Photoinduced processes in 4′-(9-anthryl)-2,2′:6′,2″-terpyridine, its protonated forms and Zn(II), Ru(II) and Os(II) complexes

Abstract

We investigated the absorption spectra and the luminescence properties of 2,2′:6′,2″-terpyridine(tpy), 4′-(9-anthryl)-2,2′:6′,2″-terpyridine (An-tpy), their protonated forms, and their Ru(II), Os(II), and Zn(II) metal complexes ([M(tpy) 2] 2+ and [M(An-tpy) 2] 2+). For both tpy and An-tpy, the addition of CF 3SO 3H to acetonitrile (or dichloromethane) solutions causes changes in the absorption spectra that indicate the formation of monoprotonated (tpyH + and An-tpyH +) and diprotonated (tpyH 2 2+ and An-tpyH 2 2+) forms. The weak fluorescence of tpy ( λ max = 338 nm) becomes much stronger and moves to lower energy ( λ max = 410 nm) upon the first protonation, and then moves back to higher energy ( λ max = 360 nm) upon the second protonation. An-tpy shows a strong anthracene-type fluorescence band with λ max = 422 nm which disappears upon the first protonation: the diprotonated form shows a very weak emission with λ max = 505 nm. Both the absorption and emission spectra of the protonated forms of An-tpy indicate that the lowest excited state can be assigned to a charge-transfer transition from the anthracene moiety to the protonated tpy moiety. In contrast, [Zn(tpy) 2] 2+ shows a very intense, ligand-centered fluorescence band with λ max = 353 nm, whereas [Zn(An-tpy) 2] 2+ shows that a much weaker emission with λ max = 543 nm, assigned to an An → [Zn(tpy)) 2] 2+ charge-transfer transition. The very weak metal-to-ligand charge transfer (MLCT) phosphorescence of [Ru(tpy) 2] 2+ is no longer present in [Ru(An-tpy) 2] 2+ because the 1MLCT level of the Ru-based unit is quenched by the lower lying T 1 excited state of the anthracene unit, as shown by the appearance of the characteristic T 1 transient absorption band with λ max = 420 nm in flash spectroscopy experiments. In aerated solutions the T 1 excited state of the anthracene unit of [Ru(An-tpy) 2] 2+ is quenched by dioxygen with formation of 1 Δ(O 2), whose emission can be observed in the near-IR region ( λ max = 1270 nm. Continued irradiation of [Ru(An-tpy) 2] 2+ in aerated solution causes the destruction of the anthracene moiety because of its reaction with 1 Δ(O 2). [Os(An-tpy) 2] 2+ shows the same 1MLCT emission as [Os(tpy) 2] 2+ ( λ max = 728 nm) since the 1MLCT level lies below the T 1 excited state of the anthracene moiety. The absorption and excitation spectra of [Os(An-tpy) 2] 2+ are almost coincident, showing that the S 1 excited state of the anthracene unit is converted efficiently to the 1MLCT level of the [Os(tpy) 2] 2+ unit.