Sonja Draxler and Max E. Lippitsch" Institut f i r Experimentalphysik, Karl-Franzens-Universitat Graz, Universitatsplatz 5, A-8010 Graz, Austria

Ingo Klimant, Meinz Kraus, and Otto S. Wolfbeis Institut @ r Organische Chemie, Karl-Franzens- Universitat Graz, Heinrichstrasse 28, A-8010 Graz, Austria Received: June 30, 1994; In Final Form: October 31, 1994@

Quenching of luminescence by oxygen of ruthenium diphenylphenanthroline in various polymers was studied by time-resolved spectroscopy. The luminescence decay was not single exponential, and the Stern-Volmer plot was nonlinear (downward-curved) in all cases. A new model for describing the nonexponential luminescence decay was developed, which considers the interaction of the fluorophore with the nonuniform environment provided by the polymer. This model has a better physical basis than the usual fits with multiple exponentials or lifetime distributions of arbitrary shape. Oxygen quenching is described by a single parameter, which, in favorable cases, depends linearly on oxygen pressure. This may be of advantage for the calibration of optical oxygen sensors based on luminescence quenching.

1. Introduction Determination of oxygen is most important in various fields of clinical analysis, environmental monitoring, or process control and is usually performed amperometrically using Clark-type electrodes. Recently, optical sensors based on luminescence measurements have gained much intere~tl-~ because of their high sensitivity and specificity. Various suggested the use of transition metal complexes, especially Ru(II), Os(11), and Re(I), in developing oxygen sensors, since the luminescence of these compounds is strongly quenched by oxygen. Another advantage is their relatively long luminescence decay time, ranging from several hundreds of nanoseconds up to 10 ps. In homogeneous media and under circumstances of purely dynamic quenching, the variation in luminescence intensity I or luminescence decay time t with externally applied oxygen partial pressure po2 is described by the Stem-Volmer equation

with

where l o is the luminescence intensity and t o the lifetime of the excited state in the absence of oxygen, ksyis the Stem-Volmer quenching constant, and k is the bimolecular quenching constant. However, when dye molecules are incorporated into a polymer matrix, deviations from the linear intensity-based Stern-Volmer plot are frequently encountered. In addition, a single time constant is no longer sufficient to describe the luminescence decay. In recent years it has been shown that fluorophores in nonuniform environments exhibit luminescence decays which are best understood by a model of continuous distributions of decay times. (See, for example, ref 10 and references therein.) The problem in this approach is, however, that the form of the distribution cannot unambiguously be deduced from the mea~~

@

Abstract published

In Advance

ACS Abstrucrs, February 15, 1995.

0022-365419512099-3162$09.00/0

sured decay data. It was s h o ~ n " - ' ~that fitting procedures cannot distinguish sufficiently between, for example, a single Gaussian distribution of lifetimes and a sum of two exponentials or a bimodal Gaussian distribution and a sum of at least three exponentials. Thus, a description based on discrete lifetime components should only be regarded as truly representing discrete molecular states if supported by supplementary data. Since the Stern-Volmer constant is the product of a bimolecular quenching constant and a lifetime, any lifetime distribution is also reflected by a distribution of Stern-Volmer constants. Again, however, the form of the distribution cannot be readily calculated from the quenching behavior. In this work, luminescence intensity and decay time measurements of ruthenium complexes in different polymers were performed. Measurements resulted in nonexponential luminescence decays and nonlinear Stern-Volmer plots. A model was developed which in a straightforward manner accounts for the nonuniformity of the microenvironment and provides a simple method for quantitative description of the quenching. For optical oxygen sensors based on luminescence quenching this model is advantageous, since it opens the possibility to describe the characteristic dependence of sensor signal on oxygen concentration with a minimum number of variable parameters.

