Narrow Results

The photophysical and two-photon absorption (TPA) properties of three homologous quadrupolar and one related three-branched chromophores were investigated. Design of the quadrupoles is based on the symmetrical functionalization of a biphenyl core. Modulation of the nonlinear absorptivity/transparency/photostability trade-off can be achieved by playing with the twist angle of the core and on the spacers (phenylene-vinylene versus phenylene-ethynylene). The quadrupolar chromophores combine high TPA cross-sections, high fluorescence quantum yield and solvent sensitive photoluminescence properties. The branched structure exhibits spectrally broadened TPA in the NIR region (up to 3660 GM at 740 nm measured in the femtosecond regime) but reduced sensitivity to the environment.

Bromination of palladium meso-tetraphenyl tetrabenzoporphyrin (Pd Ph₄TBP, 1) by Me₄NBr₃ or Me₄NBr/Br₂ was shown to proceed regioselectively to the benzo-rings annelated to main porphyrin macrocycle. Conditions for preferential mono- and octa-bromination have been established. The respective mono- and octa-bromide (Pd Ph₄TBP(Br), 2 and Pd Ph₄TBP(Br)₈, 3) have been isolated and characterized by UV-vis, NMR and LDI-TOF spectroscopy. Changes of electrochemical properties of tetrabenzoporphyrins induced by Br atoms were found to follow the same trends as the changes in analogous non-extended porphyrins. Room temperature phosphorescence is not substantially influenced by the substitution.

In this mini-review, new developments in chemistry of organometallic porphyrins and their analogs reported between 2007 and mid 2012 have been discussed. Synthetic strategies for preparation, as well as properties of metallocenyl-type compounds in which organometallic substituents connected to the porphyrinoid via (i) axial coordination; (ii) covalent bond to meso- or β-pyrrolic position; or (iii) β,β′-fused into the aromatic system as well as porphyrinoids with organometallic fragments σ-bonded in η¹-fashion were overviewed.

Two ruthenium(II) carbonyl complexes of porphycene, (carbonyl)(pyridine)(2,7,12,17-tetra-n-propylporphycenato)ruthenium(II) (1) and (carbonyl)(pyridine)(2,3,6,7,12,13,16,17-octaethylpor-phycenato)ruthenium(II) (2), have been structurally characterized by single-crystal X-ray diffraction analysis. Cyclic voltammetry has revealed that the porphycene complexes undergo multiple oxidations and reductions in dichloromethane and the reduction potentials are highly positive compared to porphyrin analogs. UV-light irradiation (400 nm or shorter wavelength region) of a benzene solution of 1 and 2 containing external pyridine leads to dissociation of the carbonyl ligand from the ruthenium(II) centers to give the corresponding bis-pyridine complexes. The identical reaction has been also studied for a porphyrin derivative (carbonyl)(pyridine)(2,3,7,8,12,13,17,18-octaethylporphyriato)ruthenum(II) (3). The first-order kinetic analysis has revealed that the photosubstitution of all of the compounds occurs in the order of 10^-3 s^-1 at 298 K but proceeds faster for complexes of porphycene (1 and 2) than that of porphyrin (3).

Recently, porphyrin derivatives having ring-fused structures at the periphery have attracted attention by virtue of their unique optoelectronic characteristics. This manuscript surveys the syntheses and properties of porphyrin derivatives having fused five-membered rings at the periphery. Porphyrin derivatives with fused five-membered rings have been revealed to exhibit unique properties, which are different from those having fused six-membered rings at the periphery. For instance, porphyrins with fused five-membered rings exhibit expanded aromatic and anti-aromatic circuits passing through the outer rim of the fused rings. The expanded aromatic and anti-aromatic circuits severely affect the electronic structures of the porphyrin derivatives.

Antipodal β-diformyl porphyrin (H₂TPP(CHO)₂) and its metal complexes (MTPP(CHO)₂, M = ${\text{Co}}^{\text{II}}$, ${\text{Ni}}^{\text{II}}$, ${\text{Cu}}^{\text{II}}$, and ${\text{Zn}}^{\text{II}})$ are synthesized and characterized by various spectroscopic techniques. Diformyl porphyrins possess two electron-withdrawing formyl substituents at their β-pyrrole positions making them electron-deficient macrocycles. UV-Vis absorption spectra of MTPP(CHO)₂ are red-shifted as compared to β-monoformyl/hydroxymethyl porphyrins and a further large anodic shift ($\mathrm{\Delta }$${\text{E}}_{1/2}$ = 0.15 - 0.52 V) was observed in their redox potentials. Hammett plots of MTPP(X)(Y) (X = CHO, CH₂OH; Y = H, CHO) showed that the HOMO-LUMO energy gap decreases with the increment in the Hammett parameter ($\sigma )$ of the substituents. DFT studies are performed for different positional isomeric diformyl porphyrins which reveal that the position of β-formyl groups affects the geometry of the porphyrin core while the symmetrical nature of 2,13-diformyl porphyrin and longer distance between two formyl groups bring quasiplanar conformation of the diformyl porphyrin macrocycle.

