Broadband Femtosecond Fluorescence Spectroscopy of [Ru(bpy)₃]²⁺

Abstract

The excited-state dynamics of metal-polypyridine complexes are of great importance in applications as diverse as solarenergy conversion [1][2][3] and information storage [4] because they can be photo-and redox-triggered. Ruthenium trisbipyridine ([Ru(bpy) 3 ] 2+ ) is the prototype for this class of complexes, the study of which has formed the basis for most photochemical applications. These complexes exhibit transitions due to charge transfer between the metal-centered d orbital and the ligand p orbital, commonly known as metal-to-ligand charge transfer (MLCT). Femtosecond transient-absorption studies on [Ru(bpy) 3 ] 2+ have shown that upon excitation of the singlet 1 MLCT state (absorption maximum 450 nm), ultrafast intersystem crossing (ISC) occurs in < 100 fs, leading to the formation of the triplet 3 MLCT state with near-unity quantum yield. [5,6] From 300 fs onwards, the transient-absorption spectrum remains unchanged.[5] The 3 MLCT state decays radiatively to the ground state with a lifetime of % 600 ns in aqueous solution at room temperature. [2,3,7] However, the issue of energy disposal and vibrational relaxation within the complex is still a subject of debate. Indeed, a 400-nm excitation corresponds to an excess energy of % 8500 cm À1 above the vibrationally relaxed 3 MLCT state, [8] which would be dissipated in % 300 fs, according to the literature. [5,6] To address this issue, Bhasikuttan et al. [9] carried out a fluorescence-upconversion study at single wavelengths that correspond to those at which the 1 MLCT (500 and 575 nm) and the 3 MLCT (620 nm) emissions are expected. Their results were interpreted in terms of fast ISC to the 3 MLCT state followed by vibrational cooling on a timescale of 0.6 to 1 ps. The emission by the 3 MLCT state could not be observed in their experiment owing to its low radiative rate. However, single-wavelength detection does not produce a complete picture of the relaxation dynamics. Consequently, Browne et al.[10] implemented a picosecond broadband detection technique and observed an emission band centered at 520 nm, which they attributed to the 1 MLCT state. Unfortunately, they could not capture the details of the relaxation dynamics within the 3 MLCT state with the time resolution (%3 ps) used.Herein we report for the first time a polychromatic femtosecond fluorescence-upconversion experiment in the 440-690 nm range, with a resolution of 110 AE 10 fs to capture the early relaxation processes leading to the steady-state emission of the 3 MLCT state of [Ru(bpy) 3 ] 2+ . The experimental procedure and the data analysis are explained in reference [11] and in the Supporting Information. Figure 1 a shows a typical 2D spectrum obtained upon excitation at 400 nm (25 000 cm À1 ). The spot at % 21 600 cm À1is the Raman line of water. Although fluorescence in the 15 000-20 000-cm À1 region was present at t = 0, it was very short-lived, converging within 200 fs to a weak emission in the 16 000-17 500-cm À1 (575-680 nm) region. Spectra at fixed 2+ under excitation at 25 000 cm À1 ...

Ultrafast Relaxation Dynamics of Osmium–Polypyridine Complexes in Solution

“…Because the up-conversion signal is proportional to the radiative rate, this corresponds to a signal that is several hundred times weaker than that of the fluorescence (assuming a 10−20 ns lifetime for it) and thus below our detection limit. This explains why the steady-state phosphorescence is not observed in ultrafast measurements (i.e., no nanosecond-lived signal appears in Figure 5A, where only the 50 ps lived transient 2+ , 15 which has a comparable radiative rate, the steady-state phosphorescence was close to the detection limit yet still measurable due to a better sensitivity of our detection system in the corresponding spectral region.…”

“…In fact, while Fe or Ru polypyridine complexes exhibited ISC times of <30 fs, 15,20 diplatinum complexes had ISC times three orders of magnitude longer. 38 An extreme case was also reported by Steffen et al for rhodium-cyclopentadienes, where the singlet fluorescence was dominant as a result of an inefficient ISC to the triplet state.…”

“…9−19 The relaxation of the electronic states of metal−polypyridine complexes from and through the 1 MLCT states down to the triplet metal-to-ligand charge transfer ( 3 MLCT) states is characterized by an ultrafast cascade of internal conversion (IC), intramolecular vibrational redistribution (IVR), and intersystem crossing (ISC) events in tens to hundreds of femtoseconds. 15,20,21 In sensitization by the commonly used Ru−polypyridine complexes, electron injection is known to occur on two different time scales: a short one, from the initially excited nonthermalized 1 MLCT state(s) within a few femtoseconds, 22−24 and a slower one, from the thermalized 3 MLCT states on the picosecond to nanosecond time scale. 25−28 The overall injection quantum yield is close to 100%.…”

Vibrational Relaxation and Intersystem Crossing of Binuclear Metal Complexes in Solution

“…30 In organic molecules of appreciable size (>30 atoms), IVR is typically very fast (<100 fs) due to the large density of vibrational states and high-frequency modes that exhibit anharmonicity already for low vibrational quantum numbers. [31][32][33] In the case of PtPOP, however, no vibrational modes of lower energy than the FC-active Pt-Pt stretch mode (150 cm -1 ) have been reported. 34,35 A 40 cm -1 sideband was superimposed on the Pt-Pt vibronic progression in lowtemperature single-crystal spectroscopic studies, but the assignment of this feature as being due to an intramolecular FC-active mode or a phonon mode of the crystal lattice was ambiguous.…”

A general method to describe intersystem crossing dynamics in trajectory surface hopping

“…[33] This large spread is difficult to explain simply by the type of central atom, as for example the heavier Pd is expected to induce more efficient ISC than Fe. Femtosecond time scales for ISC have been also observed in transition metal complexes based on ruthenium, [34] rhenium, [35] osmium, [36] and other metals. [31] More recently, also the role of ISC in organic molecules has captured the attention of researchers.…”