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Rotational dynamics of C60 and C70. Investigation by picosecond fluorescence time-resolved anisotropy decay
29
July 1994
CHEMICAL PHYSCS LETTERS
ChemicalPhysicsLetters225 (1994) 181-185
Rotational dynamics of &-, and CTO. Investigation by picosecond fluorescence time-resolved anisotropy decay Mita Roy, S. Doraiswamy Chemical PhysicsGroup, TataInstituteof Fundamental Research,Homi BhabhaRoad, Bombay 400 005, India
Received 10 August 1993;in final form 29 March 1994
Time-resolved fluorescence depolarisation spectroscopy was used for measuring rotational reorientation times ( 7r) of C&and C,,, in toluene as a function of temperature.The anisotropydecay could not be monitored at any temperature (296-l 91 K) . The
plausiblereasons could be either washingoff anisotropy due to the presence of Jahn-Teller active modes in excited states T,, and T, or an inverseviscosityeffect phenomena.
1. Introduction Recently, the rotational dynamics of buckminster fullerenes, e.g. Cho and CT0have aroused great interest. The rotational reorientation times (TV)of Ceo and C,. in the ground electronic state, have been studied by “C-NMR [ l-41. It has been observed that in solid phase, both Cso and CT0show anomalous behaviour at their phase transition. Johnson et al. [2] have measured T, of C6,,in solid phase, as 9.1 ps at 283 K and 2.1 ns below 260 K. Recently, the rotational motion of CT0 in solid phase has been reported to be about 5 ps at 340 K, and about 2 ns at 250 K [4]. The mechanism of rotational diffusion above and below the transition temperature are believed to be different [ 5 ] ; this may explain such wide variation of 2, with temperature. In solution Johnson et al. have reported tr of Cso in tetrachloroethane at 283 K as 15.5 ps, whereas Jones and Rodriguez [ 31 have determined T,of C60in deuterated toluene at 303 K as 16.9 ps. Rotational reorientation times of CT0in solution have not been determined so far. Recently, using a Elsevier Science B.V. SSDIOOO9-2614(94)00629-5
time-resolved EPR (TREPR) method, z, of C&, in methylcyclohexane (in the triplet excited state) was estimated to be of the order of less than a ps at room temperature [ 61. The authors could not attribute such rapid rotation to physical rotation and have termed it as pseudo-rotation. It is well known that a direct method for experimental determination of T, is a time-resolved fluorescence depolarisation technique. As the fullerenes, CW and C,. are both fluorescent at room temperature [7,8], we have attempted to measure the r, of Go and CT0using the above method. Extensive theoretical work has also been done on rotational dynamics of solutes in liquids, and a correlation between the experiment and theory has been found to be less than satisfactory [ 9 1. Incidentally, the Stokes-Einstein-Debye (SED) theory that had been originally proposed for understanding rotational dynamics [ 10 1, was for a spherical molecule. In general, molecules are far from spherical shape and the proximate spherical molecule admantane, is nonfluorescent. Thus, to the best of our knowledge, Cso
182
hi. Roy, S. Doraiswamy /ChemicaI Physics Letters 225 (1994) 181-185
appears to be the maiden spherical molecule on which fluorescence technique can be applied for studying rotational dynamics. This Letter deals with our effort in measuring rotational reorientation times of C6,,and C,Oin toluene by varying viscosity as a function of temperature, using a time-resolved fluorescence depolarisation method.
lyvinyl alcohol an adequate homogeneous film could not be made, and led to poor signal-to-noise ratio. Samples of Cso and CT0were provided by the Materials Science Department of Indira Gandhi Centre for Atomic Research, Kalpakkam. The 1Op6 M solutions were excited at 580 nm and the emission observed around 700 nm. The purity of the samples were checked by matching their absorption spectra and fluorescence lifetimes in toluene with that in literature [ 71.
