Big Molecules
Overview
Electronically excited molecules play pivotal roles in many chemical and biochemical reactions because the nuclei move after the electron is excited. For example, when retinal (in the eye) absorbs light it undergoes a trans to cis isomerization which distorts the protein in which it resides and sends a nerve impulse to the brain. Electronic spectroscopy is sensitive to molecular structure, and conversely, provides detailed information on how the molecule changes.
Objectives
In general terms the objective is to explore how molecular structure changes with electronic excitation. We have studied a range of halogenated benzene species and all three isomers of binaphthyl to find out how the interaction between the chromophores change with their separation and electronic state.
How the Experiment is Done
The electronic spectroscopy of large molecules is vastly simplified if the molecule can be cooled. This removes "hot bands" from the spectrum and the fewer the number of transitions, the easier the spectrum is to assign. We use a supersonic free jet to cool molecules to temperatures of ~1-5 K while still in the gas phase. This ensures that all molecules are in their lowest possible energy levels (the ground states) and free from interference from solvent or matrix effects that plague solution or solid spectroscopy. A laser crosses the supersonic jet and electronically excites the molecule. The ensuing fluorescence is detected and a spectrum obtained as the laser is tuned.
An Example: Spectroscopy of 2,2’-binaphthyl
Binaphthyl is two naphthalene molecules joined by a single C-C bond. There are three isomers of binaphthyl that depend on where the C-C linkage is for each naphthalene. The molecule at right is 2-2’-binaphthyl (2,2-BN). Naphthalene is, of course, a prototypical aromatic molecule with all 11 C-C bonds conjugated. One of the aims of this work was to explore whether the conjugation stretched across the inter-ring C- C bond. The way that we test whether the central bond is conjugated is by measuring vibrational frequencies that involve stretching or bending about the central bond, and whether the molecule is flat across the central bond. A fully conjugated BN would be flat, whereas a single bond would allow the two naphthalenes to rotate wherever they like to minimise steric overlap.
What we measured was a laser induced fluorescence spectrum as shown in the figure below. It is a spectrum rich in structure. The numbered peaks represent a single progression involving the torsional vibration about the central bond. This tells us that the ground and excited electronic states have very different structures in the torsion coordinate. A fit of a trial torsional potential energy function to the data reveals that the molecule is twisted about 30 degrees in the ground state, but completely planar in the excited state. The vibrational frequencies involving the central bond also increase in frequency. One of the vibrations, called the "butterfly" mode, is shown above. In this vibration the two naphthalenes bend out of plane together like a butterfly flapping its wings. This information, which we have supported by ab initio calculations, indicates that in the excited state 2,2-BN is a completely aromatic, fully conjugated species across the whole four rings, including the central bond. In the ground state, the two rings are completely separate. The pi-orbitals of one naphthalene do not communicate with the pi-orbitals of the other one.
The downstream application of research such as this is to understand the mechanism of electron transfer. When one part of a molecule is excited, how is this excitation (energy) passed to another part of the molecule? In BN, the excitation of one naphthalene can be passed to the other because the excitation creates a totally conjugated system.
Selected References