Photoisomerization In Solution Research Articles

2-(1,1-Dicyanomethylene)rhodanine-Functionalized Oligothiophenes: A Structure-Property Investigation of Z/E Photoisomerization Behavior.

A series of oligothiophenes singly and doubly functionalized with dicyanorhodanine (RCN) units have been investigated to understand their Z/E photoisomerization behavior upon structural modulation. Monotopic RCN target molecules (1-Z-9-Z) were designed to observe the consequences of π-conjugation, solubilizing group substitution, and formylation of the thiophene units. In all cases, the Z isomer is obtained from synthesis as the thermodynamically stable isomer, whereas the E isomer is achieved through selective irradiation (including red light, λirr = 628 nm) as a Z/E mixture in solution. For the quarterthiophene entries, photoisomerization is inhibited, with photoirradiation resulting only in degradation. The result comports with concentration-dependent studies, which show that increasing π-conjugation results in greater aggregation and muted Z/E photoisomerization. Ditopic RCN targets (10-ZZ-12-ZZ), mimicking acceptor-donor-acceptor (A-D-A) oligomers relevant to OPV materials, also show evidence of photoisomerization in solution, with formation of Z,Z/Z,E mixtures at the photostationary state (PSS). Complementary ground- and excited-state DFT calculations show excellent agreement with the experimental findings. This comprehensive structure-property analysis is expected to both guide and caution the functional materials community with respect to the usage of photoisomerizable RCN-oligothiophenes for optoelectronic applications.

Modulating the isomerization of bpp-modified azoheteroarene by transition metal ions coordination

A new photoswitchable azoheteroarene L was design and synthesized by replacing one of the phenyl rings of azobenzene moiety with a 2,6-di[pyrazol-1-yl]pyridine(bpp) unit. The assembly of L with transition metal ions results in the formation of three metal complexes, namely, M(L)2(OTf)2 (M=ZnII, FeII and CoII). These complexes are fully characterized by ESI-TOF-MS, UV–vis spectrum, 1H NMR spectrum and single-crystal X-ray diffraction. Upon irradiation with 380 nm light, L exhibits excellent E to Z photoisomerization in solution, achieving up to 82 % Z-configuration with a thermotropic isomerization half-life time of 301 days. After coordination with ZnII, FeII and CoII, the photoisomerization properties of L were significantly varied. Coordinating with the ZnII ion, the conversion of Z-L slightly decreased from 82 % to 77 %. However, this coordination significantly extends thermal isomerization half-life time to 564 days, resulting in a 1.9-fold enhancement compared to free Z-L. In contrast, the coordination with open-shell transition metal ions FeII and CoII noticeably inhibits the formation of Z-L and only yields E-L isomers. Moreover, the spin-crossover behavior of Fe(E-L)2(OTf)2 was also investigated. The analysis of variable temperature NMR indicates that Fe(E-L)2(OTf)2 shows a spin transition temperature of 265 K in solution. This work provides valuable insights into tunning conformational transitions of azoheteroarene molecules.

Azobenzene-Imidazolium ionic liquid crystals: Phase properties and photoisomerization in solution state

This is the first report of phase properties and photoisomerization behaviour of a series of amphiphilic azobenzene-imidazolium derivatives. The cationic imidazolium moiety was functionalised with varying alkyl chain length (C10-C18) and connected to an azobenzene via a flexible ether spacer. Their phase transitional properties and photoisomerization in solution were modulated by the alkyl chains in the imidazolium moiety. C14 was the shortest chain length to induce the formation of a liquid crystal phase, wherein compounds having C14-C18 exhibited stable smectic A phase. All compounds photo switched from trans-to-cis isomers in solution when illuminated with UV radiation. DFT calculations revealed that the trans isomers could adopt two geometries, however, folded geometry (global minima) was thermodynamically more stable than the open chain structure. The long conjugate (C16) had a higher trans-cis isomerization barrier due to the multilayer folding geometry that reduced its structural flexibility as compared to the C10 homologue. The photo conversion efficiency (CE) was alkyl chain length dependent. The C10 salt had a larger CE than C16 homologue due to the transition gap between S1 excited state and ground surface is smaller in the former (0.211 eV) than that in the latter (0.278 eV), thus resulting in efficient photoisomerization in the former.

