Preferential Orientation of Fulleropyrrolidine Bisadducts in E7 Liquid

Dec 7, 1999 - Marilena Di Valentin, Arianna Bisol, Giancarlo Agostini, Paul A. Liddell, Gerdenis Kodis, Ana L. Moore, Thomas A. Moore, Devens Gust, an...
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J. Phys. Chem. B 1999, 103, 11275-11281

11275

Preferential Orientation of Fulleropyrrolidine Bisadducts in E7 Liquid Crystal: A Time-Resolved Electron Paramagnetic Resonance Study L. Pasimeni,*,† U. Segre,‡ M. Ruzzi,† M. Maggini,§ M. Prato,| and K. Kordatos| Dipartimento di Chimica Fisica, UniVersita` di PadoVa, Via Loredan, 2-35131 PadoVa, Italy, Dipartimento di Chimica, UniVersita` di Modena, Via Campi, 183-41100 Modena, Italy, Dipartimento di Chimica Organica, UniVersita` di PadoVa, Via Marzolo, 1-35131 PadoVa, Italy, and Dipartimento di Scienze Farmaceutiche, UniVersita` di Trieste, Piazzale Europa, 2-34151 Trieste, Italy ReceiVed: July 30, 1999; In Final Form: October 12, 1999

The effect of the orientating potential in the E7 liquid crystal on a series of fulleropyrrolidine bisadducts has been examined. Photoexcited triplet states, populated by spin-orbit promoted intersystem crossing, have been used as spin probes to determine the preferential orientations of the guest molecules in the anisotropic medium. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy has been applied and spectra with the external magnetic field set parallel or perpendicular to the director n of the mesophase have been measured. Spectrum analysis has enabled us to determine the preferred orientation of n with respect to the principal axis frame of the dipolar interaction and to calculate the order parameters by using the orientational distribution function obtained by spectral simulation. It has been found that the macroscopic order degree induced by the mesophase on the guest molecules varies from one bisadduct to another. Bisadducts with D2h symmetry have exhibited triplet spectra due to dimers besides those of monomers. The measured zero field splitting parameters and magnetic level populations of the monomer and dimer triplet states have been analyzed, and a structure of the dimer has been postulated.

Introduction There is considerable interest in the study of nonlinear optical properties of fullerenes including optical limiting properties, based on the different cross sections for the ground and excited singlet and triplet state absorptions at a given wavelength of the incident light.1-4 The metastable nature of the excited triplet state makes time-resolved electron paramagnetic resonance (TREPR) spectroscopy5 a convenient means, because spin polarization generated at the birth of the triplet state favors detection of species even at low stationary concentrations. The analysis of the TREPR spectra gives the values of D and E parameters that express the extent of the electron dipolar interaction and information on the spin polarization originated from the spin selective population of the triplet sublevels at zero field. In principle, optical properties could be modulated by inclusion of guest molecules into liquid crystal (LC) solvents. In fact, it is known that absorption and emission phenomena are enhanced or reduced when the active components are macroscopically oriented.6 For example, rodlike conjugated oligothiophenes,7 such as R-sexithienyl, and disklike porphirin molecules,8 dissolved in E7 LC, are easily oriented in the nematic phase. Although LCs have been shown to be order imposing media to various solute molecules, it has been also demonstrated that pristine C60 is scarcely affected by the orientating potential in the nematic phase of E7 LC,9 the spherical shape of the C60 cage seeming responsible for such a behavior. However, the * Corresponding author. Phone: (+)39.049.8275107. (+)39.049.8275135. E-mail: [email protected]. † Dipartimento di Chimica Fisica, Universita ` di Padova. ‡ Dipartimento di Chimica, Universita ` di Modena. § Dipartimento di Chimica Organica, Universita ` di Padova. | Dipartimento di Scienze Farmaceutiche, Universita ` di Trieste.

