DFT studies of squarylium and core‐substituted squarylium dye derivatives: understanding the causes of the additional shorter wavelength absorption in the latter

Recent reports indicate that the core‐substituted squarylium (CSQ) dyes (obtained when one oxygen atom of the SQ moiety is replaced by an electron withdrawing group or sulfur) show bathochromic shift of the absorption maxima and an additional shorter wave length absorption in visible when compared to parent SQ dyes. To investigate this interesting property of these dyes which will be more suitable for applications in DSSC, a comparative study using computational techniques of some selected SQ and CSQ derivatives has been carried out. The effect of this core substitution on geometries is studied. The ground state charge distribution is analyzed by natural population analysis. It is noticed that the biradical character, which is normally large in SQ derivatives, is reduced in CSQ due to the substitution and the zwitterionic character is increased. The absorption maxima for both parent SQ and CSQ dyes obtained with TD‐DFT methods using various functional like B3LYP, M06‐2X and CAM‐B3LYP methods do not match the experimental results. However, results obtained using SAC‐CI method are better. Charge transfer (CT) data based on Mulliken charges of both ground and excited states is obtained from SAC/SAC‐CI studies. It is seen that on excitation substantial CT from the side groups to the central core is taking place in parent SQ molecules. In contrast, intense CT occurs from –X to side groups through central core in the case of CSQ molecules. This study will be helpful in designing and synthesizing new CSQ dyes which makes them suitable for solar cell applications. Copyright © 2012 John Wiley & Sons, Ltd.     

Tuning HOMO–LUMO levels: trends leading to the design of 9‐fluorenone scaffolds with predictable electronic and optoelectronic properties

Highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) tuning is an important consideration in the development of organic‐based semiconducting materials. A study of the specific effects and overall trends for the HOMO–LUMO tuning of a diverse series of 9‐fluorenones by means of extended conjugation and substituent effects is described. Trends were explored in a range of compounds, beginning with structures having highly electron‐withdrawing substituents and progressing to structures having highly electron‐donating substituents. Compounds with an incremental increase in conjugation were also examined. Electrochemical and optical measurements were used to calculate the HOMO–LUMO levels and HOMO–LUMO bandgap (HLG) for each structure. Results from both methods were compared and correlated with the differences in molecular structure. Increasing the electron‐donating character of the substituents was observed to decrease the HLG and increase the energy levels of the HOMO and the LUMO, whereas an increase in the electron‐withdrawing character produced the opposite results. Increasing conjugation decreased the HLG, increased the HOMO energy level, but decreased the LUMO energy level. Spectroscopic evidence of substituent influence on the carbonyl suggests that substituents directly impact the HLG by influencing the availability of nonbonding electrons within the carbonyl, which impacts the probability of an nπ* transition. The data presented not only elaborate on the HOMO–LUMO tuning of 9‐fluorenone systems but also enable the consideration of 9‐fluorenones as analogous models for HOMO–LUMO tuning in other more complex polyaromatic systems such as bifluorenylidenes. These trends may provide insight into developing materials with specifically tuned HLGs and HOMO–LUMO levels for a variety of applications. Copyright © 2011 John Wiley & Sons, Ltd.     

Theoretical studies of the structures and optical properties of the bifluorene and its derivatives

The ground and excited structures of the molecules are compared basis on the calculated by HF and CIS, respectively. The ionization potentials (IPs), electron affinities (EAs) and HOMO–LUMO gaps (ΔE_HOMO–LUMO) of the oligomers are studied by the density functional theory (DFT) with B3LYP functional while the vertical excitation energies (E_gs) and the maximal absorption wavelength λ_abs of oligomers of bifluorene and its derivatives DFE, DFA, DFBT, FDBO, and FSCHD are studied employing the time dependent density functional theory (TD‐DFT) and ZINDO. Compared with BF, the derivatives DFE, DFA, and DFBT are better conjugated, easier to give an electron or a hole, as well as get an electron or a hole. Their HOMO–LUMO gaps are narrower and they have lower vertical excitation energies. The absorption and emission spectra of them are red shifting. However, FDBO and FSCHD are in the other way round. It is important that FDBO and FSCHD are good blue emitters. Copyright © 2007 John Wiley & Sons, Ltd.     

Tautomerism in 5-Bromouracil: Relationships with Other 5-Haloderivatives and Effect of the Microhydration

A comparative analysis on the effect of the water on the molecular structure and spectroscopy of 5-halogenated uracils was carried out. Solvent effects were considered by using a variable number (1–10) of explicit water molecules surrounding the 5-halouracil derivatives in order to simulate the first hydration shell. More than 300 cluster structures with water were analyzed. B3LYP and MP2 quantum chemical methods were used. For cases where literature data are available, the computed values were in good agreement with previous experimental and theoretical studies. Several general conclusions were underlined.     