Boosting Ultrafast Trans‐Cis Photoisomerization and Intersystem Crossing in Nanocrystals of Double‐Bond Photoswitching Molecules

AbstractDouble‐bond photoswitching molecules typically can undergo trans→cis photoisomerization, but their photoisomerization mechanisms in the solid state have been rarely reported. Herein, the excited‐state evolutions of trans‐azobenzene (AB), trans‐stilbene (TS), and N‐benzylideneaniline (NBA) are studied to unveil the relationship between the double‐bond photoswitching molecules and the photoisomerization kinetics in the solution phase and in nanocrystal suspensions (NCS). The photoisomerization rate of NBA is the fastest among these three molecules in the solution phase due to its most apparent single‐bond feature of the central double bond in the S1 state. The free space ordered crystal configuration boosts the photoisomerization rates of AB and TS in NCS to be faster than that of the solution phase because the hindrance of the wrapping solvent is eliminated in the crystals. In contrast, when NBA is prepared into NCS, it becomes distorted into a nonplanar structure because of the asymmetrical C‐H···π interactions, resulting in NBA having a large spin‐orbital coupling (SOC) value which opens the intersystem crossing channel to generate the triplet state instead of undergoing photoisomerization in solution. Therefore, this work reveals that the crystal configuration and the molecular structure may critically determine the photophysical properties of photoswitching materials.

Synthesis, crystal structure, photoisomerization, and DFT studies of novel azo compounds based on imidazole

AbstractNew azo imidazole compounds 1‐3 were synthesized by condensation of 2‐hydroxy‐3‐methoxy‐5‐([aryl]diazenyl)benzaldehyde derivatives with o‐vanillin and characterized. The crystal structure of 1 crystallizes in the monoclinic system with space group P21/c and adopts an E configuration for the azo and azomethine double bonds. The photoisomerization experiments have been performed in DMSO as a function of time in order to investigate the photodynamical trans‐cis‐trans conversion of the azo (–N=N‐) chromophore of the imidazoles 1‐3. The compounds exhibited strong photoisomerization in solution and their trans to cis isomerization took about 40 min whereas the reverse process took 120 min. The experimental studies are rationalized with theoretical DFT and TD‐DFT studies successfully.

Effects of Enzyme-Ligand Interactions on the Photoisomerization of a Light-Regulated Chemotherapeutic Drug.

Molecular photoswitches permit using light to control protein activity with high spatiotemporal resolutions, thereby alleviating the side effects of conventional chemotherapy. However, due to the challenges in probing ultrafast photoisomerization reactions in biological environments, it remains elusive how the protein influences the photochemistry of the photoswitches, which hampers the rational design of light-regulated therapeutics. To overcome this challenge, we employed first-principles nonadiabatic dynamics simulations to characterize the photodynamics of the phototrexate (PTX), a recently developed photoswitchable anticancer chemotherapeutic that reversibly inhibits its target enzyme dihydrofolate reductase (DHFR). Our simulations show that the protein environment impedes the trans to cis photoisomerization of the PTX. The confinement in the ligand-binding cavity slows down the isomerization kinetics and quantum yield of the photoswitch by reshaping its conical intersection, increasing its excited-state free-energy barrier and quenching its local density fluctuations. Also, the protein environment results in a suboptimal binding mode of the photoproduct that needs to undergo large structural rearrangement to effectively inhibit the enzyme. Therefore, we predict that the PTX's trans → cis photoisomerization in solution precedes its binding with the protein, despite the favorable binding energy of the trans isomer. Our findings highlight the importance of the protein environment on the photochemical reactions of the molecular photoswitches. As such, our work represents an important step toward the rational design of light-regulated drugs in photopharmacology.

