Symposium: Applications of Inorganic Photochemistry
Symposium: Applications of Inorganic Photochemistry
Luminescence Sensors for Cations Based on “Designed” Transition Metal Complexes Yibing Shen and B. Patrick Sullivan Department of Chemistry, University of Wyoming, Laramie, WY 82071-3838 An understanding of the photophysical processes in metal complexes along with advances in preparative methods has led to applications of inorganic excited states in solar energy conversion, biological probes, and sensors. Our discussion in this paper focuses on principles that allow “design” of anion and cation sensors that are capable of discriminating one analyte over another based on the luminescence properties of inorganic chromophores. In its most rudimentary form, a discriminating sensor requires a molecular recognition component capable of binding one particular species among a mixture and a responsive fragment capable of producing a distinct signal change immediately after the binding act. Inspired by natural complexation phenomena in biological processes, chemists have successfully designed and synthesized a number of intricate abiotic molecules, for example, crown ethers, cryptands, spherands, cyclophanes, and calixarenes that are able to specifically sequester ions and molecules. These have been the basis for the new field of molecular recognition (1). Generally, crown and related compounds themselves cannot generate distinctive changes in physical properties upon complexation that would serve as a basis for signal transduction and sensing. To overcome this difficulty, a new generation of host molecules has been investigated. These substances contain responsive functions based on metal-to-ligand charge transfer (MLCT) excited states (2). In addition, a number of studies on organic-based sensors already provide an intellectual backdrop for sensor design (3). Some interesting but complex examples of inorganicbased sensors containing luminescent chromophores, or luminophores, are presented in Figure 1. The complex in
A
B
Figure 1. Examples of inorganic-based luminescence sensors. A: a pH-senstive probe; B: a complex sensitive to alkali metal cations.
Figure 1a is a pH-sensing system that combines the trisbipyridylruthenium(II) moiety as a luminophore with free phenolic units of a calix[4]arene acting as acid–base sites (4). Figure 1b shows a vinyl-linked benzocrown etherbipyridyl ruthenium(II) complex designed with conjugated linkages between the 2,2′-bipyridine moiety and the macrocyclic binding sites (5). This intriguing structure was designed for recognition of group IA and IIA metal cations. Sensor Design The structurally less complicated complexes pictured in Figure 2 illustrate the three main features of molecular design inherent to inorganic cation sensors (6). These are
Figure 2. Amine and azacrown Re(I)-based 2,2'-bipyridine and 1,10-phenanthroline luminescence sensors sensitive to both pH and heavy metal ions. A: [fac -(AZA-bpy)Re(CO)3(py)](CF3 SO3); B: [ fac -(DEAMbpy)Re(CO) 3 (py)](CF 3 SO 3 ); C: fac -(AZAphen)Re(CO)3 Cl; D: [fac-(AZA-phen)Re(CO) 3(py)](CF3SO 3).
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Symposium: Applications of Inorganic Photochemistry (i) a cation recognition site, in this case a macrocycle, (ii) a luminophoric grouping responsive to analyte binding, and (iii) a chemical linker, or spacer, that connects the recognition site and the luminophore to one another. Structures like those shown in Figures 1 and 2 are referred to as supramolecular because they combine individual functional elements into a larger structure whose chemical properties differ from that of the individual components. The elements of design necessary for cation recognition have been established in a broad sense. Five chemical features must be accounted for in order to maximize cation selectivity. Turning to the structures in Figure 2, an easily recognizable feature is the cavity of the macrocycle. It is clear that the ion being detected must “fit” in the cavity to optimally interact with the donor atoms. A second consideration is the ability of the macrocycle to adopt the preferred coordination geometry of the analyte ion. Third, the ability of the donor atoms to form strong bonds, that is, soft donors for soft ions or hard donors for hard ions, plays a crucial role. Fourth, the overall charge on the macrocycle is of importance. For example, according to coulombic considerations, higher negative charges will disproportionately favor more positively charged cations. Last, the solvation energy change between the free ion and the macrocyclecomplexed form should be favorably optimized. As far as the choice of chromophoric groupings, the use of MLCT excited states as the probe of the binding interaction is ideal owing to their thermal and photochemical stability, solubility in water, and preparative flexibility. These chromophores are predominantly derived from d6 metals and polypyridine ligands such as 2,2′-bipyridine and 1,10phenanthroline (Figs. 1 and 2). By varying the polypyridine or an ancillary ligand in the coordination sphere of, for example, an Re(I) center, the absorption and emission energies can be tuned over a ca. 8000-cm{1 range in the visible spectrum, and the excited-state lifetimes can vary from tens of nanoseconds to microseconds (7). Such flexibility is potentially important for the use of different lasers as excitation sources, and of course for variable sensitivity diode, PMT, or CCD detection. The linking chemistry that couples the recognition site to the responsive probe is important in determining the mechanism and the absolute sensitivity of the sensor (see later). In Figures 2a and 2b, a methylene spacer has been inserted between the luminophore and the recognition site. In Figures 2c and 2d the link is through a C–N bond, with a direct aromatic attachment between the recognition site and the chromophore. The chemistry necessary to achieve this latter linkage is based on a new synthetic procedure that uses the reactive precursor 1,10-phenanthroline-5,6epoxide as shown in Figure 3 (6a). Figure 4 demonstrates
Figure 3. A preparative route to an azacrown substituted 1,10phenanthroline ligand (AZA-phen) based on the ring opening of 1,10-phenanthroline-5,6-epoxide. This ligand exhibits a linking group that is directly attached to the chromophore (see text).
