Efficient Excited‐State Symmetry Breaking in a Cationic Quadrupolar System Bearing Diphenylamino Donors

We report a joint experimental and theoretical investigation of a quadrupolar D–π–A⁺–π–D system, the electron donors being diphenylamino groups and the electron acceptor being a methylpyridinium, in comparison with the dipolar D–π–A⁺ system. The emission spectra of the two compounds overlap in all the investigated solvents. This finding could be rationalized by TD‐DFT calculations: the LUMO–HOMO molecular orbitals involved in the emission transition are localized on the same branch of the quadrupolar structure that becomes the fluorescent portion, corresponding to that of the single‐arm compound. Excited‐state symmetry breaking has been rarely observed for quadrupolar systems showing negative solvatochromism and is here surprisingly revealed, even in low polarity solvents. Femtosecond transient absorption measurements revealed that an efficient photoinduced intramolecular charge transfer takes place in the quadrupolar chromophore, more efficient than in its dipolar analogue. This result is promising in view of the application of these compounds as novel two‐photon absorbing materials.     

Mechanical Behavior of Polymer Gels for RDCs and RCSAs Collection: NMR Imaging Study of Buckling Phenomena

Anisotropic NMR parameters, such as residual dipolar couplings (RDCs), residual chemical shift anisotropies (RCSAs) and residual quadrupolar couplings (RQCs or Δν_Q), appear in solution‐state NMR when the molecules under study are subjected to a degree of order. The tunable alignment by reversible compression/relaxation of gels (PMMA and p‐HEMA) is an easy, user‐friendly, and very affordable method to measure them. When using this method, a fraction of isotropic NMR signals is observed in the NMR spectra, even at a maximum degree of compression. To explain the origin of these isotropic signals we decided to investigate their physical location inside the NMR tube using deuterium 1D imaging and MRI micro‐imaging experiments. It was observed that after a certain degree of compression the gels start to buckle and they generate pockets of isotropic solvent, which are never eliminated. The amount of buckling depends on the amount of cross‐linker and the length of the gel.     

Effect of solvent on absorption and fluorescence spectra of a typical fluorinated azo dye for its acidic and basic structures

The effect of 15 polar solvents on absorption and fluorescence energies of a typical fluorinated azo dye, 4-(2,3,5,6-tetrafluoro-pyridin-4-yl azo)-phenol, was reported for its acidic, MH, and basic, M, structures.For MH, the absorption energy is described on the basis of multi-linear equation with Taft's π* (solvent polarity) and β (hydrogen bond acceptor) parameters while the fluorescence energy varies rectilinearly with free energy of transferring the proton to the surrounding solvent, ΔG_t°.For M, the hydrogen bonding donor ability of protic solvent, α, is a predominant factor which affects the absorption energy while in aprotic solvents, the absorption energy correlates linearly with Kirkwood function. As the ability of the solvent for hydrogen bonding increases, the absorption band width will increase in parallel with the transition energy.     

Novel Metal Complexes Unit of Phenanthroline Derivative for Being Used as Auxiliary Electron Acceptor in Dye Sensitizer Molecules

Donor-acceptor-π bridge-acceptor (D−A−π−A) motif dyes are promising dye sensitizers in dye-sensitized solar cells (DSSCs). In this study, to strengthen with-drawing electron force of the auxiliary electron acceptors(A) in D−A−π−A motif dye sensitizers, the metal complexes unit is be used as auxiliary electron acceptor(A) instead of organic electron-withdrawing monomer. The four polymeric metal complexes were designed, synthesized, and characterized, which used metal complexes of phenanthroline derivatives as auxiliary acceptors (A), benzodithiophene-dithiophene derivatives (BDTT) as donors (D), and 8-hydroxyquinoline derivatives as π-bridges and acceptors of the dye sensitizers, and have been used for dye sensitizers. Under AM 1.5 G (100 mW cm⁻²), the photovoltaic test results indicated that the short-circuit photocurrent density (J_sc) of the DSSCs based four polymeric metal complexes are 11.26, 13.68, 14.42 and 15.57 mA cm⁻² and power conversion efficiency (PCE) are 5.96 %, 7.83 %, 8.07 %, 9.28 % respectively. Both J_sc and PCE value of the four polymeric metal complexes increased in order. This may be due to the fact that larger radius of metal ion under the same change number can enhance the coordination bond and cause stronger electron-withdrawing ability of auxiliary acceptor and stronger charge-transfer ability between the donor and the acceptor, which results in higher J_sc and higher PCE of the polymeric complex dye sensitizer.     

