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

One new aminopyrazole, 1,3-di(pyridin-2-yl)-1 H -pyrazol-5-amine ( 2 ), and three new thiourea-functionalized compounds, 1-(4-methoxyphenyl)-3-(3-phenyl-1-(pyridin-2-yl)-1 H -pyrazol-5-yl)thiourea ( 3 ), 1-(4-methoxyphenyl)-3-(3-pyridinyl-1-(pyridin-2-yl)-1 H -pyrazol-5-yl)thiourea ( 4 ), and 1-(3-methoxyphenyl)-3-(3-pyridinyl-1-(pyridin-2-yl)-1 H -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)-1 H -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 R 2 2 (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 (Φ solid-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 S 1 state of the Zn(II) complex.