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Tetracyanoethylene. The absorption spectrum of TCNEO-, presented in Figure 3, is very similar to that observed in 2-MTHF at 77 K.I9 In the steady state, the conversion is about 12% as determined by T C N E absorption recovery upon bleaching. Complete bleaching requires a longer irradiation than was the case for COT'-, BQ'-, and NQ'-. Extended X-ray irradiation of T C N E caused the formation of a byproduct characterized by absorption peaks a t 307 and 328 nm. At high T C N E concentrations or high deposition temperatues a broad structureless absorption band at 470 nm is present already before X-ray irradiation. This is attributed to the charge-transfer absorption band of a TCNE-TEA complex.20 Efforts to generate and observe radical anions of naphthalene, anthracene, and acenaphthylene in argon matrices by X-ray irradiation have not been successful although their UV-vis ab(19) Hoshino, M.; Arai, S.; Imamura, M. Radiaf. Phys. Chem. 1980, 15, 377. (20) In n-hexane, the broad band of the TCNE-TEA complex lies at 418
nm. The -50-nm shift is most probably caused by solvent or temperature
effects.
sorption spectra are known from previous work in 2-MTHF at 77 K.3 Even a thoroughly dried sample of matrix-isolated anthracene failed to yield a detectable amount of the radical anion. W e conclude that "free" argon-matrix isolated anions can be generated by the X-ray radiolytic method, particularly when special care is taken to remove water and oxygen, but only from substrates of relatively high electron affinity. This is a far more severe restriction than those encountered for radical
Acknowledgment. This work was supported by the National Institute of Health ( G M 37929-02). Registry No. COT, 629-20-9; BQ, 106-51-4; NQ, 130-15-4; TCNE, 670-54-2; TEA, 121-44-8; DMP, 106-58-1; DABCO, 280-57-9; COY-, 34510-85-5; BQ'-, 3225-29-4; NQ'-, 20261-01-2; TCNE'-, 34512-48-6;
Ar, 7440-37-1; naphthalene, 91-20-3; anthracene, 120-12-7; acenaphth-
ylene, 208-96-8. (21) Radical cations of naphthalene and anthracene have been easily generated in argon matrices doped with CH2C12with use of the same X-ray irradiation conditions, and a steady state has been observed at about 35% conversion of the neutral parent.
Interpretation of the Absorption Spectra of Incompletely Relaxed Electrons in Polar Liquids as a Direct Measure of the Energetic Distribution of Band-Tail Localized States C. Hou6e-Levint and J.-P. Jay-Gerin*,* Groupe du Conseil de Recherches MPdicales du Canada en Sciences des Radiations et DZpartement de MZdecine NuclZaire et de Radiobiologie, FacultZ de MZdecine, UniversitP de Sherbrooke, Sherbrooke, QuZbec, Canada J l H 5N4 (Received: March 7, 1988: In Final Form: May 13, 1988)
We have analyzed the optical absorption spectra of incompletely relaxed excess electrons in liquid alcohols and water and have shown that we can extract, for these liquids, the energetic distribution of band-tail localized states below the free-electron continuum. The density of these states depends on the temperature and varies exponentially with energy for the three studied alcohols, namely, ethanol, 1-propanol, and 2-butanol. In liquid water, the conduction band tail, which seems to have two exponential components, is compared with the results of a recent molecular dynamics simulation study.
Electron solvation experiments in alcohols'-5 and in water at room temperature6 have shown the existence of an initial localization stage suggesting preexisting trap states. These states, presumed to be due to orientational fluctuations of solvent dip o l e ~ , belong ~ ~ * to a tail of localized states (called an Urbach tail), which lies within the forbidden band gap of the material below the bottom of the conduction band.9 The density of such states (DOS) is known to depend more or less exponentially on energy in a wide class of disordered systems, irrespective of the origin of the disorder.I0 This exponential behavior has recently been substantiated by the work of Monroe and Kastner,l' who reported transient photocurrents in glassy As&. These authors have shown that the inferred band-tail DOS is accurately of simple exponential form over almost 5 decades of electron density. In a recent study, Bennett and Thompsoni2have also suggested an exponentially decaying band tail of localized states for interpreting their photoinjection data in liquid NH3. It is the purpose of this work to use initially localized excess electrons as microprobes of the intrinsic local trap structure in various polar liquids, including ethanol, 1-propanol, 2-butano1, and water. In order to do so, we 'On leave of absence from the Laboratoire de Chimie-Physique, Universite Rene Descartes, 45, Rue des Saints-Ptres, 75270 Paris CCdex 06, France. *Also at the Centre de Recherche en Physique du Solide, Departement de Physique, Facult6 des Sciences, Universite de Sherbrooke, Sherbrooke, Quebec, Canada J1K 2R1.
