Photoisomerization in different classes of azobenzene

Azobenzene undergoes trans→cis isomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis→trans isomerization can be driven by light or occurs thermally in the dark. Azobenzene's photochromatic properties make it an ideal component of numerous molecular devices and functional materials. Despite the abundance of application-driven research, azobenzene photochemistry and the isomerization mechanism remain topics of investigation. Additional substituents on the azobenzene ring system change the spectroscopic properties and isomerization mechanism. This critical review details the studies completed to date on the 3 main classes of azobenzene derivatives. Understanding the differences in photochemistry, which originate from substitution, is imperative in exploiting azobenzene in the desired applications.     

Photoisomerization of azobenzene derivatives in nanostructured silica

A series of derivatized azobenzene molecules are synthesized such that one of the phenyl groups can be chemically bonded to mesostructured silica and the other, derivatized with dendrons, is free to undergo large-amplitude light-driven motion. The silica frameworks on which the motion takes place are either 150 nm thick films containing ordered hexagonal arrays of tubes (inner diameter about 2 nm) containing the bonded azobenzenes, or particles (about 500 nm in diameter) containing the same ordered arrays of functionalized tubes. The photoisomerization yields and the rate constants for the thermal cis to trans back-reaction of the azobenzenes in the tubes are measured and compared to those of the molecules in solution. The rate constants decrease with increasing size of the dendrons. Fluorescence spectra of the cis and trans isomers in the pores show that the photoisomerization in the nanostructured materials is selectively driven by specific wavelengths of light and is reversible.     

Photoisomerization of azobenzene from first-principles constrained density-functional calculations

Despite considerable work in the field, the precise mechanism for the photoisomerization of azobenzene, C(12)H(10)N(2), is still an open issue. Early theoretical studies of the problem indicated that isomerization occurs through an in-plane inversion path, and this has been used to explain recent time-resolved UV-visible spectroscopy measurements. On the other hand, a number of recent theoretical studies have concluded that a torsion of the N-N bond ("rotation path") is probably the most favorable mechanism for photoisomerization involving the first excited state. We have performed first-principles calculations using constrained density-functional theory (DFT) and time-dependent DFT in the local-density approximation, with results that also favor the rotation path mechanism. Our results are compared with other analyses, primarily based on configuration interaction.     

Photoisomerization of azobenzene derivatives confined in gold nanoparticle aggregates

Photoisomerization is an important reaction that confers photoresponsive functionality on nanoparticles. Although photoisomerization of molecules forming self-assembled monolayers on two-dimensional surfaces or three-dimensional clusters has been studied, a detailed picture of interactions of molecules undergoing isomerization with nanoparticles is not available. In this paper, we report on the photoisomerization of azobenzene derivatives spatially confined in gold nanoparticle (AuNP) aggregates. AuNP aggregates allow us to simultaneously probe the structural changes of molecules via surface-enhanced Raman spectroscopy (SERS) and the accompanying changes in interparticle interactions via surface plasmon couplings. AuNP aggregates are formed by the adsorption of synthesized azobenzene-derivatized sulfides (Az) onto the surfaces of AuNPs. The photoisomerization of the adsorbed Az from trans to cis by excitation at 365 nm causes the AuNPs to move close to each other in the aggregates, leading to a redshift of the surface plasmon coupling band in the UV-vis spectra and a concomitant rise in SERS intensity. SERS spectra reveal that the vibrational modes containing the N=N stretching character redshift upon irradiation, suggesting that the N=N bond is significantly weakened when Az is in the cis form in the AuNP aggregates. The weakening of the N=N bond is attributed to the interaction of the N=N bond, which is more exposed to the outside in the cis conformation, with the nearby AuNPs that have come closer by the isomerization of adsorbed Az. We find that backisomerization from cis to trans occurs much faster in the AuNP aggregates (k = 1.9 × 10(-2) min(-1)) than in solution (k = 1.3 × 10(-3) min(-1)) because of the reduced N=N bond order of cis-Az in the aggregates.     

