Crystals and Aggregates of a Molecular Tetrarotor with Multiple Trityl Embraces Derived from Tetraphenyladamantane

Samples of tetrakis-1,3,5,7-(4′-(3″,3″,3″-triphenylpropynyl)-phenylene)­adamantane and its trityl-deuterated isotopologue were synthesized and their crystallization and packing properties were analyzed within the context of formation of 4- or 6-fold phenyl embraces. The tetrahedral shape of these molecules with four propeller-like triphenylmethyl moieties generates several edge-to-face intermolecular interactions in the solid state that result in the formation of infinite chains of molecules that are tightly interlocked. The formation of analogous edge-to-face intermolecular interactions leading to aggregation in solution was also suggested by NMR experiments carried out in different solvents as a function of concentration. The formation of interdigitated chains was also manifested in fibrils and thin needles, which were documented by scanning electron microscopy (SEM). Single crystal X-ray diffraction studies revealed the presence of multiple 4-fold phenyl embraces and edge-to-face interactions as the leading motifs behind the formation of tightly interlocked molecular chains

Photochromic Molecular Gyroscope with Solid State Rotational States Determined by an Azobenzene Bridge

We describe the synthesis, characterization, photochemical isomerization, and rotational dynamics of a crystalline molecular gyroscope containing an azobenzene bridge (<i>trans</i>-<b>2</b>) that spans from one end of the stator to other, with the intention of exploring its function as a molecular brake. While single crystal X-ray diffraction analysis of a photochemically inactive dichloromethane solvate was used to confirm the molecular and packing structures of <i>trans</i>-<b>2</b>, a nanocrystalline pseudopolymorph was shown to be photoactive, and it was analyzed by powder X-ray diffraction (PXRD), scanning electron microscopy, and variable temperature solid state ²H NMR before and after photoisomerization. It was shown that the nanocrystalline suspension irradiated with λ = 340 nm reaches a photostationary state with 34% of <i>cis</i>-isomer, as compared to that observed in solution where the corresponding value is 74%. Line shape analysis of solid state ²H NMR spectra of a phenylene-<i>d</i>_<i>4</i> isotopologue, obtained as a function of temperature, indicated that rotation in crystals of the <i>trans-</i><b>2</b> isomer, with a mean activation energy of 4.6 ± 0.6 kcal/mol and a pre-exponential factor exp(29.4 ± 1.7), is ten times faster than that of samples containing the <i>cis-</i><b>2</b> isomer, which has a higher mean activation energy of 5.1 ± 0.6 kcal/mol and a lower pre-exponential factor of exp(27.9 ± 1.3)

Crystals and Aggregates of a Molecular Tetrarotor with Multiple Trityl Embraces Derived from Tetraphenyladamantane

Samples of tetrakis-1,3,5,7-(4′-(3″,3″,3″-triphenylpropynyl)-phenylene)­adamantane and its trityl-deuterated isotopologue were synthesized and their crystallization and packing properties were analyzed within the context of formation of 4- or 6-fold phenyl embraces. The tetrahedral shape of these molecules with four propeller-like triphenylmethyl moieties generates several edge-to-face intermolecular interactions in the solid state that result in the formation of infinite chains of molecules that are tightly interlocked. The formation of analogous edge-to-face intermolecular interactions leading to aggregation in solution was also suggested by NMR experiments carried out in different solvents as a function of concentration. The formation of interdigitated chains was also manifested in fibrils and thin needles, which were documented by scanning electron microscopy (SEM). Single crystal X-ray diffraction studies revealed the presence of multiple 4-fold phenyl embraces and edge-to-face interactions as the leading motifs behind the formation of tightly interlocked molecular chains

Photochromic Molecular Gyroscope with Solid State Rotational States Determined by an Azobenzene Bridge

We describe the synthesis, characterization, photochemical isomerization, and rotational dynamics of a crystalline molecular gyroscope containing an azobenzene bridge (<i>trans</i>-<b>2</b>) that spans from one end of the stator to other, with the intention of exploring its function as a molecular brake. While single crystal X-ray diffraction analysis of a photochemically inactive dichloromethane solvate was used to confirm the molecular and packing structures of <i>trans</i>-<b>2</b>, a nanocrystalline pseudopolymorph was shown to be photoactive, and it was analyzed by powder X-ray diffraction (PXRD), scanning electron microscopy, and variable temperature solid state ²H NMR before and after photoisomerization. It was shown that the nanocrystalline suspension irradiated with λ = 340 nm reaches a photostationary state with 34% of <i>cis</i>-isomer, as compared to that observed in solution where the corresponding value is 74%. Line shape analysis of solid state ²H NMR spectra of a phenylene-<i>d</i>_<i>4</i> isotopologue, obtained as a function of temperature, indicated that rotation in crystals of the <i>trans-</i><b>2</b> isomer, with a mean activation energy of 4.6 ± 0.6 kcal/mol and a pre-exponential factor exp(29.4 ± 1.7), is ten times faster than that of samples containing the <i>cis-</i><b>2</b> isomer, which has a higher mean activation energy of 5.1 ± 0.6 kcal/mol and a lower pre-exponential factor of exp(27.9 ± 1.3)

Transmission Spectroscopy and Kinetics in Crystalline Solids Using Aqueous Nanocrystalline Suspensions: The Spiropyran-Merocyanine Photochromic System

