Thieno[1,3,2]oxazaborinine-containing aza-BODIPYs with near infrared absorption bands: synthesis, photophysical properties, and device applications

Abstract

As part of an ongoing study of near infrared (NIR) absorbing dyes applicable to optoelectronics, thieno[1,3,2]oxazaborinine-containing aza-BODIPYs were synthesized for the first time. 3,5-Di(thiophene)-substituted N₂O₂-type 1 showed a NIR absorption band centered at 780 nm, with a molar extinction coefficient (ε_max) of 5.51 × 10⁴ M⁻¹ cm⁻¹ in CH₂Cl₂; however, it was easily hydrolyzed in solution. In comparison, 3-thiophene-containing N₂O-type analogues 2 and 3 had greater stability, and NIR absorption bands at 768 nm (ε_max = 8.62 × 10⁴ M⁻¹ cm⁻¹) and 779 nm (ε_max = 6.32 × 10⁴ M⁻¹ cm⁻¹), respectively, in CH₂Cl₂. Notably, these dyes had longer absorption bands than those of all of the structurally constraining aza-BODIPYs reported thus far. CV measurements and theoretical calculations indicated that this shift was the result of a higher-lying HOMO energy level generated by incorporation of thiophene into the aza-BODIPY core. As a potential device application, the 3-loaded film was prepared on indium tin oxide (ITO). The film showed a NIR absorption band at 827 nm, with a wide spectral range, and a λ_onset value of 897 nm; thus, its applicability as a NIR photodetector was evaluated through fabrication of a single-component device. The ITO/3 (85 nm)/Al (100 nm) device produced a photocurrent of 9.57 × 10⁻⁷ A cm⁻² at a bias potential of 1 V upon illumination at 850 nm and a fluence of 130 μW cm⁻². This demonstrated the potential of thieno[1,3,2]oxazaborinine-containing aza-BODIPY materials in optoelectronic applications such as NIR photodetectors.