ORIGINAL PAPER Photoinduced Electron Transfer Reactions of Ruthenium(II)-Complexes Containing Amino Acid with Quinones Rajkumar Eswaran & Swarnalatha Kalayar & Muthu Mareeswaran Paulpandian & Rajagopal Seenivasan Received: 3 October 2013 /Accepted: 5 February 2014 /Published online: 4 March 2014 # Springer Science+Business Media New York 2014 Abstract With the aim of mimicking, at basic level the photoinduced electron transfer process in the reaction center of photosystem II, ruthenium(II)-polypyridyl complexes, carrying amino acids were synthesized and studied their photoinduced electron transfer reactions with quinones by steady state and time resolved measurements. The reaction of quinones with excited state of ruthenium(II)-complexes, I–V in acetonitrile has been studied by luminescence quenching technique and the rate constant, k q , values are close to the diffusion controlled rate. The detection of the semiquinone anion radical in this system using time-resolved transient absorption spectroscopy confirms the electron transfer nature of the reaction. The semiclassical theory of electron transfer has been successfully applied to the photoluminescence quenching of Ru(II)-complexes with quinones. Keywords Ru(II) complex . Electron transfer . Quinone . Transient absorption Introduction Ruthenium(II)-polypyridyl complexes, [Ru(NN) 3 ] 2+ are having unique and advantageous photophysical properties [1, 2]. The application of [Ru(NN) 3 ] 2+ complexes are varied from solar energy conversion [3–5] as photocatalysts [6–8] sensors for biomolecules [9–11] to phototherapeutic agents [12, 13]. The excited state properties of these complexes can be varied systematically by changing the structure of the ligands [1, 14, 15]. For instance several amide moiety incorporated polypyridyl ligands have been synthesized to mimic the function of peptides and proteins and these were used for the synthesis of ruthenium(II)-polypyridyl complexes [16–19]. Metal containing amino acids and peptides are key components for the study of photoinduced energy and electron transfer processes [15–29]. These organic–inorganic hybrid molecules have been assembled via different routes: (i) metal complexes coordinate to the donor-containing nat- ural amino acids (e.g.histidine) [21] (ii) attachment of metal complexes to peptide termini [24, 25] (iii) synthesis of chelator ligands which contain amino acids followed by metal coordination [20, 26–29]. In Photosystem, quinones are ultimate electron acceptors, on excitation of primary electron donor chlorophyll, P 680 , with a light quantum, an electron is transferred to the primary electron acceptor, phenophytin and subsequently to the qui- nones Q A and Q B [30–33]. Quinones are appear to be predestined as electron acceptors in nature for a variety of reasons and some key factors are (i) quinones possess favor- able redox potentials, (ii) they can be converted stepwise into stable reduction products such as hydroquinones via Electronic supplementary material The online version of this article (doi:10.1007/s10895-014-1365-4) contains supplementary material, which is available to authorized users. R. Eswaran : S. Kalayar : M. M. Paulpandian : R. Seenivasan (*) School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India e-mail: rajagopalseenivasan@yahoo.com R. Eswaran (*) Department of Chemistry, Vel Tech University, Avadi, Chennai, India e-mail: rajjkumar_e@yahoo.com S. Kalayar Department of Chemistry, Manonmaniam Sundaranar University, Thirunelveli, India M. M. Paulpandian Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon, South Korea J Fluoresc (2014) 24:875–884 DOI 10.1007/s10895-014-1365-4