Extensive Electron Delocalization Research Articles

Molecular Engineering of Intensely Near-Infrared Absorbing Excited States in Highly Conjugated Oligo(porphinato)zinc−(Polypyridyl)metal(II) Supermolecules

A new series of chromophores, MPZn(n), which combine ethyne-bridged bis(terpyridyl)metal(II)-(porphinato)zinc(II) (MPZ(n)) and oligomeric, ethyne-bridged (porphinato)zinc(II) (PZn(n)) architectures, have been synthesized and characterized, along with a series of derivatives bearing pyrrolidinyl electron-releasing groups on the ancillary terpyridine units (Pyr(m)MPZn(n)). Cyclic voltammetric studies, as well as NMR, electronic absorption, fluorescence, and femtosecond pump-probe transient absorption spectroscopies, have been employed to study the ground- and excited-state properties of these unusual chromophores. All of these species possess intensely absorbing excited states having large spectral bandwidth that penetrate deep in the near-infrared (NIR) energy regime. Electronic structural variation of the molecular framework shows that the excited-state absorption maximum can be extensively modulated [lambdamax(T(1) --> T(n))] (880 nm < lambdamax < 1126 nm), while concomitantly maintaining impressively large T(1) --> T(n) absorption manifold spectral bandwidth (full width at half-maximum, fwhm, approximately 2000-2500 cm(-1)). Furthermore, these studies enable correlation of supermolecular electronic structure with the magnitude of the excited-state lifetime (tau(es)) and demonstrate that this parameter can be modulated over 4 orders of magnitude ( approximately 1 ns < tau(es) < 45 micros). Terpyridyl pyrrolidinyl substituents can be utilized to destabilize terpyridyl ligand pi(*) energy levels and diminish the E1/2 (M3+/2+) value of the bis(terpyridyl)metal(II) center: such perturbations determine the relative energies of the PZn(n)-derived 1pi-pi(*) and bis(terpyridyl)metal(II) charge-transfer states and establish whether the T(1)-state wave functions of MPZn(n) and PyrmMPZn(n) species manifest the extensive electronic delocalization and charge-separated (CS) features characteristic of long-lived triplet states that absorb strongly in the NIR.

Paramagnetic Active Site Models for the Molybdenum−Copper Carbon Monoxide Dehydrogenase

New paramagnetic, heterobimetallic Mo/Cu complexes featuring the Mo(=O)(mu-S)Cu core of O. carboxidovorans carbon monoxide dehydrogenase have been synthesized and structurally and spectroscopically characterized. The complexes exhibit EPR spectra (left graphic) indicative of extensive electron delocalization across the Mo-S-Cu core, in agreement with computational studies identifying the singly-occupied molecular orbital (right graphic).

Electronic and structural effects in muscular relaxants: Riparin I and Riparin III

Two muscular relaxants (MR), methyl ethers of N-benzoyl tyramine (Riparin I) and N-(1,6-dihydroxybenzoyl) tyramine (Riparin III) were studied with X-ray diffraction (XRD), optical spectroscopy and semi-empirical calculations. The XRD data show that the C O group in Riparin I is 34° out of plane from the closest benzene ring. The abinitio calculations using RHF/6-31G* suggest that the dihydroxybenzoyl (DHB) group in Riparin III forms intramolecular hydrogen bond between the H of the O–H group and the O of the C O group, and gives a relatively planar structure between DHB group and C O group. The spectral analysis together with the excitation energy calculations using the ZINDO program showed that the absorption band at 309 nm in Riparin III, absence in Riparin I, is assigned to the π→π* transition associated mainly from the DHB. A long range charge-transfer (CT) transition, as the low-lying excited electronic state, S 1 has been calculated for both relaxants involving the non-bonding electron on carbonyl and/or amine group and the antibonding orbitals of aromatic ring. The planar structure of Riparin III and its extensive electron delocalization may have an important role on the pharmacological action mode and on the potency this MR as compared to those of Riparin I.

