Effects of substituents on synthetic analogs of chlorophylls. Part 1: Synthesis, vibrational properties and excited-state decay characteristics

Understanding the effects of substituents on the spectra of chlorins is essential for a wide variety of applications. Recent developments in synthetic methodology have made possible systematic studies of the properties of the chlorin macrocycle as a function of diverse types and patterns of substituents. In this paper, the spectral, vibrational and excited-state decay characteristics are examined for a set of synthetic chlorins. The chlorins bear substituents at the 5,10,15 (meso) positions or the 3,13 (beta) positions (plus 10-mesityl in a series of compounds) and include 24 zinc chlorins, 18 free base (Fb) analogs and one Fb or zinc oxophorbine. The oxophorbine contains the keto-bearing isocyclic ring present in the natural photosynthetic pigments (e.g. chlorophyll a). The substituents cause no significant perturbation to the structure of the chlorin macrocycle, as evidenced by the vibrational properties investigated using resonance Raman spectroscopy. In contrast, the fluorescence properties are significantly altered due to the electronic effects of substituents. For example, the fluorescence wavelength maximum, quantum yield and lifetime for a zinc chlorin bearing 3,13-diacetyl and 10-mesityl groups (662 nm, 0.28, 6.0 ns) differ substantially from those of the parent unsubstituted chlorin (602 nm, 0.062, 1.7 ns). Each of these properties of the lowest singlet excited state can be progressively stepped between these two extremes by incorporating different substituents. These perturbations are associated with significant changes in the rate constants of the decay pathways of the lowest excited singlet state. In this regard, the zinc chlorins with the red-most fluorescence also have the greatest radiative decay rate constant and are expected to have the fastest nonradiative internal conversion to the ground state. Nonetheless, these complexes have the longest singlet excited-state lifetime. The Fb chlorins bearing the same substituents exhibit similar fluorescence properties. Such combinations of factors render the chlorins suitable for a range of applications that require tunable coverage of the solar spectrum, long-lived excited states and red-region fluorescence.     

Effects of substituents on synthetic analogs of chlorophylls. Part 2: Redox properties, optical spectra and electronic structure

The optical absorption spectra and redox properties are presented for 24 synthetic zinc chlorins and 18 free base analogs bearing a variety of 3,13 (beta) and 5,10,15 (meso) substituents. Results are also given for a zinc and free base oxophorbine, which contain the keto-bearing isocyclic ring present in the natural photosynthetic pigments such as chlorophyll a. Density functional theory calculations were carried out to probe the effects of the types and positions of substituents on the characteristics (energies, electron distributions) of the frontier molecular orbitals. A general finding is that the 3,13 positions are more sensitive to the effects of auxochromes than the 5,10,15 positions. The auxochromes investigated (acetyl>ethynyl>vinyl>aryl) cause a significant redshift and intensification of the Qy band upon placement at the 3,13 positions, whereas groups at the 5,10,15 positions result in much smaller redshifts that are accompanied by a decrease in relative Qy intensity. In addition, the substituent-induced shifts in first oxidation and reduction potentials faithfully track the energies of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), respectively. The calculations show that the LUMO is shifted more by substituents than the HOMO, which derives from the differences in the electron densities of the two orbitals at the substituent sites. The trends in the substituent-induced effects on the wavelengths and relative intensities of the major features (By, Bx, Qx, Qy) in the near-UV to near-IR absorption bands are well accounted for using Gouterman's four-orbital model, which incorporates the effects of the substituents on the HOMO-1 and LUMO+1 in addition to the HOMO and LUMO. Collectively, the results and analysis presented herein and in the companion paper provide insights into the effects of substituents on the optical absorption, redox and other photophysical properties of the chlorins. These insights form a framework that underpins the rational design of chlorins for applications encompassing photomedicine and solar-energy conversion.     

Applicability of different fluorescent dyes for isoform quantification on linear IPG gels

For biotechnological research, development, and production various analytical methods are required to determine the quality of the target product. In this context, the determination of isoforms is state-of-the-art; however, the majority of applied techniques are more qualitative than quantitative. To address this fact, we evaluated different post- and pre-electrophoretic staining dyes for their applicability on linear IPG gels using recombinant human erythropoietin as a model protein. Each evaluated dyes was able to detect all isoforms reproducibly, but CyDyes were found to be the most promising. Using CyDyes, up to three samples can be focused on the same lane under identical electrophoretic conditions, thus, a fast, reproducible, sensitive and quantitative isoform determination can be performed. To illustrate the practical relevance, quantitative CyDye technique was used for the characterization of our model protein, recombinant human Epo-Fc. This method makes it possible to determine the isoform pattern of nonpurified supernatants as well as purified proteins. We conclude that quantitative pre-electrophoretic staining IEF using CyDyes is a fast, simple, accurate method to determine isoforms, which can be used in research, development, and manufacturing for product quality analysis, e.g., clone screening, process optimization, and purification monitoring.     

