Platinated azulenes: Structures and electronic spectroscopy

Eight PtII-azulenes sigma-complexes were synthesized with the aim of understanding the effect of metalation on the electronic structures, spectroscopy and luminescence of the organic chromophore. Trans-[PtII(PEt3)2(X)]+ (Et = ethyl, X = Br- or I- ) groups were attached to the 3-position or 1,3-positions of azulene to form two mononuclear PtII-azulene complexes (1a and 1b) and two binuclear complexes (2a and 2b). Coordination of trans- [PtII(PEt3)2(X)]+ group(s) to 1-bromo-3-ethynylazulene or 1,3-diethynylazulene led to two mononuclear (3a and 3b) and two binuclear (4a and 4b) Pt-acetylide complexes.Molecular structures of the complexes were characterized by X-ray diffraction and NMR spectroscopy. The electronic structures of the complexes were elucidated by DFT calculations which showed small contributions of the Pt 5d pi orbitals in the HOMOs of the metalated complexes, whereas the LUMOs have no contribution from the Pt orbitals. Attaching the trans-[PtII(PEt3)2(X)]+ fragment directly to azulene in 1a,b and 2a,b destabilizes the HOMOs, LUMOs and LUMO+1 s to different extents. However, the bonding of the trans-[Pt(PEt3)2(X)(C---C)]-group to azulene in 3a,b and 4a,b destabilizes its HOMO, but stabilizes the LUMO and LUMO+1. The extent of orbital perturbations increases with the number of Pt-substituents, but do not depend on the halide ligands X. UV-vis absorption spectra of the complexes show red-shift of azulene-centered absorption bands with the S0 -> S2 transition more red-shifted than the S0 -> S1 transition. As a result, the "window" between the transitions is red -shifted and reduced to-8548 cm-1 in the case of 4a and 4b (c.f. 12,000 cm-1 in azulene). Despite reduction of the energy gap between the S1 and S2 excited states in all the platinated-azulene complexes, the complexes still violate Kasha's rule, displaying S2 -> S0 fluorescence in solutions which are red-shifted with respect to the fluo-rescence of azulene.