-. #4" "'7/ -7";./

OFFE-HW 65-0398 Reprinted from THE JOURNAL OF CHEMICAL PHYSICS, Vol. 42, No. 1, 430-434, 1 January 1965

Printed in U. S. A.

Absorption Spectrum of Solid Anthracene-TNB Complex under Pressure

HENRY W. OFFEN

Chemistry Department, University of -California, Santa Barbara, California

(Received 17 August 1964)

The intermolecular charge transfer (CT) and intramolecular absorption spectrum of the crystalline anthracene-TNB complex as well as (for comparison) the crystalline anthracene L band have been studied in KBr pe~ets ~s a function of pressure to 27 kbar. It is observed that the magnitu"de of the red shift in the band m~a 15 ",1450 em-I at 27 kbar for both the CT band and the L" band, but is only ",300 em-I

for the mtramolecular d~nor. (anthrai.:.ene) absorption at 16 kbar. Broadening of the CT band appears absent, but the molar extinction coefliClent at the band maximum increases by ",50% at 16-kbar applied pressure: The L" band of anthracene broadens but shows only small changes in band height. Plausible explanations of the observed results are discussed.

INTRODUCTION

I T is well known that aromatic hydrocarbons exhibit charge transfer (CT) with many acceptors (Lewis

acids) including 1,3 ,5-trinitrobenzene (TNB) .I The spectral properties of these 7r-molecular complexes at atmospheric pressure have been investigated in solu­tion2 and in polycrystalline form.3 •4 Lower Hochstrasser . " and Reld5 have studied the polarized CT absorption spectrum of the crystalline anthracene-TNB complex.

The effect of high pressures on intermolecular CT absorption has been noted by Ham6 in iodine- aromatic donor complexes dissolved in n-heptane solution (to 2 kbar), by Gott and Maisch7 in TCNE- aromatic donor complexes dissolved in methylene chloride (to 10 ~bar), and by Stephans and DrickamerB in polycrystal­hne hexamethylbenzene (HMB)-chloranil and quinhy­drone complexes (to 50 kbar). These molecular com­plexes usually show a red shift (to lower energies) in the CT absorption band, and enhancement of the band intensity results from changes in the equilibrium con­stant as well as the oscillator strength f of the CT transition. The crystalline HMB- chloranil complex develops a relatively large increase in molar extinction and little broadening.

The KBr pellet technique was used in the present work to study in detail the absorption spectrum of the solid anthracene-TNB molecular complex as a function of increasing pressure (to 27 kbar). This donor- accep-

1 G. Briegleg, Elektronen-Donator-Acceptor-Komplexe (Springer­Verlag, Berlin, 1961).

2 A. Bier, Rec. Trav. Chim. 75, 866 (1956). aM. J. S. Dewar and A. R. Lepley J. Am. Chem. Soc 83

4560 (1961). ' . ,

tor pair is representative of "loose" complexes with weak charge-transfer forces. The spectral range 350- 575 mJ.L allows comparison of the pressure effects on the CT and donor absorption in the solid complex with the solid free donor La band absorption.

EXPERIMENTAL

The spectra at various isobars (and room temper­ature) are measured in an optical cell modeled after the pioneering design of Drickamer, et al.9 Details on the present optical cell are found elsewhere.10 The window as well as pressure-transmitting medium is sodium chloride. The pressure is generated by a 50-ton capacity Blackhawk hydraulic unit located in a "simple squeezer." 11 The sample pressure corresponding to a gi:ren ram pressure, read on a Heise gauge, is deter­mmed from optical observa,tions of phase transitions in potassium halide salts. The beginning of the nuclea­tion of the high-pressure phase is accompanied by a sharp decrease in the light transmittance due to scatter­ing.l2 •13 The onset of decreased transmittance with in­creasing pressure is identified with the volume discon­tinuities measured by Bridgman.I4 Appropriate to our pressure range (0-30 kbar) are the salts KI, KBr, and KCI whose phase transitions occur, respectively, at 17.8, 18.0, and 19.6 kbar. The pressures quoted in this work are probably accurate to within 15%, but the precision is considerably higher. The pressure scale below 10 kbar is less certain in this type of apparatus as evidenced by the fact that the RbCI phase transition apparently occurs at 7 kbar (compare Ref. 13). Below

