J. Phys. Chem. 1984,88, 5868-5873
5868
Fluorescence Quenching Mechanism of Aromatic Hydrocarbons by Closed-Shell Heavy Metal Ions in Aqueous and Organlc Solutionst Hiroshi Masuhara,*Hiroshi Shioyama, Tokashi Saito, Kei Hamada, Seikichi Yasoshima, and Noboru Mataga* Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: May 22, 1984)
Fluorescence quenching rate constants of some aromatic hydrocarbons by Zn2+,Ag", Cd2", In3+,SnZ+,Cs', HgZ+,T1+, and PbZ+were determined in aqueous and N,N-dimethylformamide solutions. Paramagnetic interactions, the heavy atom effect, and electron transfer were excluded as a possible quenching mechanism. Nz gas laser photolysis studies revealed that a cation radical of the fluorescer was detected only for N-ethylcarbazolequenched by Ag", PbZ+,and Hg2+in N,N-dimethylformamide and for 1-pyrenesulfonicacid by Hg2+ in water. All other systems yielded the triplet state of the fluorescer quantitatively. The intermediates observed in the microsecond time region are the transient species with the lowest free energy. Picosecond laser photolysis of the Ag+ and PbZ+quencher systems in N,N-dimethylformamide confirmed directly that the triplet state is induced by fluorescence quenching. On the basis of these results, it has been concluded that fluorescence quenching is due to nonfluorescent complex formation followed by rapid intersystem crossing. The electronic and geometrical structures of this complex were considered and compared to the excited aromatic hydrocarbon-halogen anion systems.
Introduction Fluorescence quenching processes of aromatic hydrocarbons with an organic donor or acceptor in organic solvents have been studied in detail, and the obtained rate constants (k,) correlate well with the free energy change due to electron transfer.' This electron transfer quenching results in the formation of ion radicals of the aromatic hydrocarbon and quencher molecules in polar solvents; this has been directly confirmed by transient absorption spectral and transient photoconductivity measurements.2 For quenching by inorganic anions, such a complete one-electron transfer is energetically impossible, and a partial charge-transfer interaction was ~uggested.9~No aromatic hydrocarbon anion radical was observed as an intermediate after quenching by the flash photolysis method. Summarizing these results, it has been considered in general that a weak intermediate complex with a short lifetime plays an important role in fluorescence quenching.$ On the other hand, the quenching mechanism by metal ions still seems to be beyond our knowledge. Electron transfer operates in the quenching by lanthanides,6 while electron as well as energy transfer and paramagnetic interactions are responsible for the quenching by other transition-metal A relation between kq and the inherent intersystem crossing of aromatic hydrocarbons was reported in systems with xenon and CsCl quenchers.11q12A direct canfirmation of this enhanced intersystem crossing was given for the Ag+ quencher system by measuring the absorption spectra of the produced triplet state.13 Such an application of the nanosecond laser photolysis technique was reported for aqueous and micellar solutions including various metal ions.14 Some of the systems yielded the triplet aromatic hydrocarbon and others its cation radical. All these results indicate that the quenching mechanism depends rather strongly upon the chemical nature of the metal ions used. From this viewpoint we have focussed our efforts on fluorescence quenching by closed-shell heavy metal ions in the present work. Their spin multiplicity is singlet and no paramagnetic interactions are involved. Their absorption band is in the wavelength region shorter than that of aromatic hydrocarbons, which excludes an efficient energy transfer to metal ions. Therefore, the present work will help to clarify the differences in quenching mechanisms between metal ions and other quencher systems. Experimental Section 1-Pyrenesulfonic acid (Eastman Kodak) was used without further purification. 1-Naphthalenesulfonic acid (Kit Chemical Presented in part at the Japanese Symposium on Photochemistry (Kyoto, 1978, and Tsu, 1980).
*Present address: Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan.
