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.     

Effects of substituents on synthetic analogs of chlorophylls. Part 3: The distinctive impact of auxochromes at the 7- versus 3-positions

Assessing the effects of substituents on the spectra of chlorophylls is essential for gaining a deep understanding of photosynthetic processes. Chlorophyll a and b differ solely in the nature of the 7-substituent (methyl versus formyl), whereas chlorophyll a and d differ solely in the 3-substituent (vinyl versus formyl), yet have distinct long-wavelength absorption maxima: 665 (a) 646 (b) and 692 nm (d). Herein, the spectra, singlet excited-state decay characteristics, and results from DFT calculations are examined for synthetic chlorins and 13(1)-oxophorbines that contain ethynyl, acetyl, formyl and other groups at the 3-, 7- and/or 13-positions. Substituent effects on the absorption spectra are well accounted for using Gouterman's four-orbital model. Key findings are that (1) the dramatic difference in auxochromic effects of a given substituent at the 7- versus 3- or 13-positions primarily derives from relative effects on the LUMO+1 and LUMO; (2) formyl at the 7- or 8-position effectively "porphyrinizes" the chlorin and (3) the substituent effect increases in the order of vinyl < ethynyl < acetyl < formyl. Thus, the spectral properties are governed by an intricate interplay of electronic effects of substituents at particular sites on the four frontier MOs of the chlorin macrocycle.     

Synthesis and properties of synthetic analogs of natural humic acids

A new procedure was suggested for preparing analogs of natural humic acids using as precursor a nitrogen-containing amphoteric polyelectrolyte prepared by condensation of pyrocatechol with hexamethylenetetramine. The physicochemical and spectral properties of the product were examined.     

Synthesis and characterization of ethidium analogs: Emphasis on amino and azido substituents

A series of eight ethidium derivatives has been synthesized in which the substituents at R₃ and R₈ have been varied with hydrogen, azido and amino functions. Three of these compounds are new and their synthesis and characterization by uv-visible, ir and ¹H nmr spectroscopy are presented. The synthesis and characterization of the other compounds are also given for comparison, because the compounds served as precursors, or the synthetic route undertaken for these compounds differed from that reported previously.     

Influence of chiral ionic liquids on the excited-state properties of naproxen analogs

The synthesis and decolorization of chiral room-temperature ionic liquids based upon 1-methyl imidazole and chloromethyl menthyl ether is reported. The excellent optical quality of these solvents permits the investigation of the effects of the two enantiomers on the excited-state photophysics of (S)-N-methyl-2-pyrrolidinemethyl 2(S)-(6-methoxy-2-naphthyl)propionate [(S,S)-NPX-PYR]. Whereas in conventional bulk polar solvents such as acetonitrile, (S,S)-NPX-PYR is known to execute excited-state intramolecular electron transfer and to form exciplexes, in these chiral solvents these nonradiative processes are absent. The chiral solvents do, however, induce a small but reproducible (approximately 10%) stereodifferentiation in the fluorescence lifetime of (S,S)-NPX-PYR as well as in the parent compound, (S)-naproxen. To our knowledge, this is the first example of chiral ionic liquids inducing such an effect on photophysical properties.     

Effects of inductive substituents on electronic spectra aza analogs of fluoranthene

The conditions under which the Hückel molecular orbital (HMO) treatment of inductive substituent effects can be expected to be reliable are discussed from the viewpoint of the Pariser-Parr-Pople (PPP) model. The HMO theory fails to predict correctly the direction of the shift of the first intense band in the ultraviolet spectrum of fluoranthene (1–1 transition) on 3-aza substitution. Calculations on the five azafluoranthenes using the simple version of the PPP method suggest that this is due to strong mixing of hydrocarbon states by the effect of the substituent and predict significant changes in energies, intensities, and polarizations of fluoranthene electronic transitions on aza substitution. Transition moment directions derived from dichroic absorption of 3-azafluoranthene in stretched polyethylene sheets are in agreement with PPP predictions. Arguments are given against a previous assignment of the first transition in 1-azafluoranthene as n- ^*.

