Synthesis and Optoelectronic Properties of Perylene Diimide-Based Liquid Crystals

Abstract

Perylene diimide (PDI), initially synthesized and explored as an organic dye, has since gained significant recognition for its outstanding optical and electronic properties. Early research primarily focused on its vibrant coloration; however, the resolution of solubility challenges has revealed its broader potential. PDIs exhibit exceptional optical characteristics, including strong absorption and high fluorescence quantum yield, along with remarkable electronic properties, such as high electron affinity and superior charge carrier mobility. Furthermore, the robust π-π stacking interactions and liquid crystalline behavior of PDIs facilitate precise their self-assembly into highly ordered structures, positioning them as valuable materials for advanced applications in optoelectronics, photonics, and nanotechnology. This article provides a comprehensive review of the progress made in the design, synthesis, and optoelectronic performance of PDI-based liquid crystals. It explores how various substituents and their placement on the PDI core impact the properties of these liquid crystal molecules and discusses the challenges and opportunities that shape this rapidly evolving class of optical materials. This review is strictly focused on PDIs and does not cover their elongated or laterally extended derivatives, nor does it include monoimide or ester compounds.

Keywords: aggregation-induced emission; charge carrier mobility; liquid crystals; perylene diimide; π-π stacking.

Figure 1

Schematic diagram of the molecular structure of PDIs. The blue part represents the core of the perylene diimide molecule, while the red part indicates the common substitution positions of perylene diimide.

Figure 2

Two methods for preparing non-symmetrical PDIs with Distinct substituents at each imide position [36].

Figure 3

Molecular structure of PDI derivative. (a) Charge mobility of PDI derivative 2.1 as a function of electric field: electron mobility (1) and hole mobility (2) for annealed 2.1 films, electron mobility (3) and hole mobility (4) for pristine 2.1 films; (b) polarizing microscope image of PDI derivative 2.1 [46].

Figure 4

Molecular structure of PDI derivative 2.2: (a) transient photocurrent curve at 30 °C; (b) transient photocurrent curve at 70 °C; (c) POM image of PDI derivative 2.2 [48].

Figure 5

Molecular structures of D-PDIs and L-PDIs: (a) schematic illustration of the potential self-assembly mechanism of D-PDIs; (b) schematic illustration of the potential self-assembly mechanism of L-PDIs [49].

Figure 6

A cogwheel model of self-organization. (a) The molecular structure of rrr-PBI where r stands for racemic dm8*. The chiral methyl group is indicated in pink. (b) The two crystalline columnar hexagonal (Φ_h^k) phases are generated via the hierarchical self-organization of PBI dimers rotated around the column axis being offset from the column axis in Φ_h^k1 and centered on the column axis in Φ_h^k2. Due to its columnar shape and the absence of alkyl groups perpendicular or tilted to the long axes of the column, the intercolumnar interdigitation of alkyl chains is absent, and therefore, we denote Φ_h^k2 as the “cogwheel” assembly. (c) The formation of a Φ_h^k array with only one helical column in the unit cell drives helical deracemization between columns. (d) Key aspects of the cogwheel model. (e) Supramolecular columns assembled from r8r-, rr8-,8r8-, and88r-PBI, where 8 represents the n-octyl group, and insets illustrate the incomplete space filling by the alkyl ends from the 3 and 5 positions of the dendron. Dashed circles in insets indicate empty space on the column periphery [50].

Figure 7

(a) The molecular structure of PBI-C1-6BP (The red part represents the core of the perylene diimide molecule, while the purple part corresponds to the 1,2,3-triazole segment). (b) Packing models of the A15 phase, σ phase, BCC phase, and the two-dimensional tiling pattern of the DQC phase are illustrated. (c) A schematic representation of the hierarchical self-assembly mechanism of POSS-based PBIs, where the central red blocks denote perylene cores and the gray shell of the supramolecular spheres represents amorphous BPOSS cages [51].

Figure 8

Molecular structures of PDIs 1-3: (a) Crystalline phase of PDIs 1 at 172 °C. (b) Liquid crystalline phase of PDI 2 at 130 °C. (c) Liquid crystalline phase of PDI 3 at 155 °C. (d) Current density–voltage (J–V) characteristics of electronic devices, showing the performance of PDI 1 (black squares), PDI 2 (red circles), and PDI 3 (blue triangles) before (empty symbols) and after annealing (filled symbols) [52].

Figure 9

Intermediates of two PDI derivatives with bay region modification (The highlighted part represents the halogen substituents that are easy to modify).