2. Experimental Experimental details of time-correlated singlephoton counting are given elsewhere [ 91. In brief, the excitation source consisted of an actively mode-locked frequency-doubled cw Nd : YAG laser, that synchronously pumps a rhodamine 6G dye laser. The repetition rate of the pulses (typically 6 to 8 ps duration) was maintained at 800 kHz by a cavity dumper. Fluorescence emission was detected at right angles by an MCPPMT R 2809U-0 1 (Hamamatsu) through a polarizer. The polarization of the exciting laser pulse was vertical. The polarizer in the emission channel was positioned parallel or perpendicular to the polarization of the excitation pulse for measurement of anisotropy decay, and at 54.7” for lifetime measurements. The outputs of the time to amplitude converter were stored in a multichannel analyzer and the data was processed in a computer. The instrument response function (full width at half maximum) measured by scattering the laser light with a dilute solution of nondairy cream in distilled water, was about 100 ps. Low temperatures were obtained by making a liquid nitrogen bath of different solvents viz. carbon tetrachloride, chloroform, ethyl alcohol, and ethyl acetate. Measurements of z, and zf were made in toluene at 2 10,258,243,23 1 and 19 1 K with respective viscositiesof0.43,0.97, 1.21, 1.60and6.81 CP [ll]. The temperatures were recorded by a thermocouple placed directly inside the solution in the cuvette. Steady state anisotropy was measured in toluene at room temperature. Steady state anisotropy measurements were carried out using Shimadzu RF-540 spectroflurometer with polariser attachment. Attempts were made to measure initial anisotropy or r. in solid matrix by preparing a film of the fullerenes in polyvinyl alcohol. Both Cboand CT0being insoluble in po-
3. Results The fluorescence lifetimes (rf) of C6Oand CT0in toluene at 298 K were 0.64 f 0.02 and 1.16 + 0.02 ns, respectively, and remained invariant with the change in temperature. Rotational correlation times of C6o and CT0were measured by time-resolved fluorescence depolarisation method. The basic principle of this technique is that the molecules with transition moment parallel to the polarised laser light are preferentially excited, thereby creating an anisotropy in the excited state which then decays due to the rotational motion of the molecules. The direction of the transition moment is thus fixed in the laboratory frame. Therefore even in spherical molecules, anisotropy can be created. Timeresolved decay of anisotropy r(t) for a spherical molecule is a single exponential and is given by [ 12 ] exp( -t/z,)
r(t)=r,
(1)
where 7r is the rotation correlation time of the fluorophore and r. is the anisotropy at t = 0. The steady state anisotropy (r) is an average of r( t) weighted by I(t) which represent the time-resolved decay of total fluorescence intensity. The effects of rotational diffusion on the steady state anisotropy can be understood from r0 r=m9
(2)
where rf its fluorescence lifetime. In a solid matrix or a viscous solution where 7r B 7f, the measured anisotropy would be r,. Whereas in a solution where 7, << 7f the steady state anisotropy would be close to zero. Steady state anisotropy was measured using a
hf. Roy. S. Doraiswamy/Chemical Physics Letters 225 (1994) 181-185
fluorimeter with a single emission channel. The anisotropy is given by Z, - GIL
‘= I1 +2GI,
’
(3)
where Z denotes the fluorescence intensity and the subscripts refer to the settings of the emission polariser with respect to the polar&ion of the exciting light. G is the ratio of the sensitivities of the detection system for vertically and horizontally polarised light and is given by, G+ HH
(4)