4 H. Kuroda, K. Yoshihara, and H. Akamatu, Bull. Chem. Soc. g R. A. Fitch, T. E. Slykhouse, and H. G. Drickamer, J . Opt. Japan 35, 1604 (1962). Soc. Am. 47, 1015 (1957) .

t S. K. Lower, R. M. Hochstrasser, and C. Reid Mol. Phys 4 10 H. W. Offen, Ph.D. thesis, UCLA (1963) . 161 (1961). ,., 11 D. T: Griggs and G. C. Kennedy, Am. J. Sci. 254,722 (1956).

6 J. Ham, J. Am. Chem. Soc. 76, 3881 (1954). 12 S. Wlederhorn and H. G. Drickamer, J. Appl. Phys. 31,1665 7 J. R. Gott and W. Y. Maisch, J. Chem. Phys. 39, 2229 (1960).

(1i~: ·R. Stephens and H. G. Drickamer, J. Chem. Phys. 30, (l;lf>, Knof and W. G. Maisch, J. Phys. Chem. Solids 24, 1625

1518 (1959). 14 P . W. Bridgman, Proc. Am. Acad. Arts Sci. 74. 21 (1940). 430 .

431 SPECTRUM OF SOLID ANTHRACENE-TNB COMPLEX

10 kbar no sharp phase transition is known to be suitable for optical calibration.

The radiant energy from a headlight-type incandes­cent lamp or a xenon (Pek Labs) 75-W short arc lamp traverses the sample and is focused on the entrance slit of a 0.5-m Jarrell-Ash scanning spectrometer. The signal from a IP28 photomultiplier tube is amplified and displayed on a Varian G-14 recorder.

Anthracene was purified by repeated recrystalliza­tions and column chromatography following the general procedure employed by Sangster and Irvine.Is TNB was purified by several recrystallizations from glacial acetic acid and ethanol. The crystalline molecular com­plex was recovered as orange needles from evaporated carbon tetrachloride solutions containing equimolar amounts of anthracene and TNB. Samples were pre­prepared by mixing the solid molecular complex or anthracene with infrared-quality potassium bromide in suitable proportions (corresponding to optical densities 0.5-2.4), grinding for 15 sec, and pressing a pellet in a standard die. A rectangularly cut section of the 0.005-0.020-in.-thick pellet is placed in the optical cell. The KBr dispersing medium gives sharper spectra than other media and serves as a useful (and reproducible) internal pressure standard for each run but precludes spectral observations from 19- 23 kbar. The transmit­tance is recorded successively for the blank (KBr) and the sample at each isobar. For comparison the spectrum at atmospheric pressure is recorded before and after each run. The slitwidth is 200 p. for all

3 .6

3.4

w '" 3.2 o oJ

3.0

28 24 20

11 X 10-3 ( em-I)

FIG. 1. Absorption spectrum of crystalline anthracene--TNB at two isobars.

16 R. C. Sangste(and J. W. Irvine, Jr., ]. Chem. Phys. 24, 670 (1956) .

3.6

3.4

3.0

28 20

FIG. 2. Absorption spectrum of crystalline anthracene at two isobars.

measurements. Transmittance ratios are calculated at 1-2-mp. intervals. In the calculation of spectra it is assumed that the number of absorbing particles in the light path remains constant upon compression. The spectra calculated at atmospheric pressure compare favorably in position and general shape with those measured with the Cary 14 spectrometer.