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TABLE I: Values of the Exponential Tail Widths E,,, As Determined from the Slopes of the Straight Lines Shown in Figure 1
alcohol ethanol 1-propanol 2-butanol
temp, K
Eo,eV
157 152 261 162
1.32 1.64 2.27 1.56
determine the energetic distribution of band-tail localized states in these liquids from an analysis of existing incompletely relaxedI3 (1) Baxendale, J. H.; Wardman, P. Nature (London) 1971, 230, 449; J . Chem. SOC.,Faraday Trans. 1 1973, 69, 584. (2) Chase, W. J.; Hunt, J. W. J . Phys. Chem. 1975, 79, 2835. (3) Gilles, L.; Bono, M. R.; Schmidt, M. Can. J . Chem. 1977, 55, 2003. (4) Okazaki, K.; Freeman, G . R. Can. J . Chem. 1978,56, 2305; 1978.56,
2313. (5) Kenney-Wallace, G. A.; Jonah, C. D. J . Phys. Chem. 1982,86,2572. (6) Migus, A,; Gauduel, Y.; Martin, J. L.; Antonetti, A. Phys. Rea. Left. 1987, 58, 1559. ( 7 ) Tachiya, M . ; Mozurnder, A. J . Chem. Phys. 1974,60, 3037; 1974, 61, 3890. (8) Krohn, C. E.; Antoniewicz, P. R.; Thompson, J C. Surf. Sei. 1980, .i n r. , x- . i.. (9) Thompson, J. C.; Antoniewicz, P. R.; Krohn, C. E . Comments Solid State Phys. 1980, 9, 223. (10) Sa-Yakanit, V . ; Glyde, H. R. Comments Condens. Matter Phys. 1987, 1 3 , 35.
0 1988 American Chemical Society
Absorption Spectra of Electrons in Polar Liquids
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6455
.-.. W
u
z a
m CT 0 u)
m
5 z J
ENERGY (eV)
Figure 1. Plot of the logarithm of absorbances of incompletely relaxed electrons against energy for ethanol a t 157 K (ref 3) (0),1-propanol a t 152 K (ref 1) (W) and 261 K (ref 2) (O),and 2-butanol at 162 K (ref 4) ( 0 ) .The straight lines are least-squares fits to the data points. The zero of energy corresponds to the energy of the bottom of the conduction band at V,. Note that the absorbance scale is arbitrary for each set of data points.
I 0
electron optical absorption spectra. The basic assumption of our analysis is that optical absorption spectra of incompletely relaxed excess electrons can be interpreted in terms of electronic bound-to-free direct transitions from the localized gap states to the bottom of the conduction band, which lies a t an energy Vowith respect to vacuum (-Vo is the electron affinity of the liquid). The absorbance a t a given energy is proportional to the transition probability matrix element between the initial and final electron states considered and to the joint trap-conduction band density of states. Assuming a constant transition matrix element as is usually done in the theory of direct optical measurement^,'^ and neglecting transitions between localized states, the absorbance should simply be proportional to the band-tail DOS provided that the conduction band minimum is sufficiently flat. In fact, such a flat-band condition, which we assume to be satisfied for the liquids considered here, permits us to neglect optical transitions between localized states and conduction band extended states above Vo. In Figure 1, we show a plot of the logarithm of absorbances of incompletely relaxed electrons against energy in three liquid alcohols at low temperatures, namely, ethanol a t 157 K,3 1propanol a t 152’ and 261 K,* and 2-butanol at 162 K.4 For all considered cases, and despite a small absorption contribution from fully relaxed electrons in the case of 1-propanol at 261 K? a linear relationship is observed over the entire energy range inve~tigated.’~ (1 1) Monroe, D.; Kastner, M. A. Phys. Reu. E : Condens. Mutter 1986, 33, 8881. (12) Bennett, G. T.; Thompson, J. C. J . Chem. Phys. 1986, 84, 1901. (13) A reviewer has pointed out that the term “presolvated electron” is ambiguous and should be avoided. The literal meaning of presolvated can be either “before being solvated” or ‘already solvated”. See,on this point: Bolton, G. L.; Jha, K. N.; Freeman, G. R. Can. J . Chem. 1976,54, 1497. As suggested by Freeman, we use here the term “incompletely relaxed electron”; it refers to electrons in the initial site of solvation: Freeman, G. R. In Kinetics of Nonhomogeneous Processes; Freeman, G. R., Ed.; Wiley: New York, 1987; Chapter 6, p 277. (14) For example, see: Payson, J. S.; Guha, S. Phys. Rev. E : Condens. Mutter 1985, 32, 1326. ( 1 5 ) Experimentally, a maximum in the optical absorption spectrum of localized electrons in alcohol glasses from 12 to 115 K has been observed around 2000 nm (-0.6 eV). See: Klassen, N. V.; Teather, G. G. J . Phys. Chem. 1983, 87, 3894. This maximum could be explained by the fact that, in the region above 1800 nm, localized electrons are in weakly bound states so that they have higher possibility of escaping their shallow traps. The probability of their presence in these traps is thus lowered, and consequently, the observed optical absorption is reduced. The maximum might also be attributable to strong absorption bands in the glasses at wavelengths above 2000 nm, making measurements of the trapped electron spectrum less precise in this region. See: Buxton, G. V.; Kroh, J.; Salmon, G. A. J . Phys. Chem. 1981, 85, 2021.
