Symposium: Applications of Inorganic Photochemistry
Symposium: Applications of Inorganic Photochemistry
Photoinduced Energy or Electron Transfer in Supramolecular Systems: Applications to Molecular Wires and Light-Harvesting Sensors An Advanced Chemistry Project on Kinetics Using Fast Laser Spectroscopy Raymond F. Ziessel Laboratoire de Chimie, d’Électronique et de Photonique Moléculaires Ecole de Chimie, des Polymères et des Matériaux de Strasbourg (ECPM) 1, rue Blaise Pascal, 67008 Strasbourg, France Recent innovations in synthetic methodology, together with advances in technology, provide new opportunities for the chemist to explore classical problems and apply the results to newly emerging disciplines. A good example of this progressive advancement in scientific knowledge concerns the design of materials for use in molecular-scale electronic devices (1). Following the realization that many simple transition metal complexes with polypyridine ligands (e.g., 2,2′-bipyridine or bpy, 1,10-phenanthroline, 2,2′:6′,2′′terpyridine or terpy) exhibit interesting redox and photochemical properties (2), strenuous efforts have been made to incorporate such complexes into highly organized multicomponent molecular arrays. Subsequent examination by fast laser spectroscopy reveals that such supramolecular systems facilitate rapid energy and/or electron transfer between the components. With improved temporal resolution and application of sophisticated spectroscopic tools it becomes possible to delve deeply into the mechanism(s) of information transfer, thereby stimulating the rational design of advanced molecular systems. This in turn stimulates interesting laser spectroscopy and leads to an exciting iteration of ideas and methodology. Perhaps the most active area of current research relating to molecular electronics (1) concerns the identification of appropriate means by which to insert redox- or photoactive units into well defined and stable arrays so that they are maintained in (i) mutual electronic communication
(photoactive molecular-scale wires), (ii) electronic insulation (molecular-scale insulators), or (iii) close spatial proximity (chromophoric antennas), when under external stimulation. Future potential applications of these molecular devices could be to transfer energy or electrons across an insulating membrane or other interfaces. The gating process involves electron transfer across the insulating barrier as the basis for signal transmission (Fig. 1). The net results of such a system would be the formation of charge separation between the oxidized form of the donor D + and the reduced form of the acceptor A{. Other uses of photoactive molecular wires and chromophoric antennas are to use them to sensitize large bandgap semiconductor materials in the visible part of the solar spectrum by injecting electrons from excited states into the conduction band of the semiconductor (Fig. 2). This feature is of important relevance to the use of semiconductors to convert solar energy directly to electricity. Recently, a low-cost, highly efficient solar cell based on dye-sensitized colloidal TiO2 films has been constructed (3). Regarding the construction of linear polynuclear metal complexes having discrete units linked by short, highly conductive wires, we and others have identified polyynes as being particularly attractive molecular-scale connectors (4). Such conductive units, when used in conjunction with luminescent ruthenium(II) or osmium(II) polypyridine complexes, facilitate a wide range of light-induced processes inelectron flow
A
D hν
PS
molecular wire
O
+
A
PS, Q
N Ru
Ru N n
N
N
N
N
hν N
N N
O
N
N
O
A–
D
N
TiO2
Q
D+
N
N N
Water
N
N
O
Membrane Water
N
N Ru
Ru N n
N
N
N
N
D + + A– electron flow
Figure 1. Schematic representation of a possible use of molecular scale wires to allow electron flow across an insulating membrane; PS for photosensitizer and Q for primary acceptor.
Figure 2. Dye sensitization of semiconductors with molecular scale wires by electron injection in the conduction band of the semiconductor.
