Ghosh Rajib, Nandi Amitabha, Palit Dipak K
Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai-400085, India.
Phys Chem Chem Phys. 2016 Mar 21;18(11):7661-71. doi: 10.1039/c5cp07778h.
Organic molecules substituted with the nitro group show efficient nonlinear optical (NLO) properties, which are a consequence of the strong intramolecular charge transfer (ICT) character of the molecules because of the strong electron withdrawing nature of the nitro group and rapid responsiveness because of highly movable π-electrons. Dynamics of the ICT process in the excited states of a push-pull biphenyl derivative, namely, 4-N,N-dimethylamino-4'-nitrobiphenyl (DNBP), an efficient NLO material, has been investigated using ultrafast transient absorption spectroscopy. The experimental results have been corroborated with DFT and TDDFT calculations. In solvents of large polarity, e.g. acetonitrile, the ultrafast ICT process of DNBP is associated with the barrierless twisting of the N,N-dimethylaniline (DMA) group with respect to the nitrobenzene moiety to populate the twisted ICT (or TICT) state, and the rate of this process is solely governed by the viscosity of the medium. In solvents of moderate polarity, e.g. ethyl acetate, the rate of the twisting process is significantly slowed down and the LE and TICT states remain in equilibrium because of a low energy barrier for interconversion between these two states. By further lowering the polarity of the solvent, e.g. in dioxane, the twisting process is completely retarded. In nonpolar solvents, e.g. cyclohexane, a reverse twisting motion towards the planar geometry (i.e. the PICT process) has been evident in the excited state dynamics. In this solvent, the S1 state undergoes an ultrafast intersystem crossing to the triplet state because of its close proximity with the T2 state.
被硝基取代的有机分子表现出高效的非线性光学（NLO）性质，这是由于硝基的强吸电子性质导致分子具有强分子内电荷转移（ICT）特性以及高活性π电子带来的快速响应性的结果。使用超快瞬态吸收光谱研究了一种高效NLO材料——推-拉联苯衍生物4-N,N-二甲基氨基-4'-硝基联苯（DNBP）激发态下ICT过程的动力学。实验结果已通过密度泛函理论（DFT）和含时密度泛函理论（TDDFT）计算得到证实。在大极性溶剂中，例如乙腈，DNBP的超快ICT过程与N,N-二甲基苯胺（DMA）基团相对于硝基苯部分的无障碍扭转相关，以填充扭曲ICT（或TICT）态，并且该过程的速率仅由介质的粘度决定。在中等极性溶剂中，例如乙酸乙酯，扭转过程的速率显著减慢，并且由于这两个状态之间相互转换的能垒较低，局域激发（LE）态和TICT态保持平衡。通过进一步降低溶剂的极性，例如在二氧六环中，扭转过程完全受阻。在非极性溶剂中，例如环己烷，在激发态动力学中明显出现向平面几何结构的反向扭转运动（即平面内电荷转移（PICT）过程）。在这种溶剂中，S1态由于与T2态非常接近而经历超快系间窜越到三重态。
