A new Cu(II) complex of an asymmetrically dicondensed Schiff base (HL = N-(2-hydroxyacetophenylidene)-N'-salicylidene-1,3-propanediamine) derived from 1,3-propanediamine, salicylaldehyde, and o-hydroxyacetophenone has been synthesized. Using this complex, [CuL] (1), as a metalloligand, two new trinuclear Cu-Mn complexes, [(CuL)Mn(N)(HO)](ClO)·HO (2) and [(CuL)Mn(NCS)] (3), have been prepared. Single-crystal structural analyses reveal that complexes 2 and 3 both have the same bent trinuclear {(CuL)Mn} structural unit in which two terminal bidentate square-planar (CuL) units are chelated to the central octahedral Mn(II) ion. This structural similarity is also evident from the variable-temperature magnetic susceptibility measurements, which suggest that compounds 2 and 3 are both antiferromagnetically coupled with comparable exchange coupling constants (-21.8 and -22.3 cm, respectively). The only difference between 2 and 3 lies in the coordination around the central Mn(II) ion; in 3, two SCN groups are coordinated to the Mn(II), leaving a neutral complex, but in 2, one N group and one HO molecule are coordinated to give a positively charged species. The presence of such a labile HO coligand makes 2 catalytically active in mimicking two well-known polynuclear copper proteins, catecholase and phenoxazinone synthase. The turnover numbers (k) for the aerial oxidation of 3,5-di-tert-butylcatechol and o-aminophenol are 1118 and 6581 h, respectively, values which reflect the facility of the heterometallic catalyst in terms of both efficiency and catalytic promiscuity for aerial dioxygen activation. The mechanisms of these biomimetic oxidase reactions are proposed for the first time involving any heterometallic catalyst on the basis of mass spectral analysis, EPR spectroscopy, and cyclic voltammetry. The evidence of the intermediates indicates possible heterometallic cooperative activity where the substrates bind to a Mn(II) center and Cu(II) plays the role of an electron carrier for transformation of the phenolic substrates to their respective products with the reduction of aerial dioxygen.

A compact naphthalenediimide (NDI)–2,2,6,6‐tetramethylpiperidinyloxy (TEMPO) dyad has been prepared with the aim of studying radical‐enhanced intersystem crossing (EISC) and the formation of high spin states as well as electron spin polarization (ESP) dynamics. Compared with the previously reported radical–chromophore dyads, the present system shows a very high triplet state quantum yield (ΦT=74 %), a long‐lived triplet state (τT=8.7 μs), fast EISC (1/kEISC=338 ps), and absorption in the red spectral region. Time‐resolved electron paramagnetic resonance (TREPR) spectroscopy showed that, upon photoexcitation in fluid solution at room temperature, the D0 state of the TEMPO moiety produces strong emissive (E) polarization owing to the quenching of the excited singlet state of NDI by the radical moiety (electron exchange J>0). The emissive polarization then inverts into absorptive (A) polarization within about 3 μs, and then relaxes to a thermal equilibrium while quenching the triplet state of NDI. The formation and decay of the quartet state were also observed. The dyad was used as a three‐spin triplet photosensitizer for triplet–triplet annihilation upconversion (quantum yield ΦUC=2.6 %). Remarkably, when encapsulated into liposomes, the red‐light‐absorbing dyad–liposomes show good biocompatibility and excellent photodynamic therapy efficiency (phototoxicity EC50=3.22 μm), and therefore is a promising candidate for future less toxic and multifunctional photodynamic therapeutic reagents.

