Function-Oriented Design And Synthesis Of Ladder-Type Poly(p-phenylene)

Since the first synthesis of ladder-type poly (p-phenylene) (LPPP) by Scherf and Müllen in 1991, LPPP and its derivatives have received a great deal of attention as blue light emitting materials. LPPPs possess a rigid and fully conjugated backbone, exhibiting nice blue emission with high luminescence quantum yield. LPPP with its the two-dimensional ladder-type framework does not show any steric inhibition ofπ-electron delocalization due to the drastically reduced conformational freedom of the conjugated, ladder-type backbone. LPPP found various applications in fields such as polymer light-emitting diodes, field-effect transistors, plastic lasers and photovoltaic devices etc.Main problems in the applications of LPPP are the poor charge injection properties and the occurrence of an additional low energy emission component during the operation of organic light emitting diodes (OLEDs) or after heat treatment. This thesis containing three parts; the first two parts are focused on design and synthesis of two novel LPPPs which contribute in solving the problem of charge injection and unwanted low energy emissions. In the last part, we describe the generation of a new ladder-type polyarylene containing cyclopentadithiophene moieties which shows a red-shift of absorption/emission and may be used as donor component of organic photovoltaic device. (1) One effective route to overcome the poor charge injection in conjugated ladder polymers of the LPPP-type is the incorporation of carbazole units into the polymer backbone, due to the electron-donating capabilities associated to the nitrogen of the carbazole units. In the first part of the thesis, we describe a soluble ladder-type conjugated polymer (LPFC) with N-alkyl-2,7-carbazole units in the polymer backbone. The N-alkyl-2,7-carbazole monomers are prepared by nitration of dibromobiphenyl followed by a ring closure reaction to carbazole. The ladder polymer LPFC is prepared in a reaction sequence involving a Suzuki-type cross coupling reaction, a carbonyl reduction and subsequent ring closing to form the final ladder-type polymers. The structural characterization of LPFC and its optical and electroluminescence properties are presented. LPFC shows high molecular weight, excellent thermal stability, and good solubility in common organic solvents. It presents a blue photoluminescence (PL,λmax=465 nm) with high PL efficiency (70%). Electrochemical analysis for LPFC gave an increased HOMO level of -5.29 eV if compared to the reference LPPP (-5.5 eV) thus indicating an improved charge injection ability from ITO or PEDOT anodes. A single layer light-emitting device using LPFC as the active layer showed blue light emission (λmax=470 nm) (CIE coordinates: 0.18, 0.34) with high maximum luminescence of ~ 2000 cd/m2 and maximum luminance efficiency of 0.43 cd/A, The blue light emission is obtained with a low turn-on voltage of 4 V. These attractive properties characterize LPFC as promising material for polymer-based optical and electrooptical devices.(2) One very effective attempt to inhibit the unwanted low-energy emission component of LPPP-type polymers is the generation of a ladder backbone composed of spirobifluorene units. Spirobifluorene contains two biphenylene units connected via a tetrahedrally carbon. In this chapter we report a simple way to synthesize spiro-LPPP that is thoroughly composed of spirobifluorene building blocks. The route involves only two steps: a single-stranded precursor polymer containing bis(biphenyl-2-oyl)benzene building blocks is made in a Suzuki-type coupling reaction, followed by a subsequent two-fold cyclization cascade using methanesulfonic acid to form the target spiro-LPPP. The PL spectra of the spiro-LPPP in THF solution and thin film, respectively, indicate spiro-LPPP as a promising blue light emitting material with the main emission band peak at 455 and 456 nm, respectively. Spiro-LPPP shows a high PL quantum yield of 94% in THF solution. Annealing a thin film of spiro-LPPP to 120 oC in air for 3 to 24 hours its emission spectra keep unchanged thus reflecting an excellent thermooxidative stability of spiro-LPPP. Spiro-LPPP is a good candidate for realization of a stable blue light emitting in OLEDs and polymer lasers.(3) LPPP with rigid and fully planar conjugated backbone shows a relatively high charge carrier (especially hole) mobility of up to 2×10-3 cm2V-1s-1 in the bulk and up to 600 cm2 V-1s-1 for single polymer chains. However, the use as active material in organic solar cells is hindered by the large mismatch with the solar spectrum. To improve the absorption properties of LPPP-type ladder polymers, we have introduced bridged bithiophene building block (cyclopentadithiophene, CPDT) into the ladder-type polymer backbone. The new CPDT-based ladder polymers are generated in a sequence involving a Still-type cross coupling reaction, carbonyl reduction and subsequent ring closure to form the ladder-type framework. The new LPPP containing CPDT units (LPPT) displays a distinct red-shift the UV-Vis absorption (λmax= 525 nm) and PL (red emission,λmax= 582 nm) as well as a lower band-gap energy of 2.03 eV when compared with the corresponding all-phenylene ladder polymer LPPP (-5.5 eV). First bulk heterojuction-type photovoltaic devices with the LPPT/PCBM couple (1:1) as active blend showed a power conversion efficiency under white light illumination of 0.15% without optimization of blend composition and morphology. The promising properties of LPPT qualify it as a potential candidate for application as donor polymer in bulk heterojunction-type organic photovoltaic devices.