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Fluorescence Properties of Carboxyl Substituted Anthracenes
An Investigation of the Fluorescence Properties of Carboxyl Substituted Anthracenes T. C. Werner,. Thomas Matthews, and Babs Soller Department of Chemistiy, Union College, Schenectady, New York 12308 (Received August 1, 1975) Publication costs asslsted by the Petroleum Research Fund
The spectral properties of the three monocarboxyl substituted anthracenes have been investigated. Methyl 9-anthroate (g-COOMe), methyl 1-anthroate (1-COOMe), and methyl 2-anthroate (2-COOMe) are the specific compounds used in this study. Their fluorescent lifetimes (rf),quantum yields (4f),and spectra are shown to be quite dependent on solvent and the position of substitution. The electronic transition responsible for the long wavelength absorption and the fluorescence spectrum of all three esters is primarily the lL, anthracene transition. No evidence is found for enhancement of the long axis polarized lA1 lA1 'Lb transition or for a solvent induced inversion of the 'La and 'Lb states when the carboxyl group is in the 2 position. HMO calculations, molecular models, and electronic spectra suggest that a substantial change in the equilibrium molecular geometry occurs on excitation to the first excited singlet state (SI) for 9COOMe. By contrast, for 1 and 2 substitution the equilibrium geometry does not change much on excitation. However, a restriction in the number of conformers which can exist about the minimum energy conformation is observed in S1 relative to the ground state. The variation in & for 1-and 2-COOMe from nonpolar solvents through solvents of moderate hydrogen bonding properties is consistent with the existence of a thermally assisted intersystem crossing (ISC) pathway to a triplet level near SI. This triplet level is thought to be the second anthracene triplet rather than an n , r * triplet as previously implicated in the ISC process for carbonyl-substituted anthracenes. In strong hydrogen bonding solvents a solvent induced internal conversion pathway replaces ISC as the major nonradiative pathway.
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Introduction Anthracene has three positions for monosubstitution and the ability of a substituent to electronically interact with the anthracene ring is quite dependent on the position of substitution. In the 9 or meso position, a substituent encounters the most electron-rich position of the ring and it also lies along the polarization axis of the lowest energy anlL,). However, complete coplathracene transition (lA1 narity between a substituent a t the 9 position and the ring is often prevented by steric hindrance with the peri hydrogens a t the 1and 8 positions. For substitution in the 2 position, which is the least electron-rich position, the substituent axis is more closely aligned with the polarization direction of the second anthracene transition (lAl- 'Lb). Steric hindrance to coplanarity from ring hydrogens appears to be insignificant for 2 substitution. The electron density at the 1 position is intermediate between the other two positions. A substituent in the 1 position lies more nearly along the lL, transition and encounters consideraaxis of the lAl bly less steric hindrance from ring hydrogens than in the 9 position but more than in the 2 position. It is not surprising then that the spectral properties of monosubstituted anthracenes are dependent on the position of substitution. When the substituent is a carboxyl group, the resulting anthracene derivatives have some unusual excited state behavior. The carboxyl group, by its electron-withdrawing nature, is classified as a fluorescence inhibiting group and benzoic acid is indeed nonfluorescent.2 Other similar substituents (nitro, carbonyl) produce virtually nonfluorescent anthracene derivative^^,^ but the anthroic acids and their esters are quite fluorescent in some solvents. The 4f values for all three of the methylanthroates are quite solvent dependent and the nature of this solvent dependency is very much a function of the position of substitution. The lowest excited singlet energies of all three esters are also solvent
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dependent showing increasing red shifts as solvent polarity increases. In addition, a comparison of the absorption and fluorescence spectra of methyl 9-anthroate (9-COOMe) reveals a substantial difference in the equilibrium geometry of the ground (SO)and lowest excited singlet state (S1).5-7 For methyl 1-anthroate (1-COOMe), the spectra are consistent with a smaller difference in the equilibrium geometries of the two states while for methyl 2-anthroate (2-COOMe) the equilibrium geometry of both SO and SI appears to be quite similar. We have previously discussed the spectral properties of 9-substituted esters in some detai1.5-7 In this work, we have extended our investigation to the 1-and 2-methyl esters in order to explain the solvent and substituent position dependence observed for the fluorescence behavior of monocarboxyl-substituted anthracenes. A correction is also made in the previous assignment of the emitting state of 2-anthroic acid in polar solvents which we now believe is in error.8
