Theoretical prediction of electronic structures of fully pi-conjugated zinc oligoporphyrins with curved surface structures

A theoretical prediction of the electronic structures of fully pi-conjugated zinc oligoporphyrins with curved surface, ring, tube, and ball-shaped structures was conducted as the objective for the future development of triply meso-meso-, beta-beta-, and beta-beta-linked planar zinc oligoporphyrins. The excitation energies and oscillator strengths for the optimal ring and ball structures were calculated using the time-dependent density functional theory (DFT). Although there is an extremely small energy difference of < 0.1 eV between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the ring structure relative to the same-sized triply linked planar one, the Q and B bands of the former are smaller redshifted excitation energies and intensified oscillator strengths than those of the latter due to the structurally shortened effective pi-conjugated lengths for the electron transition. It is expected that the ball structure becomes an excellent electron acceptor and shows the highly redshifted Q' band in the near-IR region relative to the monomer. The minimum value of the HOMO-LUMO energy gaps of the infinite-length ring structures was estimated using periodic boundary conditions within the DFT, resulting in the metallic characters of both the tube structures with and without the spiral triply linked porphyrin array. The relation between the diameters and strain energies of the tube and ball structures was also examined. The present fused zinc porphyrins may become more colorful materials with new optelectronic properties including artificial photosynthesis than the carbon nanotubes and fullerenes when the axial coordinations of the central metal of porphyrins are functionally used.     

Comparison of DFT methods for molecular orbital eigenvalue calculations

We report how closely the Kohn-Sham highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) eigenvalues of 11 density functional theory (DFT) functionals, respectively, correspond to the negative ionization potentials (-IPs) and electron affinities (EAs) of a test set of molecules. We also report how accurately the HOMO-LUMO gaps of these methods predict the lowest excitation energies using both time-independent and time-dependent DFT (TD-DFT). The 11 DFT functionals include the local spin density approximation (LSDA), five generalized gradient approximation (GGA) functionals, three hybrid GGA functionals, one hybrid functional, and one hybrid meta GGA functional. We find that the HOMO eigenvalues predicted by KMLYP, BH&HLYP, B3LYP, PW91, PBE, and BLYP predict the -IPs with average absolute errors of 0.73, 1.48, 3.10, 4.27, 4.33, and 4.41 eV, respectively. The LUMOs of all functionals fail to accurately predict the EAs. Although the GGA functionals inaccurately predict both the HOMO and LUMO eigenvalues, they predict the HOMO-LUMO gap relatively accurately (approximately 0.73 eV). On the other hand, the LUMO eigenvalues of the hybrid functionals fail to predict the EA to the extent that they include HF exchange, although increasing HF exchange improves the correspondence between the HOMO eigenvalue and -IP so that the HOMO-LUMO gaps are inaccurately predicted by hybrid DFT functionals. We find that TD-DFT with all functionals accurately predicts the HOMO-LUMO gaps. A linear correlation between the calculated HOMO eigenvalue and the experimental -IP and calculated HOMO-LUMO gap and experimental lowest excitation energy enables us to derive a simple correction formula.     

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.     

