Bridge improvement work: maximising non-linear optical performance in polyoxometalate derivatives

New phenyl and stilbene-bridged polyoxometalate (POM) charge-transfer chromophores with diphenylamino donor groups produce, respectively, the highest intrinsic and absolute quadratic hyperpolarisabilities measured for such species. The β0,zzz obtained for the phenyl bridge - at 180 × 10-30 esu - is remarkable for a short conjugated system while changing to the stilbene (260 × 10-30 esu) produces a substantial increase in non-linearity for a minimal red-shift in the absorption profile. Together with TD-DFT calculations, the results show that maximising conjugation in the π-bridge is vital to high performance in such "POMophores".

Development of Donor-π-Acceptor-Type Fluorinated Tolanes as Compact Condensed Phase Luminophores and Applications in Photoluminescent Liquid-Crystalline Molecules

Fluorinated tolanes, produced by introducing fluorine atoms into one of the aromatic rings of tolane, emitted almost no fluorescence in a solution state, but the fluorescence intensity increased dramatically in the crystalline state because of intermolecular H⋅⋅⋅F hydrogen bonds. The photoluminescent (PL) colors depend on the molecular orbitals, dipole moments, and molecular aggregated structures can be varied by controlling terminal substituents along the major molecular axis. The introduction of a long alkoxy or semifluoroalkoxy unit as a flexible chain into the terminal positions along the major molecular axis induced the formation of a liquid-crystalline (LC) phase; fluorinated tolanes act both as luminophores and as mesogens, leading to the molecular design of new photoluminescent LC molecules (PLLCs). The results also indicated that a fluorinated tolane dimer, which consists of two fluorinated tolanes linked by a flexible alkylene spacer, also becomes a novel PLLC.

Influence of the Solvent Properties on Two Individuals Presented in the Excited State - Reverse Fluorosolvatochromism of CNAacDMA

The photophysical behavior of 10-((4-(dimethylamino)phenyl)ethynyl)anthracene-9-carbonitrile (CNAacDMA)  was investigated with absorption and steady-state fluorometry. Studies have shown extremely interesting properties of the analyzed derivative. The studied molecule is characterized by dual fluorescence and bidirectional fluorescence. These two observed phenomena are interrelated. Dual fluorescence analysis (part 1) explains bidirectional fluorescence. The compound in polar solvents shows only one emission band, however in moderate polar solvents shows dual emission. The appearance of the long-wavelength CT band in moderate polar solvents and its lack in polar solvents leading to bidirectional solvatofluorochromism. The solvatochrome analysis was performed in a wide range of 24 solvents. To determine the contribution of specific and nonspecific interaction of this compound with solvents linear and multi-parametric correlation of the solvent polarity were analyzed. The novelty and innovation in this article is to carry out such an analysis separately for two different individuals occurring in the excited state. Single-parameter scale with (30) as a solvent polarity parameter and multi-parameter scales such as the Catalán four-parameter solvent scale were used. The excited state dipole moment was determined based on solvatochromic method. The hydrogen bond energy change after excitation in the Franck-Condon and relaxed excited state were estimated.

Systematic Studies on the Effect of Fluorine Atoms in Fluorinated Tolanes on Their Photophysical Properties

In this study, we synthesized a series of fluorinated and non-fluorinated tolanes, in which one or more fluorine atoms were systematically introduced into one aromatic ring of a tolane scaffold, and systematically evaluated their photophysical properties. All the tolanes with or without fluorine substituents were found to have poor photoluminescence (PL) in tetrahydrofuran (THF) solutions. On the other hand, in the crystalline state, non-fluorinated and fluorinated tolanes with one or four fluorine atoms were less emissive, whereas fluorinated tolanes with three or five fluorine atoms exhibited high PL efficiencies (Ф_PL) up to 0.51. X-ray crystallographic analyses of the emissive fluorinated tolanes revealed that the position of the fluorine substituent played a key role in achieving a high Ф_PL. Fluorine substituents at the ortho (2/6) and para (4) positions led to tight and rigid packing due to plural π–π stacking and/or hydrogen bonding interactions, resulting in enhanced Ф_PL caused by the suppression of non-radiative deactivation. Additionally, fluorinated tolanes with three fluorine atoms exhibited notable aggregation-induced PL emission enhancement in THF/water mixed solvents. This demonstrates that the PL characteristics of small PL materials can be tuned depending on the usage requirements.

