Formation of vertically oriented aromatic molecules chemisorbed on

J. Phys. Chem. 1984,88, 1089-1094

Peer’s method can be used to predict E,,,(Na-,am) = 1.85 f 0.08 eV or A,, = 670 f 30 nm. Sodium/ammonia solutions are intensely absorbing at total N a concentration greater than 0.1 m, and Fourier transform photoacoustic s p e c t r o ~ c o p y may ~~-~~ be the only viable technique for Na- detection in these systems. 3. 23Na N M R in liquid ammonia’.2s5,6,26 may prove to be another useful detection technique. The method has been used by Dye in various solvents to differentiate between Na- and eNa+e-. Experiments are being constructed in our laboratory to investigate this possibility.

of solvation of metal anions are all extrapolations. 4. The concentration/activity conversion (eq 6) may become increasingly inappropriate as concentrated M/NH3 solutions begin to experience deviations from “strong” electrolyte behavior. Perhaps the only statement one may make with confidence is that Na- is probably the least unstable alkali-metal anion in liquid ammonia at 298 K; that is, given the proper experimental conditions, Na- stands the greatest chance of being detected.

Possible Detection of Na- in Liquid Ammonia 1. A detailed study of the physical and chemical properties of Na/NH, solutions may reveal anomalies with respect to other alkali-metal/”, solutions, which may be best explained in terms of Na-. 2. Peer’7d has noted that charge transfer to solvent (CTTS) absorption band maxima are linearly related between solvents.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 1, 2015 | http://pubs.acs.org Publication Date: March 1, 1984 | doi: 10.1021/j150650a012

1089

Acknowledgment. We acknowledge the generous financial support of the Robert A. Welch Foundation (Grant No. F081). The reviewers’ comments on the original manuscript have also been helpful. Registry No. Na-, 19181-13-6; NH3, 7664-41-7; Na, 7440-23-5.

Formation of Vertically Oriented Aromatic Molecules Chemisorbed on Platinum Electrodes: The Effect of Surface Pretreatment with Flat Oriented Intermediates Manuel P. Soriaga* and Arthur T. Hubbard* Department of Chemistry, University of California, Santa Barbara, California 93106 (Received: June 10, 1983; In Final Form: August 8, 1983)

The adsorption of aromatic compounds on Pt electrodes, pretreated with a layer of flat oriented intermediates at fractional or full coverages, has been studied as a function of concentration. Measurements of packing densities were based on thin-layer electrochemical methods. Four aromatic compounds, previously shown to adsorb on clean electrodes in flat and edgewise (vertical) orientations depending upon the adsorbate concentration, were studied: hydroquinone (1) 1,4-naphthohydroquinone (2), 2,3-dimethylhydroquinone (3), and 2,5-dimethylhydroquinone(4). The same edge orientations formed on the clean electrode were also obtained on the pretreated surface, but the adsorption profiles (total packing density vs. concentration) of 1-3 showed that complete formation of the edge orientation was retarded (severely for 1 and 2 slightly for 3) when the electrode was pretreated with a full monolayer of flat oriented species; the same retardation was observed even when the electrode was pretreated with less than half a monolayer. When the electrode was precoated with a layer of aromatic molecules in transition between flat and vertical orientation, the adsorption profile was remarkably similar to those for clean electrodes. The adsorption profile of 4 (which displays hindered reorientation due to the blocking effect of the methyl groups) was unaffected by surface pretreatment. These results suggest that (i) adsorption of 1-3 at concentrations above 1 mM on clean Pt leads primarily to direct attachment of edge-oriented species; (ii) for 1-3, complete reorientation from flat to vertical structures is less facile than direct adsorption in the vertical orientation; (iii) adsorption of 4 in the edge orientation involves formation of flat oriented species as an intermediate step; (iv) adsorption of aromatic molecules at high concentrations on a sparsely pretreated surface involves completion of the flat oriented layer prior to formation of edge-bonded species; and (v) reorientation of nearly isolated flat adsorbed intermediates is at least as difficult as that of closepacked molecules. Similarities and differences between results from solution and vacuum studies are discussed.

