Tuning Near-Infrared Optical Properties of Aromatic Model Compounds by Substituents

In this study, the effects of substituents on the near-infrared (NIR) absorption spectra of model compounds were examined theoretically and experimentally, including benzene (B), fluorobenzene (FB), 1,4-difluorobenzene (DFB), and 1,4-bis(trifluoromethyl) benzene (6FB). It was found that a stronger electron-withdrawing substituent resulted in a smaller C–H bond length and thus a blue-shifted overtone position. Thus, the order of B > FB > DFB > 6FB was found for the positions of C–H fundamentals and overtones. The variations of optical windows were primarily influenced by the C–H molar density as well as by the shifting of the absorption band by the substituent. A pseudo compound (PC) with the same C–H number density as the 6FB but with the same band position as B was used to address the effect of a polarized substituent on the optical window. It was found that 6FB could have the optical window at the lower wavelength regions of the S-band (1480–1530 nm) and C-band (1530–1570 nm) but that of the PC at the higher wavelength regions of the C-band and L-band (1570–1610 nm). The present study provides another approach for designing new organic polymers for optical waveguides besides the current approach of reducing the C–H number density.