Reactive Intermediate Chemistry

(typically 2 mA) for a short time. Detection from 200 nm and into the nearinfrared (NIR) is readily achieved with modern photomultipliers, although the same detector may not be suitable for both wavelength extremes. The use of fiber optic technologies and compact photomultipliers has greatly reduced the size of nLFP systems.

The system, as described above, is capable of signal detection as a function of time at a selected wavelength. Spectra are constructed by monitoring time-resolved traces at many wavelengths, and then extracting absorbance data at a given time from each trace.

An alternate approach consists of using a gated spectrally resolved detector to capture spectral data in a well-defined, and frequently narrow, time window. Image capture is achieved with either a diode array detector or a CCD camera. An intensifier located between the output window of the spectrograph and the detector provides the gating capabilities. In this approach, time resolution can be achieved by collecting spectral data at different times and then reconstructing the time profile at one or more wavelengths.

Ultimately, the two approaches mentioned above collect data in a threedimensional (3D) matrix in which the vertical axis is signal intensity (usually transient absorbance), and the other two axes are time and wavelength. The difference is simply which direction in the matrix is collected for each laser shot.

2.2. Data Acquisition and Processing

The signal from a nLFP system operating in the time-resolved mode (see above) is the voltage across a resistor (terminator) at the output of the photomultiplier. This terminator (typically 50 or 93 ) needs to be matched by the type of signal cables used. The output has the shape shown in Figure 18.3 for a continuous wave

Laser pulse causes absorbance increase

Time

It

I0

Products absorb light, signal does not return to pre-excitation level

Signal

Figure 18.3. Time evolution of the photomultiplier output as an absorbing transient is generated and decays.

852 NANOSECOND LASER FLASH PHOTOLYSIS

(CW) (i.e., not pulsed) monitoring source. Note that the voltage signals from photomultipliers are negative.

The laser fires at t ¼ 0 and causes an increase in absorbance in the sample; as a consequence the intensity of light reaching the detector decreases. While laser photolysis systems are normally single-beam spectrometers, in fact they behave as dual-beam instruments. The reference beam is separated from the sample beam in time, rather than space. Thus, the reference signal is acquired before laser excitation and leads to I0. The absorbance at time t in Figure 18.3 is given by Eq. 1:

Since the reference beam is the same signal prior to laser excitation, the technique measures changes in absorbance, normally abbreviated as OD. The abbreviation is derived from ‘‘change in optical density’’; however, the preferred IUPAC term is absorbance. It is therefore possible to acquire inverted signals, when the laser pulse causes bleaching of the sample. Naturally, bleaching can only be observed where the starting materials can absorb the monitoring beam; it is easy to confuse emission from the sample with bleaching, since both lead to inverted signals. However, bleaching requires the monitoring beam on, while fluorescence (or scattered light, phosphorescence, etc.) can also be observed with the monitoring beam off.

2.3. Transient Spectroscopy

The output from a nLFP is normally a plot of OD versus time at a given wavelength, following processing of the data as indicated in Section 2.2. The signal intensity obtainable, and indeed desirable, depends on many parameters. Some of the key parameters are outlined below.

The signal intensity depends on the energy-per-pulse delivered by the laser, however, more is not always better. A range of 5–25 mJ/pulse delivered to the sample is convenient frequently achieving reasonable signals without the problems that can be encountered at very high laser doses (see below).

The signal is proportional to the product of the quantum yield of transient formation times the extinction coefficient difference between the transient and its precursor.

Many reactive intermediates can decay via self-reactions, giving dimers or disproportionation products, as is the case of free radicals and carbenes. When these self-reactions are not the ones under study, it is desirable to keep the

transient concentration low enough to minimize this type of interference. For

example, for a radical that dimerizes with kt ¼ 3 109 M 1 s 1 and a concentration ‘‘c’’ of 10 5M, its first half-life (t1=2 ¼ 1=kc) would be 33 ms. Note that excited triplet states also undergo bimolecular decay by triplet–triplet

annihilation (TTA). Rate constants for this process usually approach or exceed 1010 M 1 s 1.

The absorbance, OD, is independent of the value of I0, the monitoring light intensity. However, the signal/noise ratio is strongly dependant on the I0 value. Very high I0 values can lead to distorted signals and nonlinear response. The I0 value that a given nLFP can tolerate depends on the photomultiplier used and on the wiring of the dynod chain. Typically, 2 mA are acceptable for short periods of time; with a 93 terminator this corresponds to 200 mV.10

2.4.Kinetic Studies

The simplest situation is that in which the reactive intermediate of interest can be directly observed and decays with clean first-order kinetics, with the signal after decay returning to the preexcitation level. There are many cases in which this situation is encountered, such as the case of many short-lived excited triplets. The decay follows simple first order kinetics, that is,

where the subscript 0 indicates initial conditions, and t after time t. Note that it makes no difference whether the transient concentration [R] or the absorbance is used for the calculation–plot.

The lifetime of the intermediate is given by t ¼ 1=k and the half-life by Eq. 3,

where 0.69 is ln 2. The k obtained from this analysis is seldom the reflection of a single elementary process. Rather, it is more frequently than not, a composite of more than one parallel mode of decay. We will usually refer to it as the experimental rate constant or kexp.

