Fluorescence of methylated derivatives of hydroxyphenylimidazopyridine. Resolution of strongly overlapping spectra and a new ESIPT dye showing very efficient radiationless deactivation.

The ground- and excited-state behaviour of the isomeric species 2-(2'-methoxyphenyl)imidazo[4,5-b]pyridine (1-OMe) and 2-(2'-hydroxyphenyl)-4-methylimidazo[4,5-b]pyridine (1-NMe) in neutral and acid media has been studied by UV-vis absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy. The new dye 1-NMe is non-fluorescent in neutral media except in trifluoroethanol, where it shows a very weak fluorescence. 1-NMe also exhibits highly solvent-dependent fluorescence intensity in acidic media. We propose that the neutral species experiences a fast excited-state intramolecular proton transfer (ESIPT), relaxing afterwards by intramolecular twisting associated with internal charge transfer (TICT) and subsequent very fast internal conversion of the proton-transferred TICT structure. The behaviour of 1-NMe in acidic media is explained by the existence of a ground-state tautomeric equilibrium between species with intramolecular hydrogen bonds N-HOH and NHO. The first type of tautomers dissociates at the hydroxyl group in water and ethanol, but fluoresces in acetonitrile and trifluoroethanol due to the inability of these solvents to accept the proton. The second type of tautomers is non-emissive due to fast radiationless deactivation through an ESIPT-TICT process. The fluorescence of 1-OMe was investigated in neutral and acidic media, demonstrating the photobasic character of the pyridine nitrogen. A ground-state equilibrium between pyridinium and imidazolium cations was found for this species, showing overlapping absorption and fluorescence spectra. We devised a method to resolve the spectra by applying principal component global analysis to a series of excitation spectra taken at different emission wavelengths, which allowed estimation of the equilibrium constant between the cations.

Rotamerism, tautomerism, and excited-state intramolecular proton transfer in 2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazoles: novel benzimidazoles undergoing excited-state intramolecular coupled proton and charge transfer.

The solvent and temperature dependence of the phototautomerization of 1-methyl-2-(2'-hydroxyphenyl)benzimidazole (4) and the novel compounds 2-(4'-amino-2'-hydroxyphenyl)benzimidazole (1), 2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (2), and 1-methyl-2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (3), together with the ground-state rotamerism and tautomerism of these new compounds, have been studied by UV-vis absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. A solvent-modulated rotameric and tautomeric equilibrium is observed in the ground state for 1, 2, and 3. In cyclohexane, these compounds mainly exist as a planar syn normal form, with the hydroxyl group hydrogen-bonded to the benzimidazole N3. In ethanol, the syn form is in equilibrium with its planar anti rotamer (for 1 and 2), with the phenyl ring rotated 180 degrees about the C2-C1' bond and with a nonplanar rotamer for compound 3. In aqueous solution, a tautomeric equilibrium is established between the anti normal form (or the nonplanar rotamer for 3) and the tautomer (with the hydroxyl proton transferred to the benzimidazole N3). The syn normal form of these compounds undergoes in all the solvents an excited-state intramolecular proton-transfer process from the hydroxyl group to the benzimidazole N3 to yield the excited tautomer. The tautomer fluorescence quantum yield of 2, 3, and 4 shows a temperature-, polarity-, and viscosity-dependent radiationless deactivation, connected with a large-amplitude conformational motion. We conclude that this excited-state conformational change experienced by the tautomer is associated with an intramolecular charge transfer from the deprotonated dialkylaminophenol or phenol (donor) to the protonated benzimidazole (acceptor), affording a nonfluorescent charge-transfer tautomer. Therefore, these compounds undergo an excited-state intramolecular coupled proton- and charge-transfer process.

Excited-state intramolecular proton transfer in 2-(3'-hydroxy-2'-pyridyl)benzoxazole. Evidence of coupled proton and charge transfer in the excited state of some o-hydroxyarylbenzazoles.

The influence of solvent, temperature, and viscosity on the phototautomerization processes of a series of o-hydroxyarylbenzazoles was studied by means of ultraviolet-visible (UV-vis) absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. The compounds studied were 2-(2'-hydroxyphenyl)benzimidazole (HBI), 2-(2'-hydroxyphenyl)benzoxazole (HBO), 2-(2'-hydroxyphenyl)benzothiazole (HBT), 2-(3'-hydroxy-2'-pyridyl)benzimidazole (HPyBI), and the new derivative 2-(3'-hydroxy-2'-pyridyl)benzoxazole (HPyBO), this one studied in neutral and acid media. All of these compounds undergo an excited-state intramolecular proton transfer (ESIPT) from the hydroxyl group to the benzazole N3 to yield an excited tautomer in syn conformation. A temperature- and viscosity-dependent radiationless deactivation of the tautomer has been detected for all compounds except HBI and HPyBI. We show that this radiationless decay also takes place for 2-(3-methyl-1,3-benzothiazol-3-ium-2-yl)benzenolate (NMeOBT), the N-methylated analog of the tautomer, whose ground-state structure has anti conformation. In ethanol, the radiationless decay shows intrinsic activation energy for HPyBO and HBO; however, it is barrierless for HBT and NMeOBT and controlled instead by the solvent dynamics. The relative efficiency of the radiationless decay in the series of molecules studied supports the hypothesis that this transition is connected with a charge-transfer process taking place in the tautomer, its efficiency being related to the strength of the electron donor (dissociated phenol or pyridinol moiety) and electron acceptor (protonated benzazole). We propose that the charge transfer is associated with a large-amplitude conformational change of the tautomer, the process leading to a nonfluorescent charge-transfer intermediate. The previous ESIPT step generates the structure with the suitable redox pair to undergo the charge-transfer process; therefore, an excited-state intramolecular coupled proton and charge transfer takes place for these compounds.

Two competitive routes in the lactim-lactam phototautomerization of a hydroxypyridine derivative cation in water: dissociative mechanism versus water-assisted proton transfer.

Ground-state tautomerism and excited-state proton-transfer processes of 2-(6'-hydroxy-2'-pyridyl)benzimidazolium in H2O and D2O have been studied by means of UV-vis absorption and fluorescence spectroscopy in both steady-state and time-resolved modes. In the ground state, this compound shows a tautomeric equilibrium between the lactim cation, protonated at the benzimidazole N3, and its lactam tautomer, obtained by proton translocation from the hydroxyl group to the pyridine nitrogen. Direct excitation of the lactam tautomer leads to its own fluorescence emission, while as a result of the increase of acidity of the OH group and basicity at the pyridine N upon excitation, the lactim species undergoes a proton translocation from the hydroxyl group to the nitrogen, favoring the lactam structure in the excited state. No fluorescence emission from the initially excited lactim species was detected due to the ultrafast rate of the excited-state proton-transfer processes. The lactim-lactam phototaumerization process takes place via two competitive excited-state proton-transfer routes: a one-step water-assisted proton translocation (probably a double proton transfer) and a two-step pathway which involves first the dissociation of the lactim cation to form an emissive intermediate zwitterionic species and then the acid-catalyzed protonation at the pyridine nitrogen to give rise to the lactam tautomer.