Evidence and mechanism of efficient thermally activated delayed fluorescence promoted by delocalized excited states.

The design of organic compounds with nearly no gap between the first excited singlet (S1) and triplet (T1) states has been demonstrated to result in an efficient spin-flip transition from the T1 to S1 state, that is, reverse intersystem crossing (RISC), and facilitate light emission as thermally activated delayed fluorescence (TADF). However, many TADF molecules have shown that a relatively appreciable energy difference between the S1 and T1 states (~0.2 eV) could also result in a high RISC rate. We revealed from a comprehensive study of optical properties of TADF molecules that the formation of delocalized states is the key to efficient RISC and identified a chemical template for these materials. In addition, simple structural confinement further enhances RISC by suppressing structural relaxation in the triplet states. Our findings aid in designing advanced organic molecules with a high rate of RISC and, thus, achieving the maximum theoretical electroluminescence efficiency in organic light-emitting diodes.

Design of weak-donor alkyl-functionalized push-pull pyrene dyes exhibiting enhanced fluorescence quantum yields and unique on/off switching properties.

We designed, synthesized, and evaluated environmentally responsive solvatochromic fluorescent dyes by incorporating weak push-pull moieties. The quantum yields of the push (alkyl)-pull (formyl) pyrene dyes were dramatically enhanced by the introduction of alkyl groups into formylpyrene (1-formylpyrene: Φ(F) =0.10; 3,6,8-tri-n-butyl-1-formylpyrene: Φ(F) =0.90; in MeOH). The new dyes exhibited unique sensitivity to solvent polarity and hydrogen-bond donor ability, and specific fluorescence turn-on/off properties (e.g., 3,6,8-tri-n-butyl-1-formylpyrene: Φ(F) =0.004, 0.80, 0.37, and 0.90 in hexane, chloroform, DMSO, and MeOH, respectively). Here, the alkyl groups act as weak donors to suppress intersystem crossing by destabilizing the HOMOs of 1-formylpyrene while maintaining weak intramolecular charge-transfer properties. By using alkyl groups as weak donors, environmentally responsive, and in particular, pH-responsive fluorescent materials may be developed in the future.

A three-dimensional assembly of pyrene dye on a tetraphenylethane scaffold enhances fluorescence quantum yield.

A three-dimensional pyrene assembly on a tetraphenylethane skeleton enhanced the fluorescence quantum yield compared to the yield obtained when using monomeric species, without changing the shape of the emission spectrum. The unique spacing of the pyrene units may prevent intramolecular fluorescence quenching or non-radiative decay.

Fluorescence enhancement of pyrene chromophores induced by alkyl groups through σ-π conjugation: systematic synthesis of primary, secondary, and tertiary alkylated pyrenes at the 1, 3, 6, and 8 positions and their photophysical properties.

We have systematically synthesized 1-, 3-, 6-, and 8-alkyl-substituted pyrene derivatives using the latest synthesis methods and investigated the effects of alkyl substitution on the photophysical properties of the pyrene chromophore. Like the trimethylsilyl group, which is known to enhance the fluorescence properties of some chromophores through σ*-π* conjugation, alkyl groups (primary, secondary, and tertiary) enhanced the fluorescence quantum yield of the pyrene chromophore through σ-π conjugation in most cases. While these enhancements in the fluorescence quantum yield were beyond expectations, the results were supported by absolute measurements. These results also indicate that ubiquitous alkyl groups can be used to tune the photophysical properties of the pyrene chromophore, as well as to improve the solubility or prevent aggregation. In other words, they can be used to develop new photofunctional materials.

The factor which determines whether excitation of charge-transfer complexes leads to final net products in comparison with the reactivity on excitation of one of the components.

Selective excitation of charge-transfer complexes of indene or acenaphthylene with various electron acceptors does or does not afford final net reaction products, depending on the free energy of the resulting radical ion pairs over the ground state, -deltaGBET, with threshold values. A similar factor governs the efficiency of the reaction on direct excitation of either the donor or the acceptor of their components, except that it does not fall to nil below the threshold and the reaction affords higher quantum yields than the selective excitation of the charge-transfer complex.