Interactions between purine derivatives: Electronic spectral studies. I. Electronic transitions in caffeine monomer and exciton coupling in caffeine dimer

The concentration dependence of the ultraviolet absorption spectrum of aqueous solutions of caffeine has been studied. Individual species spectra have been derived for the monomer, dimer, and tetramer of caffeine. The emission spectrum of caffeine in aqueous solution and the dichroic spectra in oriented poly(vinyl alcohol) and polyethylene films have been measured. The long-wavelength tail of the absorption spectrum of caffeine in non-polar environment has been found to incorporate at least one carbonyl(π*, n) transition. Dichroic spectral data and molecular orbital calculations have been used to assign transition moment directions to the (π*,π) transitions. The lowest energy (π*,π) transition, responsible for the near-ultraviolet absorption peak in aqueous solution of caffeine, has been used for the study of degenerate exciton interactions in the dimeric species of caffeine. Assuming that the caffeine molecules in the dimer are stacked in parallel planes, theoretical calculations of the ground-state interactions and of the degenerate exciton interactions have been combined with experimental data and a unique model for the dimer of caffeine has been derived. The transfer rate of energy between the molecules in the dimer is of the order of 10¹³_S^?1.     

Orientation of reaction center and antenna chromophores in the photosynthetic membrane of rhodopseudomonas viridis

Whole cells of Rhodopseudomonas viridis were oriented in a magnetic field. The degree of orientation of the cells was determined by using a photoselection technique. In order to deduce the orientation of the antennae and chromophores of the reaction centers with respect to the membrane plane, we performed linear dichroism measurements of absolute spectra and light induced difference spectra linked to states P⁺I and PI^? on oriented cells. These measurements lead to the following conclusions:The antennae bacteriochlorophyll molecular plane is nearly perpendicular to the membrane. The Q_y and Q_x transitions moments of these molecules make respectively angles of 20 and 70°ith the membrane plane. The antenna carotenoid molecules make an angle of 45°ith the membrane.The primary electron donor possesses two transition moments centered respectively at 970 and 850 nm. The 970 nm transition moment is parallel to the membrane plane, the 850 nm transition is tilted out of the plane. Upon photooxidation of this primary electron donor, a monomer-like absorption band appears at 805 nm. Its transition makes an angle smaller than 25° with the membrane. The photooxidation of the dimer also induces an absorption band shift for the two other bacteriochlorophyll molecules of the reaction center. The absorption band shifts of the two bacteriochlorophyll molecules occur in opposite direction.One bacteriopheophytin molecule is photoreduced in state PI^?. This photoreduction induces an absorption band shift for only one bacteriochlorophyll molecule. Finally, the geometry of the dimeric primary donor seems to be affected by the presence of a negative charge in the reaction center.     

Properties of reaction centers of Rhodopseudomonas sphaeroides in dried gelatin films.: Linear dichroism and low temperature spectra

Methods of preparing dried gelatin films containing purified reaction centers of Rhodopseudomonas sphaeroides are described. The spectral properties of reaction centers in solution are essentially maintained in dried gelatin films. These films are uniform and have excellent optical properties, showing little particulate scattering at temperatures down to about 4K. Film contraction on cooling to 90K is less than 1% in linear dimension. Linear dichroism spectra are reported for films at room and low temperature. Reaction centers show a moderate amount of linear dichroism in unstretched gelatin films; the magnitude of the linear dichroism becomes much greater when the films are stretched. In stretched films, linear dichroic ratios ($\text{A}{}_{\mathrm{\parallel }}\text{A}{}_{\mathrm{\perp }}$; absorbance measured with electric vector parallel and perpendicular to stretching direction) between 1.7 and 2.2 were obtained for the 860 nm absorption band of the bacteriochlorophyll component that undergoes primary photooxidation. The relative polarizations of light-induced absorption changes of reaction centers in stretched films are similar to those reported by Vermeglio and Clayton ((1976) Biochim. Biophys. Acta 449, 500–515) and support their hypothesis that absorbance decreases, maximal near 860 and 810 nm, and an increase near 790 nm are associated with the respective disappearance and appearance of discrete bands characteristic of the reduced and oxidized bacteriochlorophyll dimer. This interpretation is also supported by the polarization of the absolute absorption spectrum near 810 and 860 nm. An absorption band near 540 nm, ascribed to the Q_x transitions of two molecules of bacteriopheophytin in the reaction center, is split at low temperatures into two bands having similar polarizations. This splitting is probably not due to exciton coupling of the two molecules, since excition theory predicts different polarizations.     