2. Experimental Section The transition metal complex used in this study was ruthenium(I1) tris(4,7-diphenyl-1,lO-phenanthroline), abbreviated as Ru(dpp) in the following. It was prepared with perchlorate or lauryl sulfate (LS) as the counterion, as described in ref 14. The polyester foil (Mylar) was from Dupont (Brussels). Four different materials have been used: (1) PVC membrane ( M - I ) : 0.5 g of poly(viny1 chloride) powder (Fluka) and 1.0 g of dioctyl phthalate (DOP) were dissolved in a 10% solution of dried tetrahydrofuran (Aldrich), which was distilled before use. To 4.0 g of this solution was added 26 mg of Ru(dpp)(C104)2. The solution was spread on a polyester foil. The dried layer had a thickness of 10 pm. (2) SiZicone membrane (M-2): 4 mg of Ru(dpp)(LS), was dissolved in 10 mL of chloroform. To this solution was added 1.0 g of silicone E-4 (Wacker). A 0 1995 American Chemical Society

J. Phys. Chem., Vol. 99, No. IO, 1995 3163

Effects of Polymer Matrices on a Ruthenium Complex

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layer of 50 p m thickness was spread on a polyester foil and dried at 90 "C for 48 h. The thickness of the dried layer was 15 pm. (3) Ethylcellulose membrane (M-3): 1.0 g of ethylcellulose (46% ethoxylated, Aldrich) and 13 mg of Ru(dpp)(C104)2were dissolved in chloroform, drawn on a polyester foil, and dried at 90 "C for 2 h. The thickness of the layer was 5 pm. (4) Polystyrene membrane (M-4): 1.0 g of polystyrene (molecular weight 250 000, Aldrich) and 13 mg of Ru(dpp)(C10& were dissolved in 10 mL of chloroform, spread on a polyester foil, and dried at 90 "C for 2 h. The thickness of the layer was 10 pm. Absorption measurements were performed on a Shimadzu W 2101 PC spectrophotometer, and luminescence intensity measurements were performed on an Aminco SPF 500 spectrofluorimeter. Gas mixtures were prepared using a precision gas mixer PGM-3 (Medicor Inc.). Time-resolved luminescence measurements were performed using a nitrogen laser-dye laser system (Photonics Research Associates Inc. LN103/102) as the light source, a fast photomultiplier (Valvo T W P 56) as detector, and a digital signal analyzer (Tektronix DSA 601A) for signal acquisition. The time resolution of the whole system was below 1 ns. The decay data were fitted using various commercially available mathematical software packages.

3. Results Figure 1 shows Stem-Volmer plots for the luminescence intensities of membranes M-1 through M-4. Deviations from the linear relationship between IdI and poZcan be observed for all membranes, being strongest with the silicone and ethylcellulose membranes. The luminescence decay of Ru(dpp) was single-exponential with t = 4.7 ps in oxygen-free aqueous solution and t = 5.2 ps in ethanol. In the polymer matrices, the decays were no longer single exponential. Luminescence decay curves were fitted by trial functions consisting of a sum of two or three exponentials. Figures 2a-5a show the dependence on oxygen pressure of the time constants zi obtained from the best fits to the measured decays. In addition, a preexponential weighted mean lifetime zm is shown for each membrane. This mean lifetime is calculated as proposed by Carraway et al.I5 according

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oxygen pressure (Torr) Figure 2. Quenching of the luminescence of Ru(dpp)(ClO& in PVC (M-1): (a) Stern-Volmer plots of the decay time components 5, and preexponentially weighted mean lifetime tm;(b) preexponential factors a, of the decay time components vs oxygen pressure.

to the following equation:

Figures 2b-5b show the normalized preexponential factors of the calculated decay times ti. The luminescence decay can be described satisfactorily by a sum of two exponentials for the PVC membrane M-1 only. While the preexponential weighted mean lifetime tm (Figure 2a) agrees very well with the intensity plot (Figure l), the decay times t1 and t2 show significantly lower quenching. The experimental data of silicone membrane M-2 (Figure 3) require a sum of three exponentials for an acceptable fit. While also in this membrane the preexponential weighted mean lifetime agrees with the intensity plot, the decay times ti are not strongly affected by variations in the oxygen partial pressure. One might ai

Draxler et al.

