Titanium complexes electron spectra

VIV. As with the titanium porphyrins, these complexes occur as vanadyl complexes with an axial oxo group and produce a "normal electronic spectrum which changes to a hematin spectrum when trifluoroacetic acid is added [Bonnett (9)]. The vanadyl porphyrins contain a single unpaired electron (/i=l,4B.M.) and their esr spectra have been studied in some detail [Kivelson (113a), Roberts (152)]. The rich redox chemistry of the inorganic vanadium salts is not reflected in the vanadyl porphyrins, where the +4 state seems to be exceptionally stable. 

Most organometallic EDA complexes of arenes with titanium tetrachloride [116] in solution also follow the general reaction scheme in Eq. 15 in that no net chemical reaction is observed upon charge-transfer irradiation due to rapid back electron transfer (A et 10 ° s ). For example, the transient absorption spectrum of bro-moanthracene (BrAnt) cation radical generated by 532-nm laser excitation of the [BrAnt, TiCU] complex in cyclohexane (see Figure 7) decays completely to the spectral baseline within about 1 ns (see inset) due to back electron transfer [116], (Eq. 18) ... 

Transient terahertz spectroscopy Time-resolved terahertz (THz) spectroscopy (TRTS) has been used to measure the transient photoconductivity of injected electrons in dye-sensitised titanium oxide with subpicosecond time resolution (Beard et al, 2002 Turner et al, 2002). Terahertz probes cover the far-infrared (10-600 cm or 0.3-20 THz) region of the spectrum and measure frequency-dependent photoconductivity. The sample is excited by an ultrafast optical pulse to initiate electron injection and subsequently probed with a THz pulse. In many THz detection schemes, the time-dependent electric field 6 f) of the THz probe pulse is measured by free-space electro-optic sampling (Beard et al, 2002). Both the amplitude and the phase of the electric field can be determined, from which the complex conductivity of the injected electrons can be obtained. Fitting the complex conductivity allows the determination of carrier concentration and mobility. The time evolution of these quantities can be determined by varying the delay time between the optical pump and THz probe pulses. The advantage of this technique is that it provides detailed information on the dynamics of the injected electrons in the semiconductor and complements the time-resolved fluorescence and transient absorption techniques, which often focus on the dynamics of the adsorbates. A similar technique, time-resolved microwave conductivity, has been used to study injection kinetics in dye-sensitised nanocrystalline thin films (Fessenden and Kamat, 1995). However, its time resolution is limited to longer than 1 ns. 

Redox processes are fairly common in the formation of Z —CO— complexes of transition metals, and an example is given in Eq. (9). In this reaction, titanium is oxidized from the + 2 to the +3 state, thus becoming a better Lewis acid, and the molybdenum dimer is reductively cleaved, thus developing Z —CO— donor character (59). A characteristic low-frequency Z —CO— band is observed in the IR spectrum, and a crystal structure is available. A proposed mechanism for the redox process, based on CO mediated electron transfer, is discussed in Section IV,C. 

Titanium group allyl compounds are sensitive to air, water, and alcohols. Tetrallyl derivatives of zirconium and hafnium are dynamic compounds. The complex Hf(allyl)4 gives the AX4 type NMR spectrum even at very low temperatures (199 K). The Ti(III) complexes are paramagnetic magnetic moments correspond to one unpaired electron and, for various allyl groups, vary in the 1.47-1.75 BM range. The properties of allyl... 

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