J. Phys. Chem. 1992, 96, 95-99
95
Analysis of the Induced Rotational Strength of Mono- and Disubstituted Benzenes Included in ,&Cyclodextrin Mamoru Kamiya,* Setsuko Mitsuhashi, Masakazu Makino, and Hisashi Yoshioka Laboratory of Instrumental Analysis, Division of Environmental Health Sciences, University of Shizuoka. 395, Yada, Shizuoka-shi, Japan 422 (Received: July 15, 1991; In Final Form: August 30, 1991)
-
In order to accumulate geometrical informations about substituted benzenes included in j3-cyclodextrin, the circular dichroism induced by the T T* transitions of the included guests was analyzed by using the Kirkwood-Tinoco formula for the induced rotational strength based on the chiral exciton interaction theory. The main part of the analysis procedure was simplified by avoiding parametrization of the anisotropic bond polarizabilities which would cause some indefinite effects to the result of the analysis. Several important properties of the inclusion geometries clarified from the rotational strength analysis were considered in terms of the effects of substituent groups upon the inclusion interaction forces. Characteristic contributions to the inclusion complex formation were confirmed for the hydroxyl, carboxyl, nitro, and amino group of substituted benzenes.
Introduction Cyclodextrins are known as cyclic oligosaccharides having the ability to form inclusion complexes with many different guest molecules.'" On the basis of this fact, cyclodextrins have been studied in both basic and practical research fields as models for enzymic c a t a l y ~ i s * *and ~ ~ for ~ ~ *a variety of other p u r p ~ s e s . ~For ~~** the driving forces contributing to the inclusion complex formation, several types of interactions have been proposed in terms of the van der Waals interaction, hydrogen-bonding interaction, release of high-energy water, and decrease in strain energy in the cyclodextrin cavity.* To investigate the fundamental nature of the inclusion interactions forces, many works have been done by applying physicochemical methods to inclusion complexes of cyclodextrins with hydrophobic aromatic guests containing phenols and a n i l i n e ~ . ~ - These '~ works have given valuable information about the relationship between the nature of the interaction and the inclusion geometry in solution represented by the orientation and position of the guest molecules in the host cavity. However, it still seems to be necessary for us to accumulate more basic data of the interaction-geometry relationship of the inclusion complexes by means of appropriate spectroscopic methods. The main purpose of the present work is to investigate the geometrical factors of substituted benzenes included in the 8cyclodextrin cavity and to confirm the fundamental trends dominating them. For this purpose we pay attention to the circular dichroism induced in the included benzene derivatives because it is very sensitive to their orientations and positions in the host cavity. To estimate the geometrical factors of the included phenols (1) Griffths, D. W.; Bender, M. L. Adu. Cuful. 1973, 23, 209. (2) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; SpringerVerlag: Berlin, 1978. (31 Hinze. W. L. Sen PuriL Methods 1981. 10. 159. (4j Szejtli, J. Cyclojextrinf and Their Inclusion Complexes; Academiai Kiado: Budapest, 1982. (5) Tabushi, 1. Acc. Chem. Res. 1982, 15, 66. (6) Breslow, R. Sciences 1982, 218, 532. (7) Beraeron. R. J. J. Chem. Educ. 1977. 54. 204. ( 8 ) Saeiger, W. Angew. Chem. 1980, 19,' 344. (9) Lin, S. F.; Connors, K. A. J . Phurm. Sci. 1983, 72, 1333. (10) Connors, K. A.; Pendergast, D. D. J. Am. Chem. SOC.1984, 106, 7607. (11) Pendergast, D. D.; Connors, K. A. Bioorg. Chem. 1985, 13, 150. (12) Inoue, Y.; Okuda, T.; Miyata, Y.; Chujo, R. Curbohydr. Res. 1984, 125, 65. (1 3) Sanemasa, 1.; Mimguchi, T.; Deguchi, T. Bull. Chem. Soc. Jpn. 1984, 57, 1358. (14) Zukowski, J.; Sybilska, D.; Jurczak, J. J. Chromutogr. 1984,286, 183. (1 5) Kysl, S.; Smolkova-Keulemansova,E. J. Chromatogr. 1985,349, 167. (16) Armstrong, D. W.; Nome, F.;Spina, L. A.; Golden, T. D. J. Am. Chem. Soc. 1981, 108, 1418. (17) Shimizu, H . ; Kaito, A.; Hatano, M. Bull. Chem. SOC.Jpn. 1979,52, 2678. (18) Shimizu, H.; Kaito. A.; Hatano, M. Bull. Chem. Soc. Jpn. 1981, 54, 513.
