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Institute
Sonstige beteiligte Institutionen
Herein we devise and execute a new synthesis of a pristine boron-doped nanographene. Our target boron-doped nanographene was designed based on DFT calculations to possess a low LUMO energy level and a narrow band gap derived from its precise geometry and B-doping arrangement. Our synthesis of this target, a doubly B-doped hexabenzopentacene (B\(_{2}\)-HBP), employs six net C−H borylations of an alkene, comprising consecutive hydroboration/electrophilic borylation/dehydrogenation and BBr\(_{3}\)/AlCl\(_{3}\)/2,6-dichloropyridine-mediated C−H borylation steps. As predicted by our calculations, B\(_{2}\)-HBP absorbs strongly in the visible region and emits in the NIR up to 1150 nm in o-dichlorobenzene solutions. Furthermore, B\(_{2}\)-HBP possesses a very low LUMO level, showing two reversible reductions at −1.00 V and −1.17 V vs. Fc\(^{+}\)/Fc. Our methodology is surprisingly selective despite its implementation of unfunctionalized precursors and offers a new approach to the synthesis of pristine B-doped polycyclic aromatic hydrocarbons.
The pseudopeptide backbone provided by N-(2-aminoethyl)-glycine oligomers with attached nucleobases has been widely utilized in peptide nucleic acids (PNAs) as DNA mimics. Here we demonstrate the suitability of this backbone for the formation of structurally defined dye stacks. Toward this goal a series of peptide merocyanine (PMC) dye oligomers connected to a N-(2-aminoethyl)-glycine backbone were prepared through peptide synthesis. Our concentration-, temperature- and solvent-dependent UV/Vis absorption studies show that under the control of dipole–dipole interactions, smaller-sized oligomers consisting of one, two or three dyes self-assemble into defined duplex structures containing two up to six chromophores. In contrast, upon further extension of the oligomer, the chosen peptide backbone cannot direct the formation of a defined duplex architecture anymore due to intramolecular aggregation between the dyes. For all aggregate species a moderate aggregation-induced emission enhancement is observed.
Multichromophoric macrocycles and cyclophanes are important supramolecular architectures for the elucidation of interchromophoric interactions originating from precise spatial organization. Herein, by combining an axially chiral binaphthol bisimide (BBI) and a bay-substituted conformationally labile twisted perylene bisimide (PBI) within a cyclophane of well-defined geometry, we report a chiral PBI hetero-cyclophane (BBI-PBI) that shows intramolecular energy and solvent-regulated chirality transfer from the BBI to the PBI subunit. Excellent spectral overlap and spatial arrangement of BBI and PBI lead to efficient excitation energy transfer and subsequent PBI emission with high quantum yield (80–98 %) in various solvents. In contrast, chirality transfer is strongly dependent on the respective solvent as revealed by circular dichroism (CD) spectroscopy. The combination of energy and chirality transfer affords a bright red circularly polarized luminescence (CPL) from the PBI chromophore by excitation of BBI.
Many dyes suffer from fast non-radiative decay pathways, thereby showing only short-lived excited states and weak photoluminescence. Here we show a pronounced fluorescence enhancement for a weakly fluorescent merocyanine (MC) dye by being co-facially stacked to other dyes in hetero-folda-trimer architectures. By means of fluorescence spectroscopy (lifetime, quantum yield) the fluorescence enhancement was explained by the rigidification of the emitting chromophore in the defined foldamer architecture and the presence of a non-forbidden lowest exciton state in H-coupled hetero-aggregates. This folding-induced fluorescence enhancement (FIFE) for specific sequences of π-stacked dyes points at a viable strategy toward improved fluorophores that relates to the approach used by nature in the green fluorescent protein (GFP).
Organic dyes offer unique properties for their application as room temperature single photon emitters. By means of photon‐correlation, the emission characteristics of macrocyclic para‐xylylene linked perylene bisimide (PBI) trimers and tetramers dispersed in polymethyl methacrylate matrices are analyzed. The optical data indicate that, despite of the strong emission enhancement of PBI trimers and tetramers according to their larger number of chromophores, the photon‐correlation statistics still obeys that of single photon emitters. Moreover, driving PBI trimers and tetramers at higher excitation powers, saturated emission behavior for monomers is found while macrocycle emission is still far‐off saturation but shows enhanced fluctuations. This observation is attributed to fast singlet–singlet annihilation, i.e., faster than the radiative lifetime of the excited S1 state, and the enlarged number of conformational arrangements of multichromophores in the polymeric host. Finally, embedding trimeric PBI macrocycles in active organic light‐emitting diode matrices, electrically driven bright fluorescence together with an indication for antibunching at room temperature can be detected. This, so far, has only been observed for phosphorescent emitters that feature much longer lifetimes of the excited states and, thus, smaller radiative recombination rates. The results are discussed in the context of possible effects on the g(2) behavior of molecular emitters.
