Transient dynamical studies of ruthenium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2';6',2' '-terpyridine)2+ bis-hexafluorophosphate (Ru-PZn), osmium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2';6',2' '-terpyridine)2+ bis-hexafluorophosphate (Os-PZn), ruthenium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-15-(4'-nitrophenyl)ethynyl-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2';6',2' '-terpyridine)2+ bis-hexafluorophosphate (Ru-PZn-A), osmium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-15-(4'-nitrophenyl)ethynyl-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-(2,2';6',2' '-terpyridine)2+ bis-hexafluorophosphate (Os-PZn-A), and ruthenium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-ruthenium(II)-15-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-bis(2,2';6',2' '-terpyridine)4+ tetrakis-hexafluorophosphate (Ru-PZn-Ru), and ruthenium(II) [5-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-osmium(II)-15-(4'-ethynyl-(2,2';6',2' '-terpyridinyl))-10,20-bis(2',6'-bis(3,3-dimethyl-1-butyloxy)phenyl)porphinato]zinc(II)-bis(2,2';6',2' '-terpyridine) tetrakis-hexafluorophosphate (Ru-PZn-Os) show that these highly conjugated supermolecular chromophores feature electronically excited states that absorb over broad NIR spectral windows with considerable oscillator strength and manifest lifetimes (1-50 mus) that are extraordinarily long relative to those of classic low band-gap organic materials. The excited-state absorptive domains of these strongly coupled multipigment ensembles can be extensively modulated. For sequential one-photon absorptive processes, these compounds evince large sigmae, sigmae/sigmag, and sigmae - sigmag values. As the combination of all these properties within single chromophoric entities have heretofore lacked precedent within the NIR, these and closely related structures may find particular utility in a variety of technologically important optical-limiting applications.

A method of two-dimensional infrared (2D IR) spectroscopy called relaxation-assisted 2D IR (RA 2DIR) is proposed that utilizes vibrational energy relaxation transport in molecules to enhance crosspeak amplitudes. This method substantially increases the range of distances accessible by 2D IR and is capable of identifying longrange connectivity patterns in molecules. RA 2DIR is illustrated in interactions among CN and CO modes in 3-cyanocoumarin and 4-acetylbenzonitrile, where the distances between the CN and CO groups are Ϸ3.1 and Ϸ6.5 Å, respectively. A 6-fold increase in cross-peak amplitude was observed in 4-acetylbenzonitrile when the dual-frequency RA 2DIR method was used.dual-frequency 2D IR ͉ heat transport ͉ vibrational relaxation D evelopment of new methods for determining the 3D structures of molecules in solution is an important challenge in chemistry. Recently introduced in the pioneering works of Hochstrasser and colleagues (1, 2), two-dimensional infrared (2D IR) spectroscopy has demonstrated its strong potential for measuring molecular structures in solution under physiological conditions (2-10). 2D IR and 2D NMR correlation methods have many analogies (11). The cross-peaks in 2D IR spectra originate from pairwise interactions of vibrational modes (vs. nuclear spins for 2D NMR), offering a measure of the distances between interacting vibrational modes (vs. spins). Readily measurable anisotropy of the cross-peaks in 2D IR reveals another important type of structural information, the mutual orientation of the transition dipoles of interacting modes, vide infra (12). Although 2D IR methods have been successfully applied to the study of small molecules and peptides, the measurement of structural constraints in macromolecules such as proteins remains a challenge. Here, we propose a method that uses vibrational relaxation and vibrational energy transport in molecules to significantly enhance the cross-peaks measured with 2D IR for modes separated by distances Ͼ4-5 Å. This approach substantially increases the range of distances accessible by 2D IR and is capable of identifying vibrational modes separated by multiple bonds and of delivering the long-range bond connectivity patterns in a way similar to that of the total correlation spectroscopy method of 2D NMR (13) and its heteronuclear version, heteronuclear multiple bond correlation. Relaxation-assisted 2D IR (RA 2DIR) enhances the potential of weak modes in 2D IR and allows their convenient use as structural reporters.In general, as structural labels, vibrational modes have the advantage of being very compact compared with, for example, FRET labels. However, the choice of vibrational labels for structural measurement in macromolecules by using 2D IR is not obvious. Thus far, most 2D IR cross-peak measurements have been performed by using strong IR stretching modes, such as CAO, OOH, NOH, and CON. However, these modes are highly abundant in biopolymers, and isotope substitution is required in order to decouple the label from the rest of the modes. 13 C 1...

