Small Chromophore Research Articles

An unexpectedly simple NIR dye for 1.1 microm with a central mesoionic structure.

Predictable with the color rules? The NIR dye 1 has a novel, mesoionic structure that exhibits a “super acceptor” function and an absorption far in the near infrared within a remarkably small chromophore.

Excited-state hydrogen detachment and hydrogen transfer driven by repulsive 1πσ* states: A new paradigm for nonradiative decay in aromatic biomolecules

The combined results of ab initio electronic-structure calculations and spectroscopic investigations of jet-cooled molecules and clusters provide strong evidence of a surprisingly simple and general mechanistic picture of the nonradiative decay of biomolecules such as nucleic bases and aromatic amino acids. The key role in this picture is played by excited singlet states of πσ* character, which have repulsive potential-energy functions with respect to the stretching of OH or NH bonds. The 1πσ* potential-energy functions intersect not only the bound potential-energy functions of the 1ππ* excited states, but also that of the electronic ground state. Via predissociation of the 1ππ* states and a conical intersection with the ground state, the 1πσ* states trigger an ultrafast internal-conversion process, which is essential for the photostability of biomolecules. In protic solvents, the 1πσ* states promote a hydrogen-transfer process from the chromophore to the solvent. Calculations for chromophore–water clusters have shown that a spontaneous charge-separation process takes place in the solvent shell, yielding a microsolvated hydronium cation and a microsolvated electron. These results suggest that the basic mechanisms of the complex photochemistry of biomolecules in liquid water can be revealed by experimental and theoretical investigations of relatively small chromophore–water clusters.

Bacteriorhodopsin: a natural (nonlinear) photonic bandgap material

The angular dependence of the hyper-Rayleigh scattered light intensity from a suspension of bacteriorhodopsin has been analyzed. The observation calls for combination of the second-order nonlinear hyperpolarizability of the retinal chromophore with the linear refractive index of the apo-protein matrix. The structuring of the small nonlinear chromophore in the large linear matrix imparts properties of a photonic bandgap to bacteriorhodopsin.

Probing single molecule orientations in model lipid membranes with near-field scanning optical microscopy

Single molecule near-field fluorescence measurements are utilized to characterize the molecular level structure in Langmuir–Blodgett monolayers of L-α-dipalmitoylphosphatidylcholine (DPPC). Monolayers incorporating 3×10−4 mol % of the fluorescent lipid analog N-(6-tetramethylrhodaminethiocarbamoyl)-1,2-dihexadecanoyl-sn- glycero-3-phosphoethanolamine, triethylammonium salt (TRITC–DHPE) are transferred onto a freshly cleaved mica surface at low (π=8 mN/m) and high (π=30 mN/m) surface pressures. The near-field fluorescence images exhibit shapes in the single molecule images that are indicative of the lipid analog probe orientation within the films. Modeling the fluorescence patterns yields the single molecule tilt angle distribution in the monolayers which indicates that the majority of the molecules are aligned with their absorption dipole moment pointed approximately normal to the membrane plane. Histograms of the data indicate that the average orientation of the absorption dipole moment is 2.2° (σ=4.8°) in monolayers transferred at π=8 mN/m and 2.4° (σ=5.0°) for monolayers transferred at π=30 mN/m. There is no statistical difference in the mean tilt angle or distribution for the two monolayer conditions studied. The insensitivity of tilt angle to film surface pressure may arise from small chromophore doped domains of trapped liquid-expanded lipid phase remaining at high surface pressure. There is no evidence in the near-field fluorescence images for probe molecules oriented with their dipole moment aligned parallel with the membrane plane. We do, however, find a small but significant population of probe molecules (∼13%) with tilt angles greater than 16°. Comparison of the simultaneously collected near-field fluorescence and force images suggests that these large angle orientations are not the result of significant defects in the films. Instead, this small population may represent a secondary insertion geometry for the probe molecule into the lipid monolayer.

