5004
J. Phys. Chem. 1984,88, 5004-5008
mean ordering in the smectic phase of S2 and also to an apparent loss of coupling to the postulated cooperative chain-distortion mode. We summarize in Table IV the different ways that the different probes appear to behave in various smectic phases.
V. Conclusions We are able to describe the motion of CSL and the 1,lCstearic acid segment containing the nitroxide moiety in the smectic A phase of S2 mainly in terms of reorientation in a mean potential, in contrast to some other probes exhibiting unusual patterns, presumably due to the existence of cooperative chain distortion modes that are slow on the ESR time scale. Such differences are attributed to the different regions of the smectic layer reported on by the various probes, a feature that suggests the unique value of such studies. We have shown that the simulations of the slow-motional and orientation-dependent ESR spectra of CSL in S2 are very sensitive to the detailed form of the mean orienting potential, and this permitted its fairly complete estimation from the slow-motional spectra. The results on 1,14-stearic acid showed features that related to its average chain conformation and to deviations from a model of simple reorientation in a mean potential. This work and recent related studies illustrate the great wealth of both equilibrium and dynamic information which is, in principle, contained in slow-motional ESR spectra in oriented fluids. This warrants further development of theoretical models to compare with experimental results as well as development of newer ex-
perimental techniques to more effectively abstract out the static and dynamic information inherent in these spectra. As but a few examples, we mention in the former context ( 1 ) inclusion of better models of molecular reorientation that include localized cooperativityi6 and (2) inclusion of the effects of internal vs. overall reorientationZ2as well as (3) allowing for the possibility of noncoincidence of principal molecular axes of ordering and of diffusion. In the latter context we mention the use of spin echo methods for accurately determining relaxation even in the presence of considerable inhomogeneous b r ~ a d e n i n g , especially ~ ~ , ~ ~ the development of new two-dimensional electron spin echo methods.24 Finally, efficient nonlinear least-squares methods designed to seek out the optimum ordering and dynamic parameters (six in the present CSL study and three in the 1,14 stearic acid study) need to be developed.26
Acknowledgment. This work was supported by NSF Solid State Grant No. 81-02047, by NIH Grant No. GM-25862, and by the Charles Revson Foundation. Registry No. CSL, 18353-76-9;S2, 83488-72-6; 1,14-stearic acid, 61443-66-1. (22) Campbell, R. F.; Meirovitch, E.; Freed, J. H. J. Phys. Chem. 1979, 83, 525. (23) Stillman, A. E.; Schwartz, L. J.; Freed, J. H. J . Chem. Phys. 1980, 73, 3502. Schwartz, L. J.; Stillman, A. E.; Freed, J. H. J . Chem. Phys. 1982, 77, 5410. (24) Millhauser, G. L.; Freed, J. H. J . Chem. Phys. 1984, 81, 37. (25) Broido, M. S.; Meirovitch, E. J . Phys. Chem. 1982, 86, 4197. (26) Moro, G.; Nayeem, A.; Freed, J. H., unpublished work.
Localized Electrons in Crystalline 1,(i-Hexanediol and 1,8-0ctanediol Studied by Absorption Spectroscopy at 4.2 and 77 K Masaaki Ogasawara,* Ola Claesson, Hiroshi Yoshida, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan
and Anders Lund The Studsvik Science Research Laboratory, S-611 82 Nykoping, Sweden (Received: February 23, 1984)
In a steady-state study of the optical absorption spectrum of localized electrons in y-irradiated crystalline 1,6-hexanediol and l&octanediol, infrared absorption bands with ,,A at ca. 1300 and ca. 1000 nm, respectively, have been found at 4.2 K, in addition to the visible absorption band of the localized electron which is comprised of two or three partially resolved peaks. The effects of temperature, photobleaching, and sample preparation method have been investigated. The results altogether indicate that electrons are localized at two distinctly different trapping sites, shallow and deep.