Introduction The orientation or mode of attachment of aromatic compounds adsorbed on platinum surfaces has been the subject of numerous investigations.’-28 The first systematic studies on the chemi(1) Soriaga, M. P.; Hubbard, A. T. J . Am. Chem. SOC.1982, 104, 2735. (2) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. SOC.1982, 104, 2742. (3) Soriaga, M. P.; Hubbard, A. T. J . Am. Chem. SOC.1982,104, 3937. (4) Soriaga, M. P.; Wilson, P. H.; Hubbard, A. T.; Benton, C. S. J. Electroanal. Chem. 1982,142, 317. ( 5 ) Chia, V. K. F.;Soriaga, M. P.; Hubbard, A. T.; Anderson, S. E. J . Phys. Chem. 1983,87, 232. (6) Soriaga, M. P.; White, J. H.; Hubbard, A. T. J . Phys. Chem. 1983, 87. 3048. (7) Soriaga, M. P.; Stickney, J. L.; Hubbard, A. T. J . Electroanal. Chem. 1983,144, 207. ( 8 ) Soriaga, M. P.; Stickney, J. L.; Hubbard, A. T. J. Mol. Catal. 1983, 21, 211. (9) Soriaga, M. P.; Hubbard, A. T. J . Phys. Chem., in press. (10) Soriaga,M. P.; Hubbard, A. T. J. Electroanal. Chem. 1983,159, 101.

0022-3654/84/2088-1089$01.50/0

sorption of b e n ~ e n e , ’ ~based - ’ ~ on a wide variety of experimental techniques such as radiotracer methods and deuterium exchange (11) Stickney, J. L.; Soriaga, M. P.; Hubbard, A. T.; Anderson, S. E. J . Electroanal. Chem. 1981, 125, 73. (12) Moyes, R. B.; Wells, P. B. Adu. Catal. 1975, 23, 121. (13) Bond, G . C. “Catalysis by Metals”; Academic Press: New York, 1962. (14) Garnett, J. L.; Sollich-Baumgartner, W. A. Adu. Cutul. 1966,16, 95. (15) Palazov, A. J . Catal. 1973, 30, 13. (16) Primet, M.; Basset, J. M.; Mathieu, M. V.; Prettre, M. J . Catal. 1973, 29, 213. (17) Haaland, D. M. Surf. Sci. 1981, 102, 405. (18) Haaland, D. M. Surf. Sei. 1981, 111, 555. (19) Fischer, T. E.; Kelemen, S. R.; Bonzel, H. P. Surf. Sei. 1977.64, 157. (20) Richardson, N. V.; Palmer, N. R. Surf. Sei. 1982, 114, L1. (21) Netzer, F. P.; Matthew, J. A. D. Solid State Commun. 1979,29, 209. (22) Lehwald, S.; Ibach, H.; Demuth, J. E. Surf. Sci. 1978, 78, 577. (23) Gland, J. L.; Somorjai, G. A. Adu. Colloid Interface Sei. 1976,5, 205. (24) Stair, P. C.; Somorjai, G. A. J . Chem. Phys. 1977, 67, 4361. (25) Gavezzotti, A,; Simonetta, M. Surf. Sei. 1982, 116, L207.

