Substituent-tuned structure and luminescence sensitizing towards Al3+ based on phenoxy bridged dinuclear EuIII complexes

Abstract

To develop a Ln^III complex-supported chemsensor, two new phenoxy bridged dinuclear Eu^III complexes, [Eu₂(H₂L)₃(NO₃)₃]·3CH₃CN (EuL) and [Eu₂(H₂L′)₂(NO₃)₄]·3CH₃CN (EuL′), constructed by two new structurally related salicylamide salen-like ligands, 1-(2-hydroxy-benzamido)-2-(2-hydroxy-5-nitro-benzylideneamino)-ethane (H₃L) and 1-(2-hydroxy-benzamido)-2-(2-hydroxy-4-diethylamino-benzylideneamino)-ethane (H₃L′), have been synthesized and structural analysis shows that the different substitution groups on 2-(iminomethyl)phenol moiety have significant effects on their structures. Upon excitation of the ligand-centered absorption band at 375 nm, emissions both originating from ligands and Eu^III ions were observed in the two Eu^III compounds with the Eu^III-centered emission intensity more than three times higher than that of ligand-centered emission. The capability of EuL and EuL′ for selective detection Al³⁺ ions were evaluated and the results indicate EuL exhibits a turn-on luminescent enhancement as high as 5.7 fold with K_d = 1.53 × 10⁻⁴ in CH₃CN, but comparable compound EuL′ could not detect Al³⁺ among various cations. The considerably ‘turn-on’ luminescence response of EuL concomitantly led to the apparent color change from reddish to brilliant red, which could also be identified by naked eyes easily under UV lamp. This luminescence enhanced response can be explained in terms of the decrease of non-radiative transitions in EuL in addition to excited-state intra-molecular proton transfer (ESIPT) and photo-induced electron transfer suppression upon Al³⁺ coordination which is also rationalized by a theoretical calculation.