Experimental Section The ethanol used in the spectral measurements was obtained from U.S. Industrial Chemicals. All other spectral solvents were Matheson Coleman and Bell Spectroquality solvents. Methyl 9-anthroate was prepared from 9-anthroic acid (Aldrich Chemical Co.) and d i a z ~ r n e t h a n eMethyl .~ 2-anthroate was made by a free acid esterification of 2-anthroic acid (K & K Labs) with methanol with subsequent recrystallization from glacial acetic acid.1° Methyl 1-anthroate was synthesized in a three step procedure beginning with benzanthrone (Aldrich Chemical Co.). To remove impurities, the benzanthrone was extracted with acetone in a Soxhlet extractor. After removal of the acetone, the benzanthrone was allowed to react to form anthraquinone-lThe Journal of Physical Chemistry, Vol. 80. No. 5, 1976
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carboxylic acid.1° This acid was then reduced to 1-anthroic acid and then esterified with methanol using the free acid procedure of Coulson.lo Purification of the final ester product was performed by dissolving the ester in hot glacial acetic acid and inducing crystallization by the addition of hot water. All melting points agreed with literature values. Absorption measurements were made on either a Cary Model 14 spectrophotometer or a Beckman D.U. spectrophotometer. Fluorescence emission spectra were recorded on a Perkin-Elmer MPF-2A spectrofluorometer using excitation and emission band passes of 7 and 3 nm, respectively. The spectra were corrected for the response function of the instrument as described earlier.5a The response function was obtained by comparison of the fluorescence spectra of methyl 9-anthroate in several solvents taken on our instrument with those on an identical instrument having a corrected spectral attachment. The band passes above were identical with those used on the latter instrument. Fluorescence excitation spectra at 77 K were obtained using the phosphorescence attachment of the MPF-2A with the chopper removed. Fluorescence quantum yield (&) determinations were obtained using the following equation where r and u stand for reference and unknown, respectively:
A is the absorbance a t the exciting wavelength (> ki,. Thus we can calculate hisc from eq IV for 2-COOMe in these solvents. Figure 8 shows
a plot of these calculated ki,, values vs. the k , values from the heavy atom quenching experiments. A relatively good linear relationship exists between the two sets of rate constants. Although no direct linear relationship is predicted on theoretical grounds, there seems little doubt that the plot in Figure 8 confirms the mutual dependence of the two rate constants on the same single-triplet gap. We have tried to measure k, values for the ethyl iodide quenching of 9-substituted ester fluorescence. No quenching could be observed with the procedure used to measure k , for 2-COOMe. This is then confirmation of the predicted large A E s ~ - Tand ~ consequent lack of significant ISC for the 9-substituted esters. One note of caution must be added in interpretation of this data. Electron-withdrawing substituents in the 9 position will considerably reduce the electron-donating power of the ring. The anthracene ring apparently behaves as an electron donor in formation of the complex with ethyl iodide which leads to quenching.33 Therefore the lack of heavy atom quenching for a 9-substituted ester could be due in part to weak complex formation as well as a large A E s ~ - T ~ . Finally, it is worthwhile to compare our results obtained with carboxyl substituted anthracenes to recent work with
ESR Line Width Studies of Vanadium(1V)
carbonyl substituted anthracenes. These latter compounds are virtually nonfluorescent a t room temperature in solvents such as cyclohexane and ethanoL4 Presumably, very efficient ISC occurs from S1 to a lower T (n,r*) level of the carbonyl group.4 I t is well known that the S1 (n,n*)energies of aliphatic carboxylic acids and esters are substantially higher than the SI(n,a*) energies of aliphatic aldehydes and ketones.34Assuming the same is true for T1 (n,a*) levels, the T1 (n,a*) state of the carboxyl group should be too high in energy to influence the SI decay of the anthroate esters. In conclusion, then, it is ISC to the T2 level of anthracene rather than to the TI (n,n*) of the carboxyl group which can compete with anthroate fluorescence in nonpolar solvents.
