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1.      We have developed two synthetic approaches toward unsymmetric PDIs

In approach A, the unsymmetrically substituted PDI 1s was prepared in a high yield and under a mild condition as shown in scheme 1.  Variations of 1 were conveniently transformed to an array of unsymmetrically substituted PDIs including amphiphilic PDIs and monomers of PDI side-chain polymers.

In approach B, the versatile intermediate 2 was prepared in a good yield as presented in Scheme 2.  2 was successfully converted into a series of unsymmetrically substituted PDIs.  Moreover, the first unsymmetrically substituted perylene tetracarboxylic ester 3 was successfully prepared from 2.  Perylene tetracarboxylic esters are also good organic electron acceptors, however their applications other than simple symmetrically tetrasubstituted compounds are very rare, probably due to the difficulty in accessing unsymmetrically substituted compounds.  The method we developed may catalyze the further development of perylene tetracarboxylic tetraesters, particularly their incorporation in complicated molecular and supramolecular systems and polymers.

2. We have performed unsymmetric tuning of PDIs

Equipped with the enabling chemistry, unsymmetric PDI p-stack tuning has been performed.  This type of tuning has been done at both sub-nanometer and nanometer levels.

At sub-nanometer level, the tunability of PDI p-stacks was further enhanced by the synthesis of unsymmetrically substituted 4s. It is reasonable to expect the phase behavior of 4 to be between that of two corresponding symmetrically substituted 5₁ and 5₂.  On one hand, the expected behaviors were indeed observed for most variations of 4.  On the other hand, a few exceptions occurred.  For instance, both 5a and 5b are optically anisotropic liquid crystalline materials at room temperature.  However, at room temperature compound 4a self-assembles into a novel optically isotropic phase with p-stacked PDI moieties.  An interesting electric switching phenomenon was observed when the electric field applied to 4a is greater than a threshold value.

Intramolecular microphase segregation was utilized to control the organization of PDI p-stacks at nanometer level.  In addition to the microphase segregation between the rigid PDI core and flexible side-chains, incompatible side-chains were introduced to PDIs.  We have synthesized a number of amphiphilic PDIs with two examples given in Scheme 4.  Structural analysis of these compounds indicated that alkyl/tetraethylene glycol and alkyl/perfluoalkyl microphase segregation indeed occur and lead to the formation of novel phase structures.

3. p-Stack tuning in molecular glass

In many applications of organic conjugated materials, it is important to be able to obtain homogeneous, transparent thin films.  In this regard, amorphous glassy materials exhibit isotropic properties and it is their inherent ability to form homogenous and transparent films.  Although polymers are the most common sources of amorphous glassy materials, amorphous glassy materials derived from small molecules, i.e. molecular glasses, offer the advantage of well-defined composition, high purity and good processability.  However, molecular glasses are usually thermodynamically unstable. Thus they tend to order quickly unless the ordering process is kinetically depressed by a glass transition temperature significantly higher than the application temperature.  Despite their technique usefulness, there are few reports on PDI molecular glasses with high Tgs.  By introducing rigid side groups, we have successfully synthesized a series of amorphous glassy PDIs.  They have been thoroughly characterized by polarizing optical microscopy, X-ray diffraction, UV-Visible spectroscopy, differential scanning calorimetry, atomic force microscopy and charge carrier mobility measurements. All of them exhibit a glass transition temperature higher than 100 °C.  The electron mobility is as high as 10^-4 cm²V^-1S^-1. X-ray and UV analysis indicate that PDI p-stacks exist in these amorphous glassy solids and more importantly, both the intra-stack separation and the extent of stacks can be systematically altered.  This is the first time the degree of short-range order can be systematically controlled in a molecular glass.  The availability of this adjustability may lead to many new technologically useful molecular glassy materials.

4. Both graduate and undergraduate students were involved in this project.  One undergraduate student carried out the related research in summer 2008 for three months.  Five graduate students have been directly or indirectly supported by this grant.  This grant definitely impact them quite positively in terms of career development.