Fluorene as the π-spacer for new two-photon absorption chromophores

We report herein the design and synthesis of two new quadrupolar D-π-A-π-D chromophores containing diphenyl amine and dicyanobenzene or 2,1,3-benzothiadiazole as electron donor (D) and acceptor (A), respectively, which are bridged by fluorene linkage (π). The introduction of coplanar fluorenes is highly beneficial for the enhancement of two-photon absorption (TPA), where 1b displays a TPA cross section (σ₂) of up to 1975±207 GM.     

Thiazolo[4,5-d]thiazole—a new domain for potential optoelectronic application

A novel heterocyclic moiety based on thiazolo[4,5-d]thiazole was prepared via a six-step synthesis from butane-2,3-dione. This thiazolothiazole compound was further derivatized to afford either neutral conjugated condensation products or analogous N-alkyl salts. The obtained derivatives represent dipolar structures expressed as A-π-D (push-pull compounds) as well as quadrupolar D-π-A-π-D structures. Both types of compound show intramolecular charge transfer and therefore could be suitable for nonlinear optical applications.     

Quadrupolar Ultrafast Charge Transfer in Diaminoazobenzene-Bridged Perylenediimide Triads

Strong push-pull interactions between electron donor, diaminoazobenzene (azo), and an electron acceptor, perylenediimide (PDI), entities in the newly synthesized A−D−A type triads (A=electron acceptor and D=electron donor) and the corresponding A−D dyads are shown to reveal wide-band absorption covering the entire visible spectrum. Electrochemical studies revealed the facile reduction of PDI and relatively easier oxidation of diaminoazobenzene in the dyads and triads. Charge transfer reversal using fluorescence-spectroelectrochemistry wherein the PDI fluorescence recovery upon one-electron oxidation, deterring the charge-transfer interactions, was possible to accomplish. The charge transfer state density difference and the frontier orbitals from the DFT calculations established the electron-deficient PDI to be an electron acceptor and diaminoazobenzene to be an electron donor resulting in energetically closely positioned PDI ^δ− -Azo ^δ+ -PDI ^δ− quadrupolar charge-transfer states in the case of triads and Azo ^δ+ -PDI ^δ− dipolar charge-transfer states in the case of dyads. Subsequent femtosecond transient absorption spectral studies unequivocally proved the occurrence of excited-state charge transfer in these dyads and triads in benzonitrile wherein the calculated forward charge transfer rate constants, k_f, were limited to instrument response factor, meaning >10¹² s⁻¹ revealing the occurrence of ultrafast photo-events. The charge recombination rate constant, k_r, was found to depend on the type of donor-acceptor conjugates, that is, it was possible to establish faster k_r in the case of triads (∼10¹¹ s⁻¹) compared to dyads (∼10¹⁰ s⁻¹). Modulating both ground and excited-state properties of PDI with the help of strong quadrupolar and dipolar charge transfer and witnessing ultrafast charge transfer events in the studied triads and dyads is borne out from the present study.     

A Potential Carcinogenic Pyrene Derivative under Förster Resonance Energy Transfer to Various Energy Acceptors in Nanoscopic Environments

Picosecond‐resolved Förster resonance energy transfer (FRET) from various vibronic bands in benzo[a]pyrene (BP) shows a strong dependency on the spectral overlap of an energy acceptor in a confined environment. Our study on the dipolar interactions between BP and different acceptors, including ethidium (Et), acridine orange (AO), and crystal violet (CV), at the surface of a model anionic micelle revealed that the Förster distance (R₀) and the rate of energy transfer is dependent on the individual spectral overlap of the vibronic bands of BP with the absorption spectra of the different energy acceptors. The differential behavior of the vibronic bands is compared with that of different dyes [quantum dots (QDs)] in a “dye‐blend” (mixture) under FRET to an energy acceptor. Comparison of the FRET of the QDs with that of BP confirmed the independent nature of the dipolar interaction of the vibronic bands with other organic molecules, and the use of deconvolution techniques in the interpretation of the donor–acceptor (D –A) distance was also justified. We also showed that the consideration of differential FRET from the vibronic bands of BP and from the QDs in the dye‐blend is equally acceptable in theoretical frameworks including the Infelta–Tachiya model and D –A distribution analysis in nanoenvironments.     

