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COMMUNICATIONS TO THE EDITOR Selectlve Capture of Mlgratlng Holes by Pyrene and Naphthalene In y-Irradlated Butyl Chloride Glasses at 77 K Pubiicatlon costs assisted by The Institute of Physlcal and Chemical Research
Sir: y-Irradiation of solutions in alkyl halide glasses produces cation radicals of solute molecules,l which will be referred to as cations. The cation formation is explained in terms of capture of migrating holes by solute molec u l e ~ Spectra . ~ ~ ~ which may be associated with stabilized holes have been measured and the kinetics of hole scavenging has also been studied;'-4 however, the nature of - yet. j YRecently - it + the migrating hole has not been clarified has been suggested that vibrational excited states of solute-molecule cations may be the migrating hole.4 In this communication we present the results for hole scavenging by pyrene and naphthalene in a sec-butyl chloride glass at 77 K. The interesting finding is that each of these solutes captures a different state of the hole. Glasses of sec-butyl chloride containing both naphthalene and pyrene were y irradiated to a dose of 2 X eV kg-' at 77 K. The yield of the pyrene cation was measured as D,, the optical density for the 791-nm band of the pyrene ~ a t i o n The . ~ yield of the naphthalene cation was measured as D,, the optical density for its 703-nm band;6 D, was corrected for the superposing pyrene-cation absorption. The dependence of both D, and D, on solute concentration is shown by the Stern-Volmer plots in Figures 1 and 2 . The naphthalene-cation yield depends on the naphthalene concentration as shown in Figure 1A and on the pyrene concentration as shown in Figure 1B. The pyrene-cation yield is dependent on pyrene concentration but independent of naphthalene concentration as shown in Figure 2. The scheme that both solutes competitively trap the single state of the hole can never satisfy the four plots simultaneously. Instead, the following scheme can account for the results:
P'
other products
N+
other products
Pyrene first reacts with hl+,the hole in a certain state, and forms the pyrene cation P+. The hl+ is assumed to be unreactive with naphthalene. A portion of the holes that survive the capture by pyrene changes to h2+,another state of the hole, which selectively reacts with naphthalene to form the naphthalene cation. Using the notation of rate constants given in the above diagram, we can write the equations yh _ -1+--k d + k t
y*
k , [PI
.
.
O
O
10
[ P I / ( mrnd kg-')
[Nl-'/(rnol-'kg)
Figure 1. The dependence of the naphthalene-cation yield on naphthalene concentration (A) and on pyrene concentration (B).
4
tB
A
--
OO
100
t PI-# mol-'kg )
200 0
20
40
60
I N ] / ( rnrnol kg-')
Flgure 2. The dependence of the pyrene-cation yield on pyrene concentration (A) and on naphthalene concentration (B).
where Yh represents the hlf yield; Y,, the pyrene-cation yield; and Y,, the naphthalene-cation yield. The ratio Yh/Y in eq 1can be replaced by Dp"/D,, where Dp" is the D, vahe extrapolated on the ordinate. Consequently, the plot in Figure 2A fits eq 1,and (hd + k , ) / k , is evaluated as 2.4 X lo-' mol kg-l. The fact that the pyrene-cation yield is independent of naphthalene concentration (Figure 2B) is explained in terms of eq 1. Equation 2 can reproduce the lines of Figure 1, when it is written using the optical density ratio Dn0/D,. The plot in Figure 1A gives a value of 2.1 X 1W2 mol kg-' for k f / k , . Hamill and co-workers have studied capture of the hole in glassy solutions of 3-methylpentane containing two solute^.^,^ Most of their results are not enough to judge whether the holes captured by the two solutes are of the same state or not, since the effects of two solutes on each of the two cations were not examined. Furthermore, the processes involved in the systems they dealt with may be complicated. According to current knowledge, 2methylpentene-1 and toluene may form dimer cations in The Journal of Physical Chemistry, Vol 8 1, No. 6, 1977
Communications to the Editor
592
-
7t
3 1l
g -10
~
-11
BuCI*
P
N
Figure 3. Energy diagram of the highest occupied orbitals of butyl chloride cations, pyrene, and naphthalene.
a 3-methylpentane matrix without thermal annealing.6,7 Their results for the solute pair of toluene and carbon tetrachloride3 suggest that each of these solutes reacts selectively with a different precursor. However, these different precursors do not immediately mean the different states of the hole, since the resultant transient originating in carbon tetrachloride is not identified definitely with CC1,+. Therefore, we will not attempt to interpret their results and ours together but will discuss only ours. Figure 3 shows an energy diagram based on gas-phase ionization potentials. The above proposed scheme is incompatible with the model that the migrating hole is the ground state of the solvent cation which is depicted in Figure 3. The selective capture of the hole by the two
The Journal of Physical Chemistv, Vol. 81, No. 6, 1977
solutes suggests that an electron transfers resonantly from a solute molecule to the hole. Therefore, a simple model compatible with the scheme is as follows. Excited states of either solvent cations or solvent aggregate cations hop in a glass, losing the excitation energy. These are migrating holes. The resonant charge transfer occurs when the energy of the migrating excited state becomes as high as the energy levels of the highest occupied orbitals of the solute molecules, as indicated by dotted lines in Figure 3. This model indicates that pyrene whose ionization potential is lower than that of naphthalene captures the hole before naphthalene does. Experiments on various solute pairs are in progress.
References and Notes (1) W. H. Hamill, “Radical Ions”, E. T. Kaiser and L. Kevan, Ed., Interscience, New York, N.Y., 1968. (2) J. B. Gallivan and W. H. Hamill, J. Chem. Phys., 44, 2378 (1966). (3) D. W. Skelly and W. H. Hamill, J. Phys. Chem., 70, 1630 (1966). (4) S. Arai, A. Kira, and M. Imamura, J. Phys. Chem, 80, 1968 (1976). (5) T. Shida and S. Iwata, J. Am. Chem Soc., 95, 3473 (1973). (6) B. Badger and B. Brocklehurst, Trans. Faraday Soc, 65, 2576,2582 (1969)... (7) R. E. Buhler and W. Funk, J. Phys. Chem, 79, 2098 (1975). The Institute of Physical and Chemical Research Wako, Saitama 351, Japan Received October 28, 1976
Akira Klra’ Takashi Nakamura Masashl Imamura
