N. Mchedlov-Petrossyan, T. Cheipesh, A. Roshal
Oct 17, 2019
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Journal
The journal of physical chemistry. A
Abstract
In this paper, we examined the fluorescent properties of 4'- and 5'-aminofluorescein, un-substituted fluorescein, and its 4'-nitro derivative in a set of solvent systems. Fluorescence lifetimes, quantum yields, time-resolved fluorescence spectra, and quantum-chemical calculations allowed clarifying the reasons of the emitting properties in this dye series. In water, the dianions R2- of aminofluoresceins are practically non-fluorescent; in alcohols the quantum yields are low. In DMSO, acetonitrile and other non hydrogen bond donor-solvents, the bright fluorescence of R2- ions is quenched either on adding small amounts of water which hydrate the carboxylate group or under conditions of protonation of this group (COO- COOH). The last observation is possible owing to the peculiarities of the tautomerism of the 5'-aminofluorescein monoanion, HR-, which exists in DMSO as an equilibrium mixture of a colorless lactone and colored 'phenolate' tautomer with ionized xanthenes moiety and unionized carboxylic group. By contrast, the R2- anion of 4'-nitrofluorescein demonstrates spectral behavior different from that of the amino derivatives. It practically does not emit in aprotic solvents, however, in alcohol or water media, its quantum yield increases to some extent. Such changing spectral properties were explained in terms of the excited-state inter-fragmental charge transfer.