J. Phys. Chem. igao, 84, 57-59
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Effect of an Inefficient Electron Scavenger on Infrared- and Visible-Absorbing Electrons in an Ethanol Matrix Shoji Noda, Kiyomi Yoshida, Masaaki Ogasawara, and Hiroshi Yoshida" Faculty of Engineering, Hokkaido University, Kita-ku, Sapporo 060, Japan (Received June 1 1, 1979)
Publication costs assisted by Hokkaido University
In order to obtain a deeper insight into the initial localization and the subsequent stabilization of electrons and to unravel the detailed mechanism of the electron scavenging reaction in a glassy ethanol matrix, spectrophotometric studies have been made on this matrix with toluene, an inefficient electron scavenger, y irradiated at 4.2 K. Inhomogeneous depletion of the trapped electron spectrum by toluene indicates that IR-absorbing (Arnm = 1500 nm) and visible-absorbing(Am= = 640 nm) electrons are initially generated, but that the former are unstable at higher temperature. Toluene scavenges the IR-absorbingelectrons more efficiently by a factor of 10 than the visible-absorbing electrons. This selectivity is much higher than that of an efficient scavenger, such as benzyl chloride, as previously reported. The electron scavenging reaction results in the formation of the transient radical anion of toluene, which is readily protonated to yield the methylcyclohexadienyl radical at 77 K. generated by the electron scavenging reaction. The 4.2 K Introduction irradiation technique seems favorable for the spectroscopic There still remains controversy about the nature of the detection of such a transient intermediate. trapped electron spectra and on the initial trap population in glassy alcohol matrices. For instance, the ethanol matrix Experimental Section y irradiated at, 4.2 K shows an initial spectrum of trapped Reagent grade ethanol was degassed and sealed in a electrons with a, , A of 1500 nm, which largely shifts to Suprasil quartz absorption cell of 0.2-cm optical path. the blue (AmmL = 540 nm) upon subsequent thermal an2-Methyltetrahydrofuran (MTHF) was purified as denealing at 77 K.lp2 A similar spectral change has been observed in pulse radiolysis studies at low t e m ~ e r a t u r e . ~ - ~ scribed before.1° Reagent grade toluene was used as a solute without further purification. The sample solutions This blue shift has been attributed to either the reorienwere frozen as glasses by plunging them into liquid nitation of matrix molecules around the trapped electrons trogen and then transferring them to a liquid helium or trap-hopping of the electrons from initial shallow traps cryostat described e1sewhere.l' to deeper traps. Recently, comparison of the trapped The samples were irradiated with 6oCoy rays to a dlose electron spectra and their behavior upon thermal annealing of 0.22 Mrd at 4.2 K and subjected to absorption meain several alcohols has suggested both IR-absorbing and surements at 4.2 K with a Shimadzu MPS-5000 spectrovisible-absorbing electrons are initially generated in alcophotometer. Thermal annealing was carried out by holic matrices, and that the large blue shift observed in transferring the irradiated samples into liquid nitrogen, the ethanol matrix is primarily due to the selective decay keeping them there for 20 min, and then transferring them of the IR-absorbing electrons.6 back into the cryostat, all in complete darkness. PhotoThe distinct nature of two kinds of trapped electrons bleaching was carried out with light from a tungsten lamp has previously been suggested to result from the effect of through filters. Background absorption was corrected by electron scavengers. Higashimura et aL7v8have reported recording the absorption before irradiation and by subthat the depkkion of the trapped electron spectra at 4.2 tracting it from the absorption spectra after irradiation. K is greater at 1500 nm than at 540 nm, when an increasing concentration of benzyl chloride (an efficient electron Results and Discussion scavenger) is added to the ethanol matrix. Such an inhomogeneous depletion of the spectra is more prominent Formation of the Toluene Anion at 4.2 K. The effect for an inefficient electron scavenger, as communicated of added toluene on the initial absorption spectra of the recently by The addition of toluene eliminates the ethanol matrix irradiated at 4.2 