J. Phys. Chem. 1992, 96, 8042-8048
8042
Subpicosecond Transient Absorption Study of Intermolecular Etectron Transfer between Solute and Electron-Donating Solvents Hideki Kandori, Klaus Kemnitz,+and Keitaro Yoshihara* Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: February 14, 1992; In Final Form: May 4, 1992)
Nile blue A experiences ultrafast fluorescence quenching in diffusionless, electron-donatingsolvent systems such as N,Ndimethylaniline (DMA) or aniline (AN),in which the shortest fluorescence decay time reaches s, as revealed by f e m w n d fluorescence upconversion studies [Kobayashi, T.; et al. Chem.Phys. Lett. 1991,180,416]. In the present paper, the reaction mechanism is studied in more detail by subpicosecond transient absorption spectrmpy. By directly detecting the absorption of the products, namely, reduced nile blue (470 nm) and the solvent cation (470 nm, DMA'; 405 nm, AN'), we established that the fluorescence quenching is, as previously proposed, due to an electron-transfer reaction. In the present systems of small free energy difference for forward and backward electron transfer, the reverse electron transfer occurs on a picosecond time scale, Le., 4.0 ps in DMA and 2.7 ps in AN, and the present systems perform a full cycle of charge separation and recombination in less than 10 ps.
Introduction Intermolecular electron transfer is one of the most important reactions in chemical and biological processes and can occur extremely fast. In recent years, progress in generating ultrashort pulses led to the direct observation of ultrafast intermolecular electron transfer, which can occur in less than 1 ps in some chemical] and biological2 systems. Very recently,3a we reported the femtosecond fluorescence quenching of an excited dye molecule, nile blue A (NB = NB+ClO,-), in neat aniline (AN) and NJV-dimethylaniline (DMA), which act as weakly polar electron-donating solvent^.^ In these systems, electron donor and acceptor are in direct contact and electron transfer is not limited by translational diffusion. The fluorescence decay, observed by femtosecond fluorescence upconversion, displayed a clear nonexponential behavior.3d Since the electron-transfer rate is much faster than the longitudinal solvent relaxation time, the present system is of particular interest from both theoretical and experimental points of view. Fluorescence decay measurements, though a powerful technique to probe ultrafast phenomena, can obtain information only about excited states and are unable to identify transient reaction products. Direct evidence for electron transfer can be obtained by detecting products of charge separation. By use of time-resolved absorption spectroscopy, we can observe the formation of reduced dye and solvent cation, i.e., the formation of neutral NB radical (NB' NB') and aniline or dimethylaniline cation (AN', DMA').
-
NE (= NE'CIOd-)
AN
DMA
The forward electron transfer reaction is described as NB'ClO,--DMA
A NB'C1O4--DMAt
Transient absorption spectra of the electron-transfer systems of NB/DMA and NB/AN are compared to the system of NB/CN (CN = l-chloronaphthalene). Chloronaphthalene is used as a weakly polar reference solvent without the permeability of electron transfer. Due to its much higher ionization potential, the fluorescence of NB/CN is unquenched with a lifetime of 2.6 ns. In addition to product identification, piscosecond transient absorption studies can monitor the reverse electron transfer. We find very fast return rates on the picosecond time scale, in general agreement with theoretical consideration^.^^ Thus, the present 'Present address: Government Industrial Research Institute, Osaka, 1-8-
31, Midorigaoka, Ikeda, Osaka 563,Japan.
systems perform a complete cycle of charge separation and recombination in about 10 ps.