Multiresponsive self-assembled liquid crystals with azobenzene groups

An optical and electric field-responsive self-assembled complex containing nitril azobenzene groups and 1,3,5-triazine-2,4-diamine was obtained and characterized. Both the azobenzene precursor and the complex form a liquid-crystalline phase in a certain temperature range. The transition temperature from crystalline phase to liquid-crystalline mesophase was obviously decreased in the complex by the self-assembling. The self-assembled liquid crystals revealed good response to both stimuli of light irradiation and electric field, and the induced molecular orientation could be held even after the removal of the stimuli. The structural and mechanical investigation proved that the formation of hydrogen bonds and assembly-induced molecular dipolar change contributed to the multiresponding action. This kind of self-assembled complex thus has potential applications in imaging and data storage.     

H-bonding-driven gel formation of a phenylacetylene macrocycle

An amide-containing phenylacetylene macrocycle (PAM) has been synthesized and its gelation properties were studied in different solvents. Surprisingly, this macrocycle forms organogels at low concentration in many polar and apolar solvents. XRD and FTIR analysis suggest that this macrocycle forms stable supramolecular assemblies owing to H-bonding. Scanning electron microscopy analyses show the formation of bundles of nanofibrils, demonstrating the long-range organization of this material.     

Versatile formation of [2]catenane and [2]pseudorotaxane structures; threading and noncovalent stoppering by a self-assembled macrocycle

A self-assembled dimeric macrocycle between 4,4'-bis(4-pyridylmethoxy)biphenyl (L) and (en)Pd(NO(3))(2) was constructed, and its interactions with cyclodextrins of different cavity size resulted in the formation of [2]catenane and [2]pseudorotaxane systems, respectively. The structures were identified by 1D and 2D NMR spectroscopy and cold spray ionization mass (CSI-MS) spectrometry.     

Photoregulation of DNA triplex formation by azobenzene

Formation and dissociation of DNA triplex are reversibly photoregulated by cis <--> trans isomerization of the azobenzene tethered to the third strand. When the azobenzene takes the trans from, a stable triplex is formed. Upon the isomerization of trans-azobenzene to its cis form by UV light irradiation (300 < lambda < 400 nm), however, the modified oligonucleotide is removed from the target duplex. The triplex is re-formed on photoinduced cis --> trans isomerization (lambda > 400 nm). The photoregulating activity significantly depends on the position of azobenzene in the third strand, as well as on the geometric position (meta or para) of its amido substituent. For m-amidoazobenzene, the photoregulation is the most effective when it is tethered to the 5'-end of the third strand. However, p-amidoazobenzene should be introduced into the middle of the strand for effective regulation. In the optimal cases, the change of T(m) of the triplex, caused by the cis <--> trans isomerization of azobenzene, is greater than 30 degrees C. UV-visible and CD spectroscopy, as well as computer modeling studies, clearly demonstrate that the trans-azobenzene intercalates between the base pairs in the target duplex and thus stabilizes the triplex by stacking interactions. On the other hand, nonplanar cis-azobenzene destabilizes the triplex due to its steric hindrance against the adjacent base pairs.     

偶氮苯衍生物自组装单分子膜中的分子取响

利用反射红外光谱研究了金表面一系列具有不同碳链长度的偶氮苯巯基衍生物的自组装单分子膜.通过对比各向同性样品的透射谱和单分子膜的反射谱中各个吸收峰强度,定量地研究了分子中各部分的取向与分子结构的关系.我们分别提出了烷基链和偶氮基团取向计算的方法,利用该方法成功地求得了分子中各部分在膜的倾角.结果显示,当分子中烷基链长度增大时,碳链和偶氮苯基团相对于法线的倾斜逐渐加剧.这种倾角的变化归因于分子中碳链间范德华引力增大时,引起分子逐渐倾斜以达到最佳的范德华接触.同时研究发现,烷基链和偶氮基团受碳长度变化的影响并不相同.当分子中亚甲基数目增多时,烷基链的倾角迅速增大而偶氮苯倾角的增大则相对缓慢,这反映了它们在空间需求和本身刚性上的不同。     

Photoisomerization of an individual azobenzene molecule in water: an on-off switch triggered by light at a fixed wavelength

The alpha-hemolysin (alphaHL) pore was used as a nanoreactor for the direct observation of the reversible photoisomerization of individual tethered azobenzene molecules in an aqueous environment. alphaHL pores, PAZO, were used that had been derivatized within the lumen at a single cysteine residue with 4-((4-(2-chloroethanoamido)phenyl)diazenyl)benzenesulfonate. Trans-cis isomerizations were monitored at the single-molecule level by observing the modulation of the current passing through PAZO by electrical recording in planar bilayers. When PAZO was irradiated at 330 nm, continuous interconversion between the trans and cis states was observed. Either the trans or the cis state was maintained in the dark, depending upon which was present when the light source was shuttered. The cis state of PAZO was surprisingly stable in the dark, and no cis --> trans transitions were seen over a total observation period of more than 8 h. Therefore, based on our findings, it might be possible to make fast digital nanoscale switches operated by light of a fixed wavelength.     

