Recent Advances in Molecular Excited States, Intramolecular Electron Transfer, and Interfacial Electron Transfer

An underlying goal of our research program has been to achieve a basic mechanistic understanding of molecular excited states and interfacial electron transfer at conductive interfaces. The nanocrystalline materials prepared in our laboratories allow spectroscopic and electrical characterization of interfacial science with molecular detail that was not previously imaginable. While much of the work in this area has been directed toward solar energy conversion, fundamental research in this expanding field has far reaching implications that extend to other present and future devices that operate on a molecular level. Ultimately, we will be able to rationally design molecular-semiconductor interfaces to perform specific functions. Our research has also branched out to studies in fluid solution and insulating and biological molecular materials. Summarized below are recent breakthroughs that provide new directions for future research.

1. Photodriven Electron and Ligand Transfer from Copper Compounds. We recently reported the first examples of intramolecular charge separation in Cu(I) donor-acceptor compounds.¹ In favorable cases, a 416 nm photon is converted into ~ 1 eV of potential energy that is stored for 1.8 ms. Significantly, the rate of charge separation is more then 100 times faster then charge recombination. Solvent coordination to Cu(II) tunes the driving force for charge recombination and consequently the lifetime of the charge separated state.¹ It is particularly encouraging that our first generation charge separated states are so notably long-lived. Our ability to photoinitiate electron and/or ligand transfer reactions allows intermediates to be identified and quantified on short time scales. The geometric and coordination number changes that are novel to Cu(II/I) redox chemistry will allow intramolecular electron transfer processes with large inner-sphere reorganization energy changes to be systematically explored.

2. An Alternative Semiconductor Sensitization Mechanism. We reported the first spectroscopic characterization of an alternative reductive quenching semiconductor sensitization mechanism.² This type of interfacial electron transfer has previously been proposed to be operative in photogalvanic cells but, has never been directly observed. In the commonly accepted mechanism for the sensitization of n-type semiconductors to visible light, the excited state transfers the electron to the semiconductor and is then regenerated by an electron donor in solution. In the alternative reductive quenching mechanism the excited state is first reduced by an electron donor in solution and then transfers an electron to the semiconductor. The two mechanisms are shown for Ru(dcb)(bpy)₂²⁺/TiO₂ with an electron donor D. The reductive quenching mechanism may provide a method for increasing the potential energy stored in interfacial charge separated states.²

3. Control of Molecular Excited States at Semiconductor Interfaces. Control of molecular excited states at semiconductor interfaces was recently achieved.³ That is, we can optimize interfacial electron transfer or long-lived excited state quantum yields through environmental changes. For example, the quantum yield for electron injection from Ru(dcb)(bpy)₂^2+*, where dcb is 4,4’-(COOH)₂-2,2’-bipyridine, to titanium dioxide (anatase) can be reversibly tuned from below detection limits, ~ 0, to near unity simply by altering the ionic strength of an external acetonitrile bath.³ The model shown was proposed wherein surface adsorption by cations shifts the semiconductor acceptor states resulting in better overlap with the donor levels of the molecular excited state. We recently incorporated an optical parametric oscillator (OPO) into our laser apparatus that will allow us to more rigorously test this model and quantify electron injection yields as a function of excitation wavelength.

4. Intermolecular Energy Transfer Across Semiconductor Surfaces. We reported an unprecedented example of molecular excited states that can be switched from lateral energy transfer across a nanocrystalline semiconductor surface to orthogonal interfacial electron injection by electrolyte modification or with an externally applied potential.⁴ At negative applied potentials (or at open circuit in tetrabutyl ammonium, TBA⁺) efficient intermolecular energy transfer from Ru(dcb)(bpy)₂²⁺* to Os(dcb)(bpy)₂²⁺ across the nanocrystalline semiconductor surface is observed with a quantum yield near unity. Energy transfer can be switched off in favor if interfacial electron transfer with a positive applied potential or by changing the cation to Li⁺ at open circuit. Energy transfer may be exploited in photocatalysis for long-range sensitization of remote reactive sites. More fundamentally, with alternative energy transfer donors and acceptors, energy transfer dynamics will provide orientation and distance information on chromophores bound in the unique restricted geometry of mesoporous nanocrystalline semiconductor materials.

5. Temperature Dependent Interfacial Electron Injection. The first example of temperature-dependent electron injection yields from molecular excited states to semiconductor surfaces was recently reported by our group.⁵ This fascinating behavior was realized by ‘designing’ low-lying ligand field (LF) states into the excited state manifold. Our data provides compelling evidence that the intersystem crossing yield from upper excited states to the thermally excited (thexi) state is not unity and temperature independent as is commonly assumed. For Ru(bpy)₂(ina)₂/TiO₂, where ina is isonicotinic acid, the data is consistent with ¹MLCT à LF conversion that lowers the electron injection yield from unity at –10 ^o C to 0.5 at room temperature.⁵ The materials prepared in our labs allow thermally-sensitive electrical and optical responses to be triggered with light and fine tuned at the molecular level.

