Electronic structures of three anchors of triphenylamine on a p-type nickel oxide(100) surface: density functional theory with periodic models.

In this paper, we investigate the electronic structures of triphenylamine molecules with three different anchoring groups (pyridinyl, carboxyl, and phenyl-1,2-diol) before and after attachment with a p-type semiconductor, nickel oxide (100), surface. To understand the charge transfer characteristics of these structures commonly used in dyes of the dye-sensitized solar cells (DSSC), we use periodic models to study their configurations with density functional theory (DFT). We find that carboxyl and phenyl-1,2-diol anchors adsorb more strongly compared to pyridinyl anchor on NiO(100). Stronger binding is reflected as a bigger dipole moment and a more viable charge transfer from the anchors to NiO(100). Furthermore, the alignment of electronic levels favors charge transfer only for pyridinyl and phenyl-1,2-diol anchors. Despite its weaker binding on the NiO(100) surface, pyridinyl is a more promising anchoring group for transferring charge to NiO, as it does not create trap states.

Utilizing Furfural-Based Bifuran Diester as Monomer and Comonomer for High-Performance Bioplastics: Properties of Poly(butylene furanoate), Poly(butylene bifuranoate), and Their Copolyesters.

Two homopolyesters and a series of novel random copolyesters were synthesized from two bio-based diacid esters, dimethyl 2,5-furandicarboxylate, a well-known renewable monomer, and dimethyl 2,2'-bifuran-5,5'-dicarboxylate, a more uncommon diacid based on biochemical furfural. Compared to homopolyesters poly(butylene furanoate) (PBF) and poly(butylene bifuranoate) (PBBf), their random copolyesters differed dramatically in that their melting temperatures were either lowered significantly or they showed no crystallinity at all. However, the thermal stabilities of the homopolyesters and the copolyesters were comparable. Based on tensile tests from amorphous film specimens, it was concluded that the elastic moduli, tensile strengths, and elongation at break values for all copolyesters were similar as well, irrespective of the furan:bifuran molar ratio. Tensile moduli of approximately 2 GPa and tensile strengths up to 66 MPa were observed for amorphous film specimens prepared from the copolyesters. However, copolymerizing bifuran units into PBF allowed the glass transition temperature to be increased by increasing the amount of bifuran units. Besides enhancing the glass transition temperatures, the bifuran units also conferred the copolyesters with significant UV absorbance. This combined with the highly amorphous nature of the copolyesters allowed them to be melt-pressed into highly transparent films with very low ultraviolet light transmission. It was also found that furan-bifuran copolyesters could be as effective, or better, oxygen barrier materials as neat PBF or PBBf, which themselves were found superior to common barrier polyesters such as PET.

Electronic couplings and rates of excited state charge transfer processes at poly(thiophene-co-quinoxaline)-PC71BM interfaces: two- versus multi-state treatments.

Electronic coupling between adjacent molecules is one of the key parameters determining the charge transfer (CT) rates in bulk heterojunction (BHJ) polymer solar cells (PSCs). We calculate theoretically electronic couplings for exciton dissociation (ED) and charge recombination (CR) processes at local poly(thiophene-co-quinoxaline) (TQ)-PC71BM interfaces. We use eigenstate-based coupling schemes, i.e. the generalized Mulliken-Hush (GMH) and fragment charge difference (FCD) schemes, including 2 to multiple (3-11) states. Moreover, we study the effects of functionals, excited state methods, basis sets, surrounding media, and relative placements of TQ and PC71BM on the coupling values. Generally, both schemes provide consistent couplings with the global hybrid functionals, which yield more charge-localized diabatic states and constant coupling values regardless of the number of states, and so the 2-state schemes may be sufficient. The (non-tuned and optimally tuned) long-range corrected (LRC) functionals result in more notable mixing of the local components with the CT states. Employing multiple states reduces the mixing and thus improves the LRC results, although the method still affects the GMH CR couplings. As the FCD scheme is less sensitive, we recommend combining it with the multi-state treatment for polymer-fullerene systems when using the LRC functionals. Finally, we employ the 11-state FCD couplings to calculate the ED and CR rates, which are consistent with the experimental rates of the polymer-fullerene systems. Our results provide more insight into choosing a suitable eigenstate-based coupling scheme for predicting the electronic couplings and CT rates in photoactive systems.

