J . Phys. Chem. 1987, 91, 6359-6364 of the surface containing the quasi-liquid layer and, a t T,, yQL = ycL yLv). If ycv > yQL at T = T,, then it would be a metastable state of the system. This is only possible if there is an activation energy barrier preventing the formation of the quasi-liquid layer. The trajectory plots near T , do not support this hypothesis. The layers near the surface have considerable disorder and large diffusion rates, implying that their structure is very similar to that of a liquid. A second alternative is that ycv lies below yQLnear T , and that the surface region is in a state that is distinct from that of the liquid. Again, however, the trajectory plots and two-dimensional radial distribution functions imply that the structure of the material in the vicinity of the surface is quite similar to that of a liquid. Therefore the free energy of this material must be close to that of the liquid, and the quasi-liquid layer model should hold.
+
XI. Conclusions We have seen that at high temperatures the properties of crystalline interfaces are not all isotropic. Although surface free energies are independent of face, surface stresses are not. Indeed, surface stresses may be compressive or tensile at T,. Further, in all of the systems examined, f # y at any temperature. Surface free energies, surface stresses, trajectory plots, and in-plane correlation functions all indicate that melting does occur
6359
on LJ crystal-vapor surfaces and in at least one LJ grain boundary. Further, a quasi-liquid layer model has been examined by the use of free energies calculated from the MD data, and this model can predict which interfaces should exhibit interface melting. Our results indicate that interface melting is not a true phase transition that can be identified at a temperature below T,. The thermodynamic parameters are continuous functions of temperature, and the quasi-liquid layer must retain some crystalline symmetry because of the finite layer thickness below T,. However, experiments that probe local structure may be unable to distinguish between the quasi-liquid layer at temperatures close to T , and a true bulk liquid. Lastly, note that although our simulations support a divergence T,, we are unable to determine in the layer thickness as T whether this is a logarithmic or algebraic function. What is clear, however, is that measurable quasi-liquid layer thicknesses may only be obtained at temperatures very close to the bulk melting point, T,. An analysis of possible system size contributions to the surface free energies quoted here and an error analysis are given in ref 68.
-
Acknowledgment. J.Q.B. was supported in part by a grant from the Olin Charitable Trust Foundation of Research Corp. and by a contract from the DOE (DE-FG02-85ER45218).
ARTICLES Electronic Structure and Spectra of 9-Anthroic Acid and Its Esters in Supersonic Free Jets V. Swayambunathan and E. C. Lim* Department of Chemistry, Wayne State University, Detroit, Michigan 48202 (Received: November 14, 1986; In Final Form: April 28, 1987)
The fluorescence excitation and dispersed fluorescence spectra of 9-anthroic acid, methyl 9-anthroate, and n-butyl 9-anthroate have been measured under supersonic jet expansion conditions. The fluorescence excitation spectra of the acid and its methyl ester show a long progression in a low-frequency mode that can be attributed to the torsional vibration of the carboxyl group. In all cases, the dispersed fluorescence spectrum is diffuse and greatly Stokes shifted relative to their absorption spectrum. These results are consistent with the “perpendicular” ground-state molecule (in which the plane of the carboxyl group is perpendicular to that of the anthracene ring) changing to a more coplanar form in the electronically excited state. The calculation of the torsional potential on methyl 9-anthroate indicates that the resonance stabilization is canceled by the steric repulsion even before complete coplanarity is achieved in the lowest excited singlet state of this molecule. The study also shows that 9-anthroic acid exists mainly in its hydrogen-bonded dimer form at nozzle temperatures of 150 O C . The probable structure of this dimer in the ground and electronically excited states is briefly discussed.
