J. Phys. Chem. 1993,97, 9408-9412
9408
Contact Ion Pairs Formed from Photolyzed Solutions of N,N,N’,N/-Tetrametbylbenzidinein CCl, Hiroshi Shimamori’ and Yoshitsugu Tatsumi Fukui Institute of Technology, 3-6-1 Gakuen. Fukui 910, Japan Received: May 4, 1993’
The time variation of microwave dielectric absorption has been examined for photolyzed solutions of N,N,N’JV’tetramethylbenzidine (TMB) in C C 4 . The dielectric absorption signals show a rapid growth followed by a second-order decay approaching a flat level higher than the baseline. These are attributed to the formation of contact ion pairs (TMB+Cl-) by reaction of the excited state of TMB with C C 4 and a subsequent dimerization process. The dipole moment of the contact ion pair and the rate constant for its dimerization are determined to be 12.4$-1/2 D and 1.7 X 1094-1M-1 s-l, respectively, where $ is the quantum yield for the ion-pair formation with a value larger than 0.26 (possibly 20.5). Most properties associated with the contact ion pairs well resemble those found in similar ion pairs involving a cation of N,N,N’,N’-tetramethyl-p-phenylenediamine.
1. Introduction
A previous study’ has shown that N,N,N’,N’-tetramethyl-pphenylenediamine (TMPD), possibly in the excited singlet state, reacts with solvent CC14 molecule to form a contact ion pair TMPD+Cl-with near unit quantum yield. Even in benzene solvent a TMPD molecule in the excited triplet state can transfer an electron to a halogen-containing compound AX (AX: CC4, C ~ H S IC, ~ H S ICaH5Br, , and C6HsC1) to form a contact ion pair TMPD+X-, where X is a halogen atom.2 These processes occur one-photonically. Energetically the ionization is made possible mostly by the energy gained by forming a contact ion pair (CIP), and minor help is brought from the excess energy released by dissociative electron attachment to halogen-containing compounds. Very large electron affinities of halogen atoms seem to prevent back electron transfer. Above all, its low ionization potential (5.9-6.6 eV3) should be the main reason for such an ionization even in a nonpolar solvent. Thus it is of great interest whether this kind of ionization can occur in other compounds that have ionization energies higher than TMPD. Known oxidation potentials4 and data from charge-transfer spectra and photoionization5suggest that many aromatic diamines and aniline derivativeshave ionization potentials similar to, or slightly higher than, that for TMPD. The present study intends to examine the possibility of the formation of a stable contact ion pair for photolyzed N,N,N’,N’-tetramethylbenzidine (TMB) in C C 4 solvent. The gas-phase ionization potential of TMB is 6.1-6.8 eV6, which is comparable to that of TMPD, whereas the oxidation potentialofTMBvsSCEis0.43eV (0.53eV higher thanTMPD).4 In any event the ionization of TMB in nonpolar solvent seems to require more energy than that for TMPD. So far, long-lived (tens of microseconds) ion pairs such as TMB+.-(ACN),- or free ions TMB+ + (ACN), have been observed by measurements of transient absorption and photoconductivity in photolyzed TMB in acetonitrile (ACN) solution? Similar ion pairs, but with much shorter lifetimes, have been detected in pyridine solvent.8 In a nonpolar solvent, however, no CIPs with a long lifetime involving TMB+ have been detected. We will show that photolyzed TMB in CC14can produce an ion pair TMB+Cl- with a high quantum yield. The value of the dipole moment of the ion pair has been determined also. 2. Experimental Section
Thedetails of the time-resolved microwavedielectricabsorption technique used in this study were described previously.’ Only a brief description is made here. The sample was irradiated with ‘Abstract published in Advance ACS Abstracts, August 15, 1993.