Effect of B-ring substituents on absorption and circular dichroic spectra of colchicine analogues.

Near-ultraviolet absorption and circular dichroic spectra of several B-ring derivatives of colchicine have been obtained in a variety of solvents. The spectra of the molecules in solvent were analyzed and compared with spectra of the molecules bound to tubulin. Absorption spectra of deacetamidocolchicine, deacetylcolchicine, demecolcine, and N-methyldemecolcine [B-ring substituents = H, NH2, NHCH3, and N(CH3)2, respectively] were analyzed by multiple differentiation of the spectrum. It was found that an amine substituent at the C-7 position on the B-ring of the colchicinoid affected the higher energy transition of the near-ultraviolet spectra of the colchicinoid in the absence of tubulin in a manner consistent with a hyperconjugative alteration of this transition. The fourth derivatives of the absorption spectra of all four molecules bound to tubulin were similar to each other and to colchicine. As was true in the case of colchicine, the negative near-ultraviolet circular dichroic band of the aminoclochicinoids was relatively unaffected by solvent, but the molar ellipticity of the band was greatly reduced with tubulin binding. It is concluded that the binding site environments of the B-ring analogues of colchicine, as probed by absorption and circular dichroic spectroscopy, are equivalent.     

Rotovibrational states of the water molecule on the sun

The infrared spectrum of water observed in sunspots is complex and dense, with bands separated by approximately 0.01 cm^?1. For top asymmetrical molecules, there is no theoretical approach that allows for the calculation of rotovibrational energy with such precision. Experimentally derived rotovibracional energy levels of water at high temperatures combined with variational calculations have been used for the band assignments. These energy levels are employed to refine the analysis of a small portion of the infrared absorption spectrum. Such procedure has allowed for the identification of additional 55 bands to the 70 already identified as rotovibrational transitions of the water molecule. Our new assignments, which include pure and cross transitions, offer additional evidence of the existence of water on the sun, but above all they illustrate the complexity of the solar spectrum that involves states with higher levels of rotational excitation. Given the conditions on the sun, more molecules of water would occur in excited electronic states, which include apolar and paramagnetic states, generating intense bands in the spectrum. Since there is an analytical solution for the rotovibrational transitions of linear molecules, we were able to identify 16 bands relative to the excited electronic states ¹B₂ and ³A₁ in the sunspot spectrum. Density functional B3LYP/AUG-cc-pVTZ calculations of the electric and magnetic dipole are employed to discuss some consequences of the presence of excited states of water in the dynamics of sunspots and solar magnetic field.     

Cu(II)–poly(L-arginine) complexes. Potentiometric and spectral characterization of amine and peptide nitrogen ligands

The study of Cu(II)–poly(L -arginine) complexes by potentiometric titration, as well as by optical, circular dichroism, and infrared spectra, provides information about the nature of ligands and the coordination sphere around the metal ion. Three different complexes have been identified. The first, which is formed below pH 8, contains two guanidinium nitrogens and two water molecules at the corners of the coordination square. The constant of the overall process as determined by the Gregor method equals 2.0 ± 0.1 × 10^?9. The two other complexes form between pH 8 and 10.5 and they contain two guanidinium and two peptide nitrogens as nearest ligands. One of them is a monomer and the other probably a dimer, which differ in the symmetry of the coordination sphere around the cupric ion. The optical spectra of the three complexes show an absorption band at 260 nm that we have assigned to a charge-transfer transition between a σ metal nitrogen (amine) molecular orbital and a d_x2_?y2 metal orbital. The spectra of the two complexes containing peptide nitrogens exhibit another absorption band at 320 nm, which we have assigned to a charge transfer from a π orbital of the amide group to the d_x2_?y2 metal orbital.     

