Decay of the fluorescence anisotropy from isolated stilbene molecules

The fluorescence anisotropy of trans-stilbene molecules, free from collisions, decays on the picosecond timescale. This result is direct evidence for the occurrence of extensive vibration-rotation interactions in a spectral regime where no spectral structure is observable. The anisotropy at time zero of 0.1 ± 0.01 decays in approximately 18 ps to a value of 0.05 ± 0.01. We suggest this is caused in part by redistribution of the initially created vibronic excitation. The calculated value of the anisotropy for stilbene, considered a regular rotor, is 0.078 - significantly less than the measured value.     

Ortho-substituent effect on fluorescence and electroluminescence of arylamino-substituted coumarin and stilbene

[structure: see text] Newly synthesized arylamino-substituted coumarins and stilbenes show enhanced fluorescence emission (bluer and brighter) both in solution and in solid film. Pure blue efficient electroluminescence with 2.7% and 4.1% of external quantum efficiency can be achieved with MeC1 and XTPS, respectively.     

Synthesis, characterization and fluorescence study of new polyacrylates containing stilbene

A new urethane acrylic monomer with stilbene in its structure, trans-4-(2-methacryloyloxyethylcarbamoyloxymethyl)stilbene (SUM), was synthesized to be further free radically copolymerized with methyl methacrylate (MMA). The structures of SUM and the resulting copolymer, trans-4-(2-methacryloyloxyethylcarbamoyloxymethyl)stilbene-co-methyl methacrylate (SUMMA) were characterized by ¹H NMR, UV-vis and FTIR spectroscopy, differential scanning calorimetry (DSC), TGA and fluorescence spectroscopy. From the ¹H NMR spectrum of the formed copolymer, a composition of about 1:2.6 (SUM/MMA) was determined. The photochemical behavior of the stilbene moieties in polymer was investigated in solution (DMF) and in film state comparatively to that of SUM, following the decreasing under UV irradiation of the π-π∗ absorption band characteristic to the trans-isomer in the UV spectra. Morphologic changes in the surface of the polymeric film during the photoisomerization were visualized by means of atomic force microscopy (AFM) and it is proposed that the new formed cone-shaped structures from the irradiated surface to be attributed to J-aggregates. Analysis of fluorescence shows that the stilbene derivatives showed both monomer emission at 358 (SUM) and 357 nm (SUMMA), and excimer (aggregate) emission at 391 and 412 nm (SUM), and at 374 and 398 nm (SUMMA) in solution. Quenching of the active species (SUMMA) by aliphatic amines shows that this process involves an electron transfer from the ground-state amine to excited stilbene, more efficiently being triethylamine.     

Dual fluorescence of Methylene Blue encapsulated in silica matrix

Dynamics related to the optical effect of Methylene Blue (MB) dye molecules in solid host has been investigated. MB doped silica samples of varying concentrations are prepared by the sol–gel technique in acidic environment. Absorption and laser induced fluorescence spectra have been recorded for the samples. Interesting result of dual fluorescence at lower concentrations is explained. Attempt is done to study the dynamics of the dual fluorescence. These results may be useful for designing and developing solid-state optoelectronic devices.     

Spectral modulation of charge transfer fluorescence probe encapsulated inside aqueous and non-aqueous β-cyclodextrin nanocavities

In this work, we report the spectral modulations of a intramolecular charge transfer (ICT) molecule ethyl ester of N,N-dimethylaminonaphthyl-(acrylic)-acid (EDMANA) when encapsulated in the water and N,N-dimethylformamide (DMF) solution of β-CD nanocavities. From the nature of the Benesi-Hildebrand (B-H) plots, the stoichiometry of the host guest inclusion complexes are found to be 1:1 in water β-CD solution and both 1:1 and 1:2 in DMF β-CD solution. The preferential location and difference in orientation of EDMANA molecule inside the β-CD cavity has been accessed by analysis of the effect of acid and metal cation Ni²⁺ on the spectral characteristics in both the media. In case of 1:1 complex, the polar donor group prefers to expose to bulk aqueous phase capable of binding with H⁺ and Ni²⁺ ions and the acceptor to the hydrophobic interior. On the other hand, the acceptor group remains exposed to the non-polar bulk phase and the donor group is orientated preferentially inside the non-polar core in 1:2 inclusion complexes.     