Introduction Since the pioneering work by Bardsley et aL,l several organic crystalline compounds in which localized electrons are produced by ionizing radiation have been known. By use of ESR and ENDOR spectroscopy, irradiated single crystals of some carbohydrates have been studied extensively and the geometrical structures of the trapping sites for electrons in several sugars have become ~ l e a r . ~Yet . ~ there are only a limited number of reports (1) Bardsley, J.; Baugh, P. J.; Phillips, G. 0.J . Chem. SOC.,Chem. Commun. 1972, 24, 1335. (2) (a) Box, H. C.; Budzinski, E. E.; Freund, H. G. J . Chem. Phys. 1978, 69, 1309. (b) Box, H. C.; Budzinski, E. E.; Freund, H. G.; Potter, W. R. J. Chem. Phys. 1979, 70, 1320. (c) Budzinski, E. E.; Potter, W. R.; Potienko, G.; Box, H. C. J . Chem. Phys. 1979, 70, 5040. (d) Budzinksi, E.; Potter, W. R.; Box, H. C. J . Chem. Phys. 1980, 72, 972.
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on the optical absorption spectrum of the localized electrons in organic crystalline mat rice^^-^ and details of the energy levels and potential depth of the electrons remain unknown. Since the trapping sites for electrons in crystalline matrices must be better defined than those in amorphous matrices, a sharper and more complicated absorption spectrum had been expected. Actually observed spectra so far were, however, characterized by a very (3) (a) Samskog, P.-0.; Kispert, L. D.; Lund, A. J . Chem. Phys. 1983, 78, 5790. (b) Samskog, P.-0.; Kispert, L. D.; Lund, A. J. Chem. Phys. 1983,79,
635. (4) Samskog, P.-0.; Lund, A,; Nilsson, G.; Symons, M. C. R. J . Chem. Phys. 1980, 73, 4862. (5) Buxton, G. V.; Salmon, G. A. Chem. Phys. Lett. 1980, 73, 304. (6) Samskog, P.-0.; Lund, A,; Nilsson, G. Chem. Phys. Lett. 1981, 79,447. (7) Ichikawa, T.; Yoshida, H. Radiat. Phys. Chem. 1978, 1 1 , 173.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 5005
Localized Electrons in Crystals A Inm o,6
loo0
700
500
Alnm 300
400
1
loo0
o,6
500
700
300
400
I
4 , '
O1
10
20
30
D I 103cm-1 Figure 1. Absorption spectra observed from crystalline 1,6-hexanediol (grown from the melt) y-irradiated at 4.2 K: A, immediately after the y-irradiation; B, photobleached by 700 nm for 5 min; C, photobleached successively by the same light for an additional 5 min; D, photobleached by 525-nm light for 5 min. broad and structureless band in the visible r e g i ~ n . ~Recently -~ we reported on the absorption spectrum of the localized electron in irradiated crystalline 1,8-octanediol which comprised three resolved peaks with maxima at 740,590, and 490 nm.8 This was the first observation of a resolved structure in the spectrum of the localized electron in organic crystalline matrices. The study is extended here by y-irradiating crystalline 1,6hexanediol and l&octanediol at 4.2 and 77 K to elucidate the following questions: (1) What is the detailed shape of the spectra? (2) How many kinds of localized electrons are there? (3) What is the effect of deuteration on the spectra? (4) Does relaxation of electron traps occur in the rigid crystalline matrices? In the preliminary study we used crystals grown from melted samples exclusively.8 In the present study, crystals grown from solution were also examined. The samples prepared by this method proved to be suitable for the spectral measurement at 4.2 K, since, unlike the crystals grown from the melt, they did not become severely cracked when they were cooled in liquid helium. An additional band was observed in the infrared region by careful exqmination of the absorption spectrum of the irradiated 1,8-octanedial.
Experimental Section Commercially available 1,6-hexanediol and 1&octanediol of 97-98% purity were purified by recrystallizing from H 2 0 and ethanol solutions, respectively. Typically the single crystals grown from the solution were 8 X 15 X 2 mm3 in size, but the thickness of the crystals was different from sample to sample. The length of the light path could not be determined definitely because of the uneven surface of the crystals. Deuterated analogues were prepared by exchanging hydroxy hydrogen with deuterium in a D20 solution. The sample crystals were directly immersed in liquid helium. They were not cracked in liquid helium and were transparent enough. Occasionally crystals grown from the melt in a Suprasil cell with 2-mm light path were used for comparison, the preparation method of which being described previously.8 These samples were first cooled in liquid nitrogen and then transferred to a liquid-helium cryostat. The crystals prepared by this method were transparent enough at room and liquid-nitrogen temperature, but they became severely cracked when they were cooled to liquid-helium temperature. The irradiation was carried out with 6oCoy-rays at a dose rate of 900 Gy h-I a t 4.2 and 77 K. Naturally, the intensity of the spectra depended on the thickness of the samples. Irradiation and measurements were made in complete darkness. Absorption measurements were made by a Shimadzu MPS5000 spectrophotometer with a double-detector system usually at 4.2 K. The spectra were plotted as the difference between the (8) Claesson, 0.; Ogasawara, M.; Yoshida, H.; Lund, A. Chem. Phys. Lett. 1983, 4, 408.
