Protonation-state-dependent luminescence and excited-state electron-transfer reactions of 2- and 4-Pyridine (-ium)-substituted metallo-1,2-enedithiolates

Abstract

Luminescence and excited-state electron-transfer reactions of (dppe)Pt{S2C2(2-pyridine)(H)} and (dppe)Pt{S2C2(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 (pi)* intraligand charge-transfer transition (ILCT) becomes lower in energy than the d-to-d transition, thus giving rise to emissive 1ILCT* and 3ILCT* excited states for [(dppe)Pt{S2C2(2-pyridinium)(H)}][BF4] and [(dppe)Pt{S2C2(4-pyridinium)(H)}][BF4]. The assignment of these excited states was based on their energies and lifetimes (t) which range from t = 3 to 4 ns for the singlet and from t = 2000 to 7500 ns for the triplet, respectively. Emission quantum yields () increase with solvent polarity and range from = 0.0006 to 0.003 for the singlet and from = 0.001 to 0.03 for the triplet. The electron acceptors p-dinitrobenzene and tetracyanoquinodimethane quench the 3ILCT* with kq values of 4 × 109 and 9 × 109 M^-1 s^-1, respectively. The kq 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 (Refer to PDF file for exact formulas).