Carbazole is a polycyclic aromatic hydrocarbon with a tricyclic structure. The structure consists of two six-membered benzene rings (A rings) fused on either side of a five-membered nitrogen-containing ring (B ring). Carbazole has a large π-electron conjugation system, and its central nitrogen atom shows extensive electron delocalization. Carbazole and its derivatives not only can be used as hole transport materials, but also can be easily modified with electron transport units in different positions to make electrons and holes easier to be injected. What's more, the transition-metal complexes as a classic photoredox catalyst show great potential in organic synthetic chemistry while the carbazole and carbazole-related compounds as organic light catalysts also show a wide range of uses. Carbazoles are used as versatile photocatalysts for the rapid construction of various complicated organic molecules during the photocatalytic processes.
Fig.1 General structure of carbazoles
Carbazole catalysts are used in a wide range of applications, and examples of carbazole catalysts are as followings:
In recent years, photoredox catalysis as a branch of photochemistry, has attracted great interest among chemists due to its ability to activate small molecules. Photocatalysts such as transition-metal complexes, organic dyes, and semiconductors can convert visible light into chemical energy by engaging in single-electron transfer with organic substrates. The cationic polycarbazole networks (CPOP-28 and CPOP-29) prepared by simple oxidative coupling reaction are heterogeneous photocatalysts for cross-dehydrogenative coupling reactions. The introduction of trifluoromethyl groups into the polymer network can enhance the visible light absorption and prolong the excited-state lifetime, thereby enhancing the photocatalytic activity. The cationic polycarbazole catalyst exhibits good performance in photocatalytic organic reactions.
Fig.2 Photocatalytic cross-dehydrogenative coupling reactions using CPOPs and proposed mechanism.
1,2,3,5-Tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) is a powerful photocatalyst with many unique properties. The photophysical characteristics of 4CzIPN contain long excited state lifetime, wide redox window, and high fluorescence quantum yield. All these features are beneficial to photocatalytic activities. In addition, the redox properties of 4CzIPN analogs could also be tuned by molecular design.
Thus, 4CzIPN is irreplaceable in modern synthesis as a metal-free alternative to transition metal photocatalysts. In organic reactions, 4CzIPN, as an excellent organophotocatalyst, can catalyze radical precursors to generate various reactive radicals such as benzyl radicals, formyl radicals, silyl radicals, etc., which are captured by organic substrates, thus enabling metal-free photocatalytic methods.
Fig.3 Chemical structure, HOMOs and LUMOs of 4CzIPN.
Organic semiconductors are considered particularly attractive photocatalytic candidates. And conjugated microporous polymers (CMPs) as an organic semiconductor have been developed as innovative photocatalysts for the generation of hydrogen (H2) and O2 from water. A series of novel donor-acceptor conjugated microporous polymers (D-A CMP) have been synthesized, in which carbazole (Cz) units act as the donors and pyrene (Py), triphenyltriazine (TPT), benzothiadiazole (BT) and thiazolylthiazole (TzTz) moieties act as acceptors. These CMP-based photocatalysts have many outstanding features, such as low density, good light absorptivity, high surface areas, etc., which can effectively generate H2 and O2 from water.
Fig. 4 Cz-Py CMP for effective photocatalytic H2 and O2 generation from water.
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