A comparison of the solution and solid state thermal decay kinetics of five photochromic spiropyrans with different <i>N</i>-alkyl groups (<b>SP1</b>–<b>SP5</b>) was carried out in acetonitrile and nanocrystalline suspensions at 298 K. The change in absorbance at ca. 550 nm was measured as a function of time for the merocyanine (MC) using transmission UV–vis spectroscopy. We found that the thermal decay kinetics are slower and follow a biexponential decay in the solid state compared to a faster, monoexponential decay that was measured in solution. We observed that, while the kinetic range measured in solution varies by a factor of 13, the decay kinetics in the solid state cover a range of ca. 150, indicating that crystal packing has an influence much greater than that of the effects of <i>N</i>-alkyl substitution. A fluorescence analysis of irradiated samples of <b>SP1</b> in solution could be used to determine the formation of the MC species and its subsequent decay. By contrast, a similar analysis of nanocrystalline suspensions displayed changes as a function of time that are consistent with self-quenching

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

Synthesis, Rotational Dynamics, and Photophysical Characterization of a Crystalline Linearly Conjugated Phenyleneethynylene Molecular Dirotor

We report the synthesis, crystal structure, solid-state dynamics, and photophysical properties of 6,13-bis­((4-(3-(3-methoxyphenyl)-3,3-diphenylprop-1-yn-1-yl)­phenyl)­ethynyl)-5,7,12,14-tetrahydro-5,14:7,12-bis­([1,2]­benzeno)­pentacene (<b>1</b>), a molecular dirotor with a 1,4-bis­((4-ethynylphenyl)­ethynyl)­benzene (BEPEB) chromophore. The incorporation of a pentiptycene into the molecular dirotor provides a central stator and a fixed phenylene ring relative to which the two flanking ethynylphenylene rotators can explore various torsion angles; this allows the BEPEB fluorophore dynamics to persist in the solid state. X-ray diffraction studies have shown that molecular dirotor <b>1</b> is packed so that all the BEPEB fluorophores adopt a parallel alignment, this is ideal for the development of functional materials. Variable temperature, quadrupolar echo ²H NMR studies have shown that phenylene rotator flipping has an activation energy of 9.0 kcal/mol and a room temperature flipping frequency of ∼2.6 MHz. Lastly, with measurements in solution, glasses, and crystals, we obtained evidence that the fluorescence excitation and emission spectra of the phenyleneethynylene chromophores is dependent on the extent of conjugation between the phenylene rings, as determined by their relative dihedral angles. This work provides a promising starting point for the development of molecular dirotors with polar groups whose amphidynamic nature will allow for the rapid shifting of solid-state absorption, fluorescence, and birefringence, in response to external electric fields

Theory of Substituent Effects on the Regioselectivity of Di-π-methane Rearrangements of Dibenzobarrelenes

The regioselectivities of the di-π-methane rearrangements of unsymmetrically substituted dibenzobarrelenes have been explored with DFT (UM06-2X). Regioselectivity depends on the intramolecular hydrogen bonding and originates from specific stabilization of the triplet biradical intermediates

Photoinduced and Thermal Denitrogenation of Bulky Triazoline Crystals: Insights into Solid-to-Solid Transformation

The photoinduced and thermal denitrogenation of crystalline triazolines with bulky substituents leads to the quantitative formation of aziridines in clean solid-to-solid reactions despite very large structural changes in the transition from reactant to product. Analysis of the reaction progress by powder X-ray diffraction, solid-state ¹³C CPMAS NMR, solid-state FTIR spectroscopy, and thermal analysis has revealed that solid-to-solid reactions proceed either through metastable phases susceptible to amorphization or by mechanisms that involve a reconstructive phase transition that culminates in the formation of the stable phase of the product. While the key for a solid-to-solid transformation is that the reaction occurs below the eutectic temperature of the reactant and product two-component system, experimental evidence suggests that those reactions will undergo a reconstructive phase transition when they take place above the glass transition temperature

Synthesis, Rotational Dynamics, and Photophysical Characterization of a Crystalline Linearly Conjugated Phenyleneethynylene Molecular Dirotor

We report the synthesis, crystal structure, solid-state dynamics, and photophysical properties of 6,13-bis­((4-(3-(3-methoxyphenyl)-3,3-diphenylprop-1-yn-1-yl)­phenyl)­ethynyl)-5,7,12,14-tetrahydro-5,14:7,12-bis­([1,2]­benzeno)­pentacene (<b>1</b>), a molecular dirotor with a 1,4-bis­((4-ethynylphenyl)­ethynyl)­benzene (BEPEB) chromophore. The incorporation of a pentiptycene into the molecular dirotor provides a central stator and a fixed phenylene ring relative to which the two flanking ethynylphenylene rotators can explore various torsion angles; this allows the BEPEB fluorophore dynamics to persist in the solid state. X-ray diffraction studies have shown that molecular dirotor <b>1</b> is packed so that all the BEPEB fluorophores adopt a parallel alignment, this is ideal for the development of functional materials. Variable temperature, quadrupolar echo ²H NMR studies have shown that phenylene rotator flipping has an activation energy of 9.0 kcal/mol and a room temperature flipping frequency of ∼2.6 MHz. Lastly, with measurements in solution, glasses, and crystals, we obtained evidence that the fluorescence excitation and emission spectra of the phenyleneethynylene chromophores is dependent on the extent of conjugation between the phenylene rings, as determined by their relative dihedral angles. This work provides a promising starting point for the development of molecular dirotors with polar groups whose amphidynamic nature will allow for the rapid shifting of solid-state absorption, fluorescence, and birefringence, in response to external electric fields