Fax:

small changes introduced by the addition of small organic groups in well-defined stereospecific positions might be sufficient to induce orientating capabilities to the whole fullerene molecule. Recently,10 a series of polyadducts of C60 in isotropic solvents have been examined by TREPR spectroscopy. The zero field splitting (zfs) parameters D and E of excited triplet states were measured, and they proved to be a useful tool to investigate changes produced in the triplet wave function by stereospecific addition of substituents. TREPR spectroscopy was chosen as the investigative method, considering that this technique enables one to monitor the time dependent triplet spectra with a time resolution of 150-200 ns and important structural and dynamic features can thus be extracted. In fact, electronic relaxation kinetics results in a spin polarization of the excited triplet states that can be measured by TREPR spectroscopy, besides the zfs parameters. In the oriented phase, besides these parameters that are accessible through the simulation of the powder spectrum, the anisotropy imposed by the LC on the guest molecule is also significant for the interpretation of the orientation effects in the spectrum. In LCs, in fact, the relation between the principal axes of the zfs tensor and the director n, defined by the LC frame of reference, can be deduced. It is therefore possible to utilize the above properties to further extend the structureresolving ability of EPR methods. When the zfs tensor is expressed as located in the molecular frame, the assignment of the fine structure axes in the molecular frame bears important implications to structural and conformational data.11 In this contribution, we report on the orientation of fullerene bisadducts dissolved in E7. Preferential orientation of molecules embedded in a nematic liquid crystalline matrix has been investigated mainly with linear dichroism using linearly polarized optical absorption and emission12 and with nuclear magnetic resonance techniques.13,14 If the guest molecules are paramag-

10.1021/jp9927000 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/07/1999

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Pasimeni et al. nematic phase for more than 10 min, before cooling it down slowly to the desired temperature (5 deg/min). In the oriented glass the sample keeps the distribution of the molecular orientations unchanged, irrespective of the direction of the Zeeman field. The measurements were done at 120 K. Samples were irradiated at a repetition rate of 18 Hz by a Lambda Physik LPX 100 XeCl excimer laser that fed a Rhodamin 6G dye laser with emitting light at 580 nm (energy 2 mJ/pulse, pulse duration 20 ns). The temperature was maintained using a nitrogen variable-temperature flow dewar in the EPR resonator. Transient EPR measurements were carried out using a Bruker ER 200D X-band EPR, with the field modulation disconnected as described elsewhere.17 Field-swept transient spectra were recorded by a boxcar integrator (EG&EG, 162 mod. plug-in) for a fixed time delay (typically 0.5 µs) from the laser pulse and a time window of 50 ns with a microwave power of 1 mW. Measurements were performed with the LC director, n, either in its initial default orientation parallel to the external magnetic field (B0|n) or perpendicular to it (B0⊥n). Results

Figure 1. Positional relationships between the 6-6 bonds in C60 engaged in the substitution of R groups in 1-3 bisadducts.

netic, also EPR methods are well suited to obtain accurate structural information on their orientations. To our knowledge, no such method has been applied to study the orientating properties of fullerene adducts in LCs. In this study we apply TREPR and, to investigate the orientating properties of the fullerene adducts in E7, we use as spin probe the photoexcited triplet,15 an important transient state for the nonlinear optical properties of fullerenes. We present a description of the TREPR spectra of excited triplets in the compounds sketched in Figure 1 that are characterized by substituents with lateral side chains of increasing length. Molecular modeling of 1-3 shows that the molecules are spherical except for the substituents. Detailed line shape analyses will show that specific orientation ordering is induced in the 1-3 bisadducts dissolved in the E7 LC matrix depending on the topology of bis-addition. Oriented samples of fullerene molecules might offer the opportunity to explore the possible anisotropy in optical properties of C60 derivatives.

Triplet Line Shape Analysis. In this section we outline the procedure for computer simulating the TREPR spectra in ordered glasses. A more detailed description is given elsewhere.18 The nematic phase of a liquid crystal displays an axial symmetry around the director n of the mesophase. The molecular orientation is specified by the three Euler angles ΩLM of the transformation, which goes from the laboratory reference frame L, with zL parallel to n, to a molecule fixed frame M.19 According to the mean field model of the statistical distribution of molecular orientations, the probability of finding a molecule with orientation ΩLM is given by

for(ΩLM) ) Q-1 exp[-U(ΩLM)/kBT]

where U(ΩLM) is the orientating potential and Q is the orientational partition function. The shape of the orientating potential depends on the nature of the intermolecular interactions and the choice of the molecular frame. We will assume that it is possible to find a particular molecular frame with an axial shape of the potential, U(ΩLM) ) u(ψ) and that the dominant interactions are related to second rank tensor properties, such as the polarizability anisotropy. Therefore, we write

