Torsion Angle Analysis of a Thermally Activated Delayed Fluorescence Emitter in an Amorphous State Using Dynamic Nuclear Polarization Enhanced Solid-State NMR

The torsion angle between donor and acceptor segments of a thermally activated delayed fluorescence (TADF) molecule is one of the most critical factors in determining the performance of TADF-based organic light-emitting diodes (OLEDs) because the torsion angle affects not only the energy gap between the singlet and triplet but also the oscillator strength and spin–orbit coupling. However, the torsion angle is difficult to analyze, because organic molecules are in an amorphous state in OLEDs. Here, we determined the torsion angle of a highly efficient TADF emitter, DACT-II, in an amorphous state by dynamic nuclear polarization enhanced solid-state NMR measurements. From the experimentally obtained chemical shift principal values of 15N on carbazole, we determined the average torsion angle to be 52°. Such quantification of the torsion angles in TADF molecules in amorphous solids will provide deep insight into the TADF mechanism in amorphous OLEDs

Theoretical Determination of Rate Constants from Excited States: Application to Benzophenone

A cost-effective method of theoretically predicting electronic-transition rate constants from the excited states of molecules is reported. This method is based on density functional theory calculations of electronic states and quantitative rate constant determination with the Fermi golden rule. The method is applied to the theoretical determination of the excited-state decay mechanism of photoexcited benzophenone, a representative molecule in photochemistry and biochemistry. Calculated rate constants for benzophenone are quantitatively consistent with experimental ones, which validates the reliability of our rate constant calculation. The calculated population kinetics indicate that S1 → T2 → T1 → S0 is the predominant decay pathway

Visible-Light-Induced Reversible Complexation Mediated Living Radical Polymerization of Methacrylates with Organic Catalysts

Photoinduced reversible complexation mediated polymerization (photo-RCMP) was developed as a new photoinduced living radical polymerization (LRP). It consisted of an alkyl iodide as a dormant species and an amine as a catalyst, using visible light at 350–600 nm. The amine catalysts include such common compounds as tributylamine. Mechanistically, the polymerization is induced by the photolysis of the dormant species and dormant species/catalyst complex, which frequently occurs as the main activation process. The polymer molecular weight and its distribution (Mw/Mn = 1.1–1.4) were well controlled in the polymerizations of methyl methacrylate and some functional methacrylates up to fairly high conversions in many cases. Perfectly no polymerization took place without photoirradiation, meaning that the system is an ideal photo “on”–“off” switchable system. The polymerization rate was also finely tunable by the external irradiation power. Attractive features of photo-RCMP include the uses of inexpensive compounds and visible light, good polydispersity control, good tolerance to functional groups, and fine response to external photoirradiation

Promoting Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence via the Heavy-Atom Effect

Thermally activated delayed fluorescence (TADF) molecules are promising for realizing durable organic light-emitting diodes in all color regions. Fast reverse intersystem crossing (RISC) is a way of improving the device lifetime of TADF-based organic light-emitting diodes. To date, RISC rate constants (kRISC) of 108 s–1 have been reported for metal-free TADF molecules. Here, we report the heavy-atom effect on TADF and a molecular design for further promoting RISC. First, we reproduced all the relevant rate constants of a sulfur-containing TADF molecule (with kRISC of 108 s–1) via density functional theory. The role of the heavy-atom effect on the rapid RISC process was clarified. Our calculations also predicted that much larger kRISC (>1010 s–1) will be obtained for selenium- and tellurium-containing TADF molecules. However, a polonium-containing molecule promotes phosphorescence without exhibiting TADF, indicating that a too strong heavy-atom effect is unfavorable for achieving both rapid RISC and efficient TADF

CP/MAS ¹³C NMR Characterization of the Isomeric States and Intermolecular Packing in Tris(8-hydroxyquinoline) Aluminum(III) (Alq₃)

The isomeric states and intermolecular packing of tris(8-hydroxyquinoline) aluminum(III) (Alq3) in the α-, γ-, and δ-crystalline forms and in the amorphous state, which are important for understanding the light-emitting and electron-transport properties, have been analyzed by CP/MAS 13C NMR. This simple NMR experiment shows that the isomeric state of α- and amorphous Alq3 is meridional, whereas that of γ- and δ-Alq3 is facial. In the amorphous Alq3, the inclusion of facial isomers has been under debate. Our experiments show that meridional isomers are dominant in the amorphous Alq3, although the existence of facial isomers cannot be completely denied. The local structure of amorphous Alq3 is similar to that of α-Alq3 and is significantly different from those of γ- and δ-Alq3. Among these Alq3 samples, the effect of intermolecular interaction is not found only for γ-Alq3. This finding can explain the good solvent solubility of γ-Alq3, compared with the other crystalline forms. It is also shown that the structures are locally disordered not only for amorphous Alq3 but also for α-Alq3, although clear X-ray diffraction peaks are observed for α-Alq3. In contrast, the local structures of γ- and δ-Alq3 are well defined. A clear relation is found between the spectral patterns of CP/MAS 13C NMR and the fluorescence wavelengths; the samples, which consist of facial isomers, show blue-shifted fluorescence compared with those of meridionals

