J. Phys. Chem. 1992,96, 7635-7640
7635
Fluorescence of Excited Charge-Transfer Complexes and Absolute Dynamics of Radical-Ion Pairs in Acetonitrile Ian R. Could* and Samir Farid* Corporate Research Laboratories. Eastman Kodak Company, Rochester, New York 14650- 2109 (Received: December 30, 1991; In Final Form: March 16, 1992)
The absolute dynamics of the contact and the solvent-separated radical-ion pairs for a series of 2,6,9,1O-tetracyanoanthraccne acceptor/alkylbenzene donor systems have been determined in acetonitrile from a combined analysis of time-resolved excited charge-transferemission and radical-ionpair absorption data and measurementsof the quantum yields for formation of separated radical ions. Absolute rate data for the contact radical-ion pairs are obtained from timeresolved emission experiments. Absolute rate data for the solvent-separated radical-ion pairs are obtained for those cases in which the timeresolved emission is different from the time-resolved absorption.
Introduction Photoinduced bimolecular electron-transfer reactions are normally performed in polar solvents such as acetonitrile to facilitate the formation of separated radical ions in solution.'-3 A mechanistic scheme for such a reaction between a singlet excited-state acceptor A* and a donor D is shown in Scheme I. The primary intermediates are the encounter pair (A*(S)D), the contact radical-ion pair (CRIP, A'D*+), and the solvent-separated radical-ion pair (SSRIP, k-(S)D'+).'-3 The two radical-ion pair intermediates can be distinguished on the basis of electronic coupling, which is much higher in the CRIP compared to the SSRIPa3As a consequence, emission can be observed from the contact pairs but not from the solvent-separated pairs, and the dynamics of the solvent-separated pairs are susceptible to the application of weak external magnetic fields whereas those of the contact pairs are not.' Bimolecular photoinduced electron-transfer reactions in polar solvents have been the subject of several recent studies in which the quantum yields for formation of "freen radical ions (FRI, A'- + Do+),@ions, have been measured as a function of the energy gap between the ion-pair states and the neutral ground states.fs Of particular significance is the observation that the rates of energy wasting return electron transfer within the radical-ion pairs (kJ decrease with increasing energy difference between the ion-pair state and the neutral state; i.e., the reactions are in the Marcus "inverted" region? However, only the relative dynamics of a series of radical-ion pairs are obtained from quantum yield measurements. Absolute dynamics can only be obtained from direct timaresolved measurements such as transient absorption spectroscopy.'*'3 In transient absorption experiments, however, it is difficult to differentiate the contact and the solvent-separated radical-ions pairs due to their similar absorption spectra, and the dynamics of the two radical-ion pair intermediates have been distinguished in this way in very few cases (see for example ref 11). As indicated above, the absolute dynamics of the contact pairs can in principle be obtained from emission lifetime experiments, although there are very few reports of such emission lifetimes in acetonitrile. Ideally, for a complete analysis of the dynamics of the radical-ion pairs, a combination of timeresolved emission and absorption spectroscopiesand measurements of aims from both the SSRIP the CRIP is required. In this work we describe an analysis of the dynamics of the radical-ion pairs of a series of 2,6,9,10-tetracyanoanthracene (TCA) acceptor/ alkylbenzene donor systems, based on such combined experimental data. Results and Discussion Mechanistic Scheme. The mechanistic scheme used to analyze the data is shown in Scheme I.I4 In this work, TCA is the acceptor A and alkylbenzenes are the donors D. Excitation of the cyanoanthracene to form the first excited singlet state, A*, in the presence of D leads to an encounter pair (A*(S)D), which may collapse to form the CRIP with a rate constant k,, or direct formation of a SSRIP may occur with a rate constant ket. Ex-
SCHEME I: Intermediates in Bimolecular Photoinduced Electron-Transfer Reactions in Homogeneous Solutiona
t A(S)D
A + D
Donor/acceptor separation distance increases from left to right from ca. 3.5 A for the contact pairs to ca. 7 A for the solvent-separated pairs. Energy increases in the vertical direction. citation of a ground-state CT complex formed between the cyanoanthracene and a donor (AD) leads to the formation of a CRIP directly. Emission may be detectable from the CRIP. A higher efficiency of CRIP emission for excitation of AD compared to the efficiency via diffusive quenching of A* indicates that direct formation of SSRIP from A*(S)D occurs, Le., that the yield of CRIP from A*(S)D is less than unity.15 Free-radical ions are formed when the SSRIP further separate (k& The yields of FRI in such systems, aim,are almost always less than unity due to energy wasting return electron-transfer reactions which occur within the CRIP, (k,) ,and the SSRIP, (k-& A lower value of aiom for excitation oTAD compared to the value for diffusive quenching of A* also indicates that direct formation of SSRIP from A*(S)D occurs, since in the latter case the extra energy wasting return