STRUCTURE AND PROPERTIES OF π-π INTERACTING COMPLEXES OF CARBON NANOTUBES

Abstract

Functionalization of carbon nanotubes (CNTs) has gained wide attention during the last two decades due to their applications in various optoelectronic devices. Such functionalization has been achieved by either covalently or non-covalently. The non-covalent functionalization is further classified into endohedral and exohedral. The endohedral functionalization involves the encapsulation of guest molecules inside the cavity of host CNT while in the exohedral one the molecules are adsorbed non-covalently on the surface of CNT. The non-covalently interacting endo- and exohedral complexes of CNTs are often stabilized by weak interactions such as π-π, C-H…π and N-H…π, depending on the functional groups present in the attached molecule. Among these, the π-π interactions are more common and exist between aromatic rings and CNT. Such interactions also exist between π-donor and π-acceptor molecules. In general, the various interactions such as dispersion, electrostatic and polarization stabilize the complex while the exchange interaction destabilizes it. Such complexes exhibit distinctive optoelectronic and charge transport properties which makes them suitable for their applications in organic electronic, organic light-emitting diode (OLED) and organic transistor devices. The donor-acceptor complexes of CNTs are particularly useful in OLEDs. The organic semiconducting complexes of CNTs with good carrier mobilities are potential candidates for organic transistors. They can be of p-type, n-type or ambipolar in nature depending on the magnitude of electron and hole mobility. Most of the studies on optoelectronic properties of the non-covalent complexes of CNTs have been done without an in-depth understanding of charge transfer at the molecular level. Besides, a molecular level understanding of the charge transport properties of such complexes are also lacking. In this regard, the computational investigation of the optoelectronic as well as the charge transport properties of non-bonded complexes of CNTs with donor or acceptor molecules is of utmost important. A new type of non-covalent functionalization of CNT is the mechanically interlocked nanotubes (MINTs). In MINTs, the movement of macrocycles on the surface of CNT is possible in presence of external stimuli such as light and can be used for various applications including molecular motors. In the present thesis, dispersion-corrected density functional theoretical methods are employed to study the interaction of carbon nanotubes (CNTs) with selected macrocyclic host molecules to explore their optoelectronic properties. ii The thesis is divided into seven chapters. In chapter 1, different types of functionalization of CNTs, mechanically interlocked nanotubes, their properties and applications are discussed. The earlier reported studies on the complexes of CNTs are also briefly reviewed. The important computational methodologies used in the present thesis are described in the second chapter. Beginning with Schrödinger equation, the quantum chemical methods such as Hartree-Fock and Post-Hartree-Fock methods are briefly discussed. Apart from these wavefunction-based methods, density functional methods used in the present work are also discussed. A brief outline of different types of functionals and basis sets is presented. This chapter also provides basic concepts of ground- and excited-state electron transfer processes for donoracceptor compounds. Various charge transport parameters such as reorganization energy, transfer integral and carrier mobility are explained. In chapter 3 of the thesis, stability, optoelectronic and charge transport properties of endo- and exohedral complexes of CNT with indigo are investigated using dispersion-corrected density functional B97-D in conjunction with 6-31G(d,p) basis set. The stabilization energy, ionization energy, electron affinity, the energy gap between the highest occupied and lowest unoccupied molecular orbitals (ΔEHOMO-LUMO), and absorption spectra of the complexes as well as their free components are determined. The ΔEHOMO-LUMO of about 1 eV is obtained for the complexes indicating them as organic semiconductors. The effect of number of indigo molecules on the above mentioned properties of their exohedral complexes with CNT is examined. The dependence of diameter of CNT on the stability and properties of its endohedral complexes with indigo is investigated. The effect of hybrid functional B3LYP-GD3 and long-range corrected hybrid functional ωB97X-D on the properties of most stable endohedral complex is examined. The photoinduced charge transfer for the exohedral complexes in which CNT behaves as a donor and indigo acts as an acceptor is observed. The optical absorption spectra of the complexes are simulated using the time-dependent density functional theoretical (TD-DFT) method. The complexes show charge transfer peaks in the visible and near-infrared regions of the electromagnetic spectrum. Based on the Marcus theory, the carrier mobility is calculated from the charge hopping rate. The carrier mobility calculations reveal that the exohedral complexes exhibit p-type character due to significantly higher hole mobility than electron mobility while the endohedral complexes possess nearly the same value of hole and electron mobilities. The results for the exohedral complexes of long and closed CNTs are similar to those obtained for the complexes of CNT of relatively small length as well as with open ends. Apart from this, the iii exohedral complex in which indigo is aligned parallel to the tube-axis exhibits almost similar value of hole and electron mobilities. The structure, optoelectronic and charge transport properties of the exohedral complex of (6,6)CNT with perylene bisimide (PBI) are investigated using different dispersion-corrected density functionals (B97-D, B3LYP-GD3 and ωB97X-D) in conjunction with 6-31G(d,p) basis set and the results are discussed in chapter 4. The electron density distribution in the frontier molecular orbitals of the complex indicates the possibility of photoinduced charge transfer from donor CNT to acceptor PBI constituting a donor-acceptor complex between them. Due to inappropriate size of the cavity of (6,6)CNT to host PBI, a relatively larger diameter (8,8)CNT is used for the encapsulation. The calculations of stabilization energy reveal that the endohedral complex PBI@(8,8)CNT is more stable than the exohedral complex PBI-(8,8)CNT. The energy decomposition analysis of the complexes suggests that the dispersion and the electrostatic interactions are predominant for endo- and exohedral complexes, respectively. In chapter 5 of the thesis, the structure and properties of endo- and exohedral complexes of (6,6)CNT with electron donor molecule quaterthiophene (4T) are investigated using various dispersion-corrected density functionals. A comparative study on the charge transport properties of both types of complexes is presented. The results indicate a n-type charge transfer characteristics owing to remarkably higher electron mobility than hole mobility, irrespective of the type of functionalization. The excited state calculations of the complexes carried out in the framework of TD-DFT indicate several charge transfer transitions from donor 4T to acceptor CNT in the visible region of the electromagnetic spectrum. The complexes also show very high light-harvesting efficiency implying their possible application in solar cells. The optoelectronic properties of the complexes of guest (6,6)CNT with macrocyclic hosts [10]cycloparaphenylene ([10]CPP) and its derivatives are studied using dispersion-corrected density functional method and are discussed in chapter 6. The various derivatives of [10]CPP are modelled by doping nitrogens as well as by substituting hydrogens with electron-donating amino/electron-accepting fluorine groups. The values of stabilization energy indicate that the complexes CNT@[10]CPP and CNT@nF-[10]CPP (n = 10, 20 and 40) are energetically stable. The frontier molecular orbital analysis predicted the occurrence of photoinduced charge transfer in the complex CNT@40F-[10]CPP. The optical absorption spectrum of the complex CPP-CNT indicates absorption in the near-ultraviolet and visible regions, whereas that of the complexes CNT@nF-[10]CPP show absorption in a wide range starting from near-ultraviolet to nearinfrared region of the electromagnetic spectrum. Among the complexes, high values of lightiv harvesting efficiency are obtained for CNT@nF-[10]CPP. The change in potential energy for the translational movement of CPP over CNT for both ground and excited states is examined. The results indicate an energy barrier for the piston type movement of CNT in the complexes for the ground state, but not for its excited states. The barrier for rotation of bare and fluorinated CPP over CNT suggests the application of these complexes as components in molecular wheels and shuttles. The summary and conclusions of the thesis are provided