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.