10432
J. Phys. Chem. 1994, 98, 10432-10435
Magnetic Circular Dichroism and Circular Dichroism Spectra of Xanthones Shizuo Ishijima, Miwako Higashi,' and Hiroyuki Yamaguchi Molecular Technology, Doctor's Courses, Graduate School of Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi 316, Japan Received: June 20, 1994@
The magnetic circular dichroism (MCD) spectra of xanthones (xanthone, thioxanthone, and acridone) and circular dichroism (CD) spectra of the inclusion complexes of xanthones with P-cyclodextrin have been measured. The spectra are interpreted by results of the PPP and CNDO/S calculations. The first m*state of thioxanthone and acridone shows the intramolecular charge-transfer character. The Faraday A term is observed in the MCD spectrum for the excited triplet state (3nn*) of xanthone.
Introduction The triplet state properties of xanthone (9H-xanthen-9-one) and its substituted derivatives have been of interest for a number of The first and second excited triplet states (3nn* and 3nn*) of xanthone are extremely close lying to each other. Thus, the phosphorescence spectra are very sensitive to the temperature and solvent. Thioxanthone (9H-thioxanthen-9-one), which is the iso-n-electron system of xanthone, has similar triplet state proper tie^.^,^ The magnetic circular dichroism (MCD) spectra of some quinones have been measured and assigned by calculations of the PPP method and CNDO/S method.',* Well-parametrized PPP calculations turn out to be successful in interpreting the Faraday B terms of the z n * transitions. On the other hand, the position of the m* transition is well predicted by the CNDOIS method. For the excited triplet state of a few quinones, a very weak Faraday A term was measured in the MCD spectra. The circular dichroism (CD) spectrum of the inclusion complex of a guest molecule with P-cyclodextrin is a very simple tool to assign the polarizations of the transition^.^^'^ In this work, we study xanthone, thioxanthone, and acridone, 9( lOH)-acridinone, whcih are iso-n-electronsystems of xanthone (Figure 1). We have considered the MCD spectra of xanthones and CD spectra of the inclusion complexes of xanthones with P-cyclodextrin to assign the polarizations of the transitions.
Experimental Section Xanthone (Wako Pure Chemical Industries, Ltd.), thioxanthone (Aldrich Chemical Company, Inc.), and acridone (Kanto Chemical Co., Inc.) were recrystallized three times from ethanol. /3-Cyclodextrin (Kanto Chemical Co., Inc.) was purified by repeated recrystallizations from distilled H20. Spectral-grade solvents (cyclohexane and ethanol) were used as received. The n-pentane and 1-iodopentanewere chromatographedthrough an activated aluminum oxides column. The absorption spectra were recorded on a Hitachi U-3200 spectrophotometer. The CD spectra were measured using a Jasco J-600C spectropolarimeter. To obtain an adequate signalto-noise ratio, multiple scanning and averaging was accomplished using a microcomputer. The stoppered silica cells of 1, 5 , and 10 cm pathlengths were used. The concentration of ,L?-cyclodextrinwas maintained in all experiments at 9.00 x M. @
Abstract published in Advance ACS Abstracts, September 15, 1994.
0022-365419412098-10432$04.50/0
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S
0
xanthone
N
H
thioxanthone
acridone
Figure 1. Structures of xanthone, thioxanthone, and acridone.
The MCD spectra were recorded on a Jasco J-600C spectropolarimeter with a 1.32 T electromagnet. To measure the MCD spectra of the long-wavelength region, an SM1-7 Oxford Instruments cryomagnet (6.51 T) was mounted. The magnetic field strength was measured with a 0.05 M aqueous solution of CoSO4. All measurements were carried out at room temperature.