Designing highly efficient purely organic phosphors at room temperature remains a challenge because of fast non-radiative processes and slow intersystem crossing (ISC) rates. The majority of them emit only single component phosphorescence. Herein, we have prepared 3 isomers (o, m, p-bromophenyl)-bis(2,6-dimethylphenyl)boranes. Among the 3 isomers (o-, m- and p-BrTAB) synthesized, the ortho-one is the only one which shows dual phosphorescence, with a short lifetime of 0.8 ms and a long lifetime of 234 ms in the crystalline state at room temperature. Based on theoretical calculations and crystal structure analysis of o-BrTAB, the short lifetime component is ascribed to the T\(^M_1\) state of the monomer which emits the higher energy phosphorescence. The long-lived, lower energy phosphorescence emission is attributed to the T\(^A_1\) state of an aggregate, with multiple intermolecular interactions existing in crystalline o-BrTAB inhibiting nonradiative decay and stabilizing the triplet states efficiently.
Boric acid (BA) has been used as a transparent glass matrix for optical materials for over 100 years. However, recently, apparent room-temperature phosphorescence (RTP) from BA (crystalline and powder states) was reported (Zheng et al., Angew. Chem. Int. Ed. 2021, 60, 9500) when irradiated at 280 nm under ambient conditions. We suspected that RTP from their BA sample was induced by an unidentified impurity. Our experimental results show that pure BA synthesized from B(OMe)\(_{3}\) does not luminesce in the solid state when irradiated at 250–400 nm, while commercial BA indeed (faintly) luminesces. Our theoretical calculations show that neither individual BA molecules nor aggregates would absorb light at >175 nm, and we observe no absorption of solid pure BA experimentally at >200 nm. Therefore, it is not possible for pure BA to be excited at >250 nm even in the solid state. Thus, pure BA does not display RTP, whereas trace impurities can induce RTP.
Exciton coupling between two or more chromophores in a specific environment is a key mechanism associated with color tuning and modulation of absorption energies. This concept is well exemplified by natural photosynthetic proteins, and can also be achieved in synthetic nucleic acid nanostructures. Here we report the coupling of barbituric acid merocyanine (BAM) nucleoside analogues and show that exciton coupling can be tuned by the double helix conformation. BAM is a nucleobase mimic that was incorporated in the phosphodiester backbone of RNA, DNA and GNA oligonucleotides. Duplexes with different backbone constitutions and geometries afforded different mutual dye arrangements, leading to distinct optical signatures due to competing modes of chromophore organization via electrostatic, dipolar, - stacking and hydrogen-bonding interactions. The realized supramolecular motifs include hydrogenbonded BAM–adenine base pairs and antiparallel as well as rotationally stacked BAM dimer aggregates with distinct absorption, CD and fluorescence properties.
Utilizing Pd‐catalyzed [5+2] annulation a series of heptagon‐extended corannulenes could be synthesized from a borinic acid precursor furnished by C−H borylation strategy. Single‐crystal X‐ray analysis revealed the presence of two conformational enantiomers crystallizing in a racemic mixture. Through their embedded five‐ and seven‐membered rings these polycyclic aromatic hydrocarbons (PAHs) exhibit both negative and positive curvature and UV/Vis/NIR absorption spectroscopy as well as cyclic voltammetry experiments provided insights into the influence of larger flanking aromatic systems and electron‐donating substituents encompassing the heptagonal ring. Through [5+2] annulation of acenaphthylene an azulene‐containing PAH with intriguing optoelectronical properties including a very small bandgap and absorption over the whole visible spectrum could be obtained. Theoretical calculations were employed to elucidate the long‐wavelength absorption and aromaticity.
Water‐soluble multinuclear complexes based on ruthenium 2,2′‐bipyridine‐6,6′‐dicarboxylate (bda) and ditopic bipyridine linker units are investigated in three‐component visible light‐driven water oxidation catalysis. Systematic studies revealed a strong enhancement of the catalytic efficiency in the absence of organic co‐solvents and with increasing oligomer length. In‐depth kinetic and morphological investigations suggest that the enhanced performance is induced by the self‐assembly of linear Ru(bda) oligomers into aggregated superstructures. The obtained turnover frequencies (up to 14.9 s\(^{−1}\)) and turnover numbers (more than 1000) per ruthenium center are the highest reported so far for Ru(bda)‐based photocatalytic water oxidation systems.