Computational and spectral analysis of derivatives of 9,9-dimethyl-9,10-dihydroacridine and 10-Phenyl-10H-phenothiazine-5,5-dioxide with hybridized local and charge-transfer excited states for optoelectronic applications

Aiming to combine the advantages of both prompt fluorescence and thermally activated delayed fluorescence in single emitter, molecular design of emitters with hybridized locally excited and charge transfer states were investigated by computational approaches and optical spectroscopy. Taking into account the results of the theoretical screening, the most promising derivatives of 9,9-dimethyl-9,10-dihydroacridine and 10-phenyl-10H-phenothiazine-5,5-dioxide based with the different linking topology (meta- and para-isomers) were selected for the synthesis and experimental investigations. Both the compounds exhibit ultraviolet LE emission peaking at ca. 360 nm, green ICT peaking at ca. 510 nm, and deep-blue HLCT emission peaking at ca. 430 nm when they are molecularly dispersed in the solid media of the different polarity. The developed emitters allow to obtain deep-blue electroluminescence for the host-containing OLEDs and green electroluminescence of host-free devices with the efficiency of exciton production of 42 and 73%, respectively. Efficient exciton production is due to the spin-flip switching via thermally activated processes which is much more efficient than prompt fluorescence. Showing the impact of the linking topology, the para-isomer demonstrated more efficient triplet harvesting in OLEDs than meta-isomer. A detailed discussion on the structure-property relationships and on some discrepancies between the results of the results of theoretical calculations and spectral analysis allows to obtain important insights on the photophysical properties of these compounds.

Controlled n-Doping of Naphthalene-Diimide-Based 2D Polymers

2D polymers (2DPs) are promising as structurally well-defined, permanently porous, organic semiconductors. However, 2DPs are nearly always isolated as closed shell organic species with limited charge carriers, which leads to low bulk conductivities. Here, the bulk conductivity of two naphthalene diimide (NDI)-containing 2DP semiconductors is enhanced by controllably n-doping the NDI units using cobaltocene (CoCp₂ ). Optical and transient microwave spectroscopy reveal that both as-prepared NDI-containing 2DPs are semiconducting with sub-2 eV optical bandgaps and photoexcited charge-carrier lifetimes of tens of nanoseconds. Following reduction with CoCp₂ , both 2DPs largely retain their periodic structures and exhibit optical and electron-spin resonance spectroscopic features consistent with the presence of NDI-radical anions. While the native NDI-based 2DPs are electronically insulating, maximum bulk conductivities of >10^-4  S cm^-1 are achieved by substoichiometric levels of n-doping. Density functional theory calculations show that the strongest electronic couplings in these 2DPs exist in the out-of-plane (π-stacking) crystallographic directions, which indicates that cross-plane electronic transport through NDI stacks is primarily responsible for the observed electronic conductivity. Taken together, the controlled molecular doping is a useful approach to access structurally well-defined, paramagnetic, 2DP n-type semiconductors with measurable bulk electronic conductivities of interest for electronic or spintronic devices.

Impact of Imine Bond Orientations on the Geometric and Electronic Structures of Imine-based Covalent Organic Frameworks

Many efforts are currently devoted to improving the stability and crystallinity of imine-based two-dimensional (2D) covalent organic frameworks (COFs) given their wide range of potential applications. The variation in the relative orientations of the imine bonds has been found to be a critical factor that impacts the stacking of the 2D COF layers, leads to the formation of isomer structures, and influences the crystallinity of the final product. Most investigations to date have focused only on the structural properties, while the role of the imine orientations on the electronic properties has not been studied systematically. Here, we explore this effect by examining how the electronic band structures, electronic couplings, and effective masses evolve when considering four isomeric structures of an imine-linked tetraphenyl-pyrene naphthalene-diimide COF. Our results provide an understanding of the impact of the imine orientations and how they need to be controlled to realize COF inter-layer stackings that can lead to efficient cross-plane electron transport. They can be used to guide the design and synthesis of imine-based COFs for applications where charge transport needs to be optimized.

Interfacial versus Bulk Properties of Hole-Transporting Materials for Perovskite Solar Cells: Isomeric Triphenylamine-Based Enamines versus Spiro-OMeTAD

Here, we report on three new triphenylamine-based enamines synthesized by condensation of an appropriate primary amine with 2,2-diphenylacetaldehyde and characterized by experimental techniques and density functional theory (DFT) computations. Experimental results allow highlighting attractive properties including solid-state ionization potential in the range of 5.33-5.69 eV in solid-state and hole mobilities exceeding 10^-3 cm²/V·s, which are higher than those in spiro-OMeTAD at the same electric fields. DFT-based analysis points to the presence of several conformers close in energy at room temperature. The newly synthesized hole-transporting materials (HTMs) were used in perovskite solar cells and exhibited performances comparable to that of spiro-OMeTAD. The device containing one newly synthesized hole-transporting enamine was characterized by a power conversion efficiency of 18.4%. Our analysis indicates that the perovskite-HTM interface dominates the properties of perovskite solar cells. PL measurements indicate smaller efficiency for perovskite-to-new HTM hole transfer as compared to spiro-OMeTAD. Nevertheless, the comparable power conversion efficiencies and simple synthesis of the new compounds make them attractive candidates for utilization in perovskite solar cells.

