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Passeri, Rosita,Aloisi, Gian Gaetano,Elisei, Fausto,Latterini, Loredana,Caronna, Tullio,Fontana, Francesca,Sora, Isabella Natali Korean Society of Photoscience 2009 Photochemical & photobiological sciences Vol.8 No.11
In this work, three compounds having the same organic moiety (N-methyl-5-azahelicenium salts) but different counterions ($I^-$, $NO_3{^-}$ and $COOCF_3{^-}$) have been investigated in buffered aqueous solutions and in the presence of DNA to give information on the counterion effects on the binding. In particular, the absorption spectra, fluorescence quantum yields and fluorescence lifetimes in aqueous solution for free organic molecules have been determined by steady-state and time-resolved spectrofluorimetric measurements. The obtained values are compared with those of the chromophores in the presence of increasing concentrations of DNA. The results allow determination of the association constants ($K_a$) and the number of base couples per chromophore molecule (n) by means of the McGhee Von Hippel model. The binding parameters are strongly affected by the nature of counterions since the highest $K_a$ value was determined for the compound having $COOCF_3{^-}$; on the other hand the $NO_3{^-}$ derivative is able to interact with the highest number of binding sites. The morphology and structural properties of the DNA-chromophore complexes were investigated by circular dichroism (CD) and atomic force microscopy (AFM). The data revealed that $I^-$ and $COOCF_3{^-}$ derivatives preferentially form intercalation complexes, while the $NO_3{^-}$ salt is able to form intercalation and grove binding complexes at the same time.
Pizzoli, Giuliano,Lobello, Maria Grazia,Carlotti, Benedetta,Elisei, Fausto,Nazeeruddin, Mohammad K.,Vitillaro, Giuseppe,De Angelis, Filippo The Royal Society of Chemistry 2012 Dalton Transactions Vol.41 No.38
<P>We report a combined spectro-photometric and computational investigation of the acid–base equilibria of the N3 solar cell sensitizer [Ru(dcbpyH<SUB>2</SUB>)<SUB>2</SUB>(NCS)<SUB>2</SUB>] (dcbpyH<SUB>2</SUB> = 4,4′-dicarboxyl-2,2′ bipyridine) in aqueous/ethanol solutions. The absorption spectra of N3 recorded at various pH values were analyzed by Single Value Decomposition techniques, followed by Global Fitting procedures, allowing us to identify four separate acid–base equilibria and their corresponding ground state p<I>K</I><SUB>a</SUB> values. DFT/TDDFT calculations were performed for the N3 dye in solution, investigating the possible relevant species obtained by sequential deprotonation of the four dye carboxylic groups. TDDFT excited state calculations provided UV-vis absorption spectra which nicely agree with the experimental spectral shapes at various pH values. The calculated p<I>K</I><SUB>a</SUB> values are also in good agreement with experimental data, within <1 p<I>K</I><SUB>a</SUB> unit. Based on the calculated energy differences a tentative assignment of the N3 deprotonation pathway is reported.</P> <P>Graphic Abstract</P><P>The acid–base properties of the N3 solar cell sensitizer are investigated, identifying four separate equilibria and determining the corresponding p<I>K</I><SUB>a</SUB>s. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2dt31340e'> </P>