O-H oscillator

Flowever, there is a trade-off in using near-IR emissive lanthanides, in that luminescence lifetimes are shorter, and quantum yields lower, compared to complexes of Tb and Eu. This arises because the near-IR emissive lanthanides are quenched by lower harmonics of the O-H oscillator, increasing the Franck-Condon overlap with the metal excited state. For neodymium, matters are further complicated by the manifold of available metal-centered excited states, which leads to particularly effective quenching by C-H oscillators. Thus, complexes in which there are few C-H oscillators close to the metal are desirable if the luminescence lifetime is to be optimized (e.g. 44).76 97-101... 

A characteristic feature of hydride stretches in general [79] and the O—H oscillator in alcohols in particular [80] is its frequency isolation from other... 

Although most of the reported gas-phase experiments do not investigate the temporal evolution of alcohol clusters explicitly, the frequency-domain spectral information can nevertheless be translated into the time domain, making use of some elementary and robust relationships between spectral and dynamical features [289]. According to this, the 10-fs period of the hydrogen-bonded O—H oscillator is modulated and damped by a series of other phenomena. Energy flow into doorway states is certainly slower than for aliphatic C—H bonds [290] but on a time scale of a few picoseconds, energy will nevertheless have... 

Again, solvation numbers in methanol can be calculated by altering the constants to account for halving the number of O-H oscillators per solvent molecule, giving Eqs. (4) and (5) ... 

Because of the higher sensitivity of Ndm ions towards deactivation through O-H oscillators, the complexes with this lanthanide have much lower quantum yields and lifetimes when compared to Ybm. The best photophysical properties are obtained with phthalexon S and since complexes with PS contain 4-5 water molecules, depending on the lanthanide ion, it is quite clear that exclusion of these water molecules from the first coordination sphere will lead to much enhanced luminescent properties. This is indeed demonstrated by bis(cyclen)-substituted PS, H736 (see fig. 36), which increases quantum yields to 0.23 and 1.45% in D2O for Ndm and Ybm, respectively (Korovin and Rusakova, 2002). 

As described previously, nonradiative decay due to solvent interactions can severely reduce lanthanide luminescence through energy dissipation by vibronic modes, with the O—H oscillator being the most common and eflBcient quencher. However, if these O—H oscillators are replaced with lower-frequency O—D oscillators, the eflBciency of vibronic deactivation decreases substantially. Therefore, the rate constants for luminescence lifetimes (th o) of lanthanide excited states in water or alcoholic solvents are often much shorter than those in analogous deuterated solvents (td o)- This property can be utilized to determine the degree of solvation for luminescent lanthanides. 

The effect of H-bonding on the anharmonicity of the O—H oscillator has been extensively discussed in the literature (43-46), and analysis of the available data allows the conclusion that weak and medium H-bonds lead to a decrease of the O-H bond anharmonicity (43,44). The anharmonicity is increased when strong H-bonds are present (45,46), but this increase is not dramatic (45). To provide some values, the anharmonicity of the free O-H oscillator (OH radical) is about 90 cm whereas an anharmonicity of approximately 200 cm was reported for very strongly H-bonded hydroxyls in a phenol-pyridine complex (47). If the hydrogen acceptor is an oxygen atom, the anharmonicity can reach values of up to 120 cm ... 

Overtones appear in the spectra because of the anharmonicity of the O-H bond. It is important to consider that the Vq2(OH) bands appear always at a wavenumber lower than twice that of the fundamental hydroxyl stretching mode. The difference is used to calculate the anharmonicity of the O-H oscillator according to Equation (2.1) and also the harmonic frequencies according to Equation (2.2). In principle, accurate calculations should be based also on the next overtones but it was reported that, for OH oscillators, calculations using the first overtone always give correct results (613). In the absence of any other factors, the anharmonicity... 

In hydration studies, the relation of knjo to the number of water molecules in the primary coordination sphere of the lanthanide is usually calibrated with a series of solid compounds of known hydrate structure, e.g. crystalline Eu and Tb compounds in which n varies from zero to nine isolated from H2O and from D2O solutions. The results of lifetime studies for crystalline hydrates reported by Horrocks and Sudnick (1979) are listed in table 2. For crystals with n = 0, all the measured k values were identical whether prepared from H2O or from D2O solution. This confirms that only O-H oscillators directly bound to the metal ion can significantly affect the luminescence decay constants. 

Fig. 9.2. Overtone series of the O-H stretching mode in MgfOHlj, plotted as a function of energy (wavenumbers), corresponding to the vibrational stretching excitation of the O-H oscillator. Note the appearance of an edge at the OS tranation (with kind permission of JAI Press).

The most significant result which comes out of this work is that Mg(OH)2, Ca(OH)2 and Al(OH)3 may be protonic semiconductors that can be described by highly localized protons occupying a valence band, VB, and the presence of a conduction band, CB, providing for proton delocalization. Between VB and CB lie intermediate levels, corresponding to the vibrationally excited states of the O-H oscillator, separated by Si 0.4 eV. The gap values are 2.0-2.1 eV for Mg(OH)2,2.2 eV for the more basic Ca(OH)2 and 1.6-1.7 eV for the more acidic Al(OH)3. 

Eu and Tb, thus resulting in more efficient nonradiative decay through high-energy C-H, N-H, and O-H oscillators in the ligands and solvents [96]. 

The coefficients A and B were empirically determined and are summarised in Table 2.2. Since methanol contains one O—H oscillator, the equation can be extended for this solvent. This equation has been expanded by several authors, to include quenching effects by second sphere solvent molecules, as well as other oscillators, N—H in amines and amides and C-H [51-53]. 

As demonstrated above in Scheme 6.14a, b, anions such as salicylic acid can be detected in water with the formation of ternary complexes using Tb complexes. In a similar manner, a mono-cationic complex (Scheme 6.14e) was shown by Gray and co-workers to bind salicylic acid as well as salicylurate in water. Meanwhile Georges and Amaud reported a method of salicylic acid detection using a combination of Tb and ethylenediaminetetra-acetic acid (EDTA). The presence of the EDTA prevents the formation of insoluble terbium hydroxides and also acts to exclude water molecules from the coordination sphere of the lanthanide ion, reducing quenching caused by O—H oscillators [73]. 

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