Molecular switches can toggle between two or more stable states encoding different physical features. They allow complex systems to respond to changes in their environment defined by light intensity, pH, temperature or voltage. As such, molecular switches play a key role in biology and information technology and have become important components of advanced materials. Chemists have developed a large number of synthetic switches, complementing the many types found in nature. Given their prominence in biology, it is not surprisingly that photoswitches, which are actuated by ultraviolet, visible or infrared light, have been an especially productive field of study.
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Graphical Abstract
Scheme 1: The acid-activated switching process of PPH-1.
Scheme 2: The hydrazone-based molecular systems that were analysed in this paper, each having different rotor...
Graphical Abstract
Figure 1: para-Substituted bisazobiphenyls 1 investigated by Hecht (R1, R2 = H, Me, R3 = t-Bu) and by Woolley...
Figure 2: Synthetic strategy for the assembly of meta-substituted oligo-azobiphenyls 2.
Scheme 1: Synthesis of 2-nitro-4-tert-butyl-6-bromobenzoic acid (6).
Scheme 2: Preparation of nitroso derivative 10.
Scheme 3: Assembly of oligomer 2 by Suzuki cross-coupling and site-selective Mills reaction.
Figure 3: Isomerization studies of compound 13 (a), 15 (b), 16 (c) and 2 (d) (Irradiation at 356 nm in CHCl3)....
Figure 4: Comparison of the absorption as well as the photostationary state of compounds 13, 15, 16, 2.
Figure 5: Four different isomers of 2.
Graphical Abstract
Scheme 1: Photoisomerization of the photoswitchable click amino acid 2-amino-3-(4-((3-vinylphenyl)diazenyl)ph...
Figure 1: Histogram showing the distribution of end-to-end distances of the trans and the cis form between th...
Scheme 2: Thiol–ene click reaction of PSCaa with cysteine within the helical model peptides 1 (i,i+4) and 2 (...
Figure 2: ESI–MS spectra of fractions of the crude reaction solution of the thiol–ene click reaction of pepti...
Graphical Abstract
Figure 1: Structures of azobenzene thioesters, Nα-ligation auxiliaries and peptides for the application in li...
Scheme 1: Structural differences between the trans- and the cis-state of azopeptides with a SKV PDZ binding m...
Figure 2: Structure of the glycine-linked auxiliary conjugates 7 and 8.
Scheme 2: Solid-phase synthesis of the ligation-mediating peptides 3–5.
Scheme 3: Reduction of the diazene unit of 4,4'-AMPB thioesters and peptides during aliphatic thiol-based Nα-...
Scheme 4: Synthesis of the azopeptides 16/17 by final TFA cleavage of the Boc-protecting groups, and of the a...
Graphical Abstract
Figure 1: Schematic structure of photosystem 1 on indium tin oxide (ITO, grey) with antiparallel gradients in...
Scheme 1: Synthesis of initiator 2.
Scheme 2: Synthesis of propagators 3 and 4.
Scheme 3: Synthesis of stack exchangers 5 and 6. Compounds 5, 6, 45 and 47 are mixtures of 2,6- and 3,7-regio...
Scheme 4: Synthesis of photosystem 1, self-organizing surface-initiated polymerization (SOSIP). R1 = SH (50) ...
Scheme 5: Synthesis of photosystem 1, stack exchange. R1 = SH or oxidized derivative, R1 = CH2CHCH2. 5 and 6 ...
Scheme 6: Schematic overview over SOSIP and stack exchange.
Graphical Abstract
Scheme 1: Spiropyran as DNA base surrogate 1, DNA base modifications 2 and 3, and diarylethene-modified nucle...
Scheme 2: Synthesis of diarylethene-modified 2’-deoxyuridines 4 [30], 5 and 6.
Figure 1: Photoswitching properties of nucleosides 4–6 (each 20 mM in MeCN, rt). Top: Irradiation of 4 at 242...
Scheme 3: Synthesis of DNA building block 17 [30] and sequences of diarylethene-modified DNA1–DNA4.