Functional [6]pericyclynes: synthesis through [14+4] and [8+10] cyclization strategies

Critical analysis of possible strategies for the synthesis of novel carbo-benzene derivatives suggests several [(18-n)+n] routes for the preparation of hexaoxy[6]pericyclyne precursors. Beyond the previously attempted [9+9] symmetrical scheme (n=9), the a priori most selective strategies are those for which n=1, 4, 7, 10, 13, and 16. They involve a cyclizing double-propargylation of a C(18-n) omega-bis-terminal-skipped oligoyne containing (19-n)/3 triple bonds with a C(n) omega-dicarbonyl-skipped oligoyne containing (n-1)/3 triple bonds. To complement the previously used [11+7] strategy, the [14+4] and [8+10] strategies were thus explored. They proved to be efficient, affording seven novel hexaoxy[6]pericyclynes corresponding to six different substitution patterns. These compounds were obtained in 7-15 steps as mixtures of stereoisomers. Thus, by using dibenzoylacetylene as the C(4) electrophile, a [14+4] strategy allowed the synthesis of two hexaphenyl representatives with two or four free carbinol vertices. This approach also afforded tetraphenyl representatives in which the two remaining carbinoxy vertices were substituted with either two alkynyl or one 4-anisyl and one 4-pyridyl groups. By using the hexacarbonyldicobalt complex of butynedial as the C(4) electrophile, a [14+4] strategy also allowed the isolation of a tetraphenylhexaoxy[6]pericyclyne with two adjacent unsubstituted carbinol vertices. A regioisomer with two opposite unsubstituted carbinol vertices was obtained through an [8+10] strategy and its oxidation afforded the corresponding pericyclynedione. Several attempts at synthesizing diphenylhexaoxy[6]pericyclynes with four unsubstituted carbinoxy vertices are described. Only an [8+10] strategy allowed the generation of a fragile diphenylhexaoxy[6]pericyclyne with four adjacent unsubstituted carbinoxy vertices, which could be partly characterized. These results show that the synthesis of the nonsubstituted hexahydroxy[6]pericyclyne, the ring carbo-mer of [6]cyclitol, is a difficult challenge.     

Phosphorus-bearing axially chiral biaryls by catalytic asymmetric cross-cyclotrimerization and a first application in asymmetric hydrosilylation

A novel and efficient, two-step route to axially chiral biaryls is demonstrated. In a direct asymmetric cross-cyclotrimerization in the presence of a chiral cobalt(I) catalyst, axially chiral biaryls bearing phosphoryl moieties have been prepared, and through indirect evidence the authors have been able to clarify the origin of the stereochemical induction and the nature of the central intermediate in the catalytic cycle. By subsequent reduction of the phosphoryl moiety to the corresponding phosphine, a very efficient and atom-economical approach to chiral systems has been developed. These chiral systems clearly have great potential use as axially chiral monodentate P- or bidentate P,O-ligands, as has been demonstrated by the employment of the novel NAPHEP as a new monodentate acting ligand in an asymmetric hydrosilylation reaction.     

Isolation and structural characterization of a family of endohedral fullerenes including the large, chiral cage fullerenes Tb(3)N@C(88) and Tb(3)N@C(86) as well as the I(h) and D(5)(h) isomers of Tb(3)N@C(80)

The recent finding that isomer 2 of Tb(3)N@C(84) uses one of the 51,568 possible nonisolated pentagon rule (non-IPR) structures for the C(84) cage rather than one of the 24 cage isomers that do obey the IPR suggests that further experimental work on the structure of larger endohedrals is needed to observe the utility of the IPR rule in this uncharted territory. The structures of the newly synthesized endohedral fullerenes--Tb(3)N@C(88), Tb(3)N@C(86), and the Ih and D(5)(h) isomers of Tb(3)N@C(80)--have been determined by single-crystal X-ray diffraction on samples cocrystallized with NiII(octaethylporphyrin). In contrast to the situation for isomer 2 of Tb(3)N@C(84), the structures of Tb(3)N@C(88) and Tb(3)N@C(86) do conform to the IPR. Both Tb(3)N@C(88) and Tb(3)N@C(86) have chiral structures with D(2) symmetry for Tb(3)N@C(880 and D(3) symmetry for Tb(3)N@C(86). Within this group of endohedrals, the size of the carbon cage affects the Tb-N and Tb-C distances, the orientations of the carbon cage with respect to the porphyrin plane, the locations of the metal ions and their orientations relative to the porphyrin plane, and the degree of pyramidalization of the Tb(3)N unit.     