0'
lb
30
20
40
ii 110~cm-l
Figure 2. Effects of irradiation and measurement temperature on the absorption spectrum of y-irradiated crystalline 1,6-hexanediol (grown from the melt): A, irradiated at 4.2 K for 75 min and measured at 4.2 K; B, irradiated at 77 K for 45 min and measured at 77 K; C, irradiated at 77 K for 30 min and measured at 4.2 K D, irradiated at 4.2 K for 75 min, annealed at 77 K for 10 min, and measured at 4.2 K.
t
O210
,
25
I
30
35
I
Log ( t / s )
Figure 3. Decay of the absorbance at 590 nm at 77 K of the absorption spectra from (A) protiated and (B) deuterated 1,8-octanediolcrystals (grown from the melt) y-irradiated at 77 K.
measured spectrum and the spectrum obtained after bleaching completely with white light from an incandescent light. Photobleaching by monochromatic light was made with the analyzing light of the spectrophotometer through a wide slit width without moving the sample position. Time-dependent spectra at 77 K were taken after transferring the sample irradiated at 4.2 K into another cryostat filled with liquid nitrogen.
Results Crystals Grown from the Melt. Figure 1 shows the spectra obtained from y-irradiated crystalline 1,6-hexanediol at 4.2 K which is grown from the melt. The spectrum immediately after the y-irradiation comprises three partially resolved peaks with maxima at 720, 570, and 500 nm, a weak additional shoulder at ca. 430 nm, and a long tail extending to the UV region. Repeated bleaching with light of 700 and 525 nm gradually lowered the intensity of the whole spectrum, but the structure in the visible remained unchanged. Figure 2 shows the spectra obtained under a variety of conditions for crystalline 1,6-hexanediol. When the sample was irradiated at 77 K and measured at 77 K, the spectrum became broader and less defined and slightly shifted to the longer wavelength side (spectrum B) in comparison wjth that measured at 4.2 K after irradiating at 4.2 K (spectrum A). When the sample was irradiated at 77 K and measured at 4.2 K, the observed spectrum C showed essentially the same features as spectrum A. These temperature effects indicate the absence of any appreciable relaxation of trap conformation on warming to 77 K and reversibility of the thermally induced change of the trapping sites. The absorption spectrum is not stable when the sample temperature is kept at 77 K. The whole spectrum decays without changing its shape. The absorbance at 590 nm is plotted against the logarithm of the time after irradiating the 1,8-octanediol and
5006 The Journal of Physical Chemistry, Vol. 88, No. 21, 1984
Ogasawara et al.
A I nm 2000
loo0
700
- 0.6r 3
Figure 4. Absorption spectra observed from crystalline 1,s-octanediol (grown from the solution) y-irradiated at 4.2 K: A, immediately after the y-irradiation; B, photobleached by 1000-nm light for 4 min; C, photobleached by 730-nm light for 7 min, D, photobleached by 590-nm light for 7 min, E, the difference spectrum between A and B.
Figure 5. Absorption spectra observed from crystalline 1,6-hexanediol (grown from the solution) y-irradiated at 4.2 K: A, immediately after the y-irradiation; B, photobleached by 1000-nm light for 4 min; C, photobleached by 730-nm light for 7 min; D, the difference spectrum between A and B.
