Two-dimensional optical spectroscopy: two-color photon echoes of electronically coupled phthalocyanine dimers

Two-color photon echo peak shift spectroscopy was used to study electronic coupling in a phthalocyanine homodimer. Two optical parametric amplifiers were used to produce pulses to excite the split lower states of LuPc2-. The existence of a two-color peak shift indicates the existence of correlation between these two dipole-allowed states. The nature of this correlation is discussed based on theoretical predictions of the interactions between exciton and charge resonance states.     

Two-dimensional circularly polarized IR photon echo spectroscopy of polypeptides: four-wave-mixing optical activity measurement

A coherent two-dimensional (2D) optical spectroscopy utilizing circularly polarized (CP) beams, which was shown to be useful in studying molecular chirality in condensed phases, was theoretically proposed recently [Cho et al. J. Chem. Phys. 2003, 119, 7003]. A photon echo (PE) version of 2D optical activity spectroscopy is discussed in this paper. Considering various dipeptide and polypeptide systems, where the amide I local modes constitute the set of basis modes used to describe exciton and biexciton states as linear combinations of those basis modes, we present numerically simulated 2D circularly polarized IR PE spectra. It is shown that this novel spectroscopic method can provide additional information on the angles between the transition magnetic dipole and the transition electric dipole of two different vibrationally excited states, which are highly sensitive to the 3D structure and chirality of a given polypeptide. Also, a hierarchical relation of IR absorption, vibrational circular dichroism, 2D IR PE, and 2D CP-IR PE is discussed to show advantages of 2D optical activity spectroscopy in general.     

Tunable resonance hyper-Raman spectroscopy of second-order nonlinear optical chromophores

Two-photon-resonant hyper-Raman spectra are reported for three "push-pull" conjugated organic chromophores bearing -NO(2) acceptor groups, two dipolar and one octupolar. The excitation source is an unamplified picosecond mode-locked Ti:sapphire laser tunable from 720 to 950 nm. The linear resonance Raman spectra of the same molecules are measured using excitation from the laser second harmonic. Excitation on resonance with the lowest-lying band in the linear absorption spectrum yields nearly identical resonance Raman and resonance hyper-Raman spectra. However, excitation into a region that appears to contain more than one electronic transition gives rise to different intensity patterns in the linear and nonlinear spectra, indicating that different transitions contribute differently to the one-photon and two-photon oscillator strength. The promise of the hyper-Raman technique for examining electronic transitions that are both one- and two-photon allowed is discussed.     

Two-dimensional Raman and infrared vibrational spectroscopy for a harmonic oscillator system nonlinearly coupled with a colored noise bath

Multidimensional vibrational response functions of a harmonic oscillator are reconsidered by assuming nonlinear system-bath couplings. In addition to a standard linear-linear (LL) system-bath interaction, we consider a square-linear (SL) interaction. The LL interaction causes the vibrational energy relaxation, while the SL interaction is mainly responsible for the vibrational phase relaxation. The dynamics of the relevant system are investigated by the numerical integration of the Gaussian-Markovian Fokker-Planck equation under the condition of strong couplings with a colored noise bath, where the conventional perturbative approach cannot be applied. The response functions for the fifth-order nonresonant Raman and the third-order infrared (or equivalently the second-order infrared and the seventh-order nonresonant Raman) spectra are calculated under the various combinations of the LL and the SL coupling strengths. Calculated two-dimensional response functions demonstrate that those spectroscopic techniques are very sensitive to the mechanism of the system-bath couplings and the correlation time of the bath fluctuation. We discuss the primary optical transition pathways involved to elucidate the corresponding spectroscopic features and to relate them to the microscopic sources of the vibrational nonlinearity induced by the system-bath interactions. Optical pathways for the fifth-order Raman spectroscopies from an "anisotropic" medium were newly found in this study, which were not predicted by the weak system-bath coupling theory or the standard Brownian harmonic oscillator model.     

Synthesis of conjugated dendrons with nonlinear optical activity

New dipolar conjugated dendrons with electron-withdrawing groups on the periphery and electron-donating groups distributed at the core and throughout the dendritic skeleton have been conveniently synthesized. The pi-conjugated dendritic skeleton provides extended conjugation between each electron donor-acceptor pair, resulting in numerous traditional nonlinear optically active chromophores in one single dendron.     

