Instrumentation
Ε. F. Hilinski P. M. Rentzepis Bell Laboratories Murray Hill, N.J. 07974
PICOSECOND
SPECTROSCOPY METHODS AND RECENT APPLICATIONS Advances in laser technology and methods of optical detection have per mitted us to investigate chemical reac tions spectroscopically in increasingly greater detail. The cascade of events that occurs when light is absorbed by a molecule can be initiated and probed with the ultrashort picosecond pulses emitted by mode-locked lasers. Tran sient species and states involved in a photophysical or photochemical pro cess can be detected, and their kinet ics of formation and decay can be measured directly as the process evolves by means of picosecond emis sion, absorption, and, recently, Raman spectroscopy. On the time scale of 1 0 - 1 2 s, sufficient time resolution ex ists so that complex mechanisms of many important chemical and biologi cal reactions can be elucidated in greater detail than was previously pos sible. Laser systems based on N d 3 + / yttrium aluminum garnet (YAG) os cillators and synchronously pumped dye lasers are currently finding wide spread applications in numerous mechanistic studies. With the use of detection equipment such as the streak camera and two-dimensional photodiode arrays and the abilities of micro- and minicomputers to store, process, and average data and to present the data in a tabular or graph ical form, optical data in the picosec ond time regime can be acquired in an interprétable manner relatively easily and straightforwardly. We will describe several types of laser systems and present some examples of recent problems in organic, inorganic, and biological chemistry that have been studied by means of picosec0003-2700/83/A351-1121$01.50/0 © 1983 American Chemical Society
ond absorption and emission spectroscopy. Since the number of studies performed by means of picosecond spectroscopy has grown quite large in the years that have passed since the first picosecond laser experiment was performed (1 ), our discussion will be limited to a few studies that were performed in our laboratory including emission studies on vibrational relaxation in a polyatomic organic molecule, naphthazarin; photodissociation of haloaromatic compounds to give short-lived radicals; photoinduced intermolecular electron transfer between an acceptor molecule, chloranil, and several donor arènes; the kinetics of intramolecular electron transfer in a binuclear transition metal complex; and relaxation mechanisms of excitedstate metalloporphyrins. For other recent reviews, see References 2 and 3. Experimental The picosecond studies described in this paper were performed with laser systems in which the picosecond pulses were generated with a solidstate oscillator, either Nd 3+ /glass or Nd 3 + /YAG. Mode-locked solid-state oscillators such as Nd 3+ /glass, Nd 3 + /YAG, and ruby are responsible for laser pulse generation in a majority of the picosecond spectroscopic studies reported in the literature to date. Similar experiments have been performed with laser systems based on synchronously pumped dye lasers and passively mode-locked dye lasers, which are used in increasing numbers to probe a variety of physical and chemical phenomena. A typical laser system used for picosecond spectroscopy is illustrated in Figure 1. Al-
though the system in Figure 1 depicts a synchronously pumped dye laser system, a system based on a solidstate oscillator can be constructed by removing the argon ion pump laser, ring dye laser, and dye amplifier cells pumped by a nanosecond Nd 3 + /YAG laser and adding in their place a Nd 3 + /YAG oscillator and the number of Nd 3 + /YAG amplifiers required to achieve the desired degree of amplification of the picosecond pulse emitted from the oscillator. Therefore, after pulse generation and desired amplification are achieved, pulse direction and manipulation are performed in a very similar manner regardless of the original lasing medium. The laser system depicted in Figure 1 consists of an argon ion laser that pumps a ring dye laser to produce a train of low-energy picosecond pulses. In this particular system, the argon ion laser is actively mode-locked at 123 MHz by an acousto-optic modulator driven by an