Organic Molecules with Inverted Gaps between First Excited Singlet and Triplet States and Appreciable Fluorescence Rates

Abstract

One of the recent proposals for the design of state-of-the-art emissive materials for organic light emitting diodes (OLEDs) is the principle of thermally activated delayed fluorescence (TADF). The underlying idea is to enable facile thermal upconversion of excited state triplets, which are generated upon electron-hole recombination, to excited state singlets by minimizing the corresponding energy difference resulting in devices with up to 100% internal quantum efficiencies (IQEs). Ideal emissive materials potentially surpassing TADF emitters should have both negative singlet-triplet gaps and appreciable fluorescence rates to maximize reverse intersystem crossing (rISC) rates from excited triplets to singlets while minimizing ISC rates and triplet state occupation leading to long-term operational stability. However, molecules with negative singlet-triplet gaps are extremely rare and, to the best of our knowledge, not emissive. In this work, based on computational studies, we describe the first molecules with negative singlet-triplet gaps and considerable fluorescence rates and show that they are more common than hypothesized previously.

Data-Driven Strategies for Accelerated Materials Design

“…More importantly, it is robust, can be used in a black-box manner, and is readily deployed in simulations of tens of thousands of molecules with minimal failure rates . However, one pernicious failure mode of TD-DFT is the description of excited states with significant double-excitation character, which is, inter alia , important in describing molecules with inverted singlet–triplet gaps, , such as the INVEST emitters recently described by our group . Nevertheless, as computing power is increasing, more sophisticated ab initio approaches can be used in HTVS, allowing one to tackle ever more complicated problems and new material classes.…”

S-Heptazine N-ligand based luminescent coordination materials: synthesis, structural and luminescent studies of lanthanide–cyamelurate networks

“…The substituted derivatives of heptazines are notorious for thermally activated delayed fluorescence and the very close positioning of the triplet state relative to the singlet one is at the origin of this phenomenon. 55,56 Consequently, the fluorescence and phosphorescence bands of the ligand may be nearly perfectly superposed. The presence of thermally activated delayed fluorescence is easily demonstrated by the presence of a biphasic decay (two slopes) where the short component is in the ps/short ns time scale (fluorescence), whereas the long one is in the μs one.…”

Singlet-Triplet Inversion in Triangular Boron Carbon Nitrides

“…Consequently, traditional TD-DFT methods for studying excited states miserably fail in describing singlet-triplet inversion, as was noted early on. 2,46,47 To address ST-inversion, high-quality but computationally demanding methods are typically used, spanning from ADC(2), [2][3][4]7,8 CC2, 2,3,8,9 EOM-CCSD, 2,4,5,48,49 Mukherjee's multireference coupled-cluster with noniterative triple excitations (Mk-MRCCSD(T)), 50 CC3, 51 Green's functions methods, 10 CIS(D), 3,6 SA-CASSCF, 3,8,50 and NEVPT2. 3,4,8,50 We adopt a different strategy and conduct an extensive study of various triangular boron carbon nitrides using the Pariser-Parr-Pople (PPP) model Hamiltonian to account for correlated electrons in π-conjugated molecules.…”

“…2,46,47 To address ST-inversion, high-quality but computationally demanding methods are typically used, spanning from ADC(2), [2][3][4]7,8 CC2, 2,3,8,9 EOM-CCSD, 2,4,5,48,49 Mukherjee's multireference coupled-cluster with noniterative triple excitations (Mk-MRCCSD(T)), 50 CC3, 51 Green's functions methods, 10 CIS(D), 3,6 SA-CASSCF, 3,8,50 and NEVPT2. 3,4,8,50 We adopt a different strategy and conduct an extensive study of various triangular boron carbon nitrides using the Pariser-Parr-Pople (PPP) model Hamiltonian to account for correlated electrons in π-conjugated molecules. The model dates back to the 1950s, [52][53][54] and was able to effectively describe ground state as well as excited state properties of several small organic molecules.…”