E. F. Hilinski P. M. Rentzepis Bell Laboratories Murray Hill, N.J. 07974
Advances in laser technology and methods of optical detection have permitted us to investigate chemical reactions 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. Transient species and states involved in a photophysical or photochemical process can be detected, and their kinetics of formation and decay can be measured directly BS the process evolves by means of picosecond emission, absorption, and, recently, Raman spectroscopy. On the time scale of s, sufficient time resolution exists so that complex mechanisms of many important chemical and biological reactions can be elucidated in greater detail than was previously possible. Laser systems based on Nd3+/ yttrium aluminum garnet (YAG) oscillators and synchronously pumped dye lasers are currently finding widespread 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 graphical form, optical data in the picosecond time regime can be acquired in an interpretable 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/A351-1121501.50/0 S 1983 American Chemcal 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 arenes; 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 Nd3+/glass or Nd3+NAG. Mode-locked solid-state oscillators such as Nds+/glass, Nd3f/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 l 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 Nd3+NAG laser and adding in their place a Nd3+/YAG oscillator and the number of Nd3+NAG 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 mJ, generated at a rate of up to 10 Hz after in-
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11. SEPTEMBER 1983
1121 A
.
Reference * Cell
Dye Amplifier
..lodelocked Argon Ion Laser
t PC Lc. P
Ring Laser Figure 1. Schematic diagram of a synchronously pumped dye laser system AC, au1oconelaloT:P, pokrizer; PC.Pockds cell; CP. crossed polarizer: WP. high-voltage pulser: FU. photodiode: Sffi. second hermonic generating crystal; BS, team 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 he 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 hy 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 hy a series of flowing dye cells that are pumped hy 10-ns pulses of the second harmonic of a Nd3+NAG laser operating at a repetition rate of -10 Hz. Saturable absorbers are placed between each dye cell to exclude superradiance. This pulse, which can he amplified hy a factor of -106, has sufficient energy to he used 1122A
-
directly for sample excitation or to be converted to wavelengths other than those accessible directly from the dye laser. This can be accomplished hy means of frequency doubling and/or stimulated Raman scattering. At this point, that is, after pulse generation and desired amplification, the picosecond spectroscopicexperiment 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 Nd3+/ glass oscillators can be operated at up to -0.02 Hz and produce pulses of 1060-nmlight that are 4-8 ps FWHM in duration and have a bandwidth of -30 cm-1. Passively mode-locked Nd3+h’AG lasers can be operated at -10-20 Hz, and produce pulses that have a -0.3 em-‘ bandwidth centered at 1064 nm and a duration of -20-40 pa. 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-
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11. SEPTEMBER 1983
ers. Available excitation wavelengths, of course, depend on the type of laser employed. With a picosecond synchronouslypumped 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 Nd3+NAG laser, the fundamental 1064-nmlaser 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 cm-I 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-
The LC Connection
Injectors.Valves.Filters.Columns.Good advice. When you use HPLC, make the important connection, Rheodyne. Our components both simplify your work and extend the capabilities of your system. Just check our connections. 1.Sample injectors. The Model 7125 injects either a fully-loaded or partially-loaded sample loop. Other injectors include the pneumatically-actuated 7126, the 7410 for microsamples and the 7010 for full-loop loading only. 2.Column selection valve. Model 7066 selects one of up to five columns. 3. Switching valves. Type 70 Valves switch flow for backflushing, sample enrichment, sample cleanup, etc. 4. Solvent switches. Low-cost Teflon valves switch low-pressure
streams. 5. Pressure relief valve. Model 7037 protects equipment from overpressure. 6.Column inletfilter. The 7302 keeps particles from damaging columns. 7. Columns. Guard, presaturator and analytical columns. 8. Fittings. As for good advice, you'll do better HPLC if you ask us for free copies of Technical Notes 1,2,3 and 4. They contain practical how-to-do-it information for both the beginner and the experienced chromatographer. For Tech Notes and product literature, please address Rheodyne, Inc.. PO. Box 996, Cotati, California 94928, U.S.A. Phone (707) 664-9050
4
5
ticular photoinitiated process withal of these available types of piwseconc spectroscopy. However, experimenh limitations often prohibit such a complete investigation. Judicious application of feasible picosecond time-resolved methods is usually sufficient to provide details about the mechanism of a reaction. For emigsion spectroswpy, the picosecond 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 i vidicon, an optical multichannel analyzer (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 t o n vidicon, an OMA, and a minicomputer. In addition to emission spectroscopic data, transient absorption spectra recorded as a function of time aid in unraveling the mechanistic complexities of photoinitiated processes. Also, when possible, the structural information provided by picosecond Raman data helps in the elucidation of reaction 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 absorption spectroscopy. The continuum picosecond pulse is generated by focusing the probe pulse into a cell containing DzO/HzO. The intensities of the band of wavelengths contained in the continuum pulse are adjusted by means of filters. A double-beam picosecond absorption spectrometer is created by splitting the continuum pulse so that it passes through a sample cell and a reference cell. Difference absorption spectra are recorded at selected times relative to sample excitation by measuring transmitted light intensities of both the sample cell and reference cell as a function of wavelength when the sample is not subjected to an excitation laser pulse and when the sample is excited by a laser pulse. The light intensities as a function of wavelength at a selected time relative to sample excitation for the absorption or Raman experiment are detected by means of a polychromator coupled ton vidicon, an OMA, and a minicomputer. In the absorption experiment described above, changes in absorbance (AA)as a function of wavelength can be recorded for the spectral region 1124A
ible 1. Unrelaxed Emission Yields and Lifetimes for Several Vibrational Levels of Naphthazarln a II
yod.
