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Two N-methylpyridinium compounds and analogous N-protonated salts of 2- and 2,7-substituted 4-pyridyl-pyrene compounds were synthesised and their crystal structures, photophysical properties both in solution and in the solid state, electrochemical and spectroelectrochemical properties were studied. Upon methylation or protonation, the emission maxima are significantly bathochromically shifted compared to the neutral compounds, although the absorption maxima remain almost unchanged. As a result, the cationic compounds show very large apparent Stokes shifts of up to 7200 cm\(^{-1}\). The N-methylpyridinium compounds have a single reduction at ca. −1.5 V vs. Fc/Fc\(^+\) in MeCN. While the reduction process was reversible for the 2,7-disubstituted compound, it was irreversible for the mono-substituted one. Experimental findings are complemented by DFT and TD-DFT calculations. Furthermore, the N-methylpyridinium compounds show strong interactions with calf thymus (ct)-DNA, presumably by intercalation, which paves the way for further applications of these multi-functional compounds as potential DNA-bioactive agents.
Catalytic C−X borylation of aryl halides containing two ortho‐fluorines has been found to be challenging, as most previous methods require stoichiometric amounts of base and the polyfluorinated aryl boronates suffer from protodeboronation, which is accelerated by ortho‐fluorine substituents. Herein, we report that a combination of Pd(dba)2 (dba=dibenzylideneacetone) with SPhos (2‐dicyclohexylphosphino‐2’,6’‐dimethoxybiphenyl) as a ligand is efficient to catalyze the C‐Cl borylation of aryl chlorides containing two ortho‐fluorine substituents. This method, conducted under base‐free conditions, is compatible with the resulting di‐ortho‐fluorinated aryl boronate products which are sensitive to base.
We report four new luminescent tetracationic bis-triarylborane DNA and RNA sensors that show high binding affinities, in several cases even in the nanomolar range. Three of the compounds contain substituted, highly emissive and structurally flexible bis(2,6-dimethylphenyl-4-ethynyl)arene linkers (3: arene=5,5′-2,2′-bithiophene; 4: arene=1,4-benzene; 5: arene=9,10-anthracene) between the two boryl moieties and serve as efficient dual Raman and fluorescence chromophores. The shorter analogue 6 employs 9,10-anthracene as the linker and demonstrates the importance of an adequate linker length with a certain level of flexibility by exhibiting generally lower binding affinities than 3–5. Pronounced aggregation–deaggregation processes are observed in fluorimetric titration experiments with DNA for compounds 3 and 5. Molecular modelling of complexes of 5 with AT-DNA, suggest the minor groove as the dominant binding site for monomeric 5, but demonstrate that dimers of 5 can also be accommodated. Strong SERS responses for 3–5 versus a very weak response for 6, particularly the strong signals from anthracene itself observed for 5 but not for 6, demonstrate the importance of triple bonds for strong Raman activity in molecules of this compound class. The energy of the characteristic stretching vibration of the C≡C bonds is significantly dependent on the aromatic moiety between the triple bonds. The insertion of aromatic moieties between two C≡C bonds thus offers an alternative design for dual Raman and fluorescence chromophores, applicable in multiplex biological Raman imaging.
The synthesis, photophysical, and electrochemical properties of selectively mono-, bis- and tris-dimethylamino- and trimethylammonium-substituted bis-triarylborane bithiophene chromophores are presented along with the water solubility and singlet oxygen sensitizing efficiency of the cationic compounds Cat\(^{1+}\), Cat\(^{2+}\), Cat(i)\(^{2+}\), and Cat\(^{3+}\). Comparison with the mono-triarylboranes reveals the large influence of the bridging unit on the properties of the bis-triarylboranes, especially those of the cationic compounds. Based on these preliminary investigations, the interactions of Cat\(^{1+}\), Cat\(^{2+}\), Cat(i)\(^{2+}\), and Cat\(^{3+}\) with DNA, RNA, and DNApore were investigated in buffered solutions. The same compounds were investigated for their ability to enter and localize within organelles of human lung carcinoma (A549) and normal lung (WI38) cells showing that not only the number of charges but also their distribution over the chromophore influences interactions and staining properties.
Organoboron compounds are well known building blocks for many organic reactions. However, under basic conditions, polyfluorinated aryl boronic acid derivatives suffer from instability issues that are accelerated in compounds containing an ortho‐fluorine group, which result in the formation of the corresponding protodeboronation products. Therefore, a considerable amount of research has focused on novel methodologies to synthesize these valuable compounds while avoiding the protodeboronation issue. This review summarizes the latest developments in the synthesis of fluorinated aryl boronic acid derivatives and their applications in cross‐coupling reactions and other transformations.
