Understanding the Linear and Second-Order Nonlinear Optical Properties of UiO-66-Derived Metal–Organic Frameworks: A Comprehensive DFT Study

Periodic
density functional theory calculations have been performed
to investigate the structures, the linear and second order nonlinear
optical (NLO) properties of Zr-based UiO-66 metal–organic frameworks,
in which the 1,4-benzene-dicarboxylate (BDC) linkers are functionalized
by a series of isovalent substituents including −NH2, −OH, −SH, and halogen atoms. Our results show that
the mixed phase will be formed for the case of the incorporation of
a single hydroxyl or halogen atom, while it is more likely to synthesize
the pure phase for the homodifunctionalized compounds. The ligand
functionalization results in the appearance of the band gap states
as well as the narrowing of the band gap. For the linear optical response,
besides the redshift of the absorption edge, the ligand modification
tends to increase the dielectric constant and enhance the optical
anisotropy of UiO-66, especially when two −SH groups are incorporated
into the BDC ligand. With regard to the NLO activity, the second harmonic
generation (SHG) intensity of the pristine UiO-66 is comparable to
that of KDP. The SHG response of UiO-66 can be effectively improved
by the single-site substitution, and the obvious enhancement of the
SHG activity is observed after introducing the sulfydryl group or
the iodine atom. However, it is noted that the difunctionalizations
by two −OH and −SH have a relatively weak impact on
the SHG intensity because of the nearly centrosymmetric organization
of the homodifunctionalized organic linkers. Among all derivatives,
UiO-66-(SH)2 is the most promising candidate for use as
a near-infrared NLO material with a strong SHG effect (>20 pm/V)
and
good phase matchability. Furthermore, the primary origin of the SHG
response for UiO-66 and its derivatives is determined by analyzing
the band structure. This study reveals the possibility of designing
NLO materials with outstanding performances based on the UiO-66-derived
MOFs through rational functionalization of the organic linking unit.