Motoyuki Uejima

A light-emitting mechanism for organic light-emitting diodes: molecular design for inverted singlet–triplet structure and symmetry-controlled thermally activated delayed fluorescence

The concepts of symmetry-controlled thermally activated delayed fluorescence (SC-TADF) and inverted singlet–triplet (iST) structure are proposed. Molecules that can exhibit SC-TADF or have an iST structure can be employed as light-emitting molecules in organic light-emitting diodes. The molecular symmetry plays crucial roles in these concepts since they are based on the selection rules for the electric dipole transition, intersystem crossing, and nonradiative vibronic (electron-vibration) transitions. In addition to the symmetry conditions for the SC-TADF and iST molecules, the molecules should have small diagonal and off-diagonal vibronic coupling constants for suppressing vibrational relaxations and nonradiative vibronic transitions, respectively, and a large transition dipole moment for the fluorescence process. Analyses using the vibronic coupling and transition dipole moment densities are employed to reduce the vibronic coupling constants and to increase the transition dipole moment. The preferable point groups in the development of SC-TADF and iST molecules are discussed on the basis of the ratios of forbidden pairs of irreducible representations. It is found that the existence of the inversion symmetry is preferable for designing SC-TADF and iST molecules. On the basis of these guiding principles, we designed some anthracene and pyrene derivatives as candidate iST molecules. Their electronic structures, spin–orbit couplings, transition dipole moments, and vibronic couplings are discussed.

One-Shot Double Amination of Sondheimer–Wong Diynes: Synthesis of Photoluminescent Dinaphthopentalenes

Photoluminescent diamino-substituted dinaphthopentalenes were synthesized successfully by the treatment of in situ prepared dinaphthocyclooctadiyne with lithium amide. This reaction involves a series of transformations including the nucleophilic addition of the lithium amide to a triple bond of the cyclooctadiyne moiety, transannulation, protonation of the resulting pentalene anion, and the nucleophilic substitution of the pentalene core with the lithium amide. In this procedure, a novel double amination step plays a key role. When the diamino-substituted dinaphthopentalenes were irradiated with UV light in toluene, fluorescence was observed at around 580 nm (ΦF < 0.03).

Vibronic coupling density and related concepts

Vibronic coupling density is derived from a general point of view as a one-electron property density. Related concepts as well as their applications are presented. Linear and nonlinear vibronic coupling density and related concepts, orbital vibronic coupling density, reduced vibronic coupling density, atomic vibronic coupling constant, and effective vibronic coupling density, illustrate the origin of vibronic couplings and enable us to design novel functional molecules or to elucidate chemical reactions. Transition dipole moment density is defined as an example of the one-electron property density. Vibronic coupling density and transition dipole moment density open a way to design light-emitting molecules with high efficiency.

Electronic spectra of cycl[3.3.2]azine and related compounds: solvent effect on vibronic couplings.

Quantitative ab initio calculations are presented for the ultraviolet-visible peaks of cycl[3.2.2]azine and its mono- and dibenzannulated polycyclic compounds at the multistate CASPT2 (MS-CASPT2) level of theory, with 11 nm deviation from the experimental S0 → S1 absorption. The electrophilic substitution reactions of cycl[3.2.2]azine, benzo[a]/[g]annulated cycl[3.2.2]azines, and 6-dimethylamino[2.2.3]cyclazine-1-carboxylates with 3-cyano-4-methylthiomaleimide gave the corresponding functionalized cycl[3.2.2]azine derivatives, which exhibited the absorption maxima around 510-630 nm. The first intense peaks were investigated by means of time-dependent density functional theory (TD-DFT). These peaks were systematically underevaluated by ∼50 nm, within the acceptable accuracies of TD-DFT. Furthermore, we calculated vibronic coupling constants of the electronic excited states of cycl[3.2.2]azine and simulated absorption spectra both in vacuo and in ethanol. The solvent effect is found to enhance oscillator strengths and vibronic couplings. This is because the solvent effect gives rise to changes in the electron density difference on the phenyl ring, and in turn, the intensified overlap between the electron density difference and the potential derivative in the phenyl ring leads to enhanced vibronic couplings in ethanol.

Vibronic coupling density analysis for the chain-length dependence of reorganization energies in oligofluorenes: a comparative study with oligothiophenes

The vibronic coupling constants and reorganization energies of oligofluorenes OF(n) (n = 1-6) are calculated for their cationic states (hole transport). Those of oligothiophenes OT(2n) (n = 1-6) are also calculated for comparison. The vibronic coupling constants of OF(n) are smaller than those of OT(2n), and decrease with increasing n. For the elucidation of the small vibronic couplings of the oligofluorenes, the calculated vibronic coupling constants are analyzed on the basis of the concept of vibronic coupling density. The vibronic coupling density of OF(n) becomes small in the middle of the chain with increasing n because of the reduction in the electron-density difference between the neutral and cationic states. It is found that orbital relaxation plays a crucial role in the distribution of the electron-density difference. From the fragment molecular orbital analyses, the large orbital relaxation in OF(n) is found to originate from the small transfer integral between the fragment molecular orbitals. These findings led to a design principle for a carrier-transporting oligomer/polymer with small vibronic couplings, or small reorganization energy, as follows: the orbital interaction between the monomers should be small from the view of vibronic couplings.