Theoretical study of two‐dimensionally fused zinc porphyrins: DFT calculations

Electronic structures of D_4h square‐fused zinc porphyrin sheets of two types ( SA , SB ), where SA is a directly meso‐meso‐, β‐β‐, and β‐β‐linked array and SB is a directly β‐fused array, were compared using density functional theory (DFT). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of oligomeric SA _n are characteristically delocalized at the cyclooctatetraene‐like sites composed of β‐pyrrolic carbons and their nearest‐neighbor nitrogens. Those of oligomeric SB _n remain solitary monomeric features, reflecting weakly interacting porphyrin units. These two‐dimensionally (2D) square‐fused sheets, especially for SA _n, show effective reduction of both the HOMO–LUMO energy gaps (E_g) and the lowest Q‐like excitation energies because of LUMO's greater stabilization with increasing number of porphyrins than the corresponding one‐dimensionally (1D) linear‐fused tapes. To estimate the minimum value of E_g, the electronic band structures of the infinite‐fused SA _∞ and SB _∞ were examined in detail using modern periodic DFT. Results indicate a full metal for SA _∞, with HOMO and LUMO bands crossing the Fermi level, and a semiconductor with E_g ≈ 0.5 eV for SB _∞. Furthermore, the phonon modes and the electron–phonon coupling (EPC) constant of SA _∞ were calculated throughout the Brillouin zone using density functional perturbation theory (DFPT), yielding a weak EPC constant, λ = 0.35. Within the standard phonon‐mediated BCS mechanism, the superconducting transition temperature, T_c is demonstrated using the McMillan formula, predicting ≈0.5 K. Results show that SA _∞ will become a rare synthetic metal/superconductor without a metal‐insulator transition coming from Peierls lattice instability because it has no serious imaginary phonon modes. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Opp‐Dibenzoporphyrins as a Light‐Harvester for Dye‐Sensitized Solar Cells

New opp‐dibenzoporphyrins were prepared in a concise method that was based on a Pd⁰‐catalyzed cascade reaction. These porphyrins, which contained carboxylic‐acid linker groups on benzene rings that were fused to the porphyrin at their β,β′‐positions, were examined as sensitizers for dye‐sensitized solar cells for the first time. Whereas all of the porphyrins showed solar‐energy‐to‐electricity conversion, an opp‐dibenzoporphyrin with conjugated carboxylic‐acid linkers displayed the highest conversion efficiency and an exceptionally high J_sc value. Cyclic voltammetry of these porphyrins suggested that the fusion of two aromatic benzene rings onto the periphery of the porphyrin lowered the HOMO–LUMO energy gap; the incorporation of a conjugated carboxylic‐acid linker group decreased the HOMO–LUMO gap even further. These CV data are consistent with DFT calculations for these porphyrins and agree well with the UV/Vis absorption‐ and fluorescence spectra of these porphyrins.     

First-Principles Investigation of the Electronic and Conducting Properties of Oligothienoacenes and their Derivatives

Herein, we calculated reorganization energies, vertical ionization energies, electron affinities, and HOMO–LUMO gaps of fused thiophenes and their derivatives, and analyzed the influence of different substituents on their electronic properties. Furthermore, we simulated the angular resolution anisotropic mobility for both electron- and hole-transport, based on quantum-chemical calculations combined with the Marcus–Hush electron-transfer theory. We showed that: 1) styrene-group substitution can effectively elevate the HOMO energy level and lower the LUMO energy level, and therefore lower both the hole- and electron-injection barriers; and 2) chemical oxidation of the thiophene ring can significantly improve the semiconductor properties of the fused oligothiophenes through a decrease of the injection barrier and an increase in the charge-transfer mobility for electrons but without lowering their hole-transfer mobilities, which suggests that it may be a promising way to convert p-type semiconductors into ambipolar or n-type semiconductor materials.     

Comprehensive Analysis of Fragment Orbital Interactions to Build Highly π‐Conjugated Thienylene‐Substituted Phenylene Oligomers

π‐Conjugated thienylene? phenylene oligomers with fluorinated and dialkoxylated phenylene fragments have been designed and prepared to understand the interactions in fragment orbitals, the influence of the substituents (F, OMe) on the HOMO–LUMO gap, and the role of intramolecular non‐covalent cumulative interactions in the construction of π‐conjugated nanostructures. Their strong conjugation was also evidenced in the gas phase by UV photoelectron spectroscopy and theoretical calculations. These results can be explained by the crucial role of the relative energetic positions of the π orbitals of the dimethoxyphenylene, which was used to model the dialkoxyphenylene entity, in determining the π/π^* orbital levels of the fluorinated phenylene entity. Dialkoxyphenylenes raise the HOMO orbitals, whereas fluorinated phenylenes lower the LUMO orbitals in the oligomers. In addition, the presence of S???F and H???F interactions in the fluorinated phenylene? thienylene compounds add to the S???O interactions in the mixed targets and contribute to the full conjugation in the oligomer, inducing weak inter‐ring angles between the involved aromatic cycles. These results, which showed extended conjugation of the π system, were corroborated by a narrow HOMO–LUMO gap (according to DFT calculations) and by a relatively strong maximum wavelength (as obtained by TD‐DFT calculations and experimental UV/Vis measurements). The crystallographic data of two mixed thienylene? (fluorinated and dialkoxylated phenylene) five‐ring oligomers agree with the above results and show the formation of quasi‐planar conformations with non‐covalent S???O, H???F, and S???F interactions. These studies in the solid and gas phases show the relevance of associating dialkoxyphenylene and fluorinated phenylene fragments with thiophene to lead to oligomers with improved electronic delocalization for electronic or optoelectronic devices.     

