RESUMEN

A novel supramolecular photoactuator in the form of a thin film of centimetric size has been developed as an alternative to traditional liquid crystal elastomers (LCE) involving azobenzene (AZO) units or photochromic microcrystals. This thin film is produced through spin coating without the need for alignment or crosslinking. The photoactuator combines a photochromic dithienylethene (DTE) functionalized with ureidopyrimidinone (UPy) units, and a telechelic thermoplastic elastomer, also functionalized with UPy, allowing quadruple hydrogen bonding between the two components. Upon alternating ultraviolet (UV) and visible light exposure, the film exhibits reversible bending and color changes, studied using displacement and absorption tracking setups. For the first time, the photomechanical effect (PME) is quantitatively correlated with photochromism, showing that DTE units drive the movement under both UV (photocyclization) and visible (photoreversion) light. In situ illumination techniques reveal that the PME arises from photoinduced strain within 160 nm UPy-bonded DTE domains, which expand and contract by approximately 50% under UV and visible light, respectively. The semicrystalline nature of the elastomer and a robust supramolecular network connecting both components are critical in converting microscopic photostrain into macroscopic actuation.

RESUMEN

Time-resolved absorption spectroscopy measurements were performed to study the dynamics of photochromic 1,2-Bis(2,4-dimethylthiophene-3-yl)perfluoro-cyclopentene (DMTPF) in chloroform, including antiparallel conformer ring-closure reaction and parallel conformer photophysics. All characteristic times are given, discussed and compared to a previous publication concerning the close molecule substituted with phenyl rings. (Hamdi et al., PCCP, 2016). Apart from the expected photocyclization process, condensed ring by-product formation is observed and hypotheses concerning the origin of this by-product are presented.

RESUMEN

A dithienylethene (DTE) photochromic compound functionalized by ureidopyrimidinone (UPy) quadruple hydrogen bonding blocks was synthesized by Takeshita and coworkers [Takeshita et al., Chem. Commun., 2005, 761] in order to form a light-responsive supramolecular self-assembling system. In solution, the formation of supramolecular assemblies was only observed for one DTE isomer, namely the closed-form isomer. To rationalize this experimental finding, with the help of Molecular Dynamics (MD) and (time-dependent) DFT calculations, the behaviour of open-form and closed-form monomers, dimers, hexamers and π-stacked dimers in solution is investigated. Our simulations show that, for the open-form oligomers, the progression of the supramolecular assembly is hindered due to (i) the possible formation of a very stable cyclic dimer for the open-form parallel isomer, (ii) the relative flexibility of the open-form oligomers compared to their closed-form counterparts, and (iii) the possible existence of π-stacked dimers that constitute bottlenecks blocking the progression of the supramolecular self-assembly.

RESUMEN

The excited state dynamics of constrained photochromic benzodithienylethenes were addressed by considering the bridging with polyether chains (from x = 4 to 6 units) at the ortho and meta positions of the aryl group, named DTE-ox and DTE-mx, via time-resolved absorption spectroscopy supported with (TD)-DFT calculations. The photochromic parameters and geometrical structures of these series are discussed. A novel photocyclization pathway via a triplet state, evidenced recently (Hamdi et al., Phys. Chem. Chem. Phys., 2016, 18, 28091-28100), is largely dependent on the length and the position of the polyether chain. For the first time, by comparing the two series, we revealed, for the DTE-ox series, an interconversion not only in the ground state but also between the triplet states of the anti-parallel and parallel open form conformers.

RESUMEN

The photophysical properties of a series of dithienylethenes, free or blocked in an ideal photoactive conformation by an alkyl bridge, have been investigated by stationary, ultrafast spectroscopy and state-of-the-art time-dependent density functional theory calculations. Thanks to the clear ultrafast transient signatures corroborating NMR results, we bring strong evidence that the unreactive parallel open form conformer has been efficiently removed by the chain. For the first time, the photophysics of this species, namely an internal conversion of 120 ps is highlighted. In contradiction to the main ideas in the literature, the photocyclization mechanism is rationalized by a direct photocyclization mechanism from the Franck-Condon region passing directly through a conical intersection within ≈100 fs (not few picoseconds) while a competitive mechanism occurs through the relaxed S1 state. Relaxation processes (fluorescence and internal conversion) originating from this relaxed state are sensitive to the length of the blocking chain. Both concomitant pathways are necessary to rationalize: (i) the inverse relationship between emission and cyclization quantum yields and (ii) the non-unity value of the latter for bridged compounds.

RESUMEN

State-of-the-art experimental and theoretical tools were used to investigate the gas-phase relaxation dynamics of various photoexcited photochromic dithienylethene molecules in situations where several relaxation channels are simultaneously at play. Unconstrained and constrained dynamics were addressed by considering unbridged and bridged molecules with a polyether bridge of various sizes (from 2 to 4 units). Time-resolved ultrafast ionization spectroscopy techniques were used to probe the dynamics. This revealed the existence of several relaxation pathways from the first excited state to the ground-state. Characteristic times were determined for each process. These channels compete at an early stage of the dynamics only when the initial wavepacket splits into two parts. A striking excited state wavepacket oscillation is observed in bridged molecules. A general reaction mechanism is proposed which rationalizes the carbon-carbon distance rule which is widely used as an empirical tool to predict the photoactivity of photochromic molecules in crystals.

