Boron-based donor-spiro-acceptor compounds as novel thermally activated delayed fluorescence (TADF) emitters

Thermally activated delayed fluorescence (TADF) emitters played a pivotal role in the improvement of organic light-emitting diode (OLED) technology. Thanks to the participation of triplet excited states in the delayed emission, diodes based on TADF chromophores display high efficiencies comparable to those of diodes that make use of transition metal-based phosphorescent emitters. In addition, employing purely organic emitters presents additional advantages connected to the cost of the emitters and their environmental impact. The molecular architecture of purely organic TADF emitters is directly connected to the observation and efficiency of the delayed emission. In particular, adopting a donor-acceptor architecture where the two different fragments are tilted, one with respect to the other, has proven to be an efficient strategy for designing efficient TADF emitters. A way to guarantee this spatial disposition is introducing a spiro center between the donor and acceptor fragments, which forces the two moieties in an orthogonal arrangement. This thesis discusses the synthesis of two different classes of donor-spiro-acceptor compounds, which both include a boron atom in the molecular structure but at different positions and with a coordinations. The first part of the thesis discusses the synthesis and functionalization of emitters that contain a tetracoordinated boron atom as the spiro center. The investigated acceptor fragments were based on the phenylpyridine moiety, whereas the donors employed were based on a disulfide, di- or triarylamine fragment. By combining different donors and acceptors, nine different emitters were synthesized and analyzed, which allowed us to establish a correlation between the emission wavelength and the relative strength of the two fragments. Additionally, the characterization of the photophysical properties of these emitters showed that the adopted molecular architecture successfully induced the emission via thermally delayed fluorescence in most cases. Nonetheless, the emitters belonging to this class displayed a low quantum yield in solution, which could be caused by the relatively high flexibility of the donor moieties employed. Therefore, an improvement of the chromophores' performances can be potentially obtained by implementing a more rigid donor fragment in the molecular structure. Additionally, it was shown how the derivatization of the emitters' donor moiety can be used to synthesize different chiral emitters or to insert a transition metal center in the molecular structure, thereby expanding the potential applications of this class of compounds. The second part of the thesis discusses the development of a synthesis path toward a class of emitters in which structure the boron atom is tricoordinated and confers the electron-accepting nature to the 9-bora-9,10-dihydroanthracene-based acceptor fragment. In this class of compounds, the spiro center is constituted by a carbon atom and the donor fragment by a triphenylamine moiety. The developed synthesis path was designed with the aim of generating a common precursor which photophysical properties could be tuned by exchanging the third substituent at the tricoordinated boron center. With the developed synthesis pathway, the derivative with a 9-mesityl-9-bora-9,10-dihydroanthracene acceptor moiety was synthesized and the analysis of its photophysical properties confirmed the emission via TADF and, consequently, the potential of this class of compounds. The emission via TADF involves relatively long-lived triplet excited states, making TADF chromophores also potentially applicable in photocatalysis. To investigate the performance of the synthesized emitters in this field, one compound from each of the above-mentioned classes was tested in photosensitized and photoredox reactions. The outcome of the catalytic trial showed that both tested compounds do promote photosensitized reactions via the generation of singlet oxygen. However, the two chromophores did not display photocatalytic activity in any of the tested photoredox transformations.publishe

Boron-based donor-spiro-acceptor compounds exhibiting thermally activated delayed fluorescence (TADF)

Four boron-based donor-spiro-acceptor compounds, composed of different donor moieties and borylated 2-phenylpyridines as the acceptor, were studied. Their intense photoluminescence in the solid state can be tuned by changing the donor and long emission lifetimes on the microsecond scale indicate thermally activated delayed fluorescence (TADF).publishe

Canavanine utilization via homoserine and hydroxyguanidine by a PLP-dependent γ-lyase in Pseudomonadaceae and Rhizobiales