3164 J. Phys. Chem., Vol. 99, No. 10, 1995 5

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oxygen pressure (Torr) Figure 3. Quenching of the luminescence of Ru(dpp)(LS)2 in silicone (M-2): (a) Stern-Volmer plots of the decay time components t, and

Figure 4. Quenching of the luminescence of Ru(dpp)(ClO& in ethylcellulose (M-3): (a) Stern-Volmer plots of the decay time compo-

preexponentially weighted mean lifetime t,; (b) preexponential factors

nents t, and preexponentially weighted mean lifetime t,; (b) preexponential factors a, of the decay time components vs oxygen pressure.

a, of the decay time components vs oxygen pressure.

assume that the discrepancy between intensity and lifetime quenching data would point to static quenching as the responsible mechanism. By plotting the normalized preexponential factors vs oxygen pressure, it becomes obvious, however, that the assumption of static quenching cannot explain the observed behavior. Very similar results were found for the ethylcellulose and the polystyrene membrane (Figures 4 and 5). While two of the three exponentials do not depend on the oxygen pressure significantly, the longest decay time increases with increasing oxygen concentration up to about 30% oxygen. The preexponential factors show a similar behavior as mentioned above. All the fits are of very good quality, as judged from usual mathematical criteria k2, residuals, autocorrelation). Thus, it is possible to fit the experimental data satisfactorily with a sum of two or three exponentials. However, the preexponential

factors ai show a dependence on the oxygen pressure which cannot be described by any reasonable physical model. Even if the multiexponential fit is replaced by a Gaussian distribution of lifetimes, no reasonable dependence of the envolved parameters on oxygen concentration becomes apparent. Thus, the only conclusion can be that models with two or three components or Gaussian distributions, though yielding good-quality fits, do not have any physical meaning in the present context and are only mathematical constructs. Therefore, they cannot reasonably be used to gain any insight in the photophysics and quenching of the ruthenium complexes, and they are of little help in understanding the properties of sensor membranes. 4. Discussion

The luminescence decay function of isolated molecules usually can be described by single or multiple exponentials. For



fluorophore and the nearest interacting polymer site. The excited state of the fluorophore then relaxes with a distancedependent rate. So the differential equation describing the time evolution of the excited-state population is given by dpjldt = k‘”

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Herep, is the probability for thefi excited molecule to be found in the excited state at time t, and kc’) is the internal relaxation rate (assumed to be the same for all fluorophore molecules and given as the sum of radiative and nonradiative decay rates). kj’”)(rjk)is the additional external decay rate induced by the interaction of the jth excited molecule with the kth site on the polymer, dependent on the distance rjk between them. In the simplest case we assume the distances between fluorophores and interaction sites to be homogeneously distributed and the rate to depend on r with a power law, k&) = ri-”. The total decay function is calculated by integrating this differential equation and summing up over all sites. The lengthy calculation has been published several timesl6-I9 and shall not be repeated here. The result is that the decay obeys the following relation

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oxygen pressure (Torr) Figure 5. Quenching of the luminescence of Ru(dpp)(ClO& in polystyrene (M-4): (a) Stem-Volmer plots of the decay time components t, and preexponentially weighted mean lifetime s,; (b) preexponential factors a, of the decay time components vs oxygen pressure.

molecules embedded in a polymer matrix the decay is influenced by interactions between the fluorophores and the polymer. Most polymers are nonuniform media in a sense that there is no longrange structural order. The luminescence decay profiles of molecules in a nonuniform medium can be assumed as being an average over a distribution of relaxation rates. The theoretical treatment of this problem can be undertaken in a way shown first in a classical paper by Forster.I6 While that paper dealt only with one specific interaction (resonant energy transfer by dipole-dipole coupling), the mathematical treatment can be generalized to cover interactions of arbitrary form.”J8 The relaxation probability of a single fluorophore molecule is determined by interactions with the neighboring regions of the polymer. The influence of these interactions on the decay time can be assumed to depend on the distance between the

Here pj(0) is the initial probability for the jth molecule to be in the excited state, a is a parameter depending on the kind of interaction, z = l/kci),and A is the dimensionality of the system. Thus, by taking into account the interactions with the medium, the luminescence decay profile of molecules embedded in a nonuniform medium even in the absence of any quencher is found to deviate from the usual exponential form where the exponent is a linear function of time. The kind of interaction between fluorophore and surrounding polymer is unknown a priori. It can be readily assumed, however, that in many cases this interaction is electrical in nature. Since the polymer is not charged, the lowest term in a power series expansion represents the dipolar interaction. The distance dependence of a dipolar interaction scales with r-$ hence n = 6. Since the interacting polymer sites are assumed to be uniformly distributed in space, the problem is threedimensional, A = 3. Thus, A h = l/2, corresponding to a dipole-dipole type interaction in three dimensions, gives a very good description of the decay. When the fluorophore is quenched by a species Q , a third relaxation rate has to be added to eq 4: dp/dt = k‘”