Generation of electron spin polarization (ESP) during the bimolecular quenching of an excited chromophore by a free radical is generally explained by the radical–triplet pair mechanism, which is capable of giving the magnitudes of ESP arising from the quenching of the singlet or the triplet excited chromophore. When the chromophore and the free radical are covalently linked, although there are several mechanisms to explain the observed spin-polarized electron paramagnetic resonance signals under a variety of experimental conditions and in different chromophore–radical systems, there are no schemes that allow quantitative determination of the magnitude of ESP. In this work, we present a phenomenological scheme with this objective. In this scheme, we have incorporated several concepts of the reversed quartet mechanism of Rozenshtein et al. [J. Phys. Chem. A 109, 11144 (2005)] to our phenomenological sequential quenching scheme [V. Rane and R. Das, J. Phys. Chem. A 119, 5515 (2015)] of ESP in covalently linked chromophore–radical systems. This phenomenological reversed quartet scheme is able to explain the observed inversion of ESP with time and can also give a quantitative measure of the absorptive and emissive ESP in such systems. We have applied this scheme to the photophysical quenching of a series of newly synthesized pyrene–TEMPO molecules, where a spacer group of different lengths covalently links the pyrene chromophore and the TEMPO free radical. Given the simplicity of our scheme, reasonable estimates of the magnitudes of the ESP have been obtained in all cases.

Quenching of excited states by a free radical is generally studied in systems where these two are separate entities freely moving in a liquid solution. Random diffusive encounters bring them together to cause the quenching and leave the spins of the radical polarized. In the dynamics of the radical-triplet pair mechanism of the generation of electron spin polarization (ESP), the distance-dependent exchange interaction plays a crucial role. To investigate how the distance between the excited molecule and the radical influences the ESP, we have covalently linked a naphthalene moiety to a TEMPO free radical through a spacer group of three different lengths. We compared the ESP process of these linked compounds with that of the usual "unlinked system" of naphthalene and TEMPO through time-resolved EPR experiments at low temperature in n-hexane solution. The time evolution of both the linked and the "unlinked system" was treated on a similar footing. The time-dependent EPR signal was analyzed by combining photophysical kinetics and time-dependent Bloch equations incorporating spin dynamics. Sequential quenching of the singlet state and the triplet state of naphthalene was seen in all the systems, as revealed through the spin-polarized TREPR spectra of opposite phase. The magnitudes of the ESP in the linked molecules were higher than those of the "unlinked system," showing that when the two moieties are held together greater mixing of quartet-doublet states takes place. The magnitudes of ESP steadily decrease with increasing the length of the spacer group. The polarization magnitudes due to triplet quenching and singlet quenching are very similar, differing by a factor of only ∼2. These characteristics show that for all the linked molecules the quenching takes place in the "weak exchange" regime and at almost the same distance of separation between the two moieties. Our results also showed that observation of small absorptive TREPR signals does not necessarily imply that its magnitude of polarization is small.

Cu-doped TiO2 photocatalysts [Cu-(1 to 10) atom %] synthesized by the sol–gel method and thoroughly characterized using several techniques were evaluated for photocatalytic reduction of CO2 under ambient conditions. These photocatalysts converted the reactant mixture of CO2 and moisture selectively into methane and oxygen under visible irradiation without the use of any sacrificial agent. The Cu dopant considerably lowered the bandgap of TiO2, thereby making it feasible for doped TiO2 to absorb light in the visible region. Beyond 1% Cu doping the Cu-doped TiO2 materials were far superior as compared to anatase TiO2 for the reduction of CO2 to CH4, and the lowest doping (Cu-1%) concentration in TiO2 lattice shows the maximum production of CH4 (1081 μL/h/g), whereas the highest doping (Cu-10%) shows the least activity (200 μL/h/g). The photoreduction activity was found to decrease with increasing Cu concentration; under similar conditions without use of any cocatalysts. X-ray photoelectron spectroscopy (XPS) results along with cyclic voltammetry data (CV) indicate that the doped Cu exists in the two stable oxidation states of Cu1+ and Cu2+. The electron paramagnetic resonance (EPR) results show that there are two distinct types of Cu2+ species in the doped system and clustering of the Cu2+ states at higher doping concentrations may be detrimental for the photoactivity. The electronegative surface Cu1+ species along with O-vacancies seem to play the most important role in the photocatalytic reduction of CO2 to CH4.