Understanding the mechanisms by which molecules interconvert among their distinct electronic and/or geometric configurations in response to external perturbations constitutes an important step toward the development of molecular-based materials.1 Low-spin Fe II complexes comprise an intriguing class of compounds in this regard due to their potential utility in solar energy conversion strategies 2 as well as the basis for magneto-optical devices. 3 It is well-known that photoexcitation of such compounds results in the eventual formation of the high-spin form of the molecule; the net two-quantum spin conversion coincides with an exceptionally large structural reorganization (∆r Fe-L ∼0.2 Å, ∆V ∼25 cm 3 mol -1 ), significant attenuation of the compound's optical density (∆ ∼10, and a dramatic change in the compound's magnetic properties (S ) 0 f S ) 2). 4 Several groups have exploited various spectroscopic probes in an effort to quantify the dynamics of this spin-state conversion but thus far have only been able to define an upper limit for the time constant associated with formation of the high-spin state.5 In this report, we present femtosecond optical and stimulated Raman scattering data on a prototypical low-spin Fe II charge-transfer complex that, for the first time, kinetically resolves the formation of the high-spin state and thus defines the time scale for optical switching in this class of compounds.The compound under study is 3 )] 2+ (R ) H (1), CH 3 (2)), which is illustrated in Chart 1 along with a simplified energy level diagram showing the low-lying ligand-field excited states of a d 6 transition metal ion. As shown by Drago and coworkers, 6 this compound sits very close to the so-called spincrossover point wherein the energies of the low-spin ( 1 A 1 ) and highspin ( 5 T 2 ) forms are in close proximity. Compound 1 possesses a low-spin ground state, whereas the steric constraints imposed by the introduction of the CH 3 groups in compound 2 require an elongation of the Fe-N bond, resulting in stabilization of the highspin 5 T 2 term as its ground state. This system thus has the unique characteristic of having reciprocal ground-and lowest-energy excited states; i.e., the lowest-energy excited state of one corresponds to the ground state of the other. We have previously exploited this feature to probe the ultrafast electronic absorption spectroscopy of [Fe(tren(py) 3 )] 2+ in the visible region and have shown that charge-transfer excitation of this compound results in the formation of the 5 T 2 state in <1 ps. 5aOwing to the nature of the absorption spectra of the low-spin and high-spin forms of the molecule, more diagnostic features can be found in the ultraviolet region of the spectrum. Specifically, there exist two isosbestic points between the 1 A 1 and 5 T 2 states at 285 ( 5 nm and 320 ( 5 nm as indicated by the calculated differential absorption spectrum between compounds 1 and 2 as well as data from nanosecond time-resolved absorption spectroscopy (Figure 1). 7These observations constitute imp...

Femtosecond stimulated Raman spectroscopy (FSRS) is used to examine the structural dynamics of the para-hydroxycinnamic acid (HCA) chromophore during the first 300 ps of the photoactive yellow protein (PYP) photocycle, as the system transitions from its vertically excited state to the early ground state cis intermediate, I0. A downshift in both the C7═C8 and C1═O stretches upon photoexcitation reveals that the chromophore has shifted to an increasingly quinonic form in the excited state, indicating a charge shift from the phenolate moiety toward the C9═O carbonyl, which continues to increase for 170 fs. In addition, there is a downshift in the C9═O carbonyl out-of-plane vibration on an 800 fs time scale as PYP transitions from its excited state to I0, indicating that weakening of the hydrogen bond with Cys69 and out-of-plane rotation of the C9═O carbonyl are key steps leading to photoproduct formation. HOOP intensity increases on a 3 ps time scale during the formation of I0, signifying distortion about the C7═C8 bond. Once on the I0 surface, the C7═C8 and C1═O stretches blue shift, indicating recovery of charge to the phenolate, while persistent intensity in the HOOP and carbonyl out-of-plane modes reveal HCA to be a cissoid structure with significant distortion about the C7═C8 bond and of C9═O out of the molecular plane.

A comparison between a Fabry-Pérot etalon filter and a conventional grating filter for producing the picosecond (ps) Raman pump pulses for femtosecond stimulated Raman spectroscopy (FSRS) is presented. It is shown that for pulses of equal energy the etalon filter produces Raman signals twice as large as that of the grating filter while suppressing the electronically resonant background signal. The time asymmetric profile of the etalon-generated pulse is shown to be responsible for both of these observations. A theoretical discussion is presented which quantitatively supports this hypothesis. It is concluded that etalons are the ideal method for the generation of narrowband ps pulses for FSRS because of the optical simplicity, efficiency, improved FSRS intensity and reduced backgrounds.

Femtosecond stimulated Raman spectroscopy is used to examine the structural dynamics of photoinduced charge transfer within a noncovalent electron acceptor/donor complex of pyromellitic dianhydride (PMDA, electron acceptor) and hexamethylbenzene (HMB, electron donor) in ethylacetate and acetonitrile. The evolution of the vibrational spectrum reveals the ultrafast structural changes that occur during the charge separation (Franck-Condon excited state complex → contact ion pair) and the subsequent charge recombination (contact ion pair → ground state complex). The Franck-Condon excited state is shown to have significant charge-separated character because its vibrational spectrum is similar to that of the ion pair. The charge separation rate (2.5 ps in ethylacetate and ∼0.5 ps in acetonitrile) is comparable to solvation dynamics and is unaffected by the perdeuteration of HMB, supporting the dominant role of solvent rearrangement in charge separation. On the other hand, the charge recombination slows by a factor of ∼1.4 when using perdeuterated HMB, indicating that methyl hydrogen motions of HMB mediate the charge recombination process. Resonance Raman enhancement of the HMB vibrations in the complex reveals that the ring stretches of HMB, and especially the C-CH(3) deformations are the primary acceptor modes promoting charge recombination.

Interfacial electron transfer between sensitizers and semiconducting nanoparticles is a crucial yet poorly understood process. To address this problem, we have used transient absorption (TA) and femtosecond stimulated Raman spectroscopy (FSRS) to investigate the photoexcited dynamics of a series of triphenylamine−coumarin dye/TiO 2 conjugates. The TA decay is multiexponential, spanning time scales from 100 fs to 100 ps, while the characteristic transient Raman spectrum of the radical cation decays biexponentially with a dominant ∼3 ps component. To explain these observations, we propose a model in which the decay of the TA is due to hot electrons migrating from surface trap states to the conduction band of TiO 2 while the decay of the Raman signature is due to internal conversion of the dye molecule. Furthermore, the S 1 Raman spectrum of TPAC3, a dye wherein a vinyl group separates the triphenylamine and coumarin moieties, is similar to the S 1 Raman spectrum of trans-stilbene; we conclude that their S 1 potential energy surfaces and reactivity are also similar. This correlation suggests that dyes containing vinyl linkers undergo photoisomerization that competes with electron injection.