Metal‐Free OLED Triplet Emitters by Side‐Stepping Kasha’s Rule

Abstract

With organic light-emitting diodes (OLEDs) emerging in ever more applications, such as smart phones, televisions, and lighting, it is easy to forget that the present technology is based on a brilliantly simple patch to an inherent problem of fluorescent hydrocarbons: three quarters of the electrically generated energy is dissipated as heat by triplet excitons. Radiative decay from the triplet state via phosphorescence is generally very weak, and has only been resolved in transient spectroscopy at low temperatures in select organic semiconductors. [1] The solution to this problem has been to incorporate metal-organic emitters in OLEDs, [2] which mix spin by enhancing intersystem crossing through spin-orbit coupling: the heavy-atom effect. As this approach relies on the longevity of triplet excitons and the associated diffusion lengths, it is highly effective: in a suitably homogeneous environment even ppm concentrations of covalently bound metal atoms are sufficient to activate electrophosphorescence. [3] The second conceivable approach to harvesting energy from triplets is based on endothermic conversion [4] to a fluorescent singlet by reverse intersystem crossing. [5] This method necessitates control not only over spin-orbit coupling, requiring a heavy atom or a carefully engineered charge-transfer state, but also over the singlet-triplet exchange gap, which can be tuned by excitonic confinement. [6] Although progress has been made recently, conceptually it parallels the former approach: all excitations are converted to either triplets or singlets, thereby losing information on the underlying spin correlations of charge carriers. Evidence is emerging, however, that spin correlations in excitonic electron-hole precursor pairs can be used for exquisitely sensitive measurements of magnetic fields [7] and possibly even for quantum coherence phenomenology, [7b] with analogies to avian radical-pair photomagnetosensory processes. [8] To quantify such spin correlations, it is desirable to develop materials without heavy-atom spin mixing that show both intrinsic fluorescence and phosphorescence.The third approach to triplet harvesting has not been explored previously: tuning spin-orbit coupling without heavy atoms such that non-radiative internal conversion from the triplet excited state to the singlet ground state is suppressed and phosphorescence is the only remaining relaxation mechanism. Even in low-atomic-order-number compounds such as hydrocarbons, the orbital component of the wavefunction can give rise to substantial magnetic moments, leading to non-negligible spin-orbit energy terms. The effect is well-studied in carbon nanotubes and graphene, where zero-field splitting correlates with nanoscale curva

Intramolecular Charge Transfer Controls Switching Between Room Temperature Phosphorescence and Thermally Activated Delayed Fluorescence

“…Fast ISC is usually promoted by incorporating aheavy atom or an ICT state into the molecular structure to facilitate strong spin-orbit coupling (SOC). [3] Metal-free phosphors are relatively rare and generally suffer from extremely weak phosphorescence under ambient conditions due to the spin forbidden nature of the triplet to singlet transition. [2][3][4] Therefore,toachieve pure organic RTP, the SOC strength needs to be enhanced, and the nonradiative dissipations should be suppressed.…”

Room‐Temperature‐Phosphorescence‐Based Dissolved Oxygen Detection by Core‐Shell Polymer Nanoparticles Containing Metal‐Free Organic Phosphors

“…[7] This new class of organic emitters can be particularly useful for highly sensitive optical DO detection because of their long decay time (from millisecond to second) originating from their inefficient spin-orbit coupling (SOC) and large Stokes shifts, [8] which enables spectral separation of excitation and emission wavelengths,r esulting in simple optical set-ups and low-interference measurements.However, despite their vast potential, metal-free organic phosphors have been largely unexplored as DO indicators. [24,25] In addition, stringent conditions of rigid hosts such as crystalline solid-state structures and carefully chosen polymer matrices are commonly required for bright room-temperature phosphorescence (RTP). [24,25] In addition, stringent conditions of rigid hosts such as crystalline solid-state structures and carefully chosen polymer matrices are commonly required for bright room-temperature phosphorescence (RTP).…”

“…[9,10, 11b] Although substantial progress has been made in the field of metal-free organic phosphors since the pioneer works from the groups of Fraser, [11] Kim, [12] and Tang, [13] bright organic phosphors at room temperature are still very rare and only limited to BF 2 -chelates, [9,11,14] phenylthiobenzene, [15] benzophenone, [13,16,17] fluorene, [18,19] triazine, [20] boronic ester, [21] naphthalimide, [22] sulfone, [23] bromobenzaldehyde, [12] and polyaromatic analogues. [24,25] In addition, stringent conditions of rigid hosts such as crystalline solid-state structures and carefully chosen polymer matrices are commonly required for bright room-temperature phosphorescence (RTP). However, practical applicability of such aphosphor-doped polymer film and aphosphorescent crystal can be rather limited and, hence, development of anew versatile platform that allows achieving bright organic RTPa nd thus quantifying DO at significantly low concentrations for various applications is required.…”

Magnesium(II) Complexes with High Emission: The Distinct Charge‐Transfer Process from Transition Metal

“…To investigate the charge transfer in emission process and why the fluorescence intensity of Mg II complexes is smaller than AIP, time‐dependent density functional theory (TD‐DFT) method was employed to investigate their molecular orbital properties. The fluorescence properties of the molecules were closely related to the lowest singlet excited states (S 1 ) rather than the higher (S n ) excited states according to the Kasha's rule . Thus, three typical crystal structures ( 1 , 2 and 3 ) were used to optimize the corresponding singlet excited state geometries in chloroform medium.…”