-
I. Introduction The photophysical properties of 9-carbonyl substituted anthracenes of the type An-CO-R ( R = H, CH3, OH, NH2, etc.) have been the subject of extensive investigations for a long time. The main reason for the interest comes from the fact that the fluorescence behavior of these molecules in fluid solutions depends very much upon the nature of the group R.’v2 The carbonyl anthracenes with a strongly electron-donating R group (e.g., OH, OCH3, NHJ are modestly fluorescent even at room temperature, (1) Swayambunathan, V.: Lim, E. C. J . Phys. Chem. 1985,89,3960, and ref 7 therein. (2) Swayambunathan, V.; Lim, E. C. submitted for publication in J. Phys.
Chem.
0022-3654/87/2091-6359$01.50/0
whereas those with a weakly electron-donating R group (e.g., H, CH3) are fluorescent only at very low temperatures or in rigid matrices. The difference in their fluorescence behavior is thought to be related to the energy level disposition of the lowest energy a a * singlet state relative to the lowest energy na* singlet state which determines the efficiency of S1 So internal conversion. A strongly electron-donating R group is known to cause an increase in the energy of the na* singlet state relative to the lowest energy a a * singlet state, making the SI So internal conversion less e f f i ~ i e n t . ~ -The ~ relatively high fluorescence efficiency of 9-
-
-
(3) Lim, E. C. In Excited States; Lim, E. C., Ed.: Academic: New York, 1977: Vol. 3, p 305. (4) Wassam, W. A., Jr.; Lim, E. C. J . Chem. Phys. 1978, 68, 433.
0 1987 American Chemical Society
Swayambunathan and Lim
6360 The Journal of Physical Chemistry, Vol. 91, No. 25, 1987 carbonyl substituted anthracenes with a strongly electron-donating R group renders fluorescence a convenient probe for studying the electronic structure of these molecules in the ground state (So) and the lowest excited singlet state (S,). The absorption and fluorescence spectra of 9-anthroic acid and its esters were examined in detail in condensed phase by Werner and HerculesS6 The absorption spectra of all these molecules exhibit "anthracene-like" structure and show negligible solvent shift. The minute substituent effect on the absorption spectrum has been attributed to negligible resonance interaction between the carboxyl group and the anthracene ring. This was thought to arise largely due to steric repulsion from the peri-hydrogens which prevent molecular coplanarity in the ground state. Although the absorption spectra of these carboxyl anthracenes show similarities to that of anthracene, their fluorescence spectra in nonpolar solvents are largely Stokes shifted and diffuse. The absence of a mirror-image relationship between absorption and emission has been presented as evidence for the excited-state geometry change along the -COR torsional coordinate in these molecules. The absorption spectrum of 9-anthroic acid is concentration dependent, and the presence of many isosbestic points in the spectrum indicates that a certain equilibrium is involved in the ground state of this system. Suzuki et al.' attributes this to monomer-hydrogen-bonded dimer equilibrium. Although considerable information has been accumulated on the photophysical behavior of these molecules in condensed phase, a detailed study of their electronic structure under supersonic jet expansion conditions is highly desired. The absence of solvent and matrix effects and the added advantages of appreciable rotational and vibrational cooling achieved with such an expansion technique allow for a detailed examination of the electronic structure of these molecules. The preliminary results of the jet spectroscopic study on these molecules were reported in our previous paper.' In this paper, we present the detailed analysis of their jet spectra and discuss the electronic structure of these carboxyl anthracenes under collision-free conditions. 