Electronic absorption spectra of cis-M(CO)2(α-diimine)2 complexes (M = Mo or W; α-diimine = 2,2′-bipyridine or 1,10-phenanthroline)

Syntheses, infrared spectra, and electronic absorption spectra of cis-M(CO)₂(α-diimine)₂ (M = Mo, W; α-diimine = 2,2′-bipyridine, 1,10-phenanthroline) complexes are reported. Infrared spectra indicate carbonyl stretching frequencies in the 1700–1800 cm⁻¹ region, consistent with strong M(dπ) → (π*)CO back-bonding in these dicarbonyl complexes. Electronic absorption spectra illustrate several intense M(dπ) → (π*)α-diimine transitions throughout the visible region. A comparison of the solvent effects on the absorption spectra of the cis-M(CO)₂(α-diimine)₂ species is made with the well known M(CO)₄(α-diimine) complexes.     

Dichroism of transient absorbance changes in the red spectral region using oriented chloroplasts. II. P-700 absorbance changes

The light induced transient absorbance changes associated with the trap of photosystem I have been studied using magnetically oriented spinach chloroplasts and a polarized measuring beam. The ΔA spectra for the two polarizations parallel and perpendicular to the plane of the photosynthetic membranes have been recorded in the spectral range 630–850 nm.A dichroic ratio greater than two is observed both in the main band around 700 nm and in the radical cation band around 810 nm, leading to the conclusion that the far-red transition moment of the P-700 dimeric species is lying almost parallel to the membrane plane.Dichroic ratios smaller than one are reported in the 650–670 nm band of the ΔA spectrum. The possible attribution of this band to excitonic interactions in the dimer favors the hypothesis of a tilting out of the membrane plane of this transition. This finding ruled out an orientation parallel to the membrane plane of the two chlorophyll molecules constituting the P-700 phototrap.A small residual transient absorbance change is observed in the absence of artificial electron acceptor. Its spectrum shows significant differences as compared to the normal P-700 spectrum: the magnitude of the signal at 700 nm is only 15–25% of the normal signal, the half-band width of the band around 700 nm is nearly twice as large and the dichroic ratio in the band is only 1.5±0.1. In the presence of ferricyanide, this signal is still observed both for intact and osmotically broken chloroplasts, suggesting a heterogeneity in the population of traps in Photosystem I.     

Preparation and photophysical properties of precursors of inorganic macromolecules. Mono and binuclear complexes of Ru(II) and terpyridine derivatized with thiophene and 4-(5-bromothiophene) groups

The preparation and characterization of mono and binuclear complexes of Ru(II) with a newly synthesized derivate of the terpyridine ligand, 4^′-(5-bromothiophene)-2,2^′,6^′,2″-terpyridine, are communicated. In the binuclear complex, 2,5-bis(2,2^′,6^′,2″-terpyridine-4^′yl)thiophene was used as a bridge between two Ru(II) centers. The new compounds were characterized by H NMR, UV-Vis and IR spectroscopies. Bands at ∼500 nm for the Ru(II) to terpyridine charge transfer transition and absorption bands at λ<400 nm assigned to intraligand transitions, π^*←π, centered in the tpy moiety were observed in the UV-Vis spectra of the complexes. Irradiation of the complexes in CH₃CN at 337 or 500 nm induced luminescence with maxima at ∼670 nm and lifetimes τ?10² ns. Time-resolved absorption spectroscopy revealed the formation of long-lived species during the decay of the metal to ligand charge transfer excited states. The intermediates were tentatively assigned as unstable products of ligand-substitution or orthometalation excited state reactions.     

A facile synthesis of ceramides.

