Abstract
On the basis of planar and relatively rigid nitrogen-rich heterocyclic system of the heptazine core, heptazine-based π-conjugated materials have aroused widespread attention over the past decade by virtue of the fascinating electronic, optical, thermal, and mechanical properties in the fields of light-emitting, photocatalysis, sensors, environmental remediation, and so forth. However, there are still several obstacles to be solved before practical applications, such as low photoluminescence quantum efficiencies for light-emitting and weak visible absorption for photocatalysis. To further enhance various properties of heptazine-based π-conjugated materials, a series of strategies have been developed, including ingenious molecular design and modification, novel synthetic, and preparation methods. In this review, the significant progress of monomeric and polymeric heptazine-based π-conjugated materials and their applications typically in light-emitting are reviewed, which is beneficial for the acceleration of practical applications of heptazine-based materials and devices.
Highlights
Over the last decade, considerable progress in the fields of organic light-emitting diodes (OLEDs) and photocatalysis has triggered intensive effort to develop highly efficient lightemitting materials and photocatalysts (Wu and Ma, 2016; Zhou et al, 2017; Zhou et al, 2019; Yin et al, 2020; Zhou et al, 2020)
To better clarify the photocatalytic mechanism of heptazine-based materials, Wen et al measured the photophysical properties of melamine, melem, and g-C3N4
Based on the strong electron-withdrawing ability of heptazine core and strong electron-donating feature of triphenylamine, Li et al designed and synthesized a highly efficient heptazine-based thermally activated delayed fluorescence (TADF) emitter, HAP-3TPA (Figure 2), which exhibited a small ΔEST of 0.27 eV based on density functional theory (DFT) in view of the effective separation of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
Summary
Considerable progress in the fields of organic light-emitting diodes (OLEDs) and photocatalysis has triggered intensive effort to develop highly efficient lightemitting materials and photocatalysts (Wu and Ma, 2016; Zhou et al, 2017; Zhou et al, 2019; Yin et al, 2020; Zhou et al, 2020). The terbium melonate showed green emission with an emission peak (λem) of 545 nm due to the 5D4→7F5 transition They found that the rare-earth melonates exhibited rather low thermal stability probably deriving from the tight binding of crystal water, which resulted in hydrolytic decomposition at elevated temperatures (Makowski et al, 2012). To better clarify the photocatalytic mechanism of heptazine-based materials, Wen et al measured the photophysical properties of melamine, melem, and g-C3N4 They found that the PL intensities of melem is the highest, g-C3N4 second, and melamine the weakest, indicating that the condensation of melamine to melem makes PL stronger, while the condensation of melem to g-C3N4 results in weaker PL (Wen et al, 2018). Realized the broadband white light luminescence based on electron-deficient porous g-C3N4 constructed by supramolecular copolymerization design (Tang et al, 2020) They successfully narrowed the band gap of g-C3N4 from 2.64 to 1.39 eV. The emission wavelengths of electron-deficient porous g-C3N4 can be tuned from narrow blue to broad-band white range by the addition of 2, 4, 6-triaminopyrimidine (TAP)
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