Molecular materials with conducting and magnetic properties based on ET and [ M(tdas)2]x-dithiolenes

Two hybrid molecular materials showing a combination of magnetic and conducting properties, the charge-transfer (ET) 2 [Fe(tdas) 2 ] (1) and (ET)Ni(tdas) 2 (2), (ET=bis(ethylenedithio) tetrathiafulvalene; M=Fe, Ni; tdas=l,2,5-thiadiazole-3,4-dithiolate) salts, are characterized by vibrational (IR and Raman) and UV-VIS-NIR spectroscopies. These studies have proved to be effective and diagnostic tools in identifying the oxidation state (partial or integer) and the packing pattern (dimers or segregated stacks) of the ET donor only, since no v(C=C) group vibration sensitive to the charge of M(tdas) 2 complexes has been observed. This is ascribed to the extensive electron-delocalization inside the tdas ring in agreement with semiempirical extended Huckel calculations, indicating also that the AO's of the terminal S-atom give a high contribution to the HOMO and favour the occurrence of three-dimensional intermolecular interactions along the molecular longitudinal axis. These findings are consistent with structural results.

1-Dimethylamino-3-dimethyliminio-2-(p-methoxyphenyl)prop-1-ene perchlorate

The title compound, C14H21N2O+·ClO4−, is an organic non-linear optical material. It crystallizes in the monoclinic space group P21 and exhibits second-harmonic generation equivalent to that of urea. The structural study shows extensive electron delocalization in the vinamidinium moiety. The dihedral angle between the vinamidinium moiety and the benzene ring is 81.6 (1)°.

Photophysical properties of closely-coupled, binuclear ruthenium(II) bis(2,2':6',2''-terpyridine) complexes.

The photophysical properties of closely-coupled, binuclear complexes formed by connecting two ruthenium(II) bis(2,2':6',2''-terpyridine) complexes via an alkynylene group are compared to those of the parent complex. The dimers exhibit red-shifted emission maxima and prolonged triplet lifetimes in deoxygenated solution. Triplet quantum yields are much less than unity and the dimers generate singlet molecular oxygen with low quantum efficiency. Temperature dependence emission studies indicate coupling to higher-energy triplet states while cyclic voltammetry shows that the metal centres are only very weakly coupled but that extensive electron delocalization occurs upon one-electron reduction. The radiative rate constants derived for these dimers are relatively low, because the lowest-energy metal-to-ligand, charge-transfer states possess increased triplet character. In contrast, the rate constants for nonradiative decay of the lowest-energy triplet states are kept low by extended electron delocalization over the polytopic ligand. The poor triplet yields are a consequence of partitioning at the second triplet level.

Effect of the parent ligand on the photophysical properties of closely-coupled, binuclear ruthenium(II) tris(2,2'-bipyridine) complexes.

The photophysical properties of closely-coupled, binuclear complexes formed by connecting two ruthenium(II) tris(2,2'-bipyridine) complexes via an alkynylene group differ significantly from those of the relevant mononuclear complex. In particular, the energy of the first triplet excited state is lowered relative to the parent complex, because of the presence of the alkynylene substituent, while the triplet lifetime is prolonged, in part, because of extended electron delocalization. We now report that the triplet lifetime is also affected by the nature of the spectator 2,2'-bipyridyl ligands. Thus, replacing the parent 2,2'-bipyridine ligands with the corresponding 4,4'-dinitro-substituted ligands serves to decrease the luminescence yield and lifetime. With the corresponding carboxylate ester, the luminescence yield and lifetime are increased. Perdeuteration of the parent 2,2'-bipyridine ligands also leads to a modest increase in the luminescence yield. Such observations are indicative of electronic coupling between the various metal-to-ligand, charge-transfer excited triplet states. Temperature dependence studies confirm that these excited states are closely spaced and thermally accessible at ambient temperature. For some of the binuclear complexes, the quantum yield for formation of the lowest-energy triplet state is significantly less than unity.

