Photoluminescence switching of azobenzene-conjugated Pt(II) terpyridine complexes by trans-cis photoisomerization

Pt(II) complexes with a terpyridylazobenzene ligand (tpyAB) were newly synthesized, and their photoluminescence properties by trans-cis isomerization of the azo moiety were investigated. In these complexes, upon excitation with 366-nm light in polar solvents such as DMF, DMSO, and propylene carbonate, trans-to-cis isomerization with significant UV-vis spectral changes occurred almost completely. Cis-to-trans isomerization was observed both by irradiation with visible light and by heat. The reduction peaks due to the terpyridine and the azo group in the cyclic voltammograms of the Pt complexes were shifted in the positive direction by trans-to-cis isomerization. Emission spectral changes due to trans-cis isomerization were observed for both the tpyAB and the Pt complexes. The significant differences in the emission properties of the complex compared to tpyAB include the observation that both the excitation and emission wavelengths were shifted to lower energy, located in the visible region. Moreover, the change in emission intensity between the trans and cis forms was more significant upon excitation with UV light, because the trans form of the complexes showed absolutely no emission. Accordingly, the azobenzene-conjugated Pt(II) terpyridine complexes promise to be doubly photofunctional materials, showing complete off-on switching of emission linked to the trans-cis conformation change.     

Theoretical Study of Halogen Effect in Isomerization of 2-Halo-[9]-annulen Anion at the DFT Level

Cis-trans isomerization of [9]-annulenanion(1) and its 2-fluoro-,2-chloro-and 2-bromo-derivatives(2,3 and 4,respectively) were investigated at the HF/6-31G* and B3LYP/6-311++G** levels of theory.Cis,cis,cis,cis structures appear more stable than their corresponding cis,cis,cis,trans-isomers.The relative height of energy barriers for cis-trans isomerization is:2cis > 1cis > 3cis > 4cis.This trend for the reverse trans-cis isomerization follows the electronegativity of the substituent at C-2(2trans > 3trans > 4trans > 1trans).     

Photochemically induced disturbance of the alkyl chain packing in vesicular membranes

In previous reports, we presented the synthesis and properties of double-tailed azobenzene-substituted phosphate amphiphiles (Kuiper et al. Synthesis 2003, 695 and Kuiper et al. Langmuir 2004, 20, 1152). We also reported that an ion channel can be regulated by trans-cis isomerization of these amphiphiles, which were incorporated in the membrane (Folgering et al. Langmuir 2004, 20, 6985). In the present study, the effect of trans-cis isomerization of both single- and double-tailed azobenzene-substituted amphiphiles on the aggregation and packing behavior has been studied. The phase transition temperature of a membrane and the thermal half-life times of the cis azobenzene-substituted amphiphiles in membranes have been measured. Furthermore, the synthesis and properties of single-tailed azobenzene-substituted phosphate amphiphiles are described and compared with those of the double-tailed analogues. The single-tailed azobenzene-substituted phosphates have a low solubility in water and form micelles, sheets, and crystals. In all cases the trans-cis isomerization leads to a disturbance of the chain packing. Both single- and double-tailed cis azobenzene-substituted phosphates lowered the main phase transition temperature of bilayer membranes. The effect increased when the azobenzene moiety was situated closer to the head group. Accordingly, the half-life times of the cis azobenzene group was shorter when the azobenzene group was positioned closer to the head group for both the single- and double-tailed amphiphiles. Interestingly, the thermal cis-trans isomerization of the single-tailed azobenzene-substituted phosphates was faster in a DOPC membrane than that for the free monomer in aqueous solution.     

Peptide bond trans-cis isomerization and acylimine formation in chromophore maturation of the red fluorescent proteins

In red fluorescent proteins such as DsRed, an acylimine is formed from the Phe65-Gln66 linkage in GFP-like immature form, while it shows a cis configuration in its mature state. To date, the relationship between acylimine formation and trans-cis isomerization is still unresolved. We have calculated bond rotation profiles for mature and immature chromophores within the protein using our own n-layered integrated molecular orbital and molecular mechanism (ONIOM) approach. The results suggested that the isomerization is barrierless in acylimine formed in the mature state, suggesting that the acylimine formation precedes the trans-cis isomerization in DsRed chromophores. Further decomposition analysis of electrostatic contributions from individual residues has identified several residues and a specific water molecule which could play key roles in controlling the rate of the trans-cis isomerization of peptide bond in immature GFP-like protein. The results also highlight the importance of Gln66-like of tripeptide motif (chromophore) in the maturation of red fluorescent proteins. In view of the considerable interest in developing red fluorescent proteins for numerous biotechnological applications, these results should be useful for design of novel fluorescent proteins.     

