Electroluminescence enhancement in ultraviolet organic light‐emitting diode with graded hole‐injection and ‐transporting structure

Electroluminescent intensity and external quantum efficiency (EQE) in ultraviolet organic light‐emitting diodes (UV OLEDs) have been remarkably enhanced by using a graded hole‐injection and ‐transporting (HIT) structure of MoO₃/N,N ′‐bis(naphthalen‐1‐yl)‐N,N ′‐bis(phenyl)‐benzidine/MoO₃/4,4′‐bis(carbazol‐9‐yl)biphenyl (CBP). The graded‐HIT based UV OLED shows superior short‐wavelength emis‐ sion with spectral peak of ～410 nm, maximum electroluminescent intensity of 2.2 mW/cm² at 215 mA/cm² and an EQE of 0.72% at 5.5 mA/cm². Impedance spectroscopy is employed to clarify the enhanced hole‐injection and ‐transporting capacity of the graded‐HIT structure. Our results provide a simple and effective approach for constructing efficient UV OLEDs. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Synthesis of novel bis(β‐cyclodextrin)s linked with aromatic diamine and their molecular recognition with model substrates

Novel β‐cyclodextrin (β‐CD) dimers with aromatic diamine linkers, 1,3‐(aminomethyl)‐benzylamine‐bridged bis(6‐amino‐6‐deoxy‐β‐CD) (2) , 4,4′‐diaminodiphenylmethano‐bridged bis(6‐amino‐6‐deoxy‐β‐CD) (3) , and 4,4′‐ ethylenedianiline‐bridged bis(6‐amino‐6‐deoxy‐β‐CD) (4) , were synthesized. The inclusion complexation behaviors of these compounds, together with 4,4′‐aminophenyl ethyl‐bridged bis(6‐amino‐6‐deoxy‐β‐CD) (5) , with substrates such as acridine red (AR), neutral red (NR), ammonium 8‐anilino‐1‐naphthalenesulfonate (ANS), sodium 2‐(p‐toluidinyl) naphthalenesulfonate (TNS), rhodamine B (RhB), and brilliant green (BG), were investigated by ultraviolet, fluorescence, circular dichroism, and 2D NMR spectroscopy. The results indicated that the two linked CD units cooperatively bound to a guest, and the molecular binding affinity toward substrates, especially curved guest ANS and linear guests such as NR and AR, was increased. The linker length between two CD units played a crucial role in the molecular recognition of the hosts with guest dyes. The binding constants of the hosts for AR, TNS, ANS, and RhB decreased with increasing linker length in hosts 2‐4 . Moreover, structurally similar hosts 3 and 5 exhibited very different binding behavior for the guests. Host 5 showed much higher Ks values toward positively charged guests and lower Ks toward negatively charged guests than host 3 . The 2D NMR spectra of hosts 3 and 5 with RhB were acquired to understand the binding difference between 3 and 5 . The molecular binding ability and selectivity of model substrates by these hosts were sufficiently investigated to reveal not only the cooperative contributions of the linker group and CD cavities upon inclusion complexation with dye guest molecules, but also the controlling factors for the molecular selective binding. Copyright © 2008 John Wiley & Sons, Ltd.     

Structural,electronic, and optical properties of doubly ortho‐linked quinoxaline/diphenylfluorene hybrids

