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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.
J Phys Chem A. 2008 Jan 24; 112(3):376-87.JP

Abstract

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.

Authors+Show Affiliations

Departamento de Química Física, Facultade de Química, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain.No affiliation info availableNo affiliation info availableNo affiliation info available

Pub Type(s)

Journal Article
Research Support, Non-U.S. Gov't

Language

eng

PubMed ID

18154323

Citation

Vazquez, Sonia Ríos, et al. "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 Journal of Physical Chemistry. A, vol. 112, no. 3, 2008, pp. 376-87.
Vazquez SR, Rodríguez MC, Mosquera M, et al. 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. J Phys Chem A. 2008;112(3):376-87.
Vazquez, S. R., Rodríguez, M. C., Mosquera, M., & Rodríguez-Prieto, F. (2008). 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 Journal of Physical Chemistry. A, 112(3), 376-87.
Vazquez SR, et al. 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. J Phys Chem A. 2008 Jan 24;112(3):376-87. PubMed PMID: 18154323.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - 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. AU - Vazquez,Sonia Ríos, AU - Rodríguez,M Carmen Ríos, AU - Mosquera,Manuel, AU - Rodríguez-Prieto,Flor, Y1 - 2007/12/23/ PY - 2007/12/25/pubmed PY - 2008/3/1/medline PY - 2007/12/25/entrez SP - 376 EP - 87 JF - The journal of physical chemistry. A JO - J Phys Chem A VL - 112 IS - 3 N2 - 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. SN - 1089-5639 UR - https://www.unboundmedicine.com/medline/citation/18154323/Rotamerism_tautomerism_and_excited_state_intramolecular_proton_transfer_in_2__4'_NN_diethylamino_2'_hydroxyphenyl_benzimidazoles:_novel_benzimidazoles_undergoing_excited_state_intramolecular_coupled_proton_and_charge_transfer_ L2 - https://doi.org/10.1021/jp076634a DB - PRIME DP - Unbound Medicine ER -