Aggregation Of Chromophores Research Articles

Polymer-bound pyrrole compounds. V . On the optical properties of an insoluble polystyrene matrix bearing di- and tetrapyrroles covalently anchored

The absorption and fluorescence spectra of samples of variable particle size, consisting of an insoluble polystyrene matrix bearing different covalently attached di- and tetrapyrroles, are studied and compared with solutions of the same chromophores present as their methyl ester derivatives. The specific chromophores consist of xanthobilirubinic acid, N′-methyl xanthobilirubinic acid, bilirubin IX-α, biliverdin IX-α, mesoporphyrin IX and Zn(II) mesoporphyrin IX, anchored as esters to α-4-[(ylmethyl-3-nitrobenzamido)benzyl]-poly(styrene-co-1%-divinylbenzene). The spectral differences observed are: (1) an apparent decrease in the extinction coefficient of the high energy band relative to the low energy band of the verdin and porphyrin chromophores; and (2) variable shifts of the absorption wavelength maxima depending on the specific system. Scattering by the polymer is mainly responsible for the first, but chromophore aggregation also contributes to band shifts. Both phenomena are more important in particles of comparatively larger size, as has been confirmed by comparing spectroscopic properties of micellar and vesicular systems containing the same chromophores. The present systems are interesting because: (1) one can derive qualitative information on several aspects of the polymer matrix itself, such as the interactions of a given site with its immediate surroundings; and (2) they serve as models for investigations of chromophore-imbedded tissue optics, and in general, of energy absorption by dyes in a light-scattering medium.

Chromophore aggregation and concomitant wavelength-dependent photochemistry of a main-chain liquid crystalline poly(aryl cinnamate)

Chromophore aggregation and concomitant wavelength-dependent photochemistry of a main-chain liquid crystalline poly(aryl cinnamate)

Molar Fraction Dependence of Excitation Energy Transfer in Mixed Monolayer Assemblies of 11‐(9‐Carbazolyl)undecanoic Acid and Long‐chain Fatty Acids

AbstractThe efficient energy transfer from 11‐(9‐carbazolyl)undecanoic acid to a very small fraction (470 ppm) of benzocarbazolyl derivatives in mixed monolayer assemblies with long‐chain fatty acids was observed when the molar fraction of chromophore‐containing amphiphiles (fc) was higher than about 0.05. The fluorescence intensity ratio of acceptors and donors increased with the molar fraction. It tended to saturate when fc is larger than about 0.2. Such fraction dependence was not explained by the Förster energy transfer among randomly distributed chromophores, but was analyzed by the exciton migration model. Our results show that aggregation of chromophores in mixed monolayer assemblies does occur depending on the molar fraction fc.

Orientation evaluation of polyion complex Langmuir-Blodgett films by Fourier transform IR transmission and reflection-absorption spectroscopy

Quantitative evaluation of molecular orientation in Langmuir-Blodgett (LB) films of polyion complexes between two azobenzene-containing fatty acids with different chain lengths (the C 12 and C 8 tail with the common C 5 spacer) and two polycations were performed by comparing the absorption intensities of the Fourier transform IR transmission and reflection-absorption spectra. The results obtained were discussed in connection with the UV absorption peak maxima of the azobenzene chromophores and the limiting areas of the surface pressure-area isotherms of the spread monolayers on the water surface. For LB films of polyion complexes of the fatty acid with the C 12 tail, the self-association forces are so strong that the degree of perpendicular orientation of the molecular chain and the stacking aggregation of the azobenzene chromophores remain high and unchanged from those for its barium salt. The polymer chain is considered to be twisted or folded to fit with the compact cross-section of the fatty acid. For LB films of polyion complexes of the fatty acid with the C 8 tail, however, the degree of orientation and aggregation are degraded from those of its barium salt, the polymer chain being highly extended.

Theoretical investigation on the optical bandshape properties of chromophore aggregates by use of the incoherent dynamic interaction model

By using a modified perturbation expansion to resolve the commutator nest, the complex polarizability of chromophore aggregates was expressed in the coupled resolvent form adaptable to take account of the incoherent dynamic interaction arising from the interference between the positive- and the negative-frequency parts of the light scattering by chromophores. Furthermore, with the help of the ensemble-averaged resolvent matrix method, the imaginary part of the complex polarizability matrix needed to calculate the absorptive spectral function of chromophore aggregates was formulated as a simplified analytical function of the chromophore's bandshape distorted out of its original shape by frequency modulation coming from the incoherent dynamic interaction. The utility of this formulation was shown by applying it to calculation of the absorption intensity and the circular dichroism bandshape of chromophore aggregates modelled on the fundamental helical geometries of polynucleotides. The above calculation gave new information about the effect of the incoherent dynamic interaction upon the hypochromic change of the total absorption intensity and upon the nonconservative distortion of the circular dichroism bandshape.

Divalent Cation Dependent Spectral Change in Merocyanine Monolayer

Abstract The effect of divalent cation on chromophore aggregation in a merocyanine monolayer was studied by means of spectroscopic measurements. The absorption spectra for J-aggregates were found to be controlled by the nature of metal cation.