Refined Structure Determination of Blue-Emitting Tris(8-hydroxyquinoline) Aluminum(III) (Alq₃) by the Combined Use of Cross-Polarization/Magic-Angle Spinning ¹³C Solid-State NMR and First-Principles Calculation

The combined use of cross-polarization/magic-angle spinning (CP/MAS) ¹³C NMR experiments and gauge-including projector-augmented wave (GIPAW) isotropic chemical shift calculations is an easy and useful method for the structural refinement of organic aggregates. In this study, the method is applied to an important material for organic light-emitting diodes, tris­(8-hydroxyquinoline) aluminum­(III) (Alq₃). CP/MAS ¹³C NMR spectra include precise structural information of not only the conformation of the molecules but also the intermolecular packing. First, the structural refinements were performed for the Alq₃ in the γ- and δ-crystalline forms employing the combined method. Second, information on intramolecular structures and intermolecular structures was distinguished by comparing GIPAW calculations for crystals under periodic boundary conditions and those for isolated molecules extracted from the crystals. It was found from the analysis that the γ-Alq₃ and δ-Alq₃ crystals have similar intramolecular structures both in the facial isomeric state, whereas their intermolecular packing is significantly different. Both the γ-Alq₃ and δ-Alq₃ crystals exhibit unusual blue emission, which is different from conventional green emission, and the origin of the difference has been debated. This investigation shows that the origin of the blue-shifted emission is the isomeric states of Alq₃, not the intermolecular packing

Refined Structure Determination of Blue-Emitting Tris(8-hydroxyquinoline) Aluminum(III) (Alq₃) by the Combined Use of Cross-Polarization/Magic-Angle Spinning ¹³C Solid-State NMR and First-Principles Calculation

The combined use of cross-polarization/magic-angle spinning (CP/MAS) ¹³C NMR experiments and gauge-including projector-augmented wave (GIPAW) isotropic chemical shift calculations is an easy and useful method for the structural refinement of organic aggregates. In this study, the method is applied to an important material for organic light-emitting diodes, tris­(8-hydroxyquinoline) aluminum­(III) (Alq₃). CP/MAS ¹³C NMR spectra include precise structural information of not only the conformation of the molecules but also the intermolecular packing. First, the structural refinements were performed for the Alq₃ in the γ- and δ-crystalline forms employing the combined method. Second, information on intramolecular structures and intermolecular structures was distinguished by comparing GIPAW calculations for crystals under periodic boundary conditions and those for isolated molecules extracted from the crystals. It was found from the analysis that the γ-Alq₃ and δ-Alq₃ crystals have similar intramolecular structures both in the facial isomeric state, whereas their intermolecular packing is significantly different. Both the γ-Alq₃ and δ-Alq₃ crystals exhibit unusual blue emission, which is different from conventional green emission, and the origin of the difference has been debated. This investigation shows that the origin of the blue-shifted emission is the isomeric states of Alq₃, not the intermolecular packing

π‑Extended Planarized Triphenylboranes with Thiophene Spacers

Planarized triphenylboranes extended with thiophene or bithiophene spacers were synthesized, which showed intense fluorescences in solution and reversible redox waves for reduction in cyclic voltammetry. Organic light-emitting diodes (OLEDs) using these compounds as an electron-transporting material were fabricated

π‑Extended Planarized Triphenylboranes with Thiophene Spacers

Planarized triphenylboranes extended with thiophene or bithiophene spacers were synthesized, which showed intense fluorescences in solution and reversible redox waves for reduction in cyclic voltammetry. Organic light-emitting diodes (OLEDs) using these compounds as an electron-transporting material were fabricated

Kinetics of “Melting” of Sucrose Crystals

Kinetics of “melting” of sucrose crystals has been examined by conventional differential scanning calorimetry (DSC) and fast-scan calorimetry in terms of the possibility of a clear distinction between physical melting and chemical decomposition processes by fast scan up to 10 000 K s^–1. On the basis of a modeling of the crystal melting kinetics with superheating and the possible influence of thermal lag, the heating rate dependence of “melting” was carefully examined. The equilibrium melting point <i>T</i>_M of sucrose crystals at a zero heating rate was estimated to be <i>T</i>_M = 188.9 ± 1.2 °C by fast-scan calorimetry, and the heat of fusion of 46 kJ mol^–1 was determined by conventional DSC, which is in agreement with the reported values in the literature. The Kissinger plot of the peak temperatures by heating runs and the plot of characteristic times of isothermal runs against the inverse of absolute temperature suggested a kinetic diagram, in which the “melting” behaviors above and below <i>T</i>_M are qualitatively different with purely physical melting above <i>T</i>_M and “melting” initiated by chemical decomposition at active sites below <i>T</i>_M