where Z, corresponds to fluorescence intensity with horizontally polarised excitation and vertically polarised emission and ZHHto both horizontally polarised excitation and emission. Attempts to measure r. of the fullerenes are hindered by the insolubility, of these compounds in viscous solvents viz. ethylene glycol, polyvinyl alcohol. Steady state anisotropy of C60 and CT0was measured in toluene at room temperature. The excitation wavelength was varied from 525 to 580 nm. Weak fluorescence intensity of the fullerenes lead to large uncertainty in the data. Repeated measurements resulted in acquiring the values of r as 0.005 + 0.003 and 0.006 + 0.002 for Cm and CT0 respectively, irrespective of the excitation wavelength. Measurement of r( t ) involves measuring independently Z,(t) and II(t). Peak counts greater than 10000 were collected for each component. The two components of the fluorescence were then normalised by tail matching at times longer than about four fluorescence lifetimes, where the Z,(t) and II (t) become randomized. In measurem,ents of 7, with Cbo and CT0at ambient temperature, the Z,(t) and I, (t ) waveforms overlapped each other throughout the decay profile. Similar observations were bade at all temperatures used, down to as low as 19 1 K where the viscosity of toluene is 6.8 cP, thereby indicating that the rotation reorientation time was faster than our detection limit. 4. Discussion The SED equation for a molecule of volume V rotating in a liquid of viscosity v is given by
183
W
7r=kT’ Using the radius of C& molecule 3.55 A and that of CT03.95 A [ 131 and Eq. (5), tr of the fullerenes in toluene at 303 K (q=O.Sl cP), shouldbe 22 and 31 ps, respectively. The experimentally determined 7r (using %NMR) of Cso in deuterated toluene at the same temperature was also found to be of the same order [ 31. At 296 K the respective values of 7, of Cso and CT0in toluene (q= 0.43 cP) should be around 20 and 27 ps. However, the lowest value of 7r that has been measured with our instrument so far was 57 ps [ 91. Thus it is possible that 7, of Choand CT0at room temperature may be too short to measure in our system. However we fail to detect any difference in Z1(t) and I, (t ) waveforms for both Cm and CT0even at the highest viscosity of 6.8 1 cP, when their rotational reorientation times are expected to be more than 400 ps as per Eq. (5), and should have been easily measurable. Some possible scenarios under which the rotational anisotropy decay could not be observed even at high viscosity of the solvent are discussed below. 4.1. Instrumentation limitations 4. I. 1. Slip or subslip behaviour Probe molecules can rotate much faster than that predicted by the SED model under ‘slip’ or ‘subslip’ boundary conditions [ 141. However, since the volumes of Cso and CT0are greater than that of toluene any of the above boundary conditions are less likely to be applicable. 4.1.2. Inverse viscosity efsect It has been observed that at 283 K &-, rotates much faster in the solid state than in solution. In liquid phase, deuterated toluene at 303 K (q=O.Sl cP) 7r reported is 16.7 ps [ 31, whereas in tetrachlormethane at 283’K where the viscosity is almost doubled 7r determined is even less, i.e. 15.5 ps [2]. If these observations are interpreted as a peculiar behaviour, wherein unlike the SED model, rotational motion has ti inverse viscosity dependence, then for Cho in solution the tr values would decrease with the decrease in temperature. In other words the molecules would rotate even faster with the increase in
M. Roy, S. Lbraiswamy /Chemical PhysicsLetters 225 (1994) MI-185
184
viscosity, thereby making its measurement impossible. No information is available on rotational reorientation times of CT0other than the recent report [4] of measured rr in solid phase where like CsOit rotates isotropically at 330 K. 4. I. 3. State-dependent7, Unlike “C-NMR measurements which are carried out in the ground state, the fluorescence depolarisation experiment deals with the measurement in excited electronic state. There is a possibility that rr of the fullerenes is faster in the excited state. However, TREPR studies have shown that in the singlet excited state both ChOand CT0get distorted The 7, of the fullerenes should then be higher in the excited state than in the ground state. The literature data also indicates that in general 7r in the excited state is greater than 7, in the ground state [ 15 1. 4.2. Limitationsof the technique The term r. in Eq. ( 1) or the anisotropy observed in the absence of the depolarizing processes of rotational diffusion is given by r. =
3 co&Y - 1 5 ’
(6)