6. Semiconductor Sensitization by Two Pathways. We recently realized the first example of a molecular compound that can inject an electron into a semiconductor by two discrete pathways.⁶ The compound is Na₂[Fe(bpy)(CN)₄], which binds to TiO₂ through the ambidentate cyano ligands. By incorporating two electron transfer pathways into one molecular compound, broad spectral sensitization may be realized for solar energy conversion applications. Furthermore, since the two sensitization pathways have distinct dynamics, efficiencies, and spectral responses, the time-dependent opto-electronic properties can be systematically controlled and fine tuned at the molecular level for other applications. For example, we have shown that the excited state injection yield is ionic strength independent while the direct charge transfer yield is not.⁶

7. Bimetallic Sensitizers. We have reported the first examples of semiconductor sensitization by bimetallic coordination compounds.⁷ Studies with Re-Ru compounds demonstrate that efficient electron injection can occur in cases where the chromophoric ligand is not directly bound to the semiconductor.^7a Intervalence hopping from an excited Ru chromophore to a Rh(III) unit, to the semiconductor has been realized.^7b As an extension of previous work with inorganic-organic dyads,⁸ the bimetallic Ru-Os compound shown was designed to promote rapid intramolecular electron transfer, 2), after electron injection 1).⁹ Collectively, this work exemplifies remarkable control of excited state electron transfer at semiconductor interfaces. It is now possible to design specific molecular functionality into ‘supramolecular’ compounds to achieve desired properties.

8. Interfacial Charge Recombination Dynamics. We recently provided some new insights into the question of why charge injection into TiO₂ proceeds in the ultra-fast time regime while charge recombination requires milliseconds.¹⁰ Our observations indicate that charge recombination is slow, not because of intrinsically slow rate constants, but because the reaction follows second-order kinetics.^2b We have shown that charge recombination rate constants are insensitive to an ~ 960 mV change in the apparent thermodynamic driving force, the number of carboxylic acid groups present (1, 2, or 4), and the identity of the metal center (Re, Ru, Os, or Fe). The results suggest that after electron injection, efficient separation of the charge transfer products occurs, and recombination is rate limited by diffusional encounters of the oxidized molecular compound and the electron in the semiconductor.

9. Fixed Distance Interfacial Electron Transfer. The study of fixed distant electron transfer has provided keen insights into fundamental aspects of electron transfer. Recently, we reported a new strategy for quantifyfing fixed distance electron transfer at nanocrystalline semiconductor surfaces.¹¹ The idea was to use was to use a molecular tripod to provide a stable three-point attachment and a semi-rigid spacer to vary the distance between a sensitizer and the surface. Electron transfer studies with the Ru(II) sensitizer shown demonstrate efficient and rapid injection, k_cs > 10⁸ s^-1 over an 18 Å distance.¹¹

10. Allignment of Magnetic Nanowires. Nickel nanowires coated with fluorescent molecules allign and self-assemble in the presence of magnetic fields.¹² The allignment has been quantitatively modeled and can be used to orientate magnetic entities into desired configurations. An advantage of the nanowires is that the chemical composition can be vaired along the length scale of the wire. For example, the wire shown has a non-luminescent gold segment, visible in the upper picture, and a luminescent nickel segment. The manipulation of individual cells with magentic nanowires is now under active study.

Click on the image below to play the movie!

Bibliography

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10. a) Hasselmann, G.M.; Meyer, G.J. Zeit. Phys. Chem. 1999, 212, 39-44. b) Hasslemann, G.M.; Meyer, G.J. J. Phys. Chem. B 1999, 103, 7671-7675.

11. a) Galoppini, E.; Guo, W.; Qu, P.; Meyer, G.J. J. Am. Chem. Soc. 2001, 123, 4342-4343. b) Galoppini, E.; Guo, W.; Hoertz. P.; Qu, P.; Meyer, G.J. J. Am. Chem. Soc. 2002, 124, in press.

12. a) Tanase, M.; Bauer, L.A.; Hultgren, A.; Silevitch, D.M.; Sun, L.; Reich, D.H.; Searson, P.C.; Meyer, G.J. NanoLett 2001, 1, 155-158. b) Tanase, M.; Silevitch, D.M.; Hultgren, A.; Bauer, L.A.; Searson, P.C.; Meyer, G.J.; Reich, D.H. J. Appl. Physics 2002, 91, 8549-8551. (See also: http://www.vjnano.org, June 3 2002 issue.)