Assessing the effects of increasing conjugation length on exciton diffusion: from small molecules to the polymeric limit

Organic solar cells (OSC) generally contain long-chain π-conjugated polymers as donor materials, but, more recently, small-molecule donors have also attracted considerable attention. The nature of these compounds is of crucial importance concerning the various processes that determine device performance, among which singlet exciton diffusion is one of the most relevant. The efficiency of the diffusion mechanism depends on several aspects, from system morphology to electronic structure properties, which vary importantly with molecular size. In this work, we investigated the effects of conjugation length on the exciton diffusion length through electronic structure calculations and an exciton diffusion model. By applying extrapolation procedures to thiophene and phenylene vinylene oligomer series, we investigate their electronic and optical properties from the small-molecule point of view to the polymeric limit. Several properties are calculated as a function of oligomer size, including transition energies, absorption and emission spectra, reorganization energies, exciton coupling and Förster radii. Finally, an exciton diffusion model is used to estimate diffusion lengths as a function of oligomer size and for the polymeric limit showing agreement with experimental data. Results also show that longer conjugation lengths correlate with longer exciton diffusion lengths in spite of also being associated with shorter exciton lifetimes.

Understanding the colorimetric properties of quinoxaline-based pi-conjugated copolymers by tuning their acceptor strength: a joint theoretical and experimental approach

We report the synthesis and characterization of donor–acceptor π-conjugated copolymers leading to blue-to-green coloration according to acceptor strength.

Strain effects on the work function of an organic semiconductor

Establishing fundamental relationships between strain and work function (WF) in organic semiconductors is important not only for understanding electrical properties of organic thin films, which are subject to both intrinsic and extrinsic strains, but also for developing flexible electronic devices. Here we investigate tensile and compressive strain effects on the WF of rubrene single crystals. Mechanical strain induced by thermal expansion mismatch between the substrate and rubrene is quantified by X-ray diffraction. The corresponding WF change is measured by scanning Kelvin probe microscopy. The WF of rubrene increases (decreases) significantly with in-plane tensile (compressive) strain, which agrees qualitatively with density functional theory calculations. An elastic-to-plastic transition, characterized by a steep rise of the WF, occurs at ∼0.05% tensile strain along the rubrene π-stacking direction. The results provide the first concrete link between mechanical strain and WF of an organic semiconductor and have important implications for understanding the connection between structural and electronic disorder in soft organic electronic materials.

Bis(diphenylamino)naphthalene host materials: careful selection of the substitution pattern for the design of fully solution-processed triple-layered electroluminescent devices

Two new triarylamine-based wide bandgap small molecules differing by the position of their substituents were investigated as hosts for solution-processed organic light-emitting diodes (OLEDs).

Synthesis and properties of the derivatives of triphenylamine and 1,8-naphthalimide with the olefinic linkages between chromophores

Two donor–acceptor type molecules consisting of triphenylamine and 1,8-naphthalimide moieties with the olefinic linkages between chromophores were synthesized by Heck reaction.

Di(p-methoxyphenyl)amine end-capped tri(p-thiophenylphenyl)amine based molecular glasses as hole transporting materials for solid-state dye-sensitized solar cells

Star shaped hole conducting molecular glasses were synthetized and applied in solid state dye-sensitized solar cells.

Structure-properties relationship of carbazole and fluorene hybrid trimers: experimental and theoretical approaches

Synthesis and properties of fluorene and carbazole derivatives having three electrophores per molecule with different architectures are reported. The synthesized compounds possess high thermal stabilities with 5% weight loss temperatures exceeding 350 °C. They form glasses with glass transition temperatures ranging from 60 to 68 °C. Cyclovoltammetric experiments revealed the high electrochemical stability of the fluorene trimer. In contrast, 2- and 2,7-fluorenyl substituted carbazole derivatives show irreversible oxidation in the CV experiments. The electron photoemission spectra of the films of the synthesized compounds revealed ionization potentials of 5.65-5.89 eV. Hole drift mobilities in the amorphous layers of the synthesized compounds reach 10(-2) cm(2) V(-1) s(-1) at high electric fields, as established by a xerographic time-of-flight technique. DFT calculations show that HOMO and LUMO orbitals of the compounds are very similar in energy and shape. The similar hole mobilities observed for the three compounds are discussed in the frame of the Marcus theory. An important influence of the alkyl groups on the ionization potentials and on the hole mobilities was also observed and its origin is discussed.

