time-resolved infrared spectroscopy in studies of organometallic excited states and reactive...

Photochemistry and Photobiology, 1997, 65( 1): 57-64

Symposium-in-Print

Time-Resolved Infrared Spectroscopy in Studies of Organometallic Excited States and Reactive Intermediates

Peter C. Ford*, Jon S. Bridgewater and Brian Lee Department of Chemistry, University of California, Santa Barbara, CA, USA

Received 1 June 1996; accepted 2 October 1996

ABSTRACT

This article provides a brief overview of time-resolved infrared spectroscopy (TRIR) applied to the investigation of organometallic photochemistry in solution. In this context, some fundamental problems where flash photolysis TRIR techniques may provide information relevant to photochemical pathways and to certain thermal reaction mechanisms are described. Different approaches to obtaining TRIR spectral information are summarized and illustrated with specific applications.

PROBLEMS IN ORGANOMETALLIC CHEMISTRY-WHY TRIR?

Organometallic chemistry is extraordinarily broad given its inclusion of most compounds involving carbon-metal bonds. Transition metal carbonyls are commonly included because numerous members of that class involve other organic ligands as well. Both main group and transition metal organometallics have been the subject of photochemical studies; the latter compounds will be the principal examples drawn here because that is where the interest and expertise of the authors lie. For such complexes, one can categorize excited states (ES)t by orbital parentages such as metal-centered (e.g. d-d) states, metal to ligand (MLCT) or ligand to metal (LMCT) charge transfer states or ligand localized (e.g. TT*)

states. However, too strict adherence to simple labels can be deceiving given that the HOMO and LUMO are often de- localized and the characters of actual states are affected by electronic reorganization accompanying excitation.

Elucidating the photochemical mechanism requires iden- tifying key ES and reactive intermediates generated as the products of the primary photoreactions. Time-resolved tech-

*To whom correspondence should be addressed at: Department of Chemistry, University of California, Santa Barbara, CA 93 106, USA. Fax: 805-893-41 20; e-mail: [email protected].

tAhhreviarions: CCD, charge-coupled device; CW, continuous wave; ES, excited state; FT, Founer transform; LMCT, ligand to metal charge transfer; MLCT, metal to ligand charge transfer; OMA, optical multichannel analyzer; PC, photoconductive; PMT, photomultiplier tube: PV, photovoltaic; TRIR, time-resolved infrared; TRO, time-resolved optical.

0 1997 American Society for Photobiology 0031 -8655/97 $5.00+0.00

niques allow one to plot the temporal course of events, i.e. the rise and fall of key species, ES, reactive intermediates and final products. Infrared detection is particularly useful for spectroscopic characterization and kinetic studies of organometallic reactions because the relevant compounds often include functional groups with characteristic frequencies that are sensitive to molecular structure and conformation as well as the medium. The broad, often featureless, UV-visible absorption bands commonly seen for solution phase organometallic compounds by time-resolved optical (TRO) spectroscopy generally allow little structural interpretation, and the bands of various species often overlap. The much narrower IR bands give better resolution and often allow for direct observation of the temporal decay and appearance of individual species with minimal interference. Numerous qualitative and quantitative criteria have been elaborated for the analysis of such bands in terms of structure and bonding.

What types of species might one expect to be able to detect? Besides the ES noted above, other transient species may result from ES “unimolecular” or “bimolecular” processes. For a mononuclear metal complex ML, with a certain subset of ligands L, the most common photoreaction would be simple ligand dissociation (Eq. 1) because the M-L bonds are usually the weakest in the complex. With molecular ligands such as CO or an alkene, dissociation is heterolytic, but with metal alkyls and similar species homolytic dissociation to radicals is common. In solution, other relaxation processes are sufficiently fast that one normally expects but a single ligand to be labilized; in the gas phase, multiple dissociations are common given sufficient excitation energy. For polynuclear organometallic complexes, metal-metal bonds are often the weakest, and homolytic cleavage to metal radicals may compete with ligand dissociation (Eq. 2). For either mononuclear or polynuclear species, unimolecular processes may occur from very short-lived or even unbound dissociative states, but bimolecular reactions (e.g. electron transfers as in Eq. 3) require longer ES lifetimes. The ensemble of spectroscopic and kinetic information from TRO and time-resolved infrared (TRIR) techniques combined with theory and chemical intuition enable elucidation of structures and dynamics of key ES and reactive intermediates.

