Stepwise intramolecular photoisomerization of NHC-chelate dimesitylboron compounds with C-C bond formation and C-H bond insertion

C,C-chelate dimesitylboron (BMes(2)) compounds containing an N-heterocyclic carbene (NHC) donor have been obtained. Single-crystal X-ray diffraction analyses established that the boron atom in these compounds is bound by four carbon atoms in a distorted tetrahedral geometry. Compared to previously reported N,C-chelate dimesitylboron compounds, the new C,C-chelate boron compounds have a much larger HOMO-LUMO energy gap (>3.60 eV). They do, however, respond to UV irradiation (300 nm) in the same manner as N,C-chelate BMes(2) compounds do, undergoing photoisomerization and converting to an intensely colored (yellow or orange) isomer A quantitatively, with a high quantum efficiency (0.60-0.75). NMR and single-crystal X-ray diffraction analyses established that the structure of A is similar to the dark isomers obtained from N,C-chelate BMes(2) compounds. However, unlike the N,C-chelate dark isomers that have the tendency to thermally reverse back to the light colored isomers, the isomers A of the C,C-chelate BMes(2) are thermally stable and no reverse isomerization was observed even when heated to 80 °C (or 110 °C) for hours. The most unusual finding is that isomers A undergo further photoisomerization when irradiated at 350 nm, forming a new colorless species B nearly quantitatively. NMR and single-crystal X-ray diffraction analyses established the structure of isomer B, which may be considered as an intramolecular C-H insertion product via a borylene intermediate. Mechanistic aspects of this unusual two-step photoisomerization process have been examined by DFT computational studies.     

Influence de la longueur d'onde sur la photoisomerisation de derives de l'hexatriene-1,3,5

Kinetic studies of photoisomerization of 1-phenyl 6-methyl-cyclohexa-1,3-diene, and of the four isomeric 5-phenylhepta-1,3,5-trienes, have been performed at 313, 254, 228 and 214 nm. Photoreactions of these compounds are dependent on the wavelength of irradiation. Photolysis at short wavelength of Z,E-5-phenylhepta-1, 3,5-triene initiates a new reaction, sigmatropic [1,5] hydrogen migration, forming an ene-allenic compound. Thermal reactivity data for polyenic compounds supports the assigned structures.     

Spectral and photochemical properties of covalently linked dyads based on 2-styrylquinoline and 6-hydroxy-2-naphthoic acid

Photophysical and photochemical properties of bichromophoric covalently linked SnN dyads (n = 3, 5, 9), in which the 2-styrylquinoline (SQ) and 6-hydroxy-2-naphthoic acid (Np) moieties are linked by the dioxypolymethylene bridge–O–(CH₂)_n–O–have been studied. The properties of the dyads have been compared with those of the model compounds 2-(4-methoxystyryl)quinoline and methyl 6-hydroxy-2-naphthoate. Inductive-resonance (Förster) energy transfer (FRET) from the Np to the SQ unit with an efficiency up to 99.6% is observed in the S₁ state of the dyads. The Np unit in the neutral form does not affect the photoisomerization of the SQ unit regardless of the form of the latter, neutral or protonated (cationic). The Np moiety in the anionic (deprotonated) form hinders the photoisomerization of the SQ moiety, presumably, as a result to a combined action of several factors.     

Regioselective E(trans)-Z(cis) photoisomerization in naphthyldiene derivatives

Naphthyldiene derivatives,1-4, carrying electron-donating groups at one end and electron-withdrawing groups at the other, were synthesized to study the photoisomerization process. All the compounds showed efficient photoisomerization upon direct excitation leading to the formation of 4-Z isomer with high selectivity. Triplet sensitization studies indicated inefficientE-Z isomerization process. Room temperature fluorescence of1 and2 displayed fine structure in hexane solvent and the same was replaced by broad or structureless fluorescence in acetonitrile and methanol solvents. A mechanism involving a polarized or charge transfer singlet excited state is proposed for the observed photoisomerization in these naphthyldiene derivatives.     

Photochemical properties of amino and nitro derivatives of 2-and 4-styrylquinolines and their hydrochlorides

The trans-cis and cis-trans photoisomerization reactions of a double bond in nitro and amino derivatives of 2-and 4-styrylquinolines in the neutral and protonated (hydrochloride) forms were studied. Protonation in nitrostyrylquinolines was shown to have no effect on the photoisomerization quantum yields. In aminostyrylquinolines, the photoisomerization reaction is “switched off” for the monocationic form as a result of the competitive process of intramolecular charge transfer and is “switched on” again for the dication. In the latter case, the quantum yield of trans-cis photoisomerization decreases by a factor of 2–2.5 and the quantum yield of cis-trans photoisomerization remains practically unchanged as compared to that of the neutral compound. Upon long-term irradiation of 4-(4′-nitrostyryl)quinoline, the photocyclization reaction of the cis-isomer was observed.     

