A Benzophosphole <i>P</i>-Oxide with an Electron-Donating Group at 3-Position: Enhanced Fluorescence in Polar Solvents
作者:Eriko Yamaguchi、Aiko Fukazawa、Youhei Kosaka、Daisuke Yokogawa、Stephan Irle、Shigehiro Yamaguchi
DOI:10.1246/bcsj.20150238
日期:2015.11.15
Fluorophores with intramolecular charge-transfer (ICT) character in the excited state exhibit significant solvatochromism of their fluorescence. Here, we report an example for such compounds, a benzophosphole P-oxide bearing an electron-donating p-(diphenylamino)phenyl group at the 3-position. While this compound shows only subtle dependence of the absorption maximum on the solvent polarity (λmax = 383–392 nm), its emission maximum is significantly red-shifted upon increasing the solvent polarity (cyclohexane: λem = 457 nm; DMF: λem = 598 nm). Most notably, the fluorescence quantum yields gradually increase with increased Stokes shifts, ultimately reaching ΦF = 0.28 in DMF. This trend is fundamentally different from that observed for the corresponding 2-(diphenylamino)phenyl-substituted benzophosphole congener, for which applications as a fluorescent bioimaging probe were previously demonstrated. In this study, the origins of this striking difference are examined by a combined experimental and theoretical approach. Our results suggest that the observed difference arises from a significant contribution of quinoidal resonance forms in the ICT excited state, which suppresses nonradiative decay and hence increases the quantum yield in polar solvents.
在激发态具有分子内电荷转移(ICT)特性的荧光团,其荧光具有显著的溶解变色特性。在此,我们报告了此类化合物的一个实例,即在 3 位上带有一个电子捐献对(二苯基氨基)苯基的苯并膦唑 P-氧化物。虽然这种化合物的吸收最大值与溶剂极性(λmax = 383-392 nm)只有微妙的关系,但当溶剂极性增加时,其发射最大值会发生明显的红移(环己烷:λem = 457 nm;DMF:λem = 598 nm)。最值得注意的是,荧光量子产率随着斯托克斯位移的增加而逐渐增大,最终在 DMF 中达到 ΦF = 0.28。这种趋势与相应的 2-(二苯基氨基)苯基取代的苯并膦烯同系物的趋势截然不同,后者以前曾被证明可用作荧光生物成像探针。本研究采用实验和理论相结合的方法,对这一显著差异的根源进行了研究。我们的研究结果表明,所观察到的差异是由 ICT 激发态中的醌类共振形式造成的,它抑制了非辐射衰变,从而提高了极性溶剂中的量子产率。