The focus of investigation was directed towards designing tailor‐made liquid crystalline perylene bisimides and to elucidate a structure‐property relationship with respect to thermotropic behaviour in N‐substituted derivatives of PBIs. Therefore, symmetrically and unsymmetrically N‐substituted PBIs have been synthesized successfully by two different synthetic strategies (Route A and B in Scheme 2). The PBIs were classified into three different types of N‐substitution patterns, involving swallow‐tail and linear alkyl‐ or oligooxyethylene (OEG) substituents respectively (Scheme 2). Several PBIs exhibiting columnar hexagonal liquid crystalline (Colh) packing with broad liquid crystalline temperature‐widths were obtained via these approaches.
Scheme 2. Overview of symmetrically and unsymmetrically N‐substituted perylene bisimides (PBIs) classified according to the nature of respective N‐substituents. Type I: symmetrically N‐substituted PBIs (2 x swallow‐
tail), Type II: unsymmetrically N‐substituted PBIs (2 x swallow‐tail) and Type III: unsymmetrically N‐substituted PBIs (swallow‐tail + linear). PBIs exhibiting columnar hexagonal liquid crystalline (Colh) packing are labeled by an asterisk. Main synthetic pathways for symmetrical and unsymmetrical N‐substitution of PBIs. i) imidazole, Zn(OAc)2, 2‐4 h, 160 °C; ii) KOH, t‐BuOH, 90°C 1‐1.5 h; iii) DMAc, Zn(OAc)2, 20 Min, 160 °C, 200 Watt; iv) 1. KOH, 2. AcOH; v) NH4OH, K2CO3; vi) DMF, K2CO3, KI, 80 °C, 3‐5 d;
vii) DMF, NaH, 80 °C, 48 h. Route A: 1‐4 and 6‐9; Route B: 5, 10 and 11.
The standard procedure for the synthesis of symmetrically N‐substituted PBIs is the condensation of perylenetetracarboxylic acid dianhydride (PTCDA) with primary amines (Route A).
The synthesis of unsymmetrically substituted PBIs is more challenging and there exist two different synthetic strategies suitable for asymmetric N‐substitution of PBIs. The common method therefore follows a partial saponification of symmetrically N‐substituted PBIs to afford key intermediates 12. Subsequent condensation of these monoimide‐monoanhydrides 12 with a second amine results in unsymmetrically N‐substituted PBIs (Route A). We also developed a microwave‐assisted method for the second condensation reaction of monoimide‐
monoanhydrides 12 with amines. An alternative synthetic strategy (Route B) makes use of imide anhydride 13 as asymmetric building block. The anhydride moiety allows for condensation reactions with amines in a first step, whereas the imide moiety offers the possibility for introduction of a second N‐substituent via nucleophilic substitution SN2 under basic conditions with alkylhalides in a second step. PBIs can be regarded as a closed chromophoric system due to nodes of the HOMO and LUMO orbitals at the imide nitrogens and accordingly, show identical absorption behaviour. Hence, the UV/Vis spectra of these compounds in very dilute CHCl3 solutions exhibit the characteristic finger‐print vibronic fine structure of PBI with peaks at about 525, 490, 460 and 430 nm respectively. Due to the electronic decoupling of the perylene bisimide core and the imide substituents, also the electronic properties are similar, with LUMO energy levels of about ‐3.8 eV and HOMO energy values of ‐6.0 eV with respect to the zero energy level.
In the following, the three different N‐substitution patterns with respect to thermotropic behaviour are outlined in brief.
Type I with a symmetric N‐substitution pattern, utilized different alkyl or OEG swallow‐tail substituents to study the influence of sidechains with different spatial requirements or polarity on thermotropic behaviour. It could be shown here, that the symmetrical molecule 1 carrying alkyl substituents (R1 = ‐C7H15) exhibits a narrow monotropic hexagonal columnar Colh mesophase upon cooling from the isotropic melt with a phase‐width of only 8 °C. A simple increase in substituent length for symmetrical analogue 2 (R1 = ‐C11H23) does not expand the mesophase temperature‐
width, but surprisingly implicates crystalline behaviour. On the other hand, a change from alkyl swallow‐tail to OEG swallow‐tail side groups in case of PBIs 3 and 4, results in much broader liquid crystalline phases which are not monotropic. This can be attributed to the higher conformational freedom of the C‐O bond as compared to C‐C bond allowing for a better space filling around the discotic mesogens.