The excited-state dynamics of two conjugated bis[(porphinato)zinc(II)] (bis[PZn]) species, bis[(5,5'-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethyne (DD) and [(5,-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5',-15'-ethynyl-10',20'-bis(heptafluoropropyl)porphinato)zinc(II)]ethyne (DA), were studied by pump-probe transient absorption spectroscopy and hole burning techniques. Both of these meso-to-meso ethyne-bridged bis[PZn] compounds display intense near-infrared (NIR) transient S(1)-->S(n) absorptions and fast relaxation of their initially prepared, electronically excited Q states. Solvational and conformational relaxation play key roles in both DD and DA ground- and excited-state dynamics; in addition to these processes that drive spectral diffusion, electronically excited DA manifests a 3-fold diminution of S(1)-->S(0) oscillator strength on a 2-20 ps time scale. Both DD and DA display ground-state and time-dependent excited-state conformational heterogeneity; hole burning experiments show that this conformational heterogeneity is reflected largely by the extent of porphyrin-porphyrin conjugation, which varies as a function of the pigment-pigment dihedral angle distribution. While spectral diffusion can be seen for both compounds, rotational dynamics driving configurational averaging (tau approximately 30 ps), along with a small solvational contribution, account for essentially all of the spectral changes observed for electronically excited DD. For DA, supplementary relaxation processes play key roles in the excited-state dynamics. Two fast solvational components (0.27 and 1.7 ps) increase the DA excited-state dipole moment and reduce concomitantly the corresponding S(1)-->S(0) transition oscillator strength; these data show that these effects derive from a time-dependent change of the degree of DA S(1)-state polarization, which is stimulated by solvation and enhanced excited-state inner-sphere structural relaxation.

The structure fluctuations of the peptide bond interacting with solvent are examined through the coupling and correlations of the frequency distributions of amide I and amide II transitions. The fluctuations of the two modes are anticorrelated as a result of the solvent-induced changes in the mixing of the dominant valence-bond structures of the peptide. Significant anharmonic coupling of the two modes is seen. The results are the application of a new approach to two-dimensional infrared (2D-IR) spectroscopy in which the pulse sequences used to produce the vibrational echoes incorporate two frequencies. This dual-frequency arrangement greatly extends the capabilities of 2D-IR spectroscopy by allowing the coupling between widely separated modes to be characterized in analogy with heteronuclear NMR. The experiment exposes the cross peaks, representing the mode coupling, free of the interference of the strong diagonal peaks that typically dominate 2D-IR spectroscopy. The alignment and dephasing of coupled transitions, in this example the amide I and amide II transition dipoles, is also determined by these experiments.M ultidimensional IR spectroscopy, in particular 2D-IR spectroscopy, is a powerful new method with which to obtain the time dependence of structural features of complex molecular systems, including peptides and proteins, under ambient conditions in solutions (1-7). Furthermore, the methods allow the determination of key parameters of the anharmonic potential surfaces of peptides and hence provide important tests of theoretical calculations of molecular structure and dynamics (8, 9). These coherent nonlinear IR techniques permit experimental determination of the coupling and angular relations of vibrators by using experimental protocols that are analogous to those developed for NMR. The first such experiments concerned the amide I modes of peptides, which are mainly CAO vibrators (1)(2)(3)(4)(5)(10)(11)(12). In such cases all of the relevant frequencies of an interacting ensemble of modes could readily be bracketed by the spectral bandwidth of 120-fs IR laser pulses. The response of such a system to sequences of three pulses, each with the same center frequency in the amide I region, gave rise to coherent signals, the 2D and 3D correlation spectra of which yielded the relevant structural and dynamical information. In a recent advance, 2D-IR spectroscopy experiments incorporating two pulses with different frequencies, one in the NOH and the other in the amide I region of peptides, were successful in measuring the coupling and angular relations between NOH and CAO modes in some simple peptides (13)(14)(15).In this article we report a dual-frequency 2D-IR spectroscopy heterodyned photon-echo experiment. The 2D-IR spectra, which require manipulation of the vibrational coherences by the interaction of short laser pulses with inhomogeneously broadened distributions of transition frequencies, consist of diagonal features and cross peaks. The diagonal peaks are independent properties of each of the anharmonic m...

