Excited solute

Will the solvent react with the excited state to yield undesirable side-products Often there is a real possibility that the solvent will enter into the picture through reaction with the excited solute. A common example of this is the abstraction of hydrogen atoms from solvents by excited ketones. Several solvents often used for a preliminary examination due to their relative inertness are benzene, /-butanol, carbon disulfide, carbon tetrachloride, and cyclohexane. 

In alicyclic hydrocarbon solvents with aromatic solutes, energy transfer (vide infra) is unimportant and probably all excited solute states are formed on neutralization of solute cations with solute anions, which are formed in the first place by charge migration and scavenging in competition with electron solvent-cation recombination. The yields of naphthalene singlet and triplet excited states at 10 mM concentration solution are comparable and increase in the order cyclopentane, cyclohexane, cyclooctane, and decalin as solvents. Further, the yields of these... 

Tagawa et al (1982a) studied excited solute state formation in solutions of cyclohexane, methylcyclohexane, and isooctane. The lifetime of the excited state... 

Ware W. R., Lee S. K., Brant G. J. and Chow P. P. (1971) Nanosecond Time-Resolved Emission Spectroscopy Spectral Shifts due to Solvent-Excited Solute Relaxation, J. Chem. Phys. 54, 4729-4737. 

The products resulting from this energy absorption in aerated water are H, OH, H02, H202, and e3q in concentrations which depend upon the dose rate and upon the pH of the solution, and the nature of the solute molecules.133 These species react with dissolved nucleic acid derivatives. There is a great difference between this sort of interaction and that in photochemistry where only the solute molecule absorbs the energy, and the ensuing reactions are those of the excited solute molecule. 

The second area of activity also includes some problems in laser electrochemistry of semiconductors, which are in no way confined to the above-considered light-sensitive etching. First of all, it is threshold electrochemical reactions stimulated by intensive laser radiation. Such reactions may proceed via new routes, because both highly excited solution... 

Fluorescence depolarisation by energy transfer (rather than rotational relaxation) between donor molecules of the same type can occur. Eisenthal [174] excited solutions of rhodamine 6G (9 mmol dm-3) in glycerol with 530 nm light from a frequency-doubled neodymium laser. The polarisation... 

The elucidation of the early steps in the photosynthetic process, in the solvation of excited solute molecules, and other important reactions. 

It should be emphasized that solvation of excited electronic states is fundamentally different from the solvation of closed-shell solutes in the electronic ground state. In the latter case, the solute is nonreactive, and solvation does not significantly perturb the electronic structure of the solute. Even in the case of deprotonation of the solute or zwitterion formation, the electronic structure remains closed shell. Electronically excited solutes, on the other hand, are open-shell systems and therefore highly perceptible to perturbation by the solvent environment. Empirical force field models of solute-solvent interactions, which are successfully employed to describe ground-state solvation, cannot reliably account for the effect of solvation on excited states. In the past, the proven concepts of ground-state solvation often have been transferred too uncritically to the description of solvation effects in the excited state. In addition, the spectroscopically detectable excited states are not necessarily the photochemically reactive states, either in the isolated chromophore or in solution. Solvation may bring additional dark and photoreactive states into play. This possibility has hardly been considered hitherto in the interpretation of the experimental data. 

As a final note, we point out two special advantages of QMSTAT for this study. First, to study the fluorescence, the solvent configurations have to be sampled with the solvent interacting with the excited state of indole. The sequential approach would require classical force-field parameters valid for excited states, which are more difficult to obtain. In QMSTAT, this is not a problem. Second, the nature of the excited state in this study is an issue. With QMSTAT, the solute-solvent interactions, the solvent configurations and the properties of the excited solute are coupled, and hence the nature of the excited state is not assumed, in any instance, but follows from the simulation. 

Step 1 electronic excitation of the solute. Solute and solvent are in a nonequilibrium situation, where the solvent is only partially equilibrated with the new charge distribution of the excited solute. 

