A series of Pt(II)–Schiff
base complexes were synthesized
as triplet sensitizers for the purpose of tuning the singlet and triplet
energy levels so as to minimize energy loss during triplet–triplet
annihilation (TTA) upconversion (UC). A deep-red to blue TTA-UC was
achieved with an unprecedentedly large anti-Stokes shift of 1.08 eV.
UC quantum yields of up to 21% (with a theoretical maximum efficiency
of 50%) were observed in solution. The complexes also showed efficient
UC emission in air-saturated hydrogels with a UC quantum yield up
to 14.8%, which is much higher than the highest previously reported
value. The low threshold excitation intensity provided by the present
system offers promising potential for application in terrestrial solar
energy conversion.
Stereoisomeric β-cyclodextrin (CD) dimers linked with a sulfur atom or an arene spacer were designed to create a tethered dual CD capsule for precisely manipulating the regioand enantioselectivities of the photocyclodimerization of 2anthracenecarboxylate (AC) to four stereoisomeric classical 9,10:9′,10′-cyclodimers and two nonclassical 5,8:9′,10′-cyclodimers. Among the dimeric CD hosts prepared, exo-3-thia-β-CD dimer formed 1:1 and 1:2 host−guest complexes with AC in aqueous solutions, the former of which hindered but the latter facilitated the AC photocyclodimerization with regio-and enantioselectivities much higher than those obtained with native β-CD or the rest of the β-CD dimers. The stereochemical outcomes turned out to be highly sensitive to and hence critically manipulable by the linking position and configuration of the connected saccharide units and the linker length, as well as the external variants, such as temperature, pH, and added salt. Eventually, the photocyclodimerization of AC mediated by the dimeric β-CD host gave enantiopure syn-head-to-tail-9,10:9′,10′-cyclodimer in 97−98% yield in a pH 5.1 buffer solution at 0.5 °C and also in an aqueous CsCl solution at −20 °C.
Visible-light-driven enantiodifferentiating photodimerization of 2-anthracenecarboxylic acid (AC) sensitized by Schiff base Pt(II) complex-grafted γ-cyclodextrins leads the first triplet-triplet annihilation-based catalytic photochirogenesis. The syn-head-to-tail (HT) photodimer 2 was achieved in up to 31.4% ee at 61.0% conversion in the presence of 0.5% equiv of the photocatalyst.
Several
γ-cyclodextrin (γ-CDx) derivatives were used
as chiral hosts for the photocyclodimerization of 2-anthracenecarboxylic
acid (AC). The effect of pH on photoreactivity and stereochemical
outcome of photoproducts was investigated. Upon changing the solution
pH, the stereochemical outcome of HH cyclodimer 3 was
inverted from 25.2% to −64.4% and 41.2% to −76.2%, respectively,
in the photocyclodimerization of AC mediated by bis-quinoline-modified γ-CDx 7 and its N-methylated derivative 8.
Water-soluble 9,10-diphenylanthracene-modified γ-cyclodextrin derivatives A1 and A2, in which the γ-cyclodextrin unit serves as a molecular host for a binding sensitizer, and the 9,10-diphenylanthracene moiety plays a role as an emitter/annihilator, were synthesized to investigate the supramolecular triplet-triplet annihilation (TTA) upconversion in aqueous solution. Both A1 and A2 readily aggregate and form nanoscale assemblies in water as a combined result of host-guest complexation and π-π stacking among the 9,10-diphenylanthracenes. The aggregation behavior of the supramolecular emitters was fully characterized by using a diversity of methods, including dynamic light scattering (DLS), SEM, NMR, fluorescence, and circular dichroism studies. Fluorescence spectroscopic analysis reveals that the emitters have high fluorescence quantum yields in water (82 and 90 % for A1 and A2, respectively), thus demonstrating that aggregation does not quench the fluorescence. By using a coordinated ruthenium sensitizer, a high TTA upconversion quantum yield of up to 6.9 % was observed for this supramolecular TTA system, which is significantly higher than the value (<0.5 %) obtained with nonassembled emitters in organic solvent and in contrast to the fact that TTA upconversion emission in aqueous solution is usually low or negligible. We ascribe the strong TTA upconversion emission in the present supramolecular assembly system to an efficient TTA process, which is facilitated along the stacked emitters by triplet energy migration and improved triplet-triplet energy transfer through host-guest complexation.
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