---

Zusammenfassung Aus der Sicht des Pariser-Parr-Pople-Modells (PPP) werden die Bedingungen diskutiert, unter denen die HMO-Behandlung induktiver Substituenteneffekte als zuverlässig erscheint. Die HMO-Theorie ist nicht in der Lage, die Verschiebung der ersten intensiven Bande im UV-Spektrum des Fluoranthens (1–1-Übergang) bei 3-Aza-Substitution richtig vorauszusagen. Berechnungen an den 5 Azafluoranthenen nach der einfachen PPP-Methode lassen vermuten, daß diese Tatsache durch eine starke Vermischung von Kohlenwasserstoffzuständen — hervorgerufen durch die Wirkung des Substituenten — bedingt ist. Die PPP-Methode sagt ferner beträchtliche Änderungen in den Energien, Intensitäten und Polarisationen der Elektronenübergänge des Fluoranthens voraus, wenn man im Fluoranthen eine Aza-Substitution vornimmt. Die Richtungen des Übergangsmoments, die aus der dichroitischen Absorption des 3-Azafluoranthens in langgestreckten Polyäthylenschichten abgeleitet werden können, stimmen mit den Voraussagen von PPP-Berechnungen überein. Es werden Argumente vorgebracht, die gegen eine Kennzeichnung des ersten Übergangs in 1-Azafluoranthen als n- ^*-Übergang sprechen.

---

Résumé On discute à l'aide du modèle de Pariser-Parr-Pople (PPP) les conditions de validité du traitement par la méthode de Hückel (HMO) des effets de substituants inductifs. La méthode de Hückel ne peut prédire correctement la direction du déplacement de la première bande intense du spectre UV du fluoranthène (dite transition «1–1») dans les substitutions aza en 3. Les calculs sur 5 azafluoranthènes à l'aide de la méthode PPP ordinaire suggèrent que cet échec est du à un fort mélange des états de l'hydrocarbure sous l'effet du substituant, et prédit des changements importants dans les énergies, les intensités et les polarisations des transitions du fluoranthène par les substitutions aza. Les directions des moments de transition déduits de l'absorption dichroïque du 3 aza fluoranthène dans des feuilles de polyéthylène étiré sont en accord avec le calcul PPP. On apporte des objections à la caracterisation antérieure de la 1 ère bande du 2 azafluoranthène comme une bande n- ^*.

     

Qy-excitation resonance Raman spectra of chlorophyll a and bacteriochlorophyll c/d aggregates. Effects of peripheral substituents on the low-frequency vibrational characteristics

Low-frequency (80-700 cm-1) Qy-excitation resonance Raman (RR) spectra are reported for thin-solid-film aggregates of several chlorophyll (Chl) a and bacteriochlorophyll (BChl) c/d pigments. The pigments include Chl a, pyrochlorophyll a (PChl a), methylpyrochloropyllide a (MPChl a), methylbacteriochloropyllide d (MBChl d), [E,M] BChl cS, [E,E] BChl cF, and [P,E] BChl cF. The BChl c/d's are the principal constituents of the chlorosomal light-harvesting apparatus of green photosynthetic bacteria. Together, the various Chl a's and BChl c/d's represent a series in which the peripheral substituent groups on the chlorin macrocycle are varied in systematic fashion. All of the Chl a and BChl c/d aggregates exhibit rich low-frequency vibrational patterns. In the case of the BChl c/d's, certain modes in the very low-frequency region (100-200 cm-1) experience exceptionally strong Raman intensity enhancements. The frequencies of these modes are qualitatively similar to those of oscillations observed in femtosecond optical experiments on chlorosomes. The RR data indicate that the low-frequency vibrations are best characterized as intramolecular out-of-plane deformations of the chlorin macrocycle rather than intermolecular modes. The coupling of the out-of-plane modes in turn implies that the Qy electronic transition(s) of the aggregate have out-of-plane character. The RR spectra of the BChl c/d's also reveal that the nature of the alkyl substituents at the 8 and 12 positions of the macrocycle plays an important role in determining the detailed features of the low-frequency vibrational patterns. The frequencies of the modes are particularly sensitive to larger substituent groups whose conformations may be more easily perturbed in the tightly packed aggregates. These factors also make aggregates of pigments containing larger substituents more susceptible to structural, electronic, and vibrational inhomgeneities. Collectively, the RR studies of the various pigments delineate the factors which influence the low-frequency vibrational characteristics of chlorosomal aggregates.     

