Communication: Coherences observed in vivo in photosynthetic bacteria using two-dimensional electronic spectroscopy.

Energy transfer through large disordered antenna networks in photosynthetic organisms can occur with a quantum efficiency of nearly 100%. This energy transfer is facilitated by the electronic structure of the photosynthetic antennae as well as interactions between electronic states and the surrounding environment. Coherences in time-domain spectroscopy provide a fine probe of how a system interacts with its surroundings. In two-dimensional electronic spectroscopy, coherences can appear on both the ground and excited state surfaces revealing detailed information regarding electronic structure, system-bath coupling, energy transfer, and energetic coupling in complex chemical systems. Numerous studies have revealed coherences in isolated photosynthetic pigment-protein complexes, but these coherences have not been observed in vivo due to the small amplitude of these signals and the intense scatter from whole cells. Here, we present data acquired using ultrafast video-acquisition gradient-assisted photon echo spectroscopy to observe quantum beating signals from coherences in vivo. Experiments were conducted on isolated light harvesting complex II (LH2) from Rhodobacter sphaeroides, whole cells of R. sphaeroides, and whole cells of R. sphaeroides grown in 30% deuterated media. A vibronic coherence was observed following laser excitation at ambient temperature between the B850 and the B850(∗) states of LH2 in each of the 3 samples with a lifetime of ∼40-60 fs.

Dispersion-free continuum two-dimensional electronic spectrometer.

Electronic dynamics span broad energy scales with ultrafast time constants in the condensed phase. Two-dimensional (2D) electronic spectroscopy permits the study of these dynamics with simultaneous resolution in both frequency and time. In practice, this technique is sensitive to changes in nonlinear dispersion in the laser pulses as time delays are varied during the experiment. We have developed a 2D spectrometer that uses broadband continuum generated in argon as the light source. Using this visible light in phase-sensitive optical experiments presents new challenges in implementation. We demonstrate all-reflective interferometric delays using angled stages. Upon selecting an ~180 nm window of the available bandwidth at ~10 fs compression, we probe the nonlinear response of broadly absorbing CdSe quantum dots and electronic transitions of Chlorophyll a.

Modulation of CEST images in vivo by T1 relaxation: a new approach in the design of responsive PARACEST agents.

A novel approach for the design of responsive paramagnetic chemical exchange saturation transfer (PARACEST) magnetic resonance imaging (MRI) agents has been developed where the signal is "turned on" by altering the longitudinal relaxation time (T1) of bulk water protons. To demonstrate this approach, a model Eu(DOTA-tetraamide) complex (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) containing two nitroxide free radical units was synthesized. The nitroxide groups substantially shortened the T1 of the bulk water protons which, in turn, resulted in quenching of the CEST signal. Reduction of paramagnetic nitroxide moieties to a diamagnetic species resulted in the appearance of CEST. The modulation of CEST by T1 relaxation provides a new platform for designing biologically responsive MRI agents.

Europium(III) DOTA-tetraamide complexes as redox-active MRI sensors.

PARACEST redox sensors containing the NAD(+)/NADH mimic N-methylquinolinium moiety as a redox-active functional group have been designed and synthesized. The Eu(3+) complex with two quinolinium moieties was nearly completely CEST-silent in the oxidized form but was "turned on" upon reduction with ß-NADH. The CEST effect of the Eu(3+) complex containing only one quinolinium group was much less redox-responsive but showed an unexpected sensitivity to pH in the physiologically relevant pH range.

Europium(III) DOTA-derivatives having ketone donor pendant arms display dramatically slower water exchange.

A series of new 1,4,7,10-tetraazacyclododecane-derivatives having a combination of amide and ketone donor groups as side-arms were prepared, and their complexes with europium(III) studied in detail by high resolution NMR spectroscopy. The chemical shift of the Eu(3+)-bound water resonance, the chemical exchange saturation transfer (CEST) characteristics of the complexes, and the bound water residence lifetimes (τ(m)) were found to vary dramatically with the chemical structure of the side-arms. Substitution of ketone oxygen donor atoms for amide oxygen donor atoms resulted in an increase in residence water lifetimes (τ(m)) and a decrease in chemical shift of the Eu(3+)-bound water molecule (Δω). These experimental results along with density functional theory (DFT) calculations demonstrate that introduction of weakly donating oxygen atoms in these complexes results in a much weaker ligand field, more positive charge on the Eu(3+) ion, and an increased water residence lifetime as expected for a dissociative mechanism. These results provide new insights into the design of paramagnetic CEST agents with even slower water exchange kinetics that will make them more efficient for in vivo imaging applications.

