We report T-shaped Sb(III) and Bi(III) trisamides that exhibit phosphorescence in the near-infrared-II region (NIR-II, 1000-1700 nm) in solution at 25 °C. Geometric constraint of the pnictogen(III) centers by an NNN pincer ligand enables ligand-to-metal charge transfer upon visible-light excitation. Significant structural reorganization upon photoexcitation leads to the largest Stokes shifts (> 6000 cm-1) ever reported for Sb and Bi compounds to emit in the NIR region, reminiscent of precious metal photosensitizers. The emissive triplet states exhibit lifetimes up to the microsecond range, offering a new design principle for NIR emitters based on main group elements.

We report on the synthesis and characterization of two new macrocyclic, rectangular tetrarhenium complexes resulting from the {Re(CO)3}2(2,2 '-bis(benzimidazolate)) precursor {Re(CO)3}2(BiBzIm) and the core-rigidified 4,4 '-bipyridine-type bridging ligands thieno[2,3-c:5,4-c']dipyridine (complex 2a) or 9-(4-methylthiophenyl)-9H-pyrrolo[2,3-c:5,4-c']dipyridine (complex 2b). Both complexes undergo four consecutive one-electron reductions which are all centered on the diimine ligands. Redox splittings of the first pair of reductions are traced to electronic coupling in the mixed-valent state where the charges at the two diimine ligands, 0 and-1, (formally) differ. IR and UV/vis/NIR spectroscopic investigations on the neutral, the one-, and the two-electron reduced forms of these complexes characterize the singly reduced monoanions as moderately strongly coupled mixed-valent systems of Class II. Interestingly, the charge distribution parameter Delta rho derived from IR spectroscopy of ca. 0.25 is roughly twice as large as the optically derived values of Hushs delocalization parameter alpha. Complexes 2a,b are strongly phosphorescent at 77 K, with quantum yields of 62% for 2a and 91% for 2b Phosphorescence is retained at r. t., albeit with significantly reduced quantum yields of 1.1% and 2.0%, respectively.

We present three new intensely colored ferrocenyl- (Fc-) (Fc = (η5-C5H5)Fe(η5-C5H4), 1+) and cymantrenyl-substituted (CpMn(CO)2(L); Cp = (η5-C5H4), L = CO in 2+, PiPr3 in 3+) triarylmethylium ions, in which a sterically demanding anthryl spacer links the (half-)sandwich donor to an electron-poor diarylmethylium acceptor (4-CF3-C6H4)2C+. Single-crystal X-ray studies as well as NOESY experiments reveal that these dyads exhibit strongly bent fulvene-anthraquinodimethane (AQDM) structures in the crystalline state and in solution, in which the positive charge is efficiently transmitted from the diarylmethylium unit to the (half-)sandwich complex entity. Despite large differences between the redox potentials for metal oxidation and tritylium reduction, EPR spectroscopy detects small amounts of paramagnetic open-shell isomers (valence tautomers) with one unpaired spin at each site. Reduction to neutral monoradicals 1˙-3˙ converts the AQDM to an anthryl linker, resulting in structural transformation from a folded to a twisted form.

The concept of aromaticity is one of the most fundamental principles for understanding the properties and reactivities of organic molecules. However, in molecular electronics research, it has been shown that aromaticity is not necessarily advantageous for electron transmission across electrode-molecule-electrode junctions. In this work, we introduce formally antiaromatic, yet planar N,N'-disubstituted dihydrophenazines as a compelling building block for exploring molecular conductance properties beyond the scope of classical aromatic molecules and compare anchor group-modified dihydrophenazines and structurally closely related anthracenes. We find that molecular conductance increases by 1.5 orders of magnitude from aromatic anthracenes to their phenazine congeners, where the central ring attains partially antiaromatic character. Oxidation of the dihydrophenazine core results accordingly in conductance attenuation.