Thermochromic Langmuir-Blodgett films of a novel amphiphilic azo dye with homogeneous chromophore orientation

Langmuir-Blodgett (LB) multilayers have been prepared from a novel amphiphilic azobenzene derivative. These films were transparent and showed homogeneous polarization of visible light over the whole film area. They exhibit reversible thermochromism with a red shift of 60 nm over a temperature range of less than 10°C. This thermochromism can be explained by a “J-like” aggregation of the azo chromophores that is slightly effected by temperature.

Molecular Orientation and Photoelectric Properties of Alloxazine Monolayer Assemblies

AbstractLangmuir‐Blodgett (LB) films of an alloxazine derivative with two dodecyl chains at 1,3‐positions showed UV/vis dichroism at normal incidence. Spectral shifts indicating aggregation of chromophores were also observed. Angular dependences of polarized absorption spectra indicated that alloxazine rings were biaxially oriented with respect to the film plane and to the dipping direction. The molecular orientation of alloxazine rings in the film plane was suggested to be induced during the transferring process of monolayers. Alloxazine LB films showed photoelectric responses corresponding to the absorption spectra. The in‐plane photoconductivity was almost isotropic and was greater than that of the through‐plane one by about five orders of magnitude.

An extended theoretical formulation of the circular dichroism band shape of chromophore aggregates by use of the ensemble-averaged resolvent matrix method

A new theoretical formulation is given to the circular dichroism band shape of chromophore aggregates by means of the ensemble-averaged resolvent matrix method. This formulation can be used to calculate the circular dichroism band shape of chromophore aggregates having arbitrary strengths of the excited-state interactions between chromophores. Using the resolvent expansion rule, thermal and static effects upon the circular dichroism band shape are incorporated as the mean-frequency shifts of the normalized band shape functions used for ensemble averaging the resolvent matrix of chromophore aggregates. This formulation is applied to derive a generalized band shape expansion formula which gives a theoretical justification to the practical first-order Taylor expansion equation used to calculate the circular dichroism band shape of chromophore aggregates. Basic properties of the generalized band shape expansion formula are investigated by comparing it with the usual perturbation expansion equation for the circular dichroism band shape of chromophore aggregates.

Control of chromophore aggregation in mono- and multilayers of long-chain merocyanine dyes

Control of the orientation and aggregation of the chromophore in the mixed monolayers of long-chain merocyanine dyes with various matrices has been investigated. The mixed monolayers on water were characterized by measuring the surface pressure-area isotherms, and the organization of the chromophore in the transferred layers with different matrices was studied by measuring the polarized absorption spectra. As a result of the addition of hexadecane to the binary mixtures of the dye and arachidic acid or methyl arachidate, anisotropic aggregation of the chromophores in the mixed monolayers has been found to occur during the process of loss of hexadecane from the spread films under a constant surface pressure. These mixed films exhibit narrow and strong bands in the absorption and fluorescence spectra, which can be attributed to J aggregates.

New Fluorescence from Molecular Aggregates of 10-(1-Pyrenyl)decanoic Acid

Abstract In addition to the normal monomer and excimer fluorescence, vacuum-deposited, cast and their laser-irradiated films of 10–(1-pyrenyl)decanoic acid showed three kinds of fluorescence bands. Similar new fluorescence bands were also observed in its aqueous solution. These emissions were due to meta-stable aggregates of pyrenyl chromophores whose structures were ascribed to molecular interactions operating during the processes of film preparation.

The circular dichroism of helical chromophore aggregates as studied by a self-consistent non-linear response theory

An extended formulation of the circular dichroism of helical chromophore aggregates is given by means of the self-consistent non-linear response theory. In this formulation the helix circular dichroism is expressed in closed form in terms of the chromophore's polarizability and hyperpolarizability and in terms of the retarded helical-lattice sum of interchromophoric dipole-dipole interactions. The effects of interchromophoric dynamic interactions upon the helix circular dichroism are then absorbed self-consistently into the polarizability and hyperpolarizability band shape matrices of the perturbed chromophore. The utility of this formulation is shown by a sample calculation of the polarizability and first-hyperpolarizability contribution effects upon the circular dichroism band shape of an oriented helix modelling the B-form conformation of polynucleotides.

Photoacoustic spectra and photovoltaic effect of flavins

Abstract Photoacoustic spectra and the kinetics of the photovoltaic effect, as well as action spectra of 10-formylmethylisoalloxazine, riboflavin, flavomononucleotide and the complexes of flavins with riboflavin-bound proteins, were measured. The spectral regions of high efficiency of thermal deactivation of flavin excitation were observed. In these regions there could be high absorption of some aggregates of dye or there could be some vibronic transitions characterized by efficient thermal deactivation. The aggregation of chromophores with proteins causes the increase in the photoacoustic signal and the change in the shape of the photoacoustic spectrum. The illumination of a sample by light, causing a high photoacoustic signal, leads to low efficiency of photopotential generation. This means that the process of electron transfer from the excited pigment to the semiconducting electrode is less efficient when thermal deactivation is higher.

