Asymmetrically Substituted and π‑Conjugated 2,2′-Bipyridine Derivatives: Synthesis, Spectroscopy, Computation, and Crystallography

A new
series of monosubstituted styryl- and bistyryl-2,2′-bipyridine
luminophores (compounds 16–23) have
been synthesized via Horner–Wadsworth–Emmons reaction
involving a monophosphonate and donor aromatic aldehydes. In the title
chromophores, the amino donors are varied between acyclic and cyclic
while the alkoxy donors are varied in terms of their number and position.
The absorption maxima of these chromophores shift predominantly due
to intramolecular charge transfer (ICT) between different donor and
acceptor moieties. The title donor–acceptor molecules exhibit
intense fluorescence in solution at room temperature, and their emissive
behavior has been found to be highly sensitive to solvent polarity.
The fluorescence spectra and quantum yields of all the chromophores
were recorded in four different solvent media, and the chromophores 16, 17, 19, and 21 exhibit
fluorescence in the solid state too. The influence of the nature and
position of the donor functionalities in the conjugated backbone of
the bipyridine moiety on the electronic absorption properties of the
title chromophores (16–23) has been
demonstrated, which has further been corroborated by DFT and TD-DFT
computation both in gas phase and in solution phase. The crystal structure
of compound 18 has been described as a representative
member of the family (16–23).