Photochemistry of Phenyl Halides

We have studied fundamental aspects of photo-induced dissociation kinetics and dynamics in several phenyl halides. By combining femtosecond pump-probe measurements with ab initio calculations we are able to account for several observations. In mixed phenyl halides, the dissociation kinetics is found to be dependent on the nature, the number, and the position of the substituents, and also on the excitation wavelength. A surprisingly large reduction in the dissociation time constant, compared to that of bromobenzene (~30 ps), is observed when having two or more fluorine atoms. For example, in bromopentafluorobenzene a subpicosecond time constant is obtained. This can be explained by a significant lowering of the repulsive potential energy curves (PEC) along the C-Br bond. However, several of the experimental results cannot be accounted for by one-dimensional PECs. Therefore, we suggest a refined model for the dissociation, in which the excited states of the same spin multiplicity are coupled by employing multidimensional potential energy surfaces. This model has been explicitly evaluated by quantum dynamics simulations in the case of 3-BrFPh, and it seems to be capable of capturing the main features in the measured kinetics. Thereby we are also able to clarify the role of spin-orbit coupling in these molecules.


1 Introduction
2 Excited states of phenyl halides
2.1 Molecular orbitals
2.2 The concept of potential energy surfaces
2.3 Selection rules for transitions between electronic states
2.3.1 The role of spin multiplicity
2.3.2 The role of symmetry
2.4 Electronic states
2.4.1 Naming the electronic states
2.4.2 Optically active electronic states
2.4.3 Electronic states observed in ab initio calculations
2.5 The faith of the excited electronic states
2.5.1 Radiative processes
2.5.2 Non-radiative processes
3 Experimental methodology
3.1 Studying ultra-fast processes, the pump-probe method
3.2 Measurement of the excited state lifetimes in phenyl halides
3.2.1 Experimental setup
3.2.2 Long pulse fs-laser system
3.2.3 Long pulse light modification arrangement
3.2.4 Molecular beam apparatus
3.2.5 Long pulse data acquisition system
3.2.6 Short pulse fs-laser system
3.2.7 Short pulse light modification arrangement
3.2.8 Short pulse data acquisition system
3.2.9 External control of the experimental setup
3.3 Polarization
3.4 Analyzing the experimental data
3.4.1 First order kinetics
3.4.2 Consecutive kinetics
3.4.3 Data analysis
4 Photochemistry of phenyl halides
4.1 Previous work, monohalide phenyls
4.1.1 Photolysis of the carbon halogen bond
4.2 Polyhalide phenyls
4.2.1 Experimental results
4.2.2 Substituent effects
4.3 Channel one, 2 ps to 40 ps
4.3.1 1,3,5-triBrPh, Br-3,5-diClPh and Br-3,5-diFPh
4.3.2 Out-of-plane bending and spin-orbit effects
4.3.3 Dihalid phenyls
4.3.4 Photodissociation dynamics of 3-BrFPh
4.4 Channel two, 1 ps to 15 ps
4.5 Channel three, subpicosecond
4.6 Reinvestigating the photochemistry of chlorobenzene
5 Concluding remarks
6 Acknowledgements
7 References

Author: Karlsson, Daniel

Source: Uppsala University Library

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