Probing structural evolution along multidimensional reaction coordinates with femtosecond stimulated Raman spectroscopy

Frontiera, Renee R.; Fang, Chong; Dasgupta, Jyotishman; Mathies, Richard A.

doi:10.1039/c1cp22767j

Cited by 62 publications

(89 citation statements)

References 70 publications

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“…3c) and the 1,260 cm À 1 band that is present in the latter FSRS spectrum with a low intensity. The nature and timescale of molecular changes that FSRS is able to follow has been the subject of recent discussions [23][24][25] . This is because at early times, after electronic excitation, the molecular system may not have equilibrated and its vibrational signatures are changing during this evolutionary phase 23 .…”

Section: Resultsmentioning

confidence: 99%

Direct observation of ultrafast long-range charge separation at polymer–fullerene heterojunctions

et al. 2014

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In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This results in distinctive signatures in the vibrational modes of the polymer. Here, we probe polaron photogeneration dynamics at polymer:fullerene heterojunctions by monitoring its timeresolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 300 fs. Surprisingly, further structural evolution on t50-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not significantly different from that near equilibrium. We interpret this as suggestive that charges are free from their mutual Coulomb potential because we would expect rich vibrational dynamics associated with charge-pair relaxation. We address current debates on the photocarrier generation mechanism at molecular heterojunctions, and our work is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials.

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Section: Resultsmentioning

confidence: 99%

Direct observation of ultrafast long-range charge separation at polymer–fullerene heterojunctions

et al. 2014

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“…24 Thus, in order to better understand the structural dynamics of the photophysical states, we have explored the excited electronic state dynamics of BPEB using femtosecond transient absorption (TA) and the vibrational mode-dependent dynamics usingultrafastRamanlossspectroscopy(URLS).URLSisanalogous to femtosecond stimulated Raman spectroscopy (FSRS) but the Raman spectra are measured in the anti-Stokes region with high signal-to-noise ratios. [29][30][31][32][33][34][35][36][37][38][39] Based on the literature, it was expected that in the excited state major conformational changes could occur. The objective of this work is to study the vibrational mode-specific dynamics at femtosecond time scales within the context of structural reorganization and understand the contribution of different modes towards excited state planarization and their instantaneous frequency response.…”

Section: Introductionmentioning

confidence: 99%

Mode specific excited state dynamics study of bis(phenylethynyl)benzene from ultrafast Raman loss spectroscopy

Roy

Kayal

Ariese

et al. 2017

The Journal of Chemical Physics

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Femtosecond transient absorption (fs-TA) and Ultrafast Raman Loss Spectroscopy (URLS) have been applied to reveal the excited state dynamics of bis(phenylethynyl)benzene (BPEB), a model system for one-dimensional molecular wires that have numerous applications in opto-electronics. It is known from the literature that in the ground state BPEB has a low torsional barrier, resulting in a mixed population of rotamers in solution at room temperature. For the excited state this torsional barrier had been calculated to be much higher. Our femtosecond TA measurements show a multi-exponential behaviour, related to the complex structural dynamics in the excited electronic state. Time-resolved, excited state URLS studies in different solvents reveal mode-dependent kinetics and picosecond vibrational relaxation dynamics of high frequency vibrations. After excitation, a gradual increase in intensity is observed for all Raman bands, which reflects the structural reorganization of Franck-Condon excited, non-planar rotamers to a planar conformation. It is argued that this excited state planarization is also responsible for its high fluorescence quantum yield. The time dependent peak positions of high frequency vibrations provide additional information: a rapid, sub-picosecond decrease in peak frequency, followed by a slower increase, indicates the extent of conjugation during different phases of excited state relaxation. The CC triple (–C≡C–) bond responds somewhat faster to structural reorganization than the CC double (>C=C<) bonds. This study deepens our understanding of the excited state of BPEB and analogous linear pi-conjugated systems and may thus contribute to the advancement of polymeric “molecular wires.”

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“…Electrolysis is exploited to control the solution pH and counter-ion content precisely during cluster synthesis without using chemical reagents. The evolution of solution species is followed in situ by an improved femtosecond stimulated Raman (FSR) technique (20)(21)(22) that can detect weak signals associated with structure-defining vibrational modes. The resulting pHdependent Raman spectra are interpreted by juxtaposition to quantum mechanically computed vibrational modes to assign specific molecular structures.…”

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confidence: 99%

Electrolytic synthesis of aqueous aluminum nanoclusters and in situ characterization by femtosecond Raman spectroscopy and computations

Wang

Liu

Chang

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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The selective synthesis and in situ characterization of aqueous Alcontaining clusters is a long-standing challenge. We report a newly developed integrated platform that combines (i) a selective, atomeconomical, step-economical, scalable synthesis of Al-containing nanoclusters in water via precision electrolysis with strict pH control and (ii) an improved femtosecond stimulated Raman spectroscopic method covering a broad spectral range of ca. T he importance of Al (aluminum) in the biosphere and to human civilization is enormous. The scale of mining and production of Al compounds is second only to that of Fe (iron). Our lives are influenced by its use in electronics (1, 2), cooking and eating utensils, and food packaging, and as structural materials in the construction, automotive, and aircraft industries. Its deposition and migration as a mineral ore are controlled by its aqueous chemistry and speciation. Millions of tons of Al compounds are used worldwide each year for water treatment, and it is found in all drinking water (3). The behavior of Al in water plays significant roles in soil chemistry and plant growth (4, 5), for example, governing Al bioavailability, toxicity, and its overall impact in aquatic ecosystems (6). Meanwhile, aqueous Al clusters are gaining importance as solution precursors for the large-area deposition of Al 2 O 3 coatings with broad technological applications (7,8).Despite more than a century of study (9, 10), the complete portrait of aqueous Al chemistry remains unclear. Studies of aqueous Al chemistry are notoriously difficult because of the variety and complexity of the species that can be formed, encompassing monomeric, oligomeric, and polymeric hydroxides (11-17); colloidal solutions and gels; and precipitates. Synthesis is complicated by the fact that the counter-ions and the method and rate of pH change all have dramatic effects on product formation (18,19). Few methods exist for the in situ determination and assignment of molecular-level structures. For instance, 27 Al NMR can only identify certain Al aqueous species (15). Furthermore, unlike organic compounds, systematic spectroscopic signatures of metal hydroxide clusters are less accessible, making interpretation of experimental spectra challenging. We hereby report a combined synthesis, experiment, and theory platform for the study of aqueous metal clusters. Electrolysis is exploited to control the solution pH and counter-ion content precisely during cluster synthesis without using chemical reagents. The evolution of solution species is followed in situ by an improved femtosecond stimulated Raman (FSR) technique (20-22) that can detect weak signals associated with structure-defining vibrational modes. The resulting pHdependent Raman spectra are interpreted by juxtaposition to quantum mechanically computed vibrational modes to assign specific molecular structures. Through this integrated approach, we have discovered a speciation behavior for Al in water that has not previously been observed. We focus here on the synthesis an...

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Probing structural evolution along multidimensional reaction coordinates with femtosecond stimulated Raman spectroscopy

Cited by 62 publications

References 70 publications

Direct observation of ultrafast long-range charge separation at polymer–fullerene heterojunctions

Direct observation of ultrafast long-range charge separation at polymer–fullerene heterojunctions

Mode specific excited state dynamics study of bis(phenylethynyl)benzene from ultrafast Raman loss spectroscopy

Electrolytic synthesis of aqueous aluminum nanoclusters and in situ characterization by femtosecond Raman spectroscopy and computations

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