Characterization of the Lowest Excited States of [Rh(bpy-h(8))(n)(bpy-d(8))(3-n)](3+) by Highly Resolved Emission and Excitation Spectra

Ligand Mediated Luminescence Enhancement in Cyclometalated Rhodium(III) Complexes and Their Applications in Efficient Organic Light-Emitting Devices

A series of luminescent cyclometalated rhodium(III) complexes have been designed and prepared. The improved luminescence property is realized by the judicious choice of a strong σ-donor cyclometalating ligand with a lower-lying intraligand (IL) state that would raise the d-d excited state and introduction of a lower-lying emissive IL excited state. These complexes exhibit high thermal stability and considerable luminescence quantum yields as high as up to 0.65 in thin film, offering themselves as promising light-emitting materials in OLEDs. Respectable external quantum efficiencies of up to 12.2% and operational half-lifetimes of over 3000 h at 100 cd m^-2 have been achieved. This work demonstrates a breakthrough as the first example of an efficient rhodium(III) emitter for OLED application and opens up a new avenue for diversifying the development of OLED materials with rhodium metal being utilized as phosphors.

Theoretical Studies of Phosphorescence Spectra of Tris(2,2‘-bipyridine) Transition Metal Compounds

Phosphorescence spectra of tris(2,2'-bipyridine) metal compounds, [M(bpy)3]n+, where M = Zn(II), Ru(II), Os(II), Rh(III), and Ir(III), were calculated using a harmonic oscillator approximation of adiabatic potential surfaces obtained by density functional theory (DFT). Using the Huang-Rhys (S) factors calculated by theoretical Franck-Condon analysis of T1 and S0 geometries, we successfully reproduced the emission spectra observed under various conditions by nonempirical calculations. The simulations of well-structured spectra of the Zn(II), Rh(III), and Ir(III) compounds confirmed that the emission originated from localized ligand-centered excited states with considerably distorted geometries of C2 symmetry. The spectrum simulation revealed that the phosphorescence state of [Ru(bpy)3]2+ was localized 3MLCT both in a solution and a glass matrix. Furthermore, a highly resolved phosphorescence spectrum observed for [Ru(bpy)3]2+ doped in a [Zn(bpy)3](ClO4)2 crystal was reproduced well using the geometry of the localized 3MLCT by assuming mode-specific broadening of low-frequency intramolecular vibrational modes. The deuterium effects of the electronic origins of the doped crystal observed by Riesen et al. were in excellent agreement with those predicted for the localized 3MLCT. However, the calculated satellite structures of the localized 3MLCT involving bpy-h8 in [Ru(bpy-h8)(3-x)(bpy-d8)x]2+ (x = 1,2) exhibited only the bpy-h8 vibrational modes, inconsistent with the simultaneous appearance of both bpy-h8 and bpy-h8 modes in the observed spectra. A simulation on the basis of the geometry of the delocalized 3MLCT was in reasonable agreement with an unresolved spectrum observed for a neat crystal of [Ru(bpy)3](PF6)2, which is inconsistent with the assignments of localized 3MLCT on the basis of the electronic origins. The inconsistency of the assignment on the basis of the adiabatic model is discussed in terms of vibronic coupling between the localized 3MLCT states. The 3MLCT state in [Os(bpy)3]2+ seems to vary with the environment: a fully localized 3MLCT in a solution, partially localized in a glass matrix, and delocalized in PF6 salts.

Intraligand charge transfer in the Pd(II) oxinate complex Pd(qol)2. Site-selective emission, excitation, and optically detected magnetic resonance

The first spectroscopic investigation of Pd(qol)2 (qol- = 8-quinolinolato-N,O = oxinate) dissolved in an n-octane matrix (Shpol'skii matrix) is reported. Application of several spectroscopic methods at liquid helium temperatures (typically, T = 1.2 K), such as site-selective and highly resolved luminescence and excitation spectroscopy, time-resolved emission spectroscopy, optically detected magnetic resonance, microwave recovery, phosphorescence microwave double-resonance, and magnetic fields, allows us to characterize the lowest excited electronic states in detail. In accord with previous assignments for the related Pt(qol)2 it is shown that these lowest states represent-intraligand charge-transfer states, namely, 1ILCT and 3ILCT. The electronic origin of the 1ILCT state lies at 20,617 cm(-1) (site A). It exhibits a nearly homogeneous line width with a half-width of about 80 cm(-1) (fwhm), which corresponds to a lifetime of tau(1ILCT) approximately equals 2 x 10(-13) s. This value is even shorter than that found for Pt(qol)2, presumably due to intersystem crossings and relaxations to dd* states. The electronic origin of the 3ILCT state lies at 16 090 cm(-1) (site A), and its zero-field splittings (zfs) into three sublevels are 2E = 2356 MHz (0.0785 cm(-1)) and D - E = 5241 MHz (0.175 cm(-1)). The emission decay times of the three sublevels are determined as tauI = 90 +/- 30 ms, tau(II) = 180 +/- 10 mus, and tau(II) = 80 +/- 10 mus. (Slightly different values are found for a second site B at 16,167 cm(-1).) From the small values of zfs and the long emission decay times it is concluded that metal-d or MLCT admixtures to 3ILCT are very small. This result clearly reflects the ligand-centered character of the transition. The assignment as an ILCT transition is supported by the occurrence of relatively strong vibrational satellites of Pd-N and Pd-O character in highly resolved emission spectra. Although the transition is ascribed to a charge-transfer process, the geometry changes between the ground state and 3ILCT are very small. The results found for Pd(qol)2 are compared to those of companion studies of Pt(qol)2 and Pt(qtl)2 (qtl- = 8-quinolinethiolato-N,S).

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

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.