Vol. 74 No. 6 June 1997 • Journal of Chemical Education
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Symposium: Applications of Inorganic Photochemistry cluding triplet energy transfer, electron transfer, photon mimigration to the reaction centers are pivotal processes (12). gration, and electron delocalization (5). The polyyne spacer The fundamental principles involved in nature have also actively facilitates these processes, in some cases by way of guided the chemist in the design and construction of lumisuperexchange, while at the same time enforcing strict stenescent sensors and probes which mimic natural light-harreochemical integrity. Energy and electron (or hole) transvesting antennas or light concentrators (13). Indeed, a light fer, occurring by way of an orbital localized on the polyyne conversion process, via an absorption/energy transfer/emisbridge, has been shown to be exceptionally fast and highly sion (A/ET/E) sequence of photochemical events, is realized directional. The electronic properties and stereochemistry in photoactive lanthanide macrocyclic systems (14). These of the bridge can be modified readily, either by chemical resupramolecular complexes are considered to function as moduction of the alkyne or by incorporation of additional lecular-scale light conversion devices, transforming UV groups into the wire. The major goal of development of molight absorbed by the surrounding chelators into visible lanlecular arrays is the insertion of an intelligent bridge that thanide emission (red light from Eu3+ and green from Tb3+), can both direct electron flow and function as a switch to revia intramolecular energy transfer. This rapid migration of verse that flow. Unidirectional triplet energy transfer by electronic energy has important applications, in particular way of a small cascade of individual steps can be achieved as luminescent labels for biological species in fluoroby incorporating in the bridge polycyclic hydrocarbons (e.g., immunoassays (FIAs) (15, 16). The special interest of Eu3+ anthracene, naphthalene, pyrene). and Tb3+ compounds for FIAs is related to the very long lifeOther recent examples use alkyne spacers in molecutime of their emitting states, which induces an enhancelar photonic wires or optoelectronic gates. In these very elment of the sensitivity of the assays via time-resolved meaegant systems, absorption of a photon of light by an input surement. chromophore at one end of the molecule leads to light emisBecause of their exceptional coordinative properties, sion by an output chromophore at the opposite end. The chemical stability, and synthetic versatility, we chose to use overall efficiency of the cascade energy transfer events from oligopyridino ligands as the key molecular building blocks the input to the output is very high (> 75%) (6). A related by which to construct multicomponent wires, insulators, and sensors. type of wire allows the emission of the output light to be turned on or off in a controlled manner by an internal redox-switched site (7). Synthetic Strategy and Target Molecules The effective conductivity of a single molecular wire constructed with alkyne spacers has recently been probed The critical aspect of this work concerns the applicaby scanning tunneling microscopy (STM) and microwave tion of Pd0 catalysis (Heck-type or Sonogashira-type coufrequency measurements (8). Other electronic conduction pling reactions), oxidative Glaser-type or Eglinton-type restudies of polyphenyl structures and benzene rings conactions, or Cu(I)-catalyzed Cadiot–Chodkiewicz homologanected to alkyne spacers with a thiol (–SH) group at the tion reactions for building unsaturated –(C≡ C)n– (n = 1, 2, ends have also been carried out. These studies demon4) bridges between adjacent oligopyridino ligands in such a strated that the resistance of the molecule can be effectively manner as to permit selective metallation (17). Following lowered by tailoring the internal structure of the chain with this rationale, a general stepwise synthetic procedure was triple bonds (9). A different approach to modular molecular engineering N P involves the precise positioning N N N N P P Pt N P of prefabricated chromophoric N N Pt units around a metalloN N Pt N P synthon—for example, Pt(II)P N N N N N N N bis(trialkylphosphine) comcis-PtL4 plexes. Here σ-coordination of trans-PtL5 trans-PtL4 the alkyne connectors to the Pt(II) core allows formation of various multicomponent structures incorporating various luN N N N N minescent subunits. Such arrays, N OSO CF N SiEt N H N N SiMe H especially with the cis configuration, make effective insulators N N N N N 5 4 3 1 2 for through-bond energy-transfer processes. These structural variations allow one to switch between an extremely fast through-bond, or Dexter-type N N N N N N electron exchange (10), to a slow N N N N N N through-space, or Förster-type N N N N coulombic (11) triplet energy N N transfer mechanism. Photochemical molecular L2 L3 L1 devices are present in nature, where they perform functions essential to life—in particular, photosynthesis, photorespiration, Figure 3. Schematic representation of protocol used to assemble the building blocks for construcand vision. The harvesting of sotion of the bridging ligands and metallo-ligands. Reactants and reaction conditions are given in lar energy by antennas and its refs 17, 18, and 27. 2
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3
3
Journal of Chemical Education • Vol. 74 No. 6 June 1997
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Symposium: Applications of Inorganic Photochemistry chain. Different metal cations can now be inserted into the vacant coordination sites (Figs. 4 and 5), allowing for a cruder variation in electronic properties and also providing subtle tuning of the electronic properties.
