The Nephelauxetic Effect Becomes an Important Design Factor for Photoactive First-Row Transition Metal Complexes

The expansion of d-orbitals as a result of metal-ligand bond covalence, the so-called nephelauxetic effect, is a well-established concept of coordination chemistry, yet its importance for the design of new photoactive complexes based on first-row transition metals is only beginning to be recognized. Until recently, much focus has been on optimizing the ligand field strength, coordination geometries, and molecular rigidity, but now it becomes evident that the nephelauxetic effect can be a game changer regarding the photophysical properties of 3d metal complexes in solution at room temperature. In Cr^III and Mn^IV complexes with the d³ valence electron configuration, the nephelauxetic effect was exploited to shift the well-known ruby-like red luminescence to the near-infrared spectral region. In Fe^II and Co^III complexes with the low-spin d⁶ electron configuration, charge-transfer excited states were stabilized with respect to detrimental metal-centered excited states, to improve their properties and to enhance their application potential. In isoelectronic (3d⁶) isocyanide complexes of Cr⁰ and Mn^I, the nephelauxetic effect is likely at play as well, enabling luminescence and other favorable photoreactivity. This minireview illustrates the broad applicability of the nephelauxetic effect in tailoring the photophysical and photochemical properties of new coordination compounds made from abundant first-row transition metals.     

Effect of the Metal on Disulfide/Thiolate Interconversion: Manganese versus Cobalt

It has recently been proposed that disulfide/thiolate interconversion supported by transition‐metal ions is involved in several relevant biological processes. In this context, the present contribution represents a unique investigation of the effect of the coordinated metal (M) on the Mⁿ⁺?disulfide/M⁽ⁿ⁺¹⁾⁺?thiolate switch properties. Like its isostructural Co^II‐based parent compound, Co^II ₂ SS (Angew. Chem. Int. Ed.­ 2014 , 53, 5318), the new dinuclear disulfide‐bridged Mn^II complex Mn^II ₂ SS can undergo an M^II?disulfide/M^III?thiolate interconversion, which leads to the first disulfide/thiolate switch based on Mn. The coordination of iodide to the metal ion stabilizes the oxidized form, as the disulfide is reduced to the thiolate. The reverse process, which involves the reduction of M^III to M^II with the concomitant oxidation of the thiolates, requires the release of iodide. The Mn^II ₂ SS complex slowly reacts with Bu₄NI in CH₂Cl₂ to afford the mononuclear Mn^III?thiolate complex Mn^IIII . The process is much slower (ca. 16 h) and much less efficient (ca. 30 % yield) with respect to the instantaneous and quantitative conversion of Co^II ₂ SS into Co^IIII under similar conditions. This distinctive behavior can be rationalized by considering the different electrochemical properties of the involved Co and Mn complexes and the DFT‐calculated driving force of the disulfide/thiolate conversion. For both Mn and Co systems, M^II?disulfide/M^III?thiolate interconversion is reversible. However, when the iodide is removed with Ag⁺, the M^II ₂ SS complexes are regenerated, albeit much slower for Mn than for Co systems.     

Excitonic Bands in the Spectra of Some Organic-Inorganic Hybrid Compounds Based on Metal Halide Units

Summary.  The optical absorption, photoluminescence, and photoconductivity spectra of some compounds of the formulas [R(CH₂) _n NH₃] _x M _y X _z , [R(CH₂) _n NH(CH₃)₂] _x M _y X _z , [R(CH₂) _n S(CH₃)₂] _x M _y X _z , [R(CH₂) _n SC(NH₂)₂] _x M _y X _z , and [R(CH₂) _n SeC(NH₂)₂] _x M _y X _z (R = organic residue; M = Bi(III), Pb(II), Sn(II), Cu(I), Ag(I) etc; X = I, Br, Cl; n, x, y, z = 0, 1, 2, 3, …) are briefly reviewed, and some new results are reported. The position, intensity, and shape of the excitonic bands depend on the dimensionality and size of the inorganic network as well as on the nature of the M, X, R, and onium moieties. Received June 23, 2000. Accepted August 1, 2000     

Tris(tetraceno)triquinacenes: Synthesis and Photophysical Properties of Threefold Linearly Extended Tribenzotriquinacenes

The linear extension of the rigid, C_3v‐symmetrical carbon framework of tribenzotriquinacene (TBTQ) along its three wings is reported. The key step of the extension procedure consists of a Diels–Alder reaction of three ortho‐quinodimethane units generated in situ at the triquinacene core. The use of 1,4‐naphthoquinone provides a facile and particularly efficient access to tris(tetraceno)‐annellated triquinacenes. The steady‐state photophysical properties of these new oligotetracenes bearing three mutually orthogonal chomophores are determined and analyzed by DFT calculations.     

