Tuning fluorescence properties of imidazole derivatives with thiophene and thiazole

Synthesis, photophysics and two-photon absorption of imidazole-centred tripodal chromophores

Tripodal push-pull chromophores with D-(π-A)3 arrangement were synthesized using 1-methyl-2,4,5-triphenyl-1H-imidazole as a central electron donor, and their thermal, electrochemical, photophysical and non-linear optical properties were studied and corroborated with quantum-chemical calculations. Their facile synthesis involved Suzuki-Miyaura and Knoevenagel reactions, allowing the installation of various peripheral electron acceptors such as formyl, cyano, ester, trifluoromethyl and more complex moieties such as malonic/acetic acid derivatives, indan-1,3-dione and rhodanine. All phenyl rings appended at the central imidazole core were more or less twisted depending on the peripheral substitution. Although imidazole undergoes reversible one-electron oxidation, peripheral acceptors are reduced irreversibly in a multi-electron process. This behaviour is further seen as a variation of the LUMO, while the HOMO remained almost unaltered across the whole series. TD-DFT calculations revealed centrifugal charge transfer from the central imidazole to all C2, C4 and C5 branches occupied by the LUMO, LUMO+1 and LUMO+2. The HOMO-LUMO gap is tuneable within the range of 3.55-2.31 eV, while the longest-wavelength absorption/emission maxima were found within the broad range of 304-448/393-612 nm. Although the absorption spectra are solvent-independent, the emission depends strongly on the solvent polarity and the electron-withdrawing power of the peripheral acceptors. Extended chromophores with complex electron acceptors were investigated as two-photon absorbers, revealing relatively good cross-section values of up to 521 GM and a figure-of-merit (ΦF × δ2PA) of around 190 GM.

Fluorescence Amplification of Unsaturated Oxazolones Using Palladium: Photophysical and Computational Studies

Weakly fluorescent (Z)-4-arylidene-5-(4H)-oxazolones (1), Φ_PL < 0.1%, containing a variety of conjugated aromatic fragments and/or charged arylidene moieties, have been orthopalladated by reaction with Pd(OAc)₂. The resulting dinuclear complexes (2) have the oxazolone ligands bonded as a C^N-chelate, restricting intramolecular motions involving the oxazolone. From 2, a variety of mononuclear derivatives, such as [Pd(C^N-oxazolone)(O₂CCF₃)(py)] (3), [Pd(C^N-oxazolone)(py)₂](ClO₄) (4), [Pd(C^N-oxazolone)(Cl)(py)] (5), and [Pd(C^N-oxazolone)(X)(NHC)] (6, 7), have been prepared and fully characterized. Most of complexes 3-6 are strongly fluorescent in solution in the range of wavelengths from green to yellow, with values of Φ_PL up to 28% (4h), which are among the highest values of quantum yield ever reported for organometallic Pd complexes with bidentate ligands. This means that the introduction of the Pd in the oxazolone scaffold produces in some cases an amplification of the fluorescence of several orders of magnitude from the free ligand 1 to complexes 3-6. Systematic variations of the substituents of the oxazolones and the ancillary ligands show that the wavelength of emission is tuned by the nature of the oxazolone, while the quantum yield is deeply influenced by the change of ligands. TD-DFT studies of complexes 3-6 show a direct correlation between the participation of the Pd orbitals in the HOMO and the loss of emission through non-radiative pathways. This model allows the understanding of the amplification of the fluorescence and the future rational design of new organopalladium systems with improved properties.

V-shaped fluorophores with a 1-methyl-4,5-bis(arylethynyl)imidazole skeleton displaying solid-state fluorescence, acid responsiveness, and remarkable fluorescence solvatochromism

V-Shaped fluorophores with a 1-methyl-4,5-bis(arylethynyl)imidazole skeleton displayed both solid-state and solution fluorescence, which remarkably changed on exposure to acid vapor.

Study of Compression-Induced Supramolecular Nanostructures of an Imidazole Derivative by Langmuir-Blodgett Technique

In this communication, we report the design and synthesis as well as the supramolecular assembly behavior of a 2,4,5-triaryl imidazole derivative (compound 1) at the air-water interface and in thin films using Langmuir-Blodgett (LB) technique. The main idea for such a chemical structure is that the long alkyl chain and N-H of the imidazole core may help to form supramolecular architecture through the hydrophobic-hydrophobic interaction and hydrogen bonding, respectively. Accordingly, the interfacial behavior as well as morphology of 1 in thin films were studied through a series of characterization methods such as surface pressure-area (π-A) isotherm, hysteresis analysis, ultraviolet-visible (UV-vis) absorption and steady-state fluorescence spectroscopies, Fourier transform infrared, X-ray diffraction, Brewster angle microscopy (BAM), and atomic force microscopy (AFM) measurements, and so forth. Pressure-area isotherm is an indication toward the formation of supramolecular nanostructures instead of an ideal monolayer at the air-water interface. This has been confirmed by the hysteresis analysis and BAM measurement at the air-water interface. AFM images of 1 in the LB monolayer exhibits the formation of supramolecular nanowires as well as nanorods. By controlling different film-forming parameters, it becomes possible to manipulate these nanostructures. With the passage of time, the nanowires come close to each other and become straight. Similarly, nanorods come close to each other and form bundles of several rods in the LB films. H-bonding, J-aggregation, as well as compression during film formation might play a key role in the formation of such nanostructures. Electrical switching behavior of compound 1 was also observed because of the presence of an electron donor-acceptor system in 1. This type of organic switching behavior may be promising for next-generation organic electronics.