18119 18609 18750 19097 19206 19237 19393 19709 20023
0 490 831 978 1087 1118 1274 1590 1904
b
0: 490 631 2 x 488 1087 630 -k 488 1274 1590
Naphthazark Dt 18182 0 18665 483 18605 623 n Refrnence 5 I
ranging from -350 to -835 nm. Each of these difference absorption spectra is recorded for only one specific time of sample interrogation relative to excitation; the time between excitation and interrogation must be changed to obtain each difference spectrum. An alternate method provides LUas a function of time at one particular interrogating 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 elass or auartz dates of differing, increasing iengthi. Because of thedifferent 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 monochromator coupled to a vidicon, an OMA, and a minicomputer. Applications of Picosecond Spedroscopy With the number of variations that can be made in experimental conditions and methods of optical detection, many applications of picosecond laser svstems to nhotochemical and photophysical pioblems are possible. Vibrational Relaxation in Napbthazarin. Picosecond spectrmcopy has been used to study the mechanisms of radiationless decay from excited states of several molecules. Rerently, Rondybey et al. ( 5 ) investigated naphthazarin (5,8-dihydroxy1,4-naphthoquinone) spectroscopically in a neon matrix at 4 K. Besides investigating the ground-state structure of naphthazarin, which has been previously studied (&12), they studied vibrational relaxation of this relatively
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11. SEPTEMBER 1983
large molecule while it was matrix-isolated in neon at 4 K. Vibrational and electronic relaxation of excited molecules in condensed media has received much attention, particularly with regard to understanding the mechanism of energy dissipation in larger poly. atomic molecules. Unrelaxed fluorescence frequently is observed from excited vibrational levels of diatomic and even triatomic molecules. However, such an observation is rare in larger molecules which, because of a higher density of states, usually relax extremely rapidly. Several experiments have been performed recently that establish the existence of weak, unrelaxed fluorescence that occurs before a large molecule reaches thermal equilibrium with its surroundings. For example, Barbara et al. (13)detected the result of a vibrational relaxation process by measuring an -40-ps rive time for the SI SOfluorescence of tetracene that was excited 7000 cm-1 above the Y = 0 level of SI. Unfortunately, the vibrational structure in the unrelaxed emission 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) wavelenah-resolved fluorescence measur~mentsthat resulted from sample excitation with a tunable dye laser were combined with direct picosecond time-resolved measurements of emission intensities to provide insight into the mechanism for the vibrational relaxation of matrix-isolated naphthazarin within its first excited singlet state. A comparison of the fluorescence spectrum obtained by direct excitation of the 0-0 transition with spectra obtained by exciting higher vibrational levels revealed the presence of a (continued on p . J129A)
-
to
Shimadzu's promise
Modular design, versatile comnonents and full automation LC4A Uquld Chromatograph Shimadzu's inexpensive ternary gradient liquid chromatograph offers microprocessor control over all system
V
operational conditions. Generous selfdiagnostics are complemented by safety devices to protect against abnormal temperature, leaks and presC-R2A which can offer true 2 channel
and stored in up to 10 separate files, allowing analytical runs to be made automatically with different sets of
user-defined BASIC programming
A LC-SA Microbore Column
Uquld Chromatograph A world-first to help you to reduce
solvent consumption up to 95% and reach theoretical plate numbers higher than 200.000. The pump capacity of the Shimadzu LC-SA Microbore Column Liquid Chromatograph can be varied from 1 to 99oOpl a minute. while the total system dead volume is only @I. CIRCLE 207 I
V
A SPD-Y1A Photo-Dl& Array 8 7 0.tec(orforLC Designed forsimultaneous quantitative and qualitative analyses, the Shimadzu SPDMlA Photo-Diode Array Spectrcphotometric Detector is a new type of detector offering the advantage of threedimensional spectro-chromatograms, easily identifiable peaks, spectrum memory on-flow and single and double wavelength range chromatograms. The photodiode array sensor covers the full UV and VIS spectrum range. CIRCLE 208
RF-530 fluorescence Spectmmonltor for HPLC
wide range of excitation and emission wavelengths. The corrected excltation energy and sensitivity to give a detection limit for quinine sulphate isO.45pg t2pl An off-plane optical system and aberration-corrected concave holographic gratings decrease noise All connections are made with standard 1 1 1 6 ' 0 0. tubing forconnection toall HPLC systems. CIRCLE 209
.. .-
A S P D I A UV Spectrophotometric D e t d o r Scries for HPLC Shimadzu'sSPD3Aisequipped witha high-performance grating monochro-. mator having a wavelength range from 195 to 350nm. To ensure high stability. the SPD-2A has adouble beam optical system which cancels drifts caused by fluctuation in the intensity of the light source. Available withwl flow cell and CIRCLE 211 0.5~1micro flow cell.