Figure 2: Irradiation of dsDNA2 at 310 nm (A, left) and plot of kinetic trace of absorption changes at 450 nm...
Figure 3: UV–vis absorption spectra of ssDNA1–ssDNA4 (2.5 μM in 50 mM Na–Pi buffer, pH 7, 250 mM NaCl, rt).
Graphical Abstract
Scheme 1: Dihydroazulene (DHA)/vinylheptafulvene (VHF) photo-/thermoswitch.
Figure 1: Numbering of azulene.
Scheme 2: Bromination–elimination protocol for functionalization of DHA [3,5-8]. HMDS = hexamethyldisilazide.
Scheme 3: Bromination of VHF [11]. NBS = N-bromosuccinimide.
Scheme 4: Radical and ionic brominations of VHF.
Figure 2: 1H,1H COSY NMR spectrum of DHA 8 (CDCl3, 500 MHz). For assignments of DHA signals, see numbering in ...
Figure 3: Molecular structures (with displacements ellipsoids at 50% probability for non-H atoms) of (a) DHA 8...
Figure 4: 1H NMR spectra (C6D6, 300 MHz) of (a) DHA 1; (b) VHF 2; (c–e) VHF 2 after treatment with 2 molar eq...
Figure 5: Compound 9 was selectively brominated to furnish the product 10.
Scheme 5: Synthesis of 3,7-dibromo-DHA.
Scheme 6: Synthesis of a 3,7-dibromoazulene.
Scheme 7: Regioselective Sonogashira and Suzuki couplings. RuPhos = 2-dicyclohexylphosphino-2',6'-diisopropox...
Scheme 8: Slow conversions to azulenes in the solid state.
Figure 6: Absorption spectra of DHA 8 and VHF 7 in cyclohexane. The broken curve shows the absorption spectru...
Graphical Abstract
Figure 1: Structure of the macrocycle (R,R)-1 (top), and synthetic strategies for the production of novel ami...
Scheme 1: Reagents and conditions: (i) SOCl2, CHCl3 or (COCl)2, DMF, CH2Cl2 then amine, Et3N, CH2Cl2 or (ii) ...
Scheme 2: Structure and synthesis of the macrocycles discussed in this paper.
Figure 2: UV and CD (EtOH) spectra of macrocycles (R,R)-10, (R,R)-11 and (R,R)-12 in the range 220–400 nm.
Figure 3: Minimized molecular structures of (from top left to bottom left, clockwise): (R,R)-10, (R,R)-11, (R,...
Figure 4: Aromatic region of the 1H NMR (CDCl3, 500 MHz, 25 °C) spectra of macrocycle (R,R)-12 (2.8 mM, botto...
Graphical Abstract
Scheme 1: Principle of the switching mechanism of 2-(2-hydroxyphenyl)pyridine (2) and 2-(2-methoxyphenyl)pyri...
Figure 1: (a) Calculated energy profiles of the pyridine derivatives 1 (blue) and 2 (red) in relation to the ...
Scheme 2: Principle of the switching mechanism of 2-(2-hydroxyphenyl)-3-methylpyridine (6) and 2-(2-methoxyph...
Figure 2: (a) Calculated energy profiles of the 3-methylpyridine derivatives 5 (blue) and 6 (red) in relation...
Figure 3: UV spectral change of phenolate 5 (blue) in dichloromethane (c = 5.6 × 10−5 M) at 20 °C upon additi...
Figure 4: UV spectral change of 3-methylpyridine 7 (blue) in dichloromethane (c = 5.6 × 10–5 M) at 20 °C upon...
Figure 5: HOMO (left) and LUMO (right) of the 3-methylpyridine 7 calculated by using B3LYP/6-31G*.
Scheme 3: Synthesis of the methoxyphenylpyridine switch 10 and the hydroxypyridine switch 12; reaction condit...
Figure 6: (a) CD spectral change of pyridine switch 13 (blue) in dichloromethane (c = 5.6 × 10–5 M) at 20 °C ...
