Abstract
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.
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- 1.
For Pt(4,6-dFppy)(acac) in a PMMA film, a distinctly higher quantum yield of 40% is found. This can be explained with a destabilization of quenching dd* states in the rigid PMMA host compared to fluid solutions.
- 2.
Librations are hindered rotational modes of the doped complex in its matrix cage.
- 3.
The radial average of the function \( \xi ({r_A}) \) \( {\hbar^2} \) can be written as \( hc\zeta (A) \) [119].
- 4.
This rule is frequently addressed in the discussion of intersystem crossing (ISC) and SOC between 1(ππ*) and 3(ππ*) states of purely organic molecules (compare [113]). For these molecules, SOC is very weak. In this case, the matrix elements as expressed in (12) involve equal p-orbitals that are located on one C atom. The angular momentum operator acts on a p-orbital by rotating it by 90°. Thus, a matrix element of two orthogonal p-orbitals results, and this integral vanishes. A corresponding situation is discussed below by use of the example shown in Fig. 12b.
- 5.
It is remarked that for a quantitative description, a significant LC (ππ*) contribution cannot be ignored (e.g., compare [110]).
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Acknowledgments
The Bundesministerium für Bildung und Forschung (BMBF) is gratefully acknowledged for providing the funding of our research. We thank Prof. Dr. Mark E. Thompson (University of Southern California) for a fruitful cooperation with respect to the studied compounds.
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Appendix
Appendix
Here, we discuss how Rule E is related to the fact that angular momentum operators are purely imaginary. It further depends on the fact that, without magnetic field, the spatial orbitals are real. It is claimed that the diagonal matrix elements of the angular momentum operators vanish. For instance, in the case of l z , we have for any real orbital \( \chi = \chi (\vec r) \)
since l z is Hermitian. On the other hand, performing the complex conjugation explicitly, we have
where \( {{\rm d}}\Omega = \sin \vartheta {{\rm d}}\vartheta {{\rm d}}\varphi \) is the usual surface element in spherical coordinates \( r,\vartheta, \varphi \) and the integration is over all space. Combining the two equations yields that the matrix element can only be zero.
This proves the assertion, since the other components of \( \vec l \) can be treated similarly.
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Rausch, A.F., Homeier, H.H.H., Yersin, H. (2010). 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. In: Lees, A. (eds) Photophysics of Organometallics. Topics in Organometallic Chemistry, vol 29. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3418_2009_6
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