J. Phys. Chem. B 2000, 104, 4625-4632
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Photophysical and Electron-Transfer Properties of Mono- and Multiple-Functionalized Fullerene Derivatives Ya-Ping Sun,*,† Radhakishan Guduru,† Glenn E. Lawson,† Jason E. Mullins,† Zhixin Guo,† Jessica Quinlan,† Christopher E. Bunker,‡,§ and James R. Gord‡ Department of Chemistry and Center for AdVanced Engineering Fibers and Films, Howard L. Hunter Chemistry Laboratory, Clemson UniVersity, Clemson, South Carolina 29634-0973, and Air Force Research Laboratory, Propulsion Directorate, Wright-Patterson Air Force Base, Dayton, Ohio 45433-7103 ReceiVed: January 5, 2000; In Final Form: March 1, 2000
Mono- and multiple-functionalized C60 derivatives were synthesized and studied for their photophysical properties. Electronic absorption spectra and absorptivities of the C60 derivatives in solution were measured and compared. By recording the fluorescence spectra using a near-infrared-sensitive emission spectrometer, we quantitatively determined fluorescence quantum yields of the C60 derivatives. For the mono-functionalized C60 derivatives, the compound with a [5,6]-open fulleroid addition pattern on the fullerene cage appeared to be considerably less fluorescent than those with a [6,6]-closed cage addition pattern. Despite the disturbance of the electronic structure via multiple additions to the fullerene cage, the multiple-functionalized C60 derivatives exhibited no dramatic changes in fluorescence quantum yields in comparison with the mono-functionalized C60 derivatives. The fluorescence lifetimes of the C60 derivatives, obtained using the time-correlated single photon-counting technique, were all in the range of 1-3 ns. In addition, the dependencies of the fluorescence intensities and lifetimes of the C60 derivatives on the concentration of the quencher N,N-diethylaniline (DEA) were evaluated. Apparently, upon photoexcitation, even the C60 derivatives with a hexa-functionalized fullerene cage underwent significant electron-transfer interactions with the electron donor DEA, resulting in efficient fluorescence quenching. In a polar solvent, the contribution of static quenching also became important. The results for different derivatives and their molecular structural and mechanistic significance are discussed.
Introduction The photophysical and electron-transfer properties of fullerenes have attracted much attention, in an effort not only to obtain a fundamental understanding of the electronic characteristics of this new class of molecules but also to explore their potential technological applications.1-5 For example, fullerenes are actively investigated as nonlinear absorbers for limiting pulsed laser irradiation, where the optical limiting results are typically interpreted in terms of the photophysical parameters of the molecules.6-9 Fullerenes are also excellent electron acceptors; their use in photoinduced redox systems has been a focus of several recent investigations.2,4,5,10 Fullerene C60 can be derivatized via a large number of known cage functionalization methods.11 Several photophysical studies of fullerene derivatives have been reported for a systematic evaluation of the effects of derivatization on the photoexcited state and redox properties of fullerenes.5 Among the more widely investigated C60 derivatives have been methano-C60 and pyrrolidino-C60 compounds in which the fullerene cage is monofunctionalized. Photophysical properties of the C60 derivatives in individual compound classes are apparently similar, indicating that electronic transitions and photoexcited-state properties of the C60 derivatives are likely to be dominated by contributions of the mono-functionalized fullerene cage.5,12-14 However, there are only a few reports in the literature concerning the photo†
Clemson University. Wright-Patterson Air Force Base. § Present address: Idaho Engineering and Environmental Laboratory, P.O. Box 1625-2208, Idaho Falls, ID 83415-2208. ‡
