Ushakou Dzmitryi, Józefowicz Marek
Institute of Experimental Physics, Faculty of Mathematics, Physics and Informatics, University of Gdańsk, str. Wita Stwosza 57, 80-308 Gdańsk, Poland.
Institute of Exact and Technical Sciences, Pomeranian University in Słupsk, str. Arciszewskiego 22d, 76-200 Słupsk, Poland.
Phys Chem Chem Phys. 2024 Oct 2;26(38):25029-25047. doi: 10.1039/d4cp02493a.
To gain a more comprehensive understanding of the phenomenon of high-fluorescence anisotropy of the normal form emission of ESIPT-active compounds in protic solvents, excitation wavelength dependence of emission anisotropy was investigated for 3-hydroxyflavone (3HF) using steady-state spectroscopic technique and quantum chemical calculations. It was shown for the first time that the anisotropy of 3HF normal form emission is characterized by significant dependence on excitation energy. Experimental results indicate that the fluorescence anisotropy of 3HF in methanol (at 20 °C) changes abruptly from about 0.18 to about 0.10 with a decrease in excitation wavelength. This spectroscopic phenomenon can be explained by two factors: (1) breaking of intermolecular solute-solvent hydrogen bonds upon photoexcitation and (2) excitation of ESIPT-active fluorophores to the second singlet state (S). The results of quantum chemical calculations clearly indicate that specific hydrogen bonding solvation interactions can lead to the formation of 3HF-methanol complexes with larger molecular volumes than the volume of free 3HF molecule. High excitation energy can reform and break solute-solvent bonds, which leads to a decrease in molecular system volume. This results in a decrease in rotational correlation time and fluorescence anisotropy. As is known, the fluorescence lifetime of small-sized molecules is closely correlated with the conformational changes in the excited state, and in the case of ESIPT-active compounds, the lifetime of normal form emission is almost fully determined by the ultrafast ESIPT process. Therefore, although in general, fluorescence lifetime is considered independent of excitation energy, but because the timescale of ESIPT processes is of the same order as the timescale of internal conversion, in the case of ESIPT-active compounds, fluorescence lifetime changes caused by high-energy state excitation cannot be neglected. The emission anisotropy of the normal form of an ESIPT-active compound will decrease with an increase in fluorescence lifetime caused by an increase in excitation energy sufficient to excite molecules to higher electronic states. In this work, both hypotheses are discussed and verified using experimental data and quantum chemical calculations for 3HF in methanol.
为了更全面地理解质子溶剂中ESIPT活性化合物正常形式发射的高荧光各向异性现象，使用稳态光谱技术和量子化学计算研究了3-羟基黄酮（3HF）发射各向异性的激发波长依赖性。首次表明，3HF正常形式发射的各向异性显著依赖于激发能量。实验结果表明，甲醇中3HF（20℃）的荧光各向异性随着激发波长的减小从约0.18突然变化到约0.10。这种光谱现象可以由两个因素解释：（1）光激发时分子间溶质-溶剂氢键的断裂；（2）ESIPT活性荧光团激发到第二单重态（S）。量子化学计算结果清楚地表明，特定的氢键溶剂化相互作用可导致形成比游离3HF分子体积更大的3HF-甲醇复合物。高激发能量可重新形成并破坏溶质-溶剂键，导致分子体系体积减小。这导致旋转相关时间和荧光各向异性降低。众所周知，小分子的荧光寿命与激发态的构象变化密切相关，对于ESIPT活性化合物，正常形式发射的寿命几乎完全由超快ESIPT过程决定。因此，虽然一般来说，荧光寿命被认为与激发能量无关，但由于ESIPT过程的时间尺度与内转换的时间尺度相同，对于ESIPT活性化合物，高能态激发引起的荧光寿命变化不可忽视。ESIPT活性化合物正常形式的发射各向异性将随着由足以将分子激发到更高电子态的激发能量增加而引起的荧光寿命增加而降低。在这项工作中，使用甲醇中3HF的实验数据和量子化学计算对这两种假设进行了讨论和验证。
