Dereka Bogdan, Vauthey Eric
Department of Physical Chemistry , University of Geneva , 30 Quai Ernest-Ansermet , CH-1211 Geneva 4 , Switzerland . Email:
Chem Sci. 2017 Jul 1;8(7):5057-5066. doi: 10.1039/c7sc00437k. Epub 2017 May 16.
The fluorescence quenching of organic dyes H-bonding interactions is a well-known phenomenon. However, the mechanism of this Hydrogen-Bond Induced Nonradiative Deactivation (HBIND) is not understood. Insight into this process is obtained by probing in the infrared the O-H stretching vibration of the solvent after electronic excitation of a dye with H-bond accepting cyano groups. The fluorescence lifetime of this dye was previously found to decrease from 1.5 ns to 110 ps when going from an aprotic solvent to the strongly protic hexafluoroisopropanol (). Prompt strengthening of the H-bond with the dye was identified by the presence of a broad positive O-H band of , located at lower frequency than the O-H band of the pure solvent. Further strengthening occurs within a few picoseconds before the excited H-bonded complex decays to the ground state in 110 ps. The latter process is accompanied by the dissipation of energy from the dye to the solvent and the rise of a characteristic hot solvent band in the transient spectrum. Polarization-resolved measurements evidence a collinear alignment of the nitrile and hydroxyl groups in the H-bonded complex, which persists during the whole excited-state lifetime. Measurements in other fluorinated alcohols and in chloroform/ mixtures reveal that the HBIND efficiency depends not only on the strength of the H-bond interactions between the dye and the solvent but also on the ability of the solvent to form an extended H-bond network. The HBIND process can be viewed as an enhanced internal conversion of an excited complex consisting of the dye molecule connected to a large H-bond network.
有机染料的氢键相互作用导致的荧光猝灭是一种众所周知的现象。然而，这种氢键诱导的非辐射失活（HBIND）的机制尚不清楚。通过在红外波段探测带有氢键接受氰基的染料发生电子激发后溶剂的O-H伸缩振动，得以深入了解这一过程。先前发现，当从非质子溶剂转变为强质子性的六氟异丙醇时，这种染料的荧光寿命从1.5纳秒降至110皮秒。通过位于比纯溶剂的O-H波段更低频率处的六氟异丙醇宽正O-H波段的出现，确定了与染料形成的氢键迅速增强。在激发的氢键复合物在110皮秒内衰减至基态之前的几皮秒内，进一步增强发生。后一过程伴随着能量从染料耗散到溶剂以及瞬态光谱中特征性热溶剂波段的出现。偏振分辨测量证明了氢键复合物中腈基和羟基的共线排列，这种排列在整个激发态寿命期间持续存在。在其他氟化醇以及氯仿/混合物中的测量表明，HBIND效率不仅取决于染料与溶剂之间氢键相互作用的强度，还取决于溶剂形成扩展氢键网络的能力。HBIND过程可被视为由连接到大型氢键网络的染料分子组成的激发复合物的增强型内转换。
