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.