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k/(e)(rjk) kjQ)[Q] k

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In the most simple case we can assume that the rate k$Q)is the same for all fluorophores, and the index j can be dropped. In addition, the approximation can be made that the density of interaction sites in the polymer does not depend on the oxygen concentration. Both assumptions are not necessarily fulfilled, however. The solution of eq 6 under the above assumptions gives for the luminescence decay ~ t= )exp[-aJt/z

- t/z(l + c)]

(7)

In this equation&) is the time-dependent fluorescence intensity, which is proportional to the probability pj(t) in eqs 4-6, and c is a quenching parameter depending on the oxygen concentration and related to the term kj’Q)[Q](the two being identical when quenching has the same probability for each molecule). Equa-

3166 J. Phys. Chem., Vol. 99,No. IO, 1995

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Figure 8. Quenching parameter c as a function of external oxygen pressure in ethylcellulose membrane M-3.

Figure 6. Fluorescence decay of Ru(dpp)(ClO& in polystyrene (M4) in the absence of quencher, fitted according to eq 7. u = 1.92 and t = 11.42 ps.

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tion 7 was used in a global analysis of luminescence decays at different oxygen pressures. In the case of polystyrene, this model gave excellent fits of the luminescence decay curves with a = 1.92 and z = 11.42 ps (Figure 6). Moreover, the global analysis yielded consistent results for the parameters a and z over the full range of oxygen concentrations. The only oxygen-dependent variable was the quenching parameter c, as expected from theory. We find the linear dependence c = c'poz with c' = 3.45 x Torr-' from our data (Figure 7). In a similar way it was possible to describe the decay profiles for molecules in ethylcellulose with a = 1.87 and t = 14.37 ps. The curve fits are not as good as in the case of polystyrene,

Figure 9. Quenching parameter c as a function of external oxygen pressure in PVC membrane M- 1.

and the linear dependence of the parameter c on the oxygen pressure is not perfect, however (Figure 8). This may be attributed to spurious precipitation of the indicator due to poor local solubility. For the PVC matrix, again excellent fits of the decay curves could be obtained. The parameters were a = 0.76 and z = 8.55 ys. However, the dependence of the quenching parameter c on oxygen pressure is not perfectly linear (Figure 9). This was the case even in pure PVC without plasticizer. A tentative description with a nonlinear dependence of the oxygen concentration in the matrix on the external pressure in the form of a Langmuir adsorption isotherm gave perfect agreement between calculation and experiment. We do not know, however, whether this assumption has any physical justification. Applying the global analysis using eq 7 to the data for the indicator-lauryl sulfate ion pair in a silicone matrix gave



parameters. Moreover, it is based on a clear physical concept, in contrast to more or less arbitrarily chosen multiexponentials or lifetime distributions. This fact can be very helpful when dealing with optical oxygen sensors based on luminescence quenching, because the dependence of the optical signal on the oxygen concentration is easily described by a simple mathematical function.

Acknowledgment. Financial support of this work by the Fonds zur Forderung der wissenschaftlichen Forschung, Grants S5703 and S5702, is gratefully acknowledged. References and Notes

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acceptable fits and fairly consistent parameters, but strong deviations from a linear dependence of the parameter c on oxygen pressure (Figure 10). The reason therefore is unknown. One may speculate that the indicator-lauryl sulfate ion pairs form reverse micelles in the highly hydrophobic matrix, leading to changes in the decay function as well as the quenching dynamics.

5. Conclusion An analysis of the luminescence decay of Ru(dpp) fluorophores in various polymers reveals that in all matrices the decay is not single exponential. A fit with a sum of exponentials gives physically unreasonable dependences of the preexponential factors of the decay components on oxygen pressure. This proves the decay to be due to a distribution of relaxation rates. A model relating this distribution to a distribution of distances between fluorophores and interaction sites on the polymer proves to be well-suited to describe the relaxation of luminophores in nonuniform environments. Quenching of a luminophore by oxygen, on the other hand, is a more difficult case. In principle, a single quenching parameter is sufficient for a full description, and in favorable cases, Le., when the indicator is well dispersed in a single-phase matrix for which Henry’s law is valid, this quenching parameter varies linearly with oxygen pressure. These favorable conditions, however, obviously prevail only in certain polymer matrices, while a nonlinear dependence of the quenching parameter on oxygen pressure is found in others. Thus, the present model is not able to cope with all possible complications of nonuniform systems, but over multiexponential models it has the advantage to require a smaller number of fitting