Design, synthesis, and photochemical behavior of poly(benzyl ester) dendrimers with azobenzene groups throughout their architecture

A new class of dendrons and dendrimers containing azobenzene units (bearing up to 29 azobenzene groups for four generations) were designed and synthesized with the convergent method, which uses azobenzene derivatives as monomers and benzyl ester groups as linkages leading to photoresponsive dendrons and dendrimers with azobenzene units throughout their architecture. Photochemical isomerization experiments revealed that all of the dendrons and dendrimers undergo trans-cis isomerization by irradiation and cis-trans isomerization by either irradiation or heating.     

Structural and spatial considerations in the N,N'-diacyl- and bis(alkoxycarbonyl)bispidinone series

Diamidic and dicarbamic bispidinones show trans-cis isomerism, the relative population in solution of the cis form increasing with solvent polarity. The mutual proximity of the two amide functions in 4a has no impact on the barrier to isomerization. The system represents a peculiar case of planar chirality posing a challenge to its specification.     

“哑铃状”偶氮化合物的合成与表征

从分子设计的角度出发, 合成了末端含有两个庞大基团的哑铃状偶氮化合物. 其大的末端基团一方面可以增加顺、反两种异构体之间的能垒, 有利于两种异构体的稳定存在, 另一方面由于大的末端基团的存在, 为采用AFM或高分辨STM直接观测两种异构体分子相的相互转变降低了难度.     

Systematic control of photochemistry: the dynamics of photoisomerization of a model cyanine dye

On-the-fly CASSCF nonadiabatic dynamics have been used to model the trans-cis isomerization of a model cyanine dye. Our results show that the photochemical generation of the trans versus cis product is dynamically controlled by the presence of an extended cis-trans conical intersection seam that persists along all torsional angles. This in turn suggests that the photochemistry could be completely controlled by controlling the distribution of momentum components in a wave packet excited by laser photolysis in a coherent control experiment.     

Synthesis and properties of amphiphilic photoresponsive gelators for aromatic solvents

A sugar-based photoresponsive supergelator, N-glycosylazobenzene that shows selective gelation of aromatic solvents is described. The partial trans-cis isomerization of the azobenzene moiety allows photoinduced chopping of the entangled gel fibers to short fibers, resulting in controlled fiber length and gel-sol transition. The gelator is useful for the selective removal of toxic aromatic solvents from water.     

Folded and unfolded conformations of the omega-3 polyunsaturated fatty acid family: ch(3)ch(2)[ch=chch(2)](b)[ch(2)](m)cooh: first principles study

Polyunsaturated fatty acids (PUFA) like stearidonic acid (SDA;18:4 n-3) eicosapentaenoic acid (EPA; 20:5 n-3), and docosahexaenoic acid (DHA; 22:6 n-3) and its chain fragment models were studied at B3LYP/6-31G(d) levels of theory. Significant conformations for the cis and trans isomers were selected to obtained the thermodynamic functions (DeltaH, DeltaS, DeltaG) for the cis-trans isomerization and for folding using the B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) level of theory. The structural analysis shows that there are significant differences in thermodynamic function of the trans- and cis-PUFAs. The trans-cis isomerization energy values reinforce the consistency and the relative accuracy of theoretical model calculations. The observed flexibility of naturally cis PUFAs could be explained by a very special "smooth basin" PES of the motif of sp(2)-sp(3)-sp(2) hybrid states as reported previously (J. Phys. Chem. A 2005, 109, 520-533). We assumed that intrinsic thermodynamic functions may describe this flexible folding process. The folding enthalpy as well as the folding entropy suggests that there is a new role of the cis-PUFAs in membranes: these cis isomers may have a strong influence on membrane stability and permeability. The average length of the cis helix and beta PUFA was approximated. The difference between the lengths of these two structures is approximately 10 A.     