11. Experimental Section 9-Carbonyl substituted anthracenes such as 9-anthroic acid and its esters, 9-anthraldehyde, 9-acetylanthracene, and 9-benzoylanthracene all undergo metal-catalyzed, thermal decomposition, yielding anthracene as the major decomposition product. The fluorescence excitation and dispersed fluorescence spectra of all these carbonyl compounds are therefore virtually identical with those. of anthracene, when studied in a stainless steel nozzle with metal pinholes. All measurements described in this work were therefore carried out in a Pyrex nozzle, where such thermal decomposition does not take place. The continuous-free-jet apparatus used to record the spectra of jet-cooled molecules was described before.* The free jet was created by passing argon (-200 Torr) or helium (- 1500-4000 Torr) gas over carboxylanthracenes heated to 80-150 OC and expanding the gas through an -200-pm pinhole into the vacuum chamber. A Pyrex nozzle was used for the supersonic jet expansion. The excitation source was a Nd-YAG laser pumped dye laser (Quanta-Ray). The output of the dye laser was mixed with the YAG fundamental (1064 nm) with the aid of a Quanta-Ray wavelength extension system (WEX-1) to generate laser pulses in the 340-400-nm region. The focused laser beam crossed the free jet at 1.O cm from the pinhole. A steady walk of the UV laser beam across the free jet was maintained by the autotracking mechanism available with the WEX- 1 system. Laser-induced fluorescence excitation spectra were recorded by collecting fluorescence through an appropriate sharp cutoff Corning or Schott GG filter and focusing onto a PM tube (Hamamatsu R106). The dispersed fluorescence spectra were obtained by collecting
-
( 5 ) Wassam, W. A,, Jr.; Lim, E. C . J. Mol. Struct. 1978, 47, 129. (6) Werner, T. C.; Hercules, D. M. J . Phys. Chem. 1969, 73, 2005. ( 7 ) Suzuki, S.;Fujii, T.; Yoshiike, N.; Komatsu, S.; Iida, T. Bull. Chem. Soc. Jpn. 1978, 51, 2460. ( 8 ) Saigusa, H.; Lim, E. C . J . Phys. Chem. 1983, 78, 91.
383
388
,
I
388
372
,
376
378
vw"luw Figure 1. Fluorescence excitation spectrum of jet-cooled 9-anthroicacid.
Anthracene
Figure 2. Fluorescence excitation spectrum of jet-cooled anthracene.
fluorescence throughfll optics and focusing onto the slit of a microprocessor-controlled double monochromator. The fluorescence signals from the PM tube were averaged in a boxcar averager (PAR 162 with Model 165 gated integrators) and normalized for laser power fluctuations by using the ratio mode of the boxcar averager. The excitation and fluorescence spectra under static gas conditions were recorded on an Aminco SPF-500 spectrophotometer. The samples contained in an evacuated Pyrex tube were heated in a specially designed heat cell. 9-Anthroic acid (Aldrich Chemical Co.) was purified by dissolving it in base, precipitating with acid, and finally subliming under vacuum. The esters of 9-anthroic acid were synthesized according to the method of Parish and Stockg and purified by several recrystallizations from light petroleum (bp 60-80 "C). 111. Results and Discussion
A. 9-Anthroic Acid. Figure 1 shows the fluorescence excitation spectrum of jet-cooled 9-anthroic acid (An-9-COOH). An identical spectrum was obtained with 200 Torr of argon or 2-5 atm of helium. Note that the spectrum exhibits a long progression with an -47-cm-' separation between the members of the progression. About 12 members of the progression originating from the u" = 0 level of the ground state can be seen clearly in the spectrum. The absence of such a long progression in a low-frequency vibration in the excitation spectrum of jet-cooled anthracene (Figure 2) can be taken as evidence for the involvement of the carboxyl group in the vibrational motion. In the absence of IR or Raman spectral data or calculations on the low-frequency vibrations of this molecule, the assignment of this 47-cm-' vibration cannot be made with certainty. Such a long progression in a very low frequency mode may indicate a change in geometry along the -COOH torsional coordinate or it may indicate a change in geometry along the hydrogen-bond coordinate because aromatic carboxylic acids are known to dimerize very efficiently. The multiphoton ionization spectrum of benzoic acid dimer shows a long progression in a hydrogen-bonding mode (-70 cm-I), sug(9) Parish, R. C.; Stock, L. M. J . Org. Chem. 1965, 30, 927.
