J. Phys. Chem. 1980, 84, 1259-1262
Union Carbide Corporation. References aind Notes (1) J. K. Balrd. (J. Chem. Phys., 70, 1575 (1979). (2) I. Carmichael, J. Chem. Phys., 70, 1576 (1979).
1259
(3) (a) H. Eyring, J . Chem. Phys., 4, 283 (1936); (b) W. F. Schmidt, G. Bakale, and U. Sowada, ;bid., 61, 5275 (1974). (4) L. S. Frost, Phys. Rev., 105, 354 (1957). (5) (a) C. E. Kbts and D. R. Nelson, Bull. Am. Phys. Soc., 15,424 (1970); (b) R. E. Rubson, Aust. J. Phys., 25, 685 (1972).
Photoionization of Solutes and Conduction Band Edge of Solvents. Indole in Water and Alcohols A. Bemas,"
D. Grand, and E. Amouyal
ERA 718, St. 350, Universit6 Paris-Sud, 91405 Orsay, France (Received August 1, 1979)
Publication costs assisted by CNRS
Indole has been photoionized at neutral pW and room temperature in a few transparent solvenk tetramethylsilime (Me4%),water, methanol, and ethanol. Photoconductivity measurements in Me4%solutions and solvated electron scavenging by N20 in all solvents lead to the following results: (la) The electron affinity of the scavenger does not intervene in the relationship giving the solute optical ionization potential IliT (Ib) I1iq = 4.95,4.85,4.60, and 4.35 f 0.1 eV in Me4Si, ethanol, methanol, and water solutions, respectively. (2) The threshold energy and the empirical law describing the relative photoionization yield, 4e-,in the threshold region were found dependent on the solvent. (3) The values of the solvent conduction band edge Vodeduced from the solute Iuqare found to be -1.3, -1.0, and -0.65eV for water, methanol, and ethanol, respectively. (4)An extrapolation of Ibqvs. the alcohol chain length to 1-propanol and 1-butanol leads to VO,pfiH= -0.3 eV and VoBuoH= + 0.03 eV.
Introduction During the past decade, a great deal of experimental and theoretical work has been devoted to the photoionization of impurity atoms or molecules. However, the systems investigated have been mostly liquid and solid rare gases,' hydrocarbon and alcoholic glasses,2and dielectric whereas the studies pertaining to polar liquids have been scarce. Recently aqueous tryptophan (Trp) photoionization was reexamined arid the photodissociation channels were analyzed below aind above 111q.5 Indole (In) photoionization was then inveefigated in liquid tetramethylsilane (Me4Si) and water with particular emphasis on the conduction band edge and energy gap of liquid water.6 Indole was chosen as a solute because it serves as a model compound in Trp photochemistry studies; besides, it is soluble in both polar and nonpolar solvents and a relatively high solvated electron yield, $e- = 0.25,has been reported78upon 265-nm excitation in oxygen-free aqueous solutions at neutral pH and 25 "C. The present study deals with the photoionization of indole in water, methanol, and ethanol at neutral pH and room temperature. Me4& solutions, where both photoconductivity ineasurements and solvated electron scavenging could be performed, have been used to calibrate the aqueous and alcoholic solutions. As is well established, excess electrons injected in fluids not only constitute "microscopic probes" of the dynamical molecular structureg but the knowledge of an impurity ionization potential also allows an indirect determination of a solvent bulk property: that of the fundamental energy Vo of quasi-free e1ectrons.l Such Vo values may in turn be correlated with transport properties'O and the reactivity of the excess electron.11J2 Since Vomeasurements have not been performed, to our knowledge, i n alcoholic fluids, euen a n indirect ap0022-3654/80/2084-1259$01 .OO/O
proach seems of interest. With this objective in mind, we have considered the following in succession: (I) the influence of the solvent on the optical ionization potential of the solute; (11) an empirical law relating the photoionization yield to the exciting light frequency in the threshold energy region; (111)the energy position of the lower edge of the solvent conduction band Voas deduced from the solute photoionization threshold Iliq.
