Quantum Chain Reaction of Tethered Diarylcyclopropenones in the Solid State and Their Distance-Dependence in Solution Reveal a Dexter S₂–S₂ Energy-Transfer Mechanism

When promoted to their second singlet excited state (S₂) in benzene, alkyl-linked dimers of diarylcyclopropenone undergo a photodecarbonylation reaction with quantum yields varying from Φ = 0.7 to 1.14. Quantum yields greater than 1.0 in solution rely on an adiabatic reaction along the S₂ energy surface where the immediately formed excited-state product transfers energy to the unreacted molecule in the dimer to generate a second excited state. By determination of the quantum yields of decarbonylation for the linked diarylcyclopropenones with linkers of various lengths it was shown that S₂ → S₂ energy transfer is limited to distances shorter than ca. 6 Å. Notably, the quantum chain reaction occurs with similar efficiency for all the linked diarylcyclopropenones dimers in the solid state

Elucidating the Effects of a Rare-Earth Oxide Shell on the Luminescence Dynamics of Er³⁺:Y₂O₃ Nanoparticles

Rare-earth (RE) (Er³⁺ and Yb³⁺, Er³⁺)-doped yttrium oxide (Y₂O₃) core–shell particles were synthesized in this work using a two-step process where the cores were formed by molten salt synthesis while the shell was deposited by a sol–gel process. The cores were 100–150 nm, and a shell layer, up to 12 nm thick, was controllable based on the mass ratio between the RECl₃ salts and the Er³⁺:Y₂O₃ (1 mol %) particles. A passive Y₂O₃ shell layer, at an optimal thickness around 8 nm, passivated the surface quenching sites and resulted in a 53% increase in photoluminescence lifetimes and visible separation in Stark splitting. Optically active shell layers, such as Yb₂O₃ and Yb³⁺:Y₂O₃, not only passivated the quenching sites but also facilitated energy transfer between the spatially controlled RE ions. Furthermore, the effect of surface passivation on the upconversion luminescence was determined through the purposed dynamic processes to corroborate the effect of the hydroxyl groups on energy dissipation. The addition of a passive shell layer or a sensitizer reduced the upconversion to a two-photon process due to a decreased branching ratio at the ⁴I_11/2 energy level. Yb₂O₃ is deemed the most effective shell material due to the largest increase photoluminescence intensity at 1535 nm as a function of the pump power and the lifetime of the ⁴S_3/2 radiative transition, important in upconversion luminescence. The increased lifetime and low pump power achieved with Er³⁺:Y₂O₃|Yb₂O₃ core–shell phosphors hold promise in lighting devices for improved overall device efficiency

Reaction Mechanism in Crystalline Solids: Kinetics and Conformational Dynamics of the Norrish Type II Biradicals from α-Adamantyl-<i>p</i>-Methoxyacetophenone

In an effort to determine the details of the solid-state reaction mechanism and diastereoselectivity in the Norrish type II and Yang cyclization of crystalline α-adamantyl-<i>p</i>-methoxyacetophenone, we determined its solid-state quantum yields and transient kinetics using nanocrystalline suspensions. The transient spectroscopy measurements were complemented with solid-state NMR spectroscopy spin–lattice relaxation experiments using isotopically labeled samples and with the analysis of variable-temperature anisotropic displacement parameters from single-crystal X-ray diffraction to determine the rate of interconversion of biradical conformers by rotation of the globular adamantyl group. Our experimental findings include a solid-state quantum yield for reaction that is 3 times greater than that in solution, a Norrish type II hydrogen-transfer reaction that is about 8 times faster in crystals than in solution, and a biradical decay that occurs on the same time scale as conformational exchange, which helps to explain the diastereoselectivity observed in the solid state

Excited State Kinetics in Crystalline Solids: Self-Quenching in Nanocrystals of 4,4′-Disubstituted Benzophenone Triplets Occurs by a Reductive Quenching Mechanism

We report an efficient triplet state self-quenching mechanism in crystals of eight benzophenones, which included the parent structure (<b>1</b>), six 4,4′-disubstituted compounds with NH₂ (<b>2</b>), NMe₂ (<b>3</b>), OH (<b>4</b>), OMe (<b>5</b>), COOH (<b>6</b>), and COOMe (<b>7</b>), and benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (<b>8</b>). Self-quenching effects were determined by measuring their triplet–triplet lifetimes and spectra using femtosecond and nanosecond transient absorption measurements with nanocrystalline suspensions. When possible, triplet lifetimes were confirmed by measuring the phosphorescence lifetimes and with the help of diffusion-limited quenching with iodide ions. We were surprised to discover that the triplet lifetimes of substituted benzophenones in crystals vary over 9 orders of magnitude from ca. 62 ps to 1 ms. In contrast to nanocrystalline suspensions, the lifetimes in solution only vary over 3 orders of magnitude (1–1000 μs). Analysis of the rate constants of quenching show that the more electron-rich benzophenones are the most efficiently deactivated such that there is an excellent correlation, ρ = −2.85, between the triplet quenching rate constants and the Hammet σ⁺ values for the 4,4′ substituents. Several crystal structures indicate the existence of near-neighbor arrangements that deviate from the proposed ideal for “n-type” quenching, suggesting that charge transfer quenching is mediated by a relatively loose arrangement

Cruciforms’ Polarized Emission Confirms Disjoint Molecular Orbitals and Excited States

Steady-state and time-resolved polarized spectroscopy studies reveal that electronic excitation to the third excited state of 1,4-distyryl-2,5-bis(arylethynyl)benzene cruciforms results in fluorescence emission that is shifted an angle of ca. 60°. This result is consistent with quantum chemical calculations of the lowest electronic excited states and their transition dipole moments. The shift originates from the disjointed nature of the occupied molecular orbitals being localized on the different branches of the cruciforms