Experimental Section Nile blue A (NB) was purchased from Exiton and l-chloronaphthalene, N,N-dimethylaniline, and aniline were purchased from Wako Chemicals (Spectro grade) and used without further purification, but control experiments were performed with freshly distilled samples. The maximal absorbance of NB was kept below 5 , corresponding to 0.96 X lo4, 1.92 X lo4, and 0.86 X lo4 in CN, DMA, and AN, respectively. Absorption spectra were obtained at 2-mm light path, by using a Shimadzu PC-310 spectrophotometer, and were measured before and after the experiment to confirm the integrity of the samples. The apparatus for obtaining transient absorption spectra was a double-beam spectrometer linked with an amplified subpicosecond light source! A CW modelocked NdYAG laser (Coherent Antares 76-S) was both used to pump a dual jet hybridly mode-locked dye laser (Coherent 702-1) and to seed to regenerative amplifier (Quantel RGA60-10). The dye laser, operating with rhodamine 6G,produced SWfs pulses (autocorrelation width) at 600 nm and 76-MHz repetition rate. Pulses of about 1 nJ from the dye laser were amplified to 250 rJ at 10-Hz repetition rate in a three-stage amplifier, using sulforhodamine lOl/methanol. The width of the amplified pulse was obtained to be 0.7 ps by an autocorrelation measurement. The amplified laser output was divided by a dichroic beam splitter, one-half being used as pump to excite the samples after adequate attenuation. The energy range of the pump pulse was 25-100 pJ at a beam diameter of about 2 mm. The other half was used to generate a subpicosecond continuum probe pulse by focusing it into a 1-cm cell containing a D 2 0 / H 2 0mixture (1:2). Residual 600-nm light was removed by a blue glass filter (Melles Griot, BG28) and a cutoff filter (Schott, RG630 or RG645) respectively for measurements at wavelengths shorter and longer than 600 nm. The continuum was split into two parts by a half-mirror and then focused onto two independent 25-cm spectrographs, one for sample measurement and the other for reference. The light path of the sample cell was 2 mm, and the sample was flowed by a peristaltic pump. The continuum light intensities were detected by 5 12 multichannel photodiodes (MCPD, Hamamatsu Photonics), and the signals on the MCPD were ~ c a ~ edigitized, d , accumulated, and transferred to a computer for calculation of the difference absorption spectrum. Accumulations of 100-200 signals were made for each delay setting. Two computer-controlled shutters for pump and probe as well as one for a variable optical delay were used for automatic control of the experiment. Measurements of background and pumponly signals, to eliminate the contribution of spontaneous emission, were
0022-3654/92/2096-8042%03.OO/O 0 1992 American Chemical Society
Ultrafast Intermolecular Electron Transfer
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 8043
i
9 400
500
600
700
600
n
Wavelength / nm Figure 1. Normalized absorption spectra of NB in DMA, AN, and CN (dashed line). Samples are prepared to keep the maximal absorbance at about unity at 2-mm light path, corresponding to concentration of 0.96 X l p , 1.92 X lo-", and 0.86 X lo4 M in CN, DMA, and AN, respectively.
1 400
450
500
Wavelength / nm
TABLE I: Steady-State Abrptioa Spectral Properties of Nile Blue A in Several Solveatso spectral oscillator A,t width strength concn (lo4 M) (nm) (M-I cm-l) (cm-l) (normalized) NB/CN 0.96 646 52000 1300 1 2300 0.81 633 26000 NB/DMA 1.92 1700 1.37 651 58000 NB/AN 0.86 1900 1.54 625 67000 NB/MeOH 0.75 = absorption maximum, cmX = molar extinction coefficient, spectral width = full width at half-maxima of absorption band, and oscillator strength = calculated in the main absorption band.
followed by alternate measurements of probe-only and pumpDuring each measurement, a part of every pump pulse was monitored by a photodiode. Since the arrival time of a probe pulse is wavelength-dependent, i.e. at shorter probing wavelengths the arrival is more delayed, we made corrections by determining the time delay by observing the rise in absorbance change of NB in methanol. NB/methanol was also used to check the time resolution of the complete system by measuring the rise in absorbance change, which could be well fitted by a sech2pulse shape of fwhm = 0.7 ps. All present simulations were carried out under convolution with the pulse shape.
Results Steady-State Absorption Spectra. Figure 1 shows the peaknormalized absorption spectra of NB in CN, DMA, and AN. The
spectral properties are also shown in Table I together with those in methanol. A detailed discussion of the ooncentrationdependent and solvent-dependent band shape, Le., the influence of possible dimerization: acid-base equilibrium;7"and CT complex formation in much dilute solution, will be given elsewhere.' "htAbsorptioa Study of NB in the Inert Reference M e a t I-cblorwrphthalene. The fluorescence lifetime of NB in CN is about 2.6 ns, and the absorption of the excited state should be easily observed on the picosecond time scale. Figure 2a shows the difference absorption between 380 and 550 nm, Le., the wavelength region on the blue side of the excitation wavelength (600 nm). The obtained time-resolved spectra are identical to each other, showing two maxima at 423 and 523 nm, and are assigned to the excited-state absorption of NB. The full width at half-maximum of the main band (A- = 523 run) is about 2400 cm-I and corresponds to that in methanol (not shown). The difference absorption spectra for wavelengths longer than 630 nm are shown in Figure 2b. The difference minimum is located at around 656 nm, which is by 10 nm shifted to the red from the ground-state absorption spectrum. Since the spectral shape is dissimilar to the mirror-image type band displayed by the steady-state absorption spectrum (Figure 1, in CN), the negative absorption contains contributions from the gain signal due to stimulated emission. On the other hand, spontaneous
1
650
"
"
l
700
"
'
750
Wavelength / nm F i i 2. Difference absorption spectra of NB in CN excited at 600 nm by a 70-pJ, 0.7-ps pulse. The spectra are taken at 4, 10, 16,22,28,34, 40,and 46 ps after excitation (top to bottom) and are obtained by the accumulation of 100 laser shots. (a) The ordinate is divided into 0.1 absorbance units. Positive signals are due to the excited-state absorption of NB. (b) The ordinate is divided into 0.2 absorbance units. Negative signals are due to ground-state depletion and stimulated emiseion. o'2
1
Diff(46.p~-4ps)
0.0
4
-0.2
P
a
-0.4
n
-0.6
-0.8
650
700
750
Wavelength / nm Figure 3. Difference absorption spectra of NB in CN at 4 and 46 ps after excitation, together with difference of both spectra (46 p minus 4 ps).
emission (fluorescence) does not contribute, since it is canceled by subtracting the pumponly signal (see Experimental Section). Whereas the band shape in Figure 2a is independent of time, there seems to be a slight red shift at longer times in Figure 2b. We take a closer look at this phenomenon by showing two spectra
8044 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 1.52
s
1.0-
L
0.5-
.
