VISIBLE-LIGHT EFFECTS ON ELECTRONIC ABSORPTION BANDS the U. S. Atomic Energy Commission under Contract No. AT(l1-1)-2086. I n addition equipment support
639
was received from the Air Force Office of Scientific Research under Grant No. AFOSR-70-1852,
Visible-Light Effects on Electronic Absorption Bands of Some Cation Radicals Produced by Photoionization at 77 OK by Katsumi Kimura,* Shunji Katsumata, and Kazutoshi Sawada Physical Chemistry Laboratory, Institute of Applied Electricity, Hokkaido University, Sapporo, Japan (Receiued July 16, 1971) Publication costs assisted by the Institute of Applied Electricity, Hokkaido University
In an EPA rigid glass at 77”K, we have carried out photoionization experiments of some aromatic amines (m-aminophenol, m-anisidine, and m-phenylenediamine)yielding radical cations and trapped electrons located in their vicinity, and have observed appreciable blue shift and sharpening of electronic absorption bands of the cations by visible-light bleaching of the trapped electrons. When the biphenyl anions instead of the trapped electrons are located near the cations, the almost same spectral changes have also been observed. The observed cation-band shifts may be explained quantitatively by a perturbation theory, in which Coulomb potential terms due to the trapped electron are taken into account in the Fock Hamiltonian of the cations, suggesting that the photoejected electrons are trapped at a mean distance of about 30 b from the partner cations. Recently we have studied electronic absorption spectra of free radicals produced by uv irradiation of substituted benzenes in rigid glasses at 77°K. 1-4 When photolyzed in an EPA glass, some meta-disubstituted benzenes have been found to give very broad absorption bands peaking at about 670 nm owing to trapped electrons with an intensity as strong as the visible bands of the ~ a t i 0 n . s . ~It has also been shown that the trapped-electron band disappears completely by irradiation with red light without significantly affecting the intensity of the cation ~ p e c t r a . ~ This experimental fact seems to indicate that most of the trapped electrons react with the solvent molecule by near-ir excitation to yield a secondary negative species, e.g., CzH50-, which does not show any detectable visible absorption bands. According to recent photoionization studiess-’ on some aromatic amines in rigid glasses, it has been indicated that photoejected oelectrons are trapped a t a imean distance of 30 to 70 A apart from the partner cations depending on polarity of rigid solvents used. For instance, the mean distance is reported to be 30 A in a rigid EPA glass.6 Therefore, it seems interesting to determine whether a secondary negative ion species could be located at a similar distance. While reinvestigating the 77°K photolysis of the meta-disubstituted benzenes, we noticed that their cation radicalri showed small blue shifts and considerable band sharpenings of the visible bands when ir-
radiated by the visible light of 550-620 nm. This did not come to our attention in the previous s t ~ d y . ~ I n this note it is indicated that the spectral changes observed here may tentatively be explained in terms of a difference in electrostatic interaction between the cation-trapped electron system and the cation-secondary anion system.
Experimental Section Samples studied here are m-aminophenol, m-anisidine, and m-phenylenediamine which were purified by the same method as p r e v i ~ u s l y . ~Photoirradiation experiments of these compounds were carefully carried out using glass and solution filters in the two following steps. The rigid EPA glasses containing these compounds (about 5 X lom4M ) were first irradiated by uv light of 270-370 nm for 5-6 min in order to produce their cation radicals as well as the trapped electrons. (1) K. Kimura, K. Yoshinaga, and H. Tsubomura, J . Phys. Chem., 71, 4485 (1967).
(2) K. Kimura, H. Yamada, and H. Tsubomura, J . Chem. Phys., 48,
440 (1968).
