Reprinted from THE JOURNAL OF CHEMICAL PHYSICS, Vol. 47, No. 11, 4446-4450, 1 December 1967 Printed in U. S. A.

Spectroscopy of EDA Complexes at High Pressures. IV. Absorption of Several Haloquinone Complexes in Polymer Matrices*

H . W. OFFEN AND T. T. NAKASHIMA

Department oj Chemistry, University oj CaUjornia, Santa Barbara, Catijornia

(Received 17 July 1967)

~veral ~aloquinone complexes are subjected to high pressures (0-20 kbar) , and the resulting effects are registered 1D the charge-transfer (CT) absorption spectra. The observed shift in the CT band maxima are discussed in terms of the various interaction terms, including CT forces, which contribute to the displace­ment of the ground relative to the excited CT state of the EDA complex. The results indicate that the stronger complexes show the smaller red shift or even a blue shift in this pressure range. CT intensities in the 1-20-kbar interval are observed to change by -15% to +40% in this group of related complexes.

INTRODUCTION

The spectroscopic study of electron-donor-acceptor (EDA) complexesl is widely used to understand the charge-transfer (CT) process. The terms EDA com­plex and molecular complex are used interchangeably to designate paired aggregates which show a new elec­tronic absorption due to an intermolecular transition. Mulliken's2.8 description of CT forces in terms of a no-bond and a dative bond structure has proven to be essentially correct, although many (MO) refinements can be introduced in computing CT transition ener­gies, binding energies, and intensities. Many spectro­scopic studies have been confined to the measurement of thermodynamic quantities in liquid solutions. These studies are subject to many errors, as has been recog­nized repeatedly,4 and do not represent an ideal ap­proach to understanding CT forces. Solvent-shift stud­ies6 are equally unsuccessful in unscrambling the finer details of CT interactions, although the comparison of spectra observed in nonpolar solutions and in the vapor phase has resulted in important conclusions6•7

about the nature of CT forces. The present series of high-pressure spectroscopic

studies seeks to explore another approach to an under­standing of CT interactions because intermolecular phenomena should be particularly sensitive to the pressure parameter. This report develops a qualitative orientation regarding pressure shifts and applies these ideas to the spectral shifts in several haloquinone com­plexes. This group of 71' acceptors has received fre­quent attention in the spectral studies of 71' , 7r-molecular

complexes, especially from biologically oriented chem­ists.8 •9 Most acceptors in this class form "loose" or weak molecular complexes with aromatic hydrocarbon donors. Previous spectral studies of the haloquinone complexes under pressure are limited to measurements made in Drickamer's laboratorylG-12 on crystalline com­plexes. Papers Ips and lITH of the present series employ matrices to note the effect of the compressed "solid solvent" on EDA complexes involving TCNE and TCPA, respectively, as the acceptors of charge from aromatic hydrocarbons.

RESULTS: THE PRESSURE SHIFT

Five acceptors used in this work were the related sub­stituted benzoquinones; fluoranil, chloranil, bromanil, 2,5-dichloro-p-benzoquinone (2,5-DQ) and 2,3-di­chloro-5 ,6-dicyano-p-benzoquinone (D DQ) . The com­mercially available chemicals were recrystallized from benzene (and sublimed in the case of fluoranil and 2, 5-DQ). Bromanil was prepared by a modification of Ling's method.15 The acceptors dissolved in acetone were easily studied in the cellulose acetate (CA) ma­trix except for DDQ where a reaction ensued. It was found that DDQ in polymethylmethacrylate (PMMA) was stable over a sufficient period of time for spectral studies at room temperature. The polymer matrix was then PMMA for DDQ and CA for the remaining halo­quinone acceptors. Figure 1 illustrates the effect of pressurel6 on the acceptor absorption for the chloranil

8 R. Foster and P. Hanson, Trans. Faraday Soc. 60, 2189 (1964) .

t R. Beukers and A. Szent-Gyorgyi, Rec. Trav. Chim. 81, 255 • This work was supported in part by the U.S. Office of Naval (1962).