NIR‐Triggered Release of Nitric Oxide with Upconversion Nanoparticles Inhibits Platelet Aggregation in Blood Samples

There is a great challenge to overcome the limitation of tissue penetration depth, while maximizing the benefit of light‐triggered biochemical cascades in a well‐defined mode simultaneously. Here, a new method of near‐infrared (NIR) light‐triggered release of nitric oxide (NO) by developing upconversion nanoparticles (UCNPs)‐based conjugate chemistry is reported. As the key nanotransducer in the design, core–shell‐structured UCNPs are encapsulated with a layer of SiO₂ and then covalently linked with a potent NO‐releasing donor (S‐nitroso‐N‐acetyl‐dl ‐penicillamine, SNAP). It is featured with highly localized breakage of chemical bonds of SNAP molecules by NIR–UV upconversion, enabling simultaneous NO release in a light dosage‐dependent manner. The biological effects of NO releasing are demonstrated by cellular imaging and inhibition of platelet aggregation from blood samples. This work provides a flexible and robust platform to generate cell‐signaling gas molecules trigged by NIR laser with deep tissue penetration.     

How glucosylation triggers physical–chemical properties of curcumin: an experimental and theoretical study

In the present study, we investigate the structures of glucosylated curcumin derivatives with DFT at B3LYP/6‐31G* level. A conformational analysis is performed in order to determine the conformational minimum (GS) and rotational transition state (TS) of curcumin derivatives and then their electronic features are evaluated. HOMO and LUMO frontier orbitals and maps of electron density potential (MEPs) are plotted and compared. In order to correlate their predicted spectroscopic properties with IR, UV–vis and NMR experimental data we extended the theoretical study on electronic properties to different solvents (H₂O, MeOH, ACN, DMSO). The main finding is that the curcuminic core maintains the same geometrical and electronic structures in all compounds miming the metal coordination capability showed by curcumin. Therefore, we may confirm that the presence of glucose does not affect the electronic properties of the derivatives. Copyright © 2010 John Wiley & Sons, Ltd.     

Theoretical study of new acceptor and donor molecules based on polycyclic aromatic hydrocarbons

Functionalized polycyclic aromatic hydrocarbons (PAHs) are an interesting class of molecules in which the electronic state of the graphene-like hydrocarbon part is tuned by the functional group. Searching for new types of donor and acceptor molecules, a set of new PAHs has recently been investigated experimentally using ultraviolet photoelectron spectroscopy (UPS). In this work, the electronic structure of the PAHs is studied theoretically with the help of B3LYP hybrid density functionals. Using the ΔSCF method, electron binding energies have been determined which affirm, specify and complement the UPS data. Symmetry properties of molecular orbitals are analyzed for a categorization and an estimate of the related signal strength. While σ-like orbitals are difficult to detect in UPS spectra of condensed film, calculation provides a detailed insight into the hidden parts of the electronic structure of donor and acceptor molecules. In addition, a diffuse basis set (6-311++G**) was used to calculate electron affinity and LUMO eigenvalues. The calculated electron affinity (EA) provides a classification of the donor/acceptor properties of the studied molecules. Coronene-hexaone shows a high EA, comparable to TCNQ, which is a well-known classical acceptor. Calculated HOMO-LUMO gaps using the related eigenvalues have a good agreement with the experimental lowest excitation energies. TD-DFT also accurately predicts the measured optical gap.     

Theoretical study of the optical,electronic, and charge-transfer properties of 1,2,4,5-tetrakis(phenylethynyl)benzene derivatives for solar cell applications

A series of 1,2,4,5-tetrakis(phenylethynyl)benzene derivatives has been investigated at the CAM-B3LYP/6-31G(d) and TD-CAM-B3LYP/6-31?+?G(d,p) levels to design materials with high performance with respect to suitable frontier molecular orbitals (FMOs), broad absorption spectra, and better and balanced charge-transfer properties. The calculated results reveal that the molecule possessing benzene has the largest torsion angle of these derivatives. Different branches have a slight influence on the distributions of the FMOs of the molecules. 2-vinyl-thieno[3,2-b]thiophene branches display a small HOMO–LUMO gap corresponding to red shifts of the absorption spectra. These molecules are potential ambipolar charge-transport materials under the appropriate operating conditions.     

