Time-Resolved Resonance Raman Spectroscopy: Exploring Reactive Intermediates

Abstract

The study of reaction mechanisms involves systematic investigations of the correlation between structure, reactivity, and time. The challenge is to be able to observe the chemical changes undergone by reactants as they change into products via one or several intermediates such as electronic excited states (singlet and triplet), radicals, radical ions, carbocations, carbanions, carbenes, nitrenes, nitrinium ions, etc. The vast array of intermediates and timescales means there is no single "do-it-all" technique. The simultaneous advances in contemporary time-resolved Raman spectroscopic techniques and computational methods have done much towards visualizing molecular fingerprint snapshots of the reactive intermediates in the microsecond to femtosecond time domain. Raman spectroscopy and its sensitive counterpart resonance Raman spectroscopy have been well proven as means for determining molecular structure, chemical bonding, reactivity, and dynamics of short-lived intermediates in solution phase and are advantageous in comparison to commonly used time-resolved absorption and emission spectroscopy. Today time-resolved Raman spectroscopy is a mature technique; its development owes much to the advent of pulsed tunable lasers, highly efficient spectrometers, and high speed, highly sensitive multichannel detectors able to collect a complete spectrum. This review article will provide a brief chronological development of the experimental setup and demonstrate how experimentalists have conquered numerous challenges to obtain background-free (removing fluorescence), intense, and highly spectrally resolved Raman spectra in the nanosecond to microsecond (ns-μs) and picosecond (ps) time domains and, perhaps surprisingly, laid the foundations for new techniques such as spatially offset Raman spectroscopy.

Understanding Ultrafast Dynamics of Conformation Specific Photo-Excitation: A Femtosecond Transient Absorption and Ultrafast Raman Loss Study

“…Understanding the excited state optical properties of small organic molecules is important for many areas of science and technology, including their potential application in molecular optoelectronics. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] As compared to their inorganic analogues, they are relatively easy to synthesize at lower cost and they are gaining further attention because of their potential applications in nano scale physics. [20][21][22][23][24][25][26][27] A wide range of small organic molecules is currently under spectroscopic investigation by many groups.…”

Determination of lead ion removal from a flowing electrolyte in the presence of a magnetic field using Raman spectroscopy

“…information about the chemical structure of molecules from different vibrational transitions. 40 The Raman spectroscopy technique has important characteristics: 41 (a) the samples do not require prior preparation, which is important for complex biological systems (e.g., blood) where the preparation can affect the sample properties; 42,43 (b) the time required to obtain a complete Raman spectrum is typically less than 1 min; 44 (c) the technique is suitable to study aqueous systems because the Raman bands of OH groups are narrow and well defined, compared with IR spectroscopy, where the absorptions related to this group are strong and wide, with the possibility to mask other bands; (d) the bands used to characterize the presence of lead are strong and well defined; 44,45 (e) the amount of sample is small, typically one drop; (f) the technique can be used for in-line analysis, allowing for analysis of a considerable number of samples in a short amount of time; (g) enhanced Raman spectroscopy allows for detection of very weak signals to provide reliable analysis; and (h) Raman spectrometers have become more popular at reasonable prices and are being more frequently used for an increasing number of applications, specifically to determine the presence of chemical groups. [46][47][48] The intensity (height) of the Raman bands is proportional to the number of functional groups involved in the scattering process; thus, the band height associated with a particular group can provide information about the content of that particular group.…”

Solvent effects on the structure of the triplet excited state of xanthone: a time-resolved resonance Raman study

“…In this work, conventional Raman and time‐resolved resonance Raman spectroscopy (TR 3 ) techniques were used to examine the ground and the triplet excited states structure, respectively. TR 3 spectroscopy is a powerful technique to elucidate the vibrational structure of the transient species . When the Raman excitation wavelength is in resonance with an allowed electronic transition of the molecule, the Raman peaks coupled to the chromophore become enhanced, accounting for the high sensitivity and selectivity of the technique .…”

“…TR 3 spectroscopy is a powerful technique to elucidate the vibrational structure of the transient species. [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] When the Raman excitation wavelength is in resonance with an allowed electronic transition of the molecule, the Raman peaks coupled to the chromophore become enhanced, accounting for the high sensitivity and selectivity of the technique. [49][50][51][52][53][54] Resonance Raman spectroscopy generally involves measurement of the Raman intensities of different modes as a function of wavelength, known as Raman excitation profiles (REPs).…”