Naphthalene Diimide-Based Donor–Acceptor–Donor Small Molecules as Metal-Free Organocatalysts for Photocatalytic CO₂ Reaction

Abstract

The application of organic small molecules as metal-free photocatalysts for light-driven photoreduction of carbon dioxide (CO2) has seldom been explored. This work developed four naphthalene diimide (NDI)-derived donor–acceptor–donor small molecules with different numbers of thiophene units, namely, NDI-2T, NDI-TT, NDI-4T, and NDI-6T, as metal-free photocatalysts to catalyze the reduction of CO2 under irradiation with an air mass 1.5G solar simulator at one-sun intensity. The structure–property relationship was investigated by exploring the effects of the electron-donating ability of the donor units on the optical properties, redox potential, electron–hole distribution, and exciton lifetime. NDI-6T exhibited the most red-shifted absorption, longest exciton lifetime, and strongest electron–hole separation. However, the large upshift in oxidation potential because of the elevated electron-donating ability of the hexathiophene unit significantly reduced the driving force for catalyst regeneration, leading to poor catalytic performance. Alternatively, NDI-4T possessed proper redox potentials, reduced charge-transfer resistance, and excellent photocurrent intensity; therefore, it effectively converted CO2 to a single product of CO in the presence of water as an electron donor without a sacrificial reagent or cocatalyst with a product yield of 168.6 μmol gcat –1 24 h–1, which was considerably higher than those of NDI-TT (111.9 μmol gcat –1 24 h–1), NDI-2T (88.4 μmol gcat –1 24 h–1), and NDI-6T (40.5 μmol gcat –1 24 h–1). This study provides a practical guideline for the molecular design of conjugated organic molecules as promising photocatalysts for CO2 photoreduction.

Solution-Processable Polymer Donor-Small Molecule Acceptor Bulk Heterojunction Organocatalysts for Enhancing CO2-to-CO Photoconversion

“…Additionally, atmospheric N 2 within the needle edges during injection may contribute to the observed larger m/z = 28 peak compared to other products. As shown in Figure 4, another strong peak at m/z = 30 in the mass spectrograph indicates the presence of C 18 O product, strongly affirming the C 18 O products originated from C 18 O 2 photoreduction, 40 excluding other catalysts or catalysts degradation. Moreover, the C 18 O content observed in our isotope experiment exceeds that found in naturally occurring C 18 O in the atmosphere, providing strong evidence of BHJ's efficient generation of CO through CO 2 photoreduction.…”

Organic Donor‐Acceptor Systems for Photocatalysis

“…Naphthalene diimide (NDI) is a classical n‐type semiconductor with superior electron‐withdrawal and charge transport properties. Lai et al have designed a D‐A‐D molecule with different numbers of thiophene units, [ 60 ] namely NDI‐TT, NDI‐2T, NDI‐4T, and NDI‐6T, based on NDI (Figure 2c ). Among them, the largest redshift absorption, the strongest e − ‐h + separation efficiency, and the longest exciton lifetime are reflected in the NDI‐6T molecule.…”

“…The covalent bonding between the DTBT moiety and C 60 molecules is found to exert a pronounced impact, considerably augmenting the rate of exciton generation, and effectively extending the longevity of charge carriers. A series of small D‐A type molecules based on thiophene and NDI units are also synthesized by their group and used for photocatalytic CO 2 reduction, [ 60 ] and the molecular structures have been shown in Figure 2c . Among them, with the appropriate redox potential, the photocatalytic CO 2 to CO conversion yield of NDI‐4T is 168.6 µmol g −1 24 h −1 (Figure 19g ).…”

Naphthalenediimide‐Based Polymer Dots with Dual Acceptors as a New Class of Photocatalysts for Photocatalytic Hydrogen Generation under Visible Light Irradiation

“…One widely used approach is to design conjugated polymers with a donor–acceptor (D–A) configuration. [ 38 ] This approach achieves narrow bandgaps and efficient charge transport properties due to the intramolecular charge transfer effects between the electron‐rich (donor) and electron‐deficient (acceptor) moieties. [ 39 ] The backbone and side‐chain structures of D–A‐type conjugated polymers can also be fine‐tuned to tailor their optoelectronic properties.…”