The non-fused five-membered ring structure of 2-(1H-pyrrol-2-yl)pyridine (5-HB) is more flexible than the fused six-membered ring structure of 10,11,12,13-tetrahydro-9H-quinolino[8,7-a]carbazole (6-HB) to form the intramolecular hydrogen bond between pyrrole and pyridine, while the six-membered ring structure possesses more favorable distance and orientation between pyrrole and pyridine. In the present work, we carried out the electronic structure calculations and nonadiabatic dynamics simulations to gain insight into the effect of different structure feature on the excited-state intramolecular proton transfer (ESIPT) processes of 5-HB and 6-HB. The geometric parameters, IR vibrational spectra, bond critical point (BCP) parameters and reduced density gradient (RDG) show that the intramolecular hydrogen bond of 6-HB is much stronger than that of 5-HB. The 6-HB has a lower kinetic stability and a higher chemical activity than 5-HB through the analysis of the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The pyridine N1 atom of 6-HB shows much stronger ability than that of 5-HB to attract the hydrogen proton based on the natural population analysis (NPA). The analysis of the potential energy curves and time evolution of selected bond distances indicate that the ESIPT process of fused six-membered ring structure is easier and faster than the non-fused five-membered ring structure. Our findings might be useful for the design of relevant ESIPT systems.

2-（1 H-吡咯-2-基）吡啶（5-HB）的非稠合五元环结构比10,11,12,13-四氢- 9 H-喹啉[8,7- a]咔唑（6-HB）在吡咯和吡啶之间形成分子内氢键，而六元环结构在吡咯和吡啶之间具有更有利的距离和方向。在目前的工作中，我们进行了电子结构计算和非绝热动力学模拟，以了解不同结构特征对5-HB和6-HB的激发态分子内质子转移（ESIPT）过程的影响。几何参数，红外振动光谱，键临界点（BCP）参数和降低的密度梯度（RDG）表明6-HB的分子内氢键比5-HB的强得多。通过分析最高占据分子轨道（HOMO）与最低未占据分子轨道（LUMO）之间的能隙，6-HB具有比5-HB更低的动力学稳定性和更高的化学活性。根据自然种群分析（NPA），6-HB的吡啶N1原子显示出比5-HB强大的吸附氢质子的能力。对势能曲线和选定键距的时间演化的分析表明，稠合六元环结构的ESIPT过程比非稠合五元环结构更容易，更快。我们的发现可能对设计相关的ESIPT系统很有用。根据自然种群分析（NPA），6-HB的吡啶N1原子显示出比5-HB强大的吸附氢质子的能力。对势能曲线和选定键距的时间演化的分析表明，稠合六元环结构的ESIPT过程比非稠合五元环结构更容易，更快。我们的发现可能对设计相关的ESIPT系统很有用。根据自然种群分析（NPA），6-HB的吡啶N1原子显示出比5-HB强大的吸附氢质子的能力。对势能曲线和选定键距的时间演化的分析表明，稠合六元环结构的ESIPT过程比非稠合五元环结构更容易，更快。我们的发现可能对设计相关的ESIPT系统很有用。