Instituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior, Ciudad Universitaria, México, 04510, D.F., México.
J Phys Chem B. 2013 Aug 29;117(34):9947-55. doi: 10.1021/jp403602v. Epub 2013 Aug 20.
The photochemistry of nitro-substituted polyaromatic compounds is generally determined by the rapid decay of its S1 state and the rapid population of its triplet manifold. Previous studies have shown that such an efficient channel is due to a strong coupling of the fluorescent state with specific upper receiver states in the triplet manifold. Here we examine variations in this mechanism through the comparison of the photophysics of 2-nitrofluorene with that of 2-diethylamino-7-nitrofluorene. The only difference between these two molecules is the presence of a diethylamino group in a push-pull configuration for the latter compound. The femtosecond-resolved experiments presented herein indicate that 2-nitrofluorene shows ultrafast intersystem crossing which depopulates the S1 emissive state within less than a picosecond. On the other hand, the amino substituted nitrofluorene shows a marked shift in its S1 energy redounding in the loss of coupling with the receiver triplet state, and therefore a much longer lifetime of 100 ps in cyclohexane. In polar solvents, the diethylamino substituted compound actually shows double peaked fluorescence due to the formation of charge transfer states. Evaluation of the Stokes shifts in different solvents indicates that both bands correspond to intramolecular charge transfer states in equilibrium which are formed in an ultrafast time scale from the original locally excited (LE) state. The present study addresses the interplay between electron-donating and nitro substituents, showing that the addition of the electron-donating amino group is able to change the coupling with the triplet states due to a stabilization of the first excited singlet state and the rapid formation of charge transfer states in polar solvents. We include calculations at the TD-DFT level of theory with the PBE0 and B3LYP functionals which nicely predict the observed difference between the two compounds, showing how the specific S(π-π*)-T(n-π*) coupling normally prevalent in nitroaromatics is lost in the push-pull compound.
硝基取代的多环芳烃的光化学通常由其 S1 态的快速衰减和三重态态的快速填充决定。先前的研究表明，这种高效的通道是由于荧光态与三重态态中的特定上受体态之间的强耦合。在这里，我们通过比较 2-硝基芴与 2-二乙氨基-7-硝基芴的光物理性质来研究这种机制的变化。这两个分子之间的唯一区别是后者化合物中存在一个推拉构型的二乙氨基。本文呈现的飞秒分辨实验表明，2-硝基芴表现出超快的系间窜越，在不到一个皮秒的时间内使 S1 发射态失活。另一方面，取代的氨基硝基芴的 S1 能量发生明显位移，导致与受体三重态的耦合丢失，因此在环己烷中的寿命延长至 100 ps。在极性溶剂中，由于形成电荷转移态，二乙氨基取代的化合物实际上显示出双峰荧光。不同溶剂中斯托克斯位移的评估表明，两个带都对应于平衡的分子内电荷转移态，这些态是从原始局部激发（LE）态在超快时间尺度上形成的。本研究探讨了供电子和硝基取代基之间的相互作用，表明由于第一激发单线态的稳定和在极性溶剂中电荷转移态的快速形成，供电子氨基的添加能够改变与三重态的耦合。我们包括了在 TD-DFT 水平上的理论计算，使用了 PBE0 和 B3LYP 泛函，这些泛函很好地预测了两种化合物之间的观察到的差异，展示了通常在硝基芳烃中存在的特定 S(π-π*)-T(n-π*) 偶联如何在推拉化合物中丢失。