Heterocyclic analogs of pleiadiene 60. Synthesis and properties of 1-methyl-2-chloromethylperimidine

The previously unknown 1-methyl-2-chloromethylperimidine was synthesized. The chlorine atom in it has high lability and is readily exchanged under the influence of S, O, N, and C nucleophiles. It is shown that 1-methyl-2-cyanomethylperimidine exists in the methylidene form.See [1] for Communication 59.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 827–830, Junek 1988.     

Synthesis and vibrational spectroscopic characterisation of synthetic hydrozincite and smithsonite

Hydrozincite and smithsonite were synthesised by controlling the partial pressure of CO₂. Previous crystallographic studies concluded that the structure of hydrozincite was a simple one. However both Raman and infrared spectroscopy show that this conclusion is questionable. Multiple bands are observed in both the Raman and infrared spectra in the (CO₃)²⁻ antisymmetric stretching and bending regions of hydrozincite showing that the symmetry of the carbonate anion is reduced and in all probability the carbonate anions are not equivalent in the hydrozincite structure. Multiple OH stretching vibrations centred in both the Raman and infrared spectra show that the OH units in the hydrozincite structure are non-equivalent. The Raman spectrum of synthetic smithsonite is a simple spectrum characteristic of carbonate with Raman bands observed at 1408, 1092 and 730 cm⁻¹.     

Synthesis,stereochemistry and fluorescence properties of polystyrenes having carbazole substituents

Poly(styrene)s bearing carbazole substituents at various positions were synthesised by homopolymerisation of 3-(9-carbazolylmethyl)styrene and 4-(9-carbazolylmethyl)styrene and by chemical modification of preformed poly(styrene)s prepared in the presence of different initiators. Main chain stereochemistry of the polymer samples was investigated by ¹³C-NMR and related to the initiation process. The fluorescence emission spectra were independent of the stereochemistry of polymer backbone and exhibited only monomer emission. In no case was there any evidence of excimer fluorescence. In contrast, energy migration appeared to be very efficient, as evaluated by measurements of the degree of fluorescence polarisation relative to that of a structurally related model compound     

二茂铁基Tamoxifen类似物的合成、结构与性质

合成了一系列二茂铁基Tamoxifen类似物并作了初步生化试验(RBA试验),研究了代表化合物二茂铁基羟基Tamoxifen的晶体结构和顺-反异构化。     

Studies on nitroenamines. Part III. Synthesis and spectral properties of 4-Acyl-1-arylimidazoles

The catalytic (C/Pd) hydrogenation of 3-arylamino-2-nitro-2-enones ( 1 ) in the presence of carboxylic ortho esters ( 2 ) affords 4-acyl-1-arylimidazoles ( 3 ) in yields (20–70%) which depend on the degree of substitution of the imidazole ring. The spectral properties of compounds 3 , and particularly the ¹³C-nmr spectra, reflect both the electronic and steric effects of the substituents on the π-electron delocalization and planarity of the bicyclic system.     

Synthesis and properties of analogs of pyridoxal

A method for the synthesis of analogs of pyridoxal-2-norpyridoxal, 6-methyl-2, 4-pyridoxal, and 6-methylpyridoxal—has been worked out. The reaction of 5-ethoxyoxazoles with maleic diesters gave diesters of substituted 3-hydroxycinchomeric acids, which were converted by reduction with lithium aluminum hydride into analogs of pyridoxine. The latter were oxidized with magnesium dioxide to the corresponding analogs of pyridoxal. The oximes of the aldehydes and their Schiff's bases with p-phenetidine have been obtained. Analogs of pyridoxamine have been obtained by the hydrogenation of the oximes. The UV absorption spectra of the compounds and the ratios of the ionic forms in aqueous solutions have been studied.     

