1794
F. L. MINN,J. P. PINION, AND N, FILIPESCU
mers are formed. Thus the fact that the dipole moments in dilute cyclohexane solution are smaller than those in aromatic solvents, and show a marked increase with increased alcohol concentration, is consistent with a greater degree of alcohol self-association in cyclohexane. Solute-solvent interactions are minimal in the latter inert solvent, and alcohol self-association is the only important interaction. I n the aromatic solvents solute-solvent interactions are important and have a disassociating effecton the alcohol m~1timers.l~Somewhat larger dipole moments are to be expected in the aromatic solvents because of the solute-solvent interactionsZ6which are absent in cyclohexane solution.
Both relaxation times appear to be a composite of several processes and it is remarkable that consistent analyses in terms of two apparently Debye-like absorptions have been possible. The situation is somewhat analogous to pure liquid water2’ and alcoholsa both of which have a complex molecular structure but show a single deb ye-type relaxation.
(26) J. Crossley and C. P. Smyth, J. Amer. Chem. Soc., 91, 2482 (1969). (27) M. Davies, “Some Electrical and Optical Aspects of Molecular Behaviour,” Pergamon Press, London, 1965.
Excimer Model for Fluorene and Dibenzofuranl by Fredrick L. Minn,* Jack P. Pinion, and Nicolae Filipescu Department of Chemistry, The George Washington University, Washington,D. C. ,90006 (Received September 81, 1970) Publication costs assisted by the
U.S. Atomic Energy Commission
SCF-CI-MO calculations were carried out for excimers of naphthalene, fluorene, and dibenzofuran in a model which allowed relative rotational, translational, and shearing motions of the two parallel-stacked monomer molecules. Within the approximations only changes in the distance between the planes of the monomers were found to affect the predicted energy levels. An interplanar distance of ~ 3 . A 5 was found to give agreement with experimental excimer fluorescence wavelengths. The calculated oscillator strengths for excimer emission were all small (f < lo-*) and predicted polarizations were parallel to the in-plane long axis of monomer.
The original attempts to correlate experimentally observed red-shifted excimer emission with predictions from an acceptable theoretical model were based on exciton splitting and charge-resonance concept^.^^^ Nore recent calculations involving configuration interaction between molecular-exciton and charge-resonance states gave substantially improved Most previous theoretical work on excimers has been done on n a ~ h t h a l e n e . ~ J - lIndependent ~ of Horrocks and Brown,ll we recently observed pronounced excimer emission from dibenzofuran and substituted fluorenes. l2 Since these have lower symmetry than naphthalene or pyrene and since dibeneofuran is heteroaromatic, we were interested in testing a theoretical model on systems described by two such molecules. Here we report the results of a “supermolecule” modification to a semiempirical molecular orbital (MO) method, first suggested by Chandra and Lim,13applied to excimer aggregates of naphthalene, fluorene, and dibenzofuran. Excimer Model Since the crystalline emission of pyrene does not differ from that of its solvated excimers,14 Barnes and The Journal of Physical Chemistru, Vol. 76, No.12, 1971
Birks15 suggested that the excimer configuration comprises two adjacent molecules in parallel planes, as in the crystalline lattice. Consequently, our model places (1) Taken in part from the Ph.D. Thesis of J. P. P. at The George Washington University, 1970. (2) Th. Forster, Pure A p p l . Chem., 4, 121 (1962). (3) J. Ferguson, J. Chem. Phys., 28, 765 (1958). (4) E. Konijenberg, Dissertation, Freie Universitat Amsterdam, 1963. (5) T. Aeumi and H. Aeumi, Bull. Chem. SOC.Jap., 39, 1829, 2317 (1966) ; 40, 279 (1967). (6) J. N. Murre11 and J. Tanaka, Mol. Phys., 7, 363 (1964). (7) T. Aeumi, A. T. Armstrong, and S. P. McGlynn, J. Chem. Phys., 41, 3839 (1964). (8) S. P. McGlynn, A. T. Armstrong, and T . Asumi in “Modern Quantum Chemistry. Part 111,” 0. Sinanoglu, Ed., Academic Press, New York, N. Y., 1965, p 203. (9) B. N. Srinivasan, J. V. Russel, and S.P. McGlynn, J . Chem. Phys., 48, 1931 (1968). (10) Th. Forster, Angew. Chem., Int. Ed. Engl., 8, 333 (1969). (11) D. L. Horrocks and W. G. Brown, Chem. Phys. Lett., 5 , 117 (1970). (12) J. P. Pinion, Ph.D. Thesis, The George Washington University, 1970. (13) A. K. Chandra and E. C. Lim, J. Chem. Phys., 48,2589 (1968).