The optical and electronic properties of spiro‐fluorene‐dibenzosuberene[d](1,4‐bis(4‐t‐butylphenyl)quinoxaline) 1a , spiro‐fluorene‐dibenzosuberene[d](1,4‐bis(4‐methoxyphenyl)quinoxaline) 1b , spiro‐fluorene‐dibenzosuberene[d](1‐(4‐(N,N‐diphenylamino)‐phenyl)‐quinoxaline) 1c , spiro‐fluorene‐dibenzosuberene[d](1,4‐bis(methylphenylamino)quinoxaline) 1d , spiro‐fluorene‐dibenzosuberene[d](1,4‐bis(methyl‐(4‐methylphenyl)amino)quinoxaline) 1e , spiro‐fluorene‐dibenzosuberene[d](1,4‐bis(methyl‐(4‐methoxyphenyl)amino)quinoxaline) 1f , 5,8‐bis‐(4‐methoxy‐phenyl)‐2,3‐diphenyl‐quinoxaline 1 , and N,N,N',N'‐tetraphenyl‐ 5h‐dibenzo[a,d]cycloheptene‐3,7‐diamine 2 were investigated theoretically in this paper. The doubly ortho‐linked quinoxaline/diphenylfluorene hybrids 1a – 1f show great potential as bipolar materials for the design of optimized organic light‐emitting diodes (OLEDs). Density functional theory (DFT) and ab initio HF were employed to study the geometric and electronic structures of these molecules in the ground state, and ab initio CIS were used to investigate the lowest singlet excited states. The radiative lifetime (τ) and the maximal absorption/emission wavelength of these molecules were calculated within time‐dependent DFT (TDDFT). The results show that the LUMO energies of the bipolar molecules 1a – 1d are all lower than those of 1 and 2 , consequently, the electron‐accepting abilities of 1a – 1d are greatly improved. The HOMO energies of 1c – 1f are all higher than those of 1 and 2 , suggesting that the hole‐creating abilities of 1c – 1f become better. Also, the results reveal that the HOMO and LUMO energies, energy gaps, IP, EA, as well as the maximal absorption/emission spectra can be tuned feasibly by changing the C5‐ and C8‐substituents in the quinoxaline backbone of these molecules. As expected, these materials show different emission spectra varying from 436.11 to 715.47 nm. Copyright © 2010 John Wiley & Sons, Ltd.     

Acid‐base properties and supramolecular structure of N‐[(hydroxymethyl)triazolyl]triflamides: DFT,ab initio,and FTIR study

For N‐{[2‐(hydroxymethyl)‐2H‐1,2,3‐triazolyl‐4‐yl]methyl}triflamide 1 , N‐{[2‐(hydroxymethyl)‐2H‐1,2,3‐triazolyl‐4‐yl]methyl}‐N‐phenyltriflamide 2 , and N,N‐bis{[2‐(hydroxymethyl)‐2H‐1,2,3‐triazolyl‐4‐yl]methyl}triflamide 3 , the proton affinities of the triazole nitrogen atoms and the hydroxy and sulfonyl oxygen atoms as well as the energies of formation of the conformers with intramolecular H‐bonds and dimers with intermolecular NH?N, OH?N, OH?O═S, and NH?O═S H‐bonds were calculated by density functional theory and second‐order Møller‐Plesset perturbation methods. Quantum Theory of Atoms in Molecules analysis was performed to investigate the nature of H‐bonds. According to Fourier transform infrared spectroscopy, in CH₂Cl₂ solution, the monomeric molecules of 1 to 3 exist in the equilibrium with cyclic dimers having the OH?N hydrogen bonds.     

Molecular structure,NMR and vibrational spectral analysis of 2,4‐difluorophenol by ab initio HF and density functional theory

Quantum chemical calculations of energies, geometries and vibrational wavenumbers of 2,4‐difluorophenol (2,4‐DFP) were carried out by using ab initio HF and density functional theory (DFT/B3LYP) methods with 6‐311G(d,p) as basis set. The optimized geometrical parameters obtained by HF and DFT calculations are in good agreement with related molecules. The best level of theory in order to reproduce the experimental wavenumbers is the B3LYP method with the 6‐311G(d,p) basis set. The difference between the observed and scaled wavenumber values of most of the fundamentals is very small. A detailed interpretation of the infrared and Raman spectra of 2,4‐DFP is also reported. The entropy of the title compound was also performed at HF/6‐311G(d,p) and B3LYP/6‐311G(d,p) levels of theory. The isotropic chemical shift computed by ¹H, ¹³C NMR analyses also shows good agreement with experimental observations. The theoretical spectrograms for FT‐IR and FT‐Raman spectra of the title molecule have been constructed. Copyright © 2009 John Wiley & Sons, Ltd.     

