Donor-chromophore-acceptor triads

2013 Spring.Includes bibliographical references.The photophysical behavior of [Cu(I)P2] (P=2,9-disubstituted-1,10-phenanthroline ligands) in donor-chromophore-acceptor (D-C-A) triads and chromophore-acceptor (C-A) diads is a complex and fascinating area of under developed, yet fundamental, electron transfer chemistry. In metal polypyridyl D-C-A and C-A triads/diads, metal polypyridyl chromophores (C) in which the polypyridyl ligands are covalently linked to acceptor (A) and/or donor (D) moieties, photo-excitation of the chromophore initiates a series of electron transfer events that result in the formation of a charge separated (CS)/charge transfer (CT) state, respectively. The majority of high-performing metal polypyridyl D-C-A/C-A complexes, on which [Cu(I)P2] D-C-A/C-A research is based, incorporate ruthenium (as [Ru(II)L3] where L=polypyridyl ligand) or other rare, expensive, and sometimes toxic metals such as osmium, rhenium and platinum. Although [Ru(II)L3] D-C-A/C-A's have historically set the benchmark for metal polypyridyl D-C-A/C-A performance, it is clear that these complexes are not a practical choice if D-C-A's or C-A's were incorporated into a device for large scale production. However, bisphenanthroline complexes of copper, a much more earth abundant, cheaper and less toxic metal, exhibit very similar photophysical properties to [Ru(II)L3] and have thus gained recognition as promising new materials for D-C-A/C-A triad/diad construction. In order to understand the electron transfer (ET) events occurring in [Cu(I)P2] D-C-A/C-A triads/diads, a complex must be synthesized that is capable of forming a CS with high quantum efficiency (Φcs/ct) and a long CS/CT lifetime (τcs/ct). Therefore, the intent of the research reported herein is to synthesize novel, yet functional heteroleptic [Cu(I)P2] D-C-A/C-A triads/diads and study their fundamental, photo-initiated electron transfer chemistry, specifically the formation of a CS/CT state. Many challenges, which are not present for [Ru(II)L3], make the design and synthesis of [Cu(I)P2] D-C-A/C-A assemblies an art in itself. Therefore, a significant amount of effort was spent on fabricating ligand architectures that (1) are appended with acceptor and/or donor moieties capable of being reduced/oxidized resulting in the formation of a CS/CT, (2) are able to be easily modified so the amount of energy stored in the CS/CT can be tuned, (3) favor the self-assembly of [Cu(I)P2] complexes, (4) are able to facilitate processes that maximize the Φcs/ct. Once the ligands were obtained, the complexation equilibria behavior of these [Cu(I)P2] triads and diads were studied. Despite efforts to design ligand architectures that favor heteroleptic formation, the thermodynamic driving force for heteroleptic D-C-A triad formation is less favor-able than expected. Thus, mixing stoichiometric quantities of D, C and A results in a statistical mixture of C-A, C-D and D-C-A products. Furthermore, since the ligands are labile and will re-arrange to the most thermodynamically stable configuration of products when these complexes are dissolved, isolation of the D-C-A product is impossible. However, recent advances in ligand design have shown promise for resolving this on-going issue. Despite having a mixture of products with the D-C-A, the electron transfer processes of the [Cu(I)P2] D-C-A triads and C-A diads were investigated. Using Transient Absorption (TA) laser spectroscopy, the CT state in the constructed C-A diads and the CS state in the D-C-A triads were detected and the lifetimes were determined. However, it was found that those lifetimes could be modulated to a small degree by solvent in the C-A diads (c.a. 6x longer in polar solvents), and drastically via the application of a magnetic field in D-C-A triads (c.a. 60x longer). The ability to modulate the lifetimes enabled the deconvolution of the effects due to the C-A diad vs D-C-A triad in the statistical product mixtures. Although the response in a magnetic field was a somewhat expected result, as similar effects occur in the [Ru(II)L3 D-C-A/C-A's, the magnitude of change in the lifetime and the quantum efficiency offers new insight into the electron transfer events that occur in the CS/CT formation process for [Cu(I)P2] D-C-A/C-A complexes

What difference does a thiophene make? Evaluation of a 4,4′-bis(thiophene) functionalised 2,2′-bipyridyl copper(I) complex in a dye-sensitized solar cell

AbstractThe synthesis of a 4,4′-bis(2-thienyl-5-carboxylic acid) functionalised 2,2′-bipyridine ligand and corresponding copper(I) complex is described and its application in a dye-sensitized solar cell (DSSC) is studied. The positioning of the thiophene groups appears favourable from DFT analysis and a best efficiency of 1.41% was obtained with this dye, for a 0.3 cm2 cell area DSSC. Two absorbance bands are observed in the electronic absorption spectrum of the copper(I) complex at 316 nm and 506 nm, with ε values of 50,000 M−1 cm−1 and 9030 M−1 cm−1, respectively. Cyclic voltammetry and electrochemical impedance spectroscopy are also used to provide a detailed analysis of the dye and assess its functionality in a DSSC

Photoinduced Multistep Charge Separation in a Heteroleptic Cu(I) Bis(phenanthroline)-Based Donor–Chromophore–Acceptor Triad

A molecular triad assembly consisting of an electron donor, a bis­(phenanthroline)­copper­(I) chromophore, and an electron acceptor has been prepared. Under visible-light excitation, this assembly undergoes efficient (ca. 50%) photoinduced, multistep formation of a diradical cation charge-separated state that has a lifetime of >100 ns and stores >1.0 eV of energy. This system constitutes an earth-abundant functional analogue of related Ru­(bpy)₃ triad systems

Spin-chemical effects on intramolecular photoinduced charge transfer reactions in bisphenanthroline copper(I)-viologen dyad assemblies

Two covalently linked donor–acceptor copper phenanthroline complexes (C–A dyads) of interest for solar energy conversion/storage schemes, [Cu(I)(Rphen(OMV)24+)2]9+ = RC+A48+ with RC+ = [Cu(I)Rphen2]+ involving 2,9-methyl (R = Me) or 2,9-phenyl (R = Ph)-phenanthroline ligands that are 5,6-disubstituted by 4-(n-butoxy) linked methylviologen electron acceptor groups (A2+ = OMV2+), have been synthesized and investigated via quantum chemical calculations and nanosecond laser flash spectroscopy in 1,2-difluorobenzene/methanol (dfb/MeOH) mixtures. Upon photoexcitation, charge transfer (CT) states RC2+A+A36+ are formed in less than one ns and decay by charge recombination on a time scale of 6–45 ns. The CT lifetime of RC2+A+A36+ has a strong dependence on MeOH solvent fraction when R = Me, but is unaffected if R = Ph. This solvent effect is due to coordination of MeOH solvent in MeC+A48+ (i.e. exciplex formation) allowed by conformational flattening of the ligand sphere, which cannot occur in PhC+A48+ having bulkier Phphen ligand framework. Interestingly, the decay time of the CT state increases for both species at low magnetic fields with a maximum increase of ca. 30% at ca. 150 mT, then decreases as the field is increased up to 1500 mT, the highest field investigated. This magnetic field effect (MFE) is due to magnetic modulation of the spin dynamics interconverting 3CT and 1CT states. A quantitative modeling according to the radical pair mechanism involving ab initio multireference calculations of the complexes revealed that the spin process is dominated by the effect of Cu hyperfine coupling. The external magnetic field suppresses the hyperfine coupling induced spin state mixing thereby lengthening the CT decay time. This effect is counteracted by the field dependent processes of T0–S mixing through the Δg-mechanism and by a local mode spin–orbit mechanism. Further, the maximum MFE is limited by a finite rate of direct recombination of 3CT states and the spin-rotational mechanism of spin relaxation. This study provides a first comprehensive characterization of Cu(II)-complex spin chemistry and highlights how spin chemistry can be used to manipulate solar energy harvesting and storage materials

Dipole Moment and Charge Reorganization in Photoredox Catalysts

We report evidence of excited-state ion pair reorganisation in a cationic iridium (III) photoredox catalyst in 1,4-dioxane. Microwave-frequency dielectric-loss measurements allow us to assign both ground and excited-state molecular dipoles and excited-state polarizability volumes. These measurements show significant changes in ground-state dipole moment between [Ir[dF(CF_{3})ppy]_{2}(dtbpy)]PF_{6} (10.74 Debye) and [Ir[dF(CF_{3})ppy]_{2}(dtbpy)]BAr^{F}_{4} (4.86 Debye). Photoexcitation of each complex results in population of highly mixed ligand centered and metal-to-ligand charge transfer states. Relaxation to the lowest lying excited-state leads to a negative change in the dipole for [Ir[dF(CF_{3})ppy]_{2}(dtbpy)]PF_{6}, and a positive change in dipole for [Ir[dF(CF_{3})ppy]_{2}(dtbpy)]BAr^{F}_{4}. These observations are consistent with a sub-nanosecond reorganization with the PF_{6}^{-} counter-ion. Taken together, these observations suggest contact-ion pair formation in [Ir[dF(CF_{3})ppy]_{2}(dtbpy)]PF_{6}. The ion pair reorganisation we observe with the PF_{6}^{-} may modify both the thermodynamic potential available for electron transfer and inhibit oxidative catalysis, providing a possible mechanism for recently observed trends in similar complexes