ultrastable frequency generator. At 514.5 nm, an average power of ~ 4 W is obtained with a pulse duration of 140 ps full width at half maximum (FWHM). The ring laser usually has rhodamine 6G (R6G) in ethylene glycol as its lasing medium. Pulses with a duration of ~ 1 ps FWHM are generated at a repetition rate of 246 MHz and are tunable in wavelength between 570 and 620 nm with R6G with an average power of ~100 mW. Of course, the range of wavelengths can be varied with selection of suitable dyes. Requirements of a particular picosecond study will dictate whether pulses of high energy, ~1-10 m j , generated at a rate of up to 10 Hz after in-
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983 ·
1121 A
Figure 1. Schematic diagram of a synchronously pumped dye laser system AC, autocorrelator; P, polarizer; PC, Pockels cell; CP, crossed polarizer; HVP, high-voltage puiser; PD, photodiode; SHG, second harmonic generating crystal; BS, beam splitter; CC, continuum cell
tense amplification, or pulses of lower energy content generated at a higher repetition rate (on the order of 100 MHz) are desired. If a high-energy pulse is desired, single pulse selection can be accomplished electrooptically by means of the usual arrangement of a Pockels cell and crossed Glan polarizers. The polarizers are crossed so that the ring laser pulses are transmitted by the first polarizer and Pockels cell but are rejected by the second crossed polarizer. Application of an ~8-kV pulse to the potassium dihydrogen phosphate (KDP) crystal of the Pockels cell causes the polarization of the pulse present within the crystal to be rotated by 90°. The rotation of polarization permits the pulse to pass through the second polarizer. This selected single pulse may be amplified by a series of flowing dye cells that are pumped by 10-ns pulses of the second harmonic of a Nd 3 + /YAG laser operating at a repetition rate of ~10 Hz. Saturable absorbers are placed between each dye cell to exclude superradiance. This pulse, which can be amplified by a factor of ~10 6 , has sufficient energy to be used
directly for sample excitation or to be converted to wavelengths other than those accessible directly from the dye laser. This can be accomplished by means of frequency doubling and/or stimulated Raman scattering. At this point, that is, after pulse generation and desired amplification, the picosecond spectroscopic experiment is essentially the same for either dye laser or solid-state laser systems. For comparison with the characteristics of the dye laser system described above, passively mode-locked N d 3 + / glass oscillators can be operated at up to ~0.02 Hz and produce pulses of 1060-nm light that are 4-8 ps FWHM in duration and have a bandwidth of ~30 c m - 1 . Passively mode-locked Nd 3 + /YAG lasers can be operated at ~10-20 Hz, and produce pulses that have a ~0.3 c m - 1 bandwidth centered at 1064 nm and a duration of ~20-40 ps. For a particular set of experimental conditions, the characteristics of a certain laser oscillator, such as pulse energy, bandwidth, repetition rate, and available wavelengths, may make one type of laser more suitable than oth-
1122 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 1 1 , SEPTEMBER 1983
ers. Available excitation wavelengths, of course, depend on the type of laser employed. With a picosecond synchronously pumped dye laser system, the excitation wavelength can be tuned within the fundamental lasing region of the dye employed. With a solid-state system, such as a Nd 3 + /YAG laser, the fundamental 1064-nm laser pulse is generally not useful for direct electronic excitation of a sample and therefore may be converted to the second, third, or fourth harmonic (532 nm, 355 nm, or 266 nm, respectively) by an appropriate harmonic generating crystal. This type of higher-order harmonic generation also is possible with sufficiently energetic dye laser pulses. Other wavelengths, shifted by 500 to 4000 c m ' 1 relative to the original pulse wavelength, also can be achieved quite efficiently by focusing a sufficiently energetic pulse into an appropriate liquid or gas to induce stimulated Raman scattering. Thus far, studies in the time regime of picoseconds have been performed primarily by means of emission, absorption, and, lately, Raman spectroscopy. Ideally, one would study a par-