Figure 7: (a) CD spectral change of pyridine switch 10 (blue) in dichloromethane (c = 5.6 × 10–5 M) at 20 °C ...
Graphical Abstract
Figure 1: Some important families of photochromic compounds and their photochromic reactions.
Figure 2: Photochromism of azobenzene derivatives and energetic profile for the switching process.
Figure 3: General overview of the different types of azoderivatives presented in this review.
Figure 4: Changes in the electronic spectrum of a 3 cis-to-trans isomerising ethanol solution at 45 °C (Δt = ...
Figure 5: Chemical structure and thermal relaxation time in ethanol at 298 K, τ, for the slow thermally-isome...
Figure 6: Rotation and inversion mechanisms proposed for the thermal cis-to-trans isomerisation processes of ...
Figure 7: Effect of the presence of the electron-withdrawing cyano and nitro groups on the thermal relaxation...
Figure 8: Transient absorption generated by UV irradiation (λ = 355 nm) for azo-dyes 8 (right) and 9 (left) i...
Figure 9: Effect of the presence of a positively charged nitrogen as an electron-withdrawing group on the the...
Figure 10: Mechanism proposed for the thermal cis-to-trans isomerisation process for the push–pull azopyridini...
Figure 11: Comparison between the thermal relaxation time at 298 K, τ, for the azoderivative 4 (type-I) and th...
Figure 12: Solvent effect on the thermal relaxation time at 298 K, τ, for the type-II azophenols 11–13.
Figure 13: Transient generated by irradiation with UV-light (λ = 355 nm) for the type-II azophenol 12 in ethan...
Figure 14: Proposed isomerisation mechanisms for the thermal cis-to-trans isomerisation of the alkoxy-substitu...
Figure 15: Solvent effect on the thermal relaxation time at 298 K, τ, for the type-II ortho-substituted azophe...
Figure 16: Cooperative effect of the para- and ortho-hydroxyl groups in azophenol 17.
Figure 17: Effect of the poly-hydroxylation of the azobenzene core on the thermal relaxation time at 298 K, τ,...
Figure 18: Transients generated by irradiation with UV-light (λ = 355 nm) for the poly-substituted azophenol 18...
Figure 19: Effect of the introduction of electron-withdrawing groups in the position 4’ of the azophenol struc...
Figure 20: Transient absorptions generated by UV irradiation (λ = 355 nm) of azo-dyes 11 (type-II), 19 (type-I...
Figure 21: Effect of the introduction of the hydroxyl group in the position 2’ of the push–pull azo-dye on the...
Figure 22: Effect of the substitution of a benzene ring by a pyridine one on the thermal relaxation time in et...
Figure 23: Influence of the introduction of additional electron-withdrawing nitro groups in the pyridine ring ...
Figure 24: Chemical structure and thermal relaxation time in ethanol at 298 K, τ, for the type-III azoderivati...
Figure 25: Oscillation of the optical density of an ethanol solution of azo-dye 26 generated by UV-light irrad...
Graphical Abstract
Scheme 1: Photochromism of diarylethenes 1–3.
Scheme 2: Synthetic route for diarylethenes 1–3.
Figure 1: Absorption spectral changes of diarylethenes 1–3 by photoirradiation with UV–vis in hexane (2.0 × 10...
Figure 2: The color changes of diarylethene 1–3 by photoirradiation at room temperature: (A) in hexane; (B) i...
Figure 3: The photoconversion ratios of diarylethenes 1–3 in the photostationary state as analyzed by HPLC.
Figure 4: Fatigue resistance of diarylethenes 1–3 in hexane in air atmosphere at room temperature: (A) in hex...
Figure 5: Fluorescence emission spectra of diarylethenes 1–3 at room temperature: (A) in hexane solution (2.0...
Figure 6: Emission intensity changes of diarylethene 1 upon irradiation with UV light at room temperature: (A...