1795
EXCIMER MODEL FOR FLUORENE AND DIBENZOFURAN
...,... .,
't;
:m t *'
Figure 1.
0'
... .......,.......t .**
.
*.
*.
Assumed geometry of excimers.
the two molecules atop one another but displaced from perfect superposition, as shown in Figure 1. Intramolecular bond distances and angles were taken from X-ray crystallographic data for naphthalene16s17and fluorene;'g those of dibenzofuran were estimated by analogy with fluorene.lD The two parallel molecules were taken to be displaced along the short axis by about one carbon-carbon bond length, an arrangement suggested by the excimeremitting pyrene crystal where such a geometry exists. This configuration both minimizes intermolecular ?relectronic repulsion and maintains a high degree of symmetry. For dibenzofuran, the initial relative orientation of the two molecules was chosen such that their electric dipole moments were opposed. This geometry not only reduces the oxygen lone-pair repulsions but also staggers the aromatic 2pn orbitals. The effectsof changes in the distance between the two photoassociated molecules and of in-plane translations and rotations on the relative disposition of energy levels in the excimer and on predicted polarization of the transition dipole moments were examined.
Method of Calculation Energy-level diagrams for monomer and excimer, together with polarizations and oscillator strengths, were calculated by a semiempirical 310-SCF-GI procedure. For the excited dimers the a electrons of the two-component chromophores were treated as parts of a single extended system delocalized over both component molecules. The technique includes effects of local excitations and of electron transfer in a natural way. Unlike the method outlined by Chandra and Lirn,lo ours uses self-consistent-field orbitals instead of
Huckel MO's as basis. We expected to obtain reliable results on fluorene and dibenzofuran with SCF-MO's without having to include a large number of interacting configurations, since SCF-MO's give the ground-state energy accurate to second order with respect to oneelectron-excited configurations. Empirically in this laboratory and theoretically in others,20addition of more than the few lowest-energy configurations brings about very small changes in the smallest excitation energy; while the higher excited states could probably be improved by use of more extended CI, the concern of this work is with the excimer (lowest level) emission. I n addition, SCF orbitals, which are better approximations to actual wave functions than are HMO's, are known to give more accurate results when used for evaluating spectral properties of heteroaromatic or nonalternant systems.21*22For the alternant, highly symmetrical naphthalene excimer, our results were close to those of Chandra and Lim. The usual LT--?r separation and neglect of differential overlap were assumed. Interactions between the two ?r systems were introduced by the electrostatic repulsions between all pairs of contributing atoms and by partial bonding between sites of the two molecules near one another. Specifically, the electron-repulsion integrals y were evaluated by the Mataga-Nishimoto and the nonzero bond resonance integrals p by the distance-dependent equation, p = 9.811 exp (- 1.032~), whose form was suggested by Pariser and Parr;24this equation was constructed to give literature values for benzene and ethylene a t their experimental C-C distances.2sj26 Inter-ring p's were further adjusted to allow for the v interaction of the two p orbitals. Thus, denoting components of the unit vector between the two cores as x, y, and x , we have
+ (Y2 + m,, + (Y2 + + + + 4Yu,u,) P
Y =
X4YUUUU
=
z2Puu
x2)2Y,,71,