Experimental Section The synthesis of compounds 1 have been reported previously,10a,16 while those of bisadducts 2 and 3 will be described elsewhere. E7 is a eutectic mixture of cyano-biphenyls Rk-C6H5-C6H5 -CN with composition R1 ) C5H11 (51%), R2 ) C7H15 (25%), R3 ) C8H17O (16%), and R4 ) C5H11C6H5 (8%). It was obtained from Merck and used without further purification. It is characterized by the following phase transitions: crystalline (210 K) soft crystalline (263 K) nematic (333 K) isotropic. Compounds 1-3 have been first dissolved in E7 (≈0.5 mM); the solution was introduced into 2.8 mm i.d. quartz tubes, degassed by several freeze-pump-thaw cycles on a vacuum line, and then sealed under vacuum. The alignment of molecules in the sample was carried out by following the procedure described by Levanon et al.8 Starting from the isotropic phase the sample was cooled into the nematic phase (280 K) under an external magnetic field of 0.8 T, followed by further cooling to the experimental temperature, under the high-field conditions. The sample was kept in the

(1)

u(ψ)/kBT ) λP2(cos ψ)

(2)

where the dimensionless parameter λ measures the strength of the orientating potential. The resonance frequencies and the intensities of the transitions induced between the spin states of the triplet system are described better by using two different reference frames, the first one with the z axis aligned with the magnetic field and the second one defined on the molecule by the principal directions x, y, z of the zfs tensor. We name these additional frames B and T, respectively. In the oriented glass of a frozen nematic, the relative angle θ between the magnetic field and the director can be varied at will, as it can be made with a crystal sample. The TREPR line shape in anisotropic matrices is given by

I(B) ) Re

Pr(ΩBT) for(ΩLM)

∑∫ r)0,1 W(Ω

BT)

+ i[B - Br(ΩBT)]

dΩBT

(3)

where for(ΩLM) is given by eq 1, W(ΩBT) is the orientation

Fulleropyrrolidine Bisadducts

J. Phys. Chem. B, Vol. 103, No. 51, 1999 11277

dependent intrinsic line width, and Br(ΩBT) is the resonance field value of the rth transition, which, in the secular approximation, is given by

Br(ΩBT) ) (hν/gµB) + (r - 1/2)[D(3 cos2 β - 1) + 3E sin2 β cos 2γ] (4) where D and E are in magnetic field units. The intensity factor Pr(ΩBT) in eq 3 takes into account the non-Boltzmann populations of the triplet energy levels when they are populated from the excited singlet state via the intersystem crossing (ISC) mechanism. They are given by the differences between the diagonal elements of the spin density matrix:

Po ) 〈0|F|0〉 - 〈-1|F|-1〉

(5a)

P1 ) 〈+1|F|+1〉 - 〈0|F|0〉

(5b)

Since the relations between the Zeeman states |m〉 (m ) -1, 0, +1) and the zfs states |k〉 (k ) x, y, z) are well-known, the Pr’s are easily expressed as a linear combination of the relative populations pk of the zfs levels.20 The calculated spectrum needs as input parameters the D and E zfs parameters, the population ratio (px - pz):(py - pz) and the orientating potential strength λ. Typically, spectra were calculated when the director n is aligned with one of the principal axes for the two canonical configurations θ ) 0° (B0|n) and θ ) 90° (B0⊥n). As shown by eq 2, the value of λ determines the orientation distribution function of the guest molecules. In particular, the width of such distribution is accounted for by the order parameter S defined as the orientation average

S)

∫for(ψ) P2(cos ψ) sin ψ dψ

Figure 2. Calculated TREPR spectra for a model system. They have been computed with the same values of the zfs parameters, D ) 15 mT and E ) -2 mT, and with an intrinsic line width W ) 0.6 mT. The polarization pattern varies along the columns, while the rows correspond to different orientational degrees in the ordered arrangements. In the first column spectra with Boltzmann populations, i.e., standard EPR spectra, are shown as reference with an isotropic distribution and for two different values of the orientating potential. The two different experimental arrangements θ ) 0° (B0|n) and θ ) 90° (B0⊥n) are considered for either of them. In the second and third columns two different sets of totally polarized spectra are considered, with pX ) 1, pY ) pZ ) 0 and pX ) pY ) 0, pZ ) 1, respectively.