where (Y is the angle between the absorption and emission transition moments. Depending on (I! the value of r. varies between -0.2 to 0.4. As the orientation of the absorption transition moment differs for each absorption band, and the emission is always from the lowest singlet state, the angle (r, and therefore r. varies with wavelength. In case of Cso all singlet excited electronic states in the 4 10-620 nm region correspond to forbidden transitions, with triple and higher-order degeneracy [ 161. In these forbidden electronic transitions a vibrational mode of suitable symmetry (HenbergTeller active mode) interacts with the allowed transition, thereby lifting the orbital forbiddenness partially. The bands in the 560-620 nm region (our excitation is at 580 nm) are associated with interleaved vibronic bands of very closely spaced electronic states, viz. Ti, and T, [ 171. These states are also calculated to be quasidegenerate. The degeneracy in the triply degenerate states of T,, and Tzg is also removed by excitation of Jahn-Teller active modes. CT0 has a
similar absorption spectrum like Cao in the visible region with comparable oscillator strengths [ 181. The ground state of CT0is known to have a prolate shape with a lower symmetry ( Dlih) than Cho (4). Molecules with DSh symmetry can also show Jahn-Teller activity [ 191. The presence of closely spaced excited singlet states and Jahn-Teller active modes in both Cso and Go, would lead to excitation of these compounds into multiple excited states leading to a distribution of (Y. The fundamental question which can be raised at this point is whether r. exists in a symmetrically spherical molecule C& in which the transition dipole moment cannot be defined. Our steady state anisotropy value of 0.05 f 0.003, indicates that r. in Chodoes exist. CT0 being nonsymmetrical should be expected to have a definite r. value. The steady state anisotropy data of both Cso and CT0did not change with the wavelength which is expected as r. in these compounds would be due to superposition of (Y from several electronic states. Secondly there is a rapid interconversion between the Jahn-Teller distorted states, and as a result whenever Jahn-Teller active modes are excited there is a fast switching of transition dipole moments. Such effect would enhance the anisotropy decay drastically and lead to much low 7, values, the measurement of which would be beyond the scope of our instrument. In other words the anisotropy decay may not be due to a physical rotation, in which case our inability to observe viscosity dependent measurements of rotation correlation times of these fullerences becomes meaningful. It may be relevant to recall that in 7r measurement of Cso by TREPR method [ 61, rapid switching of the magnetic axes within the molecule as a result of the fast switching over of the Jahn-Teller active states, caused an estimation of extremely short 7r value of the order of less than a picosecond, rightly termed as pseudo rotation. In summary we note that the problem encountered in measuring 7, of these fullerenes in the excited singlet state using the fluorescence depolarisation method, is that the 7, is very fast and beyond the scope of our instrument. This may be either due to inverse viscosity effect or that the anisotropy decay is due to internal switching of transition dipoles and not a physical rotation. At present we are unable to say
M. Roy, S. Doraiswamy /Chemical PhysicsLetters225 (1994) 181-185
whether an inverse viscosity effect or anisotropy decay method is responsible for not observing 7, of the fullerenes in toluene even at as high viscosity as 6.8 1 cP. In order to arrive at a definite conclusion it is worthwhile measuring 7, of the fullerenes using ( 1) 13C-NMR method in solution phase as a function of temperature, and (2) the time-resolved fluorescence depolarisation method at room temperature on a subpicosecond time scale. ’
Acknowledgement The authors would like to thank Dr. T.S. Radhakrishnan and Dr. Y. Hariharan of the Indira Gandhi Centre for Atomic Research for providing the samples of fullerenes. We thank Dr. S.J. Wategaonkar for constructive suggestions and critical reading of the manuscript. Fruitful discussion with Dr. R. Das is gratefully acknowledged. The authors thank the Department of Science and Technology for the financial support in establishing the picosecond spectrometer facility.
References [ 1 ] R. Tycko, R.C. Haddon, G. Dabbagh, S.H. Glarum, D.C. Dot@- and A.M. Mujsce, J. Phys. Chem. 95 (1991) 518. [ 21 R.D. Johonson, C.S. Yannoni, H.C. Dom, J.R. Salem and D.S. Bethune, Science 255 (1992) 1235.