K is shown in Figure 1. IR-absorbing electrons while leaving the visible-absorbing In the absence of toluene, the absorption band of the ones. trapped electrons has a maximum at 1500 nm and a long The present investigation extends the previous study on absorption tail toward the blue, as already reported by the electron scavenging effect of tolulene in the ethanol several groups.'v2p6 The addition of toluene efficiently matrix irradiated at 4.2 K and helps to elucidate the nature depletes the IR part of the trapped electron band, and of the trapped electrons. The inefficient electron scavenger causes the buildup of a new absorption band at about 300 is expected to differentiate the depth of the electron traps; nm. The visible part of the trapped electron band is rather shallowly trapped electrons are preferentially scavenged, insensitive to the addition of toluene, so that the trapped if the scavenging reaction occurs via electron tunneling. electrons persisting at high toluene concentrations exhibit Another aim of the present investigation is the elucia band shape similar to that of the ordinary trapped dation of the elementary steps of the reaction between the electrons generated at 77 K, except for a slight red shift electron and toluene in the matrix. Shida and Hamillg (Amm of 640 nm is slightly but clearly longer than 540 nm have observed the formation of the methylcyclohexadienyl at 77 K). radical from toluene in the ethanol matrix at 77 K, and The nature of the new band at 300 nm is better illushave attributed its formation to the rapid proton transfer trated in Figure 2 for a high enough concentration of of the toluene radical anion, the primary intermediate toluene. The band (Figure 2A) appears to be the same as 0022-3654/80/2084-0057$0 1.OO/O
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The Journal of Physical Chemistry, Vol. 84, No. 1, 1980 Xlnm 500 400
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Figure 1. Optical absorption spectra of y-irradiated ethanol matrices
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Figure 2. (A) Optical absorption spectra of a y-irradiated 0.28 mol
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the spectrum recorded from the MTHF matrix containing 2 mol % of toluene irradiated at 77 K and photobleached with light of >680 nm (Figure 2B). The spectrum observed from the MTHF matrix is entirely due to toluene radical anionB generated by the electron scavenging reaction; photobleaching completely removes the trapped electron band from the spectrum. The shape of the band of the toluene anion (a peak at about 300 nm and a weak, broad absorption in the visible region) is very similar to that of benzene anion reported by Shida and Iwata.12 It is evident that in the ethanol matrix at 4.2 K toluene scavenges electrons and is transformed into the toluene radical anion, which is stably trapped at this temperature. It should be noted that in the presence of toluene the trapped electron bands (Figure 1) are underlaid with the absorption due to the toluene anions in the wavelength region 400-1000 nm. On annealing the matrix thermally at 77 K, the absorption band of the toluene anions disappears and an absorption band with resolved vibrational structure appears at about 320 nm (Figure 2A). This band has the most intense peak at 324 nm and is attributed to methylcyclohexadienyl radicals? Simultaneously,the trapped electron band is blue shifted without loss of total intensity. The toluene anion is the precursor of the methylcyclohexadienyl radical; thermal annealing promotes proton transfer from the ethanol matrix to the toluene anion. The detection of transient radical anions at 4.2 K has been reported by Namiki in the electron scavenging reaction of halobenzenes in an ethanol matrix. There, the molecular anions decompose to give phenyl radicals upon
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Figure 3. Dependence of the yields of trapped electrons monitored lo3 cm-’) and (0) at 640 nm (15.6X cm-‘) before thermal annealing, (A)at 540 nm (18.5X cm-’) after thermal annealing at 77 K, (0)the yield of methylcyclohexadienyl radical (ab-
(0) at 1500 nm (6.7X
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sorbance difference between 324 and 329 nm), on the concentration of toluene in the ethanol matrix irradiated at 4.2 K.