X2(Y2
~2)(2Yuu?r*
(14) J. B. Birks, A. A. Kazzaz, and J. A. King, Proc. Roy. Soc., Ser. A , 291, 568 (1966). (15) R.L.Barnes and J. B. Birks, ibid., 291, 570 (1966). (16) A. I. Kitaigorodskii, "Organic Chemical Crystallography," Consultants Bureau, New York, N.Y.,1955. (17) J. Trotter, Acta Crystallogr., 16, 605 (1963). (18) D. M. Burns and J. Iball, Proc. Roy. Soc., Ser. A , 227, 200 (1955). (19) The results of the calculation are insensitive to slight variations
in the atomic coordinates. (20) For example, E. Weltin, J. P. Weber, and E. Heilbronner, Theoret. Chim. Acta, 2, 114 (1964). (21) J. Fabian, A. Mehlhorn, and R. Zahradnik, J . Phys. Chem., 72, 3975 (1968). (22) M.Tichy and R. Zahradnik, ibid.,73, 534 (1969),and references therein. (23) M. Mataga and K. Nishimoto, 2.Phys. Chem., 13, 140 (1957). (24) K.Pariser and R. G. Parr, J . Chem. Phys., 21, 767 (1953). (25) M.Klessinger, Theoret. Chim. Acta, 5, 236, 251 (1956). (26) J. E.Bloor, B. R. Gilson, and N. Brearley, ibid., 8, 35 (1967). T h e Journal of Physical Chemistry, Vol. 76,No. 12, 1971
1796
F. L. MINN,J. P. PINION, AND N. FILIPESCU
3 4
5
3W
350
4W
zJ5L 450
300
350
400
450
WAVELENGTH, nm
Figure 2. Total emission spectra (uncorrected). a, 2-methoxyfluorene in dioxane a t 22’ excited a t 300 nm; 1-5, 0.05, 0.10, 0.20, 0.32, and 0.43 M. b, 2-fluorofluorene in dioxane a t 22” excited a t 293 nm; 1-5, 0.10, 0.21, 0.60, 0.81, and 1.16 M . c, dibenzofuran in dioxane a t 22” excited a t 278 nm; 1-6, 0.16, 0.21, 0.24, 0.60, 1.08, and 1.64 M. d, dibenzofuran in methylcyclohexane, 1.05 M , a t different temperatures; 1-4, 31, 11, 7, and OD, respectively. Inserts represent the excimer component of emission obtained by subtraction.
where the subscripts designate components obtained from orbitals that point directly toward one another ( g ) or are parallel to each other and perpendicular to the line of centers (T). Since numerical integration of the carbon p’s gives values which differ by only a few per we took pcu = prr;the various directional combinations of the y’s (yuuou,ynarr, etc.) were also taken to be equal. Carbon valence-state ionization potentials and one-center repulsion integrals for naphthalene were taken from the literaturejZ4the corresponding values for fluorene and dibenzofuran were taken to be the same as those for ~ h e n a n t h r e n e . ~For ~ the etheroxygen ionization potential and one-center integrals we used 33.9 and 16.2 eV, r e s p e c t i ~ e l y . ~The ~ SCF ground-state wave functions were allowed to interact with the lowest six excited-state configurations. A modification to Bloor and Gilson’s closed-shell SCF-CI programzswas used on an IBRl360/60 computer. The two parallel ring systems were displaced relative to one another by (1) variation of the interplanar distance, (2) rotation of one molecule in plane about its center, or (3) translation of one molecule along its short axis in a motion resembling shearing. The expected excimer emission energies, oscillator strengths, and polarizations were thus calculated.
Results and Discussion The monomer and excimer emissions for 2-methoxyfluorene, 2-fluorofluorene, and dibenzofuran, as determined from variable concentration and temperature spectra, are shown in Figure 2. The inserts represent the excimer emission alone, obtained by subtraction of a The Journal of Physical Chemistry, Vol, 76, rVo. 12, 1971
I
I
I
Figure 3. Variation in configuration energy with interplanar separation distance for the naphthalene excimer: 1, ~ I - Zh 0 , 1 1 ; 2, $g,il - ~ O , I Z ; 3, rC.s,11 t ~ O J S ; 4, h 1 1 ho,ia; Ic.10,~~. Energies are relative to the 5, h , 1 1 ~ I Z 6,; +SJZ dimer ground state.
