On the electronic structure of alternant conjugated organic radicals for light-emitting diode applications

Abstract

Since the demonstration in 2018 that organic radicals can be used to make highly efficient organic light-emitting diodes, there has been an explosion of interest in their capabilities and many experimental and computational studies of their performance. Here we take a theoretical view and describe the electronic structure of radicals from an algebraic perspective. By rediscovering and adapting historic investigations of organic radicals, we show how many experimentally useful properties can be determined without synthesis or computation, but simply from knowledge of the molecular structure and in particular whether or not the radical is an alternant hydrocarbon. These include the location of orbital amplitude in the SOMO, absorption and emission characteristics, and solvatochromic shift. We explain these results in the context of modern organic light-emitting diodes in order to inform future investigations.

Alternant Hydrocarbon Diradicals as Optically Addressable Molecular Qubits

“…This is known as the pairing theorem, and, consequently, the SOMO has nodes on the unstarred atoms. An excellent review on this topic has been provided by Hele, and we shall not rederive the results. Instead, we will briefly mention that, within the aforementioned mean-field treatment, the AHR’s electronic GS is a doublet described by the following (Slater) determinants: | normalΨ ; + 1 / 2 ⟩ = | · · · 2 2̅ 1 1̅ 0 ⟩ , | normalΨ ; − 1 / 2 ⟩ = | · · · 2 2̅ 1 10̅ ⟩ that is, determinants with one electron in the SOMO ( j = 0) and two electrons in each HOMO – j ( j = 1, 2, ...).…”

“…Hence, the GS, being of “–” symmetry, can only be photoexcited locally and to “+” states (an alternative interpretation is to realize that the j → 0 and 0 → j ′ transitions have identical transition dipole moments, so they cancel when taking antisymmetric combinations in eq ). , …”

“…Hence, the GS, being of "−" symmetry, can only be photoexcited locally and to "+" states (an alternative interpretation is to realize that the j → 0 and 0 → j′ transitions have identical transition dipole moments, so they cancel when taking antisymmetric combinations in eq 3). 69,73 Next, we consider perturbative effects on the approximate zeroth-order eigenstates due to V AB . Because V AB is a spinpreserving intermonomer operator of "+" particle−hole symmetry and A irrep [eq 5], only CT-type mixing between states of like symmetry can occur…”

“…46,73,77 On a related note, we did not explicitly model IC in the PPP framework but only expected them to occur without preserving particle−hole symmetry, as with most AHRs. 69,73 Finally, because spins are polarized by ; J 00 = 2.0 eV; J 10 = 1.1 eV; K 10 = 0.5 eV. shelving the triplet M S = ±1 population into the singlet state, at most 25% of the ensemble will be a triplet M S = 0, unlike the NV center where this value is 100%.…”

Luminescent Radicals

“…These absorption and emission characteristics can be qualitatively understood based on an electronic structure model for alternant π-radicals proposed by Longuet-Higgins and Pople, which was recently rediscovered by Hele, Evans, and co-workers (Figure ). , Alternant π-hydrocarbon systems are molecules whose conjugated atoms can be divided into two sets such that no adjacent atoms belong to the same set. In alternant π-radicals, the SOMO lies at an energy level exactly between that of the NHOMO and NLUMO, and the NHOMO–SOMO and SOMO–NLUMO transitions are degenerate with identical magnitude of their transition dipole moments.…”