The superior light/electronic transport capabilities of organic micro-nano crystals show great application prospects in optoelectronic fields such as optical waveguides, optical logic gates, and p-n heterojunctions. However, there is still a lack of in-depth understanding and control over the complex dynamic assembly characteristics of organic molecules. In particular, achieving controlled preparation of π-conjugated molecular micro-nano crystals in terms of composition, size, and low dispersion remains a daunting challenge.
Recently, Professor Lei Yilong and his research group from Tianjin University, along with collaborators, published a research paper titled “Vapor-Phase Living Assembly of π-Conjugated Organic Semiconductors” in ACS Nano, reporting for the first time the controlled assembly of hydrophobic π-conjugated organic semiconductors using micro-spacing physical vapor transport technology (PVT). Inspired by the active micelle growth of amphiphilic block copolymers (BCPs) and π-stacking dyes, the researchers envisioned using initially grown one-dimensional organic crystals as active seeds, followed by homo- or heteroepitaxial growth to prepare extendable or multi-block micro-nano structures with controllable lengths. Based on this, the researchers used perylene (Pe) and 9,10-dicyanoanthracene (DCA, A) as electron donor and acceptor, respectively, to first form a binary charge transfer (CT) alloy (DCA1−xPex, B). They verified the active assembly behavior of π-conjugated organic semiconductor molecules using micro-spacing physical vapor transport technology (PVT) (Figure 1).
Figure 1. Schematic diagram of homo- and hetero-active epitaxial growth of organic semiconductor materials.
Micro-spacing physical vapor transport technology (PVT) utilizes the temperature difference between the precursor substrate and the product substrate to promote the crystallization of organic semiconductor molecules through short-range vapor transport. This vapor preparation method has significant advantages such as low preparation cost, ease of operation, and high yield. By using micro-spacing physical vapor transport technology, the researchers preliminarily studied the active epitaxial growth behavior of π-conjugated organic micro-nano crystals: first, they prepared shorter length A and B one-dimensional micro-rods using this method. Subsequently, different concentrations of A and A/Pe precursor solutions were introduced as secondary growth components, using the pre-formed A and B one-dimensional micro-rods as corresponding seeds to verify the active assembly characteristics of the organic semiconductor molecules. Systematic studies found that under the same heating temperature and time, as the concentration ratio of A and A/Pe precursor solutions to A and B seed precursor increased, the length of the generated micro-rods continued to increase, and the length showed a linear relationship with the concentration ratio, achieving the active homoepitaxial growth of the organic seeds (Figure 2).
Figure 2. Optical characterization and length statistics of A and B micro-rods based on active homoepitaxial growth.
The researchers used the pre-synthesized A micro-rods as active seeds and the A/Pe precursor solution as the secondary growth component, utilizing the aforementioned vapor technology to prepare B–A–B type fluorescent three-segment heterojunctions. Conversely, A–B–A type block heterojunctions were also successfully prepared. This result indicates that heteroepitaxial growth can be achieved at the active ends of both A and B micro-rod seeds, and their lengths exhibit low dispersion (Figure 3).
Figure 3. Optical and structural characterization of B–A–B and A–B–A three-segment micro-rod heterojunctions based on active heteroepitaxial growth.
The research group achieved controlled synthesis of a series of forward and reverse three-segment micro-rod fluorescent heterojunctions by introducing more binary π-conjugated CT alloy systems, and further rationally synthesized more complex five and seven-segment micro-rod heterostructures composed of A and B (Figure 4).
Figure 4. Molecular structure and optical characterization of multi-segment heterostructures in the π-conjugated binary alloy system.
Similar to the liquid-phase active crystallization-driven self-assembly and active supramolecular polymerization processes, the simple micro-spacing physical vapor transport technology (PVT) has confirmed that organic semiconductors also exhibit active assembly behavior, revealing the universal applicability of this growth method.
Hai, T.; Feng, Z.; Sun, Y.; Wong, W.-Y.; Liang, Y.; Zhang, Q.; Lei, Y. Vapor-Phase Living Assembly of π-Conjugated Organic Semiconductors. ACS Nano 2022, DOI: 10.1021/acsnano.1c11295.
Source: Polymer Science Frontiers
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