title: Spectral response of 4-methyl-2,6-dicarbomethoxyphenol, an excited-state intramolecular proton-transfer probe in cyclohexane-ethanol mixtures: signatures of medium microheterogeneity.
abstract: In this paper, we explore the role of microscopic heterogeneity of the medium on the spectral response of an excited-state proton-transfer (ESIPT) probe, namely, 4-methyl-2,6-dicarbomethoxyphenol (CMOH) using steady-state and time-resolved emission spectroscopy. The mixtures of two solvents with widely different properties, viz., cyclohexane, a nonpolar, and ethanol, a polar protic solvent, were used as microheterogeneous media for spectroscopic studies. Dual fluorescence (normal and tautomer fluorescence) is observed in the nonpolar solvent (cyclohexane), while only a single peak is observed in the protic solvent, ethanol. The spectral responses of CMOH in the binary mixtures have been found to be dependent on the solvent composition and excitation wavelength. The emission spectral properties of CMOH in the cyclohexane-ethanol mixture have been seen to be superposition of spectral properties in their bulk counterparts, indicating the presence of microscopic heterogeneity in the system. A zwitterionic species of CMOH appears to have been detected in binary solvent mixtures with higher ethanol content only through low-energy excitations. The species is converted into an anionic species as excitation energy increases. Density functional theory calculations indicate that two intramolecularly hydrogen bonded rotamers of CMOH have a small energy difference. The formation of a hydrogen bonded 1:1 molecular cluster of CMOH with ethanol has been investigated in the ground state at the same level of theory. Our findings are expected to shed light on the mechanism of many acid-base reactions occurring in microscopically inhomogeneous media that often mimic many biologically relevant processes.
authors: Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India.
authors: Ramprasad
authors: R
authors: Misra
authors: S P
authors: SP
authors: Bhattacharyya
authors: Abhijit
authors: A
authors: Mandal
mesh_terms: D003510:Cyclohexanes / Q000737:chemistry*; D000431:Ethanol / Q000737:chemistry*; D005740:Gases / Q000737:chemistry; D006139:Guaiacol / Q000031:analogs & derivatives* / Q000737:chemistry; D011522:Protons; D012997:Solvents / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet
chemical_list: C569687:4-methyl-2,6-dicarbomethoxyphenol; D003510:Cyclohexanes; D005740:Gases; D011522:Protons; D012997:Solvents; D000431:Ethanol; C506365:Cyclohexane; D006139:Guaiacol