Synthesis of Benzothiadiazole Derivatives by Applying C-C Cross-Couplings.

The benzothiadiazole moiety has been extensively exploited as a building block in the syntheses of efficient organic semiconducting materials during the past decade. In this paper, parallel synthetic routes to benzothiadiazole derivatives, inspired by previous computational findings, are reported. The results presented here show that various C-C cross-couplings of benzothiadiazole, thiophene, and thiazole derivatives can be efficiently performed by applying Xantphos as a ligand of the catalyst system. Moreover, improved and convenient methods to synthesize important chemical building blocks, e.g., 4,7-dibromo-2,1,3-benzothiadiazole, in good to quantitative yields are presented. Additionally, the feasibility of Suzuki-Miyaura and direct coupling methods are compared in the synthesis of target benzothiadiazole derivatives. The computational characterization of the prepared benzothiadiazole derivatives shows that these compounds have planar molecular backbones and the possibility of intramolecular charge transfer upon excitation. The experimental electrochemical and spectroscopic studies reveal that although the compounds have similar electronic and optical properties in solution, they behave differently in solid state due to the different alkyl side-group substitutions in the molecular backbone. These benzothiadiazole derivatives can be potentially used as building blocks in the construction of more advanced small molecule organic semiconductors with acceptor-donor-acceptor motifs.

Intrinsic Properties of Two Benzodithiophene-Based Donor--Acceptor Copolymers Used in Organic Solar Cells: A Quantum-Chemical Approach.

Conjugated donor-acceptor (D-A) copolymers show tremendous promise as active components in thin-film organic bulk heterojunction solar cells and transistors, as appropriate combinations of D-A units enable regulation of the intrinsic electronic and optical properties of the polymer. Here, the structural, electronic, and optical properties of two D-A copolymers that make use of thieno[3,4-c]pyrrole-4,6-dione as the acceptor and differ by their donor unit-benzo[1,2-b:4,5-b']dithiophene (BDT) vs the ladder-type heptacyclic benzodi(cyclopentadithiophene)-are compared using density functional theory methods. Our calculations predict some general similarities, although the differences in the donor structures lead also to clear differences. The extended conjugation of the stiff ladder-type donor destabilizes both the highest occupied and lowest unoccupied molecular orbital energies of the ladder copolymer and results in smaller gap energies compared to its smaller counterpart. However, more significant charge transfer nature is predicted for the smaller BDT-based copolymer by natural transition orbitals than for the ladder copolymer. That is, the influence of the acceptor on the copolymer properties is "diluted" to some extent by the already extended conjugation of the ladder-type donor. Thus, the use of stronger acceptor units with the ladder-type donors would benefit the future design of new D-A copolymers.

van der Waals interactions are critical in Car-Parrinello molecular dynamics simulations of porphyrin-fullerene dyads.

The interplay between electrostatic and van der Waals (vdW) interactions in porphyrin-C60 dyads is still under debate despite its importance in influencing the structural characteristics of such complexes considered for various applications in molecular photovoltaics. In this article, we sample the conformational space of a porphyrin-C60 dyad using Car-Parrinello molecular dynamics simulations with and without empirical vdW corrections. Long-range vdW interactions, which are poorly described by the commonly used density functional theory functionals, prove to be essential for a proper dynamics of the dyad moieties. Inclusion of vdW corrections brings porphyrin and C60 close together in an orientation that is in agreement with experimental observations. The structural differences arising from the vdW corrections are shown to be significant for several properties and potentially less important for others. Additionally, our Mulliken population analysis reveals that contrary to the common belief, porphyrin is not the primary electron donating moiety for C60 . In the considered dyad, fullerene's affinity for electrons is primarily satisfied by charge transfer from the amide group of the linker. However, we show that in the absence of another suitable bound donor, C60 can withdraw electrons from porphyrin if it is sufficiently close.