Two $\beta$-substituted Pd(II) porphyrins, (PdTPP(NO${}_2)$Ph₂ and PdTPP(Ph)${}_4)$, were synthesized and characterized by various spectroscopic techniques, and their spectral and electrochemical redox properties were investigated with respect to PdTPP. The single-crystal X-ray structure of PdTPP(NO${}_2)$-Ph₂ revealed saddle-shape conformation of the macrocyclic core of porphyrin. DFT studies revealed nonplanar saddle shape conformation of PdTPP(Ph)₄ with an average deviation of $\beta$-pyrrole carbon atoms from the porphyrin mean plane ($\mathrm{\Delta }$C${}_{\beta }$ = ± 0.751 Å) and also 24 core atoms ($\mathrm{\Delta }$24 = ± 0.368 Å) as compared to PdTPP(NO${}_2)$Ph₂ ($\mathrm{\Delta }$C${}_{\beta }$ = 0.649 Å and $\mathrm{\Delta }$24 = ± 0.317 Å) due to more $\beta$-pyrrole substitution. Both PdTPP(NO${}_2)$Ph₂ and PdTPP(Ph)₄ exhibited 13-19 nm and 49-59 nm red-shift in the Soret and Q${}_{x(0,0)}$ bands as compared to PdTPP, respectively. PdTPP(NO${}_2)$Ph₂ exhibited a very high dipole moment (7.32 D) as compared to PdTPP(Ph)₄ (0.239 D) due to the “push-pull” effect of electron donor (phenyl) and acceptor (-NO${}_2)$ groups at the $\beta$-pyrrole positions of the macrocycle. The redox potentials of PdTPP(NO${}_2)$Ph₂ anodically shifted ($\mathrm{\Delta }$E = 0.2-0.4V) as compared to PdTPP and PdTPP(Ph)₄ due to the presence of $\beta$-phenyl groups and the strong electron-withdrawing effect of the nitro group. Hence the HOMO-LUMO energy gap of PdTPP(NO${}_2)$Ph₂ decreased by 0.20 V and 0.28 V as compared to PdTPP(Ph)₄ and PdTPP, respectively.

$\pi$-Extended azacoronene has been synthesized via an S${}_N$Ar reaction using acenaphthopyrrole and octafluoronaphthalene utilizing successive Scholl reactions. Although the chiral conformers could not be isolated, the new azacoronene analog exhibited longer-wavelength absorption and reversible oxidation properties, which are characteristic of pyrrole-fused azacoronenes. When combined with the results of our DFT calculations, the experimental results indicate weaker global aromaticity than that of previously reported azacoronenes.

Novel cobalt(III) complexes of N-confused porphyrin (5,10,15,20-tetrakis(penta-fluorophenyl)-2-aza-21-carbaporphyrin) and its 3,3${}^{\prime }$-linked dimer, Co1 and Co2, were synthesized and their structures were elucidated by X-ray crystallographic analysis and NMR spectroscopy. The redox properties of the complexes were examined by electrochemical approaches, and the corresponding oxidized/reduced species were analyzed by spectroelectrochemistry and DFT calculations. Notably, due to the facile redox feature of the N-confused porphyrins, ligand-centered oxidative/reductive processes were assumed for Co1 and Co2 presumably as inferred from the spectral analyses. These results indicate the unique multi-electron reservoir capability of the metal complexes of the N-confused porphyrin dimer.

Orthogonal and twisted dimers of pyrrole-fused pentaazapolycyclic hydrocarbons (pentaazacoronene analogs) were synthesized, linked either directly or by phenylenes. Although the dimers and the reference monomer did not have any substituents at the $\alpha$-positions of the terminal pyrroles, two reversible oxidation waves were observed in the cyclic voltammograms. ESR measurements of the chemically generated dications of the dimers showed typical radical cation signals, indicating successful accumulation of the redox activity of the pentaazacoronene analog.

Two new complexes, oxidovanadium(IV) 2,3,5,10,12,13,15,20-octaphenyl-7,8,17,18-tetrabromoporphyrin V(IV)O(OPP)Br₄ (2), and oxidovanadium(IV) 2,3,5,10,12,13,15,20-octaphenyl-7,8,17,18-tetrachloroporphyrin V(IV)O(OPP)Cl₄ (3) starting from oxidovanadium(IV) 2,3,5,10,12,13,15,20-octaphenylporphyrin V(IV)O(OPP) (1) have been prepared in good yields and characterized by various spectroscopic techniques. The single-crystal and DFT-optimized structures of these porphyrins showed a highly nonplanar saddle-shape conformation of the porphyrin macrocycle. In the cyclic voltammograms, the first ring oxidation potential and the first ring reduction potential of V(IV)O(OPP)Br₄ (2) and V(IV)O(OPP)Cl₄ (3) showed anodic shift as compared to V(IV)O(OPP) (1), indicating the electron-deficient nature of both the porphyrins. The oxidation state of vanadium in the synthesized complexes as +4 was confirmed by EPR spectroscopy, and these porphyrins exhibited axially compressed $d_{\text{x}\text{y}}^1$ configuration. These porphyrins were utilized as catalysts for the oxidative bromination of thymol and other phenol derivatives with excellent conversion ($\sim$ 99.9%), good selectivity, and high TOF value (86869 h${}^{-1})$.

Many interesting properties of polyoxometalates (POMs) are related to their ability to reversibly accept and release electrons. The correct computational description of metal electrons in POMs is central to fully understand the implications and mechanisms related to reduced structures. We performed DFT calculations on fully oxidized [P₂W₁₈O₆₂]^6- and α₁/α₂-[P₂MW₁₇O₆₂]^n-(M = V, Mo) anions, and their one-electron reduced partners. Atomic spin populations of the reduced forms reveal that pure GGA functionals, which overdelocalize the electron density by nature, cannot correctly describe the extra electron especially when competition between the localized and delocalized situations is very close in energy, namely for α₂-[P₂MoW₁₇O₆₂]^7-. On the other hand, hybrid functionals like B3LYP or M05 can localize the electron at the correct site. Within these "well-behaved" density functionals, only the B3LYP gives the correct ordering and relative reduction energies (REs) with respect to electrochemical measurements, although the absolute values are always less exothermic than expected. At present, this is the best density functional for computing redox properties of POMs.