electron-transfer reaction within the CRIP, ( k J , is avoided.15 In general, mixing of other electronic states has to be considered for the CRIP (indicated by A'-D'+ A*D in Scheme I), especially if A or D has low-lying excited states which m a y mix with the pure ion-pair state, as is the case for the cyanoanthracenes. However, we have found that for the TCA/alkylbenzene system in acetonitrile the pure ion-pair states are sufficientlylow in energy that such mixing are not important.I6 If there is no ground-state CT complex formed between A and D, then the term exciplex is most often used to describe the contact charge-separated state. If there is a ground-state CT complex, then the term excited CT complex is used. Both exciplexes and excited CT complexes may be CRIP if mixing of other electronic states with the pure ion-pair state is negligible. The systems studied here do form ground-state CT complexes, and thus the CRIP can also be described as excited CT complexes. The excited-state and radical-ion pair intermediates in Scheme I are singlet states for reactions of TCA with alkylbenzenes in acetonitrile. Triplet states are not considered since intersystem crossing has been found to be insignificant both for the excited singlet state of TCA and for the radical-ion pairs of these systems in acetonitrile." With TCA as the excited-state acceptor, in-
0022-365419212096-7635$03.00/0@ 1992 American Chemical Society
-
7636 The Journal of Physical Chemistry, Vol. 96, No. 19, 1992
Gould and Farid
Wavelength, nm 500
600
700
2.0
- 6 . 1
1
0 6
2.4
2.2
2.6
EDXD-
20
18
16
14
20
18
16
I I
800
14
Wavenumber, kK
Figure 1. Normalized excited CT (CRIP) emission spectra and emission maxima of 2,6,9,1O-tetracyanoanthracene(TCA) acceptor/alkylbenzene donor (0.06 M) systems in acetonitrile. Residual tetracyanoanthracene (monomer) emissions have been subtracted. The spectral range is determined at the low-energy end by the sensitivity of the spectrometer and at the high-energy end by the presence of relatively intense residual TCA monomer emissions.
tersystem crossing can only be detected in solvents less polar than acetonitrile, where the lifetimes of the intermediate radical-ion pairs are longer.’’ Exdted CX Complex Emissions. As indicated above, there are many reports of exciplex emissions in nonpolar solvents, although the emissions are usually very weak in acetonitrile when the extent of charge transfer is high, due to competing nonradiative decay proccsses and dissociation (solvation) to the SSRIP. For excited CT complexes, however, there are very few reports of emission spectra in homogeneous solution, even in solvents with low polarity. The main reason for the weakness of these emissions is nonradiative return electron transfer to form AD ((k&,, Scheme I), which generally occurs with a rate which is several orders of magnitude larger than that for the corresponding radiative process. Excitation of the ground-state CT complexes of TCA with alkylbenzenes as donors does, however, lead to detectable, though weak, emission from the excited CT complexes (CRIP) even in acetonitrile. Emissions were detected using the following donors, listed in order of decreasing oxidation ~ t e n t i a l : m-xylene, ~ mesitylene, p-xylene, 1,2,4-trimethylbenzene, 1,2,3,4-tetramethylbenzene, durene, pentamethylbenzene, and hexamethylbenzene. The CRIP emission quantum yields are small (ca. l V ) , as expected, and observation of the emissions is only possible after extensive purification of the TCA (see Experimental Section) to minimize emissions from impurities. Typical spectra obtained after subtraction of residual monomer fluorescence are shown in Figure 1. The emission maxima of exciplexes and excited CT complexes are often related to the difference in the electrochemical oxidation potential of the donor and the reduction potential of the acceptor, ExD - @*.I8 A plot showing that such a relation exists for the TCA/alkylbenzene excited CT complex emissions is shown in Figure 2.19 The emission maxima are also consistent both with those of the exciplexes of the closely related 9,lO-dicyanoanthracene (DCA) with the alkylbenzene donors in acetonitrile, which emit with approximately an order of magnitude higher efficiency (Figure 2),16 and also with the stronger emissions of the same TCA complexes in less polar solvents.I6 The DCA exciplex lifetimes are longer due to both slower rates of solvation and the more negative reduction potential of DCA (-0.91 V vs SCE)compared to TCA (4.44V vs SCE) which results in higher energy CRIP’s, which thus undergo much slower return electron
2.0
V
Figure 2. Plot of the emission maxima of the excited CT complex (CRIP) emissions of TCA with various alkylbenzene donors, together with the emission maxima of the exciplexes of DCA with alkylbenzene donors in acetonitrile at room temperature, versus the difference between the electrochemical oxidation potential of the acceptor and the reduction - pa) (redox data from ref 4). The closed potential of the donor (PxD circles refer to the TCA complexes with (1) durene, (2) 1,2,3,4-tetramethylbenzene, (3) 1,2,4-trimethylbenzene, (4) pxylene, ( 5 ) mesitylene, and (6) m-xylene. The open circles refer to the DCA exciplexes with (7) hexamethylbenzene, (8) pentamethylbenzene, and (9) durene. Wavelength, nm 400 425 450
375
475
!.