0 1984 American Chemical Society


1090 The Journal of Physical Chemistry, Vol. 88, No. 6, 1984

Soriaga and Hubbard

reactions, suggested the existence of three types of adsorbed erages were determined as the sample concentration was increased molecule orientations: one in which the aromatic ring is attached incrementally starting from lo4 M; cleaning of the electrode prior to each concentration change (and coverage measurement) was parallel to the surface (q6 orientationz9)and the other two in which not done. That is, the design of the experiments was such that the ring is adsorbed perpendicular to the surface, either in an adsorption was actually carried out on an electrode previously edgewise ( q 2 )or endwise (VI) configuration. The flat orientation has invariably been associated with n-bonded i n t e r m e d i a t e ~ , I ~ - ~ ~ treated with a layer of (flat oriented) aromatic adsorbed from a M solution. and homogeneous n-arene metal complexes abound which serve Recently, we reported thin-layer electrochemical (TLE) studies as model compounds for such surface specie^.^*^^ Vertical of about 50 aromatic compounds'-" spontaneously chemisorbed orientations have been related to a-bonded species,12*18,23,30 although from aqueous solutions onto smooth polycrystalline Pt electrodes. n-complexation through a ring double bond may also be possible These studies revealed that the orientation and packing efficiency for q2 orientation^;^^^^^ a few q2- and ql-bonded metal-aromatic compounds have been ~ h a r a c t e r i z e d . ~ ~ - ~ ~ of chemisorbed organic molecules are dependent on the molecular structure' and solution c o n c e n t r a t i ~ n ~of- ~the adsorbate, the Early infrared spectroscopic studies of benzene adsorbed on surface activity of the supporting electrolyte,z the bulkiness3 and supported Pt catalysts showed only n-bonded species;1s17 however, chiralityS of inert substituents, the pH of the and the more recent FT-IR work has provided evidence for a-bonded temperature of adsorption;6 the effects of other factors such as adsorbates.I8 Studies on well-defined surfaces under ultrahigh electrode potential, the nature of the solvent, and the structure vacuum (UHV) have been reported. Angle-resolved ultraviolet of the electrode surface are presently under investigation. The photoemission spectroscopy of benzene19 and chlorobenzeneZoon effect of orientation and mode of attachment on the electroPt single-crystal surfaces indicated r-bonded species. ElectronicZ1 chemical reactivities of adsorbed species has been demonstratand vibrational22energy-loss spectra for benzene on Pt(l1 l ) , ed.s37-11In our studies, the concentration dependence of adsorbed likewise, correlated with q6-oriented adsorbates. Packing energy aromatic orientation was in general found to be as follows: at calculationsZ5for naphthalene adsorbed on Pt(ll1) suggested flat @5 M and in the absence of competing surface-active oriented species corresponding to the surface structure implied substituents, homocyclic aromatics are adsorbed flat; at 1 by the low-energy electron diffraction (LEED) data.41 On the M, the same group of compounds are adsorbed edgewise. These other hand, LEED of benzene adsorbed on Pt( 111) and Pt( 100) results are in qualitative agreement with the benzene reorientation revealed dosage- and time-dependent structural transformations noted on polycrystallineZSand ~ e l l - d e f i n e dPt~surfaces, ~ ~ ~ ~ except of the organic superlattices; with the aid of work function change measurementsZ3and quantitative Auger electron s p e c t r o s c ~ p y , ~ ~ that, under the conditions of our experiments, the orientational transitions (as indicated in the adsorption profiles) were observed these phase transitions were rationalized in terms of reorientation to occur much more abruptly. The cause of this difference is the of the adsorbed molecules from flat to vertical structures. Such subject of the present study. The results indicate that the difreorientations may be analogous to (n-to-u) linkage isomerism ference arises mainly because in our experiments adsorption at in homogeneous organometallic compounds usually brought about a given adsorbate concentration was always begun with a clean when the number of organic ligands per metal center is inelectrode, while in the other measurements adsorbate concentration n-to-a bonding rearrangements have been invoked was increased incrementally such that the electrode surface was in the mechanistic description of homogeneous metal-catalyzed initially covered with a monolayer of flat oriented intermediates reaction^.^^-^^^^^ formed at the lower concentrations. The adsorption of aromatic compounds on Pt electrodes in Four diphenolic compounds, previously shown to reorient from aqueous electrolytes has been studied by radiotracer methods. On flat to edgewise structures as the solute concentration was inplatinized Pt, and at concentrations below M, only the flat c r e a ~ e d ,were ~ . ~ studied: hydroquinone (l),1,4-naphthohydroorientation for adsorbed benzenez6 and naphthalenez7was indiquinone (2), 2,3-dimethylhydroquinone(3), and 2,5-dimethylcated. But at concentrations above low3M, benzene, phenol and hydroquinone (4). naphthalene were found to chemisorb on smooth Pt electrodes at coverages indicative of edge-oriented species;28the possibility of Measurement of Adsorbed Amounts multilayer condensation was negated by the irreversibility of the Adsorption on Clean Electrodes. Analytical measurements of adsorption. The adsorption profiles (coverage vs. concentration packing densities using thin-layer electrochemical method^^^,^^ plots) did not show an abrupt transition but only a slow increase have been described in detai1!s6 Absolute coverage was determined in packing density. In those experiments, the Pt electrode was from the disappearance of surfactant from solution which is given constantly immersed in the aromatic solution and surface covby the difference between the charge for quinone/diphenol electrolysis (at electrode potentials characteristic of the unadsorbed material) obtained after only a single filling ( e l ) of the thin-layer (26) Heiland, W.; Gileadi, E.; Bockris, J. O'M. J. Phys. Chem. 1966, 70, cavity and that after multiple fillings (Q): 1207. (27) Bockris, J. O'M.; Green, M.; Swinkels, D. A. J. J. Electrochem. SOC. 1964, 111, 743.

