title: Dual fluorescence of ellipticine: excited state proton transfer from solvent versus solvent mediated intramolecular proton transfer.
abstract: Photophysical properties of a natural plant alkaloid, ellipticine (5,11-dimethyl-6H-pyrido[4,3-b]carbazole), which comprises both proton donating and accepting sites, have been studied in different solvents using steady state and time-resolved fluorescence techniques primarily to understand the origin of dual fluorescence that this molecule exhibits in some specific alcoholic solvents. Ground and excited state calculations based on density functional theory have also been carried out to help interpretation of the experimental data. It is shown that the long-wavelength emission of the molecule is dependent on the hydrogen bond donating ability of the solvent, and in methanol, this emission band arises solely from an excited state reaction. However, in ethylene glycol, both ground and excited state reactions contribute to the long wavelength emission. The time-resolved fluorescence data of the system in methanol and ethylene glycol indicates the presence of two different hydrogen bonded species of ellipticine of which only one participates in the excited state reaction. The rate constant of the excited state reaction in these solvents is estimated to be around 4.2-8.0 × 10(8) s(-1). It appears that the present results are better understood in terms of solvent-mediated excited state intramolecular proton transfer reaction from the pyrrole nitrogen to the pyridine nitrogen leading to the formation of the tautomeric form of the molecule rather than excited state proton transfer from the solvents leading to the formation of the protonated form of ellipticine.
authors.affiliation: School of Chemistry, University of Hyderabad, Hyderabad 500 046, India.
authors.forename: Sanghamitra
authors.initials: S
authors.lastname: Banerjee
authors.forename: Ashok
authors.initials: A
authors.lastname: Pabbathi
authors.forename: M Chandra
authors.initials: MC
authors.lastname: Sekhar
authors.forename: Anunay
authors.initials: A
authors.lastname: Samanta
mesh_terms: D000970:Antineoplastic Agents / Q000737:chemistry* / Q000494:pharmacology; D001932:Brain Neoplasms / Q000188:drug therapy; D004611:Ellipticines / Q000737:chemistry* / Q000494:pharmacology; D005453:Fluorescence; D006801:Humans; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D015394:Molecular Structure; D009584:Nitrogen / Q000737:chemistry; D011522:Protons; D011725:Pyridines / Q000737:chemistry; D012995:Solubility; D012997:Solvents / Q000737:chemistry*; D013050:Spectrometry, Fluorescence
chemical_list: D000970:Antineoplastic Agents; D004611:Ellipticines; D011522:Protons; D011725:Pyridines; D012997:Solvents; C034192:ellipticine; D009584:Nitrogen