Photoinduced Exchange Research Articles

Structural, optical, electrical, and thermal properties of PANI/[Co(NH3)2(C2H8N2)2]Cl3 nanocomposite

The study reports successful synthesis of [Co(NH3)2(C2H8N2)2]Cl3 photoadduct and its subsequent use as a filler in polyaniline (PANI) matrix to form PANI/photoadduct nanocomposite by chemical oxidative polymerisation method. The photo-induced exchange of ligands between the hexamine metal complex and ethylenediamine solution on photo irradiation resulted in the formation of photoadduct. FTIR and UV–Vis analysis show a significant interaction developed between the photoadduct and the PANI chains and hence successful nanocomposite formation. Optical band gap determination from Tauc plot shows higher value of band gap in case of PANI/photoadduct nanocomposite than that of PANI. The structural characteristics of the PANI/photoadduct nanocomposite as observed from XRD show crystalline nature of the nanocomposite. SEM analysis shows photoadduct particles lying embedded in the polymer matrix. The porous sponge like surface morphology of the nanocomposite shows potential of material to act as a catalyst. Electrical measurement of the nanocomposite revealed its less conducting nature than PANI which is attributed to charge carrier scattering by embedded photoadduct particles. This is in consistent with the higher value of band gap energy obtained in case of PANI/photoadduct nanocomposite. The thermal analysis shows improvement in the thermal stability of PANI nanocomposite with incorporated filler photoadduct, which holds the polymer chains tightly bound to each other and hence confers some extra stability to the polymer against thermal degradation.

Photoinduced ligand exchange and DNA binding of cis-[Ru(phpy)(phen)(CH3CN)2]+ with long wavelength visible light

The complex cis-[Ru(phpy)(phen)(CH3CN)2]+ (phpy=2-phenylpyridine, phen=1,10–phenanthroline) was investigated as a potential photodynamic therapy (PDT) agent. This complex presents desirable photochemical characteristics including a low energy absorption tail extending into the PDT window (600–850nm) and photoinduced exchange of the CH3CN ligands, generating a species analogous to the chemotherapy drug cisplatin. Furthermore, photochemical reactivity can be controlled through selective irradiation into the Ru-phen singlet metal-to-ligand charge transfer (1MLCT) band (λirr=500nm) of [Ru(phpy)(phen)(CH3CN)2]+ in the presence of excess t-butylammonium chloride (TBACl) resulting in efficient photoinduced production of [Ru(phpy)(phen)(CH3CN)Cl] (Φ=0.25). This lower energy irradiation resulted in greater quantum yield of photosubstitution when compared to direct irradiation into the Ru-phpy 1MLCT peak (λirr=450nm; Φ=0.08) in CH2Cl2. It was found that the lower quantum yield observed for irradiation into the Ru→phpy−1MLCT band results from significant orbital mixing of the phpy− ligand with the t2g-type filled set in the metal, giving this state significant ligand-centered character. Lastly, this complex produced a decrease in the mobility of linearized ds-DNA when irradiated with λirr≥420nm, indicative of covalent binding by the transition metal complex similar to that observed for cisplatin. No change in mobility was found for the same samples kept in the dark indicating, unlike cisplatin, DNA binding of cis-[Ru(phpy)(phen)(CH3CN)2]+ only occurs with the activation of light. These observations support the use of cis-[Ru(phpy)(phen)(CH3CN)2]+ as a potential PDT agent by the photoinduced generation of a cisplatin analog.

Enhancement of TC of the (Ga,Mn) As diluted magnetic semiconductor by photoinduced exchange coupling of the Mn2+ spins

Effect of photo-excitation on the ferromagnetic transition temperature TC of the diluted magnetic semiconductor (DMS) (Ga,Mn) As is theoretically studied. The model Hamiltonian used to describe the system consists of interaction of localized Mn2+ spins with radiation field as appropriate to ferromagnetic and/or spin wave resonance. Using second order perturbation theory, an expression for photoinduced exchange coupling energy between the Mn spins is obtained. It is argued that the coupling energy could be significantly large to provide very large TCat/near spin wave resonance. The intensity and energy of the light can further assist enhancement of TC useful for high temperature spintronic devices. An analogy with carrier mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) type interaction with magnetic ions is also established. Key words: Spintronics, (Ga,Mn)As, exchange coupling, photo-excitation, resonance, ferromagnetism, DMS.

Photoinduced exchange anisotropy in a ferromagnet/semiconductor hybrid

It is shown theoretically that the exchange coupling of a ferromagnet film with optically oriented electrons in deep centers near the ferromagnet/semiconductor interface induces an effective magnetic field acting upon the ferromagnet. The effective field with its sign depending on the helicity of circular polarized light leads to the displacement of the magnetic hysteresis loop of the film, i.e. to the phenomenon of photoinduced exchange (unidirectional) anisotropy.