Ultrafast exciton transfers in DNA and its nonlinear optical spectroscopy

We have calculated the nonlinear response function of a DNA duplex helix including the contributions from the exciton population and coherence transfers by developing an appropriate exciton theory as well as by utilizing a projector operator technique. As a representative example of DNA double helices, the B-form (dA)10-(dT)10 is considered in detail. The Green functions of the exciton population and coherence transfer processes were obtained by developing the DNA exciton Hamiltonian. This enables us to study the dynamic properties of the solvent relaxation and exciton transfers. The spectral density describing the DNA base-solvent interactions was obtained by adjusting the solvent reorganization energy to reproduce the absorption and steady-state fluorescence spectra. The time-dependent fluorescence shift of the model DNA system is found to be ultrafast and it is largely determined by the exciton population transfer processes. It is further shown that the nonlinear optical spectroscopic techniques such as photon echo peak shift and two-dimensional photon echo can provide important information on the exciton dynamics of the DNA double helix. We have found that the exciton-exciton coherence transfer plays critical roles in the peculiar energy transfer and ultrafast memory loss of the initially created excitonic state in the DNA duplex helix.     

Quadrupole contribution to the third-order optical activity spectroscopy

Time-resolved nonlinear optical activity measurement spectroscopy can be a useful tool for studying biomolecular and chemical reaction dynamics of chiral molecules. Only recently, the two-dimensional (2D) circularly polarized photon echo (CP-PE) spectroscopy of polypeptides and a photosynthetic light-harvesting complex were discussed, where the beam configuration was specifically controlled in such a way to eliminate the quadrupole contribution to the CP-PE signal. In this paper, we generalize the CP-PE spectroscopy by including the transition quadrupole contributions from peptide amide I vibrational transition and chlorophyll electronic transition. By using a density functional theory calculation method, the corresponding amide I vibrational and chlorophyll Q(y) electronic transition quadrupole tensor elements are determined. Amplitude of nonlinear optical transition pathway involving a quadrupole transition is found to be comparable to those of magnetic dipole terms for two different cases considered, i.e., dipeptides and photosynthetic antenna complex. However, due to the rotational averaging factors, the overall quadrupole contribution is an order of magnitude smaller than the magnetic dipole contribution. This suggests that the conventional 2D photon echo method and experimental scheme can be directly used to measure the 2D CP-PE signal from proteins and molecular complexes and that the 2D CP-PE signal is mainly dictated by the magnetic dipole contribution.     

The analysis of archaeological glass by inductively coupled plasma optical emission spectroscopy

Summary An analytical procedure is described for the analysis of archaeological glass by inductively coupled plasma optical emission spectroscopy (ICP-OES). Glass samples were analysed in solution after fusion with lithium metaborate at 1100°C. The analyses were performed in the sequential multielemental mode of operation, with the determination of 15 elements in four analytical runs; only elements with not too large concentration difference were analysed in a single run. The following elements were accounted for: Si, Na, Ca, Al, Fe, Mg, Mn, Ti, Sr, Ba, Cr, Ni, Cu, Co, Pb.     

Two-dimensional ultraviolet spectroscopy of proteins

Two-dimensional ultraviolet (2DUV) spectroscopy is a novel technology for probing molecular structure. We have developed a generalized quantum mechanics/molecular mechanics (QM/MM) approach to simulate the electronic transitions of protein backbones and aromatic amino acids in aqueous solution. These transitions, which occur in the ultraviolet (UV) region, provide a sensitive probe of molecular structure. The features of 2DUV spectra are accurately characterized and enable us to trace small variations in the structure and dynamics as well as evolution propensity with high accuracy. Various structures and dynamic phenomena are investigated to construct a systematic framework for 2DUV simulation mechanisms, so as to explore further applications of this technique. In this feature article, we summarize the theory and applications of 2DUV spectroscopy we have engaged in recently, present the important roles of 2DUV spectroscopy, and outline directions for future development. We hope this article can offer a platform for more scientists in different research fields to gain a clear overview of 2DUVand further attract more people to explore this promising field.     

