Evgenia Vaganova

Electrostatically self-assembled poly(4-vinylpyridine-co-vinylpyridinium-chloride)-based LED

We demonstrate the fabrication and characterization of an organic light-emitting diode (OLED) device based on self-assembled poly(4-vinyl-pyridine) (P4VPy) with poly(N-vinyl-carbazole) (PVK) and 2-(4-biphenylyl)-5-(4-tert-butyphenyl)-1,3,4-oxidiazole (PBD) as transport layers. The self-assembly is based on the electrostatic attraction of oppositely-charged polymers. We have shown here that we can use this self-assembled methodology to fabricate alternating multilayers, not only by the poly(phenyl-vinylene) (PPV) and derivatives, but also by partial protonation of P4VPy, by which the charge on P4VPy is generated by the protonation process. The multilayered structures are characterized by specular X-ray reflectivity (XRR), UV absorption, photoluminescence (PL) and electroluminescence (EL).

Photoinduced Structural Changes in Poly(4-Vinyl Pyridine): A Luminescence Study

In the present work we show a way of controlling photoluminescence (PL) properties through photoinduced quasi-crystal formation in a system based on poly(4-vinyl pyridine) (P4VPy). Under UV irradiation at 380 nm, concentrated solutions of P4VPy in pyridine turn into gel. This phase transition results in changes in the optical properties of this polymer. The position of the PL maximum can be changed continuously from 440 to 480 nm during irradiation. After several minutes of UV irradiation a new red-shifted PL at 492 nm appears upon excitation by light of a wavelength corresponding to that of the initial PL maximum, which is also red-shifted during irradiation. Solutions of P4VPy in pyrimidine show similar behavior, but those in pyridazine do not exhibit such behavior. We have found that the reason for the observed changes in the electronic properties is a photoinduced directional ordering of polymer molecules in a special quasi-crystal formation. The process originates from a structural change in the side chain of P4VPy, namely, protonation of the polymeric pyridine after solvation. During irradiation, the polymeric pyridinium ion interacts with neutral polymeric pyridine molecules. Interchain interaction through hydrogen bonds lead to an electronic property change. We observed that the process of photoinduced sol-gel transformation is reversible. Mechanical perturbation or heating can convert the gel back to a fluid solution. The red-shifted PL is not observed, and the initial PL is blue-shifted to 450 nm and stays there.

Photoinduced proton transfer in a pyridine based polymer gel.

We describe an experimental and theoretical consideration of photoexcited proton transfer in a poly(4-vinyl pyridine)/pyridine gel. Evidence was found for two states of a multiple state process analyzed by DFT modeling. According to the latter, following irradiation at 385 nm, the proton donor is the CH group of the polymer main chain and the proton acceptor is the nitrogen of the polymeric pyridine side chain. Proton transfer is made possible through the assistance of a mobile pyridine solvent molecule acting as a transfer vehicle. Proton transfer promotes both a geometrical rearrangement of the vinyl side chain as well as electronic density redistribution. The photoproduct intermediate-the hydrogen-bonded complex between the protonated solvent pyridine molecule and the deprotonated polymeric pyridine side chain-is identified by its Curie law magnetic susceptibility, ESR spectrum, and fluorescence lifetime measurements. The proton transfer from the nitrogen of the solvent pyridine molecule to the pyridine side chain nitrogen, producing pyridinium, is a thermodynamically favorable relaxation process and occurs without an energy barrier. The protonation of nitrogen on the polymeric side chain was detected by solid state NMR spectroscopy performed on a (15)N-polymer enriched gel. The calculations and experimental data suggest a central role for the gel solvent molecule as a catalytic agent and proton transfer vehicle. The process suggested by DFT modeling may have relevance for photosensitive devices in part due to the fact that we have been able to show that long-lived paramagnetism may be included among the inducible properties of soft polymer gels.

Photoactive proton conductor: poly(4-vinyl pyridine) gel.

We describe a hydrogen-bonded poly(4-vinyl pyridine)-based dielectric material, in which conductivity can be induced due to the presence of side-chain protonated species that form spontaneously when the polymer is dissolved in pyridine. The conductivity of the proton conductive gel can be controlled by direct irradiation at the proton-transfer center: a reversible change of conductivity was observed in response to the on/off switching of 385 nm wavelength radiation. Over most of the range of intensities used, the proton conductivity exhibited a bimolecular character. We present a model of the protonated pyridine side-chain unit in the ground and excited states (DFT level). In the ground state, the protonated pyridine moiety has a cyclic, conjugated structure.

Tunable emission in poly(4‐vinylpyridine)‐based gel

We have discovered a new class of smart-gels, poly(4-vinylpyridine) (PVPy) swollen with pyridine (Py), that following photoexcitation (395, 455, 557 nm) emits three independent colors: blue, green, and red (477, 527, 585 nm, respectively). These excitation wavelengths also induced large reversible changes in pH and electrical conductivity. The primary photoexcitation paths within this photo-active gel are singlet-excitons. The origin of this phenomenon is attributed to the combined properties of weak coupling via hydrogen-bonding and high polarizability. Our preliminary model suggests, that C—H N based Py associates are formed during the gelation process. Photoexcitation leads to proton transfer within three different kinds of assemblies and to a solid-state-like electronic behavior of the gel. This system, i. e. a photoconducting transparent organic gel with multiple color emission, is very attractive for a wide range of promising applications in the field of optoelectronic devices such as organic LEDs and 3D holography.