’>
2a
I
I
26
24 Wavenumber, kK
22
20
Figure 3. Absorption (smooth solid lines) and excitation (noisy solid lines) spectra of the excited CT emission of the 2,6,9,10-tetracyanoanthracene/mesitylene system, monitored at 700 nm at 0.1 and 1.0 M donor concentration, in acetonitrile at 10 OC. The dotted line in the lower figure is the absorption spectrum of the uncomplexed tetracyanoanthracene showing the effect of complex formation on the absorption spectrum.
transfer due to the inverted region e f f ~ t In . ~addition, the band shapes of the excited CT complex emissions are similar to those of the DCA/alkylbenzene exciplexes in acetonitrile as discussed in more detail below. Further evidence that the emissions are indeed due to the excited CT complexes was obtained from measurements of excitation spectra. Shown in Figure 3 are the excitation spectra for the TCA/mesitylene emission monitored at 700 nm, at two concentrations of the donor (0.1 and 1.O M), together with the absorption spectra of the same solutions. The absorption spectra consist of absorptions due to both uncomplexed TCA and to the AD complexes. The close similarity between the excitation and absorption spectra at different concentrations of the donor demonstratesthat excitation of either uncomplexed TCA or the AD complex leads
Dynamics of Radical-Ion Pairs in Acetonitrile to the emission with equal efficiency, which supports the assignment of the emission to the excited CT complex. Furthermore, the fact that the spectra agree at both low and high concentrations suggests that 2:l donor:acceptor complexes are not responsible for the emissions, at least at 700 nm. Finally, it was also observed that the emission quantum yields for the TCA/mesitylene and the TCAlpxylene systems did not depend upon the concentration of the donor in the range 0.06-0.6M, which argues further against the involvement of 2:l complexes. Therefore, despite the weakness of the observed emissions, we are confident in assigning them to the TCA/alkylbenzene excited CT complexes (CRIP) rather than, for example, emissions from impurities. To the best of our knowledge, these are the first reported examples of emission for excited CT complexes in acetonitrile, although excited CT emissions have been observed previously in less polar solvents.20 The spectra shown in Figure 3 also indicate that, for the TCA/mesitylene system, diffusional quenching of 'TCA* by this donor leads to CRIP formation with close to unit efficiency, since excitation of the complexed and the uncomplexed TCA results in CRIP emission with the same efficiency.ls For alkylbenzene donors with oxidation potentials lower than that of mesitylene, however, diffusional quenching of 'TCA* leads to CRIP formation with an efficiency which rapidly decreases with decreasing oxidation potential, approaching zero for pentamethylbenzene and he~amethylbenzene.~ In these cases, diffusive quenching of the 'TCA* does not result in CRIP emission, and the excitation and absorption spectra should be different. These issues have been discussed in more detail elsewhere.3qls In previous publications on the TCA systems we showed that the rates of nonradiative return electron-transfer increase with increasing substitution of the alkylbenzene donor for both the contact ((&-&,) and the solvent-separated ((&-& pair^.^^^ This is due to the fact that, with increasing substitution of the donor, the exothermicity of the return electron-transfer reaction decreases, and the reaction rate increases due to the Marcus "inverted region" effects9 In addition, we quantitatively analyzed the emission of the DCA/pentamethylbenzene exciplex in acetonitrile using a theory21which treated the emission as a return electron-transfer pr0cess.j It was found that the electron-transfer reorganization parameters &, 5,and v, (defined in ref 3) which fit the exciplex emission spectrum also gave a reasonable fit to the dependence of (&J on reaction exothermicity ((AGJcp) for the CRIP of TCA. ?he DCA exciplex was used since at that time we were not able to measure the TCA CRIP emission spectra with sufficient accuracy. The quality of the emission spectra we have now obtained for the TCA CRIP is significantly better, although the spectra are still not complete enough to allow accurate determination of the reorganization parameters by independent fitting. However, unlike the DCA systems, a part of the CT absorption spectrum can also be observed for the TCA systems. Although the CT absorption spectra are also incomplete, the availability of both the absorption and the emission spectral data allows a fairly rigorous testing of the spectral fitting procedure. Shown in Figure 4 are fits to the CT absorption and excited CT emission spectra of the TCA/durene system using the reorganization parameters used previously to fit the DCA/pentamethylbenzene exciplex spectrum and the TCA (k-JEP datan3The durene system was chosen for spectral fitting since this is the donor for which both the CT absorption can be clearly distinguished from the locally excited absorption and for which an emission maximum could be ~btained.'~ The fits are calculated using the procedure described in ref juZz It is clear that both the CT absorption and emission spectra are entirely consistent with the reorganization parameters used to calculate the fits. This important result supports our previous interpretation of the (&-et)cp data in terms of a strong dependence on exothermicity ((AG-Jc )3 compared to a weak dependence, as suggested elsewhere.I3 & of interest is the value of -2.28 eV for (AGJcp, which gives the best fit to the spectra. This is only ca. 60 mV higher than the difference between the electrochemical reduction potential of the acceptor and the oxidation potential of the donor (eA - EoxD),4which further s u p
The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7637 ,
"."