Figure 7: Cyclic voltammetry of diarylethenes 1–3 in acetonitrile with a scanning rate of 50 mV/s.
Graphical Abstract
Figure 1: Photoisomerization process of azobenzene.
Figure 2: Representative example of an UV spectrum of an azocompound of the azobenzene type (blue line: trans...
Figure 3: Mechanistic proposals for the isomerization of azobenzenes.
Figure 4: Representation of the photocontrol of a K+ channel in the cellular membrane based on the isomerizat...
Figure 5: (a) MAG interaction with iGluR; (b) photocontrol of the opening of the ion channel by trans–cis iso...
Figure 6: Photocontrol of the structure of the α-helix in the polypeptide azoderivative 2. Reprinted (adapted...
Figure 7: Recognition of a guanidinium ion by a cis,cis-bis-azo derivative 3.
Figure 8: Recognition of cesium ions by cis-azo derivative 4.
Figure 9: Photocontrolled formation of an inclusion complex of cyclodextrin trans-azo 5+6.
Figure 10: Pseudorotaxane-based molecular machine.
Figure 11: Molecular hinge. Reprinted (adapted) with permission from Org. Lett. 2004, 6, 2595–2598. Copyright ...
Figure 12: Molecular threader. Reprinted (adapted) with permission from Acc. Chem. Res. 2001, 34, 445–455. Cop...
Figure 13: Molecular scissors based on azobenzene 12. Reprinted (adapted) with permission from J. Am. Chem. So...
Figure 14: Molecular pedals. Reprinted by permission from Macmillan Publishers Ltd: Nature, 2006, 440, 512–515...
Figure 15: Design of nanovehicles based on azo structures. Reprinted (adapted) with permission from Org. Lett. ...
Figure 16: Light-activated mesostructured silica nanoparticles (LAMs).
Figure 17: Molecular lift.
Figure 18: Conformational considerations in mono-ortho-substituted azobenzenes.
Scheme 1: Synthesis and photoisomerization of sulfinyl azobenzenes. Reprinted (adapted) with permission from ...
Figure 19: Photoisomerization of azocompound 22 and its application as a photobase catalyst.
Figure 20: Effect of irradiation with linearly polarized light on azo-LCEs. Reprinted by permission from Macmi...
Figure 21: Chemically and photochemically triggered memory switching cycle of the [2]rotaxane 25.
Figure 22: Unidirectional photoisomerization process of the azobenzene 26.
Graphical Abstract
Scheme 1: Ring-opening polymerization (ROP) of lactide with TBD or the acyclic guanidine 1 as catalysts [16,18].
Scheme 2: Illustration of a photoswitchable guanidine catalyst for the ROP of lactide and the corresponding t...
Scheme 3: Synthesis of guanidine 2E.
Figure 1: ORTEP image of the single-crystal X-ray structure of guanidine 2E, as well as a rotated close-up of...
Figure 2: UV–vis spectra of guanidine 2 in acetonitrile, c = 3.9·10−5 mol/L. (a) E→Z isomerization with irrad...
Scheme 4: Guanidine 11 as a catalyst in the ROP of rac-lactide (catalyst/initiator/monomer ratio = 10:1:100).
Figure 3: Supposed intermediates resulting from either a cyclohexane-substituted guanidine (a) [18] or an aromati...
Graphical Abstract
Scheme 1: Previous design of diastereomeric molecules 2 and 3 with switchable chirality starting from achiral...
Scheme 2: General design of asymmetric pentadentate ligands 4 and chiroptically switchable quasi-diastereomer...
Scheme 3: Preparation of imino–carbonyl ligands 13 by desymmetrization of achiral carbonyl–carbonyl ligands 12...
Scheme 4: Preparation of complexes 14a,b and 15 by reactions of ligands 12 with glycine.
Figure 1: Crystallographic structure of complex 14a.
Scheme 5: Oxidation of enolates 16 and formation of complexes 19 and 20.
Figure 2: Crystallographic structure of complex 19.
Graphical Abstract
Scheme 1: Bisazobenzene derivatives.