Synthesis of 2,5,8,11,14,17-hexafluoro-hexa-peri- hexabenzocoronene for n-type organic field-effect transistors

A fluorine-substituted hexa-peri-hexabenzocoronene was synthesized as a thermostable active material for n-type semiconductors. The LUMO and HOMO energy levels, estimated by UV-vis and photoelectron spectroscopy, were lower by 0.5 eV than those of hexa-peri-hexabenzocoronene. A field-effect transistor fabricated by vacuum sublimation showed n-type performance with a field-effect mobility of 1.6 x 10(-2) cm(2)/Vs and an on/off ratio of 10(4). The electron-withdrawing effect of the fluorine substituents changed the polarity from p-type to n-type.     

Ruthenocene‐Type Complexes of N‐Fused Porphyrins

Ruthenocene‐type hybrid complexes with N‐fused porphyrinato ligands, [Ru(NFp)Cp] (NFp=N‐fused porphyrin, Cp=cyclopentadienyl), have been prepared and characterized by NMR and UV/Vis/NIR spectroscopy, cyclovoltammetry, and X‐ray crystallography. [Ru(NFp)Cp] is a common low‐spin ruthenium(II) complex and shows strong aromaticity. The Ru–Cp distance (1.833 Å) in [Ru(NFp)Cp] is comparable to that in [RuCp₂] (1.840 Å). DFT calculations on [Ru(NFp)Cp] showed the unequivocal contribution of the RuCp moiety as well as the NFp moiety to both the HOMO and LUMO, constructing a three‐dimensional d–π conjugated system. The HOMO–LUMO gaps of [Ru(NFp)Cp] are insensitive to the substituents on the NFp ligand, which is illustrated spectroscopically as well as theoretically. This is in sharp contrast to the ligand precursor, the N‐fused porphyrin, in which the HOMO–LUMO gap is affected by substituents in a similar manner to standard porphyrins and related macrocycles.     

Molecular designing of organic conductor based upon the model of polypyrrole

The electronic structure and properties of polypyrrole (p-Pyr) based substituting by the group of pyrazine (Pyz) and their model compounds were studied by the density functional theory (DFT) at the B3LYP level with 6-31G* basis set. The bond length, the topological analyses and nucleus-independent chemical shifts (NICS) were analyzed and correlated with the electronic properties. The bond length of all compounds is reduced with the increase in the degree of polymerization, and the band gap of these compounds is decreased. The change of NICS shows that the conjugation degree in central section of the polymeric axis is stronger than that in outer section, and the structure of central part was close to quinoid structure in polymers. The theoretical results suggest that the band gap of p-Pyz (polymer of Pyz) (0.37 eV) is much smaller than the band gap of polypyrrole (1.84 eV). The narrow band gap, large HOMO and LUMO bandwidths and small effective masses make p-Pyz have a remarkable elevation of the conductivity, so it may be considered as a very good candidate for conducting material.     

