Electronic spectroscopy and structures of the van der Waals complexes of α,ω-dihaloalkanes with anthracene Original Research Article

Anthracene has been coexpanded in helium with the α,ω-dichloroalkanes of carbon chain length n=1–6,8 and with the α,ω-dibromoalkanes of chain length n=3–5 in a supersonic jet. Van der Waals complexes of 1:1 and 1:2 stoichiometry are observed and their microscopic solvent shifts and excited state van der Waals vibrational progressions have been measured by laser induced fluorescence spectroscopy. The experimental results, together with Lennard-Jones calculations of the ground state potential energy surfaces, suggest that the 1:1 complexes exhibit structures and dynamic behaviour which are different when n is even compared with when n is odd. The most stable 1:1 complexes have structures in which the odd n dihaloalkanes are aligned parallel to the long axis of the substrate, whereas those of n=2,4,6 lie across the ring and those with n=2,4 have C2 symmetry. Isomers are found in the 1:2 complexes; only 1,2-dichloroethane forms a symmetric (1|1) complex; the other dihaloalkanes form (2|0) complexes preferentially. Evidence of intermolecular electronic quenching was found in the dibromoalkane complexes, and this was confirmed by steady-state fluorescence quenching measurements in solution.