(3) 5. Arimitsu, K. Kimura, and H. Tsubomura, Bull. Chem. SOC. Jap., 42, 1858 (1969). (4) A. Egawa, K. Kimura, and H. Tsubomura, ibid., 43, 944 (1970). (5) W. M.McClain and A. C. Albrecht, J . Chem. Phys., 44, 1694 (1966). (6) N. Yamamoto, Y. Nakato, and H. Tsubomura, Bull. Chem. SOC. Jup., 40,451 (1967). (7) K. Tsuji and E”. Williams, Trans. Faraday Soc., 65, 1718 (1969). The Journal of Physical Chemistry, Vole 76,No. 6,197.9
640
K. KIMURA, S. KATSUMATA, AND E(. SAWADA Table I: Blue Shifts of the Longest Wavelength Cation Bands by the Visible-Light Irradiation Systems
Au? cm-1
+
m-Aminophenol+ em-Aminophenol+ biphenylm-Anisidine+ em-Anisidine + biphenylm-Phenylenediamine+ em-Phenylenediamine + biphenyl-
+
+
‘A 400
4 2 0 4 4 0 460 480 WAVELENGTH (nm)
500
Figure 1. The longest wavelength cation bands observed before (-) and after (- - - - -) the visible-light irradiation: a, m-aminophenol+; b, m-anisidine +; c, m-phenylmenediaine +.
Then these uv-irradiated samples were illuminated by visible light of 550-620 nm for 2-3 min in order to bleach mainly trapped electrons without significantly affecting the cation band intensities. Absorption spectrum measurements were carried out with a Cary Model 15 spectrophotometer, using a quartz cell of 15-mm path length. The solutions in the quartz cells were degassed by the freeze-pumpthaw technique and then were sealed.
+
+ +
139 139 73
77 N O N O
Similar spectral changes were also observed when the biphenyl anion is located in the vicinity of the catM ) as an elecion. When biphenyl (about 1 X tron scavenger is added into the EPA solutions of the compounds studied here, the absorption bands of not only the cation but also the biphenyl anion appear by the uv irradiation. The successive photobleaching of the biphenyl anion by using the visible light of the 600-nm region gives rise to essentially the same spectral change observed in the case of the trapped electron, again without affecting the cation-band intensities. The resulting cation band shifts are also included in Table I, for comparison. Since as far as the cation remains the opposite charge should exist somewhere, the photobleaching of the trapped electron or of the biphenyl anion seems to transform an electron into, e.g., CzH60-, not showing any detectable visible absorption band. If the resulting negative charge retains in the same position as that of the original one (the trapped electron or the biphenyl anion), no spectral change may take place. The considerable cation-band sharpening@ observed here could indicate a reduction in electrostatic field strength acting on the cation. Therefore, the secondary negative ion should locate in a much longer distance from the cation than the initial one. (Another possibility of an ion-pair formation between the cation and anion might be considered upon the visiblelight irradiation, but this may be ruled out since such an ion-pair formation will shift the cation band to the opposite side.) Let us estimate the average strength of electric field acting on the cation by the charge of an electron trapped in the EPA glass as follows. Using the Lorentz field for the effective field strengthg and the coulombic field for the external field, then the effective field ( F ) is given by ( D 2)e/3Dr2,where D is the dielectric con-
+
Results and Discussion I n Figure 1, the absorption bands of the three cations measured before and after the photobleaching of the trapped electron are compared. Magnitudes of the observed band shifts are shown in Table I. From the previous studies, it may be sure that the electrons ejected from the molecules upon uv irradiation are trapped in the rigid The Journal of Physical Chemiatry, Vol. 76, N o . 6,1978
(8) T o investigate how much the cation-band sharpening takes place, the m-phenylenediamine + band with vibrational structure obtained before and after the visible-light irradiation was analyzed in terms of a superposition of Gaussian functions, From such a spectrum simulation, it was found that the bandwidth (at l / e of the peak height) becomes narrower from 600 to 500 om-1 by 20%. Similar band sharpenings were also observed for the other two cations. (9) H. Labhart in “Advances in Chemical Physics,” I. Prigogine, Ed., Interscience, New York, N. Y . ,1967, p 179; K. Seibold, H. Navangul, and H. Labhart, Chem. Phys. Lett., 3, 275 (1969).
