Excited solvation

Due to distribution of solvates over different intensities of electric field R, it is possible to excite the solvates with the different pure electronic, 0-0, frequencies on both branches of the excited parabola. Relaxation is accompanied by the spectral shifts of emission in time when we excite solvates with fields R Rh we have... 

The relaxation of the fluorescence spectrum takes place as a result of rearrangement of all molecules in solvate (with characteristic times of orientational relaxation) and depends essentially on the exciting radiation frequency, that is, on the type of selectively excited solvates. The following three characteristic cases are possible ... 

In this discussion, I have intentionally not discussed the solvation of the electron in water. These results are quite confusing because of the overlap of the electronic relaxation of the excited solvated electron and the rearrangement of the solvent. In an alcohol, these terms are well separated and so discussion is simplified. In addition, we are not aware of any study on the solvation of an anion in water. 

Although the mechanism of the photo-induced generation of mono- and bimetallic metal clusters, except for the photographic application (Section 20.6), has been studied with considerably less detail than for the radiolytic route, some stable clusters, mostly of noble metals (Ag, Au, Pt, Pd, Rh), have also been prepared by UV excitation of metal ion solutions [129-141]. Generally, halides and pseudo-halides counter anions are known to release, when excited, solvated electrons, which reduce the metal ions up to the zerovalent state. Oxalate excitation yields the strong reducing carbonyl radical COO [30]. Photosensitizers are likewise often added [142]. Metal clusters are photo-induced as well at the surface of photo-excited semiconductors in contact with metal ions [143,144]. 

Assel M, Laenen R, Laubereau A. (1998) Dynamics of excited solvated electrons in aqueous solution monitored with femtosecond-time and polarization resolution. /Phys Chem A 102 2256-2262. 

Even proteins that contain only a single Trp residue generally exhibit multiexponential decays. Several hypotheses have been proposed to explain why. Pirst, multiple conformational states may exist for the single Trp such as different rotameric configurations (orientations about the Trp xi or X2 C-C bond) (18). Even in the absence of mnltiple rotamers, the electron-transfer qnenching rate is extremely sensitive to the local environment, so a distribntion of local microconformational states may cause a nonexponential flnorescence decay. Other possible sonrces of nonexponential flnorescence decay inclnde the response of the protein and surronnding solvent to the change in dipole moment of Trp on excitation ( solvation ) (19). 

A number of types of calculations can be performed. These include optimization of geometry, transition structure optimization, frequency calculation, and IRC calculation. It is also possible to compute electronic excited states using the TDDFT method. Solvation effects can be included using the COSMO method. Electric fields and point charges may be included in the calculation. Relativistic density functional calculations can be run using the ZORA method or the Pauli Hamiltonian. The program authors recommend using the ZORA method. 

Excited-State Relaxation. A further photophysical topic of intense interest is pathways for thermal relaxation of excited states in condensed phases. According to the Franck-Condon principle, photoexcitation occurs with no concurrent relaxation of atomic positions in space, either of the photoexcited chromophore or of the solvating medium. Subsequent to excitation, but typically on the picosecond time scale, atomic positions change to a new equihbrium position, sometimes termed the (28)- Relaxation of the solvating medium is often more dramatic than that of the chromophore... 

Solvatochromic shifts are rationalized with the aid of the Franck-Condon principle, which states that during the electronic transition the nuclei are essentially immobile because of their relatively great masses. The solvation shell about the solute molecule minimizes the total energy of the ground state by means of dipole-dipole, dipole-induced dipole, and dispersion forces. Upon transition to the excited state, the solute has a different electronic configuration, yet it is still surrounded by a solvation shell optimized for the ground state. There are two possibilities to consider ... 

Let (Xgr and jXex be the dipole moments of the ground and excited states. Then if iXgr > the less polar excited state is surrounded by a solvation shell... 

Hayon23 studied the yields of ions and excited states in pulse radiolysis of liquid DMSO using anthracene as a solute to determine the yield of free ions and naphthalene as a solute to measure the yield of triplet excited states. Anthracene is known to react with solvated electrons to give the anthracene radical anion, A T... 

Recently, Eisenthal and coworkers have developed time-resolved surface second harmonic techniques to probe dynamics of polar solvation and isomerization reactions occurring at liquid liquid, liquid air, and liquid solid interfaces [22]. As these experiments afford subpicosecond time resolution, they are analogous to ultrafast pump probe measurements. Specifically, they excite a dye molecule residing at the interface and follow its dynamics via the resonance enhance second harmonic signal. 

Tables 11-6, 11-7, and 11-8 show calculated solvatochromic shifts for the nucle-obases. Solvation effects on uracil have been studied theoretically in the past using both explicit and implicit models [92, 94, 130, 149, 211-214] (see Table 11-6). Initial studies used clusters of uracil with a few water molecules. Marian et al. [130] calculated excited states of uracil and uracil-water clusters with two, four and six water molecules. Shukla and Lesczynski [122] studied uracil with three water molecules using CIS to calculate excitation energies. Improta et al. [213] used a cluster of four water molecules embedded into a PCM and TDDFT calculations to study the solvatochromic shifts on the absorption and emission of uracil and thymine. Zazza et al. [211] used the perturbed matrix method (PMM) in combination with TDDFT and CCSD to calculate the solvatochromic shifts. The shift for the Si state ranges between (+0.21) - (+0.54) eV and the shift for the S2 is calculated to be between (-0.07) - (-0.19) eV. Thymine shows very similar solvatochromic shifts as seen in Table 11-6 [92],...

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