AI 062000
its deuterated analogue in Figure 3. The linear relation shown in this figure indicates that the decay curves can be expressed by the following equation: absorbance = a - log (bt) Such a relationship has been commonly observed in electrontransfer processes in solid matrices at low t e m p e r a t ~ r e . ~Deuteration of the hydroxy hydrogen atoms causes practically no effect on the decay rate. Crystals Grown from the Solutions. The single crystals of 1,8-octanediol grown from ethanol solution gave rise to the spectra shown in Figure 4. When a sample was irradiated and measured at 4.2 K, the s p t r u m obtained immediately after the y-irradiation showed at least two components, a visible and an infrared band. The visible band, which is essentially the same as that observed in the crystals grown from the melt,* comprises three peaks or shoulders at 500, 590 and 730 nm. The new infrared band which is observed in the present study shows a maximum at ca. 1300 nm, but the detailed bandshape is not clear because of the strong intrinsic absorptions of the l&octanediol ranging from 1450 to 1900 nm. When the sample irradiated at 4.2 K was photobleached with 1000-nm monochromatic light, the infrared band disappeared as indicated by spectrum B in Figure 4. This band was found to be quite sensitive to light, 3-min irradiation by the analyzing light of the spectrophotometer being enough to eliminate the band completely. The difference spectrum between A and B is shown by spectrum E. It has a long tail toward the shorter wavelength side. Further photobleaching with 730-nm monochromatic light simply reduced the residual visible band without changing its shape (see spectrum C). Homogeneous depletion of the visible band is consistent with the previous result that the structure of the visible band cannot be separated by photobleaching.8 Further photobleaching by 590-nm light reduced the visible band even more an4 caused a concomitant growth in the infrared band. Although the revived infrared band is not very strong in its intensity, the bandshape and the position of the maximum are the same as before as indicated by spectrum D. A spectrum composed of similar components was observed in crystalline 1,6-hexanediol. As shown in Figure 5 the absorption spectrum obtained immediately after the y-irradiation at 4.2 K (spectrum A) exhibited essentially two peaks in the visible and a shoulder at ca. 1000 nm which extended its tail to the infrared. The long-wavelength part of the spectrum was selectively photoble%ched with monochromatic light of 1000 nm as shown by spectrum B. The difference spectrum D is similar to the long(9) See for example: Miller, J. R. J . Phys. Ckem. 1975, 79, 1070.
-
nm
500
loo0
300
0.4 Y
I
L I 10~cm-l Figure 6. Time-dependent spectra after raising the temperature from 4.2 to 77 K of the 1,s-octanediol crystals (grown from the solution) y-irradiated at 4.2 K for 75 min; the time after raising the temperature is (A) 11, (B) 25, (C) 38, (D) 93, and (E) 135 min, respectively. The wavelength was scanned from the longer wavelength side to the shorter one and the times indicated are the ones when the scanning passed the position shown by the arrow. The absorbance is not corrected for the time needed to scan the spectra. wavelength component of the spectrum observed from 1,8-octanediol except that the peak position is shifted to the shorter wavelength side so that most of the component is buried in the visible band. When the crystalline 1,6-hexanediol or 1,8-octanediol y-irradiated at 4.2 K was warmed to 77 K, the absorption spectra started to decrease gradually. The time-dependent spectra after transferring the irradiated 1,8-octanediol into liquid nitrogen are shown by Figure 6. The spectra indicate homogeneous decrease of the spectral intensity at 77 K and no indication of the selective decay in the infrared region is observed. Figure 7 shows the effect of deuterium substitution of the hydroxy hydrogen atoms of 1,6-hexanediol. The deuterated analogue y-irradiated at 4.2 K gave rise to essentially the same spectrum as that from the protiated 1,6-hexanediol except that the resoltuion of the structure and the intensity at -350 and 1000 nm were somewhat different. It can be concluded that the deuteration has no effect on the spectrum. In comparison with the 1,6-hexanediol crystals grown from the melt, the crystals grown from the solution gave the spectrum with (1) stronger peak intensity at 740 nm, (2) a weaker shoulder at ca. 500 nm, and (3) a stronger infrared component. In the case of 1,8-cctanediol the shape of the structure in the visible was more or less similar in both crystals, but the infrared band was observed preferentially from the crystals grown from the solution.
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le Journal of Physical Chemistry, Vol. 88, No. 21, 1984 5007
Localized Electrons in Crystals Alnm
m
0
loo0
10
,
20 I,/ 10~cm-I
30
Figure 7. Effect of deuterium substitution of the hydroxy hdyrogen atom of the 1,6-hexanediolcrystal (grown from the solution) on the absorption spectra observed on irradiating at 4.2 K (-) protiated and (- - -) deuterated 1,6-hexanediol.