185
[ 31 V.K. Jones and A.A. Rodriguez, Chem. Phys. Letters 198 (1992) 373. [4] R. Tycko, G. Dabbagh, G.B. Vaughan, R.M. Strongin, M.A. Cichy and A.B. Smith III, J. Chem. Phys. 99 ( 1993) 7554. [ 51 P.A. Heiny, J.E. Fischer, A.R. McGhie, W.J. Romanow, A.M. Denenstein, J.P. McCauley Jr. and A.B. Smith III, Phys. Rev. Letters 66 ( 199 1) 29 11. [6] G.L. Closs, P. Gautom, D. Zhang, P.J. Krusic, S.A. Hilland E. Wasserman, J. Phys. Chem. 96 (1992) 5228. [ 7 ] K. Do&o, L. Minyung, S.D. Yung and K.K. Seong, J. Am. Chem. Sot. 114 (1992) 4429. [8]Ya.P.SunandC.E.Bunker, J.Phys.Chem.97 (1993) 6770. [9] M. Roy and S. Doraiswamy, J. Chem. Phys. 98 (1993) 3231. [lo] G. Stokes, Trans Cambridge Phil. Sot. 9 ( 1956) 5; A. Einstein, Ann. Phys. 19 (1906) 371; P. Debye, Polar molecules (Dover, New York, 1929). [ 111 Landolt-Barnstein Series, Band II, Teil 5, Transportphitnomene I (Springer, Berlin, 1969). [ 121 G.R. Fleming, Chemical applications of ultrafast spectroscopy (Oxford Univ. Press, Oxford, 1986) p. 128. [ 13 ] M.R. Wasielewski, M.P. G’Neil, K-R. Lyldce, M.J. Pellin and D.M. Gruen, J.Am. Chem. Sot. 113 (1991) 2774. 1141 D. Kivelson, in: Rotational dynamics of small and macromolecules, eds. Th. Dorfmueller and R. Pecora (Springer, Berlin, 1986) ch. 1. [ 151 G.J. Blanchard, J. Phys. Chem. 92 (1988) 6303; G.J. Blanchard and C.A. Cihal, J. Phys. Chem. 92 (1988) 5905; G.J. Blanchard, J. Phys. Chem. 93 (1989) 4315. [ 161 ENegri, G. Orlandi and F. Zerbetto, Chem. Phys. Letters 144 (1988) 31. [ 171 S. Leach, M. Vervloet, A. Despres, E. Breheret, J.P. Hare, T.J. Dennis, H.W. Kroto, R. Taylor and D.R.M. Walton, Chem. Phys. 160 (1992) 451. [ 18 ] J.P. Hare, H.W. Kroto and R. Taylor, Chem. Phys. Letters 177 (1991) 394. [ 191 G. Her&erg, Molecular spectra and molecular structure, Vol. 3 (Van Nostrand, Princeton, 1966) ch. 1.
July 1994
CHEMICAL PHYSCS LETTERS
ChemicalPhysicsLetters225 (1994) 181-185
Rotational dynamics of &-, and CTO. Investigation by picosecond fluorescence time-resolved anisotropy decay Mita Roy, S. Doraiswamy Chemical PhysicsGroup, TataInstituteof Fundamental Research,Homi BhabhaRoad, Bombay 400 005, India
Received 10 August 1993;in final form 29 March 1994
Time-resolved fluorescence depolarisation spectroscopy was used for measuring rotational reorientation times ( 7r) of C&and C,,, in toluene as a function of temperature.The anisotropydecay could not be monitored at any temperature (296-l 91 K) . The
plausiblereasons could be either washingoff anisotropy due to the presence of Jahn-Teller active modes in excited states T,, and T, or an inverseviscosityeffect phenomena.
1. Introduction Recently, the rotational dynamics of buckminster fullerenes, e.g. Cho and CT0have aroused great interest. The rotational reorientation times (TV)of Ceo and C,. in the ground electronic state, have been studied by “C-NMR [ l-41. It has been observed that in solid phase, both Cso and CT0show anomalous behaviour at their phase transition. Johnson et al. [2] have measured T, of C6,,in solid phase, as 9.1 ps at 283 K and 2.1 ns below 260 K. Recently, the rotational motion of CT0 in solid phase has been reported to be about 5 ps at 340 K, and about 2 ns at 250 K [4]. The mechanism of rotational diffusion above and below the transition temperature are believed to be different [ 5 ] ; this may explain such wide variation of 2, with temperature. In solution Johnson et al. have reported tr of Cso in tetrachloroethane at 283 K as 15.5 ps, whereas Jones and Rodriguez [ 31 have determined T,of C60in deuterated toluene at 303 K as 16.9 ps. Rotational reorientation times of CT0in solution have not been determined so far. Recently, using a Elsevier Science B.V. SSDIOOO9-2614(94)00629-5
time-resolved EPR (TREPR) method, z, of C&, in methylcyclohexane (in the triplet excited state) was estimated to be of the order of less than a ps at room temperature [ 61. The authors could not attribute such rapid rotation to physical rotation and have termed it as pseudo-rotation. It is well known that a direct method for experimental determination of T, is a time-resolved fluorescence depolarisation technique. As the fullerenes, CW and C,. are both fluorescent at room temperature [7,8], we have attempted to measure the r, of Go and CT0using the above method. Extensive theoretical work has also been done on rotational dynamics of solutes in liquids, and a correlation between the experiment and theory has been found to be less than satisfactory [ 9 1. Incidentally, the Stokes-Einstein-Debye (SED) theory that had been originally proposed for understanding rotational dynamics [ 10 1, was for a spherical molecule. In general, molecules are far from spherical shape and the proximate spherical molecule admantane, is nonfluorescent. Thus, to the best of our knowledge, Cso
182
hi. Roy, S. Doraiswamy /ChemicaI Physics Letters 225 (1994) 181-185
appears to be the maiden spherical molecule on which fluorescence technique can be applied for studying rotational dynamics. This Letter deals with our effort in measuring rotational reorientation times of C6,,and C,Oin toluene by varying viscosity as a function of temperature, using a time-resolved fluorescence depolarisation method.