The Control of Conjugation Lengths and Steric Hindrance to Modulate Aggregation‐Induced Emission with High Electroluminescence Properties and Interesting Optical Properties

A series of novel AIE‐active (aggregation‐induced emission) molecules, named SAF‐2‐TriPE, SAF‐3‐TriPE, and SAF‐4‐TriPE, were designed and synthesized through facile reaction procedures. We found that incorporation of the spiro‐acridine‐fluorene (SAF) group, which is famous for its excellent hole‐transporting ability and rigid structure, at different substitution positions on the phenyl ring affected the conjugation lengths of these compounds. Consequently, we have obtained molecules with different emission colors and properties without sacrificing good EL (electroluminescence) characteristics. Accordingly, a device that was based on compound SAF‐2‐TriPE displayed superior EL characteristics: it emitted green light with η_{c, max}=10.5 cd A^?1 and η_{ext, max}=4.22 %, whereas a device that was based on compound SAF‐3‐TriPE emitted blue‐green light with η_{c, max}=3.9 cd A^?1 and η_{ext, max}= 1.71 %. These compounds also displayed different AIE performances: when the fraction of water in the THF solutions of these compounds was increased, we observed a significant improvement in the Φ_F of compounds SAF‐2‐TriPE and SAF‐3‐TriPE; in contrast, compound SAF‐4‐TriPE showed an abnormal phenomenon, in that it emitted a strong fluorescence in both pure THF solution and in the aggregated state without a significant change in Φ_F. Overall, this systematic study confirmed a relationship between the regioisomerism of the luminophore structure and its AIE activity and the resulting electroluminescent performance in non‐doped devices.     

Substituents modification of meso-aryl BODIPYs for tuning photophysical properties

We successfully synthesized eight meso-aryl BODIPYs with 2,6-diethyl- or 1,2,6,7-tetraethyl substituents and characterized their photophysical properties. The steric hindrance resulting from the phenolic group in the meso-aryl moiety and the ethyl groups on the BODIPY core affected the synthesis of dipyrromethanes as an intermediate as well as the UV–Vis absorption and fluorescence emission of the BODIPYs due to the constrained rotation of the aryl ring. The potential use of the meso-hydroxyphenyl BODIPY as a pH sensor was also shown by the pH-dependent fluorescence emissions.     

9,10‐Diaminoanthracenes Revisited: The Influence of N‐Substituents on Their Electronic States

The electronic and molecular structures of 9,10‐diamino‐substituted anthracenes with different N‐substituents have been re‐examined. In particular, different N‐substituents influence both the electronic and molecular structures of the oxidized species of 9,10‐diaminoanthracenes. The anthrylene moiety of 9,10‐bis(N,N‐di(p‐anisyl)amino)anthracene retains its planarity during the course of two successive one‐electron oxidations, whereas 9,10‐bis(N,N‐dimethylamino)anthracene and 9,10‐bis(N‐p‐anisyl‐N‐methylamino)anthracene undergo a substantial structural change to a butterfly‐like structure through a two‐electron oxidation process. The structural changes observed for the oxidized states are ascribed to significant differences in the frontier molecular orbitals of the above‐mentioned three kinds of 9,10‐diaminoanthracenes due to different extents of mixing between the amine‐localized and anthrylene‐localized orbitals.     

Lowest Singlet Exciton in Pentacene: Modern Calculations versus Classic Experiments

Based on simple model calculations, the expected magnitude of the field‐induced shift observable in electroabsorption is estimated for three alternative assignments proposed in the literature for the lowest singlet excitation of the pentacene crystal (pure Frenkel exciton, pure charge‐transfer exciton, or a mixture of both). The results are compared with the corresponding experimental value, which is also known from the literature. The latter turns out to be compatible only with the mixed parentage of the pertinent state, which contains the charge‐transfer contribution in the range from 25 to 70 %. The conclusion is discussed in the context of the present controversies concerning the existing experimental and theoretical evidence on this subject.     

