The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs

Triplet emitter materials present attractive possibilities for optimizations of organic/organometallic light emitting diodes (OLEDs) This is due to the significantly higher efficiencies obtainable with these compounds ascompared to organic emitters In this contribution, first a schematic introduction is given, how an OLED device is built-up and why multi-layer structures are preferred Then a basic model is presented, how electron-hole recombination, ie the exciton formation process, can be visualized and how the singlet and triplet states of the (doped) emitter compounds are populated This takes place by specific singlet and triplet paths The occurrence of such paths is explained by taking into account that the dynamical process of exciton trapping involves dopant-to-matrix charge transfer states ( 1  3 DMCT states) It is also explained, why the excitation energy is harvested in the lowest triplet state of organo-transition-metal complexes Due to spin statistics, one can in principle obtain an efficiency of a factor of four higher than using organic singlet emitter molecules Simple comparisons suggest that electron-hole recombination should preferentially occur on the triplet emitter itself, rather than on matrix molecules with subsequent energy transfer to the emitter Further, it is pointed out that essential photophysical properties of organometallic triplet emitters depend systematically on the metal participation in the triplet state and on the effective spin-orbit coupling These factors control the amount of zero-field splitting (ZFS) of the triplet state into substates Increase of ZFS corresponds to higher metal character in the triplet state Higher metal character reduces the energy difference between excited singlet and triplet states, enhances the singlet-triplet intersystem crossing rate, lowers the emission decay time, changes the vibrational satellite structure, decreases the excited state reorganization energy, etc These effects are discussed by referring to well characterized compounds Based on a new ordering scheme presented for triplet emitter materials, a controlled development of compounds with pre-defined photophysical properties becomes possible

Triplet emitters for OLED applications. Mechanisms of exciton trapping and control of emission properties

Micellar solutions as reaction media

High-resolution optical spectroscopy of organometallic triplet emitters reveals detailed insights into the lowest triplet states and the corresponding electronic and vibronic transitions to the singlet ground state. As case studies, the blue-light emitting materials Pt(4,6-dFppy)(acac) and Ir(4,6-dFppy)2(acac) are investigated and characterized in detail. The compounds’ photophysical properties, being markedly different, are largely controlled by spin–orbit coupling (SOC). Therefore, we study the impact of SOC on the triplet state and elucidate the dominant SOC and state-mixing paths. These depend distinctly on the compounds’ coordination geometry. Relatively simple rules and relations are pointed out. The combined experimental and theoretical results lead us towards structure-efficiency rules and guidelines for the design of new organic light emitting diode (OLED) emitter materials.

Organometallic Pt(II) and Ir(III) Triplet Emitters for OLED Applications and the Role of Spin–Orbit Coupling: A Study Based on High-Resolution Optical Spectroscopy

Two homologous organometallic compounds, Pd(2-thpy)2 and Pt(2-thpy)2 (with 2-thpy-=2-(2-thienyl)-pyridinate, structure formulae in Fig. 1), are chosen for case studies of photophysical properties of the lowest excited states. The triplets of these two representative compounds are marked by differences of nearly two orders of magnitude in metal/MLCT (metalto-ligand charge transfer) character. Determination of detailed photophysical properties of both compounds is possible, since highly resolved spectra are obtained when the compounds are dissolved in an n-octane matrix (Shpol’skii matrix), when the measurements are carried out at low temperature (typically at T = 1.3 K), and when modern techniques of laser spectroscopy are applied. In addition, methods of time-resolution and of microwave double-resonance, such as optically detected magnetic resonance (ODMR), microwave recovery, and phosphorescence microwave-double resonance (PMDR) are used. In particular, it is shown that with increasing metal character of the triplet, that is when Pd(2-thpy)2 is compared to Pt(2-thpy)2, many properties change characteristically and in part by orders of magnitude. For example, the following properties will be addressed: Transition probabilities, emission decay times, zero-field splittings (zfs), processes of spin-lattice relaxation (slr), intersystem crossing rates, intrastate relaxation rates, excited state binding properties as compared to those of the electronic ground state, anharmonicity effects, metal-mediated ligand-ligand coupling or spatial extensions of the excited state wavefunctions. Moreover, we focus on spin-selectivity in the vibrational satellite structures of the emission spectra as identified by the complementary methods of time-resolved emission and PMDR spectroscopy. We also discuss radiative deactivation processes, such as spin-vibronic Herzberg-Teller and Frank-Condon activities. Further, we specify sub-picosecond relaxation paths on the basis of micro-second time resolution by applying for the first time the method of time-resolved excitation spectroscopy to transition metal complexes. It is further demonstrated that the size of zero-field splitting of the triplet state can be used as an ordering parameter, that reflects the metal participation in the lowest triplet. Thus, one can relate Pd(2-thpy)2 and Pt(2-thpy)2 to a larger number of other compounds, such as [Rh(bpy)3]3+, [Pt(bpy)2]2+, Pt(qol)2, [Pt(mnt)2]2-, [Ru(bpy)3]2+, [Os(phen)3]2+, [Os(bpy)3]2+, etc. (compare Fig. 1 and Table 11) and one obtains a series that demonstrates chemical tunability of photophysical properties. — For several specific subjects, we present the basic background information in order to make the paper more easily readable, also for non-specialists.

