Protonation-State-Dependent Luminescence and Excited-State Electron-Transfer Reactions of 2- and 4-Pyridine (-ium)-Substituted Metallo-1,2-enedithiolates

Luminescence and excited-state electron-transfer reactions of (dppe)Pt{S 2 C 2 (2-pyridine)(H)} and (dppe)Pt{S 2 C 2 (4-pyridine)(H)} (dppe = diphenyldiphosphinoethane) are enabled by protonation of the appended pyridine, thus serving as a novel means of electronic switching. The neutral complexes have low-lying d-to-d transitions that lead to rapid decay of excited states by nonradiative processes. However, upon protonation, a 1,2-enedithiolate-to-heterocycle π* intraligand charge-transfer transition (ILCT) becomes lower in energy than the d-to-d transition, thus giving rise to emissive 1 ILCT* and 3 ILCT* excited states for [(dppe)Pt{S 2 C 2 (2-pyridinium)(H)}][BF 4 ] and [(dppe)Pt{S 2 C 2 (4-pyridinium)(H)}][BF 4 ]. The assignment of these excited states was based on their energies and lifetimes (τ) which range from τ = 3 to 4 ns for the singlet and from τ = 2000 to 7500 ns for the triplet, respectively. Emission quantum yields (φ) increase with solvent polarity and range from φ = 0.0006 to 0.003 for the singlet and from φ = 0.001 to 0.03 for the triplet. The electron acceptors p -dinitrobenzene and tetracyanoquinodimethane quench the 3 ILCT* with k q values of 4 × 10 9 and 9 × 10 9 M - 1 s - 1, respectively. The k q values are nearly identical for the 2- and 4-pyridinium complexes, reflecting the similarity in the thermodynamic driving forces for electron transfer from these complexes. The ability to employ a simple and reversible ground-state reaction (ligand protonation) to control access to reactive excited states should prove useful in numerous applications.