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43047-G1
Organic Electronic Materials Composed of Alternating Aryl/Perfluoroaryl Units: Synthesis and Structure-Property Relationships
During this funding cycle, synthetic methods were developed for, and structure-property studies were performed on, two classes of materials:
1. Partially fluorinated, condensed polycyclic aromatic compounds (FFPA)
2. Conjugated polymers containing pyromellitic diimide (PMDI) units
The unifying theme is the study of materials containing electron-rich and electron-poor pi-electron units which are electronically conjugated through formal single bonds or fused together. The originally proposed research focused entirely on fluorinated aromatics as the electron-poor units, but this work has expanded to include PMDI moieties. The regularly arranged electron-rich and –poor units can provide desirable (opto)electronic properties and control over supramolecular behavior.
As stated in our first annual report, this ACS-PRF-G grant truly functioned as an effective “starter” grant. Some important indicators of the impact of this grant on the careers of the P.I. and associated students:
-Sufficient preliminary results concerning fluorinated polymer synthesis and characterization were generated to complete one publication in an ACS journal and to warrant funding from a state agency and the NSF.
-Preliminary results concerning FFPAs now form the basis for a recently submitted NSF-CAREER award proposal and one manuscript in preparation.
-Based on work with fluorinated materials, the P.I. has been invited to present at a number of universities, two fluorine symposia at regional ACS meetings, and has been nominated as a candidate for the ACS Fluorine Division Executive Committee.
-One graduate student completed his M.S. based on FFPAs and is now pursuing his Ph.D. at the Max-Planck Institute for Polymer Research in Germany. He has one manuscript in preparation.
-A second graduate student has prepared a new class of PMDI-based poly(phenylene ethynylene)s (PPE) which has resulted in presentations at two conferences and a manuscript in preparation.
Fused Fluorinated Polycylic Aromatics (FFPA)
Our initial approach for preparing FFPAs followed classical routes whereby aromatic frameworks were constructed via transition-metal mediated carbon-carbon bond formation, followed by (photo)oxidative fusion to planar, fully conjugated pi-systems. Between the time that the proposal was submitted and approved for funding, we realized that all the new bonds in the target FFPAs could be formed in one laboratory step by reacting bis-metalated biaryls with fluorinated aromatics.
During this second funding cycle, we expanded the scope of this approach to prepare more extended all-carbon pi-frameworks, as well as a number of thiophene-containing FFPAs. Thiophene-containing materials were generated by reaction of 3,3'-dilithio-2,2'-bithiophenes with various fluorinated aromatics to form two or four bonds in one laboratory step. These resulted in new napthodithiophenes, anthradithiophenes, and tetrathienotetracenes. Crystallographic studies confirm that most of these thiophene-containing FFPAs assemble to face-to-face stacks, as we hoped. However, these pi-interactions were interrupted in one case when the FFPAs carried crystallizable alkyl side chains. As has been published for other highly fluorinated pi-systems (though without comment), we noted that fluorination often nearly extinguishes the lowest energy optical transitions of these materials (UV-Vis absorption). Through a collaboration established during the first funding cycle, we are obtaining calculations to delineate this effect. Significant contribution of the fluorine-based non-bonding orbitals to both the HOMOs and LUMOs have been noted.
Pyromellitic Diimide (PMDI) containing polymers
As outlined in our first annual report, we reasoned that electron-accepting PMDI-based monomers fit well into our originally proposed research plan, both from the viewpoint of developing new synthetic methodology, and for studying new materials containing electron-rich and –poor aryl units. A series of PPEs with alternating PMDI units and functionalized benzenes were prepared. These polymers have PMDI units incorporated such that the aromatic ring is in direct electronic conjugation with the polymer backbone. After failing to gain NMR-based structure-proof with in-house instrumentation (due to aggregation), we were at least able to obtain proton NMR at “extreme” temperatures through a foreign collaboration. The benzene-based comonomers carry side chains with varying electron-donating abilities. Polymers having exactly the same electronic backbone but differing side chains vary in solid-state color from yellow to red to purple to black as a consequence of decreasing intrinsic optical band-gap and likely intermolecular charge transfer. Despite the fact that more than one of these polymers displays metallic luster in the solid state, we determined that none of them are intrinsic semiconductors by simple conductivity measurements. The series of polymers was expanded to include a total of six examples. A series of analogous trimeric model compounds was prepared to compare the effects of substituents. Optical measurements have been completed and a manuscript is in preparation.
We had hoped that electrostatic attraction between the carbonyl oxygen atoms and adjacent thiophenes would result in planarization of newly-prepared PMDI-thiophene copolymers. However, optical and WAXD measurements indicate the contrary. This new work will be continued in the future using bithiophene comonomers along with PMDI.