We report the synthesis of new mononuclear styryl and dinuclear divinylphenylene-bridged diruthenium complexes, terminated on both ends with thiomethyl (-SMe) substituents at the styryl and the bidentately coordinating benzoate or beta-acetylphenolate chelate ligands. Robust binding of the SMe functionalities to surface atoms of nanoscale gold electrodes enables the modified complex molecules to establish nanojunctions in scanning tunneling microscopy break-junction experiments. Ten of the 13 complexes provided defined molecular conductance features that could be evaluated. Complexes with beta-ketoenolate ligands proved to be somewhat better conductors than those with benzoate ligands, and the dinuclear complexes, despite their considerably larger molecular lengths, proved to be more conductive than the mononuclear ones. While the molecular conductances of the present complexes are at the low end of those with rigid-rod fully pi-conjugated ethynylarylene backbones and linkages, their modular design makes this type of complex a versatile testbeds for probing the impact of every constituent on the molecular conductances of such metal complex-based wires.

The heavier Group 14 elements Si and Ge are known for their superior conductance performance over carbon in σ-channel-dominated molecular wires. Only a few studies have, however, explored their potential as C substitutes in wires with π-dominated conductance channels. We report a series of diarylamine (DA)-terminated π-conjugated wires, where C, Si, and Ge bridging atoms planarize a phenylene-vinylene linker. We find a slight impact on the conductance performance that deviates from the ordering according to atomic mass, with Si achieving a 20-30% higher conductance compared to Ge and C. DA oxidation increases conductance by 2 orders of magnitude and mitigates heteroatom-induced variations. Furthermore, the meta isomer of the Si analogue was synthesized in an effort to increase heteroatom participation in the conductance channel, revealing increased sensitivity to atomic substitution and considerable conductance at a 0.5 V bias.

Three new isomeric pyridone fluorenone hydrazone-based boron complexes, 3-BrN to 5-BrN, with bromination at the 3-, 4-, or 5-position of the pyridone ring and their corresponding σ-platinum complexes trans-Pt(PEt3)2(n-N)X (n = 3, 4, or 5; X = Cl, Br, I) resulting from oxidative addition of the aryl-Br bond of n-BrN to Pt(PEt3)2 and, for X = Cl, I, subsequent substitution of the halogenide ligand, were synthesized and characterized by NMR, UV–Vis absorption, and photoluminescence spectroscopy. The molecular structures of dyes 3-BrN to 5-BrN and of seven complexes, including the cis-isomer of the bromo complex resulting from 3-BrN, were established by single X-ray diffraction. The nearly orthogonal orientation of the Pt coordination plane with respect to the plane of the dye ligand limits intermolecular π-stacking interactions in the crystalline state while giving rise to extensive C-H···halogen and C-H···π interactions, resulting in intricate packing patterns. Electronic absorption spectra of dyes 3-BrN to 5-BrN show a prominent HOMO-LUMO absorption band at ca. 520 nm, which is red-shifted and intensifies on platination. All compounds are dual fluorescence and phosphorescence emitters in the range of 520 to 670 nm, or at ca. 1000 nm, both at room temperature and at 77 K. The population of an excited triplet state and their photostability even towards continuous light irradiation renders these compounds efficient sensitizers for singlet oxygen generation and catalysts for the photo-oxidation of triphenylphosphine.

Triarylamines (TAAs) are one of the most important classes of redox-active organic compounds, which are readily available from modular synthesis, thereby offering the possibility to easily adjust their intrinsic redox potentials. We present herein two bis(triarylamines) (BTAAs) with pi-extended 2,7-diethynylfluorene or 2,2 '-(1,3-butadiyne-1,4-diyl)-bis(7-ethynylfluorene) bridges and two benzoic acid headgroups per TAA and their (formally) mixed-valent radical cations. Owing to their amphiphilic character and favorable redox properties, these BTAAs are designed to serve as charge conduits through membranes. The lipid bilayer/BTAA systems are water-soluble, which allowed us to explore their photoactivity in aqueous solution and utilize their mixed valent form for membrane-mediated photoinduced electron transfer. Our findings will be relevant for constructing artificial nanoreactors for solar light energy conversion and light-driven redox chemistry in water.