57

58 Peter C. Ford et a/.

ML, 5 ML,-, + L (1)

L,M-ML, - (2)

(3)

h” M2L2n-I + L

2L,-,M‘ Red

hv p ML; + Red+ ML, + [ML,]* -

ML,’ + Ox- ox

It should be emphasized that transient species such as formed in Eqs. 1-3 may be the same as reactive intermediates proposed in thermal reactions of organometallics. Such intermediates are elusive for catalytic (or stoichiometric) reactions owing to the low steady-state concentrations that im- pede direct observation. In this context, preparing nonsteady- state concentrations of reactive intermediates via flash photolysis and characterizing these by TRIR has proved to be an important strategy in elucidating mechanisms of C-H and H-H bond activation, ligand substitution reactions, migratory insertion and other processes relevant to catalytic and photocatalytic activation of small molecules such as methane and carbon monoxide (1-3).

TRIR APPARATUS AND TECHNIQUES

The key issues in considering different apparatus for TRIR studies are instrumental sensitivity, spectral range and temporal response. Also important, especially given that the po- tential clientele may not be experienced practitioners of state-of-the-art laser spectroscopy, are convenience and of course (always) the cost. In principle, the term “time-resolved infrared spectroscopy” could apply to any series of temporal IR spectra recorded of an evolving system. Here we will generally focus on short-lived systems where solutions cannot be prepared by standard (or even stopped flow) mixing techniques but require flash photolysis to generate the transients of interest. An alternative to flash photolysis TRIR techniques for investigating such species is to prepare these by photolyses of low-temperature frozen solutions or solid matrices where lifetimes are sufficiently extended that spectra can be recorded by conventional spectrometers (43). Such studies provide complementary information regarding structures in these rigid media but limited insight into the dynamics in ambient solutions. Low-temperature solutions, even solutions of liquefied noble gases, have also been used as media for TRIR studies (6).

The TRIR systems illustrated here will be arbitrarily separated into three categories. The first is the conventional pump-probe system with a ns pump source, a continuous IR probe source operating at a single (but tunable) IR frequency and a fast rise time, liquid N,-cooled diode detector. The advantage of this system is relative ease of use. The second involves an ultrafast laser as the pump source and some sort of IR pulse generated in conjunction with the excitation pulse with the temporal aspects determined by small light pathlength differences. The obvious advantage is the much shorter time intervals that may be probed. The third utilizes “step scan FTIR” detection techniques with the advantage of much easier data collection over wide frequency ranges.

Figure 1. Schematic diagram of a nanosecond TRIR spectrometer using a lead salt IR diode laser as the probe source. The probe source is based on three components available from Laser Photonics Inc., a cold head containing four diode lasers cooled by a closed cycle He refrigerator, a collimating assembly that includes a HeNe laser for alignment and a monochromator to filter unwanted frequencies because at a given i and T, the diode lasers may lase at several modes separated by 1-3 cm-I. The IR beam is directed through a chopper (C), then the sample cell to a HgCdTe (PV) detector (A), the signal from which is amplified by a preamplifier (B) and recorded by a digital oscilloscope. (The chopper is used to obtain I,, then is removed.) The excitation source is either a XeCl excimer laser or a Nd:YAG laser, either of which can be operated with a dye laser. The sample cell is modified from a commercially available (McCarthy) demountable IR cell with CaF2 windows. The inlets and outlets of this cell are silver soldered to stainless-steel cannula con- nected to a pump-driven flow system designed for constant flow of deaerated solutions under inert or reactive gas atmospheres.

Conventional ns TRIR systems

In the systems described here, the pump source is a ns pulse laser such as a Nd:YAG or a XeCl excimer laser with excitation wavelengths determined by harmonic generation or dye lasers. For many metal carbonyYorganometallic complexes, the 308 nm output of the XeCl excimer and the 355 nm third harmonic of the Nd:YAG are convenient because extinction coefficients at these wavelengths are comparable to the c of carbonyl stretching bands in the IR. The high reliability of such pump sources makes for convenient signal averaging. For most organometallic systems, the repetitive excitations implied by signal averaging also requires methodology for flowing the sample through the IR and UV/visible-transmitting sample cell. (The sample cell, flow system and any sample handling hardware must be appropriate for handling air-sensitive compounds under various gas mixtures.) The probe is a continuous IR source operating at a single frequency and the IR detector is a fast rise-time photodiode. The system long used in the authors’ laboratory (7-9) is illustrated in Fig. 1. This is capable of probing the TRIR of organometallic systems over the ns to ms time regime.

