Strongly Absorbing π–π* States in Heteroleptic Dipyrrin/2,2′‐Bipyridine Ruthenium Complexes: Excited‐State Dynamics from Resonance Raman Spectroscopy

A recently reported new class of ruthenium complexes containing 2,2′‐bipyridine and a dipyrrin ligand in the coordination sphere exhibit both strong metal‐to‐ligand charge‐transfer (MLCT) and π–π* transitions. Quantitative analysis of the resonance Raman scattering intensities and absorption spectra reveals only weak electronic interactions between these states despite direct coordination of the bipyridyl and dipyrrin ligands to the central ruthenium atom. On the basis of DFT calculations and time‐dependent DFT (TD‐DFT), we propose that the electronic excited states closely resemble “pure” MLCT and π–π* states. Resonance Raman intensity analysis demonstrates that a large amplitude transannular torsional motion provides a mechanism for relaxation on the π–π* excited‐state surface. We assert that this result is generally applicable to a range of dipyrrin complexes such as boron–dipyrrin and metallodipyrrin systems. Despite the large torsional distortion between the phenyl ring and the dipyrromethene plane, π–π* excitation extends out onto the phenyl ring which may have important consequences in solar‐energy‐conversion applications of ruthenium–dipyrrin complexes.     

Synthesis of Di‐peri‐dinaphthoporphyrins by PtCl2‐Mediated Cyclization of Quinodimethane‐type Porphyrins

Di‐peri‐dinaphthoporphyrins can be regarded as a key and common substructure of fused porphyrinoids. PtCl₂‐mediated cycloisomerization reaction of quinodimethane‐type porphyrins provided these doubly fused porphyrins, which exhibit characteristic paratropic ring currents that presumably arise from 24π antiaromatic circuit as a dominant resonance contributor. UV/Vis absorption spectra, cyclic voltammetry, and excited‐state dynamics as well as theoretical calculation support this conclusion.     

Structural explanation of the spectral features of the nonsymmetrical complex {2,3,7,8,12,13,17,18‐octaethyl‐5‐[(methylimino)methyl]porphyrinato‐κ4N21,N22,N23,N24}palladium(II)

The features of porphyrins defining their functionality are related to their conformational flexibility. The degree of nonplanarity of metalloporphyrins depends directly on the number of substituents, their size and their location. The introduction of substituents in the meso positions of β‐substituted porphyrins increases the steric interaction and leads to distortions of the porphyrin core. Increasing the distortion of the porphyrin core would augment the bathochromic (red) shift of the electronic absorption spectra. A new nonsymmetrical 2,3,7,8,12,13,17,18‐octaethyl‐5‐[(methylimino)methyl]porphyrin complex of palladium(II), [Pd(C₃₈H₄₇N₅)], was synthesized and characterized by NMR, mass spectrometry and X‐ray analysis. The features of the electronic absorption spectrum of the synthesized complex are explained by the planarity of the porphyrin core and the π‐system of the imino group orthogonal to it.     

β‐to‐β 2,5‐Pyrrolylene‐Linked Cyclic Porphyrin Oligomers

β‐to‐β 2,5‐Pyrrolylene linked cyclic porphyrin oligomers have been synthesized by Suzuki–Miyaura coupling of 2,5‐diborylpyrrole and 3,7‐dibromoporphyrin. The cyclic porphyrin oligomers exhibit roughly coplanar structures, strong excitonic coupling, small electrochemical HOMO–LUMO gaps, and ultrafast excitation energy transfer between the neighboring porphyrins via the pyrrolylene bridge.     

Selective Cycloaddition of Tetracyanoethene (TCNE) and 7,7,8,8‐Tetracyano‐p‐quinodimethane (TCNQ) to Afford meso‐Substituted Phenylethynyl Porphyrins

π‐Extended TCBD‐porphyrins that contained a 1,1,4,4‐tetracyanobuta‐1,3‐diene unit were prepared by a highly efficient [2+2] cycloaddition of tetracyanoethene (TCNE) or 7,7,8,8‐tetracyano‐p‐quinodimethane (TCNQ) with meso‐substituted trans‐A₂B₂‐porphyrins that contained two phenylethynyl groups, followed by a retro‐electrocyclization reaction. Depending on the electronic properties of the arylethynyl groups, the cycloaddition reaction took place exclusively on either one or two ethynyl moieties with high yield. The addition of TCNQ proceeded with complete regioselectivity. The resulting π‐expanded TCBD‐porphyrins had a hypsochromically shifted Soret band and showed unique, broad absorption in the visible region.     