Charge delocalization through benzene, naphthalene, and anthracene bridges in π-conjugated oligomers: an experimental and quantum chemical study

To understand the influence of orthogonal conjugation pathways fused directly to π-conjugated polymer backbones, we synthesized and studied three series of thiophene-based model compounds containing benzene, naphthalene, and anthracene peri-substituted central cores as representative acenes. These models were functionalized with methyl groups at the reactive thiophene positions in order to generate and observe oxidized species without complications from follow-up polymerization. The neutral monomers and their oxidized charged counterparts were subjected to cyclic voltammetry, spectroelectrochemistry, and EPR spectroscopy as appropriate, and these results were further corroborated with thorough density functional theory studies. This joint experimental and theoretical analysis allowed us to determine that benzene-based conjugated linkers led to more delocalized charge carriers on account of the quinoidal character maintained within the benzene core. In contrast, anthracene-based linkers displayed very localized carriers due to torsional strain between the adjacent aryl groups and to the local evolution of formal aromatic sextets on the benzo-fused rings orthogonal to the backbone in the quinoidal state. In some cases, the electronics of the thiophene-based substituent dominated the electronic properties of the oxidized species regardless of the nature of the central acene linker. These results highlight the dramatic influence that orthogonal conjugation pathways can exert on the electronic properties of π-conjugated materials.

Fluorine... and pi...alkali metal interactions control in the stereoselective amide enolate alkylation with fluorinated oxazolidines (Fox) as a chiral auxiliary: an experimental and theoretical study

The alpha-alkylation of amide enolates by using a pseudo-C(2) symmetry trans 4-phenyl-2-trifluoromethyloxazolidine (trans-Fox) as a chiral auxiliary occurs with an extremely high diastereoselectivity (>99 % de). The origin of this excellent stereocontrol was investigated by an experimental and theoretical (DFT) study. With this trans chiral auxiliary, both F...metal and pi...metal interactions compete to give the same diastereomer through Re face alkylation of the enolate. A 5.5 kcal mol(-1) energy difference found between the Re face and the Si face attack transition states is consistent with the complete diastereoselectivity that has been experimentally achieved. On the other hand, in the case of the cis chiral auxiliary (cis-Fox) the competition between the F...metal and pi...metal interactions is unfavourable to the diastereoselectivity. In this case, the Re face and the Si face attack transition states were found to be nearly isoenergetic (0.3 kcal mol(-1) difference), which is in good agreement with the very low diastereoselectivity observed.

[Minimally invasive surgery and neoplasms of the inferior third of the rectum: personal technique]

In the last decade minimally invasive surgery has gained an increasing success and was firstly applied to the treatment of benign pathologies, and recently to the malignant ones. The Authors report two cases of inferior rectal neoplasm treated with Miles operation through a video-laparoscopic-perineal approach. The abdominal phase of the operation was performed only through 5 small laparoscopic incisions. The perineal phase and the stoma were performed using traditional technique. The method appears feasible and oncologically correct; however, further trials are needed to evaluate long-term results.

[Sutures without sutures in digestive surgery. Experimental study of the rat intestine]

The aim of the experimental study was to evaluate the possibility of performing an end-to-end anastomosis by using fibrin adhesive as the only means of suture. To this end, 24 Wistar rats were used, of which 16 underwent ilear resection and 8 underwent colotomy, and they were divided respectively into two groups. On the rats of the first group the anastomosis was performed by using only fibrin adhesive; on the rats of the second group operated the anastomosis was performed by using fibrin adhesive in association with non adsorbable suture material, while on those operated of colectomy the anastomosis was performed by fibrin adhesive and adsorbable suture material. The anatomo-phatological studies on the anastomosis have shown a similar healing process in the cases treated only with fibrin adhesive and by using absorbable material and has demonstrated the trange material from the suture, which are a possible causes of complication.

[Laparoscopic cholecystectomy. Our experience]

This study analysed 90 cases of symptomatic cholelithiasis treated with laparoscopic cholecystectomy. 27% of the cases studied suffered from microlithiasis, 44% from multiple calculosis with calculi measuring 1 to 2 cm in diameter, 25% from single calculi measuring up to 4 cm in diameter, and 4% from a benign proliferative pathology of the cholecystic wall. Calculosis of the main biliary tract was also found in 3 patients and was treated by preoperative endoscopic papillosphincterotomy. In addition to routine tests and ultrasonography, endovenous cholangiography, or retrograde cholangiography (ERCP) in cases of suspected calculosis of the biliary tract, was always performed prior to surgery. Mean operating time was 60 min. Two procedures were converted into laparotomy due to laceration of the cystic artery. In one case laparotomy was performed on day two due to choleperitoneum following a lesion of the cystic duct. Pneumoperitoneum could not be performed in three cases. Mean hospital stay was 48 hours. No other intra- or postoperative complications were reported. These results support the opinion that laparoscopic cholecystectomy is a safe method, with few limitations and represents the treatment of choice in the therapy of symptomatic cholelithiasis.