Three different IR probe sources have commonly been used in such systems. In terms of convenient selection of probe frequency, a globar black body source combined with a monochromator has proved quite useful in various labo- ratories (10) but has the problem that the photon flux at any frequency is relatively low (which inherently leads to poor S / N or resolution problems). To counter this problem, Po-

Photochemistry and Photobiology, 1997, 65(1) 59

liakoff and Weitz ( 1 1) described the use of CO lasers for the TRIR investigations of the photoreactions of metal carbonyls. The CO laser gives high monochromatic photon flux (50-500 mW) in a spectral region appropriate for investigating metal carbonyls, although its range is limited (2000- 1500 cm-I). A CO laser is line tunable but the spacing of the laser frequencies is 4 cm-I. Resolution is improved by using I3CO in the laser as the resulting lines fall between those observed when the laser is filled with lZCO.

A third IR probe source between the globar and the CO laser in single frequency photon flux (0.1-10 mW) is the semiconductor diode IR laser that has the advantage of continuous tunability with very high resolution (<lo-’ cm-’) (12). These miniature lasers are manufactured from single crystal “lead salt” semiconductor alloys. The band gap can be altered by varying the composition and such semiconductor diode lasers are available for a broad IR frequency range (3050-370 cm-I). However, an individual laser of this type has a maximum scanning range of 100-150 cm-I and is tuned over this region by changing the temperature and current through the diode. These operate at low temperatures (<60 K), and early systems required a closed cycle He refrigerator. A newer type can operate at liquid N, temperatures (80-1 20 K) but typically have less power. Nonetheless, not being coupled to the closed cycle refrigerator lowers the cost of these systems and has the added advantage of re- ducing a major source of noise. A cold head has four diode lasers so that a broader range of frequencies is conveniently accessible. For TRIR studies of organometallics in condensed phase, the range, resolution and continuously tunable nature of the semiconductor diode IR laser make it the most advantageous of the three probe systems described in this section.

Signal detection in the TRIR systems described in this section is typically achieved with a liquid N,-cooled Hg- CdTe or InSb diode IR detector. The material of choice is HgCdTe owing to its broad range of spectral response (5000-400 cm-I) and HgCdTe detectors can be made with rise-times near 10 ns. Photovoltaic (PV) detectors are generally better for fast response and low noise than the comparable photoconductive (PC) diodes (13). In the system described in Fig. 1 a low-noise fast rise-time preamplifier is used to boost detector signal, which is recorded on a digital oscilloscope before transferring to a dedicated computer for data workup by custom software.

Ultrafast TRIR systems

The principal differences between the various methods used for TRIR interrogation of short-lived transients by ultrafast (picosecond and femtosecond) laser flash photolysis lies in the manner by which the IR probe source is generated and detected. This topic has been the subject of several reviews (14,15), so the present discussion will limit itself to a broad- stroke survey of several representative systems.

In ultrafast TRO studies part of the excitation pulse is used to generate a continuum probe pulse that is passed through the sample into a spectrograph mounted on an optical multichannel analyzer (OMA) detector system. Spatial delay of the probe relative to the pump pulse provides the timing ( 1 ps = 0.30 mm). This optical setup results in a

Nd:YAQ Laser h Dual Jet Dye L a r r

c r

I I .

...-. &-.-...

Figure 2. Schematic of a picosecond TRIR apparatus using a single frequency IR probe pulse (dashed line) generated by frequency mixing and IR detection (adapted from Stoutland er al. (14)).

highly sensitive system with relatively good spectral resolution. Most ultrafast TRIR systems are based on similar principles but generate IR probe pulses instead.

