Mild Synthesis of Fluorosolvatochromic and Acidochromic 3-Hydroxy-4-pyridylisoquinoline Derivatives from Easily Available Substrates

Abstract

The reaction of sodium cyanate with benzo­[b]­quinolizinium substrates at room temperature gave 3-hydroxy-4-pyridyl-isoquinoline derivatives in good yields. Presumably, the overall reaction proceeds through an ANRORC-type sequence, that is, addition of the nucleophile, ring opening, and ring closure. Preliminary photophysical investigation of the parent compound revealed a pronounced sensitivity of its emission properties toward solvent effects and the pH of the medium.

Fluorimetric Detection of Zn2+, Mg2+, and Fe2+ with 3-Hydroxy-4-Pyridylisoquinoline as Fluorescent Probe

“…And the emission bands do not vary strongly over the series of metal ions, with complexes of Zn 2+ and Fe 2+ showing the most pronounced deviations. As the shifts of the emission also do not differ strongly from the ones of uncomplexed 3 in MeOH (λ em = 519 nm) or DMSO solution (λ em = 529 nm) [39] it may be deduced that the emission of the complexes in MeOH originates mainly from the ligand with only marginal influence from the metal ion and that every efficient quenching by the metal cations is suppressed in this solvent, presumably by solvation of the cation or the complex. In contrast, the emission of particular complexes is significantly quenched in 2-PrOH and acetonitrile solutions, however, with completely different trends.…”

“…From the overall results we conclude that the 3-hydroxy-4- (3) is a promising starting poing for the further development of cation-sensitive chemosensors. In particular, the parent compound and derivatives thereof [39] are readily accessible so that the selectivity of the metal ion complexation as well as the emission properties of the resulting complexes may be fine-tuned by the attachment of appropriate donor or acceptor substituents.…”

“…Considering the fact, that the 4-pyridyl-3-hydroxyisoquinoline (3) has the same 2-(2hydroxyaryl)pyridine unit as the established ligands 1, 2a and 2b (Scheme 1), we reasoned that the former may also operate as an efficient ligand in metal-ion complexes. And based on our previous findings with 3 [39] we proposed that this complex formation is accompanied by a distinct change of its emission properties, so that it may be used as fluorescent probe for metal cations. To check this hypothesis we assessed the complexing ability of 3 toward selected metal ions by means of absorption and emission spectroscopy, and we investigated the effect of the complex formation on the emission properties.…”

“…Compound 3 was synthesized according to published procedure [39]. All commercially available chemicals were reagent-grade and used without further purification unless otherwise mentioned.…”

“…In this context, we have recently observed that the absorption and emission properties of the structurally resembling 4pyridyl-3-hydroxyisoquinoline (3) and its derivatives are highly dependent on their environment in solution [39], and others have assessed these properties theoretically [40]. Namely, the changes in solvent polarity and pH affect the tautomeric equilibrium of 3 which in turn leads to changes of the emission color and intensity.…”