and anilines, we used the rotational strength analysis method based on the chiral exciton interaction theory. By using the axial symmetry of the cyclodextrin host cavity, we could in this case obtain geometrical information without the somewhat indefinite effects which would arise from parametrization of the anisotropic bond polarizabilities and the averaged bond excitation energies. The merit of this analysis method may be ascribed to the use of the relative magnitude of the dipole and rotational strength data associated with the axial and equatorial transitions of the guest molecule included in the host cavity. The analysis result has clarified several important properties with respect to the geometrical factors of the benzene derivatives included in the host cavity. The inclusion geometry data obtained here will be discussed in terms of the hydrophilic and hydrogen-bonding abilities of substituent groups which have been accepted as the important sources dominating the inclusion interaction forces for the benzene derivatives.
Experimental Section P-Cyclodextrin of high quality (99%+) was obtained from Nihon Food Technologies Inc. The benzene derivative guest compounds of analytical grade were obtained from Wako Chemical Co. and purified prior to using. Phenol was purified by freezing out from the melt, and aniline was distilled under reduced pressure. Benzoic acid was recrystallized from aqueous solution and then twice recrystallized from methanol. p-Nitrophenol was recrystallized from aqueous HCl solution and then twice recrystallized from ethanol. p-Hydroxybenzoic acid and p-hydroxyphenol were recrystallized from aqueous solution and then twice recrystallized from benzene. The recrystallized compounds were dried under high vacuum, and their purity was checked by GC/MS prior to use. Sample preparations were carried out by mixing the stock solutions of 8-cyclodextrin and of a given guest molecule, both of which were made up in phosphate buffer of ionic strength 0.2. To eliminate the effect of ionized species upon the spectra of inclusion complexes, the pH of the sample solutions was adjusted to 7.5 for the case of aniline + 8-cyclodextrin solution, and to 6.2 for the other cases of sample solutions by referring to the PK, values of the guest molecules and of the hydroxyl groups of 8cyclodextrin. The absorption and circular dichroism spectra of inclusion complexes investigated here were not affected by slight variation of pH from the adjusted value. In this work we avoided the use of excessively acidic or alkaline media since 8-cyclodextrin is sensitive to acid-catalyzed hydrolysis, and also the hydroxyl groups of it are ionized in strongly alkaline medium to cause shallowing deformation of the host cavity. The stock solution of p-hydroxyphenol was used within 2 h of preparation because a slight discoloration was observed near the end of this period. Water used as solvent through this work was distilled and deionized.
0022-3654192f 2096-95%Q3.QQ/Q0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 1, 1992
Kamiya et al.
