Electronic configuration assignment and the importance of low-lying excited states in high-spin imidazole-ligated iron(II) porphyrinates

The synthesis and characterization of six new high-spin deoxymyoglobin models (imidazole(tetraarylporphyrinato)iron(II)) are described. These have been intensively studied by temperature-dependent Mossbauer spectroscopy from 295 to 4.2 K. All complexes show a strong temperature dependence for the quadrupole splitting consistent with low-lying excited states of the same or lower multiplicity. An analysis of the data obtained in applied magnetic fields leads to the assignment of the sign of the quadrupole splitting. All model compounds as well as those of deoxymyoglobin and deoxyhemoglobin, previously studied, have a negative sign for the quadrupole splitting. Although not previously predicted, this experimental observation leads to the assignment of the ground-state electronic configuration for all high-spin imidazole-ligated iron(II) porphyrinates as (d(xz)())(2)(d(yz)())(1)(d(xy)())(1)(d(z)()()2)(1)(d(x)()()2(-)(y)()()2)(1). This is a distinctly different ground-state electronic configuration from other high-spin iron(II) porphyrinates; differences in structural details for the two classes of high-spin complexes are also discussed. The apparent anomaly of differing signs for the zero-field splitting constant between previously studied model complexes and the heme proteins is addressed; the difference appears to result from the fact that the assumptions used in the spin Hamiltonian approach that has been applied to these complexes are not adequately satisfied. Structures of four of the new five-coordinate species have been determined. Core conformations in these derivatives show variation, but these and previously studied compounds reveal a limited number of conformational patterns. The bond lengths and other geometrical parameters such as porphyrin core size and iron out-of-plane displacement support a high-spin state assignment for the iron(II).     

Two-pump-one-probe femtosecond studies of Ni(II) porphyrins excited states

Dual excited states of nickel(II) meso-tetra(4-sulfonatophenyl)porphyrin (NiTPPS) and nickel(II) meso-tetraphenylporphyrin (NiTPP) have been investigated by two-pump-one-probe transient absorption spectrometry. By dual excited states, we mean molecular entities that have absorbed two photons to generate molecular states with electronic excitation in two distinct regions of metalloporphyrin. Two successive pulses of 400 and 550 nm were used for excitation. The first pulse (400 nm) produced an S2 state of the porphyrin pi-system, which deactivated to give rise to an S1 state and subsequently produce a metal-centered (d,d) state. The second (550 nm) pulse selectively targeted an S0 --> S1 transition of those molecules having an excited metal center and was delivered to the sample approximately 40 ps after the first excitation event. At this time, the ground state of the tetrapyrrole pi-system was already regenerated and the excitation was localized at the metal center. The kinetic profiles of the NiTPPS transients in DMSO revealed biexponential decays with time constants of 0.6 and 4 ps. Photoexcitation of NiTPP in toluene and NiTPPS in water resulted in similar behavior. A mechanism for the Ni(II) porphyrin dual excited state deactivation involving the formation of an intramolecular charge transfer state has been proposed.     

Lowest electronic excited states of platinum(II) diimine complexes

Absorption and emission spectra of Pt(diimine)L2 complexes (diimine = 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (dmbpy); L = pyrazolate (pz-), 3,5-dimethylpyrazolate (dmpz-), or 3,4,5-trimethylpyrazolate (tmpz-)) have been measured. Solvent-sensitive absorption bands (370-440 nm) are attributed to spin-allowed metal-to-ligand charge-transfer (1MLCT) transitions. As solids and in 77 K glassy solution, Pt(bpy)(pz)2 and Pt(dmbpy)(pz)2 exhibit highly structured emission systems (lambda max approximately 494 nm) similar to those of the diprotonated forms of these complexes. The highly structured bands (spacings 1000-1400 cm-1) indicate that the transition originates in a diimine-centered 3(pi-->pi*) (3LL) excited state. The intense solid-state and 77 K glassy solution emissions from 3MLCT[d(Pt)-->pi*(bpy)] excited states of complexes with dmpz- and tmpz- ligands occur at longer wavelengths (lambda max = 500-610 nm), with much broader vibronic structure. These findings are consistent with increasing electron donation of the pyrazolate ligands, leading to a distinct crossover from a lowest 3LL to a 3MLCT excited state.     