thermal annealing at 77 K.13 The electron scavenging of styrene has been found to form transient styrene radical anions to which a proton transfers from a matrix alcohol molecule.1° The proton transfer in rigid matrices (and also in liquid solution) is sterically controlled, and the preexponential factor of its rate constant depends much on the shape of the molecular anions inv01ved.l~ Efficiency of Electron Scavenging by Toluene at 4.2 K. The effect of toluene indicates the inhomogeneity of the trapped electron band. It is essentially the superposition of two components: one is due to IR-absorbing electrons in shallow traps giving a A, of 1500 nm (curve A in Figure 1)and another due to visible-absorbing electrons in deep of 640 nm (curve D in Figure 1). The traps giving a ,A, generation of two kinds of trapped electrons distinctly different from each other has been indicated at 4.2 K, based mainly on the comparison of the trapped electron band in several alcohols.e The decrease in the trapped electron yields is shown as a function of the toluene concentration in Figure 3. The yields of the IR-absorbing and the visible-absorbing electrons have been monitored at 1500 and 640 nm, respectively. The absorption intensity at 640 nm has been corrected for the contribution due to the toluene anion (see Figure 2B). The IR-absorbing electrons are efficiently scavenged and disappear completely at a toluene concentration of about 0.4 mol %. The yield of the visible-absorbing electrons appears to decrease first rapidly and then slowly. However, the initial rapid decrease can be attributed to the efficient scavenging of IR-absorbing electrons, whose absorption tail extends well into the visible region and contributes to the absorption intensity at 640 and at 540 nm as well. Therefore, the efficiency of the scavenging of the visible-absorbing electrons can be known from the slope of the scavenging curve (solid circles in Figure 3) at high toluene concentrations (>0.2 mol %). It is estimated from the reciprocal of the toluene concentration required to reduce the trapped electron yields to 50% to be only one-tenth of the scavenging efficiency for the IR-absorbing electrons. The yield of the methylcyclohexadienylradical has also been studied as a function of the toluene concentration. Since direct determination of the absorption intensity around 300 nm is difficult because of overlapping absorption due to free radicals from ethanol in this wavelength region,15 the absorption due to the methylcyclohexadienyl radical has been estimated from the observed difference in absorption intensity between 329 and 324 nm (the two most intense peaks). The buildup of the methylcyclohexadienyl radical yield is counterbalanced by a
Electron Scavenging in an Ethanol Matrix
decrease in the trapped electron yield at 4.2 K. This gives further support for stepwise formation of the methylcyclohexadienyl radical via electron capture by toluene followed by proton transfer. The tenfold difference in scavenging efficiency between the two kinds of trapped electrons strongly suggests that the scavenging reaction occurs by electron tunneling from a trap to toluene. Toluene provides a shallow acceptor level because of its low electron affinity (negative in gas phase1'?; it has no appreciable chance of level matching with the deep electron trap giving the visible absorption, whereas level matching with the shallow trap facilitates the tunneling of the IR-absorbing electron. Higashimura et a l 7 s 8 have found a difference in the scavenging efficiency only of a factor of 3 between the IR and visibre parts of the trapped electron band in ethanol matrix irradiated at 4.2 K, biy using benzyl chloride, an efficient electron scavenger. This small difference is attributed to a deep acceptor level of the efficient acceptor which results in an appreciable level matching with both the IR- and visible-absorbing electrons, although the small difference does not afford the possibility to clearly distinguish these two types oftrapped electrons. Toluene is less efficient than benzyl chloride by only a factor of 3 in scavenging IR-absorbing electrons, while it is less efficient by