Scheme 2: Synthetic route for the preparation of bisazobenzene derivative 2.
Figure 1: (a) Histograms showing the relative populations of conformations having the sulfur-to-sulfur distan...
Figure 2: (a) Normalized UV–vis absorption spectra of 2 in DMSO, acetonitrile, MeOH and sodium phosphate buff...
Figure 3: (a) Photoswitching of 2 with violet light (407–410 nm) at different temperatures (4, 10 and 20 °C) ...
Graphical Abstract
Figure 1: Configurations and conformations of 5,6-dihydrodibenzo[c,g][1,2]diazocine (1), and DFT (B3LYP/6-31G...
Scheme 1: Synthesis of 3,3’-diamino-EBAB 4 and its acetamide derivative 5.
Figure 2: Crystal structures of the cis isomers of 3,3’-diamino-EBAB 4 and its acetamide derivative 5. The at...
Figure 3: UV–vis spectra of the diazocine derivatives 3,3’-diamino-EBAB 4 and its bisamide derivative 5 in ac...
Figure 4: Absorbances of solutions of 4 and 5 in acetonitrile at 405 nm (red) and 485 nm (blue) in the corres...
Figure 5: DFT-calculated structure (B3LYP/6-31+G**) of a complex of 5 with ethylenediamine as a conceivable m...
Graphical Abstract
Figure 1: The α-(1→3)-linked mannobioside α-D-Man-(1→3)-D-Man 1 (B) is a potent disaccharide ligand for the b...
Scheme 1: Synthesis of azobenzene mannoside 6 and azobenzene mannobioside 2 by glycosylation.
Figure 2: Connolly [32,33] descriptions of the FimH CRD with the docked azobenzene mannobioside 2. Top: (E)-isomer (...
Graphical Abstract
Figure 1: Change of electron distribution between HS and LS states of an octahedral iron(II) coordination com...
Figure 2: Types of spin transition curves in terms of the molar fraction of HS molecules, γHS(T), as a functi...
Figure 3: Single crystal UV–vis spectra of the spin crossover compound [Fe(ptz)6](BF4)2 (ptz = 1-propyltetraz...
Figure 4: Thermal spin crossover in [Fe(ptz)6](BF4)2 (ptz = 1-propyltetrazole) recorded at three different te...
Figure 5: (a) Mössbauer spectra of the LS compound [Fe(phen)3]X2 recorded over the temperature range 300–5 K....
Figure 6: (left) Demonstration of light-induced spin state trapping (LIESST) in [Fe(ptz)6]BF4)2 with 57Fe Mös...
Figure 7: Schematic representation of the pressure influence (p2 > p1) on the LS and HS potential wells of an...
Figure 8: χMT versus T curves at different pressures for [Fe(phen)2(NCS)2], polymorph II. (Reproduced with pe...
Figure 9: Molecular structure (a) and γHS(T) curves at different pressures for [CrI2(depe)2] (b) (Reproduced ...
Figure 10: HS molar fraction γHS versusT at different pressures for [Fe(phy)2](BF4)2. The hysteresis loop broa...
Figure 11: Proposed structure of the polymeric [Fe(4R-1,2,4-triazole)3]2+ spin crossover cation (a) and plot o...
Figure 12: Temperature dependence of the HS fraction γHS(T), determined from Mössbauer spectra of [Fe(II)xZn1-x...
Figure 13: Influence of the noncoordinated anion on the spin transition curve γHS(T) near the transition tempe...
Figure 14: Spin transition curves γHS(T) for different solvates of the SCO complexes. [Fe(II)(2-pic)3]Cl2·Solv...
Figure 15: ST curves γHS(T) of the deuterated solvates of [Fe(II)(2-pic)3]Cl2·Solv with Solv = C2D5OH and C2H5...
Figure 16: Sketch of the two-step spin transition; [LS–LS] pair is diamagnetic, [LS–HS] is paramagnetic and th...
