Mixed‐valence (MV) compounds are excellent model systems for the investigation of basic electron‐transfer (ET) or charge‐transfer (CT) phenomena. These issues are important in complex biophysical ...processes such as photosynthesis as well as in artificial electronic devices that are based on organic conjugated materials. Organic MV compounds are effective hole‐transporting materials in organic light emitting diodes (OLEDs), solar cells, and photochromic windows. However, the importance of organic mixed‐valence chemistry should not be seen in terms of the direct applicability of these species but the wealth of knowledge about ET phenomena that has been gained through their study. The great variety of organic redox centers and spacer moieties that may be combined in MV systems as well as the ongoing refinement of ET theories and methods of investigation prompted enormous interest in organic MV compounds in the last decades and show the huge potential of this class of compounds. The goal of this Review is to give an overview of the last decade in organic mixed valence chemistry and to elucidate its impact on modern functional materials chemistry.
Mix and match: Organic mixed‐valence compounds are excellent model systems to study fundamental electron‐ and charge‐transfer phenomena, as well as offering numerous perspectives for application. Particularly attractive is the possibility to systematically change the properties, since such compounds can be synthesized with a number of different redox centers and bridging units.
Symmetric‐ and asymmetric hexaarylbenzenes (HABs), each substituted with three electron‐donor triarylamine redox centers and three electron‐acceptor triarylborane redox centers, were synthesized by ...cobalt‐catalyzed cyclotrimerization, thereby forming compounds with six‐ and four donor–acceptor interactions, respectively. The electrochemical‐ and photophysical properties of these systems were investigated by cyclovoltammetry (CV), as well as by absorption‐ and fluorescence spectroscopy, and compared to a HAB that only contained one neighboring donor–acceptor pair. CV measurements of the asymmetric HAB show three oxidation peaks and three reduction peaks, whose peak‐separation is greatly influenced by the conducting salt, owing to ion‐pairing and shielding effects. Consequently, the peak‐separations cannot be interpreted in terms of the electronic couplings in the generated mixed‐valence species. Transient‐absorption spectra, fluorescence‐solvatochromism, and absorption spectra show that charge‐transfer states from the amine‐ to the boron centers are generated after optical excitation. The electronic donor–acceptor interactions are weak because the charge transfer has to occur predominantly through space. Moreover, the excitation energy of the localized excited charge‐transfer states can be redistributed between the aryl substituents of these multidimensional chromophores within the fluorescence lifetime (about 60 ns). This result was confirmed by steady‐state fluorescence‐anisotropy measurements, which further indicated symmetry‐breaking in the superficially symmetric HAB. Adding fluoride ions causes the boron centers to lose their accepting ability owing to complexation. Consequently, the charge‐transfer character in the donor–acceptor chromophores vanishes, as observed in both the absorption‐ and fluorescence spectra. However, the ability of the boron center as a fluoride sensor is strongly influenced by the moisture content of the solvent, possibly owing to the formation of hydrogen‐bonding interactions between water molecules and the fluoride anions.
If it ain't broke: Although superficially symmetric, a hexaarylbenzene system (see figure) that contains several amine and boron redox centers shows optical spectroscopic properties that only marginally differ from those of its asymmetric analogue, owing to the breaking of the symmetry in the excited state. In contrast, the properties of the materials, such as solubility or phase‐transitions, are strongly influenced by the presence/absence of symmetry.
Two shape‐persistent star mesogens with oligo(phenylene ethenylene) arms and a phthalocyanine core—one providing free space (2) and one sterically encumbered by four fullerenes attached through ...spacers (3)—have been successfully synthesized. In contrast to the smaller discotic derivative 1, mesogen 2 forms a columnar liquid crystal (LC), which can only be partially aligned without π‐stacking, while 3 is not an LC. Exceptionally, the 1:1 mixture of 2 and 3 forms an alignable columnar LC with strong π‐stacking and quadruply helically organized fullerenes by an unprecedented click process that is similar to a ball detent mechanism. The C60 units also interconnect different columns. This is driven by nanosegregation and space‐filling of the voids with fullerenes. Photophysical studies confirm the presence of a light‐collecting system that generates charge‐separated states in solution and in the solid state, which makes such highly organized materials attractive for the study of future photovoltaic devices.
Only a click away: A supramolecular click procedure analogous to a ball detent mechanism (see figure) leads to a helical columnar liquid‐crystalline separated donor–acceptor antenna system. Two shape‐persistent star mesogens are used: one provides the free space and the other is sterically encumbered by four fullerenes.
We show that by judicious choice of substituents at the 2‐ and 7‐positions of pyrene, the frontier orbital order of pyrene can be modified, giving enhanced control over the nature and properties of ...the photoexcited states and the redox potentials. Specifically, we introduced a julolidine‐like moiety and Bmes2 (mes=2,4,6‐Me3C6H2) as very strong donor (D) and acceptor (A), respectively, giving 2,7‐D‐π‐D‐ and unsymmetric 2,7‐D‐π‐A‐pyrene derivatives, in which the donor destabilizes the HOMO−1 and the acceptor stabilizes the LUMO+1 of the pyrene core. Consequently, for 2,7‐substituted pyrene derivatives, unusual properties are obtained. For example, very large bathochromic shifts were observed for all of our compounds, and unprecedented green light emission occurs for the D/D system. In addition, very high radiative rate constants in solution and in the solid state were recorded for the D‐π‐D‐ and D‐π‐A‐substituted compounds. All compounds show reversible one‐electron oxidations, and Jul2Pyr exhibits a second oxidation, with the largest potential splitting (ΔE=440 mV) thus far reported for 2,7‐substituted pyrenes. Spectroelectrochemical measurements confirm an unexpectedly strong coupling between the 2,7‐substituents in our pyrene derivatives.