Alphabet-Inspired Design of (Hetero)Aromatic Push-Pull Chromophores

Push-pull molecules represent a unique and fascinating class of organic π-conjugated materials. Herein, we provide a summary of their recent extraordinary design inspired by letters of the alphabet, especially focusing on H-, L-, T-, V-, X-, and Y-shaped molecules. Representative structures from each class were presented and their fundamental properties and prospective applications were discussed. In particular, emphasis is given to molecules recently prepared in our laboratory with T-, X-, and Y-shaped arrangements based on indan-1,3-dione, benzene, pyridine, pyrazine, imidazole, and triphenylamine. These push-pull molecules turned out to be very efficient charge-transfer chromophores with tunable properties suitable for second-order nonlinear optics, two-photon absorption, reversible pH-induced and photochromic switching, photocatalysis, and intercalation.

Self-assembly of sodium 4-(4,5-diphenyl-1H-imidazol-2-yl)benzoate into ultralong microbelts

Ultralong SDB microbelts with interesting optical and electrical properties were successfully fabricated by the poor solvent mediated self-assembly method.

Characterization, physiochemical and computational studies of the newly synthesized novel imidazole derivative

The photophysical properties of imidazole derivative were studied in several solvents. The observed fluorescence quantum yield is attributed to a loss of planarity in the excited state provided by the non co-planarity of the aryl rings attached to C(2) and N(1) atoms of the imidazole ring. The solvent effect on the absorption and fluorescence bands was analyzed by a multi-component linear regression in which several solvent parameters were analyzed simultaneously. DFT calculations were carried out in order to find out the NBO analysis, HOMO-LUMO energies, MEP studies and hyperpolarisability behaviour. This chromophore possess more appropriate ratio of off-diagonal versus diagonal β tensorial component (r=β(xyy)/β(xxx)=0.12) which reflects the in plane non-linearity anisotropy. Since they have largest μβ(0) value, the reported imidazole can be used as potential NLO material. Within this context, reasonable conclusions concerning the steric hindrance in the chromospheres, push-pull character, hyperpolarisability of the imidazole and their application as NLO materials will be drawn.

Asymmetric-based novel poly(ether-imide)s: Preparation, properties, kinetics of thermal decomposition, and study electrochemical oxidation behavior by using nanocomposite paste of polymer and multi-walled carbon nanotube

A series of novel fluorescent poly(ether-imide)s (PEIs) bearing ether linkage and diarylimidazole pendant in the backbone was successfully prepared by polycondensation reaction between a new unsymmetrical diamine and commercially available tetracarboxylic acid dianhydrides. The PEIs were characterized by Fourier transform infrared, 1H-NMR, UV-vis spectroscopy, fluorimetry, scanning electron microscopy and elemental analysis. The PEIs with inherent viscosities in the range of 0.47–0.63 dL g−1 showed excellent solubility in various aprotic solvents such as N-methylpyrrolidone, dimethyl sulfoxide, N, N-dimethylacetamide and N,N-dimethylformamide. Thermal properties of new PEIs were evaluated by differential scanning calorimetric and thermogravimetric analysis in N2 and air. These polymers showed glass transition temperatures ( Tg) in the range of 269–333 °C and 10% weight loss temperatures above 400 °C in air atmosphere. The data obtained from TGA experiments of these polymers have been applied in different kinetic equations such as Coats–Redfern, Kissinger and Criado methods, to determine the kinetic parameters ( E, ▵ S, ▵ G, ▵ H) and mechanism of thermal decomposition. Herein a method based on the adsorptive stripping cyclic voltammetry technique to investigate electrochemical oxidation of these polymers by using multi-walled carbon nano tubes modified glass electrode is also reported.

Imidazole as a parent π-conjugated backbone in charge-transfer chromophores

Research activities in the field of imidazole-derived push-pull systems featuring intramolecular charge transfer (ICT) are reviewed. Design, synthetic pathways, linear and nonlinear optical properties, electrochemistry, structure-property relationships, and the prospective application of such D-π-A organic materials are described. This review focuses on Y-shaped imidazoles, bi- and diimidazoles, benzimidazoles, bis(benzimidazoles), imidazole-4,5-dicarbonitriles, and imidazole-derived chromophores chemically bound to a polymer chain.