Structure and Spectra of 9-Anthroic Acid
The Journal of Physical Chemistry, Vol. 91, No. 25, 1987 6361
me--
,.! 341
3b
347
3w
& 383
wpvBB(GTwm
Figure 3. Fluorescence excitation spectrum of jet-cooled 9-anthroic acid in the higher energy region.
388
366
376
32
378
WAMLEM;TW NM Figure 5. Fluorescence excitation spectrum of jet-cooled methyl 9anthroate.
9-enthrcic acid
I
I
I
I
,
1
a sn8 369 3a iuwBGlwFM Figure 6. Fluorescence excitation spectrum of jet-cooled methyl 9anthroate in the higher energy region. 344
WAVELENGM/ NM Figure 4. Excitation and fluorescence spectra of 9-anthroic acid under static gas conditions. Emission spectra 1 and 2 were recorded by exciting the sample at 368 and 351 nm, respectively. Sample temperature 150
-
"C.
gesting a substantial change in geometry along this coordinate on electronic excitation.I0 The distinction between these two possibilities can be made if the jet spectrum shows different features for the monomer and hydrogen-bonded dimer that can coexist in equilibrium under supersonic jet expansion conditions. However, the jet spectrum in the wavelength range of 340-425 nm and in the temperature range of 100-200 OC shows features that can be attributed to only one species. Figure 3 shows the fluorescence excitation spectrum of jetcooled 9-anthroic acid in the higher energy region. The spectrum clearly shows a long series of bands with the same separation ( 7 4 7 cm-I) as observed in the low-energy region. Comparison of the low- and high-energy jet spectral features with the low-resolution excitation spectrum recorded under static gas conditions (Figure 4) shows that the midpoint of the progression in the low-energy region matches with the 0-0 band position and that of the progression in the high-energy region matches with the position of the next vibronic band (0 + 1400 cm-I) of the static gas sample that can be assigned to the totally symmetric ring vibration (6; of anthracene"). From this similarity, it is concluded that the series of bands seen in the higher energy region of the jet spectrum are the 47-cm-' progression built on the 6; band. The failure to detect the presence of two species under jet conditions suggests that the monomer-dimer equilibrium may be largely in favor of only one of these two species. The inability to assign the 47-cm-' vibration to either torsion or hydrogenbonding mode, therefore, necessitated the study of the jet spectra
-
(10) Tomioka, 2263.
Y.;Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1984,88,
(11) Lambert, W. R.; Felker, P. M.; Syage, J. A.; Zewail, A. H. J . Chem. Phys. 1984,81, 2195.
34
360
of the esters of 9-anthroic acid that cannot form hydrogen-bonded dimers. B. Methyl 9-Anthroate. Figure 5 shows the fluorescence excitation spectrum of the methyl ester of 9-anthroic acid under supersonic jet expansion conditions. The spectrum shows a long progression with an 37-cm-I separation between the bands. About 14 members of the progression originating from the u f f = 0 level of the ground state can be seen clearly in the spectrum. This very low frequency mode can only be assigned to the torsional motion of the carboxyl group. This assignment is supported by the results of the condensed-phase studPl2 and also by the valence force field calculations on the methyl ester of benzoic acid.I3 In addition to a long progression in -COOCH3 torsion, the spectrum also shows a series of combination bands with an 37-cm-' separation between them. The vibration that combines with torsion must be of fairly low frequency because the combination bands start appearing at -90 cm-' from the SIorigin. (The origin band is assumed to be the lowest energy feature at 26 718 cm-' or 374.3 nm. No reproducible feature was seen on the lower energy side of this band.) The lowest frequency optically active mode of anthracene is -209 cm-I, and so the vibration that combines with -COOCH3 torsion must be one of the several low-frequency modes of the carbonyl substitutent. The calculations on the methyl ester of benzoic acidI3 show that, other than the -COOCH3 torsion, only the -OCH3 torsion is of fairly low frequency (-58 cm-I) and all other vibrations are above 140 cm-'. It is, therefore, quite likely that the 4 C H 3 torsion combines with the -COOCH3 torsion and gives rise to this series of combination bands. As discussed earlier, the fluorescence excitation spectrum of jet-cooled 9-anthroic acid in the higher energy region exhibits a series of combination bands with a -47-cm-' separation between them (Figure 3). In order to see whether the jet-cooled methyl ester also exhibits a similar behavior, its fluorescence excitation spectrum was recorded in the 345-361-nm region, and it is shown
-
-
(12) Werner, T. C.; Hoffman, R. M. J . Phys. Chem. 1973, 77, 1611. (13) Boerio, F. J.; Bahl, S. K. Spectrochim. Acta, Part A 1976, 32A, 987.
