Design and Applications of Organic Charge-Transfer Complexes for Near-Infrared Light Harvesting Technologies

Abstract

Charge-transfer complex (CTC) is a kind of material that involves significant charge-transfer from an electron donor (D) to an acceptor (A) resulting in electron delocalization between the two molecules. CTCs with considerably different photophysical/physicochemical properties from their constituting components have been considered game-changers for organic optoelectronic devices. The light-harvesting capability of CTCs can be easily tuned by regulating the energy gaps of CTCs, derived from the highest occupied molecular orbital (HOMO) level of D and the lowest unoccupied molecular orbital (LUMO) level of A. However only a handful of reports exploiting CTC material with unusual near-infrared (NIR) optical properties for the light-harvesting technologies. This thesis mainly focuses on the design, development and applications of CTC materials for innovative NIR applications.

Firstly, three CTC co-crystals with the following D+A combinations, Dibenzotetrathiafulvalene (DBTTF)+ 1,2,4,5-Tetracyanobenzene (TCNB)→DTC, Tetrathiafulvalene (TTF)+TCNB→TBC and TTF+ Tetracyanoquinodimethane (TCNQ)→TQC, were prepared. By manipulating the energy gaps of donors and acceptors, the intermolecular CT absorptions of the co-crystals can be effectively tuned to achieve full-spectrum solar absorption. TQC with an excellent optical absorption shows a good solar-derived photothermal performance. To exploit its full solar spectrum harvesting properties, the co-crystals are loaded onto a three dimensional (3D) porous polymer scaffold. The porous composite is applied for seawater desalination and wastewater purification using solar energy. As the first demonstration of CTC’s application for solar seawater desalination and wastewater purification, the present system shows a promising solar-evaporation rate of 1.67 kg m^-2 h^-1 with a solar conversion efficiency of 90.3% under 1 Sun (1.0 kW m⁻²) illumination. Combining with the merits of easy preparation, and wide possible choices of numerous existing donor and acceptor molecules, CTC is a promising class of novel materials for solar energy harvesting materials.

Secondly, considering the prominent photothermal characteristics of bulk organic CTCs, it is reasonable to conceive that tuneable nano-CTC with NIR-II absorption can be a practical strategy of photothermal therapy (PTT). By employing coronene (COR)-TCNQ, perylene (PER)-TCNQ, and DBTTF-TCNQ as the CT pair to form CTC nanoparticles (CT NPs), absorption peaks of CT NPs can be controllably tuned from the NIR-I to the NIR-II region. Interestingly, we demonstrate for the first time CT NPs with absorption peaks in the NIR-II region by combing PER (donor) and TCNQ (acceptor). The CT NPs show excellent photothermal performance with an absorption peak at 1040 nm in the NIR-II therapy window and it was applied for antibacterial therapy by using NIR-II laser (1064 nm) excitation. The nanosized PER-TCNQ NPs system shows a promising bacteria inhibition efficiency of 99% with photothermal conversion efficiency (PCE) of 42% under 1064 nm irradiation. All the results suggested that CTC can be developed as efficient photothermal agents (PTAs) for PTT.

Thirdly, to address the issue of regulation of the intermolecular interaction between D and A, a polymer-matrix is introduced in the charge transfer (CT) system. A CTC pair of common TTF (donor) and TCNB (acceptor) was doped into a poly(methyl methacrylate) (PMMA) polymer matrix for achieving a charge-transfer complex filter (CTC-F). Via regulation of the intermolecular interaction between TTF and TCNB in the polymer network, the CTC-F shows tunable absorption from the visible to the NIR region with a relatively high penetration of NIR radiation (~80%). The CTC-F can successfully highlight NIR information hidden in a complex environment and allows reading of NIR security images for advanced anti-counterfeiting. Moreover, the CTC-F can be used for viewing protected NIR images with good resolution, and thus provides a convenient tool for different security applications using NIR-encoded information.

Date of Award	18 Jul 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chun Sing LEE (Supervisor)