lyvinyl alcohol an adequate homogeneous film could not be made, and led to poor signal-to-noise ratio. Samples of Cso and CT0were provided by the Materials Science Department of Indira Gandhi Centre for Atomic Research, Kalpakkam. The 1Op6 M solutions were excited at 580 nm and the emission observed around 700 nm. The purity of the samples were checked by matching their absorption spectra and fluorescence lifetimes in toluene with that in literature [ 71.
2. Experimental Experimental details of time-correlated singlephoton counting are given elsewhere [ 91. In brief, the excitation source consisted of an actively mode-locked frequency-doubled cw Nd : YAG laser, that synchronously pumps a rhodamine 6G dye laser. The repetition rate of the pulses (typically 6 to 8 ps duration) was maintained at 800 kHz by a cavity dumper. Fluorescence emission was detected at right angles by an MCPPMT R 2809U-0 1 (Hamamatsu) through a polarizer. The polarization of the exciting laser pulse was vertical. The polarizer in the emission channel was positioned parallel or perpendicular to the polarization of the excitation pulse for measurement of anisotropy decay, and at 54.7” for lifetime measurements. The outputs of the time to amplitude converter were stored in a multichannel analyzer and the data was processed in a computer. The instrument response function (full width at half maximum) measured by scattering the laser light with a dilute solution of nondairy cream in distilled water, was about 100 ps. Low temperatures were obtained by making a liquid nitrogen bath of different solvents viz. carbon tetrachloride, chloroform, ethyl alcohol, and ethyl acetate. Measurements of z, and zf were made in toluene at 2 10,258,243,23 1 and 19 1 K with respective viscositiesof0.43,0.97, 1.21, 1.60and6.81 CP [ll]. The temperatures were recorded by a thermocouple placed directly inside the solution in the cuvette. Steady state anisotropy was measured in toluene at room temperature. Steady state anisotropy measurements were carried out using Shimadzu RF-540 spectroflurometer with polariser attachment. Attempts were made to measure initial anisotropy or r. in solid matrix by preparing a film of the fullerenes in polyvinyl alcohol. Both Cboand CT0being insoluble in po-
3. Results The fluorescence lifetimes (rf) of C6Oand CT0in toluene at 298 K were 0.64 f 0.02 and 1.16 + 0.02 ns, respectively, and remained invariant with the change in temperature. Rotational correlation times of C6o and CT0were measured by time-resolved fluorescence depolarisation method. The basic principle of this technique is that the molecules with transition moment parallel to the polarised laser light are preferentially excited, thereby creating an anisotropy in the excited state which then decays due to the rotational motion of the molecules. The direction of the transition moment is thus fixed in the laboratory frame. Therefore even in spherical molecules, anisotropy can be created. Timeresolved decay of anisotropy r(t) for a spherical molecule is a single exponential and is given by [ 12 ] exp( -t/z,)
r(t)=r,
(1)
where 7r is the rotation correlation time of the fluorophore and r. is the anisotropy at t = 0. The steady state anisotropy (r) is an average of r( t) weighted by I(t) which represent the time-resolved decay of total fluorescence intensity. The effects of rotational diffusion on the steady state anisotropy can be understood from r0 r=m9
(2)
where rf its fluorescence lifetime. In a solid matrix or a viscous solution where 7r B 7f, the measured anisotropy would be r,. Whereas in a solution where 7, << 7f the steady state anisotropy would be close to zero. Steady state anisotropy was measured using a
hf. Roy. S. Doraiswamy/Chemical Physics Letters 225 (1994) 181-185
fluorimeter with a single emission channel. The anisotropy is given by Z, - GIL
‘= I1 +2GI,
’
(3)
where Z denotes the fluorescence intensity and the subscripts refer to the settings of the emission polariser with respect to the polar&ion of the exciting light. G is the ratio of the sensitivities of the detection system for vertically and horizontally polarised light and is given by, G+ HH
(4)
