Intersystem crossing rate dependent dual emission and phosphorescence from cyclometalated platinum complexes: a second order cumulant expansion based approach

Abstract

Rates of intersystem crossing (k_ISC) of two platinum(II) complexes containing acetylacetonate (acac) and extended cyclometalated ppy (Hppy = 2-phenylpyridine) (1) and thpy (Hthpy = 2-(2^' thienyl)pyridine) (2) ligands are calculated using the Condon approximation to the Golden Rule and employing the second-order cumulant expansion method. The emission wavelengths obtained at the RI-CC2 level for the lowest excited singlet (S₁) and triplet (T₁) states of the two complexes are well in agreement with the experimental results. Our analysis based on k_ISC evinces that the major pathway involved with the phosphorescence process in complex 1 arises from the S₁ → T₂ intersystem crossing while the S₁ → T₁ intersystem crossing is the key step towards the commencement of dual emission in complex 2. Furthermore, it is found that the different pathways are mostly guided by two factors namely, the energy gap and the spin–orbit interaction between the concerned states. Interestingly, the calculated k_ISC for complex 1 is found to be 10⁷ times larger than that of complex 2, which suggests a rapid depletion of the S₁ state population vis-à-vis radiative emission only by phosphorescence from the internally converted lowest excited triplet state while for complex 2, the relatively lower k_ISC is attributed to the dual emission from this complex.