Directly Diphenylborane‐Fused Porphyrins

Mono‐ and bis(diphenylborane)‐fused porphyrins were synthesized from the corresponding β‐(2‐trimethylsilylphenyl)‐substituted porphyrins through the sequence of Si–B exchange reaction, intramolecular bora‐Friedel–Crafts reaction, and ring‐closing Si–B exchange reaction. Effective electronic interactions of the empty p‐orbital of the boron atom with the porphyrin π‐circuit lead to red‐shifted absorption spectra and substantially decreased LUMO energy levels. Pyridine adds at the boron center to cause disruption of the electronic interaction of the boron atom with large association constants (1.9–17×10⁴ m ^?1) depending on the central metal at the porphyrin. The Zn^II complex behaved as a hetero‐dinuclear Lewis acid, exhibiting regioselective binding of pyridines at the boron or the zinc center.     

Pictet–Spengler Synthesis of Quinoline‐Fused Porphyrins and Phenanthroline‐Fused Diporphyrins

Doubly and quadruply quinoline‐fused porphyrins were effectively synthesized through a reaction sequence consisting of Suzuki–Miyaura coupling of β‐borylated porphyrins with 2‐iodoaniline and subsequent Pictet–Spengler cyclization. These quinoline‐fused porphyrins display red‐shifted absorption bands and higher electron‐accepting abilities. This synthetic protocol also allowed the synthesis of phenanthroline‐fused porphyrin dimers, which bound either a Ni^II or Zn^II cation. The resultant metal complexes displayed further red shifted absorption spectra and molecular twists to effect an almost perpendicular arrangement of the two porphyrins.     

Self‐Assembly of Chiral Propeller‐like Supermolecules with Unusual “Sergeants‐and‐Soldiers” and “Majority‐Rules” Effects

Chiral amplification is an interesting phenomenon in supramolecular chemistry mainly observed in complicated systems in which cooperative effect dominate. Herein, chiral, supramolecular, propeller‐like architectures have been constructed through coassembly of an achiral disk‐shaped molecule and chiral amino acid derivatives driven by intermolecular hydrogen bonding. Both the “sergeants‐and‐soldiers” principle and “majority‐rules” effect are applicable in these discrete four‐component supermolecules, which are the simplest supramolecular system ever reported that exhibit chiral amplification.     

HOMO Stabilisation in π‐Extended Dibenzotetrathiafulvalene Derivatives for Their Application in Organic Field‐Effect Transistors

Three new organic semiconductors, in which either two methoxy units are directly linked to a dibenzotetrathiafulvalene (DB‐TTF) central core and a 2,1,3‐chalcogendiazole is fused on the one side, or four methoxy groups are linked to the DB‐TTF, have been synthesised as active materials for organic field‐effect transistors (OFETs). Their electrochemical behaviour, electronic absorption and fluorescence emission as well as photoinduced intramolecular charge transfer were studied. The electron‐withdrawing 2,1,3‐chalcogendiazole unit significantly affects the electronic properties of these semiconductors, lowering both the HOMO and LUMO energy levels and hence increasing the stability of the semiconducting material. The solution‐processed single‐crystal transistors exhibit high performance with a hole mobility up to 0.04 cm² V^?1 s^?1 as well as good ambient stability.     