Research. 10 D. R. Stephens and H. G. Drickamer, J. Chern. Phys. 30, 1518 1 G. Briegleb, Elektronen-Donator-Acceptor-Komplexe (Springer- (1959).

Verlag, Berlin, 1961). 11 R. B. Aust, M. S. thesis, University of Dlinois, 1962. 2 R. S. Mulliken, J. Am. Chern. Soc. 74, 811 (1952); J. Phys. 12 W. H. Bentley and H. G. Drickamer, J. Chern. Phys. 42, 1573

Chern. 56, 801 (1952). (1965) . a R. S. Mulliken, J . Chim. Phys. 61, 20 (1964); R. S. Mulliken, 13 H. W. Offen and A. H. Kadhim, J. Chem. Phys. 45, 269

Rec. Trav. Chim. 75, 845 (1956) . (1966). 4 For example, P. J. Trotter and M. W. Hanna, J. Am. Chern. 14 H. W. Offen and J . F. Studebaker, J . Chem. Phys. 47, 253

Soc. 88, 3724 (1966). (1967). & H. W. Offen and M. S. F. A. Abidi, J. Chem. Phys. 44,4642 1& C. L. Jackson and E. K. Bolton, J. Am. Chern. Soc. 36,301

(1966). (1914). 8 J. Prochorow and A. Tramer, J. Chern. Phys. 44,4545 (1966) . 18 The high-presSl,lre technique has been described in Paper Il 7 P. J. Trotter, J. Am. Chem. Soc. 88, 5721 (19~) , (Ref. 13). .

4446

4447 S P E C T R 0 S COP Y 0 FED A COM P LEX E SAT HI G H PRE S SUR E S. I V

TABLE 1. Cornputed electron affinities of haloquinone acceptors relative to chIoranil.

Pyrene Phenanthrene

EA" CCI, CA CCI. CA

1. 2,5-DQ 1.12 1.11 1.11 1.12 1.13 2. Fluoranil 1.26 1.31 1.20 1.22 3. Brornanil 1.37 1.44 1.37 1.38 1.37 4. DDQ 1. 95 ••• b .... b.o 1.82 1.78-

" Reference 29. b Very broad band. • The plastic is PMMA.

molecule. It is seen that the n"4l'* bandI7 ,18 becomes a shoulder at higher pressures, presumably because the larger red shift and band broadening of the more in­tense 11'~* band overshadows the long-wavelength n"4l'* band.ls The base line observed for the blank (CA) in the sample chamber of the high-pressure optical cell is also shown in Fig. 1.

EDA complexes were formed using these acceptors and several 11' donors in the two polymer matrices. The molal concentrations were 5 X lO- L 1 X lQ-2m acceptor and 9X Hr-L 3.2 XIQ-Im donor. The upper limit for the concentrations is set by solubility, and the lower limit is governed by the consideration that an absorbance of 0.4-0.6 is desirable for the ,....,Q.020-in.-thick plastic films. Excess donor is found to give the most distinct CT bands, although in many cases the CT band and acceptor band continued to overlap. The acceptor ab­sorbance, measured independently at the same isobars, is subtracted from the complex absorbance to yield the CT band only in those cases where the CT band is Dot clearly developed. This procedure assumes known concentrations and a similar pressure behavior of the n-?l'* band in the absence and presence of a donor. In addition, the sloping base line (Fig. 1) made the over­lapping band analysis difficult and introduced large errors in the pressure shifts so determined. The loca­tion of CT band maxima are similar to those measured in liquid solvents,8,»-27 when allowance is made for solvent shifts. The CT transition energy is predicted, according to Eq. (6) (below), to be proportional to

17 L. E. Orgel, J. Chern. Soc. 1955, 121. 18 K. H. Hausser and R. S. Mulliken, J. Phys. Chern. 64, 367

(1960) . 11 A. H. Kadhim and H. W. Offen (unpublished work). to S. K. Chakrabarti and S. Basu, Trans. Faraday Soc. 60, 465

(1964) . 21 M. Kinoshita, Bull. Chern. Soc. Japan 35, 1609 (1962). 22 R. Foster, Tetrahedron 1960,96. 23 G. Briegleb and J. Czeka)]a, Z. Phys. Chern. (Frankfurt) 24,

37 (1960) . 2C G. Briegleb, J. Czekalla, and G. Reuss, Z. Phys. Chern.