(28) Kazarinov, V. E.; Frumkin, A. N.; Ponomarenko, E. A,; Andrew, N. V. Elektrokhimiya 1975, 11, 860. (29) Cotton, F. A. J. Am. Chem. SOC.1968, 90, 6230. (30) Ugo,R. In "Catalysis Reviews"; Heineman, H., Carberry, J. J.; Eds.; Marcel Dekker: New York, 1975; Vol. 11, p 225. (31) Zeiss, H.; Wheatley, P. J.; Winkler, H. J. S. "Benzenoid-Metal Complexes"; Ronald Press: New York, 1980. (32) Cotton, F. A.; Wilkinson, G. "Advanced Inorganic Chemistry"; Wiley: New York. 1980. (33) Muetterties, E. L.; Bleeke, J. R.; Wucherer, E. J.; Albright, T. A. Chem. Rev. 1982,82, 499. (34) Rogers, R. D.; Bynum, R. V.; Atwood, J. L. J. Am. Chem. SOC.1978, 100, 5238. (35) Calderon, J. L.; Cotton, F. A.; DeBoer, B. G.; Takats, J. J. Am. Chem. SOC.1971, 93, 3592. (36) Browning J.; Green, M.; Penfold, B. R.; Spencer, J. L.; Stone, F. G. A. J. Chem. SOC.,Chem. Commun. 1973, 31. (37) Sweet, J. R.; Graham, W. A. G. J. Am. Chem. SOC.1983,105,305. (38) Cobbledick, R. E.; Einstein, F. W. B. Acta Crystallogr., Sect. B 1978, 34, 1849. (39) Gorewitt, B.; Tsutsui, M. Ado. Catal. 1978, 27, 227. (40) Sheldon, R. A.; Kochi, J. K. Adv. Catal. 1976, 25, 274. (41) Dahlgren, D.; Hemminger, J. C. Surf. Sci. 1981, 109, L513. ~

r = (e- Qb)

-

(Qi

- Qd

(1) nFA where Qb is the appropriate background charge, A the electrode surface area,"4,45and n (equals 2 for the reversible quinone/diphenol couple) is the number of Faradays F consumed per mole of reactant. Equation 1 specifically applies for concentrations sufficiently high such that one filling not only saturates the surface with aromatic but also leads to excess dissolved material. For very dilute concentrations, multiple fillings may be necessary to saturate the surface, and the procedure follows that discussed in detail e l ~ e w h e r e . ~Equation *~,~ 1 is valid whether or not the adsorbed and unadsorbed species react ~ i m i l a r l y . ~ The packing density is related to the average area, u, occupied by an adsorbed molecule by a = (NAr)-',where N A is Avogadro's (42) Hubbard, A. T. In "Critical Reviews of Analytical Chemistry"; Meites, L., Ed.; Chemical Rubber Co.: Cleveland, 1973; Vol. 3, p 20. (43) Lai, C. N.; Hubbard, A. T. Inorg. Chem. 1972, Z I , 2081. (44) Hubbard, A. T. Acc. Chem. Res. 1980, 13, 177. (45) Hubbard, A. T. J. Vac. Sci. Technol. 1980, 17, 49.