Energy Migration and Excimer Formation in Quasi-Two-Dimensional Polymer Films As Revealed by the Time-Resolved Fluorescence Depolarization Measurement

Excitation energy migration and subsequent energy transfer to excimer-forming sites occurring in a quasi-two-dimensional plane of poly(isobutyl methacrylate) Langmuir−Blodgett films were investigated through the time-resolved analyses of anthracene fluorescence. Fluorescence anisotropy decays showed that the energy migration occurred more efficiently as the anthracene fraction increased. A further increase of the anthracene gave rise to the excimer-forming sites acting as energy traps, and finally the anthracene fluorescence was markedly quenched from the energy transfer to the traps. The migration process was quantitatively analyzed by a computer simulation based on the Monte Carlo method, which allowed estimation of the number of hopping and the mean-square displacement of excitons. The initial growth of the mean-square displacement with time became gradually gentle, while the number of hopping linearly increased with time. This result shows that the excitons are not diffusive and tend to stay within a small cluster of chromophores, especially for small chromophore density at short times.

Linear and non-linear optical studies of mono- and multilayer Langmuir-Blodgett films of N-docosanoyl para-nitroaniline

Abstract Second harmonic generation (SHG) and surface plasmon studies have been carried out on mono- and multilayer Langmuir-Blodgett (LB) films of an amphiphile containing a small chromophore (N-docosanoyl-4-nitroaniline). A quadratic variation in the second harmonic intensity was observed for Y-type multilayers (film thickness above 4 bilayers), but for the thinner films (film thickness less than 4 bilayers) the enhancement of the SHG intensity does not follow the quadratic dependence. The results indicate that the non-linear chromophores lie almost flat on the plane of the substrate with orientation preference in the dopping direction. The SHG behavior and the second-order non-linearity of the LB films are compared with those of 2-docosylamino-5-nitropyridine and N-docosyl-4-nitroaniline which also have a chromophore of similar size.

Exciton coupling in porphyrin dimers

Exciton coupling in cofacial porphyrin dimers that exhibit small chromophore separations and various orientations is predicted using point-dipole and distributed transition-monopole treatments. For these systems, the transition-monopole treatment is shown to be superior in predicting the observed spectral properties of the dimers. The properties of the dimers are in some respects analogous to those of the “special pair” bacteriochlorophyll dimer in the photosynthetic reaction centre.

Rotational diffusion coefficients of a small, spherical subunit flexibly tethered to a larger sphere

The rotational diffusion coefficients of a small spherical particle, which is flexibly anchored to the surface of a much larger sphere, are calculated using the hydrodynamic theory of segmentally flexible particles. The model is intended for representing the rotational mobility of a small residue or chromophore in the surface of a globular macromolecule. The coefficients are found to be essentially independent, or to vary slowly with the relative dispositions of the spheres. They are also insensitive to the size ratio when this ratio is high enough. These findings support the use of an approximative treatment proposed by Wegener in which the small conformation dependence is averaged out. The resulting averages are tentatively used in the Lipari-Szabo model for restricted rotational diffusion in a cone. It is concluded that the rotational relaxation of the small sphere has three components: (i) a torsional rotation with the same diffusion coefficient as the free sphere; (ii) a perpendicular wobbling with a diffusion coefficient several (five in a typical case) times smaller; and (iii) an overall rotation of the whole macromolecule, that will appear in a much longer time scale if the two spheres have quite distinct sizes.

RESONANCE RAMAN STUDIES OF VISUAL PIGMENTS

Visual pigments are composed of a small chromophore (absorption center) called retinal (the aldehyde of vitamin A) covalently linked to surrounding protein, called opsin. The pigments are situated in specialized membranes. The best studied, rhodopsin, is found in velebrate rods, the cells of the retina responsible for low light level vision as opposed to color vision. The purple membrane protein of a bacterial cell called halobacterium halobium also contains retinal as its chromophore surrounded by a colorless protein. The absorption of light by visual pigments causes, eventually, a neural response giving rise to vision. Light absorption by the purple membrane results in protons being pumped across the cell wall of the bacterium; the energy of this electral gradient is then used to produce available chemical energy for the cell in terms of high energy chemical bonds (formation of ATP).