Determination of first-order molecular hyperpolarizability of EAADCy and EDMAADCy using steady-state spectroscopic data and quantum-chemical calculations

Ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate (EAADCy) and ethyl 5-(4-dimethylaminophenyl)-3-amino-2,4-dicyanobenzoate (EDMAADCy) organic molecules containing separate electron donor and electron acceptor groups belong to biphenyl derivatives in which a large dipole moment change between ground (S₀) and the first intramolecular charge transfer excited (S₁) states, as well as a large transition moment have been noted. The existence of electronically excited states with a strong intermolecular charge transfer (ICT) character is an essential prerequisite for large non-linear optical properties. Therefore, in this paper, we present a scrupulous analysis of the first-order hyperpolarizabilities of the studied molecules using an equivalent internal field model of an organic molecule. The calculated (using semiempirical calculations, CAChe WS 5.04) additive part of the first-order hyperpolarizability, β_add, values are discussed in relationship to the experimental data of the charge transfer hyperpolarizability, β_CT, obtained from steady-state spectroscopic measurements.     

Crystal structures and transistor properties of phenyl-substituted tetrathiafulvalene derivatives

The crystal structures, thin-film properties, and field-effect transistor (FET) characteristics of tetrathiafulvalene (TTF) derivatives with two phenyl groups are systematically investigated. The highest mobility, 0.11?cm(2)?V(-1)?s(-1), is observed in biphenyl-substituted TTF (1). The correlation between the crystal structures and the FET properties demonstrates that good transistor properties are associated with two-dimensional intermolecular interaction, which is achieved when the molecules are standing nearly perpendicular to the substrate. Since these TTF derivatives are strong electron donors, the use of a metallic charge-transfer salt (TTF)(TCNQ) as the source and drain electrodes has resulted in a considerable reduction of the off current (TCNQ: tetracyanoquinodimethane).     

Molecular charge-transfer interaction with single-layer graphene

While the effect of electrochemical doping on single-layer graphene (SG) with holes and electrons has been investigated, the effect of charge-transfer doping on SG has not been examined hitherto. Effects of varying the concentration of electron donor and acceptor molecules such as tetrathiafulvalene (TTF) and tetracyanoethylene (TCNE) on SG produced by mechanical exfoliation as well as by the reduction of single-layer graphene oxide have been investigated. TTF softens the G-band in the Raman spectrum, whereas TCNE stiffens the G-band. The full-width-at-half-maximum of the G-band increases on interaction with both TTF and TCNE. These effects are similar to those found with few-layer graphene, but in contrast to those found with electrochemical doping. A common feature between the two types of doping is found in the case of the 2-D band, which shows softening and stiffening on electron and hole doping, respectively. The experimental results are explained on the basis of the frequency shifts, electron–phonon coupling and structural inhomogeneities that are relevant to molecule–graphene interaction.     

Tuning the Photoinduced Electron Transfer in a Zr‐MOF: Toward Solid‐State Fluorescent Molecular Switch and Turn‐On Sensor

The immobilization of fluorescent photoinduced electron transfer (PET) switches/sensors into solid state, which usually cannot maintain their identical properties in solution, has remained a big challenge. Herein, a water‐stable anthracene and maleimide appended zirconium‐based‐metal–organic framework (Zr‐MOF; UiO‐68‐An/Ma) is reported. Unlike the regular intramolecular “fluorophore–spacer–receptor” format, the separated immobilization of fluorescent (anthracene) and acceptor (maleimide) groups into the framework of a multivariate MOF can also favor a pseudo‐intramolecular fluorescent PET process, resulting in UiO‐68‐An/Ma with very weak fluorescence. Interestingly, after Diels–Alder reaction or thiol‐ene reaction of maleimide groups, the pseudo‐intramolecular fluorescent PET process in UiO‐68‐An/Ma fails and the solid‐state fluorescence of the crystals is recovered. In addition, UiO‐68‐An/Ma shows an interesting application as solid‐state fluorescent turn‐on sensor for biothiols, with the naked eye response at a low concentration of 50 µmol L^?1 within 5 min. This study represents a general strategy to enable the efficient tuning of fluorescent PET switches/sensors in solid state, and considering the fluorescence of the PET‐based MOFs can be restored after addition of analyte/target species, this research will definitely inspire to construct stimuli‐responsive fluorescent MOFs for interesting applications (e.g., logic gate) in future.     

