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

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 (MLCT)-M-1/(LLCT)-L-1 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 (MLCT)-M-3 for 4 and 5, and to a mixture of (MLCT)-M-3/(ILCT)-I-3/(3)pi,pi* for 6. Due to the admixture of (ILCT)-I-3/(3)pi,pi* 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.