Six centrosymmetric pyrrolo[3,2‐b]pyrroles possessing either two nitro or two CF₃SO₂ strong electron‐withdrawing groups have been synthesized in a straightforward manner from simple building blocks and their one and two‐photon properties investigated. Although two‐photon absorption maxima clearly correspond to the S₀→S₂ transition, there is always a considerable shoulder at the region related to the S₀→S₁ transition. This fact clearly points to a small degree of symmetry breaking in the ground state which becomes more prevalent in the excited state. The systems showing the strongest excited‐state symmetry breaking have also been identified by first principles calculations. Both experimental data and theoretical calculations (performed with time‐dependent density functional theory) have revealed that changing the degree of conjugation between the nitro−aryl substituents and the core enables fine‐tuning of not only emission wavelengths but also the solvent dependent fluorescence intensity. Although there is no charge‐transfer between electron‐withdrawing 4‐cyanophenyl substituents located on nitrogen atoms and the core, nevertheless, their presence modifies the absorption and emission maxima. These groups also enable a record high fluorescence quantum yield in toluene (0.97) to be reached. By probing ground‐state symmetry by two‐photon absorption and excited‐state symmetry by solvatofluorochromism, we are able to obtain insight regarding the pathway of the molecule during and after the electronic transition.

中文翻译：

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具有两个硝基的中心对称吡咯并[3,2-b]吡咯的基态和激发态对称性破坏和溶剂荧光致变色

从简单的结构单元中直接合成了六个具有两个硝基或两个CF ₃ SO ₂强吸电子基团的中心对称吡咯并[3,2- b ]吡咯，并研究了它们的一个和两个光子性质。尽管两光子吸收最大值明显对应于S ₀ →S ₂跃迁，但在与S ₀ →S ₁相关的区域始终有相当大的肩峰过渡。这个事实清楚地表明，在基态中对称性的破坏程度较小，而在激发态中对称性破坏更为普遍。表现出最强激发态对称性破坏的系统也已经通过第一原理计算得到了识别。实验数据和理论计算（根据时间相关的密度泛函理论进行的研究）均表明，改变硝基芳基取代基与核之间的共轭程度不仅可以对发射波长进行微调，还可以对溶剂相关的荧光强度进行微调。尽管位于氮原子上的吸电子的4-氰基苯基取代基与核之间没有电荷转移，但是它们的存在改变了吸收和发射的最大值。这些基团还使甲苯中的荧光量子产率达到创纪录的高（0.97）。通过通过双光子吸收探测基态对称性和通过溶剂荧光致变色探测激发态对称性，我们能够获得有关电子跃迁期间和之后分子途径的见解。