Single‐Molecule Sensing of Environmental pH—an STM Break Junction and NEGF‐DFT Approach

Sensors play a significant role in the detection of toxic species and explosives, and in the remote control of chemical processes. In this work, we report a single‐molecule‐based pH switch/sensor that exploits the sensitivity of dye molecules to environmental pH to build metal–molecule–metal (m‐M‐m) devices using the scanning tunneling microscopy (STM) break junction technique. Dyes undergo pH‐induced electronic modulation due to reversible structural transformation between a conjugated and a nonconjugated form, resulting in a change in the HOMO–LUMO gap. The dye‐mediated m‐M‐m devices react to environmental pH with a high on/off ratio (≈100:1) of device conductivity. Density functional theory (DFT) calculations, carried out under the non‐equilibrium Green’s function (NEGF) framework, model charge transport through these molecules in the two possible forms and confirm that the HOMO–LUMO gap of dyes is nearly twice as large in the nonconjugated form as in the conjugated form.     

Ab initio investigation of 2,2′‐bis(4‐trifluoromethylphenyl)‐5,5′‐bithiazole for the design of efficient organic field‐effect transistors

The initial molecular structure of 2,2′‐bis(4‐trifluoromethylphenyl)‐ 5,5′‐bithiazole has been optimized in the ground state using density functional theory (DFT). The distribution patterns of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) have also been evaluated. To shed light on the charge transfer properties, we have calculated the reorganization energy of electron λ_e, the reorganization energy of hole λ_h, adiabatic electron affinity (EA_a), vertical electron affinity (EA_v), adiabatic ionization potential (IP_a), and vertical ionization potential (IP_v) using DFT. Based on the evaluation of hole reorganization energy, λ_h, and electron reorganization energy, λ_e, it has been predicted that 2,2′‐bis(4‐trifluoromethylphenyl)‐5,5′‐bithiazole would be a better electron transport material. Finally, the effect of electric field on the HOMO, LUMO, and HOMO–LUMO gap were observed to check its suitability for the use as a conducting channel in organic field‐effect transistors. © 2015 Wiley Periodicals, Inc.     

Optical properties of (Z)‐2‐(2‐phenylhydrazinylidene)acenaphthen‐1(2H)‐one: a potential electron donor in organic solar cells

Electron‐donating molecules play an important role in the development of organic solar cells. (Z )‐2‐(2‐Phenylhydrazinylidene)acenaphthen‐1(2H )‐one (PDAK), C₁₈H₁₂N₂O, was synthesized by a Schiff base reaction. The crystal structure shows that the molecules are planar and are linked together forming `face‐to‐face' assemblies held together by intermolecular C—H…O, π–π and C—H…π interactions. PDAK exhibits a broadband UV–Vis absorption (200–648 nm) and a low HOMO–LUMO energy gap (1.91 eV; HOMO is the highest occupied molecular orbital and LUMO is the lowest unoccupied molecular orbital), while fluorescence quenching experiments provide evidence for electron transfer from the excited state of PDAK to C₆₀. This suggests that the title molecule may be a suitable donor for use in organic solar cells.     

(Dibenzoylmethanato)boron Difluoride Derivatives Containing Triphenylamine Moieties: A New Type of Electron‐Donor/π‐Acceptor System for Dye‐Sensitized Solar Cells

(Dibenzoylmethanato)boron difluoride derivatives containing triphenylamine moieties were synthesized as a new type of electron‐donor/π‐acceptor system. These new compounds exhibited long‐wavelength absorptions in the UV/Vis spectra, and reversible oxidation and reduction waves in cyclic voltammetry experiments. Their amphoteric redox properties are based on their resonance hybrid forms, in which a positive charge is delocalized on the triphenylamine moieties and a negative charge is localized on the boron atoms. Molecular orbital (MO) calculations indicate that their HOMO and LUMO energies vary with the number of phenylene rings connected to the difluoroboron‐chelating ring. This is useful for optimizing the HOMO and LUMO levels to an iodine redox (I^?/I₃^?) potential and a titanium dioxide conduction band, respectively. Dye‐sensitized solar cells fabricated by using these compounds as dye sensitizers exhibited solar‐to‐electric power conversion efficiencies of 2.7–4.4 % under AM 1.5 solar light.     