双(苯并噁唑)芪的光物理行为及其异构化反应的研究

研究了双(苯并噁唑)芪的光物理行为和光顺反异构化反应,对这类化合物的发光光谱不依赖于溶剂极性的原因进行了初步讨论。异构化反应中,发现乙二醇引入时会加速该化合物反-顺异构化和抑制顺-反过程的回复,其原因可能是与生成了顺式异构体两个苯基苯并噁唑间的氢键有关。     

A photo-responsive cholesterol capable of inducing a morphological transformation of the liquid-ordered microdomain in lipid bilayers

An azobenzene-modified cholesterol was designed and synthesized for photo-induced domain transformation in lipid bilayer membranes. Upon UV-light irradiation, the cholesterol derivative changes the conformation through photoisomerization of the azobenzene moiety from trans- to cis-form. The photoisomerization effectively occurred both in liquid-ordered (L_o) and liquid-disordered (L_d) phases. Phase-contrast and fluorescence microscopic observation revealed that photoisomerization of the azobenzene-modified cholesterol induced the shape transformation of giant unilamellar vesicle (GUV) and the reorganization of L_o domain structure. Such a photo-induced transformation of lipid domain gave two different pathways dependent on the lipid composition of GUV; disappearance of the L_o domain or appearance of a small L_d domain with in the L_o domain.     

Triplet-sensitized photobehaviour of the three stereoisomers of 1,4-distyrylbenzene and some aza-analogues

The triplet spectral properties of the three stereoisomers (EE, ZE and ZZ) of 1,4-distyrylbenzene and some symmetric EE-aza-analogues, bearing a nitrogen heteroatom in the side (2′-pyridyl, 4′-pyridyl or 2′-quinolyl) or in the central (2,5-pyridine) arene rings, have been investigated by laser flash photolysis in different solvents and compared with those of the parent hydrocarbon. The quantum yields of the triplet-sensitized photoisomerization have also been measured. Adiabatic formation of the excited ³EE^* isomer by irradiation of the other geometrical isomers has been detected by transient spectroscopy. The effect of the substrate concentration on the sensitized photoisomerization quantum yield revealed the occurrence of a quantum chain process, as known for similar arylolefins.     

Photoisomerization of naphthylquinolylethylenes in neutral and protonated forms

The quantum yield of the trans-cis photoisomerization of 1-(1-naphthyl)-2-(2-quinolyl)ethylene and 1-(2-naphthyl)-2-(2-quinolyl)ethylene is close to the theoretical limit (0.5) for diabatic photoisomerization and does not change on passing from the neutral to the protonated form. The data obtained indicate the absence of the α-effect for the test compounds, which consists in an increase in the trans-cis photoisomerization quantum yield to values of >0.5 upon protonation of some azadiarylethylenes with the nitrogen atom in the α-position to the ethylene group.     

Photoisomerization of PtII Complexes Containing Two Different Photochromic Chromophores: Boron Chromophore versus Dithienylethene Chromophore

In order to examine competitive photoisomerization, a series of novel photochromic Pt^II molecules that contain both dithienylethene (DTE) and B(ppy)Mes₂ units (ppy=2-phenylpyridine, Mes=mesityl) were successfully synthesized and fully structurally characterized. Their photochromic properties were examined by UV/Vis, emission and NMR spectroscopy. It was found that the DTE unit in all three compounds is the preferred photoisomerization site, exhibiting reversible photochromism with irradiation. The B(ppy)Mes₂ unit does not undergo photoisomerization in these molecules, but likely enhances the photoisomerization quantum efficiency of the DTE moiety through the antenna effect. Extended irradiation with UV light leads to the rearrangement of the ring-closed isomers of DTE. TD-DFT computational studies indicate that the DTE photocyclization proceeds via a triplet pathway through an efficient energy transfer process.     

Photochemical and electrochemical Z–E isomerization of 1,4-dialkoxy-2,5-bis[2-(thien-2-yl)ethenyl]benzene stereoisomers

The group of 1,4-dialkoxy-2,5-bis[2-(thien-2-yl)ethenyl]benzene stereoisomers was synthesized in which methoxy- and ethoxy-groups were used as alkoxy-substituents. These isomers were characterized as solution species both electrochemically and spectroscopically. As expected, these compounds, having a stilben like structure, are the subject of photoisomerization, which is described and discussed. It is demonstrated how electrochemical process may cause isomerization of the double CC bonds in that group of compounds. An attempt of using electrochemical methods to monitor the process of photoisomerization of these compounds is presented. Mechanism of the oxidatively induced electrochemical isomerization has been proposed and discussed. The electrochemical isomerization mechanism is verified by digital simulation, which allowed estimating basic kinetic parameters of the processes.     