The absorption and circular dichroism spectra were recorded at 25 OC by using a Beckman DU-70 spectrophotometer and a JASCO 5-600 spectropolarimeter, respectively. Calibration of the circular dichroism measurement was done by using 10-camphorsulfonic acid. Background contributions from @-cyclodextrin were not clearly observed at the circular dichroism above 220 nm. However, in order to prevent any background effects which might be slightly caused by the hydrolysis of @-cyclodextrineven in the weakly acidic or alkaline medium, the circular dichroism measurement was always done using the free @-cyclodextrinsolution as a reference system. The inclusion stability constants were estimated by means of the Benesi-Hildebrand method.I9 In this case the guest’s concentrations were held at least 10 times lower than the lowest host’s concentration in order to meet the requirement for a linear Benesi-Hildebrand plot. All of these data were analyzed with a least-squares linear regression treatment. The dipole strength (D) and the rotational strength (R) were estimated by means of the following equations
as follows by taking the sum of the rotation operations applied to the tensors Pki and Qki
96
A€
D = 0.920 X 10-38r1/2€max-
where the z axis of the Cartesian coordinate of the inclusion complex system is fixed to the symmetry axis of the host cavity. Here the suffix i refers to the bonds forming a single unit of the linked glucose residue, 0, is the inclination angle of the guest’s transition dipole from the cavity axis, and n is the number of the linked glucose residues (n = 7 for @-cyclodextrin). It is difficult in general to do accurate estimation of the averaged wavenumber (ti)of the u u* transitions of the a-bond i of the host cavity. But it is much larger than the wavenumber (5,) of the r r* transition of the benzene ring of the included guest. This allows us to use an approximationf&v,) = (al1- al)i and to calculate R, by the following equation
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R, = t,D,K(l - 3 cos2 e,)
Amax
R = 0.696
x
A0
io-w~2[ejmaxAmax
where emax and e[,] are the maximum values for the molar extinction coefficient and the molar ellipticity, respectively, and A€ and A0 are the half-bandwidth at l / e of the maximum absorption and ellipticity, respectively.
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Analysis Method The rotational strength (R,) induced by the transition 0 a of the included guest can be expressed as follows by the exciton interaction theory taking account of the anisotropic bond polarizabilitiesZ0
where t,, D,, and e, are the wave number, dipole strength, and unit direction vector, respectively, associated with the transition 0 a of the included guest, tkiis the averaged wavenumber of the electronic transitions of the bond i belonging to the linked glucose residue k (referred to as the bond ki), ekiis the unit direction vector along the symmetry axis of the bond ki, (all a& is the difference in the parallel and perpendicular components of the zero-frequency polarizability associated with the axially symmetric bond ki, rki is the distance vector pointing from the chromophore center of the inclined guest to that of the bond ki, and ikiis the unit vector defined by rki/lrki(.Equation 1 can be rearranged as follows
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Ra = -*t,D,CCfki(~kir~a)Irkil-31ea.(P,i- 3 Q k i ) d k
i
(3)
by introducing the chiral interaction tensors Pki and Qkigiven by
Qki
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Pki = eki(rki X eki)
(4)
= (eki’rki)fki(fkiX eki)
(5)
@-Cyclodextrin has the axial symmetry about the cavity axis surrounded by seven units of the a- 1,Clinked D-glucose residues. and lrkilare independent of the Because of this, t k i , (all- aL)ki, suffix k labeling the individual glucose residues. On the basis of the symmetry-adapted inclusion model, eq 3 can be then simplified (19) Benesi, H. A.; Hildebrand, J. H. J . Am. Chem. SOC.1949, 71,2703. (20) Tinoco, I. Jr. Adu. Chem. Phys. 1962, 4 , 113.
(7)
where K, in eq 6 has been replaced by the frequency-independent K. The quantity K,referred to as the geometrical chirality index, is expressed as a function of the inclusion depth of the guest’s center located on the cavity axis. Its magnitude increases as the guest approaches the top torus of the host cavity and is inserted more deeply into the host cavity. With eq 7 we can calculate the ratio of the rotational strengths induced by two different transitions (0 a and 0 b) of the benzene derivative guest. That is, we have from eq 7
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The difference in the inclination angles ea and is determined by the intramolecular polarization property of the guest’s transitions, and hence it is independent of the overall orientation of the guest molecule in the host cavity. Equation 8 is thus useful for estimation of the orientation of the guest molecule, which is expressible by either the angle 0, or Ob, from the ratios of taD,/t&, and R,/Rb. If the two transitions 0 a and 0 b of the guest molecule are perpendicular to each other by the molecular symmetry, we have from eq 8
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This is a simple and practical equation which can be used to estimate the orientation of the included guest molecule from the ratios of the spectroscopic data associated with the transitions 0 a and 0 b. In this case the orientation of the guest molecule in the host cavity can be estimated without a direct calculation of the induced rotational strength which requires somewhat difficult parametrization of the anisotropic polarizabilities and the averaged excitation energies for the individual bonds forming the host cavity.
