Charge-transfer Energy Levels Research Articles

A facile strategy for enhancing reverse intersystem crossing of red thermally activated delayed fluorescence emitters

By enhancing electron-withdrawing abilities of the secondary acceptor attachments into the main acceptor core, their charge transfer triplet ( 3 CT) energy levels are gradually reduced, while the locally excited triplet ( 3 LE A ) levels are well maintained. The well-aligned excited states, consequently, results in the enhancement of the rates of reverse intersystem crossing (RISC), and boosting the device efficiencies in red OLEDs. • A facile strategy to modulate CT states while maintaining LE state is proposed. • CT energy levels are gradually reduced, while the 3 LE A ones are well maintained. • Fast k RISC , small ΔE ST and high PLQY can be realized simultaneously. • mDPBPZ-DPXZ-based red OLED exhibits much higher EQE compared to its counterparts. Realizing highly efficient reverse intersystem crossing (RISC) process for red thermally activated delayed fluorescence (TADF) emitters remains a formidable challenge. Herein we demonstrate that charge transfer (CT) energy levels are gradually reduced with enhancing electron-withdrawing abilities of the secondary acceptor (A) attachments. Meanwhile, their local excited triplet ( 3 LE A ) state levels are well maintained due to the conjugations between the main framework and the attachments are all similar for these A segments. The optimized molecular energy level alignments are obtained in the case of mDPBPZ-DPXZ, in which 3 CT and the 3 LE A states are almost degenerate, and gives rise to a small singlet–triplet energy difference ( ΔE ST ) of 0.02 eV, a fast RISC rate of 1.54 × 10 6 s −1 and a high photoluminescence quantum yield of 93%, simultaneously. And the corresponding device achieves an external quantum efficiency of 21.6%, significantly higher than the other counterparts. This work demonstrates that the energy alignment between the CT and 3 LE A state can be easily managed by introducing secondary A attachments on original A framework, which shifts the 1 CT and 3 CT state to lower energies without affecting the 3 LE A energy. It provides a facile design strategy to promote the RISC process, which is particular useful in designing highly efficient red TADF emitters.

Studies on the local structures and the spin Hamiltonian parameters for Fe3+ in CdX (X = S, Se, Te)

The improved perturbation formulas of the spin Hamiltonian parameters (zero-field splitting D and g factors) for a 3d5 ion in trigonally distorted tetrahedra are constructed from the cluster approach by including both the crystal-field and charge-transfer contributions. These formulas are applied to the studies of the local structures and the electron paramagnetic resonance (EPR) spectra for Fe3+ in CdX (X = S, Se, Te). The impurity Fe3+ is found not to occupy exactly the host Cd2+ sites but to experience the small outward shifts 0.014 and 0.006 Å away from the ligand triangles along the C3 axis in CdS and CdSe, respectively. The charge-transfer contributions to the spin Hamiltonian parameters are important and increase significantly with increasing atomic number of the ligand (i.e., S2− < Se2− < Te2−) arising from the decreases of charge-transfer energy levels and the increases of ligand spin–orbit coupling coefficients. The results are discussed.

Theoretical calculations of spin-Hamiltonian parameters for the (MoOX5)2− (X=Cl, Br) metallic complexes in solution or frozen-glass

The spin-Hamiltonian parameters (g factors g//, g⊥ and hyperfine structure constants A//, A⊥) of the (MoOX5)2− (X=Cl, Br) metallic complexes in solution or frozen-glass are calculated from the high-order perturbation formulas based on the two-mechanism model. In these formulas, the contributions to spin-Hamiltonian parameters due to both the widely-applied crystal-field (CF) mechanism and the charge-transfer (CT) mechanism (which is neglected in CF theory) are taken into account, and the needed CF and CT energy levels are obtained from the optical spectra. The calculated results with two adjustable parameters are in reasonable agreement with the experimental values. The calculations show that (i) the relative importance of CT mechanism in (MoOBr5)2− metallic complexes is larger than that in (MoOCl5)2− ones because of the stronger covalence of Mo5+–Br− combination, and (ii) in both (MoOCl5)2− and (MoOBr5)2− metallic complexes, the contributions to spin-Hamiltonian parameters due to CT mechanism should also be taken into account because of the high valence state of Mo5+ ion.