0022-365419312097-9408$04.00/0
a 308-nm laser pulse from an excimer laser (Lamda Physik, LPX 205) or with a 355-nm pulse from a YAG laser (Quanta-Ray, DCR 2). There was no essential difference in theobserved signals between usages of these two lasers. An X-band microwavecircuit was used. A silica cell containing a sample solution was placed within theresonant cavity (TElol mode, resonant frequency e8.8 MHz). Following the laser pulse, a change in the microwave dielectric absorption is detected by a crystal diode. The signal was amplified and fed to a Tektronix 2430 digital oscilloscope. The signal was mostly taken with one shot of laser pulse, but in some cases averaged over two or several shots using the digital oscilloscope. The response time of the detection system was shorter than 100 ns. TMB (Aldrich Chemical Co.) was purified by recrystallization from ethanol or by vacuum sublimation. Diphenylcyclopropenone (Aldrich) was sublimed. Carbon tetrachloride (Wako Chemicals, Spectrograde) was dehydrated by contact with Molecular-Sieve 3A. Before irradiation all the samples were deaerated by bubbling with Ar gas for more than 20 min. Measurements were carried out at room temperature (298 K).
3. Principles of Measurements 3.1. Time-Resolved Measurements. The amplitude of the microwave dielectric absorption signal due to the production of transient species by pulsed laser irradiation is expressed by9
v = W S l N p 2 ) g(7)
(1) where K is a constant relating to the sensitivity of the cavity resulting from the dielectric constant of the solvent, temperature, the coupling factor and the unloaded Q of the cavity, and the fillingfactorofthecell in thecavity. [SIis themolarconcentration of the transient, A(p2) = p2 - p: ( p is the dipole moment of the transient (ion pairs here); pg is the dipole moment of the solute in the ground state), and AT)= w / ( l ( W T ) ~ ) + 1/07 (0is the microwave angular frequency; T is the dielectric relaxation time for the transient). When photoabsorption produces a transient that possesses dipole moments larger than those of original photoabsorbing species, one can observe increases in the signal amplitude. This is what we observe in the measurement. Although the dipole moment of the ground state TMB (pg)has not been reported, it must be small. TMPD has a dipole moment of 1.0-1.3 D,”J and the simpler molecule p-phenylenediamine, possessing amino groups instead of dimethylamino groups, has a dipolemoment of 1.5 D.*O In analogy to this, the dipolemoment of TMB may be taken as slightly smaller than that of benzidine ( p = 1.65 Dlo). Thus we estimate the value pg for TMB to be about 1.5 D or less. In the dielectric absorption measurements
+
0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 37, 1993 9409
Contact Ion Pairs the contribution of this value can be neglected in comparison with that of the ion pair that, as will be shown below, possesses a dipole moment larger than 14 D, since the dielectric loss is associated only with the square of the dipole moment. As a result A(p2) can be replaced by p 2 in eq 1. 3.2. Determinationof the Dipole Moment. The dipole moment of a transient species can be determined from analyses of the amplitude of the microwave dielectric absorption signal. The intensity of the observed signal, V,, can be related to the dipole moment p of the ion pair by eq 1. It is difficult, though, to determine the value of p directly from eq 1, since the absolute value of the unknown factor K is hard to estimate. To overcome this difficulty, we take a ratio of the amplitude of signal for the transient to that for a referencecompound, by which the common factor K can be canceled out. The ratio of the initial amplitude of the signal for the CIP to that of the reference compound is therefore expressed by9