ticular photoinitiated process with all of these available types of picosecond spectroscopy. However, experimental limitations often prohibit such a com plete investigation. Judicious applica tion of feasible picosecond time-re solved methods is usually sufficient to provide details about the mechanism of a reaction. For emission spectroscopy, the pi cosecond excitation pulse is directed into the sample cell. Time-resolved emission intensity can be measured by collecting the light emitted by the sample and focusing it into the slit of a streak camera, which is coupled to a vidicon, an optical multichannel ana lyzer (OMA), and a minicomputer. Filters or a monochromator may be used to confine the measurement of the emission lifetime to a specific spectral region. Emission spectra of transient molecular species may be obtained by directing the emitted light into the slit of a polychromator, which is also coupled to a vidicon, an OMA, and a minicomputer. In addition to emission spectroscop ic data, transient absorption spectra recorded as a function of time aid in unraveling the mechanistic complex ities of photoinitiated processes. Also, when possible, the structural informa tion provided by picosecond Raman data helps in the elucidation of reac tion paths. For absorption and Raman spectroscopy, the laser pulse is split into two parts: One is used for sample excitation and the second is a probing pulse used for Raman scattering or to generate an interrogating broadband continuum picosecond pulse for ab sorption spectroscopy. The continuum picosecond pulse is generated by fo cusing the probe pulse into a cell con taining D 2 0 / H 2 0 . The intensities of the band of wavelengths contained in the continuum pulse are adjusted by means of filters. A double-beam pi cosecond absorption spectrometer is created by splitting the continuum pulse so that it passes through a sam ple cell and a reference cell. Difference absorption spectra are recorded at se lected times relative to sample excita tion by measuring transmitted light intensities of both the sample cell and reference cell as a function of wave length when the sample is not subject ed to an excitation laser pulse and when the sample is excited by a laser pulse. The light intensities as a func tion of wavelength at a selected time relative to sample excitation for the absorption or Raman experiment are detected by means of a polychromator coupled to a vidicon, an OMA, and a minicomputer. In the absorption experiment de scribed above, changes in absorbance (ΔΑ) as a function of wavelength can be recorded for the spectral region
Table 1. Unrelaxed Emission Yields and Lifetimes for Several Vibrational Levels of Naphthazarin · T>
Αν
18119 18609 18750 19097 19206 19237 19393 19709 20023
0 490 631 978 1087 1118 1274 1590 1904
Naphthazarin D2 18182 0 18665 483 18805 623 a
II Ιο
τ(ρβ)
Og 490 631 2 X 488 1087 630 + 488 1274 1590 1274 + 630
— 0.27 0.19 0.14 0.16 0.11 0.09 0.02 0.04
(420) 110 80 60 70 45 40 8 15
Og 483 623
— 0.12 0.03
(430) 55 12
Mode
Reproduced from Reference 5
ranging from ~350 to ~835 nm. Each of these difference absorption spectra is recorded for only one specific time of sample interrogation relative to ex citation; the time between excitation and interrogation must be changed to obtain each difference spectrum. An alternate method provides AA as a function of time at one particular in terrogating wavelength. The probing continuum may be passed through an echelon to provide discrete temporal delays along the cross section of the continuum pulse (4). An echelon is a stepped optical delay that may be made of a set of optically contacted glass or quartz plates of differing, in creasing lengths. Because of the dif ferent pathlengths, the plates impress a time lag along the cross section of the laser pulse. The transmitted light intensities, obtained under conditions of sample excitation and of no sample excitation, are focused into a mono chromator coupled to a vidicon, an OMA, and a minicomputer. Applications of Picosecond Spectroscopy With the number of variations that can be made in experimental condi tions and methods of optical detec tion, many applications of picosecond laser systems to photochemical and photophysical problems are possible. Vibrational Relaxation in Naphthazarin. Picosecond spectros copy has been used to study the mech anisms of radiationless decay from ex cited states of several molecules. Re cently, Bondybey et al. (5) investi gated naphthazarin (5,8-dihydroxy1,4-naphthoquinone) spectroscopically in a neon matrix at 4 K. Besides in vestigating the ground-state structure of naphthazarin, which has been pre viously studied (6-12), they studied vibrational relaxation of this relatively