Porphyrin adsorbed on the (101[combining macron]0) surface of the wurtzite structure of ZnO--conformation induced effects on the electron transfer characteristics.

Electron transfer at the adsorbate-surface interface is crucial in many applications but the steps taking place prior to and during the electron transfer are not always thoroughly understood. In this work a model system of 4-(porphyrin-5-yl)benzoic acid adsorbed as a corresponding benzoate on the ZnO wurtzite (101[combining macron]0) surface is studied using density functional theory (DFT) and time-dependent DFT. Emphasis is on the initial photoexcitation of porphyrin and on the strength of coupling between the porphyrin LUMO or LUMO + 1 and the ZnO conduction band that plays a role in the electron transfer. Firstly, ZnO wurtzite bulk is optimized to minimum energy geometry and the properties of the isolated ZnO (101[combining macron]0) surface model and the porphyrin model are discussed to gain insight into the combined system. Secondly, various orientations of the model porphyrin on the ZnO surface are studied: the porphyrin model standing perpendicularly to the surface and gradually brought close to the surface by tilting the linker in a few steps. The porphyrin model approaches the surface either sideways with hydrogen atoms of the porphyrin ring coming down first or twisted in a ca. 45° angle, giving rise to π-interactions of the porphyrin ring with ZnO. Because porphyrins are closely packed and near the surface, emerging van der Waals (vdW) interactions are examined using Grimme's D2 method. While the orientation affects the initial excitation of porphyrin only slightly, the coupling between the LUMO and LUMO + 1 of porphyrin and the conduction band of ZnO increases considerably if porphyrin is close to the surface, especially if the π-electrons are interacting with the surface. Based on the results of coupling studies, not only the distance between porphyrin and the ZnO surface but also the orientation of porphyrin can greatly affect the electron transfer.

A TDDFT study of the fluorescence properties of three alkoxypyridylindolizine derivatives.

The fluorescence properties of three pyridylindolizine derivatives (one tricarbomethoxy-7-pyridyl-pyrrolopyridine and two dicarboethoxy-3-bromobenzoyl-7-pyridyl-pyrrolopyridines) have been investigated by applying density functional theory (DFT) and the time-dependent DFT (TDDFT). Performances of two hybrid-type functionals (BH&HLYP and B3LYP) and one generalized gradient approximation (GGA) functional (PBE) as well as three basis sets (SV(P), DZP, and TZVP) have been assessed. The solvent environment has been modeled with the conductor-like screening model (COSMO). Of the three functionals only BH&HLYP is able to yield reasonable estimates for all the studied indolizine derivatives whereas the success of the PBE and B3LYP functionals is highly dependent on the structure of the studied molecule. The SV(P) basis set provides geometrical changes as well as fluorescence maxima and Stokes shifts that agree with those obtained with DZP and TZVP. When a nonpolar solvent is used, COSMO is able to reproduce the experimental fluorescence maxima and Stokes shifts well. However, the agreement between the calculations and experiments is not as good when a solvent with higher polarity is used.

Ab initio description of photoabsorption and electron transfer in a doubly-linked porphyrin-fullerene dyad.

Structure, photoabsorption and excited states of two representative conformations obtained from molecular dynamics (MD) simulations of a doubly-linked porphyrin-fullerene dyad DHD6ee are studied by using both DFT and wavefunction based methods. Charge transfer from the donor (porphyrin) to the acceptor (fullerene) and the relaxation of the excited state are of special interest. The results obtained with LDA, GGA, and hybrid functionals (SVWN, PBE, and B3LYP, respectively) are analyzed with emphasis on the performance of used functionals as well as from the point of view of their comparison with wavefunction based methods (CCS, CIS(D), and CC2). Characteristics of the MD structures are retained in DFT optimization. The relative orientation of porphyrin and fullerene is significantly influencing the MO energies, the charge transfer (CT) in the ground state of the dyad and the excitation of ground state CT complex (g-CTC). At the same time, the excitation to the locally excited state of porphyrin is only little influenced by the orientation or cc distance. TD-DFT underestimates the excitation energy of the CT state, however for some cases (with relatively short donor-acceptor separations), the use of a hybrid functional like B3LYP alleviates the problem. Wavefunction based methods and CC2 in particular appear to overestimate the CT excitation energies but the inclusion of proper solvation models can significantly improve the results.