Swayambunathan and Lim
6362 The Journal of Physical Chemistry, Vol. 91, No. 25, 1987 methyl 9-anthroate
TABLE I: Experimental and Calculated Torsional Levels in the SI State of Methyl 9-Antbroate assignt
6
vexPl: cm-' 39.0 19.0 119.0 158.0 196.0 233.0 269.0
T'
1'
T2 T3
74 T5
T6 T7
The frequencies are reported relative to the zero-point torsional level in the S, state of this molecule, which is assumed to be the lowest energy feature at 26718 cm-I or 374.3 nm (Figure 5).
'\ 275
475
375
cm-' 42.4 83.4 123.1 161.2 197.6 232.0 263.3
ucald.
575
WAVELENGTH/ NM
-
Figure 7. Excitation and fluorescence spectra of methyl 9-anthroate under static gas conditions. Sample temperature 120 'C.
in Figure 6. The spectrum clearly shows a series of bands with the same separation (37 cm-I) as observed in the lower energy region. This series of bands can again be assigned to the -COOCH3 torsional progression built on the 6; totally symmetric ring vibration (- 1400 cm-I), by comparing the jet spectrum with the low-resolution excitation spectrum recorded under static gas conditions (Figure 7). C. Determination of Torsional Potentials. In methyl 9anthroate, one can anticipate barriers in the torsional potentials at Oo and 90°. The barrier at 90' arises due to the absence of resonance interaction between the aromatic ring and the carboxyl group, and the 0' barrier arises due to steric repulsion from the peri-hydrogens of the anthracene ring. The general potential function that will allow the anticipated barriers and enforce the required symmetry around Oo and 90° can be written as 1 v(e) = -z v,,(1 - COS ne) 2
(1)
where n can take even values. It is assumed here that the torsional vibrations are confined to a single mathematical dimension 8 and they do not interact with the other vibrational modes of the molecule. The Hamiltonian operator for this problem is d d0
d
H = --[B(O)]g+ y2zV,,(1 - cos ne) where B(8) = h/8acI(8) and I is the reduced moment of inertia which can be defined as (3)
where ZIand Z, are the moments of inertia of the two parts of the molecule (carboxyl group and anthracene ring) about the torsional axis. The rotational constant for the -COOCH3.group ( B , ) was calculated to be 0.2707 cm-I by using the Gaussian 82 program, and that for the anthracene ring (B2)I4,l5is known from the literature to be 0.0151 cm-l. Thus, the internal rotation constant for this carbonylanthracene ( B , B2) was estimated to be 0.2858 cm-I. The experimentally observed torsional spacings were used to determine the parameters of eq 1 for the SI state of this molecule. The vibrational energy levels were calculated according to the method of Lewis et a1.I6 The results of this fitting procedure are given in Table I. A basis set of 70 functions was used in the calculation. The best fit parameters for SI are V, = -235 cm-I and V, = -425 cm-'. In the most stable S I structure, the torsional angle between the two parts of this molecule and the potential
+
~~~~
~
~
(14) Yamasaki, K.; Arita, K.; Kaiimoto, 0.;Hara, K. Chem. Phys. Lett. 1986, 123, 211. (15) Syage, J. A,; Felker, P. M.; Zewail, A. H . J . Chem. Phys. 1984, 81,
370
420
470
520
McpMLBK;n-vMn
Figure 8. Dispersed fluorescence spectra of 9-anthroic acid and its methyl ester under supersonic jet expansion conditions. Monochromator resolution for both spectra was 18 A. The excitation wavelengths were 3723 and 3695 8, for the acid and ester, respectively. The spectra taken with 3.5 8, (-20-cm-I) resolution were equally diffuse.
