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B3 (Gilch)

Spectroscopy of Molecules with (Near-)Degenerate Singlet and Triplet Excitations

Principal Investigator: Prof. Peter Gilch
Doctoral Researcher: Wiebke Haselbach, Simon Zimmermann
Associated Doctoral Researcher: Oliver Nolden


Molecules serving as emitters in OLEDs usually feature singlet and triplet excitations in close energetic vicinity. In TADF-type emitters this is a prerequisite since otherwise a thermal repopulation of the emissive singlet state is not possible. In phosphorescent emitters a small singlet-triplet energy gap reduces the energy loss for singlet excitons. Obviously, for characterization and optimization of emitters a precise energetic localization of singlet and triplet states is required. Direct optical excitation (see Figure, left) combined with fluorescence and phosphorescence spectroscopy yields the energies of the lowest singlet (S1) and triplet (T1) excitations. Higher excitations – particularly the triplet ones – are more difficult to localize. Yet, these higher excitations can play an important role for rISC in emitters. Assume an upper triplet state (denoted T2 here) with a very efficient rISC channel towards the bright S1 state is located several kbT above the S1 state. The T2 state might be crucial for the harvesting of triplet-correlated electron hole pairs in an OLED. In a direct excitation experiment one is very likely to miss this state due to its energetic positioning and the concomitantly small population. With sensitized excitation such upper triplet states can sometimes be identified (see Figure, center). A molecule with a high-lying triplet state donates the energy of its triplet excitation to the molecule of interest. If this energy transfer ends up in the upper triplet state, rISC followed by fluorescence emission will occur.

The leitmotiv of project area B, the class of linear copper carbene complexes, features closely spaced excited singlet and triplet states in the vicinity of the lowest excitation. These states define the luminescence properties of the complexes. It is the aim of this project to locate these states energetically and measure their interconversion rate constants. The energetic locations of these upper excited states shall be obtained from femto- and nanosecond near infrared (NIR) spectroscopy (see Figure, right). In the analysis of the spectroscopic results close collaboration with the Marian group (project B2) will be sought. The spectroscopic and quantum-chemical findings shall produce guidelines for synthetic modifications of the complexes which will be taken up by the Ganter group (project B1).

Previous work and further information

  1. Wöll, D.; Laimgruber, S.; Galetskaya, M.; Smirnova, J.; Pfleiderer, W.; Heinz, B.; Gilch, P.; Steiner, U. E., On the mechanism of intramolecular sensitization of photocleavage of the 2-(2-nitrophenyl)propoxycarbonyl (NPPOC) protecting group. J. Am. Chem. Soc. 2007, 129, 12148-12158.
  2. Villnow, T.; Ryseck, G.; Rai-Constapel, V.; Marian, C. M.; Gilch, P., Chimeric Behavior of Excited Thioxanthone in Protic Solvents: I. Experiments. J. Phys. Chem. A 2014, 118 (50), 11696-11707.
  3. Rai-Constapel, V.; Villnow, T.; Ryseck, G.; Gilch, P.; Marian, C. M., Chimeric Behavior of Excited Thioxanthone in Protic Solvents: II. Theory. J. Phys. Chem. A 2014, 118 (50), 11708-11717.
  4. Torres Ziegenbein, C.; Fröbel, S.; Glöß, M.; Nobuyasu, R. S.; Data, P.; Monkman, A.; Gilch, P., Triplet Harvesting with a Simple Aromatic Carbonyl. ChemPhysChem 2017, 18 (17), 2314–2317.
  5. Thom, K., Gilch, P., Surprisingly simple molecules as potential OLED-Emitters? https://q-more.chemeurope.com/q-more-articles/294/surprisingly-simple-molecules-as-potential-oled-emitters.html
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