Effect of the size of the chelate ring on the stability of cyclic exciplexes between the cadmium(5 3P1) atom and diamines

Abstract

The cadmium(5 ³P₁)-sensitized luminescence of some aliphatic diamines [R₂N(CH₂)_nNR₂; R = H or CH₃, n= 1–6] has been studied under the conditions of steady-state illumination at temperatures between 493 and 609 K. It was found that the diamines of n= 2, 3, 4, and 5 have two emission bands (wavelengths at the peaks of these bands are ca. 450 and 550 nm), while the diamines of n= 1 and 6 have only a short-wavelength band. These two bands were assigned to two kinds of 1 : 1 exciplex (cyclic and acyclic ones) between an excited cadmium atom and a diamine molecule. From the temperature dependence of the equilibrium constants between two exciplexes, the enthalpy and entropy changes of the equilibrium were evaluated. The stability of the cyclic exciplexes assigned to the long-wavelength band was discussed in view of the ring size of cyclic exciplexes.