TRIR measurements

Besides TRIR, it is also possible to employ excited-state resonance Raman spectroscopy (TR or TR2) to characterize excited-states of [Re(L)(CO)3(N,N)] 1 complexes [25, 27, 37, 48, 79, 81, 82], The spectra show bands due to vibrations of the N,N ligand and provide information on its structural changes upon excitation. Picosecond TR3 spectra of Re complexes give very weak signals [37], Measurements on the ns timescale are more informative. In the case of CT and TL states of [Re(py) (CO)3(dppz)]+ and the 3LLCT state of [Re(py-azacrown)(CO)3(bpy)]+ [16, 27], complementary vibrational information was provided by TR3 and TRIR measured in the fingerprint region. 

Time-resolved infrared spectroscopy (TRIR) has been outstandingly successful in identifying reactive intermediates and excited states of both metal carbonyl [68,69] and organic complexes in solution [70-72]. Some time ago, the potential of TRIR for the elucidation of photochemical reactions in SCFs was demonstrated [73]. TRIR is particularly suited to probe metal carbonyl reactions in SCFs because v(CO) IR bands are relatively narrow so that several different species can be easily detected. Until now, TRIR measurements have largely been performed using tunable IR lasers as the IR source and this has restricted the application of TRIR to the specialist laboratory [68]. However, recent developments in step-scan FTIR spectroscopy promise to open up TRIR to the wider scientific community [74]. 

Like laser-based TRIR [68], step-scan FTIR uses repetitive UV photolysis to build up the data. The difference is that the photolysis and data collection in step-scan FTIR is fully automated. Therefore a wider spectral range can be covered much more rapidly than with laser-based TRIR. The difference between laser based and step-scan TRIR measurements is shown in Figure... 

TRIR measurements are obtained from three different recordings of the emission spectrum of the IR source. These include a spectrum of the IR source without the sample in the beam path (/q), a spectrum of the IR source with the sample in the beam path (/), and the infrared intensity changes induced by photoexcitation (A/). The absorbance spectrum of the unexcited sample is derived from /q and /, A=log (/q//). Because the IR detector is AC coupled, measurements of /q and / are performed with an optical chopper (Stanford SR540) to modulate the IR light. 

The understanding of spin-forbidden reactions of organometallic reactions continues to be the subject of continued activity, particularly the theoretical understanding of such reactions. However, there is only a limited amount of experimental data available in order to benchmark such theoretical calculations of spin crossing and reactivity. Recent picosecond TRIR measurements have provided valuable insight into how spin-forbidden reactions affect other first-row organometallic metal carbonyl species, and it is likely that the combination of matrix isolation and TRIR will continue to produce important information for this class of reactions. 

One of the earliest condensed-phase TRIR measurements monitored the recombination of photodissociated CO from carboxymyoglobin on the millisecond timescale. Since then TRIR has frequently been applied to coordination compounds, to characterize reactive intermediates for the elucidation of reaction mechanisms and to study their excited states and electron/energy transfer processes. 

Recent technical advances have greatly expanded the applicability of TRIR spectroscopy, making measurements over wide temporal and spectral ranges now feasible. The relative merits of different experimental approaches have been discussed previously. " ... 

Gas-phase studies have not been restricted to the group VI hexacar-bonyls. Fu and co-workers (54) have used TRIR to study the coordina-tively unsaturated species CpMn(CO) (x = 1 and 2) generated by 266-and 355-nm laser photolysis of CpMn(CO)3 in the gas phase. In the presence of noble gas L (L = He, Ar, or Xe), they were able to measure the rate constant for reaction of the noble gas complex CpMn(CO)2L with CO. Interestingly, they foimd that only Ar significantly perturbed the rate fi om that observed in the absence of noble gas. This was thought to be because He has too high an ionization potential and Xe is too bulky to interact with the Mn center. In light of recent TRIR experiments conducted in supercritical fluid solution, the conclusion that Xe is unable to coordinate is incorrect. 

Ultrafast TRIR. The most fundamental processes of bond making, bond breaking and electron transfer have ultrafast dynamics. Access to these ultrafast time scales by TRIR requires a different approach from real-time measurements. Instead, pulsed-laser techniques based upon optical delay for measurement of time must be used. There are several approaches to measuring TRIR on the 10 — 10 s... 

The Nottingham TRIR apparatus has been described in detail elsewhere [5]. Briefly, a pulsed Nd YAG laser (Quanta-Ray GCR-11 266 nm or 355 nm), is used to initiate the photochemical reaction and a cw infrared source, (Mutek IR diode laser) monitors the changes in infrared transmission following the UV/visible pulse. IR spectra are built up on a point-by-point basis by repeating this measurement at different infrared frequencies. The stainless steel high pressure cells for supercritical TRIR have been described previously [14]. All solutions were characterised by conventional FTIR prior to use. 

Related experiments have recently been carried out in Xe (sc) and Kr (sc) with a time-resolved infrared (TRIR) spectrometer . In this system, reaction is initiated with a laser as before, but spectra are measured one frequency at a time with a continuous diode laser as the IR source. This apparatus, which has a time resolution of ca. 10 s, has been used to observe the complete set of M(CO)5Ng complexes (M = Cr, Mo, W Ng = Kr, Xe) and has provided tentative evidence for W(CO)sAr in Ar (sc). The measurements are carried out with a small pressure of added CO chosen such that the complexes have lifetimes from 100 ns to 2 qs. The rate constants for reaction with CO increase as follows Xe < Kr < Ar W < Mo Cr. The IR spectra are supplemented by UV/visible spectra of Cr(CO)sNg, which are in satisfactory agreement with matrix spectra. 

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