Jye-Shane Yang

“Too Small, Too Big, and Just Right” − Optical Sensing of Molecular Conformations in Self-Assembled Capsules

Irradiation of dimethylbenzil within a cylindrical capsule gives bright green phosphorescence, while irradiation of benzil and dimethoxybenzil in the same capsule results in high energy blue fluorescence. This difference is likely due to the geometric restrictions imposed by the capsules space on its excited guests, forcing a trans-planar conformation in some cases and cis-skewed in others.

Site-Selective Hydrogen-Bonding-Induced Fluorescence Quenching of Highly Solvatofluorochromic GFP-like Chromophores

The unconstrained green fluorescence protein (GFP)-like chromophore m-DMABDI displays a high solvatofluorochromicity in aprotic solvents, but the fluorescence is quenched in protic solvents. According to the site-specific intramolecularly hydrogen-bonded analogs 1OH and 2OH, the hydrogen bonding to the carbonyl oxygen is more important than that to the imino nitrogen of the imidazolinone group in the fluorescence quenching.

A Pentiptycene-Derived Light-Driven Molecular Brake

A room-temperature light-driven molecular brake (1), consisting of a pentiptycene rotator, a 3,5-dinitrophenyl brake, and a photoisomerizable ethenyl spacer, is reported. The rotation rates of the rotator differ by about 9 orders of magnitude between the brake-on (cis-1) and brake-off (trans-1) states.

Effect of Ground-State Twisting on the trans → cis Photoisomerization and TICT State Formation of Aminostilbenes§

The synthesis, X-ray crystal structures, and photochemical behavior of a series of methyl- and ethylene-bridge-substituted trans-4-(N-(4-cyanophenyl)amino)stilbenes (3-8) are reported and compared to those of the parent compound 1CN. Aminostilbene 1CN displays dual fluorescence in polar solvents due to planar and twisted intramolecular charge-transfer (PICT and TICT) states. Alkyl substitution on the stilbene group of 1CN significantly perturbs its photochemistry, including fluorescence, trans --> cis photoisomerization, and TICT state formation. The alkyl substituent effect can be dissected into electronic and steric influences, and both are position dependent, which is vinyl alpha-carbon > vinyl beta-carbon > phenyl o-carbon. The main outcome of the alkyl substituent effect is to lower the barrier for the singlet-state photoisomerization. As a result, the quantum yield for photoisomerization is increased, and that for fluorescence is reduced. The corresponding quantum yield for TICT state formation in polar solvents is reduced only when significant ground-state twisting (a steric influence) is present. The alkyl substitution exerts little or no effect on the rate of intersystem crossing.

Isotruxene-Derived Cone-Shaped Organic Dyes for Dye-Sensitized Solar Cells

The synthesis, electronic properties, and performance in dye-sensitized solar cells (DSSCs) of four cone-shaped organic dyes (ITD, ITD-Th, ITD-Hx, and ITD-OM) containing the isotruxene π-scaffold are reported. Selective substitution of the unsymmetrical isotruxene core with two diarylamino donors and one cyanocarboxylic acid acceptor was achieved by using a prefunctionalized dibromoisotruxene building block. The ortho-para-branched isotruxene core allows strong electronic couplings among the donors and the acceptor, leading to red-shifted absorption profiles with significant charge-transfer character. All four isotruxene dyes display reversible anodic waves in cyclic voltammagrams with both HOMO and LUMO potentials suitable for application in DSSCs. The DSSCs fabricated with these cone-shaped organic dyes exhibited high open-circuit voltages (0.67-0.76 V) and fill factors (0.67-0.72) with a power conversion efficiency (η) up to 5.45%, which is 80% of the ruthenium dye N719-based standard cell fabricated and measured under the same conditions.

Phospholipid-Induced Aggregation and Anthracene Excimer Formation

The zinc complex of anthryl bis(dipicolylamine) (1) aggregates upon binding with long-chain aliphatic phosphates and displays anthracene excimer fluorescence, which provides a new strategy toward detection of the biologically important lysophosphatidic acid in aqueous solution.