. = . = g D . - . = l - . r - -
w
Kandori et al.
510 nm
.-
0
E 0.0-,,
ti
P
a
-0.5-
680 nm
5
-1.0- -
0 o
640 nm -1
5 7
l
'
l
'
l
'
l
'
l I
1 " " I " " l " ' ~ i
400
450
500
550
Wavelength / nm
0.0
ti
P
ga
-0.2
------L4,0-
4 400
450
500
550
Wavelength / nm 650
700
Wavelength / nm Figure 5. Difference absorption spectra of NB in DMA (630-730MI). Spectra (solid lines) are obtained at 0.8,1.9,3.0,4.0,5.1, 6.2,7.3,and 17.0p after excitation by a 100-dpulse (from bottom to top). The dotted line indicates the steady-state absorption spectrum of NB in DMA.
at 4 and 46 ps after excitation in Figure 3, which also shows the difference between both spectra (46 ps minus 4 ps), with the difference minimum being located at about 670 nm. The kinetic changes of the absorbances at 510,640, and 680 nm, monitoring mainly the excited-state absorption, ground-state depletion, and stimulated emission, respectively, are shown in Figure 4. Only the stimulated emission (680 nm) displays a kinetic change, and the decrease in absorbance (increase in emission) can be fitted by a single exponential of 18 ps. Transient Absorption Study of NB in Dimethylrmlliw. The transient absorption of N B in DMA is remarkably different from that of the nonreactive system, discussed in the previous section, as could be expected from previous fluorescence Figure 5 shows the dif€erenceabsorption spectra of NB in DMA at wavelengths longer than 630 nm. The difference spectrum at 0.8 ps after excitation looks similar to the steady-state absorption spectrum, and the negative signal is mostly due to groupd-state depletion and less to stimulated emission. The lack of any stimulated emission implies that the lifetime of the excited state of NB in DMA must be very short, in agreement with the measured fluorescence lifetimes of 0.1 ps (95%) and 2.3 ps (5%).3a On the other hand, the ground-state absorption recovers slower (Figure 5 ) than the excited state is decaying, indicating that the excited state does not decay to the ground state directly. Figure 6a shows the difference absorption spectra of N B in DMA at wavelengths shorter than 550 nm. The absorption band
Figure 6. Difference absorption spectra of NB in DMA (380-550 nm). (a) The spectra are obtained at 0.7,2.9,5.0,7.2,and 9.4ps after excitation by a 80-p.J pulse. (b) The sum of the individual spectra in Figure 6a is superimposed on the absorption spectrum of DMA' at 77 K (solid smooth line, ref 8).
at about 470 nm, which appears immediately after excitation, decays within 10 ps. The amplitude of this band is about 6 times smaller than that of the ground-state depletion. The N B excited-state absorption at around 520 nm, which is the prominent feature in nonreactive solvents like CN (Figure 2a) and methanol, cannot be captured by the present time resolution of 0.7 ps due to the very rapid depopulation (0.1 p s 9 . Figure 6b compares the transient of Figure 6a with the spectrum of DMA radical cation (observed in a Freon mixture at 77 K (A, = 473 nm)*), and the close spectral similarity makes us to assign the transient absorption band to DMA'. Since the molar extinction coefficient of DMA+ ( ~ 4 7 0 0M-l cm-') is considerably lower than that of N B (26000 M-l cm-I), the observed change at 470 nm should be smaller than the depletion signal of NB. The kinetic analysis of the change in transient absorption is shown in Figure 7,which displays the decay of the transient (467 nm) and the recovery of the original ground state (655 nm). Both t r a m can be fitted by a lifetime of 4.0 ps, indicating the direct decay of the transient back to the ground state:
C104-NB'.*.DMA+
C104-NB+-.DMA
The full reaction scheme is drawn in Figure 12a: ultrafast charge separation in about 0.1 ps is followed by very fast charge recombination in 4.0 ps. The transient absorption of the second reaction product, i.e., the reduced semiquinoid NB', has not been discussed so far. Unfortunately, the absorption spectrum of reduced NB+, namely NB', is not available. However, transient spectra of the semiquinoid forms of the closely related cresyl ~ i o l e toxinon,1° ,~ and
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 8045
Ultrafast Intermolecular Electron Transfer
a 467 nm
0.1
0.0
ui n
a
g
-0.2
-0.4
b 5
0
3 .-
15
10
Figure 7. Kinetic changes of absorbances at 467 and 655 nm of NB in DMA,monitoring the transient absorption and ground-state depletion of NE,respectively. Solid lines are fitted curves, obtained by convolution with a pulse of 0.7-psduration and a single-exponential decay process of 4.0-pslifetime.