The IR pulses can be generated by Raman scattering (16) but the more common method used involves nonlinear mixing with a material such as crystalline LiIO-,. In the latter scheme, photons of a pulse at one visible frequency are converted (with conservation of energy and momentum) to photons of a lower visible frequency plus IR photons at a frequency representing the difference. The conversion efficien- cy is enhanced by seeding the crystal with a pulse at the lower visible frequency. A broadband IR pulse can be generated if the visible seed pulse itself is broadband. Problems with group velocity dispersion cause inadvertent pulse broadening but are minimized by using very short LiI03 crystals. For IR generated from fs pulses one has to be concerned also about uncertainty broadening because a 100 fs pulse would have a bandwidth of -150 cm-I. Notably, spectral resolution lost to uncertainty broadening can be “recov- ered” by dispersing the signal through a monochromator after passing through the sample ( 17).

Figure 2 illustrates the ultrafast TRIR system described by Stoutland et al. (14), which has a pump pulse of tunable wavelength and IR probe pulses generated by the frequency difference method. In this case the probe beam is split into sample and reference beams before reaching the sample cell. The sample and reference beams are individually detected by InSb or HgCdTe diodes (77 K) and the signal ratios com- puted. With this apparatus, absorbance changes on the order of with S/N > 10 have been recorded with reasonable


d.OEtor

Figure 4. Schematic of a picosecond TRIR apparatus based on a CW IR probe beam with timing determined by frequency mixing with a visible “gate” pulse and detection of the frequency mixed signal (adapted from Wynne and Hochstrasser (15)).

Rhcdarnine 6G (RBG) Dye Laper

t Nd:Yag or DCM dye laser

Bearnspllner (BS)

Figure 3. Schematic of a dual-beam picosecond TRIR instrument using a broadband IR probe pulse (dashed line) generated by frequency mixing with detection by a CCD camera following up conversion to visible wavelengths (adapted from Dougherty and Heil- weil (19)).

signal averaging (several thousand laser shots at 30 Hz) and temporal resolution of -0.5 ps (1 8).

A somewhat different approach has been taken by Heil- weil and coworkers, whose system is illustrated in Fig. 3 (17.19). In this apparatus the IR pulse is generated with -100 cm-l bandwidth by frequency mixing in LiJO, between the second harmonic of a Nd:YAG laser (532 nm) or Nd:YAG-pumped DCM dye laser and the broadband output of a synchronously pumped dye laser also in the visible range. This pulse is split into a reference and a signal pulse, the latter being sent through the sample cell at some temporal delay after the pump pulse. The two IR pulses are then upconverted by mixing with 532 nm light in another LiIO, crystal. The resulting two visible pulses are dispersed across a charge-coupled device (CCD) detector. This method allows one to exploit the sensitivity of optical multichannel detectors to obtain transient spectra of 100 cm-I bandwidth with 4 cm-I spectral resolution, lo-) AOD sensitivity and an effective experimental time resolution of 0.4 ps. Because the dye laser is tunable, the frequency of the resulting IR pulse can also be tuned. Therefore, the IR range available should be limited only by the tuning range of the LiIO, crystal (1800-4000 cm-I).

Hochstrasser and coworkers have used a third approach that is illustrated in Fig. 4 (15,20,21). In their system, the sample is probed by a tunable diode IR laser or a line tunable CO laser at a single frequency uir. which is continuous over the time scale of the experiment. After the sample is subjected to excitation by a visible region pulse from a femtosecond pump laser, a temporal response of the IR absorption at u,, will result. Time-resolved detection of this continuous- wave (CW) IR beam is accomplished by frequency mixing in a LiIO, crystal with a femtosecond visible gate pulse at uVirr which has been time-delayed relative to the pump pulse. This mixing upconverts the IR into a pulse at frequency uVis + uir with an intensity proportional to the intensity of the IR beam transmitted through the sample. The upconverted pulse can then be selectively detected with a photomultiplier tube (PMT). The 1 kHz pump beam is synchronously chopped to give alternating pump on-pump off signals that are then de- modulated in two separate lock-in amplifiers. The demodu-

lated photolyzed vs unphotolyzed signals are then used to compute the average differential IR absorbance (AOD). The advantage of this system is that CW IR lasers have very narrow line widths. Hence the detection can be accomplished at very high spectral resolution on a sub-picosecond time scale without the complications of uncertainty broadening. However, because each experiment provides but a single point on the three dimensional IR intensity/frequency/time surface, the acquisition of a detailed TRIR spectrum is a sizable undertaking.