Write M a y for more hlomtlon on there and other ihlmadzu Insfwnmnb
WlYlDZU SCIENTIFIC INSTRUMENTS,INC 7102 Riveru d Drive. Columbia Maryland 21046. U.S.A. phone:(331)ssl-l227 SHMAON (EUROPA)GMBH Ackw Slr111.4wo Dliwldwf. F.R. Gemmany Phons (mi11668371 Telex: 08586839 1 CORP0RATW)NtNERNATlOHLL MARKEllNG DIV. ShinjUkU-MitWi Building. 1-1. Niahishinjuku Z-chom, Shinjuku-ku, Tokyo 160. Jawn.
for userchromatography: simniicity. Data integrity. b Chromatopac C R l B Recordlng
Deta Processor Designed for completely automatic
lation isa standard feature. Parameters are memory protected and 8 separate files are available for peak processing and calibration parameters. Other
appear on the same chart for onepoint ObseNatiOn. Thetwepoint curve method assures precise calibration of -2sponse factorshd multi-step cabu-
signals from FPD. selective grouping and self-diagnostic function.
cmw
212
atopac C-R2A with cessors. With high-speed calculating capability and a capacious memory capacity. it offers operational and graphic display of processed data by user-defined BASIC programming, simultaneous processing and recording of output signals from two detectOrS and CRT-display of chromatcgrams and alphanumeric data. Fully supporting its data integrity are a long list of complementing features that are first class. The design allows operational control of Shimadzu microcomputerized gas and liquid chromatographs. And automatic peak detection enables processing of up to 1.900
peaks (Chromatopac C-R2AX) with 1
R-Serlea Labomtoy R . c o ~ l m Shimadzu's R-Series Laboratory Recalculation Darameters. A wide rana corders are available in models which of calculat/on functions. including' . meeta wide range of recordingneeds. statistical calculation and quantitation. . The R-111 Recorder for GC and LC is are provided. Component name re- ; . a one-pen recorder with a fast 0.5 cording and FPD exponential signal: . . &full scale (1mV) pen speed. The processing isastandardfeature. Otherr ,' R-112 twepen recorder offers the features include freely selectable re$ ,.:, same efficient pen speed. And the Rport formats. time programming,cab il+:*>: 123 Recorder is a threepen recorder ration of response factors, selective." '< with 1.5.10 and 20mV full scale range grouping, built-in protection to retain and full scale pen speed of 1/3 second. stored data in event of power failure CIRCLE 214 and generous self-diagnostics. ,
,
CIRCLE 213
w&
today tor mom hfonnailan on these and other ihlmedru IndNnmnls.
WUMAI SCIENTIFIC INSTRUYENTS,INC. 7102 R ~ ~ r * mOrivl d nbla. MalVland 21046 U S A . PhonsIJ01)997~1217 (YI1Yu)ZU INROPA) OYBH ACLer Slraore 1 I1.4ooo Oussnoon. r R G m n y Phone lUZ11)888371 Tab% 08588839 S H W L U CORWRCnON IN-TERNATIOWL YARUETING DIV. Shmlbhb-M 044 Ed0 Dong 1-1 Nidhlshiniuku 22Mm.Shmjukb-ku To*w 180. Japan Pnonr T o k p ~ Tamxm32mi i %HOT J
alternative for auality u ~ .
ANNOUNCING .....Solit-solitless ooeration, full detector selection, full accessory availability and aoolication supoort.
v GC-8A Series Gas Chromatograph A simple construction and low cost are the key features of Shimadzu’s high-quality GC. The GC-8A Series offers digital temperature setting and digital temperature pmgramming. An on-column on-detector system, FID. constant current TCD. FPD. and ECD models are available. CIRCLE 229
VGC-MINI? Gas Chromatograph Compact and reliable, Shimadzu’s ~ C - M I N Gas I ~ Chromatograph has a jual column flow system and on-colm n on-detector system. An optional Lemperature programmer is available for programming from the near ambient point. The three temperature sensors provide excellent repeatability in resetting initial column temperature.
9 9 1
A GC-MINIS Gas Chromatograph The most compact microcomputerized GC with dual FID. A full performance new-generation GC with full keyboard operation. Features include an oncolumn on-detector system. multistage temperature-program capability with near ambient column temperature even with high injection port temperCIRCLE 228 ature. V GC-GAM Series
Gas Chromatograph Thefirst modular designed system with sample injection port. column and detector combined intoonecartridge unitquick change of analytical conditions, elimination of cross contamination between samples and shorter down time. Theonlysystemwith the complete analytical unit :in the oven simultanecmaE 227 ously.