(1) Lippitsch, M. E.; Pusterhofer, J.; Leiner, M. J. P.; Wolfbeis, 0. S. Fibre-optic oxygen sensor with the fluorescence decay time as the information carrier. Anal. Chim. Acta 1988, 205, 1. (2) Wolfbeis, 0. S., Ed. Fiber Opric Chemical Sensors and Biosensqrs; CRC: Boca Raton, FL, 1991. (3) Thompson, R. B. Fluorescence-based fiber-optic sensors. In Lakowicz, J. R., Ed. Topics in Fluorescence Spectroscopy; Plenum Press: New York, 1991, 1992; Vols. 1-3. (4) Lippitsch, M. E.; Draxler, S. Luminescence decaytime based optical sensors: principles and problems. Sensors Actuators B 1993, 11, 421. ( 5 ) Lakowicz, J. R.; Szmacinski, H. Fluorescence lifetime-based sensing of pH, Ca2+, K+ and glucose. Sensors Actuators B 1993, 11, 133. (6) Wolfoeis, 0. S.; Leiner, M. J. P.; Posch, H. E. A new sensing Material for optical oxygen measurement. Microchim. Acta (Vienna) 1986, Ill, 359. (7) Carraway, E. R.; Demas, J. N.; DeGraff, B. A,; Bacon, J. R. Photophysics and photochemistry of oxygen sensors based on luminescent transition-metal complexes. Anal. Chem. 1991, 63, 337. (8) McCraith, B. D.; McDonagh, C. M.; O’Keeffe, G.; Keyes, E. T.; Vos, J. G.; O’Kelly, B.; McGilp, J. F. Fibre optic oxygen sensor based on fluorescence quenching of evanescent-wave excited ruthenium complexes in sol-gel derived porous coatings. Analyst 1993, 118, 385. (9) Demas, J. N.; DeGraff, B. A. Applications of highly luminescent transition metal complexes in polymer systems. Makromol. Chem., Macromol. Symp. 1992, 59, 35. (10) Eftink, M. R. Fluorescence quenching: Theory and application. In Topics in Fluorescence Spectroscopy; Lakowicz, J. R., Ed.; Plenum Press: New York, 1991; Vol. 2. (11) James, D. R.; Liu, Y.-S.; De Mayo, P.; Ware, W. R. Distributions of fluorescence lifetimes: consequences for the photophysics of molecules adsorbed on surfaces. Chem. Phys. Lett. 1985, 124, 460. (12) Demas, J. N.; DeGraff, B. A. Luminescence-based sensors: microheterogeneous and temperature effects. Sensors Actuators B 1993, 11, 35. (13) James, D. R.; Ware, W. R. A fallacy in the interpretation of fluorescence decay parameters. Chem. Phys. Lett. 1985, 120, 455. (14) Klimant, I:,Entwicklung optischer Sauerstoffsensoren auf der Basis lumineszierender Ubergangsmetallkomplexe. PhD Thesis, Karl-Franzens University, Graz, Austria, 1993. (15) Carraway, E. R.; Demas, J. N.; DeGraff, B. A. Luminescence quenching mechanism for microheterogeneous systems. Anal. Chem. 1991, 63, 332. (16) Forster, Th. Experimentelle und theoretische Untersuchung des zwischenmolekularen Ubergangs von Elektronenanregungsenergie. Z . Naturforsch. 1949, 4A, 321. (17) Baumann, J.; Fayer, M. D. Excitation transfer in disordered twodimensional and anisotropic three-dimensional systems: Effects of spatial geometry on time-resolved observables. J . Chem. Phys. 1986, 87, 4087. (18) Sienicki, K.; Blonski, S.; Durocher, G. Relaxation distributions of fluorescence decays. J . Phys. Chem. 1991, 95, 1576. (19) Blumen, A,; Manz, J. On the concentration and time dependence of the energy transfer to randomly distributed acceptors. J . Chem. Phys. 1979, 71, 4694. JP94 1649R

Effects of Polymer Matrices on the Time-Resolved Luminescence of a

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