Steric shielding vs. ?-? orbital interactions in triplet-triplet energy transfer.
ABSTRACT: The influence of non-covalent ?-? orbital interactions on triplet-triplet energy transfer (TTET) through tuning of the donor excitation energy remains basically unexplored. In the present work, we have investigated intermolecular TTET using donor moieties covalently linked to a rigid cholesterol (Ch) scaffold. For this purpose, diaryl ketones of ?,?* electronic configuration tethered to ?- or ?-Ch were prepared from tiaprofenic acid (TPA) and suprofen (SUP). The obtained systems TPA-?-Ch, TPA-?-Ch, SUP-?-Ch and SUP-?-Ch were submitted to photophysical studies (laser flash photolysis and phosphorescence), in order to delineate the influence of steric shielding and ?-? orbital interactions on the rate of TTET to a series of energy acceptors. As a matter of fact, fine tuning of the donor triplet energy significantly modifies the rate constants of TTET in the absence of diffusion control. The experimental results are rationalized by means of theoretical calculations using first principles methods based on DFT as well as molecular dynamics.
Project description:Here, a comprehensive photophysical investigation of a the emitter molecule <b>DPTZ-DBTO2</b>, showing thermally activated delayed fluorescence (TADF), with near-orthogonal electron donor (D) and acceptor (A) units is reported. It is shown that <b>DPTZ-DBTO2</b> has minimal singlet-triplet energy splitting due to its near-rigid molecular geometry. However, the electronic coupling between the local triplet (<sup>3</sup>LE) and the charge transfer states, singlet and triplet, (<sup>1</sup>CT, <sup>3</sup>CT), and the effect of dynamic rocking of the D-A units about the orthogonal geometry are crucial for efficient TADF to be achieved. In solvents with low polarity, the guest emissive singlet <sup>1</sup>CT state couples directly to the near-degenerate <sup>3</sup>LE, efficiently harvesting the triplet states by a spin orbit coupling charge transfer mechanism (SOCT). However, in solvents with higher polarity the emissive CT state in <b>DPTZ-DBTO2</b> shifts below (the static) <sup>3</sup>LE, leading to decreased TADF efficiencies. The relatively large energy difference between the <sup>1</sup>CT and <sup>3</sup>LE states and the extremely low efficiency of the <sup>1</sup>CT to <sup>3</sup>CT hyperfine coupling is responsible for the reduction in TADF efficiency. Both the electronic coupling between <sup>1</sup>CT and <sup>3</sup>LE, and the (dynamic) orientation of the D-A units are thus critical elements that dictate reverse intersystem crossing processes and thus high efficiency in TADF.
Project description:Aggregation-induced emission (AIE) has proven to be a viable strategy to achieve highly efficient room temperature phosphorescence (RTP) in bulk by restricting molecular motions. Here, we show that by utilizing triphenylamine (TPA) as an electronic donor that connects to an acceptor via an sp<sup>3</sup> linker, six TPA-based AIE-active RTP luminophores were obtained. Distinct dual phosphorescence bands emitting from largely localized donor and acceptor triplet emitting states could be recorded at lowered temperatures; at room temperature, only a merged RTP band is present. Theoretical investigations reveal that the two temperature-dependent phosphorescence bands both originate from local/global minima from the lowest triplet excited state (T<sub>1</sub>). The reported molecular construct serves as an intermediary case between a fully conjugated donor-acceptor system and a donor/acceptor binary mix, which may provide important clues on the design and control of high-freedom molecular systems with complex excited-state dynamics.
Project description:MP2 (Second order approximation of Møller?Plesset perturbation theory) and DFT/TD-DFT (Density functional theory/Time-dependent_density_functional_theory) investigations have been performed on metallophilic nanomaterials of host clusters [Au(NHC)?]????[M(CN)?]<sup>-</sup>???[Au(NHC)?]? (NHC = N-heterocyclic carbene, M = Au, Ag) with high phosphorescence. The phosphorescence quantum yield order of clusters in the experiments was evidenced by their order of <i>?</i><sub>S1</sub>/?<i>E</i><sub>S1-T1</sub> values ( ? S 1 : S? ? S? transition dipole, ? E S 1 - T 1 : splitting energy between the lowest-lying singlet S? and the triplet excited state T? states). The systematic variation of the guest solvents (<b>S1</b>: CH?OH, <b>S2</b>: CH?CH?OH, <b>S3</b>: H?O) are employed not only to illuminate their effect on the metallophilic interaction and phosphorescence but also as the probes to investigate the recognized capacity of the hosts. The simulations revealed that the metallophilic interactions are mainly electrostatic and the guests can subtly modulate the geometries, especially metallophilic Au???M distances of the hosts through mutual hydrogen bond interactions. The phosphorescence spectra of hosts are predicted to be blue-shifted under polar solvent and the excitation from HOMO (highest occupied molecular orbital) to LUMO (lowest unoccupied molecular orbital) was found to be responsible for the ³MLCT (triplet metal-to-ligand charge transfer) characters in the hosts and host-guest complexes. The results of investigation can be introduced as the clues for the design of promising blue-emitting phosphorescent and functional materials.
