Localized excitation effect on dipole moments of oligophenylenevinylenes in their excited Franck–Condon state

Polarity and strong sensitivity to external electric field in azacrown oligophenylenevinylene

Complex study of quadrupolar azacrown dye (E,E)-5,5́-Bis[2-(4-(4',7',10',13',16'-pentaoxa-1 azacyclooctadecyl)phenyl)ethenyl]-2,2́-bipyridine 1 was performed. Electronic spectra of absorption and fluorescence in different solvents exhibit strong solvatochromism. Electrooptical absorption measurements (EOAM) were performed to determine the electric dipole moments. These measurements gave large values of dipole moments in the ground μ_g and Franck-Condon excited state μ_e^FC equal to 6.8 ± 0.14C m and 39.3 ± 0.3C m, respectively. Furthermore, the results of EOAM suggest the existence two conformers in the ground state with close energies of electronic transitions. Density functional theory (DFT) calculations directly show that the shape of this molecule is not planar in the ground state and also allows the existence of two stable conformers with close energies. They appeared due to different orientations of the left and right pyridine fragments of the solute. The energies, electric dipole moments and dependences of dipole moments on the strength of applied electric field were calculated for found stable conformers of 1. DFT calculations with TD / B3LYP / 3-21G and cc-pVDZ (Time Depend) approach show that external electric field increases dramatically the dipole moments of the solute under study. The higher field intensity the larger the excited electric dipole in the range intensities from zero to ∼ 2.8·× 10 ⁹ V/m.

Diethyl [(2,5-diiodo-4-methylphenyl)methyl]phosphonate

The title compound was prepared in three steps from p-xylene. Heterodimers between nearly identical mol­ecules are connected via three hydrogen bonds from benzylic and ester methyl­ene groups to phospho­nate. The dimers form chains along the a-axis direction, stabilized by C—H⋯O bridges.

The title compound, C₁₂H₁₇I₂O₃P, was prepared in three steps from p-xylene. Heterodimers between nearly identical mol­ecules are connected via three hydrogen bonds from benzylic and ester methyl­ene groups to phospho­nate. The dimers form chains along the a-axis direction, stabilized by C—H⋯O bridges.

Solvatochromy and symmetry breaking in two quadrupolar oligophenylenevinylenes

Electrooptical absorption measurements (EOAM), solvatochromic dependences and quantum chemical simulations testify to large dipole moments change of two quadrupolar oligophenylenevinylenes upon transition to Franck-Condon excited state μ_e^FC. The values of the dipole moments μ_g and μ_e^FC are in the range [(4.2 - 4.9)10³⁰] C m and (30.8 - 47.0)10³⁰C m, respectively. The relations of dipole moments in the ground and excited states determined by EOAM correlate well with results obtained via the solvatochromic method. Calculations carried out by density functional theory (DFT) show that optimized configuration of the ground state of these molecules is not planar. The results from all methods applied unequivocally show the structural symmetry breaking in the studied compounds.

X-ray structures of precursors of styrylpyridine-derivatives used to obtain 4-((E)-2-(pyridin-2-yl)vinyl)benzamido-TEMPO: synthesis and characterization

The synthesis and characterization of the precursor isomers trans-4-(2-(pyridin-2-yl)vinylbenzaldehyde (I), trans-4-(2-(pyridin-4-yl)vinylbenzaldehyde (II), trans-4-(2-(pyridin-2-yl)vinylbenzoic acid (III) and (E)-4-(2-(pydridin-4-yl)vinylbenzoic acid (IV) are reported. These compounds were prepared in order to obtain trans-4-((E)-2-(pyridin-2-yl)vinyl)benzamide-TEMPO (V). Compounds I and II were obtained by using a Knoevenagel reaction in the absence of a condensing agent and solvent. Oxidation of the aldehyde group using the Jones reagent afforded the corresponding acid forms III and IV. A condensation reaction with 4-amino-TEMPO using oxalyl chloride/DMF/CH2Cl2 provided the 4-((E)-2-(pyridin-2-yl)vinyl)benzamide-TEMPO. Single crystals of compounds I, II and III were obtained and characterized by X-ray diffraction. Compound I belongs to space group P2(1)/c, a = 12.6674(19) Å, b = 7.2173(11) Å, c = 11.5877(14) Å, b = 97.203(13)° and the asymmetric unit was Z = 4, whereas compound II was in the space group P2(1), with a = 3.85728(9) Å, b = 10.62375(19) Å, c = 12.8625(2) Å, b = 91.722 (2)° and the asymmetric unit was Z = 2. Compound III crystallized as single colorless needle crystals, belonging to the monoclinic system with space group P2(1), with Z = 2, with a = 3.89359(7) Å, b = 17.7014(3) Å, c = 8.04530(12) Å, b = 94.4030 (16)°. All compounds were completely characterized by IR, (1)H-NMR, EI-MS and UV-Vis.

