Synthesis and photophysics of 4′-R-2,2′;6′,2″-terpyridyl (R = Cl, CN, N(CH3)2) platinum(II) phenylacetylide complexes

Abstract

Platinum terpyridyl (tpy) phenylacetylide complexes with –Cl, –CN, and –NMe₂ substituents on the 4′-position of the tpy ligand were synthesized and characterized. Their photophysical properties were systematically investigated. In addition, theoretical electronic structure calculations using density functional theory (DFT) and time-dependent (TD-DFT) approaches were carried out for complexes 3 and 6; the results of these calculations provided additional information on the nature of the electronic structures of the low-lying electronic states of these complexes, including the electron density distribution and the composition of the frontier molecular orbitals. Complexes 4–6 exhibit moderately intense charge-transfer bands in the visible region, which are assigned to the ¹MLCT/¹LLCT transitions. In comparison to their corresponding chloride complexes 1–3, these charge-transfer bands become broadened and red-shifted. Complexes 4–6 emit at room temperature in CH₃CN and CH₂Cl₂ solutions and at 77 K in butyronitrile glassy solutions. At room temperature, the emission is tentatively attributed to ³MLCT for 4 and 5, and to a mixture of ³MLCT/³ILCT/³π,π* for 6. Due to the admixture of ³ILCT/³π,π* characters in its emitting state, 6 displays a much higher emission quantum yield and longer emission lifetime compared to 4 and 5. Replacing the chloride co-ligand in 1–3 by phenylacetylide co-ligand clearly enhances the emission and prolongs the lifetime. 3–6 exhibit a broad and moderately intense triplet excited-state absorption in the visible to the NIR region, with large excited-state absorption coefficients and moderately high triplet excited state quantum yields. Therefore, complexes 3–6, especially 6, have potential applications in organic light emitting devices (OLED) and as reverse saturable absorbing materials.