Photomodulation of Two-Dimensional Self-Assembly of Azobenzene–Hexa-peri-hexabenzocoronene–Azobenzene Triads

Achieving exquisite control over self-assembly of functional polycyclic aromatic hydrocarbons (PAH) and nanographene (NG) is essential for their exploitation as active elements in (nano)technological applications. In the framework of our effort to leverage their functional complexity, we designed and synthesized two hexa-peri-hexabenzocoronene (HBC) triads, pAHA and oAHA, decorated with two light-responsive azobenzene moieties at the pseudo-para and ortho positions, respectively. Their photoisomerization in solution is demonstrated by UV–vis absorption. 1H NMR measurements of oAHA suggested 23% of Z-form can be obtained at a photostationary state with UV irradiation (366 nm). Scanning tunneling microscopy imaging revealed that the self-assembly of pAHA and oAHA at the solid–liquid interface between highly oriented pyrolytic graphite (HOPG) and their solution in 1,2,4-trichlorobenzene can be modulated upon light irradiation. This is in contrast to our previous work using HBC bearing a single azobenzene moiety, which did not show such photomodulation of the self-assembled structure. Upon E-Z isomerization both pAHA and oAHA displayed an increased packing density on the surface of graphite. Moreover, pAHA revealed a change of self-assembled pattern from an oblique unit cell to a dimer row rectangular crystal lattice whereas the assembly of oAHA retained a dimer row structure before and after light irradiation, yet with a modification of the inter-row molecular orientation. Molecular mechanics/molecular dynamics simulations validated the self-assembly patterns of pAHA and oAHA, comprising azobenzenes in their Z-forms. These results pave the way toward use of suitably functionalized large PAHs, as well as NGs, to develop photoswitchable devices.

Impact of the Alkyne Substitution Pattern and Metalation on the Photoisomerization of Azobenzene-Based Platinum(II) Diynes and Polyynes.

Trimethylsilyl-protected dialkynes incorporating azobenzene linker groups, Me3SiC≡CRC≡CSiMe3 (R = azobenzene-3,3'-diyl, azobenzene-4,4'-diyl, 2,5-dioctylazobenzene-4,4'-diyl), and the corresponding terminal dialkynes, HC≡CRC≡CH, have been synthesized and characterized. The CuI-catalyzed dehydrohalogenation reaction between trans-[Ph(Et3P)2PtCl] and the deprotected dialkynes in a 2:1 ratio in iPr2NH/CH2Cl2 gives the platinum(II) diynes trans-[Ph(Et3P)2PtC≡CRC≡CPt(PEt3)2Ph], while the dehydrohalogenation polycondensation reaction between trans-[(nBu3P)2PtCl2] and the dialkynes in a 1:1 molar ratio under similar reaction conditions affords the platinum(II) polyynes, [-Pt(PnBu3)2-C≡CRC≡C-]n. The materials have been characterized spectroscopically, with the diynes also studied using single-crystal X-ray diffraction. The platinum(II) diynes and polyynes are all soluble in common organic solvents. Optical-absorption measurements show that the compounds incorporating the para-alkynylazobenzene spacers have a higher degree of electronic delocalisation than their meta-alkynylazobenzene counterparts. Reversible photoisomerization in solution was observed spectroscopically for the alkynyl-functionalized azobenzene ligands and, to a lesser extent, for the platinum(II) complexes. Complementary quantum-chemical modeling was also used to analyze the optical properties and isomerization energetics.

Photoisomerization for a model protonated Schiff base in solution: Sloped/peaked conical intersection perspective

The topographical character of conical intersections (CIs)--either sloped or peaked--has played a fundamental and important role in the discussion of the efficiency of CIs as photochemical "funnels." Here this perspective is employed in connection with a recent study of a model protonated Schiff base (PSB) cis to trans photoisomerization in solution [Malhado et al., J. Phys. Chem. A 115, 3720 (2011)]. In that study, the calculated reduced photochemical quantum yield for the successful production of trans product versus cis reactant in acetonitrile solvent compared to water was interpreted in terms of a dynamical solvent effect related to the dominance, for the acetonitrile case, of S(1) to S(0) nonadiabatic transitions prior to the reaching the seam of CIs. The solvent influence on the quantum yield is here re-examined in the sloped/peaked CI topographical perspective via conversion of the model's two PSB internal coordinates and a nonequilibrium solvent coordinate into an effective branching space description, which is then used to re-analyze the generalized Langevin equation/surface hopping results. The present study supports the original interpretation and enriches it in terms of topographical detail.