An intramolecular ionic hydrogen bond stabilizes a cis amide bond rotamer of a ring-opened rapamycin-degradation product

Rapamycin (1), a macrolide immunosuppressant, undergoes degradation into ring-opened acid products 2 and 3 under physiologically relevant conditions. The unsaturated product (3) was isolated and studied in this work. Unlike 1, which has its amide primarily in a trans conformation in solution, 3 has both cis and trans conformations in approximately a 1:1 ratio in dimethyl sulfoxide (DMSO). The amount of cis rotamer was increased dramatically in the presence of an organic base such as triethylamine. The detailed NMR results indicate that the cis rotamer is stabilized through an intramolecular ionic hydrogen bond of the carboxylate anion with the tertiary alcohol as part of a nine-membered ring system. This hydrogen bond was characterized further in organic media and the trans-cis rotamer equilibria were used to estimate the relative bond strengths in several solvents. The additional stabilization arising from this ionic hydrogen bond in the cis rotamer was determined to be 1.4 kcal mol(-1) in DMSO-d6, 2.0 kcal mol(-1) in CD3CN and 1.1 kcal mol(-1) in CD3OD.     

Synthesis, structural, spectral, and photoswitchable properties of cis- and trans-2,2,2',2'-tetramethyl-1,1'-indanylindanes

As versatile synthetic intermediates for new photoswitchable molecules with a tetramethylindanylindane (stiff-stilbene) core, the cis and trans isomers of 5,16-dibromo-2,2,13',13'-tetramethylindanylindanes 2 were synthesized by the Barton-Kellogg coupling. The bromine atoms of trans-2 could be readily replaced with alkyl (sp3), aryl (sp2), and ethynyl (sp) groups. The cis isomers of the parent tetramethylindanylindane 1 and its bromo derivative 2 were isolated, and their structural and photophysical properties were examined for the first time. Clean and efficient trans-cis and cis-trans photochemical isomerization processes were observed in 1.     

Photoisomerization of arylstilbazolium ligands in the presence of DNA

The ability of the DNA duplex to behave as an efficient organized medium for trans-cis photoisomerization has been explored. The presence of DNA affected isomerization in a variety of ways depending on the aryl moiety properties of the ligand and its DNA-binding mode. Contrary to intercalating ligands, 9-[2-(N-methylpyridinium-4-yl)vinyl]phenanthrene (2) and 9-[2-(N-methylpyridinium-4-yl)vinyl]anthracene (3), which gave only cis and trans isomers, the additional products--cyclobutane photodimers--were detected for 2-[2-(N-methylpyridinium-4-yl)vinyl]naphthalene (1), which binds cooperatively to the minor groove of DNA. Photostationary states (pss) for all ligands were seriously affected by the presence of DNA. A trans isomer-rich pss and restriction of trans --> cis process, observed for ligands 1 and 2, were explained in terms of a different binding affinity of DNA toward particular isomers. On the contrary, 9-anthryl derivative 3 isomerized against the isomer-binding preferences, showing cis isomer-rich pss and enhanced quantum yield of isomerization. The unique behavior of ligand 3-DNA complex was attributed to different isomerization mechanism that consists in quantum chain isomerization from an excited singlet state possessing a charge transfer character. This is the first example of ligand, which undergoes DNA-mediated cis-trans isomerization in the opposite direction than expected from DNA-binding preferences. The analysis of pss data based on two alternative pathways of photoisomerization showed that investigated ligands follow the model that allows isomerization of both free and DNA-bound ligands.     

An amphiphilic molecular basket sensitive to both solvent changes and UV irradiation

A molecular basket was obtained by linking four cholate units to a cone-shaped calix[4]arene scaffold through azobenzene spacers. The molecule turns its polar faces inward in nonpolar solvents to bind polar molecules such as sugar derivatives. In polar solvents, the nonpolar faces turn inward, allowing the binding of hydrophobic guests such as pyrene. The molecule can also respond to UV irradiation by trans-cis isomerization of the azobenzene spacers. Response toward both solvents and UV light is fully reversible.     

