Solvent effects on charge transfer bands of nitrogen-centered intervalence compounds

Intramolecular Through-Space Double-Electron Transfer Between A Pair of Redox-Active Guanidine Units Aligned by Dithiolate Bridges

Using unconventional synthesis protocols, two redox-active triguanidine units are connected by a dithiolate bridge, aligning the two redox-active units in close proximity. The reduced, neutral and the tetracationic redox states with two dicationic triguanidine units are isolated and fully characterized. Then, the dicationic redox states are prepared by mixing the neutral and tetracationic molecules. At low temperatures, the dications are diamagnetic (singlet ground state) with two different triguanidine units (neutral and dicationic). At room temperature, the triplet state with two radical monocationic triguanidine units is populated. At low temperature (210 K), chemical exchange by intramolecular through-space electron-transfer between the two triguanidine units is evidenced by EXSY NMR spectroscopy. Intramolecular through-space transfer of two electrons from the neutral to the dicationic triguanidine unit is accompanied by migration of the counterions in opposite direction. The rate of double-electron transfer critically depends on the bridge. No electron-transfer is measured in the absence of a bridge (in a mixture of one dicationic and one neutral triguanidine), and relatively slow electron transfer if the bridge does not allow the two triguanidine units to approach each other close enough. The results give detailed, quantitative insight into the factors that influence intramolecular through-space double-electron-transfer processes.

Molecular Engineering of Anthracene Core-Based Hole-Transporting Materials for Organic and Perovskite Photovoltaics

Anthracene core-based hole-transporting material containing TIPs (triisopropylsilylacetylene) has been spotlighted as potential donors for perovskite solar cells (SCs) due to their appropriate energy levels, efficient hole transport capacity, high stability, and high power conversion efficiency. Herein, we have efficiently designed seven new highly conjugated A-B-D-C-D molecules (AS1-AS7) containing an anthracene core. We used end-capped modifications of donor units with acceptor units on one side and then theoretically characterized them for their appropriate use for SC applications. Modern quantum chemistry techniques have theoretically described the R (reference molecule) and developed (AS1-AS7) molecules. Moreover, the proposed (AS1-AS7) molecules are explored with density functional theory (DFT) and time-dependent density functional theory (TD-DFT) employing B3LYP/6-31G(d,p), and numerous parameters like photovoltaic, optical and electronic characteristics, frontier molecular orbital, excitation, binding and reorganization (λe and λh) energies, open circuit voltage, light harvesting efficiency, transition density matrix, fill factor, and the density of states have been studied. End-capped modification causes a smaller band gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), higher UV-vis absorption maxima, tuned energy levels, lower binding and reorganizational (λe and λh) energies, and larger Voc values in proposed (AS1-AS7) molecules than R. AS5 has a remarkable absorption maximum of 495.94 nm and a narrow optimal energy gap (Eg) of 1.46 eV. Furthermore, a complex study of AS5:PC61BM has revealed extraordinary charge shifting at the HOMO (AS5)-LUMO (PC61BM) interface. Our results suggested that newly developed anthracene core-based compounds (AS1-AS7) would be effective candidates with excellent photovoltaic and optoelectronic properties and could be employed in future organic and perovskite SC applications.

Novel adamantane-based hole transport materials for perovskite solar cells: a computational approach

Adamantane derivatives have been subjected to quantum mechanical calculations to determine their capabilities as potential hole transport materials (HTMs) in perovskite solar cells (PSCs). Adamantane has been modified in two ways: multiple arm substitution and outermost substituent variation. It has been shown that tetra-substitution of adamantane gave the best characteristics as a HTM. Further modification showed tetra-ethyl substituted adamantane (ad-EtTPA) has the lowest HOMO, a small hole reorganization energy (λh), absorption in the UV region, and good stability. These appropriate properties mean that ad-EtTPA could be a promising HTM in PSCs. In addition, the relationship between λh and the electrostatic potential (ESP) maps of the cationic geometry has been studied. Three outcomes were drawn from the investigation: (1) the high positive potential in the ESP map is the region where more geometric distortions are occurring in going from the neutral to the cationic state, (2) large geometric distortions in this region lead to high λh, and (3) for tetra-substituted adamantane derivatives, delocalization of the positive potential leads to lower λh. The results showed that ESP maps can give insight into the molecular engineering of HTMs.

