Molecular Dyads of Ruthenium(II)– or Osmium(II)–Bis(terpyridine) Chromophores and Expanded Pyridinium Acceptors: Equilibration between MLCT and Charge-Separated Excited States

Abstract

The synthesis, characterization, redox behavior, and photophysical properties (both at room temperature in fluid solution and at 77 K in rigid matrix) of a series of four new molecular dyads (2-5) containing Ru(II)- or Os(II)-bis(terpyridine) subunits as chromophores and various expanded pyridinium subunits as electron acceptors are reported, along with the reference properties of a formerly reported dyad, 1. The molecular dyads 2-4 have been designed to have their (potentially emissive) triplet metal-to-ligand charge-transfer (MLCT) and charge-separated (CS) states close in energy, so that excited-state equilibration between these levels can take place. Such a situation is not shared by limit cases 1 and 5. For dyad 1, forward photoinduced electron transfer (time constant, 7 ps) and subsequent charge recombination (time constant, 45 ps) are evidenced, while for dyad 5, photoinduced electron transfer is thermodynamically forbidden so that MLCT decays are the only active deactivation processes. As regards 2-4, CS states are formed from MLCT states with time constants of a few dozens of picoseconds. However, for these latter species, such experimental time constants are not due to photoinduced charge separation but are related to the excited-state equilibration times. Comparative analysis of time constants for charge recombination from the CS states based on proper thermodynamic and kinetic models highlighted that, in spite of their apparently affiliated structures, dyads 1-4 do not constitute a homologous series of compounds as far as intercomponent electron transfer processes are concerned.

Towards Water Soluble Mitochondria-Targeting Theranostic Osmium(II) Triazole-Based Complexes

“…Unlike in the CV trace for 2, no ligand-based reductions are observed for 1 within the available The electrochemical properties of 1 were investigated by cyclic voltammetry (CV) and reveal a reversible Os(II)/Os(III) oxidation at +0.64 V (vs Fc/Fc + = 0.0 V). This is close to that exhibited by the known model complex 2 (E ox = +0.49 V) and other related osmium(II) complexes [39][40][41] indicating that the highest occupied molecular orbital (HOMO) has primarily metallic 5d orbital character. Unlike in the CV trace for 2, no ligand-based reductions are observed for 1 within the available electrochemical window (−2.0 to +1.2 V), which is indicative of the higher energy lowest unoccupied molecular orbital (LUMO) localized on the btzpy ligand compared to that for the tolterpy complex.…”

Photoinduced electron transfer across molecular bridges: electron- and hole-transfer superexchange pathways

“…As pointed out by Creutz, Newton, and Sutin, 42a a Hushtype analysis of such spectral transitions affords a direct measurement of the metal-bridge electronic couplings, and such couplings can be used to calculate the metal-metal interaction, in a superexchange formalism similar to that of eqn (6) (eqn (25)) 33,42 where the effective energy gaps can be obtained from spectroscopic measurements (eqn (26) and (27)). …”

Harnessing Reversible Electronic Energy Transfer: From Molecular Dyads to Molecular Machines

“…Modificationo ft he photophysicalp roperties of cyclometalated iridium complexes is of particular interestb ecause of their central role in ar ange of applications that require highly efficient emission such as electroluminescence, sensing and bioimaging. [23] Ac yclometalated iridium complex (14)w as recently reportedw here the complex comprises naphthylpyridine as the principal ligand and the pyrene unit is attached to a2 ,2'-bipyridine ancillary ligand by as hort saturated alkyl bridge. Time-resolved spectroscopiess howedr eversible electronic energy transfer( REET) between the close low-lyingl ying excited states (DE = 680 cm À1 ).…”

“…Molecular dyads of ruthenium(II) bis(terpyridine) chromophores and various expanded pyridinium electron acceptors and the osmium-containing homologues have been reported. [14] Molecular dyads such as 4 were designed such that their MLCT and charge-separated (CS) states are close in energy,a llowing excited-state equilibration between these levels to take place. While not directly concerning REET,the kinetic scheme is similar to that described above.C Ss tates are formed from MLCT states with time constants of af ew dozens of picoseconds.…”