The synthesis, characterization, and photophysical properties of a family of pnictogen complexes Pn(DPP)I 2 bearing dipyridine pyrrolide (DPP-) ligands and with As, Sb, and Bi as the central atom Pn are reported. Slight but systematic trends arising from the variation in the atomic number are apparent in NMR and UV-vis spectra. As confirmed by time-dependent density functional theory calculations, the electron-rich DPP- ligand endows the energetically lowest-lying electronic absorption band with partial DPP-to-Pn charge-transfer (ligand-to-metal charge transfer) character. All complexes are phosphorescent at cryogenic temperatures with high phosphorescence quantum yields of Phi phos = 50.8 to 83.9% and lifetimes in the range of tens of microseconds. In accordance with the heavy atom effect, merely As(DPP)I 2, the lightest representative of the three complexes within this work, displays weak fluorescence. It also possesses the shortest phosphorescence lifetime and the highest quantum yield for 1O2 generation. This is attributed to a higher degree of covalent character of the Pn-ligand bonds and increased orbital overlap as well as larger iodide contributions to the LUMO.

The realization of single molecule-based electronic switching devices is an intriguing perspective on the path towards ultimate device miniaturization. In particular, the integration of multiple switching centers into a single molecule will open new possibilities for device integration. Here, we report on the synthesis, characterization, and the redox and the on-surface switching properties of the triazatruxene (TAT) dimer 1 and tetramer 2 with covalent butadiynediyl or ethynediyl linkages between the TAT moieties. TAT oxidation gives rise to electronic absorption over the entire range of electromagnetic radiation in the UV/vis/NIR (NIR = near infrared), with TAT -> TAT+ charge transfer absorptions in mixed-valent redox states. For on-surface switching, both compounds were successfully deposited on an Ag(111) substrate using electrospray deposition (ESD). Compound 1 retains the low-bias three-level switching of both constituting TAT units, giving rise to six distinguishable switching states. In tetramer 2, the larger number of substrate anchoring points restricts on-surface configurations to those with only one or two non-neighbouring low-bias activatable TAT switching units. Our findings demonstrate that it is possible to realize single-molecule multi-state switches with covalently linked TAT units and pinpoint the particular impact of the substrate on their switching dynamics.

We report on the emissive properties of two related bismuth complexes, LBiBr2 and LBi, with a monoanionic 2,6-diiminophenyl N,C,N pincer ligand L. The Bi(III) complex phosphoresces at 77 K, while the bismuthinidene Bi(I) species represents the first example of a phosphorescent bismuth complex that emits in the near infrared at room temperature.

This contribution investigates the role of the metal-metal bond in paddle-wheel complexes for molecular conductance. To these ends, we compare two pairs of Mo and Rh paddle-wheel complexes M2(LN)4 and M2(LSMe)4 (M = Rh, Mo), each with four lateral 4-pyridyl-(LN-) or 4-(methylthio)-(LSMe-) functionalized benzamidinate ligands mutually disposed at 90° angles. These complexes represent bond orders of 4 (Mo) and 1 (Rh), while offering metal-based highest occupied frontier orbitals of the same δ symmetry. The structural features of the complexes were established by X-ray diffraction on single crystals. Molecular conductance measurements were performed with the aid of a scanning-tunnelling microscopy break-junction setup and revealed that the Mo complexes surpass their Rh congeners. Decoration of the paddle-wheel complexes with four laterally disposed anchor groups results in four different possible modes of molecule attachment to the Au electrodes with two, three or even four anchoring points. The conductances of possible junction geometries were assessed quantum chemically with the DFT+Σ approach using Au slab electrodes. Calculated variations by about one order of magnitude for the different anchoring geometries can explain the rather broad conductance distributions observed in our experiments. Further transport calculations considered two-point molecular attachment to sharp or blunt nanoelectrodes for mutual cis and trans dispositions of the anchor groups. Our computational results indicate that the better performance of the Mo complexes originates from superior conjugation between the Mo2 δ-binding and the ligand π orbitals as compared to the Rh2 δ* orbital, rather than from the larger metal-metal bond order. Upon increasing the bias voltage, we observed a new conductance feature associated with a nearly 100 times higher G value, which we ascribe tentatively to oxidation of the molecule inside the junction.

