Mechanofluorochromism and self-recovery of alkylsilylpyrene-1-carboxamides.

A family of (alkylsilyl)pyrene-1-carboxamides exhibits similar mechanofluorochromic responses upon grinding. However, their spontaneous fluorescence recovery processes are distinct despite their similarity in chemical structures and crystal packings. Fluorescence spectroscopy, crystallography, and nanomechanical tests revealed that the chromic direction is dominated by the packing motif, while the fluorescence recovery is driven by the intermolecular interactions and the reversibility of deformation.

Electrophile-Dependent Reactivity of Lithiated N-Benzylpyrene-1-Carboxamide.

In this paper, we describe the lithiation of N-benzylpyrene-1-carboxamide with RLi-TMEDA. We found that the reaction outcome strongly depends on the electrophile used in the quenching step. The electrophile can be introduced at either the benzylic position or at the C-2 position in the pyrene nucleus. Furthermore, when H+ was used as the quencher, the product of the intramolecular carbolithiation of the pyrene K-region was formed. Dehydrogenation of the obtained compound with DDQ allowed the synthesis of a novel nitrogen polycyclic compound with an aza-benzo[c,d]pyrene (azaolympicene) skeleton. Attempts to extend the reaction scope to the amides substituted in the phenyl ring 8a and 8b gave an unexpected result. The reaction of both compounds with BuLi gave 1-valerylpyrene (9) in good yield. Photophysical properties, including absorption spectra, emission spectra and quantum yields of the emission of selected products, were studied and discussed.

Highly Fluorescent Dyes Containing Conformationally Restrained Pyrazolylpyrene (Pyrazoolympicene) Chromophore.

The triflic-acid-promoted cyclization of 1-phenyl-3-(pyren-1-yl)-1H-pyrazole-4-carbaldehyde afforded a mixture of 9-phenyl-7,9-dihydropyreno (10,1-fg)indazole and 9-phenylpyreno(10,1-fg)indazole-7(9H)-one, readily separable by column chromatography. Both products contained a rigid six-ringed pyrazoolympicene backbone and exhibited bright fluorescence in chloroform solution and a weak fluorescence in the solid state. DFT and TD DFT calculations revealed that the lowest excited state (S1) of these compounds is populated via HOMO →LUMO π-π * transition. Furthermore, the synthesized compounds behaved as weak bases and their emission spectra showed substantial changes upon protonation. Therefore, they may be of interest for sensing of strongly acidic fluorophore environments.

Multi-Directional Mechanofluorochromism of Acetyl Pyrenes and Pyrenyl Ynones.

A series of acetyl pyrenes and pyrenyl ynones with and without tert-butyl groups showed distinct mechanofluorochromism (MFC). Four pairs of polymorphic solids were found out of six compounds and interestingly, each of them showed hypsochromic, bathochromic or off-to-on MFC. The MFC properties were rationalized by categorizing the packing schemes into herringbone, sandwich, beta and gamma motifs depending on the relative contributions of C⋅⋅⋅C (or π-π) against C⋅⋅⋅H contacts. The bulky tert-butyl and trimethylsilyl groups served not only to reduce the number of aggregation patterns but also to prohibit the complete back reactions in solid state. Our results suggest that the simple pyrene derivatives may be promising candidates for a novel group of mechanically-sensitive materials.

Triflic Acid-Promoted Adamantylation and tert-Butylation of Pyrene: Fluorescent Properties of Pyrene-Decorated Adamantanes and a Channeled Crystal Structure of 1,3,5-Tris(pyren-2-yl)adamantane.

Triflic acid-promoted 1-adamantylation and tert-butylation of pyrene at positions 2 and 2,7 along with the synthesis of compounds having one-, two-, and three-pyrenyl groups attached to the adamantane scaffold are disclosed. Fluorescent properties of these compounds and channeled crystal structure of the 1,3,5-tris(pyren-2-yl)adamantane containing chloroform as a guest are also presented.

Mechanofluorochromism of pyrene-derived amidophosphonates.

Aiming to develop a library of organic mechanofluorochromic (MFC) materials, solid-state emission properties of pyrene-derivatives with amidophosphonate groups (diethyl 1-(pyrene-1-carboxamido)alkylphosphonates, alkyl: methyl, isopropyl, phenyl) were investigated. All of these compounds were found to show strong blue fluorescence, and blue-to-green emission shift was identified upon grinding. Monomer and several excimer emissive species were identified by steady-state and time-resolved fluorescence investigations. The characteristic green-to-blue thermal back reaction was observed even at room temperature without additives, which would be related to the flexibility of side groups for molecular rearrangement. We here demonstrated that the pyrene-derived compounds can be attractive platforms to host MFC properties.

Alkylation of the K-Region in a Sterically Hindered Pyrene Carboxamide via Directed Reaction with Alkyllithiums under Air.

Sterically hindered N,2,7-tri- tert-butylpyrene-1-carboxamide treated with n-BuLi, i-BuLi, s-BuLi, and n-HexLi in THF in the presence of TMEDA and air afforded trans- N,2,7-tri- tert-butylpyrene-10-alkyl-9-hydroxy-9,10-dihydropyrene-1-carboxamides in 63-74% yield. Trifluoroacetic acid promoted dehydration of these compounds gave 10-alkyl derivatives of the starting amide in 79-89% yield. The minor products of this reaction were deamidated compounds, 4-alkyl-2,7-di- tert-butylpyrenes.

Polycyclic Aromatic N-Ethoxycarbonyl Thioamide S-Oxides and Their Triflic Acid Promoted Cyclization to Fluorescent Thiophene Imine-Fused Arenes.

