Direct Observation of Excited Singlet-Triplet (S₁ -T₁) State Inversion in Pentaazaphenalene

According to Hund’s multiplicity rule, the energy difference (singlet-triplet gap, STG) between the first excited singlet (S₁) and first excited triplet (T₁) is positive for closed shell molecules. Minimization of the STG has been a key approach to increasing the fluorescence efficiency of organic light emitting diodes. These efforts recently led to experimental evidence that inversion of the two states (i.e STG < 0) is possible for some chromophores in the azaphenalene family. However, most experiments to date rely on indirect probes of the T₁ energy ^[1–3]. Here, we present a direct spectroscopic measurement of the inverted singlet-triplet gap in an unsubstituted pentaazaphenalene (STG = -0.047(7) eV) using a combination of high-resolution cryogenic anion photoelectron spectroscopy and optical absorption of the chromophore in an argon matrix at 20 K. When accounting for zero-point energies and geometry relaxation, the EOM-CCSD/aug-cc-pVTZ high-level electronic structure method produces a qualitatively inaccurate positive STG of +0.024 eV. The inclusion of some triple excitation (EOM-CCSDt) is necessary to address this inaccuracy and yields a calculated STG of -0.022 eV. The results described here demonstrate that inherent singlet-triplet inversion is possible for an unsubstituted azaphenalene and provides insight into the level of electronic structure theory necessary to predict this phenomenon.

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[2] J. Li, Z. Li, H. Liu, H. Gong, J. Zhang, X. Li, Y. Wang, Q. Guo, Dyes and Pigments 2022, 203, 110366.

[3] N. Aizawa, Y.-J. Pu, Y. Harabuchi, A. Nihonyanagi, R. Ibuka, H. Inuzuka, B. Dhara, Y. Koyama, K. Nakayama, S. Maeda, F. Araoka, D. Miyajima, Nature 2022, 609, 502–506.