What is it about?
The violation of Hund’s rule in some N-heterocyclic chromophores (cyclazine and its derivatives) is often attributed to an inversion of the the low-lying excited singlet (S1 ) or triplet (T1 ) states. The negative energy difference between the excited states S1 and T1 , denoted as ∆EST = E(S1)-E(T1) makes them templates for the fifth generation of OLED materials which aim to use reverse intersystem crossing (RISC) to increase quantum yield leading to thermally activated delayed fluorescence (TADF). These systems, popularly known as INVEST systems, exhibit both static and dynamic correlation. Common excited state methods such as CIS, RPA or LR-TDDFT, are unable to predict the inversion of ST gap. This has been attributed to lack of doubles correction in S1. However, S0, S1 and T1 states all exhibit a large contribution from doubles and higher excited configurations that need a delicate balance of static and dynamic correlation for accurate estimation. In this work we study a systematic hierarchy of single and multi-reference wave-function based methods with the twofold goal of (a) understanding the important configurations in the states involved, (b) generating high quality benchmark numbers The role of spin-contamination and spin-polarization along with the suitable connections to minimal two and three state exciton models are drawn to identify the minimal physics governing the interactions in these molecules.
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Why is it important?
We calculate the strongly correlated excited singlet and triplet states with highly accurate wave function based theory (Fully internally contracted multi-reference configuration interation or FICMRCI), which provides a benchmark for determining a suitable cheap method like DFT with reasonable accuracy. The cheaper method can be used for screening more applicable molecules with reasonable spin-orbit or spin-vibronic coupling as well as ISC and RISC rate constants are for synthesizable larger derivatives and n-mers of these templates for OLED applications.
Perspectives
The inversion of the singlet triplet gap has recently attracted a lot of attention due to Hund's law violations in certain N-heterocyclic chromophores (cyclazine, heptazine, etc.). However, experiments have shown near degenerate levels, with both positive and negative being within the experimental error bar [J. Am. Chem. Soc. 102, 6068 (1980), J. Am. Chem. Soc. 108, 17(1986)]. In light of this difficult situation, we have conducted a thorough and methodical analysis of numerous electrical construction techniques, determining their advantages and disadvantages. We have discovered that near degeneracy can be attained through state-specific orbital corrections, including their connection with correlation, or through the appropriate balance of static and dynamic correlation in multireference theories.
Dr. Sangita Sen
Indian Institute for Science Education and Research Kolkata
Read the Original
This page is a summary of: Benchmark computations of nearly degenerate singlet and triplet states of N-heterocyclic chromophores. I. Wavefunction-based methods, The Journal of Chemical Physics, November 2024, American Institute of Physics,
DOI: 10.1063/5.0225537.
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