Preferred Conformations of Stabilized Phosphorus Ylides

The stabilized phosphorus ylides, Ph3P=C(CO.R′)CO.OR; 1, R=Et, R′=CH2P+Ph3; 2, R=R′=Me; 3, R=Et, R′=Me; 4, R=Pri; R′=Me; 5, R=But; R′=Me, adopt a near planar conformation in the crystal which allows extensive electronic delocalization. The keto and alkoxylic oxygens are oriented and align favorably with the cationoid phosphorus. These conformations bring methyl hydrogens in the ester residue into proximity with the face of a phenyl group and lead to π-shielding and upfield shifts of the 1HNMR signals of 3 over a wide temperature range (-50–95°C) in (CD3)2CO, CDCl3 and DMSOd-6. Geometries of 2 and 3, optimized by using the HF 3-21 (G*) or 6-31 (G*) basis sets, are very similar to those in the crystal, but semiempirical treatments generate structures in which either the ester or keto moiety is twisted out of plane. The authors thank the CEPEDEQ, Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile for instrumental facilities.

Bis-Adducts of Substituted Phenylethynyl on a Ru2(DMBA)4 Core: Effect of Donor/Acceptor Modifications

The reactions between arylethynes (either BuLi activated or Et3N assisted) and Ru2(RDMBA)4X2, where RDMBA is either N,N‘-dimethylbenzamidinate (R = H) or N,N‘-dimethyl-m-methoxybenzamidinate (R = m-MeO) and X is either Cl- or NO3-, resulted in the compounds Ru2(RDMBA)4(C⋮CC6H4Y)2 (Y = H (1a,b), 4-NO2 (2a,b), 4-CN (3a,b), 3-CN (4a,b), 4-NMe2 (5a,b); R = H (a), m-MeO (b)). The single-crystal X-ray diffraction study of 1b and 2a revealed the linear alignment of the arylethynyl ligands along the Ru−Ru vector. All of the compounds display two Ru-based one-electron processes, an oxidation and a reduction, and their electrode potentials correlate linearly with the Hammett constants of substituent Y. Compounds 5a,b display an additional pair of one-electron processes attributed to the oxidation of 4-NMe2 groups, on the basis of which an extensive electron delocalization along the metallayne backbone was inferred.

Steady-State and Time-Resolved Spectroscopic Characteristics of Novel Silicon-Containing Organopolymers

The spectroscopic characteristics of novel π-conjugated polymers containing four-coordinated silicon, acetylene groups and either 1,4-biphenylene or 2,7-fluorene in the main chain were investigated by steady-state and picosecond laser spectroscopy. The spectral features of absorption, fluorescence excitation spectra, fluorescence lifetime, and fluorescence polarization were explained by the existence of two kinds of inhomogeneously broadened electronic states formed in the disordered polymeric chain. The dynamics of photoinduced absorption was measured in the 400–900 nm spectral range with picosecond time resolution. The long-wavelength band with λmax ∼ 710 nm was ascribed to excited-state absorption from higher-lying electronic states created in short polymeric segments with essential conformational distortion of the subunits. The short-wavelength band with λmax ∼ 580 nm and a shoulder at 500 nm was interpreted as photoinduced absorption from a lower-lying state arisen in more planar, longer π-conjugated segments populated via direct excitation and energy migration between disordered segments of the polymeric chain. For the fluorene-containing polymer, the smaller Stokes shift and the greater degree of fluorescence polarization are consistent with more extensive electron delocalization along the backbone.

Diruthenium Metallaynes: Versatile Chromophores and Electrophores

Both the {\\rm mono} - and {\\rm bis} -alkynyl adducts on a diruthenium core supported by either diarylformamidinate, or 2-anilinopyridinate, or dimethylbenz amidinate have been synthesized and characterized. These novel Ru 2 -metallaynes are highly redox flexible, soluble in common organic solvents and stable towards air, moisture and heat. Electrochemical and spectroscopic studies revealed that the Ru 2 -metallaynes have exceptionally small solution HOMO-LUMO gaps (1.2-1.4 eV) and high electron affinities. Ru 2 -metallaynes can be oxidatively coupled at ≡C-H termini, and the resultant dimers exhibit an extensive electronic delocalization across the polyyne-diyl (-C 2m -) bridge. Study of a bis-(1-ferrocenylethynyl)-adduct revealed that the Ru 2 core facilitates electron transfer between two Fc termini.