Multicolor Fluorescence Writing Based on Host–Guest Interactions and Force‐Induced Fluorescence‐Color Memory

A new strategy is reported for multicolor fluorescence writing on thin solid films with mechanical forces. This concept is illustrated by the use of a green-fluorescent pentiptycene derivative 1, which forms variably colored fluorescent exciplexes: a change from yellow to red was observed with anilines, and fluorescence quenching (a change to black) occurred in the presence of benzoquinone. Mechanical forces, such as grinding and shearing, induced a crystalline-to-amorphous phase transition in both the pristine and guest-adsorbed solids that led to a change in the fluorescence color (mechanofluorochromism) and a memory of the resulting color. Fluorescence drawings of five or more colors were created on glass or paper and could be readily erased by exposure to air and dichloromethane fumes. The structural and mechanistic aspects of the observations are also discussed.

Pentiptycene-derived light-driven molecular brakes: substituent effects of the brake component.

Five pentiptycene-derived stilbene systems (1 R; R = H, OM, NO, Pr, and Bu) have been prepared and investigated as light-driven molecular brakes that have different-sized brake components (1 H<1 OM<1 NO<1 Pr<1 Bu). At room temperature (298 K), rotation of the pentiptycene rotor is fast (k(rot)=10(8)-10(9)  s(-1)) with little interaction with the brake component in the trans form ((E)-1 R), which corresponds to the brake-off state. When the brake is turned on by photoisomerization to the cis form ((Z)-1 R), the pentiptycene rotation can be arrested on the NMR spectroscopic timescale at temperatures that depend on the brake component. In the cases of (Z)-1 NO, (Z)-1 Pr, and (Z)-1 Bu, the rotation is nearly blocked (k(rot)=2-6 s(-1)) at 298 K. It is also demonstrated that the rotation is slower in [D(6)]DMSO than in CD(2)Cl(2). A linear relationship between the free energies of the rotational barrier and the steric parameter A values is present only for (Z)-1 H, (Z)-1 OM, and (Z)-1 NO, and it levels off on going from (Z)-1 NO to (Z)-1 Pr and (Z)-1 Bu. DFT calculations provide insights into the substituent effects in the rotational ground and transition states. The molar reversibility of the E-Z photoswitching is up to 46%, and both the E and Z isomers are stable under the irradiation conditions.

Anomalous crystal packing of iptycene secondary diamides leading to novel chain and channel networks

The crystal structures of triptycene and pentiptycene secondary diamides 1 and 2 grown from methanol solutions are reported. In addition, the crystal structure of 1 from a toluene–methanol mixed solution was also determined. The molecular structures of 1 in both crystals are folded, and an anomalous hydrogen-bonding chain motif is generated. On the other hand, the pentiptycene diamide 2 adopts an extended conformation, and the amide groups do not participate in any known amide–amide hydrogen-bonding patterns. Instead, interdigitated molecular stacking results in grid-like channels having a void volume of ca. 25–30% of the crystal. The potential of iptycene-derived building blocks in the design of new organic crystalline materials is thus demonstrated. Figure options Download full-size image Download as PowerPoint slide

A Pentiptycene-Derived Molecular Brake: Photochemical E→Z and Electrochemical Z→E Switching of an Enone Module

The synthesis and brakelike performance of a new molecular system (1) consisting of a pentiptycene rotor and a 2-methyleneindanone brake are reported. The rotation kinetics of the rotor was probed by both variable-temperature (1)H and (13)C NMR spectroscopy and DFT calculations, and the switching between the brake-on and brake-off states was conducted by a combination of photochemical and electrochemical isomerization. Because of the greater steric hindrance between the rotor and the brake units in the Z form ((Z)-1) than in the E form ((E)-1), rotation of the rotor is slowed down 500-fold at room temperature (298 K) on going from (E)-1 to (Z)-1, corresponding to the brake-off and brake-on states, respectively. The (E)-1→(Z)-1 photoisomerization in acetonitrile is efficient and reaches an (E)-1/(Z)-1 ratio of 11:89 in the photostationary state upon excitation at 290 nm, attributable to a much larger isomerization quantum efficiency for (E)-1 versus (Z)-1. An efficient (Z)-1→(E)-1 isomerization (96%) was also achieved by electrochemical treatment through the radical anionic intermediates. Consequently, the reversibility of the E-Z switching of 1 is as high as 85%. The repeated E-Z switching of 1 with alternating photochemical and electrochemical treatments is also demonstrated.