barrier at 90' were found to be -47' and -316 cm-I, respectively (Figure 9). These values are comparable to those reported for 9,9'-bianthryl (67' and 140 and 9-phenylanthracene (60' and 243 cm-').'' The inclusion of the v6 term in the potential function slightly improved the quality of the fits and did not significantly change the potentials in the well regions. The minima in the torsional potentials are caused essentially by the balance between steric repulsion and resonance stabilization. The bulkiness of the rotating group increases the steric repulsion enormously. Therefore, the resonance stabilization is canceled even before complete planarity is achieved, and this results in a shallow minimum somewhat between Oo and 90' for all of these molecules. D. Comparison and Contrast between the Jet Spectral Features of the Acid and Its Methyl Ester. The 37-cm-' vibration of the methyl ester and the 47-cm-I vibration of the acid behave identically in terms of showing a long progression built on the 0; band in the lower energy region and a long progression built on the 6; band in the higher energy region. It seems, therefore, reasonable to assign the 47-cm-I mode of the acid to the torsional motion of the -COOH group around the bond joining it to aromatic ring. Figure 8 shows the dispersed fluorescence spectra of the jetcooled acid and its methyl ester recorded by exciting one of the band of the torsional progression in the low energy region. Note that the two spectra are similar in their spectral shape and position. In each case, the fluorescence spectrum is diffuse and greatly shifted with respect to the corresponding excitation spectrum. The large Stokes shift and the diffuse character of the fluorescence spectra are consistent with the radiative transitions from the nearly planar excited state to the highly twisted ground state, which have
**em
LLJ3).
(16) Lewis, J. D.; Malloy, T. B., Jr.; Chao, T. H.; Laane, J. J . Mol. Struct. 1972, 12, 421.
(17) Werst, D. W.; Gentry, W. R.; Barbara, P. F. J . Phys. Chem. 1985, 89, 729.
The Journal of Physical Chemistry, Vol. 91, No. 25, I987 6363
Structure and Spectra of 9-Anthroic Acid
9-anuroic edd
I
I
I
I
373
376
379
382
WAVELENGTH/ nm 47'
90'
133
0 Figure 9. Torsional coordinate in methyl 9-anthroate.
their termini in the high vibrational levels of the ground state. An additional source of the spectral diffuseness in these molecules may be the presence of many low-frequency vibrations and extensive coupling (anharmonic and possible Coriolis) between the vibrational modes of the anthracene ring and the carboxyl group, which could render intramolecular vibrational redistribution (IVR) efficient even at low excess energies (for example, a few hundred wavenumbers above the S1 origin). Consistent with this assumption, the dispersed fluorescence spectra of the acid and the methyl ester recorded by exciting one of their combinations bands in the higher energy region (Le., torsional progression built on the 6; band) are very similar in shape and position to the spectra shown in Figure 8. Although the fluorescence excitation and dispersed fluorescence spectra of the acid and the methyl ester reveal many similarities, these two molecules also differ in some aspects. For example, the origin of the fluorescence excitation spectrum of the jet-cooled acid (assumed to be the feature at 26412 cm-' or 378.6 nm) is red-shifted relative to that of the jet-cooled methyl ester (26718 cm-' or 374.3 nm), even though the absorption 0-0 band of the acid monomer in cyclohexane is blue-shifted by 1 nm relative to that of the ester in the same solvent (380 and 381 nm for the acid and the methyl ester, respectively'). A difference of only 1 nm (-70 cm-') in the absorption 0-0 band position of these two molecules in cyclohexane may arise from a slightly different electron-donating inductive effect of the -OH and -OCH3 groups. But the red-shift (- 306 cm-') observed for the SI origin of the jet-cooled acid relative to that of the methyl ester is very nearly the same as the red-shift (-300 cm-l) observed for the 'AT* origin of benzoic acid dimer