Department of Chemistry Biology and Biotechnology, University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy.
Department of Chemistry Biology and Biotechnology, University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy.
Spectrochim Acta A Mol Biomol Spectrosc. 2019 May 5;214:522-530. doi: 10.1016/j.saa.2019.02.057. Epub 2019 Feb 19.
This paper reports for the first time a detailed spectroscopic investigation into the ground- and excited-state properties of α-amino-orcein (α-AO), one of the main components of the orcein dye, in solvents of different proticity and water at different pHs. In order to gain insight into the nature of the involved transitions and excited state deactivation pathways, the study was carried out by means of UV-Visible steady state and ultrafast spectroscopic techniques with the support of quantum mechanical calculations (DFT and TDDFT). The results highlight that the photophysical and photodynamic behaviour of α-AO are highly sensitive to the solvent proticity and pH. In particular, protic environment induces a red shift (55 nm) of the absorption spectrum together with a relevant decrease of the fluorescence quantum yield (from 0.19 in acetonitrile to 6.6 × 10 in methanol) and radiative rate constant (two orders of magnitude). A notable red shift is also caused by increasing the pH leading the molecule from monocationic to neutral and then monoanionic form through two deprotonation steps (pKa = 3.539 ± 0.006 and 11.180 ± 0.006). Following deprotonation, the molecule assumes spectral and photophysical properties very similar to those retrieved in protic media. The observed behaviour has been rationalized through the occurrence of hydrogen bonding, likely involving to a greater extent the carbonyl oxygen of α-AO and the protic solvent, that favours the charge delocalization on the whole chromophore as well as fast non-radiative excited state deactivation. The ultrafast spectroscopic investigation revealed in fact the presence, in protic solvent, of a short living component (tens of picoseconds), assignable to solvent complexed S state, alongside the long living component (few nanoseconds) observed in aprotic media and attributed to the solvent free S state. The results achieved in this study for α-AO provides an important contribution to the interpretation of absorption and fluorescence features of orcein dye mixture in more complex systems (protein based substrates within the many aspects of the cultural heritage and biomedical field) where hydrogen bonds are expected to play a crucial role in mediating the interaction with the environment.
本文首次详细研究了α-氨基胭脂红（α-AO）在不同质子性溶剂和不同 pH 值的水中的基态和激发态性质。为了深入了解涉及的跃迁和激发态失活途径的本质，本研究采用紫外-可见稳态和超快光谱技术，并辅以量子力学计算（DFT 和 TDDFT）进行了研究。结果表明，α-AO 的光物理和光动力行为对溶剂质子性和 pH 值高度敏感。具体而言，质子性环境会导致吸收光谱红移（55nm），同时荧光量子产率（从乙腈中的 0.19 降低到甲醇中的 6.6×10-3）和辐射速率常数（两个数量级）显著降低。随着 pH 值的增加，分子从单阳离子形式变为中性形式，然后变为单阴离子形式，也会导致明显的红移，这是通过两个去质子化步骤（pKa=3.539±0.006 和 11.180±0.006）实现的。去质子化后，分子呈现出与在质子溶剂中非常相似的光谱和光物理性质。通过氢键的发生，观察到的行为得到了合理的解释，这可能涉及到α-AO 的羰基氧和质子溶剂更大程度的相互作用，有利于整个发色团的电荷离域以及快速的非辐射激发态失活。超快光谱研究实际上表明，在质子溶剂中存在一个短寿命成分（几十皮秒），可归因于溶剂络合的 S 态，而在非质子溶剂中观察到长寿命成分（几纳秒），可归因于无溶剂的 S 态。本研究中对α-AO 的研究结果为解释胭脂红染料混合物在更复杂体系（文化遗产和生物医学领域的许多方面的基于蛋白质的底物）中的吸收和荧光特征提供了重要贡献，在这些体系中，氢键有望在介导与环境的相互作用中发挥关键作用。