26
24
22
20
18
16
14
12
Wavenumber, kK Figure 4. Normalized excited CT (CRIP) emission spectrum, after subtraction of residual monomer and impurity emissions (noisy solid tine) and CT absorption spectrum (smooth solid line) of the 2,6,9,10-tetracyanoanthracene (TCA)/durene system in acetonitrile at room temperature. The dotted line is the uncomplexed TCA absorption spectrum. The dashed lines are fits to the excited CT emission and the CT absorption spectra calculated as described in footnote 22. The electrontransfer reorganization parameters used in the fits are X, = 0.55 eV, X, = 0.2 eV, Y, = 1400 cm-I, and (AG& = -2.28 eV (cf. ref 3).
ports our previous assumption that (E",- EoXD) gives a good estimate for (AG+Jcp in these system^.^^^ The lifetimes of the emissions were obtained using the timecorrelated single photon counting technique. The excitation wavelength of 415 nm meant that excitation of both the complex AD and also of uncomplexed TCA occurred, although this was minimized by performing the experiments in the presence of high concentrations of donor. Small amounts of impurity emissions were also detected in the time-resolved experiments. Typical experimental results are shown in Figure 5 for TCA with the donors durene, 1,2,3,4-tetramethylbenzene, and mesitylene. The data were analyzed as a sum of three exponentials. A fast growth component was observed due to the diffusive quenching process ('TCA* D), although this component was generally too fast to be accurately resolved using our apparatus. A fast decay due to the excited CT complex was observed in addition to a smaller component of a slow decay due to impurity emissions (Figure 5). In general, the emission decays were analyzed at wavelengths which were not necessarily the excited CT complex emission maxima to minimize the contributions from the impurities. The measured CRIP (excited CT complex) lifetimes, T,.+, for TCA with the different donors are as follows: m-xylene (89 ps), mesitylene (89 ps), p-xylene (85 ps), 1,2,4-trimethylbenzene (84 ps), 1,2,3,4-tetramethylbenzene(107 ps), 1,2,3,5-tetramethylbenzene (95 ps), durene (96 ps), pentamethylbenzene (88 ps), and hexamethylbenzene (54 ps). From repeated measurements under differing conditions the lifetimes were found to be reproducible to ca. f 3 ps. Transient Absorption Dynamics. The dynamics of the solvent separated radical-ion pairs can in principle be obtained from time-resolved transient absorption studies, and indeed, such experiments have recently been described by Asahi et al. for TCA with hexamethylbenzene,pentamethylbenzene, durene, 1,2,3,4tetramethylbenzene, and mesitylene as donors in a ~ e t o n i t r i l e . ~ ~ These workers reported the timeresolved decay of the absorptions due to the TCA radical anions in the radical-ion pairs formed upon excitation of the ground-state CT complexes. As indicated in Scheme I, excitation into the CT absorption bands results in initial formation of the CRIP of the donor/acceptor pairs. Asahi et al. found that the radical-ion absorption decays could be fitted to single exponentials and therefore assumed that only the CRIP were observed and that contributions from the SSRIP were not important.23The TCA/hexamethylbenzene radical-ion pair was observed to decay with a lifetime of 55 ps, which is essentially identical to the CRIP emission lifetime of 54 ps determined in the present work for the same pair. The very close agreement in the lifetimes supports the suggestion that in this case mainly the dynamics of the CRIP are monitored in the transient absorption experiment, as in the emission experiment. The lifetimes
+
Gould and Farid
7638 The Journal of Physical Chemistry, Vol. 96, No. 19, I992
of the CRIP, the SSRIP, and the FRI are assumed to be similar, and the measured transient absorbance, A(t), will therefore be given by eq 2." The time dependence of A(t), which decays
40
in CHJCN
'2 .
4 t )
.,.. .e'-
.:.. . ........ -.:..-.... .-.. . .. ... .. .. .....-.. .. .:>,J . . ...... _. . . . . . . . . . . . . . . . - . . . .... .,-....I
'
..#
...
. I .
~ O C R I+P~ O S S R I+P ~ O F R I
(2) according to the mechanism given in eq 1, is accordingly given by eq 3." ET
+ B exp(-ht) + YI
= 4 O ) b exp(-&,t)
(3)
where
A..:'.
a=--(k-et)CP
kaolv(k-et)s.9
&,(&, - &SI
Acp
8=
kaolv(k-ct)s.9
- &s)
A,(Xcp
y=-
kWl"k, X,&S
A,, = ksolv + (k-et)cp
A, = k,p + (k-et)s.9 .......... . . . . . . .