Comparing a photoinduced pericyclic ring opening and closure: differences in the excited state pathways

The photochromicity of fulgimides rests on the existence of open (E) and closed ring (C) isomers. As predicted by the Woodward-Hoffmann rules both isomers can photochemically be interconverted. This interconversion has been studied by femtosecond fluorescence and transient absorption spectroscopy. For either direction (E --> C cyclization and C --> E cycloreversion) a biphasic fluorescence decay on the 0.1-1 ps time scale is observed. The longer time constants of the decays equal the formation times of the photoproducts. The time constants retrieved (0.06 and 0.4 ps for E --> C, 0.09 and 2.4 ps for C --> E) and the associated spectral signatures differ substantially. This indicates that no common excited-state pathway for the two directions exists, as one would infer from a simple Woodward-Hoffmann consideration. These findings support recent quantum dynamic calculations on the excited-state topology of fulgimides.     

Diastereoselective oxidative addition of dihydrogen to IrI(CO)((R)-BINAP) and [Ir(CO)2((R)-BINAP)][SbF6

Two chiral iridium(I) (R)-BINAP complexes, IrI(CO)((R)-BINAP) (1) and [Ir(CO)2((R)-BINAP)][SbF6] (2), have been synthesized and characterized, and their reactivity with dihydrogen has been studied. Complex 1 is formed on the addition of (R)-BINAP to [Bu4N][IrI2(CO)2] in toluene, and 2 is generated by the addition of AgSbF6 to a solution of 1 in dichloromethane under CO. A structure determination of complex 2 confirms a square planar coordination geometry, while that of 1 reveals a significant tetrahedral distortion from the expected planar coordination. Additionally, the structure of 1 shows a disorder between iodide and CO ligands. The reaction of 1 with H2 proceeds under kinetic control and shows a high degree of kinetic and thermodynamic selectivity; the kinetic product is formed by H2 addition across the P-Ir-CO axis of IrI(CO)((R)-BINAP) and yields two diastereomers which then convert over time to two more stable diastereomers which correspond to oxidative addition across the P-Ir-I axis. The kinetically favored diastereomers are formed in an initial ratio of 8.6:1, corresponding to a DeltaDeltaG* of 1.27 kcal/mol. The reaction of H2 with the C2-symmetric complex 2 also leads to the formation of two diastereomers, with one favored over the other kinetically by a 9.9:1 ratio on extrapolation to t = 0. When these reactions are followed using parahydrogen NMR methods, only one of the initially formed diastereomers in each case is found to exhibit substantial parahydrogen-induced polarization in the hydride resonances at room temperature.     

Ruthenium-terpyridine complexes with multiple ethynylpyrenyl or ethynyltoluyl subunits: X-ray structure, redox, and spectroscopic properties

Several heteroleptic and homoleptic ruthenium-terpyridine complexes bearing two and four ethynylpyrenyl or ethynyltoluyl residues have been prepared from complexes carrying reactive bromo functions. Cross-coupling promoted by low-valent palladium(0) on these preformed complexes has advantageously been used to prepare the target complexes. The structure of a bis-terpyridine complex carrying four ethynylpyrenyl subunits was determined by single-crystal X-ray diffraction, showing a distorted octahedral geometry around the metal center, with the ethynylpyrenyl fragment being slightly tilted (about 5 degrees) from the terpyridine plane. The molecular packing is characterized by intermolecular pi...pi interaction within dimers. The counteranions and the solvent molecules are entrapped in well-defined channels spanning along the a-axis. The complexes are redox active with a Ru oxidation overlapping the pyrene oxidation and two well-defined ligand-centered reduction processes. Pyrene reduction is irreversible and strongly cathodic. The new multichromophoric complexes are luminescent both in solution and in rigid matrix at 77 K, with room-temperature lifetimes and quantum yields significantly larger than those of [Ru(terpy)2]2+. At room temperature, the toluyl-substituted complexes are triplet metal-to-ligand charge-transfer (3MLCT) emitters, whereas for the pyrene-grafted complexes pyrene-centered emission is observed. For the latter complexes, the energy gap, DeltaTT, between higher 3MLCT levels and lower ligand-centered (3pipi*, 3LC) levels is in the 640-730 cm(-1) range, which results in the interstate dynamics at the basis of the observed luminescent behavior. At 77 K, for the pyrene-grafted complexes, the emission reveals features that are tentatively ascribed to intraligand interactions involving the pyrene and terpyridine units.