Spirobifluorene Bridged Ir(III) and Os(II) Polypyridyl Arrays: Synthesis, Photophysical Characterization, and Energy Transfer Dynamics

Abstract

The synthesis, characterization, photophysics, and time-dependent density functional theory (TD-DFT) calculations of spirobifluorene-bipyridine based iridium(III), osmium(II), and mixed Ir/Os complexes are presented. The preparation of the reference and mixed complexes proceeded step-by-step and microwave irradiation facilitated the complexation of osmium. The absorption of the target heterobimetallic derivative, Ir-L-Os, is described by linear combination of half of the absorption spectra of the homobimetallic analogues, Ir-L-Ir and Os-L-Os, due to the occurrence of mixed ligand and metal based transitions when the spirobifluorene-(bpy)(2) bridging ligand L is linked to the metal, confirming a negligible interaction between the substituted metallic chromophores. TD-DFT calculations on monometallic, homo- and hetero-bimetallic complexes fully disentangled the origin of the absorption features. Noticeably, in the mixed Ir-L-Os complex an almost quantitative energy transfer from the (3)Ir to the (3)Os MLCT state is occurring, with a rate constant of 4.1 × 10(8) s(-1) and nearly exclusively via a Dexter-type mechanism mediated by the orbitals of the spiroconjugated ligand. This result, together with the outcomes of the TD-DFT calculations, supports the existence of spiroconjugation and evidences the interesting role of this kind of bridge in the energy transfer dynamics of the arrays. In all the complexes, moreover, the ligand fluorescence is heavily quenched by energy transfer processes toward the metallic appended units; the rate constant is estimated in the order of 10(10) s(-1) for Ir-L-Os and higher than 10(12) s(-1) for the other complexes. In the heterometallic array, both at room temperature and at 77 K, all photons are thus funneled to the emissive Os (3)MLCT state, which acts as energy trap for the antenna cascade.

Vinyl Ruthenium-Modified Biphenyl and 2,2′-Bipyridines

“…Bipy ligands with extended π-conjugation such as 5-ethynyl-2,2′-bipyridine have been extensively used for such purposes as exemplified by the recent work of Ziessel and co-workers. − Some of us have recently reported on σ-alkynyl ruthenium complexes of the latter ligand, its coordination behavior to tris­(ketoenolato) lanthanide ions (Eu, Yb, Nd), and the on/off switching of Yb and Nd near-infrared (NIR) luminescence on oxidizing/reducing the electron-rich ethynyl ruthenium appendices. , Similar complexes of dysprosium perform as single molecular magnets whose magnetic relaxation can be further slowed down by oxidation of the remote alkynyl ruthenium sites. , Other authors have used gold or ruthenium alkynyl complexes derived from 5-ethynyl-2,2′-bipyridine as coordinating tectons for the construction of heterometallic complexes with up to six different metal atoms. ,− Building on the general interest in the electrochemical and electronic properties of bridged bis­(alkenylruthenium) complexes {Ru}-CHCH-π-linker-CHCH-{Ru} ({Ru} = Ru­(CO)­Cl­(P i Pr 3 ) 2 , − Ru­(CO)­Cl­(PMe 3 ) 3 − ), we were drawn to using 2,2′-bipy as the bridge by the prospect of being able to manipulate the electron density on that bridge and the degree of coplanarity of the interconnected pyridyl rings by coordination of an additional metal fragment to the diimine donor sites . In the course of that work, we obtained several pieces of evidence for significant ligand contributions to the oxidations of the 5,5′-divinyl-2,2′-bipyridine-bridged diruthenium complex 2 and the 5-vinyl-2,2′-bipyridine-derived complex 3 .…”

Multichromophoric Arrays Arranged around a Triptycene Scaffold: Synthesis and Photophysics

“…Notably, in the spectral portion between 300 and 400 nm, a new band associated with an 1 LC triptycene absorption transition appears. This transition stems from the coordination by the metal center of the ethynylene-linked bpy site, which is otherwise unoccupied in the free ligands, L1 and L2″ , similarly to what was previously observed for a spirobifluorene-based system . The spectrum of the dyads matches reasonably well the superposition of the spectra of the single components (Figure SI16); the only exception to an otherwise good superimposition is in the range between 300 and 400 nm, where the effect of new bpy–metal interactions introduces some distortions.…”

Photophysical study of spiro-bifluorene bridged Pt(ii), Os(ii) and Ir(iii) luminescent complexes and supramolecular arrays

“…This 1 LC spirobifluorene absorption transition is assigned, in particular, to the coordination by the metal center of bpy subunits bearing the ethynylene linker, which is otherwise unoccupied in the free ligand, L-TMS, in agreement with what was previously observed for a similar spiro-bifluorene based system. 8 Another band is placed in the spectral region between 430 and 550 nm and is originated by spin-allowed transitions of mixed metalto-ligand ( 1 MLCT) charge transfer character, related to the transitions of Os(bpy) 3 type moiety. At last, the weakest and broadest band that appears just like a tail in the range between 550 nm and 730 nm is associated to a formally spinforbidden 3 MLCT set of transitions, induced by the presence of the heavy metal.…”

“…6 Similar ligands retain attractive stereochemical properties and interesting electronic features due to the "spiroconjugation" of the spirobifluorene unit 7 and are well suited for the design of mixed-metal complexes where photoinduced intramolecular energy transfer between the ligand and the metal centers can occur. 8 On the other hand, alkynyl Pt(II) complexes with a square-planar coordination geometry exhibit intriguing luminescent properties and polyimine complexes incorporating σ-alkynyl ligands, in particular, are well-known for their rich photochemistry. 9 As such, the rigid nature of the connecting polytopic ligand combined with the peculiar geometry of Pt(II) complexes provides a rare opportunity to explore the mechanisms of the energy-transfer steps.…”