Control of Chromophore Symmetry by Positional Isomerism of Peripheral Substituents

The first example of the control of porphyrinoid chromophore symmetry based on the positional isomerism of peripheral substituents has been achieved by preparing tetraazaporphyrins (TAPs) with C_4h, D_2h, C_2v, and C_s symmetry due to the relative arrangement of peripheral tert‐butylamino and cyano groups as push and pull substituents, respectively. The four structural isomers were successfully isolated and characterized by ¹H NMR spectroscopy and X‐ray crystallography. The band morphology in the Q‐band region varies depending on the molecular symmetry due to the significant perturbation introduced into the chromophore by the push and pull substituents. The C_4h and C_2v isomers exhibit a single Q band, whereas the Q bands of the D_2h and C_s isomers show a marked splitting. The magnetic circular dichroism spectra indicate that the push–pull TAPs retain the properties of the 16‐membered 18π‐electron perimeter generally observed for porphyrinoids. Theoretical calculations have demonstrated that the perturbation introduced by the substituents lowers the D_4h symmetry of the parent TAP π‐conjugated system, and this results in significant spectral changes. A novel approach to the fine‐tuning of the spectral properties of porphyrinoids based on changes in the chromophore symmetry is described.     

Room-Temperature Photogeneration of Nitrosyl Linkage Isomers in Ruthenium Nitrosyl Complexes

The conditions for the photogeneration of NO linkage isomers at room temperature are studied. By pulsed laser irradiation in the blue spectral range, the long-lived Ru−ON isomer can be generated at room temperature, which is crucial for potential applications, such as holography and data storage. By using static and time-resolved spectroscopy (UV/Vis and IR), we give evidence that the liftime of the Ru−(η²-(NO)) isomer is a decisive parameter for the formation of the Ru−ON isomer at high temperature owing to a two-step isomerization mechanism Ru−NO→Ru−(η²-(NO))→Ru−ON. Furthermore, we report the low-temperature structures for each isomer, which were revealed by photocrystallography.     

Electronic Absorption Spectra and Redox Properties of C Type Cytochromes in Living Microbes

Shewanella is an electrogenic microbe that has significant content of c type cytochromes (ca. 0.5 mM ). This feature allows the optical absorption spectra of the cell‐membrane‐associated proteins to be monitored in vivo in the course of extracellular respiratory electron‐transfer reactions. The results show significant differences to those obtained in vitro with purified proteins.

     

Four New Thioantimonates(III) with the General Formula [TM(tren)]Sb4S7 (TM = Mn,Fe, Co,Zn) with the Transition Metal as Part of a Thioantimonate(III) Network Synthesized under Solvothermal Conditions and Tuning of the Optical Band Gap by the Transition Metal Cation

Four new thioantimonate(III) compounds with the general formula [TM(tren)]Sb₄S₇, TM = Mn 1 , Fe 2 , Co 3 and Zn 4 , were synthesized under solvothermal conditions by reacting elemental TM, Sb and S in an aqueous solution of tren (tren = tris(2‐aminoethyl)amine). All compounds crystallize in the monoclinic space group P2₁/n with four formula units in the unit cell. Single crystal X‐ray analyses of 1 [a = 8.008(2), b = 10.626(2), c = 25.991(5) Å, β = 90.71(3)°, V = 2211.4(8) ų], 2 [a = 8.0030(2), b = 10.5619(2), c = 25.955(5) Å, β = 90.809(3)°, V = 2193.69(8) ų], 3 [a = 7.962(2), b = 10.541(2), c = 25.897(5) Å, β = 90.90(3)°, V = 2173.0(8) ų] and 4 [a = 7.978(2), b = 10.625(2), c = 25.901(5) Å, β = 90.75(3)°, V = 2195.2(8) ų] reveal that the compounds are isostructural. The [Sb₄S₇]^2‐ anions are composed of three SbS₃ trigonal pyramids and one SbS₄ unit as primary building units (PBU). The PBUs share common edges and corners to form semicubes (Sb₃S₄) which may be regarded as secondary building units (SBU). The SBUs and SbS₃ pyramids are joined in an alternating fashion yielding the equation/tex2gif-stack-1.gif[Sb₄S₇^‐] anionic chain which is directed along [100]. Weaker Sb‐S bonding interactions between neighbored chains lead to the formation of layers within the (001) plane which contain pockets that are occupied by the cations. The TM²⁺ ions are in a trigonal bipyramidal environment of four N atoms of the tren ligand and one S atom of the thioantimonate(III) anion. The optical band gaps depend on the TM²⁺ ion and amount to 3.11 eV for 1 , 2.04 eV for 2 , 2.45 eV for 3 , and 2.60 eV for 4 .     

