Bio-inspired conjugated organometallic dyades, triads and polymers for photophysical studies

Abstract

The objectives pertaining to my projects were the synthesis and characterization of all organometallic polymers and hybrid materials as well as the synthesis of the model compounds for the biomimicry of the B800-carotenoid-B850 unit in the LH II (light harvesting devices) of the purple photosynthetic bacteria.The design of new conjugated polymers that have potential applications in solar cells, a man-made device that converts light energy into electricity, or light emitting diodes which use little electricity to produce light will be discussed as well as the energy flow across a material or a polymer as a function of time in a controlled manner.The latter properties, rate of exciton and energy transfer processes, are the key parameters that define the efficiency of such materials.The full understanding of the relationship between the structure and the rate of energy migration is necessary for such design. In chapter 2, electrostatically held polymer anion / polymer cation's were prepared and characterized.The intrinsic polydispersity of the starting ionic organic conjugated and luminescent polymers was believed to limit the morphology of the resulting hybrid, which in turn play an important, if not crucial, role in the efficiency of the energy migration across the material.The results from this project have been published in Inorganic Chemistry. In Chapter 3, the mono-dispersed oligomers (monomers, dimers, and timers), were synthesized and characterized taking into consideration two crucial structural parameters in the design. First, the chromophore had to be easy to synthesize and also exhibit chromophoric properties very similar to those of the bacteriochlorophylls. Secondly, the interchromophoric ?-contact responsible for the electronic communication across a material must be elucidated. We elected to use a porphyrin derivative as the aromatic macrocycle playing the role of the bacteriochlorophyll. Both consist of a cyclic tetrapyrol center but differ by the absence of the double bonds in the bacteriochlorophyll macrocycle and the substitution groups around it. We chose to covalently link these chromophores together with ethynyl-phenyl spacers directly onto the trans-positioned meso-carbons. Conjugation secures electronic communication and covalent bonding secures the integrity of the assembly in solution (in comparison with H-bonds for example). We also decided to use luminescent Pd- and Zn-containing porphyrins acting as the energy donor and acceptor molecules, respectively. This choice is based on experience where both the singlet and triplet energy transfers were demonstrated with respect to the possible electron transfer.The results of this project have been submitted to Inorganic Chemistry. Interests in improving the efficiency of current photonic devices, whether it be LED's or solar panels, has increased tremendously over the past few years. This interest, along with the efficiency of photosynthetic bacteria, has inspired us to carry out these projects. Our hypothesis of creating or keeping a"communication" (such as energy transfer and electron transfer) between donors and acceptors, for example by the addition of a heavy metal, was proven to be correct.The key feature is that incorporation of a heavy metal in the backbone of the polymers can enhance intersystem crossing, hence leading to an increase in the population of the triplet states, allowing for a second path of communication between the two chromophores.