RESULTS

The absorption spectrum of polycrystalline anthra­cene-TNB in the 350-575 mJL range at 1 atm and at 16-kbar pressure is shown in Fig. 1. Noteworthy in the spectrum at atmospheric pressure are the fine structure in the CT band, the considerable blue shift, and reduc­tion in intensity of the longest wavelength donor band relative to the solution CT spectrum. These observations have been noted previously.4.6.16 The 16-kbar spectrum shows a large red shift, an increase in intensity, and a loss of structural detail of the CT band. The donor absorption gives only a small red shift (",,300 cm-I at 16 kbar) which is in stark contrast to the uncomplexed anthracene absorption band shown in Fig. 2. The location and relative intensity distribution within the band compare favorably with thin film measurements by Perkampus.I7 The effect of pressure on the La band in solid anthracene has been observed previously by Wiederhorn and Drickamer.IB From their graph, the average red shift at 27 kbar is estimated to be 1435 cm-1

16 J. Czekalla, Ber. Bunsenges. Physik. Chem. 63,1157 (1959). 17 H. H. Perkampus, Z. Physik. Chern. (Frankfurt) 19, 206

(1959). 18 S. Wiederhom and H. G. Drickamer, J. Phys. Chern. Solids

9,330 (1959).

HENRY W. OFFEN 432

2.8

:;:­Ie .!! '1' 02.4

10 20 30

PRESSURE (kbar)

FIG. 3. Spectral shifts of (a) four vibrational subbands in the La band system of anthracene C ... ) and (b) the CT and intra­molecular absorption of anthracene-TNB (0) in the solid state.

which is in excellent agreement with the value 1450±50 cm-1 obtained in the present work for this isobar.

Less confidence is warranted in the absolute intensi­ties indicated in Figs. 1 and 2. The CT band intensity of logEmax =3.3 at 1 atm reported by Dewar and Lepley' who also used the KBr pellet technique is in sufficient agreement with the present work. However, the anthracene La band is expected to have Em"" exceed­ing 10000,17 while the values found here are 3600±1500 for all runs. In addition to the usual errors accompanying a relatively wide slitwidth, single-beam operation in the present manner, and the use of the KBr pellet technique,19 uncertainties are introduced by the optical cell geometry and the quasihydrostatic compression of the sample and windows, which in a real sense makes each run "unique." Yet the comparison of intensities in a given band system at various pressures is meaning­ful. The anthracene La absorption band (Fig. 2) shows broadening and small changes in peak heights (in essential agreement with Wiederhorn and Drickamer), while the CT band (Fig. 1) increases considerably in intensity but manifests little broadening, in contrast to the spectral behavior of solid free donor. If we assume that the CT bandwidth ~vi remains constant, then the

IJ PI. L. Kronick, H. Scott, and M. M. Labes, J. Chem. Phys. 40,890 (1964).

theoretically significant oscillator strength (f = con­stantXEm""XAvt) would have the same pressure de­pendence as the molar extinction coefficient which is

Ets kbar/ El atm = 1.5±0.3.

Intensity values above 22 kbar are not quoted because they are subject to greater uncertainty since the trans­mittance of the KBr high-pressure phase is not very reproducible.

The frequency shifts for the two systems are sum­marized in Fig. 3. It is seen that the four vibrational sub bands of the La transition show the same shift at high pressures within experimental error. The two peaks ascribed to an intramolecular transition of anthracene in the molecular complex give a small red shift and are no longer discernible above 20 kbar, probably due to a red shift in the TNB absorption edge. The red shift of the CT band maximum ("""1450 cm-I ) is comparable in magnitude to that of the anthracene La band at 27 kbar, but the rate of shift appears to be different at lower pressures. Identification of maxima are more difficult for broad bands, but the shifts are reproducible to ±250 cm-I at 27 kbar.

DISCUSSION

Mulliken20 introduced the valence bond description of charge transfer (CT) and wrote to a first approxima­tion I/;N =al/;o+lnh and I/;B=al/;l-ln/tO(a»b) for the ground and excited states of the molecular complex, where 1/;0 and 1/;1 represent the respective no-bond (D, A) and dative-bond (D+-A-) structures. The energy of the electronic CT transition may be writ­ten as hv(E+--N) =In-EA -e2/ R+C, where In is the ionization potential of the donor, EA the electron affinity of the acceptor, e2/ R the Coulomb energy representing the electrostatic attraction in the dative­bond (transferred electron) state at some average inter­nuclear separation between the donor-acceptor pair, and C contains the difference between all other energy contributions (arising from charge-transfer forces, London dispersion forces, etc.) to the states 1/;1(D+-A-) and l/;o (D, A).