Experimental Section Detailed experimental conditions have been described previou~ly.~~~ Indole (Fluka puriss.) was used as supplied. The deaerated solutions were 4 X M. T h e alcohols were doubly distilled under a N2 atmosphere, first over 2,4-dinitrophenylhydrazineand concentrated H2S04and then over a mixture of clean dry magnesium and iodine to remove traces of water. Continuous, monochromatic light illuminations (Ah N 10 nm) were performed with a xenon source (Osram XBO 2500 W)fitted to a Bausch and Lomb monochromator. The light flux measured at 265 nm was of the order of 2 X 1013photons cm" s-l. Scavenging of soluated electrons by N 2 0 was used to evaluate photoionization yields. The measured N2derives onIy from a dissociative electron attachment since N20, in contrast to ionic scavengers such as H+, NO,, ... has been shown not to quench In fluorescence.13 N20 (from Air Liquide) was used at concentrations (4.7 X and 5 X M) such that the competitive reaction e,, - + In could be neglected. %he photoproduced N2,measured by gas chromatopaphy, was corrected for a low and constant residual N2, observable in blank experiments. Photoconductiuity measurements have been performed on degassed In-Me4Si solutions, the applied electric field 0 1980 American Chemical Society
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The Journal of Physical Chemistty, Vol. 84,
10.
YI
No. 10, 1980
Bernas, Grand, and Amouyal
t
u f o r i n d o l e in Liquid S o l u t i p n s
Nb o f C in linear monoolcohols
L.35
L.50
1.85
L.95
PV
vs. exciting light frequency v for indole in liquid solutions: lo4 M, [N,O] = 4.65 X M; (A) CH,OH, [indole] = 4 X lo4 M, [N,O] = 4.71 X M; (X) C2H50H,[indole] =4X M, [N,O] = 4.71 X M; (B)Me,Si, [indole] = 4 X lo4 M, [N,O] = 5 X lo-' M; (0)Me$, photocurrent, [indole] = 2.3 x 10-4 M.
Figure 1.
$e-
(e)H20, [indole] = 4 X
Figure 2. Variation of photoionizationenergy threshold with the number of carbon atoms in linear monoalcohols: (x, curve A) indole In liquid alcohols at room temperature (the values for 1PrOH and 1BuOH have been extrapolated); (e, curve B) TMPD in solid alcohols at 77 K. Indole in TMS Log
oi
tCURVEB
I $ J3'2 A
t
being of the order of 15.6 kV cm-'.
Results and Discussion I. Ionization Thresholds. Figure 1displays the relative photoionization yield, $e-, vs. the exciting light frequency, v, for In in Me4Si, ethanol, methanol, and water; the following is noted: (ii A11 curves exhibit a threshold, defined as the lowest photon energy giving rise to a detectable signal, which are indicated by arrows in Figure 1. The same kind of empirical definition has been previously adopted in the case of photoelectron emission from solution^.'^ It could be expected a priori that such a loose definition would result in different threshold values, depending inter alia on the sensitivity of the analytical technique. However, when comparing N 2 0 scavenging and photoconductivity measurements, this does not seem to be a critical factor. Similarly, for solute ionization in solid hydrocarbon matrices, identical threshold values were obtained from photocurrents or neutralization luminescence recording.16 As indicated below an empirical law describing the photoionization cross section in the threshold energy region leads to a more accurate Ihq determination. The error on I,, may be considered to be of the order of 0.1 eV. (ii) For In in Me4Si, the same ionization cross-section curves are obtained from N 2 0 scavenging and photoconductivity measurements performed in the absence of N20. This implies that the electron affiiity of the scavenger does not affect the value of the ionization energy threshold. It also indicates that there is no direct electron transfer from the In molecule to N 2 0 and that a quasi-free delocalized electron must be the primary ionization product. Both curves give a threshold value of Ilig = 4.95 eV which means that, in the threshold spectral region, the precursor for photoionization reaction is the vibrationally excited singlet state of In. (iii) The Ihq values are solvent dependent. The sequence Iliq HzO < Iliq MeOH < I,, EtOH presently obtained for In in liquid solutions is similar to the one previously observed2a for TMPD in alcoholic glasses (Figure 2B). Iliq is found to increase linearly with the number of carbon atoms in the monoalcohol chain (Figure 2A). Such a result expresses the progressive dilution of the hydroxyl group with increasing hydrocarbon chain length. Extrapolation to l-propanol and l-butanol leads to I1iq pr0H = 5.15 eV and Iliq Bu0H = 5.4 eV. II. Empirical Laws i n the Threshold Region. A threshold law expressing the dependence of the pho-
Flgure 3. Indole In Me,% deDendence of the ohotoionization electron yleld upon the photon eiergy:,'(O,curve A) $e-'m (hv I,J3'*; (x, curve B) log 4,- a: (hv - Ih).