1124 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
large molecule while it was matrix-iso lated in neon at 4 K. Vibrational and electronic relaxation of excited mole cules in condensed media has received much attention, particularly with re gard to understanding the mechanism of energy dissipation in larger poly atomic molecules. Unrelaxed fluores cence frequently is observed from ex cited vibrational levels of diatomic and even triatomic molecules. How ever, such an observation is rare in larger molecules which, because of a higher density of states, usually relax extremely rapidly. Several experiments have been per formed recently that establish the ex istence of weak, unrelaxed fluores cence that occurs before a large mole cule reaches thermal equilibrium with its surroundings. For example, Barba ra et al. (13) detected the result of a vibrational relaxation process by mea suring an ~40-ps rise time for the Si -» So fluorescence of tetracene that was excited 7000 c m - 1 above the ν = 0 level of Si. Unfortunately, the vibra tional structure in the unrelaxed emis sion was not resolved, so only limited information about the relaxation pathways within the Si state could be obtained. In the study by Bondybey et al. (5) wavelength-resolved fluorescence measurements that resulted from sample excitation with a tunable dye laser were combined with direct pi cosecond time-resolved measurements of emission intensities to provide in sight into the mechanism for the vi brational relaxation of matrix-isolated naphthazarin within its first excited singlet state. A comparison of the fluorescence spectrum obtained by direct excita tion of the 0-0 transition with spectra obtained by exciting higher vibration al levels revealed the presence of a (continued on p. 1129A)
Figure 2. Time-resolved fluorescence intensity at 5600 ± 50 Â obtained with 530-nm excitation of naphthazarin in a neon matrix at 4 Κ The solid lines are computer fits of the data assuming a 20-ps rise time (τ>) and a 420-ps decay time. The dashed curves illustrate poorer computer fits assuming τ> = 10 ps and 30 ps. (Reproduced from Reference 5 with permission of North-Holland Publishing Company)
15 ps-.
-
65 p s \ ΔΛ 0.0
«-•*•' l-sA-^
j f j W t i V /
-r -0.1
\ f
'
T
^
t w
1
ι
400
4f 50
440 Wavelength (nm)
Figure 3. Difference absorption spectra in the Soret region taken at 15 and 65 ps after 530-nm excitation of a sample of cytochrome c maintained at pH 8.0 and 290 Κ (Reproduced from Reference 24 by copyright permission of the Biophysical Society)
number of additional sharp bands that were identified as vibrationally unrelaxed fluorescence. Bondybey et al. es timated vibrational relaxation rates on the basis of the integrated intensity of fluorescence from the excited vibra tional level (Ie) relative to the intensi ty of the relaxed fluorescence (/n) and on the lifetime of the lowest vibration al level (τ 0 ) according to re = r0(Ie/I0)
(1)
Table I lists lifetimes calculated ac cording to Equation 1 for several ex amined levels. The appearance of spectral bands, which were present in addition to those corresponding to unrelaxed and fully relaxed fluorescence, resulted from emission from interme
diate levels that are populated in the relaxation process. The presence of these additional bands indicates that vibrational relaxation involves intra molecular vibrational energy redistri bution as its first step and that it does not occur via a direct multiphoton re laxation process. Additional data that revealed the existence of unrelaxed fluorescence from excited naphthazarin in a neon matrix at 4 Κ were provided by a pi cosecond time-resolved emission study. Excitation at 532 nm with a 20-ps pulse from a Nd 3 + /YAG laser generated emission that was moni tored through a 5600 Â (~100 À FWHM) bandpass filter by means of a streak camera (Figure 2). A rise time of 20 ps and a fluorescence decay time
