Stéphanie Delbaere

A simple molecule-based octastate switch.

Dithienylethene oxazolidine hybrid system connected through an isomerizable double bond exists under eight molecular states on demand. Combinations of electrocyclization of dithienylethene, Z/E isomerization and acid-base oxazolidine change cause selective addressabilities. Two intricate gated photochromic performances allow the execution of an 8-step molecular switch, which renders this molecular system the most complex known up to date.

Gated photochromism and acidity photomodulation of a diacid dithienylethene dye.

The present study quantitatively analyses the gated photochromism and the acidity photomodulation properties of a diacid dithienylethene compound. Photoisomerisation between the open and closed isomers was investigated by UV/visible and (1)H NMR spectroscopy. It was found that the photocyclisation quantum yield of the diacid form was remarkably high (around 90%). Partial neutralisation of the open isomer revealed a gated photochromism as the photocyclisation quantum yield of the mono- and dianion were 50 and 67%, respectively. A considerable photomodulation of the acidity was observed: the closed isomer is more acid than the open one by more than one pK(a) unit. This effect has been shown to be exploitable for a reversible photo-acid generation. This is the first time that a complete quantitative investigation that allows for the determination of the main photochromic, spectral and thermodynamic parameters of a base-sensitive photochromic diarylethene has been carried out.

Bridging the visible: the modulation of the absorption by more than 450 nm.

The coexistence of 18 isomers, identified by NMR spectroscopy and gathered into six states with very different absorption properties, is observed upon irradiation with selective wavelengths of a triphotochromic system joining two naphthopyran entities through a dithienylethene bridge. The sequential electronic coupling of photochromic groups resulted in an absorption behavior reaching far into NIR.

Photochromic fused-naphthopyrans without residual color.

A series of new photochromic fused-naphthopyrans with an alkyl bridge between the pyran ring and the naphthalenic core was synthesized in several steps from 4-(bromomethyl)benzocoumarin. The presence of the alkyl bridge in these new fused-naphthopyrans prevents the formation of one long-lived photoisomer and therefore has a dramatic effect on their photochromic properties: UV irradiation of common naphthopyrans gives rise to two isomeric colored photoisomers, one of which fades very slowly and is responsible for a persistent residual color. UV excitation of these new uncolored fused-naphthopyrans leads to the formation of only one colored photoisomer that fades completely to the uncolored state in few seconds/minutes following a monoexponential decay law, thus avoiding the problem of the residual coloration typically observed with naphthopyrans.

Kinetic and structural studies of the photochromic process of 3H-naphthopyrans by UV and NMR spectroscopy

The photomerocyanines resulting from the UV irradiation of closed colourless 3,3-diphenyl-3H-naphtho[2,1-b]pyran and its 3,3-bis(4-fluorophenyl) derivative have been studied by UV–VIS and NMR spectroscopy.Kinetic bleaching studies have been carried out on a UV–VIS spectrometer at the photostationary state in acetonitrile solution. A thermal biexponential back-isomerization is observed. These two different kinetics can be attributed to two different isomers of the photomerocyanine.NMR spectroscopy allows us to obtain a 1H, 13C and 19F (where present) structural identification of these two forms: complete assignment for the predominant isomer (trans–cis) and partial assignment for the second one (trans–trans). The presence of a third form (which quickly decays) is also observed. Each open form follows a mono-exponential decay, with very different bleaching rate coefficients. This is the first time that 19F NMR has been used to identify the number and structure of isomers produced by irradiation, illustrating the advantages of this technique.

Preventing the formation of the long-lived colored transoid-trans photoisomer in photochromic benzopyrans.

A new photochromic fused benzopyran presenting a bridge between the pyran double bond and the benzenic ring was prepared. While the UV irradiation of usual benzopyrans leads to the formation of two colored photoisomers with very different thermal stabilities, studies by laser flash photolysis showed that the presence of this particular bridge prevents the formation of the undesirable long-lived colored TT isomer and therefore after laser irradiation the colored solution fades following a fast monoexponential decay.

NMR Structural and Kinetic Assignment of Fluoro-3H-naphthopyran Photomerocyanines¶

The kinetic and structural behavior of a photochromic compound, 3‐(2‐fluorophenyl)‐3‐phenyl‐3H‐naphtho[2,1‐b]pyran (F‐Py), was investigated using 1H and 19F nuclear magnetic resonance (NMR) spectroscopy. Upon irradiation, the four theoretically predicted photomerocyanines appear along with a fifth form X, whose final structure has not been elucidated. This last form and two of the photomerocyanines are thermally labile, whereas the other two do not show any signs of decay. The system has been analyzed by NMR spectroscopy. This led to the structural assignment of each photomerocyanine. The kinetics of the thermal bleaching were monitored by directly and separately measuring the concentrations of each species at regular time intervals using 19F NMR spectroscopy. We therefore propose a plausible reaction mechanism. On the basis of this mechanism, the mathematical treatment and the study of the effects of temperature led to the determination of the kinetic and thermodynamic parameters (rate coefficients, enthalpy and entropy of activation) of this photochromic system. The leading role of the labile intermediate X on the formation of trans–transoid–cis (TTC) and cis–transoid–cis (CTC) photomerocyanines is pointed out.

Photochromic C2-Symmetric Chiral Diarylethene: From the Initial State to the Final State

The behavior of 1,2-bis[(R)-2-(1-methoxymethoxyethyl)-3-benzo[b]thienyl] hexafluorocyclopentene before and upon UV irradiation and during thermal evolution of the photoirradiated solution has been thoroughly investigated by multinuclear NMR spectroscopy. A dynamic NMR study of the initial state was performed, providing a detailed description of the perceived conformational processes in the system. Before irradiation, three open conformations are in equilibrium, whereas UV irradiation generated the two expected cyclized diastereomers. The minor diastereomer was thermally less stable than the major one, thus leading to an unexpected increase in the diastereoselectivity when raising the photoirradiation temperature. In addition, a long thermal evolution induced slow rearrangement of the stable diastereomer into byproducts that were identified.

NMR proofs of the involvement of an allenyl-naphthol as a key-intermediate in the photochromic process of [3H]-naphthopyrans

The NMR characterization of an intermediate, proved to be involved in the photochromic process of fluoro-substituted-3,3-diphenyl-3H-naphtho[2,1-b]pyrans is reported. An o-allenyl-naphthol structure was deduced and a plausible reaction mechanism was proposed.

NMR characterization of allenyl-naphthol in the photochromic process of 3,3-diphenyl-[3H]-naphtho[2-1,b]pyran

Abstract Allenyl-naphthol was produced after UV irradiation followed by visible light irradiation of 3,3-diphenyl-[3 H ]-naphtho[2-1,b]pyran (CH). Elucidation of its structure was obtained by 1D and 2D NMR spectroscopy. Kinetic study showed that its thermal bleaching followed a monoexponential decay, leading to an increase of concentration of TC merocyanine.