where the subscript r corresponds to the reference compound. The values of the Ps are determined by a method described previo~sly.~ In finding thevalues of g ( q ) andg(rr), it is necessary to carry out the so-called "static" measurements that give ratio g ( ~ l ) / g ( ~ ~Since, ). however, the dipole moment of the TMB molecule in the ground state is small, it is not easy to do that measurement, and it is necessary to employ some model compound that possesses a molecular size similar to that of the transient species. In this study we chose 4-methoxy-4'-nitrobiphenyl (MNBP) as the model compound. It has a molecular structure similar to that of TMB, and its dipole moment in the ground state is known (4.98 DlO). The relative magnitudes of the ~ ( T ) ' sfor the model compound and the reference compound (DPCP in the present case) can be found from measurement of the microwave power, PI, reflected from the cavity against the concentration, [SI,of these solutes based on the following r e l a t i ~ n ; ~
where POis the microwave power incident on the cavity and C = l/2AQ07jg(r)p2. Plots based on eq 3 for MNBP and DPCP as the model (denoted by the subscript m) and the reference compounds, respectively, in CC14 are shown in Figure 3. The slope of each straight line gives the value of C. Since the ratio Cm/G is equal to (g(Tm)/g(Tr))(C1m2/CL,Z), e~ 2 reduces to
1
'
1
(
1
'
1
where a = g(q)/g(Tm), If we make measurements under the same irradiation conditions of laser intensity, absorbance of the solution, and sample cell for a pair of samples of TMB and DPCP, the ratio [ S ] o / [ S ] ,can be replaced by the ratio of the quantum yields, @/& Since = 1 and A(p?)/p? = -1 for DPCP (this molecule decomposes into diphenylacethylene ( p = 0 D) and carbon monoxide ( p = 0.1 D))? we finally obtain
(5) Observed amplitudes of the signals and eq 5 give a value of p for the transient. 4. Results and Discussion
4.1. Ion-Pair Formation for TMB inCC4. As discussed above, the dipole moment of TMB in the ground state should be small. Similarly we expect, from consideration of symmetry in the
1
/
1
1
1
1
'
1
'
1
1
1
1
\
> E 0
7
v
a, -0 3
.-
+
Q
E a
4
Time (5ps/div) Figure 1. Time dependence of apparatus output (proportional to the dielectric absorption) for solutions of TMB a
3 -
\
n
0
w
> a
. 2 -
0 ’ 0
Time
I 15
TABLE I: Parameters Relevant to the Determination of the Rate Constant of the Ion-Pair Association Reaction, and the Values of the Rate Constant (k,) Plo‘ (MI slopeb (s-1) k, (M-1 s-1) 3.73 x 10-54c 1.25 X 10’ 1.68 x 1094-1 3.99 x 10-L’@ 1.36 X 10’ 1.70 x 10944 5.19 x 1 0 - 5 v 1.74 X 10’ 1.68 x 10944 5.56 X 1 P N 1.90 x 10’ 1.71 X lO94-I The initial concentration of the ion pair, determined by a method described in the text. 4 is the quantum yield for the ion-pair formation. The slope of the straight line of the plot based on eq 10. 0.055 mM TMB, 2.0-mm cell, OD = 0.362, laser intensity 2.3 mJ/p. d0.055 mM TMPD, 3.0 mm cell, OD = 0.543, laser intensity 2.3 mJ/p. 0.055 mM TMPD, 2.0 mm cell, OD = 0.362, laser intensity 3.2 mJ/p. 10.055 mM TMPD, 3.0 mm cell, OD = 0.543, laser intensity 3.2 mJ/p.
(pus)
Figure 2. Plots of Av(o)/Av(r) as a function of time for decay curves shown in Figure 1 for different laser intensities. Av(t)corresponds to
the signal amplitude at timet measured relative to the flat level at longer times (-0.31V(O)).
.-I
c4-
\
n
1.5
0
occur from the excited singlet state of TMB. In accord with this, a measurement for an air-saturated sample gave a signal with an amplitude similar to that observed for samples without oxygen. To check the occurrence of reaction 8, we examined whether the decay portion of the signal is of second order. When both the decay of ion pairs and the formation of their dimers occur simultaneously, we have a simple relation as follows:’
AV(O)/AV(t)= 1 + 2k,[S],t
