Baweja Simran, Kalal Bhavika, Maity Surajit
Department of Chemistry, IIT Hyderabad, Kandi, Sangareddy, Telangana, 502284, India.
Phys Chem Chem Phys. 2023 Oct 11;25(39):26679-26691. doi: 10.1039/d3cp03010e.
We report the first gas phase comprehensive study of the electronic spectroscopy of 2,7-diazaindole molecule in the ground and excited states. Single vibronic level fluorescence spectroscopy (SVLF) was performed to determine the ground state vibrations of the molecule, which depicted a large Franck-Condon activity beyond 2600 cm. For the excited state characterization, laser-induced fluorescence (LIF) and two-color resonant two-photon ionization spectroscopy (2C-R2PI) were performed. The band origin (000) for S ← S transition appeared at 33910 ± 1 cm which was red shifted by 718 cm and 1322 cm compared to that of 7-azaindole and indole respectively. The Franck-Condon active vibrational modes in the spectra were seen till the (000) + 1600 cm region. The IR-UV hole burning spectroscopy confirmed the absence of any other isomeric species in the molecular beam. The ionization energy (IE) of the molecule was measured as 8.921 ± 0.001 eV, recorded using photoionization efficiency spectroscopy. The above IE value was significantly higher than that of the related indole derivatives, suggesting the higher photostability of the 27DAI molecule due to N(2) insertion. The ground and excited state N-H stretching frequencies of the molecule were determined using fluorescence-dip infrared spectroscopy (FDIR) and resonant ion-dip infrared spectroscopy (IDIR), and the values are 3523 and 3467 cm, respectively. The lower value of in the electronic excited state implied the increased photoacidity of the group. A comparative analysis of the experimental LIF/2C-R2PI spectra was done against Franck-Condon simulated spectra at three different levels of theory. The vibrational frequencies calculated at B3LYP-D4/def2-TZVPP showed the most accurate prediction in comparison with the experimentally detected symmetric modes in the ground state. However, in the excited state, the lower energy asymmetric modes simulated at the B3LYP/def-SVP level of theory provided the best agreement with the experiment. This is most probably due to the distortion observed at the pyrazolyl ring leading to the appearance of asymmetric vibrational modes. The above study highlights the possibility to appropriately tune the excitation wavelengths as well as alter the photostability of the organic chromophores additional N-insertion in the molecular systems.
我们报告了对2,7 - 二氮杂吲哚分子基态和激发态电子光谱的首次气相综合研究。进行了单振子能级荧光光谱（SVLF）以确定分子的基态振动，其显示出在2600 cm以上有较大的弗兰克 - 康登活性。对于激发态表征，进行了激光诱导荧光（LIF）和双色共振双光子电离光谱（2C - R2PI）。S←S跃迁的带起源（000）出现在33910±1 cm处，与7 - 氮杂吲哚和吲哚相比，分别红移了718 cm和1322 cm。光谱中弗兰克 - 康登活性振动模式一直到（000） + 1600 cm区域都可见。红外 - 紫外空穴烧蚀光谱证实了分子束中不存在任何其他异构体。使用光电离效率光谱测量该分子的电离能（IE）为8.921±0.001 eV。上述IE值明显高于相关吲哚衍生物的IE值，表明由于N(2)插入，27DAI分子具有更高的光稳定性。使用荧光 - 偶极红外光谱（FDIR）和谐振离子 - 偶极红外光谱（IDIR）确定了分子基态和激发态的N - H伸缩频率，其值分别为3523和3467 cm。电子激发态中较低的值意味着该基团的光酸度增加。针对三种不同理论水平的弗兰克 - 康登模拟光谱对实验LIF/2C - R2PI光谱进行了对比分析。与基态实验检测到的对称模式相比，在B3LYP - D4/def2 - TZVPP水平计算的振动频率显示出最准确的预测。然而，在激发态，在B3LYP/def - SVP理论水平模拟的较低能量不对称模式与实验结果最为吻合。这很可能是由于在吡唑基环处观察到的畸变导致了不对称振动模式的出现。上述研究突出了在分子系统中通过额外的N插入来适当调整激发波长以及改变有机发色团光稳定性的可能性。
