Rotamerism, Tautomerism, and Excited-State Intramolecular Proton Transfer in 2-(4'-N,N-Diethylamino-2'-hydroxyphenyl)benzimidazoles: Novel Benzimidazoles Undergoing Excited-State Intramolecular

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• 作者：Sonia R&iacute ; os Vá ; zquez ; M. Carmen R&iacute ; os Rodr&iacute ; guez ; Manuel Mosquera ; Flor Rodr&iacute ; guez-Prieto
• 刊名：Journal of Physical Chemistry A
• 出版年：2008
• 出版时间：January 24, 2008
• 年：2008
• 卷：112
• 期：3
• 页码：376 - 387
• 全文大小：342K
• 年卷期：v.112,no.3(January 24, 2008)
• ISSN：1520-5215

文摘

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 fluorescencespectroscopy. A solvent-modulated rotameric and tautomeric equilibrium is observed in the ground state for1, 2, and 3. In cyclohexane, these compounds mainly exist as a planar syn normal form, with the hydroxylgroup hydrogen-bonded to the benzimidazole N3. In ethanol, the syn form is in equilibrium with its planaranti rotamer (for 1 and 2), with the phenyl ring rotated 180ntities/deg.gif"> about the C2-C1' bond and with a nonplanarrotamer for compound 3. In aqueous solution, a tautomeric equilibrium is established between the anti normalform (or the nonplanar rotamer for 3) and the tautomer (with the hydroxyl proton transferred to thebenzimidazole N3). The syn normal form of these compounds undergoes in all the solvents an excited-stateintramolecular proton-transfer process from the hydroxyl group to the benzimidazole N3 to yield the excitedtautomer. 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 concludethat this excited-state conformational change experienced by the tautomer is associated with an intramolecularcharge 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.