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

Abstract

Pt(qol)(2) (qol(-) = 8-quinolinolato-O,N) is investigated in the Shpol'skii matrices n-heptane, n-octane-h(18), n-octane-d(18), 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 (3)ILCT lies at 15 426 cm(-)(1) (FWHM approximately 3 cm(-)(1)) exhibiting a zero-field splitting smaller than 1 cm(-)(1), which shows that the metal d-orbital contribution to the (3)ILCT 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 &mgr;s. 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 tau = 10 &mgr;s due to a fast slr between the triplet sublevels. From the Zeeman splitting of the (3)ILCT the g-factor is determined to be 2.0 as expected for a relatively pure spin triplet. The (1)ILCT 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 x 10(12) 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 (1)ILCT and (3)ILCT states. The vibrational satellite structures support the assignment of ILCT character to the lowest excited states.

Structural, Photophysical, and Electrophosphorescent Properties of Platinum(II) Complexes Supported by Tetradentate N2O2 Chelates

“…Because corresponding transitions appear at higher energies for Alq 3 and Rhq 3 (l max 388 and 425 nm respectively), a metal-perturbed, ligand-centered 1 (l 3 p*) (l lone pair/phenoxide) charge-transfer assignment was proposed [15] and later confirmed by Shpol×skii spectroscopic measurements. [19] Because the absorption maxima, bandshape, and intensity of the low-energy bands for Ptq 2 and especially 2 bear great resemblance, we tentatively assign the low-energy absorptions at 400 ± 500 nm in 1 and 2 to a 1 [l 3 p*(diimine)] CT transition mixed with Pt(d) 3 p*(diimine) 1 MLCT. Furthermore, like Ptq 2 , 1 and 2 also display negative solvatochromic shifts (the absorption of 2 at 24 500 cm À1 (408 nm) in C 6 H 6 /CH 2 Cl 2 (10:1) blue-shifts to 26 000 cm À1 (385 nm) in MeOH), and this is consistent with the polar character of the electron-rich oxygen/phenoxide fragment in the ground state.…”

“…At this juncture, it is appropriate to discuss the photophysical and electronic nature of the closely related bischelate Ptq 2 , which was first investigated in detail by Scandola et al [15] and more recently by Yersin et al [19] In DMF solution, Ptq 2 exhibits a low-energy band at 478 nm (e % 6900 mol À1 dm 3 cm…”

Characterization of excited electronic and vibronic states of platinum metal compounds with chelate ligands by highly frequency-resolved and time-resolved spectra

“…It is briefly mentioned that the situation for Pt(II) complexes with qol-and qtl-ligands is different, since in these compounds the lowest excited states result from intraligand-charge transfer transitions (3ILCT) with very small metal admixtures [ 195,315,318,319]. This class of compounds exhibits highly efficient red to infrared emissions.…”

“…J Resonance-enhanced Raman (RR) modes from [106,211]. protonated guests in comparison to protonated matrices (e.g., see [68,84,195] . It is extremely improbable that all these different dopants are statistically built into this matrix and have exactly the same symmetry and environment as the substituted host cation.…”

Photophysical Properties and OLED Applications of Phosphorescent Platinum(II) Schiff Base Complexes

“…2 + , with zero-field splittings of the lowest triplet state being 60, [17,18] 76, [28] and 211 cm À1 , [17,18] respectively. In contrast, complexes such as [PdA C H T U N G T R E N N U N G (thpy) 2 ] (thpy = 2-(2'-thienyl)pyridyl), [16,29] [Pd(q) 2 ] (q= 8-quinolinato), and [Pt(q) 2 ], [30] as well as [RhA C H T U N G T R E N N U N G (bpy) 3 ] 3 + [31] and [PtA C H T U N G T R E N N U N G (bpy) 2 ] 2 + [17,18] , are characterized by ZFS values significantly below 1 cm À1 . Thus, it is concluded that these complexes emit only from perturbed ligand-centered triplet states.…”