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pH dependency of the structural and photophysical properties of the atypical 2′,3-dihydroxyflavone

机译：非典型2'，3-二羟基黄酮结构和光物理性质的pH依赖性

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2′,3-Dihydroxyflavone (2′3HF) is a natural flavonol that has barely ever been studied, however the scarce studies of its physico-chemical properties have highlighted its atypical behaviour. We present a structural and spectral study of 2′3HF, performed using UV-visible absorption and fluorescence spectroscopies, coupled with DFT and TD-DFT calculations. Although its structure is close to that of 3-hydroxyflavone, 2′3HF shows a much lower p K _(a) value. We show that the origin of this particularity is the substitution by a hydroxyl group on position 2′, that induces a stronger inter-ring interaction weakening the bonding of the proton at position 3. The main absorption band of the is red-shifted upon deprotonation. The remaining proton is highly bonded in between oxygen atoms 3 and 2′, making the second deprotonation unattainable in methanol. The neutral form can undergo an excited-state intramolecular proton transfer to emit dual fluorescence by the normal and tautomer forms. We suggested five geometries to be the sources of the emission bands, and showed that the energy barriers to interconversions were almost null. The anion is also fluorescent. The Stokes shifts for the neutral normal and anion species are extremely high, that can be explained by the conformational rearrangement, as the species go from twisted in the ground-state, to planar in the excited-state. Finally, another emission band is evidenced when exciting in the vicinity of the absorption maximum of the anion species in acidic medium. We suggest an aggregate with the solvent to be the origin of the emission.
机译：2'，3-二羟基枯草（2'3HF）是一种勉强研究的天然黄酮，但其物理化学物质的稀缺研究突出了其非典型行为。我们提出了2'3HF的结构和光谱研究，使用UV可见的吸收和荧光光谱进行，与DFT和TD-DFT计算相结合。虽然其结构接近3-羟基硫酮，但2'3HF显示出低得多的Pκ_（a）值。我们表明，这种特殊性的起源是在位置2'上的羟基取代，使得较强的间环相互作用削弱质子的粘合在3.在去质子化后是红色移位的主体吸收带。剩余的质子在氧原子3和2'之间高度粘合，使得在甲醇中不可达到第二个去质子化。中性形式可以经过激发状态的分子内质子转移，通过正常和互变异构形式发射双重荧光。我们建议五个几何形状成为发射带的来源，并表明对互联的能量障碍几乎是无效的。阴离子也是荧光的。对于中性正常和阴离子物种的斯托克斯偏移非常高，可以通过构象重新排列来解释，因为物种从地面扭曲到兴奋状态下的平面。最后，当在酸性介质中阴离子物种的吸收最大值附近激发时，可以证明另一个发射带。我们建议将溶剂的骨料成为排放的起源。

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