Photon Wavelength nm

Fig. 13.19. Photoluminescence intensity at 80 K for a pristine (solid line) and a phototransformed (dotted line) Cm film (~4500 A thick) on a Suprasil fused silica substrate. The same C60 film was used for both spectra [13.128], and after (dashed curve) the phototransformation [13.128]. A ~330 cm'1 spectral red shift and peak broadening of the vibronic transitions in the whole spectrum is observed upon phototransformation. Because of the similarity of the phototransformed spectrum to that of monomeric C60, it is concluded that the features in the photopolymerized spectrum are still closely related to those observed in Fig. 13.6 for the C60 film prior to phototransformation. The red shift of the PL features for the polymer relative to the peak energies for the pristine film is consistent with the luminescence red shifts observed for oligomerization (i.e., monomer -»■ dimer; dimer ->• trimer, etc.), such as is observed in the helicene series and in poly-acenes [13.6]. Furthermore, it can be seen that the two predominant peaks at ~1.47 eV (841 nm or 11,900 cm"1) and 1.65 eV (752 nm or 13,300 cm"1) and the shoulders in the spectrum of the phototransformed sample are slightly broadened compared to those at 1.53 eV (818 nm or 12,200 cm"1) and 1.69 eV (734 nm or 13,600 cm-1) in the pristine spectrum. This broadening effect is consistent with a lifting of the degeneracies in the vibrational modes, as seen in the Raman and IR vibrational spectroscopy of the photopolymer (§11.7) [13.1], and is also consistent with the inhomogeneous broadening discussed above in connection with the OA spectrum of the phototransformed C60.

13.4. Optical Properties of Doped C60

In this section we discuss the optical properties of the MtC60 alkali metal-doped phases for C60. Of these, the M3C60 phase has been studied most extensively, and measurements have been made in both the normal and superconducting phases. Since the optical spectra of these alkali metal compounds do not depend on the alkali metal species M in most cases, it is believed that the optical properties of the alkali metal-doped materials are closely related to the optical properties of the relevant anions themselves, which can be prepared electrolytically and studied as prepared in solution. For this reason we start this section with a summary of the experimental results on the optical spectra of C£0 anions in solution in §13.4.1, which is followed by a review of the optical properties of the M6C60 compounds in §13.4.2 because of the relatively close relation of the M6C60 compounds to crystalline C60. The presentation continues with a review of the optical properties of the M3C60 compounds in the normal and superconducting phases (§13.4.3 and §13.4.4), and the section concludes with a brief summary of the optical properties of the M,C60 compounds (§13.4.5), which have some unique properties.

13.4.1. Optical Absorption of Cm Anions in Solution

C60 has been reduced electrochemically in solution into various oxidation states: C^, n = 1,2,3,4 [13.18,129-132] (see §10.3.2). Although neutral C60 in solution does not exhibit optical absorption in the near IR, distinct absorption bands have been identified in the near IR with specific C^ ions in solution [13.133]. As we shall see, these new absorption bands are a direct consequence of the electron transfer to the tlu (LUMO) level of C60 to form the anions. The Jahn-Teller distortion mechanism is invoked to split the orbitally degenerate C60-derived tlu and tlg levels, and specific transitions between these Jahn-Teller split levels are proposed to explain the observed spectra. Since the tlu - flg splitting in C6(l is ~1.1 eV [13.30,37,41,42,58], new near-IR lines would be expected near ~1000 nm in the absorption spectra for C"M ions. The spectra observed for C60 and the molecular anions Qo" (n = 1,2,3,4) are shown in Figs. 13.20 and 13.21 [13.134], The near-IR spectrum of the C6U anion has been reported by several authors [13.134— 138],

For the spectra shown in Figs. 13.20 and 13.21, the electrochemical reduction of C60 was carried out within the optical spectrometer [13.139], The Qo (~0.15 nM) was dissolved in benzonitrile with 0.1 M Bu4NPF6 used as a supporting electrolyte. Au and Pt were used as the working and counter electrodes, respectively, and the cell potentials were measured relative to a standard electrode [Ag/Ag+ (DMSO)] [13.134], Potentials of —1.1, -1.6, -2.2, and -2.9 V were found to generate the n = 1,2,3, and 4

Wavelength (nm)

Fig. 13.20. UV-visible near-IR absorption spectra for CM (trace A), C^ (trace B), and C^ (trace C). A blank sample of the supporting electrolyte solution was used for background subtraction. The feature at 840 nm in all three spectra corresponds to an instrument detector change [13.134].

Wavelength (nm)

Fig. 13.20. UV-visible near-IR absorption spectra for CM (trace A), C^ (trace B), and C^ (trace C). A blank sample of the supporting electrolyte solution was used for background subtraction. The feature at 840 nm in all three spectra corresponds to an instrument detector change [13.134].

Fig. 13.21. UV-visible near-IR absorption spectra for C^ (trace A) and (trace B). A blank sample of the supporting electrolyte solution was used for background subtraction. The feature at 840 nm in both spectra corresponds to an instrument detector change. The absorbance scale in this figure is 0.7 relative to that in Fig. 13.20 [13.134],

Fig. 13.21. UV-visible near-IR absorption spectra for C^ (trace A) and (trace B). A blank sample of the supporting electrolyte solution was used for background subtraction. The feature at 840 nm in both spectra corresponds to an instrument detector change. The absorbance scale in this figure is 0.7 relative to that in Fig. 13.20 [13.134], molecular anions, respectively, in good agreement with the literature (see §10.3.2).

To provide a quantitative optical basis for the determination of the concentrations of these C£0~ species, Lawson et al. [13.134] determined the molar extinction coefficients at wavelengths corresponding to the positions of the absorption band maxima (Amax) in the near-IR and in the red regions of the spectra shown in Figs. 13.20 and 13.21. Their results for Amax (corresponding to the peaks in the absorption) and the molar extinction ratios for C6V (n = 1,2,3,4) are summarized in Table 13.5 [13.134],

Lawson et al. [13.133,134] have proposed the schematic molecular orbital and optical transition diagrams shown in Fig. 13.22(A), (B) and (C) for Qo, Cgô and C2m and in Fig. 13.23(A) and (B) for C30" and respectively, to qualitatively explain their observed spectra. In these figures, the solid vertical arrows indicate allowed dipole transitions and the dashed arrows identify transitions which are not dipole allowed, but are activated by vibronic coupling, as described in §13.1.3 for the hu -» i,„ transitions in neutral C60. Above each level diagram is found a simulated spectrum, although the oscillator strengths and the widths of the various bands in the simulated spectra are not derived from a calculation. The optical response of isolated neutral C60 molecules in solution has been previously described in detail in §13.2.1 and §13.2.2.

The level diagram for the monoanion C^ in Fig. 13.22(A) shows the threefold degenerate tlu LUMO of C60 split into a2u and elu states by a Jahn-Teller distortion, thus removing the degeneracy of the ground state.

Table 13.5

Spectral data for various v -tli; species in benzonitrile solution [13.134],

Table 13.5

Spectral data for various v -tli; species in benzonitrile solution [13.134],

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