Information concerning the various rate constants in eq 3 for many of the donors studied here was obtained previously in experiments in which the yields of freeradical ions (A* D ' ) were measured from the CRIP, and the SSRIP, Values for (@ions), and can be obtained if the SSRIP and the CRIP can be formed selectively. The CRIP are formed directly when the ground-state AD complexes are excited. The SSRIP's can be formed directly by excitation of uncomplexed TCA. Diffusive quenching of A* by the alkylbenzene donors can then bypass the CRIP and result in direct SSRIP formation (Scheme I). In this case the yields of free-radical ions may be different for the two excitation methods, and this is indeed observed for TCA with the donors with oxidation potentials lower than mesitylenes3 As discussed in detail elsewhere,' these two free-ion yields are related to each other and to the rate constants of eq 3 by eqs 4-6. Thus, the ratio of the rate constants (k& and k,, is obtained from as indicated in eq 4. The yield of formation of SSRIP from the CRIP (&) is obtained using eq 5. Thus, together with the lifetime of the CRIP, T~~~ absolute values are obtained for (k-& and kmh.using eqs 6 and 7.3 With mesitylene (and other
+
I
\
1
In CHJCN
. . . . . . . .
0
, ............ 1
128
256
Channel number Figure 5. Time-resolved emission and instrument response functions for 2,6,9,1O-tetracyanoanthracene/alkylbenzeneexcited CT complexes in acetonitrile at room temperature. The excitation wavelength was 415 nm. The emissions were analyzed through 625- and 695-nm long-pass filters. are The calculated decay function and the weighted residual plot (R,) shown as solid lines. The emission parameters are (a) durene, A , = -34.1,T , = 0.006 ns, A2 = 31.2,i 2= 0.095ns, A , = 0.60, 7 , = 038 ns, x2 = 1.48;(b) 1,2,3,4-tetramethylbenzene,A I = -35.8, i1 = 0.006 ns, A2 = 29.8, i 2= 0.104ns, A, = 0.15,T~ = 1.11 ns, x 2 = 1.41;and (c) mesitylene, A, = -39.2,T~ = 0.008ns, A2 = 35.8,T* = 0.086 ns, A 3 = 0.13,T , = 0.93 ns, x2 = 1.02. The first growth component was found to be irreproducible upon repeated measurements and is not considered to be accurately determined. The second component represents the decay of the excited CT complexes, and the third component represents the decay of minor impurity emissions.
from the absorption and emission experiments are somewhat different for the other donors, however. The differenceis largest for the TCA/mesitylene pair, for which a lifetime of 180 ps was assigned by fitting the transient absorption data to a single exp~nential?~ whereas a lifetime of 89 f 3 ps was obtained for the emission decay of the CRIP. These differences are presumably a consequence of the fact that in the transient absorption experiment the dynamics of both the CRIP and the SSRIP contribute to the observed decay of the TCA-, whereas in the emission experiment only the dynamics of the CRIP are monitored. The part of the Scheme I which is required to explain the transient absorption data is shown in eq 1. The absorption spectra Am-D**
J
-
(k-cdap
AD
k5dv
A*-(S)D*+
1
(k-eJ55
A(S)D
k w
A'-
+
Do+ (1)
(4)
1/7cp
=
ksolv
+ (k-et)cp
(7)
donors of higher oxidation potential), however, the free-radical-ion yields are the same whether A or AD is excited, which indicates either that CRIP intermediatesare in fact formed in the diffusive quenching reaction or that return electron transfer is not an important decay mechanism for the CRIP's in this case. The constant values for the lifetimes of the CRIP's with the high oxidation potential donors (mesitylene and the xylenes) are certainly consistent with inefficient return electron transfer in these CRIP. If return-electron-transfer reactions were important in determining the CRIP lifetimes, then a dependence on donor oxidation potential would be expected since this would influence the reaction free energy, AG,,. The fact that the lifetimes are constant for these CRIP suggests that they are all controlled by the same process, i.e., solvation to form the SSRIP, kaolv.Additionally, from the dependence of (k-& for the TCA/alkylbenzene systems on AG,t, described in ref 3 (which is further supported by the spectral fitting shown in Figure 4). the predicted values for (k-ct)cpfor these high oxidation potential donors are
Dynamics of Radical-Ion Pairs in Acetonitrile
k
d
(k.et)cp k,,lv I 1.11 = 1.O X x1l@ O 1S1 OS-l
(k.&
=
7.8 x 109S-l
kp= 8 . 0 X l @ s . l
0.0
0.1
0.2 a
. h M d
time, ns
Figure 6. Time-resolved absorption data for excitation of the CT complexes of 2,6,9,10-tetracyanoanthracene with alkylbenzene donors in acetonitrile. The solid circles are the experimental data of Asahi et al. (ref 23), and the curves are the calculated decay functions obtained by convolution of the delta response function eq 3, with an instrument response function obtained by correlation of two Gaussians with 25 ps fwhm. The decay traces correspond to (a) durene, (b) 1,2,3,4-tetramcthylbenzene, and (c) mesitylene as donors. The rate constants used in the fits (defined in Scheme I) are given in each figure. The rate constants are obtained from measurements of the lifetimes of the CRIP emissions and the yields of freeradical ions as described in the text. Only one adjustable parameter, krcp,was used in the fitting procedure.
