New 2,6‐Distyryl‐Substituted BODIPY Isomers: Synthesis,Photophysical Properties,and Theoretical Calculations

A 2,6‐distyryl‐substituted boradiazaindacene (BODIPY) dye and a new series of 2,6‐p‐dimethylaminostyrene isomers containing both α‐ and β‐position styryl substituents were synthesized by reacting styrene and p‐dimethylaminostyrene with an electron‐rich diiodo‐BODIPY. The dyes were characterized by X‐ray crystallography and NMR spectroscopy and their photophysical properties were investigated and analyzed by carrying out a series of theoretical calculations. The absorption spectra contain markedly redshifted absorbance bands due to conjugation between the styryl moieties and the main BODIPY fluorophore. Very low fluorescence quantum yields and significant Stokes shifts are observed for 2,6‐distyryl‐substituted BODIPYs, relative to analogous 3,5‐distyryl‐ and 1,7‐distyryl‐substituted BODIPYs. Although the fluorescence of the compound with β‐position styryl substituents on both pyrrole moieties and one with both β‐ and α‐position substituents was completely quenched, the compound with only α‐position substituents exhibits weak emission in polar solvents, but moderately intense emission with a quantum yield of 0.49 in hexane. Protonation studies have demonstrated that these 2,6‐p‐dimethylaminostyrene isomers can be used as sensors for changes in pH. Theoretical calculations provide strong evidence that styryl rotation and the formation of non‐emissive charge‐separated S₁ states play a pivotal role in shaping the fluorescence properties of these dyes. Molecular orbital theory is used as a conceptual framework to describe the electronic structures of the BODIPY core and an analysis of the angular nodal patterns provides a reasonable explanation for why the introduction of substituents at different positions on the BODIPY core has markedly differing effects.     

Electronic Properties of Transition‐Metal‐Decorated Silicene

The electronic properties of 3d transition metal (TM)‐decorated silicene were investigated by using density functional calculations in an attempt to replace graphene in electronic applications, owing to its better compatibility with Si‐based technology. Among the ten types of TM‐doped silicene (TM–silicene) studied, Ti‐, Ni‐, and Zn‐doped silicene became semiconductors, whereas Co and Cu doping changed the substrate to a half‐metallic material. Interestingly, in cases of Ti‐ and Cu‐doped silicene, the measured band gaps turned out to be significantly larger than the previously reported band gap in silicene. The observed band‐gap openings at the Fermi level were induced by breaking the sublattice symmetry caused by two structural changes, that is, the Jahn–Teller distortion and protrusion of the TM atom. The present calculation of the band gap in TM–silicene suggests useful guidance for future experiments to fabricate various silicene‐based applications such as a field‐effect transistor, single‐spin electron source, and nonvolatile magnetic random‐access memory.     

Photoelectron Spectra and Electronic Structures of Some Acceptor‐Substituted Cyclopropanes: Linear Correlation of Substituent Effects on MO Energies with Molecular Structures

The relationship between electronic and geometrical structures in acceptor‐substituted cyclopropanes has been investigated by B3LYP DFT calculations and photoelectron (PE) spectroscopy. The spectra of cyclopropanecarbaldehyde ( 2 ), cyclopropanecarboxylic acid ( 3 ), cyclopropanecarboxylic acid methyl ester ( 4 ), nitrocyclopropane ( 5 ), isothiocyanatocyclopropane ( 6 ), cyanocyclopropane ( 7 ), and 1,1‐dicyanocyclopropane ( 8 ) have been analyzed. The first ionization potential (IP₁) of compounds 2 – 5 was found to be 0.1–0.4 eV higher than that of the analogous isopropyl derivatives indicating—contrary to expectation—that in these compounds the cyclopropyl group acts as a weaker electron donor than an isopropyl group. In the other compounds, IP₁ values are 0.4–1.1 eV lower than in the open‐chain congeners. The Walsh orbitals ω_S and ω_A of the three‐membered ring are substantially stabilized to different extents by interactions with substituent orbitals, and this is reflected in shortened distal and elongated vicinal C? C bonds. Although the nitro group in compound 5 causes large stabilizations of both ω_S and ω_A, their energy difference Δω remains rather small; this is in agreement with a relatively small difference Δr of the C? C bond lengths. For the investigated monosubstituted cyclopropanes 2 – 7 , the largest effects with respect to Δω and Δr are caused by the formyl group in carboxaldehyde 2 . Comparison of the results for nitriles 7 and 8 indicates that the effects of the cyano groups are additive. A linear relationship between Δω and Δr was established by B3LYP DFT calculations on geometrically distorted cyclopropane ( 1 ) and from the PE data of 2 – 8 .     