Discussion Two Types of Localized Electrons. The most striking feature of the present results is the infrared band produced in crystalline 1,8-0ctanediol. Previously we faund the visible absorption band in y-irradiated crystalline 1,8-octanediol at 4.2 K,* which had been assigned to localized electrons as discussed later, but no resolved infrared band had been detected. In irradiated 1,6-hexanediol, no resolved infrared band was seen, but the photobleachable component which was supposed to correspbnd to the infrared component in the irradiated 1,8-0ctanediol always appeared in the spectra observed. Photobleaching of the visible band at 590 nm induced concomitant growth of the infrared band in crystalline 1,8-octandeiol at 4.2 K. The convertibility of the visible band into the infrared band suggests that the latter is due to a kind of localized electron. It can be concluded that electrons ejected by ionizing radiation are localized at two distinctly different sites, shallow and deep in both crystals. We designate those electrons as e; and e++-, Absence of the eu- in the irradiated 1,8-octanediol grown from the melt may be explained by the selective scavenging of the electrons in the'shallow trap by impurities. The selective scavenging of electrons by inefficient scavengers has been already demonstrated in ethanol matrices.1° Spectra of e",;. The assignment of the visible band to the evi; is based on the similarity of the spectral characteristics to the earlier results6 as well as the similarity of the spectra in shape to that of (relaxed) lqcalized electrons in glassy alcohol matrices." Samskog et al. found a transient optical absorption with a maximum at 625 nm in pulse radiolsyis of single crystals of 1,8-0ctanediol in the temperature range of 318-181 Ke6 At higher temperatures, where the sample was liquid, the transient optical absorption which was also peaked at 625 nm disappeared when the liquid was saturated with N 2 0 gas. This optical absorption had been correlated to an ESR signal observed at 67 K which was assigned to the localized electron because of its single photobleachable line. The steady-state absorption spectrum obtained from irradiated 1,8-octanediol at 4.2 K in the present study was not exactly the same as the transient absorption obtained by pulse radiolysis with respect to the structure in the visible and the resolved band in the infrared. But when the sample temperature was raised to 77 K, the spectrum became less defined and similar to the transient spectrum. In addition, the Gt value of the steady-state spectra at 4.2 K is ca. 15 000 mol-' dm3 cm-' (100 eV)-', which is comparable to the Ge value determined by pulse radiolysis for the transient absorption in the temperature range from -92 to 45 O C 6 Therefore, the steady-state visible absorption (10) Noda, S.; Yoshida, K.; Ogasawara, M.; Yoshida, H. J . Phys. Chem. 1980, 84, 57. (11) Shida, T.; Iwata, S.; Watanabe, T. J . Phys. Chem. 1972, 76, 3683. 1972, 76, 369 1.
observed at 4.2 K is reasonably correlated to the transient absorption which is assigned to the localized electron. However, there still remains an open question about the thermal stability of the e,,];. According to Samskog et aL6 the singlet ESR signal of the localized electron was not detected until the temperature was lowered to 67 K. By using the kinetic data from the pulse radiolysis experiments, they estimated the half-life of the evlsto be 30 s at 77 K. However, in the present study, when the 1&octanediol and 1,6-hexanediol were irradiated and measured at 77 K, the observed half-life by steady-state optical spectroscopy was as long as 1 h. The faster decay in the pulse radiolysis measurement may be ascribed to the photobleaching effect by the intense analyzing light beam, but for the absence of the ESR signal of the localized electron at 77 K no explanation can be given. The spectra of localized electrons in glassy matrices generally consist of a broad, structureless band in the visible and infrared regions." In contrast, the spectra obtained from the crystalline alkanediols show a resolved structure with the following characteristics: (a) no peak of the structure can be selectively eliminated by photobleaching or thermal bleaching; (b) when the sample temperature is raised, the peaks become broader and less defined without shifting; (c) the relative intensity of each peak depends on the preparation method of the crystals, Le., if they are grown from the solution or from the melt; and (d) deuterium substitution of the hydroxy hydrogen causes no change in the separation between the peaks. Characteristic a strongly suggests that the peaks in the visible band