Theoretical studies on the electronic and optical properties of arene‐ versus fluoroarene‐thiophene co‐oligomer

A series of regiochemically varied and core size extension‐modulated arene‐ and fluoroarene‐thiophene co‐oligomers and the unsubstituted sexithiophene α6T were investigated theoretically to explore their electronic and optical properties. These phenylene‐thiophene oligomers show great potential for application in organic light‐emitting diodes (OLEDs), organic diode lasers, and organic thin‐film transistors (OTFTs) because of their feasible tuning of optical and electronic properties by the various structural tunings. Density functional theory (DFT) and the ab initio HF were employed to investigate the geometric and electronic structures of the oligomers in the ground state, and the singles configuration interaction (CIS) methods were used to study the lowest singlet excited state. The lowest excitation energies (E_gs), the radiative lifetime τ, and the maximal absorption/emission wavelength of the oligomers were studied within time‐dependent DFT (TDDFT). All calculations were performed using the 6‐31G(d) basis set. The results show that the HOMOs, LUMOs, energy gaps, ionization potentials (IPs), electron affinities (EAs), and reorganization energies are significantly affected by the various structural tunings in these co‐oligomers, which is important for the improvement of the hole and electron injection into OLEDs. Interestingly, the LUMO energy of 1b , 2b , and 3b is lower than that of α6T and 1a , 2a , 3a by about 0.12 ～ 0.47 eV, indicating that the fluorophenyl‐substitution has significantly improved the electron injection properties of the oligomers. Copyright © 2008 John Wiley & Sons, Ltd.     

Thick junction broadband organic photodiodes

Inorganic semiconductor‐based broadband photodetectors are ubiquitous in imaging technologies such as digital cameras and photometers. Herein a broadband organic photodiode (OPD) that has performance metrics comparable or superior to inorganic photodiodes over the same spectral range is reported. The photodiode with an active layer comprised of a poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)]:[6,6]‐phenyl‐C₇₁‐butyric acid methyl ester bulk heterojunction blend had a dark current < 1 nA/cm², specific detectivity of ∼10¹³ Jones, reverse bias −3 dB frequency response of 100 kHz to 1 MHz, and state‐of‐the‐art Linear Dynamic Range for organic photodiodes of nine orders of magnitude (180 dB). The key to these performance metrics was the use of a thick junction (700 nm), which flattened the spectral response, reduced the dark current and decreased performance variations. The strategy also provides a route to large area defect free “monolithic” structures for low noise integrated photo‐sensing, position determination, or contact, non‐focal imaging.     

Theoretical design study on multifunctional triphenyl amino‐based derivatives for OLEDs

The use of triphenyl amino‐based derivatives in organic light‐emitting diodes (OLEDs) can significantly improve their efficiency and stability and especially their electroluminescence characteristics – most of the new hole‐transport materials have this feature. In this study, a series of triphenyl amino‐based compounds were computed, including two newly designed molecules. They can function as charge transport materials and emitters with high efficiency and stability. To reveal the relationship between the properties and structures of these bifunctional and multifunctional electroluminescent materials, the ground and excited state geometries were optimized at the B3LYP/6‐31G(d), HF/6‐31G(d), TD‐B3LYP/6‐31G(d), and CIS/6‐31G(d) levels, respectively. The ionization potentials (IPs) and electron affinities (EAs) were computed. The lowest excitation energies, the maximum absorption, and emission wavelengths of these compounds were calculated by employing the time‐dependent density functional theory (TD‐DFT) method. Also, the mobilities of holes and electrons were studied computationally based on the Marcus electron transfer theory. The CH₂Cl₂ solvent effect on the absorption spectra of N,N′‐di‐1‐naphthyl‐N,N′‐diphenylbenzidine ( NPB ) was considered by polarizable continuum model (PCM). The results obtained for these compounds are in good agreement with the experimental values. These data show that the proposed compounds 1 and 2 (N,B‐di‐1‐naphthyl‐N,B‐diphenylbenzidine and Mes₂N[p‐4,4′‐biphenyl‐NPh(1‐naphthyl)]), are multifunctional and bifunctional materials similar to Mes₂B[p‐4,4′‐biphenyl‐NPh(1‐naphthyl)] ( BNPB ) and NPB , respectively. Copyright © 2009 John Wiley & Sons, Ltd.     