Hyperpolarizability studies of some nonconjugated twin donor-acceptor molecules

Extensive theoretical calculation on the effects of spacer length enhancement on the second-order NLO properties of twin donor acceptor molecules having two amide units bridged by the CH₂ spacers was performed. The role of such aliphatic bridges on the Donor-Acceptor groups was computed by ZINDO/CV quantum chemical formalism. The odd-even effects were observed in twin donor acceptor systems (with two aliphatic units) linked by an alkyl spacer of varying length from n = 1 to n = 12. The system considered for the present study was N,N′-alkane-(1, n) diyl bis-4-hydroxy hexanamides. For an odd number of CH₂ spacers, the β value was an order of magnitude higher than that for the even number of CH₂ spacers. The origin for such oscillation is attributed to the similar oscillations in the dipole moment difference between the ground state and the dipole allowed state and to some extent on the variation in the oscillator strength.     

Fine‐Tuning the Energy Levels of a Nonfullerene Small‐Molecule Acceptor to Achieve a High Short‐Circuit Current and a Power Conversion Efficiency over 12% in Organic Solar Cells

Organic solar cell optimization requires careful balancing of current–voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor–donor–acceptor (A–D–A)‐type small‐molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near‐IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the V_oc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT‐ and NFBDT‐based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge‐pair states is observed, followed by effective charge extraction. As a result, the PBDB‐T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%—among the best for solution‐processed organic solar cells.     

Nanoscale “Noise‐Source Switching” during the Optoelectronic Switching of Phase‐Separated Polymer Nanocomposites

A method is developed to directly map nanoscale “noise‐source switching” phenomena during the optoelectronic switching of phase‐separated polymer nanocomposites of tetrathiafulvalene (TTF) and phenyl‐C₆₁‐butyric acid methyl ester (PCBM) molecules dispersed in a polystyrene (PS) matrix. In the method, electrical current and noise maps of the nanocomposite film are recorded using a conducting nanoprobe, enabling the mapping of a conductivity and a noise‐source density. The results provide evidence for a repeated modulation in noise sources, a “noise‐source switching,” in each stage of a switching cycle. Interestingly, when the nanocomposite is “set” by a high bias, insulating PS‐rich phases shows a drastic decrease in a noise‐source density which becomes lower than that of conducting TTF‐PCBM‐rich phases. This can be attributed to a trap filling by charge carriers generated from a TTF (donor)–PCBM (acceptor) complex. In addition, when the film is exposed to UV, an optical switching occurs due to chemical reactions which lead to irreversible changes on the noise‐source density and conductivity. The method provides a new insight on noise‐source activities during the optoelectronic switching of polymer nanocomposites and thus can be a powerful tool for basic noise research and applications in organic memory devices.     

Efficient and Stable Deep-Blue Fluorescent Organic Light-Emitting Diodes Employing a Sensitizer with Fast Triplet Upconversion

Multiple donor–acceptor-type carbazole–benzonitrile derivatives that exhibit thermally activated delayed fluorescence (TADF) are the state of the art in efficiency and stability in sky-blue organic light-emitting diodes. However, such a motif still suffers from low reverse intersystem crossing rates (k_RISC) with emission peaks <470 nm. Here, a weak acceptor of cyanophenyl is adopted to replace the stronger cyano one to construct blue emitters with multiple donors and acceptors. Both linear donor–π–donor and acceptor–π–acceptor structures are observed to facilitate delocalized excited states for enhanced mixing between charge-transfer and locally excited states. Consequently, a high k_RISC of 2.36 × 10⁶ s⁻¹ with an emission peak of 456 nm and a maximum external quantum efficiency of 22.8% is achieved. When utilizing this material to sensitize a blue multiple-resonance TADF emitter, the corresponding device simultaneously realizes a maximum external quantum efficiency of 32.5%, CIE_y ≈ 0.12, a full width at half maximum of 29 nm, and a T80 (time to 80% of the initial luminance) of > 60 h at an initial luminance of 1000 cd m⁻².     