+
+
-
dilute normalized emission of the monomer. These spectra are included mainly to exhibit the excimer emission of these compounds and to locate the wavelengths at which the two component fluorescences are found .30-33 Some general remarks can be made regarding the influence of relative displacements of monomer molecules on predicted excimer emission energy. The calculations indicate that both the in-plane rotation of one molecule and translation along its short axis (shearing) have only a very limited influence on the dimer energy levels. Thus, x displacements from 1.3 to 3 A in any of the compounds tested caused a maximum shift of 2500 cm-’ in calculated excimer levels. In-plane rota-
(27) C. C. J. Roothaan, J. Chem. Phys., 19, 1445 (1951). (28) M. J. S. Dewar, “The Molecular Orbital Theory of Organic Chemistry,” McGraw-Hill, New York, N. Y., 1969, p 380. (29) Quantum Chemistry Program Exchange, Indiana University, Program 71.3. (30) The emission spectra of substituted fluorenes are used instead of that of fluorene because the excimer fluorescence of the former is far more pronounced than that of the unsubstituted molecule, which is not unusual. See, for example, ref 31-33 for a compa;ison of the excimer emission of naphthalene and methylnaphthalenes. (31) Th. Forster, Pure A p p l . Chem., 7, 73 (1963). (32) B. Stevens and T. Dickenson, J . Chem. Soc., 5492 (1963). (33) J. B. Birks and J. B. Aladekomo, Spectrochim. Acta, 20, 15 (1964).
1797
EXCIMER MODEL FOR FLUORENE AND DIBENZOFURAN
MONOMER
MONOMER
EXCl MER
EXCIMER
5.0 -
?i
& 4.0a W
Figure 4. Correlation diagram for the naphthalene monomer-naphthalene excimer energy levels.
MONOMER
*6,8'
5.0
A
2 >-
CL7
4,O
z W YBSORPTION
3.0
II
Figure 6. Correlation diagram for the dibenzofuran monomer-dibenzofuran dimer energy levels.
yr,,, to make their ratio with yTIIW, roughly the same as the values obtained from carbon 4{ wave functionsa4 led t o nonconvergence of the self-consistency iterations or to energies two orders of magnitude too large. We therefore settled on the equal-y approximation given in the previous section by the following rationale. The value of yaWwais given by a formula depending only on distance and the value of the one-center repulsion integral, but the one-center integrals (2pZ2p,12p,2p,) and (2p,2pg]2p,2p,) are equal and hence the empirical formulation of any y is probably not too different from that of Yanarr* I n contrast with the shearing and rotation motions, variation of the interplanar separation produces profound alterations in expected energy levels. Again, this was found to be the case for naphthalene, fluorene, and heteroaromatic dibenzofuran. To illustrate the magnitude of this effect, we have plotted in Figure 3 the predicted energy levels of the excimer, including the expected emission energy, as a function of the interring distance for naphthalene. Since curve 1 represents the variation of the lowest excited state, it also represents possible variations of energy for excimer fluorescence t o a repulsive portion of the monomermonomer ground state. From this diagram one can see that the experimentally observed excimer ymax380 nm corresponds to an intermolecular separation of 3.4 A. It is interesting that the best correlation for fluorene and for dibenzofuran isofound at essentially the same interplanar distance of 3.5 A. Physically, this is indeed
2 FLU OR ESCENCE
0
"
I I
EXCIMER
'5,7
'6,7(B2u)
I
Figure 5. Correlation diagram for t h e fluorene monomer-fluorene excimer energy levels.
tions of up to h45" from the staggered relative positions of Figure 1were found to affect the energies of predicted excited states even less, no more than 800 cm-I. These are principally but not completely artifacts of the calculation. The large inter-ring C-C distances, coupled with the assumptions of equal p's and y's, give energies which are sensitive to distance but almost invariant with angle. Attempts t o correct this insensitivity by altering the quantities yeeee, yrTar, and
(34) J. Jortner, S. A . Rice, J. L. Kate, and S.-I. Choi, J . Chem. Phys., 42, 309 (1965).