Theoretical investigations of pyridine derivatives as potential high energy density materials

Density function theory has been employed to study pyridine derivatives at the B3LYP/6‐31 G(d,p) and B3P86/6‐31 G(d,p) levels. The crystal structures were obtained by molecular mechanics methods. The heats of formation (HOFs) were predicted based on the isodesmic reactions. Detonation performance was evaluated by using the Kamlet–Jacobs equations based on the calculated densities and HOFs. The thermal stability of the title compounds was investigated by the bond dissociation energies and the energy gaps (ΔE_LUMO?HOMO) predicted. It is found that there are good linear relationships between detonation velocity, detonation pressure, and the number of nitro group. The simulation results reveal that molecule G performs similar to the famous explosive HMX and molecule D outperforms HMX. According to the quantitative standard of energetics and stability as high energy density materials, molecule D essentially satisfies this requirement. These results provide basic information for molecular design of novel high energetic density materials. Copyright © 2012 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering from gold‐coated germanium oxide nanowires

We utilized bulk‐synthesized nanowires (NWs) of germanium dioxide as nanoscale structures that can be coated with noble metals to allow the excitation of surface plasmons over a broad frequency range. The NWs were synthesized on substrates of silicon using gold‐catalyst‐assisted vapor–liquid–solid (VLS) growth mechanism in a simple quartz tube furnace setup. The resulting NWs have diameters of ∼100–200 nm, with lengths averaging ∼10–40 µm and randomly distributed on the substrate. The NWs are subsequently coated with thin films of gold, which provide a surface‐plasmon‐active surface. Surface‐enhanced Raman scattering (SERS) studies with near‐infrared (NIR) excitation at 785 nm show significant enhancement (average enhancement > 10⁶) with good uniformity to detect submonolayer concentrations of 4‐methylbenzenethiol (4‐MBT), trans‐1,2‐bis(4‐pyridyl)ethylene (BPE), and 1,2‐benzendithiol (1,2‐BDT) probe molecules. We also observed an intense, broad continuum in the Raman spectrum of NWs after metal coating, which tended to diminish with the analyte monolayer formation. Copyright © 2008 John Wiley & Sons, Ltd.     

Electronic structure and molecular orbital study of hole-transport material triphenylamine derivatives

Recently, triphenylamine (TPA), 4,4′-bis(phenyl-m-tolylamino)biphenyl (TPD), 4,4′-bis(1-naphthylphenylamino)biphenyl (NPB) and their derivatives are widely used in the organic light-emitting diode (OLED) devices as a hole-transporting material (HTM) layer. We have optimized twenty different structures of HTM materials by using density functional theory (DFT), B3LYP/6-31G method. All these different structures contain mono-amine and diamine TPA derivatives. The energies of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) along with molecular orbitals for these HTMs are also determined. We have found that the central amine nitrogen atom and the phenyl ring, which is next to the central amine nitrogen atom, show significant contribution to the HOMO and LUMO, respectively. The sum of the calculated bond angles (α+β+γ) of the central amine nitrogen atom has been applied to describe the bonding and the energy difference for HOMO and LUMO in these TPA derivatives. Electronic structure calculations have been performed for these TPA derivatives. Again, the LCAO-MO patterns of HOMO and LUMO levels of these derivatives are used to investigate their electron density. A series of electron-transporting steps are predicted for these compounds employing these calculated results.     

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.     

Visible absorbing symmetrical squaraine and croconine dye derivatives: A comparative computational study

Visible absorbing C–N bonding squaraines (SQ1‐SQ5) and croconines (CR1‐CR5) with an increase in conjugation at donor groups and heteroaromatic donor substituents have been studied by density functional theory and time‐dependent density functional theory methodologies. In these molecules, croconines always have absorption nearly 100‐nm red shift than its corresponding squaraines (it is in consistent with C–C bonding, near infrared absorbing squaraine and croconines). The reason behind this drastic red shift, by changing the central acceptor of 4‐membered squarate ring with 5‐membered croconate ring, has been analyzed by considering the concept of diradical character and variation in central C–C–C angle. It is also observed that within the same series of molecules (either in squaraine or in croconines), with an increase in donor capacity (conjugation), absorption increases towards longer wavelength region because of destabilization of HOMO and stabilization of LUMO levels. A small blue shift was observed for heteroaromatic donor groups when compared with aromatic donor group.     

Theoretical study of one‐ and two‐photon absorption properties of expanded donor–acceptor calix[4]arenes

Two types of donor(D)–acceptor(A) calix[4]arenes have been theoretically studied using DFT//B3LYP/6‐31G(d) method and ZINDO/CISD method. The calculations show that the substitution of C? C by the conjugation bridge C?C and N?N plays an important part in altering one‐photon absorption (OPA) and two‐photon absorption (TPA) properties. The maximum OPA wavelengths of all studied compounds are less than 400 nm, which means high transparency. The geometry of the calixarenes strongly influences the TPA properties of the studied compounds. In addition, the nitro derivatives have a wider TPA response range than other non‐nitro derivatives. The tetrasubstituted calix[4]arenes (type B calixarenes) have a larger TPA cross‐section values than the bisubstituted calix[4]arenes (type A calixarenes). Copyright © 2009 John Wiley & Sons, Ltd.     