4 GC-9A Series Gas ChromatograPh A
Shimadzu’s GC-SA Series Gas Chromatograph with built-in microprocessor saves valuable laboratory time without lossof analytical excellence. It makesavailableeasy user-defined programming, multiple files with linking. computer interface and generous selfdiagnostics. Now available with full detector options (FID. TCD. ECD. CRCLE 230
AOC-9 Automatlc Sampler for GC-SA A new accessory designed to enlarge the capability of the GC-SA, the AOC9 offers sequence control through the GC-SA keyboard. Sample volume is selectable in four steps of 10,20.40 and 80% of the plunger stroke: repat sampling for up to a maximum of 10 samples: and sample vial detection eliminating the need for batch Step sample placement. CIRCLE 231 f o m l l o n on thee and OW-
S H I Y A O N SCIENTIFIC INSTRUMENTS, INC. 7102 RiverWood Dnve. Columbia. Maryland 21046. U.SA. PhOnB:(3011W-l22i W I W N ( E U R 0 P A ) GMBH Ackw Straue 111.4wo O O d d a f . F.R. Germany Phone (0211)868371 Telex 0868681 SHIMADZU CORPORATION INTERNATIONALMARKETING DIV. Shinjuku-MiBUi Building, 1-1, NishiShinjukU Z~harne. Shinjuku-ku, TOkW 180. J a w Phone:Tokyo CQ1TeleX:OZ324291 SHMOT J
For True Microsamding: CapabilityonEvery PT-fR -
Introducingthe first Microsampling Accessory for your FT-IR.Now you can measure the infrared spectra of samples as small as 20 microns in diameter. This accessory works with Digilab's QUALIMAT1C'"and FTSFas well as most IBM,Nicolet and other FFIR instruments. Before. if you wanted to measure
be continuouslyvaried from 250 microns down to a minimum of 20 microns or diffraction limit. A Microsampling Accessory is available for either reflectance or transmittance studies,permiiting analysis of virtually any material. Also avi able is an X-Y stage with stepping motor drive for computer-controlled mapping of sample sulface areas. Now you can do sophisticated microsampling faster and easier than ever before. For detailed informationon how the new MicrosamplingAccessory can help your laboratory, look to Digilab,the longtime industr leader in FT-IR. To add a dedi cated FT-IR microsampling capability, ask about Digilabs cost-effectiveconfiguration includingthe QUALIMATIC, Microsampling Accessoi and 4-color plotter.
Look tc Digilab.
sampling Accessory, you can visibly locate the sample of interest under the high-powered microscope objective and, with the flip of a switch, obtain the infrared spectra in seconds. This visual alignment capability assures that you are measuringonly the area of interest. Data collect times are reduced due to matching M C T and optimized mating optlcs. The sampling size ca-
BIORAL:
I
.
.
... . .,. . i" . .
.'.I,
,.. . . .
Flgure 2. Time-resolved fluorescence intensity at 5600 f 50 A obtained with 530-nm excitation of naphmazarin in a neon matrix at 4 K The solid 1 1 m are mrnputar tils 01 the data assuming a 209s rise time (r,)and a 42decay time. The dashed CYNBS illusbate poaer computer f i assuming T , = 10 ps and 30 ps. (Reproduoed tmm Referace 5 wim permission 01 NWmnOlland Publishing Company)
Vavelength (nm)
Figure 3. Difference absorption spectra in the Soret region taken at 15 and 65 F after 530-nm excitation 01 a sample of cytochrome c maintained at DH8.0 and 290 K (Repod&
tmm Rehxence 24 oy mpwighl prmission 01 the B(0physical Society1
number of additional sharp bands that were identified as vibrationally unrelaxed fluorescence. Bondybey et al. estimated vibrational relaxation rates on the basis of the integrated intensity of fluorescence from the excited vibrational level (I.) relative to the intensity of the relaxed fluorescence (IO)and on the lifetime of the lowest vibrational level ( T O )according to To
= To(IeII0)
(1)
Table I lists lifetimes calculated according to Equation 1 for several examined 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 proceas. The presence of these additional bands indicates that vibrational relaxation involves intramolecular vibrational energy redistribution as its first step and that it does not occur via a direct multiphoton relaxation process. Additional data that revealed the existence of unrelaxed fluorescence from excited naphthazarin in a neon matrix at 4 K were provided by a picosecond time-resolved emission study. Excitation at 532 nm with a 20-ps pulse from a NdB+NAG laser generated emission that was monitored through a 5600 A (-100 A FWHM) bandpass filter by means of a streak camera (Figure 2). A rise time of 20 ps and a fluorescencedecay 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 NdS+NAG 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-Abandpass 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 v8. 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(I1) Porphyrins. In addition to such studies of energy dissipation pathways as that described for naphthazarin, which are motivated by purely physical interests, picosecond spectroscopicstudies 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 spectroscopicand 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 VI T Itransition of copper(I1) protoporphyrin IX dimethyl ester was accessible in their experimental time range, they selected the copper(I1) derivative of cytochrome c for their study. Difference absorption spectra of
-
ANALYTICAL CHEMISTRY. VOL. 55, NO. 11. SEPTEMBER 1983
1129A
samples of Cu(I1) cytochrome e a t several pH values were recorded at several times after excitation at 532 nm with a 30-ps pulse and were interpreted 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 excited-state band of the sample maintained at pH 8.0 disappeared between 30 and 80 ps (Figure 3). The decay kinetics of the sample maintained at pH 8 revealed the presence of two components: a slow one with a lifetime, r, of at least a few hundred picoseconds that may represent an -10% impurity of Cu(I1) cytochrome e "B" form and a faster component ( r = 10 5 ps) corresponding to the "A" form of cytochrome c. On the basis of previous work (25,26), it was expected that the acidic form would correspond to a five-ligand complex of Cu(I1) 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 residue, which creates an octahedral metalloporphyrin configuration. A t pH 13.0, the methionine ligand is removed, which regenerates a pentacoordinate Cu(I1) porphyrin complex. The marked difference between the octahedral and pentacoordinated complexes indicates changes in the energy relaxation mechanism. The picosecond absorption data reveal that Cu(I1) cytochrome c at pH 8.0 has a 10-ps lifetime whereas Cu(I1) protoporphyrin IX dimethyl ester, the copper porphyrin without the prosthetic protein group present, has an excitedstate lifetime of at least 450 ps in benzene. 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 considered was that the octahedral com-
*
1130A
.
