Synthetic Strategies Towards the Synthesis of Twistacenes, Oligoacenes, Polydibenzophoshole and Pentiptycene Rotor Derivatives

Author(s): Jimenez, Miguel Angel | Advisor(s): Garcia-Garibay, Miguel; Wudl, Fred | Abstract: Polycyclic aromatic hydrocarbons (PAHs) are a group of widely studied organic molecules due to their interesting electronic properties. Polyacene (1), has produced the most interest from this group, both because of the synthetic challenges it poses as well as its prediction to be highly conductive. Thus far a number of oligoacene precursors have been reported in the literature, but to date no system where n is greater than six (e.g. nonacene) has been reported. We have undertaken the synthetic challenge to synthesize heptacene as well as other extended oligoacenes, including polyacene. Additionally, we have synthesized tetrabenzoheptacene (2), a twisted acene, which despite its twisted nature is quite stable and displays intereresting electronic properties. The development of full color red-green-blue (RGB) displays has been hindered by the lack of stable blue polymer light emitting diodes (PLEDs). We propose the synthesis of poly(dibenzophosphole) (3) as a potential candidate for a blue PLED. Organic compounds possessing aromatic groups that are linearly conjugated through acetylene linkages have also received much attention. The cylindrical symmetry of the alkyne group maintains conjugation with the adjacent phenyl groups and results in low energy barriers in the ground state. However, rotation in the excited state displays a large energy difference as a function of the angle between the aromatic groups. The twisting and planarization of these aryl-alkynyl conjugated systems ultimately results in changes in both color and emission properties. We have synthesized several p-phenylene- ethynylene pentiptycene rotor derivatives (4) to probe the effects of chromophore rotation in the ground state and in the excited state both in solution and in the solid state. By incorporating pentipycene into the rotor it provides a protecting stator and a fixed central phenylene ring. The flanking ethynylphenylene rotators allow for free rotation to study the electronic properties as a function of angle. The crystal structure shows that pentiptycene rotor 4 (R=H, R1 = OMe) packs in a way that the conjugated backbone of these rotors is aligned in a parallel manner, which is desirable for the development of novel functional materials.