(6)

The order parameter vanishes for an isotropic distribution (λ ) 0). Instead, a positive value occurs when n is preferentially aligned along zM (λ < 0) while S takes negative values when such an axis lies perpendicular to n (λ > 0). Simulated TREPR spectra of a model system are displayed in Figure 2. Each spectrum is characterized by a set of six features situated at the resonant field values corresponding to the principal components X, Y, Z of the zfs interaction, which are named in the order |Z| > |Y| > |X|. The features are polarized in absorption (a) or in emission (e) and positive ordinate values are conventionally assumed for absorption. The relative intensities of the spectral features depend on the orientational degree of order and on the experimental arrangement. Since we used rather low microwave power in detecting spectra, it is assumed that EPR intensities are independent of the microwave polarization with respect to molecular orientation. TREPR Spectra of Bisadducts. We first report the triplet EPR spectrum of the N-methylfulleropyrrolidine monoadduct in order to appreciate the different capabilities of bisadducts to orient in the LC mesophase as the regiospecific addition of the second substituent changes its topology. As shown in Figure 3, from B0|n to B0⊥n configurations there is no change in the whole width of the spectrum, but rather there is a change in the polarization pattern that turns from aea eae to aaa eee. Parameters employed to simulate the spectrum are reported in Table 1. The spectra of the excited triplet states of 1 compounds detected in isotropic toluene matrix have been already reported.10a The spectra recorded for the triplets of 1 compounds in E7 are reported in Figure 4 along with their computer simulations. The parameters used in the simulation are collected in Table 1. The

Figure 3. Experimental (solid line) and simulated (dashed line) TREPR spectra of the excited triplet state in the N-methylfulleropyrrolidine monoadduct of C60 at 120 K dissolved in E7 measured at 120 K with B0|n and B0⊥n configurations. The principal axes of the zfs tensor are marked on the triplet spectra.

TABLE 1: Principal Values of the zfs Tensor X, Y, Z (in mT), Population Ratio (px - pz):(py - pz), and Order Parameter S As Obtained from TREPR Triplet Spectrum Simulation of N-Methylfulleropyrrolidine Monoadduct and of 1 Series Bisadducts monoadduct 1trans3 1trans4 1eq

X

Y

Z

(px - pz):(py - pz)

S

-1.8 -1.7 -2.0 -4.9

-3.5 -4.2 -4.8 -8.4

5.3 5.9 6.8 13.3

1.0:0.29 1.0:0.0 1.0:0.0 1.0:0.1

0.38 0.44 0.44 -0.40

striking changes in the spectral line shape are clearly visible when the external magnetic field is rotated with respect to the director of the nematic phase. The spectra of 1trans3 and 1trans4

11278 J. Phys. Chem. B, Vol. 103, No. 51, 1999

Pasimeni et al.

Figure 4. Experimental (solid line) and simulated (dashed line) TREPR spectra of series 1 bisadducts dissolved in E7 recorded at 120 K when B0|n and B0⊥n.