Lateral diffusion of phosphatide molecules in liquid crystalline bilayers has been analysed as a case of co-operative lattice diffusion. The potential energy of interaction between two molecules is assumed to arise from Van der Waals interactions of the hydrocarbon chains, and to have the form suggested by Salem [6]. From the observed values of the self-diffusion constant (of the order of 10^?8 cm² sec^?1) the depth of the potential “well” for two molecules at the equilibrium separation was estimated to have a lower limit of 1.95 kcal per mole, and the energy barrier to lateral motion was estimated to have an upper limit of 7.21 kcal per mole.     

Electronic spectra and fluorescence properties of multichromophoric sulfonylureas

The multichromophoric character of two sulfonylurea herbicides, SMT and BNS, has been investigated in its manifestations in the electronic absorption spectra and in some fluorescence properties through a combined experimental and theoretical approach. After a theoretical analysis of the most stable structures of these flexible systems, the UV absorption spectra of the two multichromophoric compounds have been analysed both experimentally and theoretically, and most transitions have been assigned to individual chromophores, also by comparison with four suitable reference compounds (5-8). Finally, some experimental information concerning the fluorescence spectra and quantum yields have been analysed with reference to the contributions from single fluorophores and the role of low-lying n → π^∗ states.     

Luminescence of bacteriorhodopsin from Halobacterium halobium and its connection with the photochemical conversions of the chromophore

Red luminescence of purple membranes from Halobacterium halobium cells in suspension, dry film or freeze-dried preparations was studied and its emission, excitation and polarization spectra are reported. The emission spectra have three bands at 665–670, 720–730 and at 780–790 nm. The position (maximum at 580 nm) and shape of the excitation spectra are close to those of the absorption spectra. The spectra depend on experimental conditions, in particular on pH of the medium. Acidification increases the long wavelength part of the emission spectra and shifts the main excitation maximum 50–60 nm to the longer wavelength side. Low-temperature light-induced changes of the absorption, emission and excitation spectra are presented. Several absorbing and emitting species of bacteriorhodopsin are responsible for the observed spectral changes. The bacteriorhodopsin photoconversion rate constant was estimated to be about 1 · 10¹¹ s^?1 at ? 196°C from the quantum yields of the luminescence (1 · 10^?3) and photoreaction (1 · 10^?1). The temperature dependence of the luminescence quantum yield points to the existence of two or three quenching processes with different activation energies. High degree of luminescence polarization (about 45–47%) throughout the absorption and fluorescence spectra and its temperature independence show that there is no energy transfer between bacteriorhodopsin molecules and no chromophore rotation during the excitation lifetime. In carotenoid-containing membranes, energy migration from the bulk of carotenoids to bacteriorhodopsin was not found either. Bacteriorhodopsin phosphorescence was not observed in the 500–1100 nm region and the emission is believed to be fluorescence by nature.     

Self-association phenomenon specific to the copper complexes with ADP,dADP, and IDP

Circular dichroism spectra between 200 and 800 nm are presented for solutions of nucleotides with pyrophosphate group derived from adenine and inosine in the presence of the Cu₂₊ cation. A study as a function of the pH, temperature, and concentration has shown the existence of a self-association, which implies a strong interaction between the bases of the two nucleotides. The interpretation of the dichroic spectra allows us to show that two oligomer species are formed successively in low concentrations in the presence of the Cu²⁺ cation. The first, constituted by Cu(ADP)^? molecules, appears at pH 5.3 and gives rise to a nonconservative excitonic term. The second, responsible for a conservative excitonic term, is a heterooligomer formed at pH 7.3 by the assembling of a Cu(ADP)^? and a Cu(ADP)(OH)^2? molecule. Furthermore, this self-association is destabilized if the nucleotide derived from adenine is replaced by the corresponding deoxyribonucleotide or by IDP.     