Excited State Intramolecular Proton Transfer in Ethynyl‐Extended Regioisomers of 2‐(2′‐Hydroxyphenyl)benzothiazole: Effects of the Position and Electronic Nature of Substituent Groups

Although the organic dyes based on excited state intramolecular proton transfer (ESIPT) mechanism have attracted significant attention, the structure‐property relationship of ESIPT dyes needs to be further exploited. In this paper, three series of ethynyl‐extended regioisomers of 2‐(2′‐hydroxyphenyl)benzothiazole (HBT), at the 3′‐, 4′‐ and 6‐positions, respectively, have been synthesized. Changes in the absorption and emission spectra were correlated with the position and electronic nature of the substituent groups. Although 4′‐ and 6‐substituted HBT derivatives exhibited absorption bands at longer wavelengths, the keto‐emission of 3′‐substituted HBT derivatives was found at a substantially longer wavelength. The gradual red‐shifted fluorescence emission was found for 3′‐substituted HBT derivatives where the electron‐donating nature of substituent group increased, which was opposite to what was observed for 4′‐ and 6‐substituted HBT derivatives. The results derived from the theoretical calculations were in conformity with the experimental observations. Our study could potentially provide experimental and theoretical basis for designing novel ESIPT dyes that possess unique fluorescent properties.     

Electronic structure and spectroscopic properties of 6‐aminophenanthridine and its derivatives: Insights from density functional theory

6‐Aminophenanthridine (6AP) and its derivatives show important biological activities as antiprion compounds and inhibitors of the protein folding activity of the ribosome. Both of these activities depend on the RNA binding property of these compounds, which has been recently characterized by fluorescence spectroscopy. Hence, fundamental insights into the photophysical properties of 6AP compounds are highly important to understand their biological activities. In this work, we have calculated electronic structures and optical properties of 6AP and its three derivatives 6AP8CF₃, 6AP8Cl, and 6APi by density functional theory (DFT) and time‐dependent density functional theory (TDDFT). Our calculated spectra show a good agreement with the experimental absorption and fluorescence spectra, and thus, provide deep insights into the optical properties of the compounds. Furthermore, comparing the results obtained with four different hybrid functionals, we demonstrate that the accuracy of the functionals varies in the order B3LYP > PBE0 > M062X > M06HF. © 2015 Wiley Periodicals, Inc.     

Ion‐Pair SN2 Substitution: Activation Strain Analyses of Counter‐Ion and Solvent Effects

The ion‐pair S_N2 reactions of model systems M_nF^n?1+CH₃Cl (M⁺=Li⁺, Na⁺, K⁺, and MgCl⁺; n=0, 1) have been quantum chemically explored by using DFT at the OLYP/6‐31++G(d,p) level. The purpose of this study is threefold: 1) to elucidate how the counterion M⁺ modifies ion‐pair S_N2 reactivity relative to the parent reaction F^?+CH₃Cl; 2) to determine how this influences stereochemical competition between the backside and frontside attacks; and 3) to examine the effect of solvation on these ion‐pair S_N2 pathways. Trends in reactivity are analyzed and explained by using the activation strain model (ASM) of chemical reactivity. The ASM has been extended to treat reactivity in solution. These findings contribute to a more rational design of tailor‐made substitution reactions.     

Theoretical Study of the Substituent Effects on the Nonlinear Optical Properties of a Room‐Temperature‐Stable Organic Electride

Excess‐electron compounds can be considered as novel candidates for nonlinear optical (NLO) materials because of their large static first hyperpolarizabilities (β₀). A room‐temperature‐stable, excess‐electron compound, that is, the organic electride Na@(TriPip222), was successfully synthesized by the Dye group (J. Am. Chem. Soc. 2005 , 127, 12416). In this work, the β₀ of this electride was first evaluated to be 1.13×10⁶ au, which revealed its potential as a high‐performance NLO material. In particular, the substituent effects of different substituents on the structure, electride character, and NLO response of this electride were systemically studied for the first time by density functional theory calculations. The results revealed that the β₀ of Na@(TriPip222) could be further increased to 8.30×10⁶ au by introducing a fluoro substituent, whereas its NLO response completely disappeared if one nitryl group was introduced because the nitro‐group substitution deprived the material of its electride identity. Moreover, herein the dependence of the NLO properties on the number of substituents and their relative positions was also detected in multifluoro‐substituted Na@(TriPip222) compounds.     