where Z, corresponds to fluorescence intensity with horizontally polarised excitation and vertically polarised emission and ZHHto both horizontally polarised excitation and emission. Attempts to measure r. of the fullerenes are hindered by the insolubility, of these compounds in viscous solvents viz. ethylene glycol, polyvinyl alcohol. Steady state anisotropy of C60 and CT0was measured in toluene at room temperature. The excitation wavelength was varied from 525 to 580 nm. Weak fluorescence intensity of the fullerenes lead to large uncertainty in the data. Repeated measurements resulted in acquiring the values of r as 0.005 + 0.003 and 0.006 + 0.002 for Cm and CT0 respectively, irrespective of the excitation wavelength. Measurement of r( t ) involves measuring independently Z,(t) and II(t). Peak counts greater than 10000 were collected for each component. The two components of the fluorescence were then normalised by tail matching at times longer than about four fluorescence lifetimes, where the Z,(t) and II (t) become randomized. In measurem,ents of 7, with Cbo and CT0at ambient temperature, the Z,(t) and I, (t ) waveforms overlapped each other throughout the decay profile. Similar observations were bade at all temperatures used, down to as low as 19 1 K where the viscosity of toluene is 6.8 cP, thereby indicating that the rotation reorientation time was faster than our detection limit. 4. Discussion The SED equation for a molecule of volume V rotating in a liquid of viscosity v is given by
183
W
7r=kT’ Using the radius of C& molecule 3.55 A and that of CT03.95 A [ 131 and Eq. (5), tr of the fullerenes in toluene at 303 K (q=O.Sl cP), shouldbe 22 and 31 ps, respectively. The experimentally determined 7r (using %NMR) of Cso in deuterated toluene at the same temperature was also found to be of the same order [ 31. At 296 K the respective values of 7, of Cso and CT0in toluene (q= 0.43 cP) should be around 20 and 27 ps. However, the lowest value of 7r that has been measured with our instrument so far was 57 ps [ 91. Thus it is possible that 7, of Choand CT0at room temperature may be too short to measure in our system. However we fail to detect any difference in Z1(t) and I, (t ) waveforms for both Cm and CT0even at the highest viscosity of 6.8 1 cP, when their rotational reorientation times are expected to be more than 400 ps as per Eq. (5), and should have been easily measurable. Some possible scenarios under which the rotational anisotropy decay could not be observed even at high viscosity of the solvent are discussed below. 4.1. Instrumentation limitations 4. I. 1. Slip or subslip behaviour Probe molecules can rotate much faster than that predicted by the SED model under ‘slip’ or ‘subslip’ boundary conditions [ 141. However, since the volumes of Cso and CT0are greater than that of toluene any of the above boundary conditions are less likely to be applicable. 4.1.2. Inverse viscosity efsect It has been observed that at 283 K &-, rotates much faster in the solid state than in solution. In liquid phase, deuterated toluene at 303 K (q=O.Sl cP) 7r reported is 16.7 ps [ 31, whereas in tetrachlormethane at 283’K where the viscosity is almost doubled 7r determined is even less, i.e. 15.5 ps [2]. If these observations are interpreted as a peculiar behaviour, wherein unlike the SED model, rotational motion has ti inverse viscosity dependence, then for Cho in solution the tr values would decrease with the decrease in temperature. In other words the molecules would rotate even faster with the increase in
M. Roy, S. Lbraiswamy /Chemical PhysicsLetters 225 (1994) MI-185
184
viscosity, thereby making its measurement impossible. No information is available on rotational reorientation times of CT0other than the recent report [4] of measured rr in solid phase where like CsOit rotates isotropically at 330 K. 4. I. 3. State-dependent7, Unlike “C-NMR measurements which are carried out in the ground state, the fluorescence depolarisation experiment deals with the measurement in excited electronic state. There is a possibility that rr of the fullerenes is faster in the excited state. However, TREPR studies have shown that in the singlet excited state both ChOand CT0get distorted The 7, of the fullerenes should then be higher in the excited state than in the ground state. The literature data also indicates that in general 7r in the excited state is greater than 7, in the ground state [ 15 1. 4.2. Limitationsof the technique The term r. in Eq. ( 1) or the anisotropy observed in the absence of the depolarizing processes of rotational diffusion is given by r. =
3 co&Y - 1 5 ’
(6)
