Unexpected change in charge transfer behavior in a cobalt(II) porphyrin-fullerene conjugate that stabilizes radical ion pair states

Two cobalt(II) porphyrin-C(60) malonate-linked conjugates, the mono-connected Co1 and the bis-connected trans-2 isomer Co3, have been synthesized for the first time either by direct cyclopropanation with the precursor malonate Co4 or by metalation of the bisadduct H(2)3. For the investigation of the interaction between the porphyrin donor and fullerene acceptor within these dyads, electrochemical and photophysical investigations have been carried out. Compared to Zn3 and trans-2 bisadduct 7, the first reduction of the fullerene moiety within Co3 becomes easier (40 mV in dichloromethane and 20 mV in benzonitrile), indicating significant interactions between the pi-system of the fullerene and the d-orbitals of the central Co atom. Compared to the Co complexes 9, Co4, and Co1, the first oxidation of Co3 is considerably shifted to more positive potentials, if benzonitrile instead of dichloromethane is used as solvent. At the same time, the oxidation is no longer centered on the Co(II) center but on the porphyrin macrocycle, as corroborated by spectroelectrochemistry. A similar solvent dependence was observed in transient absorption spectroscopic measurements. In toluene, benzonitrile and anisole photoinduced electron transfer within Co3 leads to the formation of a charge-separated state Co(II)P.+ -C(60).- with a lifetime of 560 +/- 20 ns in benzonitrile, whereas in other solvents such as THF, nitrobenzene, ortho-diclorobenzene, and tert-butylbenzene the formation of a Co(III)P-C(60).- as transient was detected, which is, however, short-lived (860 +/- 40 ps in THF) and exhibits charge recombination dynamics that are in the Marcus inverted region. Particularly important is the fact that the electronic coupling (V) in Co(III)P-C(60).- is 18 cm(-1) substantially smaller than the V value of 313 cm(-1) in ZnP.+ -C(60).- .     

Surface-hopping model for near-resonant electronic energy transfer

A surface-hopping model is applied to near-resonant electronic energy transfer in the NFBi and O₂I systems. Multiple surface crossings occur in NFBi at ca. 8 A, corresponding well with measured transfer cross section of 200 A². A Landau-Zener model yields the temperature dependence of the thermally averaged cross section for the laser pumping reaction, O^*₂(a¹Δ) + I(²P_{$\text{3}\text{2}$}) → O₂(X³Σ^?_g) + 1^*(²P_{$\text{1}\text{2}$}).     

Prediction and measurement of effect of chelating selectivity on precipitation reactions

A theoretical treatment of precipitation equilibrium in the presence of a chelating agent has been expanded to include the common ion effect on precipitation reactions. The extent of precipitation can be predicted over the full range of pH for any metal for which the solubility product of the precipitate and the stability constants for a complex are known. The expanded equation has been used in this investigation to predict the extent of precipitation for several metals in the presence of ethylenediaminetetra-acetic acid (EDTA) with common précipitants such as hydroxide, oxalate, sulphate, sulphide and 8-hydroxyquinoline, but may be applied to any system containing other chelating agents. The limits of precipitation in terms of pM', the negative logarithm of the concentration of unprecipitated metal ion, are presented graphically as a function of pH for a system containing axed amounts of precipitating and complexing agents. Measurements for observed reactions at two pH levels, 5 and 10, are compared with the calculated predictions of the equation. Thirty-five metals were investigated.     

Poly α-ester degradation studies. V. Thermal degradation of polyglycollide

The thermal degradation of polyglycollide [poly(methylene carboxylate)] has been studied over the temperature range 250–400°C by using the combined kinetic and analytical technique previously described. The results obtained from thermogravimetry and product analysis were in many ways similar to those from previous work involving higher poly-α-esters. Thus the reaction was predominantly first-order and the major degradation product was glycollide monomer. This confirms the observations of Carothers and is best interpreted in terms of an intramolecular ester interchange process. Kinetic studies have shown that the first-order rate constant k is related to temperature T by the expression: The results from gas evolution analysis showed consistent and marked deviation from first-order behavior. This is interpreted in terms of the greater sensitivity of this technique to traces of acidic degradation products. Solution viscometry was used to demonstrate the effect of degradation conditions on molecular weight change in both thermogravimetric and gas evolution techniques.     

Pyrroles and related compounds—XXXIV : Acrylic esters in the porphyrin series

Treatment of imidazolylporphyrins (12) with sodium borohydride in methanol affords high yields of the corresponding hydroxymethylporphyrins (14) which can be oxidised with chromium trioxide in pyridine to give high yields of formylporphyrins (11). When heated with methyl hydrogen malonate in pyridine, good yields of porphyrin trans-acrylic esters (15) are produced; a milder method involving reaction of the formylporphyrin (11a) with a phosphonium ylid (20) or better, a phosphonate ester (21), gives good yields of the porphyrin trans-acrylic ester (16a).Porphyrin β-keto-esters (e.g. 8b) are reduced with sodium borohydride in methanol, to give the 6-hydroxypropionate ester (17b); prolonged treatment with borohydride results in over-reduction at the 7-propionate side-chain. The hydroxypropionate porphyrin (17b) is readily dehydrated to give a high yield of the porphyrin trans-acrylic ester (15b). Methods for the magnesiation and preparation of β-keto-ester and trans-acrylic ester porphyrin 7-propionic acids are described; hydrolysis of porphyrin β-keto-esters (e.g. 8b) can be controlled such that only the propionic function is hydrolysed. However, prolonged alkaline hydrolysis and re-esterification furnishes a mixture of 2-vinylrhodoporphyrin-XV dimethyl ester (9b) and the 2-vinyl-6-acetylporphyrin (18b). The latter porphyrin can also be prepared by treatment of the acid chloride (10b) from 2-vinylrhodoporphyrin-7-methyl ester with sodio di-t-butyl malonate, followed by treatment with trifluoroacetic acid.     

Microwave-assisted synthesis of rhodamine derivatives

The microwave synthesis of 12 rhodamine-derived imines is described. The present work involves condensation of rhodamine hydrazide with various aromatic aldehydes in ethanol under microwave irradiation. The results obtained indicate that, unlike classical heating, microwave irradiation results in higher yields, shorter reaction time, mild reaction condition and simple work-up procedure. The structures of synthesized compounds were confirmed by ¹H-NMR, ¹³C-NMR, FT-IR and high-resolution mass spectra data.