(Frankfurt) 30, 313 (196l) . 26 G. Briegleb, J. Czekalla, and G. Reuss, Z. Phys. Chern.

(Frankfurt) 3D, 333 (1961) . 26 M. J. S. Dewar and A. R. Lepley, J. Am. Chern. Soc. 83, 4560

(1961) . 27 R. D. Srivastave and G. Prasad, Spectrochim. Acta 22, 1869

(1966) ,

trans-Stilbene Indole HMB

CCI. CA CCI. CA CCI. CF

0.90 1.15 1.14 1.29 1.16 1.08 1. 21 1.22 1.25 1.66 1.21 1.16 1.39 1.44 1.43 1.10 1.29 1.32 1.77 1.86' ••• d 1. 76

d Reaction.

I n- EA .8,9 ,26,28 This correlation was tested for several complexes. Table I shows the computed electron af­finities EA for four acceptors relative to chloranil EA = 1.37.29 It is seen that the order in plastics is consistent with the EA values deduced by Briegleb.29

The magnitude of the pressure shift is summarized in Table II. Smaller uncertainties are quoted for the well-developed CT band maxima. The observed spec­tra are illustrated in Fig. 2 for a few representative cases. The data usually show a red shift in the band maxima except for DDQ complexes where a net blue shift is observed. In most cases the rate of shift per pressure interval decreases at higher pressures. The pressure shift of the bromanil complexes is slightly greater than the shift of the corresponding chloranil complexes at 20 kbar. By comparison, the crystal spectra show the opposite response to pressure.ll The pressure shift of the (corrected) higher energy compo­nent of the multiple CT band observed for pyrene­tetrahaloquinone complexes24 has the same or a slightly smaller magnitude than the lowest energy CT band listed in Table II and shown in Fig. 2. The indole­bromanil complex possesses a CT band with two ill­defined maxima separated by ,....,2200 cm-I . At higher pressures, this apparent splitting is smeared out. The DDQ complexes manifest an unusual pressure response, illustrated in Fig. 2, in that the shorter wavelength intensity decreases above 480 m", and the steep ab­sorption edge becomes a gradual, structured absorption tail which cannot be attributed to the DDQ compo-

w o

! ~ 0 .4

'" III ct

0 .2

\ \ "-

CHLORANIL

..... , , \~20KBAR

"- ...... _------ ----BASELINE

400 600

WAVELENGTH (m~)

FIc. 1. Absorption of chloranil in CA at 1 and 20 kbar.

18 R. Foster, D. L. Hammick, and B. N . Parsons, J. Chern. Soc. 1956,555.

21 G. Briegleb, Angew. Chern. Intern. Ed. Eng!. 3, 617 (1964) .

H. W. OFFEN AND T. T . NAKASHIMA 4448

TABLE II. Pressure shifts t!J.v and absorbance ratios· of CT band maxima at 20 kbar.

t!J.v (em-I) A (20)/A (1)

2,S-DQ

Pyrene -1200±300 Phenanthrene -14S0±400 trans-Stilbene -1800±400 1.08 HMB -1950±300 1.37 Indole -1SS0±600 1.28

Chloranil Pyrene -1100±300 1.24 Phenanthrene -8S0±300 1.10 trans-Stilbene -1100±200 1.28 HMB -9S0±200 1.36 Indole -SOO±SOO

DDQ

Phenanthrene +700±500 ",0.7 trans-Stilbene +100±300 0.96

• The estimated uncertainties are ±O.IO units.