Doing the orbital shuffle: 2,7‐D‐π‐D‐ and unsymmetric 2,7‐D‐π‐A‐pyrene derivatives, based on a julolidine‐like moiety as very strong donor (D) and Bmes2 (mes=2,4,6‐Me3C6H2) as acceptor (A), have been prepared and show unusual photophysics, such as large bathochromic shifts and high radiative rate constants, and unprecedented green light emission for the D/D system. Spectroelectrochemical measurements indicate strong coupling between the 2‐ and 7‐substituents.
The spin chemistry of photoinduced charge-separated (CS) states of three triads comprising one or two triarylamine donors, a cyclometalated iridium complex sensitizer and a naphthalene diimide (NDI) ...acceptor, was investigated by transient absorption spectroscopy in the ns-μs time regime. Strong magnetic-field effects (MFE) were observed for two triads with a phenylene bridge between iridium complex sensitizer and NDI acceptor. For these triads, the lifetimes of the CS states increased from 0.6 μs at zero field to 40 μs at about 2 T. Substituting the phenylene by a biphenyl bridge causes the lifetime of the CS state at zero field to increase by more than 2 orders of magnitude (τ = 79 μs) and the MFE to disappear almost completely. The kinetic MFE was analyzed in the framework of a generalized Hayashi-Nagakura scheme describing coherent (S, T0 ↔ T±) as well as incoherent (S, T0 ⇌ T±) processes by a single rate constant k±. The magnetic-field dependence of k± of the triads with phenylene bridge spans 2 orders of magnitude and exhibits a biphasic behavior characterized by a superposition of two Lorentzians. This biphasic MFE is observed for the first time and is clearly attributable to the coherent (B < 10 mT) and incoherent (10 mT < B < 2 T) domains of spin motion induced by isotropic and anisotropic hyperfine coupling. The parameters of both domains are well understood in terms of the structural properties of the two triads, including the effect of electron hopping in the triad with two donor moieties. The kinetic model also accounts for the reduction of the MFE on reducing the rate constant of charge recombination in the triad with the biphenyl bridge.
A novel all‐organic host–guest system for emission in the NIR is introduced and investigated with respect to its opto‐electronic processes. The good agreement between theoretical and experimental ...results highlights the model character of this system and its potential for electroluminescent application. Comparative measurements provide access to the recombination mechanisms on molecular length scale and show that the emission behavior of the device under operation is controlled by charge carrier dynamics.
Up to three polychlorinated pyridyldiphenylmethyl radicals bridged by a triphenylamine carrying electron withdrawing (CN), neutral (Me), or donating (OMe) groups were synthesized and analogous ...radicals bridged by tris(2,6‐dimethylphenyl)borane were prepared for comparison. All compounds were as stable as common closed‐shell organic compounds and showed significant fluorescence upon excitation. Electronic, magnetic, absorption, and emission properties were examined in detail, and experimental results were interpreted using DFT calculations. Oxidation potentials, absorption and emission energies could be tuned depending on the electron density of the bridges. The triphenylamine bridges mediated intramolecular weak antiferromagnetic interactions between the radical spins, and the energy difference between the high spin and low spin states was determined by temperature dependent ESR spectroscopy and DFT calculations. The fluorescent properties of all radicals were examined in detail and revealed no difference for high and low spin states which facilitates application of these dyes in two‐photon absorption spectroscopy and OLED devices.
In combination with steady state and time resolved fluorescence spectroscopy the fluorescence of chlorinated triphenylmethyl diradicals was characterized both from the high spin state and from the low spin state. This was rather unexpected and therefore opens the way to use diradicals as emitters in molecular devices.
Abstract
Fluorogenic RNA aptamers are synthetic functional RNAs that specifically bind and activate conditional fluorophores. The Chili RNA aptamer mimics large Stokes shift fluorescent proteins and ...exhibits high affinity for 3,5-dimethoxy-4-hydroxybenzylidene imidazolone (DMHBI) derivatives to elicit green or red fluorescence emission. Here, we elucidate the structural and mechanistic basis of fluorescence activation by crystallography and time-resolved optical spectroscopy. Two co-crystal structures of the Chili RNA with positively charged DMHBO
+
and DMHBI
+
ligands revealed a G-quadruplex and a
trans
-sugar-sugar edge G:G base pair that immobilize the ligand by π-π stacking. A Watson-Crick G:C base pair in the fluorophore binding site establishes a short hydrogen bond between the N7 of guanine and the phenolic OH of the ligand. Ultrafast excited state proton transfer (ESPT) from the neutral chromophore to the RNA was found with a time constant of 130 fs and revealed the mode of action of the large Stokes shift fluorogenic RNA aptamer.
The radical cation of substituted hexaphenylbenzene derivatives displays toroidal delocalization (see picture). Owing to the face‐to‐face orientation of six aryl groups, the hole is (almost) ...completely delocalized over these π systems, as has been proved by the deconvolution and analysis of the Vis/NIR absorption bands.
PDF