appear to be quite similar although the symmetry point group of the molecules is different (1trans3 belongs to C2 and 1trans4 to Cs). In the default orientation B0|n the magnetic field appears aligned to the direction corresponding to the intermediate value Y of the zfs tensor. Consequently, when B0⊥n the whole range of orientations extending from the outer z to the inner x direction are probed by the magnetic field with the aaa eee polarization pattern. In the case of 1 bisadducts the different ordering corresponding to different values of θ manifests with large variations of the spectral extension, in addition to the modification of the polarization patterns. The values of the zfs components and of the triplet transition polarization of 1 bisadducts in E7 do not differ from those in toluene.10a The orientation dependence of the 1eq triplet spectra differs completely from that of 1trans4 although they belong to the same Cs group symmetry. In 1eq the x direction lies perpendicular to the LC director (ae spectrum) with B0⊥n, and hence, when B0|n, the magnetic field sees a spread of molecular orientations contained in the xz principal plane of the zfs interaction. The triplet spectra of 2 compounds are shown in Figure 5. The spectra of 2trans2 and 2cis3 compounds are reported beside the analogous bisadducts of 1 shown in Figure 4. Spectra of 2trans2 resemble those of the 2eq bisadduct although the two compounds have C2 and Cs symmetry, respectively, in the ground state. As shown by the B0⊥n spectra, the inner x principal axis becomes almost completely aligned with n, and consequently, in the B0|n configuration the field spans the yz principal plane. On comparison of the spectra shown in Figures 4 and 5, it appears that trans3 and trans4 bisadducts display quite similar triplet spectra. A different ordering has been observed for the zfs principal axes of the 2cis3 bisadduct that belongs to the C2 group symmetry in the ground state, the same as 1trans3. When B0|n, almost exclusively the z direction is probed by the field; instead, when B0⊥n all the molecular orientations that lie in the xy principal plane are visited. Spectra recorded for the triplets of 2 compounds dissolved in the isotropic toluene matrix, except for 2trans1 bisadduct, are very similar to those of 1 compounds.10a A separate analysis is reserved for the spectra of the 2trans1 bisadduct in E7 that are shown in Figure 6. They are characterized by unusually narrow spectral extension and also by an unusual polarization pattern. In fact, the maximum spectral extension, which is detected with B0|n, does not exceed 5 mT, to be compared with the value of 16 mT measured for the triplet of 1trans1 in toluene glass.10a To get more information on the nature of such a triplet species, we have measured the triplet

Figure 5. Experimental (solid line) and simulated (dashed line) TREPR spectra of series 2 bisadducts in E7 recorded at 120 K with B0|n and B0⊥n.

Figure 6. Experimental (solid line) and simulated (dashed line) TREPR spectra of 2trans1 and 3trans1 bisadduct dissolved in E7 recorded at 120 K with B0|n and B0⊥n.

spectrum of 3trans1 bisadduct, differing from 2trans1 in the length of the side chains bonded to the pyrrolidine groups. The spectrum of 3trans1, also shown in Figure 6, consists of the superposition of two spectra. A first species gives a triplet spectrum, denoted as A, that is similar to the one observed for 2trans1. In addition, we have found that the triplet spectrum of the second species, denoted as B, is characterized by zfs and population parameters akin to those used to fit the spectrum of 1trans1 in the toluene matrix, and it has been assigned to the triplet of 3trans1 monomer. The polarization pattern of species A is also unusual since it gives an ea doublet for B0 parallel to the x principal axis. This result contrasts with the ae pattern observed for the whole series of bisadducts when B0|x. The smaller spectral width of the A spectrum points to a larger delocalization of the triplet wave function including presumably more than one molecule and also to a change in the spin polarization pattern. The latter spectral feature could be accounted for by a rotated principal axis frame with respect to that of the B species. By comparison, the triplet spectra of 2trans1 and 3trans1 detected in the isotropic toluene matrix are shown in Figure 7. Similarity of the spectral width and

Fulleropyrrolidine Bisadducts

J. Phys. Chem. B, Vol. 103, No. 51, 1999 11279 TABLE 4: Principal Directions and Principal Planes Probed by the Resonant Field in the B0|n and B0⊥n Configurations trans1 trans2 trans3 trans4 eq cis3

B0|n

B0⊥n

symmetry

yz yz y y yz z

x x xz xz x xy

D2h C2 C2 Cs Cs C2

Symmetry point group of molecules in the ground state is also shown.

Figure 7. TREPR spectra of 2trans1 and 3trans1 bisadducts dissolved in toluene matrix detected at 120 K.