of 410 ps were observed. The observation of this rise time agreed qualitatively with the dye laser fluorescence study: Vibrational relaxation for naphthazarin occurs relatively slowly. The lack of quantitative agreement between the two experiments was attributed to a failure of excitation, in the case of a Nd 3 + /YAG laser experiment, to be in direct resonance with any of the major vibronic transitions of matrix-isolated naphthazarin. Another experimental limitation was that, while the 5600-À bandpass filter transmits several of the strongest fluorescence bands of vibrationally relaxed naphthazarin, numerous unrelaxed bands also lie in this spectral region. These two experimental shortcomings could lead to a distortion of the emission intensity vs. time measured by means of the streak camera and result in the appearance of a shorter rise time than was measured in the dye laser study. This study of matrix-isolated naphthazarin revealed a surprisingly large amount of unrelaxed fluorescence for a molecule of its size. Studies on the Relaxation Mechanisms of Cu(II) Porphyrins. In addition to such studies of energy dissipation pathways as that described for naphthazarin, which are motivated by purely physical interests, picosecond spectroscopic studies have been performed that investigate a number of biologically relevant reactions. For example, in many biological systems energy transfer from a porphyrin ring system to other reaction sites is a key step in a sequence of events that enables such important processes as photosynthesis and oxidative phosphorylation to occur. Learning the role that the chemical environment plays in the involved relaxation mechanisms is crucial to our understanding of these important energy transfer processes. In addition to previous spectroscopic and theoretical studies, (14-23) picosecond spectroscopy recently has allowed the direct measurement of ultrafast excited-state relaxation rates, has provided absorption spectra of the intermediates, and, in general, has verified and extended our understanding of excited metalloporphyrin relaxation. Reynolds et al. (24) recently investigated the effect of the prosthetic protein group of a hemoprotein such as myoglobin or cytochrome c on the relaxation rate of the photoexcited metalloporphyrin. Since the lifetime of the 2 Ti —• 4 Tj transition of copper(II) protoporphyrin IX dimethyl ester was accessible in their experimental time range, they selected the copper(II) derivative of cytochrome c for their study. Difference absorption spectra of
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983 · 1129 A
samples of Cu(II) cytochrome c at sev eral pH values were recorded at sever al times after excitation at 532 nm with a 30-ps pulse and were interpret ed in terms of ground-state bleaching and excited-state absorption. While the spectra recorded at 30 and 80 ps after excitation for samples at pH 2.5 and 13.0 revealed no measurable change over this time period, the ex cited-state band of the sample main tained at pH 8.0 disappeared between 30 and 80 ps (Figure 3). The decay kinetics of the sample maintained at pH 8 revealed the pres ence of two components: a slow one with a lifetime, τ, of at least a few hundred picoseconds that may repre sent an ~10% impurity of Cu(II) cyto chrome c " B " form and a faster com ponent (τ = 10 ± 5 ps) corresponding to the "A" form of cytochrome c. On the basis of previous work (25, 26), it was expected that the acid ic form would correspond to a five-ligand complex of Cu(II) porphyrin with methionine sulfur in one of the axial positions. As the pH is increased, the sixth ligand position is taken by an imidazole nitrogen of a histidine resi due, which creates an octahedral metalloporphyrin configuration. At pH 13.0, the methionine ligand is re moved, which regenerates a pentacoordinate Cu(II) porphyrin complex. The marked difference between the octahedral and pentacoordinated complexes indicates