Laboratório de Sólidos e Superfícies, Universidade Federal do Rio Grande do Sul-Instituto de Química, Porto Alegre-RS, Brazil.
J Phys Chem A. 2011 Nov 24;115(46):13390-8. doi: 10.1021/jp206905f. Epub 2011 Oct 27.
Two Schiff bases were synthesized by reaction of 2-(4'-aminophenyl)benzoxazole derivatives with 4-N,N-diethylaminobenzaldehyde. UV-visible (UV-vis) and steady-state fluorescence in solution were applied in order to characterize its photophysical behavior. The Schiff bases present absorption in the UV region with fluorescence emission in the blue-green region, with a large Stokes' shift. The UV-vis data indicates that each dye behaves as two different chromophores in solution in the ground state. The fluorescence emission spectra of the dye 5a show that an intramolecular proton transfer (ESIPT) mechanism takes place in the excited state, whereas a twisted internal charge transfer (TICT) state is observed for the dye 5b. Theoretical calculations were performed in order to study the conformation and polarity of the molecules at their ground and excited electronic states. Using density functional theory (DFT) methods at theoretical levels BLYP/Aug-SV(P) for geometry optimizations and B3LYP/6-311++G(2d,p) for single-point energy evaluations, the calculations indicate that the lowest energy conformations are in all cases nonplanar and that the dipole moments of the excited state relaxed structures are much larger than those of the ground state structures, which corroborates the experimental UV-vis absorption results.
两个席夫碱是由 2-(4'-氨基苯基)苯并恶唑衍生物与 4-N,N-二乙基氨基苯甲醛反应合成的。为了研究其光物理性质，采用了紫外可见（UV-vis）和稳态荧光光谱法。席夫碱在紫外区域有吸收，在蓝绿色区域有荧光发射，具有较大的斯托克斯位移。紫外可见数据表明，每种染料在基态下在溶液中表现为两种不同的发色团。染料 5a 的荧光发射光谱表明，在激发态下发生了分子内质子转移（ESIPT）机制，而对于染料 5b，则观察到扭曲的内电荷转移（TICT）态。为了研究分子在基态和激发电子态下的构象和极性，进行了理论计算。使用密度泛函理论（DFT）方法，在 BLYP/Aug-SV(P)理论水平上进行几何优化，在 B3LYP/6-311++G(2d,p)理论水平上进行单点能量评估，计算表明，最低能量构象在所有情况下都是非平面的，并且激发态弛豫结构的偶极矩远大于基态结构的偶极矩，这与实验的 UV-vis 吸收结果相符。