Two-dimensional, pyrazine-based nonlinear optical chromophores with ruthenium(II) ammine electron donors

Six new nonlinear optical (NLO) chromophores with pyrazinyl-pyridinium electron acceptors have been synthesized by complexing a known pro-ligand with electron donating {Ru(II)(NH(3))(5)}(2+) or trans-{Ru(II)(NH(3))(4)(py)}(2+) (py = pyridine) centers. These cationic complexes have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. The visible d → π* metal-to-ligand charge-transfer (MLCT) absorptions gain intensity on increasing the number of Ru(II) centers from one to two, but remain at constant energy. One or two Ru(III/II) redox processes are observed which are reversible, quasi-reversible, or irreversible, while all of the ligand-based reductions are irreversible. Molecular first hyperpolarizabilities β have been determined by using hyper-Rayleigh scattering (HRS) at 1064 nm, and depolarization studies show that the NLO responses of the symmetric species are strongly two-dimensional (2D) in character, with dominant "off-diagonal" β(zyy) components. Stark (electroabsorption) spectroscopic measurements on the MLCT bands also allow the indirect determination of estimated static first hyperpolarizabilities β(0). Both the HRS and the Stark-derived β(0) values increase on moving from mono- to bimetallic complexes, and substantial enhancements in NLO response are achieved when compared with one-dimensional (1D) and 2D monometallic Ru(II) ammine complexes reported previously.     

A relaxation study of poled nonlinear optical polymers by infrared reflection-absorption spectroscopy

The relaxation of poled nonlinear optical (NLO) chromophores in polymer films was characterized by infrared (i.r.) reflection-absorption spectroscopy. Both a guest-host system and a photocrosslinkable polymer system were investigated. Polymethylmethacrylate doped with either 2-methyl-4-nitroaniline or 4(4′-nitrophenylazo)aniline was studied. The photocrosslinkable polymer system, polyvinylcinnamate doped with 3-cinnamoyloxy-4-[4-(N,N-diethylamino)-2-cinnamoyloxy phenyl azo]nitrobenzene was also investigated. Doped NLO active molecules were aligned using the corona poling technique. i.r. spectra as a function of time were used to monitor the relaxation behavior of the oriented dyes after poling. Relaxation of NLO molecules was followed at various characteristic vibrational frequencies. The relaxation behavior of both systems were found to be consistent with those studied by the second harmonic generation technique.     

Recent development of the nonlinear optical active material system

New nonlinear optical active materials have been highlighted to apply them to practical applications since about two decades ago. During this period, a number of materials have been developed and studied in academic and industry field. Particularly, the second-order nonlinear optical properties are facile to approach in the laboratory. We can consider the possibility of the device application by investigating the macroscopic second-order nonlinear optical properties using the second harmonic generation and linear electro-optic study. Nowadays, the absolute value of the nonlinear optical coefficient of organic material overcame the value of conventional inorganic materials due to quick research progress. Therefore, the new organic materials systems showed some promising motives to fabricate the optical device.     

A multiscale description of molecular adsorption on gold nanoparticles by nonlinear optical spectroscopy

Nonlinear optical Sum and Difference-Frequency spectroscopies are used to probe and model the surface of thiophenol-functionalised gold nanoparticles grafted on a Si(100) substrate through two different silanization procedures. By scanning the [980-1100 cm(-1)] infrared spectral range with the CLIO Free Electron Laser, ring deformation vibrations of adsorbed thiophenol are investigated. Quantitative data analysis addresses three levels of organization: microscopic, nanoscopic and molecular. Grafting with p-aminophenyl-trimethoxysilane shows an increase of around 40% in surface density of nanoparticles (N(s)) as compared to 3-aminopropyl-triethoxysilane. The relative amplitudes of the resonant and nonresonant contributions to the SFG and DFG spectra are discussed in terms of N(s), Fresnel reflectivity factors and local amplification of the nonlinear signals by coupling to the surface plasmon of the particles. They are shown to quantitatively scale with N(s), as measured by atomic force microscopy. Vibration mode assignment is performed through a critical analysis of literature data on IR and Raman spectroscopies coupled to DFT calculations, for which a methodology specific to molecules adsorbed on gold atoms is discussed.     