In the absence of any specific theory for the pressure dependence of the electronic levels in solids, the discus­sion of the present work necessarily will be both qualitative and speculative. It is generally thought that in 1: 1 crystalline molecular complexes such as anthra­cene-TNB the predominant intermolecular forces are local and pairwise, so that the crystalline field may be considered as a perturbation on the energy states of the isolated discrete molecular complex. The evidence for this viewpoint stems chiefly from (a) the similarity of the CT band in solution and in the solid state, (b)

20 R. S. Mulliken, J. Am. <;:hem. Soc. 74, 811 (1952); J. Phys. Chem. 56, 801 (1952); J. Chim. Phys. 61,20 (1964).

433 SPECTRUM OF SOLID ANTHRACENE-TNB COMPLEX

the polarization studies of the E<c-N transition by Nakamot021 which are always cited as evidence for the essential validity of Mulliken's theory, (c) the investigation of the Davydov splitting in the CT spectrum,5 and (d) the results of x-ray studies. Wallwork22 has shown that the molecules are stacked alternately in columns and tilted by 6° from the per­pendicular to that axis. The molecular planes for a pair are parallel; their perpendicular seRaration is 3.28 A (considerably less than the usual 3.5-1 van der Waals distance), but the molecule on the other side differs in inclination by 12°. To this extent we may regard the donor-acceptor pair to be discrete. The reduction in the molecular planar separation R produced by 27-kbar external pressure is of course not known since anisot­ropic compressibilities of crystalline complexes are not available. However, the fractional volume change may be estimated from the investigation of various aromatic molecular crystals by Samara and Drickamer23 to be 15% at 27 kbar which may reduce the interplanar distance to Rrv3.1 A.

A prominent effect of the reduced intermolecular separation is the red shift of the CT band maximum. This result is in accord with predictions based on potential-energy curves made by Mulliken20 and based on considerations of the energy states in a one-dimen­sional free-electron description discussed by Shuler.24 In focusing attention on the energy states connected by the E<c-N electronic transition, four conceivable situations leading to a net decrease in their separation may arise from the raising and lowering of both levels with one more so than the other. The closer proximity at high pressures results in greater overlap of the donor and acceptor electron orbitals producing a greater mixing of >/10 with >/11 and thus increasing the charge­transfer forces and hence resonance energies leading to greater splitting of the two energy states. This is opposed by the increased Coulomb attraction and stronger covalent binding in the dative-bond state. If these two effects strike a balance,22 then the possibility emerges that the red shift with increasing pressure may be attributed in large part to conventional London dispersion forces which would stabilize the excited state more so than the ground state, since the dipole moment in the excited state is larger. This would rationalize the similar magnitude of the red shift (i.e., Fig. 3) for the CT band in the solid complex and for the La band in solid anthracene (the oscillator strengtb is about the same for the bands in both systems). The observed difference in the rate of frequency shift of the CT and La bands at lower pressures may be expected from the different crystal structures and compressi-

21 K. Nakamoto, J. Am. Chem. Soc. 74, 1739 (1952). 22 s. C. Wallwork, J. Chem. Soc. 1961, 494. 23 G. A. Samara and H. G. Drickamer, J. Chem. Phys. 37,

474 (1962). uK. E. Shuler, J. Chem. Phys.lO, 1865 (1952); 21, 765 (1953).

bilities of the two systems. For example, the initial compression of the 7I"-molecular complex probably would only reduce the distance between stacks and have less effect on the planar separation. It is tempting to believe that the comparison of the red shift in the band maxima at 1 atm of the complex ( ~JI"'1200 cm-I)4 and of uncomplexed anthracene (~JI"'1500 cm-l)17 in the solid relative to that in solution again points to similar forces responsible in each case. It has been argued4

that the observed red shift supports the conception that the E state in the crystallline complex is localized for a relatively long time since a delocalized electron would decrease the Coulomb energy and hence increase the E<c-N transition energy. The resulting ion pair would see the surrounding molecules as a dielectric medium similar to the situation envisioned for solu­tions.25 Conversely, further experimental work may demonstrate the significance of charge-transfer forces in pure aromatic crystals under pressure in the sense suggested by Mulliken20 and support the solution-solid blue shift of 1750 cm- l for the postulated Q-{) band of the complex.5