Optically active polymers containing side-chain azobenzene moieties: Photochromic and photoresponsive behavior of copolymers of N-(4-azobenzene) maleimide with (−)-menthyl vinyl ether and (+) (S)-2-methylbutyl vinyl ether

Photochromic behavior and photoisomerization kinetics of optically active copolymers of trans-N-(4-azobenzene) maleimide (ABM) with (?) -menthyl vinyl ether (MtVE) and (+) (S) -2-methylbutyl vinyl ether were studied by UV spectroscopy under irradiation at 348 nm. The resulting data have been compared with those of the corresponding copolymers containing also trans-N-(4-azobenzene) maleamic acid co-units as well as of low molecular weight model compounds. The photoresponsive behavior has been also investigated by circular dichroism measurements at various extents of photoisomerization. The results are discussed in terms of structural requirements of the macromolecules. © 1994 John Wiley & Sons, Inc.     

Conformational analysis of 2-anthryl-ethylene derivatives: Photochemical and computational investigation

2-Anthrylethylene derivatives 1 E?C5 E and 1 Z are synthesized to study the cis-trans photoisomerization. Interestingly, unlike 9-anthrylethylene derivatives, 2-anthrylethylene derivatives 1 E to 5 E do not exhibit E(trans) to Z (cis) photoisomerization upon direct and triplet sensitization. One-way Z (cis) to E (trans) photoisomerization of 1 Z is found to be very efficient under direct and triplet sensitization conditions, demonstrating the involvement of both singlet and triplet states. 1 E?C5 E exhibits excitation wavelength dependent fluorescence indicating the existence of conformers (rotamers) at room temperature, which is confirmed by fluorescence lifetimes measurements of compounds 1 E and 2 E. Theoretical studies are carried out at DFT and ab initio methodology and the calculated relative energy difference of the conformers is very small; it ranges between 2.9 kJ·mol^?1 to 6.3 kJ·mol^?1 for both ground and excited states.     

pH-Dependent Photoinduced Interconversion of Furocoumaric and Furocoumarinic Acids

Photo-controlled or photo-regulated molecules, especially biologically active and operating in physiological conditions, are in steady demand. Herein, furocoumaric and furocoumarinic acids being (Z/E)-isomers relative to each other were obtained in two stages starting from psoralen: the alkaline solvolysis of psoralen led to furocoumaric acid, which was further Z → E photoisomerized (365 nm) to furocoumarinic acid. The kinetics of Z → E photoisomerization was monitored by HPLC and UV-vis spectrophotometry. Photophysical characteristics in the aqueous phase for both acids, as well as the reversibility of (Z/E) photoisomerization process, were also assessed. Furocoumarinic acid was found to be visibly fluorescent at pH 2.0–12.0, with the maxima of fluorescence emission spectra being pH-dependent. The reverse E → Z photoisomerization predicted by quantum chemistry calculations as energetically favorable for the monoanionic form of furocoumarinic acid was proved in the experiment while being complicated by pyrone ring closure back to psoralen in acidic and neutral conditions. The preparative synthesis of furocoumarinic acid outlined in this work is particularly valuable in view of a wide range of pharmacological effects previously predicted for this compound.     

Photochemical synthesis of isomeric (E/Z)-3-alkylidene-3H-isobenzofuranones

The synthesis of isomeric (E/Z)-3-alkylidene-3H-isobenzofuranones by photoisomerization of 2-aroyl-2-methyl/benzylindan-1,3-diones in high yields is described.     

Photoisomerization of 2-styrylquinoline in neutral and protonated forms

The quantum yields of the trans-cis and cis-trans photoisomerization of 2-styrylquinoline (2SQ) and its several derivatives were measured in neutral, protonated, and quaternized forms. It was shown that electron-donor substituents in the styryl moiety increase the quantum yield of trans-cis photoisometization ?_tc in the neutral form as a result of stabilization of the intermediate zwitterionic perpendicular conformer. On passing from the neutral to the positively charged forms (protonated or quaternized), an increase in the quantum yields to ?_tc > 0.5 was observed, thus suggesting in terms of the classical diabatic mechanism of photoisomerization via the perpendicular conformer the shift of the minimum on the potential energy surface (PES) of the S ₁ state relative to the maximum of the S ₀ state PES to the cis-isomer or a possible contribution of the adiabatic route to the photoisomerization of the 2SQ cations.     

