Internal B ← O Bond Facilitated Photo/Thermal Isomerization of Tetra-Coordinated Boranes.

A new series of O∧C-chelate tetra-coordinated boranes with naphtha-aldehyde as the chelate backbone have been synthesized. Their photophysical and photochemical properties have been examined, which show that all of the compounds can undergo both photo and thermal transformations, generating aryl-migrated [1,2]oxaborinine derivatives as the major products. 1,3-Sigmatropic shifts and an intramolecular nucleophilic addition mechanism are proposed for the photochemical and thermal conversion pathways, respectively.

A New Platform of B/N-Doped Cyclophanes: Access to a π-Conjugated Block-Type B3 N3 Macrocycle with Strong Dipole Moment and Unique Optoelectronic Properties.

We herein describe a new design principle to achieve B/N-doped cyclophane where an electron-donor block of three triarylamines (Ar3 N) and an acceptor block of three triarylboranes (Ar3 B) are spatially separated on opposite sides of the π-extended ring system. DFT computations revealed the distinct electronic structure of the block-type macrocycle MC-b-B3N3 with a greatly enhanced dipole moment and reduced HOMO-LUMO energy gap in comparison to its analogue with alternating B and N sites, MC-alt-B3N3. The unique arrangement of borane acceptor Ar3 B and amine donor Ar3 N components in MC-b-B3N3 induces exceptionally strong intramolecular charge transfer in the excited state, which is reflected in a largely red-shifted luminescence at 612 nm in solution. The respective linear open-chain oligomer L-b-B3N3 was also synthesized for comparison. Our new approach to donor-acceptor macrocycles offers important fundamental insights and opens up a new avenue to unique optoelectronic materials.

Internal B-O Bond-Facilitated Photoisomerization of Boranes: Ring Expansion Versus Oxyborane Elimination/Intramolecular Diels-Alder Addition.

Boron compounds (1-4) containing an internal B-O bond have been found to undergo facile multistructural transformations upon irradiation at 365 or 410 nm, generating rare 8-membered B,O-heterocycles (1c-4c). In addition, 2 and 3 also undergo an intramolecular Diels-Alder addition and oxyborane elimination concomitantly, via intermediates 2b/3b, producing 2d/3d. The pathways to isomer c and product d were found to be a thermal process and a photo process, respectively.

Stabilising fleeting intermediates of stilbene photocyclization with amino-borane functionalisation: the rare isolation of persistent dihydrophenanthrenes and their [1,5] H-shift isomers.

The key intermediate, 4a,4b-dihydrophenanthrene (DPH), involved in the photocyclization of stilbene and derivatives is known to be unstable, and is therefore poorly characterized/understood. We have found that functionalising stilbenes with NMe2 and BMes2 groups can greatly enhance the stability of 4a,4b-DPHs, allowing quantitative isolation and full characterization of these rare species. Furthermore, we discovered that the new amino-borane decorated 4a,4b-DPHs can undergo thermal [1,5] H sigmatropic shift, forming isomers 4a,10a-DPHs. Both 4a,4b-DHPs and 4a,10a-DHPs are stable towards air and moisture, while only the former were found to undergo oxidative dehydrogenation upon irradiation at 365 nm under air, yielding brightly blue/green fluorescent NMe2 and BMes2 functionalised phenanthrene analogues. Control studies established that the trans-Mes2B-Ph-NMe2 unit is responsible for the stability of these isolated 4a,4b-DHPs and their [1,5]-H shift isomers.

A simple multi-responsive system based on aldehyde functionalized amino-boranes.

A simple aldehyde functionalized amino-triarylborane donor-acceptor system (BO-1) was found to display distinct responses toward multiple external stimuli including solvent, temperature and pressure, with emission colours changing from blue to red. The operating mechanism most likely involves reversible switching between closed and open structures based on an intramolecular B ← O bond. Imbedded donor-acceptor charge transfer transitions played a key role in the multi-coloured fluorescent response of this new boron system to external stimuli. Multi-coloured and erasable fluorescent images on solid substrates based BO-1's "turn-on" response toward solvents, particularly water, are demonstrated.

Photoswitchable and Water-Soluble Fluorescent Nano-Aggregates Based on a Diarylethene-Dansyl Dyad and Liposome.

In this work, a unique approach is developed to generate photoswitchable and water-soluble fluorescent nano-aggregates. Initially, a new light-controlled diarylethene-dansyl dyad DAE 1 is formed by linking two dansyl fluorophores covalently to a symmetrical dithienylethene backbone, whose photophysical properties can be reversibly switched by optical stimuli. Subsequently, the water insolubility of the molecular switch 1 is overcome by incorporating it into the bilayer of liposome DPPC (1,2-dihexadecanoyl-sn-glycero-3-phosphocholine) in water. This strategy creates stable fluorescent nano-aggregates OF-1@DPPC (≈25 nm diameter) that are soluble in an aqueous medium. The nano-aggregates OF-1@DPPC retain and even improve the photoswitchable fluorescence properties of DAE 1. More importantly, OF-1@DPPC exhibits a remarkable photostability and fatigue resistance after 5 cycles of irradiation with UV and visible light, which is crucial for its practical application.

Photocontrolled Reversible Luminescent Lanthanide Molecular Switch Based on a Diarylethene-Europium Dyad.

A new europium complex coordinated between a Eu(III) ion and an unsymmetrical diarylperfluorocyclopentene yields a light-controlled diarylethene-europium dyad, DAE@TpyEu(tta)3, whose photophysical properties can be reversibly switched by optical stimuli. When DAE@TpyEu(tta)3 is exposed to 365 nm UV light, an efficient intramolecular photochromic fluorescence resonance energy transfer (pc-FRET) occurs between the emission of the Eu(3+) donor (D) and the absorption of the diarylethene acceptor (A) in closed-form DAE@TpyEu(tta)3 accompanied by luminescence quenching. However, the pc-FRET process could be effectively inhibited by visible light (λ > 600 nm) irradiation, and the lanthanide emission of DAE@TpyEu(tta)3 is rapidly recovered. Furthermore, this luminescent lanthanide molecular switch could serve as a highly reliable and sensitive "turn on" fluorescent marker in living cells irradiated by red light without any optical interference.