Structure, Z/E photoisomerization and an effect of (phenylene-)vinylene conjugation extension on absorption and fluorescence of methylidene-pyrrolinones

Six methylidene-pyrrolinones with a conjugation chain extended by vinylene and phenylene-vinylene groups were studied; five of them were synthesized for the first time. Their structures were estimated theoretically by density functional theory (DFT) and proved by X-ray diffraction techniques in two cases. Structural changes upon Z/E photoisomerization in solution were studied by a combination of 1H NMR spectroscopy and DFT GIAO method. Both types of conjugation extension caused a bathochromic shift in absorption. Vinylene extension produced the compounds fluorescent in solution. N-methylated derivatives have shown solid-state fluorescence.

Conical intersection structure and dynamics for a model protonated schiff base photoisomerization in solution

AbstractHere, we present some highlights of our recent study [Malhado et al. J. Phys. Chem. A 115, 3729 (2011)] of the photoisomerization involving a conical intersection for a model protonated Schiff base (PSB) in modeled water and acetonitrile solvents, in which the inclusion of energy‐ and momentum‐transfer effects is described via a generalized Langevin equation (GLE) frictional approach and surface‐hopping dynamics. Short‐time GLE frictional effects on the model's three coordinates, the intramolecular bond length alternation, and torsional PSB coordinates and a solvent coordinate, have a number of important features. Among these are the general feature of accelerating nonadiabatic transitions to the ground electronic state and a predicted preferential higher proportion of ground‐state trans isomer, that is, a successful cis to trans photoreaction. This preference is solvent‐dependent and is enhanced in water solvent with its higher friction intensity and short time scales. © 2012 Wiley Periodicals, Inc.

9-Anthracenyl-substitued pyridyl enones revisited: photoisomerism in ligands and silver(i) complexes

In solution, (E) to (Z)-isomerism is facile both in 3-(9-anthracenyl)-1-(pyridin-4-yl)propenone, 2, and in its silver(I) complex [Ag(2)(2)](+). The crystal structures of (E)-2, (Z)-2 and [Ag{(E)-2}(2)][SbF(6)] are presented, and the roles of edge-to-face and face-to-face π-interactions in the lattice are discussed. Solution NMR spectroscopic data suggest that the driving force for (E) to (Z) isomerization is intramolecular π-stacking of the pyridine and anthracene domains. The reversed enone 3-(9-anthracenyl)-1-(pyridin-4-yl)propen-3-one, (E)-3, and the silver(I) complex [Ag{(E)-3}(2)][SbF(6)] have been prepared and characterized, including a single crystal X-ray determination of the latter. Surprisingly, no π-stacking between anthracene or pyridine domains is observed in the solid state, and the crystal packing is dominated by Ag···F, CH(anthracene)···π-pyridine and CH···F interactions. In contrast to (E)-2 and [Ag{(E)-2}(2)](+), neither (E)-3 nor [Ag{(E)-3}(2)](+) undergoes photoisomerization in solution.

ChemInform Abstract: Photoisomerization of 3-Benzoylacrylates on a Silica Gel Surface: Effect of Reaction Environment on Selectivity.

On irradiation on a dry silica gel surface, 2-(N, N-dialkylamino)ethyl 3-benzoylacrylates undergo selective E-Z isomerization. Competing processes, such as remote-hydrogen abstraction via a charge-transfer state, with the photoisomerization in solution are completely suppressed on the surface. The yield of the Z - from the E- isomer increases with increase in the coverage of the E-isomer on the surface, reaching a limiting value. Thermal isomerization of the Z- to the E- isomer occurs easily on the silica gels surface when the alkyl group on the nitrogen atom is small. Benzoylacrylates without any amino group also undergo selective photoisomerization.