Flash photolysis with step-scan FTIR detection

The nanosecond and ultrafast techniques described above suffer from relative inconvenience in acquiring data over a broad IR spectral range. With an apparatus such as described in Fig. 1, the frequencies of the IR diode lasers commercially available cover most of the infrared range typically used by inorganic and organic chemists for compound characterization. However, it is clear that when TRIR spectra are obtained from one manually tuned frequency at a time, the spectral resolution is dependent more upon the patience of the researcher than upon instrumental limitations. Further- more, the tuning range conveniently available with the four lasers in a single cold head is likely to be <500 cm-I. Black- body sources provide more convenient tunability because all wavelengths occur in their emission. As noted above these were among the first sources used in TRIR studies in point- by-point studies (10). A somewhat different way to utilize a black-body IR source is step-scan interferometry.

An infrared spectrum of a static material can be obtained by scanning u and recording transmittance of the sample (ratioed against a reference to correct for variations in the source) as a function of the frequency. Alternatively, an interferogram can be generated by recording the intensity of the signal from a black-body source passed through a sample and an interferometer as a function of the displacement of a moving mirror. This intensity information can be converted to a normal spectrum by taking the Fourier transform (FT) and ratioing to similar data generated for a reference. Such an interferogram could also be generated, point by point, if the mirror were precisely “stepped” from one fixed position to another and the intensity measured accurately as a function of that mirror position. If the sample is subjected to laser flash photolysis precisely coupled to the detection electronics, then one can record the time-dependent intensity at each step of the mirror scan (Fig. 5 ) . A Fourier transform of the intensities recorded at a specific time delay for the entire collection data set of mirror step positions gives the IR spectrum for that time, and an ensemble of these spectra as a


( >(jBW ( >+j ( C(jH,4>

Photoconductive MCT Detector (250na riaetlme)

Fixed mirror I

I‘ movlng mlrror

Laser Excitation (354.7 nm, 10 na 200 pJ /pu i~ )

. .

Step-Scan Interferometer

Figure 5. Schematic of modified BioRad FTS 60M896 (step-scan interferometer). The IR beam from the spectrometer is aimed out of the sampling port to an external optical train and then focused to a spot 0.3-1 mm onto the IR sample cell. The pump beam is then brought in at a small angle to the probe or made colinear with a dichroic (adapted from Schoonover er al. (24)).

function of time provides the three dimensional surface of frequency v s time vs absorbance.

A step-scan FTIR has a modified interferometer that allows the moving mirror to be stepped to an accurately pre- determined site where one point of the interferogram is recorded after a given event (laser pulse). The mirror is then “stepped” to the next position and another point of the interferogram is collected. The spectral resolution is determined by number of interferometer steps. The temporal resolution is determined by the rise time and sensitivity of the detector and electronics. For example Schoonover and coworkers (22) have recently described a system using a HgCdTe PV detector with a rise time of 250 ns. Step-scan FTIR spectrometers are commercially available but require significant modification for applications in flash photolysis TRIR studies (23.24).

A disadvantage of the step-scan method is that it does not allow setting the system at a single well-defined observation frequency to accumulate accurate kinetics data for the rise and/or fall of species absorbing at that frequency. Major advantages are that this is a methodology ideal for computer- controlled automation and that the experiment collects TRIR over the full frequency range of the spectrometer. The use- fulness of such data is still limited by the intensities of the bands affected by the flash photolysis experiment, but the advantages of full spectral accumulation are likely to be re- alized as more sensitive detectors become available.

SURVEY OF SOME ORGANOMETALLIC TRIR STUDIES Three examples relevant to the thermal and photochemistry of organometallics will be discussed in order to illustrate the application of TRIR techniques. In each case, the IR frequencies probed correspond to the metal carbonyl region be-

n 0 -4

2(

I = y ) z 333 .3 - ‘ ps

2 0 0 ps

\ I

\ I - 6 .67 P S I u I

-

‘ I ’ ( I .O ’ 20’00 1960 19’20 1 8 8 0

W a v e n u m b e r s Figure 6. The TRIR difference spectra in the picosecond Rash photolysis of W(CO), in hexane (adapted from Dougherty and Heilweil (28) ) .

cause u,, are the marker bands in the vast majority of systems examined to date. The carbonyl stretching bands usually have very high absorption cross sections and are quite sensitive to the stereochemistry and electronic nature of the metal coordination sphere. Furthermore, the IR wavelength region around 5 pm (2000 cm-I) has been conveniently accessible both for generation and for detection.