relative to that of its monomer under supersonic jet expansion conditions.1° This striking observation seems to indicate that the jet-cooled sample of 9-anthroic acid may have a large concentration of its hydrogen-bonded dimer and only a very small concentration of its monomer, even though the nozzle temperature is close to 200 OC. Consistent with the assumption of the presence of one dominant emitting species, the fluorescence decay was found to be exponential for excitation in the range of 370-377 nm. A simple thermodynamic consideration suggests that the assumed dominance of the dimer over the monomer may not be unreasonable. Suzuki et al.' reported the dimerization constant (KD)of 9-anthroic acid to be -6.6 X lo4 mo1-l.L in ethanol at 298 K. In ethanol, there is a competition between dimerization and hydrogen-bonding interaction between the solvent and the solute. It is therefore reasonable to expect the value of KD in non-hydrogen-bonding solvents, or in gas phase, will be greater than that calculated for the ethanol solution. If the value of KD in the gas phase at the nozzle temperature of -200 O C is comparable to KD in ethanol (-lo3 mol-l.L at 200 "C), the concentration of the dimer would be about the same as that of the monomer (at the estimated partial pressure of 10 Torr at 200 "C). If KD is much greater in the gas phase than in ethanol, as we expect, then the dimer concentration could far exceed the monomer concentration. Unfortunately, we were not able to deduce the value of KD in aprotic solvents due to the low solubility
Figure 10. Fluorescence excitation spectrum of jet-cooled 9-anthroic acid in the region of SIorigin.
Figure 11. Probable configuration of the hydrogen-bonded dimer of 9-anthroic acid in the ground electronic state.
of 9-anthroic acid in these solvents. If the hydrogen-bonded dimer is the dominant species of 9anthroic acid in the supersonic jet, one may expect the observation of some very low frequency hydrogen-bond (HB) modes in the fluorescence excitation spectrum. Consistent with this expectation, the fluorescence excitation spectrum of jet-cooled sample in the region of S1 origin displays a long progression in the 16-cm-' mode that can be attributed to the H B vibration (Figure 10). The appearance of a long progression in 47-cm-' torsion indicates a change in geometry from nonplanar to a more coplanar configuration, while the appearance of a long progression in the 16-cm-' mode indicates a change in geometry along the intermolecular hydrogen-bonding coordinate. On the basis of these observations, it is possible to say that, in the ground state, the dimer has a configuration in which the two anthracene rings lie in one plane and the plane of the dimer bridge is almost perpendicular to that of the aromatic rings as shown in Figure 11. This is in agreement with the results of Suzuki et al.,' who propose a similar geometry for the ground-state dimer on the basis of the observation of "anthracene-like" absorption spectrum for the dimer in ethyl alcohol. The prediction of the geometry of the hydrogen-bonded dimer in the excited state is somewhat difficult because the exact nature of the 16-cm-' mode is not known. All that can be said, on the basis of the present results, is that electronic excitation leads to the rotation of the dimer bridge into the plane of the aromatic rings, leading probably to a configuration in which all the four units lie in one plane. A similar planar structure has been proposed for the hydrogen-bonded dimer of benzoic acid in the ground electronic state.lg It has also been established that, upon electronic excitation, the geometry of the benzoic acid dimer is distorted along the hydrogen-bond stretch and in-plane bending coordinate and this distortion results in the destruction of the center of The appearance of a long progression in the 16-cm-' mode in the fluorescence excitation spectrum of 9-anthroic acid may indicate a similar distortion along the hydrogen bond (stretch + in-plane bend) coordinate which can result in the
-
(18) Sim, G.A.; Robertson, J. M.; Goodwin, T. H. Acta. Crystallog?. 8,
157.
