Solvatochromism of pyranine-derived photoacids.
Solvatochromism of pyranine-derived photoacids.
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DOI：
10.1039/c3cp53082e
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发表时间：
2013-10
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影响因子：
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通讯作者：
Christian Spies;Björn Finkler;Nursel Acar;Gregor Jung
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作者：
Christian Spies;Björn Finkler;Nursel Acar;Gregor Jung
光酸性经常出现在芳香醇中，其平衡解离常数在电子激发下增加几个数量级。在这项研究中，我们研究了最近合成的超光酸家族及其基于芘的甲基化对应物的溶剂致色性。一方面，这些分子的化学相似性和它们不同的光酸性（pKa*值在-0.8和-3.9之间）使我们能够深入了解激发态质子转移的机制。本研究分别使用了Lippert-Mataga、Kamlet-Taft和Catalán三种不同的溶剂量表，得到了一致的结果。我们发现激发态酸度与激发态的双极性（介电常数范围从-1775 cm（-1）到-2500 cm(-1)）之间的最强相关性，以及伴随的永久偶极矩的变化大约为14 Debye。由于溶剂碱度的变化而引起的光谱变化，探测溶质的共轭性质，被发现较少指示激发态酸度的梯度，即bem值在-700和-1200 cm（-1）之间。溶剂酸度是唯一对去质子化产物的电子谱有明显影响的参数。aem ~ 400 cm（-1）的值较aabs和aexc小3-4倍，表明这两种物质在激发态下的碱度较低。由半经验理论计算触发，两个最低激发态之间的能量分裂可能与激发态酸度有关，并指出由取代基引起的局部激发态和电荷转移态电子混合的改变。本文还讨论了三倍带负电的吡氨酸与新型中性光酸的区别。
Photoacidity is frequently found in aromatic alcohols where the equilibrium dissociation constant increases by some orders of magnitude upon electronic excitation. In this study we investigated the solvatochromism of a family of recently synthesized super-photoacids and their methylated counterparts based on pyrene. The chemical similarity of these molecules on the one hand and their differing photoacidity with pKa* values between -0.8 and -3.9 on the other allow for gaining insights into the mechanisms contributing to excited-state proton transfer. Three different solvent scales, namely Lippert-Mataga, Kamlet-Taft and Catalán, were independently employed in this study and gave consistent results. We found the strongest correlation of the excited-state acidity with the dipolarity of the excited state, pem ranging from -1775 cm(-1) to -2500 cm(-1), and a concomitant change in the permanent dipole moment of roughly 14 Debye. Spectral changes due to varying basicity of the solvent, which probes the conjugated property of the solute, are found to be less indicative of the graduation of excited-state acidity, i.e. bem values between -700 and -1200 cm(-1). The solvent acidity is the only parameter with a distinct influence on the electronic spectra of the deprotonated species. The low values of aem ~ 400 cm(-1), which are 3-4× smaller than aabs and aexc, indicate the low basicity of these species in the excited state. Triggered by semiempirical theoretical calculations, the energetic splitting between the two lowest excited states could be related to the excited-state acidity and points to alterations in the electronic mixing of locally excited and charge-transfer states, caused by the substituents. Differences between the threefold negatively charged pyranine and the new neutral photoacids are also discussed.