Recently, Prof. Guoqing Zhang's team at the National Research Center for Microscale Materials Science, University of Science and Technology of China (USTC) in Hefei, China, reported the discovery of a new mode of interaction between organic molecules, i.e., the formation of a stable photo-induced charge-transfer complex between aromatic imides and fatty amines, and systematically investigated the properties and formation process of this complex by steady state and time-resolved emission and absorption spectroscopy, mass spectrometry, and paramagnetic resonance spectroscopy. The properties and formation process of the complexes were systematically investigated by steady state and time-resolved emission spectroscopy, absorption spectroscopy, mass spectrometry, paramagnetic resonance spectroscopy, etc., and it was demonstrated that the complexes can be used in the fields of photo-induced polymerization, carbon dioxide photo-reduction, and UV energy storage. The results were published in Chem under the title of “Trapping Highly Reactive Photo-Induced Charge-Transfer Complex Between Amine and Imide by Light”.
Intermolecular charge transfer, which is the movement of electrons from a donor molecule to an acceptor molecule, is the most important physical process in matter interactions and chemical reactions. It is ubiquitous in nature, playing an indispensable role in processes such as photosynthesis and respiration; charge transfer also has a wide range of applications in fields such as organic synthesis and energy conversion. Therefore, unlocking new charge transfer mechanisms is crucial for understanding the complex photochemical photophysical processes in nature and developing efficient organic synthesis methodologies and energy conversion technologies.
If a strong interaction between two organic molecules occurs through charge transfer, the pair of molecules is often referred to as a charge transfer complex. Organic charge transfer complexes are generally categorized into two types. One is that the electron donor and acceptor interact in the ground state to form a ground-state electron donor-acceptor complex (EDAcomplex); the other is that one of the electron donors or acceptors is in an electronically excited state and interacts with the other in the ground state to form an exciplex, which is relatively common in light-emitting diodes, semiconductors, and photovoltaic devices. These complexes are common in light emitting diodes, semiconductors and photovoltaic devices. Exciplexes are very unstable, and as the complex decays from the excited state to the ground state, it usually dissociates into electron donor and acceptor molecules in the ground state within microseconds.
Theoretically, by rationally regulating the energy levels of the organic electron donor and acceptor molecules and constructing higher binding energy barriers, it is possible to realize that the two do not interact in the ground state, whereas the donor-acceptor pairs are capable of interacting via charge transfer in the electronically excited state and are stable even in the ground state after the electronically excited state fades. But so far, organic electronic complexes in the ground state that can be formed only through excited states have not been realized. Based on the previous research on the photophysical properties of imide molecules, Prof. Zhang Guoqing's group found that when using imide as the electron acceptor and tertiary amine as the electron donor, there is no obvious ground state interaction between the two in solution at room temperature, and obvious interactions occur under the light conditions, with spectroscopic features similar to those of the excited state complexes, and the ground state of the emerging complexes can be stabilized for a long period of time in an inert atmosphere (Figure 1). 1).
1.Formation and regulation of photoinduced charge transfer complex (PCTC)
Professor Zhang Guoqing's group firstly selected naphthimide and triethylamine as model compounds, and determined the existence of photo-induced charge transfer complex (PCTC) by measuring the spectroscopic properties of the mixed system of naphthimide and triethylamine before and after illumination (Fig. 2), and studied the formation of PCTC by means of high-resolution mass spectrometry, time-resolved spectroscopy, changing substituent groups of naphthimide molecules, and replacing the electron donor mechanism, proving that it indeed needs to be formed by charge transfer in the excited state and subsequent fading excitation of the electronically excited state.
2. Spectroscopic characteristics of the mixed system of naphthylimide and triethylamine before and after illumination
The group successfully applied this system to the photo-induced polymerization of acrylate monomers, the photoreduction of carbon dioxide, and the storage and release of light energy. By releasing the light energy stored during illumination under dark conditions, the process that originally requires light to be carried out can be carried out under dark conditions (Figure 3). According to the authors, it is hypothesized that this mode of intermolecular interaction, the formation of stable ground state complexes in solution via electronically excited states, should not be limited to imide and amine molecules, but is likely to be a more general but under-appreciated interaction, which is expected to be discovered in more molecular structures and to be able to be used in new photochemical reactions.
Link to paper: https://www.sciencedirect.com/science/article/pii/S2451929424002262
(Hefei National Research Center for Microscale Matter Science, Hefei National Laboratory, Department of Scientific Research)