The role of sulfur oxidation in controlling the electronic properties of sulfur-containing host molecules for phosphorescent organic light-emitting diodes

In this study a series of dibenzothiophene (DBT) derivatives having different valence states of sulfur atoms have been reported as host materials for blue phosphorescent organic light-emitting diodes. Their electronic properties have also been thoroughly investigated to develop structure-property relationships which include the consideration of the effect of various oxidation states of the sulfur atom in the core moiety and linking (C-N linkage) of subunits with the core at different positions. The results obtained from the electronic structure calculations highlight that the triplet energy (E_T), singlet-triplet energy difference (ΔE_ST), reorganization energy for the hole and the injection barrier for the electron decrease with an increase in the oxidation state of the sulfur atom in DBT. On the other hand, the injection barrier for the hole and the reorganization energy for the electron increase upon increasing the oxidation state of the sulfur atom present in the DBT.

Molecular Engineering of Tetraphenylbenzidine-Based Hole Transport Material for Perovskite Solar Cell

Experimental and theoretical HOMO energy correlation of tetraphenylbenzidine (TPB)-based hole transport materials (HTMs) was successfully achieved through adiabatic ground-state oxidation potential calculation using LC-ωPBE. Similarly, trends in the computed excitation energies and hole reorganization energies of the HTMs are in agreement with the experimental band gaps and hole mobilities, respectively. Using these established correlations, the calculated properties of novel TPB-based HTMs were analyzed, and among the derivatives, TPB with attached fluorene (Fl) has less absorption in the visible region, a lower hole reorganization energy, and a deeper HOMO level compared to the reference. These properties signify that Fl could be a promising HTM in perovskite solar cells because this material will not compete with the perovskite absorption, will be efficient for hole transport due to its better hole mobility, and will eventually enhance the open-circuit voltage of the device. All of these factors could improve the efficiency of the perovskite solar cell.

Rational design of carbazolyl and aryl phosphine oxide (APO) based ambipolar host materials for blue electrophosphorescence: a density functional theory study

Carbazolyl and Aryl Phosphine Oxide (APO) based ambipolar host materials.

Rational design of cyclopenta[b]naphthalenes for better optoelectronic applications and their photophysical properties using DFT/TD-DFT methods

The optical properties of cyclopenta[b]naphthalenes (CPNs) can be fine-tuned by suitable substitutions and DFT calculations show that they can make efficient OLEDs.

Push-pull effect on the geometrical, optical and charge transfer properties of disubstituted derivatives of mer-tris(4-hydroxy-1,5-naphthyridinato) aluminum (mer-AlND3)

To design innovative and novel optical materials with high mobility, two kinds of disubstituted derivatives for mer-tris(4-hydroxy-1,5-naphthyridinato) aluminum (mer-AlND3) with push (EDG)–pull (EWG) substituents have been designed. The structures of mer-tris(8-EDG-2-EWG-4-hydroxy-1,5-naphthyridinato) aluminum (type I) and mer-tris(8-EWG-2-EDG-4-hydroxy-1,5-naphthyridinato) aluminum (type II) in the ground and first excited states have been optimized at the B3LYP/6-31G(D) and CIS/6-31G(D) level of theory, respectively. It can be seen from frontier molecular orbitals analysis, in all these complexes, the highest occupied molecular orbital (HOMO) is localized on the pyridine-4-ol ring of A-ligand while lowest unoccupied molecular orbital (LUMO) is on the pyridyl ring of B-ligand in ground state irrespective of electron donor/acceptor substitution present on the ligands similar to that of mer-tris(8-hydroxyquinoline) aluminum (mer-Alq3) and parent mer-AlND3.The absorption and emission wavelengths have been evaluated at the TD-PBE0/6-31G(D) level and it can be see that all the type I derivatives show blue shift while most of the type II derivatives show red shift compared to mer-AlND3. All the disubstituted complexes have showed hypsochromic shifts in both the absorption and emission spectra when compared with the calculated absorption and emission spectra respectively of mer-Alq3. It can be seen that the reorganization energies of some of the disubstituted derivatives are comparable with mer-Alq3 and these derivatives might be good candidates for emitting materials in OLED.