changes in the en ergy relaxation mechanism. The pi cosecond absorption data reveal that Cu(II) cytochrome c at pH 8.0 has a 10-ps lifetime whereas Cu(II) proto porphyrin IX dimethyl ester, the cop per porphyrin without the prosthetic protein group present, has an excitedstate lifetime of at least 450 ps in ben zene. In this study, Reynolds et al. concluded that the presence of both axial ligands to give an octahedral complex must increase either the rates in which two excited states, a tripdoublet state and a tripquartet state, reach thermal equilibrium or the rates of relaxation from these states to the ground state. Another possibility con sidered was that the octahedral com-
Figure 4. Plot of a b s o r b a n c e c h a n g e at 5 7 0 n m v s . t i m e for a s a m p l e of 4 m M N B E T A - C o l " - N C - F e " ( C N ) 5 ~ e x c i t e d at 5 3 0 n m w i t h a 6-ps laser p u l s e (Reproduced from Reference 27)
plex may provide an alternate relaxa tion pathway such as the 2 Si -» 2Sa„ transition that is observed for Ni(II) protoporphyrin IX dimethyl ester (23). Electron Transfer in Binuclear Metal Complexes. Recently picosec ond absorption spectroscopy has prov en useful in elucidating detailed mechanisms of electron transfer reac tions between metal centers of organometallic compounds and between or ganic electron donor and electron ac ceptor molecules. An example in the area of organometallic chemistry is the picosecond absorption study by Reagor et al. (27), which measured the ki netics of intramolecular electron transfer in binuclear, bridged (NC) 5 Fe n -CN-Co n l (chelate) com plexes. The reduction of hexacyanoferrate(III) ion (Fe(CN)|-) by ethylenediaminetetraacetatocobaltate(II) (CoEDTA 2 - ) had been shown previ ously to undergo reaction by the mechanism depicted in Scheme I (28, 29). F e n - C N - C o m bridged species are formed when two com
1130 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
plexes are prepared by mixing either N-hydroxyethylethylenediaminetriacetatocobaltate(II) (Co»HEDTA) or N-benzylethylenediaminetriacetatocobaltate(II) (Co n NBETA) with Fe(CN)6~. For these two complexes no further reactions to give final products were observed. Since conventional stop-flow and temperature-jump tech niques have been unsuccessful in mea surement of the inner-sphere electron transfer rate (30), Reagor et al. stud ied this reaction by means of picosec ond absorption spectroscopy. A 530-nm, 7-ps laser pulse from a Nd 3 + /glass laser was used to excite the !Tig *- xAig transition in Co111 of the (CN) 5 Fe I I -CN-Co m X (X = HEDTA or NBETA). The spectral regions of interest that were monitored were the F e m absorption band at 420 nm and both the Co11 and Co111 absorption bands at 480-610 nm; Fe 11 has no visi ble absorption. Figure 4 illustrates that ground-state depopulation of the ( C N ) 5 F e n - C N - C o m X complex occurs as revealed by the appearance of a negative absorption or bleach at
Figure 5. Plot of absorbance change at 420 nm vs. time for a sample of 4 mM NBETA-Co" l -NC-Fe"300 ps and 440 nm, CHLT. (Reproduced from Reference 34)
had been measured to be near unity (35). Irradiation of CHL/NAP, CHL/DHP, and CHL/IN in acetonitrile with a 355-nm, 25-ps laser pulse, which excited CHL, induced electron transfer between A and D molecules and permitted, at particular times after excitation, the spectroscopic observation of such species as 1 CHL*, 3 CHL*, 2 CHL~, arene cation radical (ARt), and, in the case of NAP, (AR)2"!\ Examples of the difference absorption spectra obtained at selected times after excitation are depicted in Figure 7. In the electron transfer reaction between CHL and each of the arènes, the acceptor CHL is excited to 1 CHL*, which intersystem-crosses to 3 CHL* faster than diffusion, a process that proceeds with a bimolecular rate constant of ~10 1 0 M - 1 s - 1 . As in the previous nanosecond time-resolved study of CHL/NAP in acetonitrile (35), the picosecond absorption study revealed that, as expected, electron transfers between CHL* and the arènes—NAP, DHP, and IN—proceed with diffusion-controlled rate constants. Hilinski et al. (34) did not observe any absorption bands in the early stages of the reaction between D and A in the