-
toionization yield on the photon energy hv not only provides a more precise definition of the threshold value itself but it might also ultimately lead to a better understanding of the mechanism(s) and intermediates involved or even to a theoretical interpretation of the ionization mechanism. For TMPD photoionized in Me4&, cyclopentane, npentane, and n-hexane, an exponentiallaw has been shown to apply, whereas for pure Me4Si,the photocurrent intensity u has been found l6 to obey the relationship u 0: (hv - lfiq)3/2 (1) A 312 law, initially established for a gas-phase photodetachment process,17has recently been found to hold also for electron photodetachment from 02in solution.18 As shown in Figure 3, (1) seems to describe In photoionization in Me4Sibetter than an exponential law. This confirms the threshold value of Iliq = 4.95 eV. On the other hand, for In in aqueous solutions, a 312 law fits the data only in a limited energy range, giving = 4.35 eV, whereas an exponential law seems to cover the whole photon energy range investigated. It can then be remarked (a) that different threshold laws may apply, depending on the energy range considered; (b) that the difference in threshold laws previously reported for pure Me4Si, on one hand, or TMPD photoionized in Me4Si, on the other, does not stem from the fact that a solute molecule is photoionized in the latter case; and (c) that from the observations that $e- obeys a different law when the electron ejection occurs either in Me4Sior HzO, it may be concluded that a threshold law does not only characterize the solute electronic transition but, as the ionization threshold energy itself, depends also on the nature of the solvent.
The Jaurnal of Physical Chemistry, Val. 84, No. 10, 1980
Photoionlzation of Solutes
6261
TABLE I
a
Flgure 4. Indole in H20;dependence of the photoionizatlon electron curve A) dNza (hu - Iliq)3’2; (x, yield upon the photon energy: (0, curve B) log q5N2 0: (hv - Ih).
I t should also be emphasized that the plots considered here refer to experimentally determined ionization yields. Contrary to thle gas phase situation, these yields are in fact complex quantities since they contain not only a primary photoelectron yield but also an electron escape probability, both terms being energy and solvent dependent.l9 For liquid water, the escape probability would be close to 1;20 it would be notably smaller for nonpolar solvents.21 III. Conduction Band Edge in Water and Alcohols. The excess electrons ejected from the absorbing solute molecules ma.y also serve as probes of the solvent conducting properties. In effect, pirovided that the photoelectron is promoted to a conduction state, the optical ionization potential Iliq of an impurity atom or molecule in a condensed phase system may be expremedlain terms of the conduction band edge energy Vo of the solvent Ifiq= Ig P+ (2) where P+ is the adiabatic electronic polarization of the medium by the positive ion and Igthe ionization potential of the solute molecule in the gaseous phase. The heat of immersion of the solute should also be considered. ]!€owever,the term mi,, to be inserted in eq 2 should represent the variation upon ionization of the heat of immersion and such a short-range interaction with the solvent of the neutral solute dipole is expected to be preserved for the solute cation. Hence, AHimmwould be close to zero. Various methods, direct and indirect, experimental and theoretical, have been proposed in the past to estimate Vo for water, and conflicting data have been reported (cf. references given in ref 6). For liquid alcohols no Vovalues are available, to our knowledge. Hence, a semiempirical approach based on eq 2 may reveal fruitful. The following procedure was adopted. For Me4Si solutions,Vo is known from the l i t e r a t ~ r eIg ; ~is known from the literature;22IKqis experimentally determined (Figure 1); P+ can then be obtained by difference. On the other hand, P+ may be calculated from Born’s expression -e2 p+ = -(1 - top-l) (3) 2r, where r+ is an “effective”ionic radius and top the solvent
+ + vo
indole in
Zfiq, eV
P+, eV Vo, uq, e V Vo,
Me,Si H,O MeOH EtOH 1-PrOH 1-BuOH
4.95 4.35 4.60 4.85 (5.15) (5.40)
-2.36 -2.25 -2.22 -2.37 -2.46 -2.51
Reference 3.
Reference 2c.
-0.6a -1.3 -1.0 -0.65 (-0.3) (+0.03)
eV’
+0.50b (-0.4)’ 0.05b 0.34 0.38 0.47
Reference 23.
optical dielectric constant. The effective radius for indole derived from ref 3 turns out to be 1.4 A, which is notably smaller than the radius estimated from the molar volume. This has been considered to indicate that the positive charge remains localized around the nitrogen atom,6 in which case r+ can plausibly be assumed not to vary much with the nature of the solvent. Conversely, for aqueous and alcoholic solutions, P+ was calculated by using r+ = 1.4 A and Voinferred. Table I gathers the results obtained from a vertical ionization potential value of 7.9 eV for indole.22 For 1-propanol and 1-butanol solutions, Iliqis notably blue shifted compared to EtOH solutions and the Xe source can no longer be used. The values quoted for 1PrOH and 1-BuOH have thus been attained by extrapolating the straight line relating I1iqto the alcohol chain length (Figure 2A). For comparison, the Vovalues previously obtained by the same m e t h ~ d for ~ ” TMPD ~~ photoionized in alcohlolic glasses at 77 K are also given. The paren theses indicate that the enclosed figures are extrapolated.