Synthesis,Spectroscopy, and Computational Analysis of Photoluminescent Bis(aminophenyl)‐Substituted Thiophene Derivatives

Substituted oligothiophenes have a long history in the field of organic electronics, as they often combine outstanding electro‐optical properties with the ease of synthesis. To assist the rational selection of the most promising structures to be synthesized, there is the demand for tools that allow prediction of the properties of the materials. In this study, we present strategies for synthesis and computational characterization, with respect to the fluorescence behavior of oligothiophene‐based materials for organoelectronic applications. In a combined approach, sophisticated computational methodologies are directly compared to experimental results. The M06‐2X functional in combination with the polarizable continuum model in a state‐specific formulation for excited‐state solvation proved to be particularly reliable. In addition, a semiclassical approach for describing the vibrational broadening of the spectra is employed. As a result, a robust procedure for the prediction of the fluorescence spectra of oligothiophene derivatives is presented.     

Highly Emissive 9-Borafluorene Derivatives: Synthesis,Photophysical Properties and Device Fabrication

A series of 9-borafluorene derivatives, functionalised with electron-donating groups, have been prepared. Some of these 9-borafluorene compounds exhibit strong yellowish emission in solution and in the solid state with relatively high quantum yields (up to 73.6 % for FMesB-Cz as a neat film). The results suggest that the highly twisted donor groups suppress charge transfer, but the intrinsic photophysical properties of the 9-borafluorene systems remain. The new compounds showed enhanced stability towards the atmosphere, and exhibited excellent thermal stability, revealing their potential for application in materials science. Organic light-emitting diode (OLED) devices were fabricated with two of the highly emissive compounds, and they exhibited strong yellow-greenish electroluminescence, with a maximum luminance intensity of >22 000 cd m⁻². These are the first two examples of 9-borafluorene derivatives being used as light-emitting materials in OLED devices, and they have enabled us to achieve a balance between maintaining their intrinsic properties while improving their stability.     

Tuning the Photophysical Properties of BODIPY Molecules by π‐Conjugation with Fischer Carbene Complexes

The synthesis, structure, and photophysical properties of novel BODIPY–Fischer alkoxy‐, thio‐, and aminocarbene dyads are reported. The BODIPY chromophore is directly attached to the carbene ligand by an ethylenic spacer, thus forming donor–bridge–acceptor π‐extended systems. The extension of the π‐conjugation is decisive in the equilibrium geometries of the dyads and is clearly reflected in the corresponding absorption and emission spectra. Whereas the BODIPY fragment is mainly isolated in aminocarbene complexes, it is fully conjugated in alkoxycarbene derivatives. The former thus exhibit the characteristic photophysical properties of BODIPY units, whereas complete suppression of the BODIPY fluorescence emission is observed in the latter, as a direct consequence of the strong electron‐accepting character of the (CO)₅M?C moiety. As the π‐acceptor character of the metal–carbene group can be modified, the electronic properties of the conjugated BODIPY can be tuned. Density functional calculations have been carried out to gain insight into the photophysical properties.     

Excited State Tuning of Bis(tridentate) Ruthenium(II) Polypyridine Chromophores by Push–Pull Effects and Bite Angle Optimization: A Comprehensive Experimental and Theoretical Study