nent. Similar behavior in the case of phenanthrene­DDQ results in the submergence of the CT band into the absorption tail at shorter wavelength. Further study may reveal the possible connection between the unusual intensity behavior and the chemical reactivity of the DDQ complexes. The 2,S-DQ complexes, which contain the acceptor of lowest EA , show the larger red shifts. However, general trends are more difficult to uncover than the obvious feature that each donor­acceptor pair has an individualistic pressure depend­ence, as previously observed for aromatic hydrocarbon­TCNE complexes.13

DISCUSSION: THE PRESSURE SHIFT

The term "pressure shift" is used, analogous to "solvent shift" in solution studies, to mean the spec­tral displacement, i.e., a red or blue shift of the ab­sorption band, produced by increasing pressures on the solute plus solvent system. In many respects, the pressure shifts can be interpreted within the same frame­work customarily used in solvent-shift theories.30-34 It is immediately clear, then, that the experimentally observed effects can be extremely varied and only qualitative theories have applicability to several sets

ao W. W. Robertson, S. E. Babb, Jr. ) and F. A. Matsen, J. Chern. Phys. 26, 367 (19S7).

11 W. W. Robertson, O. E. Weigang, Jr., and F. A. Matsen, J . Mol. Speetry I, 1 (1957).

32 W. W. Robertson and A. D. King, Jr., J . Chern. Phys. 34,1511 (1961) .

13 W. W. Robertson, A. D. King, Jr., and O. E . Weigang, Jr., J . Chern. Phys. 35, 464 (1961).

U O. E. Weigang, Jr., and W. W. Robertson, High Pressure Physics mid Chemistry, R. S. Bradley, Ed. (Academic Press Inc., New York,)963).

t!J.v (em-I) A (20)/ A (1)

F1uoranil

Pyrene -12S0±600 Phenanthrene -1800±300 1.32 trans-Stil bene -300±SOO HMB -1100±400 0.98 Indole -630±300

Bromanil Pyrene -13S0±300 1.24 Phenanthrene -1S00±500 trans-Stilbene -1300±300 1.33 HMB -1200±300 1.34 Indole -1200±300 1.38

of solvents or compounds.3s •36 The same is true for the observed pressure shifts; nonetheless, it is useful to summarize some qualitative ideas in order to under­stand pressure perturbations of the solute's electronic structure in compressed media.

PHENANTI-flENE -FLUORANIL

PYRENE - BROMANIL

I~ ~-Vf CD c(

'" > HMB - CHLORANIL ~ c( .J

'" cr

700 400

WAVELENGTH (mjl)

600

FIG. 2. CT spectra of four haloquinone complexes at 1 kbar and 20 or 2S kbar. Arrows identify the locations of the CT band maxima.

16 H. W. Offen, J. Chern. Phys. 42, 2523 (1965). 36 H. W. Offen and E. H. Park, J. Chern. Phys. 43, 1848 (196S).

4449 S PEe T R 0 S COP Y 0 FED A COM P LEX E SAT H I G H PRE S SUR E S. I V

For nonpolar 7I"-conjugated organic solutes in non- tionality of HOI and S, polar solution, the pressure shift of the solute's longest wavelength electronic transition may be written as f31 = S(k- EI), (5)

&o=p(P)-p(O)=~(s"-s'), (1)

where the pressure shift ~P (cm-l ) is given as the difference of the transition frequency at P and at 0 kbar (1 atm). The quantity ~(s" -s') represents the pressure dependence of the solvation energies in the two electronic states. A considerable body of informa­tion on the pressure dependence of 71", 71"* absorption bands confirms a general red shift with increasing pres­sures. An initial blue shift may arise if the dipole mo­ment is considerably smaller in the excited state.32.33.37.33

The interest in the present work concerns the pres­sure shift of charge transfer (CT) transitions in weak (1: 1) 71", 7I"-molecular complexes. This intermolecular CT transition is expected to depend sensitively on pressure because charge-transfer forces between D and A as well as van der Waals interactions, which were involved in the one-component solutes discussed above, are influencing dIlCT/dP. McGlynn in his review article39

pointed out the danger of naive interpretations for pressure shifts of CT transitions in EDA complexes. However, insight into the role of CT forces in environ­mental perturbation effects may be obtained from a comparison of the pressure shifts in ' one component and two-component systems.

