High Resolution Laser Induced Fluorescence Spectroscopy
Introduction
Molecular structures in the electronic ground state can be determined via a large variety of well established techniques like microwave spectroscopy, X-ray diffraction, neutron diffraction, or NMR spectroscopy. Geometries of electronically excited states are much more difficult to obtain. In our group rotationally resolved fluorescence spectroscopy is applied to extract the rotational constants of the molecule under investigation. Further information can be gather from the spectra, like centrifugal distortion constants, orientation of the transition dipole moment, or life time of the excited state. Other complementary techniques, that also yield structural information of electronically excited states are the resonant ionization variant of rotationally resolved vibronic spectroscopy, which has been pioneered in the group of Neusser in Munich, and rotational coherence spectroscopy, introduced by Felker, and improved considerably in the group of Brutschy and Riehn in Frankfurt. A resolution of 1 part in 108 over the whole UV range is necessary to rotationally resolve the spectra of large molecules.
Experimental
The experimental setup for the rotationally resolved laser induced fluorescence (LIF) consists of a single frequency ring dye laser pumped with either with 6 W of the 514 nm line of an Ar+ laser or with 7 W of the 515 nm line of a diode pumped cw Yb:YAG laser (ELS MonoDisk-515). The light is coupled into an external folded ring cavity for second harmonic generation (SHG). The molecular beam is formed by expanding a mostly heated sample seeded typically in 200 - 1000 mbar of argon through a 100 - 300 µm hole into the vacuum. The molecular beam machine consists of three differentially pumped vacuum chambers which are linearly connected by two skimmers (1 mm and 3 mm, respectively) in order to reduce the Doppler width. The molecular beam is crossed at right angles in the third chamber with the laser beam 360 mm downstream of the nozzle. The resulting fluorescence is collected perpendicularly to the plane defined by laser and molecular beam by an imaging optics setup consisting of a concave mirror and two plano-convex lenses. The resulting Doppler width in this setup is 15 MHz (FWHM). In some experiment the Doppler width was 25 MHz, because of a slightly different arrangement of the optical system. The integrated molecular fluorescence is detected by a photo multiplier tube whose output is discriminated and digitized by a photon counter and transmitted to a PC for data recording and processing. The relative frequency is determined with a calibrated quasi conical Fabry-Perot interferometer with a free spectral range (FSR) of about 150 MHz . The absolute frequency is determined by recording the iodine absorption spectrum and comparing the transitions to the tabulated lines.
Example: Phenol-water
The phenol-water cluster serves as a model system for intermolecular hydrogen bonding between a polar aromatic and a solvent molecule.
The analysis of the spectrum shows a splitting into two subbands due to the internal motion of the water moiety. The barrier for this torsion could be determined to be 180 cm-1 in the electronic ground and 130 cm-1 in the electronically excited state. From an analysis of the rotational constants, the structure could be determined to be translinear, with a slight shortening of the hydrogen bond upon electronic excitation.
G. Berden, W. Leo Meerts, M. Schmitt and K. Kleinermanns High Resolution UV spectroscopy of phenol and the hydrogen bonded phenol-water cluster J. Chem. Phys. 104, 972 (1995).
Example: Phenol-methanol
The hydrogen bonded phenol-methanol cluster is an interesting model system to study the balance between hydrogen bonding of the hydroxy groups of phenol and methanol, respectively and the van der Waals interactions between the methyl group and the aromatic ring, which both determine the structure of the cluster. Since three rotational constants are not sufficient to fully characterize the intermolecular structure, we measured the rovibronic spectra of several isotopomers. Below, the rotationally resolved LIF-spectra of three different isotopomers, differently deuterated in the phenol and the methanol moiety are presented, which have been analyzed using the genetic algorithm automated fitting technique. Further details can be found in: Determination of the structures and barriers to hindered internal rotation of the phenol-methanol cluster in the S0 and S1 state; Chem. Phys. 254 (2000) 349 - 361.