Composite Hydrogels Using Bioinspired Approach with in Situ Fast Gelation and Self-Healing Ability as Future Injectable Biomaterial.

Biopolymers are attractive candidates to fabricate biocompatible hydrogels, but the low water solubility of most of them at physiological pH has hindered their applications. To prepare a water-soluble derivative of chitosan (WSC) biopolymer, it was grafted with a small anionic amino acid, l-glutamic acid, using a single-step 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide coupling reaction. This resulted in a zwitterion-tethered structure onto the polymer backbone. The degree of substitution range was 13-16 ± 1.25%, which was controlled by varying the feeding reagent ratios. Differential scanning calorimetry- and X-ray diffraction-based analysis confirmed a transition from  amorphous into a moderately amorphous/crystalline morphology after amino acid grafting, which made the derivative water-soluble at physiological pH. Composite hydrogels gelated within 60 s when using this WSC together with benzaldehyde-terminated 4-arm poly(ethylene glycol) as cross-linker. The compressive modulus of these hydrogels could be easily tuned between 4.0 ± 1.0 and 31 ± 2.5 kPa, either by changing the cross-linker concentration or total solid content in the final gel. The gels were injectable at the lowest cross-linker as well as total solid content, due to the enhanced elastic behavior. These hydrogels showed biodegradability during a 1 month incubation period in phosphate-buffered saline with weight remaining of 60 ± 1.5 and 44 ± 1.45% at pHs 7.4 and 6.5, respectively. The cytocompatibility of the gels was tested using the fibroblast cell line (i.e., WI-38), which showed good cell viability on the gel surface. Therefore, these hydrogels could be an important injectable biomaterial for delivery purpose in the future.

UV-Blocking Synthetic Biopolymer from Biomass-Based Bifuran Diester and Ethylene Glycol.

A furan-based synthetic biopolymer composed of a bifuran monomer and ethylene glycol was synthesized through melt polycondensation, and the resulting polyester was found to have promising thermal and mechanical properties. The bifuran monomer, dimethyl 2,2'-bifuran-5,5'-dicarboxylate, was prepared using a palladium-catalyzed, phosphine ligand-free direct coupling protocol. A titanium-catalyzed polycondensation procedure was found effective at polymerizing the bifuran monomer with ethylene glycol. The prepared bifuran polyester exhibited several intriguing properties including high tensile modulus. In addition, the bifuran monomer furnished the polyester with a relatively high glass transition temperature. Films prepared from the new polyester also had excellent oxygen and water barrier properties, which were found to be superior to those of poly(ethylene terephthalate). Moreover, the novel polyester also has good ultraviolet radiation blocking properties.

Molecular dipole effects on tuning electron transfer in a porphine-quinone complex: a DFT and TDDFT study.

The effect of a strong electric field generated by molecular dipoles on the ground state electronic structure and the Q and B states as well as the lowest charge transfer (CT) excited state of porphine-2,5-dimethyl-1,4-benzoquinone (PQ) complex has been investigated theoretically. Density functional theory DFT and time-dependent DFT (TDDFT) with the BH&HLYP hybrid functional have been applied in these calculations. The molecular dipole effect was generated by imposing one or two helical homopeptides consisting of eight α-aminoisobutyric acid residues (Aib(8)) close to the PQ complex. The molecular dipoles in a close proximity to the PQ complex expose it to an electric field of the order of magnitude of 10(9) V/m. The presence of the ambient molecular dipoles affects mainly the energy of the lowest CT state and barely the energies of the Q and B states. The molecular dipoles affect the energies of the excited states in a similar way as an external electrostatic field. Hence, the electric field induced by the molecular dipoles of the helical peptides could be used analogously to the external electrostatic field to control electron transfer (ET) in the PQ complex.

