Carl Trindle

Aromatic Difluoroboron β‑Diketonate Complexes: Effects of π‑Conjugation and Media on Optical Properties

Aromatic difluoroboron β-diketonate complexes (BF2bdks) are classic fluorescent molecules that have been explored as photochemical reagents, two-photon dyes, and oxygen sensors. To gain a better understanding of their emissive properties in both solution and polymer matrices, BF2bdks with varying aromatic groups were synthesized and their photophysical properties were investigated in both methylene chloride and poly(lactic acid) (PLA). Absorption spectra showed systematic variations that are well correlated with structural features, including the size of the aryl substituent and the presence of a para electron-donating methoxy substituent. Computational modeling of the absorption spectra with the TD-B3LYP/6-311+G(d)//B3LYP/6-31G(d) formulation of density functional theory and a polarizable continuum model of dichloromethane solvent shows that all systems show intense π-π* one-electron excitations, usually from one of the highest occupied molecular orbitals (HOMO - k, k = 0, 1, 2) to the lowest unoccupied molecular orbital (LUMO). Emission properties are sensitive to the dye structure and medium. Based on spectroscopic and lifetime studies, BF2bdks exhibit comparable fluorescence properties in both solutions and polymers when the diketonate group is functionalized with smaller aromatic ring systems such as benzene. For BF2bdks with larger arene ring systems, such as anthracene, emission from a strong intramolecular charge-transfer (ICT) state was also noted in both solution and in PLA. There are differences in relative intensities of peaks arising from π-π* and ICT excitations depending upon dye loading in PLA. Substituent effects were also observed. Electron-donating methoxyl groups on the aromatic rings lead to enhanced fluorescence quantum yields. For certain dyes, phosphorescence is detected at low temperature or under a nitrogen atmosphere in PLA matrices.

Versatile Room-Temperature-Phosphorescent Materials Prepared from N-Substituted Naphthalimides: Emission Enhancement and Chemical Conjugation

Purely organic materials with room-temperature phosphorescence (RTP) are currently under intense investigation because of their potential applications in sensing, imaging, and displaying. Inspired by certain organometallic systems, where ligand-localized phosphorescence ((3) π-π*) is mediated by ligand-to-metal or metal-to-ligand charge transfer (CT) states, we now show that donor-to-acceptor CT states from the same organic molecule can also mediate π-localized RTP. In the model system of N-substituted naphthalimides (NNIs), the relatively large energy gap between the NNI-localized (1) π-π* and (3) π-π* states of the aromatic ring can be bridged by intramolecular CT states when the NNI is chemically modified with an electron donor. These NNI-based RTP materials can be easily conjugated to both synthetic and natural macromolecules, which can be used for RTP microscopy.

Communication: Frequency shifts of an intramolecular hydrogen bond as a measure of intermolecular hydrogen bond strengths.

Infrared-ultraviolet double resonance spectroscopy has been applied to study the infrared spectra of the supersonically cooled gas phase complexes of formic acid, acetic acid, propionic acid, formamide, and water with 9-hydroxy-9-fluorenecarboxylic acid (9HFCA), an analog of glycolic acid. In these complexes each binding partner to 9HFCA can function as both proton donor and acceptor. Relative to its frequency in free 9HFCA, the 9-hydroxy (9OH) stretch is blue shifted in complexes with formic, acetic, and propionic acids, but is red shifted in the complexes with formamide and water. Density functional calculations on complexes of 9HFCA to a variety of H bonding partners with differing proton donor and acceptor abilities reveal that the quantitative frequency shift of the 9OH can be attributed to the balance struck between two competing intermolecular H bonds. More extensive calculations on complexes of glycolic acid show excellent consistency with the experimental frequency shifts.

Frequency dependence of spin‐lattice relaxation times of lipid bilayers

2H and 13C spin‐lattice (T1) relaxation time studies of 1,2‐dipalmitoyl‐sn‐glycero‐3‐phospho‐ choline (DPPC) in the lamellar, liquid crystalline (Lα) phase are discussed. It is shown that the T−11 results as a function of Larmor frequency ω0 are statistically better described by an ω−1/20 dependence than by an ω−10 or ω−20 dependence.

Waterborne Polyurethanes with Tunable Fluorescence and Room-Temperature Phosphorescence.

Single-component materials with both fluorescence and room-temperature phosphorescence (RTP) are useful for ratiometric sensing and imaging applications. On the basis of a general design principle, an amino-substituted benzophenone is covalently incorporated into waterborne polyurethanes (WPU) and results in fluorescence and RTP single-component dual-emissive materials (SDMs). At different aminobenzophenone concentrations, the statistical, thermal, and optical properties of these SDMs are characterized. Despite their similar thermal behaviors, the luminescence properties as a function of the chromophore concentration are quite different: increasing concentrations led to progressively narrowed singlet-triplet energy gaps. The tunability of fluorescence and RTP via chromophore concentration is explained by a previously proposed model, polymerization-enhanced intersystem crossing (PEX). The proposal of PEX is based on Kashas molecular exciton theory with a specific application in polymeric systems, where the polymerization of luminophores results in excitonic coupling and enhanced forward and reverse intersystem crossing. The mechanism of PEX is also examined by theoretical calculations for the WPU system. It is found that the presence of K1 aggregates indeed enhances the crossover from singlet excited states to triplet ones.

