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.

Highly resolved emission, excitation, and resonantly line-narrowed spectra, as well as emission decay properties of [Rh(bpy-h(8))(n)(bpy-d(8))(3-n)](3+) (n = 0, 2, 3; bpy = 2,2'-bipyridine) doped into [Zn(bpy-h(8))(3)](ClO(4))(2) are presented for the first time. [Rh(bpy-h(8))(3)](3+) and [Rh(bpy-d(8))(3)](3+) exhibit one low-lying triplet T(1) at 22 757 +/- 1 and 22 818 +/- 1 cm(-1), respectively (blue shift 61 cm(-1)), while [Rh(bpy-h(8))(2)(bpy-d(8))](3+) has two low-lying triplets at 22 757 +/- 1 and 22 818 +/- 1 cm(-1). The well-resolved vibrational satellite structures show, that the equilibrium positions of the triplet and the singlet ground S(0) state are not very different and that the force constants in T(1) are mostly slightly smaller than in S(0). Moreover, the vibrational satellite structure is strongly dominated by vibrational ligand modes, which demonstrates the pipi character of the corresponding transition. However, the occurrence of several very weak vibrational modes of metal-ligand character displays a small influence of the metal ion. This is supported by the emission decay behavior. [Rh(bpy-h(8))(2)(bpy-d(8))](3+) exhibits an emission which is clearly assignable to the protonated ligand(s), even when the deuterated ligand is selectively excited. Obviously, an efficient intramolecular energy transfer from the deuterated to the protonated ligand(s) occurs, presumably mediated by the small Rh(3+) d-admixture. A so-called "dual emission" is not observed. Moreover, a series of spectroscopic properties of the lowest excited state of [Rh(bpy)(3)](3+) (energies of electronic origins, emission decay times, zero-field splittings, structures of vibrational satellites, etc.) is compared to properties of bpy, [Pt(bpy)(2)](2+), [Ru(bpy)(3)](2+), and [Os(bpy)(3)](2+). This comparison displays in a systematic way the increasing importance of the metal d and/or MLCT character for the lowest excited states and thus provides guidelines for an experimentally based classification. In particular, the lowest excited states of [Rh(bpy)(3)](3+) may be ascribed as being mainly of (3)pipi character confined to one ligand in contrast to the situation found for [Ru(bpy)(3)](2+) where these states are covalently delocalized over the whole complex.

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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.