Related Experiment Video
Updated: Jul 15, 2026
10:35
Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals
Published on: May 29, 2018
Using internal coordinates to describe photoinduced geometry changes in MLCT excited states
Mark R Waterland1, Sarah L Howell, Keith C Gordon
1Institute of Fundamental Sciences, Massey University, Private Bag 11 222, Palmerston North, New Zealand. M.Waterland@massey.ac.nz
The Journal of Physical Chemistry. A
|May 10, 2007
Summary
This study analyzes the metal-to-ligand charge-transfer (MLCT) transition in a rhenium compound (RePQX) using resonance Raman spectroscopy. It reveals significant photoinduced geometry changes in the rhenium coordination sphere and ligand structure upon excitation.
Area of Science:
- Inorganic Chemistry
- Photochemistry
- Computational Chemistry
Background:
- Metal-to-ligand charge-transfer (MLCT) transitions are crucial in photochemistry.
- Understanding photoinduced geometry changes is key to controlling excited-state dynamics.
- Rhenium polypyridyl complexes are important in catalysis and light-emitting applications.
Purpose of the Study:
- To investigate the photoinduced geometry changes of the rhenium compound RePQX during its MLCT transition.
- To correlate resonance Raman spectral features with specific vibrational modes and structural rearrangements.
- To elucidate the electronic and structural consequences of photoinduced electron transfer in metal polypyridyl systems.
Main Methods:
- Resonance Raman intensity analysis of the MLCT transition in RePQX.
- Density Functional Theory (DFT) calculations for ground-state properties and Raman spectra.
- Time-Dependent Density Functional Theory (TD-DFT) for excited-state geometry optimization.
- Analysis using vibrational normal modes and redundant internal coordinates.
Main Results:
- Normal modes nu(37) (rhenium coordination sphere distortion) and nu(75) (ligand skeletal stretch) exhibit the largest photoinduced geometry changes.
- A single carbonyl mode is enhanced in the resonance Raman spectra.
- Excited-state calculations reveal displacements in all CO bond lengths, Re-C, and C-C ligand bond lengths.
- Photoinduced electron transfer significantly affects bond angles and torsional coordinates, primarily within the rhenium coordination sphere.
Conclusions:
- Internal coordinates provide a clearer picture of geometry changes during photoinduced electron transfer compared to normal modes alone.
- The study provides insights into the vibronic coupling and structural dynamics following MLCT excitation in rhenium complexes.
- The findings contribute to a fundamental understanding of electron transfer processes in metal polypyridyl systems.
Related Concept Videos
01:17Chemical Shift: Internal References and Solvent Effects
In an NMR sample, precise measurement of the absolute absorption frequencies of nuclei is difficult. A standard internal reference compound is added, and the frequency difference between the reference signal and sample signals is measured.
The internal reference compound generally used in NMR spectroscopy is tetramethylsilane (TMS). TMS is preferred because it is chemically inert, soluble in NMR solvents, and easily removable. Also, the highly shielded methyl protons in TMS yield an intense...
The internal reference compound generally used in NMR spectroscopy is tetramethylsilane (TMS). TMS is preferred because it is chemically inert, soluble in NMR solvents, and easily removable. Also, the highly shielded methyl protons in TMS yield an intense...
01:16UV–Vis Spectroscopy: Molecular Electronic Transitions
In Ultraviolet–Visible (UV–Vis) spectroscopy, the absorption of electromagnetic radiation is used to probe the electronic structure of molecules. This technique provides insights into molecular electronic transitions, particularly the movement of electrons between different molecular orbitals. Radiation is absorbed if the energy of the electromagnetic radiation passing through the molecule is precisely equal to the energy difference between the excited and ground states. During this process,...
01:26Photochemical Electrocyclic Reactions: Stereochemistry
The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
Selection Rules: Photochemical Activation
01:02¹H NMR: Interpreting Distorted and Overlapping Signals
Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are slanted or...
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are slanted or...
01:27Inductive Effects on Chemical Shift: Overview
The protons in unsubstituted alkanes are strongly shielded with chemical shifts below 1.8 ppm. Methine, methylene, and methyl protons appear at approximately 1.7, 1.2 and 0.7 ppm, while the proton signal from methane appears at 0.23 ppm. An electronegative substituent, such as chlorine, withdraws the electron density from the protons, increasing their chemical shift. Progressive substitution of the hydrogens in methane by chlorine shifts the proton signals increasingly downfield, to 3.05 ppm in...
02:36Molecular Geometry and Dipole Moments
The VSEPR theory can be used to determine the electron pair geometries and molecular structures as follows: