PDI Derivatives Research Articles

A comparative theoretical study of exciton-dissociation and charge-recombination processes in oligothiophene/fullerene and oligothiophene/perylenediimide complexes for organic solar cells

The exciton-dissociation and charge-recombination processes in donor–acceptor complexes found in α-sexithienyl/C60 and α-sexithienyl/perylenetetracarboxydiimide (PDI) solar cells are investigated by means of quantum-chemical methods. The electronic couplings and exciton-dissociation and charge-recombination rates have been evaluated for various configurations of the complexes. The results suggest that the decay of the lowest charge-transfer state to the ground state in the PDI-based devices: (i) is faster than that in the fullerene-based devices and (ii) in most cases, can compete with the dissociation of the charge-transfer state into mobile charge carriers. This faster charge-recombination process is consistent with the lower performance observed experimentally for the devices using PDI derivatives as the acceptor.

Synthesis and Photophysical Properties of Glass-Forming Bay-Substituted Perylenediimide Derivatives

A series of perylenediimide-based small molecules (PDI1-PDI5) containing electron-deficient groups in the bay region were synthesized and characterized. The PDI derivatives were found to be capable of forming molecular glasses with glass transition temperatures ranging from 50 to 102 degrees C. Detailed investigations of the optical properties of the synthesized derivatives were performed and compared with those obtained from quantum chemical calculations. Optimized molecular structures of the PDI derivatives exhibited core-twisting by 16 degrees and torsional angle between the bay substituent and the perylene core in the range of 60-72 degrees. The PDI derivatives exhibited absorption maxima in the range of 2.27-2.36 eV and emission maxima in the range of 2.10-2.28 eV. The impact of the bay substituents on the emission, fluorescence quantum yield, and lifetimes in solutions and thin films was established. The red shift of emission maxima (from 2.282 to 2.095 eV) observed for various PDIs in solutions was accompanied by significant reduction in the emission quantum yield (from 0.73 to 0.44) and corresponding increase of the fluorescence lifetime (from 4.5 to 6.8 ns). This was in agreement with quantum chemical calculations indicating decrease of the radiative relaxation rate due to reduction of the oscillator strength and remarkable decrease of the torsional activation barrier. The spectral properties of the wet-casted perylenediimide films featuring different bay substituents were also studied. The variation in the emission peak (of 0.25 eV) and the considerable increase of the Stokes shift (of 0.4 eV) are explained in terms of the formation of the amorphous state. The influence of the bay substituents on the thermal and spectral properties of the films are discussed.

Structures and properties of 1,7-disubstituted perylene tetracarboxylic diimides: The substitutional effect study based on density functional theory calculations

Density functional theory (DFT) calculations were carried out to describe the molecular structures, molecular orbitals, atomic charges, UV–vis absorption and IR spectra of 1,7-disubstituted-perylene-3,4;9,10-tetracarboxylic diimide (1,7-disubstituted PDI) with different substituents, namely unsubstituted PDI (PDI-0), 1,7-dimethylthio PDI (S-a), 1,7-dithiophenyl PDI (S-b), 1,7-dimethoxy PDI (O-a), 1,7-diphenoxy PDI (O-b), 1,7-dimethylamino PDI (N-a), 1-methoxy-7-methylthio PDI (OS-a), 1-phenoxy-7-thiophenyl PDI (OS-b), and 1-methoxy-7-methylamino PDI (ON-a). Good consistency was found between the calculated results and experimental data. The substitutional effects of the side groups on both the structure twisting and the electronic absorption spectra depend mainly on the linking atoms rather than the side groups as a whole. The charge distribution, the ionization energy (IE) and electronic affinity (EA) are found to be varied significantly along with the change of the electron donating abilities of the side groups. The effects of the side groups on the vibration spectra of these compounds are also discussed based on both the simulated and experimental results. The present work, representing the first systemic DFT study on the PDI derivatives, sheds further light on clearly understanding structure and spectroscopic properties of PDI compounds.

Localization-independent Regulation of Homocysteine Secretion by Phosphatidylethanolamine N-Methyltransferase

Genetic ablation of phosphatidylethanolamine N-methyltransferase (PEMT) in mice causes a 50% reduction in plasma homocysteine (Hcy) levels. Because hyperhomocysteinemia is an independent risk factor for cardiovascular disease, resolution of the molecular basis for this reduction is of significant clinical interest. The PEMT pathway is a metabolically channeled process localized to the endoplasmic reticulum (ER). To assess the importance of PEMT localization for Hcy homeostasis, we identified and ablated the minimal ER targeting motif. Mutagenesis of a conserved, C-terminal lysine residue (197) relocalized the enzyme to the Golgi, demonstrating that Lys-197 is essential for targeting PEMT to the ER. To evaluate the functional significance of PEMT localization, hepatoma cell lines were generated that stably expressed either ER- or Golgi-localized PEMT only. Intriguingly, stable expression of PEMT in either the ER or the Golgi caused increased Hcy secretion. Moreover, PEMT-mediated Hcy secretion correlated with the methyltransferase activity of the enzyme, independently of subcellular localization. Thus, our data suggest that Hcy homeostasis is regulated concomitantly with PEMT activity but independently of PEMT localization.