Calculations The molecular structures of xanthone and thioxanthone were determined from gas electron-diffraction data on the basis of the assumption that the benzene ring is a regular hexagon."J2 The experimental geometry of acridone is unknown. In the absence of relevant experimental structural data, the geometries of xanthone, thioxanthone, and acridone were optimized by using the MNDO method in MOPAC Ver. 6.01.13 With the geometry obtained by the MNDO method, we calculated the electronic transition energies, oscillator strengths, and Faraday B terms by using the PPP method.14 The ionization potentials and one-center repulsion integrals for the heteroatoms were those given in refs 15-17. The resonance integrals were calculated by the Wolfsberg-Helmholz method18 with a parameter k = 0.86. The two-center repulsion integrals were obtained by the Mataga-Nishimoto f o r m ~ l a . 'The ~ nn* transition energies were obtained by using the CNDOIS method.20
Results and Discussion The absorption and MCD spectra of xanthone, thioxanthone, and acridone are shown in Figures 2, 3, and 4, respectively, together with the CD spectrum of the inclusion complex with P-cyclodextrin. In Figures 2-4, the oscillator strength values and (- 1) x (Faraday B values) are presented as bars. The MCD and CD spectra below 210 nm cannot be observed due to the absorption of solvents. According to the CNDO/S calculation of xanthone, there are a forbidden lA2 (nn*) transition at 367 nm and an allowed lB1 (m*)transition at 212 nm. The MCD spectrum of xanthone shows the negative Faraday B term around 360 nm. In ketones of CzV symmetry, vibrations provide essentially the only source of a Faraday B term of electronically
0 1994 American Chemical Society
Circular Dichroism Spectra of Xanthones
J. Phys. Chem., Vol. 98, No. 41, 1994 10433
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Figure 2. MCD spectrum (a) and absorption spectrum (c) of xanthone in cyclohexane, and CD spectrum (b) of the P-cyclodextrin complex with xanthone in -10%aqueous ethanol solution. The bars are calculated oscillator strengths (b) and calculated ( - E ) values (c) for the ZZ* transitions. For the oscillator strengths, the negative bars represent the z-polarized transitions and positive bars represent the y-polarized transitions.
forbidden nn* transitions. It is shown that the bl and a2 vibrational modes contribute a positive Faraday B term and the b2 mode contributes a negative Faraday B term in the C2" ketones.21s22 The negative Faraday B term around 360 nm comes from the interaction between the forbidden 'A2 (nn*) transition and the allowed 'B1 (nn*) transition through the b2 vibrations. The geometrical structure of P-cy~lodextrin~~ excludes the formation of an equatorial inclusion complex in the case of xanthone, thioxanthone, and acridone. Thus, the long molecular axis (y axis) of xanthones is parallel to the molecular axis of P-cyclodextrin. It has been shown that the transition of the guest molecules with a transition dipole moment parallel to the molecular axis of P-cyclodextrin results in a positive CD value. On the other hand, the transition with a transition dipole moment perpendicular to the axis of P-cyclodextrin results in a negative CD value. The CD spectrum of xanthone shows that the transition at 378 nm is polarized perpendicular to the y axis. Yamaguchi et al. show that the symmetry species of the vibrational modes in vibronic states are determined from the signs of CD spectra.24 A b2 vibration couples the A2 (nn*) electronic state and makes the B1 vibronic state. The B1 vibronic state has the transition dipole moment perpendicular to the y axis of xanthone. Thus, the CD spectrum shows the negative CD value. In xanthone, the b2 vibrational mode contributes significantly to the MCD and CD value of the forbidden lA2 (nn*) transition. In the case of thioxanthone and acridone, the CNDO/S and PPP calculations show the first forbidden 'A2 (nn*) transition and the second allowed 'A1 (nx*)transition. However, the peak of the nx* transition is not observed in the MCD and CD spectra. When the a2 vibrational model couples the first
200
300
400
h /nm Figure 3. MCD spectrum (a) and absorption spectrum (c) of thioxanthone in cyclohexane, and CD spectrum (b) of the B-cyclodextrin complex with thioxanthone in 10%aqueous ethanol solution. The bars are calculated oscillator strengths (b) and calculated (-B) values (c) for the ZZ* transitions.