Conformational analysis: Part 42. A modelling and LIS/NMR investigation of the conformations of some aromatic SO compounds

A refined Lanthanide‐Induced‐Shift Analysis (LISA), in which both the paramagnetic and the diamagnetic lanthanide induced chemical shifts are normalised separately, is used with molecular mechanics (MMFF94) and ab initio (RHF/6‐31G, RHF/6‐311G** and B3LYP/6‐311G**) calculations to investigate the conformations of methyl para‐tolyl sulphoxide 1 , sulphone 2 and sulphonate 3 and the conformational equilibria of methyl 2,4‐dimethylphenyl sulphoxide 4 and sulphone 5 . In 1 acceptable agreement factors (AFs) in the LISA analysis were only obtained when the SO/phenyl dihedral angle was increased from that (10–15°) of the computed geometries to 30–40°. The calculated geometries for 2 had the methyl group orthogonal to the phenyl ring and gave good AF in the LISA analysis. In both compounds, the AF were in the order MMFF94 ≤ B3LYP < RHF with the larger basis sets giving better agreement than the limiting 6‐31G* function. In 3 all the geometries again had the methoxy group orthogonal to the phenyl ring but, while the ab initio geometries still gave good AFs, the MMFF94 structure gave an AF which was only just acceptable. The unsymmetrical compounds 4 and 5 display conformational equilibria. In 4 two energy minima were found, both with the SMe group orthogonal to the phenyl ring, with the SO oxygen cis or trans to the 2‐methyl group. The energy difference (cis–trans) was calculated as 1.6 kcal/mol (MMFF94), 2.7 (6‐31G*), 1.9 (6‐311G**) but ?0.11 (DFT). The LISA analyses on the separate conformers gave poor agreement (ca. 3%) for the trans conformer but increasing the SO/Ph dihedral angle again gave an acceptable value (ca. 1.4%). Inclusion of even small amounts of the cis conformer in the LIS analysis gave poorer results. Thus, the LISA analysis confirms the MM and RHF prediction that the trans conformer is more stable and suggests that the conformer energy difference in solution is equal to or greater than the largest calculated value of 2.7 kcal/mol. In 5 all the calculations gave two mirror image conformations with the SMe orthogonal to the phenyl ring. The LISA analysis in this case is complicated by the fact that in the stable conformer the complexing oxygen atoms are not equivalent. This was resolved by considering both coordination sites in one Z‐matrix and varying the % coordination on each oxygen atom. This gave an acceptable AF of ca. 1.0% for all the geometries with ca. 2:1 coordination on the oxygen atoms trans and cis to the C2‐methyl. The difference in the geometries and conformer energies found by LISA for these molecules in solution compared to the theoretical values may be due to interactions with the chloroform solvent for these polar compounds. Copyright © 2010 John Wiley & Sons, Ltd.     

DFT/TDDFT study on the electronic structures and optoelectronic properties of a series of iridium(III) complexes based on quinoline derivatives in OLEDs

The electronic structures and photophysical properties of several heteroleptic iridium(III) complexes Ir(C^∧N)₂(acac) with acetylacetonate (acac) ligand, including 1 [C^∧N = 2‐phenylisoquinoline], 2 [C^∧N = 2‐biphenyl‐4‐yl‐quinoline], 3 [C^∧N = 2‐(fluoren‐2‐yl)‐quinoline], 4 [C^∧N = 2‐dibenzofuran‐3‐yl‐quinoline], 5 [C^∧N = 2‐dibenzothiophen‐3‐yl‐quinoline], and 6 [C^∧N = 2‐phenanthren‐2‐yl‐quinoline], have been investigated by density functional theory (DFT) and time‐dependent DFT. They show a wide color tuning of photoluminescence from orange–red (λ = 601 nm) to saturated red (λ = 685 nm). The calculated absorption and emission properties of complexes 1 and 2 are in good agreement with the available experimental data. Complex 6 has the smallest ionization potentials (IP) value, which is consistent with its highest occupied molecular orbital energy level, and thus its hole injection is easiest. Corresponding to its lowest unoccupied molecular orbital energy level, the assumed complex 5 has the large electron affinities value and enhanced electron injection ability compared to the others. These calculated results show that the assumed complex 3 may possess better charge transfer abilities than others and is the potential candidate for an efficient electrophosphorescent polymer‐based red‐emitting material. Copyright © 2013 John Wiley & Sons, Ltd.     