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A series of 9-borafluorene derivatives, functionalised with electron-donating groups, have been prepared. Some of these 9-borafluorene compounds exhibit strong yellowish emission in solution and in the solid state with relatively high quantum yields (up to 73.6 % for FMesB-Cz as a neat film). The results suggest that the highly twisted donor groups suppress charge transfer, but the intrinsic photophysical properties of the 9-borafluorene systems remain. The new compounds showed enhanced stability towards the atmosphere, and exhibited excellent thermal stability, revealing their potential for application in materials science. Organic light-emitting diode (OLED) devices were fabricated with two of the highly emissive compounds, and they exhibited strong yellow-greenish electroluminescence, with a maximum luminance intensity of >22 000 cd m\(^{-2}\). These are the first two examples of 9-borafluorene derivatives being used as light-emitting materials in OLED devices, and they have enabled us to achieve a balance between maintaining their intrinsic properties while improving their stability.
With the rapid development of the hexadehydro‐Diels‐Alder reaction (HDDA) from its first discovery in 1997, the question of whether a concerted or stepwise mechanism better describes the thermally activated formation of ortho‐benzyne from a diyne and a diynophile has been debated. Mechanistic and kinetic investigations were able to show that this is not a black or white situation, as minor changes can tip the balance. For that reason, especially, linked yne‐diynes were studied to examine steric, electronic, and radical‐stabilizing effects of their terminal substituents on the reaction mechanism and kinetics. Furthermore, the influence of the nature of the linker on the HDDA reaction was explored. The more recently discovered photochemical HDDA reaction also gives ortho‐arynes, which display the same reactivity as the thermally generated ones, but their formation might not proceed by the same mechanism. This minireview summarizes the current state of mechanistic understanding of the HDDA reaction.
A nickel/N-heterocyclic carbene (NHC) catalytic system has been developed for the borylation of aryl sulfoxides with B\(_{2}\)(neop)\(_{2}\) (neop=neopentyl glycolato). A wide range of aryl sulfoxides with different electronic and steric properties were converted into the corresponding arylboronic esters in good yields. The regioselective borylation of unsymmetric diaryl sulfoxides was also feasible leading to borylation of the sterically less encumbered aryl substituent. Competition experiments demonstrated that an electron-deficient aryl moiety reacts preferentially. The origin of the selectivity in the Ni-catalyzed borylation of electronically biased unsymmetrical diaryl sulfoxide lies in the oxidative addition step of the catalytic cycle, as oxidative addition of methoxyphenyl 4-(trifluoromethyl)phenyl sulfoxide to the Ni(0) complex occurs selectively to give the structurally characterized complex trans-[Ni(ICy)\(_{2}\)(4-CF\(_{3}\)-C\(_{6}\)H\(_{4}\)){(SO)-4-MeO-C\(_{6}\)H\(_{4}\)}] 4. For complex 5, the isomer trans-[Ni(ICy)\(_{2}\)(C\(_{6}\)H\(_{5}\))(OSC\(_{6}\)H\(_{5}\))] 5-I was structurally characterized in which the phenyl sulfinyl ligand is bound via the oxygen atom to nickel. In solution, the complex trans-[Ni(ICy)\(_{2}\)(C\(_{6}\)H\(_{5}\))(OSC\(_{6}\)H\(_{5}\))] 5-I is in equilibrium with the S-bonded isomer trans-[Ni(ICy)\(_{2}\)(C\(_{6}\)H\(_{5}\))(SOC\(_{6}\)H\(_{5}\))] 5, as shown by NMR spectroscopy. DFT calculations reveal that these isomers are separated by a mere 0.3 kJ/mol (M06/def2-TZVP-level of theory) and connected via a transition state trans-[Ni(ICy)\(_{2}\)(C\(_{6}\)H\(_{5}\))(η\(^{2}\)-{SO}-C\(_{6}\)H\(_{5}\))], which lies only 10.8 kcal/mol above 5.
Persistent room-temperature phosphorence from purely organic molecules and multi-component systems
(2021)
Recently, luminophores showing efficient room-temperature phosphorescence (RTP) have gained tremendous interest due to their numerous applications. However, most phosphors are derived from transition metal complexes because of their intrinsic fast intersystem crossing (ISC) induced by strong spin–orbit coupling (SOC) constants of the heavy metal.
Metal-free RTP materials are rare and have become a promising field because they are inexpensive and environmentally friendly. This review summarizes organic molecular materials with long triplet lifetimes at room temperature from the perspective of whether they stem from a molecular or multi-component system. Among purely organic phosphors, heteroatoms are usually introduced into the backbone in order to boost the singlet–triplet ISC rate constant.
In multi-component systems, useful strategies such as host–guest, polymer matrix, copolymerization, and supramolecular assembly provide a rigid matrix to restrict nonradiative pathways thus realizing ultralong RTP.
Phenylpyridyl-fused boroles [TipPBB1]\(_4\) and TipPBB2 were synthesized and their properties investigated. [TipPBB1]\(_4\) forms a tetramer in both the solid state and solution. TipPBB2 contains a 4-coordinate boron atom in the solid state but dissociates to give a 3-coordinate boron species in solution. TipPBB2 shows interesting temperature-dependent dual fluorescence in solution because of the equilibrium between 3- and 4-coordinate boron species due to weak N⋅⋅⋅B intermolecular coordination.