Creating Graphitic Carbon Nitride Based Donor‐π–Acceptor‐π–Donor Structured Catalysts for Highly Photocatalytic Hydrogen Evolution

Conjugated polymers with tailored donor–acceptor units have recently attracted considerable attention in organic photovoltaic devices due to the controlled optical bandgap and retained favorable separation of charge carriers. Inspired by these advantages, an effective strategy is presented to solve the main obstructions of graphitic carbon nitride (g‐C₃N₄) photocatalyst for solar energy conversion, that is, inefficient visible light response and insufficient separation of photogenerated electrons and holes. Donor‐π–acceptor‐π–donor polymers are prepared by incorporating 4,4′‐(benzoc 1,2,5 thiadiazole‐4,7‐diyl) dianiline (BD) into the g‐C₃N₄ framework (UCN‐BD). Benefiting from the visible light band tail caused by the extended π conjugation, UCN‐BD possesses expanded visible light absorption range. More importantly, the BD monomer also acts as an electron acceptor, which endows UCN‐BD with a high degree of intramolecular charge transfer. With this unique molecular structure, the optimized UCN‐BD sample exhibits a superior performance for photocatalytic hydrogen evolution upon visible light illumination (3428 µmol h^?1 g^?1), which is nearly six times of that of the pristine g‐C₃N₄. In addition, the photocatalytic property remains stable for six cycles in 3 d. This work provides an insight into the synthesis of g‐C₃N₄‐based D‐π–A‐π–D systems with highly visible light response and long lifetime of intramolecular charge carriers for solar fuel production.     

Thin-film phases of organic charge-transfer complexes formed by chemical vapor deposition

Thin films of organic charge-transfer salts, (TTF)(TCNQ) and (TTF)(DMDCNQI), are prepared by the chemical vapor deposition method, where TTF is tetrathiafulvalene, TCNQ is tetracyanoquinodimethane, and DMDCNQI is dimethyldicyanoquinonediimine. Depending on the substrate temperatures, we have obtained randomly oriented polycrystalline phases composed of relatively large crystals and microcrystalline thin-film phases, which sometimes contain well-grown nanowires. The latter shows much different conducting properties from the bulk crystals, and particularly the (TTF)(DMDCNQI) film is nearly as conductive as the (TTF)(TCNQ) film in spite of the bulk insulating property coming from the mixed-stack crystal structure.     

Crystal Structure of C60 and C70 Compounds

Fullerene intercalated compounds are the most intensively examined molecular materials to exhibit superconducting, ferromagnetic, optical non-linear and other properties. the fullerene C₆₀ or C₇₀ serve usually as electron acceptors in these materials. Although the electron acceptor properties of the fullerene are similar to those of the weak organic acceptors, the fullerene forms various C₆₀-based materials, namely clathrates, charge-transfer complexes and weak, molecular complexes. the search for cation species for fullerene-based materials is one of the routes towards progress in the design of materials with interesting physical properties.

Physical properties of the fullerene-derived molecular compounds are determined mainly by their crystal structure packing. Relatively large cavities in the fullerene solid can easily accommodate small units like solvent molecules or electron-donor organic compounds. An intercalation with these species is usually accompanied either by a lowering in crystal symmetry or by a change in the stacking arrangement of the C₆₀ spheres. These factors influence the interactions between fullerene (host) and an organic molecule (guest). Charge transfer between the electron donor molecule and the fullerene is usually weak and is hindered by unfavourable steric factors; it does not correlate with the ionization potential of the donor.

In this paper we present characteristic structures of one-, two-, and three-component fullerene compounds or else the structures of fullerene clathrates, neutral (van der Waals) complexes and ionic charge-transfer complexes. One can conclude that the stability and properties of the fullerene-based derivatives are defined by the steric compatibility between the three-dimensional donor and the spherical or elongated fullerene.     

Crystal Structure of C60 and C70 Compounds

Abstract

Fullerene intercalated compounds are the most intensively examined molecular materials to exhibit superconducting, ferromagnetic, optical non-linear and other properties. the fullerene C₆₀ or C₇₀ serve usually as electron acceptors in these materials. Although the electron acceptor properties of the fullerene are similar to those of the weak organic acceptors, the fullerene forms various C₆₀-based materials, namely clathrates, charge-transfer complexes and weak, molecular complexes. the search for cation species for fullerene-based materials is one of the routes towards progress in the design of materials with interesting physical properties.

Physical properties of the fullerene-derived molecular compounds are determined mainly by their crystal structure packing. Relatively large cavities in the fullerene solid can easily accommodate small units like solvent molecules or electron-donor organic compounds. An intercalation with these species is usually accompanied either by a lowering in crystal symmetry or by a change in the stacking arrangement of the C₆₀ spheres. These factors influence the interactions between fullerene (host) and an organic molecule (guest). Charge transfer between the electron donor molecule and the fullerene is usually weak and is hindered by unfavourable steric factors; it does not correlate with the ionization potential of the donor.

