A theoretical investigation of substituent effects on the absorption and emission properties of a series of terpyridylplatinum(II) acetylide complexes

A comprehensive calculational investigation has been carried out on a series of complexes of the type [(terpyridyl-R-1)-Pt(C C-R-2)], where terpyridyl-R-1 is a series of substituted 2,2':6',2"-terpyridyl ligands and C C-R-2 is a series of substituted acetylide ligands. In one series of complexes (1), the energy of the electronic excited state is varied by changing the substituents on the terpyridyl ligand (R-1). In a second series of complexes (II), this electronic structure variation is obtained by changing the para substituents (R-2) of the acetylide ligand. The effect of varying the substituents on the lowest-energy excited states of the complexes has been assessed by calculating their electronic structures and excitation energies. We anticipated that introduction of electron-withdrawing substituents on the terpyridyl ligand will benefit the LLCT (or MLCT) and prohibit the nonradiative pathways via d-d transitions in these complexes; introduction of electron-donating substituents on the acetylide ligand can also prohibit the nonradiative pathways by increasing the energy gaps between the HOMO-LUMO and d-d transitions. The results also reveal that the lowest-energy excitations of all complexes of series I and IIa-b complexes are dominated by a pi(C C) ->pi*(terp) (LLCT) transition mixed with some energetically d pi(Pt)-> terpyridyl (MLCT) transition. However, for the complexes IIc-IId, in which phenyl rings are introduced on the acetylide ligand, the lowest-lying absorptions of IIc and IId are predominately LLCT in character, with less MLCT mixture, due to a lower contribution of the Pt(d) orbital to the HOMO, while for He, with a stronger donor on the acetylide, the lowest-lying absorption is completely LLCT in character. The absorption and emission calculations using the TDDFT method are based on the optimized geometries obtained at the B3LYP/LanL2DZ and CIS/LanL2DZ levels, respectively.