Photoinduced symmetry-breaking intramolecular charge transfer in a quadrupolar pyridinium derivative

We report here a joint experimental and theoretical study of a quadrupolar, two-branched pyridinium derivative of interest as a potential non-linear optical material. The spectral and photophysical behaviour of this symmetric system is greatly affected by the polarity of the medium. A very efficient photoinduced intramolecular charge transfer, surprisingly more efficient than in the dipolar asymmetric analogue, is found to occur by femtosecond resolved transient absorption spectroscopy. TD-DFT calculations are in excellent agreement with these experimental findings and predict large charge displacements in the molecular orbitals describing the ground state and the lowest excited singlet state. The theoretical study also revealed that in highly polar media the symmetry of the excited state is broken giving a possible explanation to the fluorescence and transient absorption spectra resembling those of the one-branched analogous compound in the same solvents. The present study may give an important insight into the excited state deactivation mechanism of cationic (donor-π-acceptor-π-donor)(+) quadrupolar compounds characterised by negative solvatochromism, which are expected to show significant two-photon absorption (TPA). Moreover, the water solubility of the investigated quadrupolar system may represent an added value in view of the most promising applications of TPA materials in biology and medicine.

(E)-N,N-Diethyl-2,6-diisopropyl-4-[2-(4-nitrophenyl)ethenyl]aniline

The title compound, C24H32N2O2, was prepared by Horner olefination of 4-diethylamino-3,5-diisopropylbenzaldehyde and diethylp-nitrobenzylphosphonate. There are two independent molecules (AandB) in the asymmetric unit. Their main axes, defined by the line connecting the N atoms of the nitro and amino groups, open an angle of 79.42 (3)°. Steric hindrance around the amino group is reflected in a long aryl C—N bond [1.434 (3) Å for moleculeAand 1.440 (3) Å for moleculeB], a pyramidal geometry [angle sum = 350.0 (2)° for moleculeAand 349.6 (2)° for moleculeB], and dihedral angles between the phenylene group and the plane defined by the CH2—N—CH2unit of 86.9 (3)° for moleculeAand 88.3 (3)° for moleculeB. This gives structural support for the electronic decoupling of the amino group from the nearly planar nitrostilbene moiety (r.m.s. deviation for C, N and O atoms = 0.097 for moleculeAand 0.107 Å for moleculeB).

Extended Stokes shift in fluorescent proteins: chromophore-protein interactions in a near-infrared TagRFP675 variant

Most GFP-like fluorescent proteins exhibit small Stokes shifts (10-45 nm) due to rigidity of the chromophore environment that excludes non-fluorescent relaxation to a ground state. An unusual near-infrared derivative of the red fluorescent protein mKate, named TagRFP675, exhibits the Stokes shift, which is 30 nm extended comparing to that of the parental protein. In physiological conditions, TagRFP675 absorbs at 598 nm and emits at 675 nm that makes it the most red-shifted protein of the GFP-like protein family. In addition, its emission maximum strongly depends on the excitation wavelength. Structures of TagRFP675 revealed the common DsRed-like chromophore, which, however, interacts with the protein matrix via an extensive network of hydrogen bonds capable of large flexibility. Based on the spectroscopic, biochemical, and structural analysis we suggest that the rearrangement of the hydrogen bond interactions between the chromophore and the protein matrix is responsible for the TagRFP675 spectral properties.

Tristriazolotriazines with π-Conjugated Segments: Star-Shaped Fluorophors and Discotic Liquid Crystals

C3-symmetrical tristriazolotriazines substituted with phenylene rings carrying lateral flexible alkoxy side chains were prepared via condensation/ring transformation of cyanuric chloride and tetrazoles. These star-shaped, planar compounds can form broad thermotropic mesophases. Due to the extensive π-conjugation, these compounds are highly emissive and the octupolar donor-acceptor electronic structure results in non-linear optical properties like solvatochromism. Brønstedt acids provoke halochromism of the absorption and of the fluorescence.