Synthesis and properties of analogs of pyridoxal

A method for the synthesis of analogs of pyridoxal-2-norpyridoxal, 6-methyl-2, 4-pyridoxal, and 6-methylpyridoxal—has been worked out. The reaction of 5-ethoxyoxazoles with maleic diesters gave diesters of substituted 3-hydroxycinchomeric acids, which were converted by reduction with lithium aluminum hydride into analogs of pyridoxine. The latter were oxidized with magnesium dioxide to the corresponding analogs of pyridoxal. The oximes of the aldehydes and their Schiff's bases with p-phenetidine have been obtained. Analogs of pyridoxamine have been obtained by the hydrogenation of the oximes. The UV absorption spectra of the compounds and the ratios of the ionic forms in aqueous solutions have been studied.     

Synthesis,vibrational, and conformational properties of novel polyazadioxime chain molecules

Novel large polyazadioxime molecules 4,5,8,9-tetraaza-3,6,7,10-tetramethyl -3,5,7,9-dodecatetraene-2, 11-dione-2, 11-dioxime (H₂doxN₄) and 4,5,8,9,12, 13-hexaaza-3,6,7,10,11,14-hexamethyl-3, 5,7,9,11,13-hexadecahexane-2,15-dione-2, 15-dioxime (H₂doxN₆) were synthesized. The molecular geometries of these molecules as well as smaller dioxime molecules, H₂dox and H₂doxN₂ were optimized by using modified intermediate neglect of differential overlap (MNDO) calculations. The optimized conformations for all the molecules under study are close to the all-E, all-s trans conformation of C_2h symmetry group. However, the energy barriers of internal rotation around the N-N single bonds were found to be low. Therefore some distortions of the polyazabackbone through internal rotation the N-N bonds have been evidenced. By infrared and Raman spectroscopies in the solid state as well as in solution. From the MNDO calculations and vibrational spectroscopy, the polyaza chain molecules under study appear as a poorly conjugated system and can be represented as a sequence of single and double bond alternation.     

Synthesis, structure, and properties of benzoquinone dimer and trimers bearing t-Bu substituents

Highly soluble and stable quinone dimer and trimers were successfully yielded by introduction of t-Bu substituents. In X-ray structure analysis, the dimer quinone moiety was distorted into the boat shape, which was clarified by MO calculations. X-ray and UV/vis studies indicated that the covalently linked quinone moieties bear a large torsional angle. Nevertheless, the reduction potentials rose significantly with the order of monomer < dimer < trimer, indicating that the negative charge was efficiently delocalized within the radical anions.     

Synthesis of new analogs of valinomycin and their properties. I

Conclusions 1. The synthesis of five valinomycin analogs differing by the configurations of the amino-acid and hydroxy-acid residues and also by the volumes of the radicals of the hydroxy-acid residues has been effected.2. The stability constants of the complexes of these analogs with potassium ions in ethanol solution have been determined.3. The antimicrobial activities of the compounds obtained have been studied and a correlation has been found between the degree of activity and stability of the complexes with K⁺.M. M. Shemyakin Institute of the Chemistry of Natural Compounds, Academy of Sciences of the USSR. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 233–240, March–April, 1974.     

Synthesis, structure, and redox properties of crowded triarylphosphines carrying 2,6-diarylphenyl substituents

Crowded triarylphosphines carrying 2,6-diarylphenyl and 2,4,6-trialkylphenyl groups were synthesized by the reaction of arylcopper(I) reagent with the chlorophosphines. The triarylphosphines had large bond angles and lengths around the phosphorus and were reversibly oxidized at significantly low potentials. X-ray crystallography of bis(2,4,6-triisopropylphenyl)[4-methyl-2,6-di(1-naphthyl)phenyl]phosphine revealed that two 1-naphthyl groups took anti conformation in the crystal.