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the use of an amide link in a fashion that produces a structure where the recognition site is attached to an ancillary ligand, in this case a 4-substituted pyridine (8). Luminescence Spectroscopy of Inorganic Cation Sensors Among optical methods such as absorption, emission, or reflection spectroscopy, which are commonly employed for measurements in optical sensors, emission techniques have high inherent sensitivity. Since the emission wavelength is always longer than that of the incident light, it is possible to read a signal versus zero or near-zero background. Upon perturbation of the emitting state the emission signal can be read in the form of intensity, intensity ratio, or lifetime. Although there are numerous luminescent transition metal complexes, only ruthenium tris(α-diimine) complexes and facial rhenium tricarbonyl (α-diimine) complexes have been used as luminescent probes. As noted earlier, their emission features are associated with the lowest-lying MLCT excited state involving a metal-centered dπ-type orbital and a π* orbital centered on the polypyridine ligand. These transitions appear in the visible region of the electronic spectrum, and usually photoexcitation is not accompanied by degradation. Since these excited states are polar, crucial excited state characteristics such as emission quantum efficiency, lifetime, and redox potential are greatly affected by the ancillary ligands, solvent polarity, and other environmental factors. Illustrated in Figure 5 is an example of the binding of Pb 2+ (aq) for the sensor complex [fac-(AZAbpy)Re(CO)3 (py)](CF3SO3 ) (see the structure in Figure 2a, where AZA-bpy is the macrocyclic ligand in the structure) where the binding event causes a large increase in emission quantum efficiency for the sensor (6). For sensors that operate in aqueous solution, the variations in emission intensity as a function of cation concentration permit determination of the stability constants, Kb , by using a mass action equation of the form of eq 1. log [(φ fin – φ) / (φ – φini)] = { log [M] – log K b
(1)
where φ fin is the emission quantum efficiency of the sensor when completely complexed by the cation; φ ini is the emission quantum efficiency of the uncomplexed sensor; φ is the emission quantum efficiency in the presence of a given quantity of cation; [M] is the concentration of free cation; and Kb is the stability constant. The inset in Figure 5 graphically presents the data for Pb2+(aq) (and also that for Hg2+(aq)) in the nonlogarithmic form of eq 1 (at pH 6.0 at 22 °C). The data in the figure clearly show
Figure 4. A Re(I)-based sensor with the analyte binding site attached to a ligand that is ancillary to the MLCT excited-state chromophore.
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Symposium: Applications of Inorganic Photochemistry that the K b value for Hg 2+(aq) is substantially larger (by ca. 500 times). Independent measurements of the absolute emission quantum efficiency (φfin in eq 1) show that Pb2+(aq) is 2.0 times that of Hg2+(aq). We can term the larger Kb value a quantitative measure of the selectivity of the sensor; and the quantum efficiency preference, a measure of the sensitivity of the luminophore response. Together, these quantities produce the ability of a luminescence sensor to discriminate one analyte in the presence of another. The data presented in Figure 5 are potentially sensitive to external physical and chemical variables, the most important of which are solution pH and ionic strength. The interrelationship between the protonation and the lead-binding equilibria for [fac-(AZAbpy)Re(CO)3 (py)](CF3SO 3) is clearly seen in the titration curves where the apparent pKa of the system drops from ca. 5.5 (no Pb2+[aq]) to 4.1 (5-fold excess of Pb2+[aq]) to 1.6 (200-fold excess of Pb 2+[aq]) as more lead ion is added to the system. Because of effects such as this, the use of computer analysis with input variables being the original Pb2+(aq) concentration, the measured pH of the solution, the independently measured pKa of the sensor, and the emission intensity is needed. Once such an analysis has been accomplished the equations that express the equilibria may be written as in eqs 2–4. In the case [fac-(AZAbpy)Re(CO) 3(py)](CF3SO3 ) these are, [Re] + + H+ [ReH]2+ + Pb2+(aq)
[ReH]2+ [RePb]3+ + H+
Ka = 2.5 ×105
(2)
KH,Pb = 0.1
(3)
been demonstrated experimentally to be small (9). This is also a result of Debye–Hückel calculations, where Kb can be partitioned into an electrostatic contribution that accommodates the charge on the analyte and on the metal center of an MLCT excited state luminophore [in this case Re(1+)], and a contribution from cavity binding Kcavity, which includes both bond breaking and making for the ligated metal ion and the associated outer-sphere solvation free energy changes. Mechanisms of Sensing A key issue in development of luminescence-based sensors is the mechanism of sensing. Thus, in order to design new sensors and optimize the existing systems, a firm understanding of sensing mechanisms, that is, the chemical and physical reasons for the absolute change in emission quantum efficiency and/or excited state lifetime, is necessary. Little is known concerning the binding modes (geometry, bond distances, etc.) of the analytes in the sensors discussed earlier; however, in selected cases, some understanding of the photophysics has been gleaned. For the cases of the Ru and Re-based luminophores of Figures 1–3, the photophysical picture is that of Scheme I.
