Static and Time-Resolved Spectroscopic Studies of Low-Symmetry Ru(II) Polypyridyl Complexes

Abstract

The spectroscopic and electrochemical properties of a series of four Ru II polypyridyl complexes are reported. Compounds of the form [Ru(dmb) x (dea) 3-x ] 2+ (x ) 0-3), where dmb is 4,4′-dimethyl-2,2′-bipyridine and dea is 4,4′-bis(diethylamino)-2,2′-bipyridine, have been prepared and studied using static and time-resolved electronic and vibrational spectroscopies as a prelude to femtosecond spectroscopic studies of excited-state dynamics. Static electronic spectra in CH 3 CN solution reveal a systematic shift of the MLCT absorption envelope from a maximum of 458 nm in the case of [Ru(dmb) 3 ] 2+ to 518 nm for [Ru(dea) 3 ] 2+ with successive substitutions of dea for dmb, suggesting a dea-based chromophore as the lowest-energy species. However, analysis of static and time-resolved emission data indicates an energy gap ordering of [Ru(dmb) , at variance with the electronic structures inferred from the absorption spectra. Nanosecond time-resolved electronic absorption and time-resolved step-scan infrared data are used to resolve this apparent conflict and confirm localization of the long-lived 3 MLCT state on dmb in all three complexes where this ligand is present, thus making the dea-based excited state unique to [Ru-(dea) 3 ] 2+ . Electrochemical studies further reveal the origin of this result, where a strong influence of the dea ligand on the oxidative Ru II/III couple, due to π donation from the diethylamino substituent, is observed. The electronic absorption spectra are then reexamined in light of the now well-determined excited-state electronic structure. The results serve to underscore the importance of complete characterization of the electronic structures of transition metal complexes before embarking on ultrafast studies of their excited-state properties.

Probing excited state charge transfer dynamics in a heteroleptic ruthenium complex

“…26 The absorption feature in the UV region (below 320 nm) originates from the ligand centred transition as well as the higher energy MLCT transitions. However, the assignment of these higher energy transitions is not important for the present study.…”

“…22 Though ultrafast dynamics of the prototype [Ru(bpy) 3 ] 2+ have been extensively studied to discern ISC, vibrational relaxation and charge localization processes, real time dynamics studies of mixed ligand (heteroleptic) complexes have not been extensively undertaken and studies on such complexes has gained impetus in recent years. [23][24][25][26][27][28] As mentioned earlier, heteroleptic ruthenium complexes have gained a lot of interest in photovoltaic systems due to the broadened MLCT absorption envelope covering the solar spectrum due to multiple MLCT bands corresponding to different ligands. Effective use of the photoexcitation energy depends on the temporal kinetics of charge distribution around different ligands.…”

Ruthenium (II) polypyridyl complexes based on bipyridine and two novel diimine ligands with carrier-transporting unit: synthesis, photoluminescence and redox properties

“…3 observe that the complexes with two novel diimine ligands [Ru (bpy) 2 L]X 2 (L ¼ L1, L2, X ¼ Cl À , PF 6 À , SCN À ) show higher intensities than the reference complexes [Ru(bpy) 2 (Pybm)]X 2 (X ¼ Cl À , PF 6 À , SCN À ). After introducing the carrier-transporting groups into the Pybm the derivatives become much more rigid and also can provide efficient shielding of the Ru (II) core towards external quenching the ligand it replaced [44,45]. The replaced ligands in our case are the ones with electron-accepting groups (oxadiazole) or electrondonating groups (carbazole).…”

A Theoretical Investigation into the Photophysical Properties of Ruthenium Polypyridine‐Type Complexes

“…[36,37,40] Nonetheless, we cannot exclude the description of a localised state with such a spectrum. Indeed, it is probable that both states would absorb around the same energies.…”