6.2 This Is How It Happens: Time-Resolved Spectroscopy in Photochemistryin Photochemistry
6.2.1 Flash Photolysis with Lamps
The history of flash photolysis is essentially that of increasing time resolution. As Lord Porter remarked in his Nobel address [14], science and technology have steadily extended the strict limits of man senses, “so as to enable him, despite the
Flash Excited
State
Intermediate Matrix Time
Window Fig. 6.1 Highly reactive
intermediates may be detected either by creating a high local concentration by a strong flash and probing it within a short time window or by hindering further evolution by confining it in a matrix
6.2 This Is How It Happens: Time-Resolved Spectroscopy in Photochemistry 143
very limited sense with which he was endowed, to observe phenomena with dimen- sions very different from those he can normally experience[. . .]Thus, in the realm of the very small, microscopes and microbalances have permitted him to observe things which have smaller extension or mass that he can see or feel. In the dimension of time. . .most of the fundamental processes and events, particularly those in the molecular word that we call chemistry”, occur in ms or much below that time, although the fastest event perceived by human eye is around 1/20th s. Entering in the realm of the short times thus had meant obtaining a better understanding of the way molecules form and react.
Immediately after World War 2, the flash photolysis method was elaborated in England. Millisecond flash lamps were developed at that time and absorption of a flash caused a dramatic perturbation. This made possible the direct observation by a relatively insensitive physical method such as UV-visible absorption both of excited states (at least of the long-lived ones, which Lewis and Kasha had observed by more sensitive emission techniques; see below) and, most typically, of interme- diates formed from them, in particular of radicals. For each flash, the change of absorbance at a fixed wavelength or the total spectrum at a fixed delay from the flash was registered. The method was versatile and could be used with both large and small specimens, in the gas or condensed phase. The time delay was introduced by inserting a rotating sector or electronically [14,15].
A typical example was the study of the generation of the biatomic radical ClO• by combination of atomic chlorine and oxygen (see Fig. 6.2) [16]. Notice that although in a 1947 Faraday Society meeting devoted to “The labile molecule” [17]
there was almost no indication of pulse methods, progress was quite fast, and in 1954 a following Faraday meeting was organized on “The study of fast reactions”
[18,19] and was based on the new results on radicals derived from flash photolysis, pulse radiolysis, mass spectrometry, and other methods.
Fig. 6.2 Photographic plates showing a sequence of spectra of ClO•after flash photolysis of a chlorine–
oxygen mixture and the bimolecular decay of this species; reprinted by permission from [16]
144 6 Photochemistry, a Powerful Science
Likewise, organic radicals were detected, giving an unprecedented support to the understanding of the nature and reactivity of such intermediates. As an example, benzyl radicals, characterized by a strong absorption around 300 nm, were gener- ated by homolytic fission of alkylbenzenes and benzyl halides (see Fig.6.3). Thus, flashing of toluene vapor showed some distinct absorptions, in particular one around 305 nm, which were observed also from benzyl chloride and, with very little difference, from ethylbenzene and was thus attributed to the benzyl radical (see also the following section) [19]. The improvement of lamps for a short flash (2 μs in the spectra in Fig. 6.4) allowed a more detailed examination and the detection of further bands [20].
As mentioned, also long-lived aromatic triplets could be observed and their interaction could be studied by using flash lamps. In particular, the decay of triplet naphthalene and anthracene was observed and found to depend strongly from viscosity (Figs.6.5 and 6.6). This fact was attributed to the deformation of the triplet with respect to the ground state [21]. Notice that the high energy of the flash generates a high local concentration of excited states, much higher than in prepar- ative experiments. Thus, under these conditions, diffusion-controlled bimolecular triplet–triplet annihilation may be the predominant process, overcoming monomo- lecular decay [21]. Triplet–triplet annihilation (see Chap.7) may in part lead to the singlet and “delayed” fluorescence (that is, the emission spectrum corresponding to the S1!S0emission, but lifetime corresponding to that of T1), a phenomenon well known from fluorescence studies [22].
Fig. 6.3 Photographic plates of the original experiment by Porter for the identification of benzyl radical. Spectra detected before (upper trace) and during the photolysis of (a) benzyl chloride and (b) toluene, showing the appearance of the benzyl radical signals. Reprinted by permission from [19]
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Many radicals absorb in the visible and near UV region, and thus photographic detection is suitable. For those that do not, the development of appropriate lasers made it possible to observe high-resolution IR spectra within a short timescale for the study of radicals in the gas phase [23]. With the availability of relatively cheap
DELAY 500 μ sec 200 ..
150 ..
100 ..
50 ..
20 ..
10 ..
5 ..
2 ..
C6H5CH2Br
λλ
23 24 25 26 27 28 29 30 31 32
230.2 nm 255 nm 305.3 nm
230 250 290 310
Wavelength, (nm)
Optical density
1.5
0.8
0.0
Fig. 6.4 Benzylic radicals generated by a much shorter flash duration than in Fig.6.3afrom benzyl bromide, photographic trace and diagram of the spectrum; by permission from [20]
146 6 Photochemistry, a Powerful Science
and solid laser sources and the appropriate electronics for detection, resolution got down to a few nanoseconds with flash photolysis and then picoseconds and below with the pump-probe spectroscopy. With the last technique, the sample volume is repeatedly excited by a succession of short laser pulses. The detection system registers the combined accumulated signal at variable time delays between pump and probe, so that a kinetic profile can be obtained. In this way, it is possible to observe and characterize singlet and triplet states [15].
Fig. 6.5 Dependence of triplet lifetime on viscosity. Oscillographic records of the decay of triplet naphthalene in (a)n-hexaneη¼0.31 cp, (b) waterη¼0.89 cp, (c) ethylene glycolη¼35.7 cp, (d) paraffinη¼33 cp, (e) paraffinη¼167 cp. Time units, ms; cp, centipoise; by permission from [21]
6.2 This Is How It Happens: Time-Resolved Spectroscopy in Photochemistry 147