Para-derivatized pybox ligands as sensitizers in highly luminescent Ln(III) complexes.

New complexes of pyridine-bis(oxazoline) derivatized with -H, -OMe, and -Br at the para position of the pyridine ring with Eu(III) and Tb(III) have been isolated. These are highly luminescent in the solid state, regardless of the ligand-to-metal ratio. Several of the metal complexes were isolated and characterized by single crystal X-ray diffraction, showing the rich diversity of structures that can be obtained with this family of ligands. [Eu(PyboxOMe)(3)](NO(3))(3)·3CH(2)Cl(2), 1, crystallizes in the monoclinic space group P2(1)/n and has the cell parameters a = 14.3699(10) Å, b = 13.4059(9) Å, c = 25.8766(18) Å, ß = 95.367(1)°, and V = 4963.1(6) Å(3). The isostructural [Tb(PyboxOMe)(3)](NO(3))(3)·3CH(2)Cl(2), 2, crystallizes with the parameters a = 14.4845(16) Å, b = 13.2998(15) Å, c = 25.890(3) Å, ß = 94.918(2)°, and V = 4969.1(10) Å(3). 3, a 1:1 complex with the formula [Eu(PyboxBr)(NO(3))(3)(H(2)O)], crystallizes in the monoclinic P2(1)/c space group with a = 11.649(2) Å, b = 8.3914(17) Å, c = 20.320(4) Å, ß = 100.25(3)°, and V = 1954.5(7) Å(3). 4, a product of the reaction of PyboxBr with Tb(NO(3))(3), is [Tb(PyboxBr)(2)(η(2)-NO(3))(η(1)-NO(3)](2)[Tb(NO(3))(5)]·5H(2)O. It crystallizes in the monoclinic space group P2(1) with a = 15.612(3) Å, b = 14.330(3) Å, c = 16.271(3) Å, ß = 92.58(3)°, and V = 3636.5(13) Å(3). [Tb(Pybox)(3)](CF(3)SO(3))(3)·3CH(2)CN, 5, crystallizes in the triclinic space group P1̅ with a = 12.3478(2) Å, b = 15.0017(2) Å, c = 16.1476(4) Å, α = 100.252(1)°, ß = 100.943(1)°, γ = 113.049(1)°, and V = 2594.80(8) Å(3). Finally, compound 6, [Tb(Pybox)(2)(NO(3))(H(2)O)](NO(3))(2)·CH(3)OH, crystallizes in the triclinic P1̅ space group with a = 9.7791(2) Å, b = 10.1722(2) Å, c = 15.3368(3) Å, α = 83.753(1)°, ß = 78.307(1)°, γ = 85.630(1)°, and V = 1482.33(5) Å(3). In solution, the existence of 3:1, 2:1, and 1:1 species can be observed through absorption and luminescence speciation measurements as well as NMR spectroscopy. The stability constants in acetonitrile, as an average obtained from absorption and emission titrations, are log ß(11) = 5.4, log ß(12) = 8.8, and log ß(13) = 12.8 with Eu(III) and log ß(11) = 4.5, log ß(12) = 8.4, and log ß(13) = 11.7 for the Tb(III) species with PyboxOMe. Pybox displayed stability constants log ß(11) = 3.6, log ß(12) = 9.1, and log ß(13) = 12.0 with Eu(III) and log ß(11) = 3.7, log ß(12) = 9.3, and log ß(13) = 12.2 for the Tb(III) species. Finally, PyboxBr yielded log ß(11) = 7.1, log ß(12) = 12.2, and log ß(13) = 15.5 for the Eu(III) species and log ß(11) = 6.2, log ß(12) = 11.0, and log ß(13) = 15.4 with Tb(III). Photophysical characterization was performed in all cases on solutions with 3:1 ligand-to-metal ion stoichiometry and allowed determination of quantum yields and lifetimes of emission for PyboxOMe of 23.5 ± 1.6% and 1.54 ± 0.04 ms for Eu(III) and 21.4 ± 3.6% and 1.88 ± 0.04 ms for Tb(III). For Pybox these values were 25.6 ± 1.1% and 1.49 ± 0.04 ms for Eu(III) and 23.2 ± 2.1% and 0.44 ± 0.01 ms for Tb(III) and for PyboxBr they were 35.8 ± 1.6% and 1.46 ± 0.03 ms for Eu(III) and 23.3 ± 1.3% and a double lifetime of 0.79 ± 0.05/0.07 ± 0.01 ms for Tb(III). A linear relationship between the triplet level energies and the Hammett σ constants was found. Lifetime measurements in methanol as well as the NMR data in both methanol and acetonitrile indicate that all complexes are stable in the 3:1 stoichiometry in solution and that there is no solvent coordination to the metal ion.