Astonishing is the large blue shift (",1200 em-I) of the intramolecular donor absorption in the crystalline complex relative to crystalline anthracene at atmos­pheric pressure. Identification of the complex absorp­tion in the 350-380 mJ,L range with the La band of anthracene is supported by (a) polarization studies of the complex which indicate that this band is pre­dominantly polarized in the plane of the molecule5

and (b) the observed vibrational frequency interval which is only ",100 cm-l less than in crystalline un­complexed donor. Such a small decrease is also found in the phosphorescence spectra of similar complexes.26 A blue shift is also reported for the triplet-singlet emission of this complex in solid solution.27 The blue shift of the intramolecular absorption in the crystal at 1 atm contrasts greatly with Czekalla's16 studies in solid solution in which no displacement of the La band is apparent for the anthracene-TNB complex. The in­creased energy of the 7I"-?T* donor transition probably originates from the close approach of the two molecules. Since the E<c-N and La<c-A transitions may be thought of as originating essentially from the same molecular orbital, it appears likely that the charge-transfer forces lower this highest occupied MO of anthracene. Turning to the small red shift of this band upon compression, three plausible contributing factors may be cited for the great contrast to the pressure sensitivity of the anthracene La band: (a) CT repulsive interaction raising the 71"* state, (b) the decreased transition mo-

26 For a review of most solvent shift theories, see O. E. Weigang, Jr., and W. W. Robertson, in High Pressure Physics and Chem­istry, edited by R. S. Bradley (Academic Press Inc., New York, 1963).

26 J. Czekalla, G. Briegleb, W. Herre, and H. J. Vahlensieck, Ber. Bunsenges. Physik. Chem. 63, 715 (1959).

27 S. P. McGlynn, J. D. Boggus, and E. Elder, J. Chem. Phys. 32,357 (1960).

HENRY W. OFFEN 434

ment resulting in a smaller influence of dispersion forces, and (c) the difference in intermolecular separa­tion and compressibility of the two crystals at a given pressure leading to different spectral shifts per unit pressure interval. It is evident from the work of Wiederhorn and Drickamer18 that at pressures above 60 kbar (where molecular separation may be reduced to values found in the CT complex at 1 atm) , the red shift as a function of pressure levels off considerably.

Variations in band intensities induced by high pres­sures again display an interesting contrast between the CT band and the La band for the two crystals. The broadness of the CT band at 1 atm is related to the shallowness of the ground-state potential-energy curve. It is expected that in sharpening the potential curves, compression would produce little broadening as ob­served. Contributions to broadening and increase in molar extinction, such as contact charge transfer and multiple configurational isomerism which are important in solution, probably have little influence in the solid. The large increase in the CT band height may be attributed to the increased overlap of the donor and acceptor electron orbitals. This increase is not antici­pated for the inplane La transition in agreement with observation. It is possible that the broadening of the La band of anthracene compressed crystals is linked

to the emergence of charge-transfer forces (self­complexes) .20

It is clear that cooperative effects involving more molecules must become increasingly important at higher pressures and that the band theory of solids should provide a more satisfactory description. Conceivably, the interaction between the ionic dispersing phase and the organic microcrystals, so far neglected, may require consideration. However, such effects appear to be of secondary importance in the pressure range accessible in the present work. Further experimental work on the effect of varying lattice parameter on the intermolecular CT as well as intramolecular spectra of solid 1r-molec­ular complexes may not only reveal the applicability of band theory but also establish more information about potential-energy curves. More data are required for the quantitative extension of present theories of electronic levels in molecular crystals to environmental effects.

ACKNOWLEDGMENTS

The financial support of the Research Committee of the University of California, Santa Barbara campus, is gratefully acknowledged. The author thanks W. Hoff­man, S. Schutz, and R. Eliason for the construction and assembly of the high-pressure apparatus.