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Time-Resolved Resonance Raman Spectroscopy: Exploring Reactive Intermediates

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focal point review SANGRAM KESHARI SAHOO AND SIVA UMAPATHY DEPARTMENT OF INORGANIC AND PHYSICAL CHEMISTRY INDIAN INSTITUTE OF SCIENCE BANGALORE -560012, INDIA ANTHONY W. PARKER CENTRAL LASER FACILITY RESEARCH COMPLEX AT HARWELL SCIENCE &TECHNOLOGY FACILITIES COUNCIL RUTHERFORD APPLETON LABORATORY HARWELL OXFORD DIDCOT OXFORDSHIRE, UK OX11 0QX Time-Resolved Resonance Raman Spectroscopy: Exploring Reactive Intermediates The study of reaction mechanisms involves technique. The simultaneous advances in con- efficient spectrometers, and high speed, highly systematic investigations of the correlation temporary time-resolved Raman spectroscopic sensitive multichannel detectors able to collect between structure, reactivity, and time. The techniques and computational methods have a complete spectrum. This review article will challenge is to be able to observe the chemical done much towards visualizing molecular provide a brief chronological development of changes undergone by reactants as they change fingerprint snapshots of the reactive interme- the experimental setup and demonstrate how into products via one or several intermediates diates in the microsecond to femtosecond time experimentalists have conquered numerous such as electronic excited states (singlet and domain. Raman spectroscopy and its sensitive challenges to obtain background-free (remov- triplet), radicals, radical ions, carbocations, counterpart resonance Raman spectroscopy ing fluorescence), intense, and highly spectrally resolved Raman spectra in the nanosecond to carbanions, carbenes, nitrenes, nitrinium ions, have been well proven as means for determin- microsecond (ns–ls) and picosecond (ps) time etc. The vast array of intermediates and ing molecular structure, chemical bonding, domains and, perhaps surprisingly, laid the timescales means there is no single ‘‘do-it-all’’ reactivity, and dynamics of short-lived inter- mediates in solution phase and are advanta- foundations for new techniques such as spatial- geous in comparison to commonly used time- ly offset Raman spectroscopy. Received 7 July 2011; accepted 25 July 2011. resolved absorption and emission spectroscopy. Index Headings: Time-resolved resonance Ra- * Authors to whom correspondence should be sent. E-mail: [email protected]; a.w. Today time-resolved Raman spectroscopy is a man spectroscopy; TR3 spectroscopy; Qui- [email protected]. mature technique; its development owes much nones; Charge transfer complex; Photobiology; DOI: 10.1366/11-06406 to the advent of pulsed tunable lasers, highly Isomerization; Inorganic complexes. APPLIED SPECTROSCOPY 1087 focal point review INTRODUCTION studying biomolecular dynamics. How- fluorescence spectra occurs, the more ever, recently the time-resolved struc- intense fluorescence obscures the weak- he chemical mechanism of a tural elucidation techniques, such as er Raman features. This is particularly single reaction or a set of similar time-resolved Raman and infrared spec- so when working on biological mole- reactions is first hypothesized T troscopies, have provided dynamical cules in the visible region; however, and then predicted by a patterned information from a few femtoseconds moving into the ultraviolet (UV) or deep analysis of the reactants used and the final products formed. Due to the lack of to microseconds and days. The first UV region can avoid such problems complete experimental evidence, this experimental study of inelastically scat- whilst still benefitting from resonance tered light was reported by Raman and enhancement. The enhanced RR bands procedure can overlook the details of 7 the intermediates formed during the Krishnan in 1928. The contemporary are sensitive to structural and environ- reaction. Chemistry textbooks are full technique at that time received world- mental change and are selective in of descriptions of proposed mechanisms wide attention, though it had limited nature, which is essential for probing (albeit based on rational basic principles application due to technical limitations. different sites of a multichromophoric of chemistry) leading to the key inter- The invention of lasers and multichannel chemical/biological system, for example detectors in the 1960s and 1970s, macromolecular systems such as por- mediates formed within simple as well 9–11 as complex reactions, yet frequently respectively, led the renaissance in phyrins and proteins, independently. concrete structural evidence is lacking Raman spectroscopy. Since then we Thus, the first time-resolved reso- to support the hypotheses put forward. It have witnessed an explosion in Raman nance Raman (TR3) experiment probed is the responsibility of time-resolved modernization. Continuous wave (CW) the