两个具有强双光子荧光的有机硼化合物

以二米基硼为电子受体, 苯乙烯基噻吩为共轭桥, 合成了两个新的稳定的有机硼化合物: 反式,反式-2-二米基 硼-5-{2-[4-(2-噻吩乙烯基)苯基]乙烯基}噻吩(1)和反式,反式-1,4-二-[2-(5-二米基硼噻吩)乙烯基]苯(2). 前者为不对称结构的偶极分子, 后者为对称的A-π-A型四极分子. 对称性不同的化合物表现出不同的双光子吸收性质. 对于偶极分子1, 单双光子吸收达到的激发态能级接近, 而对于四极分子2, 双光子吸收达到的激发态则比单光子吸收所达到的激发态高出0.35 eV. 在波长为710到 900 nm范围的飞秒脉冲激光激发下, 化合物1和2在THF溶液中都可以发出很强的绿色上转换荧光 (1, λ_max＝505 nm; 2, λ_max＝513 nm). 用双光子荧光法测得A-π-A型化合物2在775 nm处的双光子吸收截面达1340 GM.     

Head‐to‐Tail Zig‐Zag Packing of Dipolar Merocyanine Dyes Affords High‐Performance Organic Thin‐Film Transistors

Attachment of bulky substituents at both thiophene donor (D) and thiazole acceptor (A) heterocycles of a dipolar (μ_g=10.4 D) D‐π‐A merocyanine dye affords a more than 1 Å expansion of the common antiparallel supramolecular dimer motif in the solid state, enabling very close π‐contacts (3.36 Å) to two other neighbor molecules on each of the two remaining π‐faces. This unusual packing motif leads to three‐dimensional percolation pathways for hole transport and affords thin‐film transistors with mobility up to 0.64 cm² V^?1 s^?1.     

Synthesis and specific solvatochromism of D–π–A type pyridinium dye

Donor–π–acceptor (D–π–A) type pyridinium dye bearing an iodide ion as the counter anion that has been newly synthesized showed specific solvatochromism, leading to large bathochromic shift of absorption band in halogenated solvents: the bathochromic shifts of the D–π–A type pyridinium dye in halogenated solvents are larger than those of the non-halogenated solvents of low ?_r values. Investigation of absorption spectral measurement, ¹H NMR measurements, and semi-empirical molecular calculations (AM1 and INDO/S using the SCRF Onsager Model) revealed that the intramolecular charge transfer (ICT) characteristics of the D–π–A type pyridinium dye became stronger in the halogenated solvents. On the basis of the experimental results and the theoretical calculations, the influences of halogenated solvent on the large bathochromic shift of D–π–A type pyridinium dye are discussed. ^©2012 Elsevier Science. All rights reserved.     

2H QUOSY 2D-NMR Experiments in Weakly Aligning Systems: From the Conventional to the Ultrafast Approach

We describe three anisotropic ultrafast (UF) QUadrupolar Ordered SpectroscopY (QUOSY) 2D-NMR experiments (referred to as ADUF 2D NMR spectroscopy) designed for recording the ²H homonuclear 2D spectra of weakly aligned (deuterated) solutes in sub-second experiment times. These new ADUF 2D experiments derive from the Q-COSY, Q-resolved and Q-DQ 2D pulse sequences (J. Am. Chem. Soc. 1999 , 121, 5249) and allow the correlation between the two components of each quadrupolar doublet, and then their assignment on the basis of ²H chemical shifts. The UF 2D pulse sequences are analyzed by using the Cartesian spin-operator formalism for spin I=1 nuclei with a small quadrupolar moment. The optimal experimental/practical conditions as well as the resolution, sensitivity and quantification issues of these ADUF 2D experiments are discussed on comparison to their conventional 2D counterparts and their analytical potentialities. Illustrative ADUF 2D experiments using deuterated achiral/prochiral/chiral solutes in poly-γ-benzyl-L-glutamate based chiral liquid crystals are presented, as well as the first examples of natural abundance deuterium (ANADUF) 2D spectrum using 14.1 T magnetic field and a basic gradient unit (53 G.cm⁻¹) in oriented solvents.     