a factor of 30 in scavenging the visible-absorbing electrons. Relaxation of Electron Traps by Thermal Annealing. Figure 3 also shows the yield of trapped electrons after thermal annealing at 77 K. The yield appears to decrease first rapidly and then slowly in a manner similar to that for the visible-absorbing electrons. This rapid decrease of the yield of the trapped electrons relaxed by thermal annealing can also be attributed to efficient scavenging of IR-absorbing electrons. Therefore, IR-absorbing electrons partly contribute to the enhancement of the visible absorption observed after thermal annealing. In pure ethanol, about 30% of the relaxed, trapped electrons are estimated to originate from IR-absorbing electrons. The absorption maximum of the trapped electron band at 540 nm is more intense than the absorption at 640 nm (Arn, for the visible-absorbing electrons) before thermal annealing, regardless of the concentration of added toluene. This indicates that the enhancement of the visible absorption upon thermal annealing can occur even in the absence of IR-absorbing electrons. In alcohol matrices, both IR- and visible-absorbing electrons are stabilized through the rearrangement of the matrix molecules around the trap by thermal annealing and the shifts of the absorption maxima of both IR- and visible-absorbing electrons are at most -3 x lo3 cm-1.6
The Journal of Physical Chemistty, Vol. 84, No. 1, 1980 59
Therefore, the conversion of a part of the IR-absorbing electrons to the trapped electrons relaxed by thermal annealing will not be attributed to the rearrangement of the matrix molecules but mainly to redistribution to deeper traps. In conclusion, the scavenging effect provides evidence for the initial formation of unrelaxed IR-absorbing electrons and the unrelaxed visible-absorbing ones in eth,anol matrix when irradiated at 4.2 K. The relaxation of the traps occurs at high temperatures for the visible-absorbing electrons, and causes a blue shift of A,, from 640 to 540 nm and the enhancement of the absorption maximum intensity. The IR-absorbing electrons are trapped so shallowly that they are readily released from the traps and partly converted to relaxed trapped electrons by thermal annealing at 77 K. In the presence of toluene, the electron scavenging reaction occurs preferentially for IR-absorbing electrons. The formation of methylcyclohexadienyl radical from toluene proceeds stepwise. The simple electron attachment to toluene generating the toluene anion is followed by proton transfer, which is inhibited at 4.2 K; by matrix rigidity. Acknowledgment. The authors thank Dr.B. G. Ershow for a discussion which was invaluable for the initiation of the present investigation.
References and Notes (1) H. Hase, M. Noda, and T. Higashimura, J . Chem. Phys., 54, 2975 (197 1). (2) H. Hase, T. Warashina, M. Noda, A. Namlkl, and T. Higashimura, J. Chem. Phvs., 57. 1039 (1972). (3) N. V. Klassen, H. A. Gillis; 0. G. Teather, and L. Kevan, J . Chem. Phys., 62, 2474 (1975). (4) J. H. Baxendale and P. H. 0. Sharpe, Chem. fhys. Lett., 39, 401 (1976). (5) J. R. Mlller, B. E. Cllfft, J. J. Hlnes, R. F. Runowski, and K. W. Johnson, J. Phys. Chem., 80, 457 (1976). (6) M. Ogasawara, K. Shimlzu, K. Yoshlda, J. Kroh, and H. Yoshida, Chem. Phys. Lett., 64, 43 (1979). (7) T. Higashimura, A. Namiki, M. Noda, and H. Hase, J . Phys. Chom., 76, 3744 (1972). (8) T. Higashimura, Int. J. Radlat. Phys. Chem., 6, 393 (1974). (9) T. Shida and W. H. Hamili, J . Am. Chem. Soc., 88, 3669 (1966). (10) S.Noda, Y. Fujii, and H. Yoshlda, Bull. Chem. SOC.Jpn., 50, 2226 (1977). (1 1) J. Kroh, S.Ncda, K. Yoshlda, and H. Yoshlda, Bull. Chem. Soc. Jpn., 51, 1961 (1978). (12) T. Shida and S. Iwata, J. Am. Chem. Soc., 95, 3473 (1873). (13) A. Namlkl, J . Chem. Phys., 62, 990 (1975). (14) L. M. Dorfman. Acc. Chem. Res.. 3. 224 (1970). ( i 5 j A. Habersbergerov6,L. ~osimovi~, and 2. Tep$ TM&. Faraday W., 66. 669 (1970). (16) F. Gutman andL. E. Lyons, "Organic Semiconductors",Wiley, New York, 1967, pp 698-705. There Is no electron afflntty value available for toluene itself, but the electron afflnlty for benzene Is confirmed to be negative in the gas phase.