-
rhodamine B" are known. All these dyes display the radical absorption bands blue-shifted by 5000-5600 cm-l from the SI So absorption. If we assume a similar blue shift in the case of NB, we amve at 465-480 nm for the absorption of the reduced species, which is corroborated by the results of NB in AN, which suggests that the semiquinoid NB' has an absorptisn at about 470 nm (see below). T w i e n t Absorption Study of NB in Aniline. Figure 8a shows the difference absorption spectra of NB in AN, which contrasts to that of DMA (Figure 5). Immediately after excitation (1.74 ps), the minimum is red-shifted by 10 nm compared to the steady-state absorption of NB in AN (Figure l), indicating that stimulated emission is contributing to the spectrum. The difference minimum clearly shifts to the blue with time, indicating that NB in the excited state is not returning to the ground state directly and hints to the presence of an intermediate state. This effect can even more clearly be seen in the kinetics, displayed in Figure 8b. The decay of the excited state, monitored by the stimulated emission signal at 685 nm, occurs faster than the ground-state recovery, monitored at 645 nm. The excited-state decay can be fitted by a single-exponential decay of 1.5-ps lifetime, after convolution with the excitation pulse of 0.7-ps duration. Fluorescence decay experiments, which have a better time resolution, showed a nonexponential decay with a tentative two-exponential analysis of 0.4 ps (56%) and 2.5 ps (44%) lifetime^.^ The present experiment yields an intermediate value due to limited time resolution. Thus, the following reaction scheme emerges, considering the formation of an intermediate state:
-
rI = 1.5 ps N B ~ Y (NB)+ -
intermediate
0.c
N
Time I ps
72
NB
The time constant of the second reaction, 7 2 , can be obtained from the kinetics of the ground-state recovery at 645 nm in Figure 8b and is obtained as 2.7 ps. Thus, spectral and kinetic data from wavelengths longer than the pump wavelength suggest the presence of an intermediate product state, which rises with 1.5 ps and decays to the original state with 2.7 ps. The transient absorption spectra of NB in AN at wavelengths shorter than 550 nm are shown in Figure 9a and look, at first glance, similar to those obtained for NB in CN (Figure 2a), indicating the excited state of NB. A strict comparison of the spectra in AN and CN demonstrates, however, the presence of still another component. Figure 9b compares the difference spectra at 2.5 ps, with both spectra being normalized at about 520 nm. The clear difference in absorption in the region of 380-500 nm is probably due to the absorption of the transient product state, as suggested above. Since the transient absorption and NB ex-
B
E
5 -O.! ui
9
I -2
0
2
4
6
8
Time / ps Figure 8. Transient absorption of NB in AN (610-770 nm) after excitation by a 30-pJ pulse. (a) Difference absorption spectra. Sixteen spectra obtained from 1.7 ps (bottom) to 17.9 ps (top) are shown at 1.08-psinterval. The difference minimum (659 nm) at 1.74 ps shifts to blue with time. The dotted line indicatea the steady-state absorption spectrum of NB in AN,normalized to the minimum at 1.7 ps. The ground state recovers as quickly as in DMA,whereas the stimulated emission signal is mixed with the ground-state depletion, in contrast to DMA. (b) Kinetic change of the absorbance at 645 and 685 nm, monitoring mainly ground-state depletion at 645 nm (open circle) and the stimulated emission at 685 nm (full circle). The solid line at 685 nm is the best fitting curve, obtained by convolution with a pulse of 0.7-ps duration and a singlc-exponential decay of 1.5-pslifetime. Analogously, the solid line at 645 nm can be fitted by the simulation curve taking into account sequential decay process as shown in the text (section 4 in Results).
cited-state absorption are overlapping, it is not easy to separate both spectrally and kinetically. We attempt to show an isolated spectrum of the product state under the following two assumptions: (1) the excited-state absorption of NB in AN is similar to that in CN; (2) the difference absorbance at 520 nm is only due to the excited-state absorption. Figure 1Oa shows the difference spectra, obtained by subtracting the spectra of the NB/CN system (Figure 2a) from that of the respective NB/AN system (Figure 9 ) , by normalizing the a b sorptions at 520 nm. The resultant spectra display two absorption bands at 470 and 405 nm, both decaying on the picosecond time scale. The aniline cation, AN+,is known to have two a m t i o n peaks at 412 and 430 nm at 77 K,*and at 406 and 423 run at room temperature,I2the latter being shown as thick solid line in Figure lob. Although the fine structure of AN+ is not observed in the transient spectrum, which in addition is bluashifted by about 10 nm, we assign the transient band at 405 nm to AN+. The sccond absorption band at 470 nm, which is also observed in the NB+/DMA system (Figure 6b), is due to reduced nile blue, Le., NB'. Figure 11 shows the kinetic behavior of both transient specimen, Occurring in the NB/AN system. The plotted absorbaces (open circle, AN+;full circle, NB') were obtained by subtracting the excited-state absorption from the absorbance at 390-420 and 470
8046 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
1
Kandori et al.
