No abstract

The use of low-intensity NIR light to operate molecular switches offers several potential advantages including enhanced penetration into bulk materials, in particular biological tissues, and reduced radiation damage due to the limited photon energies. The latter, however, pose a challenge for designing reasonably bistable systems. We have developed a general design strategy for direct one-photon NIR photoswitches based on negative photochromic dihydropyrenes carrying opposing strong donor−acceptor substituents either along the long axis of the molecule or across it. Thus, two series of 2,7-and 4,9-disubstituted dihydropyrenes were synthesized, and their photothermal properties investigated as a function of the type, strength, and position of the attached donor and acceptor substituents as well as the polarity of the environment. By shifting the excitation wavelength deep into the NIR, both NIR one-photon absorption cross-section and photoisomerization efficiency could be maximized while retaining a reasonable thermal stability of the metastable cyclophanediene isomer. Thus, the lowest optical transition was shifted beyond 900 nm, the NIR cross-section was enhanced by two orders of magnitude, and the thermal half-lives vary between milliseconds and hours. These unique features open up ample opportunities for noninvasive, optically addressable materials and material systems.

Proton-responsive photochromic molecules are attractive for their ability to react on non-invasive rapid optical stimuli and the importance of protonation/deprotonation processes in various fields. Conventionally, their acidic/basic sites are on hetero-atoms, which are orthogonal to the photoactive p-center. Here, we incorporate azulene, an acid-sensitive pure hydrocarbon, into the skeleton of a diarylethene-type photoswitch. The latter exhibits a novel proton-gated negative photochromic ring-closure and its optical response upon protonation in both open and closed forms is much more pronounced than those of diarylethene photoswitches with hetero-atom based acidic/basic moieties. The unique behavior of the new photoswitch can be attributed to direct protonation on its p-system, supported by 1 H NMR and theoretical calculations. Our results demonstrate the great potential of integrating non-alternant hydrocarbons into photochromic systems for the development of multi-responsive molecular switches.

We use cryogenic ion trap vibrational spectroscopy in combination with quantum chemical calculations to study the structure of mono-and dialuminum oxide anions. The infrared photodissociation spectra of D 2 -tagged AlO 1-4 − and Al 2 O 3-6 − are measured in the region from 400 to 1200 cm −1 . Structures are assigned based on a comparison to simulated harmonic and anharmonic IR spectra derived from electronic structure calculations. The monoaluminum anions contain an even number of electrons and exhibit an electronic closed-shell ground state. The Al 2 O 3-6 − anions are oxygen-centered radicals. As a result of a delicate balance between localization and delocalization of the unpaired electron, only the BHLYP functional is able to qualitatively describe the observed IR spectra of all species with the exception of AlO 3 − . Terminal Al-O stretching modes are found between 1140 and 960 cm −1 . Superoxo and peroxo stretching modes are found at higher (1120-1010 cm −1 ) and lower energies (850-570 cm −1 ), respectively. Four modes in-between 910 and 530 cm −1 represent the IR fingerprint of the common structural motif of dialuminum oxide anions, an asymmetric four-member Al-(O) 2 -Al ring. Published by AIP Publishing. [http://dx

The transfer of stereoinformation is at the heart of asymmetric reactions. By incorporating the natural monoterpene l‐menthone into the backbone of a diarylethene, we achieved efficient chirality transfer upon photocyclization, resulting in the preferred formation of one major closed isomer in a diastereomeric ratio (d.r.) of 85:15. More significantly, we were able to completely reverse the diastereomeric outcome of the ring closure simply by altering the chemical environment or the irradiation conditions. As a result, we could selectively accumulate the less favored minor closed isomer, with remarkable d.r. values of >99:1 and 74:26, respectively. Computations revealed that a stability inversion after photocyclization is the basis for the observed unprecedented control over diastereoselectivity.

Quantum chemical evidence is produced to show that dimerization of linear butenes and pentenes at zeolitic Brønsted sites in H‐MFI yields alkanes featuring cyclohexane rings rather than branched alkenes. The absence of any C=C double bond in the formed cyclic alkane explains the observations that oligomerization stops at the dimer. The calculated reaction enthalpies for the dimerization of 2‐pentene in the gas phase are −84 kJ mol−1 for branched alkenes, but −153 and −154 kJ mol−1 for alkyl‐cyclopentane and ‐hexane, respectively. Together with calculated adsorption enthalpies of the dimers, −111 and −127 kJ mol−1, respectively, this implies surface dimer formation enthalpies of −264 and −281 kJ mol−1, respectively, in close agreement with the experimental value of −285 kJ mol−1. In contrast, the predicted enthalpy for formation of branched alkoxides, −198 kJ mol−1, deviates by 87 kJ mol−1 from experiment. Calculated IR spectra for the Brønsted OH group show the observed conversion of the band at approximately 3000 cm−1 (hydrogen bond with alkene) to a less intense band at approximately 3450–3500 cm−1 (interaction with alkane).

For the adsorption of methane to hexane on acidic zeolites with varying pore sizes (Socony Mobil-5 (MFI), chabazite (CHA), faujasite (FAU)) and Brønsted acid site concentrations, heats of adsorption are predicted. The widely applied “standard model” of computational catalysis, density functional theory with some account of dispersion (DFT-D) and the harmonic approximation for local sampling of the potential energy surface (PES), leads to a large mean absolute deviation (MAD) from experiment of 17.2 kJ mol–1, far outside chemical accuracy limits (±4 kJ mol–1). Passing either to molecular dynamics (MD) at the DFT-D level or to wave function-based electron correlation methods (second-order Møller–Plesset perturbation theory  MP2) for energies at local minimum structures reduces the MAD to 8.7 and 5.9 kJ mol–1, respectively, still outside the chemical accuracy range. We present MD simulations on an MP2 quality PES, which strongly reduces the MAD to 1.9 kJ mol–1. This has been achieved by finding two descriptors for the MP2–DFT-D energy differences, which reduces the total number of required MP2 calculations to only 36 and, hence, the computational demand by several orders of magnitude. The predicted heats of adsorption at reaction temperatures (650 K) support experimental results derived from spectroscopic measurements. They show that the observed decrease of experimental apparent barriers from propane to pentane for alkane cracking in MFI (28 kJ mol–1) is largely due to increasing adsorption strengths (21 kJ mol–1) and to a much smaller extent to decreasing intrinsic barriers (7 kJ mol–1).

We use cryogenic ion trap vibrational spectroscopy in combination with density functional theory to probe how the structural variability of alumina manifests itself in the structures of the gas-phase clusters (Al O ) AlO with n=1-6. The infrared photodissociation spectra of the D -tagged complexes, measured in the fingerprint spectral range (400-1200 cm ), are rich in spectral features and start approaching the vibrational spectrum of amorphous alumina particles for n>4. Aided by a genetic algorithm, we find a trend towards the formation of irregular structures for larger n, with the exception of n=4, which exhibits a C ground-state structure. Locating the global minima of the larger systems proves challenging.