High-fidelity self-assembly pathways for hydrogen-bonding molecular semiconductors

Abstract

The design of molecular systems with high-fidelity self-assembly pathways that include several levels of hierarchy is of primary importance for the understanding of structure-function relationships, as well as for controlling the functionality of organic materials. Reported herein is a high-fidelity self-assembly system that comprises two hydrogen-bonding molecular semiconductors with regioisomerically attached short alkyl chains. Despite the availability of both discrete cyclic and polymeric linear hydrogen-bonding motifs, the two regioisomers select one of the two motifs in homogeneous solution as well as at the 2D-confined liquid-solid interface. This selectivity arises from the high directionality of the involved hydrogen-bonding interactions, which renders rerouting to other self-assembly pathways difficult. In thin films and in the bulk, the resulting hydrogen-bonded assemblies further organize into the expected columnar and lamellar higher-order architectures via solution processing. The contrasting organized structures of these regioisomers are reflected in their notably different miscibility with soluble fullerene derivatives in the solid state. Thus, electron donor-acceptor blend films deliver a distinctly different photovoltaic performance, despite their virtually identical intrinsic optoelectronic properties. Currently, we attribute this high-fidelity control via self-assembly pathways to the molecular design of these supramolecular semiconductors, which lacks structure-determining long aliphatic chains.

Figure 1. Hierarchical self-assembly of 1 and 2 in different phases.

(a,b) Chemical structures and molecular models of hydrogen-bonding regioisomeric semiconductors 1 and 2 and their different hydrogen bonding motifs. (c–h) Schematic representation of the hierarchical organization of 1 and 2 in different phases.

Figure 2. NMR studies of 1 and 2.

(a) Concentration-dependent ¹H NMR spectra of 1 (c = 0.1–400 mM) in CDCl₃ at 25 °C. Inset: fraction of aggregated molecules (α) calculated from the chemical shift changes of the NH protons of 1 as a function of Kc_T (c_T: total concentration of the molecules). Black solid, red solid and black dashed curves represent simulated curves according to dimer, isodesmic, and cooperative (σ = K₂/K = 0.1) models, respectively. (b) Concentration-dependent ¹H NMR spectra of 2 (c = 0.1–100 mM) in CDCl₃ at 25 °C. (c,d) DOSY NMR spectra of 1 (c = 10 mM) (c) and 2 (c = 10 mM) (d) in CDCl₃. D = diffusion coefficient.

Figure 3. STM studies of 1 and 2.

(a) Schematic diagram of STM measurements on chiral liquid-solid interface. (b,d) STM images of 1 (b) and 2 (d) at the (S)-limonene–HOPG interface with the following tunneling conditions: I = 1.5 pA, V = −1000 mV; I = 2.0 pA, V = −1000 mV. Concentration of solution is 0.005 mM for both cases. Scale bar, 4 nm. (g,i) STM images of 1 (g) and 2 (i) at (R)-limonene–HOPG interface with the following tunneling conditions: I = 1.5 pA, V = −1000 mV; I = 3.0 pA, V = −900 mV. Concentration of solution is 0.006 mM for both cases. Scale bar, 4 nm. (c,e,f,h) Packing models of CW rosettes of 1 (c), CCW tapes of 2 (e), CCW rosettes of 1 (f) and CW tapes of 2 (h).

Figure 4. Molecular modeling of 1 and 2.

(a–d) Geometry-optimized structures of hexameric rosettes (a,b) and monomers (c,d) of 1 (a,c) and 2 (b,d). The molecular modeled structures are shown in vertical (left) and perpendicular (right) direction with respect to the rosette plane. Barbituric acid, oligothiophene, and alkyl chain moieties are colored in light blue, purple, and gray, respectively. The twisting between the barbituric acid, T1, and T2 planes in the monomer structures is shown by a schematic cartoon.

Figure 5. Self-organized structures of 1 and 2 in the bulk.

(a,d) PXRD patterns of bulk samples of 1 (a) and 2 (d) at 25 °C in a glass capillary (diameter: 1.0 mm). Values in parenthesis denote Miller indices. (b,e) Molecular models of rosettes of 1 (b) and tapes of 2 (e). (c,f) Schematic representations of a proposed packing structures of 1 (c) and 2 (f) with lattice parameters. In (c), only left-handed helical columns are used to show the packing structure. As 1 does not contain a chiral center, both left- and right-handed helical columns should be formed.

Figure 6. Morphology and photovoltaic properties of p–n heterojunction structures.

(a–d) AFM images of as-cast (a,c) and annealed (b,d) thin films of 1:PC₆₁BM (a,b) and 2:PC₆₁BM (c,d); scale bar, 200 nm. Thin film samples were prepared by spin-coating CHCl₃ solutions of 1:PC₆₁BM and 2:PC₆₁BM (c_total = 20 mg mL⁻¹) onto substrates. Annealing conditions: T = 80 °C, t = 10 min. (e–h) Schematic illustration of morphological change of 1:PC₆₁BM (e,f) and 2:PC₆₁BM (g,h) upon annealing. (i) Current–voltage (J–V) characteristics of BHJ solar cells using 1:1 (w:w) blend films of 1:PC₆₁BM (red lines) and 2:PC₆₁BM (blue lines) before (dotted lines) and after annealing at 80 °C (solid lines). (j) PCE and J_sc values of devices containing 1 (red) and 2 (blue) as a function of the annealing temperature. (k) UV-vis absorption (dashed curves) and EQE spectra (solid curves) of thermally annealed (T = 80 °C) 1:1 (w/w) blend films of 1:PC₆₁BM (red) and 2:PC₆₁BM (blue). Film thickness: 100–120 nm. Average values of four cells with standard deviation.