Colorless Near‐Infrared Absorbing Dyes Based on B‐N Fused Donor‐Acceptor‐Donor π‐Conjugated Molecules for Organic Phototransistors

Abstract

The introduction of a colorless function to organic electronic devices allows responses to light in the near‐infrared (NIR) region and is expected to broaden the applications of these devices. However, the development of a colorless NIR dye remains a challenge due to the lack of a rational molecular design for controlling electronic transitions. In this study, to suppress the π‐π* transitions in the visible region, polycyclic donor‐acceptor‐donor π‐conjugated molecules with boron bridges (Py‐FNTz‐B and IP‐FNTz‐B) are designed and synthesized, which contain pyrrole or indenopyrrole as donor units with fluorinated naphthobisthiadiazole (FNTz) as an acceptor unit. The pyrrole end‐capped Py‐FNTz‐B shows an absorption band in the NIR region without distinct visible‐light absorption, which has led to the establishment of colorless characteristics. The indenopyrrole end‐capped IP‐FNTz‐B shows a narrow optical energy gap of 0.87 eV in films. Time‐resolved microwave conductance and field‐effect transistors demonstrate the semiconducting characteristics of these molecules, and Py‐FNTz‐B‐based devices function as NIR phototransistors. Theoretical analyses indicate that the combination of a polyene‐like electronic structure with orbital symmetry is important to obtain NIR wavelength‐selective absorption. This study suggests that a molecular design based on electronic structures can be effective in the development of colorless NIR‐absorbing dyes for organic electronics.