3,4‐Dithiaphosphole and 3,3′,4,4′‐Tetrathia‐1,1′‐Biphosphole π‐Conjugated Systems: S Makes the Impact

Conjugated systems based on phospholes and 1,1′‐biphospholes bearing 3,4‐ethylenedithia bridges have been prepared using the Fagan–Nugent route. The mechanism of this organometallic route leading to intermediate zirconacyclopentadienes has been investigated by using theoretical calculations. This study revealed that the oxidative coupling leading to zirconacyclopentadienes is favored over oxidative addition within the S? C≡C bond both thermodynamically and kinetically. The impact of the presence of the S atoms on the optical and electrochemical behavior of the phospholes and 1,1′‐biphospholes has been systematically evaluated both experimentally and theoretically. A comparison with their “all‐carbon” analogues is provided. Of particular interest, this comparative study revealed that the introduction of S atoms has an impact on the electronic properties of phosphole‐based conjugated systems. A decrease of the HOMO–LUMO separation and a stabilization of the LUMO level were observed. These general trends are also observed with 1,1′‐biphospholes exhibiting σ–π conjugation. The P atom of the 3,4‐ethylenedithiaphospholes can be selectively oxidized by S₈ or O₂. These P modifications result in a lowering of the HOMO–LUMO separation as well as an increase of the reduction and oxidation potentials. The S atoms of the 3,4‐ethylenedithia bridge of the 2,5‐phosphole have been oxidized using m‐chloroperoxybenzoic acid. The resulting 3,4‐ethylenesulfoxide oxophosphole was characterized by an X‐ray diffraction study. Experimental and theoretical studies show that this novel chemical manipulation results in an increase of the HOMO–LUMO separation and an important decrease of the LUMO level. The electropolymerization of 2‐thienyl‐capped 3,4‐ethylenedithiathioxophosphole and 1,1′‐biphosphole is reported. The impact of the S substituents on the polymer properties is discussed.     

A Triphenylamine with Two Phenoxy Radicals Having Unusual Bonding Patterns and a Closed‐Shell Electronic State

Reported herein is the structure and the electronic properties of a novel triphenylamine derivative having two phenoxy radicals appended to the amino nitrogen atom. X‐ray single crystal analysis and the magnetic resonance measurements demonstrates the unexpected closed‐shell electronic structure, even at room temperature, of the molecule and two unusual C? N bonds with multiple‐bond character. The theoretical calculations support the experimentally determined molecular geometry with the closed‐shell electronic structure, and predicted a small HOMO–LUMO gap originating from the nonbonding character of the HOMO. The optical and electrochemical measurements show that the molecule has a remarkably small HOMO–LUMO gap compared with its triphenylamine precursor.     

Absolute structure of the chiral pyrrolidine derivative (2S)‐methyl (Z)‐5‐(2‐tert‐butoxy‐1‐cyano‐2‐oxoethylidene)pyrrolidine‐2‐carboxylate,a compound with low resonant scattering