Theoretical investigations of the electronic structures of carbazole-based triphenylphosphine oxide derivatives, potential bipolar host materials in blue-phosphorescent devices

Density functional theory calculations were performed to investigate the electronic structures of bipolar host materials comprising a backbone of linked acceptor moieties where each acceptor was also linked to a pendant donor moiety. The acceptor was triphenylphosphine oxide with two of its phenyls substituted with fluorine atoms or nitrile groups (CN). The donor was carbazole (CZ) substituted, or not, with t-butyl groups. The HOMO and LUMO energy levels of these host molecules were mainly influenced by their respective hole- and electron-transport units. The t-butyl substituents on the CZ moieties had an adverse effect on the triplet energies (E T) of the host molecules, especially for molecules where the phenyls of the backbone chain were substituted with CN groups. While introducing CN substituents onto the backbone chain decreased the energy difference between the lowest singlet and triplet excited states (ΔE ST), it also caused the energy gap between the HOMO and LUMO to narrow. Among the host molecules investigated, that in which one of the phenyls in the acceptor moiety was linked to the donor while the other two phenyls in the acceptor were substituted with CN substituents exhibited the highest E T, balanced charge transport, a low charge-injection barrier, and a small ΔE ST, and is therefore a promising candidate host material for use in blue-phosphorescent devices. Graphical Abstract Density functional theory calculations were performed to explore the electronic properties of bipolar host materials comprising a backbone of linked acceptor moieties where each acceptor was also linked to a pendant donor moiety. All of the designed molecules with high triplet energies were found to be suitable for use as host materials when matched with a blue-light guest material. The results demonstrate that the host molecule in which one of the phenyls in the triphenylphosphine oxide acceptor moiety was linked to the carbazole donor while the other two phenyls in the acceptor moiety were substituted with CN substituents yields the highest blue-phosphorescent device performance, as this host molecule has interesting features such as a high E T, balanced charge transport, a low charge-injection barrier, and a small ΔE ST.

The influence of numbers and ligation positions of the triphenylamine unit on the photophysical and electroluminescent properties of homoleptic iridium(III) complexes: a theoretical perspective

A DFT/TDDFT investigation was carried out on a series of homoleptic triphenylamine-featured Ir(iii) complexes [: (fac-tris[2-phenyl-4-(2-(N,N-diphenylamino)phenyl)pyridine]iridium(iii)); : (fac-tris[2-phenyl-4-(3-(N,N-diphenylamino)phenyl)pyridine]iridium(iii)); : (fac-tris[2-phenyl-4-(4-(N,N-diphenylamino)phenyl)pyridine]iridium(iii))] with one triphenylamine unit in the 2-phenylpyridine (ppy) ligand and [: (fac-tris[2,4-bis(2-(N,N-diphenylamino)phenyl)pyridine]iridium(iii)); : (fac-tris[2,4-bis(3-(N,N-diphenylamino)phenyl)pyridine]iridium(iii)); : (fac-tris[2,4-bis(4-(N,N-diphenylamino)phenyl)pyridine]iridium(iii))] with two triphenylamine units in the ppy ligand, respectively, aiming to gain insight into the influence of number and ligation position of triphenylamine units on the photophysical and electronic properties of the studied complexes. Complexes have been synthesized recently. For comparison, the parent complex Ir(ppy)3 was also investigated. The calculated results reveal that the introduction of the triphenylamine unit leads to enhanced charge-injection abilities and a balanced charge-transfer process compared with Ir(ppy)3. The different ligation positions of triphenylamine unit have an obvious effect on the absorption intensities for these complexes. The emissions of and undergo significant red shift with the introduced triphenylamine unit in ppy ligands compared with that of Ir(ppy)3, while the extent of red shift shows an apparent dependence on the number of triphenylamine units. The factors that might affect the quantum yield have been discussed.