(10) where AV(t) is the difference between the total signal height and the amplitude of the first flat level of the signal, [SI0 (=[TMB+Cl-10) is the initial concentration of TMB+Cl-, and k, is the rate constant of reaction 8. Shown in Figure 2 are plots of AV(O)/AV(t)as a function of time t for two sets of data shown in Figure 1. The linear relationship appears to be good for both plots. The slope of each line corresponds to 2kr[TMB+C1-]o. The value of k, can be estimated by knowing the initial concentration [TMB+Cl-10. This concentration may be replaced by [TMB*]o4, where [TMB*]o is the initial concentration of the excited TMB and 4 is the quantum yield of the CIP formation. We have assumed that the number of TMB* molecules is equal to the number of photons absorbed by thesolution. The absolutenumber of photons absorbed by each solution can be determined by the number of decomposed DPCP molecules (determined by decrease in the absorbance at 345 nm) for the same laser intensity as that used in each measurement for a TMB-CCl4 system. The details of the procedure are described elsewhere.’ Four sets of measurements have been done, with each set comprised of a TMBCCb solution and a DPCP-CC4 solution for different [TMB*]o)s, which were attained by changing O D and laser intensity (2.3 and 3.2mJ). Parameters obtained from thesemeasurements arelisted in Table I. The rate constant k, determined for four different [TMB+Cl-]{s agree well with each other, and we conclude k, = 1.7 X 1094-1 M-1 s-1. The diffusion-limited rate constant for the association reaction for two ion pairs is expected to be higher than the usual diffusion limit in C C 4 (7.3 X 109 M-1 s-1). This has been shown in the case of dimerization of ion pairs TMPD+CI-, where the rate constant is 9.1 X 109 M-I s-1.1 The value obtained for TMB+Cl- appears to be much lower than this diffusion-limited rate if the quantum yield 4 is close to unity. For the experimental value to be compatible to the diffusion-limited rate the value of 4 must be about 0.2. That this value for 4 is too low will be shown in the next section. 4.2. Dipole Moment of the Contact Ion Pair. The dipole moment of the CIP has been determined by a procedure described in the section 3.2. 4-Methoxy-4/-nitrobiphenyl (MNBP) and diphenylcyclopropenone (DPCP) were used for the model and
a L‘
a
W
I ?
Y
1 .o
0
5
10
15
C o n c e n t r a t i o n (mM) Figure 3. Plot of appropriate function of reflected microwave power P, against concentration of 4-methoxy-4’-nitrobiphenyl(MNBP) and diphenylcyclopropenone(DPCP) in CCL solvent.
Time ( 2 p s / d i v ) Figure 4. Comparison between time dependence of apparatus output for
TMB-CCL solution and that for DPCP-CCL solution observed by irradiation of 355-nm laser pulses. The concentrationsare 1.14 mM for TMB and 0.26 mM for DPCP. The sample cell is of 2-mm optical path length, and resulting optical densities are about 1.0.
the reference compounds, respectively. Plots based on eq 3 for MNBP and DPCP in CC4 are shown in Figure 3. The slope of each straight line gives the corresponding value of Cin eq 3. Then it is necessary to observe dielectric absorption signals for a pair of samples of TMB and DPCP under the same irradiation condition of laser intensity, absorbance of the solution, and the sample cell. We have made two sets of measurements using different cells using 355-nm laser pulses. An example of the comparison of both signals is shown in Figure 4. The values of Cs,@-1’s, and measured amplitudes Vs are shown in Table 11. Using the value pm = 4.98 D, we can calculate the value of p from eq 5; that is, 11.8(cu$~)-’/~and 1 3 . 0 ( ~ ~ 4 ) -D, ’ / ~or an averaged value of p = (12.4 f 0.6)(4a)-I/* D. The effective molecular size (with rotational motion) of the ion pair may be close to that of
The Journal of Physical Chemistry, Vol. 97,No. 37, 1993 9411
Contact Ion Pairs
TABLE II: Values of Parameter CObtained from the "Static"Measurements Using Different Sample Cells (A and B) for the Model Compound (MNBP) and the Reference Compound (DPCP), the Amplitudes of Signals for the Ion Pair ( V,) and the Reference Compound (K), and the Dipole Moment (1)of the Ion Pair Determined from Ea 5
cm
cr
v,
cell (M-I)
(M-I)
42.9 35.9
88.0
1
77.2
1
A
B
&Ia
0.98 0.98
(mV) 42.1' 49.9'
vrb
(D)
(mV) -15.W
11.8($(~)-'/2
-15.5'
13.0($~~)-'/~
p
+
a 8-l = (1 2C[S])-I corresponds to the coupling factor for a solution with concentration [SIof polar solute. For a nonpolar or near nonpolar solute = 1. As direction of the signal for DPCP is downward, the amplitude is a negative value. Each value was obtained by averaging four different values. The errors were less than A5%.
