title: Unusually slow intramolecular proton transfer dynamics of 4'-N,N-dimethylamino-3-hydroxyflavone in high n-alcohols: involvement of solvent relaxation.
abstract: Excited state intramolecular proton transfer (ESIPT) time-constants of 4'-N,N-dimethylamino-3-hydroxyflavone (DMA3HF) in high n-alcohols--1-butanol, 1-hexanol and 1-decanol--were measured to be 90 ps, 130 ps and 190 ps, respectively, which are unusually slow. At the same time, the solvation time-constants of the DMA3HF enol in the same set of solvents were measured as 100 ps, 150 ps and >300 ps, respectively. Thus, both the ESIPT and enol solvation time-constants in high n-alcohols increase monotonically with the alkyl chain-length of the solvent, although the increase is not strictly proportional. It appears that the H-bonding capacity of the solvent is the single major factor influencing both processes, causing them to become closely correlated. Solvation causes a drastic change in the solvent molecular configuration around the excited enol, E*, inducing the breakage of DMA3HF···solvent inter-molecular H-bonding, which in turn promotes ESIPT. Following previously reported theoretical work on ESIPT, a qualitative description of the S1 potential energy surface can be formulated, where the involvement of solvent relaxation with the ESIPT process is explained.
authors.affiliation: Department of Chemistry, University College of Science & Technology, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India. dmandal.chemistry@gmail.com.
authors.forename: Deborin
authors.initials: D
authors.lastname: Ghosh
authors.forename: Shaikh
authors.initials: S
authors.lastname: Batuta
authors.forename: Naznin Ara
authors.initials: NA
authors.lastname: Begum
authors.forename: Debabrata
authors.initials: D
authors.lastname: Mandal
mesh_terms: D020001:1-Butanol / Q000737:chemistry*; D000586:Amination; D005233:Fatty Alcohols / Q000737:chemistry*; D005419:Flavonoids / Q000737:chemistry*; D000441:Hexanols / Q000737:chemistry*; D007700:Kinetics; D008745:Methylation; D008958:Models, Molecular; D011522:Protons; D012997:Solvents / Q000737:chemistry; D013050:Spectrometry, Fluorescence
chemical_list: D005233:Fatty Alcohols; D005419:Flavonoids; D000441:Hexanols; D011522:Protons; D012997:Solvents; C036260:1-hexanol; C029383:n-decyl alcohol; D020001:1-Butanol; C041477:3-hydroxyflavone