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Results and Discussion The absorption and circular dichroism spectra of the included benzene derivatives investigated are illustrated in Figure 1. The mono- and para-substituted benzene guests exhibit characteristic patterns of the induced circular dichroism composed of the positive and negative bands. These bands have been ascribed to the A r* transitions polarized in axial and equatorial directions, respectively, in the @-cyclodextrin host cavity. However, their relative intensities are found to be remarkably dependent on the difference in the substituent groups of the guest molecules. This suggests that the geometrical factors of the included guests are actually sensitive to the shapes, sizes, and polarities of the sub-
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Rotational Strengths of Substituted Benzenes
The Journal of Physical Chemistry, Vol. 96, No. 1 , 1992 97 d
I
L
,
I
200
300
400
WAVELENGTHlnm)
-5
I
WAVELENGTH(nm) WAVELENGTHlnm)
-2
3 [-g5 'Y $1 200
250 WAVELENGTHlnal
300 WAVELENGTH(nal
Figure 1. (a) Circular dichroism (upper) and absorption (lower) spectra of the inclusion complex between 8-cyclodextrin (1.26 X M)and phenol (1.49 X IO4 M) in phosphate buffer solution of pH = 6.2. (b) Circular dichroism-(upper) and absorption (lower) spectra of the inclusion complex between 8-cyclodextrin (1.45 X M) and aniline (1.76 X IO4 M) in phosphate buffer solution of pH = 7.5. (c) Circular dichroism (upper) and M) and benzoic acid (1.92 X IO4 M) in phosphate buffer absorption (lower) spectra of the inclusion complex between @-cyclodextrin (1.05 X solution of pH = 6.2. (d) Circular dichroism (upper) and absorption (lower) spectra of the inclusion complex between 8-cyclodextrin (1.37 X M) and p-nitrophenol (2.05 X IO4 M) in phosphate buffer solution of pH = 6.2. (e) Circular dichroism (upper) and absorption (lower) spectra of the inclusion complex between 8-cyclodextrin (1.26 X M) and p-hydroxyphenol (1.78 X IO4 M) in phosphate buffer solution of pH = 6.2. (f) Circular dichroism (upper) and absorption (lower) spectra of the inclusion complex between 0-cyclodextrin (1.52 X M) and p-carboxyphenol (2.14 X IO4 M) in phosphate buffer solution of pH = 6.2.
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The Journal of Physical Chemistry, Vol. 96, No. 1 1992 X
x
N 02
OH
TABLE II: Geometrical Analysis Results and Inclusion Stability Constants
Figure 2. The reference axes of the guest molecules. The inclination angle (e) is defined as the angle between the guest’s reference axis and the symmetry axis of the @-cyclodextrincavity. TABLE I: Spectroscopic h t a Used in the Geometrical Analysis -V a,
auests phenol aniline
lo3 cm-l 37.2 (B2) 47.8 (A,) 35.1 (B2) 43.9 (Ai) 36.2 (A’) 43.0 (A’) 31.2 (A,) 44.1 (B2) 34.9 (B2,,) 45.2 (Bl,,) 38.7 (A’) 48.3 (A’) 35.3 (A’) 44.7 (A’) 36.6 (B2) 46.5 (A,) 34.6 (B2) 42.0 (A,)
D,b D2 1.42 6.30 1.35 6.97 0.94 8.79 19.30 9.64 2.20 4.23 19.20 14.58 7.64 27.09 1.34 1.75 2.04 7.53
Kamiya et al.
R,C cgs units -0.06 1.29 -0.10 1.62 -0.14 6.31 8.20 -2.89 -0.38 2.15 6.01 -2.82 1.25 -1.01 0.25 -0.18 0.12 -0.14
guests phenol aniline benzoic acid p-nitrophenol p-h ydroxphenol p-carboxyphenol o-nitroaniline m-hydroxyphenol m-aminoaniline
K.0 --.
cgs units 3.1 3.2 10.0 6.8 6.3 4.1 2.6 2.9 1.1
B,b deg
Kin: M-,
27.5 19.6 24.3 1.9 15.5 5.3 19.4 13.8 3.2
102 f 15 125 & 18 312 f 45 230 f 26 156f24 247 f 37
Geometrical chriality index given with reversed sign. Inclination angle of the guest’s reference axis from the cavity axis. ‘Inclusion stability constant.
