Polyhedral Oligomeric Silsesquioxane Bound Perylene Diimides And Their Bay Area Substituted Derivatives:Synthesis, Self-Assembly Behavior And Electrochemical Properties

Perylene diimides (PDI) derivatives are organic dyes and pigments which have traditionally been used for paints and coatings. In recent years, PDI derivatives have been found to be the best organic n-type material available. This unique electronic property along with other superior properties has seen great research interest in PDI as a raw material for optoelectronic application. However, like many other organic electronic chromophores, PDI derivatives lose their fluorescence emission when concentrated from solution to solid. Stacking of PDI during aggregation is another major drawback to its application as a photoluminescence and light harvesting material..The stacking and solubility problem is due to the fact that PDI have strong intermolecular π-π interactions. These interactions can be reduced by use of bulky substituents on the N,N’-imides. Various shallow tails have been used for this purpose, trialkylphenyl chains being the most commonly applied. Much as various PDI properties have been improved using various trialkyl chain lengths and branching there still remains the limitation of the phenyl ring only accommodating a maximum of three chains thus necessitating the use of very long chains which become difficult to control. There has been a desire to find another linker which unlike phenyl ring can allow more alkyl chains thus maximize the bulkiness without necessarily making the chains very long. Polyhedral oligomeric silsesquioxane (POSS) is the smallest well-defined cage-like silica nanoparticle with the idealized empirical formula (RSiO1.5)n. Unlike phenyl ring POSS has eight corner vertices which can be used to attach alkyl chains thus easily making it bulky with short chains. POSS is non toxic, biocompatible, chemically inert, mechanically stable, and nano sized. POSS in the recent past have attracted a large number of applications in many research fields.The fact that POSS is a nanoparticle which can allow attachment of up to eight chains is an important aspect which can be exploited to develop bulky substituents for PDI without necessarily using very long chains. In this research we have used POSS with seven isobutyl alkyl chains on its corner vertices as an N,N’-imide substituent on PDI. To know how the use of POSS as a bulky substituent would affect the PDI self assembly and photophysical properties, in solution and solid state we have synthesized POSS bound PDI and its derivatives and carried out a detailed investigation on various properties.A detailed description of PDI chemistry is reviewed in Chapter1, these include the electronic properties of PDI, and how the properties can be modified through N,N’-imide substitution and bay area substitutions. Examples of different substitutions and their respective effects have been outlined. POSS nanoparticle as a new substituent for hybrid materials has also been discussed. Finally the purpose of integrating POSS on PDI to develop molecular structure with better electronic properties has been explained.Chapter2. Bulky POSS nanoparticle has been condensed on PDI to form POSS bound PDI (POSS-PDI-POSS) molecule. To have a better understanding on the contribution of POSS to PDI photophysical properties more specifically the possibility developing PDI material with solid fluorescence have been explored. PDI with short linear alkyl substituent (PDI-1) was used for comparison. POSS addition did not change the characteristic absorption bands of PDI, also fluorescence emissions bands were mirror image of the absorption band as expected. Low tendency of POSS-PDI-POSS to aggregate in various non polar organic solvents was observed with concentrations as high as10"3M in chloroform having no aggregation signs.POSS-PDI-POSS crystals were found to be triclinic with dimmers as building blocks, the molecules in the dimmers had traverse offset as high as2.66A compared to0.67A for PDI-1. The high traverse offset directly weakened the binding strength in the dimmers to0.78eV as compared to1.34eV for PDI-1. Also due to steric hindrance POSS-PDI-POSS dimmers did not form continuous packing. The weak binding strength in crystal contributed greatly to high POSS-PDI-POSS solid fluorescence quantum yield (18.9%) which was several times greater than that of PDI-1.Chapter3. To know the effect of bay area substitution on photophysical properties of POSS-PDI-POSS, POSS-PDIBr-POSS with1,7-bromine substitution at bay area was synthesized and compared to POSS-PDI-POSS. The PDI characteristic absorption bands and fluorescence bands largely resembled those of POSS-PDI-POSS. The POSS-PDIBr-POSS aggregated at1.1×10-3M in chloroform as compared to3.5×10-4M for POSS-PDI-POSS in similar conditions. No new.aggregation peak at575nm was observed on the concentration dependant absorption spectra as it was the case for POSS-PDI-POSS in1:1propyl alcohol/1,4-dioxane bisolvent. Also more methanol (70%by volume) was required to lower the solvation effect of chloroform so as to allow aggregation, for POSS-PDI-POSS only33.3%was required. Solid crystals of POSS-PDIBr-POSS also were belt like with a smoother surface, the crystals had fluorescence emission at606nm as compared to625nm for POSS-PDI-POSS. The XRD revealed increase in d-spacing from3.58A for POSS-PDI-POSS to3.63A for POSS-PDIBr-POSS. Surprisingly the solid fluorescent quantum yield (Φf) of POSS-PDIBr-POSS was much higher than that of POSS-PDI-POSS,Φf values as high as83.9%was achieved from POSS-PDIBr-POSS; this was more than four times greater than POSS-PDI-POSS. The resulting crystals of POSS-PDIIBr-POSS were also able to emit red to yellow fluorescence when excited under different lights. The fluorescence were stable in high temperature, however addition of bromine on the POSS-PDI-POSS reduced the glass transition temperature from107℃to77℃and endothernic peak temperatures from366℃to342℃.Chapter4Materials which can absorb light in the near infrared region (NIR) are highly demanded for light harvesting. To develop POSS bound PDI with NIR we have substituted bromine on POSS-PDIBr-POSS with pyrrollidine to develop POSS-PDIpy-POSS. The substitution resulted to color change from red to green and maximum absorption was red shifted from525nm to653nm with a long tail extending into the NIR. The POSS-PDIpy-POSS was highly soluble in organic solvents and low aggregation which was reversible on heating and cooling was observed.Comparing the electrochemical properties of POSS-PDIpy-POSS with POSS-PDI-POSS and POSS-PDIsr-POSS revealed that the pyrrollidine substitution reduced the oxidation potential to0.8eV making it an easy material to oxidize. Also unlike POSS-PDI-POSS and POSS-PDIBr-POSS which had energy gaps greater than2eV, POSS-PDIprPOSS had1.81eV, this is a very good material for light harvesting. Despite the fact that the DSSC cell was constructed without an anchoring group, an efficiency of0.298%was achieved. Chapter5. The solid fluorescence in POSS-PDI-POSS and POSS-PDIBr-POSS among other favorable properties is desirable properties which can make the materials have wide application in the optoelectronic applications. Definitely there will be desire to use this kind of materials as pedants in other polymeric materials so as to take advantage of these properties. To enable their use as pedants, a reactive group is required, however introduction of new substituents on the POSS bound PDI as reactive group may alter the properties of the chromophore. We have introduced a reactive group on POSS-PDIBr-POSS by simply removing one POSS end so that we remain with a monoimide POSS-PDIBr-The photophysical properties remained largely similar to POSS-PDIsr-POSS with some slight increase in aggregation. Unlike POSS-PDI-POSS and POSS-PDIBr-POSS, POSS-PDIBr did not form belt like crystals, spherical shaped particles were formed by crystallization procedure. The particles just like POSS-PDIsr-POSS emitted red to yellow fluorescence in different light wavelength. When excited at525nm the particles gave fluorescence emission at622nm, an emission in between that of POSS-PDIsr-POSS (606nm) and POSS-PDI-POSS (627nm). This indicates that the monoimidization did not alter the solid.fluorescence properties greatly.Chapter6. Presents the conclusions of the research contained within this thesis and examines possible avenues of further research.