devised that permits isolation of novel multisite ligands (L1, L 2, L3) or multisite metallo-ligands (trans-PtL4 , cis-PtL4, trans-PtL5) having various coordination domains along the molecular axis. The synthetic methodology is sketched in Figure 3. This piecewise synthetic approach, although tedious in operation, permits the engineering of a plethora of multicomponent molecular systems (18, 19) embodying preferential binding properties, the use of pre-organized modules, and the programmed layering of successive molecular units. This preparative strategy, in turn, provides for a gradient of redox-active or photoactive metal complexes arranged in a logical and predetermined sequence along the
Basic Considerations of Laser Spectroscopy and the General Concept of Intramolecular Energy Transfer
Excitation of the various compounds with a short-duration laser pulse at a wavelength corresponding to a reasonably strong absorption transition results in rapid establishment of the lowest-energy excited triplet state. This metastable species may luminesce in solution, as is the case with most Ru II and Os II polypyridine complexes, and, if N N N N N N so, the triplet lifetime is conveN N N N N N N N N N + N N N niently measured by temporally N N N N N N resolving the luminescence decay. N N This technique works very well L Ru , L Ru L Ru , L Ru 1 2 2 2 1 2 L1 , n = 1 L2 , n = 2 provided that the triplet lifetime is not too short (τ > 100 ps) and that the luminescence can be properly resolved from back6+ N ground light. In other cases, the N N N N N N N N N N N N N 6 PF N N N N N N triplet state can be detected by N N N N N N N N N N transient absorption spectroscopy. Here, a second photon, de(L1)2Ru2M, M = Fe, Co, Zn L1RuOs , L2RuOs livered by a delayed laser pulse (L2)2Ru2M or from a continuous light source, is absorbed by molecules in the 4+ triplet state, giving rise to a tripN N N N N N let–triplet absorption transition. N N N N N N N 4 PF N This absorption spectrum can be N N N N recorded in the same way as L4 , n = 1 ground-state absorption spectra L4Ru2 , L5Ru2 L5 , n = 2 are measured, while kinetic measurements can be made at fixed wavelengths. This technique is Figure 4. Selection of mono- and homo-dinuclear and hetero-trinuclear complexes used during the also appropriate for recording the photoactive molecular-scale wire program. Reaction conditions and reactants are in refs 22, 24, 25. absorption spectra of other reaction intermediates and for determining their lifetime under particular conditions. Such measurements can be made on time scales as short as 100 fs (100 fs = 10{13 4+ s) and other detection modes (e.g., N P P N N N resonance Raman, infrared, cir(i) N N N Ru N Pt Pt 4 PF6N Os N N cular dichroism, or EPR) can be NN N N (ii) P P N used. In this manner, it becomes possible to follow the course of a photochemical reaction, to detertrans-PtL6 trans-(PtL6)RuOs mine kinetic parameters for important intermediates, and to identify major reaction pathways. Transient absorption spectra can 4+ also be recorded by means of this N P P experimental technique. N (i) N N Ru N Pt P Pt P 4 PF6The rate constant (kobs) for N N N (ii) light-induced electron transfer or triplet energy transfer can be determined by measuring the tripN N N let lifetime (τt ) of the appropriate N Os N N N multicomponent compound and N cis-PtL7 comparing it with that of the reference compound (τo), both in cis-(PtL7)RuOs deoxygenated solution: 4+
2+
2 PF6-
Ru
4 PF6-
Ru
Ru
n
n
n
4+
Os
Ru
4 PF6-
Ru
M
Ru
n
Ru
6
n
n
n
Ru
n
-
6
Figure 5. Selection of hetero-dinuclear complexes used during the photoactive molecular scale insulators program: (i) [Ru(bpy)2Cl 2].2H2O; (ii) [Os(bpy) 2Cl2].