The enantiopure monopyrrolidine derivative (2S)‐methyl (Z)‐5‐(2‐tert‐butoxy‐1‐cyano‐2‐oxoethylidene)pyrrolidine‐2‐carboxylate, C₁₃H₁₈N₂O₄, ( 1 ), represents a potential ligand and an attractive intermediate for the synthesis of chiral metal complexes. At the molecular level, the compound features an intramolecular N—H…O hydrogen bond; neighbouring molecules interact via N—H…N contacts to form chains along [100]. Due to its elemental composition, resonant scattering of the target compound is entirely insignificant for diffraction experiments with Mo Kα and small even for Cu Kα radiation. A preliminary study with the harder radiation type confirmed the chiral space group and the suitability of the single crystal chosen; as expected, the results concerning the absolute structure remained completely inconclusive. A second data collection with the longer wavelength gave satisfactory quality indicators for the correct handedness of the molecule, albeit with high standard uncertainties. The absolute configuration has been assessed independently: CD spectra for both enantiomers of the target molecule were calculated and the spectrum for the S‐configured stereoisomer was in agreement with the experiment. The Cotton effect of ( 1 ) may be ascribed to π–π* transitions from HOMO to LUMO and from HOMO to LUMO+1. As both independent techniques agree with respect to the handedness of the target molecule, the absolute structure may be assigned with a high degree of confidence.     

Triarylboranes with a 2‐Dimesitylboryl‐2’‐(N,N‐dimethylamino)biphenyl Core Unit: Structure–Property Correlations and Sensing Abilities to Discriminate Between F− and CN− Ions

A series of triarylboranes, in which different substituents are introduced at the para position of the dimethylamino group of a 2‐dimesitylboryl‐2’‐(N,N‐dimethylamino)biphenyl core unit, have been comprehensively investigated to explore the effect of structural modification on photophysical properties. The introduction of electron‐accepting substituents would facilitate the HOMO→LUMO charge transfer (CT) transition. In contrast, the intramolecular CT transition is significantly prohibited when electron‐donating substituents are incorporated. Notably, the HOMO→LUMO CT transition mainly consists of the transition from the electron‐donating amino group to an electron acceptor other than boryl when a strong electron acceptor such as the dicyanovinyl group is present. This dicyanovinyl‐substituted compound displays sensing abilities to discriminate fluoride and cyanide ions. In solution in THF, the fluoride ions first bind to the boron center, then attack the α‐carbon atom of the dicyanovinyl group, whereas the cyanide anion acts on the electron‐accepting centers in the reverse sequence. As a result, the absorption and emission change in different manners upon addition of fluoride and cyanide ions.     

Charge Transport and Conductance Switching of Redox‐Active Azulene Derivatives

Azulene (Az) is a non‐alternating, aromatic hydrocarbon composed of a five‐membered, electron‐rich and a seven‐membered, electron‐poor ring; an electron distribution that provides intrinsic redox activity. By varying the attachment points of the two electrode‐bridging substituents to the Az center, the influence of the redox functionality on charge transport is evaluated. The conductance of the 1,3 Az derivative is at least one order of magnitude lower than those of the 2,6 Az and 4,7 Az derivatives, in agreement with density functional theory (DFT) calculations. In addition, only 1,3 Az exhibits pronounced nonlinear current–voltage characteristics with hysteresis, indicating a bias‐dependent conductance switching. DFT identifies the LUMO to be nearest to the Fermi energy of the electrodes, but to be an active transport channel only in the case of the 2,6 and the 4,7 Az derivatives, whereas the 1,3 Az derivative uses the HOMO at low and the LUMO+1 at high bias. In return, the localized, weakly coupled LUMO of 1,3 Az creates a slow electron‐hopping channel responsible for the voltage‐induced switching due to the occupation of a single molecular orbital (MO).     

A π‐Stacked Porphyrin–Fullerene Electron Donor–Acceptor Conjugate That Features a Surprising Frozen Geometry

A “frozen” electron donor–acceptor array that bears porphyrin and fullerene units covalently linked through the ortho position of a phenyl ring and the nitrogen of a pyrrolidine ring, respectively, is reported. Electrochemical and photophysical features suggest that the chosen linkage supports both through‐space and through‐bond interactions. In particular, it has been found that the porphyrin singlet excited state decays within a few picoseconds by means of a photoinduced electron transfer to give the rapid formation of a long‐lived charge‐separated state. Density functional theory (DFT) calculations show HOMO and LUMO to be localized on the electron‐donating porphyrin and the electron‐accepting fullerene moiety, respectively, at this level of theory. More specifically, semiempirical molecular orbital (MO) configuration interaction (CI) and unrestricted natural orbital (UNO)‐CI methods shed light on the nature of the charge‐transfer states and emphasize the importance of the close proximity of donor and acceptor for effective electron transfer.     