where [Re]+ signifies the sensor complex. The sum of eqs 2 and 3 is the equilibrium of interest: Scheme I [Re]+ + Pb2+
[RePb]3+
K a = 2.5 × 104
(4)
that is, the binding constant of lead for the unprotonated form of the sensor (eq 4). The dependence of Kb on ionic strength in dilute solution of sensor and analyte (in the range 10 {6 to 10{4 M) has
Figure 5. Left: the emission response for binding of Pb 2+(aq) to the sensor complex [fac-(AZA-bpy)Re(CO)3(py)](CF3SO3) (see the structure in Figure 2a; where AZA-bpy is the macrocyclic ligand in the structure; right, the leftmost binding curve is for the data shown and the rightmost is for Hg2+ (aq) under identical conditions.
Note that absorption produces a charge-separated “singlet” state that decays via intersystem crossing with a rate constant, kisc . The intersystem crossing rate constant for these systems is so fast that following excitation both radiative and radiationless decay occur solely from the lowest excited “triplet” state. For such a system, the emission quantum efficiency (Φ) and lifetime (τ) of the emitting state are given by eqs 5 and 6, respectively. Φ = kr / (kr + ∑knr)
(5)
1 / τ = kr + ∑knr
(6)
In eqs 5 and 6, kr is the radiative decay rate constant and ∑knr is the sum of all nonradiative decay rate constants. Inspection of these equations reveals that change in either kr or knr will lead to a sensing response for the probe. An electron-transfer modulation mechanism, where the cation bound in the macrocycle controls a nonradiative, intramolecular electron transfer rate, has been implicated in studies of the complexes [fac-(DEAM-bpy)Re(CO) 3 (py)](CF 3 SO 3 ) and [fac-(AZA-bpy)Re(CO) 3 (py)](CF 3 SO 3 ) (see Fig. 2a, b) (6). Thus τ for [fac-(AZAbpy)Re(CO)3(py)](CF3 SO3 ) is 73 ns in N2 -degassed water (pH 6.0), but upon binding of Pb2+(aq) or Hg2+(aq) the lifetime of the complex increases to 123 and 129 ns, respectively. That is, the systems are partially quenched in their native forms. Following metal coordination or protonation, the oxidation potential of the amine group is shifted to more
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Symposium: Applications of Inorganic Photochemistry positive potentials because the lone pair is involved in coordination or protonation. By this mechanism, the intramolecular electron transfer process is prevented and luminescence is restored. In support of this idea, cyclic voltammetry studies for [fac-(DEAM-bpy)Re(CO) 3 (py)](CF3SO3 ) show that the amine group oxidizes at 1.1 V, which is sufficient to cause a detectable rate of electron transfer quenching of the excited state (10). A number of other sensor systems are proposed to exhibit intramolecular photoinduced electron transfer mechanisms. Most notable are the calixarene-Ru system in Figure 1a and several intriguing Re(I)-based supramolecular ensembles where crown cavities separate the luminophore from a nitroaromatic oxidative quencher. In one of the latter cases, K+ binding causes a lifetime decrease (11). Returning to the complex in Figure 4, a comprehensive study has identified a mechanism that can be termed “state switching”, illustrated in Scheme II. Here, excitation of the sensor complex leads to the indirect population of an excited state that is ligand-to-ligand in nature (LLCT) localized on the pyridine ligand. Spectroscopic studies suggest that the initially formed state in the complex is MLCT in nature, but that rapid internal conversion occurs to the LLCT excited state, which is not emissive owing to rapid radiationless decay to the ground state. Upon binding of an alkali or alkaline earth cation, the LLCT excited state is raised in energy to the point where a Re(I)-bpy MLCT excited state is now lowest. Since the MLCT excited state is emissive, the emission quantum yield and lifetime of the sensor increases with increasing cation concentration.
understood. Recently, the study by McQueen and Schanze has revealed the important mechanistic role of cation binding kinetics. For the complex in Figure 4, they found that both φ and τ varied in the order Ca 2+ > Ba2+ > Na+. This can be explained by the kinetic model shown in Scheme III.
