307 nm for both TBI and MTBI) were selected taking into account the absorption
spectra of the compounds in ethanol, the time-dependent DFT (TD-DFT) calculated
UV spectra for the two tautomeric forms of the compounds, as well as literature
data for similar compounds [11, 12, 33, 34].
In the result of the performed irradiations, the thione tautomers were converted
into the corresponding thiol forms (Fig. 7.6). Subsequent irradiation of the matrices
with shorter wavelength UV light (246 nm) successfully converted the previously
generated thiol tautomers of both benzo-substituted molecules (TBI, MTBI) back
to the corresponding thione forms (Fig. 7.6). On the other hand, the thiol ! thione
photochemical back conversion could not be induced for MTI, upon irradiation
with UV light of wavelengths ! 230 nm.
On the whole, the experimental results clearly demonstrate that the two tautomers of the benzo-substituted mercaptoimidazoles can be photochemically
interconverted in a selective way, with appropriate choice of the excitation wavelength, i.e., the populations of the two tautomers can be optically controlled in an
efficient manner through UV-driven reactions taking place under different excitation
conditions. Another important observation is that both tautomers were found to
show great stability regarding photodecomposition (no signals of photodecomposition products could be observed in the spectra along all performed irradiation
experiments), what makes these chemical systems promising candidates for acting
as molecular switches.
The observed tautomeric reactions could be rationalized in terms of the
PhotoInduced Detachment Association mechanism (PIDA), first proposed in the
theoretical studies of Chmura et al. [35]. This mechanism involves the hydrogen
photodetachment and subsequent recombination of the radical species formed upon
excitation. According to the PIDA mechanism, the thiyl radical is the common
intermediate species in the thione ! thiol and thiol ! thione phototautomerizations. Absorption of the UV light photons takes the reactant species to an excited
state of (n/p)p* type, with subsequent internal conversion to a singlet state of (n/p)r
* type, leading to dissociation through cleavage of the NH or SH bonds, thus
generating a hydrogen atom and the corresponding thiyl radical. The recombination
of these might lead to the original species or attachment of the H atom to the S atom
or N atom (depending if one is considering the thione ! thiol or the inverse
process) yielding the corresponding photoproduct.
After formation of the intermediate radical, its subsequent recombination with
the H atom to form the final products can be expected to follow a product branch
that should depend essentially on the relative stability of the products, which favors
the thione tautomer and justifies the observed higher efficiency of the thiol !
thione process compared to the thione ! thiol isomerization. Note also that results
of DFT calculations support the proposed mechanism, since for both direct and
reverse tautomerizations, the energy of the radical pair stays below the excitation
energies. For example, in the case of TBI [12], for the thione ! thiol isomerization
E[(radical + H) − thione] = 381 kJ mol
−1 and the used excitation wavelength was
7 Hydantoins and Mercaptoimidazoles: Vibrational …
209
spectra of the compounds in ethanol, the time-dependent DFT (TD-DFT) calculated
UV spectra for the two tautomeric forms of the compounds, as well as literature
data for similar compounds [11, 12, 33, 34].
In the result of the performed irradiations, the thione tautomers were converted
into the corresponding thiol forms (Fig. 7.6). Subsequent irradiation of the matrices
with shorter wavelength UV light (246 nm) successfully converted the previously
generated thiol tautomers of both benzo-substituted molecules (TBI, MTBI) back
to the corresponding thione forms (Fig. 7.6). On the other hand, the thiol ! thione
photochemical back conversion could not be induced for MTI, upon irradiation
with UV light of wavelengths ! 230 nm.
On the whole, the experimental results clearly demonstrate that the two tautomers of the benzo-substituted mercaptoimidazoles can be photochemically
interconverted in a selective way, with appropriate choice of the excitation wavelength, i.e., the populations of the two tautomers can be optically controlled in an
efficient manner through UV-driven reactions taking place under different excitation
conditions. Another important observation is that both tautomers were found to
show great stability regarding photodecomposition (no signals of photodecomposition products could be observed in the spectra along all performed irradiation
experiments), what makes these chemical systems promising candidates for acting
as molecular switches.
The observed tautomeric reactions could be rationalized in terms of the
PhotoInduced Detachment Association mechanism (PIDA), first proposed in the
theoretical studies of Chmura et al. [35]. This mechanism involves the hydrogen
photodetachment and subsequent recombination of the radical species formed upon
excitation. According to the PIDA mechanism, the thiyl radical is the common
intermediate species in the thione ! thiol and thiol ! thione phototautomerizations. Absorption of the UV light photons takes the reactant species to an excited
state of (n/p)p* type, with subsequent internal conversion to a singlet state of (n/p)r
* type, leading to dissociation through cleavage of the NH or SH bonds, thus
generating a hydrogen atom and the corresponding thiyl radical. The recombination
of these might lead to the original species or attachment of the H atom to the S atom
or N atom (depending if one is considering the thione ! thiol or the inverse
process) yielding the corresponding photoproduct.
After formation of the intermediate radical, its subsequent recombination with
the H atom to form the final products can be expected to follow a product branch
that should depend essentially on the relative stability of the products, which favors
the thione tautomer and justifies the observed higher efficiency of the thiol !
thione process compared to the thione ! thiol isomerization. Note also that results
of DFT calculations support the proposed mechanism, since for both direct and
reverse tautomerizations, the energy of the radical pair stays below the excitation
energies. For example, in the case of TBI [12], for the thione ! thiol isomerization
E[(radical + H) − thione] = 381 kJ mol
−1 and the used excitation wavelength was
7 Hydantoins and Mercaptoimidazoles: Vibrational …
209