Photo-Alignment Characteristics of Charge-Transfer Complexes Forming Polyimides with Tetramethyl Phenylene Diamine

The photo-alignment characteristics of 2,3,5,6-tetramethyl-1,4-phenylene diamine (TeMPD)- based polyimides with a good electron-releasing property were systematically investigated, such as 3,3,4,4-diphenylsulfonetetracarboxylic dianhydride-TeMPD, 4,4-(hexafluoroisopropylidene) diphthalic anhydride-TeMPD, and cyclobutane-1,2,3,4-tetracarboxylic dianhydride-TeMPD. TeMPD-based polyimides with various charge-transfer energy levels, varying from the lowest occupied molecular orbital in the diamine component of the polyimide to the highest occupied molecular orbital in the acid component, were used. All of the TeMPD-based polyimide films irradiated with linearly polarized ultraviolet light exhibited photo-alignment characteristic with the same direction, regardless of the different imidization ratios. The chemical structure and electronic state depended on the formation of charge-transfer complexes and did not affect the apparent photo alignment.

A study of the spin-Hamiltonian parameters for Cr5+ at the rhombically-distorted tetrahedral P5+ site of Ca2PO4Cl crystal using a two-mechanism model

The complete high-order perturbation formulae of spin-Hamiltonian (SH) parameters (g factors gi and hyperfine structure constants Ai , where i = x, y, z) containing contributions from both the crystal-field (CF) and charge-transfer (CT) mechanisms (the latter mechanism is neglected in the widely-used CF theory) are established for d1 ions in rhombic tetrahedra. From these formulae, the SH parameters of Cr5+ ion at the rhombically-distorted tetrahedral P5+ site of Ca2PO4Cl crystal are calculated. The CF and CT energy levels used in the calculation are obtained from the optical spectra of the studied Ca2PO4Cl : Cr5+ crystal. The calculated results showed reasonable agreement with the experimental values. The signs of the hyperfine structure constants Ai and the relative importance of the CT mechanism to SH parameters are acquired from the calculations.

Singlet oxygen quenching and the redox properties of hydroxycinnamic acids

The singlet oxygen quenching rate constants (k q) for a range of hydroxycinnamic acids in acetonitrile and D 2O solutions were measured using time resolved near infrared phosphorescence in order to establish their antioxidant activity. The magnitude of k q observed depends on both the nature of the substituent groups and solvent polarity. The variations in k q depend on the energy of the hydroxycinnamic acid/molecular oxygen charge transfer states, (O 2 δ−...HCA δ+). In D 2O the values of k q range from 4 × 10 7 M −1 s −1 to 4 × 10 6 M −1 s −1 for caffeic acid and o-coumaric acid respectively. In acetonitrile, the charge transfer energy levels are raised and this is reflected in lower singlet oxygen quenching rate constants with a k q value of 5 × 10 6 M −1 s −1 for caffeic acid. The phenoxyl radical spectra derived from the hydroxycinnamic acids were determined using pulse radiolysis of aqueous solutions and the reduction potentials were found to range from 534 to 596 mV. A linear correlation is observed between reduction potential, and hence free energy for electron transfer, and log k q. These correlations suggest a charge transfer mechanism for the quenching of singlet oxygen by the hydroxycinnamic acids.

Charge-transfer interactions, exciplex formations and ionic dissociations in singlet oxygen reactions

Intermolecular perturbation and configuration interaction calculations have been carried out to elucidate the attacking modes of singlet molecular oxygen ( 1O 2) to allyl olefins and electron-rich olefins, which are classified into four groups from their molecular structures. It is found: (1) that the attacking modes are dependent on the molecular structure of substrates ; (2) that the charge-transfer (CT) interactions between 1O 2 and substrates are particularly important for the formation of exciplexes through which the ene and (2+2) reactions of 1O 2 proceed ; and (3) that the CT energy levels are important in governing the fraction of ionic dissociation to produce Superoxide anion and the relative ratio between the (4+2) and (2+2) reactions of 1O 2 with dienes, heterocycles and related species.