Periodic Fluorescence Variations of CdSe Quantum Dots Coupled to Aryleneethynylenes with Aggregation-Induced Emission

CdSe nanocrystals and aggregates of an aryleneethynylene derivative are assembled into a hybrid thin film with dual fluorescence from both fluorophores. Under continuous excitation, the nanocrystals and the molecules exhibit anticorrelated fluorescence intensity variations, which become periodic at low temperature. We attribute this to a structure-dependent aggregation-induced emission of the aryleneethynylene derivative, which impacts the rate of excitation energy transfer between the molecules and nanocrystals. This work highlights that combining semiconductor nanocrystals with molecular aggregates, which exhibit aggregation-induced emission, can result in emerging optical properties.

Thermo-driven self-assembly of a PEG-containing amphiphile in a bilayer membrane

Self-assembly of lipid molecules in a plasma membrane, namely lipid raft formation, is involved in various dynamic functions of cells. Inspired by the raft formation observed in the cells, here we studied thermally induced self-assembly of a synthetic amphiphile, bola-AkDPA, in a bilayer membrane. The synthetic amphiphile consists of a hydrophobic unit including fluorescent aromatic and aliphatic components and hydrophilic tetraethylene glycol chains attached at both ends of the hydrophobic unit. In a polar solvent, bola-AkDPA formed aggregates to show excimer emission. In a lipid bilayer membrane, bola-AkDPA showed intensified excimer emission upon increase of its concentration or elevation of the temperature; bola-type amphiphiles containing oligoethylene glycol chains likely tend to form self-assemblies in a bilayer membrane triggered by thermal stimuli.

A synthetic multi-block amphiphile containing oligoethylene glycol chains formed a self-assembly in a bilayer membrane triggered by thermal stimuli.

Determination of Hyperfine Coupling Constants of Fluorinated Diphenylacetylene Radical Anions by Magnetic Field-Affected Reaction Yield Spectroscopy

Magnetic field-affected reaction yield (MARY) spectroscopy is a spin chemistry technique for detecting short-lived radical ions. Having sensitivity to transient species with lifetimes as short as nanoseconds, MARY spectroscopy usually does not provide detailed information on their magnetic resonance parameters, except for simple systems with equivalent magnetic nuclei. In this work, the radical anions of two fluorinated diphenylacetylene derivatives with nonequivalent magnetic nuclei and unknown hyperfine coupling constants ( A_HF) were investigated by MARY spectroscopy. The MARY spectra were found to be resolved and have resonance lines in nonzero magnetic fields, which are determined by the A_HF values. Simple relationships between the positions of resonance MARY lines and the A_HF values were established from the analysis of the different Hamiltonian block contributions to the MARY spectrum. The obtained experimental A_HF values are in agreement with the results of quantum chemical calculations at the density functional theory level.

X-ray Generated Recombination Exciplexes of Substituted Diphenylacetylenes with Tertiary Amines: A Versatile Experimental Vehicle for Targeted Creation of Deep-Blue Electroluminescent Systems

Customizable and technology-friendly functional materials are one of the mainstays of emerging organic electronics and optoelectronics. We show that recombination exciplexes of simple substituted diphenylacetylenes with tertiary amines can be a convenient source of tunable deep-blue emission with possible applications in organic electroluminescent systems. The optically inaccessible exciplexes were produced via recombination of radiation-generated radical ion pairs in alkane solution, which mimics charge transport and recombination in the active layer of practical organic light-emitting diodes in a simple solution-based experiment. Despite varying and rather poor intrinsic emission properties, diphenylacetylene and its prototypical methoxy (donor) or trifluoromethyl (acceptor) monosubstituted derivatives readily form recombination exciplexes with N,N-dimethylaniline and other tertiary amines that produce emission with maxima ranging from 385 to 435 nm. The position of emission band maximum linearly correlates with readily calculated gas-phase electron affinity of the corresponding diphenylacetylene, which can be used for fast computational prescreening of the candidate molecules, and various substituted diphenylacetylenes can be synthesized via relatively simple and universal cross-coupling reactions of Sonogashira and Castro. Together, the simple solution-based experiment, computationally cheap prescreening method, and universal synthetic strategy may open a very broad and chemically convenient class of compounds to obtain OLEDs and OLED-based multifunctional devices with tunable emission spectrum and high conversion efficiency that has yet not been seriously considered for these purposes.

