he use of donor–π–acceptor (D–π–A) skeletons is an effective strategy for the design of fluorophores with red-shifted emission. In particular, the use of amino and boryl moieties as the electron-donating...

We recently reported that fluorescent dye PB430, which consisted of a 2-phenyl-substituted benzophosphole P-oxide skeleton that was reinforced by a methylene bridge, showed pronounced photostability and, thus, high utility for applications in super-resolution stimulated emission depletion (STED) microscopy. Herein, we replaced the methylene bridge with another P=O group to 1) investigate the role of the bridging moieties; and 2) further modulate the fluorescence properties of this skeleton. We synthesized a series of phospholo[3,2-b]phosphole-based dyes-trans-PO-PB430, cis-PO-PB430, and trans-PO-PB460-all of which showed sufficient water solubility. Moreover, trans-PO-PB430 and trans-PO-PB460 exhibited intense green and orange fluorescence, respectively, and a high photostability that was comparable to that of PB430. In contrast, cis-PO-PB430 underwent rapid photobleaching upon continuous photoirradiation, which demonstrated the importance of steric shielding of the polycyclic skeleton by the substituents on the bridging moieties. The fluorescence properties of these dyes were insensitive to concentration, pH value, and polarity changes of the environment in solution. In addition, even in the solid state, these dyes showed strong green to orange emissions. These results demonstrate the potential utility of trans-PO-PB430 and trans-PO-PB460 as highly photostable fluorescent dyes.

The desulfonylative radical addition of tertiary alkyl groups to gem-difluoroalkenes by photoredox Ir-catalyst is described. This method exhibits broad substrate scope, affording structurally diverse (E)-fluoroalkene derivatives in a highly stereoselective manner. The resulting (E)-fluoroalkenes were converted into complex fused cyclic compounds by intramolecular cyclization reactions. Control experiments and theoretical calculations are consistent with a single Ir catalyst playing the dual role of generating radical species from sulfones via single electron transfer and mediating Z/E isomerization via energy transfer. A subset of fluoroalkenes provided Z stereoisomers with >90% selectivity, but the same alkenes could also be obtained as E isomers with high selectivity by taking advantage of a secondary Z to E photoisomerization.

The fluorescence quantum yield for fluorescent organic molecules is an important molecular property, and tuning it up is desired for various applications. For the computational estimation of the fluorescence quantum yield, the theoretical prediction of the nonradiative decay rate constant has become an attractive subject of study. The rate constant of thermally activated nonradiative decay is related to the activation energy in the photoreaction; thus, the accuracy and reliability of the excited-state potential energies in the quantum chemical computation are critical. In this study, we employed a second-order multireference perturbation wavefunction theory for studying the thermally activated decay via conical intersection (CI) of 1,1-dimethyldibenzo­[b,f]­silepin derivatives. The correlation between the computed activation energy to reach the CI geometry in the S1 state and the experimentally determined fluorescence quantum yield implied that silepins nonradiatively decay via the CI triggered by the twisting of the central C–C bond. Geometry optimization of the transition state using multireference perturbation theory drastically reduced the estimated activation energy. Our computation gave reasonable predictions of the activation free energies of photoexcited 1,1-dimethyldibenzo­[b,f]­silepin. The energy profiles and geometry optimizations using proper quantum chemical methods played a critical role in reliable estimation of the rate constant and fluorescence quantum yield.

Cinnamate derivatives show a variety of photo-induced reactions. Among them is trans−cis photoisomerization, which may involve the nonradiative decay (NRD) process. The extended multistate complete active space second-order perturbation (XMS-CASPT2) method was used in this study as a suitable theory for treating multireference electronic nature, which was frequently manifested in the photoisomerization process. The minimum energy paths of the trans−cis photoisomerization process of cinnamate derivatives were determined, and the activation energies were estimated using the resulting intrinsic reaction coordinate (IRC) paths. Natural orbital analysis revealed that the transition state's (TS) electronic structure is zwitterionic-like, elucidating the solvent and substituent effect on the energy barrier of photoisomerization paths. Furthermore, it was found that the charge on the pyramidalized carbon atom at the TS structure was strongly correlated with the activation energy barrier for the cinnamate derivatives. Thus, it seemingly provided a physical picture of the photoisomerization of cinnamates and was a good descriptor potentially applicable to molecular design for controlling the rate constant of the photoisomerization reaction.