Reversible wettability of optothermal responsively perfluoroalkyl azobenzene self-assembled monolayers

In this study, two perfluoroalkyl azobenzene trichlorosilanes were synthesized and then characterized by Fourier transform infrared spectroscopy (FT-IR), ¹H NMR, and ¹⁹F NMR. Subsequently, these fluorine containing trichlorosilanes were applied to form self-assembled monolayers (SAMs) on silicon substrates by the method of chemical deposition in liquid phase. The optothermal responsively isomerization of the azobenzene was achieved via UV irradiation and heat treatment. The surface structures of the SAMs were investigated by X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDS), and atomic force microscopy (AFM). The results showed that the thermal migration of the terminal fluoroalkyl groups promoted the isomerization of the azo-groups. Moreover, the reversible contact angles of the SAMs demonstrated a good reversibility of surface wettability, which was consistent with the optothermal responsive isomerization of the azo-groups.     

Photoisomerization dynamics of azobenzene in solution with S1 excitation: a femtosecond fluorescence anisotropy study

Measurements of anisotropy of femtosecond fluorescence after direct excitation of the S1(n,pi*) state of azobenzene in hexane and ethylene glycol solutions have been carried out to address the controversy about inversion and rotation in the mechanism of photoisomerization. The observed anisotropies in hexane decay to a nonzero asymptotic level with a relaxation period the same as that for slow decay of the corresponding biexponential transient; this effect is attributed to involvement of the out-of-plane CNNC-torsional motion on approach to a twisted conical intersection along the "rotation channel" that depolarizes the original in-plane transition moment. In contrast, when the rotational channel becomes substantially hindered in ethylene glycol, the anisotropies show no discernible decay feature, but the corresponding transients show prominent decays attributed to involvement of in-plane symmetric motions; the latter approach a planar-sloped conical intersection along a "concerted inversion channel" for efficient internal conversion through vibronic coupling. The proposed mechanism is consistent with theoretical calculations and rationalizes both results on quantum yields and ultrafast observations.     

Structure and properties of a 1D supramolecule self-assembled by a copper(II) macrocycle with bicarbonate ligands

A new complex of composition [Cu(L3)(HCO^– ₃)₂] _n (1) (L3 = 3, 14-dimethyl-2, 6, 13, 17-tetraazatricyclo [14.4.0^1.18.0^7.12]docosane) has been synthesized and structurally characterized by a combination of analytical, spectroscopic, and crystallographic methods. The copper(II) coordination geometry in (1) consists of an axially elongated octahedron with four nitrogen atoms from the macrocycle and two oxygen atoms from the bicarbonate ligands. The [Cu(L3)(HCO₃ ^–)₂] unit extends its structure to form a novel 1D supramolecule (1) through intermolecular hydrogen bonding interactions between the bicarbonate ligands. Thus, the [Cu(L3)( HCO₃ ^–)₂] motif works as a metal complex synthon for the formation of the supramolecule (1).     

An evanescent-field-driven self-assembled molecular photoswitch for macrocycle coordination and release

