Excited state tautomerism

At its best, the study of solvent kies by the formalism given can be used to learn about proton content and activation in the transition state. For this reason it is known as the proton inventory technique. The kinetics of decay of the lowest-energy electronic excited state of 7-azaindole illustrates the technique.25 Laser flash photolysis techniques (Section 11.6) were used to evaluate the rate constant for this very fast reaction. From the results it was suggested that, in alcohol, a double-proton tautomerism was mediated by a single molecule of solvent such that only two protons are involved in the transition state. In water, on the other hand, the excited state tautomerism is frustrated such that two water molecules may play separate roles. Diagrams for possible transition states that can be suggested from the data are shown, where of course any of the H s might be D s. 

Langer H, Doltsinis NL (2003) Excited state tautomerism of the DNA base guanine a restricted open-shell kohn-sham study. J Chem Phys 118 5400-5407... 

Mentus S, Maroncelli M (1998) Solvation and the excited-state tautomerization of 7-azaindole and 1-azacarbazole computer simulations in water and alcohol solvents. J Phys Chem A 102 3860-3876... 

Fig. 5 Absorption and fluorescence emission spectra of the 3-hydroxychromone dye F4N1 in the absence (black) and presence (red) of a local electric field, which promotes the excitation charge transfer leading from the ground state to the N state. In the presence of the local electric field, the energy of the N state is reduced, causing a red shift of the N emission peak and an increase in its intensity relative to the T emission peak. The change in relative intensities of the N and T peaks reflects a shift in the excited state tautomeric equilibrium toward the N state...

Rodrigues Prieto, F., Rios Rodriguez, M. C., Mosquera Gonzalez, M. and Rios Fernandez, M. A. Ground- and excited-state tautomerism in 2-(3 -Hydroxy-2 -pyridyl)benzimidazole, J. Phys. Chem., 98 (1994), 8666-8672... 

Rotational isomerization has been described for the phototautomer formed by proton transfer in the excited singlet state of 2,2 -bipyridin-3-ol in hydrocarbon solvents. At room temperature both the primary phototautomer and its rotamer fluoresce, allowing the activation energy for internal rotation to be determined. Excited state tautomerization has also been described for camp-tothecin in acidic aqueous solution and for derivatives of hypericin. Light-induced keto-enol tautomerism has been invoked to explain the fluorescence behaviour of certain benzimidazole compounds. Interconversion of confor-mers of constrained tryptophan derivatives takes place in the first-excited singlet state.The excited state behaviour has been reported for conformationally-distorted porphyrin derivatives. The lifetimes of the S] states of these... 

The depolarization of fluorescence has been observed at temperatures around 100 K not only for 1, but also for the two derivatives, lb and Ic. in contrast, the measurements performed under the same conditions for le revealed no sign of depolarization. The textbook values of the anisotropy were obtained, i.e. 0.4 and about -0.2 for excitation into Sj and S2, respectively (Fig. 8.11). The octaethyl derivative le is the porphycene with the largest separation (2.80 A) between the hydrogen-bonded nitrogen atoms and should therefore exhibit the slowest tautomerization kinetics. It was thus concluded that the reduced anisotropy values observed in three different porphycenes are caused by excited state tautomerization [30, 80]. As shown in Fig. 8.12, the interconversion between the two trans tautomers changes the direction of the transition moment. Therefore, only a part of the excited state population emits fluorescence polarized parallel to that of the transition moment in Sg-Sj absorption (this fraction should approach 0.5 if the tautomerization is fast compared to the excited state lifetime). For the remaining frac-... 

An independent verification of this procedure was possible for lb [82]. The four n-propyl substituents ensure that this molecule can be oriented to a high degree in stretched polymer sheets. Linear dichroism (LD) measurements on the aligned samples resulted in the determination of transition moment directions. A pattern similar to that of parent 1 was obtained. Both LD and emission anisotropy procedures yielded similar values, which, additionally, confirmed that the assignment of fluorescence depolarization to excited state tautomerization was correct. 