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-6
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300
400
h / nm Figure 4. MCD spectrum (a) and absorption spectrum (c) of acridone in ethanol, and CD spectrum (b) of the P-cyclodextrin complex with acridone in 10% aqueous ethanol solution. The bars are calculated oscillator strengths (b) and calculated ( - E ) values (c) for the ZZ* transitions.
forbidden 'A2 (nn*) band and the second allowed 'A1 (nn*) band, the positive Faraday B term and negative CD value (of
Ishijima et al.
10434 J. Phys. Chem., Vol. 98, No. 41, 1994 AE 1o* - 21 n . O i
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Figure 5. Energy levels and coefficients of the PPP MO's of xanthones.
the z-polarized A1 vibronic state) will be observed. In that case, the MCD or CD band of the forbidden nn* transition may overlap with that of the second n n * transition. In such a case, it is very difficult to measure the MCD and CD band of the forbidden nn* transition. The CD spectra of xanthones show that the second absorption band (288-360 nm, A, = 336 nm in xanthone; 310-400 nm, A, = 377 nm in thioxanthone; 318-420 nm, A,, = 398 nm in acridone) consists of a z-polarized electronic transition. The PPP calculation shows that the second band is the nn* 'A1 transition and agrees with the sign of the CD spectrum. The sign of the MCD band of xanthone is opposite to the sign of the MCD bands of the thioxanthone and acridone. The value of the MCD band of xanthone is fairly small compared to that of thioxanthone or acridone. The 'A1 (nn*) transition is mainly due to the one-electron transition from the highest occupied MO ( q g ) to the lowest unoccupied MO (99.). Figure 5 shows the coefficients and energy levels of MO's for xanthones. The oneelectron transition qg q 9 * shows a charge transfer from S and NH to C=O for thioxanthone and acridone, respectively. For xanthone, the charge transfer from 0 to C=O does not contribute to the transition v)g q 9 * . The dominant term in of the 'A1 (G 1) transition is the Faraday B
-
- -
(Ek- ~ ~ ) - l ~ ~ ( ( l l m l ~x )(klPlG)) ~~l~ll) where m and p are the magnetic and electric dipole moment operator, respectively. For xanthone, thioxanthone, and acridone, the electronic states ( k ) are 3, 5 , and 2, respectively. The electronic states k are due to the one-electron v)S v)10* transition for thioxanthone and acridone and the 4)6 q9. transition for xanthone. On the one-electron level, (p9*lmlqlo*)(q81p1q9*)x (qglplqlo*) is the dominant matrix element for thioxanthone and acridone. The signs of (v)9*lmlql0*)and (qgIpIp19*) x (qslplplo:) of thioxanthone are the same as those of acridone. Thus Faraday B terms in thioxanthone and acridone have the identical signs. For xanthone, (qglmlq&(p,slplv)9*) x (4)61p1p9*)is the dominant matrix element. The distinction of the matrix element may affect the differences of sign and intensities of the MCD spectra. According to the PPP calculation, the third and fourth bands of xanthones are 'Bz (nn*)transitions polarized along the y
--
380
390
0 400
?. / nm Figure 6. MCD and absorption spectra of xanthone in n-pentane.