Influence of the hydrogen bonding on the basicity of selected macrocyclic amines

The optimized minimum‐energy geometries of different macrocyclic amines and their protonated structures were determined by using ab initio and density functional theory (DFT) calculations. All the gas phase optimizations and energy calculations were performed at the DFT/B3LYP/6‐311++G(d,p) level of theory. The HF/6‐31 + G(d,p) level was used for all single point calculations in the solution phase. Geometry optimizations indicate that the most stable structures are stabilized by intramolecular hydrogen bonds. The proton affinity (PA) of macrocyclic amines is controlled by the strength of intramolecular hydrogen bonds of macrocyclic amines. These hydrogen bonds strongly influence the basicity of heteroatoms in macrocycles. The highest PA value among the studied macrocyclic amines was found to be 264.9 kcal mol^?1 for structure 7. This is comparable with PA of proton sponges such as 1,8‐bis(dimethylamino)naphthalene. The solution phase calculations were carried out in the dimethyl sulfoxide solution as a commonly used solvent in organic reactions. Natural bond orbital analysis was performed to calculate the charge transfers and the second‐order interaction energies (E⁽²⁾) between the donor and acceptor. Quantum theory of atoms in molecules (QTAIM) was also applied to determine the nature of hydrogen bonds. QTAIM studies showed that the intramolecular hydrogen bonds in these structures are electrostatic (closed‐shell) interactions as well as partially covalent and partially electrostatic in nature. Copyright © 2012 John Wiley & Sons, Ltd.     

Cyclacenes and short zigzag nanotubes with alternanting Ge―C bonds: theoretical impacts of Ge on the ground state,strain, and band gap

[6]_n Cyclacenes and short zigzag [6]_n carbon nanotubes (n = 5–10) have unstable singlet open‐shell (S_os) ground states. We have boosted their stability by implementing altering Ge―C bonds that acquire S_cs ground states with larger band gap (ΔE_LUMO–HOMO) at B3LYP and BPW91 levels of theory. Fascinatingly, homodesmic calculations indicated release of almost two folds of strain energy upon substitution of germanium for carbon. This may turn the green lights for synthesis of germanium–carbon cyclacenes and short zigzag nanotubes. Copyright © 2014 John Wiley & Sons, Ltd.     

FT‐Raman and FT‐IR spectral and quantum chemical studies on some flavonoid derivatives: Baicalein and Naringenin

In this study, the experimental and theoretical results on the molecular structures of some flavonoid derivatives (Baicalein and Naringenin) are presented. The FT‐IR and FT‐Raman spectra of the compounds have been recorded together for the first time between 4000–400 cm⁻¹ and 3500–5 cm⁻¹ regions, respectively. The molecular geometry and vibrational wavenumbers of the compounds have been also calculated in their ground states by using ab initio HF and DFT/B3LYP functional with 6‐31G(d,p) basis set used in calculations. The calculations were utilized to the C₁ symmetries of the molecules. All calculations were performed with Gaussian 98 software. The obtained vibrational wavenumbers and optimized geometric parameters were seen to be in good agreement with the experimental data. Scale factors have been used in order to compare how the calculated and experimental data are in agreement. Theoretical infrared intensities were also reported. Copyright © 2008 John Wiley & Sons, Ltd.     

A density functional theory investigation of the stability,aromaticity, and photophysical behavior for the highly conjugated macrocycles containing 4 pyrroles

Porphyrin ( Pr ), porphycene ( Pc ), and [22]porphyrin(2.2.2.2) ( P[22] ) have been theoretically investigated. We design 2 highly conjugated macrocycles containing 4 pyrroles with different linkage bridges, which are named for 4 pyrrole ( Pf ) and methylene‐dipyrrolidine ( Pm ), as the theoretical model so as to investigate the stability, aromaticity, and photophysical behavior of these porphyrin derivatives, and the influence of getting or losing electron to the neutral molecule. The geometric structures of the molecules are optimized by density functional theory method. The absorptions are calculated by the time‐dependent density functional theory method. Based on the optimized structures, the nucleus‐independent chemical shifts (NICS) are calculated. The molecule with negative NICS value possesses larger highest occupied molecular orbital (HOMO)‐lowest occupied molecular orbital (LUMO) gap than that with positive NICS value, the molecule with bigger positive NICS value possesses smaller HOMO‐LUMO gap, and the molecule with bigger negative NICS value (in absolute value) possesses bigger HOMO‐LUMO gap. The current density indicates that the π‐electron delocalization is more effective in Pr and Pc than in Pf , Pm , and P[22] and corresponds to the stability of molecules. The absorptions of the molecules are all in the UV‐visible and infrared regions. The major transitions for most of the molecules are all from HOMO to LUMO. Compared with Pf ^2? , Pr ^2? , Pc ^2? , and P[22] ^2? , Pm ^2? shows distinctive photophysical properties, which is due to the reduced HOMO‐LUMO gap, structural distortion, and strong antiaromaticity.