Electronic excited states of Si(100) and organic molecules adsorbed on Si(100)

The electronically excited states of the Si(100) surface and acetylene, benzene, and 9,10-phenanthrenequinone adsorbed on Si(100) are studied with time-dependent density functional theory. The computational cost of these calculations can be reduced through truncation of the single excitation space. This allows larger cluster models of the surface in conjunction with large adsorbates to be studied. On clean Si(100), the low-lying excitations correspond to transitions between the pi orbitals of the silicon-silicon dimers. These excitations are predicted to occur in the range 0.4-2 eV. When organic molecules are adsorbed on the surface, surface --> molecule, molecule --> surface, and electronic excitations localized within the adsorbate are also observed at higher energies. For acetylene and benzene, the remaining pipi* excitations are found to lie at lower energies than in the corresponding gas-phase species. Even though the aromaticity of 9,10-phenanthrenequinone is retained, significant shifts in the pipi* excitations of the aromatic rings are predicted. This is in part due to structural changes that occur upon adsorption.     

Electronic structure of molecular complexes in their ground and excited states. II.

Using a modified INDO method the potential energy curves for ground and some excited states (with an appreciable amount of charge transfer character) of wide range molecular complexes have been calculated. The calculation performed for CT, HB, CTTS, etc. systems shows that potential energy curves could be regarded as criterion for classification of molecular complexes into several groups with different properties. Analysis of proton motion in ground and excited states of H₃N-HCl and hydrogen maleate anion have also been performed.     

Static and dynamic quenching of Ru(II) polypyridyl excited states by iodide

The metal-to-ligand charge-transfer (MLCT) excited states of Ru(bpy)(2)(deeb)(PF(6))(2), where bpy is 2,2-bipyridine and deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine, in dichloromethane were found to be efficiently quenched by iodide at room temperature. The ionic strength dependence of the UV-visible absorption spectra gave evidence for ion pairing. Iodide was found to quench the excited states by static and dynamic mechanisms. Stern-Volmer and Benesi-Hildebrand analysis of the spectral data provided a self-consistent estimate of the iodide-Ru(bpy)(2)(deeb)(2+) adduct in dichloromethane, K = 59 700 M(-1). Transient absorption studies clearly demonstrated an electron-transfer quenching mechanism with transient formation of I(2)(*)(-) in high yield, phi = 0.25 for 355 or 532 nm excitation. For Ru(bpy)(2)(deeb)(PF(6))(2) in acetonitrile, similar behavior could be observed at higher iodide concentrations than that required in dichloromethane. The parent Ru(bpy)(3)(2+) compound also ion pairs with iodide in CH(2)Cl(2), and light excitation gave a higher I(2)(*)(-) yield, phi = 0.50. X-ray crystallographic, IR, and Raman data gave evidence for interactions between iodide and the coordinated deeb ligand in the solid state.     

FT-IR spectroscopic investigation of transition metal (II) 2-aminopyridine tetracyanonickelate complexes

A series of Hofmann-type clathrate host molecules containing two 2-aminopyridine (2-Apy) groups attached to transition metal (II) (M) tetracyanonickelate frame, with the formula: M(2Apy)₂Ni(CN)₄ (where M=Mn, Co, Cu or Zn), have been synthesised for the first time. Their FT-IR spectra are reported in the 400–4000 cm⁻¹ region. The spectral features suggest that the compounds are substantially isostructural to that of already known Hofmann type pyridine complex; M(py)₂Ni(CN)₄. Moreover, 2Apy pyridine molecules are found to involve coordination through the ring nitrogen. The coordination effect on the 2Apy modes was analysed.     

Ligand localized triplet excited states in platinum(II) bipyridyl and terpyridyl peryleneacetylides