Excitation
I
0
-50 T
.oo
Time (ps)
50 I
1
100 I I
150
2w
250
I
I
I
I
-.05-AA
A
6ps
Echelon
20 ps Echelon
-.lo--
-.15-,I
--
-
I Figure 4. Plot of absorbance change at 570 nm vs. time for a sample of 4 mM NBETA-Colll-NC-Fel'(CN):excited at 530 nm with a 6 p s laser pulse (Repmduced h m Reference 27)
-
plex may provide an alternate relaxa tion pathway such as the 'SI lSdr transition that is observed for Ni(I1) protoporphyrin IX dimethyl ester (23). Electron Transfer in Binuclear Metal Complexes. Recently pic-ond absorption spectroscopyhas proven useful in elucidating detailed mechanisms of electron transfer reactions between metal centers of organometallic compounds and between organic electron donor and electron acceptor molecules. An example in the area of organometallic chemistry is the picosecond absorption study by Reagor et al. (27), which measured the kinetics of intramolecular electron transfer in binuclear, bridged (NC)6Feu-CN-Con'(chelate) complexes. The reduction of hexacyanoferrate(II1) ion (Fe(CN)gf) by ethyl-
enediaminetetraacetatocobaltate(I1) (CoEDTA*-) had been shown previously to undergo reaction by the mechanism depicted in Scheme I (28,29). Fe"-CN-Co"' bridged species are formed when two corn-
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11, SEPTEMBER 1983
plexes are prepared by mixing either N-hydroxyethylethylenediaminetriacetatocobaltate(l1) (Co"HEDTA) or N- benzylethylenediaminetriacetato. onhaltntdlll ICo'INBETA) with ~~, .~ Fe(CN)g-. For theae two complexes no further reactions to give final products were observed. Since conventional stop-flow and temperature-jump techniques have been unsuccessful in measurement of the inner-sphere electron transfer rate (30),Reagor et al. studied this reaction by means of picosecond absorption spectroscopy. A 530-nm, 7-ps laser pulse from a Nd3+/glass laser was used to excite the IT1, 'A], transition in Cd" of the (CN)5Fe"-CN-Cd11X (X = HEDTA or NBETA). The spectral regions of interest that were monitored were the Fe"' absorption band at 420 nm and both the Co" and Co"' absorption bands at 48MlO nm; Fe" has no visible absorption. Figure 4 illustrates that ground-state depopulation of the (CN)SFe'WN-CdllX complex occurs as revealed by the appearance of a negative absorption or bleach at ~
-
The All New Radiometer ION85 In touch with tomorrow
0 / Radiometer's new ION85 Ion Analyzer is today's precision pHlion meter with all of tomorrow's versatility built right in. It features a unique in-line keyboard that lets you select any of nine measuring modes at the touch of a single key and through the exclusive "shift" key, use the same keyboard for all subsequent calculations. But, the ION85 does even more. mV, pH, pX, temperature and ion concentration measurements Standard and analate additions and subtractions Gran's plot technique Calibration with up to 5 standards Microprocessorcontrolled All calibration data stored permanently in independent memory
A 20-character alphalnumeric readout displays all functions and results and even helps guide the operator through measurements and calibration. The optional ABUBO Autoburette provides reagent additions from 1p1 to 1OOml in up 10 99 increments. And, you can easily up-grade the ION85 to a Radiometer end-point titration system at any time. In short, the ION85 has it all. So, why wait? Put your lab in touch with tomorrow today. For your free ION85 brochure, contact Radiometer America, Inc. Call, toll free (800) 321-9484. In Ohio, call (216) 871-8900.