1136 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
studied AR/CHL systems in acetonitrile that could be attributed to an encounter complex or exciplex. In this polar solvent, ion formation is very rapid. However, they did observe time-dependent changes in the appearances and position of several radical ion absorption bands. For the NAP (0.070 M)/CHL (0.029 M) system, vibrational intensities within the absorption band of NAP+ were changing in the time ranging from 200 to 600 ps after excitation. Also, for DHP (0.080 M)/CHL (0.027 M) in acetonitrile, a red shift of ~10 mm occurred for the absorption maximum of DHP^ in the time from ~75 ps to ~600 ps after excitation. No such changes could be observed for the CHL T absorption band in any of the CHL/AR systems or for the I N t absorption band, possibly as a result of overlap with other absorption bands that obscured discernment of such changes in spectral appearance. Hilinski et al. (34) attributed these spectral changes to a diffusional process associated with relaxation of the initial cationanion-solvent orientation to that of an equilibrium orientation. The dramatic red shift of the DHP+ absorption maximum indicates that the initially formed D H P t / C H L ~ ion pair is in
Figure 8. Two-color fluorescence spectra of 1-naphthylmethyl and 2-naphthylmethyl radicals Spectra were generated by excitation of the sample with a 25-ps, 266-nm pulse followed by a 25-ps, 355-nm pulse delayed by ~ 6 0 ps. The (halomethyl)naphthalenes were: (a) 1-(chloromethyl)naphthalene; (b) 2-(chloromethyl)naphthalene; (c) 1-(bromomethyl)naphthalene; (d) 2-(bromomethyl)naphthalene. The fluorescence in the blue region of spectrum c is due to impurities that contaminated the sample of 1-(bromomethyl)naphthalene. (Reproduced from Reference 45)
closer contact than in the final equilibrium geometry, which minimizes the energy for CHL^ and D H P t in acetronitrile. The interpretation of this shift was based on previously reported data (43). In their picosecond study of the photoreduction of benzophenone by amines, Simons and Peters (44) observed blue shifts of the absorption maximum of the benzophenone radical anion at increasing times after excitation and ascribed this shifting to the diffusion of the initially formed ion pair toward each other to form a contact ion pair. A comparison of these studies by Simons and Peters (44) and by Hilinski et al. (34) indicates the varied behavior of ion pairs in solution. Photodissociation of Haloaromatic Compounds. By means of picosecond emission spectroscopy, Kelley et al. (45) have been able to detect and identify aryl radicals generated by cleavage of the carbon-halogen (C-X) bond in the photodissociation reaction of haloaromatics. An earlier paper by Huppert et al. (46) described the predissociative pathway followed by several haloaromatic compounds after excitation at 266 nm. The recent work by Kelley et al.
(45) employed a two-color fluorescence technique to detect the radicals. The compounds investigated in the emission study were l-(bromomethyl)naphthalene, l-(chloromethyl)naphthalene, 2-(bromomethyl)naphthalene, and 2-(chloromethyl)naphthalene. In the two-color fluorescence experiment, the aryl halide was excited at 266 nm to induce dissociation, and emission from the generated aryl radicals was measured after excitation by a second laser pulse of 355-nm light that was delayed relative to the first 266-nm pulse. When the haloaromatics were excited at 266 nm, each of the four compounds exhibited a broad blue fluorescence centered at ~400 nm with lifetimes that ranged from 100 to 3500 ps, depending on the particular compound. Excitation of the radical at 355 nm with a second laser pulse, which was delayed relative to the first 266-nm pulse, produced a fluorescence centered at ~600 nm with a lifetime of ~ 1 0 ns. Since the emission spectra resulting from excitation of l-(chloromethyl)naphthalene and l-(bromomethyl) naphthalene are identical in the region from 550 to 750 nm (Figure 8),
1138 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