Luminescent Ln3+ nitrobenzoato complexes: first examples of sensitization of green and red emission.

The first examples of luminescent lanthanide complexes with an o-nitrobenzoic acid-based ligand, 2-nitro-4-thiophen-3-yl benzoic acid, have been isolated. The structural and preliminary photophysical characterization is presented.

Unusual nitro-coordination of europium(III) and terbium(III) with pyridinyl ligands.

A new ligand family based on picoline, bipyridine and terpyridine containing a nitro moiety has been synthesized and its coordination and sensitization ability for lanthanide ions has been studied. Three new complexes were characterized by X-ray single crystal diffraction and all three show uncommon coordination of the nitro moiety to the lanthanide ion. 5cTb, a terpyridine-nitro derivative with Tb(NO(3))(3), crystallizes in the orthorhombic space group Pbca with a = 15.125(3), b = 13.776(3), c = 18.716(4) Å, and V = 3899.8(13) Å(3) and is isostructural with its Eu(III) analog (5cEu) with cell parameters a = 15.1341(4), b = 13.7070(4), c = 18.8277(5) Å. 6Eu, a tripodal amine with a nitro-derivatized pyridine with Eu(CF(3)SO(3))(3), crystallizes in the triclinic space group P1 with a = 11.067(2), b = 11.633(2), c = 12.772(3) Å, α = 110.94(3), ß = 97.49(3), γ = 91.42(3)° and V = 1518.1(5) Å(3). Finally, ligand 5a, a bipyridine-nitro derivative, crystallizes in the orthorhombic space group P2(1)/n with a = 3.7128(3), b = 11.7806(8), c = 19.9856(14) Å, ß = 92.925(2)° and V = 873.01(11) Å(3). All four ligands show sensitization of Eu(III) and Tb(III) luminescence.

Improved synthesis of DOTA tetraamide ligands for lanthanide(III) ions: a tool for increasing the repertoire of potential PARACEST contrast agents for MRI and/or fluorescent sensors.

The synthesis of new DOTA tetraamide (DOTAMR(4)) compounds is of great interest given their application in the formation of Ln(III) complexes as potential PARACEST contrast agents in MRI or fluorescent molecular probes. In this context amino acid and peptide DOTAMR(4) derivatives are particularly attractive since the amino-acid and/or peptide moiety can show responsive properties dependent on a given stimuli which might translate to changes in water exchange rates of the corresponding Ln(III) complex. Current synthesis of DOTAMR(4) derivatives is typically carried out by reacting haloacetamide intermediates with cyclen. However, this method fails to generate the tetra-substituted products when bulky substituents are present in the haloacetamide and in some cases this intermediate cannot be prepared by conventional acylation procedures limiting the number of DOTAMR(4) compounds available for study. As a solution to these limitations, an improved methodology for the synthesis of DOTAMR(4) by coupling DOTA to an appropriate amine containing reagent (i.e. protected amino-acids with the alpha-amino group free) is presented in this work. Several DOTAMR(4) derivatives which are difficult or impossible to prepare with the traditional methodologies were easily obtained starting with DOTA. A new protocol was derived using this methodology for the solution-phase synthesis of DOTA peptide derivatives. With this methodology, many other DOTAMR(4) peptide and non-peptide derivatives have been prepared in our laboratories with several of these new compounds showing interesting properties for molecular imaging.

