Excited states of porphyrin isomers and porphycene derivatives: a SAC-CI study

Excited states of free-base porphyrin isomers, porphycene (Pc), corrphycene (Cor), and hemiporphycene (hPc), were studied by the Symmetry-Adapted Cluster (SAC)/SAC-Configuration Interaction (CI) method. The absorption peaks of the porphyrin isomers were assigned on the basis of the SAC-CI spectra. The X, Y, X', and Y' bands of the porphyrin isomers, which have weak intensities, are identified. The differences in the Q-band absorptions among the isomers were clearly explained by the four-orbital model. In Cor and hPc, the wave function of the B-band corresponds to the mixture of the four-orbital excitations and the optically forbidden excitation of free-base porphin (P), due to the molecular symmetry lowering in the isomers. The B-band character is described by the five-orbital model in Pc and the six-orbital model in Cor and hPc. Two tetrazaporphycenes and two ring-extended (dibenzo) porphycenes were designed, and the Q-band transition moment was successfully controlled. These examples show that the control of the four-orbital energy levels is the guiding principle for pigment design in porphyrin compounds.     

On the electronic states of macrocycles of the extended porphyrin family and their coordination compounds

The electronic absorption spectra of Ni, Zn and Mg hemiporphyrazine derivatives are presented and discussed together with theoretical results obtained by INDO/S computations. The absorption spectra of all the metal derivatives show marked red shifts of the lowest energy absorption bands with respect to those of the metal free hemiporphyrazine. The possible explanation that in metal derivatives low lying excited states with a fully conjugated π electron system are present is supported by theoretical computations.     

Solubility of porphyrin macrocycles in mixed solvents

A method based on the statements of molecular association theory and a simple lattice model (ASL = Associated Solution + Lattice) is used to calculate the solubility of a series of porphyrin macrocycles (blood porphyrins) in binary solvents (tetrachloromethane-ethyl acetate, tetrachloromethane-methanol). Separate contributions to solubility are identified and the relative role of different factors determining the solubility dependence on the mixed solvent composition is analyzed. The calculated solubility values are in good agreement with the experimental data obtained for the studied systems by the isothermal saturation method with a spectrophotometric control of concentrations.     

Excited states of indoloquinoxalines

Electronic transitions of low-symmetry molecules indolo[2,3-b]quinoxaline and its two methyl derivatives—were studied by linear dichroism, fluorescence polarization and magnetic circular dichroism methods. Transition moment directions were determined for low-lying excited states and found to be in good agreement with the results of INDO/S calculations.     

Dianionic and trianionic macrocycles in cobalt N-confused porphyrin complexes

We report the syntheses of cobalt N-confused porphyrins; this work completes the series of the late first-row transition metals that have been incorporated into the core of N-confused porphyrin, and in these compounds the macrocycles can act as either a -2 or -3 anion.     

Excited electronic states of NiCO

The results of ab initio SCF and CI calculations on the electronic states of NiCO are reported. The ¹Σ⁺ ground state is a mixture of two primary configurations associated with the Ni 3d¹⁰ and 3d⁹4s states, and is bound by 18 kcal mol⁻¹ with respect to Ni and CO at r_nic =1.77 Å. The excited states (within 22000 cm⁻¹ of the ground state) can be divided into a lower manifold, principally involving the Ni(3d⁹4s) electronic configuration, and a higher manifold, formally associated with the charge transfer configuration Ni⁺ (3d⁹)CO⁻ (π1).     

Single molecule visualization of coordination-assembled porphyrin macrocycles reinforced with covalent linkings

Coordination-assembled porphyrin macrocycles reinforced with covalent bondings were deposited on a metal surface by a pulse injection method, and their scanning tunneling microscopy (STM) images were recorded under ultrahigh vacuum conditions at liquid nitrogen temperature. The decamer ring consisting of 30 porphyrins gave clear circular STM images with hollow structure, whereas that without covalent linking did not give clear circular images, showing that covalent linking of the coordination pairs by ring-closing metathesis reaction was effective to reinforce the supramolecular structure on a metal surface. This strategy will be applicable to a variety of supramolecular assemblies.     