TABLE 2: Principal Values of the zfs Tensor X, Y, Z (in mT), Population Ratio (px - pz):(py - pz), and Order Parameter S As Obtained from TREPR Triplet Spectrum Simulation of 2 Series Bisadducts 2trans2 2trans3 2trans4 2eq 2cis3

X

Y

Z

(px - pz):(py - pz)

S

1.5 1.7 2.2 4.3 0.7

-2.7 -4.3 4.7 9.4 5.0

4.2 6.0 6.9 13.7 5.7

1.0:0.17 1.0:0.0 1.0:0.0 1.0:0.17 1.0:1.0

-0.38 0.48 0.48 -0.28 0.40

TABLE 3: Principal Values of the zfs Tensor X, Y, Z (in mT), Population Ratio (px - pz):(py - pz), and Order Parameter S As Obtained from TREPR Triplet Spectrum Simulation of 2trans1 and A and B Species of 3trans1 Bisadducts 2trans1 3trans1 (A) 3trans1 (B)

X

Y

Z

(px - pz):(py - pz)

S

-0.6 -0.6 -2.7

-1.8 -2.4 -5.7

2.4 3.0 8.4

-0.44:0.55 -0.44:0.55 1.0:0.74

-0.38 -0.38 0.63

polarization pattern with respect to those in E7 when B0⊥n is manifested and well accounted for by spectral simulation. The overall picture that emerges from the observed spectra is that there is a variety of arrangements of the zfs principal axes with respect to the director of the LC phase, each arrangement being peculiar to the particular topology of the attack of substituents on the fullerene sphere. Discussion The comparative study of the triplet spectra in E7 carried out on the 1-3 bisadducts leads us to conclude that their ability to become oriented in the mesophase depends on the topology of the saturated double bonds rather than on the nature of the substituents. An important result is that, for each bisadduct, it is possible to arrange the sample in such a way that guest molecules are preferentially oriented with a zfs principal axis aligned to a desired direction in the laboratory frame. In fact, we have shown how this result can be achieved by rotating the director n with respect to the magnetic field B0, which is fixed in the laboratory frame. Specifically, it is shown in Table 4 that for trans1, trans2, and eq bisadducts the magnetic field probes the principal axis x corresponding to the smallest component of the zfs tensor in the perpendicular arrangement B0⊥n. Instead, for trans3 and trans4 the intermediate y principal axis is parallel to the magnetic field when B0|n while, in the same arrangement, the principal axis z of the cis3 bisadduct is parallel to B0. To achieve the complete identification of the preferred

molecular orientation in the anisotropic medium we should be able to assign the zfs principal axis frame with respect to a molecular frame chosen on the basis of the molecule symmetry in its ground state. In general, EPR data do not enable us to assess the reciprocal orientation of the two frames. Nevertheless, symmetry considerations on the molecular symmetry joined to the requirement of cylindrical symmetry of the mesophase around the n direction are useful to infer the molecular orientation in the mesophase. In Table 4 we have summarized the results, indicating the principal directions probed by the resonant field in the B0|n and B0⊥n configurations. Accordingly, bisadducts can be divided in two groups: (i) those having one principal direction aligned to n and (ii) those with n lying in a principal plane of the zfs interaction. Trans3 and cis3 bisadducts belong to the first group both having C2 symmetry. In this case one principal axis coincides with the C2 axis and the plane normal to it contains the other two axes, although it is uncertain which of them is aligned to the binary axis. For trans4 of Cs symmetry the mirror plane coincides with a zfs principal plane while the remaining principal axis is aligned to n. When B0|n and a single principal axis is probed, the latter guarantees cylindrical symmetry around n in the mesophase. In the second group of bisadducts trans1, trans2, and eq present the common feature that the plane normal to n coincides with the yz principal plane, although they belong to the D2h, C2, and Cs symmetry point groups, respectively. This suggests a possible lowering of symmetry of trans1 in the excited state. The constraint of cylindrical symmetry around n imposed by orientating potential in the mesophase forces molecules to align their x axis to the magnetic field in the B0⊥n configuration. From this analysis it emerges that in general determination of the principal axes in the molecular frame cannot not be achieved without uncertainty by EPR measurements. However, it is important to stress that for all the examined bisadducts it is found that high ordering is induced in the guest molecules either with one molecular direction, corresponding to one zfs principal axis, preferentially aligned to the director of the mesophase or with one molecular direction aligned to B0 in the B0⊥n configuration. This result might have important effects in exploring the anisotropy in the optical properties. Spectra of 2trans1 bisadduct are characterized by an unusually smaller D parameter than for other bisadducts, while even the set of population rates needed to simulate the spectra has to be modified. A comparison of the spectra of 3trans1 bisadduct with those of 2trans1 made possible the assignment of the latter. A dramatic reduction of the D value is accompanied by an increase in the spread of the triplet wave function over more than one molecule. A change in the population rates and hence in polarization pattern is frequently observed in the triplet spectra of dimers with respect to those of monomers. When two identical interacting molecules A and B are excited into a triplet state in which the Coulombian interaction VAB between the molecules in the dimer is larger than the zfs