Distortion-Controlled Redshift of Organic Dye Molecules

It is shown, quantum chemically, how structural distortion of an aromatic dye molecule can be leveraged to rationally tune its optoelectronic properties. By using a quantitative Kohn–Sham molecular orbital (KS-MO) approach, in combination with time-dependent DFT (TD-DFT), the influence of various structural and electronic tuning parameters on the HOMO–LUMO gap of a benzenoid model dye have been investigated. These parameters include 1) out-of-plane bending of the aromatic core, 2) bending of the bridge with respect to the core, 3) the nature of the bridge itself, and 4) π–π stacking. The study reveals the coupling of multiple structural distortions as a function of bridge length and number of bridges in benzene to be chiefly responsible for a decreased HOMO–LUMO gap, and consequently, red-shifting of the absorption wavelength associated with the lowest singlet excitation (λ≈560 nm) in the model cyclophane systems. These physical insights together with a rational approach for tuning the oscillator strength were leveraged for the proof-of-concept design of an intense near-infrared (NIR) absorbing cyclophane dye at λ=785 nm. This design may contribute to a new class of distortion-controlled NIR absorbing organic dye molecules.     

Tuning of the electronic properties of H‐passivated armchair graphene nanoribbons by mild border oxidation: Theoretical study on periodic models

We report the results of a DFT study of the electronic properties, intended as highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies, of periodic models of H‐passivated armchair graphene nanoribbons (a‐GNRs) as that synthetized by bottom‐up technique, functionalized by vicinal dialdehydic groups. This material can be obtained by border oxidation in mild and easy to control conditions with ¹Δ_g O₂ as we reported in our previous paper (Ghigo et al., ChemPhysChem 2015, 16, 3030). The calculations show that the two models of border oxidized a‐GNRs (model A, 0.98 nm and model B, 1.35 nm wide) present LUMO and HOMO energies lowered by an extend roughly linearly dependent on the amount of oxygen chemically bound. The frontier orbital energy variations dependence on the % wt of oxygen bound are, for model A: ?0.12 eV for the LUMO and ?0.05 eV for the HOMO; for model B: ?0.15 eV (HOMO) and ?0.06 eV (LUMO). © 2016 Wiley Periodicals, Inc.     

The effect of conjugated spacer on novel carbazole derivatives for dye‐sensitized solar cells: Density functional theory/time‐dependent density functional theory study

The ground‐state structure and frontier molecular orbital of D‐π‐A organic dyes, CFT1A, CFT2A, and CFT1PA were theoretically investigated using density functional theory (DFT) on B3LYP functional with 6‐31G(d,p) basis set. The vertical excitation energies and absorption spectra were obtained using time‐dependent DFT (TD‐DFT). The adsorptions of these dyes on TiO₂ anatase (101) were carried out by using a 38[TiO₂] cluster model using Perdew–Burke–Ernzerhof functional with the double numerical basis set with polarization (DNP). The results showed that the introduction of thiophene–thiophene unit (T–T) as conjugated spacer in CFT2A could affect the performance of intramolecular charge transfer significantly due to the inter‐ring torsion of T–T being decreased compared with phenylene–phenylene (P–P) spacer of CFP2A in the researhcers' previous report. It was also found that increasing the number of π‐conjugated unit gradually enhanced charge separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of these dyes, leading to a high‐efficiency photocurrent generation. The HOMO–LUMO energy gaps were calculated to be 2.51, 2.37, and 2.50 eV for CFT1A, CFT2A, and CFT1PA respectively. Moreover, the calculated adsorption energies of these dyes on TiO₂ cluster were ～14 kcal/mol, implying that these dyes strongly bind to TiO₂ surface. Furthermore, the electronic HOMO and LUMO shapes of all dye–TiO₂ complexes exhibited injection mechanism of electron via intermolecular charge‐transfer transition. © 2012 Wiley Periodicals, Inc.     