kobs = (1/τt) – (1/τo)
Vol. 74 No. 6 June 1997 • Journal of Chemical Education
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Symposium: Applications of Inorganic Photochemistry In this method it is assumed that the difference in triplet lifetimes between the two compounds arises solely from the quenching process (i.e., electron or energy transfer) of interest. Two basic mechanisms could be involved in this energy transfer process (Scheme I). LUMO
energy transfer D*—spacer—A
Collisional Interaction Double Exchange Dexter Mechanism
Coulombic Interaction Dipole-Dipole Förster Mechanism
D—spacer—A*
HOMO
A
D*
≠ 2V 2 π 3 ∆G exp − h λRT RT
light absorption hν D
D—spacer—A
kF = 8.8X10
light emitted – h νA
−25
kD =
Φ L JF K 6 n4τ L RMM 2
≠ (λ − ∆E)2 ∆G = 4λ
ground state
The so-called Förster mechanism (11) could take place via a dipole–dipole interaction and the so-called Dexter mechanism (10) via a double electron exchange (Fig. 6). In the first mechanism, the excited state D* behaves as an oscillating dipole, creating an electric field. When A is driven in the collisional space of D* it will enter in resonance, resulting in simultaneous formation of A* and D (Fig. 6a). No exchange of electrons occurs. The rate of triplet energy transfer (kF) is proportional to E2 (E being the energy of the oscillating dipole); it is weakly dependant on the distance RMM between the donor D* and the acceptor A (dependence on RMM{6). However, kF strongly depends on the spectral overlap integral between the emission of the donor and the absorption of the acceptor. The Förster mechanism is effective mostly with highly colored compounds and when singlet excited states are involved (11). In the second energy transfer mechanism the rate of energy transfer (kD) strongly depends on the distance between the donor and acceptor subunits [dependence in exp({RMM)]. In this mechanism spectral overlap is not necessary; however, (spatial) overlap of the molecular orbitals involved in the electron or energy transfer processes is required (10). A double exchange of electrons is involved in this process (Fig. 6b). If quenching involves through-bond electron and/or hole transfer (in the case of triplet energy transfer this requires that the transfer occur by electron exchange rather than by dipole–dipole transfer), the derived rate constant can be related to the distance (RMM) separating the two reactants:
Figure 6. Schematic representation of energy transfer with a Förster-type or Dexter-type mechanism and the equation used to calculate the rates: kF = Förster rate constant, φ L = quantum yield, τL = lifetime of the triplet excited state of L1Ru or L2Ru, K = reorientation factor of the optical transition dipoles, JF = spectral overlap integral, n = solvent refractive index, RMM = metal–metal separation distance, V = electronic coupling matrix, λ = reorganization energy for energy transfer, ∆E = difference between the excited state energies of the donor and acceptor.
energy
Scheme I
2+
N N N
N
RuIII
N
-.
X
MC/LC
antibonding
N
hν
ET
hν bonding 2+
N N N
N
RuII
N
X
L
RuII
L
X
L
RuIII [L
X ] -.
N
kobs = kD = ko exp [{βRMM] In this expression, ko refers to the rate constant when the reactants are within orbital contact (i.e., RMM = 0) and β is an attenuation factor (expressed in units of Å{1 ) that describes the electronic “resistivity” of the intervening medium. When the bridge can be considered as a single chemical entity (e.g., an acetylenic function) or a homologous series of identical groups (e.g., a polyyne chain), β is representative of the individual molecular functions that combine to form the bridge and can be expressed in terms of groups, atoms, or bonds, rather than distance. If the bridge is formed by a combination of several different functionalities, as is the case with protein matrices, β cannot be assigned to individual molecular segments but must be considered as representative of the whole structure. Excellent detailed discussions referring to the mechanism of energy transfer (20) or electron transfer (21) have previously been published.
676
Figure 7. (a) Selective excitation of the ruthenium complex generating the corresponding triplet excited state. ET = excitation energy. (b) Differential absorption spectra recorded in deoxygenated acetonitrile at 20°C for the triplet excited states of (i) [Ru(terpy)2] 2+, (ii) L2Ru, and (iii) L2 Ru2.
