Photo-induced energy transfer and its switching in dyad and triad chromophore systems composed of coumarin, Ru(ii) and Os(ii) terpyridine-type complexesElectronic supplementary information (ESI) available: 1H NMR and ES mass spectra of the new dyad and triad coumarin-containing complexes, (C151)2-Os, (C151)2-Ru-ph-Os and (C151)2-Ru-azo-Os and the emission spectra of (C151)2-tpy and (C151)2-Os. See http://www.rsc.org/suppdata/dt/b2/b212274j/

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Abstract
A new dyad and two triad chromophore compounds containing coumarin, Ru(II) and Os(II) terpyridine-type complexes were synthesized. The dyad is composed of coumarin and Os(II) units, while the triads are composed of coumarin, Ru(II) and Os(II) units. One of the triads has a phenylene spacer to connect the Ru(II) unit and the Os(II) unit, while the other has an azo moiety. Energy transfer in these multichromophoric systems has been probed by electronic absorption and luminescence spectroscopy. The switching behavior of photo-induced energy transfer by redox stimuli in the latter triad has been examined. Photophysical and electrochemical analysis indicates that the contribution of the energy transfer from the coumarin chromophores and the Ru(II) center to the Os(II)-centered emission in the ‘switch-on state’ is estimated to be about 70%. This contribution is larger than that in the previously reported Ru(II)/Os(II) dyad system, which was evaluated to be 40%. Thus, it is concluded that an improved switching of directional energy transfer has been achieved from the coumarin moiety to the Os(II) center in this new triad chromophore system.

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