Ground and excited state properties of polyamine chains bearing two terminal naphthalene units

A series of compounds bearing two naphthalene units linked through methylene groups to both ends of different open-chain polyamines has been investigated. The fluorescence emission studies show the presence of an excimer species whose formation depends on the protonation state and length of the polyamine chains and implies the existence of a bending movement in the excited state allowing the two naphthalene units to approach and interact. This interpretation was clearly proven by time-resolved fluorescence with the appearance of double exponential decays with a rise time observed at the excimer emission wavelength. For comparison purposes one bis-chromophoric compound containing a rigid chain, and two mono-chromophoric analogs bearing a single terminal naphthalene unit were studied. Their emission spectra show a unique band and the fluorescence decays, are single exponential at pH values where just one species is present in the ground state, showing that in these cases no excimer or exciplex species is formed. The kinetics of excimer formation was used to evaluate the influence of the length and the pH in the flexibility of the polyamine chain.