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this work. The authors also wish to thank Dr. Fred Lytle of Purdue University for the lifetime measurements and Dr. Peter Frosch of Union College for developing the computer program for molecular orbital calculations. References and Notes (1) Presented in part at the 169th National Meeting of the American Chemical Society, Philadelphia, Pa., April, 1975. (2) R. Williams, J. Roy. lnst. Chem., 83, 611 (1959). (3) T. Vember, L. Kizanskaya, and A. S. Cherkasov, J. Gen. Chem. USSR, 33, 2281 (1963). (4) T. Matsumoto, M. Sato, and S. Hirayama, Chem. Phys. Len., 13, 13 (1972). ( 5 ) (a) T. C. Werner and R. M. Hoffman, J. Phys. Chem., 77, 1611 (1973);
541 (b) F. E. Lytle, D. R. Storey, and M. E. Juricich, Spectrochim. Acta, Part A, 29, 1357 (1973). (6) T. C. Werner, R. Fisch, and G. Goodman, Spectrosc. Lett., 7, 385 (1974). (7) T. C. Werner and D. M. Hercules, J. Phys. Chem., 73, 2005 (1969). (8) T. C. Werner and D. M. Hercules, J. Phys. Chem., 74, 1030 (1970). (9) J. A. Moore and D. E. Reed, Org. Syn., 41, 16 (1961). (IO) C. A. Coulson, J. Chem. SOC., 1932 (1930). (11) R. F. Chen, Anal. Biochem., 20, 339 (1967). (12) K. Dimroth, C. Reiehert, T. Siermann, and F. Bohlman, Ann., 861, 1 (1963). (13) I. B. Berlman, “Handbook of Fluorescence Spectra of Aromatic Molecules,” 2nd ed, Academic Press, New York, N.Y., 1971, Chapter 1. (14) S.J. Strickler and R. A. Berg, J. Chem. Phys., 37, 814 (1962). (15) A. Streitwieser, Jr., “Molecular Orbital Theory for Organic Chemists”, Wiley, New York, N.Y., 1961. (16) J. R. Platt, J. Chem. Phys., 17, 484 (1949). (17) D. M. Friedrich, R. Mathies, and A. C. Albrecht, J. Mol. Spectrosc., 51, 166 (1974). (18) A. Bergman and J. Jortner, Chem. Phys. Len., 15, 309 (1972). (19) J. G. Foss and M. E. McCarville. J. Chem. Phys., 44, 4350 (1960). (20) J. P. Larkindale and D. J. Simkin, J. Chem. Phys., 5 5 , 5668 (1971). (21) A. E. Lutskii and L. A. Antropova, Zh. Fiz. Khim., 39, 1131 (1965). (22) Reference 13, Chapter 3. (23) T. C. Werner and D.M. Hercules, unpublished studies. (24) S. Suzuki and H. Baba, J. Chem. Phys., 38,349 (1963). (25) J. M. McKelvey and A. D. King, Jr., J. Chem. Phys., 43,3178 (1965). (26) R. 0. C. Norman and P. D. Ralph, J. Chem. SOC.,2221 (1961). (27) Y. Lui and S. P. McGlynn, J. Mol. Spectrosc., 49, 214 (1974). (28) J. Trotter, Acta Crystalogr.,13, 732 (1962). (29) R. S. Becker, “Theory and Interpretation of Fluorescence and Phosphorescence”, Wiley-Interscience, New York, N.Y., 1969, Chapter 11. (30) F. Tanaka and J. Osugi, Chem. Phys. Lett., 27, 133 (1974). (31) We have tried unsuccessfully to measure the phosphorescence of all three esters. Also, Professor D. M. Hercules of the University of Georgia has been unable to detect triplet-triplet absorption for 9-COOMe in several solvents. (32) H. Dreeskamp, E. Koch, and M. Zander, Chem. Phys. Len., 31, 251 (1975). (33) T. C. Werner and B. Soller, unpublished studies. (34) S. Suzuki, “Electronic Absorption Spectra and Geometry of Organic Molecules”, Academic Press, New York, N.Y., 1967, Chapter 21.
Electron Spin Resonance Line Width Studies of Vanadium(lV) in Acidic and Basic Aqueous Solutions’ Melanie M. lannuzzi, Clifford P. Kubiak, and Philip H. Rieger’ Metcalf Research Laboratories, Brown University, Providence, Rhode lsland 029 12 (Received July 23, 1975) Publication costs assisted by the National lnstitutes of Health
ESR line widths of vanadium(1V) in acidic and basic media were determined. Analysis of the mpdependent line width contributions gave rotational correlation times which were linear in TI7 and resulted in radius estimates of 0.34 f 0.01 nm for VO(H20)5*+ and 0.27 f 0.02 nm for YO(OH)3(H20)2-. Spin-rotation line width contributions were found to be in satisfactory agreement with the theory of Atkins and Kivelson. Comparison of residual line widths obtained in HzO and D20 solutions showed that the average contribution of unresolved proton hyperfine structure was about 0.4 G for the aquo vanadyl ion and 6.5 G for the trihydroxo anion. Line shape simulations show the contribution for the aquo ion to be consistent with the proton coupling of 1.1 G determined by NMR methods. A much larger proton coupling is implied for the trihydroxo anion.
Introduction Vanadium(1V) has recently been shown to exist in basic aqueous solution as the trihydroxo anion, VO(OH)S(OH&-.~This species exhibits an eight-line ESR spectrum qualitatively similar to that of the aquo ion but
distinguished by a smaller isotropic vanadium nuclear hyperfine splitting and a less anisotropic g tensor.2 ESR paiameters-for th; trihydroxo anion and