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Can ΔSCF and ROKS DFT-Based Methods Predict the Inversion of the Singlet-Triplet Gap in Organic Molecules?

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Inverted singlet-triplet gap (INVEST) materials show promise for OLEDs. Delta Self-Consistent Field (ΔSCF) calculations, particularly with PBE0, accurately predict their inverted energy gap, unlike ROKS methods.

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Area of Science:

  • Computational Chemistry
  • Materials Science
  • Organic Electronics

Background:

  • Inverted singlet-triplet gap (INVEST) materials possess a unique electronic structure with potential for Organic Light Emitting Diodes (OLEDs).
  • These materials exhibit a negative singlet-triplet energy gap (ΔEST), deviating from Hund's rule, which is crucial for their emissive properties.

Purpose of the Study:

  • To computationally evaluate the accuracy of Restricted Open-Shell Kohn-Sham (ROKS) and Delta Self-Consistent Field (ΔSCF) methods for predicting the ΔEST in INVEST molecules.
  • To benchmark these methods against advanced wavefunction-based calculations like EOM-CCSD, NEVPT2, and SCS-CC2.

Main Methods:

  • Calculation of ΔEST for 15 INVEST molecules using ROKS and ΔSCF methods with various functionals.
  • Benchmarking against EOM-CCSD, NEVPT2, and SCS-CC2 wavefunction-based methods.
  • Analysis of orbital relaxation and spin polarization effects contributing to the inverted gap.

Main Results:

  • ROKS consistently predicts a positive ΔEST, irrespective of the functional used, failing to capture the inverted gap.
  • ΔSCF generally predicts a negative ΔEST, with the PBE0 functional showing the best agreement (lowest Mean Squared Deviation and Mean Absolute Deviation) with EOM-CCSD.
  • The singlet-triplet inversion is attributed to differential orbital relaxation between singlet and triplet states, which ΔSCF captures effectively.

Conclusions:

  • ΔSCF methods, particularly PBE0, are more suitable than ROKS for predicting the inverted singlet-triplet gap in INVEST materials.
  • While ΔSCF captures the essence of the inverted gap through orbital relaxation, caution is advised due to limitations in quantitative accuracy compared to EOM-CCSD.
  • Further investigation into the nuances of ΔSCF calculations is recommended for reliable screening of potential INVEST materials for OLED applications.