Mixed‐Valence Ruthenium Complexes Rotating through a Conformational Robin–Day Continuum

The conformational energy landscape and the associated electronic structure and spectroscopic properties (UV/Vis/near‐infrared (NIR) and IR) of three formally d⁵/d⁶ mixed‐valence diruthenium complex cations, [{Ru(dppe)Cp*}₂(μ‐C≡CC₆H₄C≡C)]⁺, [ 1 ]⁺, [trans‐{RuCl(dppe)₂}₂(μ‐C≡CC₆H₄C≡C)]⁺, [ 2 ]⁺, and the Creutz–Taube ion, [{Ru(NH₃)₅}₂(μ‐pz)]⁵⁺, [ 3 ]⁵⁺ (Cp=cyclopentadienyl; dppe=1,2‐bis(diphenylphosphino)ethane; pz=pyrazine), have been studied using a nonstandard hybrid density functional BLYP35 with 35 % exact exchange and continuum solvent models. For the closely related monocations [ 1 ]⁺ and [ 2 ]⁺, the calculations indicated that the lowest‐energy conformers exhibited delocalized electronic structures (or class III mixed‐valence character). However, these minima alone explained neither the presence of shoulder(s) in the NIR absorption envelope nor the presence of features in the observed vibrational spectra characteristic of both delocalized and valence‐trapped electronic structures. A series of computational models have been used to demonstrate that the mutual conformation of the metal fragments—and even more importantly the orientation of the bridging ligand relative to those metal centers—influences the electronic coupling sufficiently to afford valence‐trapped conformations, which are of sufficiently low energy to be thermally populated. Areas in the conformational phase space with variable degrees of symmetry breaking of structures and spin‐density distributions are shown to be responsible for the characteristic spectroscopic features of these two complexes. The Creutz–Taube ion [ 3 ]⁵⁺ also exhibits low‐lying valence‐trapped conformational areas, but the electronic transitions that characterize these conformations with valence‐localized electronic structures have low intensities and do not influence the observed spectroscopic characteristics to any notable extent.     

Synthesis, spectroscopic, and electrochemical properties of three tetrathiafulvalenes attached to perylene

Three donor–acceptor dyads 1–3 comprising of a tetrathiafulvalene (TTF) unit linked with perylene by a simple σ-bond were synthesized and characterized. Spectroscopy and cyclic voltammetry provided an indication that intramolecular charge-transfer interactions in their ground states between TTF and perylene for dyads 1–3 are negligible. Compared with the compound perylene, dyads 1–3 exhibited large fluorescence quenching, which might be ascribed to photo-induced electron transfer interaction between TTF and perylene units in the excited state. Correspondence: Yongjia Shen, Laboratory of Advanced Materials, Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, P.R. China.     

π-π* Molecular charge transfer complexes of antipyrineSchiff bases with 1,4-benzoquinone derivatives as π-acceptors

Summary Spectrophotometric studies of compounds formed by several substituted antipyrineSchiff bases as electron donors with 2,3-dichloro-5,6-dicyano-benzoquinone, chloranilic acid, and chloranil as electron acceptors have given results that are consistent with 1:1 charge transfer complexes. Transition energies, oscillator strengths, and dipole moments of the complexes as well as the effect of solvent upon their stability are discussed. Also their free energies and absorption cross sections have been determined.

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Charge-transfer-KomplexeSchiffscher Basen vom Antipyrintyp mit 1,4-Benzochinon-Akzeptoren

Zusammenfassung Spektrophotometrische Untersuchungen von ausSchiffschen Basen des Antipyrintyps als Elektronendonatoren und 2,3-Dichlor-5,6-dicyanobenzochinon, Chloranilsäure und Chloranil als Elektronenakzeptoren gebildeten Verbindungen zeigen die Entstehung vonCharge-transfer-Komplexen der Stöchiometrie 1:1. Neben der Berechnung von freien Energien und Absorptionsquerschnitten werden Übergangsenergien, Oszillatorstärken, Dipolmomente und der Einfluß des Lösungsmittels auf die Stabilität der Komplexe diskutiert.