Design of novel tetra-hetero[8]circulenes: a theoretical study of electronic structure and charge transport characteristics

Tetra-hetero[8]circulenes were theoretically designed and emerged as good candidates for ambipolar organic semiconductors.

Stable mixed-valent radicals from platinum(II) complexes of a bis(dioxolene) ligand

Three diplatinum(II) complexes [{PtL}₂(μ-thea)] (H₄thea=2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene) have been prepared, with diphosphine or bipyridyl “L” co-ligands. One-electron oxidation of these complexes gave radical cations containing a mixed-valent [thea]³⁻ ligand with discrete catecholate and semiquinonate centers separated by quaternary methylene spacers. The electronic character of these radicals is near the Robin–Day class II/III border determined by UV/Vis/NIR and EPR spectroscopies. Crystal-structure determinations and a DFT calculation imply that oxidation of the thea⁴⁻ ligand may lead to an increased through-space interaction between the dioxolene π systems.

Theoretical study of photophysical properties of 1,4-dihydropyrrolo[3,2-b]pyrrole-cored branched molecules with thienylenevinylene arms toward broad absorption spectra for solar cells

A series of oligo(thienylenevinylene) derivatives with 1,4-dihydropyrrolo[3,2-b]pyrrole as core has been investigated at the PBE0/6-31G(d) and the TD-PBE0/6-31+G(d,p) levels to design materials with high performances such as broad absorption spectra and higher balance transfer property. The results show that position and amount of arm affect the electronic density contours of frontier molecular orbitals significantly. The molecule with four arms owns the narrowest energy gap and the largest maximum absorption wavelength, and the molecule with two arms in positions a and c has the broadest absorption region among the designed molecules. Calculated reorganization energies of the designed molecules indicate that the molecules with two arms can be good potential ambipolar transport materials under proper operating conditions.

Theoretical design study on photophysical property on oligomers based on spirobifluorene and carbazole-triphenylamine for PLED applications

The photophysical properties of five blue light-emitting polymers based on spirobifluorene applied in polymer light-emitting diodes (PLED) materials have been studied by quantum chemistry. In order to understand the intrinsic reasons for the different performances displayed by the polymers, we carried out density functional theory (DFT) and Marcus theory investigations on their oligomers in terms of structure and properties stability, absorption and emission properties, and carrier injection and transport properties. Especially, some important parameters which had not been reported to our knowledge were given in this contribution, such as the ionization potentials (IPs), electron affinities (EAs), reorganization energies (λ), ke/kh (the ratio between the electron transfer rate (ke) and hole transfer rate (kh)), and the radiative lifetimes (τ). The main results indicate that the co-oligomers of PCC-1, PCC-2, and PCC-3 with push-pull interactions produced by the existing D-A segments have better carrier injection and transport properties than the oligomers of PSF and PCF. Especially PCC-2 co-oligomer, its large radiation lifetime (7.46 ns) and well balanced and adequate carrier transport guarantee its champion performance for PLED. The calculated results coincide with the experimental ones. Besides, PNF structurally similar to PCC-2 has similar photoelectric properties to PCC-2 in theory, and the fluorescence emission of PNF co-oligomer is superior to PCC-2 co-oligomer. Therefore, we predict that PNF is a promising candidate for PLED.