Photochemical behavior of azobenzene-conjugated CoII, CoIII, and FeII bis(terpyridine) complexes

Azobenzene-conjugated mononuclear and dinuclear terpyridyl complexes of Co(II), Co(III), and Fe(II) were synthesized, and their photoisomerization behavior was investigated. Co(II) and Co(III) complexes, [tpyCo(tpy-AB)]X(n) and [(Cotpy)(2)(tpy-AB-tpy)]X(n) (tpy-AB = C(15)N(3)H(10)-C(6)H(4)-N=NC(6)H(5), tpy-AB-tpy = C(15)N(3)H(10)-C(6)H(4)-N=NC(6)H(4)-C(15)N(3)H(10), X = PF(6) or BPh(4)), exhibit trans-to-cis photoisomerization by irradiation at 366 nm, and this behavior is dependent on solvents and counterions. For the Co(II) complexes, BPh(4) salts undergo cis-to-trans isomerization in propylene carbonate by both photoirradiation with visible light (435 nm) and heat, indicating that reversible trans-cis isomerization has occurred. [Co(tpy-AB)(2)](BPh(4))(2) shows a two-step trans-to-cis isomerization process. The trans-cis isomerization behavior of Co(III) complexes was observed only in the solvents with a low donor number such as 1,2-dichloroethane. Fe(II) complexes, [tpyFe(tpy-AB)]X(n) (X = PF(6) or BPh(4)), exhibit slight trans-to-cis photoisomerization due to the energy transfer from the azobenzene moiety to Fe(tpy)(2) moieties.     

Photoisomerization of the Graft Copolymer of Starch and Azo-dye

Optical data storage materials are mainly based on organic molecules. To be suited for as an optical data storage material, molecules have to be photochromatic, they must be capable of being reversibly switched be. tween two spectroscopically distinct forms by use of light[1].Photoisome rization (trans ～cis) of azoaromatic compounds has been investigated extensively using UV-Vis spectroscopy. In 1972[2], the isomerization of various azobenzenes bound on the side chain or within the main chain of poly (methyl methacrylate), poly(ethyl methacrylate),polystyrene, a polyamide, and polyester had. been demonstrated. There are three processes: a trans-cis photochemical isomerization, a cis-trans photochemical isomerization, and a cis-trans thermal isomerization, which occurs due to the higher stability of the trans configuration, since that pioneering study, two main directions of research developed and are very active in the eurrent literature.     

Conformational preferences of non-prolyl and prolyl residues

The conformational study on Ac-Ala-NHMe (the alanine dipeptide) and Ac-Pro-NHMe (the proline dipeptide) is carried out using ab initio HF and density functional methods with the self-consistent reaction field method to explore the differences in the backbone conformational preference and the cis-trans isomerization for the non-prolyl and prolyl residues in the gas phase and in the solutions (chloroform and water). For the alanine and proline dipeptides, with the increase of solvent polarity, the populations of the conformation tC with an intramolecular C(7) hydrogen bond significantly decrease, and those of the polyproline II-like conformation tF and the alpha-helical conformation tA increase, which is in good agreement with the results from circular dichroism and NMR experiments. For both the dipeptides, as the solvent polarity increases, the relative free energy of the cis conformer to the trans conformer decreases and the rotational barrier to the cis-trans isomerization increases. It is found that the cis-trans isomerization proceeds in common through only the clockwise rotation with omega' approximately +120 degrees about the non-prolyl and prolyl peptide bonds in both the gas phase and the solutions. The pertinent distance d(N...H-N(NHMe)) can successfully describe the increase in the rotational barriers for the non-prolyl and prolyl trans-cis isomerization as the solvent polarity increases and the higher barriers for the non-prolyl residue than for the prolyl residue, as seen in experimental and calculated results. By analysis of the contributions to rotational barriers, the cis-trans isomerization for the non-prolyl and prolyl peptide bonds is proven to be entirely enthalpy driven in the gas phase and in the solutions. The calculated cis populations and rotational barriers to the cis-trans isomerization for both the dipeptides in chloroform and/or water accord with the experimental values.     

