The pi-nature of a CF3 group can be understood through analysis of its bond orbitals (BOs) mixed into the pi-type molecular orbitals of CF3-substituted Ir(ppy)2MDPA+ complexes (ppy=2-phenyl-pyridine and MDPA=methylated 2,2′-dipyridyl amine). It has been found that, through this natural bond orbital analysis, the parent's molecular orbitals (MOs) can be stabilized by σ∗CF BO via negative hyperconjugation and, simultaneously, destabilized by electron lp(F) BO. Since these two competing pi-effects are virtually counterbalanced as indicated by the vanishing values of crystal orbital overlap populations, the chemical substitution strategy originated from lowering of HOMO by using this electron-withdrawing CF3 group has been found effective in color-tuning to blue region. Based on reduced shielding effect due to de- creased σ-electron density, the reported position dependent CF3-substitution effects on pi-type MOs can also be understood through HOMO/LUMO wavefunction analysis.
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