Fluorescence anisotropy measurements can also be used to obtain the rates of the excited state tautomerization. Two variants can be applied. The first is based on the analysis of time-resolved anisotropy curves. These are constructed from measurements of the fluorescence decay recorded with different positions of the polarizers in the excitation and emission channels. The anisotropy decay reflects the movement of the transition moment and thus, the hydrogen exchange. For molecules with a long-lived Sj state, the anisotropy decay can also be caused by rotational diffusion. In order to avoid depolarization effects due to molecular rotation, the experiments should be carried out in rigid media, such as polymers or glasses. When the Sj lifetime is short compared to that of rotational diffusion, tautomerization rates can be determined even in solution. This is the case for lb, for which time-resolved anisotropy measurements have been performed at 293 K, using a... 

Figure 29.18 Calculated structures of the stationary points along the reaction coordinate for the excited state tautomerization in 7-azaindole complexes with one, two and five water molecules [17].

Nanosecond and picosecond photochemical kinetics of quinoid fluorescence produced by excitation of the enol form salicylideaniline have been investigated in various environments.An excited-state tautomeric proton transfer occurs within 5 ps at temperatures above 4 K in both protic and aprotic solvents. 

Ground- and excited-state tautomerism in porphycenes 06ACR945. 

Fig. 52 Optimized geometries of the 7-azaindole homodimer in So, Si (normal species) and Si (tautomer) states The double proton exchange path leading to tautomeric conversion is denoted by arrows,Reprinted from M, Mukherjee, B. Bandyopadhyay and T. Chakraborty Ultraviolet and infrared spectroscopy of matrix-isolated 7-azaindole dimer Matrix effect on excited state tautomerization, Chem. Phys. Lett., 2012. 546. 74-79. Copyright (2012), with permission from Elsevier,...

Cyclic imino esters were originally developed as PET chain extenders (see Section 6.4.2) but the UV-stabilising ability of some members of the family was noted. The mechanism of energy dissipation is not totally clear, but may involve ring opening and closing cycles in the excited state to achieve a similar effect to the excited state tautomerism of other UV absorbers. 

Keywords On-the-lly dynamics simulation Excited-state proton transfer Excited-state tautomerization Water-assisted proton transfer Hydrogen bond rearrangement 7-Azaindole ADC(2)... 

Eased on aU those previous studies mentioned above, especially the isomerization of 7AI(H20)3 by Yu et al. [46], it occurred to us that the ab initio molecular dynamics simulations of 7AI(H20) with different isomers and different cluster sizes would be a very interesting way of testing the reaction paths for ultrafast ESPT, specially the role of the second hydration shell. Therefore, the purpose of this work is to carry out a systematic study of ESPT dynamics in 7AI(H20)i 5 complexes. The main goals are to determine the dynamic behavior of different water clusters around 7AI, the dependence of the ultra-fast excited-state tautomerization probability with the cluster size, the effect of hydrogen bond arrangement for complexes with the same number of water but different conformations, and the role played by water in different cluster sizes. 

Fig. 20. Schematic representation of the reaction coordinate for tryptophan fluorescence quenching induced by hydrogen transfer and aborted decarboxylation. The electronic nature of the Si surface changes character along the Si path due to two avoided crossings between jSi and S2 The first one occurs between the covalent state and the ionic La state along the reaction coordinate that interconverts the i9i-Min and. Si-Exc minima. The second one occurs between the ionic La state and the biradical Bi, state along the tautomerization coordinate that leads to the excited-state tautomerized form S -Taut. This point does not corresponds to a minimum on the potential-energy surface and it is found that evolution along a decarboxylation coordinate leads to a -Si /-So conical intersection, where efficient radiationless decay to the ground state takes place. The values of the relevant structural parameters are given in A. Data from Ref. 102.

Figure 3.10 Role of conjugation of extra phenyl rings in the excited-state tautomerization of 7-azaindole dimers (16 and 17) [48],...

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