The Journal of Physical Chemistry, Vol. 88, No. 6, 1984 1091


Orientation of Aromatics on Pt Electrodes

02

number. The predominant adsorbate orientations have been deduced via the traditional m e t h ~ d ~of, ~comparing ' the observed u with those calculated for various possible orientations;' molecular area calculation^'^^ were based on covalent and van der Waals distances tabulated by P a ~ l i n g . This ~ ~ method proved highly consistent for all oriented-adsorbed aromatic compounds thus far studied.' Adsorption on Pretreated Electrodes. Electrode surface pretreatment with a full monolayer of oriented adsorbate consisted of rinsing the Pt thin-layer electrode several times with surfactant solution of a concentration at which adsorption in the desired orientation occurs, based upon previously determined packing density vs. concentration curves. For example, the adsorption profile for hydroquinone showed that the surface was saturated with $-adsorbed species at concentrations in the range from M.334 The packing density at this selected pretreatment to concentration (C,) will be referred to as the "initial" coverage r,. The Pt electrode thus modified was then exposed to another solution of the same compound but at different concentrations, and any additional adsorption at the new concentration was determined. The measurement procedure was otherwise identical with that for a clean electrode. That is, (corrected) single-filling and multiple-filling charges for electrolysis of unadsorbed material were obtained; the difference between both charges is a measure of the additional adsorption Ar (cf. eq 1):

I

,

,

06 ,

,

08 I

,

,

2,3-D~m~ihylhydroqulnone

I

e,

AI' =

el'

(Q - Qb)

- (Qi' - Qib') 2FA

where represents the single-filling electrolytic charge for the the corresponding background charge. pretreated surface, and Equation 2 is applicable even if desorption, rather than adsorption, occurs [which is conceivable when the adsorption concentration is lower than the pretreatment concentration (C,)]; in such cases, a negatiue value of Ar would be obtained since removal of initially adsorbed material would make (Q,'- Ql{) larger than (Q - Qb). In any event thefinal or total packing density (I?) is AF. given by r = I', For 1, packing density measurements were also obtained for an electrode pretreated with less than half a monolayer of flat oriented intermediates. The pretreatment in this case involved a single rinse of the Pt electrode with 3.72 X M hydroquinone solution; since the adsorption is complete (as verified by the absence of a quinone/diphenol redox peak for unadsorbed material in a subsequent voltammetric scan), the initial packing density r, for this single rinse is given by3v4s6

el,,'

(e)

+

rl = VCJA

POTENTIAL.VOLT V S . AgCl

(2)

(3)

where V is the thin-layer cell volume. For the thin-layer cell employed in this study, V = 4.08 pL and A = 1.18 cm2; so C, = 3.72 X M led to I?, = 0.129 nmol cm-2. Since a full mon01ayer'-~of q6-oriented hydroquinone corresponds to r,6 = 0.322 nmol cm-2, this r, was equivalent to 0.40 monolayer.

Figure 1. Thin-layer current-potential curves for (A) dissolved (0.15 mM) and (B) chemisorbed 2,3-dimethylhydroquinone. A: (---) First filling on a clean electrode; first filling on an electrode pretreated with the subject compound at 0.05 mM; (-) multiple fillings. B: (-) Presaturated surface (C, = 0.05 mM) rinsed to remove dissolved reactant; (---) clean Pt surface. Solutions contained 1 M HC104 electrolyte. Thin-layer cell volume, V = 4.08 kL; Pt electrode area, A = 1.18 cm2. Rate of potential sweep, r = 2.00 mV s-]; solution temperature, T = 298 f 0.5 K. (..a)

