Photoelectron spectroscopy study of substituent effect of halogens on the thiophene ring

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The UV photoelectron spectra of 21 halogen (chloro- bromo- and iodo ) -substituted thiophenes have been studied. The assignment was based on band shapes, correlation of band positions of related compounds and CNDO/S calculations. In the case of compounds bearing iodine in the three and four positions the above considerations gave contradictory results and only the relative intensities in the HeI and HeII spectra provided decisive proof for the assignment. The substituent effect in the α positions differs considerably from that in the β position. To explain these effects a simple conjugative model was proposed. Not only does the achieved assignment fit the correlation along the chloro-bromo-iodo series well, but it also provides a simple method for constructing the electronic structure of polyhalogenated systems from simpler building blocks of molecules with a lower degree of substitution (e.g. the band structure of the tetrachloro derivative from those of 2,5-dichloro- and 3,4-dichloro-thiophenes). This proves that a consistent picture was obtained of the electronic structure of halogen thiophenes.