Irradiation triad

Time-resolved spectroscopy establishes that the 25-ps laser irradiation of the relatively persistent charge-transfer complex of p-bromoanisole with iodine monochloride generates the contact ion pair (see Fig. 15b) in which the metastable ICP undergoes mesolytic fragmentation to form the reactive triad, i.e.,... 

Recently it has been reported (3 ) that in a triad molecule where a porphyrin is juxtaposed between a carotenoid and a quinone, a charge transfer donor-acceptor pair with a lifetime similar to that found experimentally in biological systems was produced on light irradiation. It was suggested that an electrical potential similar to the type developed in this donor-acceptor pair may be important in driving the chemical reactions in natural photosynthesis. 

Poly(PO) formed with 17 as initiator under irradiation showed virtually the same NMR pattern as that for the polymer formed in the dark at 70 C, where the resonance due to the methyl group was very simple, indicating that the polymer consists of regular head-to-tail linkages. The diad and triad tactic-ities of the polymer, as determined by NMR [82], indicated the atactic struc-... 

To design photo-induced electron transfer devices, the group of Nishino [69] reported the preparation of a bis-a-helical nanostructure, 74. The peptidic framework was designed to orient rigidly in space the redox triad, composed of a Ru2 + complex, an anthraquinone, and two propylviologens, when incorporated in a lipid bilayer (Fig. 28). Although the compound exhibited a strong a-helical content in methanol, its conformation in a vesicle bilayer was different and undetermi ned. Nevertheless, irradiation of the Ru(II) complex of 74 resulted in a slow electron transfer. 

When the interfacial supramolecular triad is irradiated in the presence of I- under solar cells conditions, appreciable photocurrents are obtained. The profile of the photoaction spectrum shows clearly that photoinjection into TiC>2 takes place upon excitation of the ruthenium center. However, the IPCE values obtained are lower than those observed for the model compound, thus suggesting that injection is less efficient in the heterotriad. Of major interest is the mechanism for charge injection. Two different pathways can be envisaged. First, the charge injection may be a two-step process and takes place via the rhodium center as shown in the following equations ... 

Further functionalizations are obtained via the electron transfer— radical cation fragmentation pathway a typical example is side-chain nitration by irradiation of methyaromatics with tetranitromethane. Aromatics form charge-transfer complexes with C(N02)4 irradiation leads to electron transfer and fragmentation of the C(N02)4 radical anion to yield the triad [Ar + C(NO)J N02], followed by combination between the arene radical cation and the trinitromethanide anion. Thus, cyclohexadienes are formed that generally eliminate and rearomatize at room temperature yielding ring-functionalized products [234] (Sch. 21). 

Tetranitromethane (TNM) readily forms CT complexes with aromatic systems. Irradiation of these complexes leads to rapid generation of radical ca-tion/TNM - pairs [117]. The radical anion fragments rapidly (t < 2 ps) generating a triad of reactive intermediates that include the radical cation of the donor, trinitromethanide ("C(N02)3), and N02 as illustrated in Scheme 9 for methylanisole. The fragmentation is so rapid that no diffusional separation... 

Another interesting model involves the irradiation of bicumenes in the presence of tetranitromethane. Both the radical cation and the radical anion of the initial diad fragment, and the observed products arise from the resulting triad and tetrad [213-215]. 

A simple synthesis of pyrazolo[4, 5 ][60]fullerenes from pyrazolyl hydrazones with [60]fullerene was achieved with microwave irradiation evidence of intramolecular charge-transfer interaction was shown <1999TL1587>. Novel C6o-fused isoxazolines have been synthesized from 1,3-dipolar cycloadditions of pyrazole nitrile oxides to Ceo under thermal or microwave irradiation <1999T4889>. A new triad based on pyrazolino[60]fullerene and a conjugated... 

A, A -dimethylaniline group has been synthesized by a copper-free Sonogashira cross-coupling reaction using microwave irradiation as the source of energy <2006EJO2344>. The electrochemical and photophysical properties of the triad were systematically investigated by techniques such as time-resolved fluorescence and transient absorption spectroscopy. 

The photosubstitution studies of the Co, Rh, Ir triad are concerned with the hexacoordinated d -M(IIl) complexes however, photoreactions of the d -[IrClj] and a few d -M(I) square-planar complexes also are known. The d -amine complexes of Co(Ill), e.g., [Co(NH3) ] , are poor models for the photosubstitution reactions of other low-spin d complexes, which, like such Co(III) cyano ions as [Co(CN) ] "", have lowest-energy excited states (ES) that are ligand-field (LF) triplets with the (t p fe electronic configuration (Fig. 1). For the Co(III) amines, LF quintets with the (tjP (ep configuration are the lowest states. Regardless of the theoretical implications of such electronic differences, the LF photochemistry of the Co(III) amines contrasts with that of analogous Rh(III) and Ir(III) complexes and of the Co(III)-cyano complexes. For the latter systems in solution, irradiation into LF absorption bands leads to photosubstitution with moderate (ca. 0.1) quantum yields independent of the irradiation wavelength, A. ... 

Figure 25. (A) Structure of an artificial photosynthetic reaction center, the molecular triad C-P-Q, and the proton-shuttling quinone, Qsl (B) Schematic diagram showing orientation of the triad In the liposome and the sequence of events after photoexcitation (see table at right and text for details) (C) Fluorescence excitation spectra of the pH-indicator dye pyraninetrisulphonate as a measure of the concentration of the protonated form of the indicator dye (D) Fluorescence excitation-band intensity as a function of irradiation time in the absence and in the presence of FCCP. Figures adapted from Steinberg-Yfrach, Liddeii, Hung, (AL) Moore, Gust and (TA) Moore (1997) Conversion of light energy to proton potential in liposomes by artificial photosynthetic reaction centres. Natu re 385 239-241.

Ultraviolet irradiation of pyridines can prodnce highly strained species that can lead to isomerised pyridines or can be trapped. The three picolines and the three cyano-substituted pyridines constitute photochemical triads irradiation of any isomer, in the vaponr phase at 254 nm, results in the formation of all three isomers. From pyridines and from 2-pyridones 2-azabicyclo[2.2.0]-hexadienes and -hexenones can be obtained in the case of pyridines these are nsnally nnstable and revert thermally to the aromatic heterocycle. Pyridone-derived bicycles are relatively stable, 4-alkoxy- and -acyloxy-pyridones are converted in particnlarly good yields. Irradiation of iV-methyl-2-pyridone in aqueous solution prodnces a mixture of regio- and stereoisomeric 4n pins 4n photo-dimers. ... 

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