The first example discussed is concerned with the events that define the primary photosubstitution reaction of the complexes M(CO)6 (M = Cr, Mo or W) (Eq. 4). The transients formed by Rash photolysis of these have been studied by numerous workers as models for “unsaturated” organometallics (l-3), and it is now recognized that the principal primary photoproduct of UVhisible excitation is the solven- to complex M(CO),S (S = solvent) (Eq. 4). This undergoes subsequent reaction with various ligands to give M(CO),L.

(4)

Ultrafast laser excitation with TRO detection has been used to examine the sequence of events leading from initial excitation to the solvento species. In this way, several groups have demonstrated that while loss of CO from the initially excited metal hexacarbonyl occurs in <1 ps, there is a period of some 10’s of picoseconds before a constant optical spectrum for the solvento species is obtained (25,26). The slower process has been interpreted in terms of solvent coordination and vibrational cooling during this interval (27). Dougherty and Heilweil (28) reinvestigated this process for each of the hexacarbonyls using 289 nm light for femtosecond excitation and the ultrafast broadband multichannel transient absorption instrument described in Fig. 4 for TRIR detection. Their results with W(CO)6 in hexane are illustrated in Fig. 6.

M(C0)6 --% M(CO),S + CO

62 Peter C. Ford eta/.

100 5

Figure 7. IR spectral changes following 308 nm photolysis of A in cyclohexane in the presence of 6.0 mM 4-phenylpyridine. Spectra are spaced at 4 ps intervals (modified from Boese and Ford (32)).

The key features of this figure are the TRIR spectrum at t = 1.67 ps clearly shows the TI, symmetry v,, band of W(CO)6 to be bleached, but there are only broad and nearly featureless absorbances for any product this soon after excitation. After 16.67 ps, the product spectrum has sharpened into four bands at 1956, 1942, 1928 and -1908 (w) cm-I. Over a longer period of several hundred picoseconds, the bands at 1942 and 1908 cm-l disappear whereas the other two, assigned to the W(CO),(hexane) photoproduct grow and sharpen. The former transient bands, for which a lifetime -160 ps can be calculated, are attributed to v = 1 + 2 transitions of the photoproduct vco bands. Notably, the 289 nm excitation energetically is more than twice the W-CO bond dissociation energy, yet only one CO is labilized (in contrast to gas-phase systems where multiple CO dissociation might be expected (29-31)). The excess energy leads to formation of vibrationally excited photoproducts, and the longer relaxation process can be attributed to the vibroni- cally excited carbonyls that decay over several hundred picoseconds. The first decay process, i.e. the initial band sharp- ening, was attributed to the relaxation of low-frequency modes coupled to the CO stretches.

A second example is drawn from studies in this laboratory (32) using the TRIR system described in Fig. 1 to elucidate the reaction dynamics of reactive intermediates in the migratory insertion of CO into metal alkyl bonds. Transients with stoichiometries corresponding to the intermediates proposed for thermal mechanisms were prepared by running the reaction backward. For example, 355 or 308 nm photolysis of the acyl species A results in CO dissociation to give an intermediate I with the stoichiometry (CH3CO)Mn(C0),, which can undergo isomerization to the methyl complex M, i.e., the reverse of migratory insertion, in competition with other reactions (Eq. 5) .

O M 0

A

Such studies allowed elucidation of the TRIR spectra and reaction dynamics of I as functions of the solvent medium

t 2 \ I v t-5 I

1 . . . . , . . . . 1 . . . . 1 . . . . , . . . . .IOIIO' -1 0 V

1800 1850 1900 1950 Zoo0 2050 2100

cm-l

Figure 8. A: Ground-state ( I ) and 600 ns TRIR difference ES spectra (2) of the rhenium(1) carbonyl complex F. B: Ground-state (1 ) and 600 ns TRIR difference ES spectra (2) of the rhenium(1) car- bony1 complex D (adapted from Schoonover er d. (22)).

and other solution variables such as trapping agents L that can capture I to give cis-Mn(CO),(CH,CO)L before methyl migration has a chance to occur. An example of the TRIR spectra in the presence of a trapping agent is shown in Fig. 7. Studies of this type led to the conclusion that I exists as the chelated acyl complex C in weakly coordinating solvents such as cyclohexane but as the solvento species S in tetrah- ydrofuran.