Calculated polar substituent effects on the stability of 3‐substituted(X) bicyclo[1.1.1]pent‐1‐yl cations and 4‐substituted(X) bicyclo[2.2.1]hept‐1‐yl cations: a DFT study

The effects of substituents on the stability of 3‐substituted(X) bicyclo[1.1.1]pent‐1‐yl cations (3) and 4‐substituted(X) bicyclo[2.2.1]hept‐1‐yl cations (4), for a set of substituents (X = H, NO₂, CN, NC, CF₃, CHO, COOH , F, Cl, HO, NH₂, CH₃, SiH₃, Si(CH₃)₃, Li, O^?, and NH₃⁺) covering a wide range of electronic substituent effects were calculated using the DFT theoretical model at the B3LYP/6‐311 + G(2d,p) and B3LYP/6‐31 + G (d) levels of theory, respectively. Linear regression analysis was employed to explore the relationship between the calculated relative hydride affinities (ΔE, kcal/mol) of the appropriate isodesmic reactions for 3/4 and polar field/group electronegativity substituent constants (σ_F and σ_χ, respectively). The analysis reveals that the ΔE values for both systems are best described by a combination of both substituent constants. The result highlights the importance of the σ_χ dependency of charge delocalization in these systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Chemically triggered C–ON bond homolysis in alkoxyamines. Part 7. Remote polar effect

In a recent work (Org. Lett. 2012, 14, 358–361), we showed that the activation by benzylation of alkoxyamine 1 (diethyl (1‐(tert‐butyl(1‐(pyridin‐4‐yl)ethoxy)amino)‐2,2‐dimethylpropyl)phosphonate) afforded a surprisingly large C–ON bond homolysis rate constant k_d. Taking advantage of the easy preparation of para‐X‐benzyl‐activated alkoxyamines 2 and of the presence of a shielding methylene group between the two aromatic moieties, we investigated the long range (10 bonds between the X group and the C–ON bond) polar effect for X = H, F, OMe, CN, NO₂, NMe₂, ⁺NHMe₂,Br^?. It was observed that the effect was weak (4‐fold) and mainly due to the zwiterionic mesomeric forms generated by the presence of group X on the para position, i.e. k_d increased for CN and NO₂ and decreased for OMe, NMe₂ and ⁺NMe₂H,Br^?. DFT calculations at the B3LYP/6‐31G(d,p) level were performed to determine orbital interactions (natural bond orbital (NBO) analysis), Mulliken and NBO charges which support the reactivity described. Copyright © 2014 John Wiley & Sons, Ltd.     

Basicity of trifluoromethylsulfonylformamidines. DFT and FTIR study and NBO analysis

The effect of the electron–acceptor substituent CF₃SO₂ at the imine nitrogen atom on the basicity and the electron distribution in N,N‐alkylformamidines ( 1 , 2 , 3 , 4 , 5 ) was studied experimentally by the FTIR spectroscopy and theoretically at the DFT (B3LYP/6‐311+G(d,p)) level of theory, including the natural bond orbital (NBO) analysis. The calculated proton affinities of the imine nitrogen atom and the sulfonyl oxygen (PA_N′ and PA_O) depend on the atomic charges, the C?N′ and N′―S bond polarity and on the energy of interaction of the amine nitrogen and the oxygen lone pairs with antibonding π* and σ*‐orbitals. The basicity of the imine nitrogen atom is increased with the increase of the electron‐donating power of the substituent at the amine nitrogen atom due to stronger interaction n_N → π*_C?N′, but is decreased for the electron‐withdrawing groups MeSO₂ and CF₃SO₂ at the imine nitrogen atom in spite of the increase of this conjugation. Protonation of ( 1 , 2 , 3 , 4 , 5 ) in CH₂Cl₂ solution in the presence of CF₃SO₃H occurs at the imine nitrogen atom, while the formation of hydrogen bonds with 4‐fluorophenol takes place at the sulfonyl oxygen atom, whose basicity is lower than that of N,N′‐dimethylmethanesulfonamide but higher than of N,N′‐dimethyltrifluoromethanesulfonamide. Copyright © 2015 John Wiley & Sons, Ltd.     

