Emissive excited states

Figure 8.2e shows the dependence of the fluorescence intensity on the excitation power of the NIR light for the microcrystals measured with a 20x objective. In this plot, both axes are given in logarithmic scales. The slope of the dependence for the perylene crystal is 2.8, indicating that three-photon absorption is responsible for the florescence. On the other hand, slopes for the perylene and anthracene crystals are 3.9 for anthracene and 4.3 for pyrene, respectively. In these cases, four-photon absorption resulted in the formation of emissive excited states in the crystals. These orders of the multiphoton absorption are consistent with the absorption-band edges for each crystal. The four-photon absorption cross section for the anthracene crystal was estimated to be 4.0 x 10 cm s photons by comparing the four-photon induced fluorescence intensity of the crystal with the two-photon induced fluorescence intensity of the reference system (see ref. [3] for more detailed information). 

It has been shown that some phosphine or phosphite Ni° complexes have long-lived emissive excited states at room temperature and possess photochemical reactivity towards organic substrates.2520... 

Three classes of polynuclear complexes containing metal-metal bonds possess emissive excited states that undergo oxidation-reduction reactions in solution the prototypes are Re2Cl T(dlt d lt),... 

Figure 7 Energy level diagram of lowest energy excited states of europium and terbium ions. Only the emissive excited states within the 5Dj manifold are shown.

The most widely studied and most exploited of inorganic ECL reactions, however, is that of tris(2,2 -bipyridine) ruthenium(II), [(bpy)3Ru]2+. The emissive excited state is a metal-to-ligand charge transfer state of triplet spin multiplicity the emission yield is approximately 10% and oxygen does not significantly quench the luminescence [22], The complex can exhibit ECL following sequential oxidation at the metal center and reduction of the coordinated 2,2 -... 

The lower energy luminescence of Os(II) complexes relative to the corresponding Ru(II) complexes has led to a number of studies seeking complexes that absorb throughout the visible (black chromophores) and have emissive excited states in the near-infrared. Kol and Barigelletti reported the photophysical properties of Os(II) complexes of eilatin (eil, Scheme 1) and isoeilatin (ieil) bis-phenanthroline derivatives. The eil com-... 

For the Ir(III) complexes reviewed here, the lowest-lying (and emissive) excited state can be of 3LC, 3MLCT, 3ILCT, 3MLLCT, and 3LLCT nature. Some attention should also be paid to the 3MC levels, Fig. 3. As is well known and has been widely investigated for Re(I)-, Ru(II)-, and Os(II)-polyimine... 

The relaxation processes from the MLCT state can be evaluated quantitatively by measuring the temperature dependence of the emission yield and lifetime because the MLCT state is both the lowest and emissive excited state. Higher temperature caused larger 0. - However, 0g and Xe became smaller and shorter at higher temperature, respectively. These experimental results could be well simulated by assuming that the photochemical reactions occur via the thermally accessible higher excited... 

A Study of the photophysical and photochemical properties of alkylidyne complexes such as W(CPh)Cl(CO)2(tmeda) was reported by Bocarsly et al. (see also Section II,B) 24). Quenching experiments with aromatic hydrocarbons indicate that a significant amount of triplet character is associated with the emissive excited state. Bimolecular oxidative and reductive quenching is observed as well. Methylviologen dichloride is reduced and iV.A, A, A, -tetramethyl-p-phenylenediamine is oxidized by W(CPh)Q-(CO)2(tmeda) on irradiation with 488 nm light. Photosubstitution of two CO ligands in W(CPh)Cl(CO)(dppe) by dppe to give W(CPh)Cl(dppe)2 was observed as well. 

This simplest route is the most important case, because the experimental values of the ECL efficiencies may be compared with those theoretically predicted. However, it should be noted that the case of only two reaction pathways, i.e., formation of the emissive excited state and the ground state, has been found only for the ECL systems of some coordination compounds of transition metals (e.g., [98,99]). 

In an ECL experiment the application of a triple-potential-step sequence causes very intense and bright emission. The ECL emission bands, exhibiting charge-transfer character, agree well with those observed with photoexcitation. The Feldberg plot analysis indicates that the emissive excited state is formed directly by electron transfer between A -D and A -D. In most cases, the emitted light intensities were found to be linear vs. according to Eq. (43) thus in the time scale of ECL... 

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