Photoexchange products of cytosine and 5-methylcytosine with Nα-acetyl- l-lysine and l-lysine

The photoinduced exchange reactions of cytosine ( Ia) and 5-methylcytosine ( IIa) with Nα-acetyl- l-lysine, a derivative of the common amino acid l-lysine, were studied. These reactions of Ia and IIa at pH 7.5 produce, respectively, 2- N-acetylamino-6-(1-cytosinyl)hexanoic acid ( Ib) and 2- N-acetylamino-6-(1-(5-methylcytosinyl))hexanoic acid ( IIb) as major final products. In addition, small amounts of the corresponding deamination products were formed in the 5-methylcytosine- Nα-acetyl- l-lysine and cytosine- Nα-acetyl- l-lysine systems, namely 2- N-acetylamino-6-(1-thyminyl)-hexanoic acid and 2- N-acetylamino-6-(1-uracilyl)hexanoic acid. The compounds Ib and IIb were deacetylated by acid hydrolysis to yield the corresponding lysine products: 2-amino-6-(1-cytosinyl)hexanoic acid ( Ic) and 2-amino-6-(1-(5-methylcytosinyl))hexanoic acid ( IIc). The compound Ic was identified as a product in the photoreaction of cytosine with l-lysine at near neutral pH, while IIc is found as a product in the corresponding reaction of 5-methylcytosine. The occurrence of the above photoexchange reactions at, pH values near those found in physiological systems could have biological implications; in particular, our observations suggest that cytosine and 5-methylcytosine residues, contained in DNA, might react with the ϵ-amino groups of lysine residues in proteins upon UV irradiation of nucleosomes and other DNA-protein complexes under physiological conditions.

Photochemical reactions of cytosine and 5-methylcytosine with methylamine and n-butylamine.

AbstractThe photoinduced exchange reactions of cytosine (la) and 5‐methylcytosine (IIa) with two primary amines have been studied. The reactions of Ia and IIa with methylamine lead, respectively, to 1‐methylcytosine (Ib) and 1,5‐dimethylcytosine (IIb) as final products; the reactions of the same two starting materials with n‐butylamine gives the corresponding n‐butyl compounds. The reactions of IIa with the two amines also gave small amounts of 1‐methylthymine and 1‐n‐butylthymine. An opened ring intermediate, N‐(N'‐methylcarbamoyl)‐3‐arnino‐2‐methylacrylamidine (IVa), was isolated from the IIa‐methylamine system irradiated at pH 7 and shown to be a precursor of both IIb and 1‐methylthymine. The pH profiles for rate of production of IIb, 1‐methylthymine and IVa in the reaction of IIa with methylamine were determined and found to be similar in shape. All three profiles show a maximum in reactivity at about pH 8.7 with some reactivity being detectable at pH values as low as 5.

Photochemical exchange reactions of thymine, uracil and their nucleosides with selected amino acids.

AbstractThe photoinduced exchange reactions of thymine with lysine at basic pH, using 254 nm light, have been studied. Three products have been isolated, namely, 6‐amino‐2‐(1‐thyminyl)hexanoic acid (Ia), 2‐amino‐6‐(1‐thyminyl)hexanoic acid (IIa) and 1‐amino‐5‐(1‐thyminyl)pentane (IIIa). Compound IIIa was shown to be a secondary product, produced by photochemical decarboxylation of Ia. Photochemical reaction of thymine with glycine and alanine at basic pH led, respectively, to formation of 2‐(1‐thyminyl)acetic acid (Ic) and 2‐(1‐thyminyl)propionic acid (Id). Compounds Ic and Id underwent photolysis to produce the decarboxylated secondary products 1‐methylthymine and 1‐ethylthymine, respectively. Thymidine reacts photochemically with glycine and alanine to produce the same products.Irradiation of DNA in the presence of lysine at basic pH led to the formation of the same products formed in the thymine‐lysine system, namely Ia, IIa and IIIa.Uracil was found to undergo analogous photochemical exchange reactions with lysine to form 6‐amino‐2‐(1‐uracilyl) hexanoic acid (Ib), and 2‐amino‐6‐(1‐uracilyl)hexanoic acid (IIb). Compound Ib was found to undergo photodecarboxylation to form 1‐amino‐5‐(1‐uracilyl)pentane (IIIb), analogous to the secondary photoreaction of Ia. Photoreaction of uracil with 1,5‐diaminopentane (cadaverine) likewise led to formation of IIIb.