much smaller than the rates of decay of the CRIP’s. For the TCA/mesitylene system, (k,,),, is calculated using the returnelectron-transfer reorganization parameters given in ref 3 and Figure 4. The calculated value is ca. 1 X lo8 s-I, which is very small compared to l/rq (1.12 X 1O1Os-l). Therefore, the lifetime of the CRIP in this case is dominated by the solvation process, i.e., kml, = 1.11 X 1O1Os-l. Shown in Figure 6 are fits to the transient absorption experimental data of Asahi et al. obtained by convolution of the delta response function of eq 3 with an instrument response function obtained by correlation of two Gaussian functions of 25 ps fwhm, representing the excitation and monitoring light pulses. As indicated above, values for ( k J q and kWkand the ratio (k&Jk,, were either determined or calculated from the emission lifetimes and the ion yields and were therefore fixed. The only variable used in the fitting procedure was the absolute value of kscp,the value of (k,,),, being determined from its ratio to kscp. The absorbance decay of the TCA/mesitylene pair, and to a lesser extent those of the TCA/ 1,2,3,4-tetramethylbenzeneand the TCA/durene pairs, is sensitive to the choice of kv since the yield of SSRIP from the CRIP (&J is appreciable in these three c a m s 3 The fits to the transient absorption decays for the TCA/hexamethylbenzene and the TCA/pentamethylbenzene pairs (not shown) are dominated by the CRIP, and therefore no additional information is obtained compared to the emission decays alone. The values of the rate constants which give the best fits to the data are given in Figure 6. An interesting number which comes from the fitting procedure is a value for the rate of separation of the SSRIP to give the FRI, kw,,. For the limited data set that we are able to analyze here, a value of 8 X lo8 s-I gives a good fit to the data for each donor. This is close to the value of 5 X lo8 s-l which we previously assumed for these systems? based upon the results of product analysis s t ~ d i e s ,and ~ ~ the , ~ ~magnetic field effect work of Weller et a1.l For the different alkylbenzenes studied here, k,, appears to be constant, whereas k& was previously found to increase with decreasing substitution of the donor, from ca. 2 X IO9 s-l for hexamethylbenzene and pentamethylbenzene to ca. loios-l for
The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7639 mesitylene and the xylene^.^ An explanation of this effect may lie in the relative energies of the CRIP, SSRIP, and FRI in acetonitrile. Kochi et al. have suggested that the solvent stabilization of alkylbenzene radical cations in a polar solvent may depend upon the number of alkyl substituents, decreasing with increasing substitution.26 The solvation of the radical ions in the contact pair is significantly lower than in the solvent-separated pairs due to both the additional solvent molecules between the ions and the increased separation distance in the latter.3 Thus, the solvation process in which a SSRIP is formed from a CRIP may be more exothermic for the less substituted donors and thus faster. Solvation differences between the SSRIP and the FRI are likely to be small, however, which is consistent with a similar rate of separation for all of the SSRIP. The relative rates of return electron transfer within the SSRIP of DCA and TCA with the alkylbenzene donors had previously been inferred from studies of @ions.3*4 The conclusions concerning the dependence of (k,& on exothermicity obtained from those studies are unchanged by the present results and so are not discussed further here. The only result of using a value for k,, of 8 X lo8 s-I instead of 5 X lo8 s-l as assumed previously4 is to slightly increase our previous estimate for the electronic coupling matrix element for electron transfer within the SSRIP from ca. 11 to ca. 14 cm-’. Experimental Section