a
f 9
-
. I 400 450 500
1
0.100.104
J
6.8 ps
\"I
NB in AN at 2.5 ps
uj
Q
---#
Ih\
P
a g
5.4 ps
0.05: ,-
450
400
500
Wavelength / nm Figure 9. Transient absorption spectra of NB in AN (380-520 nm). (a) Difference absorption spectra: 12 spectra from 1.1 ps (top) to 9.0 ps (bottom) are shown at 0.72-ps time interval. (b) Comparison of the transient of NB in AN with the excited-state absorption of NB in CN.
nm, respectively. The absorbance of both species was simulated (solid line) with a rise of 1.5 ps and a decay of 2.7 ps, obtained from a convolution with a pulse of 0.7-ps duration. Thus, the spectral and kinetic results indicate that the following scheme is appropriate for the NB/AN system:
-
C104-(NB+)*.**AN
1.5 p
-
C104-NB'***AN+
400
2.7 pa
C104-NB+..*AN
The full reaction scheme is drawn in Figure 12b. Different from NB/DMA, the rise to the excited state of NB/AN could be eked, since it is within the time resolution of the present system.
450
500
Wavelength / nm Flppc 10. (a) Absorption spectra of the transient species in the NB/AN system. The spectra are obtained by subtraction of the excited-state absorption of NB from the spectra in Figure 9a, as d d b e d in the text. (b) The sum of the spectra in (a) is compared to the absorption spectrum of AN+ at room temperature (ref 12).
Excited-State Relaxation Processes of NB in Nonreactive
0
Solvents. Nile blue (NB) has been studied intensely, and the
excited-state relaxation dynamics have been investigated by many authors. Recent progress in generating ultrashort pulses allows the detailed understanding of the vibronic relaxation dynamics of the dye in solution. When the NB molecule is excited, intramolecular vibrational redistribution (IVR)occurs within 60-85 fs (dephasing time Tz),as determined by femtosecond p u m p probe,13 hole buming,I4 and photon echoIs studies. Vibrational cooling, the consecutive step, occurs in the subpicosecond regime and was investigated by the pumpprobe method16 and by the fluorescence upconversion technique." Two decay times were reported by the fluorescence measurements of NB in methanol, one on the subpicosecond time scale, ascribed to vibrational relaxation, and a slower one of 20 ps, ascribed to solvent re1axati0n.l~ The latter is appearing somewhat larger than the longitudinal relaxation time of methanol (TL = 8.7 ps), which is considered to regulate polarization relaxation. Monitoring the stimulated emission in the present work yielded a similar time constant of 18 ps in the weakly polar solvent CN (Figure 4). The observed value in the NB/CN system is closer to T~ of CN (24 ps)18 than that in the NB/methanol system, indicting that the solvent relaxation is a dominant factor of the observed relaxation and that the NB/CN system is better described by the continuum theory than the NB/methanol system.
-2
0
2
4
6
8
Time 1 ps Figure 11. Kinetic changes of both transient specimen in the NB/AN system: calculated absorbances at 390-420 nm (open circle) and 468472 nm (full circle) arc plotted, which are obtained by subtraction of the excited-state absorption from the total signal (see text). The solid line indicates the simulation curve, which rises with a time constant of 1.5 ps and decays with 2.7 ps, obtained by convolution with a pulse of 0.7-ps duration.
The SIabsorption of NB in CN (Figure 2a) is almost identical to that in methanol (data not shown), and both relaxation times
of the SIstates are similar. Thus, spectral and kinetic results
The Journal of Physical Chemistry, Vol. 96, NO. 20, 1992 8047
Ultrafast Intermolecular Electron Transfer
(NE')'
a-
DMA
marizing, the dominant photochemistry of NB in DMA and AN is governed by intermolecular electron transfer, which performs a full cycle and recovers to the original state within 10 ps.
