PREFACE
1.3. Perylene as a core for the liquid crystalline organic semiconductor
1.3.3. Liquid crystals based on perylene bisimides
1.3.3.8. Perylene based oligomers
(Figure 1.22). They incorporated two polarizable thiophenic sulfur atoms in the molecular structure (23-24). Incorporation of racemic doubly branched alkyl chains brought down the melting temperature in the case of 24 and increased the mesophase range.
Wang et al. reported four perylene ester triphenylene dimers separated by spacers of dimethylene, hexamethylene, decamethylene and dodecamethylene units.60 These donor-acceptor dyads preserved the genuine electrochemical behaviors of the donor and acceptor units (26a-d). The fluorescence quenching of the acceptor unit varies with the spacer length, which is ascribed to the ground state charge transfer from donor to acceptor unit. Dimers 26a and b were monomesomorphic and exhibited unidentified Col phase, while dimers 26c and d were bimesomorphic exhibiting a low temperature Col phase and high temperature Colh phase. All the dimers exhibited a RT Col phase and the clearing temperatures were increased with the spacer length.
Figure 1.24. Molecular structures and bar graph of PBI based oligomers (27a-e).
Yang et al. synthesized trimers (27a-b) and pentamers (27c-e) derived from 1,7-bay substituted perylene (Figure 1.24).61 The trimers were prepared by connecting two cholesterol moieties at the bay-position or four cholesterol moieties on both bay-position and imides position. All the oligomers exhibited good fluorescence in comparison to similar PBIs with alkyl units on imides position. The presence of more cholesterol units lowered the melting temperature, widened the mesophase range and enhanced the fluorescence.
Increase in the spacer length was found to be favorable for improved thermal behavior and luminescence.
Figure 1.25. (a) Molecular structures of PBI-triphenylene oligomers (28a-c); (b) Schematic representation of (left) the self-organization within the Colh mesophases of the D–A dyad and (right) the hexagonal lattice (blue lozenge) formed by undifferentiated columns (Reproduced from reference [63]); (c) Schematic representation of (left) the self-organization within the Colob mesophases of the D–A–D triad after annealing and (right) the oblique lattice (blue parallelogram) formed by intermingled distinct columns located at the nodes of distorted hexagonal lattices (blue dashed distorted hexagon) (Reproduced from reference [63]); (d) Graphical representation of the thermal behavior of PBI based oligomers (28a-c).
Cammidge et al. reported triphenylene-perylene-triphenylene triads separated by the spacers of 2, 4, 6 and 10 methylene units (Figure 1.25).62 Hexahexyloxy triphenylene connected through 2, 4 and 6 methylene units to central PBI unit turned to be crystalline, while the one with decamethylene spacer 28a exhibited enantiotropic Col phase. Doping hexahexyloxytriphenylene, which exhibited a Col phase between 70-100 oC with 0.1% of mesogenic triad 28a gives a Col phase below 76 oC with no separation of the trimer from its host matrix. This suggests that the trimer can act as a vehicle to introduce functional constituents into the columnar matrices of parent hexaalkoxy triphenylenes without any phase separation. No evidence was found for the charge transfer or other ground state
interactions. The stability of the Col phase is likely to result from the favorably matched core-core separations. In this trimer, the luminescence of the central perylene core remains preserved, and the excitation spectrum clearly establishes that the excitation of the triphenylene leads to emission from the perylene. Lee et al. reported donor (D)- acceptor (A) diads (28c) and D-A-D triads (28b) where triphenylene and PBI units are connected covalently through flexible decycloxy spacers.63 The diad 28c self-assembles to form Colh
phase while the triad 28b self-assemble to form Colob phase. Fluorescence and subpicosecond transient absorption measurements in films confirmed a complete quenching of the singlet excitons through a highly effective photo-induced charge transfer process. On photo-excitation, they observed that photo-induced charge transfer states, that are formed in both dimer and trimer in a time of about 0.2–0.3 ps. The charge recombination process was significantly slower with characteristic time constants ranging between 150 and 360 ps. Such features are important for the development of organic photovoltaics.
Figure 1.26. (a) Molecular structures of Perylene/core extended PBI-triphenylene oligomers (29-32); (b) High-resolution STM image (20 nm×20 nm) of 29. Iset=349.1 pA, Vbias=599.1 mV; (c) Illustrated molecular model for (b). In domains A, 29 molecules from three neighbouring rows associate with close contacts between PI segments, while only two rows in domains B. Sub-image in (c) shows a schematic illustration of the molecular packing in domains B; (Reproduced from reference [64] with the permission of Wiley VCH) (d) Graphical representation of the thermal behavior of PBI based oligomers (29-32).
Zhao et al. reported D-A-D triads, where the donor triphenylene units are connected to central PBI (29), benzo[ghi]perylenediimide (30), or coronene bisimide acceptor units (31, 32) (Figure 1.26).64 All D–A–D triads self-organize to form a lamello-columnar oblique mesophase. This LC phase shows a well-separated donor-acceptor (D–A) heterojunction organization, resulting from efficient molecular self-sorting. Here, the donor and acceptor molecules form separated conducting pathways for the movement of holes
and electrons. The electron mobility was found to be about 15 times higher than the hole mobility due to the co-existence of two different inter-locked and immiscible carrier pathways. Such developments are exciting from the viewpoint of organic solar cells.
Asha et al. reported dimeric molecules of PBIs connected through central alkyl spacer where the length was varied from methylene to dodecamethylene spacer.65 Here, the PBI unit with an ethyl hexyl chain at one end and pentadecyl phenol at the other end. The phenolic -OH was connected through the polymethylene spacer to form the dimers. The differential packing allowed by the spacer parity (odd and even number central methylene segments) led to an odd-even oscillation of the melting points and associated enthalpies.
Higher values were observed for the even-spacered dimers. This odd-even effect was prominent for the spacers up to heptamethylene spacer from the lower end after which this effect was reduced. Lower homologues with methylene and trimethylene spacers exhibited inclinations to stabilize smectic phases while most of the dimers with longer spacers exhibited tendencies to show high temperature nematic phases.