On the origin of the inverted singlet–triplet gap of the 5th generation light-emitting molecules

Abstract

Excitation energies of the lowest singlet and triplet state of molecules whose first excited singlet state lies energetically below the first triplet state have been studied computationally at (time-dependent) density...

Gold(i)-containing light-emitting molecules with an inverted singlet–triplet gap

“…65–71 Previously, we showed that ΔSCF calculations at the DFT level of theory predict singlet–triplet inversion of heptazine and five other related INVEST molecules. 22 However, the SCF wavefunctions of the S 1 state suffer from severe spin contamination ( ŝ 2 > 1), and Δ E ST depends heavily on the employed functional. Similar problems arose for the molecules studied here.…”

“…However, we recently proposed that the ca 1%-higher percentage of doubles in S 1 is the reason for its energy stabilization with respect to the T 1 state. 22 …”

“…There is a growing interest in experimental and computational studies of the energy order of S 1 and T 1 (Δ E ST < 0) in nitrogen- and/or boron-doped derivatives of the phenalenyl diradical, 9–25 that violates Hund's spin-multiplicity rule. 26 In such an unusual situation, reverse intersystem crossing (RISC) is a downhill process, which is expected to increase the quantum yield of fluorescence with respect to state-of-the-art TADF emitters.…”

“…This phenomenon is termed as delayed uorescence from inverted singlet-triplet gaps (DFIST) 24 and the molecules displaying it are called INVEST molecules. 11,12,17,25 Although the origin of the singlet-triplet inversion is still not fully understood, it is known that contributions from doublyexcited congurations increase the spatial separation of the frontier orbitals, 22 which explains the stabilisation of S 1 with respect to T 1 and the inability of computational methods like time-dependent density functional theory (TD-DFT) or conguration interaction singles (CIS) to predict the inverted gap. The correct energy ordering of the S 1 and T 1 states of INVEST molecules can be obtained by using correlated wavefunctionbased methods that include double excitations, such as approximate second-order coupled cluster (CC2), 11,13,14,22,23 algebraic diagrammatic construction of second order (ADC(2)), 11,[13][14][15]18,22 equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) aka linear-response CCSD, 11,24 stateaveraged complete active space self-consistent eld (SA-CASSCF), 13,14,22,23 CASSCF second-order perturbation theory (CASPT2), 11,22 and strongly contracted N-electron valence state second-order perturbation theory (SC-NEVPT2) methods, 13,14,23 to mention the most commonly used ones.…”

Aromaticity-driven singlet-triplet inversions

“…22 With the prospect of superior emissive properties, it is imperative to find new classes of molecules with singlet-tripet inversions. Given the historic lack of focus on the possible existence of Hund's rule violations among excited states, and because inverted singlet-triplet gaps can only be modelled using high-level quantum chemical methods, 14,[23][24][25][26][27][28] we suggest that there may be many cases of excited state singlet-triplet inversions among already synthesized molecules.…”