Consideration of dipole–quadrupole interactions in molecular based equations of state

The thermodynamic behaviour of a number of real substances is determined by dipolar as well as quadrupolar interactions of the molecules. In equations of state (EOS) like, e.g. BACKONE separate contributions to the Helmholtz energy for the dipolar and the quadrupolar interactions are considered but no cross contributions. Here, the concept of effective dipole and quadrupole contributions is suggested in which the effective dipole strength μ_e is influenced by the quadrupole cross interaction. Similarily, the effective quadrupole strength Q_e takes into account the dipole cross interaction. In order to arrive at these effective dipolar and quadrupolar strengths, molecular simulations are performed. From the simulation results correlation equations are derived which are used in combination with BACKONE for the calculation of vapour–liquid equilibria (VLE) of real mixtures. By using these effective moments, the only required binary mixing rule parameter k_ij tends to small values of about 0.01 and becomes temperature-independent. Moreover, the VLE pressures are predicted now considerably better than without consideration of the cross contributions.     

Photophysical and Photoacoustic Properties of π‐Extended Curcumin Dyes. Effects of the Terminal Dimethylamino Electron‐donor and the Bridging Aryl Ring

The synthesis, photophysical and photoacoustic characterization for a series of nine π‐extended quadrupolar curcumin dyes is presented. A systematic evaluation of the π‐bridging unit including the p‐phenyl, naphth‐4‐yl, thien‐2‐yl and hybrid 4‐naphthathien‐2‐yl groups is presented. Furthermore, evaluation of the strongly donating donor‐π‐acceptor‐π‐donor quadrupolar dimethylamino terminated derivatives is also included. Select dyes exhibit excited state absorption at increased laser fluence which translates to the production of a nonlinear enhanced photoacoustic response. In particular, the bis‐4‐dimethylaminonaphtha‐2‐thien‐5‐yl curcuminBF₂ contrast agent DMA‐5 exhibits an excellent molar photoacoustics (PA) emission at both low (9.4 × 10³ V M^?1) and high (1.47 × 10⁵ V M^?1) laser fluence which is confirmed by its strong contrast by photoacoustic tomography (PAT). In summary, the strong absorbance and enhanced photoacoustic properties of naphthyl and thienyl curcuminoids here presented provides great promise for future photoacoustic imaging applications as demonstrated by preliminary PAT studies.     

Selective double-quantum nmr in solids

A simple method for selective double-quantum NMR in solids is described. The spin system is first prepared in a state having only dipolar, or quadrupolar, order. Selective excitation and detection of double-quantum coherence is then achieved by the 90°_x,y-t-45°_y pulse sequence.     

Structural analysis of boron compounds using 11B-3QMAS solid state NMR

The structural analysis of three boron compounds, boron carbide (B₄C), silicon tetraboride (SiB₄) and hexagonal-boron nitride (h-BN), were performed using 2D ¹¹B-triple quantum MAS (3QMAS) solid state NMR capable of averaging the second-order quadrupolar interaction of ¹¹B that cause line broadening, splitting and low frequency shift of the central transition (−1/2, 1/2). The coordination number around the boron atom and structural symmetry of each boron compound is discussed by means of the isotropic chemical shift Δσ and the quadrupolar coupling constant C_Q calculated from 3QMAS spectra. Δσ of SiB₄ is quite larger than that of B₄C, which is thought to be caused by its structural distortion and distribution. Δσ of h-BN was found to be higher frequency shift obviously than that of B₄C and SiB₄ because of the difference of the boron coordination number, three-coordinated in h-BN and six-coordinated in B₄C and SiB₄. h-BN has very large C_Q compared to other two boron compounds since the h-BN forming a two-dimensional network has less structural symmetry than B₄C and SiB₄.     