The simplest view of EDA complexes2 considers the resonance interaction between the no-bond structure "lJro(D, A) and one dative bond structure "lJr1(D+-A-) so that the ground and excited states are given, re­spectively, by

"lJrN= a"lJro(D, A) +bW1(D+- A-), (2)

"lJrE= a*"lJrI(D+-A-) -b*"lJro(D, A). (3)

For weak EDA complexes, 11/'-'1, b<O.l, a*"""a, and b*,....,b. Also, the energy of the dative bond structure exceeds that of the no-bond structure, i.e., El»Eo. The expression for the CT transition energy hCPCT of a noninteracting molecular complex is40

hCIICT= [W + EI +f3U (EI- Eo) ]

-[W+Eo-f3N(EI -Eo)]' (4)

where the subscripts identify the two structures, and the two terms represent the relative energies of "lJrE and "lJrN, respectively. W is the energy of the compo­nents at infinite separation. For an assumed propor-

37 E. G. McRae, J. Phys. Chern. 61, 562 (1957) . 38 R. B. Aust, W. H. Bentley, and H. G. Dric.karner, J. Chern.

Phys. 41, 1856 (1964). ~ S. P. McGlynn, Chern. Rev. 58, 1113 (1958). 40 R. S. Mulliken and W. B. Per!'On . Ann Rp.v. Phys. Chern. 13,

107 (1962).

where k»l is the proportionality constant between the interaction matrix HOI and the overlap integral S. A similar expression may be written for the resonance interaction in the ground state.

The term Eo includes the classical electrostatic inter­actions, London dispersion interactions, and steric or repulsive interactions.1·2•4o The intermolecular separa­tion RDA of the D and A components in the complex is about 0.1-0.4 A less than ordinary van der Waals distances, depending upon the strength of the CT forces. Hence, steric repulsion can make a significant contribution to Eo. Hence Eo could be positive, but Eo + f3N (E1- Eo) <Wand would equal the heat of formation of the complex (,....,0.15 eV) in an inert solution. The energy term EI consists of van der Waals interactionsl,4O and other interactions which are not of charge-transfer origin. The other interactions include the exchange interactions due to the formation of a very weak chemical bond, but are dominated by the Coulombic ion-pair attraction e2/ RDA . El is frequently approximated by the expression

C includes all interactions between the molecular pair other than the Coulombic and CT interactions.

When the EDA complex is placed in solution and squeezed, the resulting pressure shift in the CT transi­tion frequency PCT is

(7)

The last term represents the difference in solvation energies of the molecular pair by the surrounding medium at ordinary van der Waals distances R. This term is expected to make a red shift contribution which is larger than ~lJ for one-component solutes [Eq. (1) ] with comparable oscillator strengths31 because J.l.E» J.l.N(~J.I.""'lO D)25 for this CT process.

The first term in Eq. (7) represents the changes in the energy difference between the two structures as pressure is applied to the compressible complex.2 •7.41

A red shift contribution is expected because Eo in­creases and El decreases at higher pressures. The sec­ond term in Eq. (7) arises from CT interactions, i.e., resonance between the two structures, and can be ap­proximated by

41 H. McConnell, B. M. Hoffrnan, and R. M. Metzger, Proc. Natl. Acad. Sci. U.S. 53, 46 (1965).

H. W. OFFEN AND T. T. NAKASHIMA 4450

",0.8

~ :5 0.6 V> II) <t

0.4

.,,-- ..... / ,

/' 298°K "

I ' I '

I ' I ' / ,

" 400 500

WAVELENGTH (mil)

FIG. 3. The absorption spectrum of phenanthrene-<:hloranil in CA at room and near-liquid-nitrogen temperatures.

Since the denominator is seen to decrease for E1> Eo, and the overlap is expected to increase upon compres­sion, a blue shift contribution is predicted for the changing CT interactions under pressure.