Time-resolved emission upon two-photon excitation of bis-N-carbazolyl-distyrylbenzene: mapping of water molecule distribution in the mouse brain

We present a method of mapping the water molecule distribution in mouse brain tissues using injected bis-N-carbazolyl-distyrylbenzene and the FLIM technique. The fluorescence lifetime of this two-photon absorbing chromophore diminishes when the amount of water in the surrounding area increases. The fluorescence lifetime of the injected in vivo chromophore strongly depends on the content of water in different areas. Thus, lifetimes of 900 ± 50 ps in the hippocampus (extracellular fluid), 520 ± 50 ps in the lateral ventricle (choroid plexus, cerebrospinal fluid), and 400–150 ps in blood vessels were observed. Moreover, the fluorescence lifetime distribution undergoes drastic changes when mice are deprived of water. Statistical analysis of the investigated samples showed that upon water deprivation water content decreased at the border of the hippocampus/lateral ventricle areas and increased in blood vessels.

Enhanced photoemission of the complex Eu(III)/poly(4-vinyl pyridine)–pyridine photoproduct

Abstract Liquid pyridine and polymeric solutions of poly(4-vinyl pyridine)/pyridine/water doped with Europium exhibit a broad blue emission centered at 460 nm with low intensities emissions at 590 and 614 nm under excitation of 380 nm wavelength via a pyridine electronic state. Under excitation of 395 nm wavelength to the 5L6 state effective quenching of Eu(III) 5D0 emission in both cases was observed. The polymeric solution is a photosensitive material and becomes a gel under UV-irradiation. In a gel the longer-wavelength yellow-colored emission under excitation of 460–480 nm appears. The presence of Eu(III) significantly increases the intensity of this emission. The maximum of the peak position is blue shifted from 524 to 504 nm and its quantum efficiency is higher by a factor of five. We make a first effort to interpret the results.

Photoinduced Pyridine Cleavage-Closure in Viscous Polymer Solutions

We have found that photoinduced pyridine ring cleavage-closure occurs in polymeric viscous solution, because in certain polymer solutions, pyridine can serve as a photo-modulated crosslinker. We suggest this reaction as a way to control a polymers optical properties. Irradiation of the system: poly(4-vinyl pyridine)/pyridine/water with 250-nm wavelength range leads to the appearance of a new absorption band centered at 360 nm, new red-shifted emission, and HOMO-LUMO band gap changes. The subsequent irradiation with 360 nm (the new absorption band maximum) leads to reversion almost to the initial stage. A main active product of the photoreaction is aldehyde enamine, which has two active groups: primary amine and aldehyde, which can associate with the polymer molecules to form a physical crosslinked supramolecular structure. We evaluated the activation energy of the pyridine ring cleavage and back reaction depending on the polymer/pyridine/water ratio and by changing the polymer structure. The activation energy of pyridines ring cleavage in viscous polymeric solutions is in the range of 0.6–3.2 Kcal/mol. The activation energy of the back reaction is significantly lower and is in the range of 0.05–0.15Kcal/mole.

Modelling of poly(4-vinyl pyridine) and poly(4-vinyl pyridine)/pyridine composites: structural and optical properties

The interactions of the polymer poly(4-vinyl pyridine) moieties with free pyridine molecules in concentrated solution develop protonated and hydrogen-bonded species on the polymer backbone and turn the viscous solution to gel. Direct irradiation at proton transfer centre on the protonated polymer moiety promotes an amorphous-to-crystalline transition. The polymer crystals exhibit completely different optical properties when compared to the amorphous material. The proposed mechanism of the photoinduced crystallisation is the following: direct excitation to the proton transfer centre generates in abundance protonated polymer moieties, which have rigid quinone structure. Rigid quinone conformations stimulate the crystallisation of the polymer chains; in their turn, increasing polymer ordering stabilises the photoinduced protonated species. Photoinduced phase transition is reversible, meaning, that crystalline phase is metastable. To clarify the mechanism of the phase transition, in the present issue, using molecular modelling, we investigate the conformational behaviour of the polymer species depending on the state of protonation, interaction with adjacent solvent molecules and polymer side-chain units. The Density Functional Theory (DFT) calculations show the protonated pyridine moiety as a quinone structure that is clearly stable, thus emphasising the ability of such structure to play a key role as a ‘working’ species.

Effect of spontaneous diffusion in micro/nanoporous chemically crosslinked poly (N-vinyl imidazole) gel on the conformational changes of acetylcholine

Interdependent structural properties such as molecular conformation, flexibility and charge redistribution control the intermolecular interactions of acetylcholine (ACh) with adjacent molecules. This paper reports the results of an investigation of the effect of the diffusion of ACh through a nano/microporous poly (N-vinylimidazole) (PVI) gel on its structural properties, namely on changes in its conformation. To investigate the conformational changes of ACh during spontaneous diffusion through the gel, the fluorescence lifetime of the label molecule - fluorescein - was monitored. To clarify the results, analogous experiments were conducted with nicotinic acid and dopamine. In contrast to the nicotinic acid and dopamine, ACh can play the role of a regulator in molecular transport.