Synthesis and characterization of a novel high luminescent gold-2-mercapto-1-methyl-imidazole complex

Synthesis and characterization of a new gold-2-mercapto-1-methyl imidazole are reported. This new organic material shows an extraordinary fluorescence activity (superfluorescence) up to 220°C with an unusual quantum yield of 0.2. Both fluorescence and NMR spectroscopy were applied to understand the behavior of the gold-2-mercapto-1-methylimidazole complex. Results suggest that the superfluorescence activity can be attributed to the shrinking of the HOMO-LUMO band gap energy following complexation of the organic imidazole system with gold.

Determination Co2+ in vitamin B12 based on enhancement of 2-(4-substituted-phenyl)-4,5-di(2-furyl) imidazole and H2O2 chemiluminescence reaction

In this paper, three kinds of imidazole derivatives, 2-(4-methylphenyl)-4,5-di(2-furyl) imidazole (MDFI), 2-(4-nitrophenyl)-4,5-di(2-furyl) imidazole (NDFI), and 2-(4-tert-butylphenyl)-4,5-di(2-furyl) imidazole (t-BDFI) were synthesized. In an alkaline medium, the chemiluminescence (CL) reaction of imidazole derivatives with H(2)O(2) has been investigated. It was also found that MDFI/H(2)O(2) and t-BDFI/H(2)O(2) systems gave strong CL. When Co(2+) was added into the two CL systems, the CL intensity was remarkably enhanced. In the optimum conditions, the CL intensity is linearly related to the logarithm of concentration of Co(2+). The linear ranges are 5×10(-9)-2.5×10(-7) mol/L for MDFI/H(2)O(2) system and 5×10(-9)-2.5×10(-7) mol/L for t-BDFI/H(2)O(2) system, and the corresponding detection limits are 1.2×10(-9) mol/L and 1.1×10(-9) mol/L, respectively. The method was applied to the determination of Co(2+) in vitamin B(12) injection. Furthermore, the CL mechanism was also discussed.

Physiochemical properties of organic nonlinear optical crystal from combined experimental and theoretical studies

2-(4-fluorophenyl)-1-(4-methoxyphenyl)-4,5-dimethyl-1H-imidazole (fpmpdi) was synthesised and its structure was characterized by 1H, 13C, mass, elemental analysis and X-ray techniques. It belongs to monoclinic crystal system having the lattice dimensions of monoclinic with point group, P21/c; a=8.5132 (1) Å, b=9.5128 (2) Å, c=19.2610 (3) Å, β=96.798 (2). Since (fpmpdi) is highly sensitive to different external stimulations such as UV irradiation, heat, increasing pressure and changing the environmental pH causing color change and so they can be used as a "multi-way" optically switchable material. A prominent fluorescence enhancement was found in the presence of transition metal ions such as Hg2+, Pb2+ and Cu2+ and this was suggested to result from the suppression of radiationless transitions from the n-π* state in the chemosensors. The optimized geometrical parameters obtained by DFT calculation is in good agreement with single crystal XRD data. The Mulliken, NBO charge analysis and the HOMO-LUMO energies were also calculated. The molecular electrostatic potential surface (MEP) and first static hyperpolarisability have also been employed theoretically which reveal that the fpmpdi have non-linear optical (NLO) behavior with non-zero values. Ground and excited states DFT calculations were carried out in order to find out dipole moment and energy.

Synthesis and characterization of the ground and excited states of tripodal-like oligothienyl-imidazoles

Six new thiophene oligomers, here designated as tripodal-like oligothienyl-imidazoles, were synthesized and have been investigated in ethanol solution at room and low temperature. The oligomers bear a common core where two or more thiophenes are linked to one or more imidazole units that further links through its alpha-position to a different number of incremental thiophene units. The study involves a comprehensive spectral and photophysical investigation where the properties of the singlet and triplet states have been investigated regarding absorption, fluorescence and phosphorescence, transient triplet-triplet absorption together with all relevant quantum yields (fluorescence, phi(F), internal conversion, phi(IC), intersystem crossing, phi(T,) and singlet oxygen, phi(Delta)) and lifetimes. In addition, density functional theory quantum chemical calculations were performed to gain a detailed understanding of the molecular geometry and optical properties of the investigated oligomers. From the overall data, the radiative (k(F)) and radiationless (k(NR), k(IC), and k(ISC)) rate constants have been determined and it is shown that, in contrast with the parent oligothiophenes, the radiative competes with the radiationless deactivation channels. The results show that, by comparison with the oligothiophene counterparts, there is an augment of the relative contributions of the internal conversion and fluorescence processes relative to the S(1)~~-->T(1) intersystem crossing. Phosphosphorescence emission was found for the simplest member of the investigated compounds with a low quantum yield (phi(Ph) = 0.009) and a lifetime of 8 micros. The data also show that the introduction of a 4,5-dithienyl-imidazole moiety in a bi- or terthiophene oligomer results in, respectively, a 20-fold and a 3-fold increase of the fluorescence quantum yield relative to their oligothiophene counterparts. The synergy of the structural and photophysical properties, combined with the exceptional thermal stabilities, opens new perspectives related to the copolymerization of oligothiophenes with thienyl-imidazole units with potential application as organic light emitting devices.