Two color femtosecond infrared pump/probe spectroscopy has been used to study the vibrational dynamics and vibrational mode coupling of amide-A and amide-I/II modes within the same amide unit and those connected by a hydrogen bond for several model dipeptides: AcAla(H)OMe, AcAla(D)OMe, and AcProNHMe. Three spectral ranges were explored as follows: around 3 µm (amide-A(H) mode), 4 µm (amide-A(D) mode), and 6-8 µm (amide-I/II modes). The lifetime of the excited amide-A mode in nonhydrogen-bonded amide is found to be strongly dependent upon deuteration: 4.0 ps in AcAla(H)OMe and 0.58 ps in AcAla(D)OMe. The diagonal anharmonicities are found to be ∆ N-D ) 110 cm -1 and ∆ N-H ) 144 cm -1 , respectively, both much larger than the ca. 15 cm -1 for the amide-I mode. By pumping the amide-A band and probing the amide-I/II mode region, direct coupling between the amide-A and the amide-I/II modes has been observed. The off-diagonal anharmonicity between amide-A(D) and amide-I within the same amide unit has been measured to be 1.6 cm -1 , and this anharmonicity is larger (4.6 cm -1 ) when a Fermi resonance component in the amide-I mode is involved. A mixed mode coupling with an apparent time-dependent off-diagonal anharmonicity is observed as a result of the relaxation of the amide-A mode to lower frequency modes, which are in turn coupled to the amide-I mode. This is found to be significant within the first half picosecond after the pump in the case of the deuterated AcAlaOMe since the T 1 of the amide-A(D) vibration in that example is short. The amide-A/amide-I coupling across the hydrogen bond in the self-hydrogen-bonded C 7 conformation of AcProNHMe is 1.4 cm -1 . The alignment of the transition dipoles (amide-I/II modes) relative to the known directions for N-H and N-D was measured and used to obtain structural information on the chemical structure of peptides. The direction of the amide-I transition moment in the molecular frame changes upon deuteration of the amide and upon hydrogen bonding of the N-H group of the amide. The angle between the amide-I transition moment and N-H in the same amide is determined to be 23 ( 3°for AcAla(H)OMe where the N-H group of the amide is weakly hydrogen-bonded in the C 5 conformation. This angle changes to 13 ( 4°for the deuterated compound and to 34.5 ( 3°when the N-H group is involved in strong intramolecular hydrogen bonding in the C 7 conformation (AcProNHMe). Two different conformers were observed for AcAla(H)OMe based on cross-peak anisotropy measurements. The structures of these conformers are assigned to C 5 (carbonyl) and C 5 (ester) conformations, where a five-membered ring is formed by the carbonyl or ester oxygen atom. A first principles simulation of the two color pump/probe signal based on the third order nonlinear polarization, assuming a homogeneous line width for the amide-I and amide-A modes, reproduces the data satisfactorily.