Two-dimensional optical three-pulse photon echo spectroscopy. I. Nonperturbative approach to the calculation of spectra

The nonperturbative approach to the calculation of nonlinear optical spectra of Seidner et al. [J. Chem. Phys. 103, 3998 (1995)] is extended to describe four-wave mixing experiments. The system-field interaction is treated nonperturbatively in the semiclassical dipole approximation, enabling a calculation of third order nonlinear spectroscopic signals directly from molecular dynamics and an efficient modeling of multilevel systems exhibiting relaxation and transfer phenomena. The method, coupled with the treatment of dynamics within the Bloch model, is illustrated by calculations of the two-dimensional three-pulse photon echo spectra of a simple model system-a two-electronic-level molecule. The nonperturbative calculations reproduce well-known results obtained by perturbative methods. Technical limitations of the nonperturbative approach in dealing with a dynamic inhomogeneity are discussed, and possible solutions are suggested. An application of the approach to an excitonically coupled dimer system with emphasis on the manifestation of complex exciton dynamics in two-dimensional optical spectra is presented in paper II Pisliakov et al. [J. Chem. Phys. 124, 234505 (2006), following paper].     

Enzyme-based online monitoring and measurement of antioxidant activity using an optical oxygen sensor coupled to an HPLC system

It is estimated that up to 50 % of the adult population take antioxidant products on a daily basis to promote their health status. Strangely, despite the well-recognized importance of antioxidants, currently there is no international standard index for labeling owing to the lack of standardized methods for antioxidant measurement in complex products. Here, an online high-performance liquid chromatography (HPLC)-based method to detect and measure the total antioxidant capacity of antioxidant samples is presented. In this approach, complex samples containing antioxidants are separated by the HPLC system, which is further coupled to an antioxidant measuring system consisting of an optical oxygen sensor, laccase, and tetramethoxy azobismethylene quinone (TMAMQ). The antioxidants, separated via HPLC, reduce TMAMQ to syringaldazine, which is then reoxidized by laccase while simultaneously consuming O₂. The amount of consumed oxygen is directly proportional to the concentration of antioxidants and is measured by the optical oxygen sensor. The sensor is fabricated by coating a glass capillary with an oxygen-sensitive thin layer made of platinum(II) meso-tetra(4-fluorophenyl)tetrabenzoporphyrin and polystyrene, which makes real-time analysis possible (t ₉₀?=?1.1 s in solution). Four selected antioxidants (3 mM), namely, catechin, ferulic acid, naringenin (used as a control), and Trolox, representing flavonol, hydrocinnamic acid, flavanone, and vitamin E, respectively, were injected into the online antioxidant monitoring system, separated, and then mixed with the TMAMQ/laccase solution, which resulted in oxygen consumption. This study shows that, with the use of such a system, the antioxidant activity of individual antioxidant molecules in a sample and their contribution to the total antioxidant activity of the sample can be correctly assigned.     

Electronic interactions in a branched chromophore investigated by nonlinear optical and time-resolved spectroscopy

The third order nonlinear optical properties of a trimer branched chromophore system and its linear molecule analog are investigated. Two-photon absorption and degenerate four wave mixing measurements were carried out on both systems. An enhancement in the nonlinear optical effect is observed for the branched trimer molecule in comparison to the linear chromophore system. Ultrafast time-resolved measurements were carried out to probe the excited state dynamics in the branched structures. The time-resolved measurements suggest that the two important processes affecting the nonlinear optical properties in the trimer system, charge transfer stabilization and initial electronic delocalization, occur on two different time scales.     

Synthesis of novel nonlinear optical chromophore to achieve ultrahigh electro-optic activity

A new diene-conjugated chromophore WJ1 was synthesized with high yield of 36% through an H-bonding induced Vilsmeier reaction. By simple guest-host doping, a large electro-optic efficiency of 337 pm V(-1) at 1310 nm and excellent temporal stability at 75 °C have been achieved in poled films of WJ1/APC with a high loading density of 40 wt%.     

Two-dimensional coherent double resonance electronic spectroscopy

A new form of coherent 2D spectroscopy involving a pair of electronic resonances appears to provide several advantages over more established techniques. It can resolve congested peaks and sort them by vibrational quantum number, rotational quantum number, and isotopomer. The high degree of symmetry in the resulting spectra facilitates the ability to assign the quantum numbers and isotopomer for each peak. Quantitative results are demonstrated using an isotopomeric mixture of bromine.