Probing the excited state properties of the highly phosphorescent Pt(dpyb)Cl compound by high-resolution spectroscopy

Detailed photophysical studies of the emitting triplet state of the highly phosphorescent compound Pt(dpyb)Cl based on high-resolution optical spectroscopy at cryogenic temperatures are presented {dpyb = N^C²^N-coordinated 1,3-di(pyridylbenzene)}. The results reveal a total zero-field splitting of the emitting triplet state T₁ of 10 cm⁻¹ and relatively short individual decay times for the two higher lying T₁ substates II and III, while the decay time of the lowest substate I is distinctly longer. Further evidence for the assignment of the T₁ substates is gained by emission measurements under high magnetic fields. Distinct differences are observed in the vibrational satellite structures of the emissions from the substates I and II, which are dominated by Herzberg-Teller and Franck-Condon activity, respectively. At T = 1.2 K, the individual spectra of these two substates can be separated by time-resolved spectroscopy. For the most prominent Franck-Condon active modes, Huang-Rhys parameters of S ca. 0.1 can be determined, which are characteristic of very small geometry rearrangements between the singlet ground state and the triplet state T₁. The similar geometries are ascribed to the high rigidity of the Pt(N^C^N) system which, unlike complexes incorporating bidentate phenylpyridine-type ligands and exhibiting similar metal-to-ligand charge transfer admixtures, cannot readily distort from planarity. The results provide new insight into strategies for optimizing the performance of platinum-based emitters for applications such as organic light-emitting diode (OLED) technology and imaging