     

Effects of Spatial Constraints and Brønsted Acid Site Locations on para‐Terphenyl Ionization and Charge Transfer in Zeolites

The locations of Brønsted acid sites (BAS) in the channels of medium‐pore zeolites have a significant effect on the spontaneous ionization of para‐terphenyl (PP₃) insofar as spatial constraints determine the stability of transition states and charge‐transfer complexes relevant to charge separation. The ionization rates and ionization yield values demonstrate that a strong synergy exists between the H⁺ polarization energy and spatial constraints imposed by the channel topology. Spectroscopic and modeling results show that PP₃ incorporation, charge separation, charge transfer and charge recombination differ dramatically among zeolites with respect to channel structure (H‐FER, H‐MFI, H‐MOR) and BAS density in the channel. Compartmentalization of ejected electrons away from the initial site of ionization decreases dramatically the propensity for charge recombination. The main mode of PP₃^.+ decay is hole transfer to form AlO₄H^.+ ??? PP₃ charge‐transfer complexes characterized by intense absorption in the visible range. According to the nonadiabatic electron‐transfer theory, the small reorganization energy in constrained channels explains the slow hole‐transfer rate.     

Chiral interactions in azobenzene dimers: a combined experimental and theoretical study

To investigate interchromophore interactions in azobenzene polymers, we have undertaken a thorough spectroscopic analysis of the azodye [(S)-3-pivaloyloxy-1-(4'-nitro-4-azobenzene)pyrrolidine] by modeling the repeating unit of poly[(S)-3-methacryloyloxy-1-(4'-nitro-4-azobenzene)pyrrolidine) and its dimeric derivative whose synthesis is presented here. The analysis of the electronic and Raman spectra of the azodye in several solvents is based on a previously proposed model for polar chromophores in solution. Electronic and CD spectra of the dimeric unit are collected and analyzed within the framework of a new model. On the basis of the information collected from the spectroscopic analysis of the solvated dye, this model accounts for interchromophore interactions in the dimer. The large CD signal measured for the dimer (amounting to about a third of the signal measured for the polymer) suggests the presence of important chiral interactions in the dimeric unit, and is modeled in terms of a right-handed relative orientation of the two chromophores.     

Exciton <Emphasis Type="Italic">Cotton</Emphasis> Effects of Benzoates in the <Superscript>1</Superscript>B Transition Region. Demonstration and Applications

Summary. The benzoate ¹B region exciton Cotton effects, hitherto unexplored, were analyzed for their use in stereochemical assignments in both acyclic (conformationally flexible) and cyclic molecules. It was found that a strong, allowed ¹B_a transition, polarized longitudinally, dominates the ¹B region (185–210nm) both in the UV and the CD spectra. The exciton Cotton effects due to this transition have the same sign (but differing magnitude) as those due to the ¹L_a (CT) band. The other component of the nearly degenerate ¹B system, i.e. ¹B_b transition, polarized orthogonally to the ¹B_a transition, gives a Cotton effect in the case of di- and poly(4-chlorobenzoate) chromophoric system, the sign of which is sensitive to the configuration of di- or polyol. In rigid 5-steroid skeleton ¹B_b transition couplings appear responsible for strong exciton Cotton effects due to nearly parallelly oriented benzoate chromophores. Whereas ¹B_a transition excitation energy appears insensitive to the nature of a substituent in 4-position of the benzoate chromophore, substitution with a donor group (methoxy, dimethylamino) brings about a red shift of the ¹B_b band, although less pronounced than the red shift of the ¹L_a (CT) band.     

Porphyrins Fused to N‐Heterocyclic Carbenes (NHCs): Modulation of the Electronic and Catalytic Properties of NHCs by the Central Metal of the Porphyrin

We report herein a detailed study of the use of porphyrins fused to imidazolium salts as precursors of N‐heterocyclic carbene ligands 1 M . Rhodium(I) complexes 6 M – 9 M were prepared by using 1 M ligands with different metal cations in the inner core of the porphyrin (M=Ni^II, Zn^II, Mn^III, Al^III, 2H). The electronic properties of the corresponding N‐heterocyclic carbene ligands were investigated by monitoring the spectroscopic changes occurring in the cod and CO ancillary ligands of [( 1 M )Rh(cod)Cl] and [( 1 M )Rh(CO)₂Cl] complexes (cod=1,5‐cyclooctadiene). Porphyrin–NHC ligands 1 M with a trivalent metal cation such as Mn^III and Al^III are overall poorer electron donors than porphyrin–NHC ligands with no metal cation or incorporating a divalent metal cation such as Ni^II and Zn^II. Imidazolium salts 3 M (M=Ni, Zn, Mn, 2H) have also been used as NHC precursors to catalyze the ring‐opening polymerization of L ‐lactide. The results clearly show that the inner metal of the porphyrin has an important effect on the reactivity of the outer carbene.