An investigation of the photophysics of two complexes, [Pt((t)Bu3tpy)(C triple bond C-perylene)]BF4 (1) and Pt((t)Bu2bpy)(C triple bond C-perylene)2 (2), where (t)Bu3tpy is 4,4',4'-tri( tert-butyl)-2,2':6',2'-terpyridine, (t)Bu2bpy is 4,4'-di( tert-butyl)-2,2'-bipyridine, and C triple bond C-perylene is 3-ethynylperylene, reveals that they both exhibit perylene-centered ligand localized excited triplet states ((3)IL) upon excitation with visible light. These complexes do not display any significant photoluminescence at room temperature but readily sensitize (1)O2 in aerated CH2Cl2 solutions, as evidenced by its characteristic emission near 1270 nm. The transient absorption difference spectra were compared to bi- and tridentate phosphine peryleneacetylides intended to model the (3)IL peryleneacetylide excited states in addition to the related phenylacetylide-bearing polyimine analogues, with the latter model being the respective triplet charge-transfer ((3)CT) excited states. The transient difference spectra of the two title compounds display excited-state absorptions largely attributable to perylene localized (3)IL states yet exhibit somewhat attenuated excited-state lifetimes relative to those of the phosphine model chromophores. The abbreviated lifetimes in 1 and 2 may suggest the involvement of the energetically proximate (3)CT triplet state exerting an influence on excited-state decay, and the effect appears to be stronger in 1 relative to 2, consistent with the energies of their respective (3)CT states.     

The crystal structures of α,ω-diaminoalkanecadmium(II) tetracyanonickelate(II)-aromatic molecule inclusion compounds. IV. (1,6-Diaminohexane)cadmium(II) tetracyanonickelate(II)-m-toluidine(1/1),-p-toluidine(1/1), and-2,4-xylidine(1/1) clathrates,and the coordination complex bis(p-toluidine) (1,6-diaminohexane)cadmium(II) tetracyanonickelate(II)

The crystal structures of the four title compounds have been analyzed by single crystal X-ray diffraction methods at room temperature. Three with a general formula Cd[NH₂(CH₂)₆NH₂]Ni(CN)₄·G (G=m-toluidine,Im;p-toluidine,Ip; and 2,4-xylidine,Ix) are the inclusion compounds of the respective aromatic molecules in the three-dimensional metal complex host (1,6-diaminohexane)cadmium(II) tetracyanonickelate(II). The remaining one is a coordination complex ofp-toluidine, bis(p-toluidine) (1,6-diaminohexane)cadmium(II) tetracyanonick-elate(II),II, Im, Ix, andII crystallize under similar experimental conditions;Ip is obtained using thep-toluidinemesitylene mixture at higher dilution than that used forII. Im crystallizes in the tri linic space group \(P\bar 1\) , witha=9.725(2),b=7.598(1),c=7.177(1) Å, α=90.44(1), β=98.80(1), γ=95.70(1)^o, andZ=1 (the final conventionalR=0.037 for 3526 reflections);Ip: monoclinic,P2/m,a=9.540(2),b=7.611(1),c=7.120(1) Å, β=100.95(1)^o, andZ=1 (R=0.027 for 1700 reflections);Ix: monoclinic,P2/m,a=9.628(2),b=7.613(1),c=7.122(1) Å, β=100.01(1)^o, andZ=1 (R=0.049 for 2704 reflections);II: monoclinic,P2₁/n,a=12.107(3),b=10.117(2),c=12.471(3) Å, β=113.67(2)^o, andZ=2 (R=0.037 for 2616 reflections). The structures ofIm, Ip andIx are similar to that of theo-toluidine inclusion compound of the same metal complex host. InII atrans pair of thep-toluidine molecules to the cadmium atom in the two-dimensional network formed by thecatena-μ-linkages of ?Cd?NH₂(CH₂)₆NH₂?Cd? and ?NC?Ni?CN?Cd?NC?Ni?CN?intersecting at each Cd atom; two cyanide groups of the tetracyanonickelate(II) moiety have free N-ends.     

Multiple emissions from charge transfer excited states of ruthenium(II)—polypyridine complexes

The complex cis-[(bpy)₂Ru

₂]⁴⁺ (bpy is 2,2′-bipyridine) has been prepared by methylation of (bpy)₂Ru

₂]²⁺. Electrochemical studies show that introduction of the bound pyridinium group creates a chemically attached electron acceptor site (E_1/2 = ?0.76 V in 0.1 M [N(n-C₄H₉)₄]PF₆-acetonitrile versus the SSCE). Evidence for a low-lying dπ — π^*

charge transfer (CT) state has been obtained by the appearance of a low energy emission at λ_max 680 nm in ecetonitrile (τ₀ = 104 ns) and for an upper dπ — π^* (bpy) state by a higher energy emission at 580 nm in a methanol glass at 77 K (τ₀ = 7.59 μs). Both emissions appear in a water—ethylene glycol solution containing 5% by weight polyvinyl alcohol at room temperature.     