Conformations of cycloundecane

Cycloundecane (1) was shown to exist at -183.1 degrees C as a mixture of the [12323] (approximately 59%) and [335] (approximately 41%) conformations. Populations were determined from the (13)C NMR spectrum, and assignments were based on the (13)C spectra, calculated free energies and chemical shifts, and information from the literature, including X-ray studies of solid derivatives and calculated barriers.     

Conformations of Biopolymers

Discussion of the history of biopolymers is focused on proteins and polypeptides. Rubber elasticity is discussed from the early days onward, when the first and second laws of thermodynamics were established. Insight in the elasticity of elastin, an amorphous protein occurring in ligaments and arteries, is followed against this background. Denatured proteins also fit in this category. At present, the random-coil state that underlies the elasticity is rather well understood as a result of the new methods of analyzing the dimensions in terms of the conformations of the residues. Subsequently, the discovery of the α-helix, as well as that of the other helical structures of DNA and collagen, is described. The conversion to random coils is followed with emphasis on our insight into the cooperative nature of the transition. Finally, the least understood globular proteins are considered. Major progress was made with the successful analysis of x-ray patterns. The native state is characterized by closely packed residues in complicated, but unique, patterns. The conversion to random coils (denaturation) is sharp, not unlike a phase transition. Although the native state is rather closely packed, some mobility still exists. The implication of this mobility for enzymatic action is hinted at.     

Conformations of antipyrines

Reliable conformational energetics is essential in interpreting and predicting structures of molecular crystals. We provide a combined density functional theory (DFT)-structural database study, demonstrating that this combination can be used as a foundation for this purpose. A subtle problem of nitrogen pyramidalization is used as the example in antipyrines, a group of bioactive molecules. Nitrogen pyramidalization on the two adjacent sp(3) nitrogens directly affects the orientation of the methyl and phenyl substituents, which tend toward opposite sides of the heteroaromatic ring, affecting crystal packing. Accordingly, the overwhelming majority of the structures of antipyrines in the Cambridge Structural Database (CSD) are either nearly planar or have substituents on the opposite sides of the ring. Recent powder X-ray structures by Lemmerer et al. identified propyphenazone, an antipyrine, to have two substituents on the same side in an apparently sterically crowded conformation. We show that the new structure, although counterintuitive, is not an outlier on the conformational map. A distribution of the conformations of all antipyrines listed in the CSD is in good agreement with the computed conformational map. We also examine the role of the hysteretic property of the phenyl torsion in propyphenazone and its indirect effects on its overall conformation.     

Conformations of cyclobutane

The literature concerning the structures of compounds containing saturated, 4-carbon rings is reviewed critically, and the variety of conformations (dihedral angles of 0° to 30° ± 6°) of the cyclobutane ring are tabulated and discussed.     

Water-soluble stilbene dendrimers

The third generation of novel photo-responsive water-soluble stilbene dendrimer (trans- and cis-G3 WSD) undergoes unusual one-way trans-to-cis isomerization to give 100% of cis isomer at the photostationary state on UV irradiation in water.     

The photo-isomerization of stilbene

The S₀ potential of stilbene has minima in the trans (t) and cis (c) configurations and a maximum in the perp (p) configuration. The S₁ potential has minima at t, p and c due to crossing of the ¹B_u and ¹A_g potentials, and it is proposed that the T₁ potential has similar minima due to crossing of the ³B_u and ³A_g potentials. Five alternative isomerization mechanisms are considered, the yields of which depend on competition between vertical radiative and radiationless transitions to lower states, and horizontal radiationless transitions between the t, p and c configurations of S₁ and T₁.     

Parametric method in the theory of vibronic spectra of complex molecules. Excited state structure and fluorescence spectrum of stilbene

The parameters of the adiabatic model of the stilbene molecule in the excited state and the vibrational structure of the fluorescence spectrum are calculated by the parametric method of the theory of vibronic spectra of complex molecules. The first and second orders of theory are used. The molecular models are constructed by the fragment method. Satisfactory agreement with experiment is obtained. The parameters of molecular fragments are shown to be highly transferable. The model and the spectrum max be refined in the second order of theory using only one additional parameter for CCC angles (optimal values were obtained). The parametric method under discussion makes it possible to predict variations of structural parameters due to excitation and to calculate the vibrational structure of the electronic spectra of complex molecules, including characteristic variations of the spectra in series of related molecules (stilbene-di-phenylpolyenes). This method surpasses all previous procedures (in particular, the correlation method) in completeness and accuracy of results. Translated fromZhurnal Strukturnoi Khinii, Vol. 41, No. 2, pp. 369–378, March–April, 2000.     

