558 research outputs found
Ultrafast charge transfer dynamics in supramolecular Pt(II) donor-bridge-acceptor assemblies: the effect of vibronic coupling
Thanks to major advances in laser technologies, recent investigations of the ultrafast coupling of nuclear and electronic degrees of freedom (vibronic coupling) have revealed that such coupling plays a crucial role in a wide range of photoinduced reactions in condensed phase supramolecular systems. This paper investigates several new donorâbridgeâacceptor charge-transfer molecular assemblies built on a trans-Pt(II) acetylide core. We also investigate how targeted vibrational excitation with low-energy IR light post electronic excitation can perturb vibronic coupling and affect the efficiency of electron transfer (ET) in solution phase. We compare and contrast properties of a range of donorâbridgeâacceptor Pt(II) trans-acetylide assemblies, where IR excitation of bridge vibrations during UV-initiated charge separation in some cases alters the yields of light-induced product states. We show that branching to multiple product states from a transition state with appropriate energetics is the most rigid condition for the type of vibronic control we demonstrate in our study
Spectroscopic study of optically induced ultrafast electron dynamics in gold
Copyright Š 2007 The American Physical SocietyUsing a supercontinuum pulse as a probe, we have measured the transient reflectivity spectra of a thin film of gold for different values of the pump-probe time delay. The wavelength lambda(x) at which the measured transient reflectivity changes sign has been found to depend upon the time delay, leading to bipolar time resolved signals. The time dependence of lambda(x) has been shown to be consistent with calculations that take into account the full dependence of the reflectivity upon the electron occupation number, and to contradict qualitatively a model in which the signal is assumed to be directly proportional to the occupation number. The shift of lambda(x) has been found to persist at time delays that are much longer than the time required for the electrons to thermalize. Therefore the bipolar reflectivity signals do not necessarily contain a contribution from nonthermalized electrons, as has been previously assumed
Picosecond time-resolved infrared spectroscopy of rhodium and iridium azides
Picosecond time-resolved infrared spectroscopy was used to elucidate early photochemical processes in the diazido complexes M(Cp*)(N3)2(PPh3), M = Rh (1), Ir (2), using 266 nm and 400 nm excitation in THF, CH2Cl2, MeCN and toluene solutions. The time-resolved data have been interpreted with the aid of DFT calculations on vibrational spectra of the singlet ground states and triplet excited states and their rotamers. While the yields of phototransformations via N2 loss are low in both complexes, 2 cleaves a N3 ligand under 266 nm excitation. The molecular structure of 1 is also reported as determined by single crystal X-ray diffraction
Ultrafast 2D-IR spectroscopy of intensely optically scattering pelleted solid catalysts
ACKNOWLEDGMENTS This work was supported by a UKRI Future Leaders Fellowship grant (Grant No. MR/S015574/1), STFC-UKRI program access to CLF-ULTRA (Grant No. LSF1828), direct access to CLF-ULTRA (Grant Nos. Apps 17330043 and 19130012), and a group residency in the Research Complex at Harwell (RCaH). The authors are grateful to Kathryn Welsby, Ivalina Minova, and Santhosh Matam for support early in the project with samples and the Linkam cell. Mr. John Still of the School of Geosciences, University of Aberdeen is thanked for the SEM images, and Kieran Farrell/Martin Zanni is thanked for the discussion about the polarizations of the beams creating the thermal transientsPeer reviewedPublisher PD
Ultrafast Wiggling and Jiggling: Ir_2(1,8-diisocyanomenthane)_4^(2+)
Binuclear complexes of d^8 metals (Pt^(II), Ir^I, Rh^I,) exhibit diverse photonic behavior, including dual emission from relatively long-lived singlet and triplet excited states, as well as photochemical energy, electron, and atom transfer. Time-resolved optical spectroscopic and X-ray studies have revealed the behavior of the dimetallic core, confirming that MâM bonding is strengthened upon dĎ* â pĎ excitation. We report the bridging ligand dynamics of Ir2(1,8-diisocyanomenthane)_4^(2+)(Ir(dimen)), investigated by fsâns time-resolved IR spectroscopy (TRIR) in the region of CâĄN stretching vibrations, ν(CâĄN), 2000â2300 cm^(â1). The ν(CâĄN) IR band of the singlet and triplet dĎ*pĎ excited states is shifted by â22 and â16 cm^(â1) relative to the ground state due to delocalization of the pĎ LUMO over the bridging ligands. Ultrafast relaxation dynamics of the ^1dĎ*pĎ state depend on the initially excited FranckâCondon molecular geometry, whereby the same relaxed singlet excited state is populated by two different pathways depending on the starting point at the excited-state potential energy surface. Exciting the long/eclipsed isomer triggers two-stage structural relaxation: 0.5 ps large-scale IrâIr contraction and 5 ps IrâIr contraction/intramolecular rotation. Exciting the short/twisted isomer induces a âź5 ps bond shortening combined with vibrational cooling. Intersystem crossing (70 ps) follows, populating a ^3dĎ*pĎ state that lives for hundreds of nanoseconds. During the first 2 ps, the ν(CâĄN) IR bandwidth oscillates with the frequency of the ν(IrâIr) wave packet, ca. 80 cm^(â1), indicating that the dephasing time of the high-frequency (16 fs)^(â1) CâĄN stretch responds to much slower (âź400 fs)^(â1)IrâIr coherent oscillations. We conclude that the bonding and dynamics of bridging di-isocyanide ligands are coupled to the dynamics of the metalâmetal unit and that the coherent IrâIr motion induced by ultrafast excitation drives vibrational dephasing processes over the entire binuclear cation
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 650â285 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)^(â1) than in imidazole (~150 fs)^(â1) 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. UVâvis 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^(â˘â) â 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 ĎĎ*(bpy^(â˘â)) transitions. For phen and dmp, the UV excited-state absorption occurs at 305 nm, originating from a series of mixed ĎĎ* and Re â CO;N,Nâ˘â MLCT transitions. UVâvis transient absorption features exhibit small intensity- and band-shape changes occurring with several lifetimes in the 1â5 ps range, while TRIR bands show small intensity changes (â¤5 ps) and shifts (~1 and 6â10 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 (âĽ15 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
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