Thermally-induced lateral assembly of a PEG-containing amphiphile triggering vesicle budding

A macrocyclic amphiphile consisting of a thermo-responsive octaethylene glycol chain with hydrophobic aromatic and aliphatic units undergoes lateral self-assembly in a liquid-disordered-state phospholipid bilayer membrane upon heating, which further leads to vesicle budding.

Estimation of the Fraction of Spin-Correlated Radical Ion Pairs in Irradiated Alkanes using Magnetosensitive Recombination Luminescence from Exciplexes Generated upon Recombination of a Probe Pair

We suggest a convenient probe exciplex system for studies in radiation spin chemistry based on a novel acceptor-substituted diphenylacetylene, 1-(phenylethynyl)-4-(trifluoromethyl)benzene that has a very short fluorescence lifetime (<200 ps) and low quantum yield (0.01) of intrinsic emission, provides efficient electron capture in alkanes and efficient exciplex formation upon recombination in pair with DMA radical cation, while exhibiting a shifted to red exciplex emission band as compared to the parent system DMA – diphenylacetylene. After chemical, luminescent, radiation and spin-chemical characterization of the new system we used the magnitude of magnetic field effect in its exciplex emission band for experimental estimation of the fraction of spin-correlated radical ion pairs under X-irradiation with upper energy cutoff 40 keV in a set of 11 alkanes. For linear and branched alkanes magnetic field effects and the corresponding fractions are approximately 19–20% and 0.28, while for cyclic alkanes they are lower at 16–17% and 0.22, respectively.

Experimental and computational evaluation of the barrier to torsional rotation in a butadiyne-linked porphyrin dimer

The barrier to torsional rotation in a butadiyne-linked porphyrin dimer has been determined in solution using variable temperature UV-vis-NIR spectroscopy: ΔH = 5.27 ± 0.03 kJ mol(-1), ΔS = 10.69 ± 0.14 J K(-1) mol(-1). The value of ΔH agrees well with theoretical predictions. Quantum chemical calculations (DFT) were used to predict the torsion angle dependence of the absorption spectrum, and to calculate the vibronic fine structure of the S0 → S1 absorption for the planar dimer, showing that the absorption band of the planar conformer has a vibronic component overlapping with the 〈0|0〉 absorption of the perpendicular conformer. The torsion barrier in the porphyrin dimer is higher than that of 1,4-diphenylbutadiyne (calculated ΔH = 1.1 kJ mol(-1)). Crystallographic bond lengths and IR vibrational frequencies confirm that there is a greater contribution of the cumulenic resonance form in butadiyne-linked porphyrin dimers than in 1,4-diphenylbutadiyne. The DFT frontier orbitals of the twisted conformer of the porphyrin dimer are helical, when calculated in the absence of symmetry. The helical character of these orbitals disappears when D2d symmetry is enforced in the 90° twisted conformer. Helical representations of the frontier orbitals can be generated by linear combinations of the more localised orbitals from a symmetry-constrained calculation but they do not indicate π-conjugation. This work provides insights into the relationship between electronic structure and conformation in alkyne-linked conjugated oligomers.

Experimental and theoretical studies of spectroscopic properties of simple symmetrically substituted diphenylbuta-1,3-diyne derivatives

A series of symmetrically substituted diphenylbuta-1,3-diyne (DPB) derivatives possessing electron-donating (N,N-dimethylamino or methoxy) or electron-accepting (nitrile, ester or aldehyde) groups have been prepared and studied with emphasis on their spectral and photophysical properties. The photophysical characteristics of these compounds have been studied in relation to their structures and influence of solvents or temperature. The observed spectral and photophysical properties are explained with the help of potential energy maps in the ground and excited states obtained from density functional theory (DFT, B3LYP, def2TZVP basis set) calculations. The structure-property relationship of all of the compounds is discussed and compared with appropriate diphenylacetylene derivatives.

Photophysical properties of symmetrically substituted diarylacetylenes and diarylbuta-1,3-diynes

A series of symmetrically substituted diarylacetylenes and diaryl-1,3-butadiynes were prepared and studied with an emphasis on their spectral and photophysical properties.