cleaned by electrochemical oxidation in 1 M HC104 at 1.20 V [Ag/AgCl (1 M C1-) reference] and reduction at -0.20 V. AqueousSosurfactant solutions contained 1 M HC104 electrolyte.' Adsorption was carried out at controlled potential, 0.200 V. A waiting period of 180 s was allowed for adsorption. However, no significant changes were noted when adsorption was carried out without control of potential or when the adsorption time was ~ adsorption kinetics has not yet been varied from 120 to 300 s ; the studied in detail. 1 and 2 were studied at 5 and 25 OC, 3 was studied at 25 OC, and 4 was studied at 5 OC. The adsorption temperature was controlled by immersing the H cells in a thermostatted water bath and monitroed by a thermometer placed in the reference compartment of the H cell; the temperature fluctuated less than 2~0.5 OC. Circuitry for linear potential sweep voltammetry and potential-step coulometry was of conventional design based on operational amplifier^.^^

Experimental Section Results Smooth p ~ l y c r y s t a l l i n ePt ~ ~electrodes ~~~ were prepared as Figure 1 shows thin-layer current-potential curves for adsorbed described p r e v i o u ~ l y . ' ,The ~ ~ ~geometric ~ ~ ~ ~ area was taken to be and unadsorbed 2,3-dimethylhydroquinone(3). The dashed curve the electrode surface area; verification of smoothness was made in Figure 1A corresponds to the reversible two-electron redox of by hydrogen electrodeposition5' and iodine a d ~ o r p t i o n ~ J ~ - ~3,~ after only one filling of a clean Pt thin-layer electrode, according measurements. In between experimental trials, the electrode was to eq 4: (46) Langmuir, I. Proc. R. Soc. London, Ser. A 1939, 170, 1. (47) Adam, N. K. "The Physics and Chemistry of Surfaces"; Oxford

University Press: London, 1941. (48) Pauling, L. C. "The Nature of the Chemical Bond"; Cornell University Press: New York, 1960. (49) Ptekaru, J. Y.; Hershberger, J.; Garwood, G. A.; Hubbard, A. T. Surf. Scr. 1982, 121, 396. (50) Conway, B. E.; Angerstein-Kozlowska, H.; Sharp, W. B. A.; Criddle, E. E. Anal. Chem. 1973, 45, 1331 (51) Ishikawa, R. M.; Katekaru, J.; Hubbard, A. T. J. Electroanal. Chem. 1978, 86, 27 1. (52) Felter, T. E.; Hubbard, A. T. J. Electroanal. Chem. 1979, 100,473. (53) Garwood, G. A.; Hubbard, A. T. Surf. Sci. 1980, 92, 617.

0

OH

The dotted curve was obtained after a single filling of an electrode pretreated with q6-oriented intermediate [C, = 5.0 X M;Ti = 0.252 nmol cmM2;u, = 65.9 A2; = 67.8 A2].'s4 The solid curve was obtained after multiple rinses. The area under the dashed curve (after one filling) is smaller than that after several rinses because the subject compound is bound through the elec-

1092 The Journal of Physical Chemistry, Vol. 88, No. 6, 1984

Soriaga and Hubbard

TABLE I: Adsorption Data on Pretreated Pt Electrode Surfacesn ~~~~

Co,

mM nmol cm-* r :Pi + A r T, "C rl,nmol cm-2 mM nmol cmT2 r rl + A r Hydroquinone 1.56 0.287 0.536 25 0.322 0.040 CJ, Figure 2 . This suggests that in the transition region the adsorbed monolayer consists of “tilted” molecules rather than a mixture of flat and edge oriented species. The retardation effects observed for 1-3 indicate that reorientation of a layer of flat adsorbed intermediates to edgewise structures is a much less facile process than direct adsorption in the vertical orientation. The fact that the retardation is independent of whether initial pretreatment consisted of a full or fractional monolayer provides evidence that (i) adsorption on a sparsely precoated electrode at high concentration involves com-