the cyclodextrin cavity with the carboxyl end first. The stable inclusion of benzoic acid may be then explained in terms of a balanced cooperation of the van der Waals and hydrogen-bonding p-nitrophenol forces. The former acts between the guest’s benzene ring and the p-hydroxyphenol host cavity. The latter acts between the deeply inserted carboxyl group and the hydroxyl groups surrounding the bottom torus of p-carboxyphenol the host cavity. Compared with phenol and benzoic acid, aniline is included o-nitroaniline with less deviation from the axial orientation and with intermediate inclusion stability. This finding seems acceptable since aniline m-hydroxyphenold has less ability for the formation of hydrogen bond requiring some preferred orientation of the amino group. Positive information m-aminoanilined on this can be obtained from the thermodynamic study of the cyclodextrin-substitutedbenzene inclusion According Transition wavenumber. *Transition dipole strength. Induced to it, the hydrogen-bonding contribution of the amino group to rotational strength. dTaken from ref 18. e The polarization directions the complex formation enthalpy is not so remarkable as the case of the first and second transitions are taken as 87.9’ and 4.7’ (clockof the hydroxyl and carboxyl groups. wise from the reference axis) on the basis of the P-P-P MO calculaFor the para-disubstituted phenols, the inclination angle is in tion data. fThe polarization directions of the first and second transitions are taken as 3.9O and 81.0’ in the same way as above stated. the range of 1’-15’. We can then find that there is some deviation from the axial inclusion even for these guests. The analysis result stituent groups as well as to the effective space and flexibility of indicates that the para-disubstituted phenols are included more the host cavity. axially and more deeply than the mono-substituted guests. This Table I shows the spectroscopic data used in the analysis. The is a common trend seen a t the para-substituted phenols except data of ortho- and meta-disubstituted benzenes were taken from p-hydroxyphenol. This is in accord with the N M R information the literature valuesi8 to investigate the applicability of the analysis on the inclusion complexes of cyclodextrin with para-substituted method. Table I1 shows the geometrical analysis result and the phenols.12 According to it, a number of these guests form the inclusion stability constants. The stability constants estimated inclusion complexes by penetrating the host cavity. In this case, here satisfy the same trend as found in the literature values,i7J8~24~28 the unaxial motions of the cavity-penetrating guests may be realthough some deviations are seen possibly due to the difference stricted by strong steric interactions of the aromatic protons ortho in pH and ionic strength of solvents. and meta to the phenolic hydroxyl group with the H-3 and H-5 For the mono-substituted benzene guests, the inclination angle protons of the cyclodextrin cavity. of the molecular long axis is in the range of 19’-27’. That is, The thermodynamic study on the inclusion complexes indicates these guests are not included so axially as presumed by the symthat p-hydroxyphenol is included in 0-cyclodextrin with a negative metry-adapted inclusion model. Deviation from the axial inclusion value of the complex formation entropy.24 This is contrast to is in the order of aniline C benzoic acid C phenol. On the other the case of other para-substituted phenols. It is then probable hand, the inclusion stability constant is in the order of phenol C that the exceptional inclusion geometry found for p-hydroxyphenol aniline < benzoic acid. This indicates absence of direct correlation comes from the strongly restricted configuration due to the hybetween the inclination angle and the inclusion stability constant. drogen bonds with the cyclodextrin hydroxyl groups at both the To consider this problem, we take notice of the fundamental top and bottom of the host cavity. information on the complex formation.2i According to it, the @Nitroaniline is included with less depth and stability compared phenolic hydroxyl group with high hydrophilic property is reluctant with the mono-substituted and para-disubstituted benzenes. This to enter the hydrophobic cyclodextrin cavity. That is, it remains seems acceptable because the inclusion of o-nitroaniline is prein the vicinity of the top torus of the host cavity with some prevented by steric effects of the two adjacent substituents. However, ferred orientation. In this case the nonaxial and shallow inclusion the orientation of the nitro group is not so unaxial as expected of phenol may be understood in terms of the hydrogen bonds with from the ordinary inclusion model. If o-nitroaniline enters the the peripheral hydroxyl groups surrounding the cavity top torus. host cavity with the benzene ring first, both the amino and nitro Compared with phenol, benzoic acid is included with slightly more groups would be located outside the cavity with a larger inclination axial orientation and with much larger stability constant and angle (possibly about 30’) from the cavity axis. This indicates chirality index. That is, the carboxyl group is found to make a that o-nitroaniline enters the host cavity not with the benzene ring positive contribution to the cavity affinity of the guest molecule. first but with the nitro group first, just as have been accepted in This finding is compatible with the inclusion mechanism proposed the case of inclusion of p-nitrophenols in cy~lodextrins.~~-~’ for benzoic acid.22 According to it, benzoic acid is included in benzoic acide
(21) Hirata, K. Bull. Chem. SOC.Jpn. 1977, 50, 1410. (22) Bergeron, R.; Channing, M. A.; McGovern, K. A. J . Am. Chem. SOC. 1978, 100, 2878.
