A 24-Crown-8 Bearing Naphthalene and Naphthoquinone Chromophores

Abstract

The novel bichromophoric crown ether N24C8NQ 3 has been synthesised in good yield by the conjugate addition of a polyether diol 1 to 2,3-dichloronaphthoquinone in the presence of base. Crown ether 3 was further reacted with malononitrile in the presence of TiCl4 to prepare the tetracyano-1,4-napthoquinodimethane derivative 4. Electrochemical and photophysical studies on 3 and 4 have been undertaken. The addition of alkali metal cations to the crown ether 3 causes significant changes in the first and second reduction potentials of the naphthoquinone (NQ) chromophore. In the case of the first process, a correlation exists between the reversible potential and cation fit. Crown ethers 3 and 4 exhibit red-shifted absorption bands attributed to intramolecular charge transfer interactions between the chromophores. Fluorescence from the naphthalene chromophore in 3 and 4 is quenched substantially (>80%) as compared to the model 5. The weak nature of the complex between 3 and the guest dibenzylammonium hexafluorophosphate (K a < 50 M− 1) was compared to another electron deficient crown, tetranitrodibenzo-24-crown-8, the results of which suggest that the oxygens within the crown ether macrocycle, bearing electron withdrawing chromophores, play an important role in stabilising such pseudorotaxane complexes.