Synthesis of 4-(2-arylvinyl)-8-hydroxyquinolines via anhydrous Heck coupling reaction and the PL properties of their Al complexes

Anhydrous Heck coupling between 2 and eight different arylvinyl compounds using 3 mol % of PdCl 2/2PPh 3 catalyst system occurs selectively at position-4 of 2 affording 4-(2-arylvinyl)-8-tosyloxyquinolines 3 in good to high yields. The tosyloxy protecting group showed high stability under anhydrous coupling conditions. Deprotection of the resulting 4-arylvinyl-8-tosyloxyquinolines produced 4-(2-arylvinyl)-8-hydroxyquinolines 4 in high yields. The aluminium complexes of the resulting 8-hydroxyquinolines have been synthesized and their photoluminescence (PL) properties have been studied. One of the complexes with a 2,4,6-trimethoxystyryl substituent at position-4 of the quinolate shows higher relative PL quantum yield than the other complexes due to the minimization of the cis–trans photoisomerization in solution.

Femtosecond TRIR Studies of ClNO Photochemistry in Solution: Evidence for Photoisomerization and Geminate Recombination

The photochemistry of nitrosyl chloride (ClNO) in the solution phase is investigated using Fourier transform infrared (FTIR) and ultrafast time-resolved infrared (TRIR) spectroscopies. The NO-stretch fundamental transition for ClNO dissolved in cyclohexane, carbon tetrachloride, chloroform, dichloromethane, and acetonitrile is measured, with the frequency and line width of this transition demonstrating a strong dependence on solvent polarity. Following the photolysis of ClNO dissolved in acetonitrile at 266 nm, the subsequent optical-density evolution across the entire width of the NO-stretch fundamental is measured. Analysis of the optical-density evolution demonstrates that geminate recombination of the primary photofragments resulting in the reformation of ground state ClNO occurs with a quantum yield of 0.54 +/- 0.06. In addition, an increase in optical density is observed at 1860 cm(-1) that is assigned to the NO-stretch fundamental transition of the photoisomer, ClON, having a formation quantum yield of 0.07 +/- 0.02. This work represents the first definitive observation of ClNO photoisomerization in solution. Finally, essentially no evidence is observed for significant vibrational excitation of the NO fragment following photodissociation, in marked contrast to the behavior observed in the gas phase. An environment-dependent dissociation scheme is proposed in which the interplay between solvent polarity and the location of the ground state potential-energy-surface minimum along the Cl-N coordinate provides for the optical preparation of different excited states thereby affecting the extent of NO vibrational excitation following photolysis.

Single Component Self‐Assembled Monolayers of Aromatic Azo‐Biphenyl: Influence of the Packing Tightness on the SAM Structure and Light‐Induced Molecular Movements

AbstractAiming at modulating the packing density within functional self‐assembled monolayers (SAMs), two azo‐biphenyl derivatives AZO1 and AZO2 comprising a terminal sulfur anchor group have been designed and synthesized. While AZO1 allows for a coplanar arrangement of both biphenyl subunits, additional steric repulsion due to two methyl side groups attached to the footing biphenyl of AZO2 results in an increased intermolecular distance within the SAM, providing additional free volume. SAMs of both derivatives on gold and platinum substrates have been formed and thoroughly investigated by photoelectron (XPS) and near‐edge absorption fine structure (NEXAFS) spectroscopy as well as cyclic voltammetry and scanning tunneling microscopy. These measurements confirmed the formation of tightly packed SAMs for AZO1, while AZO2 formed SAMs consisting of less organized and more loosely packed molecules. Optical investigations of both azo derivatives in solution as well as their SAMs displayed efficient photoisomerization in solution and in SAMs. Comparable maximal cis/trans ratios of ca. 0.9 have been observed in all cases upon irradiation at λ = 370 and 360 nm for AZO1 and AZO2, respectively. The thermally induced cis → trans back reaction on AZO1 was found to be slower by a factor of 3 in SAMs as compared to solution, while AZO2 displayed comparable rates of the back reaction in both environments. This behavior can be explained by the different nature of molecular isomerization in the two SAM systems: whereas the isomerization in AZO1 SAMs takes place in a highly coordinated, collective way and involves many adjacent molecules, AZO2 species behave rather individually even packed in SAMs, such that their isomerization process is similar in SAMs and in solutions.