(23) Harata, K. Eioorg. Chem. 1B1, I O , 255. (24) Bertrand, G . L.; Faulkner, J. R. Jr.; Han, S.M.; Armstrong, D. W. J . Phys. Chem. 1989, 93, 6863. (25) Bergeron, R.; Rowan, R. Eioorg. Chem. 1976, 5 , 425.
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J. Phys. Chem. 1992,96, 99-104
The meta-disubstituted benzenes are included with less depths compared with the mono- and para-substituted benzenes. Moreover, they are not so symmetrically included as expected from the molecular symmetry (CzOfor both of m-hydroxyphenol and m-aminoaniline). Actually one of the two identical groups at the meta positions is more axially included than expected from the symmetrical inclusion model. This trend is clearer as m-aminoaniline than in m-hydroxyphenol. This may be explained in terms of the difference in the abilities of the hydroxyl and amino groups (26) Bergeron, R.; Channing, M. A.; Gilbeily, G. J.; Pillor, D. M. J. Am. Chem. Soc. 1977, 99, 5146. (27) Bergeron, R.; Channing, M. A. Bioorg. Chem. 1976, 5,437. (28) Buvari, A.; Barcza, L. J . Chem. Soc., Perkin Trans. 2 1988, 543.
to form the hydrogen bonds with the peripheral hydroxyl groups of the host cavity. In conclusion we may say that the rotational strength analysis method is sufficiently useful for studying the inclusion geometry of substituted benzene guests. That is, this method enables us to obtain basic information on the substitution effect on the inclusion orientations and depths. Registry No. j3-Cyclodextrin-phenol complex, 7362 1-01-9; j3-cyclodextrinaniline complex, 73621-02-0; j3-cyclodextrin-benzoic acid complex, 68419-51-2; j3-cyclodextrin-p-nitrophenol complex, 61955-24-6; j3-cyclodextrin-hydroquinone complex, 78 153-75-0; j3-cyclodextrin-psalicylic acid complex, 80065-26-5; 8-cyclodextrin-o-nitroanilinecomplex, 781 53-70-5; j3-cyclodextrin-m-benzenediolcomplex, 78153-74-9; j3-cyclodextrin-m-benzenediaminecomplex, 78 153-77-2.
Electronic Spectra of 0 - , m-, and p-Dlfluorobenzene Cations: Striking Similarity in Vibronic Coupling between the Neutral Molecule and Its Cation Yuko Tsuchiya, Ken Takazawa, Masaaki Fujii, and Mitsuo Ito* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: July 16, 1991)
The D(r,r).- Do transitions of m- and edifluorobenzene (DFB) cations prepared by two-color REMPI have been observed by dissociation spectroscopy. All the spectra due to the transitions from different vibrational levels in Do showed well-resolved vibrational structures. The spectral analysis indicates the existence of a strong vibronic coupling between the D(*,r) state and a nearby D(u,r) state for both m- and eDFB cations. The D(r,r) state was found to be the lowest excited state, contrary to the generally accepted criterion that the lowest excited state of the nonemissive fluorobenzene cation is D(u,r). It was found that the out-of-plane vibration responsible for the vibronic coupling is exactly the same as that of the corresponding neutral molecule in S,for all the difluorobenzene (ortho, meta, and para) cations, indicating similarity in their electronic states.