Tuning the Photoisomerization of a N^C‐Chelate Organoboron Compound with a Metal?Acetylide Unit

To examine the impact of metal moieties that have different triplet energies on the photoisomerization of B(ppy)Mes₂ compounds (ppy=2‐phenyl pyridine, Mes=mesityl), three metal‐functionalized B(ppy)Mes₂ compounds, Re‐B , Au‐B , and Pt‐B , have been synthesized and fully characterized. The metal moieties in these three compounds are Re(CO)₃(tert‐Bu₂bpy)(C?C), Au(PPh₃)(C?C), and trans‐Pt(PPh₃)₂(C?C)₂, respectively, which are connected to the ppy chelate through the alkyne linker. Our investigation has established that the Re^I unit completely quenches the photoisomerization of the boron unit because of a low‐lying intraligand charge transfer/MLCT triplet state. The Au^I unit, albeit with a triplet energy that is much higher than that of B(ppy)Mes₂, upon conjugation with the ppy chelate unit, substantially increases the contribution of the π→π* transition, localized on the conjugated chelate backbone in the lowest triplet state, thereby leading to a decrease in the photoisomerization quantum efficiency (QE) of the boron chromophore when excited at 365 nm. At higher excitation energies, the photoisomerization QE of Au‐B is comparable to that of the silyl–alkyne‐functionalized B(ppy)Mes₂ ( TIPS‐B ), which was attributable to a triplet‐state‐sensitization effect by the Au^I unit. The Pt^II unit links two B(ppy)Mes₂ together in Pt‐B , thereby extending the π‐conjugation through both chelate backbones and leading to a very low QE of the photoisomerization. In addition, only one boron unit in Pt‐B undergoes photoisomerization. The isomerization of the second boron unit is quenched by an intramolecular energy transfer of the excitation energy to the low‐energy absorption band of the isomerized boron unit. TD‐DFT computations and spectroscopic studies of the three metal‐containing boron compounds confirm that the photoisomerization of the B(ppy)Mes₂ chromophore proceeds through a triplet photoactive state and that metal units with suitable triplet energies can be used to tune this system.     

降冰片二烯衍生物的光异构化 Ⅱ.2-萘胺甲酰基-2,5-[2.2.1]-双环庚二烯的光异构化

合成了降冰片二烯衍生物——2-萘胺甲酰基-2,5-[2.2.1]-双环庚二烯(B)。研究了它在太阳光直接照射下的光异构化反应。(?) 此外,还确定了化合物(B)和(C)的分子结构。它们都是文献中未曾报道过的新化合物。     

Influence of helical twisting power on the photoswitching behavior of chiral azobenzene compounds: applications to high-performance switching devices

Five photochromic chiral azobenzene compounds and one nonphotochromic chiral compound were synthesized and characterized by IR, 1H NMR spectroscopy, and elemental analysis. Cholesteric liquid crystalline phases were induced by mixing of the nonphotochromic chiral compound and one of the photochromic chiral azobenzene compounds in a host nematic liquid crystal (E44). The helical pitch of the induced cholesteric phase was determined by Cano's wedge method and the helical twisting power (HTP) of each sample was thus determined. The helical twisting powers of azobenzene compounds were decreased upon UV irradiation, due to trans-->cis photoisomerization of azobenzene molecules. Among the azobenzene compounds synthesized in our study, Azo-5, with isomannide (radical) as chiral photochromic dopant, showed the highest HTP and contrast ratio (Tmax/Tmin). Photoswitching between compensated nematic phase and cholesteric phase was achieved through reversible trans<-->cis photoisomerization of the chiral azobenzene molecules through irradiation with UV and visible light, respectively. Transmission rates (contrast ratios) increased with decreasing helical pitch length in the induced cholesteric phase. The influence of helical twisting power on the photoswitching behavior of chiral azobenzene compounds is discussed in detail.     

Acylhydrazone-based supramolecular assemblies undergoing a converse sol-to-gel transition on trans → cis photoisomerization

Photoisomeric supramolecular assemblies have drawn enormous attention in recent years. Although it is a general rule that photoisomerization from a less to a more distorted isomer causes the destruction of assemblies, this photoisomerization process inducing a converse transition from irregular aggregates to regular assemblies is still a great challenge. Here, we report a converse sol-to-gel transition derived from the planar to nonplanar photoisomer conversion, which is in sharp contrast to the conventional light-induced gel collapse. A well-designed acylhydrazone-linked monomer is exploited as a photoisomer to realize the above-mentioned phase transition. In the monomer, imine is responsible for trans–cis interconversion and amide generates intermolecular hydrogen bonds enabling the photoisomerization-driven self-assembly. The counterintuitive feature of the sol-to-gel transition is ascribed to the partial trans → cis photoisomerization of acylhydrazone causing changes in stacking mode of monomers. Furthermore, the reversible phase transition is applied in the valves formed in situ in microfluidic devices, providing fascinating potential for miniature materials.

A converse sol-to-gel transition system based on trans → cis photoisomerization of acylhydrazone-based supramolecular assemblies has been sucessfully established, which was applied in the gel-based microvalves that can in situ control flow by light.