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

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 CF3SO3H 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 (lambda(max) = 338 nm) becomes much stronger and moves to lower energy (lambda(max) = 410 nm) upon the first protonation, and then moves back to higher energy (lambda(max) = 360 nm) upon the second protonation. An-tpy shows a strong anthracene-type fluorescence band with lambda(max) = 422 nm which disappears upon the first protonation; the diprotonated form shows a very weak emission with lambda(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-centred fluorescence band with lambda(max) = 353 nm, whereas [Zn(An-tpy)(2)](2+) shows a much weaker emission with lambda(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 (MLCT)-M-3 level of the Ru-based unit is quenched by he lower lying T-1 excited state of the anthracene unit, as shown by the appearance of the characteristic T-1 transient absorption band with lambda(max) = 420 nm in hash 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)Delta(O-2), whose emission can be observed in the near-IR region (lambda(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)Delta(O-2). [Os(An-tpy)(2)](2+) shows the same (MLCT)-M-3 emission as [Os(tpy)(2)](2+) (lambda(max) = 728 nm) since the (MLCT)-M-3 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 (MLCT)-M-3 level of the {Os(tpy)(2)}(2+) unit. (C) 1998 Elsevier Science S.A. All rights reserved.