Direct observation of the potential distribution within organic light emitting diodes under operation

We show the first direct measurement of the potential distribution within organic light emitting diodes (OLEDs) under operation and hereby confirm existing hypotheses about charge transport and accumulation in the layer stack. Using a focused ion beam to mill holes in the diodes we gain access to the cross section of the devices and explore the spatially resolved potential distribution in situ by scanning Kelvin probe microscopy under different bias conditions. In bilayer OLEDs consisting of tris(hydroxyquinolinato) aluminum (Alq₃)/N, N ′‐bis(naphthalene‐1‐yl)‐N,N ′‐bis(phenyl) benzidine (NPB) the potential exclusively drops across the Alq₃ layer for applied bias between onset voltage and a given transition voltage. These findings are consistent with previously performed capacitance–voltage measurements. The behavior can be attributed to charge accumulation at the interface between the different organic materials. Furthermore, we show the potential distribution of devices with different cathode structures and degraded devices to identify the cathode interface as main culprit for decreased performance. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Fast‐response organic–inorganic hybrid light‐emitting diode

We demonstrated important changes produced on the modulation frequency of hybrid organic–inorganic light‐emitting diodes to examine the applicability as a light source for visible optical communications. The fabricated device structure was 4,4′‐bis[N ‐(1‐napthyl)‐N ‐phenyl‐amino]biphenyl/4,4′‐(bis(9‐ethyl‐3‐carbazovinylene)‐1,1′‐biphenyl:4,4′‐bis[9‐dicarbazolyl]‐2,2′‐biphenyl/ZnS/LiF/MgAg. This device showed an improvement in the modulation frequency using ZnS instead of an organic material, tris(8‐hydroxyquinoline)aluminum. A maximum cutoff frequency of 20.6 MHz was achieved.

     

Time‐resolved resonance Raman and density functional theory study of the photochemistry of 4‐benzoylpyridine in acetonitrile and 2‐propanol

A nanosecond time‐resolved resonance Raman (ns‐TR³) spectroscopic investigation of the intermolecular hydrogen‐abstraction reaction of the triplet state of 4‐benzoylpyridine (4‐BPy) in 2‐propanol solvent is reported. The TR³ results reveal a rapid hydrogen abstraction (<10 ns) by the 4‐BPy triplet state (nπ*) with the 2‐propanol solvent, leading to formation of a 4‐BPy ketyl radical and an associated dimethyl ketyl radical partner from the solvent. The recombination of these two radical species occurs with a time constant about 200 ns to produce a para‐N‐LAT (light absorbing transient). The structure, major spectral features, and identification of the ketyl radical and the para‐N‐LAT coupling complex have been determined and confirmed by comparison of the TR³ results with results from density functional theory (DFT) calculations. A reaction pathway for the photolysis of 4‐BPy in 2‐propanol deduced from the TR³ results is also presented. The electron‐withdrawing effect of the heterocyclic nitrogen for 4‐BPy on the triplet state makes it have a significantly higher chemical reactivity for the hydrogen abstraction with 2‐propanol compared to the previously reported corresponding benzophenone triplet reaction under similar reaction conditions. In addition, the 4‐BPy ketyl radical reacts with the dimethyl ketyl radical to attach at the para‐N atom position of the pyridine ring to form a cross‐coupling product such as 2‐[4‐(hydroxy‐phenyl‐methylene)‐4h‐pyridin‐1‐yl]‐propan‐2‐ol instead of attacking at the para‐C atom position as was observed for the corresponding benzophenone reaction reported in an earlier study. Copyright © 2008 John Wiley & Sons, Ltd.     

Syntheses,X‐ray crystal structures,and emission properties of diprotonated tetrapyridylpyrazine and triprotonated terpyridine

