Siwei Leng

From crystals to columnar liquid crystal phases: molecular design, synthesis and phase structure characterization of a series of novel phenazines potentially useful in photovoltaic applications

It is known that in photovoltaic applications, columnar discotic liquid crystal (LC) phases of conjugated compounds are useful to align the molecules for improving their charge mobilities. However, conjugated compounds are usually either crystalline or amorphous. For compounds to form columnar discotic LC phases, specific molecular design is required for their ordered structural packing. In our recent report, a series of conjugated compounds, 6,7,15,16-tetrakis(alkylthio)quinoxalino-[2′,3′:9,10]-phenanthro[4,5-abc]phenazine (TQPP-[SCn]4) (n = 6, 8, 10 and 12), which display p-channel characteristics, were synthesized and characterized. This series of compounds was crystalline and did not exhibit LC behavior (S. Leng, B. Wex, L. H. Chan, M. J. Graham, S. Jin, A. J. Jing, K.-U. Jeong, R. M. Van Horn, B. Sun, M. Zhu, B. R. Kaafarani and S. Z. D. Cheng, J. Phys. Chem. B, 2009, 113, 5403–5411). In order to create a columnar LC phase with the lowest free energy within a broad applicable temperature region, we specifically designed and synthesized several series of electron-deficient phenazine derivatives to disrupt the molecular crystal packing and force the compounds to enter the columnar LC phase. These phenazine derivatives were designed to control the fused rigid ring size and shape as well as the location, lengths, and chemical structures of their flexible tails. These series include a series of 2,11-bis(1-methylethyl)-6,7,15,16-tetrakis(alkoxy)quinoxalino[2′,3′:9,10]phenanthro-[4,5-abc]-phenazines (TQPP-[t-Bu]2-[OR(B)]4), a series of 2,13-bis(1-methylethyl)-7,8,18,19-tetrakis(alkoxy)pyrazino[2,3-i]pyrazino[2″,3″:6′,7′]quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]-phenazines (TPPQPP-[t-Bu]2-[OR(B)]4), and a series of 3,4,11,12,19,20-hexaalkoxy-2,5,7,8,10,13,15,16,18,21,23,24-dodecaazatri-anthracenes (HDATAN-[OR]6), where R is the alkyl chain in the substituents and B represents that they are branched structures. The different phase structures and transition behaviors of these series of compounds were studied, and based on the experimental results, we can conclude that tailoring the alkyl tail size, the core size, and the core shape leads to a promising way to design molecules that exhibit the columnar LC phase. In particular, changes in alkyl tail architecture affect the phase behaviors more significantly than changes in its length.

Phase behaviors and supra-molecular structures of a series of symmetrically tapered bisamides

A series of symmetrically tapered 1,4-bis[3,4,5-tris(alkan-1-yloxy)benzamido] benzene bisamides (CPhBA, where is the number of carbon atoms in the alkyl chains, = 10, 12 and 16), was synthesized in order to investigate the effect of alkyl chain length on supra-molecular ordered structures induced by hydrogen (H)-bonding and micro-phase separation. These bisamides consist of a rigid aromatic bisamide core with three flexible alkyl chains at each end of the core. Major phase transitions and their origins in CPhBA bisamides were studied with differential scanning calorimetry, one-dimensional (1D) wide angle X-ray diffraction (WAXD), infrared spectroscopy, and solid-state carbon-13 nuclear magnetic resonance experiments. The structures of these compounds in different phases were identified using 2D WAXD from oriented samples and were also confirmed by selected area electron diffractions in transmission electron microscopy from stacked single crystals and by computer simulations. All of the CPhBA bisamides in this series formed a highly ordered oblique columnar () phase and a low-ordered oblique columnar () phase, similar to a recent report on C14PhBA. The two main driving forces in the formation of these two supra-molecular columnar structures were identified: One was the H-bond formation between N-H and C[double bond, length as m-dash]O groups, and the other was the micro-phase separation between the bisamide cores and the alkyl chains. With increasing the length of alkyl tails, the isotropization temperature decreased, while the disordering temperature of the alkyl tails increased. The 2D lattice structures perpendicular to the columnar axis also increasingly deviated from the pseudo-hexagonal packing with increasing the alkyl tail length. However, the alkyl tail length did not have a significant influence on the packing along the columnar axis direction. Utilizing polarized optical microscopy, the phase identifications were also supported by the observation of texture changes and molecular arrangements inside of the micro-sized domains.

Alkyl Tail Length Dependence of Structures in a Series of Symmetrically Tapered Bisamides Exhibiting Self‐Assembled Supramolecular Columnar Phases

Previously, we synthesized a series of symmetrically tapered 1,4‐bis[3,4,5‐tris(alkan‐1‐yloxy)benzamido] benzene bisamides (CnPhBA, where n is the number of carbon atoms in the alkyl chains, n=10, 12, 14, and 16). These bisamides consist of a rigid aromatic core with three flexible alkyl chains at each end of the core. It was found that all of these CnPhBA bisamides formed a highly ordered oblique columnar (ΦOK) phase and a low‐ordered oblique columnar (ΦOB) phase.[1,2] To study the dependence of structure on alkyl chain length in this series of symmetrically tapered bisamides, we focus on the structural and phase transitions of CnPhBA (n=6 and 8) via differential scanning calorimetry, one‐dimensional (1D) and 2D wide angle X‐ray diffraction (WAXD) and Fourier transform infrared spectroscopy. Structural evolutions and phase identifications were also supported by texture changes observed by polarized optical microscopy. It was interesting that there was only one ordered structure in C6PhBA which contains the shortest alkyl tails in this series of bisamide. Its 2D lattice (a=1.94 nm, b=3.43 nm, and γ=63.5°) was different from the samples with longer alkyl chains (n=10, 12, 14, and 16, n≥10, even).[1,2] However, the alkyl tail length did not have a significant influence on the packing along the column (c=0.96 nm). No liquid crystalline (LC) columnar phase was observed. When the length of alkyl tails contains eight carbons (C8PhBA), there was a sudden jump in the isotropization temperature of its LC phase compared with that of molecules with longer alkyl tails (n≥10, even).[1,2] Dramatic structure and phase behavior differences between C8PhBA and other CnPhBA bisamides were observed. This finding indicated that the C8PhBA having eight carbons in its alkyl tails represents a transition from a weak nano‐phase separation effect of the alkyl chains with respect to the rigid cores in CnPhBA with n≤6 (even) to a strong nano‐phase separation effect in CnPhBA (n≥10, even). On the occasion of Prof. Phil Geils 75th birthday for his pioneering contributions in polymer crystal physics.