Changes in the circular dichroic spectrum of calf thymus solubilized chromatin caused by ultraviolet irradiation

Summary UV irradiation of the chromatin caused an increase of the positive circular dichroic band in the vicinity of 275 nm (corresponding to DNA) and a deepening of the negative band of proteins at about 225 nm. These changes in the circular dichroic spectrum are monotonous in the range of doses studied (< 6 × 10⁴ J.m^–2). The increase of the positive circular dichroic band probably reflects the occurrence of local conformational changes in DNA, which include changes in base position (tilting, distance from helix axis) in the close neighbourhood of photoproducts. The presence of photoproducts in chromatin reduces changes in its circular dichroic spectra with temperature.     

Nitrile‐Substituted QA Derivatives: New Acceptor Materials for Solution‐Processable Organic Bulk Heterojunction Solar Cells

The development of non‐fullerene‐based electron acceptors (especially organic molecules with sufficient absorption property within the solar spectrum region) for bulk‐heterojunction (BHJ) organic solar cells (OSCs) is an important issue for the achievement of high photoconversion efficiency. In this contribution, a new class of organic acceptors di‐cyan substituted quinacridone derivatives (DCN‐nCQA, n = 4, 6 and 8) for BHJ solar cells was designed and synthesized. DCN‐nCQA molecules possess facile synthesis, solution processability, visible and near‐IR light absorption and relatively stable characteristics. The DCN‐8CQA molecule exhibited a proper LUMO energy level (–4.1 eV), small bandgap (1.8 eV) and moderate electron mobility (10^?4 cm² V^?1 S^?1), suggesting that this molecule is an ideal acceptor material for the classical donor material regio‐regular poly (3‐hexylthiophene) (P3HT). A photovoltaic device with a structure of [ITO/PEDOT:PSS/P3HT:DCN‐8CQA/LiF/Al] displayed a power conversion efficiency of 1.57% and a fill factor of 57% under 100 mW cm^?2 AM 1.5G simulated solar illumination. The DCN‐nCQA molecules showed remarkable absorption in the region from 650 to 700 nm, where P3HT has a weak absorption promoting overlap with the solar spectrum and potentially improving the performance of the solar cell.     

On the mutual orientation of pigments in Photosystem I particles from green plants

The mutual orientation of pigments in Photosystem I reaction centers from spinach is evaluated by polarized photochemistry. The photoinduced linear dichroism of the absorption changes of chlorophyll a₁ at 701 nm is studied as function of the excitation wavelength. The Photosystem I reaction center particles contain about 100 and if depleted about 40 chlorophylls, respectively. To prevent their rapid Brownian rotation they were immobilized on DEAE-Sephadex.The excitation spectrum of the linear dichroism reveals a high degree of order between the long axis of β-carotene and the Q_y transition moments of those chlorophyll a molecules absorbing at the red end of the spectrum. The latter are the most endangered ones for destructive oxidation via their triplet state. Hence, the location of β-carotene in close proximity to and in parallel with these chlorophylls seems to be most favourable for the protective role of β-carotene within the antennae system I. It is observed that the dichroic ratio of the absorption changes of chlorophyll a₁ does not exceed a figure of $\text{4}\text{3}$, which characterizes a circularly degenerate system, even at far red excitation (724 nm). This will hit selectively those few chlorophyll a molecules with their peak absorption at about 700 nm (including the photooxidizable dimer). We conclude, if the dimer is the only species peaking at 700 nm then the two chlorophyll a within the dimer have their y-axes oriented perpendicular to each other. If there are some antennae in addition to the dimer, the y-axes of all chlorophyll-a peaking at 700 nm form a star which accounts for the circular degeneracy of absorption.     

The flavin dimers I. The application of absorption in anti-stokes excitation region to investigate the flavin dimer formation

The dimer formation process of the flavin in aqueous solution has been studied. The difference absorption spectra with the change of concentration in Stokes and anti-Stokes excitation region of the flavomononucleotide and riboflavin were measured. The highest temperature in which the dimers still appear is discussed. It is suggested that this temperature T_d can be treated as one of the empirical parameters which describe the dimer formation process of the dyes in solutions. The aqueous solution of flavins with the concentration c?5·10^?5 M at room temperature can be treated as a flavin monomers solution. For higher concentrations the flavin monomers and dimers exist in a solution at room temperature.     

