Progress in Fast and Red Plastic Scintillators

Radiological detection where Cherenkov residual background can be prominent requires scintillators with increased emission wavelength. Cherenkov residual background precludes the use of UV-emitting sensors such as plastic scintillators. However, the literature is scarce in red-emitting plastic scintillators and only one commercial scintillator is currently available (BC-430, from Saint-Gobain Crystals and Detectors). In addition, X-ray imaging or time-of-flight positron emission tomography (ToF-PET) applications are also demanding on this type (color) of scintillators, but such applications also require that the material displays a fast response, which is not particularly the case for BC-430. We present herein our latest developments in the preparation and characterization of fast and red plastic scintillators for this application. Here, ‘fast’ means nanosecond range decay time and ‘red’ is an emission wavelength shifted towards more than 550 nm. At first, the strategy to the preparation of such material is explained by decomposing the scintillator to fundamental elements. Each stage is then optimized in terms of decay time response, then the elemental bricks are arranged to give plastic scintillator formulations that are compatible with the abovementioned characteristics. The results are compared with the red-emissive BC-430 commercial plastic, and the ultra-fast, violet-emitting BC-422Q 1% plastic. In particular, the first-time use of trans-4-dimethylamino-4′-nitrostilbene in the scintillation field as a red wavelength shifter allowed preparing plastic scintillators with the following properties: λemmax 554 nm, photoluminescence decay time 4.2 ns, and light output ≈ 6100 ph/MeV. This means a scintillator almost as bright as BC-430 but at least three times faster. This new sensor might provide useful properties for nuclear instrumentation.

A Boron Difluoride Hydrazone (BODIHY) Polymer Exhibits Aggregation-Induced Emission

Polymers that exhibit aggregation-induced emission (AIE) find use, for example, as cell-imaging agents and as fluorometric sensors due to their unique optical properties. However, the structural diversity of AIE-active polymers has not necessarily advanced at the same rate as their applications. In this work, ring-opening metathesis polymerization is used to synthesize the first example of a polymer (M_n  = 61,600 g mol^-1 , Đ = 1.32) containing boron difluoride hydrazone (BODIHY) heterocycles in its repeating unit. The BODIHY monomer and polymer described absorb and emit in the visible region in solution (λ_abs  = 428 and 429 nm, λ_em  = 528 and 526 nm) and as thin films (λ_abs  = 443 and 440 nm, λ_em  = 535 and 534 nm). Monomer (Φ_Film  = 10%) and polymer (Φ_Film  = 6%) exhibit enhanced emission as thin films compared to solution (Φ_Soln  ≤ 1%) as well as AIE upon the addition of water to DMF solutions as a result of restriction of intramolecular motion. Enhancement factors for the monomer and polymer are determined to be 58 and 15, respectively. The title BODIHY polymer exhibited an earlier onset of AIE and enhanced sensitivity to solution viscosity when compared to the parent monomer.

Luminescent color tuning with polymer films composed of boron diiminate conjugated copolymers by changing the connection points to comonomers

The boron diiminate polymers with variable connection points and optical properties were synthesized.

Development of solid-state emissive o-carboranes and theoretical investigation of the mechanism of the aggregation-induced emission behaviors of organoboron “element-blocks”

This paper presents the aggregation-induced emission (AIE) properties of o-carborane derivatives and proposes a potential strategy for constructing AIE-active organoboron complexes via the enhancement of freedom of intramolecular mobility. Initially, the optical properties of o-carborane derivatives with or without the fused ring structure at the C–C bond in o-carborane in which elongation should be induced by photo-excitation according to theoretical calculations were compared. Accordingly, it was shown that large mobility at the C–C bond in o-carborane should be responsible for the annihilation of emission in solution, leading to the AIE property. From this result, it was presumed that by enhancing the freedom of intramolecular mobility in conventional luminescent organoboron complexes, the deactivation of the excited state in solution and emission recovery in the aggregate can be induced. Based on this idea, we have performed several studies and introduce two representative results. Firstly, the decrease in luminescent properties of boron dipyrromethene (BODIPY) in solution by introducing a movable functional group is explained. Next, the AIE behaviors of boron ketoiminates and the potential mechanism concerning conformational changes for the deactivation of the excited state in the solution state are illustrated. It is proposed that enhancement of the freedom of mobility in the excited state of luminescent organoboron complexes could be a potential strategy for realizing AIE behaviors.

Heat-Resistant Mechanoluminescent Chromism of the Hybrid Molecule Based on Boron Ketoiminate Modified Octasubstituted Polyhedral Oligomeric Silsesquioxane

The development of a heat-resistant mechanoluminescent chromic (MLC) dye based on an organic-inorganic hybrid molecular scaffold is reported. Luminescent organoboron complexes were introduced into polyhedral oligomeric silsesquioxane (POSS) at each vertex through the rigid linker structure in the Suzuki-Miyaura cross-coupling reaction. From optical measurements, it was found that the crystalline state of the modified POSS showed orange emission. By adding mechanical forces to the sample, regular structures collapsed. Correspondingly, the emission color turned to yellow. Interestingly, the yellow-emissive amorphous state can be preserved during heating. By radially distributing luminescent chromophores on the rigid cube, both optical and thermal properties can be modulated to realize heat- resistant MLC behavior. POSS can contribute not only to expressing the MLC behavior, but also to improving thermal stability in the amorphous state. This is the first example, to the best of our knowledge, to offer the rational design of a heat-resistant MLC material.

Preservation of main-chain conjugation through BODIPY-containing alternating polymers from electronic interactions with side-chain substituents by cardo boron structures

This manuscript describes the synthesis and electronic structures of the modified boron dipyrromethene derivatives containing cardo boron.