axis and have mutually opposite Faraday B terms. A MCD band is observed at about 267-290 nm in xanthone, 279-306 nm in thioxanthone, and 278-319 nm in acridone. The positive CD band is measured in the above wavelength region. The absorption band with two vibrational peaks in the above wavelength region is assigned to the third lB2 (nn*) transition. In xanthone, a 252-270 nm band appears in the absorption spectrum and a positive band (about 250-270 nm) in the CD spectrum. Thus, the 252-270 nm absorption band is assigned to the fourth lB2 (nn*) transition. These assignments of the x n * transitions of xanthone are consistent with those of Minegishi et aL2' The Faraday B term of the fourth transition may be hidden by the next strong MCD band. In the MCD spectrum, a 275 nm negative shoulder is observed in thioxanthone and a 272 nm strong negative peak in acridone. The shoulder or peak is the fourth 'B2 (nn*) transition. In the absorption spectrum of thioxanthone and acridone, the fourth transition may overlap with the strong absorption band (225280 nm in thioxanthone and 228-278 nm in acridone). In the CD spectrum of acridone, the negative and positive strong bands appear in the 228-278 nm absorption band. The PPP calculation and CD spectrum show that the negative (A, = 263 nm) and positive (Amm = 252 nm) bands are assigned to the 'A1 and lB2 (nn*) transitions, respectively. These assignments are consistent with those of Inoue et aLZ8 However, they do not observe the fourth lB2 (nn*) transition at 272 nm. In the CD spectrum, the positive bands are observed below 240 nm in xanthone and below 270 nm in thioxanthone. Since the PPP calculations show that there are many transition states in those regions, it is impossible to assign the spectra in the short wavelength regions. The MCD spectra of xanthone in n-pentane show the very weak bisignal at 384 nm (Figure 6). This transition has already been observed in the electronic spectra.29 On the basis of polarized phosphorescence excitation spectra, Pownall et al. assign this transition to the 'A1 3A2(z polarization) transition. The phosphorescence spectra of xanthone are very sensitive to the temperature and to the host matrix e m p l ~ y e d .Connors ~ and Christian2have recorded and analyzed the phosphorescence spectra in a nonpolar solvent (n-hexane). They indicate that at 4.2 K only emission from T1 (xx*) is observed. As the temperature is increased, emission from T2 (nn*) can be observed. The measurement of the MCD spectrum was carried out at room temperature. By using 1-iodopentane instead of n-pentane, the MCD signal was not enhanced, in accordance with the m* character of the transition. We conclude that the MCD signal is the Faraday A term for the 3A2(nn*) transition.
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References and Notes (1) Griesser, H. J.; Bramely, R. Chem. Phys. 1982,67, 373. (2) Connors, R. E.; Christian, W. R. J . Phys. Chem. 1982,86, 1524.
Circular Dichroism Spectra of Xanthones (3) Connors, R. E.; Sweeney, R. J.; Cerio, F. J . Phys. Chem. 1987, 91, 819. (4) Koyanagi, M.; Terada, T.; Nakashima, K. J. Chem. Phys. 1988, 89, 7349. (5) Dalton, J. C.; Montgomery, F. C. J . Am. Chem. SOC.1974,96,6230. (6) Griesser, H. J.; Bramley, R. Chem. Phys. Lett. 1982, 86, 144. (7) Meier, A. R.; Wagnikre, G . H. Chem. Phys. 1987, 113, 287. (8) Frei, J.; Yamaguchi, H.; Tsunetsugu, J.; Wagnikre, G.J. Am. Chem. SOC.1990, 112, 1413. (9) Yamaguchi, H.; Abe, S. J . Phys. Chem. 1981, 85, 1640. (10) Higashi, M.; Yamaguchi, H.; Machiguchi, T.; Hasegawa, T.; Baumann, H. Helv. Chim. Acta 1992, 75, 1880. (11) Iijima, K.; Misu, T.; Ohnishi, S.; Onuma, S. J . Mol. Struct. 1989, 213, 263. (12) Iijima, K.; Oonishi, I.; Fujisawa, S.; Shibata, S. Bull. Chem. SOC. Jpn. 1987, 60, 3887. (13) MOPAC Ver. 6: Stewart, J. J. P. QCPE Bull. 1989,9, 10. Revised as Ver. 6.01 by Hirano, T., University of Tokyo, for the HITAC machine: JCPE Newsl. 1991, 2, 26. Revised as Ver. 6.02 by one of the authors (M.H.). (14) Meier, A. R.; Wagnibre, G.H. Chem. Phys. 1987, 113, 287. (15) Bossa, M. J . Chem. SOC. B 1969, 1182.
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