physical and redox properties of multiple-functionalized fullerene derivatives,5,15-18 despite the fact that these compounds may offer new opportunities in the search for fullerene material properties that are amenable to potential technological applications. For example, a hexapyrrolidine derivative of C60 was reported to possess interesting electroluminescence characteristics.18 As an extreme in C60 cage multiple functionalization, the hexakis addition fundamentally changes the π-electronic system of the parent C60 molecule and produces a unique class of C60 derivatives.5,17-20 In these C60 derivatives, with all additions being [6,6]-closed, the fullerene cage is left with eight individual phenyl rings.5,17,18 As a result, these derivatives may be considered as special π-conjugated cage molecules that are different from both fullerenes and traditional polyenes. The photophysical and electron-transfer properties of these molecules are of significant interest. In this paper, we report a systematic study of the absorption and emission properties of a series of C60 derivatives with monoand multiple-functionalized fullerene cages. Absorption spectra and absorptivities of the C60 derivatives in solution were measured and compared. By recording the fluorescence spectra using a near-infrared-sensitive emission spectrometer (extending to 1200 nm), we quantitatively determined fluorescence quantum yields of the derivatives. Fluorescence lifetimes of the derivatives were obtained using the time-correlated single photoncounting technique. In addition, quenchings of fluorescence intensities and lifetimes of the C60 derivatives by N,N-diethylaniline (DEA) were evaluated. Apparently, upon photoexcitation even the hexakis-adducts undergo electron-transfer inter-
10.1021/jp0000329 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/22/2000
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TABLE 1: Absorption Spectral Parameters compd solvent 1 2 3 4 5 6 7 8 9 10 11 12 a
toluene toluene CS2 toluene toluene toluene CS2 toluene toluene toluene toluene hexane
abs0-0 0-0 (nm) (M-1 cm-1) 690 694 694 705 697
256 173 260 274 246
703 687
285 136
a
absmax (nm)
max (M-1 cm-1)
467 487 487 497 483 538 480 458 482 477 ∼40 at 500 nm 539(sh)
2000 1600 1970 1900 2050 1250 2390 1070 2790 2930 443
No peak in the visible region.
actions with the donor DEA, resulting in significant fluorescence quenching. The results for different derivatives are compared and discussed.
Figure 1. Absorption spectra of 1-3 and 5-7 in toluene and 4 in CS2 at room temperature.
Results UV/Vis Absorption. UV/vis absorption spectra of mono- and multiple-functionalized C60 derivatives 1-12 were measured in room-temperature (22 °C) solutions. The spectral parameters are summarized in Table 1. For the mono-functionalized methano-C60 derivatives 1-5, the absorption spectra are quite similar (Figure 1). The derivative 6 has a [5,6]-open fulleroid structure; its absorption spectrum is obviously different from that of the corresponding methano-C60 derivative 5 ([6,6]-closed structure). The difference is not only in spectral shape but also in absorptivity (Figure 1 and Table 1). On the other hand, the absorption spectrum of the carboxylic acid 7 is similar to that of the ester 5 (Figure 1). For the C60 derivative 8, the absorption spectrum is similar to those of other C60 derivatives that are functionalized with a five-member cycloether ring and also somewhat similar to those of pyrrolidino-C60 derivatives (Figure 2).5,12,13
Figure 2. Absorption spectrum of 8 in room-temperature toluene, compared with those of the C60 derivatives functionalized with a fivemember ring.12-14
Absorption spectra of the multiple-functionalized methanoC60 derivatives 9-12 are compared in Figure 3. The spectra of both the bis-adduct 9 and the tris-adduct 10 are different from that of the corresponding mono-adduct, exhibiting no clear band at the red onset (690-710 nm). Whereas the spectrum of 9 consists of a small peak at ∼425 nm, there is no similar absorption peak in the spectrum of 10 (Figure 3). For the hexakis-adducts 11 and 12, the absorption spectra are not only significantly different from those of the lower-order adducts but also different from each other. In the visible spectral region,
Mono- and Multiple-Functionalized Fullerene Derivatives
Figure 3. Absorption spectra of 9-11 and the corresponding monoadduct5 in toluene and that of 12 in hexane at room temperature.
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Figure 5. Fluorescence spectra of (a) 9 (- ‚ - ‚ -), 10 (____), and the corresponding mono-adduct (‚‚‚‚‚)5 in toluene; and (b) 11 (- ‚‚ - ‚‚ -) in toluene and 12 (- - -) in hexane at room temperature.
