Department of Chemistry and Biology, University of Siegen, Siegen, Germany.
Photochem Photobiol Sci. 2012 Dec;11(12):1914-28. doi: 10.1039/c2pp25167a.
The solvatochromic emission properties of five naphthoylurea derivatives with different substitution patterns at the naphthoylurea functionality were investigated, with a particular focus on the influence of inter- and intramolecular H-bonding interactions. The bathochromic shifts of the emission maxima correlate well with the acceptor number or Catalán's acidity of the solvent (Δλ = 47-86 nm), indicating an excited species with a pronounced negative charge that is stabilized by H-bond donating (HBD) solvents. In media with restricted free volume the formation of the charged species is not favored, because the required conformational change to establish an intramolecular charge transfer (ICT) between the fluorophore and the acylurea substituent is hindered, and the emission mainly originates from the locally excited state. This relationship between the alignment of the naphthoyl carbonyl functionality relative to the naphthyl ring and the spectroscopic shift was confirmed by the comparison of the ground state conformation and the emission spectra of the naphthoylurea derivatives in the solid state. Time-resolved experiments revealed different excited entities, whose lifetimes are significantly influenced by the HBD properties and the temperature of the environment. With few exceptions the naphthoylurea derivatives exhibit only two emissive species in the nanosecond range. All experimental data point to conformational relaxation and solvent reorganization leading to the cis and trans isomers of one preferential conformer with respect to the acylurea unit. The structure of the preferred conformation is mainly determined by the possible inter- or intramolecular H-bonds and is therefore also strongly influenced by the HBD and H-bond accepting (HBA) properties of the polar solvents. As the NH groups of the acylurea functionality contribute mainly to the entire inter- and intramolecular H-bond arrangement the variation of the substitution pattern of the urea unit, specifically the presence and position of the NH groups, leads to derivatives with significantly different steady-state and time-resolved emission properties.
五种具有不同取代模式的萘酰基脲衍生物的溶剂化变色发射性质进行了研究，特别关注了分子内和分子间氢键相互作用的影响。发射最大值的红移与溶剂的接受数或 Catalán 酸度（Δλ = 47-86nm）很好地相关，表明激发态物种带有明显的负电荷，被供质子（HBD）溶剂稳定。在自由体积受限的介质中，不有利于形成带电物种，因为建立荧光团和酰基脲取代基之间的电荷转移（ICT）所需的构象变化受到阻碍，并且发射主要源于局部激发态。萘酰基羰基功能相对于萘基环的排列与光谱位移之间的这种关系通过萘酰基脲衍生物在固态中的基态构象和发射光谱的比较得到了证实。时间分辨实验揭示了不同的激发实体，其寿命受到 HBD 性质和环境温度的显著影响。除了少数例外，萘酰基脲衍生物在纳秒范围内仅显示两种发光物种。所有实验数据都指向构象弛豫和溶剂重组，导致相对于酰基脲单元的顺式和反式异构体。优先构象的结构主要由可能的分子内或分子间氢键决定，因此也受到极性溶剂的 HBD 和 HBA 性质的强烈影响。由于酰基脲官能团的 NH 基团主要贡献于整个分子内和分子间氢键排列，因此脲单元的取代模式的变化，特别是 NH 基团的存在和位置，导致具有显著不同的稳态和时间分辨发射性质的衍生物。