The Origin of the Improved Efficiency and Stability of Triphenylamine‐Substituted Anthracene Derivatives for OLEDs: A Theoretical Investigation

Herein, we describe the molecular electronic structure, optical, and charge‐transport properties of anthracene derivatives computationally using density functional theory to understand the factors responsible for the improved efficiency and stability of organic light‐emitting diodes (OLEDs) with triphenylamine (TPA)‐substituted anthracene derivatives. The high performance of OLEDs with TPA‐substituted anthracene is revealed to derive from three original features in comparison with aryl‐substituted anthracene derivatives: 1) the HOMO and LUMO are localized separately on TPA and anthracene moieties, respectively, which leads to better stability of the OLEDs due to the more stable cation of TPA under a hole majority‐carrier environment; 2) the more balanceable hole and electron transport together with the easier hole injection leads to a larger rate of hole–electron recombination, which corresponds to the higher electroluminescence efficiency; and 3) the increasing reorganization energy for both hole and electron transport and the higher HOMO energy level provide a stable potential well for hole trapping, and then trapped holes induce a built‐in electric field to prompt the balance of charge‐carrier injection.     

Influence of Backbone Chlorination on the Electronic Properties of Diketopyrrolopyrrole (DPP)‐Based Dimers

Chlorination of π‐conjugated backbones is garnering great interest because of fine‐tuning electronic properties of conjugated materials for organic devices. Herein we report a synthesis of thiophene‐based diketopyrrolopyrrole (DPP) dimers and their chlorinated counterparts by introducing a chlorine atom in the outer thiophene ring to investigate the influence of the chlorination on charge transport. The backbone chlorination lowers both the HOMO and the LUMO of the dimers and leads to a blue‐shift of maximum absorption in compared to unsubstituted counterparts. X‐ray analysis reveals that the chlorine atom prompts the outer thiophene ring out of the planarity of the backbone with a relatively large torsional angle. The chlorinated dimers exhibit slipped one‐dimensional packing decorated with multiple intermolecular interactions, because of a combination of a negative inductive effect and a positive mesomeric effect of the halogen atom, which might facilitate charge transport within the oligomeric backbones. The mobility in the single‐crystal OFET devices of the chlorinated dimers is up to 1.5 cm² V^?1 s^?1, which is two times higher than that of the non‐chlorinated DPP dimers. Our results indicate that the chlorine atom plays a key role in directing non‐covalent interactions to lock the slipped stacks, enabling electronic coupling between adjacent molecules for efficient charge transport. In addition, our results also demonstrate that these DPP dimers with straight n‐octyl chains exhibit higher mobilities than the dimers with branched 2‐ethylhexyl chains.     

Charge Transport through Molecular Rods with Reduced π‐Conjugation

A series of oligophenylene rods of increasing lengths is synthesized to investigate the charge‐transport mechanisms. Methyl groups are attached to the phenyl rings to weaken the electronic overlap of the π‐subsystems along the molecular backbones. Out‐of‐plane rotation of the phenyl rings is confirmed in the solid state by means of X‐ray analysis and in solution by using UV/Vis spectroscopy. The influence of the reduced π‐conjugation on the resonant charge transport is studied at the single‐molecule level by using the mechanically controllable break‐junction technique. Experiments are performed under ultra‐high‐vacuum conditions at low temperature (50 K). A linear increase of the conductance gap with increasing number of phenyl rings (from 260 meV for one ring to 580 meV for four rings) is revealed. In addition, the absolute conductance of the first resonant peaks does not depend on the length of the molecular wire. Resonant transport through the first molecular orbital is found to be dominated by charge‐carrier injection into the molecule, rather than by the intrinsic resistance of the molecular wire length.     