where (Y is the angle between the absorption and emission transition moments. Depending on (I! the value of r. varies between -0.2 to 0.4. As the orientation of the absorption transition moment differs for each absorption band, and the emission is always from the lowest singlet state, the angle (r, and therefore r. varies with wavelength. In case of Cso all singlet excited electronic states in the 4 10-620 nm region correspond to forbidden transitions, with triple and higher-order degeneracy [ 161. In these forbidden electronic transitions a vibrational mode of suitable symmetry (HenbergTeller active mode) interacts with the allowed transition, thereby lifting the orbital forbiddenness partially. The bands in the 560-620 nm region (our excitation is at 580 nm) are associated with interleaved vibronic bands of very closely spaced electronic states, viz. Ti, and T, [ 171. These states are also calculated to be quasidegenerate. The degeneracy in the triply degenerate states of T,, and Tzg is also removed by excitation of Jahn-Teller active modes. CT0 has a
similar absorption spectrum like Cao in the visible region with comparable oscillator strengths [ 181. The ground state of CT0is known to have a prolate shape with a lower symmetry ( Dlih) than Cho (4). Molecules with DSh symmetry can also show Jahn-Teller activity [ 191. The presence of closely spaced excited singlet states and Jahn-Teller active modes in both Cso and Go, would lead to excitation of these compounds into multiple excited states leading to a distribution of (Y. The fundamental question which can be raised at this point is whether r. exists in a symmetrically spherical molecule C& in which the transition dipole moment cannot be defined. Our steady state anisotropy value of 0.05 f 0.003, indicates that r. in Chodoes exist. CT0 being nonsymmetrical should be expected to have a definite r. value. The steady state anisotropy data of both Cso and CT0did not change with the wavelength which is expected as r. in these compounds would be due to superposition of (Y from several electronic states. Secondly there is a rapid interconversion between the Jahn-Teller distorted states, and as a result whenever Jahn-Teller active modes are excited there is a fast switching of transition dipole moments. Such effect would enhance the anisotropy decay drastically and lead to much low 7, values, the measurement of which would be beyond the scope of our instrument. In other words the anisotropy decay may not be due to a physical rotation, in which case our inability to observe viscosity dependent measurements of rotation correlation times of these fullerences becomes meaningful. It may be relevant to recall that in 7r measurement of Cso by TREPR method [ 61, rapid switching of the magnetic axes within the molecule as a result of the fast switching over of the Jahn-Teller active states, caused an estimation of extremely short 7r value of the order of less than a picosecond, rightly termed as pseudo rotation. In summary we note that the problem encountered in measuring 7, of these fullerenes in the excited singlet state using the fluorescence depolarisation method, is that the 7, is very fast and beyond the scope of our instrument. This may be either due to inverse viscosity effect or that the anisotropy decay is due to internal switching of transition dipoles and not a physical rotation. At present we are unable to say
M. Roy, S. Doraiswamy /Chemical PhysicsLetters225 (1994) 181-185
whether an inverse viscosity effect or anisotropy decay method is responsible for not observing 7, of the fullerenes in toluene even at as high viscosity as 6.8 1 cP. In order to arrive at a definite conclusion it is worthwhile measuring 7, of the fullerenes using ( 1) 13C-NMR method in solution phase as a function of temperature, and (2) the time-resolved fluorescence depolarisation method at room temperature on a subpicosecond time scale. ’
Acknowledgement The authors would like to thank Dr. T.S. Radhakrishnan and Dr. Y. Hariharan of the Indira Gandhi Centre for Atomic Research for providing the samples of fullerenes. We thank Dr. S.J. Wategaonkar for constructive suggestions and critical reading of the manuscript. Fruitful discussion with Dr. R. Das is gratefully acknowledged. The authors thank the Department of Science and Technology for the financial support in establishing the picosecond spectrometer facility.
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