The synergy of push–pull substitution and enlarged ligand bite angles has been used in functionalized heteroleptic bis(tridentate) polypyridine complexes of ruthenium(II) to shift the ¹MLCT absorption and the ³MLCT emission to lower energy, enhance the emission quantum yield, and to prolong the ³MLCT excited‐state lifetime. In these complexes, that is, [Ru(ddpd)(EtOOC‐tpy)][PF₆]₂, [Ru(ddpd‐NH₂)(EtOOC‐tpy)][PF₆]₂, [Ru(ddpd){(MeOOC)₃‐tpy}][PF₆]₂, and [Ru(ddpd‐NH₂){(EtOOC)₃‐tpy}][PF₆]₂ the combination of the electron‐accepting 2,2′;6′,2′′‐terpyridine (tpy) ligand equipped with one or three COOR substituents with the electron‐donating N,N′‐dimethyl‐N,N′‐dipyridin‐2‐ylpyridine‐2,6‐diamine (ddpd) ligand decorated with none or one NH₂ group enforces spatially separated and orthogonal frontier orbitals with a small HOMO–LUMO gap resulting in low‐energy ¹MLCT and ³MLCT states. The extended bite angle of the ddpd ligand increases the ligand field splitting and pushes the deactivating ³MC state to higher energy. The properties of the new isomerically pure mixed ligand complexes have been studied by using electrochemistry, UV/Vis absorption spectroscopy, static and time‐resolved luminescence spectroscopy, and transient absorption spectroscopy. The experimental data were rationalized by using density functional calculations on differently charged species (charge n=0–4) and on triplet excited states (³MLCT and ³MC) as well as by time‐dependent density functional calculations (excited singlet states).     

Tuning the Electronic Coupling in Cyclometalated Diruthenium Complexes through Substituent Effects: A Correlation between the Experimental and Calculated Results

A common bridging ligand, 3,3′,5,5′‐tetrakis(N‐methylbenzimidazol‐2‐yl)biphenyl, and four terpyridine terminal ligands with various substituents (amine, tolyl, nitro, and ester groups) have been used to synthesize ten cyclometalated diruthenium complexes 1 ²⁺– 10 ²⁺. Among them, compounds 1 ²⁺– 6 ²⁺ are redox nonsymmetric, and others are symmetric. These complexes show two Ru^III/II processes and an intervalence charge transfer (IVCT) transition in the one‐electron oxidized state. The potential separation (ΔE) of 1 ²⁺– 10 ²⁺ has been correlated to the energy difference ΔG⁰, the energy of the IVCT band E_op, and the ground‐state delocalization coefficient α². Time‐dependent (TD)DFT calculations suggest that the absorptions in the visible region of 1 ²⁺– 6 ²⁺ are mainly associated with the metal‐to‐ligand charge‐transfer transitions from both ruthenium ions and to both terminal ligands and the bridging ligand. However, the energies of these transitions vary significantly. DFT calculations have been performed on 1 ²⁺– 6 ²⁺ and 1 ³⁺– 6 ³⁺ to give information on the electronic structures and spin populations of the mixed‐valent compounds. The TDDFT‐predicted IVCT excitations reproduce well the experimental trends in transition energies. In addition, three monoruthenium complexes have been synthesized for a comparison study.     

Pyridyl‐Functionalised 3H‐1,2,3,4‐Triazaphospholes: Synthesis,Coordination Chemistry and Photophysical Properties of Low‐Coordinate Phosphorus Compounds

Novel conjugated, pyridyl‐functionalised triazaphospholes with either tBu or SiMe₃ substituents at the 5‐position of the N₃PC heterocycle have been prepared by a [3+2] cycloaddition reaction and compared with structurally related, triazole‐based systems. Photoexcitation of the 2‐pyridyl‐substituted triazaphosphole gives rise to a significant fluorescence emission with a quantum yield of up to 12 %. In contrast, the all‐nitrogen triazole analogue shows no emission at all. DFT calculations indicate that the 2‐pyridyl substituted systems have a more rigid and planar structure than their 3‐ and 4‐pyridyl isomers. Time‐dependent (TD) DFT calculations show that only the 2‐pyridyl‐substituted triazaphosphole exhibits similar planar geometry, with matching conformational arrangements in the lowest energy excited state and the ground state; this helps to explain the enhanced emission intensity. The chelating P,N‐hybrid ligand forms a Re^I complex of the type [(N^N)Re(CO)₃Br] through the coordination of nitrogen atom N² to the metal centre rather than through the phosphorus donor. Both structural and spectroscopic data indicate substantial π‐accepting character of the triazaphosphole, which is again in contrast to that of the all‐nitrogen‐containing triazoles. The synthesis and photophysical properties of a new class of phosphorus‐containing extended π systems are described.     

