Derivatives of the[Ru(bipy)(CN)4]2- chromophore with pendant pyridyl-based binding sites: Synthesis, pH dependent-luminescence, and time-resolved infrared spectroscopic studies

S. Encinas, A. F. Morales, F. Barigelletti, A. M. Barthram, C. M. White, S. M. Couchman, J. C. Jeffery, M. D. Ward, David Grills, M. W. George

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Abstract

Reaction of K4[Ru(CN)6] with 2,2′:4′ ,4″-terpyridine (L1) or 2,2′:3′ ,2″:6″ ,2‴-quaterpyridine (L2) in acidic aqueous methanol affords the complexes K2[Ru(L1)(CN)4] and K2[Ru(L2)(CN)4] respectively, both containing the {Ru(bipy)(CN)4}2- chromophore but with pendant pyridyl or bipyridil units, respectively. Time-resolved IR analysis of K2[Ru(CN)4(L2)] in MeCN-D2O showed that the most intense CN stretching vibration shifted to higher energy by ca. 50 cm-1 after laser excitation, consistent with formation of a Ru(III)/(L2)·- MLCT excited state for which the lifetime measurement (38 ± 5 ns, measured by TRIR) agrees reasonably well with the value measured by luminescence methods (30 ± 2 ns). Study of the pH dependence of the absorption and emission spectra of the two complexes revealed the presence of two different effects arising from protonation of the pendant pyridyl/bipyridyl site (which occurs with pKa ≈ 3.1 in each case) and protonation of the cyanide ligands (which occurs with pKa ≈ 2 in each case). For K2[Ru(L1)(CN)4], protonation of the pendant pyridyl unit results in the 1MLCT excited state being lowered in energy by ca. 1000 cm-1, whereas at lower pH values (2.5-1), protonation of the cyanide ligands raises the 1MLCT excited state energy by over 2000 cm-1. For K2[Ru(L2)(CN)4] in contrast, protonation of the pendant bipyridyl unit has no detectable effect on the 1MLCT energy, as the pendant site is electronically decoupled from the complex core by a substantial twist between the free and coordinated bipy components of L2; protonation of the cyanides at lower pH values however destabilises the 1MLCT excited state. Protonation of the pendant pyridyl sites results in complete (for [Ru(L1(CN)4]2-) or near-complete (for [Ru(L2)(CN)4]2-) quenching of the luminescence; possible reasons for this behaviour are discussed. The crystal structures of the two related complexes [Ru(tBu2bipy)2(L1)][PF 6]2 and [Cl2Pt(μ-L2)Ru(bipy)2][PF6] 2 are also described to illustrate arguments about the conformations of L1 and L2.

Original languageEnglish
Pages (from-to)3312-3319
Number of pages8
JournalJournal of the Chemical Society, Dalton Transactions
Issue number22
Publication statusPublished - 2001

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ASJC Scopus subject areas

  • Chemistry(all)

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Encinas, S., Morales, A. F., Barigelletti, F., Barthram, A. M., White, C. M., Couchman, S. M., Jeffery, J. C., Ward, M. D., Grills, D., & George, M. W. (2001). Derivatives of the[Ru(bipy)(CN)4]2- chromophore with pendant pyridyl-based binding sites: Synthesis, pH dependent-luminescence, and time-resolved infrared spectroscopic studies. Journal of the Chemical Society, Dalton Transactions, (22), 3312-3319.