11280 J. Phys. Chem. B, Vol. 103, No. 51, 1999

Pasimeni et al. TABLE 5: Principal zfs Components (in mT) of 2trans1 Monomer (X, Y, Z) and Dimer (X*, Y*, Z*) and Corresponding Populating Rate Constants pi and pi* (i ) x, y, z)a direction cosines lxA ) 0.8811 ) lxB lyA ) 0.1880 ) lyB lzA ) 0.4474 ) lzB mxA ) 0.0919 ) -mxB myA ) 0.8725 ) -myB mzA ) -0.4798 ) -mzB nxA ) -0.4639 ) -nxB nyA ) 0.4639 ) -nyB nzA ) 0.7547 ) -nzB a

zero-field splittings

populating rate constants

X ) -2.7 Y ) -5.7 Z ) 8.4 X* ) -0.6 Y* ) -2.4 Z* ) 3.0

px ) 1.0 py ) 0.75 pz ) 0.0 px* ) 0.0 py* ) 1 pz* ) 0.45

Calculated direction cosines fitting eq 8 are also reported.

either py*(+) ) pz*(+) ) 0 or px*(-) ) 0. The data of Table 5 indicate clearly that the latter condition, corresponding to the antisymmetric wave function combination, is satisfied in our case, supporting the assumption of 2-fold symmetry for the dimer. Under this condition the fine structure principal values X*, Y*, Z* of the dimer are given by the expression22,23 Figure 8. Sketch of the possible structure of the dimer of the 2trans1 and 3trans1 bisadducts.

interaction, the spin Hamiltonian of the dimer is simply the average of the spin Hamiltonians of the isolated molecules, forcing the dimer spin system out a new principal axis system whose orientations are essentially the averaged directions obtained from the relative positions of the two monomer principal systems. Principal axes of the dipolar interaction within triplet excited molecules are related through precise geometrical expressions to those of dimers. The triplet dimer wave functions group into two sets, the symmetric Ψi*(+) (i* ) x*, y*, z*) and antisymmetric Ψi*(-) combinations of the functions describing excited states on either of the dimer molecules, their energy levels being separated by 2VAB.21,22 The spin-orbitpromoted ISC triplet populations of the spin sublevels from the singlet excited state to the dimer triplet state may be written in terms of the dimer geometry and the monomer ISC populations pi (i ) x, y, z)

px*(() ) 1/2[(lxA ( lxB)px1/2 + (lyA ( lyB)py1/2 + (lzA ( lzB)pz1/2]2 py*(() ) 1/2[(mxA ( mxB)px1/2 + (myA ( myB)py1/2 + (mzA ( mzB)pz1/2]2 (7) pz*(() ) 1/2[(nxA ( nxB)px1/2 + (nyA ( nyB)py1/2 + (nzA ( nzB)pz1/2]2 where plus and minus refer to the symmetric and antisymmetric states and we indicate with liA, liB, miA, miB, niA, and niB the director cosines of angles between magnetic principal axes (x, y, z) of monomer triplet state and averaged principal axes (x*, y*, z*) of the dimer. The fine structure principal values X, Y, Z of the monomer are also related to those of the dimer and a measurement of the monomer and dimer triplet zero-field splittings and spin sublevel population rates can be utilized to determine features of the dimer geometry.23 We assume, as shown in Figure 8, that the dimer has a 2-fold axis of rotation along x*. As a consequence, the direction cosines appearing in eq 7 will have the property that liA ) liB, miA ) -miB, and niA ) -niB. This geometrical feature implies that