Syntheses,Spectroscopic and DFT Studies of Two New D‐π‐A Compounds

Two novel 1,3‐dithiole‐2‐ylidene derivatives with a push–pull structures, 3‐(4,5‐dicarbomethoxy‐1,3‐dithiol‐2‐ylidene)naphthopyranone 1 and 3‐(4,5‐dimethylthio‐1,3‐dithiol‐2‐ylidene)naphthopyranone 2 , have been synthesized and characterized by ¹H NMR, IR, MS. The UV–vis spectra of 1 , 2 in CH₂Cl₂, the lowest‐energy absorption bands, are centered at 280, 316, and 430 nm for 1 and 284, 317, and 450 nm for 2 , respectively, which are caused by the HOMO → LUMO single electron promotion. Furthermore, the steady‐state fluorescence originating states of 1 , 2 from the excited charge‐transfer were observed. To estimate the position and energies of frontier orbitals for 1 , 2 , DFT calculations were performed using the Gaussian 03 program at the B3LYP/6‐31 G* level. The calculated vertical excitation energies are in good agreement with the experimental data. The high HOMO–LUMO gaps of 1 (3.08 eV) and 2 (3.00 eV) indicate high kinetic stability of the title compounds.     

Are water–aromatic complexes always stabilized due to π–H interactions? LMP2 study

Eight complexes of various aromatic molecules with water have been studied theoretically at the local Møller–Plesset 2nd order theory (LMP2)/aug‐cc‐pVTZ(‐f)//LMP2/6‐31+G* level of theory. Two types of complexes can be formed, depending on the electronic structure of aromatic molecules. Donor hydrocarbons form A‐type complexes, while aromatics bearing electron‐withdrawing substituents form B‐type complexes. A‐type complexes are stabilized due to π–H interactions with the OH bond pointing to the aromatic molecule plane, while B‐type complexes have geometry with the oxygen atom pointing to the aromatic molecule plane stabilized by the interaction of highest occupied molecular orbital (HOMO) of water molecule with π* orbitals of the aromatics. It has been found that a (? HOMO–lowest unoccupied molecular orbital (LUMO)/2 value of aromatic molecule, which can be called “molecular electronegativity,” is useful to predict the type of complex formed by aromatic molecule and water. Aromatic hydrocarbons with “molecular electronegativity” of <0.15 tend to form A‐type complexes, while aromatic molecules with “molecular electronegativity” of <0.15 a.u. form B‐type complexes. The binding energy of water–aromatic complexes undergoes a minimum in the area of switching from A‐type to B type complexes, which can be rationalize in terms of frontier orbital interactions. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Molecular design of a π‐conjugated single‐chain electronically conductive polymer

This study demonstrates that single‐chain π‐conjugated systems can be made electrically conductive by modifying the molecular structures of both ends of the oligomers making up a polymer. That is, the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps of a fairly long polyyne‐type oligomer with appropriately modified molecular structures at both ends are found to be on the order of thermal energy by calculations using density functional theory (DFT) with B3LYP functionals. This result applies to molecular structures with characteristic bond alternations. The peculiar bond alternations are caused by competition between two effects of the bond alternations of the two mutually perpendicular π‐conjugated systems, which partially cancel each other out. It is probable that we can design one‐dimensional polymers with HOMO–LUMO gaps small enough to be conductive by combining the above‐mentioned oligomers with each other as monomer units in the polymer. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Highly Pure Synthesis,Spectral Assignments,and Two‐Photon Properties of Cruciform Porphyrin Pentamers Fused with Benzene Units