Properties of excited states and one-electron reduced forms of Ruthenium(II)-diimine photosensitizers

In this paper we compare the spectral, acid-base, electrochemical, and kinetic properties of the excited states and one-electron reduced forms of homo- and heteroleptic Ru(II)-diimine photosensitizers that contain 2,2’-bipyridine, 2,2’-bipyrazine, and 2,2’-bipyrimidine ligands. These species, represented as *(RuIIIL.-)2+ and (RuIIL.-)+, respectively, are orbital analogues in that the intra- or intermolecularly transferred electron resides on the same uniquely reduced ligand. The variations in the properties of these species, resulting from changes in the coordinating ligands, permit the fine-tuning of photochemical systems.     

Electronic excited states of protonated aromatic molecules: Protonated Fluorene

The photo-fragmentation spectrum of protonated fluorene has been recorded in the visible spectral region, evidencing an absorption that appears largely red shifted in comparison to that of the neutral molecule fluorene. The spectrum shows two different vibrational progressions, separated by 0.19 eV. As in the case of protonated linear polycyclic aromatic hydrocarbons (PAHs), comparison of the measured spectra with ab initio calculations allows to associate the observed absorption shift with the charge transfer character of the excited state. The spectra can be properly simulated by geometry optimization of the ground and excited states, followed by Franck Condon analysis. The two vibrational bands progressions observed are assigned, with relatively good confidence, to the existence of two different conformers.     

Theoretical investigation of excited states of C(3)

In this work, we present ab initio calculations for the potential energy surfaces of C(3) in different electronic configurations, including the singlet ground state [X (1)Sigma(g) (+),((1)A(1))], the triplet ground state [a (3)Pi(u),((3)B(1), (3)A(1))], and some higher excited states. The geometries studied include triangular shapes with two identical bond lengths, but different bond angles between them. For the singlet and triplet ground states in the linear geometry, the total energies resulting from the mixed density functional--Hartree-Fock and quadratic configuration interaction methods reproduce the experimental values, i.e., the triplet occurs 2.1 eV above the singlet. In the geometry of an equilateral triangle, we find a low-lying triplet state with an energy of only 0.8 eV above the energy of the singlet in the linear configuration, so that the triangular geometry yields the lowest excited state of C(3). For the higher excited states up to about 8 eV above the ground state, we apply time-dependent density functional theory. Even though the systematic error produced by this approach is of the order of 0.4 eV, the results give different prospective to insight into the potential energy landscape for higher excitation energies.     

Triplet excited states of nitronaphthalenes. II. b-Nitronaphthalene

Nanosecond flash photolysis of b-nitronaphthalene (b-NO₂C₁₀H₇) in nonpolar and polar solvents shows a transient species with maximum absorption and lifetime dependent on solvent polarity. In deaerated n-hexane the absorption maximum and lifetime (1/k) are 425 nm and 530 nsec, while in deaerated ethanol the corresponding values are 470 nm and 1.7 ·sec. This transient absorption is attributed to the triplet excited state of b-NO₂C₁₀H₇, and the observed red shift as well as its longer lifetime in polar solvents are indicative of the intramolecular charge transfer character of this state. The change of dipole moment accompanying the transition T₁ → T_n, as well as rate constants for electron and proton transfer reactions involving the T₁ state of b-NO₂C₁₀H₇, were determined. The spectroscopic and kinetic data obtained in this work indicate that the triplet state of b-NO₂C₁₀H₇ behaves like a n-π* state in nonpolar media, while in polar solvents the n-π* character of the state is reduced with a simultaneous increase in the charge transfer character.     

Electronic structure and hyperpolarizability of some conjugated molecules in excited states

For the examples of aromatic and antiaromatic five-membered heterocycles, the static electronic polarizabilities and hyperpolarizabilities are determined in the ground and first singlet- and triplet-excited electronic states. The theoretical calculations are carried out in the SOS formalism and the correlation effects are taken into account using all mono- and biexcited configuration in the PPP approximation. It is shown that the singlet excitation of the molecules for the antiaromatic case is connected to an significant decrease of both polarizabilities and hyperpolarizabilities. Their values are discussed in terms of the index of average bond-order alternation for the ground and excited states and the localization of the electronic transitions in the molecules. © 1993 John Wiley & Sons, Inc.