Development of thermally-stable NIR absorbing films based on heptamethine cyanine dyes with bistriflimide anion

Abstract

Near-infrared (NIR) absorbing film is an essential component of CMOS image sensor modules. In order to exploit the intense NIR absorbing performance of heptamethine cyanine dyes for this application, their thermal stability must be improved. In this study, a series of heptamethine cyanine dyes were synthesized in which the meso-substituent and the counter anion were varied. Furthermore, the effects of the substituent and the counter anion on the material properties were investigated. The molar extinction coefficients of the cyanine dyes were enhanced by replacing the iodide ion with the bulky bistriflimide (TFSI) anion. The TFSI anion, in which the negative charge is dispersed over a large volume, stabilized the cyanine cation by charge delocalization and greatly improved the thermal stability. NIR absorbing films with outstanding transmittance in the visible region were fabricated using the synthesized cyanine dyes and a transparent polysulfone (PSU) binder. The thermal stability was evaluated by post-baking the prepared films at a temperature of 150 °C for 3 h. The films made with TFSI-substituted cyanine dyes showed excellent thermal stability, and they maintained their initial absorbance even after the baking process compared to films made with unsubstituted cyanine-based dyes.