Optical absorption and luminescence in Langmuir films of merocyanine dye

Optical absorption and luminescence spectra were investigated for Langmuir films of a merocyanine dye. The absorption spectra of the films of 6-methyl merocyanine diluted with arachidic acid exhibit a J-like band at 606 nm at room temperature. For the low-energy tail of the absorption band, absorption coefficients at measured temperatures between 79 K and 270 K were expressed by the Urbach rule with a steepness parameter σ′ of 1.29. The exciton-phonon coupling parameter from the present σ′ value was shown to be smaller than unity. In addition, the observed luminescence appeared to be F-type. These results suggest that the exciton-phonon interaction is weak and two-dimensional in the chromophore aggregates.

Modification of optical and photoelectric characteristics by vapour phase treatments in Langmuir-Blodgett films of merocyanine dyes

The effects of acid and basic vapour phase treatments (AT and BT respectively) have been studied on Langmuir-Blodgett films thicker than 9 monolayers of a merocyanine (DI) and its 6-CH 3, substituted product (DII). Optical measurements indicate that the red-shifted (J-like) band structure, which is dissociated on AT as well as by heat treatment, can be restored by applying BT. A polymorphism is seen for DI among the as-deposited and the restored J-like structures. The absence of polymorphism for DII indicates a stereospecific role of the 6-CH 3 group in the chromophore aggregation. The lateral photoconductivity Δσ | is controlled in a range of nearly two decades by these treatments and their combinations. The largest Δσ | is attained by a successive application AT → BT.

Absorption of methylbacteriopheophorbide alpha single crystals: spectral shifts due to pi-pi interactions.

Abstract— Polarized absorption spectra were obtained for a single crystal of methylbacteriophorbide a (MeBPhide a). The Qy band is red‐shifted ? 1660 cm‐1 (? 110 nm) relative to MeBPhide a in a CH2Cl2/benzene solution. This is equivalent to the largest red shifts observed for in vivo bacteriochlorophyll a. The Soret band exhibits a smaller red shift and a significant reduction in intensity, and the Qx band is not observed. The crystal spectra are qualitatively similar to spectra reported for several other aggregated (bacterio)chlorophyll and bacteriopheophytin systems. Since crystalline MeBPhide a contains no Mg, water or hydrogen bonding (Barkigia etal., 1981), these results demonstrate that the spectral changes associated with the aggregation of photosynthetic chromophores can arise solely from IT‐IT interactions between macrocycles.

Generalization of the bandshape expansion formula for the circular dichroism of chromophore aggregates

A generalized expression of the bandshape expansion formula for the circular dichroism of chromophore aggregates is derived using an extended Green's function method. Basic properties of the generalized bandshape expansion formula are studied in comparison with the perturbation expansion equation given by the usual decorrelation approximation.

A resolvent expansion theorem for the Fourier transform of the retarded Green function and its application to the absorption bandshape of chromophore aggregates

A resolvent expansion theorem is developed for the Fourier transform of the retarded Green function. The resolvent expansion theorem coupled with a decorrelation approximation for a statistical average over the vibrational sublevels of the electronic ground state is applied to a theoretical formulation of the averaged Green function of chromophore aggregates. Consequently, thermal and static effects upon the aggregate's absorption bandshape are incorporated in terms of the correction shift of the individual chromophore's bandshapes. The utility of the theoretical formulation is shown by an illustrative calculation of the absorption bandshape of a model dimer.

A theoretical study for the dynamic interchromophoric interaction effects upon the susceptibility and hypersusceptibility of chromophore aggregates

The dynamic interchromophoric interaction effects upon the susceptibility and hypersusceptibility of the current–charge density of chromophore aggregates are formulated to infinite order and in closed form by means of a decoupling approximation, whereby the induced field operator contained in the dynamic perturbation Hamiltonian is replaced by its averaged form satisfying the generalized Maxwell’s equation for the current-charge source density. This decoupling approximation includes a basic condition that the total momentum and hence the total current of identical charged particles belonging to any chromophore remain as nearly constant in time throughout the dynamic perturbation interactions. The physical meaning of this condition is surveyed by making reference to the local conservation equation which can be derived for the chromophoric current and charge densities. Contributions of the aggregate’s susceptibility and hypersusceptibility to the mean rate of energy absorption caused by a single-frequency external field and to the mean rate difference generated by right and left circularly polarized light are studied in terms of the frequency reversal symmetries of the susceptibility and hypersusceptibility.

Band structure of electronic circular dichroism spectra and the born‐oppenheimer approximation

AbstractTheory is presented for determining the effects of band structure on electronic CD spectra. In this development, the effect of the first‐order correction to the Born‐Oppenheimer approximation is considered. This removes the unrealistic feature of identical CD and absorbance spectral band structures present in previous models. The predicted order of magnitude of this correction is consistent with experimentally observed differences in CD and absorbance band structure. A Green function formalism is used to determine the contribution of the intrinsic CD to the total CD of an aggregate of chromophores. The intrinsic CD can be determined from the monomeric CD that has been corrected for effects of the intermolecular interactions in the aggregate.