Stokes’ shift hydrogenation

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

When Stokes shifts are plotted as a function of the orientation polarizability A f (Lippert s plot, see Section 7.2.2), solvents are distributed in a rather complex manner. A linear relationship is found only in the case of aprotic solvents of relatively low polarity. The very large Stokes shifts observed in protic solvents (methanol, ethanol, water) are related to their ability to form hydrogen bonds. 

At low temperatures the PLE of hydrogen-terminated PS reveals that phonons and the exciton exchange splitting contribute significantly to the observed Stokes shift [Ca6, Ku4, Ro5, Ka8, Kol3]. For oxidized PS the picture is not usually so clear, due to a recombination path that may involve surface states. 

Fig. 2 shows the effect of creating hydrogen-bonding complexes between HPTA and oxygen-bases on the solvation correlation function of HPTA, C(t) [10]. Utilizing a pump-probe set-up described elsewhere [11], with 400 nm excitation, the dynamic stokes shift of HPTA was analyzed with about 50fs time-resolution. The hydrogen-bonded HPTA exhibited much faster dynamics than the solvation dynamics of the uncomplexed HPTA in pure DCM. 

The ESIPT of 2-(2 -hydroxyphenyl)-4-methyloxazole (HPMO) (27) has been explored by Douhal and co-workers [166] for its probe characteristics in a variety of organized media which include cyclodextrin, calixarene, micelle, and HSA. The incorporation of HPMO into hydrophobic cavities in an aqueous medium involves the rupture of its intermolecular hydrogen bond to water and formation of an intramolecular hydrogen bond in the sequestered molecule. Upon excitation (280-330 nm) of this entity, a fast intramolecular proton-transfer reaction of the excited state produces a phototautomer (28), the fluorescence of which (Xm = 450 170 nm) shows a largely Stokes-shifted band. Because of the existence of a twisting motion around the C2—C bond of this phototautomer, the absorption and emission properties of the probe depend on the size of the host cav-... 

Table 2 Stokes Shift values for hydrogenated Si clusters present work versus theoretical data present in literature. All values are in eV...

Tables 1 and 2 summarize Stokes-shift (Av) and lifetime data, respectively, for a series of AF probes in DSPC and human SC. Since Stokes-shift data in hexane should reach a minimum value due to the absence of a dipole moment and hydrogen-bonding in this solvent [7], findings with hexane were compared to those of SC and DSPC in Table 1. The data show that while the Av of 2-AF through 12-AF in DSPC were similar to values obtained in hexane, much smaller values were obtained in fully hydrated SC. Consequently, the...

Av in SC cannot be explained by polarity and hydrogen-bonding of the surrounding medium alone. Rather the results are explained better in terms of hindered fluorophore reorientation in the excited state, similar to results obtained with other lipid systems [7,11-13]. Thus, the unusual Stokes shifts seen with AF probes in SC suggests a rather rigid environment in the lipid lamellae, which hinder the fluorophore reorientation. 

The absorption and emission maxima of SA in different states of protonation are compared in Table 3. The data show that Stokes shifts of about 6000 cm are usual for compounds in which Intramolecular proton transfer does not occur. Concentrated solutions of the acids where dimerization can occur show very large shifts of about 11,000 cm l, regardless of whether Intramolecular hydrogen bonds are formed or not. (The association of SA has not been studied in alcoholic solvents in benzene there Is considerable association even at 80°C (apparent M/M = 1.4 in 0.01 molar solution)(33). 

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