Nitrogen-Based Delocalized and Heterocyclic Ligands

Although the photophysical and photochemical properties of platinum complexes with nitrogen-based heterocyclic ligands has received less attention than has ruthenium (Chapter 5), some recent studies have shown that a chemistry for PtX2(L-L) similar to Ru(bpy)3 has the potential of being developed. For complexes of platinum(II), three classifications of complexes can be categorized based on the nature of their emitting states. The three types of emissive states are, n-n, dn-n (MLCT), or complexes that emit from metal-metal (M-M ) excited states.  

Another class of compounds that have n-n excited states and act as good sensitizers for photoredox reactions are the luminophores Pt(CN)2(L-L) (where L-L is an aromatic diimine). An example of such a compound is Pt(CN)2(5,5 -Meibpy), whose excited n-n triplet is both a powerful one-electron oxidant ( 1.0 V) and reductant pP -1.5 V vs. AgNOa/Ag). The compound undergoes oxidative quenching with methyl viologen [Eq. (3.13)], and reductive quenching withiV,A/,iV, iV -tetramethyl-p-phenylenediamine [Eq. (3.14)]  

The excited state n- n triplet is also quenched by alkyl halides k 10 M s ) to give Pt(CN)2(5,5 -Me2bpy) and RI by an electron transfer pathway 

For the analogue compound Pt(CN)2(4,7-Ph2-1,10-phen) it has been shown that the interligand n- n excited state is deactivated both by emission and by nonradioac-tive deactivation. With increasing concentration of the complex, association to an excimer takes place, with the excimer emitting at lower energies. The occurrence of this excimer is believed to be due to the formation of a metal-metal bond between the axial coordination positions of a planar ground state complex and the pseudoaxial coordination position of the distorted planar excited state. 

The complex Pt(NH3)2(bpy) also has a tt — n triplet excited state, which results in the observation of a structured phosphorescence at 488 nm that is closely 

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