Excited-state behavior of trans and cis isomers of stilbene and stiff stilbene: a TD-DFT study

The first part of the isomerization path on the two lowest excited states of trans and cis isomers of stilbene and stiff stilbene is investigated by means of TD-PBE0 calculations in the gas phase and in heptane solution. Solvent effects are taken into account by the PCM model. The excited-state optimized structures and the computed absorption and emission frequencies are in good agreement with the available experimental results. In all of the examined compounds, the isomerization process before barrier crossing occurs on the HOMO --> LUMO bright state, whereas the role played by other single-excitation states appears negligible. The relative energy barriers on the isomerization paths are consistent with the experimental excited-state lifetimes, suggesting a unifying picture of the isomerization process in stilbene-like molecules.     

Conformations and vibrational spectra of methyl-pyrazines

Molecular geometries of 2-methyl as well as 2–3-, 2–5- and 2–6-dimethyl-pyrazine have been optimized in the MINDO/3 approximation. Potential energy barriers concerning internal rotation of the methyl groups have been computed also for each molecule and relevant periodic functions have been adjusted to fit the respective results. Finally, the theoretical spectrum for the hindered rotation in 2-methyl-pyrazine has been calculated too.     

Conformations of hydrophobic polyelectrolytes

Recent studies on the (hypercoiled) compact conformation and the pH-induced conformational transition to the extended coil of the hydrophobic poly(acids or bases), which have been well used in industrial and medical circles, are summarized, especially on the basis of our studies on poly(alpha-alkyl acrylic acids) and maleic acid copolymers with hydrophobic aromatic monomers. The stability of the compact conformation and values of various thermodynamic parameters of the transition of the hydrophobic polyelectrolytes suggest the importance of hydrophobic interaction among the side chains to form the compact conformation. Characterization of various physicochemical methods, including 1H-NMR measurements, gives a model of the compact conformation with a more expanded structure than in globular proteins, a free-draining property of the solvent, the conformation fluctuation, etc. Also, the apparent two-state character of each segment in the transition region, a life-time in each state and the cooperative parameter for the compact form formation are discussed. A review of our conformational studies on the hydrophobic-hydrophilic polypeptides is also given. For the anion-induced conformation transition in basic homopoly(alpha-amino acids), hydrophobic characters of the anion and the side chain in the polypeptide are shown to be important, especially in terms of solvation. The difference in the induced conformation by the anion between random and alternating basic copolypeptides is explained by introducing a model which shows the importance of the sequence of hydrophobic and hydrophilic residues in the polypeptide to induce the ordered conformation of the anions. Also, we attempt to explain the difference between the induction of ordered conformation in the basic homo- or copolypeptide in reversed micelles with a large sulfonate (AOT) vs. that in aqueous AOT in terms of the hydrophobic character of the side chains in the polypeptides, AOT and the medium.     

Conformations of hydrophobic polyelectrolytes

Recent studies on the (hypercoiled) compact conformation and the pH-induced conformational transition to the extended coil of the hydrophobic poly(acids or bases), which have been well used in industrial and medical circles, are summarized, especially on the basis of our studies on poly(α-alkyl acrylic acids) and maleic acid copolymers with hydrophobic aromatic monomers. The stability of the compact conformation and values of various thermodynamic parameters of the transition of the hydrophobic polyelectrolytes suggest the importance of hydrophobic interaction among the side chains to form the compact conformation. Characterization of various physicochemical methods, including ¹H-NMR measurements, gives a model of the compact conformation with a more expanded structure than in globular proteins, a free-draining property of the solvent, the conformation fluctuation, etc. Also, the apparent two-state character of each segment in the transition region, a life-time in each state and the cooperative parameter for the compact form formation are discussed.A review of our conformational studies on the hydrophobic-hydrophilic polypeptides is also given. For the anion-induced conformation transition in basic homopoly(α-amino acids), hydrophobic characters of the anion and the side chain in the polypeptide are shown to be important, especially in terms of solvation. The difference in the induced conformation by the anion between random and alternating basic copolypeptides is explained by introducing a model which shows the importance of the sequence of hydrophobic and hydrophilic residues in the polypeptide to induce the ordered conformation of the anions. Also, we attempt to explain the difference between the induction of ordered conformation in the basic homo- or copolypeptide in reversed micelles with a large sulfonate (AOT) vs. that in aqueous AOT in terms of the hydrophobic character of the side chains in the polypeptides, AOT and the medium.