RESUMEN

We investigate an inverse (I) dithienylethene, the bis(3,5-dimethyl-2-thienyl) perfluorocyclopentene, using absorption, emission and NMR spectroscopies as well as state-of-art first-principles (TDDFT) calculations. First, we find in addition to the expected antiparallel and parallel conformers, a new stable antiparallel conformer , but its energy is too high to be significantly populated at working temperature. More importantly, we demonstrate that, instead of an equal proportion of an AP and a P conformer as in normal (N) diarylethenes, the AP conformer is present in large excess. This result is confirmed by both DFT thermodynamical analysis and temperature-dependent NMR experiments modelized with an ↔ fast interconversion model. With the latter, the relative populations are estimated to be ca. 3/1 for /. Furthermore, the 0-0 energies simulated with a model that accounts for both vibrational and state-specific media effects of the ground and the excited states indicate that and have very similar absorption signatures while only the conformer should give rise to emission. Eventually, within excited state manifold, important topological points along the ring-closure reaction coordinate, and more specifically the unprecedented S1(opt) of the closed isomer, have been identified.

Asunto(s)

RESUMEN

Thiophenophan-1-enes with chiral polyether bridges were prepared and their diastereospecific photochromic reactions were studied. The coupling reaction of substituted dithienylethenes and various chiral synthons afforded thiophenophan-1-enes, namely, bridged dithienylethenes, as single enantiomers without optical resolution, thus indicating that these reactions occurred diastereoselectively. Upon UV irradiation, each optically active thiophenophan-1-ene isomerized to the corresponding enantiomer of the closed form and returned to the initial enantiomer of the open form upon visible irradiation. Because thiophenophan-1-enes never isomerized to other diastereomers even at a high temperature, they underwent diastereospecific photochromic reactions. Large changes were observed in the measurement of the optical rotations of the solutions of thiophenophan-1-enes at 588 nm according to their photochromic reactions. As there was no absorption at this wavelength for both isomers of each thiophenophan-1-enes, the nondestructive readout of the photochromic reaction could be carried out by using these chiral thiophenophan-1-enes.

RESUMEN

Two photochromic diarylethenes blocked by alkyl bridges in an ideal conformation for photocyclization are studied by stationary and femtosecond transient spectroscopy in order to depict the photocyclization processes: the bistable 1,2-dicyano[2.n]metacyclophan-1-ene with n = 2, abbreviated as [2.2], and its non-bistable analogue with n = 4, abbreviated as [2.4]. The data are interpreted in the light of AM1-CIS calculations and state correlation diagrams based on conclusive TD-DFT calculations. For [2.2], a solvent-sensitive excitation wavelength threshold governing the photocyclization yield is clearly evidenced between the S(1) and S(2) singlet states. Excitation above and beyond this threshold induces two distinct photochemical pathways. The S(1) vertical excitation induces direct efficient (phi approximately = 0.9-1), and ultrafast (approximately 120 fs) photocylization from S(1) open form that leads to a ground-state transition structure, probably through a conical intersection, then to a hot cyclized ground state that relaxes by vibrational cooling. Upon higher excitation energy, the system undergoes internal conversion to the hot S(1) state, then evolves toward the cyclized S(1) state and relaxes by ultrafast S(1)-S(0) internal conversion. Alternatively, the possibility for a second conical intersection near hot S(1) state is discussed. This second photoclosure reaction is less efficient and both the photocylization yield and overall kinetics depend on solvent polarity (phi = 0.49, tau = 2.5 ps in nonpolar solvent; phi = 0.7, tau = 1.5 ps in polar solvent). In the case of [2.4], for which the distance between the two reactive carbons is larger, a unique photoclosure mechanism is found and a structural effect is reported. Indeed, this mechanim is similar to the above second reaction of [2.2] but characterized by much slower kinetics ranging from 12 to 20 ps (depending on the excitation wavelength and solvent polarity). All polarity effects are rationalized in terms of stabilization of the transient states of charge-transfer character involved in the photocyclization process.

RESUMEN

Novel thiophenophan-1-enes, which are composed of a dithienylethene unit and a chiral bridge, were synthesized and obtained as optically-active single diastereomers. Upon UV irradiation, the open-form isomers afforded single diastereomers of the closed form in 100% diastereo-excess, indicating that this system undergoes a fully diastereospecific photochromic reaction.

RESUMEN

The electrochemical oxidation of either open or closed metacyclophanene isomers, which is a positive-photochromic system due to open isomers being more stable than closed ones, quantitatively yields a stable fluorescent dihydropyrene intermediate, a well-known negative T-photochromic system where the closed form is more stable than the open one. This is one of the first examples in which two different molecular switching systems can be electrochemically triggered.

RESUMEN

The structure assigned to the antitumor antibiotic BU-4664L from Micromonospora sp. was revised to 5,10-dihydro-4,6,8-trihydroxy-10-(3,7,11-trimethyl-trans-2,trans-6,10-dodecatrienyl)-11H-dibenzo[b,e] [1,4]-diazepin-11-one based on the NMR analysis.

Asunto(s)

RESUMEN

Particle sizes of a supramolecular polymer composed of diarylethene having two quadruple hydrogen bonding moieties were changed photoreversibly.

Asunto(s)