Asymmetric topology architecture improves polarity sensitivity of nitrogen and boron containing donor–acceptor luminogens: theoretical prediction and experimental validation

Abstract

Polarity-sensitive responsiveness is a crucial property for fluorescent probes, yet developing probes with high spectral sensitivity to polarity remains challenging. In this work, we systematically designed a series of donor–acceptor (D–A) type luminogens NNB and their derivatives with symmetric and asymmetric structures by fine-tuning the proportion of nitrogen-containing D moieties and boron-containing A moieties to elucidate the structure–property relationships governing their spectral responsiveness to solvent polarity. Our findings revealed a clear trend in polarity sensitivity of these dyes: symmetric dyes < asymmetric D–D–A dyes < asymmetric D–A–A dyes. The polarity responsive trend of these dyes was rigorously verified through combined theoretical and experimental approaches across various solvent conditions and polarity-tunable polymer-doped thin films. Importantly, our calculations revealed that for these dyes the conformational planarization and solute–solvent interactions synergistically induce the polarity-dependent fluorescence redshift. Crucially, we established a robust quantitative descriptor (ΔΔE_SSI-es), which represents the difference in solute–solvent interaction energies in different solvent environments. And ΔΔE_SSI-es shows strong correlation (R² = 0.98) with the corresponding emission wavelength shifts (Δλ_em). This work not only offers fundamental insights for designing D–A type high-performance polarity-sensitive fluorescent probes with nitrogen-containing donors and boron-containing acceptors, but also delivers a practical descriptor for assessing spectral sensitivity to environmental polarity.