VISIBLE-LIGHT EFFECTSON ELECTRONIC ABSORPTIONBANDS stant and r is the distance between the cation and the trapped electron. With a reasonable value of D = 2 for the 77°K glass, the effective field strength is estimated to be about 1070 kV/cm when at r = 30 8. This may be compared with an external field of 500 kV/cm used by Sauter and Albrecht'O for studying an electronic absorption broadening of azulene in 3-methylpentane glass at 77°K With the model mentioned above for the secondary negative ion, the observed cation-band shifts (Table I) may be interpreted qualitatively by the following simple theoretical treatment. For simplicity, here, the secondary negative ion is assumed to be at a far distance from the cation so that its electric field acting on the cation is negligibly weak compared t o that due to the initial negative charge. Since perturbation terms are givlen by x Z P e 2 / D r P xin the Fock HamiltonP
ian, the excitation energies are approximately given by AE,+
= AeL - Ae, =
c ( c i P 2- c,,2)ZPxe2/Dr,x (1)
641
-'E 2000
V Y
ILL
f 180v)
a w
a! 0'
I
20
I
I
I
I
I
40
60
80
100
120
DISTANCE
(i)
Figure 2. Calculated shifts of the cation bands plotted against distance between the cation and the trapped electron (or the biphenyl anion): a, m-aminophenolf; b, m-phenylenediamine +.
P
Here A el= E,' - E ~ ,and ea' and e$ are ith molecular orbital energies expressed by the use of the perturbed and nonpertlurbed Hamiltonian, respectively, and p and X denote the pth atomic orbital and the trapped electron, respectively. I n this approximation, the transition-energy shifts can be calculated from a relation of
AW
=
xatjAE,+ ij
(2)
in which adj'sare coefficients in the state wave functions. According to our recent theoretical calculation" with the open shell SCF-MO G I method, the cation bands under consideration are assigned to electronic transitions to the second excited states from the ground one. This calculation also indicates that the transitions to the first excited states may occur at 0.782 and 0.724 eV for m-aminophenol+ and m-phenylenediamine+, respectively, with very small oscillator strengths (0.004 and 0.010, respectively). No absorption bands assigned to these first transitions have been observed in the near-infrared region. Taking these spectral assignments into account, the above eq 2 , in which a main contribution comes from AE3-6, yields the cation-band shifts plotted in Figure 2 with D = 2 . The marked differences of curves a and b arise .from mainly the differences of the coefficients in the third and fifth MO's of the two cations.12
It is of interest to indicate that red shifts of 130, and 30 cm-l are estimated at a distance of r = 30 A for m-aminophenol+ and m-phenylenediamine +, respectively, in fairly good agreement with the experimental shifts. The present experimental information seems thus to support that the photoejected electron is located in the vicinity of the partner cation, and by the visible light irradiation it reacts with the solvent molecule giving rise to a secondary anion species which is finally located at far distance from the cation. Changes in the dipole moment between the ground and excited states of the cations ( A p = p, - pg) may roughly be estimated from the band shifts (Au) by the following equation: Ap = (hc/F)Au, with neglect of a polarizability change in the both states. Then, from the band shifts observed here, the dipole moment changes were evaluated to be about 7.8, 4.2, and 0 D for m-aminophenol f, m-anisidine +, and m-phenylenediaminec, respectively. Acknowledgment. We wish to thank Mrs. T. Yama5aki for her help in the spectral measurement and Mr. Y. Achiba for his help in computation. (10) H. Sauter and A. C. Albrecht, Chem. Phys. Lett., 2, 8 (1968). (11) S. Katsumata and K. Kimura, t o be published. (12) Our calculation (ref 11) has shown that caN = -0.38, e30 = -0.29, C ~ N= +0.41, C 5 0 = +0.19 for m-aminophenol+, and C8N = - 0.32, C6N = f0.33 for m-phenylenediamine+.
The Journal of Physical Chemistry, Vol, 76, N o . 6,1972