the model compound MNBP, because most of the molecular size is determined by the biphenyl group and both the substituted groups C H 3 0 and NO2 are similar to the dimethylamino group in linear length. Thus we can put a = g(q)/g(rm)= rm/71 1 to obtain p = (12.4 f 0.6)(4)-'/' D (11) This value can be compared to p = 11D for the ion pair TMPDTldetermined previously.' To estimate the value of the quantum yield 4, we consider the free energy change for the CIP formation. It can be expressed by
unreasonable. If IP > 6.8 eV and/or E* < 3.1 eV, we obtain a higher value for 4. There is evidence13that the CIPcan be formed in a photolyzed mixture of TMB and CCl4 in benzene solvent where the main path of the formation of CIP is via the excited triplet state of TMB, whose energy is only 2.7 eV.14 Should we regard that the CIP could be formed via the triplet state even in C C 4 solvent, a calculation as made above with E* = 2.7 eV would lead to $ 1 0.50, which is consistent with the quantum yield (0.52) for the intersystem crossing estimated from the transient absorption spectrum in cyclohexane. There is, however, no clear evidence of the CIP formation via the triplet state in CC4. The calculations as made above are, of course, rough estimates, but the results give a good basis to conclude that $ is rather high. This high quantum yield is similar to that for the TMPD+Cl- formation for which the quantum yield is close to unity.' Substituting 0.5 < $ < 1.0 in eq 11, we obtain 18 D > p > 12D. Similarly,using theabovevaluesfor$, therateconstant for the dimerization of ion pairs becomes 1.7 X lo9 M-1 s-1 < k, < 3.4 X 109 M-I s-l. These values are definitely lower than the diffusion limit (about 9 X 109 M-l s-l), but the rate constant will be near this limiting value when $ takes the lowest value (0.26). The presence of a nonzero flat level in the observed signals as shown in Figure 1 implies that the ion-pair dimer has a large dipole moment. It can be shown that the amplitude at the flat level, denoted by V,(m), and the initial amplitude V,(O)have the following re1ationship:l
AG (eV) = IP - E* - (EA - BD) ( p 2 / p 3 ) ( e- 1)/(2e
+ 1) - e2/d
(12) where IP is the ionization potential of TMB, E* is the excitation energy, EA - BD is the difference between the electron affinity of C1 and the dissociation energy of the bond cl-cc13 corresponding to the exothermicity of the electron capture by CC14, p is a radius of the ion pair, e is the dielectric constant of the solvent, and d is the distance between the opposite charges. Note that the energy due to a small entropy change has been neglected in eq 12. It is possible that the CIP is formed via the singlet state. As the fluorescence maxima for TMB is at about 390 nm in cyclohexane,ll the excited singlet state can be about 3.1-3.2 eV above the ground state. The fact that we could observe the CIP formation with a laser pulse of 355-nm wavelength (3.5 eV) supports this. For the ionization potential of TMB we assume 6.8 eV (slightly higher than that of TMPD (6.6 eV3)). Using IP = 6.8 eV, E* = 3.1 eV, EA - BD = 0.58 eV1, p = 12.4$-1/2D, e = 2.24, and p = 7.2 A (a half of the molecular length), we have
AG (eV) = 3.1 - 0.0586-'
- 14.4/d
(13) with d in angstroms. For the ion-pair formation to be exothermic
(AG I O ) d I 14.4/(3.1 - 0.0584-')
(14)
Note that eq 11 is equivalent to
d = 2.584-'/' A (15) Substituting eq 15 into eq 14, we obtain $ 1 0 . 2 6 or $ 50.03. If we put $ = 0.03, the dipole moment obtained from eq 11 is 72 D, which corresponds to 15 A for the separation between two opposite charges in the ion pair. The semiquinoidal structure of the TMB radical cation has been well characterized by resonance Raman spectroscopy,lzthat is, the positive charge is symmetrically distributed in the radical cation and the electrons are delocalized over the two benzene rings and the N-ring bonds. Although the exact position of the C1- ion in the ion pair is not certain, it is reasonable to locate it at a distance close to one of the two benzene rings of the TMB cation. As the atomic radius of C1- ion is 1.8 1 A, the distance between TMB+ and C1- cannot be much. Therefore d 2 15 A suggested from 4 I0.03 is unrealistic. On the contrary, d I 5.0 A corresponding to 4 1 0.26 is not
where y = g(q)/g(rZ) = 72/71. Observed signals show that the ratio V,(m)/V,(O)is 0.31 f 0.4. Then we obtain the following expression for the value of the dipole moment pz for the ion-pair dimer using eq 16: c12 =
(0.31 X 2y)'/'p1D
(17) The value of y depends on the structure (or the molecular size) of the ion-pair dimer. According to the rigid-sphere model, we have the relation 72/r1 = uZ3/al3,where az and al are molecular radii associated with the dielectric relaxation for the dimer and monomer ion pairs, respectively. The exact geometrical structure of the dimer is hard to know, but the ratio az/al may be between 1 and 2. Therefore we can conclude from eq 17 that the dipole moment of the ion-pair dimer is comparable to, or slightly larger than, that of the monomer ion pair. Similar behavior has been suggested for the ion-pair dimers involving the TMPD cations.'
