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8562 J. Chem. Phys., Vol. 112, No. 19, 15 May 2000 Coheur et al. FIG. 4. Experimental UV–visible absorption thin line and fluorescence thick line spectra of the C 60 derivatives dissolved in cyclohexane. The inset window displays for each derivative the fluorescence spectrum expressed in arbitrary units together with the lowest absorption features in the absorption spectrum. to one or a set of electronic transitions; in turn, each transition can be associated with a one-electron excitation between a given occupied and unoccupied molecular orbital. Such an analysis is reported for C 60 Ph 5 H in Table IV. In view of their spectral similarities see Table III, the optical features observed in the absorption spectra of C 60 Cl 6 , C 60 Ph 5 Cl, and C 60 Ph 5 OH-isomer 1 are assigned in a similar way. The foregoing data therefore lead to the following conclusions re- Downloaded 28 Jun 2002 to 128.196.184.24. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/jcpo/jcpcr.jsp
J. Chem. Phys., Vol. 112, No. 19, 15 May 2000 Hexa-functionalized C 60 derivatives 8563 TABLE III. Photophysical properties collected for C 60 and the hexa-functionalized derivatives; the band labeling refers to that given in Fig. 4. C 60 Cl 6 C 60 Ph 5 Cl C 60 Ph 5 H C 60 Ph 5 OH isomer 1 C 60 Ph 5 OH Isomer 2 C 60 a Absorption bands , nm i 213 209 214 214 214 214 (G) 230 (F) h 255 259 259 258 256 258 (E) g 274 272 270 274 294 (D) f 339 340 334 330 (C) e 351 351 348 378 (B 0 ) 392A d 388 394 394 392 388 3 404 A 0 A 1 c 438 435 432 445 422 b 470 474 475 472 a 516 515 510 504-620 () a 0 666 Fluorescence band , nm 624 624 622 629 675 742 688 718 Excitation energy S 0 S 1 eV Measured 2.25 2.25 2.29 2.19 1.85 1.95 b Calculated 2.45 2.45 2.46 2.45 1.96 2.32 S 1 lifetime ns 0.5 1.2 1.2 1.17 c a The band notation for C 60 is that proposed by Leach et al. Ref. 75. b Following Sassara, Zerza, and Chergui Ref. 72. c Following Kim for C 60 in toluene Ref. 76. garding the optical properties of the C s and pseudo-C s symmetrical hexa-functionalized derivatives examined. 1 The first dominant absorption band b originates from the superposition of three optical transitions between S 0 and the seventh, eighth, and ninth excited states; the weak shoulder a is attributed to the S 0 →S 2 electronic transition. 2 The electronic transition from S 0 to the first excited state S 1 is calculated at 2.450.1 eV and displays a vanishingly small oscillator strength. The very weak intensity of this transition rationalizes the reason whereby the lowest excited state cannot be located in the absorption spectrum and is detected only through its weak fluorescence signal. The calculated S 0 – S 1 excitation energy is in very good agreement with the value of 2.240.5 eV estimated from the absorption and fluorescence spectra; the matching is further improved when considering the systematic redshift of 1200 cm 1 adopted to simulate the spectra of the derivatives. The calculations also indicate that the 1,2,4,11,15,30- FIG. 5. a Experimental UV–visible absorption spectra of C 60 Ph 5 H in cyclohexane thick line and of C 60 in similar conditions, given as a reference thin line; b measured solid line and calculated ‘‘synthetic’’ dotted line absorption spectra of C 60 Ph 5 H. The calculated ‘‘bar’’ spectrum is also shown in the visible region, where the most intense electronic transitions are assigned to the corresponding excited states S n . The calculated spectrum is rigidly redshifted by 1200 cm 1 and normalized to the intensity of the experimental band h. Downloaded 28 Jun 2002 to 128.196.184.24. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/jcpo/jcpcr.jsp
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