That is, the ordered structure of the cholesteric mesophase affects the formation of the traTO-adduct advantageously. Furthermore, the trans/cis product ratio depends significantly on the initial acenaphthylene concentration. In isotropic solutions, the dimerization of singlet-excited acenaphthylene molecules is known to yield exclusively the czv-adduct, whereas a mixture of cis- and traTO-adducts results from triplet-excited solute molecules. The lowering of cu-adduct production in the mesophase has been attributed to the enhanced efficiency of the triplet reaction in comparison with the singlet reaction, as shown by quantum yield measurements [732]. The increase in triplet reaction efficiencies has been ascribed to the increase in the fraction of acenaphthylene-acenaphthylene collisions which have coplanar or parallel-plane orientations with respect to the surrounding solvent molecules, and not to the increase in the total number of collisions per unit time [732]. See references [713, 732, 733] for a more detailed discussion of this photodimerization reaction. 

In solutions of moderate concentration, there are two types of excited state solute-solute interaction, which are common. An excited polymer, or excimer, may form through the aggregation of excited solute molecules with ground-state molecules of the same type. Fluorescence quenching and/or a spectral red shift may result. Also, a heteropolymeric excited-state complex may form between two different solute molecules. Such a complex is referred to as an exciplex. 

In recent years, there have been many attempts to combine the best of both worlds. Continuum solvent models (reaction field and variations thereof) are very popular now in quantum chemistry but they do not solve all problems, since the environment is treated in a static mean-field approximation. The Car-Parrinello method has found its way into chemistry and it is probably the most rigorous of the methods presently feasible. However, its computational cost allows only the study of systems of a few dozen atoms for periods of a few dozen picoseconds. Semiempirical cluster calculations on chromophores in solvent structures obtained from classical Monte Carlo calculations are discussed in the contribution of Coutinho and Canuto in this volume. In the present article, we describe our attempts with so-called hybrid or quantum-mechanical/molecular-mechanical (QM/MM) methods. These concentrate on the part of the system which is of primary interest (the reactants or the electronically excited solute, say) and treat it by semiempirical quantum chemistry. The rest of the system (solvent, surface, outer part of enzyme) is described by a classical force field. With this, we hope to incorporate the essential influence of the in itself uninteresting environment on the dynamics of the primary system. The approach lacks the rigour of the Car-Parrinello scheme but it allows us to surround a primary system of up to a few dozen atoms by an environment of several ten thousand atoms and run the whole system for several hundred thousand time steps which is equivalent to several hundred picoseconds. 

An investigation of the excited state tautoaerisation of 7-hydroxyquinoline using ps tiae-resolved fluorescence has revealed the presence of two different excited state tautoaerisation processes.The faster of the two occurs within electronically excited li2 solute-alcohol coaplexes and the slower is associated with tautoaerisation in excited solute-alcohol coaplexes containing aore than two solvent aolecules. 

The excited solute molecules may then be formed by energy transfer ... 

According to Scheme 2, Law [30] suggested that three different excited states of Sq4 in toluene were detected. Because the decay at 645 nm is primarily from the free squaraine, the 2.4-ns decay was assigned to the excited Sq4, the 3.5-ns decay to the excited solute-solvent complex, and the 2.7-ns decay to the relaxed excited state. The fact that the solute-solvent complex has a longer lifetime is consistent with the geometry of the complex, which is shown to be rigidized upon complexation due to n-rt interaction (Scheme 4b). The three lifetimes recorded for Sq4 in toluene provide kinetic evidence for the existence of three different excited states of squaraine in solution. 

Direct energy transfer from excited solvent molecules (A ) to solute molecules (B) to form excited solute molecules may also take place ... 

With this interpretation it is clearly useful to use the SCF perturbation equations to check that any solution of the SCF equations is, indeed, of the type we seek. Conversely, it is often useful to look for excited solutions of the SCF equations. 

A suitable set of functions are the excited solutions of the unperturbed Schrodinger equation. Why not correct the ground state function in the presence of a perturbation by addition of pieces of the excited state functions ... 

When electronic excited states in the QM region are considered, each electronic state possesses unique electronic density and thus interacts differently with polarizable environment. Purely electrostatic (Coulomb) interactions between solvent and electronically excited solute are automatically taken into account due to a presence of the one-electron Coulomb term in the QM Hamiltonian (Eq. 5.10). A part of polarization interactions is also represented in a similar way due to an explicit inclusion of EFP induction terms in the QM Hamiltonian, as given by Eq. 5.11. 

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