Polarons and Triplet Excitons in C 60 : An Optically Detected Magnetic Resonance (ODMR) and Light-Induced ESR (LESR) Stu
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POLARONS AND TRIPLET EXCITONS IN C6 0 : AN OPTICALLY DETECTED MAGNETIC RESONANCE (ODMR) AND LIGHT-INDUCED ESR (LESR) STUDY P. A. Lane, L. S. Swanson, Q.-X. Ni, and J. Shinar Ames Laboratory - USDOE and Physics Department, Iowa State University, Ames, IA
50011 J Engel and T. J. Barton Ames Laboratory - USDOE and Chemistry Department,Iowa State University, Ames, ]A 50011 J. Wheelock and L. Jones Ames Laboratory - USDOE, Iowa State University, Ames, 1A 50011 ABSTRACT The photoluminescence (PL), X-band ODMR. and LESR of C6 films and C 60 isolated in a toluene /polystyrene glass matrix (C60:T/PS) is described. The delocalized triplet LESR and ODMR of C 6 0 :T/PS are similar to previously reported LESR. In films, however, the ODMR indicates that the delocalized triplet is larger and distorted by neighboring molecules. In addition, another, localized triplet, and a narrow PL-enhancing line at g = 2.0017 ± 0.0005 are observed. These features are similar to those observed in several a-conjugated polymers. The former is attributed to a localized triplet of size similar to a 5- or 6-membered ring, tentatively on a face adjacent to another C 60 molecule; the latter is believed to result from intermolecular polaron recombination. NTTRODUCIION The novel fullerenes in general, and C60 in particular, are attracting intense attention. Recent studies include optical absorption [1-5], photoluminescence (PL) [2,5], ESR [4,6-8], and light-induced ESR (LESR) [9] of films, solutions, and glasses. The absorption spectrum of isolated C6 0 in a toluene/polystyrene (C60 :T/PS) glass matrix [2] and the PL spectra of C60 films [10] display two vibronic progressions. These are the totally symmetric 0.18 eV pentagonal pinch vibrations and additional 0.09 eV (-740 cm-1) lines believed to result from nontotally symmetric phonons [5,11] possibly associated with a Jahn-Teller distortion [2]. While X-band ESR studies yielded varying g-values and satellite structures, the main narrow line was invariably attributed to the radical anion C 60.- [6-8]. The g-value of Cq"" trapped in a molecular sieve was 1.9995, with satellites at ±10 G [8]. The interpretation of the satellites was inconclusive, since they appeared to be consistent with both a hyperfine interaction with five equivalent neighboring 13C nuclei, or with a triplet state of a pair of C6 0 - ions. The intense triplet powder pattern LESR [9] of C6 0 :T/PS at 9 K yielded zero field parameters D = 122 G (0.0114 cr-1 ) and E = 7.4 G (0.00069 cur'1) indicating a nearly axially symmetric triplet delocalized over the C60 molecule. Transient LESR yielded a lifetime of -0.4 ms [9]. The triplet level had been determined to be -1.6 eV [1,3,12]; the absence of observable phosphorescence suggested that its decay is dominated by nonradiative processes [9]. In addition to novel PL spectra and LESR of films described in this work, the PL (i.e., optically) detected magnetic resonance (ODMR) reveal novel features of excited states strikingly reminiscent of those in n-conjugated polymers [13-15]. They include (i
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