We present the synthesis of a series of polycyclic aromatic-N-ethoxycarbonylthioamide S-oxides and their triflic acid-promoted cyclization to thiophene imine-fused arenes having 2H-naphtho[1,8-bc]thiophen-2-imine, 3H-pyreno[10,1-bc]thiophen-3-imine, 4H-pyreno[1,10-bc]thiophen-4-imine, 3H-pyreno[10,1-bc]thiophen-3-imine, 4H-pyreno[1,10-bc]thiophen-3-imine, 3H-pyreno[10,1-bc]thiophen-3-imine, and 4H-peryleno[3,4-bc]thiophen-4-imine cores. The proposed reaction mechanism involves the intermediacy of a novel type of electrophilic sulfur species, namely protonated iminosulfenic acid or iminosulfenium cations. These species may attack the peri- or ipso-position of the arene, leading in some cases to regioisomeric products. The reaction affords in high yields novel polycyclic fluorophores emitting in the range of 500-606 nm with quantum yields of 0.025-0.64. Comparison with the parent arenes reveals that the fused iminothiophene moiety brings about significant bathochromic shifts of the absorption and emission bands.

Regioselective (thio)carbamoylation of 2,7-di-tert-butylpyrene at the 1-position with iso(thio)cyanates.

It has been found that 2,7-di-tert-butylpyrene reacts with aliphatic iso(thio)cyanates in the presence of trifluoromethanesulfonic acid to exclusively afford the corresponding 1-substituted (thio)amides in high yields. For aromatic iso(thio)cyanates the reaction is less regioselective, although substitution at the 1-position prevails. For ethoxycarbonyl isothiocyanate, apart from the 1-substituted thioamide, 1,8-disubstituted thioamide and 2,7-di-tert-butylpyrene-1-carbonitrile are formed (especially at longer reaction times).

Synthesis, fluorescence properties and the promising cytotoxicity of pyrene-derived aminophosphonates.

A large series of variously substituted amino(pyren-1-yl)methylphosphonic acid derivatives was synthesized using a modified aza-Pudovik reaction in 20-97% yields. The fluorescence properties of the obtained compounds were investigated revealing that N-alkylamino(pyren-1-yl)methylphosphonic derivatives are stronger emissive compounds than the corresponding N-aryl derivatives. N-Benzylamino(pyren-1-yl)methylphosphonic acid displayed strong fluorescence (ΦF = 0.68) in phosphate-buffered saline (PBS). The influence of a series of derivatives on two colon cancer cell lines HT29 and HCT116 was also investigated. The most promising results were obtained for N-(4-methoxyphenyl)amino(pyren-1-yl)methylphosphonate, which was found to be cytotoxic for the HCT116 cancer cell line (IC50 = 20.8 µM), simultaneously showing weak toxicity towards normal lymphocytes (IC50 = 230.8 µM).

Friedel-Crafts-type reaction of pyrene with diethyl 1-(isothiocyanato)alkylphosphonates. Efficient synthesis of highly fluorescent diethyl 1-(pyrene-1-carboxamido)alkylphosphonates and 1-(pyrene-1-carboxamido)methylphosphonic acid.

Friedel-Crafts-type reaction of pyrene with diethyl 1-(isothiocyanato)alkylphosphonates promoted by trifluoromethanosulfonic acid afforded diethyl 1-(pyrene-1-carbothioamido)alkylphosphonates in 83-94% yield. These compounds were transformed, in 87-94% yield, into the corresponding diethyl 1-(pyrene-1-carboxamido)alkylphosphonates by treatment with Oxone(®). 1-(Pyrene-1-carboxamido)methylphosphonic acid was obtained in a 87% yield by treating the corresponding diethyl phosphonate with Me3Si-Br in methanol. All of the synthesized amidophosphonates were emissive in solution and in the solid state. The presence of a phosphonato group brought about an approximately two-fold increase in solution fluorescence quantum yield in comparison with that of a model N-alkyl pyrene-1-carboxamide. This effect was tentatively explained by stiffening of the amidophosphonate lateral chain which was caused by the interaction (intramolecular hydrogen bond) of phosphonate and amide groups. The synthesized phosphonic acid was soluble in a biological aqueous buffer (PBS, 0.01 M, pH 7.35) and was strongly emissive under these conditions (λem = 383, 400 nm, τ = 18.7 ns, ΦF > 0.98). Solid-state emission of diethyl 1-(pyrene-1-carboxamido)methylphosphonate (λmax = 485 nm; ΦF = 0.25) was assigned to π-π aggregates, the presence of which was revealed by single-crystal X-ray diffraction analysis.

Substituent effects on dynamics at conical intersections: cycloheptatrienes.

Using selective methyl substitution, we study the effects of vibrational dynamics at conical intersections in unsaturated hydrocarbons. Here, we investigate the excited state nonadiabatic dynamics of cycloheptatriene (CHT) and its relation to dynamics in other polyenes by comparing CHT with 7-methyl CHT, 7-ethyl CHT, and perdeuterated CHT using time-resolved photoelectron spectroscopy and photoelectron anisotropy. Our results suggest that, upon ππ*-excitation to the bright 2A" state, we observe an early intersection with the dark 2A' state close to the Franck-Condon region with evidence of wavepacket bifurcation. This indicates that the wavepacket evolves on both states, likely along a planarization coordinate, with the majority of the flux undergoing nonadiabatic transition via conical intersections within 100 fs following light absorption. In CHT, large amplitude motion along the planarization coordinate improves the intra-ring π-overlap, yielding a delocalized electronic density. However, substitutions in 7 position, chosen to modify the inertia of the planarization motion, did not markedly alter the first step in the sequential kinetic scheme. This suggests that there is a crossing of potential energy surfaces before planarization is achieved and, thus, nonadiabatic transition likely takes place far away from a local minimum.