Ultrafast surface-enhanced Raman spectroscopy (SERS) with pico- and femtosecond time resolution has the ability to elucidate the mechanisms by which plasmons mediate chemical reactions. Here we review three important technological advances in these new methodologies, and discuss their prospects for applications in areas including plasmon-induced chemistry and sensing at very low limits of detection. Surface enhancement, arising from plasmonic materials, has been successfully incorporated with stimulated Raman techniques such as femtosecond stimulated Raman spectroscopy (FSRS) and coherent anti-Stokes Raman spectroscopy (CARS). These techniques are capable of time-resolved measurement on the femtosecond and picosecond time scale and can be used to follow the dynamics of molecules reacting near plasmonic surfaces. We discuss the potential application of ultrafast SERS techniques to probe plasmon-mediated processes, such as H2 dissociation and solar steam production. Additionally, we discuss the possibilities for high sensitivity SERS sensing using these stimulated Raman spectroscopies.
Copper may be recovered as evidence in high-profile cases such as thefts and improvised explosive device incidents; comparison of copper samples from the crime scene and those associated with the subject of an investigation can provide probative associative evidence and investigative support. A solution-based inductively coupled plasma mass spectrometry method for measuring trace element concentrations in high-purity copper was developed using standard reference materials. The method was evaluated for its ability to use trace element profiles to statistically discriminate between copper samples considering the precision of the measurement and manufacturing processes. The discriminating power was estimated by comparing samples chosen on the basis of the copper refining and production process to represent the within-source (samples expected to be similar) and between-source (samples expected to be different) variability using multivariate parametric- and empirical-based data simulation models with bootstrap resampling. If the false exclusion rate is set to 5%, >90% of the copper samples can be correctly determined to originate from different sources using a parametric-based model and >87% with an empirical-based approach. These results demonstrate the potential utility of the developed method for the comparison of copper samples encountered as forensic evidence.
Charge transfer reactions are frequently accompanied by large changes in dipole and are of instrumental importance in many biological and chemical processes. Understanding the mechanism and dynamics associated with charge transfer reactions has been the focus of decades of investigations; however, most studies have been done with solvated analytes in which the measurements can be significantly impacted by environmental broadening or solvent-mediated effects. Here, we study photoinduced intramolecular charge transfer in crystalline betaine-30, thus dramatically reducing environmental heterogeneities and solvent contributions to the charge transfer process. We use femtosecond stimulated Raman spectroscopy in a reflective microscope configuration to probe the ground and excited state structures in a well-characterized betaine-30 crystal, with a unit cell consisting of four nearly orthogonal betaine-30 molecules and six water molecules. This study highlights how changing the crystal orientation and laser polarization can impact the excited- and ground-state Raman signals due to differences in the unit cell net dipole moment along different crystal axes. We find that the forward charge transfer process (τ ∼ 500 fs) is unaffected by the crystalline environment, while the back electron transfer process (τ ∼ 20 ps) is delayed in the crystalline environment as compared to lifetimes obtained in a range of solvents. This work highlights the role of the net transition dipole orientation on signals resulting from charge transfer in an anisotropic system and should aid in guiding the rational design of solid-state photoactivated devices.
Femtosecond stimulated Raman spectroscopy (FSRS) is a useful technique for uncovering chemical reaction dynamics by acquiring high-resolution Raman spectra with ultrafast time resolution. However, in FSRS, it can be challenging to discern Raman features from signals arising from transient absorption and other four-wave mixing pathways. To overcome this difficulty, we combine the principles of shifted excitation Raman difference spectroscopy with a simple fixed frequency comb to perform dualfrequency Raman pump FSRS. Through the addition of only two mirrors and a slit to the standard FSRS setup, this method provides Raman spectra at two different excitation wavelengths that can be processed by an automated algorithm to reconstruct the Raman spectrum. Here, we demonstrate the utility of dual-frequency Raman pump FSRS to easily identify Raman signatures by visual inspection for excited-state and ground-state spectra, both on-and off-resonance. We show that previously assigned short-lived vibrations of photoexcited β-carotene are actually not vibrational in nature. We also use crystalline betaine-30 as a challenging test case for this method, as the FSRS spectra contain a number of narrow transient vibronic and non-SRS features. By reliably reducing interference from background signals, the interpretation is substantially more quantitative and enables the future study of new systems, particularly those with small Raman cross-sections or solid-state samples with narrow vibronic features.
Femtosecond stimulated Raman spectroscopy (FSRS) is a chemically specific vibrational technique that has the ability to follow structural dynamics during photoinduced processes such as charge transfer on the ultrafast timescale. FSRS has a strong background in following structural dynamics and elucidating chemical mechanisms; however, its use with solid-state materials has been limited. As photovoltaic and electronic devices rely on solid-state materials, having the ability to track the evolving dynamics during their charge transfer and transport processes is crucial. Following the structural dynamics in these solid-state materials will lead to the identification of specific chemical structures responsible for various photoinduced charge transfer reactions, leading to a greater understanding of the structure–function relationships needed to improve upon current technologies. Isolating the specific nuclear motions and molecular structures that drive a desired physical process will provide a chemical blueprint, leading to the rational design and fabrication of efficient electronic and photovoltaic devices. In this perspective, we discuss technical challenges and experimental developments that have facilitated the use of FSRS with solid-state samples, explore previous studies that have identified structure–function relationships in charge transfer reactions, and analyze the future developments that will broaden and advance the field.
The synthesis of the first terminal group 9 hydrazido(2−) complex, Cp*IrN(TMP) (6) (TMP = 2,2,6,6-tetramethylpiperidine) is reported. Electronic structure and X-ray diffraction analysis indicate that this complex contains an Ir-N triple bond, similar to Bergman’s seminal Cp*Ir(NtBu) imido complex. However in sharp contrast to Bergman’s imido, 6 displays remarkable redox noninnocent reactivity owing to the presence of the Nβ lone pair. Treatment of 6 with MeI results in electron transfer from Nβ to Ir prior to oxidative addition of MeI to the iridium center. This behavior opens the possibility of carrying out facile oxidative reactions at a formally IrIII metal center via a hydrazido(2−)/isodiazene valence tautomerization.
The synthesis of the first terminal Group 9 hydrazido(2-) complex, Cp*IrN(TMP) (6)( TMP = 2,2,6,6tetramethylpiperidine) is reported. Electronic structure and Xray diffraction analysis indicate that this complex contains an IrÀNt riple bond, similar to Bergmanss eminal Cp*Ir(N t Bu) imido complex. However,i ns harp contrast to Bergmans imido, 6 displays remarkable redoxn on-innocent reactivity owingtothe presence of the N b lone pair.T reatment of 6 with MeI results in electron transfer from N b to Ir prior to oxidative addition of MeI to the iridium center.This behavior opens the possibility of carrying out facile oxidative reactions at af ormally Ir III metal center through ah ydrazido(2À)/isodiazene valence tautomerization.
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