(19) Baum, J. C.; McClure, D. S. J . Am. Chem. SOC.1980, 102, 720. (20) Poeltl, D. E.; McVey, J. K. J . Chem. Phys. 1984, 80, 1801.
The Journal of Physical Chemistry, Vol. 91, No. 25, 1987
6364
Swayambunathan and Lim
w-
370
470
420
520
VIWaaKirwFM Figure 13. Dispersed fluorescence spectrum of jet-cooled n-butyl 9anthroate. Monochromator resolution and excitation wavelengths were 18 and 3702 A, respectively.
I
383
I
I
I
I
I
366
368
372
376
378
w 4 " w F M Figure 12. Fluorescence excitation spectra of the methyl and n-butyl esters of 9-anthroic acid under supersonic jet expansion conditions.
destruction of the center of symmetry of its hydrogen-bonded dimer in the excited state. Another difference between 9-anthroic acid and its methyl ester is in the number of combination bands that appear in their fluorescence excitation spectra. In the 365-373-nm region, the spectrum of the acid shows many series of combination bands with an -47-cm-' separation between the bands of each series, whereas that of the methyl ester shows only one series of combination bands with an -37-cm-l separation between the bands. The dimeric nature of the acid is very likely responsible for the appearance of many series of combination bands in its spectrum, and two of the modes that combine with the 47-cm-' torsion can be identified with the 33- and 16-cm-' hydrogen-bonding modes. E . n-Butyl 9-Anthroate. Figure 12 shows the fluorescence excitation spectrum of the n-butyl ester of 9-anthroic acid under supersonic jet expansion conditions. Also shown in the figure is the excitation spectrum of the jet-cooled methyl ester. In sharp contrast to the methyl ester, the fluorescence excitation spectrum of the n-butyl ester shows very little structure. The diffuse excitation spectrum closely resembles that of the same molecule studied under static gas or effusive conditions. The failure to observe the progression in the -COO(CH2)3CH3torsion is very likely related to the coupling of the low-frequency torsional mode
to the lower frequency alkyl chain modes which leads to a severe spectral broadening and congestion. Even so, the change in geometry from nonplanar to a nearly coplanar configuration is also indicated in this molecule by its dispersed fluorescence spectrum (Figure 13), which is greatly Stokes shifted relative to the corresponding excitation spectrum. The emission spectrum is very similar in spectral shape and position to the spectra of the acid and the methyl ester shown in Figure 8. IV. Conclusion The excited-state properties of 9-carboxyl substituted anthracenes were investigated by using the free jet expansion technique and laser spectroscopy. The appearance of a long progression in the fluorescence excitation spectra of jet-cooled 9-anthroic acid and its methyl ester coupled with their highly Stokes shifted and diffuse emission spectra provide conclusive spectroscopic evidence for the conformational change from a nonplanar to a nearly coplanar form in the lowest excited singlet state of these molecules. The analysis of their excitation spectra showed that the frequency of carboxyl torsion is -47 cm-I in acid dimer and -37 cm-' in methyl ester. A similar change in geometry along the torsional coordinate is shown to occur even in the case of the (bulky) n-butyl ester of 9-anthroic acid. The torsional potential calculations on methyl 9-anthroate show that the resonance stabilization is canceled by the steric repulsion even before complete coplanarity is achieved in the S, state of this molecule. The probable structure of the hydrogen-bonded dimer of 9-anthroic acid in the ground and electronically excited states is also briefly discussed. Acknowledgment. This work was supported by a grant from the Department of Energy. Registry No. 9-Anthroic acid, 723-62-6; methyl 9-anthroate, 150439-8; n-butyl 9-anthroate, 57516-92-4.