(NE')'... AN
k ! % 2.3 ps) c
i
/ i
4.0 ps
i / i
2.1 ps
/
NE' -* DMA NE' AN Figure 12. Electron-transfer reaction scheme of NB in DMA (a) and in AN (b). The forward electron transfer (charge separation) could not be observed in DMA (ND= not determined) because of the limited time rmlution of the present system (excitation pulse width of 0.7 ps), while that in AN can be approximately fitted by a single exponential of 1.5-ps lifetime. The reverse electron transfer occurs with 4.0 and 2.7 ps in DMA and AN,respectively. The values in parentheses indicate the decay times
of the excited state obtained by the fluorescence upconversion technique of higher time rt~01ution:~~ 0.1 p (95%) and 2.3 p (5%) in DMA: 0.4 p (56%) and 2.5 ps (44%) in AN. The counteranion (Clod-)of dye is not shown in the figure. indicate that the interaction between aromatic solute and aromatic solvent does not affect very much the properties of the SIstate. It should be noted that we observed neither spectral nor kinetic changes in the SI-S,absorption region (380-550 nm) in CN (Figure 4) and methanol. This observation suggests that the potential surface of the SIand the S, electronicstates has a similar curvature along the relaxation coordinate. A similar observation was made by another group for NB in methan01.l~ Electron Transfer of the NB/DimethyWKne and NB/Aniline System. Subpicosecond transient absorption spectroscopy was applied to study the ultrafast photochemical reaction of NB in DMA and AN. In both cases, we observed the transient absorption of the solvent cation (DMA+ and AN'). The transient absorption spectra of the reduced NB and of DMA' are located in the same wavelength region, which could be directly elucidated by observing both transients in the NB/AN system. The observation of both products of charge separation clearly demonstrates that the ultrafast fluorescence quenching is caused by intermolecular electron transfer. In addition, kinetic studies have revealed the rate constant of the back-electron-transfer reactions. The reaction scheme in Figure 12 is sufficient to explain the major features of the reaction processes of NB in DMA and AN. However, small discrepancies remain and are discussed in the following. In Fwre 5, a slight deviation of the depletion spectrum at 0.8 ps from the absorption spectrum (dotted curve) is observed between 660 and 710 nm and might be due to (a) contributions of a ground-state charge-transfer complex and/or (b) due to the nonexponential nature of the observed fluorescence decays. (a) The red tail in the steady-state absorption spectrum of NB in DMA at wavelengths longer than 650 nm (Figure la) might be due to small contributions of a ground-state charge-transfer complex, not excited by the pump beam, which is exclusively exciting NB locally. Accordingly, the observed depletion displays a narrowed spectral shape. (b) A small positive absorption, which remains even after 90% recovery, is observed at wavelengths longer than 680 nm (Figure 5). This absorption, which may be due to product states, appears even at 0.8 ps. It may be due to the minor component of fluorescence decay of 2.3-pslifetime,3awhich has a contribution of about 5% in the NB/DMA system and might contribute a slight positive absorption at 660-710 nm. Measurements of excitation-wavelength dependence will answer the question. The same argument holds for NB in AN. In Figure 8b, the absorbance change at 685 nm does not recover to zero but remains positive, even after 8 ps, and may be due to the slow component of the fluorescence decay. A third possibility, to explain the above-mentioneddeviation from the simple model, might be seen in the presence of dissociated long-lived free radicals. More accurate measurements are necessary to explain these minor features of the present transient absorption measurement. Sum-
DifMon-~WealdyPolvIntermdecuhr-E~~ System. The present intermolecular-electron-transfersystems, comprised of organic dye molecule and solvent donor, had recmtly been d e v e l ~ p e din~the ~ ~quest ~ for a homogeneous contact system, analogous to the previously studied heterosystem of organic crystal and adsorbed and turned out to be of ultrafast nature,3' if the energetic requirements are met. The above systems display the following salient features: (1) Absence of translational diffusion, due to the fact that each individual solvent molecule can act as electron donor or acceptor, guarantees the Observation of ultrafast electron transfer, unlimited by the mutual diffusion of donor and acceptor.21 (2)Due to the weakly polar character of the system, the solvent reorientation energy is small and comparable to that of intramolecular reorientation, X, = leading to ultrafast transfer rate, faster than and unlimited by solvent relaxation?2 (3) Due to the high value of the electron-exchange matrix element of two aromatic molecules in a face-to-face configuration, the transfer rate can reach 1/(100 fs), if the free energy assumes favorable values. (4) Absent or weak ground-state interactions distinguish the present systems from typical chargetransfer systems.23 (5) Electron transfer in the fastest systems might compete with vibrational redistribution and relaxation, leading to nonexponential decay. (6)The present transient absorption studies yielded the rate constant of back electron transfer of 4.0 ps in DMA and 2.7 ps in AN. It is an illustrative exercise to compare the present ultrafastelectron-transfer system with that of the photosynthetic reaction center (RC).% The efficiency of the charge separation, as defmed by the ratio of forward and backward electron transfer, is several hundred times higher in the RC. This is an even more astonishing accomplishment, since the forward electron transfer is slower by about 10 times due to smaller mr overlap. The big advantage of the RC lies in the solvent-free nature of its protein environment, resulting in a total reorientation energy of only about 0.2 eV. In contrast, the present system, though only weakly polar, has a total reorientation energy of about 0.6 eV, with the solvent contributing about 0.3 eV. The small redentation energy of the RC results in a very slim, asymmetric Marcus parabola, placing the back-reaction of electron transfer, occurring at a free energy of about 1 eV, far in the inverted region, where electron transfer is slowed down by several orders of magnitude. On the other hand, the back electron transfer in the present system is placed, almost adjacent to that of the forward reaction, right on top of a rather broad parabola, resulting in fast electron transfer in both the forward and backward direction.25 Acknowledgment. We are grateful to Dr. H. Petek for his advice in handling the lasers, Dr. T. Kobayashi for helpful discussions, and Dr.H. Miyasaka for his advice in constructing the transient absorption spectrometer. This work was supported in part by a Grant-in-Aid for Scientific Research on New Program (03NP0301) by the Department of Education, Science, and Culture of Japan.