About the concentration dependence of the anisotropy in the magnetic susceptibility of some aromatic compounds in the liquid phase,studied by NMR

Magnetic field induced quadrupolar line splittings in the ²H NMR spectra of pyridine and metadinitrobenzene are reported. From the splittings the anisotropy in the magnetic susceptibility Δ_x has been deduced. The concentration dependence of the quadrupolar magnetic field effect, and hence of Δ_x, of nitrobenzene, metadinitrobenzene and naphthalene has been investigated. The effect of different solvents has also been studied. Environmental effects of Δ_x have been found to be substantial in several cases. The relation of the present experiments with the Cotton-Mouton effect is discussed in detail.     

Transitions between the B2 phase and more usual smectic phases in binary systems of banana-shaped with calamitic mesogens

Deuterium NMR spectroscopy is used to study a ring-deuteriated chiral liquid crystal 4-(2-methylbutyl)oxycarbonylphenyl 4-(10-undecenyloxy)benzoate. The quadrupolar and proton-deuteron dipolar splittings, and deuteron quadrupolar and Zeeman spin-lattice relaxation times were measured as a function of temperature in the smectic A phase at two different Larmor frequencies. The derived spectral densities of motion at different temperatures were analysed simultaneously using a rotational diffusion model which also includes internal ring rotations. Motional parameters (D _⊥, D _∥, D _R) and order parameter tensors (S_zz , S_xx -S_yy ) were obtained. Although the present data seem insufficient to draw a definitive conclusion, we believe that it is possible for this particular chiral molecule to have D_⊥ >D_∥ , which is different from non-chiral rod-like liquid crystals.     

Quadrupolar Cyclopenta[hi]aceanthrylene‐Based Electron Donor‐Acceptor‐Donor Conjugates: Charge Transfer versus Charge Separation

Cyclopenta[hi]aceanthrylenes (CPAs) have been functionalized at two of the peripheral positions with electronically inert trimethylsilylethynyl ( 1 ), as well as with electron‐donating 4‐ethynyl‐N,N‐dimethylaniline ( 2 ), ethynyl Zn^IIphthalocyanine ( 3 ), and ethynyl Zn^IIporphyrin ( 4 ) units. Consistent with X‐ray crystal structures of 2 and 4 , analyses of absorption and fluorescence of 2 – 4 point to strong electronic communication between the CPA and the peripheral units, affording quadrupolar electron donor‐acceptor‐donor charge‐transfer conjugates. By virtue of their quadrupolar/dipolar charge‐transfer characters in the excited state, 2 – 4 exhibit fluoro‐solvatochromism. Transient absorption spectroscopy confirmed delocalized quadrupolar ground states and formation of weakly solvent stabilized quadrupolar singlet excited states. The latter transform into strongly stabilized dipolar excited states before deactivating to the ground state in 2 and give rise to a fully charge separated state in 3 and 4 .     

Solvation and temperature effect on the charge-transfer complex between 2-amino-4-picoline with 2,5-dihydroxy-p-benzoquinone

The charge-transfer (CT) complex between the donor 2-amino-4-picoline (2A4P) and the acceptor 2,5-dihydroxy-p-benzoquinone (DHBQ) was studied spectrophotometrically in different polar and non-polar solvents. The molecular composition of the complex, in all solvents, was determined by Job's method of continuous variation and photometric titrations to be 1:1. Benesi–Hildebrand equation has been applied to estimate the formation constant (K _CT) and molecular extinction coefficient (ε) of the formed complex. The variation in K _CT was rationalised based on Taft–Kamlet and electric permittivity parameters of the used solvents. Thermodynamic parameters ΔH°, ΔG° and ΔS° were estimated, they were all negative so the studied complex is reasonably stable and exothermic in nature. In addition, the thermodynamic properties were observed to be sensitive to the nature of the solvent. Moreover, the solid 1:1 CT complex between 2A4P and DHBQ was isolated and characterised using elemental analysis, FTIR and ¹H NMR measurements.