Self-assembly, binding, and dynamic properties of heterodimeric porphyrin macrocycles

A series of heterodimeric tetralactam macrocycles have been self-assembled using two kinetically labile zinc porphyrin-pyridine interactions. The stability constants have been determined by UV-vis titrations in CHCl3. The stability constants depend on the degree of preorganization of the linker units connecting the interacting groups. The ability of the self-assembled macrocycles to bind a terephthalamide guest was also investigated. One of the macrocycles was used for the construction of a [2]rotaxane. The dynamic properties of this system provide insight into the exchange mechanisms that operate in complex noncovalent assemblies.     

Excited states of the benzyl radical

The CI method is used in the -electron approximation with orbitals for closed and open shells to calculate the properties of excited doublet states with allowance for all singly excited configurations and some doubly excited ones, and also for the first quartet and sextet states, which are calculated in the one-configuration approximation via the open-shell theory. The energies and transition moments agree satisfactorily with the available experimental evidence. A classification and assignment is given for the excited terms. Truncation of the complete set of singly excited configurations greatly distorts the calculated spectrum. Inclusion of doubly excited configurations in the CI also produces a substantial change in the spectrum; in some cases it alters the order of adjacent terms. Conversion in CI from basis closed-shell orbitals to open-shell ones produces a considerable lowering of all terms in the spectrum. As in the case of triplet terms for molecules, weakening of electron interaction brings the lowest excited term of the radical closer to the ground-state term. The electron-density and spin-density distributions are calculated for the excited states.     

Excited states of phenyl carbonyl compounds

In an attempt to clarify the origin of the dual phosphorescence in phenyl alkyl ketones, we have made some calculation (within the C.I.P.S.I. method in an excitonic scheme) to elucidate the conformation of both ground states and excited states of propiophenone. Our calculations have shown the presence of two stable isomers in the ground state, first n ^* state, and first ^* singlet and triplet states. So our work suggests that the origin of the dual phosphorescence of propiophenone could be related to the conformational change of the molecule in the n ^* state, because the most stable conformations in the n ^* state and in the ground state are different.     

Excited electronic states of 1,3,5-cycloheptatriene

The electron-impact energy-loss spectrum of 1,3,5-cycloheptatriene has been measured at impact energies of 30,50, and 75 eV, and scattering angles varying from 5° to 80 °. Singlet → triplet transitions were observed at 3.05 eV and 3.95 eV. No evidence for the weak transition at 2.1 eV previously reported on the basis of threshold electron-impact studies was found. Single → singlet transitions were observed at 4.85 eV and 6.40 eV in good agreement with the optical spectrum and semi-empirical calculations.     

Excited states of antiaromatic systems1

SINDO1 studies were performed to optimize the geometry of excited states of some antiaromatic molecules. It is discussed how such states can exhibit aromatic character upon suitable electronic excitation. The nodal pattern of the molecular orbitals involved in the electronic excitation are used to invoke bond equilization in excited states. We have investigated singlet and triplet excited states of five-membered rings C₄H₅B, C₅H₅⁺ and C₅H₄O containing four π electrons and bicyclic systems bicyclo-(1,1,0)-butadiene, bicyclo-(2,2,0)-hexatriene and benzocylobutadiene. It is seen that in bicyclo-(2,2,0)-hexatriene, both the bicylic structure and the 1,4-diradical structure determine the equilibrium geometry.     

1,3,5,7-Tetrasubstituted adamantanes as frameworks in the design of assemblies of porphyrin macrocycles

A new strategy for the synthesis of adamantane-containing polyphenols of a porphyrin series has been suggested on the basis of Suzuki–Miyaura reaction between 1,3,5,7-tetrakis(4'-iodophenyl)adamantane and a monoboryl derivative of substituted porphyrin. The synthesis of this porphyrin assembly on the adamantane core followed by chemical transformations of this compound has provided a possibility to synthesize the corresponding polyphenol derivative. The latter has been used in the development of positive photoresists for nanolithography with exposure to radiation at a wavelength 13.5 nm capable of producing topological structures with a resolution of 16 nm.     

Excited states and photodissociation of hydroxymethyl hydroperoxide

The structure of hydroxymethyl hydroperoxide (HOCH(2)OOH) (HMHP) has been examined using coupled cluster and multireference configuration interaction methods to study the excited states and probable photodissociation products. The results are compared to experiments. The vertical excitation energies for several excited states of HOCH(2)OOH are presented as well as the excited state energies along the O-O, O-H, C-O, and C-H dissociation pathways. The results help in the interpretation of experimental UV absorption spectra and elucidate the photodissociation mechanism of HMHP under tropospheric conditions.     