RADIOMETER^ :m D : COPENHAGEN L A
0
Quality electronic instruments for Science, Medicine and Industry
WORLD HEADQUARTERS: RADIOMETER AIS. EMDRUPVEJ 72, DK2400 COPHENHAGEN. DENMARK IN THE US.: RADIOMETER AMERICA, INC. 811 SHARON DRIVE, WESTLAKE, OHIO 44145 CIRCLE 184 ON READER SERVlCE CARD
AkALYTICA- CHEMISTRY, VOL 55. NO 11. SEPTEMBER 1983
* 1131A
in the 2E(k,6eg)state. The cobalt doublet to quartet spin flip accompanies the eg t 2 transition to the 4Tlg(tzgSe:) state (Scheme 11, reaction 3), which is known to be the ground state of Cd' with all but the strongest of field ligands (32).Their 420-nm absorption data suggested that the 2E(kg%,) state of Co" in Scheme 11, reaction 3 has a lifetime of -75 ps and that spin-allowed backelectron transfer occurs in -95 ps (Scheme 11, reaction 4). The backelectron transfer results in the production of Cd" in an excited 3T1or T2(tzg5e,)state that relaxes to the singlet ground state (Scheme 11, reaction 5). This study by Reagor et al. (27)revealed that the Fel"-CN-Co" system differs from other bimetallic complexes that undergo electron transfer via resonance transfer between the metal atoms, which are influenced somewhat by the bridging ligands (32,33).Reagor et al. also found the rate of electron transfer to be dependent on the polarity of the solvent system, which was inferred to reflect changes in charge stabilization of the bridging Fe"-CN-Co"' and Fe"'CN-Co" species. Transient Species in Organic Electron Transfer Reactions. In addition to studies on intramolecular electron transfer within organometallic complexes, picosecond absorption spectroscopy recently has been used to investigate the dynamics of photoinitiated electron transfer between organic donor (D)and acceptor (A) molecules. Hilinski et al. (34)were able to
-
AA 0O. 1.0 l I
0.00
0 -50 . 0
5
t
' 0i
50
Tlme 100 (pa)
200
150
50
Exciiation Flgure 5. Plot of absorbance change at 420 nm vs. time for a sample of 4 mM NBETA-Co"'-NCFe"(CN):- excited at 530 nm with a S p s laser pulse he solid cwve comesponds IO me nwmaiizd optical density caka~latedon me basis of me mechanism given by Scheme 11. RBBC~IOT. 1-5. (Reproduced horn Reference 27)
575 nm, which persists for a time greater than 500 ps after excitation. The bleaching was found to be reversible and recovered in the -30 s that passed between laser shots. Accompanying the bleaching at 575 nm was a transient absorption at 420 nm that exhibited a < 10 ps rise and decayed with a half-life of -150 ps (Figure 5). Scheme I1 outlines the mechanism 'iat was proposed on the basis of the
-
picosecond absorption data. Excitation of the 'TI, 'AI, transition of Co"' (Scheme 11, reaction 1) increases the electron affinity of Co"', which causes electron transfer (Scheme 11, reaction 2) to occur. Alternatively, Reagor et al. proposed the direct excitation in the charge transfer band at 530 nm, which generated Fe"'-CNCdl** that gives rise to the Fe"' absorption and to the production of Co"
Figure 6. Difference absorption spectra obtained with 355-nm excitation of a 0.035 M solution of CHL in acetonitri (a) 0. (b) 15. (c)40. (d) 65. and (e)290 ps an- excbllon. (Reproduced horn Reference34) 1132A
ANALYTICAL CKMISTRY. VOL. 55, NO. 11, SEPTEMBER 1983
Automated Liquid Chromatography
n gets rid of tedium It iknproves In chmmatcgrapny however many people still tend to reqard automation only ,n terms of large, expeGve research-or!entedsystems. Big mislaw. Our smart nstrumenis are competitively priced And they're already making routine. day-to-day laboratory analysis a lot easier and more cost effective. Were going to tell you about one of these instruments-the SP8100 Liquid Chromatograph from Spectra-Physics.
each analysis IS prepfogrammed you dorit need a skilled technician
while operatingcomplet
Automated *If-eheeking.
Modular or total spectra-Physicscan all your chromatography n
operate this way.
You'll never ruin a batch of samples because the system wasn't running properly If any step is not DroDerlv executed, the svstem will stOD and tell you whais wrong YOJ mJSI correct tne problem beforethe system can cont nLe on
The SP8lOO offers remarkable on-board intelligencethat facilitates and simplifies the analytical process.
Automated single-vial calibration. Iml I YOJ get even more samp e Irn
AU chromatographs will one day
Automated sampling with on capability. -
I ! ~
Each vial or group of vials
' can be pre-programmed ~
to use a different set of
analysis parameters, up to 100 parameters in all. The system permits one microliter injection out of a five rnicroliter sample.
space oecause YOJ oont need mLitfpe ca ioratlon vials Whenever requ reo the system adtomatica ty tracds oack to tne Same ca ibralton vra
&J
Automated priority sampling. If you have a rush sample, the SP8100 can handle it without interrupting operation Once the priority sam$is c o m w d , the analyzer automaticallv continues on with its original analyses without missing a beat
Spectra-Physics W e G k e automated chromatography simple. CIRCLE 189 ON READER SERVICE CAR0
Most people know us lor Labnet." our toially aulomated chromatography laboratory But they someilmes foroet Inat odr DrOauciline 1s modular too. All &r components are designed to function as part 01 a fully integrated system. Including our modular Ternary Solvent Delivery system and lsocratic pump with automated flow control. Automation. It's more than a good idea. It' the only idea that makes sense in the 80s. Contact your nearest Spectra-Physics regional sales off ice for more information o theSP8100 HPLCandtheworldkmostauto~ mated line of chromatography products. Spectra-Physics, 3333 North First Street, San Jose, CA 95134.