Scheme III Thus, under the assumption that the internal conversion rate for decay from the MLCT excited state to an intraligand state (kic) is much faster than the binding rate of the cation in the excited state, kon*[Mn+], the off rate (koff*) in the excited state can play a role in determining the emission quantum yield and lifetime. Under these limiting conditions, τ and φ can be written as in eqs 7 and 8. φ = kr/(kr + knr + koff*)
(7)
τ = 1 / (kr + knr + koff*)
(8)
In this view, koff* is just another nonradiative rate constant, since analyte loss results in rapid cascade of excited state energy through the MLCT excited state to the now lowest-lying, nonemissive intraligand state. In a more practical sense, by being able to manipulate on and off rates of the analytes, rational design of reversible sensors will be possible. Conclusions Scheme II A similar phenomenon was proposed for the proton and lead sensitivity of fac-(AZA-phen)Re(CO)3 Cl (Fig. 2c). For this complex, molecular orbital calculations support the idea that the lowest energy transition is π-π* with substantial amino-nitrogen character. In addition, spectroscopic studies on fac-(AZA-phen)Re(CO) 3 Cl, [fac-(AZAphen)Re(CO)3 (py)](CF3 SO3) (Fig. 2d), and Zn(DMAphen) 3(PF6)2 show that the absorption and emission energies are very similar, indicating that the same state is the lowestlying. Protonation of the two Re complexes, but not the Zn, gives a new emissive state. For Re(I), these data are consistent with a ligand-based state on the phen ligand being lowest in energy, but on protonation a new MLCT excited state becomes the lowest. Thus state-switching is induced on the same ligand framework. Although much of our discussion has focused on the thermodynamics of analyte binding, kinetic processes involving binding are of equal importance, but are not well
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In recent years, a small but growing number of designed inorganic complexes have been shown to be viable luminescence sensors for metal cations in solution. By designed we mean supramolecular systems composed of a metal recognition site and a luminophore that have been constructed by fusing the principles of crown or cavitand chemistry with that of inorganic excited-state chemistry. The studies thus far indicate that molecules such as these are robust and can be used in aqueous solution. Crucial to this union is the role of the chemical linker between the analyte binding site and the luminophore. As was discussed, similar molecules can exhibit drastically different recognition mechanisms simply by changing the nature of the spacer between the luminophore and the binding site. The future of luminescence sensing using transition metal complexes is bright, especially in the areas of excited state mechanism and kinetics of analyte binding. In addition, studies on the immobilization of such sensor arrays on surfaces (12) are highly desirable, since they could lead to stable, reversible, fiber optic–based devices using designed transition metal systems.
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Symposium: Applications of Inorganic Photochemistry Acknowledgment BPS would like to thank the National Science Foundation (EPSCoR program) for support. Literature Cited 1. Dietrich, B.; Viout, P.; Lehn, J.-M. Macrocyclic Chemistry: Aspects of Organic and Inorganic Supramolecular Chemistry; VCH: Weinheim, New York, 1993; Crown Compounds: Toward Future Applications; Cooper, S. R. Ed.; VCH: New York, Weinheim, 1992; Izatt, R. M.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Chem. Rev. 1991, 91, 1721–2085; Calixarenes, a Versatile Class of Macrocyclic Compounds; Vicens, J.; Böhmer, V., Eds.; Kluwer: Dordercht, Boston, London, 1991; Diederich, F. Angew. Chem. Int. Ed. Engl. 1988, 27, 362–386; Synthesis of Macrocycles; Izatt, R. M.; Christensen, J. J., Eds.; Prog. Macrocyclic Chem. Vol. 3; Wiley: New York, 1987; Top. Curr. Chem. 1992, 161; Macrocycles; Weber, E.; Vögtle, F., Eds.; Springer: Berlin, Heidelberg, 1992. 2. Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic: London, 1992; Lees, J. Chem. Rev. 1987, 87, 711–743, and references therein; Meyer, T. J.; Pure Appl. Chem. 1986, 58, 1193. 3. Fluorescent Chemosensors for Ion and Molecule Recognition; Czarnik, A. W., Ed.; ACS Symp. Ser. 538; American Chemical Society: Washington DC, 1993; Czarnik, A. W. Acc.
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