Bright Solvatochromic Fluorophores With Dual Emission Enabled by a Donor-Engineering Strategy

Single-component dual-emission fluorophores have recently attracted considerable attention. However, achieving dual emission from two independent emissive states within a simple molecular framework remains challenging. Herein, we report bright dual-emission fluorophores arising from the coexistence of a locally excited (LE) state and a planar intramolecular charge-transfer (PICT) state, and propose a donor-engineering design principle that enables this functionality through straightforward structural modification. Specifically, we screened tertiary amines for installation on the donor unit of highly emissive donor-acceptor-type solvatochromic fluorophores based on a 9,9-dimethyl-2-phenyl-9H-fluorene scaffold, with the aim of enabling two bright emissive states to coexist. As a result, the azetidine derivative [4] and the azepane derivative [7] exhibited clear dual emission with high fluorescence quantum yields (> 0.7) over a wide range of solvents. Notably, only one of the two emission bands displayed pronounced fluorescence solvatochromism, as evidenced by substantial shifts in its peak position and distinct lifetime components. Accordingly, these fluorophores are applicable to ratiometric polarity sensing using the intensity ratio of the two emission bands. Overall, our donor-engineering strategy provides a promising approach to the molecular design of bright dual-emission solvatochromic fluorophores.