Development of new approaches for measuring three-dimensional structures and dynamics of structural changes is important for a number of natural sciences, including structural biology, where it can lead to understanding the physical bases of molecular recognition and catalysis. A two-dimensional infrared (2DIR) spectroscopy method permits measuring pairwise interactions among vibrational modes in molecules providing a molecular scale ruler for delivering structural constraints, such as the distances between the vibrational modes, angles between their transition dipoles, and the energy-transfer rates between them. While there is a large variety of systems that have recently been interrogated using 2DIR, questions remain of how to measure structural features of larger molecules. The challenges of working with larger molecules, such as proteins, include very congested vibrational spectra, a small range of distances accessible by the 2DIR method, and sensitivity issues. This Account describes the efforts of our laboratory to overcome some of these challenges. First, we discuss the dual-frequency 2DIR approach, which provides the highest selectivity to a particular pair of vibrational reporters and highest sensitivity. Second, we describe our steps in developing vibrational labels, novel for 2DIR, such as C identical withN and C-D stretching modes that have frequencies in the water transparency region, as well as the modes in the fingerprint region. The schemes suitable for labeling amino acids are discussed. Next, we describe the novel relaxation-assisted 2DIR (RA 2DIR) method, developed in our laboratory. The method uses vibrational relaxation and vibrational energy transport in molecules and the thermalization process on a molecular scale, to generate stronger cross-peaks. An 18-fold cross-peak amplification was observed for the modes separated by about 11 A using the RA 2DIR method, and larger amplifications are expected for larger distances between the modes. Large amplification provided by the RA 2DIR method enhances the sensitivity of 2DIR spectroscopy and permits longer range structural measurements. In addition to generating stronger cross-peaks, a correlation of the energy transport time with the intermode distance is demonstrated. This correlation permits measurements of mode-connectivity patterns in molecules much similar to those available in total correlation spectroscopy (TOCSY) and heteronuclear multiple-bond correlation (HMBC) methods of 2D nuclear magnetic resonance (NMR) spectroscopy. It is our hope that, with a proper calibration, the RA 2DIR method will permit speedy assessments of distances and the bond connectivity patterns in molecules and reach the level of an analytical method.

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Nonadiabatic electron transfer (ET) measures the rate of electron tunneling, often facilitated by intervening covalent and nonbonded interactions. [1][2][3][4] Bridge structure therefore influences the ET kinetics, and bridge thermal fluctuations are predicted to modulate the tunneling propensity. 4,5 Structure defines the coupling pathways, and thermal fluctuations enable the system to find configurations that enhance the interaction strength.6 An open and crucial question is whether or not bridge motion can be manipulated (driven) by an external field to control pathway interactions and ET kinetics. Here, we show that mid-IR driving of bridge vibrations produces ET kinetic slowing of a photoinduced charge separation reaction.Recent theoretical analysis indicates that ET kinetics can be changed by controlling the coherence of inelastic tunneling pathway interferences in a molecular analogue of the double-slit experiment. [1][2][3][4]7 In systems with two interfering ET pathways, the excitation of a pathway-specific bridge vibration, which may induce electron-vibration energy exchange, labels the ET pathway and therefore modifies pathway interferences. [1][2][3][4]7 Affecting ET rates using mid-IR radiation 3 is generally attractive because ultrashort laser pulses offer subpicosecond perturbation, and radiation in the mid-IR is chemically innocent. In addition to inelastic tunneling, excitation of bridge vibrations can perturb elastic-tunneling kinetics. The donor-acceptor (DA) coupling may be modulated by exciting a bridge vibrational mode without electron-phonon energy exchange. Related ideas for the control of currents in molecular wires are being addressed in the context of molecular electronics and inelastic-tunneling spectroscopy. 8 Here, we report the first real-time observation of ET rate modulation by mid-IR excitation in a donor-bridge-acceptor (DBA) ensemble. 9This ensemble consists of an anthracene-derived acceptor linked to a dimethylaniline-containing donor by guanosine-cytidine (GC) hydrogen bonding ( Figure 1A). The ET is probed in a 3-pulse experiment, performed with a sequence of UV, mid-IR, and visible pulses ( Figure 1B), each of ca. 100 fs duration. The first pulse at 400 nm creates the acceptor-localized electronic excited state (ES) that then "captures" an electron from the donor with an ET rate constant, k CS , of ca. (30 ps) -1 . 9 After a small time delay, τ, the second pulse centered at 1670 cm -1 (and ∼120 cm -1 in width) is applied, targeting vibrational modes in DBA labeled in Figure 1A. The third pulse in the visible spectral region probes the sample absorbance as a function of the probe's delay time, T ( Figure 1B).The absorbance changes in the 3-pulse measurements were calculated using the equation ∆abs ) D IR -D ) log(I/I IR ), where D IR and D are the optical densities and I IR and I are the probe signals with the IR pump on and off, respectively. Since the mid-IR pulses were chopped at half of the laser repetition rate, and the two consecutively recorded spectra were processed...