X* ) lx2X + ly2Y + lz2Z Y* ) mx2X + my2Y + mz2Z

(8)

Z* ) nx2X + ny2Y + nz2Z Solution of eq 8 accompanied by additional relations that guarantee orthonormality conditions gives the complete set of direction cosines that are collected in Table 5. They can be expressed through the three Euler angles that specify the relative orientation between the two principal axes frames and the results can be envisioned by means of the sketch shown in Figure 8. If the z principal axis in the monomer coincides with the D2h symmetry axis in the ground-state molecule, from the value of nz ) cos(zˆz*) ) 0.7547 one obtains 41° for the angle between the two axes. It should be noted that dimer formation is revealed in the spectra of 2trans1 and 3trans1 in E7 as well as in toluene, as shown in Figures 6 and 7, suggesting that aggregation is related to the nature of the substituents. The long chains present in 2 and 3 compounds are allowed by the LC potential to be aligned as sketched in Figure 8. Finally, it is worth noting that within the point dipole approximation, D/gµB ) -3gµB/2r3,24 and the experimental value of D ) -3.6 mT corresponds to 9.1 Å for the interspin distance r, which compares well to the value of 8.8 Å for two closely packed fullerene spheres.25 This result seems to support that we are dealing with dimers rather oligomers because in the latter case a larger spread of the triplet wave function would produce even smaller values of the zfs parameters. Conclusions We have investigated the macroscopic orientation of fullerene bisadducts dissolved in an E7 liquid crystal. Preferential alignment of the molecules along the director of the mesophase has been examined by applying the time-resolved EPR technique to their excited triplet states. Spectral line shape analysis has allowed us to measure the degree of order induced by the orientating potential and to get information on the orientation of the zfs principal axes with respect to the laboratory frame. By considering the ground-state symmetry of bisadducts, the molecular orientation with respect to the director n of LC could be inferred.

Fulleropyrrolidine Bisadducts It has been found that the preferential orientation of fullerene bisadducts is strictly related to the symmetry group (C2, Cs, or D2h) they belong to, in the ground state, suggesting that their capability of orientating in the LC phase can be exploited also as an analytical tool to discriminate among different isomers of fullerene derivatives. It has been also found that trans1 bisadducts of 2 and 3 series, but not of 1 series, exhibit a marked tendency to form dimers. The analysis of the triplet spectra of monomers and dimers have allowed us to envision the structure of aggregated fullerene molecules. It is shown that substituents in 2trans1 and 3trans1 play an essential role in dimer formation. Finally, the TREPR method, applied to the excited triplet states of fullerene bisadducts dissolved in the LC phase, have revealed that the latter manifest excellent orientating capabilities in the nematic phase. This result seems promising for the exploration of the anisotropy in the fullerene optical properties useful for potential applications. Acknowledgment. This work was in part supported by CNR (Program Materiali InnoVatiVi, legge 95/95) through Centro di Studio sugli Stati Molecolari Radicalici ed Eccitati and Centro Meccanismi Reazioni Organiche, and MURST (Contract no. 9803194198). References and Notes (1) Miles, P. A. Appl. Opt. 1994, 33, 6965-6979. (2) (a) Henari, F. Z.; Cazzini, K. H.; Weldon, D. N.; Blau, W. J. Appl. Phys. Lett. 1996, 68, 619. (b) Cha, M.; Sariciftci, N. S.; Heeger, A. J.; Hummelen, J. C.; Wudl, F. Appl. Phys. Lett. 1995, 67, 3850. (3) Maggini, M.; Scorrano, G.; Prato, M.; Brusatin, G.; Innocenzi, P.; Guglielmi, M.; Renier, A.; Signorini, R.; Meneghetti, M.; Bozio, R. AdV. Mater. 1995, 7, 404. (4) Sun, Y.-P.; Riggs, J. E. Int. ReV. Phys. Chem. 1999, 18, 43. (5) Modern Pulsed and Continuous-WaVe Electron Spin Resonance; Kevan, L., Bowman, M. K., Eds.; Wiley: New York, 1990.

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