I 2
I 1
ENERGY
I
3
(eV)
Figure 2. Plot of the logarithm of the incompletely relaxed electron absorbance against energy for pure liquid water at 21 OC. The experimental data ( 0 ) are taken from Figure 1 (curve labeled “0 ps”) of ref 6. The two straight lines are least-squares fits to the data points below and above an energy in the region of 1.6 eV, respectively. The logarithm of the calculated energy distribution of electron localization sites in liquid water (taken from Figure 2 of ref 17, after a shift of the energy scale by about 1.1 eV to higher energies) (+) is also shown for comparison (see text). The zero of energy corresponds to the energy of the bottom of the conduction band at V,.
This indicates that these liquids exhibit the same exponential conduction band tails as do disordered materials. Accordingly, the trap-state DOS can be expressed as
DOS
exp(-E/Eo)
(1)
where E is the depth of the localized state below V, and Eois the exponential tail width. The values of Eo determined from the slopes of the straight lines shown in Figure 1 are given in Table I. Considering the different alcohols around 160 K, the values of Eo appear to be correlated with the degree of linearity of the molecules, which varies as follows: 1-propanol > 2-butanol > ethanol. Such results suggest that the energy dependence of the trap-state distribution in these solvents is related to a molecular shape effect, with more spherelike molecules displaying less significant orientational disorder and, therefore, shallower electron traps. Additional data are needed, however, to better document and quantify this effect. Table I also shows a marked temperature effect for 1-propanol. A temperature dependence of the localized portion of the density of states below Vo has been proposed for polar fluids by Bennett and Thompson.’* According to these authors, the gap-state DOS is described by a Boltzmann factor exp(-E,,/kBT), where E, = LYE is the orientational reorganization energy required to create a potential well with an electron energy level a t E. Following this model, we have The two values of Eo reported in Table I for 1-propanol a t 152 and 261 K indicate that Eo increases with T . However, more experimental data are necessary to ascertain the relationship between Eo and T . Figure 2 shows a plot of the logarithm of the incompletely relaxed electron absorbance data of Migus et aL6 recorded a t “0 ps” (see Figure 1 of ref 6) as a function of energy for water at 21 O C . We observe that the overall curve can be decomposed
J . Phys. Chem. 1988, 92. 6456-6460 I
I
+
.
+*
I
LIOUID WATER
.+
++
**
w .L
.
++
++
*++ i t
1
1.5
2
2.5
ENERGY (eV)
Figure 3. DOS-vs-energy comparison between the two sets of calculated (+) and measured ( 0 )data shown in Figure 2 for liquid water. The zero of energy corresponds to the energy of the bottom of the conduction band at V,,.
reasonably well into two straight lines whose inverse slopes Eo are equal to 0.81 eV below, and to 0.27 eV above, an energy in the region of 1.6 eV. This two-exponential band-tail behavior seems to be rather unusual and might suggest that there exist two different types of electron trapping sites in liquid water. We note, however, that the DOS distribution found here differs from that of Thompson et a1.,I6 who reported in a short abstract a single conduction band tail in water decaying exponentially below Vo with a characteristic energy near 0.2 eV. A molecular dynamics simulation study aimed at identifying microscopic electron localization sites in pure liquid water has recently been published by Schnitker et al.” These authors calculated the energy distribution of preexisting traps for a scattering quasifree electron. Figures 2 and 3 compare their results (16) Thompson, J. C.; Antoniewicz, P. R.; Bennett, G. T. Bull. Am. Phys. SOC.1985, 30, 600. (17) Schnitker, J.; 1986, 85, 2986.