pletion of the flat oriented monolayer prior to formatiod of the v2-oriented layer; and (ii) reorientation of nearly isolated flat adsorbed intermediates is at least as difficult as that of closepacked molecules. These findings are consistent with the general observation that the strength of adsorption of an individual species (in a given orientation) tends to decrease as the adsorbate population is i n ~ r e a s e d that ; ~ ~ is, disruption of the original mode of attachment of isolated molecules would be more difficult than that of close-packed, less strongly bound species. The barrier to flat-to-vertical reorientation observed in this work may account for at least some of the differences between results from various studies on the adsorption of aromatic molecules from aqueous solution^.^-^^^^^ However, the lack of evidence for vertically oriented species in vacuum studies remains. It is likely that this is due to differences in adsorbate concentrations. In the solution s t ~ d i e s ~packing - ’ ~ ~ ~densities ~ indicative of $-orientations were obtained at C? > M. For qualitative comparison with gas-metal systems, this concentration may be expressed in terms of sample pressiure P by the ideal gas law P = (n/V)RTi= @RT, where TI is the sample mole number, V the volume, R the gas M constant, and T the absolute temperature. At 25 “C, translates to about 18 torr. The cited UHV studies which showed only *-bonded species’9-22~25~29 utilized pressures near torr. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Air Force Office of Scientific Research for support of this research. Registry No. 1, 123-31-9; 2,571-60-8; 3,608-43-5; 4,615-90-7; Pt, 7440-06-4. (54)Wieckowski, A.;Rosasco, S. D.; Schardt, B. C.; Stickney, J. L.; Hubbard, A. T. Inorg. Chem., in press. (55) Somorjai,G.A.“Chemistry in Two Dimensions: Surfaces”; Cornel1 University Press: New York, 1981.

Eiectrochromism in Viscous Systems. Excited-State Properties of all-trans-Retinal Ake Davidsson,* Department of Inorganic Chemistry 1, University of Lund, Chemical Centre, S-220 07 Lund, Sweden

and Lennart B.

Johansson

Department of Physical Chemistry, University of Umeb, S-901 87 UmeB, Sweden (Received: June 13, 1983)

-

A model for electric-field-inducedabsorption spectra in viscous systems is presented in this work. This model has been applied

in the investigations of the lBU+ So transition of all-trans-retinal in polyethylene and polypropylene matrices. From the anomalous electric-field dependence of the electrochromism, we conclude that retinal interacts with its surroundings in a way explained by a proposed model. Excited-state properties of all-trans-retinal have been evaluated. The all-trans-retinal molecule has a differencein the dipole moment between the ground state and the lBIUstate of about 15 D, and the corresponding transition moment is directed along this dipole moment change. A very large polarizability change of about 1000 A3 upon excitation is also found. In order to explain the electrochromismof all-trans-retinal, both first- and second-orderperturbations of the electric transition dipole moment are necessary.

Introduction When an electric field is applied to a system of chromophores, the electronic charge distribution, energy levels, and population of molecules will be affected. This will in turn affect the absorption coefficient of the chromophores. The difference in absorption obtained with and without an applied electric field is called the electrochromism (EC). The measurement of field-induced spectral changes provides a useful way for determining excited-state properties like dipole moments, polarizabilities, parameters describing intermolecular interactions, and the orientation of the 0022-3654/84/2088-lO94$01.50/0

transition dipole moments. The theoretical basis necessary for such an analysis has been developed by Liptay’ and Lin.2 Most investigations of chromophoric molecules in condensed media have hitherto been made in nonpolar liquids.’~~ Polymers like polyethylene and polypropylene can also be used as matrices, which has been shown r e ~ e n t l y . ~One advantage with this technique (1) W. Liptay, Ber. Phys. Chem., 80,207 (1976). (2) S.H.Lin, J . Chem. Phys., 62,4500 (1975). (3) A. Davidsson, Chem. Phys., 35,413 (1978).

0 1984 American Chemical Society

Formation of vertically oriented aromatic molecules chemisorbed on

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