Understanding the influence of geometric and electronic structure on the excited state dynamical and photoredox properties of perinone chromophores.

In this work, a series of eight similarly structured perinone chromophores were synthesized and photophysically characterized to elucidate the electronic and structural tunability of their excited state properties, including excited state redox potentials and fluorescence lifetimes/quantum yields. Despite their similar structure, these chromophores exhibited a broad range of visible absorption properties, quantum yields, and excited state lifetimes. In conjunction with static and time-resolved spectroscopies from the ultrafast to nanosecond time regimes, time-dependent computational modeling was used to correlate this behavior to the relationship between non-radiative decay and the energy-gap law. Additionally, the ground and excited state redox potentials were calculated and found to be tunable over a range of 1 V depending on the diamine or anhydride used in their synthesis (Ered* = 0.45-1.55 V; Eox* = -0.88 to -1.67 V), which is difficult to achieve with typical photoredox-active transition metal complexes. These diverse chromophores can be easily prepared, and with their range of photophysical tunability, will be valuable for future use in photofunctional applications.

Blue-shifted aggregation-induced enhancement of a Sn(iv) fluoride complex: the role of fluorine in luminescence enhancement.

A new type of blue-shifted aggregation-induced emission enhancement was observed for a Sn(iv) fluoride complex, resulting in strong emission in the solid state as compared with that in solutions. The fluorinated Sn(iv) complex has a significantly more intense emission efficiency compared to the chlorinated Sn(iv) complex, which is attributed to stronger σ bonding.

A Facile Electrochemical Reduction Method for Improving Photocatalytic Performance of α-Fe2O3 Photoanode for Solar Water Splitting.

Electrochemical reduction method is used for the first time to significantly improve the photo-electrochemical performance of α-Fe2O3 photoanode prepared on fluorine-doped tin oxide substrates by spin-coating aqueous solution of Fe(NO3)3 followed by thermal annealing in air. Photocurrent density of α-Fe2O3 thin film photoanode can be enhanced 25 times by partially reducing the oxide film to form more conductive Fe3O4 (magnetite). Fe3O4 helps facilitate efficient charge transport and collection from the top α-Fe2O3 layer upon light absorption and charge separation to yield enhanced photocurrent density. The optimal enhancement can be obtained for <50 nm films because of the short charge transport distance for the α-Fe2O3 layer. Thick α-Fe2O3 films require more charge and overpotential than thinner films to achieve limited enhancement because of the sluggish charge transport over a longer distance to oxidize water. Electrochemical reduction of α-Fe2O3 in unbuffered pH-neutral solution yields much higher but unstable photocurrent enhancement because of the increase in local pH value accompanied by proton reduction at a hematite surface.

Organophotocatalysis: Insights into the Mechanistic Aspects of Thiourea-Mediated Intermolecular [2+2]†Photocycloadditions.

Mechanistic investigations of the intermolecular [2+2] photocycloaddition of coumarin with tetramethylethylene mediated by thiourea catalysts reveal that the reaction is enabled by a combination of minimized aggregation, enhanced intersystem crossing, and altered excited-state lifetime(s). These results clarify how the excited-state reactivity can be manipulated through catalyst-substrate interactions and reveal a third mechanistic pathway for thiourea-mediated organo-photocatalysis.

From containers to catalysts: supramolecular catalysis within cucurbiturils.

Cucurbiturils are a family of molecular container compounds with superior molecular recognition properties. The use of cucurbiturils for supramolecular catalysis is highlighted in this concept. Both photochemical reactions as well as thermal transformations are reviewed with an eye towards tailoring substrates for supramolecular catalysis mediated by cucurbiturils.

Fun with photons: selective light induced reactions in solution and in water soluble nano-containers.

Two distinct strategies for controlling selectivity, in particular stereoselectivity in photochemical reactions are reviewed. In the first strategy, supramolecular approach using cucurbituril nano-containers in catalytic amounts is employed to control selectivity during photochemical transformations. In the second approach, a generalized methodology for carrying out light-induced transformations in solution at ambient conditions is detailed where axially chiral motifs are employed to enantiospecifically transfer the axial chirality in the reactant to point chirality in the photoproduct(s).

Supramolecular photocatalysis: insights into cucurbit[8]uril catalyzed photodimerization of 6-methylcoumarin.

Guest induced shape change of the cucurbit[8]uril cavity is likely rate limiting in the supramolecular photocatalytic cycle for CB8 mediated photodimerization of 6-methylcoumarin.

Supramolecular photocatalysis by confinement--photodimerization of coumarins within cucurbit[8]urils.

Cucurbit[8]uril (as low as 10 mol%) acts as a supramolecular catalytic nanoreaction vessel and facilitates the photodimerization of coumarins in water leading to syn dimers. Saturation kinetics shows a sigmoidal dependence with a turnover number of 3.4 min(-1) and a Hill constant of 1.8 indicating a co-operative mechanism in the catalytic process.

Photodimerization and complexation dynamics of coumarins in the presence of cucurbit[8]urils.

Coumarin derivatives with non-polar substituents at the 6 or 7 position undergoes photodimerization in the presence of CB[8] in water to give the syn dimer as the major product. It is postulated that these neutral coumarins form dynamic complexes in the presence of CB[8] and the product selectivity is reflective of the type of complex, available volume in the CB[8] cavity and relative rate of photodimerization inside and outside the CB[8] cavity.

Manipulating photochemical reactivity of coumarins within cucurbituril nanocavities.

Coumarin derivatives that are either cationic (7-ammonium) or neutral (7-hydroxy, 7-methoxy, 6-methyl) form a 1:2 host-guest complex with cucurbit[8]uril (CB[8]). Direct irradiation of these coumarin@CB[8] complexes in water gives head-to-tail (HT) adduct as the major product. The nature of the functional group (polar or nonpolar) at the 6 or 7 position on the coumarin dictates the type of HT adduct ( syn- or anti-). It is postulated that the available free volume and the hydrophobic confined environment are responsible for the observed selectivity.