The Journal of Physical Chemistry, Val. 76, No. 12, 1971
1798
L O W ~ LM. L SCHWARTZ AND LELAND 0.HOWARD
sensible, since one would expect similar types of bonding interaction. Figures 4, 5 , and 6 are energy level diagrams for monomer and excimer calculated for naphthalene, fluorene, and dibenzofuran, respectively, for the geometry giving best agreement with spectral data. The SCF levels were numbered from the lowest occupied to the highest vacant orbital; CI wave functions correspond to one-electron excitations from the ground state, the first subscript designating the vacated orbital and the second the terminal. I n the correlation diagrams the ground states of monomer and excimer were placed at zero energy. Excimer fluorescence was as-
sumed t o occur in times short compared to the separation motion of the repulsive ground-state monomers. Although informative, the disposition of the higher excited levels of the excimer cannot easily be verified experimentally. The present theoretical analysis also predicts that the excimer emission should be weak (near-zero oscillator strength) and that it should be polarized parallel to the in-plane long axis of the monomer.
Acknowledgment. This work was supported in part by the U. S. Atomic Energy Commission under Contract AT-(40-1)3797.
Conductance Study of Squaric Acid Aqueous Dissociation by Lowell M. Schwartz* and Leland 0. Howard Department of Chemistry, University of Massachusetts, Boston, Massachusetts
03116
(Received January 7, 1071)
Publication costs borne completely by T h e Journal of Physical Chemistry
The primary dissociation constant of squaric acid in aqueous solution has been measured conductometrically and found to be pK1 v 0.5 at 25'. The precise value depends on which semiempirical equation is used to predict the dependence of equivalent conductance on concentration. Measuring pK1 as a function of temperature yields AH1" of about -1.5 kcal/mol and A&' about -7.5 cal/mol deg. These thermodynamic functions, when compared with corresponding values for the aqueous dissociation of weak organic acids, lead to the conclusion that the unusual strength of squaric acid is attributable to a relatively small entropy change accompanying the primary dissociation.
Introduction 1,2-Dihydroxycyclobutenedione,commonly known as %quaric acid" (HzSq),is a member of a series of cyclic oxocarbon dibasic acids whose dianions are strongly stabilized by n-electron delo~alization.~This stability is believed to account for the unusually high acid strengths of some of these acids in aqueous solution. The primary dissociation constant of H,Sq has been estimated from pH potentiometric measurements variously as pK1 = 1.7 f 0.3,2 1.2 f 0.2,3and 0.55 h O.lj4 at room temperature. The relatively large uncertainties in these values are due to difficulties inherent in the potentiometric method as applied to highly dissociated acids. To obtain accuracy in pK1 by this technique requires extremely pure reagents and also highly concentrated solutions to ensure a reasonable concentration of undissociated acid. The limited solubility of H2Sq in water precludes reaching high concentrations and consequently the pK1 values are uncertain. I n this work we seek to avoid some of these difficulties by using conductance measurements to determine pK1. We were encouraged to attempt this approach by the T h e Journal of Ph.ysica1 Chemistry, Vol. 76,No.12, 1971
results of Darken6 working on oxalic acid which is similar to squaric acid in its aqueous dissociation. Patton and West6 have summarized the reported dissociation constants of the oxocarbon acid series and note that whereas Huckel molecular orbital calculations predict that squarate ion (Sq2-) enjoys a greater delocalization energy than croconate ion (C&62-) , croconic acid is stronger (pK1 = 0.32)' than squaric acid. Our determination of HzSq (pK1 = 0.55) was done after Patton and West made their observation and this lower value, if indeed it is more reliable than those previously reported, tends to reduce the degree of discrepancy. Nevertheless, it is well established that dissociation equilibria depend in a major way on solute-solvent in(1) R . West and D. L. Powell, J . Amer. Chem. Soc., 85, 2577 (1963). (2) D. T. Ireland and H. F. Walton, J . Phys. Chem., 71,751 (1967). (3) D. J. MacDonald, J . Org. Chem., 33, 4559 (1968). (4) L . M . Schwartz and L. 0. Howard, J. Phys. Chem., 74, 4374 (1970). (5) L. S. Darken, J . A m e r . Chem. Soc., 63, 1007 (1941). (6) E. Patton and R. West, J . P h y s . Chem., 74, 2512 (1970). (7) B. Carlqvist and D. Dyrssen, Acta Chem. Scand., 16, 94 (1962).
