Comprehensive spectroscopic analysis of energy inversion of the lowest singlet and triplet excited states in the heptazine HzTFEX2
Year: 2026
Type: article
Abstract: Detailed photophysical measurements of the non-symmetric heptazine, HzTFEX2, previously reportedly as having inverted S1-T1 levels, show complex behaviour, highly dependent on environment, concentration and aggregation. Single crystals of HzTFEX2 show anti-Kasha S0S2 and excimer emission. Solution state absorption shows hypsochromic shifts of the π-π* S0•S1, 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 S0•S2, we observe a consistent phosphorescence signal at 425 nm, lying 110±20 meV above the S1 singlet emission. Importantly, exciting S0•S1 does not give phosphorescence or delayed emission (DF). We conclude that coupling between S2 and S1 is very weak, S2-TN intersystem crossing (ISC) is effective, whereas S1 T1 intersystem crossing is highly ineffective because vibronic coupling is negligible at 77 K, fulfilling the conditions required for inverted phosphorescence to be observable. At room temperature, no long-lived DF is observed because vibrational coupling is now strong enough to drive exothermic T1•S1 ISC, such that in an OLED high efficiency can be achieved.
Source: Nature Communications
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $6,990
OpenAlex ID: W7223001464