University of Siegen, Organic Chemistry II, Adolf-Reichwein-Str. 2, D-57068, Siegen, Germany.
Photochem Photobiol Sci. 2012 Apr;11(4):752-67. doi: 10.1039/c2pp05386a. Epub 2012 Feb 16.
The influence of the substituent of the N-aroylurea functionality on the solvatochromic properties of this class of compounds was investigated with eight examples. The absorption spectra of these compounds exhibit the characteristic spectroscopic properties of the corresponding arene fragment and are only slightly dependent on the solvent. In contrast, all investigated aroylurea derivatives exhibit a strong solvatochromism with a good linear correlation between the emission energy and the acceptor numbers (AN) of the solvents; that is, the emission maximum shifts bathochromically (Δλ = 50-93 nm) with increasing AN. Furthermore, in media with increasing viscosity, as established in glycerol or ethanol solutions with decreasing temperature, the emission maxima are significantly shifted to shorter wavelengths and the full width at half maximum (FWHM) changes. All experimental data point to two emitting states, namely the locally excited (LE) state and the charge-transfer (CT) state. Thus, after initial photoexcitation to the LE state an internal charge transfer (ICT) takes place due to the donor-acceptor interplay between the arene unit and the N-acylureido functionality, mainly assisted by the intramolecular hydrogen bond between the terminal NH group and the aryl-substituted carbonyl functionality, hence interconverting the latter to a stronger acceptor. In the polarized CT state the acylurea unit develops a negative charge, which, after solvent relaxation, is stabilized by solvents with high acceptor number. Time-resolved emission spectroscopy revealed additional conformational changes in the excited state. Two emissive species were identified at room temperature, whose lifetimes depend strongly on the chemical environment. In addition, time-resolved emission spectra (TRES) showed red-shifted emission bands at longer delays after the excitation pulse in polar solvents. These findings are rationalized by the presence of two different emitting rotational conformers.
研究了 N-酰基脲官能团的取代基对这类化合物溶剂化变色性质的影响，共涉及 8 个实例。这些化合物的吸收光谱表现出相应芳基片段的特征光谱性质，并且仅略微依赖于溶剂。相比之下，所有研究的酰基脲衍生物都表现出强烈的溶剂化变色，其发射能与溶剂的接受数（AN）之间存在良好的线性相关性；也就是说，随着 AN 的增加，发射最大值发生红移（Δλ=50-93nm）。此外，在甘油或乙醇溶液中随着温度降低建立的具有较高粘度的介质中，发射最大值明显向较短波长移动，且半峰全宽（FWHM）发生变化。所有实验数据都指向两种发射态，即局域激发（LE）态和电荷转移（CT）态。因此，在最初光激发到 LE 态之后，由于芳基单元和 N-酰基脲官能团之间的供体-受体相互作用，发生了内部电荷转移（ICT），主要通过末端 NH 基团和芳基取代的羰基官能团之间的分子内氢键辅助，从而将后者转化为更强的受体。在极化 CT 态中，酰基脲单元带负电荷，在溶剂弛豫后，它被具有高接受数的溶剂稳定化。时间分辨发射光谱揭示了激发态中的附加构象变化。在室温下鉴定出两种发光物种，其寿命强烈依赖于化学环境。此外，在极性溶剂中，在激发脉冲后的较长延迟时间，时间分辨发射光谱（TRES）显示出红移的发射带。这些发现通过存在两种不同的发光旋转构象得到了合理化解释。