References and Notes (1) Asahi, T.; Mataga, N. J. Phys. Chem. 1989,93,6575.
(2) (a) Fleming, G. R.; Martin, J. L.; Breton, J. Nature 1988, 333, 190. (b) Holzapfel, W.; Finkele, U.;Kaiser, W.; Oesterhelt, D.; Schetr, H.; Zinth, W. Proc. Natl. Acad. Sci. U.S.A. 1990,87, 5168. ( 3 ) (a) Kobayashi, T.; Takagi, Y.; Kandori, H.; Kemnitz, K.; Yoshihara, K. Chem. Phys. Lett. 1991, 180,416. (b) Kemnitz, K.Chem. Phys. Lett. 1988, 152, 305. (c) Kemnitz, K.; Yoshihara, K. Chem. Lett. 1991,645. (d) Yoshihara, K.; Y a w , A.; Nagasawa, Y.; Kandori, H.; Douhal, A.; Kemnitz, K. Ulrrafmt Phenomena VIM Martin, J.-L.,Migus, A., Eds.; Springer-Verlag: Berlin, in press. (4) Petek, H.; Yoshihara, K.; Fujiwara, Y.; Frey, J. G. J . Opr. SOC.Am. B 1990, 7, 1540. ( 5 ) Since some transient absorption appeared at the shorter wavelength side of the pump wavelength (600 nm) in solute-free solvent, the solvent background was determined after each measurement and subtracted. (6) Kemnitz, K.; Yoshihara, K. J . Phys. Chem. 1991, 95, 6095. (7) (a) Douhal, A,; Yoshihara, K. To be published. (b) Kemnitz, K.; Nagasawa, Y.; Douhal, A.; Yoshihara, K. To be published.
8048
J. Phys. Chem. 1992, 96, 8048-8053
(8) Shida, T. Electronic Absorprion Spccrra of Radical Ions; Elsevier: Amsterdam, 1988 (in Physical.8cience data). (9) Kreller, D. I.; Kamat, P. V. J. Phys. Chem. 1991, 95, 4406. (10) Iwa, P.; Stciner, U. E.; Vogclmann, E.; Kramer, H. E. A. J . Phys. Chem. 1982,86, 1277. (1 1) Stevens, B.;Sharpe, R. P.; Bmgham, W. S.W. Photochem. Photobiol. 1967, 6, 83. (12) Qin, L.; Tripathi, G. N. R.; Schuler, R. H. 2.Narurforsch. 1985, 40A. 1026. (13) Fragnito, H. L.; Bigot, J.-Y.; k k r , P. C.; Shank,C. V. Chem.Phys. Lett. 1989, 160, 101. (14) Brito Cruz, C. H.; Fork, R. L.; Knox, W. H.; Shank, C. V. Chem. Phys. Let?. 1986, 132, 341. (15) k k e r , P. C.; Fragnito, H. L.; Bigot, J.-Y.; Brito Cruz, C. H.; Fork, R. L.; Shank, C . V. Phys. Rcv. Let?. 1989,63, 505. (16) Weiner, A. M.; Ippen, E. P. Chem. Phys. Le??.1985, 114, 456.
(17) Mokhtari, A.; Chesnoy, J.; Laubereau, A. Chem. Phys. Leu. 1989, 155, 593-598. (18) Forest, E.; Smyth, C. P. J . Phys. Chem. 1%5,69, 1302. (19) Martin, M. M. Private communication. (20) Kemnitz, K.; Nakashima, N.; Yoshihara, K. J . Phys. Chem. 1988, 92, 3915. (21) (a) Rehm, D.; Weller, A. Isr. J . Chem. 1970,8, 259. (b) Mataga, N.; Asahi. T.; Kanda, Y.; Okada, T.; Kakitani, T. Chem. Phys. 1988,127, 249. (22) Sumi, H.; Marcus, R. A. J. Chem. Phys. 1986,84,4894. (23) (a) Asahi, T.; Mataga, N. J . Phys. Chem. 1989,93,6575. (b) Ojima, S.; Miyasaka, H.; Mataga, N. J . Phys. Chem. 1990, 94, 4147, 5834,7534. (24) Marcus, R. A. Chem. Phys. Lett. 1987, 133, 471. (25) The plot of rate constant vs free energy in ref 3a is based on the approximationthat reorientation energits and electron-exchange matrix elcmenu are identical for the forward and kckward electron-transfer reaction.