Electric-Field-Assisted Electron Transfer in a Porphine-Quinone Complex: A Theoretical Study.

The effects of a static external electric field on the ground state electronic structure of a porphine-quinone (PQ) complex have been studied by using density functional theory (DFT). The energies of the excited states have been calculated with time-dependent density functional theory (TDDFT) and with the approximate coupled cluster singles and doubles (CC2) method. The geometries of porphine and quinone have been optimized with B3LYP. The influence of the external electric field on the PQ complex has been studied at six different intermolecular distances between 2.5 and 5.0 Å with the BH&HLYP functional. An external electric field clearly affects the orbitals localized mostly on quinone but not the orbitals localized on porphine. Additionally, the effect of the external field increases with the increasing intermolecular distance. The optical absorption spectrum of porphine obtained by using the BH&HLYP functional is consistent with the Gouterman model and with the spectrum previously calculated with CAM-B3LYP. The potential energy curves of the Q and B states and the lowest charge transfer (CT) states of the PQ complex calculated by using the BH&HLYP with TDDFT functional have also been compared with those obtained with the CC2 method. Both methods show that the lowest CT state is clearly above the Q states when no external field is applied. Therefore, when the Q states of a porphine-quinone system are excited, the conical intersection is not possible and cannot thus provide a path for electron transfer (ET). The calculations show that the Q and B states are affected by the field much less than the lowest CT state. Consequently, the calculations show that the CT state crosses the Q and B states at certain field strengths. Thus, it is possible that the external electric field triggers ET in porphine-quinone systems via conical intersection.

A density functional theory (DFT) and time-dependent density functional theory (TDDFT) study on optical transitions in oligo(p-phenylenevinylene)-fullerene dyads and the applicability to resonant energy transfer.

The optical transitions of three different size oligo(p-phenylenevinylene)-fullerene dyads (OPV(n)-MPC(60); n = 2-4) and of the corresponding separate molecules are studied using density functional theory (DFT) and time-dependent density functional theory. The DFT is used to determine the geometries and the electronic structures of the ground states. Transition energies and excited-state structures are obtained from the TDDFT calculations. Resonant energy transfer from OPV(n) to MPC(60) is also studied and the Fermi golden rule is used, along with two simple models to describe the electronic coupling to calculate the energy transfer rates. The hybrid-type PBE0 functional is used with a split-valence basis set augmented with a polarization function (SV(P)) in calculations and the calculated results are compared to the corresponding experimental results. The calculated PBE0 spectra of the OPV(n)-MPC(60) dyads correspond to the experimental spectra very well and are approximately sums of the absorption spectra of the separate OPV(n) and MPC(60) molecules. Also, the absorption energies of OPV(n) and MPC(60) and the emission energies of OPV(n) are predicted well with the PBE0 functional. The PBE0 calculated resonant energy transfer rates are in a good agreement with the experimental rates and show the existence of many possible pathways for energy transfer from the first excited singlet states of the OPV(n) molecules to the MPC(60) molecule.

Modeling the interaction between two- and four-ring progestin models and a silicone-based polymer model: a density functional theory study.

In this paper, we introduce a relatively fast and reliable method for determining the feasibility of drug delivery from transdermal and implant materials. We are using density functional theory for modeling the interaction of progestins, that is, progesterone and six of its hydroxyl derivatives, with a silicone-based polymer. The silicone-based polymer model is a linear molecule, which consists of four dimethylsiloxane units. The progestin models are (1) complete progestin structures, which are called four-ring models, and (2) their two-ring models, which are comprised of the C and D rings of the basic steroid skeletons. We are investigating the interaction between the four- and two-ring models and the polymer model in three different interaction configurations. Altogether, 42 different equilibrium geometries of progestin-polymer model complexes and the corresponding interaction energies have been calculated. Our computational results are in very good agreement with the experimental findings reported previously in the literature, which state that the release rates and permeabilities of progestin pharmaceuticals in silicone-based drug delivery systems decrease when the number of hydroxyl groups is increased in the steroid skeleton. The four-ring models take the total interaction of the steroid into account slightly better than the two-ring models. However, the two-ring models are very good for predicting the local interactions between the steroid and the polymer model.

