Diphenyldihydropentalenediones: Wide Singlet-Triplet Energy Gap Compounds Possessing the Planarly Fixed Diene Subunit

2,2,5,5-Tetramethyl-3,6-diphenyl-2,5-dihydropentalene-1,4-dione (PD-H) and its dimethoxy (PD-OCH₃) and bis(trifluoromethyl) derivatives (PD-CF₃) were developed as a new class of compounds possessing a wide excited singlet-triplet energy gap. The PD derivatives would also have a high energy level of the triplet-excited state (E _T) due to the planarity of the fused-diene subunit. The results of photophysical studies revealed that the energy level of the singlet-excited state (E _S) and E _T of PD-H are 2.88 and 1.43 eV, respectively. These values indicate that PD-H has the energy relationship, E _S > 2E _T, required for it to be a singlet fission (SF) material. Moreover, the introduction of electron-donating or -withdrawing groups on the benzene rings in PD-H enables fine-tuning of E _S and E _T. The results of transient absorption spectroscopic studies show that PD-H, PD-OCH₃, and PD-CF₃ in CH₂Cl₂ have respective T₁ lifetimes of 71, 118, and 107 μs, which are long enough to utilize its triplet exciton in other optoelectronic systems. These findings suggest that the PDs are potential candidates for SF materials with high E _T levels.

Facile strategy for obtaining luminescent polymorphs based on the chirality of a boron-fused azomethine complex

A chloro-substituted boron-fused azomethine complex (BAmCl) having a stereogenic boron center was synthesized for obtaining a luminescent chiral crystal. We succeeded in isolating the (R)- and (S)-enantiomers of BAmCl and preparing the homochiral polymorphic crystal, while we obtained the racemic crystal with rac-BAmCl. Single crystal X-ray diffraction analyses suggest that a variety of intermolecular interaction patterns and intrinsic flexibility of the molecular framework should play a significant role in stabilizing the homochiral crystal. We found the difference in molecular arrangements between the racemic and the homochiral crystals, and we observed distinctly different emission colors. In particular, we observed heat-initiated homogeneous racemization without the need for a solvent or catalyst in the molten state of the homochiral crystal (R)-BAmCl. Our results mean that chiral resolution of a flexible fused-skeleton having a stereogenic boron center can be a platform for creating luminescent polymorphic materials.

Synthesis of a Fluorescent Solvatochromic Resin Using Suzuki-Miyaura Cross-Coupling and Its Optical Waveguide Spectra to Measure the Solvent Polarity on the Surface

We have established a novel analytical method for solvent polarity on resin surface by combining the synthesis of fluorescent solvatochromic resin with optical waveguide spectrometry. The fluorescent solvatochromic resin was obtained via Suzuki-Miyaura cross-coupling between 4-iodobenzoic acid immobilized on Wang resin and 5-[4-(N,N-dihexylamino)phenyl]-2-thienylboronic acid N-methyl-iminodiacetic acid (MIDA) ester. The optical waveguide spectrometry studies on the resin showed a strong fluorescent solvatochromism in various organic solvents.

Vapochromic crystals: understanding vapochromism from the perspective of crystal engineering

Vapochromic materials, which undergo colour and/or emission changes upon exposure to certain vapours or gases, have received increasing attention recently because of their wide range of applications in, e.g., chemical sensors, light-emitting diodes, and environmental monitors. Vapochromic crystals, as a specific kind of vapochromic materials, can be investigated from the perspective of crystal engineering to understand the mechanism of vapochromism. Moreover, understanding the vapochromism mechanism will be beneficial to design and prepare task-specific vapochromic crystals as one kind of low-cost 'electronic nose' to detect toxic gases or volatile organic compounds. This review provides important information in a broad scientific context to develop new vapochromic materials, which covers organometallic or coordination complexes and organic crystals, as well as the different mechanisms of the related vapochromic behaviour. In addition, recent examples of supramolecular vapochromic crystals and metal-organic-framework (MOFs) vapochromic crystals are introduced.

Three differently coloured polymorphs of 3,6-bis(4-chlorophenyl)-2,5-dipropyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione

In this paper, the conformational polymorphism of a chlorinated diketopyrrolopyrrole (DPP) dye having flexible substituents in a non-hydrogen-bonding system is reported. The propyl-substituted DPP derivative (PR3C) has three polymorphic forms, each showing a different colour (red, orange and yellow). All polymorphs could be obtained concomitantly under various crystallization conditions. The results of the crystal structure analysis indicate that PR3C adopts different conformations in each polymorph. The packing effect caused by the difference in the arrangement of neighbouring molecules was found to play an important role in the occurrence of the observed polymorphism. The thermodynamic stability relationship between the three polymorphs was identified by thermal analysis and indicates that the yellow polymorph is the thermally stable form. The results indicate that the yellow form and orange form are enantiotropically related, and the other polymorph is monotropically related to the others.