molecular structure of the reactive spectroscopists to provide experimental and short-pulse lasers, sophisticated intermediates generated from a 30 ns data that can be used to verify these spectrometers, charge coupled devices pulse radiolytic pump source. The work hypotheses. A recent example of such a (CCD), intensified CCDs, electron mul- was reported in 1976 by Weisberg and 12 case is the study of SN2 reaction tiplying CCDs, streak cameras, Rayleigh co-worker. In the early 1980s nano- - filters, and fluorescence rejection tech- second and picosecond (ps) TR3 exper- involving Cl and CH3I. The textbook and the research papers agree with the niques have provided today’s multiple iments were demonstrated using single 13–15 formation of a five-coordinated transi- approaches to Raman spectroscopy that and double laser pulses. With the tion state during one-step nucleophilic are applicable to all sample phases under technology developed across the nano- substitution. However, a recent finding any condition. Raman spectroscopy, second and picosecond time domain, using crossed molecular beam velocity which can be used to understand the TR3 has since been successfully applied map imaging suggests a roundabout molecular structure and the subtle to probe short-lived reactive intermedi- 1 mechanism involving CH3 rotation. changes in the bond length and bond ates produced in numerous chemical and 16–20 A typical photochemical reaction in angles during a reaction, is now consid- biological systems. Figure 1 pre- solution phase involves the evolution ered the ‘‘must have’’ technique in sents schematically the theme of RR, TR and decay of various short-lived reactive academia and industry.8 spectroscopy, and regions of TR3 spec- intermediates that must navigate through If lasers led to the renaissance in troscopy in the electromagnetic spec- a series of parallel competitive pathways Raman spectroscopy, it was short-pulsed trum and their application for important to produce products. Depending on the and tunable lasers that revolutionized it! photo-processes studied in different time detail required and timescales involved, Further, the resonance Raman (RR) domains. the experimentalist has an array of time- condition means the intensity of inher- In this article we focus on the detailed resolved (TR) techniques with which to ently weak Raman bands is selectively study of structure, reactivity, and dy- probe the reaction. Any time-resolved enhanced, up to 106 times, by tuning the namics of photogenerated intermediates technique uses an activation procedure excitation line into an allowed electronic probed by time-resolved resonance Ra- to initialize (pump) a change (reaction) transition. RR scattering occurs through man (TR3) spectroscopy in the nano- and the spectra of the intermediates are coupling of electronic and vibrational second to picosecond time domain. recorded using a suitable probe source at transitions. The Raman spectral profile Time-resolved absorption (TRA) and variable delay times with respect to the obtained under the resonance condition fluorescence (TRF) on the fast and pump. Apart from photolysis, pulse is different in comparison to the non- ultrafast scale are the oldest techniques radiolysis,2–4 thermal, chemical, and resonant Raman condition. In addition, and the most commonly applied for electrochemical methods have also been because of the resonance enhancement, studying kinetics of photochemical re- used to activate change. The chemically detection of components in a very dilute actions.21–23 TR3 spectroscopy has a initiated reactions typically involve rap- solution is possible. RR spectroscopy is major advantage over the UV-Visible id mixing, stopped-flow methods,5 or used primarily in the visible wavelength transient absorption method because it oxidation and reduction6 to form inter- region; however, the UV region is can selectively probe multiple interme- mediates. Though mixing methods only important for studying biomolecules diates with greater accuracy as well as allow time resolution in the microsecond and highly conjugated chemical sys- provide bond-specific structural details to millisecond (ls–ms) domain, tems. In certain spectral regions the of the intermediate. The structural stopped-flow remains, even in today’s resonance Raman laser causes fluores- changes and the kinetics (reactivity) ultrafast world, an excellent tool for cence. When overlap of Raman and information can be obtained by analyz- 1088 Volume 65, Number 10, 2011 ing the band shift and intensity of RR bands, respectively. The RR bands are also highly sensitive to changes in the substrate structure and the environment, which helps in studying microscopic molecular interactions. Of course, other time-resolved Raman methods are being employed and the reader is referred to various recent reviews
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