Intersystem-Crossing Dynamics in Heterodinuclear Polypyridyl-Bridged Complexes

Abstract

The excited-state properties of [(L)4MII(dpp)RuII(bpy)2](PF6) n (bpy = 2,2‘-bipyridine; dpp = (2,3-bis(2-pyridyl)pyrazine); (L)4MII = (NH3)4RuII (n = 4) or (CN)4FeII (n = 0)) have been investigated by steady-state and time-resolved picosecond and nanosecond transient luminescence and absorption spectroscopic methods. The [(NH3)4RuII(dpp)RuII(bpy)2]4+* complex is nonemissive in room-temperature H2O solution. Picosecond laser flash photolysis of [(NH3)4RuII(dpp)RuII(bpy)2]4+ reveals the formation of a transient species that decays exponentially (τ = 290 (±80) ps) independent of excitation wavelength. The excited-state behavior of [(NC)4FeII(dpp)RuII(bpy)2]* display a complicated dependence on excitation wavelength. Following 591-nm pulsed laser excitation into the 1[(dπ) (π*) ] ← 1[(dπ) ] metal-to-ligand charge transfer (MLCT) band, a short-lived transient is formed with τ < 80 ps. Laser excitation at 354.7 nm into the 1[(dπ) (π*) ] ← 1[(dπ) ] MLCT transition produces a transient species with τ > 200 ns. One explanation for the wavelength-dependent photophysics of the [(NC)4FeII(dpp)RuII(bpy)2] is that the FeII metal center undergoes a low- to high-spin interconversion following 1[(dπ) (π*) ] ← 1[(dπ) ] excitation, a process not accessible via direct 1[(dπ) (π*) ] ← 1[(dπ) ] excitation.

Photophysics and Excited-State Properties of Cyclometalated Iridium(III)–Platinum(II) and Iridium(III)–Iridium(III) Bimetallic Complexes Bridged by Dipyridylpyrazine

“…All of the complexes exhibited quasi-reversible reduction waves from −0.9 to −2.31 V, among which the first and second waves can be attributed to the dpp 0/– and dpp –/2– couples, respectively, as reported for related Ru­(dpp) complexes. ,,, The third reduction wave of the bimetallic complexes should be due to the C^N 0/– couple, ,,, whereas the monometallic counterparts ( 5 and 6 ) showed only two waves. Presumably, the dpp –/2– and C^N 0/– couples might be overlapped in the second wave of 5 and 6 .…”

“…While the total or extensive quenching of phosphorescence at room temperature might follow more or less the energy-gap law, we assumed that the thermally activated nonradiative decay process would participate in a manner associated with their photochemical behavior. A crossing to a metal-to-metal charge-transfer (MM′CT) state ,, or energy transfer from the main metal center to the other site , has been proposed to explain the weaker and shorter-lived emissions of ruthenium­(II)-based bimetallic complexes compared to their monometallic counterparts. Provided that the MM′CT state can be represented by a simple electronic structure of Ir IV -dpp-Pt I , its energy level would be roughly equal to E (Ir III/IV ) – E (Pt II/I ) above the ground state.…”

“…Among such systems, binuclear complexes with two metal centers connected by a π-conjugated bridging ligand provide a typical prototype that consists of molecular components with characteristic functions . A typical light-harvesting metal center that has been extensively investigated is Ru­(bpy) n (bpy = 2,2′-bipyridine and related ligands) because of its excellent photophysical and electrochemical properties, whereas the other metal center is usually a Rh III , Pt II , or Pd II -based unit that can catalyze chemical processes. , Bridging ligands that connect the two metal centers are typically 2,3-bis­(2-pyridyl)­pyrazine (dpp), 2,2′-bipyrimidine, and related polyazine ligands. Extensive studies have been performed, particularly by Brewer and co-workers, on the electrochemical, photophysical, and photocatalytic aspects of such bimetallic complexes.…”

Spectroscopic and Photophysical Properties of Complexes of 4‘-Ferrocenyl-2,2‘:6‘,2‘ ‘-terpyridine and Related Ligands

“…Electronic Coupling. The factors which influence the strength of electronic coupling between covalently bound donor−acceptor complexes have been the focus of intense study. ,− The strength of electronic coupling is reflected by the magnitude of the electronic matrix element, H RP . In the weak coupling limit where the magnitude of H RP is small, the donor and acceptor moieties are essentially isolated and the resulting donor−acceptor complex displays properties which are essentially a sum of the individual components.…”

Elucidation of Excited-State Properties of Bimetallic Ir(III)–Pt(II) Complexes with Conjugated Bridging Ligands

“…CV data is summarized in Table , from which their oxidation and reduction potentials versus Fc + /Fc were estimated. All complexes exhibited quasireversible reduction waves from −0.9 to −2.31 V, among which the first and second waves could be attributed to BL 0/– and BL –/2– couples (BL: bridged ligand, dpp and ppz ), respectively. ,,,,− The third reduction wave of the bimetallic complexes should be due to dfppy 0/– couple. ,,,,− The electrochemical oxidation of Ir- dpp and Ir- ppz gave quasireversible waves at 1.13 and 1.17 V, respectively, attributable to Ir III/IV couple. ,, However, the cathodic peak was considerably weaker than the anodic peak, suggesting involvement of a significant irreversible nature. Similarly, the bimetallic complexes revealed irreversible oxidation behaviors with different features.…”