N,N-Dihexyl-4-[2-(4-nitro-phen-yl)vin-yl]aniline

The title compound, C(26)H(36)N(2)O(2), was prepared by Horner olefination of p-dihexyl-amino-benzaldehyde and diethyl p-nitro-benzyl-phospho-nate. It crystallizes with two independent mol-ecules in the asymmetric unit. Both have similar geometries of the π-systems but the conformations of all hexyl chains are different. Whereas one hexyl chain of the first mol-ecule shows the typical all-anti conformation, the second is arranged in a gauche-anti-gauche-anti conformation with N-C-C-C, C-C-C-C, C-C-C-C and C-C-C-C torsion angles of -65.1 (4), 167.3 (3), 63.3 (4), and 179.4 (3)°. One of the hexyl chains in the other mol-ecule has an anti-anti-gauche-anti conformation [N-C-C-C, C-C-C-C, C-C-C-C and C-C-C-C torsion angles = 179.6 (3), -179.8 (3), -68.7 (5) and -178.8 (4)°], the other starts with an anti-gauche-gauche sequence. Molecules A and B are composed of five planar subunits. The angle sums around the N atoms are in the range 356 (2)-360.0 (2)°. Torsion angles between these segments do not exceed 4.9 (4)°, except for one of the alkyl chains each [molecule A = 26.2 (4)°; molecule B = -6.0 (4)°]. The high planarity of the molecules and the short aniline C-N bonds [1.385 (3) Å in molecule A and 1.378 (3) Å in molecule B] indicate a strong electronic coupling through the stilbene unit. One methylene group is disordered over two positions with an occupancy ratio of 0.72:0.28.

2,5-Bis[4-(dimethyl-amino)-phen-yl]-3,6-dimethyl-pyrazine

The title compound, C₂₂H₂₆N₄, was prepared from p-dimethyl­amino­propiophenone in six steps. The mol­ecule has no crystallographic symmetry. The dihedral angles between the pyrazine ring and the phenyl rings are 35.81 (6) and 37.11 (8)°. The dimethyl­amino groups are essentially planar (sum of the bond angles at N = 359.3 and 359.9°) and nearly coplanar with the adjacent aromatic ring [dihedral angles = 5.54 (11) and 7.40 (3)°]. This effect and the short aniline C—N bonds can be rationalised in terms of charge transfer from the amino groups to the central pyrazine ring.

(E,E,E,E)-2,3,5,6-Tetra-kis{2-[4-(dimethyl-amino)-phen-yl]ethen-yl}pyrazine

In the title compound, C(44)H(48)N(6), the essentially planar mol-ecule [maximum deviation from the mean plane of the π system of 0.271 (3) Å] is located on a crystallographic centre of inversion. The almost planar (angle sums at N atoms = 357.6 and 357.1°) dimethyl-amino groups and short C-N bonds of the aniline groups [both 1.379 (4) Å] indicate strong electronic coupling between these groups and the central pyrazine ring.

Monoclinic polymorph of 2,5-bis[4-(dimethyl-amino)-styr-yl]-3,6-dimethyl-pyrazine

The title compound, C₂₆H₃₀N₄, was prepared by condensation of tetra­methyl­pyrazine and dimethyl­amino­benzaldehyde and crystallizes from chloro­form/methanol in two different forms. Block-shaped crystals belong to the monoclinic crystal system and plates to the triclinic system. The two crystal forms differ in the arrangement of the centrosymmetric mol­ecules, which have nearly identical geometries. In the monoclinic crystals reported here, planar mol­ecules [maximum deviation = 0.062 (2) Å], with a transoid arrangement of the (E)-styryl units and completely planarized dimethylamino groups [sum of the C—N bond angles = 359.9 (2)°], form layers connected via H–π-stacking. The dihedral angle between the central and pendant rings is 1.30 (8)°. The triclinic polymorph contains two half molecules, both completed by crystallographic inversion symmetry.

2,5-Bis{2,2-bis-[4-(dimethyl-amino)-phen-yl]ethen-yl}-N,N'-diphenyl-N,N'-dipropyl-benzene-1,4-diamine

The title compound, C₆₀H₆₈N₆, was prepared by Horner olefination of a terephthaldialdehyde and a diaryl­methyl phospho­nate. There is one half-mol­ecule, located on an inversion centre, in the asymmetric unit. The dihedral angle between the plane of the vinyl­ene unit and the central ring is 36.79 (15)°, while those between the vinyl­ene unit and the lateral phenyl rings are 53.04 (10) and 53.74 (9)°.

Linear and angular distyrylpyrazines with terminal donor groups: synthesis, solvatochromism, and acidochromism of the electronic spectra

A series of linear and angular distyrylpyrazines and lateral donor groups has been prepared by aldol condensation between dimethylpyrazines and the appropriate aromatic aldehyde. The optical absorption and emission properties of these systems were studied in different solvents and media. The materials display a strong solvatochromism of the emission that is reflected by large red shifts of their fluorescence emission maxima on increasing the solvent polarity. This behaviour suggests a highly polar emitting state, which is characteristic of compounds that undergo an internal charge transfer upon excitation. Upon protonation, the UV-vis spectra are altered, and the fluorescence intensity of the neutral compound vanishes. These molecules can be used as colorimetric and luminescence polarity and pH sensors.