The broad, structureless band, large Stokes' shift, and large red shift from the monomer absorption bands are typical features of exciplex bands, because they indicate a very large displacement of the excited state relative to the ground state. Also, exciplexes are usually characterized by a charge transfer nature. We assign this new, broad, structureless band around 432 nm as due to a heterogeneous cluster containing Pt(CN)42- and Cu(CN)2- ions and having a charge transfer nature. The sharp decrease and disappearance of the strong characteristic bands of Cu(CN)2- aqueous solutions at 379nm and the characteristic bands of Pt(CN)42- aqueous solutions at about 406 and 482 nm after mixing indicates evidence of the breakdown of the original homogeneous clusters of [CuI]2 and [PtII]2; further, the appearance of a new peak around 426~438 nm after mixing indicates the formation of d8 and d10 heterogeneous CuI-PtII metal-metal nanoclusters at room temperature. The luminescence intensity is linearly dependent on oligomer concentration. Plots of [CuI]n[PtII]m vs. stoichiometric ratio for different possible values of m and n should give the empirical formula of the mixed metal oligomer when compared to the experimental plot of oligomer concentration vs. stoichiometric ratio. The values n = 2 and m = 3 provide the best match, in terms of both curvature and overall change in luminescence intensity from ratio = 0.3 to ratio = 0.8, to the experimental curve.
The CuI-PtII exciplex exhibits greater efficiency of energy transfer with Tb3+ acceptor ions in comparison to Cu(CN)2--Tb3+ or Pt(CN)42--Tb3+ aqueous solutions at room temperature.