Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-hiroshima, 739-8527, Japan.
Org Biomol Chem. 2010 Jun 21;8(12):2756-70. doi: 10.1039/c003526b. Epub 2010 Apr 29.
Heteropolycyclic donor--acceptor (D--A) structural isomers of benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type (2a-f) and benzofuro[2,3-c]oxazolo[5,4-a]carbazole-type fluorescent dyes (3a-f), which differ in the position of oxygen and nitrogen atoms of the oxazole ring, have been synthesized, and their photophysical and electrochemical properties have been investigated. The fluorescent dyes 2a-f exhibit much stronger intramolecular charge transfer (ICT) absorption and fluorescence bands than 3a-f. Both of the D-pi-A structural isomers showed bathochromic shifts of the fluorescence band and a decrease in the fluorescence quantum yields with increasing solvent polarity (i.e., positive fluorescence solvatochromism), and the bathochromic shifts for 3a-f were larger than those for 2a-f. The fluorescent dyes 2a-c and 3a-c, which have cyano and carboxyl groups, and carboxylic acid ester as electron-withdrawing substituents, exhibited significant fluorescence solvatochromic properties, compared to 2e, 2f, 3e and 3f without an electron-withdrawing substituent. The fluorescence solvatochromism of the fluorescent dyes were analyzed with the Lippert-Mataga correlation. Moreover, to elucidate the differences of photophysical properties among the fluorescent dyes, we have performed time-resolved fluorescence spectroscopic measurements, and a change of the electronic or molecular structures between the ground and excited states was evaluated by a comparison of the experimental radiative rate constant (k(r)) with the Strickler-Berg rate constant (kSB(r)). The electrochemical properties of 2a-f and 3a-f were determined by cyclic voltammetry, which demonstrated that these fluorescent dyes have similar HOMO energy levels, but different LUMO energy levels. Semi-empirical molecular orbital calculations (AM1 and INDO/S) have been carried out to elucidate the effects of the substituents and chromophore skeleton on photophysical properties of the two D-pi-A structural isomers. These studies indicated that the ICT characteristics in the excited state are different among the fluorescent dyes, which is a principal reason for the differences of the fluorescence solvatochromism and the substituent dependent fluorescence properties among the two D-pi-A structural isomers.
苯并呋喃[2,3-c]恶唑并[4,5-a]咔唑型（2a-f）和苯并呋喃[2,3-c]恶唑并[5,4-a]咔唑型荧光染料（3a-f）的杂多环给体-受体（D-A）结构异构体，其不同之处在于恶唑环的氧和氮原子的位置，已经被合成，并研究了它们的光物理和电化学性质。荧光染料 2a-f 表现出比 3a-f 更强的分子内电荷转移（ICT）吸收和荧光带。两种 D-π-A 结构异构体的荧光带均发生红移，随着溶剂极性的增加，荧光量子产率降低（即正的荧光溶剂化变色），并且 3a-f 的红移大于 2a-f。具有氰基和羧基以及羧酸酯作为吸电子取代基的荧光染料 2a-c 和 3a-c 与没有吸电子取代基的 2e、2f、3e 和 3f 相比，表现出显著的荧光溶剂化变色性质。荧光染料的荧光溶剂化变色用 Lippert-Mataga 相关分析进行分析。此外，为了阐明荧光染料之间光物理性质的差异，我们进行了时间分辨荧光光谱测量，并通过比较实验辐射速率常数（k(r)）与 Strickler-Berg 速率常数（kSB(r)）来评估电子或分子结构在基态和激发态之间的变化。2a-f 和 3a-f 的电化学性质通过循环伏安法确定，这表明这些荧光染料具有相似的 HOMO 能级，但不同的 LUMO 能级。进行了半经验分子轨道计算（AM1 和 INDO/S），以阐明取代基和生色团骨架对两种 D-π-A 结构异构体光物理性质的影响。这些研究表明，在激发态中 ICT 特性在荧光染料之间不同，这是荧光溶剂化变色和两种 D-π-A 结构异构体之间取代基依赖荧光性质差异的主要原因。
