The lowest excited electronic states of triplet character and related vibronic properties are discussed in detail on the basis of highly frequency-resolved and time-resolved emission and excitation spectra of per-protonated, per-deuterated, and partially deuterated [Pt(bpy)2] 2+, [Rh(bpy)3] 3+, [Ru(bpy)3] 2+, and [Os(bpy)3] ~+. Emphasis is placed on the use of the enormous amount of information displayed in well-resolved vibrational satellite structures. For comparison, IR data and Raman spectra are also used. In addition, data are given for [Ru(bpz)Pt(3-thpy) 2, Pt(2-thpy)(CO)(Cl), Pt(2-thpy) 2, Pt(qol)2, Pt(qtl)2. Trends and effects are also addressed, which are related to the amount of metal d-orbital mixing. In particular, we discuss the role of traps and sites in the context of high-resolution, site-selective, and line-narrowed spectra of chromophores doped into matrices; the interplay between states of ligand-centered 3rtrr* and ~MLCT character; localization versus delocalization behavior; radiative decay properties; zero-field splittings; spin-lattice relaxations via direct and Orbach mechanisms; Arrhenius behavior after time delay; Franck-Condon activities and Huang-Rhys factors; Franck-Condon versus Herzberg Teller activities and tunability of these activities under high magnetic fields; isotope marking and deuteration effects; aggregate formation of [Ru(bpy)3]2+; and radiationless energy transfer in neat [Ru(bpy)3](PF6) 2. These effects are in part treated in detail, but the aim is to use easy-to-follow descriptions. In particular, it is emphasized throughout this review that chemical tunability opens fascinating possibilities for controlled variation of physical properties.

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The title compound consists of two-dimensional layers of [Au(CN)2]- complexes alternating with layers of Eu3+ ions. Due to this structure type, the lowest electronic transitions of the dicyanoaurates(I) exhibit an extreme red shift of Δν̄max/Δp = −130 ± 10 cm-1/kbar under high-pressure application at least up to ≈60 kbar (T = 20 K), while the shifts of the different Eu3+ transitions lie between −0.70 and −0.94 cm-1/kbar. At ambient pressure, the usually very intense emission of the dicyanoaurates(I) is completely quenched due to radiationless energy transfer to the Eu3+ acceptors. As a consequence, one observes a strong emission from Eu3+, which is assigned to stem mainly from 5D0 but also weakly from 5D1. At T = 20 K, 5D3 seems to be the dominant acceptor term. It is a highlight of this investigation that, with increasing pressure, the emission from the dicyanoaurate(I) donor states can continuously be tuned in by tuning off the resonance condition (spectral overlap) for radiationless energy transfer to 5D3. With further increase of pressure, successively, 5D2 and 5D1 become acceptor terms, however, being less efficient. Interestingly, 5D0 does not act as an acceptor term even with maximum spectral overlap. Between 30 and 60 kbar, when only the 7F0 → 5D1 acceptor absorption overlaps with the donor emission, one finds a linear dependence of the (integrated) 5D0 emission intensity on the spectral overlap integral, as is expected for resonance energy transfer. As the dominant transfer mechanism, the Dexter exchange mechanism is proposed. Besides the high-pressure studies of the Eu3+ line structure at T = 20 K, the Eu3+ emission is also investigated at T = 1.2 K (p = 0 kbar) by time-resolved emission spectroscopy, which strongly facilitates the assignments of the emitting terms.

The emitting triplet state of cyclometalated Pt(thpy)(CO)(Cl) monomers ((thpy)(-) = 2-(2'-thienylpyridinate), frequently also abbreviated as (2-thpy)(-)) is investigated at T = 1.2 K (typically) by use of the complementary methods of high-resolution optical spectroscopy and of optically detected magnetic resonance (ODMR) spectroscopy. Such a complimentary investigation is carried out for the first time for a Pt(II) compound. In solution, oligomer or short linear chain formation is also observed. However, the monomers can be investigated selectively, when they are dissolved in a relatively inert n-octane matrix (Shpol'skii matrix). This allows us to determine the energies of the T(1) triplet substates I, II, and III relative to the electronic ground state S(0)(0), the zero-field splittings (ZFSs) of T(1), and emission decay time constants (I/II <--> 0, 18012.5 cm(-1); III <--> 0, 18016.3 cm(-1); DeltaE(I,II) = 0.05437 cm(-1) (1.631 GHz), DeltaE(I,III) = 3.8 cm(-1) (114 GHz); tau(I) = 120 micros, tau(II) = 45 micros, tau(III) = 35 micros; spin-lattice relaxation time for the processes III --->I/II, tau(SLR) = 3.0 micros). The vibrational satellite structure observed in the emission of the T(1) state to the singlet ground state S(0) is also discussed. Moreover, it is possible to estimate the intersystem crossing time from the excited singlet state S(1) at 22952 cm(-1) to the triplet state T(1) to approximately 5 ps. The T(1) state is assigned as a thpy-ligand-centered (3)pipi* state with small metal-to-ligand charge-transfer (MLCT) admixtures. A comparison of Pt(thpy)(CO)(Cl) to a series of other organometallic Pt(II) compounds, such as heteroleptic Pt(ppy)(CO)(Cl) ((ppy)(-) = phenylpyridinate), Pt(dppy)(CO) ((dppy)(2-) = diphenylpyridinate), and Pt(i-biq)(CN)(2) (i-biq = 2,2'-bisisoquinoline) and homoleptic Pt(thpy)(2) and Pt(ppy)(2), is carried out. (The structures are shown in Figure 7.) Trends of photophysical properties are discussed. In particular, by chelation of two equal ligands the pattern of ZFS is strongly altered, resulting in a significant increase of the MLCT participation in the lowest triplet state of these organometallic compounds. This new observation represents an interesting further step concerning chemical tunability of photophysical properties.