Nonthermalized excited states in Ru(II) polypyridyl complexes probed by ultrafast transient absorption spectroscopy with high photon energy excitation

Abstract

The picosecond excited state dynamics of a series of homoleptic Ru(II) polypyridyl complexes (where LL = bpy, dmb, dmeob, dfmb, or dttb) have been investigated in aqueous solution at room temperature using femtosecond transient absorption spectroscopy with high photon energy excitation. All of the complexes studied produced similar spectroscopic signatures: a near-instantaneous bleach centered at 470–500 nm corresponding to the static absorption spectrum, as well as an intense absorption (475–650 nm) that decayed within the instrument response function (IRF) to form a broad, low-level absorption extending from 500–650 nm. Detailed analyses of both kinetic and spectral parameters by singular value decomposition (SVD) indicate that the excited state difference spectra contain contributions from at least three distinguishable species that have been assigned as ligand-based π* ← π* and ligand-to-metal-charge-transfer (LMCT) transitions concomitant with the loss of the ground state metal-to-ligand-charge-transfer (MLCT) transition. Kinetic information extracted at 530 nm (an optical marker for the fully intraligand-delocalized 3MLCT state) or 660 nm (LMCT transitions) appear to be biphasic in some cases with the amplitude of the IRF-limited component becoming larger with shorter wavelength excitation. Further, rise dynamics were observed at redder probe wavelengths for Ru(bpy)32+ and Ru(dttb)32+. These observations are different from those obtained using lower photon energy excitation and show that excitation wavelength strongly influences the early photophysical events in these Ru(II) complexes.Key words: ruthenium, photophysics, lasers, transient absorption spectroscopy, excited states, singular value decomposition (SVD).

Excited State Dynamics of Two New Ru(II) Cyclometallated Dyes: Relation to Cells for Solar Energy Conversion and Comparison to Conventional Systems

“…A 2 nm blue-shift is observed for the 395 nm peak of 5 from 1 to 5 ps, consistent with vibrational cooling within the 3 MLCT state. Similar small shifts and absence of sharpening has been observed by us and others for related Ru(II) complexes, with vibrational cooling times that typically range from 2 to 20 ps. ,,− This timescale for vibrational cooling is supported by time-resolved vibrational techniques for ruthenium(II) and rhenium(I) complexes. , It is not clear why a similar vibrational cooling component is not present in 4 ; however, it is possible that because of the shorter excited state lifetime, both vibrational cooling in the 3 MLCT state and decay to the ground state take place in the same timescale such that the two processes cannot be resolved.…”

Water-soluble ruthenium complex-pyrene dyads with extended triplet lifetimes for efficient energy transfer applications

“…Initial control experiments under our conditions with the reference complexes Rubpy and Ruphen established essentially instantaneous formation (within less than 300 fs) of the 3 MLCT signatures in the TA spectra, i.e. ground state bleaches in the blue spectral region and positive TA signals in the near UV, 65,66 which persist on the timescale of the fs-TAS experiments (see ESI, Fig. S9–S13†).…”

Ultrafast Electron Transfer Dynamics in Ruthenium Polypyridyl Complexes with a π-Conjugated Ligand

“…Many investigators have used ultrafast laser spectroscopy to study the photophysics of transition metal polypyridyl complexes. − Complexes with elongated ligands that position the ruthenium core farther away from the eventual charge acceptor have been studied as a strategy for slowing the charge recombination that quenches the CS state. − ,− Increasing this distance slows charge recombination but has the undesirable side effect of decreasing the rate of forward ET so greatly that it frequently falls below the rate of back ET, resulting in insignificant population of the CS state …”