IMCI Automatic Moisture Meter.
I 3
Reliable, Fast and Easy.
....
l i
MITSUBISHI CHEMICAL INDUSTRIEF I m CIRCLE 136 ON READER SERVICE CARD
n
I
investigate the mechanism of intermolecular electron transfer in greater detail than in previous nanosecond timeresolved studies (%,%). In general, light-induced electron transfer can result from three distinct types of excitation: absorption of light by D in the presence of A, by A in the presence of D, or by an electron donor-acceptor (EDA) complex formed between D and A in their ground state. Previous data obtained by means of nanosecond absorption spectroscopy for electron transfer from donor naphthalene (NAP) to acceptor chloranil (CHL) in acetonitrile suggested that a Weller-type (38,39) quenching mechanism was followed by this electron transfer reaction. According to results obtained by Weller and eo-workers ( 3 8 4 2 )a fluorescent exciplex, 13(D? A:)*, is formed from the initial encounter complex, ',3(D.. .A)*, which is formed when a photoexcited molecule, D* or A*, in a singlet or triplet state approaches a complementary ground-state molecule. The initial encounter complex, 1.3(D. ..A)*, first becomes a nonrelaxed exciplex, '33(D?A;)**, which may relax to '93(D?A;)* or, in polar solvents, ionize directly to a solventshared ion pair, 1,3(2Dt.. .2A:). The ground-state ion pair, 1,3(*D?.' ,zA;), subsequently may diffuse apart to give separated, solvated ions, *D,f and *A,;, as was previously observed for the CHL/NAP system in acetonitrile (35). Hilinski et al. (34) extended the investigation of the mechanism of photoinduced electron transfer between acceptor CHL and the donor arenesNAP, 9,lO-dihydrophenanthrene (DHP), and indene (IN)-to gain a clearer understanding of the dynamic events that occur in the early stages of this reaction. Excitation of a solution of chloranil in acetonitrile at 355 nm with a 25-ps laser pulse generated a transient absorption band in the region from 420 to 540 nm that evolved with time into a spectrum exhibiting maxima at 490 and 515 nm (Figure 6). On the basis of previous assignments (35),the latter absorption band, which developed within 300 ps after excitation and remained unchanged for a time longer than 8 ns, was attributed to the T, TItransition of CHL. They assigned the broad, initially ohserved absorption band to an S, SI transition of CHL. The rates of decay of the S. SI hand and the growth of the T. TIband permitted them to obtain an intersystem crossing rate constant of -3 X lO'Os-' for 'CHL* BCHL*. Although the rate of intersystem crossing was unknown prior to their measurement, the quantum efficiency of singlet-triplet intersystem crossing for CHL' previously
--
CIRCLE 1 ON READER SERVICE CARD
1134 A
ANALYTICAL CHEMISTRY, VOL. 55, NO. 11. SEPTEMBER 1983
-
..
MORETHAN .. 30,000 . VOLATILE ORGANIC COMPOUNDS LIVE HERE-,
. . .
'
a .
.
WE'RE ON A FIRST NAME BASIS WITH EVERY ONE OF THEMNot that all those compounds are friends of ours. some are so hazardous you wouldn't want t o share a time zone with one. But it's our business t o know everything we can about them. Because we're the largest, fastest and most comprehensive CC/MS analytical laboratory in the world. We're Mead CompuChem Laboratories.. No other analytical laboratory comes close t o matching Mead CompuChem's"capabilities.We have 24 CC/MS instruments. Our Staff of experienced and degreed spectroscopists work around the clock, seven days a Week, guided by quality control procedures that set standards for the industry. For every analytical purpose, Compuchem~ Produces comprehensive and definitive findings.With DisposalSiteScreeningwetest
for hazardous pollutants in wastes.Cround water, waste water, influents and effluents, and other NPDES concerns are all addressed by our dedicated CC/MS instruments and professional customer service. Our Diagnostic A i r Screening tests for specific volatile and semi-volatile organic compounds.Weeven find, name, and quantify the most mysterious compounds through Library Search, which uses our computerized library of over 31,000 mass spectra.And for heavy metals and other inorganics,we offer /CAP analysis.our capabilities are hard t o match. For down-to-earthanswers you can trust without question, depend on the most trustworthy analytical laboratory inthe world. Depend on Mead CompuChem.Thefirst name in analytical laboratories.