2,6,9,10-Tetracyanoanthracenewas available from previous s t ~ d i e s . ~It, was ~ purified by column chromatography on silica gel eluting with methylene chloride and further purified by repeated preparative thin-layer chromatography on silica gel again using methylene chloride as the eluting solvent. The alkylbenzene donors were the same as those used p r e v i ~ u s l y .Acetonitrile ~~~ (Baker HPLC grade) was used as received. A luminescent product is formed from a reaction of excited TCA with the alkylbenzene donors in acetonitrile. This emission, however, is readily suppresstd in the presence of traces of acid. The fluorescence measurements described here were carried out, therefore, in an acetonitrile solution containing ca. 2 X lo4 M trifluoroacetic acid. Steady-state emission spectra were recorded using a Spex Fluorolog 212 spectrometer. Corrected emission spectra were obtained by using correction factors generated using a standard tungsten lamp using the method described by the spectrometer manufacturer. Emission lifetimes were measured using timecorrelated single-photon counting.*’ The excitation source consisted of a Coherent Innova 100-10 argon ion laser which pumped a Coherent Model 710-3D dye laser. The dye laser output was cavity dumped (Coherent Model 7220) at a repetition rate of ca. 1 MHz. The CRIP emissions were isolated using long-pass and interference fdters. The emission was detected using a Hamamatsu Model R1564U-07 MCP-PMT. The output of the PMT was amplified using a Hewlett-Packard 8447F ampWier/8495B/8494B attenuator combination and passed to a Tennelec Model TC-455 constant fraction discriminator. The discriminator was modified by Tennelec by removing the input protection and using an internal delay cable.28 An Ortec Model 457 time-to-amplitudeconvertor (TAC) was used in the ?eversen mode. The stop pulse was provided by an OptoelectronicsModel PD-10 diode which monitored a small portion of the excitation light. The TAC was calibrated by averaging several calibrated delay cables (Ortec 425A). The instrument response function was measured by scattering from a solution of nondairy creamer.*’ Typically, an instrument response function of ca. 60 ps fwhm was obtained. Acknowledgment. We thank R. H. Young (Eastman Kodak Co.) for helpful discussions concerning the fitting of the absorption and emission spectra and also the kinetic analysis, E. F. Hilinski (Florida State University) for advice concerning the data fitting, and F. C. DeSchryver for kindly providing us with the singlephoton-counting lifetime deconvolution software. Registry No. TCA/durene, 130563-78-9; TCA/ 1,2,3,4-tetramethylbenzene, 130563-79-0; TCA/1,2,4-trimethylbenzene,142723-93-1; TCAlp-xylene, 142723-94-2; TCA/mesitylene, 130563-80-3; TCAImxylene, 142723-95-3; DCA/hexamethylbenzene, 17263-12-6; DCA/
J . Phys. Chem. 1992, 96, 7640-7645
7640
(15) Gould, I. R.; Mueller. L. J.; Farid, S. Z . Phys. Chem. (Munich) 1991, 170, 143. (16) Gould, I. R.; Mueller, L. J.; Young, R. H.; Farid, S.; Albrecht, A.
pentamethylbenzene, 58964-22-0; DCA/durene, 58964-23- 1.
References and Notes (1) Weller, A. Z. Phys. Chem. (Munich) 1982, 130, 129. (2) Mataga, N.; Okada, T.; Kanda, Y.; Shioyama, H. Tetrahedron 1986, 42, 6143. (3) (a) Gould, I. R.; Young, R. H.; Moody, R. E.; Farid, S. J . Phys. Chem. 1991, 95, 2068. (b) Gould, I. R.; Farid, S.; Young, R. H. J. Photochem. Photobiol. A: Chem. 1992, 65, 133. (4) Gould, I. R.; Ege, D.; Moser, J. E.; Farid, S. J . Am. Chem. Soc. 1990, 112, 4290. (5) Vauthey, E.; Suppan, P.; Haselbach, E. Helu. Chim. Acta 1988, 71, 93. (6) Ohno, T.; Yoshimura, A.; Mataga, N.; Tazuke, S.; Kawanishi, Y.; Kitamura, N. J . Phys. Chem. 1989, 93, 3546. (7) Kikuchi, K.; Takahashi, Y.; Koike, K.; Wakamatsu, K.; Ikeda, H.; Miyashi, T. 2.Phys. Chem. (Munich) 1990, 167, 27. (8) Lewitzka, F.; Lhhmannsrbben, H.-G. Z . Phys. Chem. (Munich) 1990, 169, 203. (9) Marcus, R. A. Annu. Rev. Phys. Chem. 1964, 15, 155.
C. Unpublished results. (17) Boiani, J.; Goodman, J. L.; Gould, I. R.; Farid, S. Unpublished results. (18) Weller, A. In The Exciplex; Gordon, M., Ware, W. R., Eds.; Academic Press: New York, 1975. (19) Emission maxima could not be determined for the complexes of pentamethylbenzene and hexamethylbenzene with TCA in acetonitrile since they occurred at wavelengths close to or longer than 800 nm, beyond which we are unable to obtain accurately corrected emission spectra using our fluorimeter. (20) (a) Czekalla, J.; Meyer, K.-0. Z . Phys. Chem. (Munich) 1961, 27, 185. (b) Romberg, H. M.; Eimutis, E. C. J. Phys. Chem. 1966, 70, 3494. (c) Short, G. D.; Parker, C. A. Spectrochim. Acta 1967, 23A, 2487. (21) Marcus, R. A. J . Phys. Chem. 1989, 93, 3078. (22) Equation 6 of ref 3a is used for calculating the emission spectrum. The absorption spectrum is calculated in an analogous manner as described in ref 21. (23) Asahi, T.; Mataga, N.; Takahashi, Y.; Miyashi, T. Chem. Phys. Lett.