Canavanine, the δ-oxa-analogue of arginine, is produced as one of the main nitrogen storage compounds in legume seeds and has repellent properties. Its toxicity originates from incorporation into proteins as well as arginase-mediated hydrolysis to canaline that forms stable oximes with carbonyls. So far no pathway or enzyme has been identified acting specifically on canavanine. Here we report the characterization of a novel PLP-dependent enzyme, canavanine-γ-lyase, that catalyzes the elimination of hydroxyguanidine from canavanine to subsequently yield homoserine. Homoserine-dehydrogenase, aspartate–semialdehyde–dehydrogenase and ammonium–aspartate–lyase activities are also induced for facilitating canavanine utilization. We demonstrate that this novel pathway is found in certain Pseudomonas species and the Rhizobiales symbionts of legumes. The findings broaden the diverse reactions that the versatile class of PLP-dependent enzymes is able to catalyze. Since canavanine utilization is found prominently in root-associated bacteria, it could have important implications for the establishment and maintenance of the legume rhizosphere.publishe

Synthesis, Structures, and Photophysical Properties of a Series of Rare Near-IR Emitting Copper(I) Complexes

Herein, we report on the synthesis and structural characterization of a series of trigonal and tetrahedral cationic copper­(I) complexes, bearing phosphine or N-heterocyclic carbene ligands as donors, with benzthiazol-2-pyridine (pybt) and benzthiazol-2-quinoline (qybt) acting as π-chromophores. The compounds are highly colored due to their ¹MLCT (MLCT = metal-to-ligand charge transfer) states absorbing between ca. λ_abs = 400–500 nm, with ¹ILCT (ILCT = intraligand charge transfer) states in the UV region. The relative shifts of the S₀→S₁ absorption correlate with the computed highest occupied molecular orbital–lowest unoccupied molecular orbital gaps, the qybt complexes generally being lower in energy than the pybt ones due to the larger conjugation of the quinoline-based ligand. The compounds exhibit, for Cu^I complexes, rare intense long-lived near-IR emission with λ_max ranging from 593 to 757 nm, quantum yields of up to Φ = 0.11, and lifetimes τ of several microseconds in the solid state as well as in poly­(methyl methacrylate) films. Although a bathochromic shift of the emission is observed with λ_max ranging from 639 to 812 nm and the lifetimes are greatly increased at 77 K, no clear indication for thermally activated delayed fluorescence was found, leaving us to assign the emission to originate from a ³(Cu→pybt/qybt)­MLCT state. The red to near-IR emission is a result of incorporation of the sulfur into the chromophore ligand, as related nitrogen analogues emit in the green to orange region of the electromagnetic spectrum. The photophysical results and conclusions have further been corroborated with density functional theory (DFT)/time-dependent DFT calculations, confirming the nature of the excited states and also the trends of the redox potentials

Discovery of a Ni²⁺-dependent guanidine hydrolase in bacteria

Nitrogen availability is a growth-limiting factor in many habitats1, and the global nitrogen cycle involves prokaryotes and eukaryotes competing for this precious resource. Only some bacteria and archaea can fix elementary nitrogen; all other organisms depend on the assimilation of mineral or organic nitrogen. The nitrogen-rich compound guanidine occurs widely in nature2-4, but its utilization is impeded by pronounced resonance stabilization5, and enzymes catalysing hydrolysis of free guanidine have not been identified. Here we describe the arginase family protein GdmH (Sll1077) from Synechocystis sp. PCC 6803 as a Ni2+-dependent guanidine hydrolase. GdmH is highly specific for free guanidine. Its activity depends on two accessory proteins that load Ni2+ instead of the typical Mn2+ ions into the active site. Crystal structures of GdmH show coordination of the dinuclear metal cluster in a geometry typical for arginase family enzymes and allow modelling of the bound substrate. A unique amino-terminal extension and a tryptophan residue narrow the substrate-binding pocket and identify homologous proteins in further cyanobacteria, several other bacterial taxa and heterokont algae as probable guanidine hydrolases. This broad distribution suggests notable ecological relevance of guanidine hydrolysis in aquatic habitats.publishe