Comprehensive spectroscopic analysis of energy inversion of the lowest singlet and triplet excited states in the heptazine HzTFEX2
Abstract
Detailed photophysical measurements of the non-symmetric heptazine, HzTFEX ₂ , previously reported as having inverted S ₁ -T ₁ levels, show complex behaviour, highly dependent on environment, concentration and aggregation. Here we show that, single crystals of HzTFEX ₂ show anti-Kasha S ₀ ←S ₂ and excimer emission. Solution state absorption shows a hypsochromic shift of the π-π* S ₀ →S ₁ , linearly dependent on solvent polarizability. In most environments, emission is dominated by excimer species, only in toluene and frozen 2-MeTHF do monomeric properties dominate. In frozen solutions, aggregation phenomena are solvent dependent. Given such complexity, time-dependent emission spectra are crowded, making it difficult to resolve monomeric phosphorescence. However, by exciting S ₀ →S ₂ at 77 K, we observe a consistent phosphorescence signal at 425 nm, 110 ± 20 meV above the S ₀ ←S ₁ emission, whereas exciting S ₀ →S ₁ does not give phosphorescence or delayed fluorescence (DF). We conclude that coupling between S ₂ and S ₁ is very weak, S ₂ -T N intersystem crossing is effective, whereas S ₁ ↔T ₁ intersystem crossing is thermally activated, fulfilling the conditions required for the observation of inverted phosphorescence. At room temperature, no long-lived DF is observed because exothermic T ₁ →S ₁ ISC becomes allowed, such that in an OLED, high efficiency can be achieved.
Related
- Correlation-Driven Singlet--Triplet Inversion and Optical Spectra of N-Substituted Phenalenyls: Insights from PPP-FCI and TDDFT
- Multifaceted Control of Triplet-Pair Spin Dynamics in Intramolecular Singlet Fission at Room Temperature
- Mechanical Tuning of Singlet-Triplet Resonance for Efficient Triplet-Triplet Annihilation Upconversion.
- Probing Triplet Photochemistry in Ru(II) Porphyrins Using Singlet Oxygen Generation and Triplet–Triplet Annihilation Upconversion
- When optimized geometries are not optimal: Implications for inverted singlet-triplet gap predictions
BibTeX
@article{francener2026comprehensive,
title = {{Comprehensive spectroscopic analysis of energy inversion of the lowest singlet and triplet excited states in the heptazine HzTFEX2}},
author = {Carolina Francener and Hector Miranda-Salinas and Christopher J. Riggs and Naoya Aizawa and Andrew Monkman},
journal = {Nature Communications},
year = {2026},
month = oct,
doi = {10.1038/s41467-026-78831-z},
url = {https://www.nature.com/articles/s41467-026-78831-z},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Comprehensive spectroscopic analysis of energy inversion of the lowest singlet and triplet excited states in the heptazine HzTFEX2
AU - Carolina Francener
AU - Hector Miranda-Salinas
AU - Christopher J. Riggs
AU - Naoya Aizawa
AU - Andrew Monkman
PY - 2026
DA - 2026/10/20
JO - Nature Communications
DO - 10.1038/s41467-026-78831-z
UR - https://www.nature.com/articles/s41467-026-78831-z
AB - Detailed photophysical measurements of the non-symmetric heptazine, HzTFEX ₂ , previously reported as having inverted S ₁ -T ₁ levels, show complex behaviour, highly dependent on environment, concentration and aggregation. Here we show that, single crystals of HzTFEX ₂ show anti-Kasha S ₀ ←S ₂ and excimer emission. Solution state absorption shows a hypsochromic shift of the π-π* S ₀ →S ₁ , linearly dependent on solvent polarizability. In most environments, emission is dominated by excimer species, only in toluene and frozen 2-MeTHF do monomeric properties dominate. In frozen solutions, aggregation phenomena are solvent dependent. Given such complexity, time-dependent emission spectra are crowded, making it difficult to resolve monomeric phosphorescence. However, by exciting S ₀ →S ₂ at 77 K, we observe a consistent phosphorescence signal at 425 nm, 110 ± 20 meV above the S ₀ ←S ₁ emission, whereas exciting S ₀ →S ₁ does not give phosphorescence or delayed fluorescence (DF). We conclude that coupling between S ₂ and S ₁ is very weak, S ₂ -T N intersystem crossing is effective, whereas S ₁ ↔T ₁ intersystem crossing is thermally activated, fulfilling the conditions required for the observation of inverted phosphorescence. At room temperature, no long-lived DF is observed because exothermic T ₁ →S ₁ ISC becomes allowed, such that in an OLED, high efficiency can be achieved.
SN - 2041-1723
ER -