It is common for many second-order reactions to exhibit pseudo-first-order behavior under conditions of nLFP. This is due to the fact that, while reactive intermediates are present in micromolar concentrations, (typically 10–50 mM), the molecules with which they react are present in concentrations several orders of magnitude larger. As a result, the concentration of these reagents remains essentially constant during the decay of the transient species. An example is shown in Figure 18.4, where triplet benzophenone is quenched by melatonin.12

The decay of the benzophenone triplet follows pseudo-first-order kinetics, and the experimental rate constant is related to the parameters of interest by Eq. 4,

854 NANOSECOND LASER FLASH PHOTOLYSIS

0.03 OD

0.02

0.01

0

Time (s)

Figure 18.4. Quenching of benzophenone triplet by 0.24-mM melatonin in acetonitrile, monitored at 600 nm following 355-nm laser excitation. The spectrum was recorded at room temperature, under a nitrogen atmosphere.12

where k0 is the rate constant for triplet decay in the absence of quencher (k0 ¼ t0 1) and kX is the rate constant for triplet quenching, in this example by melatonin. By plotting the values of kexp for different concentrations of quencher, as shown in Figure 18.5, it is possible to separate k0 (interpret) from kX (slope).

The trace in Figure 18.5 returns approximately to the original, preexcitation baseline. This trace was recorded at 600 nm (away from the 525-nm triplet

Figure 18.5. Quenching of benzophenone triplet by 0.24-mM melatonin in acetonitrile, monitored at 600 nm following 355-nm laser excitation. Each point is derived from a trace such as that shown in Figure 18.4. The spectrum was recorded at room temperature, under a nitrogen atmosphere.12

maximum) in order to minimize interference by the benzophenone ketyl radical, that has an absorption maximum 545 nm. It is not uncommon to observe traces that do not return to the same baseline, but rather to a higher OD level, showing that the products of reaction have absorbance at the monitoring wavelength.

A case in point was described in detail by Encinas et al. in 1980.13 This example of a biradical is derived from g-methylvalerophenone, which can be scavenged by octanethiol according to the mechanism of Scheme 18.1. In this case, the triplet state is very short lived ( 2 ns) and for practical purposes laser excitation leads to the biradical (BR) as the first detectable intermediate.

n-C8H17S-H

Scheme 18.1

The absorbance from the biradical is known to be exclusively due to the ketyl radical center,14 and therefore trapping by the thiol to produce R. not only preserves the chromophore, but in fact it makes it longer lived. Since the biradical lives100 ns, and the ketyl radical (R.) many microseconds, the latter can be regarded as a stable product in the time scale of the experiment; that is, the residual absorbance following biradical decay appears flat, as shown in Figure 18.6. In this example we have a biradical decaying completely with pseudo-first-order kinetics. The radical R. forms concurrently with biradical decay, and since both processes are directly linked, the decay (BR) and growth (R.) follow the same kinetics. Figure 18.6 shows the calculated traces for BR and R., and the sum of both signals, corresponding to the experimental observable.

858 NANOSECOND LASER FLASH PHOTOLYSIS

A first-order rate constant for the growth can be derived from the same analysis of Eq. 5; the values of kgrowth are then related to the rate constants of interest according to Eq. 9.

Thus, a plot of kgrowth versus [X] yields k0 from the intercept and kx from the slope.

2.5. The Probe Technique

The problem with the methodologies discussed above is that they require that either the reaction intermediate under study, or the product of its reaction with the substrate, give a signal in the spectral region accessible. Could one employ nLFP to determine rate constants where all the reagents and all the products are invisible to the detection technique employed? The answer to this question is affirmative, and the technique that we introduced in the late 1970s is now known as ‘‘the probe method.’’ Interestingly, when we first used this technique we did not realize that it was a new method and no claim was made of its originality; it was simply a straightforward outcome of the kinetic expressions involved.4,15,17

There are numerous reactions of interest that fit the description mentioned above. For example, the reaction of tert-butoxyl with aliphatic alcohols is essentially invisible or ‘‘silent’’ in nLFP.15 A practical example would be the reaciton of t-BuO. with 2-propanol, in which the reaction with benzhydrol (diphenyl methanol) could be used as a probe. The mechanism is shown in Scheme 18.3 for this example.

Signal carrier

Scheme 18.3

The reaction can be generalized, as illustrated in Scheme 18.4, for the case of tert-butoxyl radicals.

k 0

t-BuO Decay

k X

k P

t-BuO + P— H t-BuOH + P

Signal carrier

Scheme 18.4

Thus, according to Scheme 18.4 the rates of consumption of alkoxyl (RO.) and of formation of the probe radical (P.) will be given by Eqs. 9 and 10.

It is important to understand the relationship between d[P.] and d[RO.], given by Eq. 11.

The term in square brackets in Eq. 11 determines which fraction of the alkoxyl radicals will actually react with the probe PH. Thus, the increase in [P.] equals the decrease in [RO.], corrected by the fraction of reaction with PH, relative to all other processes in which the alkoxyl radicals participate.

When the reaction is complete the concentration of P. in the plateau region, [P.]1, will be given by Eq. 12: