Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, PIN-208 016, India.
J Phys Chem A. 2011 Aug 4;115(30):8335-43. doi: 10.1021/jp2005524. Epub 2011 Jul 13.
Ultrafast excited-state relaxation dynamics of a nonlinear optical (NLO) dye, (S)-(-)-1-(4-nitrophenyl)-2-pyrrolidinemethanol (NPP), was carried out under the regime of femtosecond fluorescence up-conversion measurements in augmentation with quantum chemical calculations. The primary concern was to trace the relaxation pathways which guide the depletion of the first singlet excited state upon photoexcitation, in such a way that it is virtually nonfluorescent. Ground- and excited-state (singlet and triplet) potential energy surfaces were calculated as a function of the -NO(2) torsional coordinate, which revealed the perpendicular orientation of -NO(2) in the excited state relative to the planar ground-state conformation. The fluorescence transients in the femtosecond regime show biexponential decay behavior. The first time component of a few hundred femtoseconds was ascribed to the ultrafast twisted intramolecular charge transfer (TICT). The occurrence of charge transfer (CT) is substantiated by the large dipole moment change during excitation. The construction of intensity- and area-normalized time-resolved emission spectra (TRES and TRANES) of NPP in acetonitrile exhibited a two-state emission on behalf of decay of the locally excited (LE) state and rise of the CT state with a Stokes shift of 2000 cm(-1) over a time scale of 1 ps. The second time component of a few picoseconds is attributed to the intersystem crossing (isc). In highly polar solvents both the processes occur on a much faster time scale compared to that in nonpolar solvents, credited to the differential stability of energy states in different polarity solvents. The shape of frontier molecular orbitals in the excited state dictates the shift of electron density from the phenyl ring to the -NO(2) group and is attributed to the charge-transfer process taking place in the molecule. The viscosity dependence of relaxation dynamics augments the proposition of considering the -NO(2) group torsional motion as the main excited-state relaxation coordinate.
超快激发态弛豫动力学的非线性光学（NLO）染料，（S）-（-）-1-（4-硝基苯基）-2-吡咯烷甲醇（NPP），进行了飞秒荧光上转换测量制度与量子化学计算的增强。主要关注的是跟踪的弛豫途径指导的第一单线态激发态的消耗后光激发，以这样一种方式，它是几乎无荧光。地面和激发态（单线态和三线态）势能面计算作为一个函数的 -NO（2）扭转坐标，这表明了垂直方向的 -NO（2）在激发态相对于平面基态构象。飞秒瞬态荧光显示双指数衰减行为。第一个几百飞秒的时间分量归因于超快扭曲的分子内电荷转移（TICT）。电荷转移（CT）的发生由激发过程中较大的偶极矩变化证实。构建强度和面积归一化时间分辨发射光谱（TRES 和 TRANES）在乙腈中的 NPP 表现出代表局部激发（LE）态衰减和 CT 态上升的双态发射，具有 2000 厘米（-1）的斯托克斯位移在 1 ps 的时间尺度上。几个皮秒的第二个时间分量归因于系间窜越（isc）。在高极性溶剂中，两个过程都比在非极性溶剂中发生得快得多，这归因于不同极性溶剂中能量状态的不同稳定性。激发态前线分子轨道的形状决定了电子密度从苯基环向-NO（2）基团的转移，并归因于分子中发生的电荷转移过程。弛豫动力学的粘性依赖性增强了考虑-NO（2）基团扭转运动作为主要激发态弛豫坐标的命题。