In this paper we present characteristic structures of one-, two-, and three-component fullerene compounds or else the structures of fullerene clathrates, neutral (van der Waals) complexes and ionic charge-transfer complexes. One can conclude that the stability and properties of the fullerene-based derivatives are defined by the steric compatibility between the three-dimensional donor and the spherical or elongated fullerene.     

Study of photocatalytic properties of pure and doped ZnO powders

In this paper pure ZnO and doped Bi–ZnO powders annealed at 200 °C were prepared using conventional solid state reaction and were applied as photocatalysts for Cr(VI) reduction. Structural properties were studied using X‐ray diffraction (XRD) and optical properties were investigated using photoluminescence (PL) spectrophotometry and ultraviolet‐visible (UV‐vis) absorbance spectroscopy. Based on the obtained results, it was found that bismuth (Bi) had a strong impact in varying the electronic structure of ZnO. This was mainly referred to Bi that existed in different states as an acceptor, donor and plasmonic nanostructure metal. According to the literature Bi was known to be acted as an acceptor and donor without knowing how it acted in this manner. A new mechanism was proposed describing the different states of Bi as waves having different orientations when interacting within ZnO. The time at which an interaction of incident UV light with the oriented Bi wave within ZnO occurred must be less than the duration of existence of such wave at this orientation.     

Optical and Magnetic Properties of Half-metallic (Zn, Mn)O Behaviors with LDA and LDA-SIC Approximations

Based on first-principles spin-density functional calculations, the optical and magnetic properties of Mn-doped ZnO diluted magnetic semiconductors with acceptor defects by Zn vacancies (V_Zn) are studied, using the Korringa–Kohn–Rostoker method (KKR) combined with the coherent potential approximation (CPA). Ferromagnetic and half-metallic behaviors were observed and confirmed with the local-moment-disordered (LMD) state energy for local density approximation (LDA) and local density approximation-self-interaction correction (LDA-SIC). The mechanism of hybridization and the interaction between magnetic ions in P-type (Zn,?Mn)O are also investigated. Moreover, the optical absorption spectra obtained by ab-initio calculations confirm the ferromagnetic stability based on the charge state of magnetic impurities.     

Synthesis, characterization, and nonlinear optical properties of donor–acceptor conjugated polymers and polymer/Ag nanocomposites

Two new donor–acceptor (D–A) conjugated polymers P1 and P2 containing 3,4-didodecyloxythiophene and 1,3,4-oxadiazole units are synthesized via Wittig reaction methodology. Cyclic voltammetry studies reveal that the polymers are both p and n dopable, and possess low-lying LUMO energy levels (?3.34?eV for P1 and ?3.46?eV for P2) and high-lying HOMO energy levels (?5.34?eV for P1 and ?5.27?eV for P2). The optical band gap of the polymers is in the range of 2.25–2.29?eV, calculated from the onset absorption edge. The polymers emit orange to yellow light in the film state when irradiated with a UV light. The synthesized polymers are used to prepare polymer nanocomposites with different wt% of silver nanoparticles. The polymer nanocomposites are characterized by UV–Vis absorption spectroscopy, field emission scanning electron microscopy, and thermogravimetric analysis. Both polymers and polymer/Ag nanocomposites show good thermal stability with onset decomposition temperature around 300?°C under nitrogen atmosphere. The nonlinear optical properties of polymers and polymer/Ag nanocomposites are measured by Z-scan technique. Both polymers and polymer nanocomposites show a good optical limiting behavior. Nearly five times enhancement in the nonlinear optical properties is observed for polymer/Ag nanocomposites. The value of effective two-photon absorption coefficient (β) is in the order of 10^?10–10^?11?m/W. These results indicate that the synthesized polymers (P1 and P2) and their Ag nanocomposites are expected to be good candidates for application in photonic devices.     

