Solid-state photochromism and photoreactivity of o- and p-anisaldehydes. Remarkable stabilization of o-xylylenols

Multifunctional Aryl Sulfonium Decavanadates: Tuning the Photochromic and Heterogeneous Oxidative Desulfurization Catalytic Properties Using Salicylaldehyde-type Functional Moieties on Counterions

Multifunctional materials based on polyoxovanadates (POVs) have rarely been reported. Herein, we used aryl sulfonium counterions (ASCIs) bearing a salicylaldehyde-type functionality to tune the properties of decavanadate ([V10O28]6-)-based hybrids for their application in photochromism and heterogeneous oxidative desulfurization (ODS) catalysis. The counterions FHPDS ((3-formyl-4-hydroxyphenyl)dimethylsulfonium), DFHPDS ((3,5-diformyl-4-hydroxyphenyl)dimethylsulfonium), and EFPDS ((4-ethoxy-3-formylphenyl)dimethylsulfonium) were clubbed with the decavanadate cluster to generate the hybrids (FHPDS)4[H2V10O28](H2O)4 (HY1), (DFHPDS)4[H2V10O28](H2O)3 (HY2), and (EFPDS)4[H2V10O28](H2O)6 (HY3). The photochromic properties of these hybrids were tested under 365 nm irradiation, which showed a color change from yellow to green. Different hybrids exhibited different photocoloration half-life (t1/2) values in the range of 0.77-28.38 min, suggesting the dependence of the photocoloration properties upon functional groups on the counterions. The hybrid HY2, having a 2,6-diformyl phenol moiety on the ASCI, exhibited an impressive t1/2 of 0.77 min. UP to 70% reversibility of photocoloration was achieved for the best photochromic hybrid HY2 in 48 h at 70 °C under an oxygen atmosphere. Theoretical and experimental data suggested that some of these aryl sulfonium POVs follow a different e--h+ stabilization mechanism than traditional sulfonium POM hybrids. Further, the salicylaldehyde-type ASCIs control the solubility of the decavanadate hybrids, which enables their application as heterogeneous catalysts for the selective oxidation of various sulfides. The nature of the substituents on the ASCIs also affected their catalytic activities; the counterion that facilitates the reversible V4+/V5+ switching enhances the catalytic ODS efficiency of the hybrids. Using HY2 as the catalyst, up to 99% conversion and 96% selectivity toward sulfones were achieved in dibenzothiophene (DBT) oxidation. The present study suggests a new promising approach for controlling POVs' photoresponsive and catalytic properties by using ASCIs bearing salicylaldehyde-type functional moieties.

Regioisomerism in Symmetric Dimethyl Dialdehydes Dictates their Photochemical Reactivity

We herein report the first light-driven selective monoderivatization (desymmetrization) of two chemically equivalent carbonyl groups in a single chromophore. By comparing of four symmetric regioisomers, featuring two equivalent ortho-methylbenzaldehyde units, we identify dimethyltherephtalaldehydes (DMTAs) which can be activated in a dual wavelength-selective fashion. Under visible light and UV-light irradiation, DMTAs undergo two consecutive Diels-Alder reactions exhibiting near-quantitative endo-selectivity (>99%) and provide excellent yields (96-98%). The influence of the regioisomerism of the dialdehydes on their photochemical behavior is profound, evidenced by an in-depth investigation of their photochemical performance. We exemplify the capability of the photosystems via the synthesis of complex Diels-Alder adducts with various dienophiles, including alkynes.

Wavelength-Gated Photochemical Synthesis of Phenalene Diimides

Herein, we pioneer a wavelength‐gated synthesis route to phenalene diimides. Consecutive Diels–Alder reactions of methylisophthalaldehydes and maleimides afford hexahydro‐phenalene‐1,6‐diol diimides via 5‐formyl‐hexahydro‐benzo[f]isoindoles as the intermediate. Both photoreactions are efficient (82–99 % yield) and exhibit excellent diastereoselectivity (62–98 % d.r.). The wavelength‐gated nature of the stepwise reaction enables the modular construction of phenalene diimide scaffolds by choice of substrate and wavelength. Importantly, this synthetic methodology opens a facile avenue to a new class of persistent phenalenyl diimide neutral radicals, constituting a versatile route to spin‐active molecules.

