Bulkiness Makes the Difference: Modulating Solid-State Emission of Anticoronavirus Active Pt(tpy)C(O)NHCH2R Complexes Featuring Adamantyl vs Propargyl R Groups.

Solid-state photoluminescence (PL) of Pt(II) terpyridine-type (tpy) complexes relies on intermolecular Pt···Pt metallophilic interactions, and it can be tuned through bulky substituents disturbing them. We took advantage of this property by designing the Pt(II) 2,2':6'2"-tpy complexes [L1PtCl]PF6 and [L2PtCl]PF6 bearing either propargyl or 1-adamantyl connected to the tpy ligand. At 293 K, PL spectra of both complexes are characterized by broadband featureless orange-red emission (λPL ≈ 650 nm) characteristic of 3MMLCT excited states. This is an indication of short Pt···Pt contacts between individual molecules in powder. The PL spectra of [L1PtCl]PF6 and [L2PtCl]PF6 are significantly different from each other at 77 K. The former complex emits at 688 nm while the latter at 665 nm. This can be rationalized by the less sterically bulky ligand L1 promoting the formation of larger aggregates, while the more bulky L2 hinders this behavior. Unlike [L2PtCl]PF6, the monohydrate [L2PtCl]PF6·H2O shows a vibronically resolved 3MMLCT+ 3LC emission in powder form at λPL = 567 nm at 293 K and 560 nm at 77 K. Such behavior indicates relaxation of the Pt···Pt contacts caused by trapped water molecules. [L2PtCl]PF6 shows superior activity to [L1PtCl]PF6 against the HCoV-OC43 and influenza A viruses.