Synthesis and Characterization of Cyclotri- and Tetrasiloxanes with Pyrenyl Groups

Cyclosiloxanes directly bearing polyaromatic groups on silicon atoms have scarcely been reported. Herein, hexa(1-pyrenyl)cyclotrisiloxane (2) and octa(1-pyrenyl)cyclotetrasiloxane (3) were successfully prepared from di(1-pyrenyl)silanediol (1) in the presence of a weak base such as tetraethylammonium acetate and triethylamine in MeCN. The structure of the cyclosiloxanes bearing multiple pyrenyl groups in the solid and solution states was evaluated by NMR, X-ray crystallography, and density functional theory (DFT) calculations. All pyrenyl groups of 2 were oriented outward, and no π-π stacking of adjacent pyrenyl groups was observed. However, all pairs of adjacent pyrenyl groups at 1- and 3-positions in 3 are oriented in the same direction and were π-π stacked with respect to each other. The UV-vis spectra of 2 and 3 in organic solvents showed a slight broadening of the peaks, as observed for typical pyrene derivatives. Interestingly, the fluorescence spectra of 2 showed small monomer and strong excimer emissions; however, those of 3 showed only a strong excimer emission in all solvents. Partially pyrenylated cyclotri- and tetrasiloxanes (compounds 4 and 5) showed solvent-dependent monomer and excimer spectra as observed for di(1-pyrenyl)silane derivatives, implying that the excimer emissions of 2 and 3 arise from mainly geminal and vicinal pyrenyl groups, respectively.

Understanding and Tailoring Excited State Properties in Solution-Processable Oligo(p-phenyleneethynylene)s: Highly Fluorescent Hybridized Local and Charge Transfer Character via Experiment and Theory

Rod-shaped oligo(p-phenyleneethynylene) (OPE) offers an attractive π-framework for the development of solution-processable highly fluorescent molecules having tunable hybridized local and charge transfer (HLCT) excited states and (reverse) intersystem crossing ((R)ISC) channels. Herein, an HLCT oligo(p-phenyleneethynylene) library was studied for the first time in the literature in detail systematically via experiment and theory. The design, synthesis, and full characterization of a new highly fluorescent (Φ_PL-solution ∼ 1) sky blue emissive 4',4‴-((2,5-bis((2-ethylhexyl)oxy)-1,4-phenylene)bis(ethyne-2,1-diyl))bis(N,N-diphenyl-[1,1'-biphenyl]-4-amine) (2EHO-TPA-PE) was also reported. The new molecule consists of a D'-Ar-π-D-π-Ar-D' molecular architecture with an extended π-spacer and no acceptor unit, and detailed structural, physicochemical, single-crystal, and optoelectronic characterizations were performed. A high solid-state quantum efficiency (Φ_{PL-solid state} ∼ 0.8) was achieved as a result of suppressed exciton-phonon/vibronic couplings (no π-π interactions and multiple (14 per dimeric form) strong C-H···π interactions). Strong solution-phase/solid-state dipole-dependent tunable excited state behavior (local excited (LE) → HLCT → charge transfer (CT)) and decay dynamics covering a wide spectral region were demonstrated, and the CT state was observed to be highly fluorescent despite extremely large Stokes shift (∼130 nm)/fwhm (∼125 nm) and significant charge separation (0.75 charge·nm). Employing the Lippert-Mataga model, along with detailed photophysical studies and TDDFT calculations, key relationships between molecular design-electronic structure-exciton characteristics were elucidated with regards to HLCT and hot exciton channel formations. The interstate coupling between CT and LE states and the interplay of this coupling with respect to medium polarity were explored. A key relationship between excited-state symmetry breaking process and the formation of HLCT state was discussed for TPA-ended rod-shaped OPE π-systems. (R)ISC-related delayed fluorescence (τ ∼ 2-6 ns) processes were evident following the prompt decays (∼0.4-0.9 ns) both in the solution and in the solid-state. As a unique observation, the delayed fluorescence could be tuned and facilitated via small dielectric changes in the medium. Our results and the molecular engineering perspectives presented in this study may provide unique insights into the structural and electronic factors governing tunable excited state and hot-exciton channel formations in OPEs for (un)conventional solution-processed luminescence applications.

Recent developments in the field of photoluminescent organically modified cyclosiloxanes

Herein, we present a brief overview of the recent developments in the field of photoluminescent cyclosiloxanes, with a special focus on the synthesis and unique photophysical properties of newly reported silole-based cyclosiloxanes with pronounced aggregation-induced emission (AIE) behaviour. Comparisons of their photophysical data as well as the results of computational studies of various types of silole-based cyclosiloxanes are presented and their potential applications are briefly discussed.

The synthesis of chiral triphenylpyrrole derivatives and their aggregation-induced emission enhancement, aggregation-induced circular dichroism and helical self-assembly

AIEE-active chiral triphenylpyrrole derivatives possess aggregation-induced circular dichroism and circularly polarized luminescence features with self-assembling helical nanofibers.

AEE-active cyclic tetraphenylsilole derivatives with ∼100% solid-state fluorescence quantum efficiency

Two new strongly AEE active (I/I0 ≈ 94) tetraphenylsilole-containing cyclosiloxanes with cyan emissions (λem = 500 nm) and ∼100% solid state fluorescence quantum yields are reported. The intra- and intermolecular C-Hπ interactions in the crystal play a major role in the observed high solid state fluorescence quantum yields.