Journal of Chemical Education • Vol. 74 No. 6 June 1997
Symposium: Applications of Inorganic Photochemistry Photoinduced Energy Transfer along the Molecular Wire The first hint that a polyyne spacer provides highly beneficial properties when used in conjunction with terpy complexes came from the observation that the triplet lifetimes (τT) of binuclear Ru(II) complexes L1Ru2 and L 2Ru2 were greatly prolonged by about 3000-fold with respect to [Ru(terpy)2]2+ (22). This has the effect of converting a poor photosensitizer into a viable one. The presence of the alkyne substituent also induces a red shift in the emission maximum (λem), indicative of a lower triplet energy as well as an easier one-electron reduction of the ligand. This effect ensures that the bridging ligand is reduced in preference to the parent terpy ligands and that the lowest-energy triplet state, being of metal-to-ligand charge-transfer (MLCT) character, is formed by charge injection from the Ru to the bridging ligand (Fig. 7a). This is further confirmed in the triplet differential absorption spectra (Fig. 7b), which exhibit an additional broad featureless absorption band around 620–650 nm for L1Ru and L2Ru assignable to the π-radical anion of the bridging ligand and bleaching of the ground-state MLCT absorption band located around 460–500 nm. The new absorption band around 860–880 nm (absent in the [Ru(terpy)2]2+ and the mononuclear complexes) that characterizes the dinuclear complexes L1Ru2 and L2Ru2 contains a contribution from an intervalence charge-transfer process associated with electron migration between the two metal centers (22). Lowering of the triplet energy inhibits mixing between the MLCT triplet and higher-energy metal-centered (MC) or ligand-centered excited states (LC) (Fig. 7a). This decoupling of the states is largely responsible for the observed prolongation of the triplet lifetime (23). Another important contribution is electron delocalization over an extended π*-orbital that encompasses much of the bridging ligand. The same effect is apparent in the trinuclear complexes arranged around a central Zn(II) cation as in (L1)2Ru2Zn or (L2)2Ru2Zn. It is clear that this is a general phenomenon caused by introduction of the alkynyl moiety (24). Apart from stabilizing the triplet state, very efficient and fast triplet energy transfer along the molecular axis could be achieved in the alkyne-bridged mixed L1RuOs and L2 RuOs complexes (Scheme II). It was observed that triplet energy transfer from the Ru part of the complex to the appended Os subunit was quantitative, at 298 and 77 K, occurring on a time-scale of ca. 20 ps (kobs = 7.1 × 1010 s{1 for L1 RuOs and 5.0 × 1010 s{1 for L2RuOs). λ exc. 440 nm
Ru*
Os
< 20 picoseconds
Ru n
n
metal centers (see above for the ruthenium/cobalt complexes). The small decrease in rate observed for L2 RuOs relative to L1RuOs corresponds roughly to an RMM{2 rather than RMM{6 dependence (where RMM is the metal-to-metal separation distance in the dinuclear complexes). In these complexes energy transfer essentially occurs by a through bond Dexter-type mechanism. The attenuation factor (β) for energy transfer [kD = k0 exp({ βRMM)], of the order of 0.17 Å{1, is one of the smallest values when compared to most other bridging modes (26). Calculation of the rates of energy transfer (ET) according to the Förster-type mechanism, after determination of the spectral overlap integrals, gives rates of 1.2 × 10 7 s {1 for L1RuOs and 4.4 × 106 s{1 for L 2RuOs (time scale in the range of 80 to 250 ns) (25). Subsequently, the ability of alkyne spacers to facilitate electron- and holeexchange by way of the superexchange mechanism was confirmed by measurements on use of the corresponding trinuclear (L1 )2 Ru2Fe and (L2 )2 Ru2Fe complexes (24). Our understanding of electron exchange in such alkyne-bridged metal complexes is that electron transfer between terminal coordination sites of ethynyl-bridged bridging ligands is facilitated by electron delocalization over the central bridging ligand within the triplet excited state. Hole transfer between the metal centers, which must take place via HOMO orbitals localized on the bridging ligand, might be the limiting step since this occurs over larger distances and involves interactions between d, sp2, and sp orbitals. This viewpoint was strengthened by transient spectroscopic measurements made on the mixed (L1 )2Ru2Co and (L2 )2 Ru2Co complexes, in which we were able to isolate the electron- and hole-transfer steps. Laser excitation into the Ru(II) subunit generates the localized triplet excited state, which rapidly abstracts an electron from the appended Co(II) center by way of a hole-transfer process (Fig. 8). The attenuation factor for this step is ca. 0.12 Å {1 and the reaction occurs on the time-scale of a few picoseconds. Charge recombination to restore the ground state occurs more slowly (time-scale of ca. 100 ps) by way of electron-transfer and is characterized by a β value of only ca. 0.04 Å{1 (Fig. 8). These are rather small attenuation factors when compared to other bridging units, suggesting the possibility for engineering molecular systems based on polyyne wires that are capable of long-range hole and/or electron transfer (24). In fact, given that the inherent triplet lifetime of a terminal RuII complex is ca. 175 ns and that β{h} ≈ 0.12 Å{1, we can calculate that light-induced hole transfer could occur over a distance of 85 Å with an efficiency of 50% in such systems. Related calculations show that under these conditions the electron could travel with 50% efficiency over a distance of ca. 250 Å. These studies are entirely consistent
Os*
n = 1; k = 7.1 x 1010 s–1 n = 2; k = 5.0 x 1010 s–1
*
RuII
-.
ca. 7 ps CoII
RuIII
RuII
CoII
RuII
N
=
N N
λ emis. 750 nm RuII
Scheme II
hole transfer ∆G = –0.37 eV β {h} = 0.12 Å–1
hν
CoII
RuII
RuII
-.
