Heavy-Cation-Induced Phosphorescence of Alkanones and Azoalkanes in Zeolites As Hosts: Induced S1 (nπ*) to T1 (nπ*) Intersystem Crossing and S0 to T1 (nπ*) Absorption

Origin of Intense Luminescence from Supramolecular 2D Molecular Crystals

Luminescence enhancement in 2D molecular crystals (2D crystals) is promising for a variety of optical applications, yet the availability is limited because of unclear mechanism and inefficient design strategy of luminescence control. Herein, the room temperature phosphorescence from micron long molecular thin free-standing 2D crystals of a mono-cyclometalated Ir(III) complex designed at the water surface is reported. A large luminescence enhancement is observed from the 2D crystals at 300 K, which is comparable with the rigidified solution at 77 K suggesting room temperature phosphorescence origin of the luminescence. In situ synchrotron grazing incidence X-ray diffraction measurements determine the constituent centered rectangular unit cells with precise molecular conformation that promotes the formation of 2D crystals. The molecular crystal design leads to a reduced singlet-triplet energy gap (ΔE_ST ) and mixing of singlet-triplet states by spin-orbit coupling (SOC) for efficient intersystem crossing, which explains the phosphorescence origin at room temperature and luminescence enhancement. The supramolecular assembly process provides an elegant design strategy to realize room temperature phosphorescence from 2D crystals by rigid intermolecular interactions.

Room-Temperature Phosphorescence from Encapsulated Pyrene Induced by Xenon

Phosphorescence from pyrene especially at room temperature is uncommon. This emission was recorded utilizing a supramolecular organic host and the effect due to the heavy atom. Poor intersystem crossing from S₁ to T₁, small radiative rate constant from T₁, and large rate constant for oxygen quenching hinder the phosphorescence of aromatic molecules at room temperature in solution. In this study, these limitations are overcome by encapsulating a pyrene molecule within a water-soluble capsule (octa acid, OA) and purging with xenon. While OA suppressed oxygen quenching, xenon enabled the intersystem crossing from S₁ to T₁ and radiative process from T₁ to S₀ through the well-known heavy atom effect. The close interaction facilitated between the pyrene and the heavy atom perturber xenon in the three-component supramolecular assembly (OA, pyrene, and xenon) resulted in phosphorescence from pyrene. Computational modeling and NMR studies supported the postulate that pyrene and more than one molecule of xenon are present within a confined space of the OA capsule.

Theoretical Study on the Photolysis Mechanism of 2,3-Diazabicyclo[2.2.2]oct-2-ene

A CASPT2/CASSCF study has been carried out to investigate the mechanism of the photolysis of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) under direct and triplet-sensitized irradiation. By exploring the detailed potential energy surfaces including intermediates, transition states, conical intersections, and singlet/triplet crossing points, for the first excited singlet (S(1)) and the low-lying triplet states (T(1), T(2), and T(3)), we provide satisfactory explanations of many experimental findings associated with the photophysical and photochemical processes of DBO. A key finding of this work is the existence of a significantly twisted S(1) minimum, which can satisfactorily explain the envelope of the broad emission band of DBO. It is demonstrated that the S(1) (n-pi*) intermediate can decay to the T(1) (n-pi*) state by undergoing intersystem crossing (rather inefficient) to the T(2) (pi-pi*) state followed by internal conversion to the T(1) state. The high fluorescence yield and the extraordinarily long lifetime of the singlet excited DBO are due to the presence of relatively high barriers, both for intersystem crossing and for C-N cleavage. The short lifetime of the triplet DBO is caused by fast radiationless decay to the ground state.

Visual observation of contact-induced intercrystalline migration of aromatic species adsorbed in zeolites by fluorescence microscopy

Intercrystalline migration and a migration-assisted chemical reaction of adsorbed aromatic species between zeolite particles in physical contact were visualized by fluorescence microscopy coupled with a particle manipulation technique. The luminescence color characteristics of particular zeolite particles originating from the specific photochemistry of adsorbed species was exploited to follow the migration of the molecules. Two examples are shown that are relevant to the visualization of the time-dependent migration process: A one guest-two sets of zeolite crystals system: chrysene (Chry)-loaded zeolite Na+ -X (the sodium form of zeolite X) crystals were placed in contact with unloaded Tl+ -X (thallium-exchanged X) crystals and allowed to stand at room temperature. Initially, the blue fluorescence of Chry was detected only from the Na+ -X particles, but later, the development of green phosphorescence emission was discernible from the Tl+ -X which suggests that Chry migrated from the Na+ -X to the Tl+ -X crystals. A two guest-species systems: Electron-donating Chry-loaded Na+ -X crystals were placed in contact with electron-accepting 1,2,4,5-tetracyanobenzene (TCNB)-loaded Na+ -X or Na+ -Y crystals. With time, the former system (Chry/Na+ -X and TCNB/Na+ -X) gave rise to the emission of Chry-TCNB charge-transfer complexes resulting mainly from the migration of Chry while the latter system (Chry/Na+ -X and TCNB/Na+ -Y) afforded the same emission resulting largely from the migration of TCNB. The present investigation reveals that there is a certain direction for guest migration depending on the zeolite host and the nature of host-guest or guest-guest interaction.

Chemically Induced Dynamic Electron Spin Polarization-Detected Energy Transfer. Substrate Size Effects and Solvent Dependence

Time-resolved electron paramagnetic resonance spectroscopy is used to probe energy transfer from aromatic photoexcited triplet states to azo compounds in liquid solution. The observation of chemically induced dynamic electron spin polarization in the spectra gives precise information regarding the spin physics and mechanism of the energy transfer process. The substrate size is varied by altering the chain length of alkyl chains covalently attached to the azo compounds via ester or amide linkages. The solvent dependence of the energy transfer process is also investigated. The results are discussed in terms of Dexter and Förster mechanisms for energy transfer, the properties of the excited states, and the diffusive properties of the molecules in the solvents of interest. Decomposition rate studies and fluorescence measurements are also reported.

Observation of azo chromophore fluorescence and phosphorescence emissions from DBH by applying exclusively the orbital confinement effect in siliceous zeolites devoid of charge-balancing cations

The molecular orbital (MO) confinement theory predicts that the HOMO and LUMO of an organic guest may become distorted when the dimensions of the host cavity approach closely those of the MO (Corma, A.; García, H.; Sastre, G.; Viruela, P. M. J. Phys. Chem. B 1997, 101, 4575-4582). Generally, quantum chemical calculations assume that the MO extends over all the space. However, the fact that a molecule is confined within a restricted cavity might impose certain limits to its MO. In agreement with this theory, we have observed for the first time the phosphorescence emission from 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) upon incorporation in zeolites devoid of charge-balancing cations, even at room temperature. This observation may be rationalized on the basis of two possible effects: (i) Decrease of the HOMO-LUMO gap induced by confinement of the azo np* chromophore favors ISC, and (ii) inhibition of radiationless deactivation pathway by the immobilization of DBH within the zeolite host favors long lifetimes.