Synthesis,Characterization, and Properties of [4]Cyclo‐2,7‐pyrenylene: Effects of Cyclic Structure on the Electronic Properties of Pyrene Oligomers

A cyclic tetramer of pyrene, [4]cyclo‐2,7‐pyrenylene ([4]CPY), was synthesized from pyrene in six steps and 18 % overall yield by the platinum‐mediated assembly of pyrene units and subsequent reductive elimination of platinum. The structures of the two key intermediates were unambiguously determined by X‐ray crystallographic analysis. DFT calculations showed that the topology of the frontier orbitals in [4]CPY was essentially the same as those in [8]cycloparaphenylene ([8]CPP), and that all the pyrene units were fully conjugated. The electrochemical analyses proved the electronic properties of [4]CPY to be similar to those of [8]CPP. The results are in sharp contrast to those obtained for the corresponding linear oligomers of pyrene in which each pyrene unit was electronically isolated. The results clearly show a novel effect of the cyclic structure on cyclic π‐conjugated molecules.     

Neutral Nickel Ethylene Oligo‐ and Polymerization Catalysts: Towards Computational Catalyst Prediction and Design

DFT calculations have been used to elucidate the chain termination mechanisms for neutral nickel ethylene oligo‐ and polymerization catalysts and to rationalize the kind of oligomers and polymers produced by each catalyst. The catalysts studied are the (κ²‐O,O)‐coordinated (1,1,1,5,5,5‐hexafluoro‐2,4‐acetylacetonato)nickel catalyst I , the (κ²‐P,O)‐coordinated SHOP‐type nickel catalyst II , the (κ²‐N,O)‐coordinated anilinotropone and salicylaldiminato nickel catalysts III and IV , respectively, and the (κ²‐P,N)‐coordinated phosphinosulfonamide nickel catalyst V . Numerous termination pathways involving β‐H elimination and β‐H transfer steps have been investigated, and the most probable routes identified. Despite the complexity and multitude of the possible termination pathways, the information most critical to chain termination is contained in only few transition states. In addition, by consideration of the propagation pathway, we have been able to estimate chain lengths and discriminate between oligo‐ and polymerization catalysts. In agreement with experiment, we found the Gibbs free energy difference between the overall barrier for the most facile propagation and termination pathways to be close to 0 kcal mol^?1 for the ethylene oligomerization catalysts I and V , whereas values of at least 7 kcal mol^?1 in favor of propagation were determined for the polymerization catalysts III and IV . Because of the shared intermediates between the termination and branching pathways, we have been able to identify the preferred cis/trans regiochemistry of β‐H elimination and show that a pronounced difference in σ donation of the two bridgehead atoms of the bidentate ligand can suppress hydride formation and thus branching. The degree of rationalization obtained here from a handful of key intermediates and transition states is promising for the use of computational methods in the screening and prediction of new catalysts of the title class.     

1,8‐Naphthalimide‐Substituted BODIPY Dyads: Synthesis,Structure, Properties,and Live‐Cell Imaging

A set of 1,8‐naphthalimide (NPI)‐substituted 4,4‐difluoroboradiaza‐s‐indacene (BODIPY) dyads 1 a – 1 c were designed and synthesized by the Pd‐catalyzed Sonogashira cross‐coupling reaction of ethynyl substituted NPI 1 with the meso‐, β‐, and α‐halogenated BODIPYs a , b , and c , respectively. The BODIPY 1 c exhibits redshifted absorption, which suggests better electronic communication with substitution at the α‐position of BODIPY compared with at the meso and β positions, which was further supported by time‐dependent DFT calculations. The optical band gap follows the order 1 a > 1 b > 1 c . The single‐crystal X‐ray structures of dyads 1 a – 1 c are reported, which reflect planar orientations of the BODIPY units with respect to the NPIs. The DFT‐optimized structures show good correlation with the experimental data obtained from the single‐crystal X‐ray structures. The packing diagram of 1 a shows a sheet‐like arrangement, 1 b forms a ladder‐like structural motif, and 1 c forms a complex 3D structural arrangement. The dyads 1 a – 1 c show low cytotoxicity (IC₅₀>100 μm ). The confocal microscopy studies with HeLa and A375 cells (when treated with dyads 1 a – 1 c ) show that all the dyads easily enter the cell membrane and show significant multicolor intracellular fluorescence covering the entire visible range with clear emissions in blue, green, and red channels.