BN Embedded Polycyclic π-Conjugated Systems: Synthesis, Optoelectronic Properties, and Photovoltaic Applications

Abstract

In the periodic table of elements, boron (B, atomic number, 5) and nitrogen (N, atomic number, 7) are neighboring to the carbon (C, atomic number, 6). Thus, the total electronic number of two carbons (12) is equal to the electronic sum of one boron (5) and one nitrogen (7). Accordingly, replacing two carbons with one boron and one nitrogen in a π-conjugated structure gives an isoelectronic system, i.e., the BN perturbed π-conjugated system, comparing to their all-carbon analogs. The BN embedded π-conjugated systems have unique properties, e.g., optical absorption, emission, energy levels, bandgaps, and packing order in contrast to their all-carbon analogs and have been intensively studied in terms of novel synthesis, photophysical characterizations, and electronic applications in recent years. In this review, we try to summarize the synthesis methods, optoelectronic properties, and progress in organic photovoltaic (OPV) applications of the representative BN embedded polycyclic π-conjugated systems. Firstly, the narrative will be commenced with a general introduction to the BN units, i.e., B←N coordination bond, B-N covalent bond, and N-B←N group. Then, the representative synthesis strategies toward π-conjugated systems containing B←N coordination bond, B-N covalent bond, and N-B←N group will be summarized. Afterwards, the frontier orbital energy levels, optical absorption, packing order in solid state, charge transportation ability, and photovoltaic performances of typical BN embedded π-conjugated systems will be discussed. Finally, a prospect will be proposed on the OPV materials of BN doped π-conjugated systems, especially their potential applications to the small molecules organic solar cells.

Keywords: BN-embedded unit; device performance; isoelectronic structure; organic solar cell; π-conjugated material.

GRAPHICAL ABSTRACT Design motif of BN embedded Pi-units for photovoltaic application.

Figure 1

Requirements for high-performance OPV devices (A) and representative D and A π-electronic units (B).

Figure 2

Isoelectronic structures of CC and BN units (A) and schematic diagram of π-electronic units containing B←N coordination bond (B), B-N covalent bond (C), and N-B←N group (D). X, Y, and Z represent the halogen atoms, alkyl chains, or phenyl groups.

Figure 3

Examples of synthesis routes toward B←N embedded units involving alkyl lithium / aryl boron reaction system.

Figure 4

Examples of synthesis routes toward B←N embedded units involving BX₃/ hindered base reaction system.

Figure 5

Examples of synthesis routes toward B-N embedded units via electrophilic cyclization between boron and aromatic units.

Figure 6

Examples of synthesis routes toward B-N embedded units via chelation of aromatic N and B precursor.

Figure 7

Examples of synthesis routes toward π-units containing N-B←N groups via BF₃•OEt₂/Et₃N reaction condition.

Figure 8

Lewis acid-base coordination (A–C) and the variation of UV-Vis absorption spectra of 74 upon coordinating with BCF (B), Reprinted with permission from Welch et al. (2009). Copyright (2009) American Chemical Society. Manipulation of optical absorption spectra of DPP molecules upon coordinating with BCF (D), Reprinted with permission from Huang et al. (2018). Copyright (2018) Elsevier Ltd.

Figure 9

Molecular structures of B←N embedded π-units and their precursors before cyclization.

Figure 10

Absorption spectra of 20/21 (upper) and 22/23 (bottom) in chloroform (A) and Single crystal structure of 8 (B). Reprinted with permission from Job et al. (2010) and Zhu et al. (2016). Copyright (2010, 2016) American Chemical Society.

Figure 11

Molecular structures of P-CC and P-BN and the alignment of frontier orbital energy levels (A) and parameters of all-polymer solar cells based on PTB7-Th and P-BN-IID (B).

Figure 12

Absorption and energy levels of anthracene and B-N embedded anthracene (A) and tetracene and B-N embedded tetracene (B). Reprinted with permission from Ishibashi et al. (2014, 2017). Copyright (2014, 2017) American Chemical Society.

Figure 13

Molecular structures, energy levels, and single crystal parameters for B-N embedded 2D π-conjugated units (A) and photovoltaic applications of compound 43 as an electron donor or additive (B). Reprinted with permission from Wang et al. (2014). Copyright (2014) American Chemical Society.

Figure 14

Molecular structures of BODIPY-based photovoltaic materials.

Figure 15

Molecular structures of BNBP-based photovoltaic materials.