Ultrafast Excited-State Dynamics of Rhenium(I) Photosensitizers [Re(Cl)(CO)_(3)(N,N)] and [Re(imidazole)(CO)_(3)(N,N)]^+: Diimine Effects

Femto- to picosecond excited-state dynamics of the complexes [Re(L)(CO)_(3)(N,N)]^n (N,N = bpy, phen, 4,7-dimethyl-phen (dmp); L = Cl, n = 0; L = imidazole, n = 1+) were investigated using fluorescence up-conversion, transient absorption in the 650−285 nm range (using broad-band UV probe pulses around 300 nm) and picosecond time-resolved IR (TRIR) spectroscopy in the region of CO stretching vibrations. Optically populated singlet charge-transfer (CT) state(s) undergo femtosecond intersystem crossing to at least two hot triplet states with a rate that is faster in Cl (~100 fs)^(−1) than in imidazole (~150 fs)^(−1) complexes but essentially independent of the N,N ligand. TRIR spectra indicate the presence of two long-lived triplet states that are populated simultaneously and equilibrate in a few picoseconds. The minor state accounts for less than 20% of the relaxed excited population. UV−vis transient spectra were assigned using open-shell time-dependent density functional theory calculations on the lowest triplet CT state. Visible excited-state absorption originates mostly from mixed L;N,N^(•−) → Re^(II) ligand-to-metal CT transitions. Excited bpy complexes show the characteristic sharp near-UV band (Cl, 373 nm; imH, 365 nm) due to two predominantly ππ*(bpy^(•−)) transitions. For phen and dmp, the UV excited-state absorption occurs at 305 nm, originating from a series of mixed ππ* and Re → CO;N,N•− MLCT transitions. UV−vis transient absorption features exhibit small intensity- and band-shape changes occurring with several lifetimes in the 1−5 ps range, while TRIR bands show small intensity changes (≤5 ps) and shifts (~1 and 6−10 ps) to higher wavenumbers. These spectral changes are attributable to convoluted electronic and vibrational relaxation steps and equilibration between the two lowest triplets. Still slower changes (≥15 ps), manifested mostly by the excited-state UV band, probably involve local-solvent restructuring. Implications of the observed excited-state behavior for the development and use of Re-based sensitizers and probes are discussed

A femtosecond fluorescence study of vibrational relaxation and cooling dynamics of UV dyes

We present a femtosecond broad-band fluorescence up-conversion study of the vibrational relaxation dynamics of two UV chromophores, 2,5-diphenyloxazole (PPO) and para-terphenyl (pTP), pumped with a large excess of vibrational energy (>2000 cm(-1)). The band narrowing of the transient fluorescence spectrum reflects a biphasic cooling process in a few hundreds of fs and a few ps. In the sub-ps regime, our data suggest a structural rearrangement in the excited state, followed by thermalization of the excess energy. These dynamics affect the fluorescence spectra of PPO and pTP in different ways. In PPO, the damping of a low frequency vibrational wavepacket and a significant sub-ps narrowing of the band characterize the vibrational relaxation. In pTP, the latter is faster and appears as a red shift with distortion of the band in <200 fs

Femtosecond UV Studies of the Electronic Relaxation Processes in Cytochrome c

We report on an experimental study with UV and visible ultrafast time-gated emission and transient absorption of the early photodynamics of horse heart Cytochrome c in both ferric and ferrous redox states. A clear separation in time and energy of tryptophan and haem emission is observed. Excitation of the haem via resonant energy transfer from the tryptophan residue is observed in the subsequent haem electronic relaxation. Different Trp haem energy transfer time constants of the ferrous and ferric forms are obtained. An almost instantaneous relaxation to the lowest singlet excited state (corresponding to the so-called Q band) characterizes the earliest electronic dynamics of the haem, independent of excitation energy, while dark intermediate states govern the ground-state recovery. The information gathered in these two experiments and in the literature allows us to propose a simple scheme for electronic relaxation leading to ligand dissociation