Charge-transfer interaction in single-walled carbon nanotubes with tetrathiafulvalene and their applications

We observed that single-walled carbon nanotube (SWNT) was aligned in the presence of TTF This alignment was induced by a specific interaction between SWNT and tetrathiafulvalene (TTF), a well-known organic donor. The interaction between the two molecules can be explained by a charge-transfer, which was confirmed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The binding energies of S (2P1/2) and S (2P3/2) were shifted from 163.0 eV and 164.1 eV to 163.9 eV and 165.1 eV, respectively. In Raman spectra of the SWNT-TTF, three peaks of SWNT in radial breathing mode were also upshifted by 4-5 cm(-1). The charge-transfer interaction also contributed in modifying the electronic structure of SWNT and furthermore enhanced the electrical conductivity of SWNT. A more conductive thin film was fabricated using the SWNT-TTF Four-probe measurement revealed that the surface resistance of the SWNT-TTF film was reduced to 4.359 omega at room temperature while that of SWNT film was 6.894 omega. These results enable carbon nanotubes to be utilized more for practically for industrial applications in fabricating peculiar nano-sized building blocks.     

Nonlinear optoelectronic materials formed by push–pull (bi)thiophene derivatives functionalized with di(tri)cyanovinyl acceptor groups

Novel nonlinear optical (NLO) materials based on six novel NLO chromophores featuring di(tri)cyanovinyl acceptor linked to (bi)thiophene heterocyclic donor system were fabricated for the first time in polymethyl methacrylate matrices with a 1,064 nm laser working in the 20 ns time pulse regime. Absorption spectra and DFT calculations were also done. This multidisciplinary study showed that tayloring of the optical (linear and nonlinear) properties in the desired direction can be achieved by increasing the length of the π-conjugated heterocyclic system (thiophene vs. bithiophene), the strength of the electron donor groups (H → MeO/EtO → Et₂N) as well as the strength of the electron acceptor moieties (dicyanovinyl vs. tricyanovinyl, two vs. three electron withdrawing cyano groups). Due to the relatively high and tunable second-order susceptibilities (0.08–6.45 pm/V at wavelength 1,064 nm), the studied push–pull chromophores can be denoted as promising second-order NLO chromophores.     

Electronic and optical properties of monoclinic and rutile vanadium dioxide

The electronic and optical properties of vanadium dioxide are investigated in the frameworks of density functional theory and GGA+U, in detail. It is found that, the metal–insulator transition in VO₂ is induced by the on-site correlation effects, accompanied with a distinct charge-transfer. Unlike that in rutile phase, the energy gap in the monoclinic phase opens suddenly and abruptly, which is consistent with the experimental observation. The calculated indirect energy gap (0.32 eV) and the direct energy gap (0.58 eV) can be used to theoretically interpret the experimental optical transmission at 0.31 eV and the optical energy gap 0.6 eV, respectively. Consequently, both of them are confirmed by our optical calculation. Furthermore, our calculated optical absorption peaks agree with the experiment very well.     

Ab Initio Quantum Chemical Methods for Investigations of Fullerene C60, CS2 and Tetrathiafulvalene Molecules and Their Complexes

Abstract

Geometry and energy of formation of single molecules: fullerene C₆₀, CS₂ and tetrathiofulvalene (TTF) and their complexes: C₆₀ +CS₂ and C₆₀ +TTF were obtained using Hartree-Fock (HF) and Density Functional Theory methods in various basis sets. Weak chemical interactions were estimated enough well using HF/6-31G for a comparison of various geometrical conformations of these complexes. Energy of formation evaluation in charge-transfer complex C₆₀ +TTF is performing additionally calculating complex with far-separated molecules.     

Triplet Harvesting with 100% Efficiency by Way of Thermally Activated Delayed Fluorescence in Charge Transfer OLED Emitters

Organic light‐emitting diodes (OLEDs) have their performance limited by the number of emissive singlet states created upon charge recombination (25%). Recently, a novel strategy has been proposed, based on thermally activated up‐conversion of triplet to singlet states, yielding delayed fluorescence (TADF), which greatly enhances electroluminescence. The energy barrier for this reverse intersystem crossing mechanism is proportional to the exchange energy (ΔE_ST) between the singlet and triplet states; therefore, materials with intramolecular charge transfer (ICT) states, where it is known that the exchange energy is small, are perfect candidates. However, here it is shown that triplet states can be harvested with 100% efficiency via TADF, even in materials with ΔE_ST of more than 20 kT (where k is the Boltzmann constant and T is the temperature) at room temperature. The key role played by lone pair electrons in achieving this high efficiency in a series of ICT molecules is elucidated. The results show the complex photophysics of efficient TADF materials and give clear guidelines for designing new emitters.