Eu(III) and Tb(III) luminescence sensitized by thiophenyl-derivatized nitrobenzoato antennas.

Thiophenyl-derivatized nitrobenzoic acid ligands have been evaluated as possible sensitizers of Eu(III) and Tb(III) luminescence. The resulting solution and solid-state species were isolated and characterized by luminescence spectroscopy and X-ray crystallography. The Eu(III) complex with 2-nitro-3-thiophen-3-yl-benzoic acid, 1, crystallizes in the monoclinic space group C2/c with a = 28.569(3) A, b = 17.7726(18) A, c = 17.7073(18) A, beta= 126.849(2) degrees, and V = 7194.6(13) A3. The Tb(III) complex with this ligand, 2, is isostructural, and its cell parameters are a = 29.755(6) A, b = 18.123(4) A, c = 19.519(4) A, beta= 130.35(3) degrees, and V = 8021(3) A3. Eu(III) crystallizes with 3-nitro-2-thiophen-3-yl-benzoic acid as a triclinic complex, 3, in the space group P1 with a = 11.045(2) A, b = 12.547(3) A, c = 15.500(3) A, alpha = 109.06(3)degrees, beta = 94.79(3) degrees, gamma = 107.72(3) degrees. and V = 1893.5(7) A3. With the ligand 5-nitro-2-thiophen-3-yl-benzoic acid, Eu(III) yields another molecular compound, 4, triclinic P1, with a = 10.649(2) A, b = 14.009(3) A, c = 15.205(3) A, alpha= 112.15(3) degrees, beta = 100.25(3) degrees, gamma = 106.96(3) degrees, and V = 1900.5(7) A3. All compounds dissolve in water and methanol, and the methanolic solutions are luminescent. The solution species have a metal ion-to-ligand ratio of 1:1. The quantum yields have been determined to be in the range of 0.9-3.1% for Eu(III) and 4.7-9.8% for Tb(III). The highest values of these correspond to the most intense luminescence reported for Ln(III) solutions with this type of sensitizer. The lifetimes of luminescence are in the range of 248.3-338.9 micros for Eu(III) and 208.6-724.9 micros for Tb(III). The stability constants are in the range of log 11 = 2.73-4.30 for Eu(III) and 3.34-4.18 for Tb(III) and, along with the energy migration pathways, are responsible for the reported efficiency of sensitization.

Nitro-functionalization and luminescence quantum yield of Eu(III) and Tb(III) benzoic acid complexes.

In our pursuit of luminescent lanthanide ion-based coordination polymers, we have isolated several complexes with nitrobenzoic acid ligands and characterized these by X-ray crystallography and luminescence spectroscopy. 2-Nitrobenzoic acid reacts with Eu(III) to form 1, which crystallizes in the P-1 space group, with a = 12.385(3), b = 12.912(3), c = 17.889(4) A, alpha = 97.49(3), beta = 109.64(3) and gamma = 101.99(3) degrees . 3-Nitrobenzoic acid forms a one-dimensional coordination polymer with Eu(III), 2, which crystallizes in the triclinic space group P-1 with a = 9.7100(19), b = 10.579(2), c = 13.361(3) A, alpha = 77.41(3), beta = 88.78(3) and gamma = 88.16(3) degrees. Structures 3 and 4 correspond to the isostructural one-dimensional coordination polymers of Eu(III) and Tb(III), respectively, with the 4-nitrobenzoato anion. These crystallize in the triclinic space group P-1 with a = 9.2242(18), b = 15.102(3), c = 18.587(4) A, alpha = 75.93(3), beta = 82.88(3) and gamma = 79.00(3) degrees for 3 and a = 9.2692(19), b = 15.369(3), c = 18.353(4) A, alpha = 75.37(3), beta = 81.32(3) and gamma = 78.15(3) degrees for 4. Potentiometry, absorption and NMR spectroscopy indicate that in solution only 1 : 1 species are present. All compounds are weakly luminescent as solids and the photophysics of solutions of ligands with Ln(III) in 1 : 1 stoichiometry were studied. Quantum yields around 1 and 3% for Eu(III)-and Tb(III)-containing methanolic solutions were measured.