This paper describes a study of excited-state properties of naphthalimide (NI) and four 4-substituted derivatives: 4-chloronaphthalimide (Cl-NI), 4-methylthionaphthalimide (MeS-NI), 4-nitronaphthalimide (O(2)N-NI), and 4-(N,N-dimethylaminonaphthalimide (Me(2)N-NI). Steady-state absorption and fluorescence spectra were collected in solvents of varying polarity to determine the excited-state character of NI derivatives. Furthermore, the excited-state dynamics were studied using femtosecond transient absorption spectroscopy. The experimental findings were compared to calculated data obtained using time-dependent density functional (TD-DFT) methods. We found that light absorption by all NI derivatives leads to the production of the second excited state (S(2)), which was found to have a n,pi* character. Within approximately 40 ps, the S(2) state undergoes internal conversion to produce the S(1) state. The S(1) state is relatively long-lived (approximately 4 ns) and has charge-transfer character in NI derivatives with electron-withdrawing and electron-donating groups (MeS-NI, O(2)N-NI, and Me(2)N-NI). In the case of NI and Cl-NI, the S(1) state has a pi,pi* character and undergoes intersystem crossing to produce the T(1) state within 400 ps.

Magnetite nanoparticles in the size range of 3.2-7.5 nm were synthesized in high yields under variable reaction conditions using high-temperature hydrolysis of the precursor iron(II) and iron(III) alkoxides in diethylene glycol solution. The average sizes of the particles were adjusted by changing the reaction temperature and time and by using a sequential growth technique. To obtain γ-iron(III) oxide particles in the same range of sizes, magnetite particles were oxidized with dry oxygen in diethylene glycol at room temperature. The products were characterized by DLS, TEM, X-ray powder diffractometry, TGA, chemical analysis, and magnetic measurements. NMR r1 and r2 relaxivity measurements in water and diethylene glycol (for OH and CH2 protons) have shown a decrease in the r2/r1 ratio with the particle size reduction, which correlates with the results of magnetic measurements on magnetite nanoparticles. Saturation magnetization of the oxidized particles was found to be 20% lower than that for Fe3O4 with the same particle size, but their r1 relaxivities are similar. Because the oxidation of magnetite is spontaneous under ambient conditions, it was important to learn that the oxidation product has no disadvantages as compared to its precursor and therefore may be a better prospective imaging agent because of its chemical stability.

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We investigated the electronic properties of N(5)-ethyl flavinium perchlorate (Et-Fl(+)) and compared them to those of its parent compound, 3-methyllumiflavin (Fl). Absorption and fluorescence spectra of Fl and Et-Fl(+) exhibit similar spectral features, but the absorption energy of Et-Fl(+) is substantially lower than that of Fl. We calculated the absorption signatures of Fl and Et-Fl(+) using time-dependent density functional theory (TD-DFT) methods and found that the main absorption bands of Fl and Et-Fl(+) are (π,π*) transitions for the S(1) and S(3) excited states. Furthermore, calculations predict that the S(2) state has (n,π*) character. Using cyclic voltammetry and a simplistic consideration of the orbital energies, we compared the HOMO/LUMO energies of Fl and Et-Fl(+). We found that both HOMO and LUMO orbitals of Et-Fl(+) are stabilized relative to those in Fl, although the stabilization of the LUMO level was more pronounced. Visible and mid-IR pump-probe experiments demonstrate that Et-Fl(+) exhibits a shorter excited-state lifetime (590 ps) relative to that of Fl (several nanoseconds), possibly due to faster thermal deactivation in Et-Fl(+), as dictated by the energy gap law. Furthermore, we observed a fast (23-30 ps) S(2) → S(0) internal conversion in transient absorption spectra of both Fl and Et-Fl(+) in experiments that utilized pump excitations with higher energy.

The presence of long-lived charge-separated excited states in metal−organic frameworks (MOFs) can enhance their photocatalytic activity by decreasing the probability that photogenerated electrons and holes recombine before accessing adsorbed reactants. Detecting these chargeseparated states via optical transient absorption, however, can be challenging when they lack definitive optical signatures. We investigate the long-lived excited state of a MOF with such vague optical properties, MIL-100(Fe), composed of Fe 3 -μ 3oxo clusters and trimesic acid linkers, using Fe K-edge X-ray transient absorption (XTA) spectroscopy to unambiguously determine its ligand-to-metal charge-transfer character. Spectra measured at time delays up to 3.6 μs confirm the long-lived nature of the charge-separated excited state. Several trinuclear iron μ 3 -oxo carboxylate complexes, which model the trinuclear cores of the MOF structure, are measured for comparison using both steady-state X-ray absorption spectroscopy and XTA to further support this assignment and corresponding decay time. The MOF is prepared as a colloidal nanoparticle suspension for these measurements, so both its fabrication and particle size analysis are presented as well.

A highly porous graphitic carbon material, dually-doped with P and S, was studied as a metal free catalyst for aerobic oxidation reactions. Catalytic mechanism studies suggest that the active centers, originated from P-and S-doping, additively/synergistically catalyze the aerobic oxidation of benzylic alcohols but with different pathways. For the first time, catalytic centers stemming from S-doping were experimentally identified to be exocyclic S species (C-S-C, sulfur out of the carbon ring), which are different from those proposed for electrochemical oxygen reduction reactions (ORR) with a 4epathway and oxygen evaluation reactions (OER). Notably, all the catalytic sites from both P and S doping share a similar "protruding out" pyramid structure, which is in contrast to the planar structure of the catalytic sites in Nor B-doped graphitic materials. The unique geometric structure of the catalytic sites can minimize substrate steric hindrance effects, endowing the P, S co-doped catalysts with a wide substrate scope and functional group tolerance. Furthermore, the unambiguous distinguishment of the catalytic sites from those in OER and ORR provides valuable guidance for designing and developing carbon materials with controlled active sites to satisfy different catalytic applications.

An iron-porphyrin-based metal organic framework PCN-222(Fe) is investigated upon postsynthetic reduction with piperidine. Fe K-edge X-ray absorption and Kβ mainline emission spectroscopy measurements reveal the local coordination geometry, oxidation, and spin state changes experienced by the Fe sites upon reaction with this axially coordinating reducing agent. Analysis and fitting of these data confirm the binding pattern predicted by a space-filling model of the structurally constrained pore environments. These results are further supported by UV-vis diffuse reflectance, IR, and resonance Raman spectroscopy data.

Ultralong afterglow emissions due to room‐temperature phosphorescence (RTP) are of paramount importance in the advancement of smart sensors, bioimaging and light‐emitting devices. We herein present an efficient approach to achieve rarely accessible phosphorescence of heavy atom‐free organoboranes via photochemical switching of sterically tunable fluorescent Lewis pairs (LPs). LPs are widely applied in and well‐known for their outstanding performance in catalysis and supramolecular soft materials but have not thus far been exploited to develop photo‐responsive RTP materials. The intramolecular LP M1BNM not only shows a dynamic response to thermal treatment due to reversible N→B coordination but crystals of M1BNM also undergo rapid photochromic switching. As a result, unusual emission switching from short‐lived fluorescence to long‐lived phosphorescence (rad‐M1BNM, τRTP=232 ms) is observed. The reported discoveries in the field of Lewis pairs chemistry offer important insights into their structural dynamics, while also pointing to new opportunities for photoactive materials with implications for fast responsive detectors.