https://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Physikalische_Chemie/Yersin/topics_2001.pdf

Low-Lying Electronic States and Photophysical Properties of Organometallic Pd(II) and Pt(II) Compounds. Modern Research Trends Presented in Detailed Case Studies

CIDNP and CIDEP studies on intramolecular hydrogen abstraction reaction of polymethylene-linked xanthone and xanthene. Determination of the exchange integral of the intermediate biradicals

Triplet mechanism of CIDNP in the photolysis of benzoquinones as studied by dynamic nuclear polarization

Zero-field splittings of the emitting ligand-ligand charge-transfer (LLCT) states were observed for Zn(4-CH{sub 3}O-PhS){sub 2}(phen) and Zn(4-Cl-PhS){sub 2}(phen). The spin-axis-independent zero-field-splitting parameters D{sup *} are 1.63 GHz for Zn(4-CH{sub 3}O-PhS){sub 2}(phen) and 2.25 GHz for Zn(4-Cl-PhS){sub 2}(phen). The measurements confirm the triplet nature of the emitting state at low temperature. The calculated splittings due to spin dipole-spin dipole interaction are much smaller than the experimental values. Mixing with the phenanthroline-localized {sup 3}{sub {pi}{pi}}{sup *} state is suggested to account for the large measured values of D{sup *}.

Zero-field splitting of the ligand-ligand charge-transfer emitting manifold of zinc(II) complexes

Photoinduced electron transfer from the zinc tetraphenylporphyrin moiety to the viologen unit in linked compounds in reversed micelles generated radical pairs, which afforded chemically induced dynamic electron polarization (CIDEP) spectra at room temperature. The CIDEP spectra varied with the number of methylene groups (n) in the alkyl spacer of the linked compounds. In the case of longer spacers (n = 6 and 8), common E/A/E/A patterns were observed throughout the measurement (0.7-5 [mu]s). The signal pattern could be simulated by a formula for the well-known spin-correlated radical pair mechanism, and the corresponding exchange coupling (J) was evaluated ([minus]0.4 mT). In the case of ZP4V, the CIDEP signals rapidly changed with elapsed time (0.5-3 [mu]s) from the absorptive to emissive mode. The observation was explained on the basis of a strong contribution of the triplet mechanism in the photogeneration of the radical pair in the early period of measurement. 15 refs., 2 figs.

Time-resolved electron spin resonance spectra of linked radical pairs on laser excitation of zinc tetraphenylporphyrin-viologen systems

The intramolecular hydrogen abstraction reaction in a series of polymethylene-linked xanthone and xanthene systems is investigated from the viewpoint of the magnetic field effect of the low-field chemically induced dynamic nuclear polarization (CIDNP) spectra. The magnetic field effect can be successfully simulated with the stochastic Liouville equation, and the effective exchange integral J between the two terminal radicals is determined from the peak of the magnetic field dependence. The effective J values thus determined are much larger than those obtained by the CIDEP method

Determination of the exchange integral by the low-field CIDNP in intramolecular hydrogen abstraction reaction of polymethylene-linked xanthone and xanthene

Tohru Azumi

Papers

CIDNP and CIDEP studies on intramolecular hydrogen abstraction reaction of polymethylene-linked xanthone and xanthene. Determination of the exchange integral of the intermediate biradicals

Triplet mechanism of CIDNP in the photolysis of benzoquinones as studied by dynamic nuclear polarization

Zero-field splitting of the ligand-ligand charge-transfer emitting manifold of zinc(II) complexes

Time-resolved electron spin resonance spectra of linked radical pairs on laser excitation of zinc tetraphenylporphyrin-viologen systems

Determination of the exchange integral by the low-field CIDNP in intramolecular hydrogen abstraction reaction of polymethylene-linked xanthone and xanthene