Time-Resolved Femtosecond Stimulated Raman Spectra and DFT Anharmonic Vibrational Analysis of an Electronically Excited Rhenium Photosensitizer

Abstract

Time-resolved femtosecond stimulated Raman spectra (FSRS) of a prototypical organometallic photosensitizer/photocatalyst ReCl(CO) 3 (2,2'-bipyridine) were measured in a broad spectral range ~40-2000 (4000) cm-1 at time delays from 40 fs to 4 ns after 400 nm excitation of the lowest allowed electronic transition. Theoretical ground-and excited-state Raman spectra were obtained by anharmonic vibrational analysis using second-order vibrational perturbation theory on vibrations calculated by harmonic approximation at DFT-optimized structures. A good match with anharmonically calculated vibrational frequencies allowed for assigning experimental Raman features to particular vibrations. Observed frequency shifts upon excitation ((ReCl) and (CC interring) vibrations upwards; (CC, CN) and (Re-C) downwards) are consistent with the bonding/antibonding characters of HOMO and LUMO involved in the excitation and support the delocalized formulation of the lowest triplet state as ReCl(CO) 3 bpy charge-transfer. FSRS

Tracking Ultrafast Structural Dynamics in a Dual-Emission Anti-Kasha-Active Fluorophore Using Femtosecond Stimulated Raman Spectroscopy

“…In this work, we utilized blue-side femtosecond stimulated Raman spectroscopy (FSRS) with the TA technique, assisted by quantum chemical calculations, to study the ultrafast structural dynamics of the anti-Kasha processes in open-form DCM-IFC molecules at pH = 11.3 (the synthetic route and characterization of the open-form DCM-IFC are shown in Section S2 and Figure S4 of the Supporting Information; the detailed sample preparation method can be found in Section S1 of the Supporting Information). The potential of combining FSRS, TA, and quantum calculations for the description of ultrafast structural dynamics in molecular systems has been extensively demonstrated in the previous reports. − In this work, analysis of the calculations and TA experimental data suggested that the short-wavelength emissions at 520 nm are due to depopulation of the S 2 excited state localized on the fluorescein fragment, and the long-wavelength emissions at 710 nm arise from the excitation energy from the transfer state S 1 localized on the DCM unit and chromene fragment, as shown in Figure a and b. The emergence of two fluorescing states is due to the large energy gap between the S 2 and S 1 states, which can mostly be ascribed to anti-Kasha behavior .…”

Photochemistry of transition metal carbonyls

“…2,32,147 Recent examples of IR, Raman and X-ray methods also involve DFT calculations. 148–150

Theory and dynamics: key points

(1) Theoretical structures of metal carbonyl fragments M(CO) n agree with experiment and depend on n and the d-electron count.(2) CO stretching frequencies difficult to reproduce accurately by computation.(3) Lower symmetry and reduced n results in low-lying excited states for 16e and 14e-M(CO) n compared with 18e species.(4) Photodissociation of CO depends on excitation into MLCT excited state followed by ultrafast (femtosecond) crossing into dissociative LF state.(5) Dissociation proceeds through excited electronic states of the photoproduct.(6) The wavelength dependence of the photochemistry of Cr(CO) 5 L and Mn 2 (CO) 10 depends on ultrafast branching leading to dissociation down competing channels.(7) The spin state may change after dissociation as in Fe(CO) 4 but not before.(8) Femtosecond dissociation is not universal for metal carbonyls. Three classes of exception:(a) (η 6 -arene)M(CO) 3 (M = Cr, Mo) lose CO more slowly(b) metal monocarbonyl dihydrides lose H 2 on irradiation(c) complexes with low-lying excited states involving charge transfer to non-carbonyl ligands such as diimines exhibit equilibrated excited states

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Real-time observation of tetrapyrrole binding to an engineered bacterial phytochrome

“…To assess the BV binding, one needs a structurally sensitive time-resolved method that is sufficiently sensitive in solution. A key quality of our recently developed watermarked stimulated Raman method [16][17][18][19][20][21][22] is that high-quality ground-state Raman spectra can be obtained within seconds of signal averaging while being entirely insensitive to fluorescence. This means that BV binding process, ranging from minutes to tens of minutes 23 , can be followed in real time after mixing apoprotein with BV.…”