Intraligand Charge Transfer in Pt(qol)2. Characterization of Electronic States by High-Resolution Shpol'skii Spectroscopy

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Abstract
Pt(qol)2 (qol- = 8-quinolinolato-O,N) is investigated in the Shpol'skii matrices n-heptane, n-octane-h18, n-octane-d18, n-nonane, and n-decane, respectively. For the first time, highly resolved triplet phosphorescence as well as triplet and singlet excitation spectra are obtained at T = 1.2 K by site-selective spectroscopy. This permits the detailed characterization of the low-lying singlet and triplet states which are assigned to result mainly from intraligand charge transfer (ILCT) transitions. The electronic origin corresponding to the 3ILCT lies at 15 426 cm-1 (FWHM ≈ 3 cm-1) exhibiting a zero-field splitting smaller than 1 cm-1, which shows that the metal d-orbital contribution to the 3ILCT is small. At T = 1.2 K, the three triplet sublevels emit independently due to slow spin−lattice relaxation (slr) processes. Therefore, the phosphorescence decays triexponentially with components of 4.5, 13, and 60 μs. Interestingly, two of the sublevels can be excited selectively, which leads to a distinct spin polarization manifested by a biexponential decay. At T = 20 K, the decay becomes monoexponential with τ = 10 μs due to a fast slr between the triplet sublevels. From the Zeeman splitting of the 3ILCT the g-factor is determined to be 2.0 as expected for a relatively pure spin triplet. The 1ILCT has its electronic origin at 18 767 cm-1 and exhibits a homogeneous line width of about 12 cm-1. This feature allows us to estimate a singlet−triplet intersystem crossing rate of about 2 × 1012 s-1. This relatively large rate compared to values found for closed shell metal M(qol)n compounds displays the importance of spin−orbit coupling induced by the heavy metal ion. Moreover, this small admixture leads to the relatively short emission decay times. All spectra show highly resolved vibrational satellite structures. These patterns provide information about vibrational energies (which are in good accordance with Raman data) and shifts of equilibrium positions between ground and excited states. These shifts are different in the 1ILCT and 3ILCT states. The vibrational satellite structures support the assignment of ILCT character to the lowest excited states.