do not correspond to a difference of the localization sites of electrons but arise from different transitions inherent to each localized electron. Different from the localization sites of electrons in amorphous matrices, those in crystalline matrices are homogeneous in nature, so that the absence of inhomogeneous broadening of the spectra facilitates resolution of the component peaks. As suggested by characteristic b, the preexisting sites in the crystalline matrices are comparatively rigid, so that the sites which capture the electrons undergo little relaxation after accommodating the electrons. This agrees with the conclusion reached by the analysis of the ENDOR2 and ESR3s4 data for the trapping sites in crystalline carbohydrates. At present no definite interpretation of the resolved structure can be given. Previously we argued that vibrational sublevels in the excited state of the localized electron* might determine the structure of the spectra. This argument was based on the fact that the separations between the peaks in the spectra from crystalline 1,8-octanediol grown from the melt were roughly equidistant and the separation of about 3000 cm-' was rather close to the value of the fundamental frequency of the C O H bending motion or O H stretching motions.12 However, it was found in the present study that the deuteration of the hydroxy hydrogen atoms in the 1,6-hexanediol had no effect on the evl; spectrum. Therefore, the structure of the e,,]; spectrum is not due to vibratiqnal but electronic origin. The ew- spectrum is very probably due to a bound-continuum transition in which the electron is excited to levels in continuum, since some of the photoliberated electrons transfer to the shallow traps. The peaks at 590 and 500 nm in the ens- spectrum of 1,8-octanediolseem, therefore, to reflect a variation of the density of states in the continuum. Spectra of e!;. The spectra of el; in irradiated crystalline 1,6-hexanediol and l&octanediol, seen in Figures 4 and 5, respectively, are similar to the absorption spectra found for irradiated alkane glasses at low temperature. The infrared band which was observed in the 1,8-octanediolis probably due to electrons localized in an effectively nonpolar environment. This speculation will explain the very efficient photobleaching of the infrared band by weak monochromatic infrared light, since in the absence of long-range forces, bound-free transitions will make a major contribution to the oscillator strength." The yield of the el; in 1,8-0ctanediol depends very much on the preparation method of the crystals. It seems to be sensitively (12) Bush, R. L.; Funabashi, K. J . Chem Soc., Furuduy Trans. 2, 1977, 73, 214
5008
J. Phys. Chem. 1984,88, 5008-5012
affected by a small amount of impurity in the sample. The time between preparation of the crystal and measurement also affects the yield of ei;. We think that the trapping site of the el; is very shallow and not well-defised. Tentatively we attribute the e,; to the electron trapped in a cavity or a defect which is accidentally formed in the crystals when they are grown. In the case of the crystalline 1,6-hexanediol, the yield of the e,; is less dependent on the preparation method. However, the trapping sites of the e,; appear to be similar to those in 1,8-octanediol because of the similarity in its spectral shape. It is of interest to compare the ei; with the infrared absorbing electrons observed in other crystals or amorphous matrices. Ichikawa and Yoshida found in y-irradiated a-cyclodextrin an absorption spectrum with a broad maximum at about 600 nm, which was photobleached selectively in its long-wavelength region with light of X > 670 nm.' The photobleached component has its maximum at 730 nm, a shorter wavelength than that of the ei,- in irradiated alkanediols, but the spectral shape itself is quite similar to that of the eu-. In irradiated crystalline D20a transient infrared absorption band has been found and it was assigned to shallowly trapped electrons. Buxton et al. suggested that this electron was localized at a natural cavity in the very open ice lattice.I3-l5 A similar infrared band has been observed in irradiated D20 glasse~.'~,'~ The absorption maxima found in (a) 50% by volume ethylene glycol, (b) 9.5 M LiCI, and (c) 2.5 and 4 M MgC12 glasses are located beyond 3200 nm, so that the trapping site must be much shallower than the e; in irradiated alkanediols. (13) Buxton, G. V.; Gillis, H. A.; Klassen, N. V. Chem. Phys. Lett. 1975, 32, 533. (14) Buxton, G. V.; Gillis, H. A.; Klassen, N. V. Can. J . Chem. 1976, 54, 367. (15) Buxton, G. V.; Gillis, H. A.; Klassen, N. V. Can. J. Chem. 1977, 55, 2385.