We present the synthesis, characterization, and photophysical properties of two pyrene-modified (NCN) pincer bismuth complexes, where the pyrenyl residues are either part of the cyclometalating pincer ligand (1) or bound as monodentate ligands to the BiIII ion (2). Both complexes are dually emissive at 77 K. For complex 2, pyrenyl phosphorescence persists at r.t. in degassed CH2Cl2, albeit with reduced intensity. This renders 2 one of only a handful of Bi complexes showing this property.

The controlled manipulation of electronic and magnetic states in single-molecule magnets (SMMs) is crucial for their implementation in molecular electronics, spintronics, and quantum computation. In typical SMMs, key properties like magnetic anisotropy and slow magnetic relaxation are imposed by complex ligand shells, whose bulky and three-dimensional structures hamper efficient manipulation of the molecular magnetism by chemical methods. This work demonstrates highly selective redox doping of an Fe4 nanomagnet on a Pb(111) surface using lithium atoms. Scanning tunneling microscopy, x-ray absorption spectroscopy, and ab initio calculations reveal the cooperative incorporation of three Li atoms per Fe4 molecule, resulting in a selective, threefold reduction of its iron-based magnetic core. The doping modifies the intramolecular exchange interaction, turning from antiferromagnetic to ferromagnetic, and changes the molecular magnetic anisotropy from easy-axis to easy-plane. This study demonstrates successful chemical redox doping of individual polynuclear molecular magnets, exploits a rare showcase of cooperative binding, and highlights a route for tuning magnetic properties of complex SMMs.

We present two triazatruxene-triarylmethylium TAT-Tr+ donor-acceptor dyads 1+ and 2+ with either 4-CF3- (1+) or 4-F-substituted phenyl rings (2+) at the tritylium (Tr+) site. In spite of rather large differences between the redox potentials for TAT oxidation and Tr+ reduction, the diamagnetic TAT-Tr+ forms of these dyads coexist with their paramagnetic TAT+•-Tr• valence tautomers with one unpaired spin at every redox site. The major quantity of the diradical isomers is trapped as dimers with concomitant loss of the unpaired spin density of the Tr• entity. The dimers were also observed by cyclic and square wave voltammetry and form readily by one-electron reduction of the dyads to the corresponding trityl radical. Dimerization of the neutral radicals occurs at a much faster rate than dissociation of their two-electron oxidized forms, indicating hysteretic behavior.

We present six bismuth complexes (NCRN)BiX2 (X = Cl, I) with diarylamine-modified pincer ligands (NCRN = (4-R-C6H4)2N-C6H2-(CH2NMe2)2-1,3; R = Me, O, NMe2) and report on their optoelectronic, photophysical, and electrochemical properties. The complexes exhibit intriguing photophysical behavior, with the p-tolyl and p-anisyl derivatives showing phosphorescence at 77 K in frozen solvent matrices and at room temperature (r.t.) in the solid state. In THF solutions at r.t., only ligand-based fluorescence is observed with strongly reduced quantum yields compared to free proligands NCHRN. Electrochemical studies reveal up to three reversible one-electron oxidations. The NMe2-substituted complexes display the lowest oxidation potentials and the largest number of redox waves. Radical cations [NCHNMe2N]+ and [(NCNMe2N)BiX2]+ are chemically stable and fluoresce weakly in the near-infrared (NIR) at ca. 1200 nm.

