Pyrazolylthiourea Ligands for Tunable Solid-State Photoluminescence and Quenching-Based Zinc(II) Sensing

One new aminopyrazole, 1,3-di(pyridin-2-yl)-1H-pyrazol-5-amine (2), and three new thiourea-functionalized
compounds, 1-(4-methoxyphenyl)-3-(3-phenyl-1-(pyridin-2-yl)-1H-pyrazol-5-yl)thiourea (3), 1-(4-methoxyphenyl)-3-(3-pyridinyl-1-(pyridin-2-yl)-1H-pyrazol-5-yl)thiourea (4), and 1-(3-methoxyphenyl)-3-(3-pyridinyl-1-(pyridin-2-yl)-1H-pyrazol-5-yl)thiourea (5) were synthesized
and characterized. The crystal structures of all four aminopyrazole-based
frameworks, as well as a minor side-product, O-ethyl(3-phenyl-1-(pyridin-2-yl)-1H-pyrazol-5-yl)carbamothioate (6), and the
zinc(II) chloride complex with 4, were determined. For
each pyrazolylthiourea, all strong hydrogen-bond donors are utilized,
while the pyrazole nitrogen hydrogen-bond acceptors are unengaged.
This is attributed in part due to the formation of intermolecular R22(8) N–H···S hydrogen-bonded rings that incorporate
the weaker sulfur acceptor, leaving an open Lewis base site available
for metal coordination. Excitation and emission properties were evaluated
in both solution and solid-state for the three methoxyphenyl-substituted
ligands (3, 4, and 5) revealing
bright solid-state fluorescence for 3 and 4. Enhanced solid-state fluorescence was observed for 4 relative to 3 (Φsolid‑state = 7.63 vs 4.28%) consistent with increased molecular rigidity and
a higher proportion of N···H/H···N interactions
(9.7 vs 5.0% of the Hirshfeld surfaces). Upon coordination to Zn(II),
fluorescence from 4 was quenched, demonstrating its potential
as a metal-responsive sensor. DFT and TD-DFT calculations indicate
that quenching likely arises from a diminished transition dipole associated
with the ligand–ligand charge-transfer S1 state
of the Zn(II) complex.