We present temperature-dependent fluorescence quantum yield and lifetime measurements on the LH-1 and LH-2 complexes of Rhodobacter sphaeroides and on the isolated B820 subunit of Rhodospirillum rubrum. From these measurements the superradiance is calculated, which is related to the delocalization of excitations in these complexes. In the B820 preparation we find a radiative rate that is 30% higher than that of monomeric bacteriochlorophyll, in agreement with a dimer model of this subunit. At room temperature both LH-1 and LH-2 are superradiant relative to monomeric Bchl-a with enhancement factors of 3.8 and 2.8, respectively. In LH-2 the radiative rate does not change significantly upon lowering the temperature to 4 K. LH-1 however exhibits a strong temperature dependence, giving rise to a 2.4 times higher radiative rate at 4 K relative to room temperature. From modeling of the superradiance using a Hamiltonian based on the LH-2 structure and including site inhomogeneity, we conclude that the ratio of inhomogeneity over the coupling between the pigments is around 1 for LH-1 and 2-3 for LH-2. From the Monte Carlo simulations we estimate the delocalization length in LH-1 and LH-2 to be on the order of 3-4 pigments at room temperature.

The ultrafast relaxation of aqueous iron(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band (1MLCT) has been characterized by femtosecond fluorescence up-conversion and transient absorption (TA) studies. The fluorescence experiment shows a very short-lived broad 1MLCT emission band at approximately 600 nm, which decays in < or =20 fs, and a weak emission at approximately 660 nm, which we attribute to the 3MLCT, populated by intersystem crossing (ISC) from the 1MLCT state. The TA studies show a short-lived (<150 fs) excited-state absorption (ESA) below 400 nm, and a longer-lived one above 550 nm, along with the ground-state bleach (GSB). We identify the short-lived ESA as being due to the 3MLCT state. The long-lived ESA decay and the GSB recovery occur on the time scale of the lowest excited high-spin quintet state 5T2 lifetime. A singular value decomposition and a global analysis of the TA data, based on a sequential relaxation model, reveal three characteristic time scales: 120 fs, 960 fs, and 665 ps. The first is the decay of the 3MLCT, the second is identified as the population time of the 5T2 state, while the third is its decay time to the ground state. The anomalously high ISC rate is identical in [RuII(bpy)3]2+ and is therefore independent of the spin-orbit constant of the metal atom. To reconcile these rates with the regular quasi-harmonic vibrational progression of the 1MLCT absorption, we propose a simple model of avoided crossings between singlet and triplet potential curves, induced by the strong spin-orbit interaction. The subsequent relaxation steps down to the 5T2 state dissipate approximately 2000 cm-1/100 fs. This rate is discussed, and we conclude that it nevertheless can be described by the Fermi golden rule, despite its high value.

No 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 ...