Ultrafast Excited-State Dynamics and Dispersion Studies of Third-Order Optical Nonlinearities in Novel Corroles

Ultrafast nonlinear optical (NLO) properties of four novel Corroles in the visible spectral range (660–800 nm) were evaluated using picosecond Z-scan technique. Ultrafast excited state dynamics have also been appraised with picosecond (ps) and femtosecond (fs) degenerate pump–probe techniques using excitation wavelengths of 800 and 600 nm, respectively. The excitation by 800 nm photons resulted in two-photon absorption at adequately high peak intensities, thereby facilitating the access to higher excited states (S_<i>n</i>). The nonradiative relaxation mechanisms from these states, reflected in the pump–probe data, consisted of double-exponential decay with a slow component in the range of 54–277 ps and faster component in the range of 2.0 to 2.5 ps. When excited with 600 nm photons (unfocused), photoinduced absorption was observed with the first excited state S₁ being populated, and as a consequence single decay was observed in the data of all molecules studied. These retrieved lifetimes were analogous to those obtained with picosecond pump–probe data. The long lifetime is attributed to nonradiative decay from the S₁ state with possible contribution from triplet states, whereas the shorter lifetime is attributed to the internal conversion (S₂ to S₁*), followed by vibrational relaxation (S₁* to S₁) processes. Time-resolved fluorescence lifetime measurements revealed the magnitude of radiative lifetimes to be in the nanosecond regime. NLO coefficients were evaluated from the Z-scan data at wavelengths of 660, 680, 700, 740, and 800 nm. Large two-photon absorption coefficients (β)/cross-sections (σ₂) at 740 nm/680 nm were recorded for these molecules, making them apposite for applications such as two-photon induced photodynamic therapy and lithography. Figure of merit, <i>T</i>, was <1 for all molecules at 740 and 800 nm, suggesting that these molecules find use in photonic device applications

High Capacity Na–O₂ Batteries: Key Parameters for Solution-Mediated Discharge

The Na–O₂ battery offers an interesting alternative to the Li–O₂ battery, which is still the source of a number of unsolved scientific questions. In spite of both being alkali metal–O₂ batteries, they display significant differences. For instance, Li–O₂ batteries form Li₂O₂ as the discharge product at the cathode, whereas Na–O₂ batteries usually form NaO₂. A very important question that affects the performance of the Na–O₂ cell concerns the key parameters governing the growth mechanism of the large NaO₂ cubes formed upon reduction, which are a requirement of viable capacities and high performance. By comparing glyme-ethers of various chain lengths, we show that the choice of solvent has a tremendous effect on the battery performance. In contrast to the Li–O₂ system, high solubilities of the NaO₂ discharge product do not necessarily lead to increased capacities. Herein we report the profound effect of the Na⁺ ion solvent shell structure on the NaO₂ growth mechanism. Strong solvent–solute interactions in long-chain ethers shift the formation of NaO₂ toward a surface process resulting in submicrometric crystallites and very low capacities (ca. 0.2 mAh/cm²_(geom)). In contrast, short chains, which facilitate desolvation and solution-precipitation, promote the formation of large cubic crystals (ca. 10 um), enabling high capacities (ca. 7.5 mAh/cm²_(geom)). This work provides a new way to look at the key role that solvents play in the metal–air system