Figure 10

Molecular structure of PDI derivative 2.3: (a) Sum of minimum isotropic charge carrier mobilities as determined by PR-TRMC for PDI derivative 2.3 (circular). (square and triangular shapes represent the test results of the precursors of PDI derivative 2.3). (b) Photoconductive transients obtained from FP-TRMC measurements of 60:40 molar ratio (perylene–HBC) thin films at 500 nm, with approximately 1 × 10¹⁵ absorbed photons, for PDI derivative 2.3 (solid line) blended with HBC-PhC12 (dotted line represent the test results of the precursors of PDI derivative 2.3). (c) Optical liquid crystal textures at crossed polarizers of PDI derivative 2.3 cooled from the isotropic phase [23].

Figure 11

Molecular structure of MEH-PDIs (The black part represents the core structure of the perylene diimide molecule, while the red part corresponds to the substituent groups): (a) POM image of MEH-PDIs at 230 °C. (b) Schematic representation of MEH-PDIs assembled into a columnar hexagonal LC phase, highlighting the orthogonal orientation of the PBI units. (The blue arrows indicate the direction of the main transition dipole moment (mag) of the PDI molecules in MEH-PDIs) [76].

Figure 12

Molecular structures of PDI derivatives 2.4 and 2.5 (The black part represents the core structure of the perylene diimide molecule, while the red part corresponds to the substituent groups): (a) POM image of PDI derivative 2.4, showing the textures on cooling at 130 °C (×400); (b) POM image of PDI derivative 2.5, displaying the textures on cooling at 130 °C (×400); (c) schematic illustration of the hexagonal columnar phase stacking for both derivatives [77].

Figure 13

Molecular structures of PDI derivatives 2.6–2.9 and their corresponding schematic representations of self-assembly mechanisms [78].

Figure 14

Fluorescence images of DDPD (10 mM) solutions/suspensions in THF/water mixtures with varying water contents [79].

Figure 15

Molecular structures of the mono- and di-TPE-substituted PBIs (The black part represents the core structure of the perylene diimide molecule, while the blue part corresponds to the AIE molecule substituent group) [105].

Figure 16

Molecular structures of PDI derivative 3.1: (a) POM texture of PDI derivative 3.1 obtained upon cooling at 150 °C, with magnification at 200× for the large image and 500× for the smaller image. (b) Emission spectra of compound 6 in THF/H₂O mixtures with varying water fractions (2 × 10⁻⁶ M) excited at λ = 330 nm. (Inset: variation in fluorescence intensity as a function of water fraction in THF/H₂O mixtures) [109].

Figure 17

Molecular structures of PDI derivative 3.2 (The black part represents the core structure of the perylene diimide molecule, while the blue part corresponds to the AIE molecule substituent group): (a) POM image of PDI derivative 3.2 in the mesophase at 120 °C. (b) Emission spectra of PBI 5 in a 10 mM THF–water system with varying water fractions (f_w), excited at λ_ex = 330 nm. (Inset: variation in fluorescence intensity as a function of water fraction, f_w) [110].

Figure 18

(a) Chemical structures of the dyad and triad molecules. (b) A schematic representation of (left) the self-organization within the Col_hex mesophases of the D–A dyad and (right) the hexagonal lattice (blue lozenge) formed by undifferentiated columns. The distance d^dyad ≈1.9 nm (green line) corresponds to the average center-to-center spacing distance between D and A columns. (c) A schematic representation of (left) the self-organization within the Col_obl 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). The distances d₁^triad ≈ 1.4 and d₂^triad ≈ 2.5 nm (respectively green solid and dashed line) correspond to the two average center-to-center spacing distances between D and A columns coexisting within the triad columnar arrangement [124].

Figure 19

(a) Molecular structures of the 5D9An series of compounds (The black part represents the core structure of the perylene diimide molecule, while the red part corresponds to the substituent groups of the disc-shaped liquid crystal molecules); (b) POM images of all dyads cooling from the isotropic liquid at 0.5 °C min⁻¹ ((a′) 5D9A4 in 100 °C; (b′) 5D9A5 in 100 °C; (c′) 5D9A6 in 100 °C; (d′) 5D9A7 in 100 °C; (e′) 5D9A8 in 100 °C; (f′) 5D9A4 in 30 °C; (g′) 5D9A5 in 30 °C; (h′) 5D9A6 in 30 °C; (i′) 5D9A7 in 30 °C); (c) Colh molecular stack diagram of 5D9A8; (d) Colr molecular stack diagram of 5D9A4 [125].