Shun Omagari

Luminescent Mechanochromic 9-Anthryl Gold(I) Isocyanide Complex with an Emission Maximum at 900 nm after Mechanical Stimulation

Upon mechanical stimulation, 9-anthryl gold(I) isocyanide complex 3 exhibited a bathochromic shift of its emission color from the visible to the infrared (IR) region, which is unprecedented in its magnitude. Prior to exposure to the mechanical stimulus, the polymorphs 3α and 3β exhibit emission wavelength maxima (λem,max) at 448 and 710 nm, respectively. Upon grinding, the λem,max of 3αground and 3βground are bathochromically shifted to 900 nm, i.e., Δλem,max (3α) = 452 nm or 1.39 eV. Polymorphs 3α and 3β thus represent the first examples of mechanochromic luminescent materials with λem,max in the IR region.

Critical Role of Energy Transfer Between Terbium Ions for Suppression of Back Energy Transfer in Nonanuclear Terbium Clusters

Lanthanide (Ln(III)) complexes form an important class of highly efficient luminescent materials showing characteristic line emission after efficient light absorption by the surrounding ligands. The efficiency is however lowered by back energy transfer from Ln(III) ion to the ligands, especially at higher temperatures. Here we report a new strategy to reduce back energy transfer losses. Nonanuclear lanthanide clusters containing terbium and gadolinium ions, TbnGd9−n clusters ([TbnGd9−n(μ-OH)10(butylsalicylate)16]+NO3−, n = 0, 1, 2, 5, 8, 9), were synthesized to investigate the effect of energy transfer between Tb(III) ions on back energy transfer. The photophysical properties of TbnGd9−n clusters were studied by steady-state and time-resolved spectroscopic techniques and revealed a longer emission lifetime with increasing number of Tb(III) ions in TbnGd9−n clusters. A kinetic analysis of temperature dependence of the emission lifetime show that the energy transfer between Tb(III) ions competes with back energy transfer. The experimental results are in agreement with a theoretical rate equation model that confirms the role of energy transfer between Tb(III) ions in reducing back energy transfer losses. The results provide a new strategy in molecular design for improving the luminescence efficiency in lanthanide complexes which is important for potential applications as luminescent materials.

Effective Photosensitized Energy Transfer of Nonanuclear Terbium Clusters Using Methyl Salicylate Derivatives

The photophysical properties of the novel nonanuclear Tb(III) clusters Tb-L1 and Tb-L2 involving the ligands methyl 4-methylsalicylate (L1) and methyl 5-methylsalicylate (L2) are reported. The position of the methyl group has an effect on their photophysical properties. The prepared nonanuclear Tb(III) clusters were identified by fast atom bombardment mass spectrometry and powder X-ray diffraction. Characteristic photophysical properties, including photoluminescence spectra, emission lifetimes, and emission quantum yields, were determined. The emission quantum yield of Tb-L1 (Φ(ππ*) = 31%) was found to be 13 times larger than that of Tb-L2 (Φ(ππ*) = 2.4%). The photophysical characterization and DFT calculations reveal the effect of the methyl group on the electronic structure of methylsalicylate ligand. In this study, the photophysical properties of the nonanuclear Tb(III) clusters are discussed in relation to the methyl group on the aromatic ring of the methylsalicylate ligand.

Fully Conjugated Porphyrin Glass: Collective Light-Harvesting Antenna for Near-Infrared Fluorescence beyond 1 μm

Expanded π-systems with a narrow highest occupied molecular orbital–lowest unoccupied molecular orbital band gap encounter deactivation of excitons due to the “energy gap law” and undesired aggregation. This dilemma generally thwarts the near-infrared (NIR) luminescence of organic π-systems. A sophisticated cofacially stacked π-system is known to involve exponentially tailed disorder, which displays exceptionally red-shifted fluorescence even as only a marginal emission component. Enhancement of the tail-state fluorescence might be advantageous to achieve NIR photoluminescence with an expected collective light-harvesting antenna effect as follows: (i) efficient light-harvesting capacity due to intense electronic absorption, (ii) a long-distance exciton migration into the tail state based on a high spatial density of the chromophore site, and (iii) substantial transmission of NIR emission to circumvent the inner filter effect. Suppression of aggregation-induced quenching of fluorescence could realize collective light-harvesting antenna for NIR-luminescence materials. This study discloses an enhanced tail-state NIR fluorescence of a self-standing porphyrin film at 1138 nm with a moderate quantum efficiency based on a fully π-conjugated porphyrin that adopts an amorphous form, called “porphyrin glass”.