Novel low-bandgap organic semiconductors for near-infrared (NIR)- based applications Adibah Zamhuri, Bob C. Schroeder University College London, UK Cancer is the second leading cause of death worldwide, hence detecting tumours at an early stage will allow effective treatment and increase the survival rate of cancer patients. In terms of tumour detection, photoacoustic imaging (PAI) has been a preferred alternative to conventional optical and ultrasound imaging due to its higher image resolution and better tissue depth penetration.Although, detecting tumours residing deeper in the body is still quite challenging due to tissue autofluorescence. 1 Hence, exogenous contrast agents are commonly used to enhance image contrast, and researchers are now focused on developing contrast agents that absorb in the near-infrared (NIR) region (700 – 2500 nm). Recent progress in in vivo biological imaging demonstrates reduced light scattering and tissue autofluorescence at longer wavelengths, which would ultimately lead to better spatial resolution and enhanced contrast at deeper imaging depth. 2 Researchers are also interested in employing organic semiconductors as contrast agents, replacing traditional inorganic/metal nanoparticles that are highly toxic. 3 Organic semiconductors are known for their unique optoelectronic properties, which can be tuned by appropriately modifying their molecular design. They also possess excellent photostability and great biocompatibility, which are essential for creating an ideal contrast agent for PAI. 4
Figure 1 Chemical structure design of novel molecule, diketopyrrolopyrrole-dithienopyrrole (DPP-DTP), with varying alkyl chains. Herein we report a series of novel organic semiconductors known as DPP-DTP, diketopyrrolopyrrole (DPP) as the acceptor unit anddithienopyrrole (DTP) as the donor unit.In this work, we synthesized and analysed DTP-DPP molecules with varying alkyl chains on the DTP moieties (Figure 1). Upon extensive photophysical and electronic studies, we observe that these molecules exhibit intriguing photophysics, which were deduced by spectroscopic studies in solution and thin films. The peak emission observed at >gt; 700 nm provides promising results in developing NIR-based DPP polymeric semiconductors for applications in PAI. Furthermore, initial theoretical studies on the DPP-DTP tetramers provided promising results, where the DPP-DTP homopolymer having predicted wavelength of maximum absorbance of ~900 nm and calculated HOMO-LUMO gap of ~1.5 eV. With these encouraging results, we later extended our series of compounds to include polymers with potential ultra-low bandgap. Overall, our study underlines the huge potential of these novel compounds as a new class of NIR-based organic materials. References 1. Smith, A. M., Mancini, M. C., Nie, S. (2009). Bioimaging: Second window for in vivo imaging. Nature Nanotechnology , 4 (11), 710–711. 2. Yang, Q., Ma, Z., Wang, H., Zhou, B., Zhu, S., Zhong, Y., Wang, J., Wan, H., Antaris, A., Ma, R., Zhang, X., Yang, J., Zhang, X., Sun, H., Liu, W., Liang, Y., Dai, H. (2017). Rational Design of Molecular Fluorophores for Biological Imaging in the NIR-II Window. Advanced Materials , 29 (12). 3. Weber, J., Beard, P. C., Bohndiek, S. E. (2016). Contrast agents for molecular photoacoustic imaging. Nature Methods , 13 (8), 639–650. 4. Wang, Y., Zhang, H., Wang, Z., Feng, L. (2020). Photothermal Conjugated Polymers and Their Biological Applications in Imaging and Therapy. ACS Applied Polymer Materials , 2 (10), 4222–4240.
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