Stockett M H, Houmøller J, Brøndsted Nielsen S
Department of Physics and Astronomy, Aarhus University, Aarhus, Denmark.
J Chem Phys. 2016 Sep 14;145(10):104303. doi: 10.1063/1.4962364.
Nile blue is used extensively in biology as a histological stain and fluorescent probe. Its absorption and emission spectra are strongly solvent dependent, with variations larger than 100 nm. The molecule is charged due to an iminium group, and it is therefore an obvious target for gas-phase ion spectroscopy. Here we report the absorption and emission spectra of the mass-selected bare ions isolated in vacuo, and based on our results we revisit the interpretation of solution-phase spectra. An accelerator mass spectrometer was used for absorption spectroscopy where the absorption is represented by the yield of photofragment ions versus excitation wavelength (action spectroscopy). The luminescence experiments were done with a newly built ion trap setup equipped with an electrospray ion source, and some details on the mass selection technique will be given which have not been described before. In vacuo, the absorption and emission maxima are at 580 ± 10 nm and 628 ± 1 nm. These values are somewhat blue-shifted relative to those obtained in most solvents; however, they are much further to the red than those in some of the most non-polar solvents. Furthermore, the Stokes shift in the gas phase (1300 cm(-1)) is much smaller than that in these non-polar solvents but similar to that in polar ones. An explanation based on charge localization by solvent dipoles, or by counterions in some non-polar solvents, can fully account for these findings. Hence in the case of ions, it is nontrivial to establish intrinsic electronic transition energies from solvatochromic shifts alone.
尼罗蓝在生物学中被广泛用作组织学染色剂和荧光探针。其吸收光谱和发射光谱强烈依赖于溶剂，变化幅度超过100纳米。该分子因一个亚胺离子基团而带电，因此是气相离子光谱的一个明显研究对象。在此我们报告了在真空中分离出的质量选择裸离子的吸收光谱和发射光谱，并基于我们的结果重新审视了对溶液相光谱的解释。使用一台加速器质谱仪进行吸收光谱研究，其中吸收由光碎片离子产率相对于激发波长来表示（作用光谱）。发光实验是在一个新搭建的配备电喷雾离子源的离子阱装置上进行的，文中将给出一些此前未描述过的质量选择技术细节。在真空中，吸收和发射最大值分别在580±10纳米和628±1纳米处。这些值相对于在大多数溶剂中获得的值有一定程度的蓝移；然而，它们比在一些最非极性溶剂中的值更偏向红色。此外，气相中的斯托克斯位移（1300厘米⁻¹）比在这些非极性溶剂中的要小得多，但与在极性溶剂中的相似。基于溶剂偶极或某些非极性溶剂中的抗衡离子导致的电荷局部化的解释，能够完全说明这些发现。因此，对于离子而言，仅从溶剂化显色位移来确定本征电子跃迁能量并非易事。
