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 π* 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 (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 3ILCT* with k q values of 4 × 109 and 9 × 109 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.

Dithiolene dimetallic molybdenum(v) complexes displaying intraligand charge transfer (ILCT) emission

“…[33][34][35][36][37] Specifically considering dithiolene derived compounds, the group of Pilato has reported a series of complexes of the type (dppe)Pt{S 2 C 2 (heterocycle)(H)} showing well characterized ILCT absorptions and emissions (both from 1 ILCT* and 3 ILCT*). [38][39][40][41][42] For instance, the value reported for the ILCT absorption λ max of a 1,2-eneditiolate-quinoxaline complex with Pt(II) is 442 nm (CH 2 Cl 2 ), and the emission λ max from the 1 ILCT* is 636 nm (CH 2 Cl 2 ) 40 in close analogy with our values. The nature of the charge-transfer states in the aforementioned Pt(II) complexes has been described as 1,2enedithiolate → heterocycle which supports our description of the ILCT* state as dithiolene → bis(2-pyridyl) for the complexes herein reported.…”

“…Closely related luminescent complexes are reported to emit with significantly lower emission quantum yields (in the range 0.0006-0.06 but typically 0.01-0.02). [38][39][40][41][42] The Φ em value of 3 in acetonitrile lies between those for 1 2− and 2 2− . The low value of the emission quantum yield measured for 2 2− could be tentatively associated with a fast charge separation in the excited state induced by the bis-(2pyridyl) Re(I) complex, as reported for other Re(I) derivatives.…”

Pterin chemistry and its relationship to the molybdenum cofactor

“…N-heterocycle protonation also has a strong effect on electronic transitions in these complexes where the relative energies of d-based and ligand-based unoccupied orbitals can be switching, accessing luminescent emissive Inter Ligand Charge Transfer (ILCT) excited states [150]. Similar effects can be expected for complexes of pterin-dithiolenes, though parallel studies have not yet been accomplished.…”

Syntheses of Metal Dithiolene Complexes from Thiometalates by Induced Internal Redox Reactions

“…Some dithiolene complexes have been reported as conducting [6][7][8] and magnetic materials; 9,10 others as solution lumophores. [11][12][13][14][15][16] It has been established that all molybdenum enzymes, except for nitrogenase, and all tungsten enzymes contain the pterin cofactor designated as Mo-co (or W-co) in which a metal-dithiolene structural motif is present. 17,18 Nickel bis(dithiolene) complexes have been used as molecular models for the nickel site in the active center of hydrodesulfurization catalysts.…”