Excited electronic states of small water clusters

The lowest electronic states that are initially formed upon excitation of small water clusters having a central water molecule with one stretched OH bond are studied with electronic structure methods. It is found that in water dimer, trimer, and pentamer the lowest excited singlet and triplet states are each nondissociative for stretching of an OH bond that is hydrogen bonded in an icelike configuration to a neighboring water molecule. This is in marked contrast to the behavior of an isolated gas phase water monomer, where it is well known that the lowest excited state is strongly dissociative upon OH stretching. The conclusions of this study may serve as a basis to interpret recent experimental evidence that suggests a significant lifetime for excited water in irradiated thin ice films, and may also have important implications for the behavior of excitation of liquid water.     

Iron(II) complexes with amide-containing macrocycles as non-heme porphyrin analogues

Iron(II) complexes of macrocyclic pentadendate ligands 3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),14,16-triene-2,13-dione (H2pydioneN5) and 16-chloro-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),14,16-triene-2,13-dione (H2pyCldioneN5) were synthesized and fully characterized. Complexes with one or two deprotonated amide groups of H2pydione were both isolated. In the former case the metal ion has a distorted octahedral coordination sphere; in the latter case the complex adopts a pentagonal-bipyramidal geometry. NMR experiments show that the protonation state of the ligand is preserved in a dimethyl sulfoxide (DMSO) solution. The complexes maintain a high-spin state even at low temperatures. Detailed kinetic studies of oxygenation of the iron(II) complexes showed that the deprotonation state of the complex has a profound effect on the reactivity with dioxygen. Oxygenation of the dideprotonated complex of iron(II), Fe(pydioneN5), in aprotic solvents proceeds via a path that is analogous to that of iron(II) porphyrins: via iron(III) superoxo and diiron(III) peroxo species, as evidenced by the spectral changes during the reaction, which is second-order in the concentration of the iron(II) complex, and with an inverse dependence of the reaction rate on the concentration of dioxygen. The final products of oxygenation are crystallographically characterized iron(III) mu-oxo dimers. We have also found that the presence of 1-methylimidazole stabilizes the diiron peroxo intermediate. The reaction of Fe(pydioneN5) with dioxygen in methanol is distinctly different under the same conditions. The reaction is first-order in both iron(II) complex and dioxygen, and no intermediate is spectroscopically observed. Similar behavior was observed for the monodeprotonated complex Fe(HpydioneN5)(Cl). The presence of an accessible proton either from the solvent (reactions in methanol) or from the complex itself (in Fe(HpydioneN5)(Cl)) proves sufficient to alter the oxygenation pathway in these macrocyclic systems, which is reminiscent of the properties of iron(II) porphyrin complexes. The new amidopyridine macrocycles can be considered as new members of the "expanded porphyrin analogue" family. The expansion of the cavity provides control over the spin state and availability of protons. These macrocyclic systems also allow for easy synthetic modifications, paving the way to new, versatile metal complexes.     

Excited states and electronic spectrum of ferrocene

The excited states of ferrocene have been calculated using the ab-initio singly excited configuration interaction and self-consistent field methods. The results obtained are used to assign the bands of the electronic absorption spectrum of the molecule.     

Solvent triggering between conformational states in amphiphilic shape-persistent macrocycles

The amphiphilic shape-persistent macrocycle 1 containing four phenol-OH groups as polar side groups and four hexyloxy groups as nonpolar side groups in an adaptable arrangement was recrystallized from solvents of different polarity. X-ray crystallography reveals that the conformation of the macrocycle is solvent dependent such that in the pyridine solvate only two of the nonpolar side groups point outward while in the THF solvate all four of them point outward. Moreover, in the latter case the three-dimensional packing leads to the formation of a supramolecular channel structure with a large pore size.     

Excited biradical states in oxirane ring opening

The mechanism of oxirane ring opening was studied by ab initio [RHF, ROHF, GVB/DH, RHF/SBK(p, d)] calculations. The strained structure of oxiranes and their complexes with aliphatic alcohols and amines is characterized by low-lying biradical states whose thermal population leads to the ring opening. The examined oxirane ring opening reacitons have low activation energy (<10 kcal mol-¹) and are catalyzed by labile hydrogen atoms in hydroxy and amino groups of the reaction complex.