The supramolecular structure ofN-(6-amino-3,4-dihydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl)glycylglycinate contains a unique O—H...N(nitroso) hydrogen bond

The molecular structure of the title compound, C9H12N6O5, shows extensive electronic delocalization involving the pyrimidine ring and the attached N atom of the glycyl­glycinate, the amino and the nitro­so groups. A unique O—H⋯N(nitro­so) hydrogen bond of 2.748 (2) Å is found in the structure. The supramolecular structure is a three-dimensional network, derived from two sets of antiparallel chains, which link chains of alternating dimers which are formed by the action of two different crystallographic centres of symmetry.

Electrochemical and In Situ SNIFTIR Spectroscopic Studies of Palladium(II) and Zinc(II) Schiff Base Complexes: Structural Control of the Electronic Communication between Two Identical Redox Sites

Abstract The mechanisms of the electron transfer of S-benzyl-N-(ferrocenyl-1-methyl-methylidene)-dithiocarbazate palladium(II)/zinc(II) complexes [Pd(lsb)2]/[Zn(lsb)2] were studied by cyclic voltammetry, differential pulse voltammetry, digital simulation and in-situ subtractively normalized interfacial Fourier transform infrared (SNIFTIR) spectroelectrochemistry. The results indicate that [Pd(lsb)2], which has a square-planar configuration, involved two consecutive one-electron steps in the redox process, while the tetrahedral configuration of Zn(II) involved a two-electron step. The [Pd(lsb)2] complex exhibits a moderately strong electronic communication between the two-ferrocene moieties, which occurs through the skeleton chain of the ligand due to extensive electron delocalization of the whole molecule during the redox process, while the [Zn(lsb)2] complex shows low electron delocalization, and has two almost identical ferrocene moieties.

Study on the spectroelectrochemical properties of S-benzyl- N-(ferrocenyl-1-methyl-metylidene)-dithio-carbazate nickel (II) complex

Mechanism of electron transfer of a bridge biferrocene complex, Ni(LSB) 2, derived from the Schiff base ligand, HLSB, S-benzyl- N-(ferrocenyl-1-methyl-metylidene)-dithio-carbazate Ni(LSB) 2, during redox processes is illuminated by in-situ Fourier transform infrared (FTIR) spectroelectrochemistry. The results indicated that two consecutive one-electron steps were involved which gives the corresponding mono- and di-ferricenium cations, respectively. This complex exhibits moderately strong degree of electronic communication between the two-ferrocene moieties, taking place through the skeleton chain of the ligand due to an extensive electron delocalization in the molecule. Possible pathways of electron transfer of Ni(LSB) 2 during the redox processes are postulated.

Electrochemical and in situ FTIR studies of biferrocene platinum(II) complex

Electron transfer mechanism during redox process of Pt(LSB) 2, a derivative of S-benzyl- N-(ferrocenyl-1-methyl-metylidene)-dithiocarbazate (HLSB), is studied by cyclic voltammetry, differential pulse voltammetry, digitally simulation and in situ FTIR spectroelectrochemistry. Electrochemistry results indicated the redox process of Pt(LSB) 2 involves two consecutive one-electron steps. Coordination of Pt to HLSB resulted in the increase of electron-withdrawing of the dithiocarbazate group, which is demonstrated by the positive shift of redox potentials of Pt(LSB) 2 compared with those of HLSB. The peak–peak separation of 129 mV revealed a strong degree of electronic communication between the two-ferrocene moieties in Pt(LSB) 2. The results of in situ FTIR indicated that electronic communication takes place through the skeleton chain of HLSB due to the extensive electron delocalization in the whole molecule.