Project description:Photon upconversion based on sensitized triplet-triplet annihilation (TTA) presents interest for such areas as photovoltaics and imaging. Usually energy upconversion is observed as p-type delayed fluorescence from molecules whose triplet states are populated via energy transfer from a suitable triplet donor, followed by TTA. Magnetic field effects (MFE) on delayed fluorescence in molecular crystals are well known; however, there exist only a few examples of MFE on TTA in solutions, and all of them are limited to UV-emitting materials. Here we present MFE on TTA-mediated visible and near infrared (NIR) emission, sensitized by far-red absorbing metalloporphyrins in solutions at room temperature. In addition to visible delayed fluorescence from annihilator, we also observed NIR emission from the sensitizer, occurring as a result of triplet-triplet energy transfer back from annihilator, termed "delayed phosphorescence". This emission also exhibits MFE, but opposite in sign to the annihilator fluorescence.
Project description:The high driving voltage of blue organic light-emitting diodes (OLEDs) based on emitters with thermally activated delayed fluorescence (TADF) remains a constraint for their portable application. A major reason for this is that the high triplet (T1) of the host required to match the blue TADF emitters would always lead to inferiority in terms of carrier injection. Therefore, a suitable host should possess not only a high T1 but also a relatively low singlet (S1) for improved carrier injection, indicating that small singlet-triplet splittings (?ESTs) are highly desired. Here, four carbazolyl benzonitrile derivatives are facilely prepared in a one-step approach with restrained conjugate lengths to maintain high triplet energies while their highly twisted structures spatially separate the frontier orbital distribution to achieve relatively low ?ESTs. Meanwhile, the charge transporting mobilities of these hosts are effectively tuned by the different linker types of the host moieties. Consequently, high-triplet-energy hosts with favorable carrier injection/transporting abilities are realized, endowing blue TADF devices with a maximum external quantum efficiency of 21.5%, a maximum power efficiency of 42.0 lm W-1 and an ultra-low onset voltage of 2.8 V. It is noteworthy that a driving voltage of 4.9 V is achieved at a practical luminance of 1000 cd m-2, which is the lowest among the doped blue TADF OLEDs reported until now. This work suggests that manipulation of the molecular topologies not only leads to the flexible and feasible design of novel bipolar host materials, but also affords a promising method for fine-tuning physical properties and thus obtaining state-of-the-art device performances.
Project description:Understanding the excited-state dynamics and conformational relaxation in thermally activated delayed fluorescence (TADF) molecules, including conformations that potentially support intramolecular through-space charge transfer, can open new avenues for TADF molecular design as well as elucidate complex photophysical pathways in structurally complex molecules. Emissive molecules comprising a donor (triphenylamine, TPA) and an acceptor (triphenyltriazine, TRZ) bridged by a second donor (9,9-dimethyl-9-10-dihydroacridin, DMAC, or phenoxazine, PXZ) are synthesized and characterized. In solution, the flexibility of the sp3-hybridized carbon atom in DMAC of DMAC-TPA-TRZ, compared to the rigid PXZ, allows significant conformational reorganization, giving rise to multiple charge-transfer excited states. As a result of such a reorganization, the TRZ and TPA moieties become cofacially aligned, driven by a strong dipole-dipole attraction between the TPA and TRZ units, forming a weakly charge-transfer dimer state, in stark contrast to the case of PXZ-TPA-TRZ where the rigid PXZ bridge only supports a single PXZ-TRZ charge transfer (CT) state. The low-energy TPA-TRZ dimer is found to have a high-energy dimer local triplet state, which quenches delayed emission because the resultant singlet CT local triplet energy gap is too large to mediate efficient reverse intersystem crossing. However, organic light-emitting diodes using PXZ-TPA-TRZ as an emitting dopant resulted in external quantum efficiency as high as 22%, more than two times higher than that of DMAC-TPA-TRZ-based device, showing the impact that such intramolecular reorganization and donor-acceptor dimerization have on TADF performance.