Infrared spectra and UHF SCF calculations of HF complexes with NO, (NO)2, and NO2

HF complexes with nitric oxide and nitrogen dioxide were prepared in argon matrices and studied using IR spectroscopy and unrestricted Hartree–Fock SCF calculations. The results indicate the formation of three different complexes NO‐‐HF, ONNO‐‐HF, and ONO‐‐HF, in which the hydrogen atom of HF is bound to an oxygen atom of the base in each complex. Perturbations in the N–O stretching base submolecule modes were observed in each complex and all were blue shifted with respect to the free base. From the calculations, the unpaired electron was found to be contained in a π antibonding orbital which is in the plane of the NO–HF and ONO–HF complexes.

Structure, Binding Energies, and IR-Spectral Fingerprinting of Formic Acid Dimers.

We describe equilibrium structures for a variety of species likely to be formed as intermediate species in the dimerization of formic acid to produce the stable C2h-symmetric doubly H-bonded dimer and perhaps produced as the vapor is irradiated. For several low-lying species the rearrangement pathways to the stable form are characterized at the MP2/6-311+G(d,p) level of theory, with optimized structures and vibrations computed with full counterpoise corrections for basis set superposition error. Estimates of vibrational frequencies with corrections for anharmonicity suggest that infrared transitions (CO stretches and OH out-of-plane motions) could signal the presence of species less stable than the C2h dimer, observable in irradiation studies of formic acid vapor.

Aggregation-Induced Emission from Fluorophore–Quencher Dyads with Long-Lived Luminescence

Aggregation-induced emission (AIE) is an important photophysical phenomenon in molecular materials and has found broad applications in optoelectronics, bioimaging, and chemosensing. Currently, the majority of reported AIE-active molecules are based on either propeller-shaped rotamers or donor-acceptor molecules with strong intramolecular charge-transfer states. Here, we report a new design motif, where a fluorophore is covalently tethered to a quencher, to expand the scope of AIE-active materials. The fluorophore-quencher dyad (FQD) is nonemissive in solutions due to photoinduced electron-transfer quenching but becomes luminescent in the solid state. The intrinsic emission lifetimes are found to be within the microseconds domain at both room and low temperatures. We performed single-crystal X-ray diffraction measurement for each of the FQDs as well as theoretical calculations to account for the possible origin of the long-lived AIE. These FQDs represent a new class of AIE-active molecules with potential applications in organic optoelectronics.

Complexes with Tunable Intramolecular Ferrocene to Ti(IV) Electronic Transitions: Models for Solid State Fe(II) to Ti(IV) Charge Transfer.

Iron(II)-to-titanium(IV) metal-to-metal-charge transfer (MMCT) is important in the photosensitization of TiO2 by ferrocyanide, charge transfer in solid-state metal-oxide photocatalysts, and has been invoked to explain the blue color of sapphire, blue kyanite, and some lunar material. Herein, a series of complexes with alkynyl linkages between ferrocene (Fc) and Ti(IV) has been prepared and characterized by UV-vis spectroscopy and electrochemistry. Complexes with two ferrocene substituents include Cp2Ti(C2Fc)2, Cp*2Ti(C2Fc)2, and Cp2Ti(C4Fc)2. Complexes with a single ferrocene utilize a titanocene with a trimethylsilyl derivatized Cp ring, (TMS)Cp, and comprise the complexes (TMS)Cp2Ti(C2Fc)(C2R), where R = C6H5, p-C6H4CF3, and CF3. The complexes are compared to Cp2Ti(C2Ph)2, which lacks the second metal. Cyclic voltammetry for all complexes reveals a reversible Ti(IV/III) reduction wave and an Fe(II/III) oxidation that is irreversible for all complexes except (TMS)Cp2Ti(C2Fc)(C2CF3). All of the complexes with both Fc and Ti show an intense absorption (4000 M(-1)cm(-1) < ε < 8000 M(-1)cm(-1)) between 540 and 630 nm that is absent in complexes lacking a ferrocene donor. The energy of the absorption tracks with the difference between the Ti(IV/III) and Fe(III/II) reduction potentials, shifting to lower energy as the difference in potentials decreases. Reorganization energies, λ, have been determined using band shape analysis (2600 cm(-1) < λ < 5300 cm(-1)) and are in the range observed for other donor-acceptor complexes that have a ferrocene donor. Marcus-Hush-type analysis of the electrochemical and spectroscopic data are consistent with the assignment of the low-energy absorption as a MMCT band. TD-DFT analysis also supports this assignment. Solvatochromism is apparent for the MMCT band of all complexes, there being a bathochromic shift upon increasing polarizability of the solvent. The magnitude of the shift is dependent on both the electron density at Ti(IV) and the identity of the linker between the titanocene and the Fc. Complexes with a MMCT are photochemically stable, whereas Cp2Ti(C2Ph)2 rapidly decomposes upon photolysis.

Conformational complexity of melatonin in water and methanol.

The magnetic circular dichroism (MCD) spectra of melatonin in water and methanol solutions is compared to the MCD spectra of indole and five melatonin conformations observed in low temperature jet spectroscopy. Based on a survey of indole compounds using Slater type orbitals-6G(d,p) and B3LYP/6-31G(d) energies, and CNDO/S-D calculations of MCD spectral bands, a dominant structure with a water molecule bridging the amide-keto oxygen and indole Nz-H atoms is proposed as the best fit for the MCD of aqueous melatonin. In methanol an additional band appears at 310 nm which is supported only by solvated structures in which the alkyl-amide arm is extended away from the indole moiety.