Tetrapalladium(II) Bisazobenzene and Azoazoxybenzene Complexes: Syntheses, Electronic Structures, and Optical Properties

Three new tetranuclear, chloride-bridged Pd-II complexes have been prepared by using 1,4-bis[(E)-phenylazo]benzenes (BPABs) and an azoazoxybenzene derivative. The latter was obtained unexpectedly by reacting N,N-dimethy-1-4-nitrosoaniline with 2,3,5,6-tetramethylphenylenediamine in glacial acetic acid. Characterization includes H-1 NMR. and UV-vis spectroscopies, and single-crystal X-ray structures have been obtained for two of the proligands and one complex As expected, the methyl groups on the central aryl rings cause severe departures from planarity. The proligands show an intense UV absorption (lambda(max) approximate to 350 nm) due to pi -> pi* intramolecular charge transfer, together with weaker n -> pi* bands in the visible region. The complexes display an intense visible band (lambda(max) = 449-475 nm) and multiple near-UV absorptions. While the electronic absorption spectra of the BPAB complexes are quite similar, the azoazoxybenzene complex shows a doubly intense visible band. Dichroic ratios (DRs) measured in two different liquid crystal (LC) hosts show that complexation is not beneficial, as noted for dinuclear azobenzene complexes previously. The shape of the new complexes appears to give relatively poor alignment within the LC. Time-dependent density functional theory reveals differing origins of the low-energy transitions for BPAB and azoazoxybenzene complexes, those for the latter involving charge transfer within the N=N(O)C6H3NMe2 moieties. The origin of the low-energy transition of the BPAB complex also differs from that in the related dinuclear complexes. The directions of the mu(12) vectors calculated with respect to the long molecular axis generally correlate with the observed DR values, with better alignment for the low-energy transitions that are relatively more dichroic.