this fluorescence was attributed to the 1-naphthylmethyl radical. In a similar manner, the fluorescence spectra resulting from two-color excitation of 2-(chloromethyl)naphthalene and 2-(bromomethyl)naphthalene were identical but different from the emission spectra of the 1-halo compounds and, therefore, attributed to the 2-naphthylmethyl radical. The twocolor fluorescence technique has proven very useful, not only for the study of the evolution of a photoreaction, but also for the detection and identification of minute concentrations of short-lived species that possess low absorption cross sections. Summary
In the examples of studies presented here, we have demonstrated the variety of photophysical and photochemical events that can be investigated by means of picosecond spectroscopy. These studies and many other picosecond spectroscopic investigations have provided data that permit detailed understanding of the cascade of events that occurs on the ultrashort time scale when light is absorbed by a chromophore. With everimproving laser systems and methods
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of optical detection and, of course, the ingenuity of the spectroscopist, the number of reactions that feasibly can be studied and the amount of detailed mechanistic information that can be provided by means of picosecond spectroscopy surely will continue to increase. References (1) Rentzepis, P. M. Chem. Phys. Lett. 1968,2,117. (2) Hilinski, E. F.; Rentzepis, P. M. Na ture 1983, 302, 481. (3) Hilinski, E. F.; Rentzepis, P. M. Ace. Chem. Res. in press. (4) Topp, M. R.; Rentzepis, P. M.; Jones, R. P. J. Appl. Phys. 1971,42, 3451. (5) Bondybey, V. E.; Milton, S. V.; En glish, J. H.; Rentzepis, P. M. Chem. Phys. Lett., 1983,97, 130. (6) Merian, E. Chimia 1959,73,181. (7) Josien, M. L.; Fuson, N.; Lebas, J. M.; Gregory, T. M. J. Chem. Phys. 1953,21, 331. (8) Schmand, H.L.K.; Kratzin, H.; Boldt, P. Liebigs Ann. Chem. 1976,1560. (9) Schiau, W. I.; Duesler, Ε. Ν.; Paul, I. C ; Curtin, D. Y.; Blann, W. G.; Fyfe, C. A. J. Am. Chem. Soc. 1980,102, 4546. (10) Bratan, S.; Strohbusch, F. J. Mol. Struct. 1980,67,409. (11) Anoshin, A. N.; Shigorin, D. N.; Gorelik, M. V. Russ. J. Phys. Chem. 1979,53, 431. (12) Anoshin, A. N.; Gastilovich, Ε. Α.; Shigorin, D. N. Russ. J. Phys. Chem. 1980,54,1409. (13) Barbara, P. F.; Rentzepis, P. M.; Brus, L. E. J. Chem. Phys. 1980, 41, 269. (14) Allison, J. B.; Becker, R. S. J. Chem. Phys. 1960,32,1410. (15) Becker, R. S.; Kasha, M. J. Am. Chem. Soc. 1955, 77, 3669. (16) Eastwood, D.; Gouterman, M. J. Mol. Spectrosc. 1969,30,437. (17) Hopf, F. R.; Whitten, D. G. In "Por phyrins and Metalloporphyrins"; Κ. Μ. Smith, Ed.; Elsevier: New York, 1972; p. 667. (18) Magde, D.; Windsor, M. W.; Holten, D.; Gouterman, M. Chem. Phys. Lett. 1974 29 183 (19) Tsvirko, M. P.; Stelmakh, G. F.; Pyatosin, V. E. Chem. Phys. Lett. 1980, 73, 80. (20) Ake, R. L.; Gouterman, M. Theor. Chim. Acta 1969,15,20. (21) Gouterman, M.; Schwarz, F. P.; Smith, P. D.; Dolphin, D. J. Chem. Phys. 1973 59 679 (22) Huppert, D.; Straub, K. D.; Rentze pis, P. M. Proc. Natl. Acad. Sci. U.S.A. 1977, 74,4139. (23) Kobayashi, T.; Huppert, D.; Straub, K. D.; Rentzepis, P. M. J. Chem. Phys. 1979, 70,1720. (24) Reynolds, A. H.; Straub, K. D.; Rent zepis, P. M. Biophys. J. 1982,40, 27. (25) Theorell, H.; Akesson, A. J. Am. Chem. Soc. 1941,63,1812. (26) Boeri, E.; Ehrenberg, Α.; Paul, K. G.; Theorell, H. Biochem. Biophys. Acta 1953 12 273 (27) Reagor, β! T.; Kelley, D. F.; Huchital, D. H.; Rentzepis, P. M. J. Am. Chem. Soc. 1982,704,7400. (28) Huchital, D. H.; Lepore, J. lnorg. Chem. 1978,77, 1134. (29) Rosenheim, L.; Spencer, D.; Haim, A. lnorg. Chem. 1974, 73, 1571. (30) Huchital, D. H.; Wilkins, R. G. lnorg. Chem. 1967,6,1022. (31) Blair, J. C., Jr.; Emeteus, H. J.; Nyholm, R.; Trotman-Dickenson, A. F., Eds.; "Comprehensive Inorganic Chem-
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Edwin F. Hilinski received his BS in chemistry from Wilkes College. With Jerome Berson as his research advi sor, he obtained his PhD from Yale University in 1982. Currently he is a postdoctoral member of the technical staff at Bell Laboratories.
Peter M. Rentzepis is head of the Physical and Inorganic Chemistry Research Department at Bell Labora tories, Murray Hill, N.J. He currently is involved in picosecond spectrosco py research using lasers.
