TUNGSTEN AND MOLYBDENUM OXO COMPLEXES OF TETRAKIS(PHENYLDIAZENYL)CALIX[4]ARENE SUBSTITUTED DERIVATIVES - EHMO CALCULATIONS, SPECTROSCOPIC CHARACTERIZATION, AND PERTURBATIONS OF THE PHOTOPHYSICAL PROPERTIES BY NEUTRAL GUEST MOLECULES

Bowl-shaped chromophores fabricated with tungsten and molybdenum oxo c omplexes of calix[4]arenes substituted at the para positions with phen yldiazenyl (R(2)C(6)H(3)N(2)) derivatives are investigated. The nature of the lowest energy excited states has been addressed theoretically using Extended Huckel Molecular Orbital (EHMO) calculations and experi mentally using UV-visible, luminescence, and polarized emission spectr oscopy at 77 K. The lowest energy excited states are intraligand (azob enzene fragment) pi pi (S-2 and T-2) and pi pi* states (S-1 and T-1). The compounds are weakly luminescent in the 600-630 nm range exhibiti ng emissions arising from the (1)(pi pi) states (S-2-->S-0, 1 < tau(e ) < 4 ns, and 3 x 10(-4) < phi < 3 x 10(-3)). The region located above 900 nm was not investigated. Evidence for guest-host chemistry has be en demonstrated by H-1-NMR spectroscopy, and the binding constants for two of the compounds with pyridine in toluene as a solvent were obtai ned by UV-visible spectroscopy. The presence of a guest molecule insid e the luminescent metal oxo complex of a calix[4]arene hosts decreases the emission lifetimes.