Proton-Induced Phase Transformations of Alkylated Diimine Platinum Complexes with Pyrazine Dithiolato

A series of platinum complexes, [Pt(pdt)(Rbpy)] (pdt = 2,3-pyrazine dithiolato; Rbpy = 4,4′-di-tert-butyl- (Ptpdt^Bu), 4,4′-dinonyl- (Ptpdt^C9), and 4,4′-bis(3-octyltridecyl)-2,2′-bipyridyl (Ptpdt^C8,10)), were synthesized and found to exhibit proton-responsive behavior in solution, crystalline, and liquid crystal (LC) states. All three complexes display distinct solvatochromic properties, ambipolar redox behavior, and reversible acid–base responsiveness. Upon treatment of Ptpdt^R with HCl in solution, monoprotonated species [Pt(Hpdt^R)(Rbpy)]⁺·Cl^– (R = Bu, C9, and C8,10) are formed, wherein each Hpdt ligand features a C═S, a C–S, and a protonated nitrogen atom. Interestingly, similar monoprotonation is also induced in the condensed phases (crystalline phase for R = Bu and C9, or smectic LC phase for R = C8,10) upon exposure to HCl vapor, resulting in [PtHpdt^R]⁺·Cl^– in the crystalline (R = Bu), low-crystalline (R = C9), and hexagonal columnar LC phases (R = C8,10). This work demonstrates a rare example of phase-dependent protonation behavior across molecular and supramolecular levels, offering a strategy for designing functional materials capable of dynamic phase modulation in response to acidic stimuli.