Core Halogenation as a Construction Principle in Tuning the Material Properties of Tetraazaperopyrenes

A detailed study on the effects of core halogenation of tetraazaperopyrene (TAPP) derivatives is presented. Its impact on the solid structure, as well as the photophysical and electrochemical properties, has been probed by the means of X‐ray crystallography, UV/Vis and fluorescence spectroscopy, high‐resolution electron energy loss spectroscopy (HREELS), cyclic voltammetry (CV), and DFT modeling. The aim was to assess the potential of this approach as a construction principle for organic electron‐conducting materials of the type studied in this work. Although halogenation leads to a stabilization of the LUMOs compared to the unsubstituted parent compound, the nature of the halide barely affects the LUMO energy while strongly influencing the HOMO energies. In terms of band‐gap engineering, it was demonstrated that the HOMO–LUMO gap is decreased by substitution of the TAPP core with halides, the effect being found to be most pronounced for the iodinated derivative. The performance of the recently reported core‐fluorinated and core‐iodinated TAPP derivatives in organic thin‐film transistors (TFTs) was investigated on both a glass substrate, as well as on a flexible plastic substrate (PEN). Field‐effect mobilities of up to 0.17 cm² Vs^?1 and on/off current ratio of >10⁶ were established.     

Controllable and reversible inversion of the electronic structure in nickel N-confused porphyrin: a case when MCD matters

Nickel N-confused tetraphenylporphyrin, 1, and nickel 2-N-methyl-N-confused tetraphenylporphyrin, 1-Me, exhibit unusual sign-reversed (positive-to-negative intensities in ascending energy) MCD spectra in the Q-type band region, suggesting a rare ΔHOMO < ΔLUMO relationship between π and π* MOs in the porphyrin core. Simple and reversible deprotonation of the external NH proton in 1 dramatically changes the electronic structure of the porphyrin core into the ΔHOMO > ΔLUMO combination characteristic for the meso-(tetraaryl)porphyrins. DFT, time-dependent DFT, and semiempirical ZINDO/S calculations on 1, 1-Me, and 1(-) confirm the experimental finding and successfully explain the MCD pattern in the target compounds.     

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.     

New materials based on thiazolothiazole and thiophene candidates for optoelectronic device applications: theoretical investigations

We report theoretical analysis on the geometries and electronic properties of new conjugated compounds based on thiazolothiazole synthesized by Ando et al. (Synth. Met., 156:327 [13]). The theoretical ground-state geometry and electronic structure of the studied molecules were investigated by the density functional theory (DFT) method at Becke’s three-parameter functional and Lee–Yang–Parr functional (B3LYP) level with 6-31G(d,p) basis set. The effects of the ring structure and the substituents on the geometries and electronic properties of these materials are discussed to investigate the relationship between molecular structure and optoelectronic properties. This investigation was used to drive further syntheses towards compounds more useful as active optoelectronic materials. Theoretical knowledge of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the components is basic in studying organic solar cells, so the HOMO, LUMO, and gap energy V _oc (open-circuit voltage) of the studied compounds are calculated and discussed. These properties suggest these materials as good candidates for use in organic dye-sensitized solar cells.     

Phthalocyanine‐Based Organometallic Nanocages: Properties and Gas Storage

Phthalocyanine (Pc) molecules are well‐known flexible structural units for 1D nanotubes and 2D nanosheets. First‐principles calculations combined with grand canonical Monte Carlo simulations are used to obtain the geometries, electronic structures, optical properties, and hydrogen‐storage capacities of nanocages consisting of six Pc molecules with six Mg or Ca atoms. The primitive Pc cage has T_h symmetry with twofold degeneracy in the highest occupied molecular orbital (HOMO), and threefold degeneracy in the lowest unoccupied molecular orbital (LUMO); the corresponding HOMO–LUMO gap is found to be 0.97 eV. The MgPc and CaPc cages have O_h symmetry with a HOMO–LUMO gap of 1.24 and 1.13 eV, respectively. Optical absorption spectra suggest that the Pc‐based cages can absorb infrared light, which is different from the visible‐light absorption in Pc molecules. We further show that the excess uptake of hydrogen on MgPc and CaPc cages at 298 K and 100 bar (1 bar=0.1 MPa) is about 3.49 and 4.74 wt %, respectively. The present study provides new insight into Pc‐based nanostructures with potential applications.