CoIII
RuII
ground state
These processes are among the fastest ever resolved for intramolecular triplet energy transfer, and because reaction must occur by way of through-bond electron exchange, they attest to the remarkable superexchange properties of the carbon bridge (25). It should be noted that electron exchange requires concomitant transfer of an electron and a positive hole, the latter process taking place between the
ca. 130 ps electron transfer ∆G = -1.75 eV β {e} = 0.04 Å-1
RuII
-.
CoIII
RuII N
=
N N
N
n
N N
n = 1 or 2
Figure 8. Schematic representation of the activation and deactivation processes for the (L 1)2Ru2Co and (L2 )2Ru2Co complexes.
Vol. 74 No. 6 June 1997 • Journal of Chemical Education
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Symposium: Applications of Inorganic Photochemistry with the notion that electron transfer along the carbon wire is slowly attenuated with increasing length of the alkyne bridge due to conjugation. Inserting phenyl rings into the spacer, however, forming complexes L4Ru2 and L5 Ru2 , curtails the enhancement of the triplet lifetime (22). This insulation reflects restricted electronic overlap between antibonding orbitals localized on the phenyl ring and on the acetylenic function.
O
Photoinduced Energy Transfer in Molecular-Scale Insulators
Scheme IV
σ-Coordination of alkyne groups to a Pt(II) core can be used in the preparation of linear trans-(PtL6 )RuOs or “Lshaped” cis-(PtL7)RuOs multicomponent structures (Fig. 5). It is noteworthy that the resultant complexes are photoactive, yet kinetically stable in solution and resistant to thermal and photochemical isomerization (27). The central Pt(II) unit has a dramatic effect on the electronic properties of the bridges as can be seen by comparing the rates of intramolecular triplet energy transfer for the linear L2 RuOs and trans-(PtL 6)RuOs complexes (see Schemes II and III). The metal-to-metal distances are comparable in these two complexes, yet energy transfer occurs about 3000 times faster in the absence of the Pt(II) unit. Clearly, insertion of a Pt(II)bis(trialkylphosphine) unit into the connecting bridge restricts the electronic coupling between the terminal Ru and Os complexes.
λexc 440 nm
N N
O O
O
N
N
N
L8 Ln
N N
N
Ln = Eu, L8Eu Ln = Tb, L8Tb
The calix[4]arene complexes exhibit very high values for molecular extinction coefficients and the luminescence quantum yield values are indicative of a strong complexation ability of the ligand and an efficient shielding of the Eu and Tb cations from interaction with solvent molecules. The efficiency of the incident-light/emitted-light conversion, defined as the product of the absorption efficiency and the luminescence quantum yield, is very high (30). The corresponding Tb complexes exhibit lower luminescence quantum yields owing to the presence of low-lying CT states involving the calixarene core, which may efficiently deactivate both the excited states of the ligand and also the terbium 5D -emitting state by thermally activated decay processes o (Fig. 9). Conclusion
Ru*
Pt
Os
ca 80 nanoseconds kobs = 1.25 x 107 s–1
Ru
Pt
Os*
for 2,2'-bipyridine
λemis 625 nm
λemis 745 nm
for 4'-ethynyl-2,2'-bipyridine
Scheme III
Another interesting feature is that the rate of Ru to Os energy transfer is slightly slower in cis-(PtL 7)RuOs than in trans-(PtL6)RuOs, despite the fact that the Ru to Os separation distance is smaller in the former complex. This discrepancy indicates that energy transfer is mediated by through-bond electronic coupling (i.e., σ- and π-conjugation) across the trans-[ethynyl-Pt-ethynyl] spacer more effectively than across the cis-[ethynyl-Pt-ethynyl] spacer (28). Spectroscopic measurements (31P-NMR and IR) confirm the increased conjugation for the trans- stereochemical disposition of the two ethynyl groups. This increased conjugation is apparently due to mixing of the π-symmetry orbitals on the ethynyl ligands with d xy and dxz orbitals at Pt(II). Photoluminescent Molecular Devices Another fascinating way to use intramolecular energy transfer is to design luminescent lanthanide complexes for use as molecular sensors in fluoroimmunoassays (FIAs). Good photochemical properties can be obtained with cagetype or hemi-cage-type ligands that are able to encapsulate the ion into the molecular cavity, reducing interactions with external species such as solvent molecules (29). One straightforward example is given in Scheme IV.
