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Pyrene bichromophores composed of polyaminopolycarboxylate interlink: pH response of excimer emission

Fluorescent response to pH has been studied on water-soluble pyrene-based bichromophores, (edtapy)H2 and (dtpapy)H3, in which two pyrenyl groups are linked by an ethylenediaminetetraacetate (EDTA) and a diethylenetriaminepentaacetate (DTPA) unit, respectively, through amide linkages. The excimer emission of the EDTA derivative is strengthened sharply with increasing pH at two steps; the first step is associated with the dissociation of acidic hydrogen from amino nitrogen in partially protonated species (edtapy)H− and the second step is attributable to the amide group. The excitation spectra have evidenced the formation of a static excimer in the ground state of completely deprotonated species (edtapy)2 − . The close contact between pyrenyl groups in (edtapy)2 − has been confirmed by density functional theory, which has also shown that the close contact is broken when amino nitrogen is protonated. The DTPA derivative exhibits a strong excimer emission, which shows an intensity–pH profile of an ‘off–on–off–on’ type. This rare pH response is ascribable to multiple protonation sites in the DTPA chain, as confirmed by 1H NMR. The novel pH-sensing capabilities in specific pH regions are due to the combination of the fluorescent group with the polyaminopolycarboxylate chains whose conformations are reversibly altered by protonation–deprotonation processes.