Switching of molecular insertion in a cyclic molecule via photo- and thermal isomerization

Two new cyclic ligands were synthesized: a ligand with two trans-azobenzene moieties and one bipyridine moiety, trans(2)-oAB-O13, and a ligand with two trans-azobenzene moieties and two bipyridine moieties, trans(2)-oAB-bpy. Both ligands underwent reversible trans-cis isomerization at the azobenzene moieties. The mole ratios of the trans(2) form:trans-cis form:cis(2) form, evaluated by (1)H NMR spectroscopy of the photostationary states prepared by 1 h illumination, were 0.13:0.27:0.60 (365 nm irradiation) and 0.41:0.47:0.12 (436 nm irradiation) for oAB-O13, and 0.18:0.12:0.70 (365 nm irradiation) and 0.36:0.43:0.21 (436 nm irradiation) for oAB-bpy. When trans(2)-oAB-O13 was mixed with Cu(I), both the bipyridine units and the polyether chains coordinated to the copper center. Addition of a noncyclic bipyridine ligand, trans(2)-oAB-2OH, afforded a bis(bipyridine)copper(I) complex, [Cu(trans(2)-oAB-O13)(trans(2)-oAB-2OH)]BF(4). The bis(bipyridine) ligand, trans(2)-oAB-bpy, formed a 1:1 complex with Cu(I), [Cu(trans(2)-oAB-bpy)]BF(4). [Cu(cis(2)-oAB-bpy)]BF(4) did not undergo the ligand substitution reaction with a noncyclic ligand with two azobenzene moieties and one bipyridine moiety, oAB, whereas its thermal isomerization in the presence of oAB caused the formation of [Cu(trans(2)-oAB-bpy)(trans(2)-oAB)]BF(4), indicating that the isomerization and ligand exchange reactions synchronized via a conformational change of the cyclic ligand.     

Spectral identification of specific photophysics of cy5 by means of ensemble and single molecule measurements

The triplet-state characteristics of the Cy5 molecule related to trans-cis isomerization are investigated by means of ensemble and single molecule measurements. Cy5 has been used frequently in the past 10 years in single molecule spectroscopic applications, e.g., as a probe or fluorescence resonance energy transfer acceptor in large biomolecules. However, the unknown spectral properties of the triplet state and the lack of knowledge on the photoisomerization do not allow us to interpret precisely the unexpected single molecule behaviors. This limits the application of Cy5. The laser photolysis experiments demonstrate that the trans triplet state of Cy5 absorbs about 625 nm, the cis ground state absorbs about 690 nm, and the cis triplet state also absorbs about 690 nm. In other words, the T1-Tn absorptions largely overlap the ground-state absorptions for both trans and cis isomers, respectively. Furthermore, the observation of the cis triplet state indicates an important isomerization pathway from the trans-S1 state to the cis-T1 state upon excitation. The detailed spectra presented in this article let us clearly interpret the exact mechanisms responsible for several important and unexpected photophysical behaviors of single Cy5 molecules such as reverse intersystem crossing (RISC), the observation of dim states with a lower emission intensity and slightly red-shifted fluorescence, and unusual energy transfer from donor molecules to dark Cy5 molecules acting as acceptors in single molecule fluorescence resonance energy transfer (FRET) measurements. Spectral results show that the dim state in the single molecule fluorescence intensity time traces originated from cis-Cy5 because of a lower excitation rate, resulting from the red-shifted ground-state absorption of cis-Cy5 compared to that of the trans-Cy5.     

Optimized and far-red-emitting variants of fluorescent protein eqFP611

Fluorescent proteins (FPs) emitting in the far-red region of the spectrum are highly advantageous for whole-body imaging applications because scattering and absorption of long-wavelength light is markedly reduced in tissue. We characterized variants of the red fluorescent protein eqFP611 with bright fluorescence emission shifted up to 639 nm. The additional red shift is caused by a trans-cis isomerization of the chromophore. The equilibrium between the trans and cis conformations is strongly influenced by amino acid residues 143 and 158. Pseudo monomeric tags were obtained by further genetic engineering. For the red chromophores of eqFP611 variants, molar extinction coefficients of up to approximately 150,000 were determined by an approach that is not affected by the presence of molecules with nonfunctional red chromophores. The bright fluorescence makes the red-shifted eqFP611 variants promising lead structures for the development of near-infrared fluorescent markers. The red fluorescent proteins performed well in cell biological applications, including two-photon imaging.