Bis{methylNβ-[4-(dipropylamino)benzylidene]dithiocarbazato}nickel(II)

The Schiff base ligand in the title complex, [Ni(C15H22N3S2)2], lost a proton from its tautomeric thiol form and coordinated to Ni(II) via the mercapto S and β-N atoms. The geometry around the Ni atom is square planar with two equivalent Ni—N and Ni—S bonds. The two phenyl rings and the coordination moieties are in one plane forming an extensive electronic delocalization system. The structure is governed by C—H⋯S and C—H⋯N hydrogen bonds, leading to the formation of centrosymmetric dimers.

Novel (Heteromolecular) π-Complexes of Aromatic Cation Radicals. Isolation and Structural Characterization

Extensive (electron) delocalization in the novel heteromolecular pi-complex of the hindered naphthalene cation radical (OMN(+)(*)) with naphthalene (NAP) accompanies the pronounced charge-transfer absorption band at approximately 1100 nm in the near-IR. X-ray crystallography establishes the viability of the unusual "club sandwich" structure despite the repulsive electrostatic forces inherent to the dicationic unit.

Redox reactivity and reorganization energy of zinc cytochrome c cation radical.

Little is known about transient intermediates in photoinduced electron-transfer reactions of metalloproteins. Oxidative quenching of the triplet state of zinc cytochrome c, 3Zncyt, is done at 20 degrees C, pH 7.00, and ionic strength of 1.00 M, conditions that suppress the thermal back-reaction and prolong the lifetime of the cation radical, Zncyt+. This species is reduced by [Fe(CN)6]4-, [W(CN)8]4-, [Os(CN)6]4-, [Mo(CN)8]4-, and [Ru(CN)6]4- complexes of similar structures and the same charge. The rate constants and thermodynamic driving forces for these five similar electron-transfer reactions were fitted to Marcus theory. The reorganization energy of Zncyt+ is lambda = 0.38(5) eV, lower than that of native cytochrome c, because the redox orbital of the porphyrin cation radical is delocalized and possibly because Met80 is not an axial ligand to the zinc(II) ion in the reconstituted cytochrome c. The rate constant for electron self-exchange between Zncyt+ and Zncyt, k11 = 1.0(5) x 10(7) M(-1) s(-1), is large owing to the extended electron delocalization and relatively low reorganization energy. These results may be relevant to zinc(II) derivatives of other heme proteins, which are often used in studies of photoinduced electron-transfer reactions.

Ranitidine hydrochloride, a polymorphic crystal form.

In the title compound, dimethyl(¿5-[2-(1-methylamino-2-nitroethenylamino)ethylthiometh yl]-2- furyl¿methyl)ammonium chloride, C(13)H(23)N(4)O(3)S(+).Cl(-), protonation occurs at the dimethylamino N atom. The ranitidine molecule adopts an eclipsed conformation. Bond lengths indicate extensive electron delocalization in the N,N'-dimethyl-2-nitro-1, 1-ethenediamine system of the molecule. The nitro and methylamino groups are trans across the side chain C=C double bond, while the ethylamino and nitro groups are cis. The Cl(-) ions link molecules through hydrogen bonds.

Metal-Carborane Multidecker Sandwiches as Building Blocks for Metallopolymers and Nanostructured Materials

The ability of cyclic planar [RR'C2B3H2X]4− carborane ligands (R, R′ = alkyl, aryl, SiMe3, H; X = alkyl, Cl, Br, I, H) to bind tightly to transition and main-group metals on both sides of the C2B3 ring plane allows the construction of a wide range of polymetallic multidecker sandwich complexes involving different metals, metal oxidation states, and molecular architectures. Although these ligands are isoelectronic and isosteric analogues of C5H5 −, they form stronger covalent bonds to metal centers, and can stabilize many robust, isoluhle structural types that are not readily accessible (or are completely unknown) in metallocene chemistry. Many of these metallacarborane sandwich complexes are paramagnetic and exhibit extensive electron-delocalization that can be “tuned” by redox action or by the introduction of appropriate substituents to the carborane ligands or the metal centers. This remarkable versatility suggests that there is considerable potential for the creation of novel polymeric and solid state materials that can be tailored to have desired electronic, optical, or other properties. This review summarizes recent efforts in the designed syntheses of oligomeric and polymeric systems and the systematic exploration of structure-property relationships.