Theoretical investigation on radical anion promoted electrocyclization in photochromes

Rapid electrocyclization is proposed under radical anionic conditions in organic photochromes. DFT calculations have been performed to investigate the radical anion mediated electrocyclization in different organic photochromes. Furthermore, the activation barriers under radical anionic conditions are compared with those in neutral and radical cationic conditions. The nuclear independent chemical shift (NICS(0)) and synchronicity calculations have been performed for the confirmation of concerted nature and aromatic character of transition states, respectively. The activation barrier for thermal return of cyclophanediene (CPD) to dihydropyrene (DHP) under radical anionic conditions is very lower (ΔH = 5.92 kcal/mol, ΔG = 6.97 kcal/mol) than under neutral conditions, but higher than that in radical cationic conditions (ΔH = 3.13 kcal/mol, ΔG = 4.0 kcal/mol). Similarly, the other prominent classes of photochromes; dithienylethene (ΔH = 20.12 kcal/mol, ΔG = 21.55 kcal/mol) and vinylheptafulvene (ΔH = 23.72 kcal/mol, ΔG = 24.82 kcal/mol) have shown decreased activation barrier under radical anionic condition. However, activation barrier of fulgide under radical anionic conditions is not different than those under neutral and radical cationic conditions. Synchronicity and NICS(0) values for organic photochromes also show significant changes under radical anionic conditions.

Reversible Shifting of a Chemical Equilibrium by Light: The Case of Keto-Enol Tautomerism of a β-Ketoester

Manipulating the equilibrium between a ketone and an enol by light opens up ample opportunities in material chemistry and photopharmacology. By incorporating β-ketoester into the ethene bridge of a photoactive diarylethene, we achieved reversible light-induced tautomerization to give thermally stable enol. In a pristine state, the tautomeric equilibrium is almost completely shifted toward the ketone. Photocyclization of diarylethene results in a new equilibrium containing a significant fraction of the enol tautomer.

Solid-state photochromic behavior of pyrazolone 4-phenylthiosemicarbazones

Pyrazolone 4-phenylthiosemicarbazones show excellent solid-state photochromic properties, thermal stability and fatigue resistance upon the UV/Vis irradiation.

Synthesis and Solid-State Photochromism of 1,3-Diphenyl-4- (2-chlorobenzal)-5-hydroxypyrazole 4-Methylthiosemicarbazone

A new photochromic compound containing a pyrazole-ring unit, 1,3-diphenyl-4-(2-chlorobenzal)-5-hydroxypyrazole 4-methylthiosemicarbazone, was synthesized. Its structure, photochromic properties, and photochemical kinetics were characterized. The results show that the title compound exhibits reversible enol-keto photoisomerization, excellent photostability, and high fatigue resistance. An intra- and intermolecular proton-transfer mechanism is proposed.

Substituent Electronic Effects on the Persistence and Absorption Spectra of (Z)-o-Xylylenols. A Nanosecond Laser Flash Photolysis Study

[reaction: see text] A systematic investigation on a broad set of aldehydes reveals that the lifetimes of (Z)-photoenols can be modulated by variation of the substituents. We have found that the lifetimes of (Z)-enols (in benzene) can be varied by more than 1 order of magnitude with a judicious choice of the substituents that exert mesomeric and inductive effects as, for example, in the case of pentamethylbenzaldehyde (tau = 35 ns) and dicyanomesitaldehyde (tau = 760 ns). This study thus points to the fact that the electronic factors in conjunction with hydrogen bonding stabilization can considerably broaden the uni- as well as bimolecular chemistry based on photoenolization. Further, we have shown that the photoenols exhibit dramatic shifts in their absorption properties with variation of the substituents; although the photoenols have long been considered to be colored, their absorption properties have not been heretofore comprehensively examined.

Highly Diastereoselective Tandem Photoenolization−Hetero-Diels−Alder Cycloaddition Reactions of o-Tolualdehydes in the Solid State

The (E)-photoenols generated in situ by photolysis of o-tolualdehydes 1-5 in the solid state react with the precursor aldehydes as dienophiles in a hetero-Diels-Alder cycloaddition fashion to afford trans-3-arylisochromanols in excellent yields and in a high diastereoselectivity. An examination of the reactivity of three different classes of rationally designed aldehydes shows that the tandem enolization-Diels-Alder cycloaddition occurs in the solid state efficiently for aldehydes whose (E)-photoenols (i) are more stable than their corresponding benzocyclobutenols and (ii) are not sterically congested. However, rapid cyclization to benzocyclobutenols is found to be the sole pathway for sterically encumbered (E)-enols derived from aldehydes 6-8. Given that the execution of heteromolecular reactions in the solid state is a challenge, the results obtained with simple crystalline o-tolualdehydes are remarkable and the strategy involving in situ generation of a dienol in the crystal lattice of a dienophile to achieve hetero-Diels-Alder reaction in a highly diastereoselective manner is heretofore unprecedented. In the context of enormous interest in tandem/domino reactions in contemporary synthetic organic chemistry, the results observed with o-tolualdehydes exemplify successful execution of tandem reactions in the solid state.