Absolute. UncompromisingDedication to QualityData Call now a t 800-334-8525 (in North Carolina call 919-549-8263)or write PO. BOX 12652, ResearchTriangle Park, NC 27709 Regional offices in Chicago. lllinois. Cleveland, Ohio Dallas.Texas New York. New York RiChfnOnU, Virginia
-
CIRCLE 132 ON READER SERVICE CARD
An Expanding Experience. . .
The American Chemical Society or over 100 years, the American Chemical Society has added more publications, new services, scores of local sections, divisions by the dozen. and over 120.000 unique members. Some are Nobel Laureates. some have just received their degrees. Today, we would like to add you. We think you would enjoy association with experienced chemical scientists who take their profession seriously. You can start today by becoming a member or national affiliate of the Society. The benefits are immediate: reduced fees at meetings weekly issues of C&EN member rates on ACS journals local section activities 31 specialized divisions employment aids group insurance plans Send the coupon below today for an application, or call (202) 872-4437.
F
m 1 1 1 1 1 - 1 1 1 1 Yes, Iz m inicicacd in joinlng the ACS. PICSMsend infomition and an
appliciiion. Name
I I
Address
Cil).
Phone(
SIIC
zinicuuntrr
)
American Chemical Society Membership Development I155 Sixteenth Street, N.W. Washington. D.C. 20036
m-==m=mmmmJ
1136A
I I I I I I I
I
0.627 M CHL/O.O80 M DHP in acetonitrile Assignments: 0-25 ps and 420-540 nm, ' C W : 75 ps and 420-560 nm, 'CHL': 570-650 nm. DHP?; >300 ps and 440 nm. CHL:. (Reproduced from Reference34)
had been measured to he near unity (35). Irradiation of CHLINAP, CHLDHP, and CHLDN 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 ohservation of such species as 'CHL*, ZCHL", ZCHL;, arene cation radical (ARt), and, in the case of NAP, (AR)2t.Examples of the difference absorption spectra obtained at selected times after excitation are deoicted in Figure I. In the electron transfer reaction between CHL and each of the arenes, the acceptor CHL is excited to 'CHL*, which intersystem-crosses to 3CHL* faster than diffusion, a process that proceeds with a bimolecular rate constant of -1O'O M-' s-'. As in the previous nanosecond time-resolved study of CHLINAP in acetonitrile (35),the picosecond absorption study revealed that, as expected, electron transfers between CHL* and the arenes-NAP, DHP, and IN-proceed with diffusion-controlled rate constants. Hilinski et al. (34) did not observe any ahsorption bands in the early stages of the reaction between D and A in the
ANALYTICAL CHEMISTRY, VOL. 55. NO. 11. SEPTEMBER 1983
studied ARKHL 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 hands. 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 he observed for the CHL: absorption hand in any of the CHL/AR systems or for the IN+ absorption band, possibly as a result of overlap with other absorption bands that ohscured 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 DHP?/CHL; ion pair is in
The world of XRF .will never be the same aga.in.
Announcing the all new SPECTRACE"4000 XRF Analysis System from Tracor Xray. Energy dispersive x-ray fluorescence (EDXRF) revolutionized elemental analysis. Now, Tracor Xray revolutionizes EDXRF, w i t h t h e SPECTRACE 4000 -a compact XRF analysis system that puts the power and versatility of EDXRF to work for you with a combination of hardware, software and performance features you won't find on any other system. And it does it all at a price that makes EDXRF a cost effective alternative or complement to other elemental analysis methods.
A simpler world The SPECTRACE 4000 takes the confusion out of instrumental analysis. It uses menu-driven commands t o help you select and set-up the analysis methods best suited to your objectives and provides a compre-
hensive set of programs ranging from the classical empirical methods to the simplest element identification. And once you've established procedures for your applications, the SPECTRACE 4000 makes it easy. by automatically storing them for future use.
A more flexible world Use the SPECTRACE 4000 in the laboratory, in the warehouse or on the production line. Its modular design lets you convert the system from a fully programmable, multi-sample. laboratory configuration to a simple push-bution.
[email protected] QC system in minutes. You can even remove
-
all you need is mistake-proof, pushbutton operation. CIRCLE 206 ON READER SERVICE CARD
A world of new possibilities. When you're not using the SPECTRACE 4000 for XRFanalysis.use it as a general purpose computer. The software operating system is CP/M86 based, so you can use off-the-shelf programs for word processing and data base management. as well as programming Contact Trdcor Xray for complete information on the SPECTRACE 4000. We'll show you what's new in the world of XRE Trdcor Xrdy 345 Ea.t Middlefield Road Mountain Vicw. C A 94043, t ISA ( 4 1 5 ) 967-0350.
E
Es -c
%
c
500
700
800
500
€QO
700
800
'"avelength (nm)
l-napf!$!methyl
- g l ~
~
and 2 - n a p h t h i i y l radicals
Specba were generated by excitation of he sample w i h a 25-p. 266-nm pulw followed by a 2S-ps, 355nm pulse delayed by 4 0 p.The (hal0memyl)naph malenes wwe: (a) l - ( c h l w m l h y l ) ~ l h a b n e ;(b) 2-(chloromethyl)naphmalene:(e) l-(bmmm&yl)naplene; (d) 2