Conclusions The Vovalue obtained for liquid water by the semiempirical method just described is -1.3 eV, whereas a semicontinuumexpression for P+ resulted in Vo= -1.2 eV.6 These figures are in excellent agreement with the photoelectrochemical findings and Vo= -1.2 f 0.1 eV should thus be retained for liquid water at room temperature. The Vo values for liquid water and alcohols appear significantly smaller than the values deduced for the corresponding glasses at 77 K. This is the situation prevailing also for liquid and glassy hydrocarbons, which has been ascribed to a density effect. The same interpretation would apply to alcohols but not to water. It should be noted, however, that the Vovalue adopted for glassy ice (Table I) may have to be revised. On the basis of X-ray photoelectron spectroscopy experiments and on the assumption that amorphous and cubic ice have the same energy gap, Vo = -0.9 eV has been recently proposed.24 The same systematic variation of Vowith solvent polarity is observed for the liquid and solid solutions: Vo increases when the solvent static dielectric constant decreases. Such a regular trend is not presently accounted for by the theoretical models describing solvated electrons. In particular, in the semicontinuum model, Vois treated as a limited (fleV being the limits) adjustable parameter. As suggested earlier,2cthe regular trend observed experimentally may require that the theoretical models be refined. It might also be mentioned that for liquid ammonia at -55 OC, a value Vo= -1.13 eV has recently been reporbed.% However, from the observation that it is easier to inject electrons in ammonia than it is in water, VO,”* would be expected to be more negative than VOHzO. The last and basic point to be considered concerns the mere significance of Voin polar liquids. In other terms, should the experimentally determined Volevel be alsso-
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J. Phys. Chem. 1980, 84, 1262-1266
ciated with a quasi-free extended state or with a localized S. Tames, and A. Kennedy, ibid., 79, 2857 (1975). (4) S. H. Peterson, M. Yaffe, J. A. Schukz, and R. C. Jarnagin, J. Chem. state of the excess electron? The completely solvated state Phys., 63, 2625 (1975). can be excluded both on time scale and energetic grounds. (5) E. Amouyal, A. Bernas, and D. Grand, Photochem. Photobiol., 29, Electron localization in polar liquids has been thoroughly 1071 (1979). (6) D. Grand, A. Bernas, and E. Amouyal, Chem. Phys., 44, 73 (1979). analyzed and the role of preexisting density fluctuations or preexisting clusters has been considered as c r ~ c i a l . ~ ~ (7) ~ ~H. ~1. Joschek ~ ~ and L. I. Grossweiner, J . Am. Chem. Soc., 88, 3261 (1966). The depth of such localization sites is, however, difficult (8) D. V. Bent and E. Hayon, J. Am. Chem. Soc., 97, 2612 (1975). (9) G. A. Kenney-Wallace, Acc. Chem. Res., 11, 433 (1978). to assess, as well’as the time required for localization, and (10) R. A. Holroyd and N. E. Cipollini, J. Chem. Phys., 69, 501 (1978). the present results are unable to elucidate these questions. (11) A. 0. Allen, T. E. Gangwer, and R. A. Holroyd, J. Phys. Chem., 79, From a pragmatic point of view, it is, however, striking 25 (1975). (12) A. Henglein, Can. J. Chem., 55, 2112 (1977). and significant that the same final energy state is attained (13) T. R. Hopkins and R. Lumry, Photochem. Photobiol., 15, 555 (1972). from quite different experimental conditions, Le., when the (14) P. Delahay, P. Chartier, and L. Nemec, J. Chem. Phys., 53, 3126 excess electron is emitted from a metallic electrode into (1970). (15) J. Bullot and M. Gauthier, Can. J. Chem., 55, 1821 (1977). a concentrated electrolyte solution or when ejected from (16) W. F. Schmidt, W. Doldissen, U. Hahn, and E. E. Koch, 2.Mturforsch. a solute molecule diluted in the pure solvent. A , 33, 1393 (1978).