Azobenzene–Helical Peptide Conjugate: Electronic Structure and Photoisomerization in Solution and on Surface

Abstract A novel helical peptide derivative with an intervening azobenzene unit was synthesized, and the effects of the peptide segments on the electronic properties of the azobenzene unit, immobilization on solid surfaces, and photoisomerization in solution and on surface were studied.

Electrochemical switch based on the photoisomerization of a diarylethene derivative

Abstract 3,4-Bis[5-(2′-thiophene)-2-methylthien-3-yl]-2,5-dihydrothiophene 1a was synthesized and underwent excellent ring-opening and ring-closing photoisomerization in solution with UV/Vis light irradiation. Both ring-open isomer and ring-closed isomer of diarylethene showed electrochemical behavior, and the oxidation/reduction potential was reversible based on the ring-opening and ring-closing photoisomerization of diarylethene with UV/Vis light irradiation. Besides, a combination of photochromism and electrochromism of 1a was also observed.

Structure Control for Fine Tuning Fluorescence Emission from Side-Chain Azobenzene Polymers

New fluorescent azobenzene dyes and side-chain polymers have been synthesized and characterized and their photophysical properties studied. A series of azobenzene dyes having different fluorophores such as phenol (S1), phenylphenol (S2) and naphthol (S3) incorporated in them were synthesized. S2 had unusually high fluorescence with a quantum yield of phi f = 0.2 recorded in dichloromethane (DCM), whereas S1 and S3 were found to be weakly fluorescent. The azobenzene dyes were converted into methacrylate monomers having short ethyleneoxy spacers and then free radically polymerized. Phenylphenol-based azobenzene polymer (P2) continued to show fluorescence, whereas fluorescence was completely quenched in the case of phenol (P1)- and naphthol (P3)-based polymers. Phenylphenol, though twisted in the ground state is known to have a more planar geometry in the excited state--a factor that enables it to retain its fluorescence behavior even when it is incorporated as part of an azobenzene unit. In contrast, naphthol, which is a better fluorophore compared to phenylphenol, loses much of its emissive behavior upon coupling to the azobenzene unit. The extent of trans to cis photoisomerization in solution was very low (approximately 17%) for P2 after 30 min of continuous irradiation using 365 nm light, in contrast to approximately 40% for P1 under identical conditions. This is attributed to the steric repulsion brought about by the bulky phenylphenol units that restrict rotation. A 2-fold enhancement in fluorescence emission was observed for P2 upon irradiation by UV light at 360 nm, which relaxed to the original intensity in about 7 day's time. The higher emission of the cis azobenzenes is generally attributed to an inhibition of photoinduced electron transfer (PET) mechanism. The emission of P2 showed a concentration dependence which increased initially and then decreased in intensity with the formation of a new red-shifted peak at higher concentration due to aggregation. Irradiation of the fluorescence quenched highly concentrated (1 x 10(-3) M) sample of P2 showed an enhancement in emission from aggregates at 532 nm.

Photochemical Behavior of Some p-Styrylstilbenes and Related Compounds: Spectral Properties and Photoisomerization in Solution and in Solid State

The spectral properties and Z,E-photoisomerizations of three 4-styrylstilbenes, a 4,4′-bis(β-methylstyryl)benzene and a 4,4′-distyrylstilbene were investigated in solution and in the solid state. Some notable features of the absorption and fluorescence spectra due to the structures and the phases (solution or solid) were observed, especially the Stokes shifts. Interesting photo-chemical behaviors in solution and in the solid state were also found. While the (E,Z)-4-styrylstilbenes undergo a one-way photoisomerization to their E,E-isomers with almost equal rates in solution, their quantum yields in the solid state decrease with increasing substituent size at the terminal aromatic rings. On the other hand, the 4,4′-bis(β-methylstyryl)benzene undergoes a mutual photoisomerization in solution, but its E,Z-isomer isomerizes in a one-way manner to the E,E-isomer in the solid state. Additionally, the (Z,E,Z)-4,4′-styrylstilbene was found to undergo a one-way photoisomerization to the E,E,E-isomer via the E,E,Z-isomer in solution and the crystalline E,E,Z-isomer to the E,E,E-isomer in the solid state.