Tunable photophysical properties of phenyleneethynylene based bipyridine ligands

Abstract

A bipyridine-based system with phenyleneethynylene at the 4,4′ positions (1) and its p-methyl (2) and p-methoxy (3) substituted derivatives were synthesized via Sonogashira coupling reactions. The photophysical properties of 1–3 and their related H⁺ and Zn²⁺ adducts (1:H⁺–3:H⁺ and 1:Zn²⁺–3:Zn²⁺) were investigated, as a function of solvent polarity, by using steady-state and time-resolved spectroscopic techniques. Molecular systems 1–3 exhibit trans conformation, whereas adducts with H⁺ and Zn²⁺ are conformationally locked cis species. The unsubstituted compound 1 emits at 360 nm with low fluorescence quantum yield (ϕ_fl = 0.2%) regardless of the solvent polarity. Fluorescence spectra of 2 and 3 are bathochromically shifted in polar solvents, and the p-methoxy (3) derivative possesses ϕ_fl as high as 12%. Complexation of 1–3 with H⁺ or Zn²⁺ in acetonitrile causes red-shift of the lowest energy absorption bands, whereas dramatic changes of the emission properties are found as a function of the electron donating ability of the substituents on the phenyleneethynylene moiety (–CH₃ or –OCH₃), suggesting a charge-transfer character of the lowest electronic transition of 1–3. 1:H⁺, 1:Zn²⁺, 2:H⁺ and 2:Zn²⁺ exhibit intense fluorescence with ϕ_fl up to 33% (1:Zn²⁺) whilst 3:H⁺ and 3:Zn²⁺ are found to be weakly emissive. The singlet radiative and non-radiative rate constants of compounds and complexes were determined, along with triplet parameters, via phosphorescence and transient absorption spectroscopy. More conclusive evidence regarding the protonation of bipyridine nitrogen atoms of compounds 1–3 were obtained through ¹H NMR titration studies. These studies indicate that the conjugate molecular systems based on 2,2′-bipyridine and phenyleneethenylenes possess tunable optical properties which can be further utilized for preparing organic and inorganic luminophores with potential application in optoelectronic systems