5. Conclusion It has been shown that photolyzed TMB moleculecan produce long-lived contact ion pairs in CCld. The difference in the ionization potential between TMB and TMPD, probably larger than 0.5 eV, does not reflect any significant difference in the CIP formation. Most of the properties associated with the CIP, that is, the Occurrenceof dimerization process, the magnitude of dipole moment of the CIP, and a high value of the quantum yield for theCIPformation, wellresemblethosefoundinthecaseofTMPD. The difference from the TMPD case might lie in that both the quantum yield of the CIP formation and the rate constant of dimerization process for TMB+Cl- are slightly lower than those for TMPD+Cl-, and that the dimers and ion-pair clusters are not stable compared to those for the TMPD case. It is known that many aromatic amines such as N,N-dimethylaniline, N,Ndiethylaniline,diphenylamine,triethylamine, etc., have ionization potentials similar to that of TMB.6 We expect that these amines can also produce long-lived contact ion pairs when photolyzed in CC14. Acknowledgment. The author is very grateful to K. Hanamuro for his technical assistance. This work was partly supported by
9412 The Journal of Physical Chemistry, Vol. 97, No. 37, 1993
a Grant-in-Aid for Special Project Research and a grant for ScientificResearch (C) from The Ministry of Education, Science and Culture.
References and Notes (1) Shimamori, H.;Uegaito, H. J . Phys. Chem. 1991,95,6218. (2) Shimamori, H.; Hanamuro, K.; Tatsumi, Y . J. Phys. Chem. 1993, 97,3545. (3) Briegleb, G.; Czekalla, J. Z . Elektrochem. 1956,63,6.Holroyd, R. A.; Russel, R.L. J . Phys. Chem. 1974,78,2128.Faidas, H.; Christophorou, L. G.; Datskos, P. G.;McCorkle, D. L. J . Chem. Phys. 1989,90,6619. (4) Weinberg, N. L.; Weinberg, H. R. Chem. Rev. 1968, 68,449. (5) Murov, S.L. Handbook of Photochemistry; Marcel Dekker, Inc.: New York, 1973. Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T.J . Phys. Chem. Re$ Data 1977,6,Suppl. 1.
Shimamori and Tatsumi (6) Fulton, A.; Lyons, L. E. Ausr. J . Chem. 1968,21,873. Nakajima, Chem. Jpn. 1969p*" 3030* (7) Hirata, Y.;Takimoto, M.; Mataga, N. Chem. Phys. Lett. 1983,97, 569. (8) Hirata, Y.;Mori, Y . ;Mataga, N. Chem. Phys. Lett. 1990,169,427. (9) Fessenden, R. W.;Carton, P. M.; Shimamori, H.; Scaiano, J. C. J . Phys. Chem. 1982,86,3803. (10) McClellan, A. L. Tables of Experimental Dipole Momenfs;Rabara Enterprises: El Cerrito, CA, 1974;Vol. 11. (11) Hashimoto, S.;Thomas, J. K. J. Phys. Chem. 1984,88,4044. (12) Guichard, V.; Bourkba, A.; Point, 0.;Buntinx, G. J . Phys. Chem. 1989,93,4429. (13) Shimamori, H.; Okuda, T., to be published. (14) Alkaitis, S. A.; Graetzel, M. J. Am. Chem. Soc. 1976,98,3549. A.; Akamatsui H.