Two self-assembled monolayer (SAM) films containing the photoswitchable 4-pyridylazophenoxy chromophore have been deposited onto a gold-coated glass substrate. One film contains the chromophore as a single component, 1 SAM, and the other is doped with a nonphotoactive component as a 1:1 mixture, 2 SAM. The reversible photoswitching performances of 1 SAM and 2 SAM via the evanescence field using light of appropriate wavelengths have been investigated by UV spectroscopic and electrochemical monitoring. In principle, the trans-form SAMs present a coordinating surface, the "on" state, that can be switched "off" in the cis form. This has been illustrated by immersing both the as-deposited (trans form) SAMs and the photoswitched (predominantly cis form) SAMs into solutions of cobalt and zinc tetraphenylporphyrin (CoTPP and ZnTPP, respectively) and an octaoctyl-substituted cobalt phthalocyanine. In a further phase of this study, the remote control of binding events at the surface of the SAMs has been demonstrated through evanescent-field-driven photoswitching of trans-form SAMs coordinated at the surfaces with examples of these metallomacrocycles. This photoswitching was undertaken with the constructs immersed in neat toluene, and the macrocycles were released from the surface into the solvent. The release was measured by spectroscopic monitoring of the material remaining on the constructs. The study was extended to develop an in situ release/coordination cycle. Thus, irradiation of a construct of ZnTPP bound to the surface of trans-form 2 SAM using waveguided light at 365 nm releases the macrocycle into a toluene solution of ZnTPP. Further irradiation of the SAM, now in its cis form, with waveguided 439 nm light regenerates the trans form, which recoordinates ZnTPP from the solution. The results demonstrate the potential for using waveguided light to control molecular events within and at the surfaces of SAM constructs.     

Intense fluorescence from light-driven self-assembled aggregates of nonionic azobenzene derivative

We describe here the highly fluorescent self-assembled spherical aggregates of an azobenzene molecule without a specific ionic component in organic solution under UV light illumination. The first stage of trans-to-cis photoisomerization by UV light at 365 nm was followed by a significant enhancement, up to about 1000 times, of the emission from an azobenzene molecule (CN2Azo) with a long alkyl chain, which is due to the spontaneous formation of spherical organic aggregates. Fluorescence emission was further enhanced in the dark, and the quantum yield increased to about 0.3. We also report the significant size and structural changes of the aggregates, from nanometer-scale micelle-like aggregates to micrometer-scale vesicular aggregates, obtained only from the variation in the concentration of an azobenzene derivative. The light-driven azobenzene aggregates show the size and structure dependences of emission wavelength from violet-blue to green-yellow.     

Surface-enhanced Raman scattering (SERS) from different azobenzene self-assembled monolayers and sandwiches

Self-assembled monolayers (SAMs) of functionalized azobenzene thiols (RAzoCnSH, n=3-6 for R=H, abbreviated as AzoCnSH; and n=4 for R=CH(3)CONH, abbreviated as aaAzoC4SH) on different substrates RAzoCnSz.sbnd;z.sfnc;S (S represents substrates of vacuum-deposited gold (Au), silver foil (Ag), HNO(3) etched silver foil (EAg), and silver mirror (mAg)) have been studied by SERS in the near-infrared region. SERS of the SAMs on EAg and/or mAg exhibit SERS effects that vary with etching time and/or deposition time. The most appropriate time is 5 s for etching in 1:1 HNO(3) and 40 s for deposition in 0.1 M Ag(NH(3))(2)NO(3). Further, a layer of Ag mirror was conveniently deposited on the top of the SAMs on different substrates, yielding a more efficient SERS-active system possessing a "sandwiched" structure of mAgz.sfnc;RAzoCnS-z.sfnc;S. An appropriate surface roughness is required for the strongest SERS effect. Scanning electron microscopy (SEM) indicates that there exist a large number of projects around 100 nm on the surface showing the strongest SERS effect. When the surface roughness is decreased or increased, the SERS effect decreases sharply. The relationship between the SERS effect and the structural nature was investigated and showed that the enhancement factor decays exponentially with increasing in distances of the azobenzene group from the underlying substrate or the overlying silver mirror. This result reveals that the SERS effect may be the result of the electromagnetic coupling effect between two metal layers.     

Photoswitching in azobenzene self-assembled monolayers capped on zinc oxide: nanodots vs nanorods

We report the synthesis and spectroscopic characterization of nanohybrid structures consisting of an azobenzene compound grafted on the surface of zinc oxide nanoparticles. Characteristic bathochromic shifts indicate that the azobenzene photochromic molecules self-assemble onto the surface of the nanocrystals. The extent of packing is dependent on the shape of the nanoparticle. ZnO nanorods, with flat facets, enable a tighter organization of the molecules in the self-assembled monolayer than in the case of nanodots that display a more curvated shape. Consistently, the efficiency of photochromic switching of the self-assembled monolayer on ZnO nanoparticles is also shown to be strongly affected by nanoparticle shape.