A series of metal‐free compounds, that is, planar diprotonated tetraH₂(PF₆)₂ ( 1 ), planar monoprotonated bppzHPF₆ ( 2 ), nonplanar diprotonated bppzH₂(PF₆)₂ ( 3 ), and planar triprotonated terpyH₃Cl(PF₆)₂ ( 4 ), were prepared and characterized by electrospray ionization mass spectrometry, Ultraviolet–visible spectroscopy and cyclic voltammetry. Abbreviations used are tetra = tetra‐2‐pyridylpyrazine, bppz = 2,3‐bis(2‐pyridyl)pyrazine, and terpy = 2,2′:6′,2″‐terpyridine. The X‐ray crystal structures of the four compounds 1 , 2 , 3 , and 4 were determined. Both protonated pyridine rings are hydrogen bonded intramolecularly to the adjacent pyridine ring in compounds 1 and 3 . The π–π* absorption bands in the ultraviolet region for 1 , 2 , 3 , and 4 in acetonitrile were red‐shifted relative to those of the corresponding neutral unprotonated compounds. All the cyclic voltammetry for the protonated species, 1 , 2 , 3 , and 4 , showed the first reduction waves from ?0.37 to ?1.18 V, that were more positive than those of the neutral ones. Density functional theory was applied to interpret the planarity in 1 . The attachment of two protons to the two terminal tetra nitrogens in 1 leads to the remarkable emission (Φ = 0.031). The attachment of three protons to the three terpy nitrogens in 4 also gives the large quantum yield (Φ = 0.48). Copyright © 2015 John Wiley & Sons, Ltd.     

Hydrolytic reactions of cyclic bis(3′‐5′)diadenylic acid (c‐di‐AMP)

Hydrolytic reactions of cyclic bis(3′‐5′)diadenylic acid (c‐di‐AMP) have been followed by Reversed phase high performance liquid chromatography (RP‐HPLC) over a wide pH range at 90 °C. Under neutral and basic conditions (pH ≥ 7), disappearance of the starting material (first‐order in [OH^?]) was accompanied by formation of a mixture of adenosine 2′‐monophosphate and 3′‐monophosphate (2′‐AMP and 3′‐AMP). Under very acidic conditions (from H₀ = ?0.7 to 0.2), c‐di‐AMP undergoes two parallel reactions (first‐order in [H⁺]): the starting material is cleaved to 2′‐AMP and 3′‐AMP and depurinated to adenine (i.e., cleavage of the N‐glycosidic bond), the former reaction being slightly faster than the latter one. At pH 1–3, isomerization to cyclic bis(2′‐5′)diadenylic acid competes with the depurination. Copyright © 2012 John Wiley & Sons, Ltd.     

Calculated polar substituent effects on the stability of 4‐substituted(X) ‐ cub‐1‐yl and ‐bicyclo[2.2.2]oct‐1‐yl cations: a DFT study

The effects of substituents on the stability of 4‐substituted(X) cub‐1‐yl cations ( 2 ), as well as the benchmark 4‐substituted(X) bicyclo[2.2.2]oct‐1‐yl cation systems ( 7 ), for a set of substituents (X = H, NO₂, CN, NC, CF₃, COOH , F, Cl, HO, NH₂, CH₃, SiH₃, Si(CH₃)₃, Li, O^?, and NH) covering a wide range of electronic substituent effects were calculated using the DFT theoretical model at the B3LYP/6‐311 + G(2d,p) level of theory. Linear regression analysis was employed to explore the relationship between the calculated relative hydride affinities (ΔE, kcal/mol) of the appropriate isodesmic reactions for 2 / 7 and polar field/group electronegativity substituent constants (σ_F and σ_χ, respectively). The analysis reveals that the ΔE values of both systems are best described by a combination of both substituent constants. This highlights the distinction between through‐space and through‐bond electronic influences characterized by σ_F and σ_χ, respectively. Copyright © 2010 John Wiley & Sons, Ltd.     

Excess polarizabilities upon the first dipole‐allowed excitation of some conjugated oligomers

This paper presents theoretical predictions for the excess polarizabilities upon excitation from the ground state to the first dipole‐allowed excited state (1¹B_u) of some conjugated oligomers. The excess polarizability was obtained by simulating the Stark shift, which was predicted by the time‐dependent density functional theory (TDDFT) with the hybrid Becke‐3 Lee–Yang–Parr (B3LYP) potential. The Stark shift in solution was simulated by employing the non‐equilibrium integral equation formalism polarizable continuum model (IEFPCM). All the model molecules considered in this study were fully optimized by the Hartree–Fock (HF) method and the density functional theory (DFT) with the B3LYP potential, respectively. For diphenylpolyenes, the excess polarizabilities displayed by the DFT/B3LYP‐optimized geometries are more reasonable than those displayed by the HF‐optimized geometries when compared with the experimental results. However, this feature is not clearly demonstrated by our results in the cases of oligo(phenylenevinylene)s (OPVs). Copyright © 2008 John Wiley & Sons, Ltd.