Title:

Functionalization of endohedral fullerenes and their application in quantum information processing

Quantum information processing (QIP), which inherently utilizes quantum mechanical phenomena to perform information processing, may outperform its classical counterpart at certain tasks. As one of the physical implementations of QIP, the electronspin based architecture has recently attracted great interests. Endohedral fullerenes with unpaired electrons, such as N@C_{60}, are promising candidates to embody the qubits because of their long spin decoherence time. This thesis addresses several fundamental aspects of the strategy of engineering the N@C_{60} molecules for applications in QIP. Chemical functionalization of N@C_{60} is investigated and several different derivatives of N@C_{60} are synthesized. These N@C_{60} derivatives exhibit different stability when they are exposed to ambient light in a degassed solution. The cyclopropane derivative of N@C60 shows comparable stability to pristine N@C_{60}, whereas the pyrrolidine derivatives demonstrate much lower stability. To elucidate the effect of the functional groups on the stability, an escape mechanism of the encapsulated nitrogen atom is proposed based on DFT calculations. The escape of nitrogen is facilitated by a 6membered ring formed in the decomposition of the pyrrolidine derivatives of N@C_{60}. In contrast, the 4membered ring formed in the cyclopropane derivative of N@C_{60} prohibits such an escape through the addends. Two N@C_{60}porphyrin dyads are synthesized. The dyad with free base porphyrin exhibits typical zerofield splitting (ZFS) features due to functionalization in the solidstate electron spin resonance (ESR) spectrum. However, the nitrogen ESR signal in the second dyad of N@C_{60} and copper porphyrin is completely suppressed at a wide range of sample concentrations. The dipolar coupling between the copper spin and the nitrogen spins is calculated to be 27.0 MHz. To prove the presence of the encapsulated nitrogen atom in the second dyad, demetallation of the copper porphyrin moiety is carried out. The recovery of approximately 82% of the signal intensity confirms that the dipolar coupling suppresses the ESR signal of N@C_{60}. To prepare ordered structure of N@C_{60}, the nematic matrix MBBA is employed to align the pyrrolidine derivatives of N@C_{60}. Orientations of these derivatives are investigated through simulation of their ESR spectra. The derivatives with a –CH3 or phenyl group derived straightforward from the Nsubstituent of the pyrrolidine ring are preferentially oriented based on their powderlike ESR spectra in the MBBA matrix. An angle of about is also found between the directors of fullerene derivatives and MBBA. In contrast, the derivatives with a –CH₂ group inserted between the phenyl group and the pyrrolidine ring are nearly randomly distributed in MBBA. These results illustrate the applicability of liquid crystal as a matrix to align N@C_{60} derivatives for QIP applications.
