Thermally-activated Fluorescence Research Articles

Narrowband emission from fully-bridged triphenylamine derivatives: insights into effects of structure modification and pressure.

Triphenylamine derivatives with narrowband emission have attracted growing attention in purely organic thermally-activated fluorescence (TADF) materials owing to their enhanced color purity and flexible molecular design strategy. Combined time-dependent density functional theory (TD-DFT) and ONIOM (QM/MM) calculations indicate that the excellent planarity of the experimentally developed DQAO could result in gradually decreased intermolecular interactions in the aggregated state at ambient pressure and upon compression, which is unfavorable for suppressing structural relaxation and achieving narrowband emission in its non-doped practical application. Therefore, three structure-modified derivatives, DQAO-Cb, DQAO-Ph, and DQAO-PhCb, were theoretically designed by introducing the spherical o-carborane and dangling phenyl units positioned para to the N atom of the DQAO to provide additional geometrical distortion and steric hindrance. The explorations on the reported DQAO, OQAO, and SQAO found that small structural relaxations, suppressed low-frequency vibrations, and noticeable short-range charge-transfer (SR-CT) natures of DQAO and OQAO are responsible for their much narrower emission spectral full-width at half-maxima (FWHMs) compared to that of SQAO. Introducing the o-carborane unit directly at the para position of the N atom could result in additional scissoring and stretching vibrations of the corresponding DQAO-Cb while the presence of the phenyl unit in DQAO-Ph is beneficial for suppressing the high-frequency vibrations of the pristine DQAO. More importantly, the bridged phenyl unit incorporated in DQAO-PhCb is of particular importance to inhibit the undesired low-frequency scissoring and high-frequency stretching vibrations of the o-carborane unit, which is crucial to reduce the reorganization energy of DQAO-PhCb and achieve narrowband emission. Also, the phenyl unit in DQAO-Ph and DQAO-PhCb helps to shorten charge transfer distances and improve ISC and RISC processes. Since the o-carborane unit is an adopted building block to achieve piezochromic behaviors, the theoretically structure-modified DQAO-PhCb is expected to exhibit narrowband emission, TADF, and piezochromic features all together. Our findings will hopefully provide ideas for designing triphenylamine-based TADF emitters with narrowband emission and piezochromic behaviors.

Thermally Activated Fluorescence vs Long Persistent Luminescence in ESIPT-Attributed Coordination Polymer.

Excited-state intramolecular proton transfer (ESIPT) molecules demonstrating specific enol-keto tautomerism and the related photoluminescence (PL) switch have wide applications in displaying, sensing, imaging, lasing, etc. However, an ESIPT-attributed coordination polymer showing alternative PL between thermally activated fluorescence (TAF) and long persistent luminescence (LPL) has never been explored. Herein, we report the assembly of a dynamic Cd(II) coordination polymer (LIFM-101) from the ESIPT-type ligand, HPI2C (5-(2-(2-hydroxyphenyl)-4,5-diphenyl-1H-imidazol-1-yl)isophthalic acid). For the first time, TAF and/or color-tuned LPL can be achieved by controlling the temperature under the guidance of ESIPT excited states. Noteworthily, the twisted structure of the HPI2C ligand in LIFM-101 achieves an effective mixture of the higher-energy excited states, leading to ISC (intersystem crossing)/RISC (reverse intersystem crossing) energy transfer between the high-lying keto-triplet state (Tn(K*)) and the first singlet state (S1(K*)). Meanwhile, experimental and theoretical results manifest the occurrence probability and relevance among RISC, ISC, and internal conversion (IC) in this unique ESIPT-attributed coordination polymer, leading to the unprecedented TAF/LPL switching mechanism, and paving the way for the future design and application of advanced optical materials.

Exciplex energy transfer through spacer: White electroluminescence with enhanced stability based on cyan intermolecular and orange intramolecular thermally activated delayed fluorescence

Capability of exciplex energy transfer through a spacer was investigated using three exciplex-forming solid mixtures which contained the well-known electron accepting 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine and appropriately designed bipolar cyanocarbazolyl-based derivatives functionalized by attachment of carbazolyl, acridanyl or phenyl units. These novel cyanocarbazolyl-based derivatives were used as both the spacer and exciplex-forming donor. Efficient organic light-emitting diodes with electroluminescence in cyan-yellow region and maximum external quantum efficiency of up to 7.7% were fabricated owing to efficient thermally activated fluorescence (TADF) of the newly discovered exciplexes. An approach of exciton separation by the spacer between the studied exciplexes and selected orange TADF emitter was proposed for the fabrication of white electroluminescent devices with prolonged lifetime comparing to that of single-color exciplex-based devices. Exciplex-forming systems were tested for exciton separation between inter- and intramolecular TADF. Exciplex energy transfer through a spacer was observed on relatively long distance for one system due to the energy resonance between triplet levels of the exciplex and spacer. First time observed here exciplex energy transfer through a spacer can be useful for both improvement of device stability and obtaining of white electroluminescence.

Polymorph‐Dependent Thermally Activated Delayed Fluorescence Emitters: Understanding TADF from a Perspective of Aggregation State

AbstractCurrent research on thermally activated fluorescence (TADF) emitters is mainly based on the molecular levels, while the aggregation states of TADF emitters are to be explored deeply. Now two multifunctional emitters are reported with simultaneous TADF, aggregation induced emission (AIE), and multicolor mechanochromic luminescence (MCL) features. Both emitters also show polymorph‐dependent TADF emission. Crystal structure analysis reveals that the polymorphism is ascribed to the mutable conformations in different aggregation states. This work brings new insight to TADF emitters from a perspective of aggregation states.

Polymorph-Dependent Thermally Activated Delayed Fluorescence Emitters: Understanding TADF from a Perspective of Aggregation State.

Current research on thermally activated fluorescence (TADF) emitters is mainly based on the molecular levels, while the aggregation states of TADF emitters are to be explored deeply. Now two multifunctional emitters are reported with simultaneous TADF, aggregation induced emission (AIE), and multicolor mechanochromic luminescence (MCL) features. Both emitters also show polymorph-dependent TADF emission. Crystal structure analysis reveals that the polymorphism is ascribed to the mutable conformations in different aggregation states. This work brings new insight to TADF emitters from a perspective of aggregation states.

Exciplex-forming derivatives of 2,7-di-tert-butyl-9,9-dimethylacridan and benzotrifluoride for efficient OLEDs

Abstract Strongly medium dependent capability to exhibit not only prompt/delayed thermally-activated fluorescence but also aggregation-induced phosphorescence or exciplex emission is demonstrated either for solutions or for solid samples of acridan-based luminophores. Multicolour emissions of different nature were detected and studied for donor-acceptor derivatives of 2- or 4-(trifluoromethyl)benzene and 2,7-di-tert-butyl-9,9-dimethylacridine containing trifluoromethyl groups in ortho and para positions. For the demonstration of one of possible applications, both compounds were tested in the emissive layers of exciplex-based OLEDs. In the best case, their high maximum current, power and external quantum efficiences of 29.5 cd/A, 29.1 lm/W and 8.2% respectively were achieved for the device based on exciplex electroluminescence resulting from recombination of intermolecular charge transfer states formed on the interface of electron-donating and electron-accepting molecules used.