Rossky, P. J.; Kenney-Wallace,G. A. J . Chem. Phys.
to the DOS obtained here. As we can see from Figure 2, the DOS obtained from the simulation can also be decomposed into two straight lines whose inverse slopes are almost identical with the values reported above. However, in order to have the calculated and the measured sets of data coincide, we had to shift, by about 1 . 1 eV to higher energies, the energy scale of the calculated electronic trap distribution. In our opinion, the striking agreement between the two sets of data (see Figures 2 and 3) seems to support the existence of two different types of microscopic electron localization sites in liquid water. The shift in the energy scale, however, remains a problem. In fact, this shift cannot be. explained by a Voeffect since the origin of the energies should be the same in both cases and corresponds to the bottom of the conduction band. One possible explanation could be that the calculation was performed with a finite-size simulation system consisting of 216 water molecules; that is, the importance of long-range interactions could have been underestimated.’* In summary, the main conclusions of this work are as follows: (i) it is possible to interpret the incompletely relaxed electron absorption spectrum in polar liquids as a direct measure of the density of localized states below the conduction band edge;19 (ii) for the studied liquid alcohols, this density of states tails off exponentially into the forbidden gap; and (iii) liquid water seems to behave in a peculiar way in that the energetic gap-state distribution below Voshows a two-exponential behavior. This result could help to bring a new insight into the understanding of the structural properties of this liquid.
Acknowledgment. We thank Drs. C. Tannous and B. Hickel for valuable discussions and Profs. G. R. Freeman and C. Ferradini for useful comments on the manuscript. The work reported here was supported by the Medical Research Council of Canada and the Natural Sciences and Engineering Research Council of Canada. One of us (C.H.-L.) benefited from a grant from the France-Quzbec (INSERM-FRSQ) exchange program in health sciences. This support is gratefully acknowledged. Registry No. Ethanol, 64-1 7-5; I-propanol, 7 1-23-8; 2-butano1, 7892-2. (18) Schnitker, J.; Motakabbir, K.; Rossky, P. J.; Friesner, R. Phys. Rev. Lett. 1988, 60, 456. (19) As a consequence, the incompletely relaxed electron molar absorptivity r(X) at a given wavelength X should be regarded as proportional to the preexisting trap-state DOS of the considered liquid at the corresponding energy E [ E (eV) = 1240/X (nm)]: a(X) = a DOS(E), where a is a constant characteristic of the absorbing trapped electron only.
Relation between Geometry and Charge Transfer in Low-Dimensional Organic Salts Timothy C. Urnland,? Sharon Allie,+ Tom Kuhlmann,+ and Philip Coppens* Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14214 (Received: April 7, 1988) The Cambridge Data Base has been used to examine the relation between charge transfer and geometry in salts containing the TCNQ (tetracyanoquinodimethanide)anion and the TTF (tetrathiofulvalene), TSF (tetraselenofulvalene), and BEDT-TTF [bis(ethylenedithio)tetrathiofulvalene]cations. The correlation is based on either a bond length ratio or a bond length difference function and is calculated both for an extended data set, including charge transfers based on stoichiometry, and on a more restricted set based on neutral molecules and experimentally measured charge transfers. A two-parameter linear least-squares fit is found to be adequate; inclusion of a third (quadratic) coefficient does not give a significant improvement with the data available. The bond length difference function tends to give somewhat smaller standard deviations in predictions based on the derived equations. The curves for BEDT-TTF are not significantly different from those for the larger TTF set of entries. Introduction Low-dimensional solids containing planar organic cations and/or anions can have remarkable electrical transport properties, in particular when the ions are arranged in homogeneous stacks with Undergraduate Research Participants.
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intermolecular overlap of valence molecular orbitals. Partial filling of the resulting electron band leads to an electronic stabilization of the homogeneous stack structure. The conductivity is particularly high when charge transfer between the donor and acceptor molecules is less than complete, so that a mixed valency exists in each off the stacks, which increases the electron mobility. The
0 1988 American Chemical Society
