Steady-State and Time-Resolved Investigations on Pyrene-Based Chemosensors

Two novel fluorescent probes bearing a single (<b>P</b>) and two (a podand-like structure, <b>L</b>) pyrene units derived from 1,5-bis­(2-aminophenoxy)-3-oxopentane have been synthesized and investigated in dioxane using UV–vis absorption, and steady-state and time-resolved (in a picosecond time scale) emission spectroscopy; in the gas phase, matrix-assisted laser desorption ionization mass spectrometry was employed. In dioxane, the absorption and emission spectra of <b>P</b> present a unique band with maxima at 361 and 392 nm, which have been associated with the monomer absorption and emission bands, respectively. In dioxane, for compound <b>L</b>, an additional band with a maximum at ∼525 nm is observed; upon the addition of water, an emissive band (with maxima varying from 405 to 490 nm) appears in both <b>P</b> and <b>L</b> spectra; this is discussed in terms of the emission of a species with charge character. Upon metal addition (Cu<sup>2+</sup>, Zn<sup>2+</sup>, and Ag<sup>+</sup>) to <b>P</b>, a gradual quenching effect of the monomer emission is observed and found to be more pronounced with Cu<sup>2+</sup>. In the case of <b>L</b>, upon the addition of metal cations, the long emission band (∼550 nm) decreases and the monomer emission band increases (with an isoemissive point at ∼450 nm) and no evidence for the intermediate band (at ∼405–490 nm) now exists. Time-resolved data in dioxane/water mixtures showed that for <b>P</b> and <b>L</b> these two fit double- and triple-exponential decay laws, respectively. With <b>P</b>, this has been attributed to a two-state system, which involves the monomer and a charged species, with its emission maxima varying with the polarity of the media (here mirrored by its dielectric constant), which can potentially be addressed to an exciplex-like species, whereas with <b>L</b>, it has been attributed to a three-state system involving, in addition to these two species, an excimer. From absorption and fluorescence excitation and time-resolved data, evidence is given for the presence of intramolecular dimer formation in the ground state.