Unusual solvent-dependent photophysical and self-assembly properties of NO2 substituted T-shaped phenazines.

This paper investigates the importance of substituent placement when designing low-molecular mass π-organogelators. The electron-deficient NO2 substituent was systematically added to novel T-shaped phenazines to examine electronic as well as assembly properties. This T-shaped molecular platform promotes selective electronic tuning, which can be theoretically analyzed by examining the system's frontier molecular orbitals. Electronic properties were characterized by UV-vis spectroscopy and cyclic voltammetry, and comparisons were made based on number and placement of the NO2 group. Computational chemistry (B3LYP/6-31G*) was employed for geometry optimizations, and to generate molecular orbital diagrams for all systems. The most noticeable influence of NO2 position was found for two molecules with four NO2 groups placed at different locations about the molecule (T-34dNT and T-35dNT). A 0.13 eV difference in ELUMO was observed while EHOMO was not significantly impacted by this change only in NO2 placement. Interestingly and unexpectedly, the photophysical properties and solvent-dependent gelation properties were considerably different for T-34dNT and T-35dNT. T-34dNT exhibited a unique fluorescence (FL) solvatochromism, with FL intensity and maxima dependent on solvent polarity. This result is indicative of intramolecular charge transfer. In addition, long tailing at the solid-state absorption of T-34dNT suggests the presence of intermolecular charge transfer. The gelation of T-34dNT produced chromism ranging from red to orange to yellow when the solvents changed from acetonitrile to ethyl acetate to cyclohexane, respectively. T-35dNT gels in these solvents did not exhibit any of the same properties. Xerogel morphology characterizations were carried out using three different solvents for both T-34dNT and T-35dNT. In the case of T-34dNT, striking differences in the morphology were detected by field-emission scanning electron microscopy (FE-SEM). We conclude that numbers of substituents are not the only consideration in effective molecular design for organogelators, but that substituent position plays a critical role in certain fundamental properties of these systems.

Morphological effects on the small-molecule-based solution-processed organic solar cells.

We report a proof-of-concept study on solution-processed organic solar cells (OSCs) based on [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and structurally compact donor molecules which have dithiophene-phenazine-dithiophene (TH-P) and dithiophene-quinoxaline-dithiophene (TH-Q) configurations with decyloxy and methyl side groups, respectively. These molecules formed one-dimensional fibers through self-assembly via weak nonbonding interactions such as π-π and van der Waals interactions even during a fast solvent removal process such as spin-casting. Photophysical and thermal properties of the new donor molecules were characterized with UV-vis absorption and fluorescence spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. The electrochemical data determined experimentally were correlated well with theoretical evaluations. The fibers from the two donor molecules showed distinct morphological differences, allowing for in-depth investigations into their influence on the OSC performance. A continuous three-dimensional network of endless one-dimensional nanofibers, with a width of 300-400 nm, were formed from TH-P regardless of the presence of PC61BM, affording spontaneous nanoscale phase separation that facilitates a large donor/acceptor interfacial area. Bulk (BHJ) and planar heterojunctions (PHJ) from TH-P/PC61BM showed a power conversion efficiency (PCE) of 0.38% and 0.30%, respectively, under optimum device conditions. Post thermal annealing led to the increased domain size and a major decrease in Jsc. Meanwhile, shorter, more rigid needles with a large thickness variation were formed from TH-Q. A continuous network of TH-Q was obtained by spin-coating only in the presence of PC61BM, and the PCE of TH-Q/PC61BM BHJ was found to be 0.36%. However, the PHJ showed poor device performance due to TH-Q's inability to form a continuous film by spin-coating. The present study suggests a basic molecular architecture to drive one-dimensional assembly and demonstrates the significance of fibrillation for small-molecule-based OSCs.

Controlling the electron-deficiency of self-assembling pyrazine-acenes: a collaborative experimental and theoretical investigation.

This paper reports novel pyrazine-acenes containing electron-deficient heteroaromatic π-extenders, such as pyridine, pyrazine, and benzothiadiazole, directly fused with pyrazine. Electronic properties of these systems were characterized by UV-Vis, fluorescence spectroscopy, and cyclic voltammetry. Computational electronic property evaluation of all experimentally synthesized compounds is provided, and is coupled with electronic calculations of closely related compounds that were not synthetically feasible. Our theoretical results provide insight into the overall analysis and interpretation of the experimentally observed trends. In this study, we found a systematic decrease in the LUMO energy (E(LUMO)) with an increasing number of imine functions in the π-extender. Additionally, when comparing the pyrazine-acene containing pyrazine π-extender to a reference compound with C≡N peripheral substituents, we found that the imine function is comparable to the C≡N substituent in lowering E(LUMO). The most dramatic E(LUMO) lowering was experimentally observed using dibromobenzothiadiazole as a π-extender. In all cases, the HOMO energy (EHOMO) was negligibly affected, thus we found options for electronic property control based solely on E(LUMO) manipulation. This is computationally validated by an examination of the molecular orbitals in which the LUMO orbital was found predominantly on the π-extender section of the molecules, while the HOMO orbital was localized away from the π-extender. Interestingly, the self-assembly of all the experimentally synthesized compounds showed excellent one-dimensional fiber formation in spite of their large π-core framework. These fibers were characterized by atomic force microscopy and UV-Vis spectroscopy.

Self-assembly of pyrazine-containing tetrachloroacenes.

In this paper, we report self-assembly of tetrachloroacenes containing pyrazine moieties. The title compounds, phenazine and bisphenazine substituted with four chlorine atoms for increased electron deficiency and alkyloxy side groups for solubility, demonstrated excellent gelation ability in select organic solvents. The assembled structure of these two series of compounds exhibited a morphological difference. Tetrachlorophenazine containing hexadecyloxy side groups induced rigid microbelts, while more extensive entanglement of thinner, more flexible fibers was observed from tetrachlorobisphenazine compounds, characterized by scanning electron microscopy. Tetrachlorophenazine and tetrachlorobisphenazine gels showed quite different emission behavior compared to their solution state. A strong, red-shifted emission compared to that of its diluted solution state was observed from the gel of tetrachlorophenazine. We have ascertained this is a result of J-aggregate formation. From the crystal structure of a model compound, it was found that tetrachlorophenazine cores adopt π-π stacking with a short stacking distance of 3.38 Å, enabling significant intermolecular π-orbital overlap. In addition, the π-cores were displaced longitudinally, indicative of J-aggregate formation. Surprisingly, the gel of tetrachlorobisphenazine showed fluorescence comparable to that of its dilute solution, suggesting that such a close packing of the π-cores may not be possible due to the bulky tert-butyl substituents.