Tuning the Photophysical Properties of BODIPY Molecules by π‐Conjugation with Fischer Carbene Complexes

The synthesis, structure, and photophysical properties of novel BODIPY–Fischer alkoxy‐, thio‐, and aminocarbene dyads are reported. The BODIPY chromophore is directly attached to the carbene ligand by an ethylenic spacer, thus forming donor–bridge–acceptor π‐extended systems. The extension of the π‐conjugation is decisive in the equilibrium geometries of the dyads and is clearly reflected in the corresponding absorption and emission spectra. Whereas the BODIPY fragment is mainly isolated in aminocarbene complexes, it is fully conjugated in alkoxycarbene derivatives. The former thus exhibit the characteristic photophysical properties of BODIPY units, whereas complete suppression of the BODIPY fluorescence emission is observed in the latter, as a direct consequence of the strong electron‐accepting character of the (CO)₅M?C moiety. As the π‐acceptor character of the metal–carbene group can be modified, the electronic properties of the conjugated BODIPY can be tuned. Density functional calculations have been carried out to gain insight into the photophysical properties.     

β‐Functionalized meso Tetrahalothien‐2‐ylporphyrins: Synthesis,Spectral, and Electrochemical Properties

Two types of β‐ functionalized (mono nitrated and perbrominated) meso tetrakis(5‐halothien‐;2‐yl)porphyrins, which can be used as precursors for the synthesis of other asymmetric and highly substituted porphyrins, have been synthesised and characterized. Introduction of a nitro group at the β‐ position shifted soret band 11–16 nm to the red region and redox potentials to > 170 mV for oxidation and > 250 mV for reduction anodically. Perbromination of halothienylporphyrins lead to enhanced bathochromically shifted uv‐visible spectral bands, but had only marginal influence on oxidation potentials. Effect of mono nitro group and eight bromo groups on the electronic properties of the porphyrins is attributed, respectively to, the electron deficiency created in the porphyrin π‐ system and the nonplanar conformation induced by the bulky bromo groups.     

π‐Extended Indenofluorenes

A series of π‐extended aromatic indenofluorene (IF) analogues with naphthalene and anthracene cores have been synthesized through acid‐catalyzed intramolecular cyclization. The regioselectivity of the reaction is controlled by a combination of steric and electronic factors and in some cases several possible regioisomers have resulted from the same precursor. The effects of ring connectivity on the optoelectronic properties were investigated by DFT calculations, absorption/emission spectroscopy, cyclic voltammetry, and spectroelectrochemical studies. All regioisomers exhibited a redshift of their absorption/emission bands relative to the parent IF analogues, but the magnitude of this shift and other optoelectronic properties (luminescence quantum yield, etc.) depends on the ring connectivity in a less obvious manner.     

Transition‐Metal‐Free Synthesis of Carbazoles and Indoles by an SNAr‐Based “Aromatic Metamorphosis” of Thiaarenes

Dibenzothiophene dioxides, which are readily prepared through oxidation of the parent dibenzothiophenes, undergo nucleophilic aromatic substitution with anilines intermolecularly and then intramolecularly to yield the corresponding carbazoles in a single operation. The “aromatic metamorphosis” of dibenzothiophenes into carbazoles does not require any heavy metals. This strategy is also applicable to the synthesis of indoles. Since electron‐deficient thiaarene dioxides exhibit interesting reactivity, which is not observed for that the corresponding electron‐rich azaarenes, a combination of a thiaarene‐dioxide‐specific reaction with the S_NAr‐based aromatic metamorphosis allows transition‐metal‐free construction of difficult‐to‐prepare carbazoles.     

“Dumbbell”‐ and “Clackers”‐Shaped Dimeric Derivatives of Monocarba‐closo‐dodecaborate

We designed, synthesized, and characterized two types of dimeric forms of monocarba‐closo‐dodecaborate, namely, a “dumbbell”‐shaped dianion having a C?C bond and a “clackers”‐shaped monoanion having an iodonium linker. The unique architectures of these anionic molecules were established by X‐ray analysis. Spectroscopic analysis, DFT calculations, and reactivity experiments revealed high anionic and chemical stability of both anions, which are crucial properties for weakly coordinating anions.     