Formation of two centre three electron bond by hydroxyl radical induced reaction of thiocoumarin: evidence from experimental and theoretical studies

Radiation chemical studies of thioesculetin (1), a thioketone derivative of coumarin, were performed by both pulse radiolysis technique and DFT calculations. Hydroxyl (^?OH) radical reaction with 1 resulted transients absorbing at 320, 360 and 500?nm. To identify the nature of the transients, the reaction was studied with specific one-electron oxidant (N₃^?) radical, where 360?nm band was absent. The transient absorption at 500?nm was concentration-dependent. The overall impression for ^?OH radical reaction was that the transient absorbing at 320, 360 and 500?nm was due to sulphur centred monomer radical, hydroxysulfuranyl and dimer radical of 1 respectively. The equilibrium constant between the monomer to dimer radical was 3.75?×?10⁴ M^?1. From the transients’ redox nature, it was observed that 57 and 24% of ^?OH radical yielded to oxidising and reducing products respectively. Further, the product analysis by HPLC suggested that the dimer radical disproportionate to esculetin and thioesculetin. DFT energy calculation for all the possible transients revealed that dimer radical has the lowest energy. The HOMO of 1 and its monomer radical suggested that the electron density was localised on the sulphur atom. The bond length between the two sulphur atoms in dimer radical was 2.88 Å which was less than the van der Waals distance. Bond order between the two sulphur atoms was 0.55, suggesting that the bond was two centre three electron (2c–3e). From TD-DFT calculation, the electronic transition of dimer radical was at 479?nm which was in close agreement with the experimental value. The nature of the electronic transition was σ → σ* from a 2c???3e bond.     

Spectroscopic studies of metal complexes containing π-delocalized sulfur ligands. The pre-resonance Raman spectra of the antitumor agent 2-formylpyridine thiosemicarbazone and its Cu(II) and Zn(II) complexes

The pre-resonance Raman spectra of 2-formylpyridine thiosemicarbazone have been measured at three pH values corresponding to the fully protonated (H₂FPT⁺), half protonated (HFPT) and deprotonated (FPT⁻) forms of the ligand. Assignments of the vibrations coupled with the π→π^* transition have been made by comparison with the spectrum of the deuterated form (DFPT). The pre-resonance Raman spectra of the Zn(II) and Cu(II) complexes, [ZnFPT]⁺, [CuFPT]⁺ and [CuHFPT]²⁺, have also been measured. The spectral pattern of the Cu(II) complexes shows resonance enhancement of vibrations coupled with the π→π^*, as well as with the ligand to metal charge transfer transitions. In addition, it is consistent with coordination through thiolate sulfur in [CuFPT]⁺ and thione sulfur in [CuHFPT]²⁺.     

Bipyrimidine-bridged rhenium(I)/rhenium(I) and ruthenium(II)/rhenium(I) complexes. Synthesis,electronic absorption and emission spectra

The binuclear complexes [Cl(OC)₃Re^I(bipym)Re^I(CO)₃Cl] (bipym=2,2′-bipyrimidine), [(bipy)₂Ru^II(bipym)Re^I(CO)₃Cl](PF₆)₂ (bipy=2,2′-bipyridine) and their mononuclear component [Re(bipym)(CO)₃Cl] were prepared. The electronic absorption spectra of these complexes display low-energy Re(I) →π*(bipym) and Ru(II)→π*(bipym) charge transfer (CT) bands. While [Re(bipym)(CO)₃Cl] shows a strong emission from its lowest CT state, the dimer [Cl(OC)₃Re(bipym)Re(CO)₃Cl] is not luminescent. The cation [(bipy)₂Ru(bipym)Re(CO)₃Cl]²⁺ emits from the lowest-energy Ru→bipym CT state. The emission behavior of the binuclear complexes is described in terms of intramolecular excited state electron or energy transfer.