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M. S.Wrighton, L. Pdungsap, and D. L. Morse
(5) D.W. Thomas and A. E. Martell, J. Amer. Chem. Soc., 78, 1338 (1956). (6) H. W. Whitlock, R. Hanauer, M. Y . Oester, and D. K. Bower, J. Amer. Chem. Soc., 91, 7485 (1969). (7) L. Epstein. D. K. Stoaub, and C. Maricondi, Inorg. Chem., 6, 1720 (1967). (8) J. M. Assour, private communication. (9) E. A. Lucia, F. D. Verderame. and G. Taddei, J. Chem. Phys., 52, 2307 (1970). (10) A. Yamamoto, L. Philips, and M. Calvin, lnorg. Chem., 7, 847 (1968). (11) G. D. Dorough. J. R. Miller, and F. M. Huennekens, J. Amer. Chem. SOC., 73, 4315 (1951). (12) G. W. Robinson, J. Mol. Spectrosc., 6, 58 (1961). (13) M.Gouterman, J. Chem. Phys., 30, 1139 (1959).
(14) B. S. Anex and R. S.Umans, J. Amer. Chem. Soc., 86, 5026 (1964). (15) (a) J. M. Robertson, J. Chem. Soc., 1195 (1936); (b) J. M. Robertson, ibid., 1809 (1939). (16) J. Webb and E. B. Fleischer, J. Chem. Phys., 43, 3100 (1965). (17) B. Pullman, C. Spanjaard, and G. Berthier, Proc. Nat. Acad. Sci. U.S., 73, 1011 (1960). (18) K. Ohno. Y. Tanabe, and F. Sasaki, Theor. Chim. Acta, 1, 378 (1963). (19) M. Zerner and M. Gouterman, Theor. Chim. Acta, 4, 44 (1986). (20) M. Zerner. M. Gouterman, and H. Kobayashi, Theor. Chim. Acta, 6, 363 (1966). (21) E. Antonini and M. Brunori, "Hemoglobin and Myoglobin in Their Reactions with Ligands," North-Holand Publishing Co.. Amsterdam, 1971, Chapter 1.
Excited State Quenching Activity of d6 Metallocenes and a Detailed Study of Ruthenocene Luminescence Mark S. Wrighton,* Laddawan Pdungsap, and David L. Morse Department of Chemistry, Massachusetts lnstitute of Technology, Cambridge, Massachusetts 02139 (Received June 17, 1974) Publication costs assisted by the National Science Foundation
Quenching of electronically excited benzil and ruthenium(2,2'-bipyridine)32f by d6 metallocenes of Fe, Ru, Os, and Co has been investigated. Consistent with quenching by electronic energy transfer, the quenching rates correlate with the position of the lowest accessible triplet state in absorption in the metallocene compared to the excitation energy available in the donor. Luminescence from pure solids of ruthenocene and 1,l'-diacetylruthenocene a t 25'K has been measured and the emission maximum at -17,000 cm-l is red shifted by nearly 10,000 cm-l from the first singlet triplet absorption maximum. The emission intensity in each case is temperature dependent and for ruthenocene both lifetimes (127 psec at 28'K) and quantum yields (0,027 f 0.005 a t 28'K) have been measured revealing that the decreasing emission efficiency a t higher temperatures is due principally to a faster rate of nonradiative decay. The structured emission of ruthenocene shows two vibrational progressions separated by 162 10 cm-l with vibrational spacings of 327 f 10 and 340 f 10 cm-l associated with the ring-metal stretch found a t 330 cm-' in the Raman.
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*
Since the geometrical structural characterization of ferrocene in 1952l investigations of the electronic structure of metallocenes have been numerous,2-6 and it is now established* that the lowest excited states of the d6 complexes are ligand field states. Even so, important aspects of the photochemical and photophysical behavior of these substances remain u n r e ~ o l v e d .The ~ effective energetic position of the lowest excited state is key to further progress in understanding the excited state chemistry. Ferrocene itself has been the object of most detailed studies and it now appears that early literature reports3a35 of its luminescence are in error.8 The quenching ability of ferrocene has also created some interest in that anthracene triplet excited states (triplet energy, E T = 14,700 ~ m - l are ) ~ deactivated by ferrocene at a diffusion-controlled ratelo even though triplet absorption maximum in ferrothe lowest singlet cene is at approximately 18,600 cm-1.2 We have undertaken two lines of investigation to begin further characterization of the lowest excited states in metallocenes: (1) determination of the quenching ability of several metallocenes having large variations in excited state energy in comparison to the donor and (2) study of metallocenes which are found to luminesce upon optical excita-
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The Journalof Physical Chemistry, Vol. 79, No. 7, 1975
tion. In this report we describe our results for several metallocenes of the d6 electronic configuration.
Experimental Section Spectra. All uv-visible absorption spectra were recorded on a Cary 17 spectrophotometer. Low-temperature (WOK) absorption spectra were obtained using an all quartz dewar with optical quality flats for windows. The sample was dissolved in EPA and placed in a round quartz cell placed in direct contact with liquid nitrogen. The emission spectrometer used for the emission studies is an Aminco-Bowman spectrophotofluorometer set up for emission measurements in the 300-900-nm range and equipped with a grating blazed a t 750 nm. The detectors used were either a Hamamatsu R136 P M T operated a t 800 V and 25' or an RCA 7102 P M T operated a t 1200-1500 V and cooled with a Dry Ice-2-propanol bath, Both the emission and excitation monochromators were calibrated using a low-pressure Hg lamp and readings are found to be within 3 nm of the expected values. The relative sensitivity of the entire detection system (and for both P M T detectors) has been calibrated in the range 300-600 nm using the Rhodamine B quantum counterll and over the entire 300-900-nm range
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Excited State Quenching Activity of d6 Metallocenes TABLE I: Quenching Activity of Several de Metallocenes
Buencher Ferrocene Acetylferrocene Benzoylferrocene Cobalticinium perchlorate Ruthenocene 1,l’-Diacetylruthenocene
Acetylosmocene
Lowest absptn max quencher cm-1 x 10-3 ( e \
h, for b e n d (solvent), M - 1 sec-1 rt 10%
k , for Ru(bipy)az+(solvent), M-1 sec-1 d= 10%
22.72 (90) 22.37 (290) 21.74 (740) 24.69 (200) 31.25 (220) 30.30 (770) 29.76 (770)
1 . 2 x 10lO(isooctane) 9 . 3 x lO3(isooctane) 1 , 1 x 101O(isooctane) 2 . O X 1Og(CH3CN) 1 . 4 x 1O9(isooctane) 2 . 5 x 107(isooctane) 4 .2 X lo8(isooctane)
5 . 9 x lOg(Et0H) 3 . 4 X 10g(EtOH) 7 . 4 X 10g(EtOH) 1 . 5 X 10B(EtOH:H20, 1:1 V / V )
