Three Reversible Polymorphic Copper(I) Complexes Triggered by Ligand Conformation: Insights into Polymorphic Crystal Habit and Luminescent Properties

Three luminescent polymorphs based
on a new copper­(I) complex Cu­(2-QBO)­(PPh3)­PF6 (1, PPh3 = triphenylphosphine, 2-QBO = 2-(2′-quinolyl)­benzoxazole)
have been synthesized and characterized by FT-IR, UV–vis, elemental
analyses, and single-crystal X-ray diffraction analyses. Each polymorph
can reversibly convert from one to another through appropriate procedures.
Interestingly, such interconversion can be distinguished by their
intrinsic crystal morphologies and colors (namely α, dark yellow
plate, β, orange block, γ, light yellow needle) as well
as photoluminescent (PL) properties. X-ray crystal structure analyses
of these three polymorphs show three different supramolecular structures
from 1D to 3D, which are expected to be responsible for the formation
of three different crystal morphologies such as needle, plate, and
block. Combination of the experimental data with DFT calculations
on these three polymorphs reveals that the polymorphic interconversion
is triggered by the conformation isomerization of the 2-QBO ligand
and can be successfully controlled by the polarity of the process
solvents (affecting the molecular dipole moment) and thermodynamics
(affecting the molecular total energy). It is also found that the
different crystal colors of polymorphs and their PL properties are
derived from different θ values (dihedral angle between benzoxazolyl
and quinolyl group of the 2-QBO ligand) and P–Cu–P angles
based on TD-DFT calculations. Moreover, an interesting phase interconversion
between γ and β has also been found under different temperature,
and this result is consistent with the DFT calculations in which the
total energy of β is larger than that of γ. This polymorphism
provides a good model to study the relationship between the structure
and the physical properties in luminescent copper­(I) complexes as
well as some profound insights into their PL properties.