Determination of the Dipole Moment of the Charge-Shift Triplet Exciplex from zinc phthalocyanine tetrasulfomorpholide and p-methoxy-benzenediazonium tetrafluoroborate

Triplet exciplexes are known to have usually relatively low polarities, in contrast to singlet exciplexes. It is shown that triplet quenching rate constants of the title system can be used to determine the exciplex dipole moment if it is (plausibly) assumed that the dissociation rate constants of charge-shift exciplexes only depend on the viscosity but not on the polarity of the solvent. The quenching rate constants kq of the title system in various acetonitrile/acetone or acetonitrile/ethylacetate mixtures are determined and treated by an adapted McRAE equation developed for UV-VIS absorption spectroscopic data. The dipole moment of p-methoxy-benzenediazonium tetrafluoroborate necessary for the solution of the McRAE equation is estimated by a Hammett correlation of known data which gives by extrapolation μ(p-MeOC6H4N2⊕BF4⊖) = 31.4 × 10−30 Cm (9.4 Debye). Using this value and a cavity radius of 3.7 × 10−10m (cp. [1]) μex3(ZnPcTSM…p-MeO–C6H4N2⊕BF4⊖)* = 16.0 × 10−30 cm (4.8 Debye) is obtained. According to this value the triplet exciplex studied has a lower polarity than the starting diazonium salt.