Figure 17: (left) Temperature dependence of χMT for {[Fe(L)(NCX)2]2bpym}(L = bpym or bt and X = S or Se). (rig...
Figure 18: Temperature dependence of χMT for [bpym, NCS−] (left) and [bpym, NCSe−] (right) at different pressu...
Figure 19: 57Fe Mössbauer spectra of [bpym, NCSe−] measured at 4.2 K at zero field (a) and at 5 T (b) (see tex...
Figure 20: Temperature dependence of χMT for [Fe2(L)3](ClO4)4·2H2O showing a complete two-step spin conversion...
Figure 21: (a) View of the dinuclear unit in the crystal structure of [Fe2(Hsaltrz)5(NCS)4]·4MeOH. (b) Tempera...
Figure 22: (left) AFM pattern recorded in tapping mode at room temperature on hexagonal single crystals of [Fe3...
Figure 23: (right) Stepwise SCO in an Fe4 [2 × 2] grid, which reveals a smooth magnetic profile under ambient ...
Figure 24: (left) View of the discrete nanoball made of Fe(II) SCO units as well as Cu(I) building blocks. (ri...
Figure 25:
(left) Linear dependency between T1/2 in the heating (Δ) and cooling () modes versus the anion volu...
Figure 26: (left) View of the linear chain structure of [Fe(1,2-bis(tetrazol-1-yl)propane)3]2+ along the a axi...
Figure 27: (left) View of the 2D layered structure of [Fe(btr)2(NCS)2]·H2O (at 293 K). The water molecules (in...
Figure 28: (left) Three interpenetrated square networks for [Fe(bpb)2(NCS)2]·MeOH. (right) χMT versus T plot s...
Figure 29: Part of the crystal structure of [Fe{N(entz)3}](BF4)2 (T = 293 K) [335,336]. (Reproduced with permission fro...
Figure 30: (left) Projection of the crystal structure of [Fe(btr)3](ClO4)2 along the c axis revealing a 3D str...
Figure 31: Size-dependent SCO properties in [Fe(pz)Pt(CN)4] (left), change of color upon spin state transition...
Figure 32: Schematic showing the epitaxial growth of polymer {Fe(pz)[Pt(CN)4]} and the spin transition propert...
Figure 33: Microcontact printing (μCP) of nanodots on Si-wafer of [Fe(ptz)6](BF4)2 after deposition of crystal...
Figure 34: (left) Projection of the two independent cations of [Fe(C6–trenH)]2+ with atom numbering scheme (15...
Figure 35: (a) χMT versus T for [Fe(C16-trenH)]Cl2·0.5H2O and variation of the distance d with temperature (T)...
Figure 36: Schematic illustration of the structure of compounds [Fe(Cn-tba)3]X2 adopting a columnar mesophase ...
Figure 37: Temperature dependence of the magnetic moment (M) at 1000 Oe and DSC profiles (inset; 5 °C/min) of ...
Figure 38: Porous structure of the SCO-PMOFs {Fe(pz)[M(II)(CN)4]} (left), representation of the host–guest int...
Figure 39: Porous structure of the guest-free SCO-PMOF’s {Fe(pz)[M(II)(CN)4]} (left), magnetic properties of t...
Figure 40: (left) The 3D porous structure of {Fe(pz)[Pt(CN)4]}·0.5(CS(NH2)2) (1) and {Fe(pz)[Pd(CN)4]}·1.5H2O·...
Figure 41: Top: The 3D porous structure of {Fe(dpe)[Pt(CN)4]}·phenazine in a direction close to [101] emphasiz...
Figure 42: View of the segregated stacking of [Ni(dmit)2]− and [Fe(sal2-trien)]+ in [Fe(qsal)2][Ni(dmit)2]3·CH3...
Figure 43: Thin films based on Fe(III) compounds coordinated to Terthienyl-substituted QsalH ligands [434] together...
Figure 44: Left: Temperature-dependent emission spectra for [Fe2(Hsaltrz)5(NCS)4]·4MeOH at λex = 350 nm over t...