Example: The phenol dimer
The phenol dimer is an ideal model system to study the very sensitive equilibrium between hydrogen bonding and dispersion interaction. The rotationally resolved UV spectra of the electronic origins of five isotopomers of the phenol dimer have been measured. The complex spectra are analyzed using a genetic algorithm based fitting strategy. From the inertial parameters, the intermolecular geometry parameters have been determined for both electronic states and compared to the results of ab initio calculations. In the electronic ground state a larger hydrogen bond length than in the ab initio calculations is found and a smaller tilt angle of the aromatic rings, showing a more pronounced dispersion interaction. In the electronically excited state the hydrogen bond length decreases, as has been found for other hydrogen bonded clusters of phenol and the two aromatic rings are tilted less toward each other. Further details can be found in: 
Schmitt, M., Böhm, M., Ratzer, C., Krügler, D., Kleinermanns, K., Kalkman, I., Berden, G. and Meerts, W. L.: Electronic excitation in the phenol dimer: The intermolecular structure in the S0 and S1 state determined by rotationally resolved electronic spectroscopy. ChemPhysChem accepted (2006)
Example: The benzonitrile dimer
The rotationally resolved UV spectrum of the electronic origin of the benzonitrile dimer has been measured and analyzed using a genetic algorithm based fitting strategy. For the electronic ground state a C2h symmetric structure is found in which the permanent dipole moments of the benzonitrile monomers are aligned anti-parallel. The orientation of the transition dipole moment could be shown to be parallel to the orientation in the monomer moiety. The distance between the two monomer moieties decreases slightly upon electronic excitation and the symmetry of the benzonitrile dimer changes from C2h in the electronic ground state to Cs in the electronically excited state.This break of symmetry is probably caused by the local excitation of only one benzonitrile moiety in the cluster due to the weak electronic coupling between the cluster moieties. Further details can be found in: Schmitt, M., Böhm, M., Ratzer, C., Siegert, S., van Beek, M. and Meerts, W. L.: Electronic excitation in the benzonitrile dimer: The intermolecular structure in the S0 and S1 state determined by rotationally resolved electronic spectroscopy. J. Mol. Struc. accepted (2006).
Example: Azaindole
7-azaindole (7AI) has been subject to numerous experimental and theoretical studies in the last decades, mainly because the 7-azaindole dimer serves as a model system for tautomeric processes in DNA base pairs. Furthermore, 7-azaindole is the chromophore of amino acid analog 7-azatryptophan, that is used to replace tryptophan in proteins. The Figure below shows the high resolution LIF spectrum of the electronic origin of 7AI at 34630.74 cm-1. The spectrum was automatically assigned using a genetic algorithm (GA) based fit described in [1] - [3].
[1] Meerts, W. L., Schmitt, M. und Groenenboom, G.: New applications of the Genetic Algorithm for the interpretation of High Resolution Spectra. Can. J. Chem. 82 (2004), 804.
[2] Meerts, W. L. and Schmitt, M.: A new automated assign and analyzing method for high resolution rotational resolved spectra using Genetic Algorithms. Phys. Scripta 73 (2005), C47.
[3] Schmitt, M., Ratzer, C., Kleinermanns, K. und Meerts, W. L.: Determination of the structure of 7-azaindole in the electronic ground and excited state using high resolution ultra-violet spectroscopy and an automated assignment based on a genetic algorithm. Mol. Phys. 102 (2004), 1605.
Example: Tryptamine
Tryptamine itself, but even more its analogs serotonin (5-hydroxytryptamine) and melatonin (5-methoxy-N-acetyl-tryptamine) are known as neurotransmitters. Their conformational stabilities and
preferences are of great importance for quantitative structure-activity relationships in neurotransmitter receptor interactions. Since all biological processes take place in an aqueous environment, the interaction with a defined number of solvent molecules is of great interest. The question arises how many water molecules are necessary to lock the large variety of energetically accessible conformations to the biologically active one(s). The spectrum shown below is one of the most impressive example for the power of the genetic algorithm based automated assignment. Eight overlapping isotopomers of the tryptamine B conformer have been fit simultaneously using the genetic algorithm automated technique. Further details can be found in: Schmitt, M., Böhm, M., Ratzer, C., Vu, C., Kalkman, I. and Meerts, W. L.: Structural selection by microsolvation: conformational locking of tryptamine. J. Am. Chem. Soc. 127 (2005), 10356.