(IO) Mataga, N.; Asahi, T.; Kanda, Y.; Okada, T.; Kakitani, T. Chem. Phys. 1988, 127, 249. (1 1) Ojima, S.; Miyasaki, H.; Mataga, N. J . Phys. Chem. 1990,94,7534. (12) Goodman, J. L.; Peters, K. S . J . Am. Chem. SOC.1986, 108, 1700. (13) Masnovi, J. M.; Kochi, J. K.; Hilinski, E. F.; Rentzepis, P. M. J . Am. Chem. Soc. 1986,108, 1126. (14) Gould, I. R.; Young, R. H.; Farid, S. In Photochemical Processes in Organized Molecular Systems; Honda, K., Ed.; Elsevier: Amsterdam, 1991.
1990, 171, 309. (24) Mattes, S. L.; Farid, S. J . Chem. Soc., Chem. Commun. 1980, 126. (25) Mattes, S. L.; Farid, S. J . Am. Chem. SOC.1986, 108, 7356. (26) Howell, J. 0.; Gonclaves, J. M.; Amatore, C.; Klasinc, L.; Wightman, R. M.; Kochi, J. K. J. Am. Chem. SOC.1984, 106, 3968. (27) Chang, M. C.; Courtney, S. H.; Cross, A. J.; Gulotty, R. J.; Petrich, J. W.; Fleming, G. R. Anal. Instrum. (N.Y.) 1985, 14, 433. (28) Holtom, G. R. Proc. SPIE 1990, 1204, 2.
Temperature Dependence of the Reaction NO, 4- NO, from 296 to 332 K
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NO 4- NO, 4- O2 in the Range
I. Wangberg,*.+E. Ljungstrom,+B. E. R. 01sson,t and J. Davidssont Departments of Inorganic Chemistry and of Physical Chemistry GU, University of Gdteborg and Chalmers University of Technology, S-412 96 Goteborg, Sweden (Received: January 23, 1992; In Final Form: May 5. 1992)
The temperature dependence of the [NO]/[N02] ratio in N205/N02/N2gas mixtures has been determined in the range 296 K 5 T 5 332 K. The experiments were made at 50 mbar in a static reactor. Tunable diode laser spectroscopy was used to measure [NO] while [NO,] and [N20,] were determined by FTIR spectroscopy. The use of a steady-state assumption for NO in the gas mixtures leads to the expression k l / k 2 = [NO]/[NO,] where k, and k2 are the rate coefficients for the title reaction and the reaction NO + NO3 2N02,respectively. The relation k l / k 2= 3.3 X exp(-1598/T) was found to describe the experimental data. The present result, in conjunction with a recently reported value for k2 [J.Phys. Chem. 1991, 95,43811, yields kl = 5.4 X exp(-1488/T) cm3 molecule-l SKI.The estimated accuracy at the 95% confidence level is f29%. The error in activation energy of kl due to the functional dependence on k2 is expected to be small due to the low activation energy of the latter.
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Introduction The importance of the nitrate radical, NO3, for atmospheric chemistry has been increasingly evident ever since this species began to attract attention some 20 years ago.," The nitrate radical is formed in the atmosphere by the reaction between ozone and nitrogen dioxide, and the radical normally exists in thermal equilibrium with nitrogen dioxide and dinitrogen pentoxide. Due to its high absorption cross section' in parts of the visible spectrum, the nitrate radical is easily photolyzed and may therefore be present in significant concentrations only at night. Unsaturated organic compounds, e.g., alkenes, isoprene, and a variety of terpenes, as well as several reduced sulfur compounds may react at high rates with the nitrate radical during the dark In some cases, the turnover of unsaturated hydrocarbons by nitrate radicals at night is expected to exceed that by hydroxyl radicals during d a ~ t i m e . ~ 'Department of Inorganic Chemistry. :Department of Physical Chemistry GL
Channels which compete with the title reaction for NO3 radicals in the atmosphere are reaction with N O and with unsaturated hydrocarbons.'+* Disappearance of NO3via Nz05hydrolysis on wet aerosols could also be importanLs The natural lifetime may be used to make an estimate of the importance of the various processes. Assuming an NO2 concentration of 1.2 X 10l2molecules (50 ppb) and using k, = 6.3 X 10-16 cm3 molecule-' s-l at 298 K' gives an NO3 lifetime of about 22 min with respect to reaction 1. The rate coefficient for the reaction between N O and NO3 is roughly 5 orders of magnitude larger than k,,and if N O is present in concentrations above 3 X lo7 molecules cm-3 this path will dominate over reaction 1. Values of NO3-alkene rate coefficients of lW5and cm3molecule-l s-' may be taken as typical for a small, slowly reacting alkene and a larger, branched alkene, respectively. A 22-min lifetime with respect to the N03-alkene reaction then corresponds to an alkene concentration of 7.5 X 10" (30 ppb) and 7.5 X 108 (30 ppt), respectively. Higher concentrations would proportionally reduce the lifetime. If the NO2concentration is assumed to be constant, then loss of the NO3
0022-3654/92/2096-7640%03.00/0 0 1992 American Chemical Society