The novel coordination polymers [Zn(PZDB)(DEF)2]n (Zn-PZDB) and [Co(HPZDB)2(DEF)2]n (Co-HPZDB) (H2PZDB = 4,4 '-(phenazine-5,10-diyl)dibenzoic acid, DEF = N,N-diethylformamide) are synthesized solvothermally from the metal nitrate salts and the linear H2PZDB linker with the redox-active phenazine-5,10-diyl core. Zn-PZDB is composed of zigzag chains with the Zn ion tetrahedrally coordinated by two diethylformamide (DEF) molecules and two carboxyl O-atoms from the bridging PZDB2- linker. The crystal structure of Co-PZDB represents a two-dimensional (2D) coordination grid. The Co2+ ion is octahedrally coordinated by two DEF molecules and four carboxyl oxygen atoms of the semi-deprotonated HPZDB- linker molecules. Yellow Zn-PZDB and red Co-PZDB turn green upon air exposure, which is due to linker oxidation, forming the (H)PZDB+ radical cation. UV/Vis/NIR spectroelectrochemistry reveals that Me2PZDB undergoes two reversible one-electron oxidations, producing characteristic absorption bands. Similar spectroscopic changes are observed upon oxidation of Zn-PZDB and Co-HPZDB. Chemical oxidation with SbCl5 yields the same color changes together with electron paramagnetic resonance signals typical of ligand-based radical cations. These findings indicate that the PZDB linker is the primary redox site. Ligand oxidation is followed by disintegration of the coordination polymers.

We report on the new star-shaped triazatruxene (TAT) tetrad 1, where three peripheral TAT moieties connect to a central TAT through propargylic spacers. The insulating nature of the linkers results in separate, partially overlapping blue and yellow-orange emissions from the peripheral and central TATs. This renders 1 a multicolor emitter, whose emission color can be tuned by the choice of the excitation wavelength and the solvent. In frozen solution, dual fluorescence emission from the two different types of TATs is complemented by dual phosphorescence with radiative lifetimes in the range of seconds. Organic light-emitting devices (OLEDs) constructed with 1 as the emissive layer achieved a peak irradiance of 5.87 µW/nm/m2 at 580 nm at an operation voltage of 6.7 V. Remarkably, the emission color of the electroluminescence can be varied from yellow over different hues of orange to red or even to near infrared (NIR) emission, depending on the applied voltage. Tetrad 1 is also redox-active, indicating that 1 may simultaneously serve as a hole conductor and emitter. Its oxidized forms are panchromatic absorbers from the near UV to the NIR due to intra- and inter-TAT charge-transfer (CT) absorptions.

We report on the emissive properties of four new pyridine dipyrrolide bismuth complexes Bi(R1PDPR2)Br, modified with electron-donating (-OMe) or electron-withdrawing (-CF3) substituents at the ligand. These complexes exhibit red phosphorescence at cryogenic temperatures with emission wavelengths ranging from 602 to 642 nm as a response to the electronic character of the substituents R1 and R2. A profound similarity between the pairs Bi(CF3PDPCF3)Br and Bi(CF3PDPOMe)Br, and Bi(OMePDPCF3)Br and Bi(OMePDPOMe)Br is attributed to improved conjugation between the respective phenyl ring and the PDP core, as based on TD-DFT calculations and crystallographic studies. The larger quantum yield of Bi(CF3PDPCF3)Br is attributed to the enhancement of the 3LMCT character of the electronic transition through the introduction of the electron-withdrawing -CF3 substituents.

With the progressing miniaturization of electronic device components to improve circuit density while retaining or even reducing spatial requirements, single molecules employed as electric components define the lower limit of accessible structural width. To circumvent the typical exponential conductance decay for increasing length in molecule-based wires, topological states, which describe the occurrence of discontinuities of a bulk material's electronic structure confined to its surface, can be realized for molecules by the introduction of unpaired spins at the molecular termini. The resulting high conductance and reversed conductance decay are typically only observed for shorter molecules, as the terminal spins must be within the electronic coupling range to produce the desired effects. We expand the realm of long and exceptionally conductive molecular wires by employing highly conjugated, planarized carbon-bridged oligo(phenylene-vinylene)s as conduits between readily oxidizable diarylamine termini. This yields molecular wires of already decent conductance values and small conductance decay in the neutral state. Upon the introduction of topological states, the conductance can be increased by a factor of up to 1800 for a 3 nm long molecule, and the conductance decay becomes inverted, together with an excellent signal intensity at concentrations as low as 0.01 mM.