Blanco-Rodríguez Ana Maria, Ronayne Kate L, Zalis Stanislav, Sýkora Jan, Hof Martin, Vlcek Antonín
School of Biological and Chemical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, United Kingdom.
J Phys Chem A. 2008 Apr 24;112(16):3506-14. doi: 10.1021/jp710442v. Epub 2008 Mar 29.
Excited-state dynamics of [Re(Etpy)(CO)3(bpy)]+ was studied in three imidazolium ionic liquids by time-resolved IR and emission spectroscopy on the picosecond to nanosecond time scale. Low-lying excited states were characterized by TD-DFT calculations, which also provided molecular dipole moment vectors in the relevant electronic states. TRIR spectra in ionic liquids show initial populations of two excited states: predominantly bpy-localized 3IL and 3MLCT, characterized by nu(CO) bands shifted to lower and higher frequencies, respectively, relative to the ground state. Internal conversion of 3IL to the lowest triplet 3MLCT occurred on a time scale commensurate with solvent relaxation. The nu(CO) IR bands of the 3MLCT state undergo a dynamic shift to higher wavenumbers during relaxation. Its three-exponential kinetics were determined and attributed to vibrational cooling (units of picoseconds), energy dissipation to the bulk solvent (tens of picoseconds), and solvent relaxation, the lifetime of which increases with increasing viscosity: [EMIM]BF4 (330 ps) < [BMIM]BF4 (470 ps) < [BMIM]PF6 (1570 ps). Time-resolved phosphorescence spectra in [BMIM]PF6 show a approximately 2 ns drop in intensity due to the 3IL --> 3MLCT conversion and a dynamic Stokes shift to lower energies with a lifetime decreasing from 1.8 ns at 21 degrees C to 1.1 ns at 37 degrees C, due to decreasing viscosity of the ionic liquid. It is proposed that solvent relaxation predominantly involves collective translational motions of ions. It drives the 3IL --> 3MLCT conversion, increases charge reorganization in the lowest excited-state 3MLCT, and affects vibrational anharmonic coupling, which together cause the dynamic shift of excited-state IR bands. TRIR spectroscopy of carbonyl-diimine complexes emerges as a new way to investigate various aspects of solvation dynamics, while the use of slowly relaxing ionic liquids offers new insight into the photophysics of Re(I) carbonyl polypyridyls.
通过时间分辨红外光谱和发射光谱，在皮秒到纳秒时间尺度上研究了[Re(Etpy)(CO)3(bpy)]+在三种咪唑鎓离子液体中的激发态动力学。通过TD-DFT计算对低激发态进行了表征，该计算还提供了相关电子态中的分子偶极矩矢量。离子液体中的TRIR光谱显示出两个激发态的初始布居：主要是bpy局域化的3IL和3MLCT，其特征在于相对于基态，ν(CO)带分别向更低和更高频率移动。3IL到最低三重态3MLCT的内转换发生在与溶剂弛豫相当的时间尺度上。3MLCT态的ν(CO)红外带在弛豫过程中经历向更高波数的动态位移。确定了其三重指数动力学，并将其归因于振动冷却（皮秒量级）、向本体溶剂的能量耗散（几十皮秒）和溶剂弛豫，溶剂弛豫的寿命随着粘度增加而增加：[EMIM]BF4（330皮秒）<[BMIM]BF4（470皮秒）<[BMIM]PF6（1570皮秒）。[BMIM]PF6中的时间分辨磷光光谱显示，由于3IL→3MLCT转换，强度下降约2纳秒，并且由于离子液体粘度降低，动态斯托克斯位移向更低能量，寿命从21℃时的1.8纳秒降至37℃时的1.1纳秒。有人提出，溶剂弛豫主要涉及离子的集体平移运动。它驱动3IL→3MLCT转换，增加最低激发态3MLCT中的电荷重组，并影响振动非谐耦合，这共同导致激发态红外带的动态位移。羰基二亚胺配合物的TRIR光谱成为研究溶剂化动力学各个方面的一种新方法，而使用缓慢弛豫的离子液体为Re(I)羰基多吡啶的光物理提供了新的见解。