Flexible Tuning of Unsaturated β‐Substituents on Zn Porphyrins: A Synthetic,Spectroscopic and Computational Study

A series of zinc porphyrins substituted at adjacent β‐positions with a CN group and para‐substituted ethenyl/ethynyl‐phenyl group have been studied using electronic absorption spectroscopy, resonance Raman spectroscopy and DFT calculations. The oxidative nucleophilic substitution of hydrogen was utilized for the introduction of a cyano substituent on the porphyrin ring. This modification has a remarkable electronic effect on the ring. The resulting porphyrin cyanoaldehyde was further modified in Wittig condensations to give series of arylalkene‐ and arylalkyne‐substituted derivatives. This substitution pattern caused significant redshifting and broadening of the B band, tuning from 433–446 nm. Additionally the Q/B band intensity ratios show much higher values than observed for the parent porphyrin ZnTPP (0.20 vs. 0.03). Careful analysis of the electronic transitions using DFT and resonance Raman spectroscopy reveal that the substituent does not significantly perturb the electronic structure of the porphyrin core, which is still well described by Gouterman’s four‐orbital model. However, the substituents do play a role in elongating the conjugation length and this results in the observed spectral changes.     

4,4′′′′‐Azobis[2,2′: 6′,2″‐terpyridine] and its Metal Complexes

By two different routes, 4,4′′′′‐azobis[2,2′: 6′,2″‐terpyridine] was synthesized. Its ruthenium complexes show interesting metal‐to‐ligand charge transfer (MLCT) absorption maxima in the electronic spectra. They represent the first ruthenium complexes of terpyridine units to give blue solutions.     

Cyclometalated Mono‐ and Dinuclear IrIII Complexes with “Click”‐Derived Triazoles and Mesoionic Carbenes

Orthometalation at Ir^III centers is usually facile, and such orthometalated complexes often display intriguing electronic and catalytic properties. By using a central phenyl ring as C?H activation sites, we present here mono‐ and dinuclear Ir^III complexes with “click”‐derived 1,2,3‐triazole and 1,2,3‐triazol‐5‐ylidene ligands, in which the wingtip phenyl groups in the aforementioned ligands are additionally orthometalated and bind as carbanionic donors to the Ir^III centers. Structural characterization of the complexes reveal a piano stool‐type of coordination around the metal centers with the “click”‐derived ligands bound either with C^N or C^C donor sets to the Ir^III centers. Furthermore, whereas bond localization is observed within the 1,2,3‐triazole ligands, a more delocalized situation is found in their 1,2,3‐triazol‐5‐ylidene counterparts. All complexes were subjected to catalytic tests for the transfer hydrogenation of benzaldehyde and acetophenone. The dinuclear complexes turned out to be more active than their mononuclear counterparts. We present here the first examples of stable, isomer‐pure, dinuclear cyclometalated Ir^III complexes with poly‐mesoionic‐carbene ligands.     

Hole Mobility Modulation in Single‐Crystal Metal Phthalocyanines by Changing the Metal–π/π–π Interactions

Weak intermolecular interactions in organic semiconducting molecular crystals play an important role in determining molecular packing and electronic properties. Single crystals of metal‐free and metal phthalocyanines were synthesized to investigate how the coordination of the central metal atom affects their molecular packing and resultant electronic properties. Single‐crystal field‐effect transistors were made and showed a hole mobility order of ZnPc>MnPc>FePc>CoPc>CuPc>H₂Pc>NiPc. Density functional theory (DFT) and 1D polaron transport theory reach a good agreement in reproducing the experimentally measured trend for hole mobility. Additional detail analysis at the DFT level suggests the metal atom coordination into H₂Pc planes can tune the hole mobility via adjusting the intermolecular distances along the shortest axis with closest parallel π stackings.     

π‐Extended Dihydrophenazines with Three‐State NIR Electrochromism Involving Large Conformational Changes

π‐Extended dihydrophenazines were successfully prepared by oxidation of 2‐(N ‐arylamino)anthracenes. Their roof‐type conformations were revealed by X‐ray diffraction analysis, and the analysis of the optical properties indicated the presence of intramolecular charge‐transfer processes. Upon chemical oxidation, the electronic absorption dramatically changed in a two‐step fashion. The electron spin resonance (ESR) analysis revealed that, depending on the amount of oxidant added, either a paramagnetic radical cation or a diamagnetic dication was generated. The NMR analysis revealed a conformational change upon oxidation, which was supported by theoretical calculations. A three‐state electrochromic behavior was observed during the electrochemical oxidation and reduction cycles, showing sequential switching between visible and near‐infrared (NIR) absorption properties upon application of electrochemical stimuli.