0022-365418412088-5868SOl .50/0
Co. Ltd., EP grade) was twice recrystallized from water. Other aromatic hydrocarbons were the same as used before.', The used quenchers were Zn(C104)2-6H20,AgC1O4-HZ0,CdCl2-2.5H,O, InCl,, SnS04,CsCl, Hg(C104),, TINO,, and Pb(C104),~3H,0. They have no appreciable absorption band in the wavelength region longer than 300 nm and the counteranions were confirmed not to affect the quenching behavior. These were carefully dried and weighted. For Pb(C104)2.3Hz0 which is barely dried, the concentration of Pbz+ in water was determined by titration. Deionized water was purified by distillation below the boiling point. The pH of aqueous solutions was adjusted to be zero by adding HC104 (Kishida GR grade) which made it possible to solubilize Hg(C104)*and Sn04 without chemical change. The fluorescence intensity of 1-naphthalenesulfonic acid was increased by 5% at this pH. N,N-Dimethylformamide was distilled under a reduced N2 pressure. Solutions were deaerated by N2 gas bubbling or degassed by the freeze-pump-thaw method. Fluorescence spectra were measured with an Aminco-Bowman spectrophotofluorometer. The method for measuring the fluorescence decay curves was the same as used before.15 Na(1) Rehm, D.;Weller, A. Ber. Bunsenges. Phys. Chem. 1969, 73, 843; Isr. J. Chem. 1970,8, 259. (2) Knibbe, H.; Rehm, D.; Weller, A. Ber. Bunsenges. Phys. Chem. 1968, 72,257. Grellmann, K. H.; Watkins, A. R.; Weller, A. J. Phys. Chem. 1972, 76,469, 3132. Taniguchi, Y.; Nishina, Y.; Mataga, N. Bull. Chem. SOC.Jpn. 1972,45,764. Masuhara, H.; Hino, T.; Mataga, N. J. Phys. Chem. 1975, Masuhara, H.; Mataga, N. J. Phys. Chern. 79,994. Hino, T.; Akazawa, H.; 1976, 80, 33. (3! Watkins, A. R. J. Phys. Chem. 1973,77,1207; 1974,78,1885,2555. Treinin, A.; Hayon, E. J. Am. Chem. SOC.1976, 98, 3884. (4) Shizuka, H.; Nakamura, M.; Morita, T. J. Phys. Chem. 1980,84,989. ( 5 ) Mataga, N.; Okada, T.; Yamamoto, N. Chem. Phys. Lett. 1967,1,119. Masuhara, H.; Mataga, N. Acc. Chem. Res. 1981, 14, 312. Hirata, Y.; Kanda, Y.; Mataga, N. J. Phys. Chem. 1983,87, 1659. Mataga, N. Radiat. Phys. Chem. 1983,21,83. Weller, A., In "Light-Induced Charge Separation in Biology and Chemistry"; Gerischer, H., Katz, J. J., Eds.; Verlag Chemie: Weinheim, West Germany, 1979; p 131. Weller, A. Z . Phys. Chem. (Frankfurt am Main) 1982, 130, 129. (6) Ricci, R. W.; Kilichowski, K. B. J. Phys. Chem. 1974, 78, 1953. (7) Varnes, A. W.; Dodson, R. B.; Wehry, E. L. J. Am. Chem. SOC.1972, 94, 946. (8) Kelmo, J. A.; Shepherd, T. M. Chem. Phys. Lett. 1977, 47, 158. (9) Formosinho, S. J. Mol. Photochem. 1976, 7, 13. (10) Wehry, E. L. In 'Practical Fluorescence"; Guilbault, G. C., Ed.; Marcel Dekker: New York, 1973; p 120. Lamola, A. A.; Eisinger, J. In "Molecular Luminescence"; Lim, E. C., Ed.; W. A. Benjamin: New York, 1969; p 801. (11) Horrocks, A. R.; Kearvell, A,; Tichle, K.; Wilkinson, F. Trans. Faraday SOC.1966,62, 3393. (12) Patterson, L. K.; Rzad, S. J. Chem. Phys. Lett. 1975, 31, 254. (13) Saito, T.; Yasoshima, S.; Masuhara, H.; Mataga, N. Chem. Phys. Leu. 1978, 59, 193. (14) Nosaka, Y.; Kira, A.; Imamura, M. J. Phys. Chem. 1981,85, 1354. Nakamura, T.; Kira, A.; Imamura, M. J . Phys. Chem. 1982,86, 3359.
0 1984 American Chemical Societv
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 5869
Fluorescence Quenching of Aromatic Hydrocarbon
TABLE I: Fluorescence Quenching Rate Constants of 1-Pyrenesulfonicand 1-Naphthalenesulfonic Acid-Metal Ion Systems in Aqueous Solutions electronic -E(M(Fl)+/ ion Z configuration Zpr(l eV him+): v' kq(PyS),CM-' s-' k,(NapS),d M-I s-I Zn2+ 30 3d1° 39.70 -1.98