Introduction Fluorobenzene cations have attracted a great interest with respect to their emissive properties. Fluorobenzene cations having more than two fluorine atoms are highly fluorescent, while monoand difluorobenzene cations are nonemi~sive.'-~In the cation, there exist two kinds of low-lying electronic excited states. One is the D(u,?r) state due to the transition of an electron from the u-bonding orbital localized mainly in the C-F bond to the highest half-occupied ?r-bonding orbital. The other is the D(?r,r) state arising from the transition of an electron in the second highest *-bonding orbital to the highest half-filled r-bonding orbital. The relative energies of the D(u,?r) and D ( r , r ) states change greatly by the number of the fluorine atoms in the cation. It is well established that in monofluorobenzene cation the D(U,T) state is the lowest excited state and for the cations having more than two fluorine atoms (tri-, tetra-, penta-, and hexafluorobenzene cations) the D ( r , r ) state becomes l ~ w e s t . Phenomenologically, ~ the nature of this lowest excited state is closely related with the emissive property mentioned above. That is, the lowest excited state of the emissive cation is D(*,r) and the nonemissive cation has D(U,T) as the lowest excited state. Difluorobenzene (e, m-, and p-difluorobenzene (DFB)) cations are at a critical position with respect to the energy order of the two electronic states. Difluorobenzene cations are nonfluorescent in and also in the low-temperature matrix.5 Therefore, it has been anticipated that the lowest excited state is D(u,?r). (1) Allan, M.; Maier, J. P. Chem. Phys. Lett. 1975, 34, 442. (2) Allan, M.; Maier, J. P.; Marthaler, 0. Chem. Phys. 1977, 26, 131. (3) Cossart-Magas, C.; Cossart, D.; Leach, S . Mol. Phys. 1979,37,793. (4) Bieri, G.; Asbrink, L.; Von Niessen, W. J . Electron Spectrosc. 1981, 23, 28 1. (5) Bondybey, V. E.; English J. H.; Miller, T. A. J . Mol. Specrrosc. 1980, 81. 455.
-
In a previous work: the electronic spectra of p D F B cation due to the Z B s U ( r , ~ ) Do(2B2,) transition were observed by massselected ion dip spectroscopy. The spectral analysis showed that there exists the strong vibronic coupling between the 2B3U(a,r) state and a nearby ZBzg(u,r)state. It was concluded from the effect of the vibronic coupling on the vibrational frequency that the ZB3u(r,r)state is the lowest excited state in spite of the nonemissive property of this cation. The result obviously contradicts the generally accepted criterion that the lowest excited state of the nonemissive cation is D(u,r). Therefore, the criterion needs to be reexamined. The result obtained for p-DFB cation suggests that the emissive property is not determined simply by the nature of the lowest excited state. It might be possible that the strong vibronic coupling also plays an important role in the emissive property. In this work, we extended the study to e and m-DFB cations. The spectra due to the D(r,*) Do transition were observed by multiphoton dissociation spectroscopy utilizing the photodissociation of the cation in a highly excited state. The cations in the ground state were generated by two-color 1 1' REMPI of the jet-cooled neutral molecule via a particular vibronic level in S1. By suitable selection of the vibronic level in S1 and choice of the laser frequencies, we can populate the ground-state cations to a specific vibrational level. The cations in the selected level were subjected to the measurement of the electronic spectra by dissociation spectroscopy. The electronic spectra of 0-and m-DFB cations exhibit well-resolved vibrational structures. The analysis of the structure shows the existence of strong vibronic coupling between the D(r,*) and D ( u , r ) states similar to the case of the p-DFB cation. It was also found that the lowest excited state is D(r,*) for both 0-and m-DFB cations. +
+
(6) Tsuchiya, Y.; Fujii, M.; Ito, M. Chem. Phys. Lett. 1990, 168, 173.
0022-3654/92/2096-99%03.00/0 0 1992 American Chemical Society