CH3

'I C S

The third example to be considered is the use of the step- scan technique to probe the TRIR spectrum of the relatively long-lived electronic ES of the rhenium(1) carbonyl complexes D and F, each of which have microsecond ambient temperature lifetimes in solution (22). The IR spectra ac- quired for these compounds by step-scan FUR 600 ns after excitation (Fig. 8) shows diagnostic differences. For both complexes, the carbonyl bands of the ground state are bleached as shown by the negative bands in the spectrum. For D* these CO stretching bands are shifted to higher frequencies. This would be consistent with the assignment of the electronic ES as an MLCT ES where the metal is for- mally oxidized by charge transfer into the phenanthroline ligand. The higher positive charge on the metal reduces


Table 1. to timing methods

Summary of TRIR apparatus based on optical pump lasers with different probe sources and detection techniques listed according

Timing (method) IR probe Detection References

Millisecond to nanosecond (point-by-point CW monitoring of interferogram vs t: transient digitizers)

Microsecond to nanosecond (CW monitoring of I vs t: transient digitizers)

Picosecond to femtosecond (delay train)

Picosecond to femtosecond (delay train)

CW source Globar

CW source including Globar CO laser Pb salt diode lasers

CW IR diode laser

IR pulse by frequency mixing ( v , ~ = u , - v2. single frequency of broad band)

Step-scan F l l R with Hg- 22-24 CdTe diode detector

IR diode detector ( e g HgCdTe)

Upconversion, i.e. IR mixed with optical laser and detected with PMT (vdelecled = yo + 4 conversion detected with a PMT or CCD

IR diodes detectors or up-

10, 11, 7, 8

15, 20, 21

14, 17-19

backbonding into ligand P* orbitals, hence v,, values in- crease. In contrast, there is little change in the Y,, values between F* and the respective ground state. This indicates that there is little change in the metal to ligand backbonding, which is consistent with the assignment of the long-lived ES in this case being a ligand-localized ITP* state.

I co I co

D F

SUMMARY The above discussion has outlined different methodologies for accessing the time-resolved infrared spectra of transient species generated by flash photolysis (Table 1) and has illustrated these with several examples taken from the organometallic literature. Each of these examples and the vast majority of organometallic as well as organic (33) systems examined by this technique has principally focused on spectral changes in the u,, region. Clearly the strong carbonyl stretching bands are valuable markers when examining the chemistry of transient species having these functional groups present. Although some studies have used the v,, bands of co- ordinated cyanides, changes in the imine bands of metallo proteins etc.. it is clear that there is a much wider range of functional group frequencies that might be exploited using the TRIR techniques. Improved sensitivities, greater convenience of use over a broader range of frequencies and cre- ative application of spectroscopic analysis promise to make TRIR a technique of growing importance in the investigation of photochemistry and reactive intermediates of thermal processes.

One might speculate briefly on the directions that new technology might take. Several of the techniques noted above allowed for extending the spectral ranges for TRIR investigations. Although it requires considerable signal averaging (hence would be a problem for photosensitive ma- terials) the relative ease by which the step-scan FTIR instrumentation is automated promises to make this a convenient method for obtaining spectra over broad spectral ranges down to the nanosecond time regime. However, the temporal resolution of step-scan FTIR is certainly limited by the speed and sensitivities of the detectors used and intensities of the black-body probe sources. For the ultrafast systems, the upconversion of a broadband IR source allows the recording of a range of IR frequencies to be recorded with highly sensitive visible range CCD detectors. It would be especially attractive if such a broadband IR source could be recorded directly with an IR diode array device or other multichannel detector. It seems that an attractive direction for future development in the area of TRIR detectors would be to exploit the known technology of infrared imaging used in IR cam- eras for military and civilian applications.

Acknowledgements-Studies at UCSB have been sponsored by a grant to P.C.F. from the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy (DE-FG03- 85ER133 17). Support for instrumentation development came from a U.S. Department of Energy University Research Instrumentation Grant (no. DE-FG05-91ER79039) and the US. National Science Foundation Instrumentation Grant (CHE-9413030). We thank Leroy Laverman of this department for his help in preparing graphics.

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