TABLE 2: Fluorescence Properties compd
solvent
FLSC0-0 (nm)
ΦF × 10-3
τF (ns)
1 2 3 4 5 6 7 8 9
toluene toluene toluene CS2 toluene toluene CS2 toluene toluene CH2Cl2 toluene CH2Cl2 toluene CH2Cl2 methanol hexane
707 714 716 715 715 712 722 706 780a
1.3 0.85 1.2 1.2 1.4 0.4 1.5 1.4 1.5
1.4 1.5 1.4 1.3 1.4 1.2 1.35 1.8
723a
0.8 0.7 2.4 2.3
10 11 12 a
Figure 4. Fluorescence spectra of (a) 1 (- ‚‚ - ‚‚ -), 2 (- ‚ - ‚ -), 4 (‚‚‚‚‚), 5 (____), and 8 (- - -); and (b) 5 (____), 6 (- - -), and 7 (‚‚‚‚‚) in toluene at room temperature.
the absorption of 12 is significantly stronger than that of 11 (Figure 3). Fluorescence Spectra. Fluorescence spectra of the fullerene derivatives 1-12 were measured in room-temperature solutions (2 × 10-5 to 10-4 M). As shown in Figure 4, all monofunctionalized C60 derivatives 1-8 have quite similar spectra, except for the [5,6]-open fulleroid 6, which has a broader fluorescence spectrum (Figure 4b). For the multiple-functionalized C60 derivatives 9-12, the fluorescence spectra are apparently different (Figure 5). The bis-
694a 664a
1.8
2.95 2.45 1.9 2.1 1.85 2.95 2.0
Band maximum.
adduct 9 has a fluorescence spectral profile similar to that of the mono-adduct, but the structured bands are at longer wavelengths.21 The fluorescence spectra of the tris-adduct and the hexakis-adducts are broad and structureless, overall blueshifted from the spectra of the mono- and bis-adducts (Figure 5). Fluorescence Quantum Yields. Fluorescence quantum yields of the C60 derivatives 1-12 in room-temperature solutions (2 × 10-5 to 10-4 M) were determined quantitatively in reference to the yield of C60 (3.2 × 10-4 M), which was obtained using rhodamine 101 in ethanol as a fluorescence standard (ΦF ) 1.0).22 Like other mono-functionalized methano-C60 derivatives reported in the literature,5 compounds 1-5 and 7 have fluorescence yields several times higher than that of C60 (Table 2). The fluorescence quantum yields of these C60 derivatives show no significant changes in the more polar solvent methylene chloride. For the mono-functionalized C60 derivative 8, the fluorescence yield is also similar to those of the methano-C60 derivatives (Table 2). However, the [5,6]-open fulleroid 6 is
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significantly less fluorescent, with only less than one-third the yield of the corresponding methano-C60 derivative 5 (Table 2). In fact, the fluorescence quantum yield of 6 is close to that of the parent C60. The multiple-functionalized C60 derivatives 9-12 have different fluorescence quantum yields (Table 2). The bis-adduct 9 is slightly more fluorescent than the corresponding mono-adduct, but the symmetric tris-adduct 10 is actually less fluorescent than the mono-adduct (Table 2). For the hexakis-adducts, the fluorescence yield of 11 is somewhat higher than that of 12 after the correction for solvent changes is applied (Table 2). Fluorescence Lifetimes. Fluorescence decays of the C60 derivatives in room-temperature solutions (2 × 10-5 to 10-4 M) were measured using the time-correlated single photoncounting method. The decays were deconvoluted well from their corresponding instrumental response functions using a monoexponential equation. The fluorescence lifetimes of the monofunctionalized methano-C60 derivatives 1-5 and 7 are similar, varying in the range of 1.2-1.5 ns (Table 2). For the [5,6]open fulleroid 6, the fluorescence lifetime is similar to that of the corresponding methano-C60 derivative, despite the fact that 6 is significantly less fluorescent (Table 2). The derivative 8 has a somewhat longer fluorescence lifetime in room-temperature toluene solution. The multiple-functionalized C60 derivatives 9-12 all have longer-lived excited singlet states than does the corresponding mono-adduct. However, changes in the fluorescence lifetime do not correlate with the degree of cage functionalization (Table 2). The fluorescence lifetime of the hexakis-adduct 11 is close to that of the symmetric tris-adduct 10, but shorter than that of the bis-adduct 9. For the two hexakis-adducts, 11 and 12 have similar fluorescence lifetimes, despite their fluorescence quantum yields being different (Table 2). Fluorescence Quenching. The photoexcited states of monoand multiple-functionalized C60 derivatives are quenched efficiently by the electron donor N,N-diethylaniline (DEA) in room-temperature solutions. In the range of DEA concentrations under consideration (