678
We have demonstrated that polyynes can operate as effective molecular-scale wires for promotion of through-bond electron exchange, at least over short distances. This process is facilitated by direct injection of charge into the bridging ligand from the MLCT triplet excited state of the donor. However, when the polyyne contains a Pt(II) metallo-ligand, it ceases to function as a molecular wire and, instead, serves to isolate the energy at the site of the donor. In this respect, the central Pt(II) core does much more than merely provide a high barrier for electron tunneling, as can be obtained with saturated hydrocarbons, since it directs charge injection into an unsubstituted bpy ligand in the MLCT triplet state. In the extreme case, this has the effect of switching the transfer mechanism from electron exchange (through bonds, Dexter-type mechanism) to dipole–dipole (through space, Förster-type mechanism), drastically altering the rate of energy transfer and overcoming the inherent difficulties of incorporating an ethynyl substituent into one of the coordinated ligands. Such platinum-bis(σ-acetylide) complexes can be used to produce elaborate molecular and macromolecular architectures of well-defined stereochemistry. By judicious incorporation of chromophores into these networks it might be possible to generate artificial light-harvesting complexes in which energy migration occurs exclusively by way of a Förster-type dipole–dipole mechanism. The design of luminescent molecular-scale devices based on lanthanide complexes must take into account both the absorption efficiency (number of antennas) and the metal luminescence quantum yield obtained upon ligand excitation, which is determined by the strength of ligand-tometal interaction and by the effect of the ligand on the nonradiative deactivation of the lanthanide emitting states. Ligand tailoring has been used for the preparation of highly
Journal of Chemical Education • Vol. 74 No. 6 June 1997
Symposium: Applications of Inorganic Photochemistry hν 1
1
pp*
pp*
A
3+
CT
3
3
pp*
pp*
ET 5
D4
Ln 5
hν
D0
hν
hν '
hν "
hν ' or h ν "
[EuL8]3+
E
Ln = Eu, Tb
[TbL8]3+
for 5,5'-substituted 2,2'-bipyridine
A hν absorption of the bipy ET energy transfer
E hν ' emission from the Ln cation
Figure 9. Schematic representation of the A/ET/E process and the energy-level diagrams for the Eu and Tb complexes. Only the most relevant deactivation paths are indicated.
luminescent lanthanide complexes in which long lifetimes (ms scale) and high quantum yields (exceeding 30%) have been obtained. The recent developments of organometallic carbon chains in which Cn chains (n = 8, 10, 12, 16, and 20 atoms) stabilized at the ends by redox active metal fragments are also fascinating in terms of synthetic chemistry and materials research (31). Similarities between photoactive wires and complexed carbon chains suggest attractive synthetic targets that will allow the study of optical, electronic, and electrochemical interactions between metal centers (32). There is hope to find answers in the near future to the following questions: (i) how large could be the maximum interaction between two metallic centers separated by Cn carbon atoms, in both the ground and excited states; (ii) what will be the effect of related spacers (e.g., double or cumulene bonds, azo bridges, condensed polyaromatic relays) on the rate and efficiency of triplet energy or electron transfer across bridging ligands, and finally (iii) what will be the electronic resistivity of these various spacers. Acknowledgments I express my sincere appreciation to Anthony Harriman and Nanda Sabbatini, who performed the kinetic and spectroscopy work and took part in helpful discussions. It is a pleasure to express my appreciation to a group of highly talented students, whose names are mentioned in the references, for their contributions to the development of this research program. I am also indebted to the Engineer School of Chemistry (ECPM) for continuous financial support and to Mohamed Kurmoo for reading and commenting on the manuscript. Literature Cited 1. Petty, M. C.; Bryce, M. R.; Bloor, D. An Introduction to Molecular Electronics; Edward Arnold: London, 1995. 2. Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem. Rev. 1996, 96, 759–833. 3. O’Regan, B.; Grätzel, M. Nature 1991, 353, 737–740. 4. Grosshenny, V; Harriman, A.; Hissler, M.; Ziessel, R. Platinum Metal Rev. 1996, 40, 26–35; 1996, 40, 72–77.
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