Conformational control and photoenolization of pyridine-3-carboxaldehydes in the solid state: stabilization of photoenols via hydrogen bonding and electronic control

We have investigated the solid-state photobehavior of a broad set of pyridine-3-carboxaldehydes 1-5. The introduction of a heteroatom into mesitaldehydes as in aldehydes 1 raises the question of conformational preference in the solid state. The preferred conformations have been unequivocally established from X-ray crystal structure analyses of two of the aldehydes, 1c and 2c; it is shown that intramolecular hydrogen bonding could be utilized to achieve conformational control. In contrast to mesitaldehydes, which undergo efficient photocyclization to benzocyclobutenols in the solid state, the heteroatom analogues 1b and 1c exhibit a perceptible color change (from colorless to pale yellow for 1b and yellow-orange for 1c) upon UV irradiation; the color attributed to (E)-enols is persistent for several hours. Continued irradiation leads to an intractable polymeric material. The AM1 calculations, which have been reliably applied to the thermal cyclization of xylylenols to benzocyclobutenols, reveal that the (E)-enols of 1 are more stable than those of the mesitaldehydes relative to their corresponding benzocyclobutenols. The stabilization is interpreted as arising from the possibility of engaging the heteroatom in resonance delocalization. That the contribution from such a role of the nitrogen atom is so pronounced is elegantly demonstrated by forming the fluoroborate salts; 1a-HBF(4) and 1b-HBF(4) readily exhibit highly red-shifted absorption upon exposure to UV radiation as a result of stabilization of the photoenols. Notably, such a remarkable stabilization via electronic control of the photoenols is unprecedented. All of the 2-methoxy- and 2-chloro-substituted aldehydes 2-5 exhibit photochromism. Ab initio calculations show that the methoxy group in aldehydes 2 and 3 stabilizes the (E)-enols via O[bond]H...O hydrogen bonding as compared to those of 1 by 5-6 kcal/mol relative to their corresponding benzocyclobutenols. Thus, the presence of methoxy and halo groups at position 2 serves not only to direct the formyl oxygen toward the methyl group for H-abstraction but also to stabilize the (E)-enols.

Solid-state diphotocyclization of iso- and terephthalaldehydes via dihalogen substitution

The supramolecular nonbonded C-H...X interactions between formyl hydrogens and ortho-halogen atoms (Br/Cl) have been exploited to achieve conformational control in the solid state of dimethyl-substituted iso- and terephthaladehydes (1-3) for unprecedented diphotocyclization. It is shown that the dihalogen substitution also contributes to the stability of the benzocyclobutenols relative to their precursor photoenols, so that the solid-state photolysis of dialdehydes 2b, 2c, and 3b leads to diphotocyclization to afford respectable yields of bis-benzocyclobutenols.

Helical self-assembly of substituted benzoic acids: influence of weaker X...X and C-H...X interactions

The X-ray crystal packing analyses of the sterically encumbered halogen-substituted benzene carboxylic acids 1-4 reveal a novel and unprecedented crystal packing in that the association of the carboxyl groups through O-H...O bonds results in the generation of a helix along the 41-screw axis. Such an organization of the acids is shown convincingly to be a result of the close packing, which exploits the weaker X...X and C-H...X interactions in conjunction with the stronger O-H...O hydrogen bonds. In contrast, the chloro- and bromo-substituted durene carboxylic acids 6 and 7 exhibit a pattern that is akin to tape/ribbon involving the centrosymmetric-dimer motif and X...X short intermolecular interactions. The structural investigations demonstrate the ability of the weaker interactions in modifying the supposedly "robust" centrosymmetric-dimer motif of the carboxyl groups in a decisive manner.

Efficient photocyclization of o-alkylbenzaldehydes in the solid state: direct observation of E-xylylenols en route to benzocyclobutenols

The photocyclization to benzocyclobutenols of o-alkyl aromatic aldehydes that are predestined for gamma-hydrogen abstraction is found to occur efficiently in the solid state; in contrast, solution-phase photolysis is known to afford a mixture of several products. It is shown that mesitaldehyde, which is a liquid, also undergoes efficient cyclization when subjected to photolysis as a solid inclusion complex. The marginal energy differences in the relative energies of the E-enols and the corresponding cyclobutenols in the case of cyano-substituted mesitaldehydes has permitted direct observation, for the first time, of the E-enols en route to benzocyclobutenols. The AM1 calculations suggest that the cyano-substitution causes intrinsic stabilization of the E-enols relative to the corresponding cyclobutenols, while the bromo groups do the opposite. The lack of observation of the red color in bromo- and formyl-substituted aldehydes is attributed to rapid cyclization of the E-enols to the their respective cyclobutenols even at low temperatures.