Ogasawara et al. have proposed that the infrared bands observed in irradiated ethanol and 1-propanol glasses at 4.2 K are due to a class of electrons localized in an alkyl environment.16 These infrared bands resemble the ei; in irradiated alkanediols in their spectral shape. It is not yet clear whether the ei; in alkanediols has a structure similar to that of infrared absorbing electrons in irradiated alcohols, however. Conclusion The main conclusions obtained in the present studies of localized electrons in irradiated 1,8-octanediol and 1$-hexanediol crystals are summarized as follows: (1) Electrons are localized in the crystalline alkanediols and their deuterated analogues at 4.2 K with yield GE 15000-20000 mol-' dm3 cm-' (100 eV)-' at their visible absorption maximum, which is comparable to the values for the localized electrons in irradiated glassy alcohols. (2) Two distinctly different traps, shallow and deep, correspondigg to ei; and evi;, exist in the crystalline alkanediols. No continuous distribution of trap depth is indicated between the shallow and deep traps. (3) The el; is not the precursor of the evi;; Le., the latter is not produced by the conformational relaxation of the former. (4) Some of the peaks of the ev,; spectra may arise from a variation of the density of states in the continuum; it is not due to a vibrational structure because of the absence of any isotope effect.
-
Acknowledgment. A part of this work was carried out in the Research Reactor Institute, Kyoto University. Registry No. 1,6-HexanedioI,629-1 1-8; l,S-octanediol, 629-41-4. (16) Ogasawara, M.; Shimizu, K.; Yoshida, H. Radiat. Phys. Chem. 1981, 17, 331.
Temperature Dependence of Excimer Formation and Excimer Fluorescence Polarization in Micellar Dispersions: Surfactants as Intrinsic Probes M. Aoudia,t%M. A. J. Rodgem,*$ and W. H. Wadel Center for Fast Kinetics Research, and Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712 (Received: February 29, 1984)
Experiments have been carried out in which the temperature dependence of the fluorescencespectrum of alkylbenzenesulfonate surfactan!$ has been studied. Excitation of the benzene chromophore above the critical micelle concentration in aqueous dispersions showed the presence of the spectral components due to both monomer and excited dimer species. The ratio of the intensities of the two fluorescent peaks allowed an examination of the monomer-excimer equilibrium in the micellar phase. The activation energy of excimer formation for different isomers was obtained and comparisons drawn with the behavior of sterically hindered benzenes in homogeneous solution. In other studies, pyrene molecules were incorporated in the surfactant micelles as extrinsic probes. Temperature studies of its monomer-excimer behavior enabled both excimer binding energy and activation energy to be evaluated. It was concluded that the pyrene probe and the benzene intrinsic probe occupy the micellar assembly such that their environments exhibit different viscosities.
Introduction ~
The study of intermolecular excimer formation in aqueous micellar solutions was first attempted by Forster and Selinger' using pyrene and 2-methylnaphthalene as fluorescent probes. Since then, pyrene has been used to investigate systems of anionic:+ cationic:,5s6 and nonionic*surfactants. Excimer formation and decay usually occur over a time scale measured in tens and +Present address: SONATRACH, END (ORD), 10 Rue du Sahara Hydra, Algiers, Algeria. *Center for Fast Kinetics Research. A Department of Chemistry.
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hundreds of nanosecond^;^ surfactant micelles, on the other hand, are stable for periods of tenths of milliseconds.* The excimer formation and decay process thus provides a suitable probe for
,
(1) Forster, T.; Selinger, B. K. Z . Naturforsch., A 1964, 19, 38. (2) Khuanga, W.; Selinger, B. K.; McDonald, R. Aust. J . Chem. 1976, 29,
(3) Gratzel, M.; Thomas, J. K. J . A m . Chem. S o t . 1973, 95, 6885. (4) Gupta, D.; Basu, S. Ind. J . Chem., Sec. A 1976, 14, 543. (5) Dorrance, R. C.; Hunter, J. K. J . Chem. SOC.1973, 70, 1572. (6) Pownall, H. J.; Smith, L. C. J . Am. SOC.1973, 95, 3136. (7) Birks, J. B. "Photophysics of Aromatic Molecules"; Wiley Interscience: London, 1970. (8) Nakagowa, T.; Tori, K. Kolloid Z . Z . Polym. 1964, 194, 143.
0 1984 American Chemical Society
