Tautomerism in the Excited State

One way of proving this tautomeric reaction in the excited state is by measurement of fluorescence anisotropy depolarization. For porphycene, this turns out to be temperature dependent. From these measurements it could be demonstrated 

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Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

The keto-enol tautomerization in the excited triplet state of 2-methylacetophenone involves the transfer of an H atom in the CHO fragment... 

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...

Fig. 6 The 2-(2-hydroxyphenyl)benzothiazole (HBT) unit that represents the keto-enol equilibrium (tautomerism). Normally, the enol (keto) form is rather stable in the ground state (in the excited state), respectively...

The fluorescence spectrum of the nonsteroidal anti-inflammatory agent piroxicam 21 has been determined in a variety of solvents (Scheme 7) <1999PCP4213>. The key observations are that the molecule exists with a strong H-bond between the phenolic OH and the adjacent amide. A very high Stokes shift in the excited state was observed and attributed to the proton-transfer event (tautomerization) between the phenolic and amide oxygens (cf. 21 —>63). In the case of protic solvents, such as water, the open conformation 64 was observed. 

Sukhorukov et al. 99 measured the displacement of the near ultraviolet absorption bands of several cytosines upon transition from the neutral to the ionic form and evaluated the ionization constants in the excited state of the compounds. They concluded that on excitation there was no considerable shift in the tautomeric equilibrium of the forms 2 and 6 (in the excited state) = 1.55 x 104]. They also... 

Fluorescence properties of lumazine (3) and its 1- and 3-methyl derivatives were studied in aqueous solution and in dependence of the pH to obtain information about the possible tautomeric forms of the various ionic species. Emissions were found for the neutral form at 380 nm, the monoanion at 467 nm, the dianion at 483 nm, and the cation at 505 nm. From similar emissions of 3-methyllumazine (but different ones of the 1-methyl isomer), it was concluded that the emitting species is the A(8)-H phototautomer (34) resulting from an N-l to N-8 proton transfer in the excited state (Scheme 4) <87MI 718-08). 

The comparison between the several purine species discussed above suggests a fine tuning of the structure/tautomerism on the excited state electronic properties. Because several excited states of various polarisabilities are involved, solvation is expected to play a role on this dynamics. Complexes of DNA bases with various solvent or with other bases, easily isolated in the gas phase, are an ideal laboratory... 

Some experimental evidence for a DPT process in the excited state leading to tautomerization not observed in the ground state has been reported (Chang et al., 1980 Sepiol and Wild, 1982) for solutions of 1-azacarbazole in 3-methylpentane and mixed l-azacarbazole/7-azaindol solutions in the same solvent. For these systems the tautomeric forms that are unstable in the ground state are formed adiabatically from the excited state hydrogen-bonded dimers via a DPT and detected in the fluorescence spectra. These results are especially relevant to excited state DPT processes possible for nucleic acid base pairs. Clearly, further experimental and theoretical studies on heterocyclic dimer systems are needed. 

Photophysical experiments and theoretical calculations have been used to determine the tautomeric equilibrium between IH- and 2/f-indazoles (15a) and (15b) both in the ground (So) and in the excited state (S,) <94JPC10606>. The measurement of sublimation and solution enthalpies in water at 25°C, basicities in the excited state of 1-methylindazole (pAa = 1.61) and 2-methylindazole (pA a = 3.00), and MP2//6-31G //6-31G calculations of the difference in energy between their conjugated acids in the ground state allows the complete thermodynamic cycle to be established. Indazole in the excited state is 3.0 pA a units more acid (pA a = 1108) and 1.8 pK units more basic... 

Extended Hiickel calculations of the (hydroxy)quinolone 145 (X = NH R = Me) showed that while the tautomer 145a is more stable in the ground state, the tautomer 145b becomes the favored one in the excited state (80IJC(B)989). A similar keto-enol tautomerism was observed in fluorescence emission spectra of (hydroxy)quinolone 146 (98TAL1065). 

Mishra and coworkers have reported spectral oscillation in the absorption spectra of adenine solution in water when air was passed through the solution and exposed to UV-radiation intermittently over a long period of time. The main absorption peak near 260 nm (4.77 eV) was shifted to near 300 nm (4.13 eV) region which was explained in terms of the formation of the N3H tautomer of adenine,the process of which is closely related to the tautomerization of 7-azaindole. This assertian (the formation of the N3H tautomer in the excited state) is also supported fixim a recent theoretical study of adenine tautomers at the MCQDPT2/CASSCF levels of theory. ... 

R,. Under these conditions, DCI tautomerization followed by decomposition of hydroperoxides, formed in the excited state (Scheme 2, reactions 3 and 4), seems most probable. 

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