An ab Initio Mdecular Orbital Study of the Thermal Decompodtlon Mechanisms of C2H,AsH2 and C2H,GaH2 Charles W. Bock* and Mendel Tracbtman Chemistry Department, Philadelphia College of Textiles and Science, Philadelphia, Pennsylvania 19144 (Received: February 20, 1992)
Ab initio molecular orbital calculations were used to study the pyolysis of ethylarsme and ethylgallane. Within the computational models selected, the dominant decomposition channel for ethylarsine was found to be the unimolecular elimination of ethane, which requires approximately 55.0 kcal/mol. However, an ethyl free radical mechanism requiring 56.0 kcal/mol was found to be competitive. The dominant decomposition channel for ethylgallane was found to be the &elimination of ethylene, which has a barrier of only 35.8 kcal/mol. In this case alternative decomposition channels all required more than 60 kcal/mol.
1. Introduction Trialkylgallanes, such as trimethylgallane (TMGa) or triethylgallane (TEGa), and arsine have been used extensively as group III/V sources in the organometallicvapor-phase epitaxial (OMVPE) growth of GaAs layers.'** tert-Butylarsine (tBAs) is an alternative group V source that has recently been found to yield GaAs layers with morphologies and carrier concentrations similar to layers produced using a r ~ i n e . ~tBAs , ~ and TEGa have lower decomposition temperatures than arsine and TMG, respectively, and can produce GaAs layers with less carbon contamination.+' tBAs has the added advantage of being somewhat less toxic and easier to handle than arsine? and its high vapor pressure makes it an ideal non-hydride group V source for the OMVPE process? The gas-phase chemistry associated with the use of alkyl-sub stituted arsines and gallanes in the OMVPE growth of GaAs layers is certainly complex and involves both adduct formation and thermal decomposition. Some progress has been made in understanding the structures, vibrational spectra and stability of various simple Lewis acid/base galliumarsenide adducts, such as H3Ga:AsH3,(CH3)3Ga:AsH3,and C13Ga:AsH3.e'6 For example, computational studies at the MP2/HUZSP*//RHF/ HUZSP' level have shown that the stability of these three adducts, relative to their precursors, are approximately 11.6,6.1, and 15.7 kcal/mol, respecti~ely.'~-'~ However, the role of such adducts in the overall OMVPE process is not yet fully understood,' and no detailed molecular level data are currently available for similar adducts formed using either tBAs or TEGa. On the other hand, thermal decomposition data are available for some of the more interesting alkyl-substituted arsine and gallane molecules. Specifically, tBAs has been studied by Larsen et al.5 using timesf-flight mass spectrometry, both in an isolated environment and in the presence of TMGa. It was shown that the expected tert-butyl free radical mechanism is not the dominant 0022-3654/92/2096-8048S03.00/0
route in the pyrolysis process. In fact, their results were adequately explained in terms of two unimolecular processes,, i.e.
-
C4H9AsH2 C4HI0+ ASH (coupling) and
C4H9AsH2 C4Hs
+ ASH,
(8-elimination)
(1) (2)
Reaction 1, in which isobutane is produced, is an example of intramolecular co~pling,~J'J* whereas reaction 2, in which isobutene is produced, is an example of &elimination.'l At all temperatures, reaction 1 was found to predominate, although at higher temperatures &elimination and radical formation became significant. To date, no ab initio molecular orbital calculations have been reported on the thermodynamics or kinetics of the possible decomposition modes of tBAs. The thermal decomposition of TEGa is significantly different from that of tBAs. The main hydrocarbon product in metalloorganic chemical vapor deposition (MOCVD) reactors is ethylene: suggesting that a &elimination mechanism, analogous to reaction 2, has a lower barrier than a coupling mechanism for ethane elimination. Using ab initio molecular orbital calculations and the smaller molecule ethylgallane as a model, Oikawa et a1.6 showed that the barrier for &elimination is quite low, approximately 41.2 kcal/mol, which is some 20.0 kcal mol lower in energy than that required to break the Ga-C bond.' These authors also found a stable *-complex, 10.2 kcal/mol below the energy of the products, GaHl and C2H4, and a second stable conformer of ethylgallane in which the GaH2 moiety is rotated 90° about the Ga-C bond. No TS for the elimination of C2Hsor H2 was given in this paper. The purpose of the present paper is to use all-electron ab initio molecular orbital calculations to investigate pyrolysis mechanisms in alkyl-substituted arsine and gallane molecules, using ethylarsine
/
0 1992 American Chemical Society