Tetrameric porphyrin formation of 2‐hydroxymethylpyrrole fused with porphyrins through a bicyclo[2.2.2]octadiene unit gave bicyclo[2.2.2]octadiene‐fused porphyrin pentamers. Thermal conversion of the pentamers gave fully π‐conjugated cruciform porphyrin pentamers fused with benzene units in quantitative yields. UV/Vis spectra of fully π‐conjugated porphyrin pentamers showed one very strong Q absorption and were quite different from those of usual porphyrins. From TD‐DFT calculations, the HOMO level is 0.49 eV higher than the HOMO?1 level. The LUMO and LUMO+1 levels are very close and are lower by more than 0.27 eV than those of other unoccupied MOs. The strong Q absorption was interpreted as two mutually orthogonal single‐electron transitions (683 nm: 86 %, HOMO→LUMO; 680 nm: 86 %, HOMO→LUMO+1). The two‐photon absorption (TPA) cross section value (σ⁽²⁾) of the benzene‐fused porphyrin pentamer was estimated to be 3900 GM at 1500 nm, which is strongly correlated with a cruciform molecular structure with multidirectional π‐conjugation pathways.     

Self‐Assembly of “Chalcone” Type Push‐Pull Dye Molecules into Organic Single Crystalline Microribbons and Rigid Microrods for Vis/NIR Range Photonic Cavity Applications

A novel supramolecular fluorescent donor–acceptor type dye molecule, (2E,4E)‐1‐(2‐hydroxyphenyl)‐5‐(pyren‐1‐yl)penta‐2,4‐dien‐1‐one (HPPD) self‐assembles in a mixture of ethanol/chloroform through intermolecular π–π stacking (distance ca. 3.384 Å) to form J‐aggregated single‐crystalline microribbons displaying Fabry–Pèrot (F‐P) type visible‐range optical resonance. The corresponding borondifluoride dye (HPPD‐BF), with a reduced HOMO–LUMO gap, self‐assembles into crystalline microrods acting as an F‐P type resonator in the near‐infrared (NIR) range.     

HOMO Stabilisation in π‐Extended Dibenzotetrathiafulvalene Derivatives for Their Application in Organic Field‐Effect Transistors

Three new organic semiconductors, in which either two methoxy units are directly linked to a dibenzotetrathiafulvalene (DB‐TTF) central core and a 2,1,3‐chalcogendiazole is fused on the one side, or four methoxy groups are linked to the DB‐TTF, have been synthesised as active materials for organic field‐effect transistors (OFETs). Their electrochemical behaviour, electronic absorption and fluorescence emission as well as photoinduced intramolecular charge transfer were studied. The electron‐withdrawing 2,1,3‐chalcogendiazole unit significantly affects the electronic properties of these semiconductors, lowering both the HOMO and LUMO energy levels and hence increasing the stability of the semiconducting material. The solution‐processed single‐crystal transistors exhibit high performance with a hole mobility up to 0.04 cm² V^?1 s^?1 as well as good ambient stability.     

Impact of replacement of the central benzene ring in anthracene by a heterocyclic ring on electronic excitations and reorganization energies in anthratetrathiophene molecules

The impact of changing the central benzene ring on the electronic excitations and reorganization energies (λ) of the anthratetrathiophene (ATT) molecules is studied by density functional theory (DFT) and time‐dependent DFT (TD‐DFT) quantum chemical calculations. The effect of changing the position of the sulfur atom at the periphery of anthracene on the optical and charge transfer properties is also studied. The calculated results suggest that the HOMO, LUMO, HOMO–LUMO energy gap, ionization potential (IP), electron affinity (EA), hole extraction potential (HEP), electron extraction potential (EEP), and reorganization energies (λ) are affected by replacing the central ring with different heterocyclic rings and the position of the sulfur atom. In addition, all molecules show good hole‐ and electron‐transport properties. This work may be helpful for future design and preparation of high‐performance charge‐transport materials.