Project description:A broadband visible light-absorbing [<sup>70</sup>]fullerene-BODIPY-triphenylamine triad (<b>C<sub>70</sub>-B-T</b>) has been synthesized and applied as a heavy atom-free organic triplet photosensitizer for photooxidation. By attaching two triphenylmethyl amine units (TPAs) to the ?-core of BODIPY via ethynyl linkers, the absorption range of the antenna is extended to 700 nm with a peak at 600 nm. Thus, the absorption spectrum of <b>C<sub>70</sub>-B-T</b> almost covers the entire UV-visible region (270-700 nm). The photophysical processes are investigated by means of steady-state and transient spectroscopies. Upon photoexcitation at 339 nm, an efficient energy transfer (ET) from TPA to BODIPY occurs both in <b>C<sub>70</sub>-B-T</b> and <b>B-T</b>, resulting in the appearance of the BODIPY emission at 664 nm. Direct or indirect (via ET) excitation of the BODIPY-part of <b>C<sub>70</sub>-B-T</b> is followed by photoinduced ET from the antenna to C<sub>70</sub>, thus the singlet excited state of C<sub>70</sub> (<sup>1</sup>C<sub>70</sub><sup>*</sup>) is populated. Subsequently, the triplet excited state of C<sub>70</sub> (<sup>3</sup>C<sub>70</sub><sup>*</sup>) is produced via the intrinsic intersystem crossing of C<sub>70</sub>. The photooxidation ability of <b>C<sub>70</sub>-B-T</b> was studied using 1,5-dihydroxy naphthalene (DHN) as a chemical sensor. The photooxidation efficiency of <b>C<sub>70</sub>-B-T</b> is higher than that of the individual components of <b>C<sub>70</sub>-1</b> and <b>B-T</b>, and even higher than that of methylene blue (MB). The photooxidation rate constant of <b>C<sub>70</sub>-B-T</b> is 1.47 and 1.51 times as that of <b>C<sub>70</sub>-1</b> and MB, respectively. The results indicate that the C<sub>70</sub>-antenna systems can be used as another structure motif for a heavy atom-free organic triplet photosensitizer.
Project description:Developing organic optoelectronic materials with desired photophysical properties has always been at the forefront of organic electronics. The variation of singlet-triplet splitting (?EST) can provide useful means in modulating organic excitons for diversified photophysical phenomena, but controlling ?EST in a desired manner within a large tuning scope remains a daunting challenge. Here, we demonstrate a convenient and quantitative approach to relate ?EST to the frontier orbital overlap and separation distance via a set of newly developed parameters using natural transition orbital analysis to consider whole pictures of electron transitions for both the lowest singlet (S1) and triplet (T1) excited states. These critical parameters revealed that both separated S1 and T1 states leads to ultralow ?EST; separated S1 and overlapped T1 states results in small ?EST; and both overlapped S1 and T1 states induces large ?EST. Importantly, we realized a widely-tuned ?EST in a range from ultralow (0.0003 eV) to extra-large (1.47 eV) via a subtle symmetric control of triazine molecules, based on time-dependent density functional theory calculations combined with experimental explorations. These findings provide keen insights into ?EST control for feasible excited state tuning, offering valuable guidelines for the construction of molecules with desired optoelectronic properties.
Project description:A molecular self-assembly approach is developed to resolve an outstanding issue in triplet energy migration-based photon upconversion (TEM-UC), that is, air-stable TEM-UC in water. Amphiphilic cationic acceptor (emitter) molecules self-assemble in water via hydrophobic and hydrogen bonding interactions, with which anionic donor (sensitizer) molecules are integrated through electrostatic interactions. Triplet energy is quantitatively transferred from the excited donor to the acceptor, which is followed by effective triplet energy migration among the pre-organized acceptors. It leads to TTA and concomitant UC emission in water. The dense acceptor chromophore arrays with extended hydrogen bonding networks show efficient barrier properties against molecular oxygen, as demonstrated by the stable UC emission even in air-saturated water.
Project description:Three diazafluorene derivatives triphenylamine (TPA)(PDAF) <i><sub>n</sub></i> (<i>n</i> = 1, 2, 3) serving as small molecular elements are designed and synthesized via concentrated sulfuric acid mediated Friedel-Crafts reaction. With highly nonplanar topological configuration, TPA(PDAF)<sub>3</sub> shows weaker intermolecular interaction in the solid states and thus exhibits single nanomolecular behavior, which is crucial for charge stored and retained in an organic field-effect transistor (OFET) memory device. Furthermore, diazafluorene derivatives possess a completely separate highest occupied molecular orbital/lowest unoccupied molecular orbital, which offers ideal hole and electron trapping sites. As charge storage elements, triphenylamine groups provide the hole trapping sites, while diazafluorene units provide the electron trapping sites and act as a hole blocking group to restrain the leakage of stored holes trapped in triphenylamine. The pentacene-based OFET memory device with solution-processing TPA(PDAF)<sub>3</sub> shows a good hole-trapping ability, high hole trapping density (4.55 × 10<sup>12</sup> cm<sup>-2</sup>), fast trapping speed (<20 ms), a large memory window (89 V), and a tunable ambipolar memory behavior. The optimized device shows a large ON/OFF current ratio (2.85 × 10<sup>7</sup>), good charge retention (>10<sup>4</sup> s), and reliable endurance properties. This study suggests that diazafluorene based donor-acceptor small molecular elements have great promise for high-performance OFET memory.