Acknowledgment. The authors are endebted to Drs. A. Henglein, N. Kestner, L. Kevan, J. Sass, and R. Schiller €or stimulating discussions and correspondence. References and Notes (1) (a) 8. Raz and J. Jortner, Chem. Phys. Lett., 4, 155 (1969); Proc. R. Soc. London, Ser. A , 317, 113 (1970); (b) 2 . Ophir, B. Raz, J. Jortner, et al., J. Chem. Phys., 62, 650 (1975); (c) I. Messing, B. Raz, and J. Jortner, Chem. Phys., 23, 23 (1977). (2) (a) A. Bernas, M. Gauthier, D. Grand, and G. Parbnt, Chem. Phys. Lett., 17, 439 (1972); (b) A. Bernas, J. Blais, M. Gauthier, and D. Grand, /bM., 30, 383 (1975); (c) D. Grand and A. Bernas, J , Phys. Chem., 81, 1209 (1977). (3) (a) R. A. Holroyd, J. Chem. Phys., 57, 3007 (1972); (b) R. A. Holroyd and R. L. Russel, J. phys. Chem., 78, 2128 (1974); (c) R. A. Holroyd,
(17) D. S. Burch, J. J. Smith, and L. M. Branscomb, Phys. Rev., 112, 171 (1958). (18) U. Sowada and R. A. Holroyd, J . Chem. Phys., 70, 3586 (1979). (19) J. Bulbt, P. Cord&, and M. Oauthii, J. Chem. phys., 69, 1374(1978), (20) H. Sano and M. Tachyia, J. Chem. Phys., 71, 1276 (1979). (21) Yu. A. Berlin, P. Corder, and J. A. Debire, J. Chem. Phys., in press. (22) H. Gusten, L. Klasinc, J. V. Knop, and N. Trinajstic, in “Excited States of Biological Molecules”, J. B. Birks, Ed., Wiley-Interscience, New York, 1976. (23) S. N d a , L. Kevan, and K. Fueki, J . Phys. Chem., 79, 2866 (1975). (24) B. Baron, D. Hoover, and F. Williams, J. Chem. Phys., 68, 1997 (1978). (25) K. Itaya, R. E. Malpas, and A. J. Bard, Chem. Phys. Lett., 63, 411 (1979). (26) J. Jortner and A. Gaathon, Can. J. Chem., 55, 1801 (1977). (27) G. Kenney-Wallace and C. D. Jonah, Chem. Phys. Lett., 47, 362 (1977).
Electrochemical and Photoelectrochemical Studies of Excess Electrons in Liquid Ammonia Allen J. Bard,* Kingo Itaya, Richard E. Malpas, and Towfik Teherani Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712 (Received July 17, 1979) Publication costs assisted by the University of Texas
The applications of electrochemical techniques (voltammetry, coulometry, current step) to studies of the generation and properties of excess electrons in liquid ammonia are discussed. A study of the variation of the equilibrium potential with total concentration of solvated electrons placed the standard potential at -2.74 V vs. Ag/Agt (0.1 M) and provided evidence for a dimeric electron species. Studies of the photoemission of electrons from a platinum electrode under laser irradiation are also described and the existence of an “injection level” at -2.91 V vs. Ag/Ag+ (0.1 M) is reported. The reaction of excess electrons with N-tosylcarbazole which leads to strong emission of light (electrogeneratedchemiluminescence) characteristicof carbazole anion fluorescence is also discussed.
Introduction Electrochemical experiments involving reductions in liquid ammonia with the probable generation of solvated Although electrons date back to at least the 1890’~.’*~ several later studies which involved the cathodic generation of excess electrons (e;) at mercury and solid metal electrodes were r e p ~ r t e d , there ~ - ~ have been surprisingly few studies on such Faradaic reactions employing modern electrochemical techniques with NH8 solutions prepared and examined under rigorously pure and dry condition^.^^^ The electrochemical generation (and oxidation) of excess electrons is especially convenient because solutions with 0022-3654/80/2084- 1262$0 1.OO/O
known concentrations of e; can be prepared coulometrically and then investigated without opening the system or adding additional reactants. Moreover, electrochemical techniques are very well suited to the collection of accurate data which can provide information about the thermodynamics, kinetics, and transport properties of excess electrons. Additional information can be obtained from studies of the photoejection of electrons from metal and semiconductor electrodes into liquid ammonia solutions. In this paper we briefly review the results of recent work in our laboratory on electrochemical studies of excess electrons in ammonia, highlighting, as appropriate, the 0 1980 American Chemical Society