These considerations are too crude to predict the relative magnitude of the three contributions to the pressure shift. Since CT forces cause the iT! and iTo state to separate as opposed to the other dielectric effects, experimental data can be employed to diagnose their relative importance. In the event that the effects from CT forces are comparable to, or dominate di­electric effects, a smaller red shift or net blue shift will be observed in the CT transition energy.

The results in Table II indicate that the pressure shift is generally red, indicating that the first and last term in Eq. (7) outweigh the second term. The strong DDQ complexes show the smallest red shift or even a net blue shift. This result is consistent with our con­siderations which suggest stronger contributions from the CT-interaction term when the CT forces are al­ready strong at atmospheric pressure. The stronger the complex, the smaller is E1- Eo, the greater is k and S, and hence the larger the CT forces separating the EE and EN levels and opposing the dielectric red shift. For complexes with predominantly ionic character in the ground state, a blue shift is predicted.6•41 •42 The acceptor 2,5-DQ has the smallest electron affinity in this group and forms the weakest complex. In this case, dielectric effects predominate and the largest red shift at a given isobar is expected, as observed for several donor-haloquinone complexes. The stilbene­fiuoranil complex exhibits an exceptionally small red shift, i.e., large contributions from CT forces. This may be cited as evidence that not only the strenoth

• 0

at atmosphenc pressure must be considered but also the relative size and orientation of the donor-acceptor pair in predicting the shift contribution from Eq. (8). These latter contributions would be especially signifi­cant for TCNE complexes,13 and aid in the interpreta­tion of the unique behavior of the HMB-TCNE rela-

(! K. M. C. Davis and M. C. R. Symons, J. Chem. Soc. 1965, 2079.

tive to other TCNE and HMB-haloquinone complexes. In summary, the observed pressure shifts serve as a guide in estimating the role of CT forces. The weakest complex shows the smallest involvement of CT forces toward the observed pressure shift.

INTENSITIES

The other parameter that is affected by pressure is the absorption strength. As long as the bandwidth is independent of pressure, and this appears to be true for most EDA complexes,1O·U.13 the change in the (cor­rected) extinction at the band maximum can be di­rectly related to the transition moment at higher pres­sures. The absorbance at 20 kbar relative to 1 kbar varied between +40% and -15% for the complexes listed in Table II. The largest decrease in absorption intensity is observed for phenanthrene-DDQ complex. The CT intensity of the HMB-fiuoranil complex de­creases slightly, while the other fiuoranil complexes increase in intensity. It is observed that the percent increase in absorbance is directly proportional to the magnitude of the molar extinction coefficient E (at 1 atm) for those complexes listed by Briegleb and Czekalla.23 On the other hand, the observation E(chlor­anil complexes) > E(bromanil complexes)20·22 appears to be inversely related to the pressure-induced intensifi­cation for these two complexes.

The influence of pressure on the CT transition mo­ment is a direct demonstration of CT forces because 7r, 7r* transition intensities in one-component solute­solid-solution systems are relatively insensitive to pres­sure. Approximate expressions for the CT transition moment reveal a pressure dependence of the coeffi­cients mixing'lro and iT!, RDA and the overlap integral S. According to Trotter,7 the compressibility of weak com­plexes may be the decisive factor in the increase in intensity. The stronger complexes are less compressible and therefore yield a smaller change in E. An actual decrease in E may occur when the mixing coefficients of the no-bond and dative bond structures approach numerical equality, i.e., for complexes with consider­able ionic character in the ground state.

The consequence of compression is not only a de­crease in RDA but also possible reorientation,13 depend­ent upon the relative size of the planar 7r systems. Orientational forces8 are expected to be sensitive to temperature and may be mainly responsible for the large temperature dependence of these complexes. The greatest effect is seen in phenanthrene-chloranil as illustrated in Fig. 3. The effects of the increased den­sity and internal pressure7.14 at low temperatures are comparable to those observed for external pressures as high as 10 kbar.14 The separation of density effects from other thermal effects on spectral intensities will be greatly aided by higher accuracies in the intensity measurements above atmospheric pressures.