TD-DFT description of photoabsorption and electron transfer in a covalently bonded porphyrin-fullerene dyad.

Structure, photoabsorption, and excited states of a covalently bonded porphyrin-fullerene dyad H(2)P-O34-C(60) are studied using DFT and TD-DFT approaches. Charge transfer from the donor (porphyrin) to the acceptor (fullerene) and the excited-state geometrical relaxation are of special interest. An analysis of differences in the description of these delicate phenomena due to the different exchange-correlation functionals is presented. We compare the results given by LDA, GGA, and hybrid functionals (i.e., SVWN, PBE, B3LYP, and PBE0). The ground-state center-to-center (cc) equilibrium distance between the donor and the acceptor moieties is 6.3, 7.1, and 7.9 Angstrom with SVWN, PBE, and B3LYP, respectively. The associated charge transfer of 0.15, 0.11, and 0.09 electrons is shown to depend on this distance but not directly on the functional itself. The same trend is seen in the HOMO-LUMO difference results, and further, in the lowest excitation energies, except for the hybrid functional calculations that yield the largest HOMO-LUMO gap and the highest energy for the lowest electronic excitation. The hybrid functionals were not found practical for excited-state conformational relaxation with the present computing resources. With LDA, the relaxation increases the cc distance by about 0.2 Angstrom, which is associated with a 0.14 eV decrease in energy. As compared to the ground-state dipole moment of about 4 D, the relaxed excited-state charge-transfer complex dipole moment turns out to become about 20 D. A local excitation of the porphyrin donor is considered, as well, and based on all these results, the nature and interpretation of the photoinduced electron-transfer process is discussed.

DFT and TDDFT study related to electron transfer in nonbonded porphine...C60 complexes.

Spectroscopic properties of a ground state nonbonded porphine-buckminsterfullerene (H2P...C60) complex are studied in several different relative orientations of C60 with respect to the porphine plane by using the density functional (DFT) and time-dependent density functional (TDDFT) theories. The geometries and electronic structures of the ground states are optimized with the B3LYP and PBE functionals and a SVP basis set. Excitation energies and oscillator strengths are obtained from the TDDFT calculations. The relative orientation of C60 is found to affect the equilibrium distance between H2P and C60 especially in the case of the PBE functional. The excitation energies of different H2P...C60 complexes are found to be practically the same for the same excitations when the B3LYP functional is used but to differ notably when PBE is used in calculations. Existence of the states related to a photoinduced electron transfer within a porphyrin-fullerene dyad is also studied. All calculations predict a formation of an excited charge-transfer complex state, a locally excited donor (porphine) state, as well as a locally excited acceptor (fullerene) state in the investigated H2P...C60 complexes.

Molecular simulations for the conformational assessment of a porphyrin-fullerene dyad in different environments.

Conformational space of a porphyrin-fullerene dyad with the donor and acceptor connected by a relatively flexible linker is studied by molecular dynamics simulations in both non-polar and polar solvents, as well as in vacuum. The most probable conformations obtained from the vacuum MD simulations were optimized with semi-empirical (SE) and density functional theory (DFT) methods and the extent of the structural changes is assessed. The computational results indicate the co-existence of different conformers in both polar and nonpolar solvents showing agreement with experimental results. The most probable vacuum conformations at 300 K are similar to the ones at 0 K, while the structures most often observed in the solvents show less compact conformations. Optimization with SE and DFT calculations leads to structures, which represent relatively well the folded conformations in solvent, which validates the electronic structure calculations relevant to describing photoinduced electron-transfer in H2P-O34-C60.