Type II uses an unsymmetrical N‐substitution pattern comprising different alkyl swallow‐tail substituents in combination with OEG swallow‐tail or alkyl swallow‐tail substituents with distinct spatial demands. Here an unsymmetrical N‐substitution is used to study the impact of asymmetry of the substitution profile on thermotropic behavior. Generally, unsymmetrical PBIs with one OEG swallow‐tail substituent and one alkyl swallow‐tail substituent (6‐9) exhibit thermotropic liquid crystalline packing. Moreover, in the unsymmetrical molecules 6‐9, an increase in OEG length decreases the clearing temperature considerably, whereas a corresponding increase in alkyl chain
shown for these materials, that OEG sidechains pack more tightly than alkyl substituents. It is also interesting to note that unsymmetrical PBI 5 carrying two different alkyl swallow‐tail chains remains a crystalline material (Fig. 3b, d and e) with a melting point of 99 °C. Thus a comparison of 1, 2 and 5 clearly shows that the Colh‐phase observed for 1 could not be broadened for 2 or 5, both by extending the length of alkyl‐substituents or by introducing unsymmetrical alkyl N‐substitution.
Figure 3. (a) Molecular structure of PBIs 5 and 6 with an identical number of sidechain atoms but different flexibility of the R2‐N‐substituents. (b‐c) Polarization optical microscopy images of 5 and 6 (under crossed polarizers). (b) Crystals of 5 formed upon annealing at 63 °C. (c) Dendritic texture of 6 at 153 °C in the Colh phase. (d) DSC‐thermogramms of 5 and 6 measured at 2 Kmin‐1. (e) X‐ray diffraction patterns of 5 in the crystalline phase (RT) and 6 in the Colh phase (120 °C).
An insightful comparison can be drawn between PBIs 5 and 6, as both bear an identical number of side chain atoms but differ in flexibility of the respective R2‐substituent (Fig. 3a).
As described above, PBI 5 is a crystalline material but on the other hand 6 exhibits a Colh mesophase which can only be attributed to the presence of an OEG‐substituent. Fig. 3e shows the typical diffraction pattern of a 2D‐lattice with ahex = 21.23 nm and a π‐π stacking distance dππ= 3.48 Å for 6 in the Colh phase at 120 °C. The dendritic texture for 6 observed in POM is consistent with a Colh‐packing in the mesophase as well (Fig. 3d). In short, the comparison of 5 and 6 clearly indicates the ability of OEG substituents to promote the liquid crystallinity of the perylene bisimide moiety.
The influence of a reduction of branched swallow‐tail substituents to linear ones is investigated for PBIs of Type III, as it is of fundamental interest to design PBI semiconductors with a high content of the electronically active perylene chromophore for device applications.
Thus, unsymmetrical PBIs 10 and 11 with one swallow‐tail and one linear substituent were synthesized. Both 10 and 11 are crystalline materials as can be seen from DSC‐thermograms and POM textures. Both compounds recrystallize on cooling and do not exhibit any supercooling effect. The absence of any mesophase indicates that the linear substituents are spatially less demanding and may not be able to fill the space sufficiently around the columnar stacked PBI molecules.
In a nutshell, it could be shown that incorporation of flexible OEG swallow‐tail substituents alone or in combination with alkyl swallow‐tail substituents, efficiently supports Colh packing arising from π‐π interactions between cofacially orientated perylene molecules. Thus, both N‐substituents have to be branched in nature. A reduction to linear N‐substituents resulted in strongly crystalline behaviour. This molecular design was crucial to obtain liquid crystallinity and intracolumnar long‐range order. Additionally, the melting point to the liquid crystalline phase as well as clearing temperature could be controlled very efficiently by an unsymmetrical N‐substitution pattern. Upon cooling the liquid crystalline phase, the crystallization process of these liquid crystalline PBIs is strongly supercooled. The N‐substituents did not influence the electronic energy levels and optical properties.
SYNTHESIS AND STRUCTURE ELUCIDATION OF DISCOTIC LIQUID CRYSTALLINE PERYLENE