Donor–Acceptor–Acceptor-Type Near-Infrared Fluorophores Featuring Thieno[3,2-<i>b</i>]thiophene <i>S</i>,<i>S</i>-Dioxide and Diarylboryl Units as Dual Acceptors

A new class of donor-acceptor-acceptor-type fluorophores was developed, incorporating thieno[3,2-<i>b</i>]thiophene <i>S</i>,<i>S</i>-dioxide and dimesitylboryl units as dual acceptors, along with aminoaryl groups serving as donors. These compounds exhibit deep-red to near-infrared fluorescence in low-polarity media, such as toluene and poly(methyl methacrylate) films. Substantial structural relaxation occurs in the excited state, even under such low-polarity conditions, resulting in a strongly red-shifted emission. Wavelength conversion films incorporating these fluorophores were fabricated via Förster resonance energy transfer from suitable host materials.