Photo- and protonation-induced changes in structures and physical properties of azo-conjugated metal complex systems

Novel photo- and proton-coupled behavior of azo-conjugated metal complexes is described. The trans-to-cis isomerization of azoferrocene proceeds not only by the photoexcitation of the π-π* transition band but also by photoexcitation of the MLCT (d-π* transition) band, by a protonation-deprotonation cycle (proton-coupled isomerization), and by an oxidation-reduction cycle (redox isomerization). A platinum tolylazophenylenedithiolato complex shows reversible trans-to-cis photoisomerization, protochromism, and novel proton-coupled cis-to-trans isomerization.     

Photoisomerization and thermal isomerization of arylazoimidazoles

Photoisomerization and thermal isomerization behaviors of an extensive series of arylazoimidazoles are investigated. Absorption spectra are characterized by a structured pipi* absorption band around 330-400 nm with a tail on the lower energy side extending to 500 nm corresponding to an npi* transition. The trans-to-cis photoisomerization occurs on excitation into these absorption bands. The quantum yields are dependent on the excitation wavelength, as observed for azobenzene derivatives, but are generally larger than those of azobenzene. The thermal cis-to-trans isomerization rates are also generally larger than that of azobenzene and are comparable to those of 4-N,N-dimethylaminoazobenzene and 4-nitroazobenzene. Arylazoimidazoles with no substituent on the imidazole nitrogen are unique in that the quantum yield for the trans-to-cis photoisomerization and the rate of thermal cis-to-trans isomerization are particularly large. It is proposed that the fast thermal isomerization is due to an involvement of self-catalyzed and protic molecule-assisted tautomerization to a hydrazone form.     

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.     

Photochromism of 2-(phenylazo)imidazoles

The isomerization behaviors of 2-(phenylazo)imidazole (Pai-H) and 1-N-methyl-2-(phenylazo)imidazole (Pai-Me) have been investigated. The crystal structure of trans-Pai-Me was determined, revealing that key structures around the azo group are nearly identical among azobenzene, Pai-H, and Pai-Me. Pai-Me undergoes reversible cis/trans photoisomerization, whereas Pai-H responds poorly to irradiation. The quantum yields of trans-to-cis isomerization of Pai-Me on 454 and 355 nm excitation are 0.35 +/- 0.03 and 0.25 +/- 0.03, respectively, in toluene. The wavelength-dependent isomerization quantum yield is well-known for azobenzene, but these values are substantially higher than those of azobenzene. The activation energy of thermal cis-to-trans isomerization of Pai-Me in toluene is 79.0 +/- 3.5 kJ mol(-1), which is lower than that of azobenzene by 15 kJ mol(-1). The thermal cis-to-trans isomerization of Pai-H is even faster. Density functional theory calculations were performed, revealing that the energy gaps between the azo n-orbital and the highest pi-orbital of azoimidazoles are much narrower than that of azobenzene. Finally, a preliminary study suggested that metal ions can modulate the absorption spectrum of Pai-Me without a loss of the gross photochromic behavior.     

Synthesis, characterization, and photochemical properties of azobenzene-conjugated Ru(II) and Rh(III) bis(terpyridine) complexes

We synthesized azobenzene-conjugated bis(terpyridine) Ru(II) and Rh(III) mononuclear and dinuclear complexes and investigated their photochemical properties on excitation of the azo pi-pi band upon 366 nm light irradiation. The Ru mononuclear complex underwent trans-to-cis photoisomerization to reach the photostationary state with only 20% of the cis form, while the Ru dinuclear complex did not isomerize at all photochemically. On the other hand, the mononuclear and dinuclear Rh complexes showed almost complete trans-to-cis photoisomerization behavior. Cis forms of the Rh complexes thermally returned to the trans form at a much slower rate than those of organic azobenzenes, but they did not isomerize photochemically. The reduction potential of the cis forms was 80 mV more negative than that of the trans forms. The photoisomerization quantum yields of the Rh complexes were strongly dependent on the polarity, viscosity, and donor site of the solvents as well as the size of the counterions. We investigated the photoisomerization process of these complexes using femtosecond absorption spectroscopy. For the Rh complexes, we observed S(n) <-- S(2) and S(n) <-- S(1) absorption bands similar to those of organic azobenzenes. For the Ru complexes, we observed very fast bleaching of the MLCT band of the Ru complex, which indicated that the energy transfer pathway to the MLCT was the primary cause of the depressed photoisomerization. The electronic structures, which were estimated from ZINDO molecular orbital calculation, supported the different photochemical reaction behavior between the Ru and Rh complexes.     

Photon-, electron-, and proton-induced isomerization behavior of ferrocenylazobenzenes

3-, 4-, and 2-ferrocenylazobenzenes, 1, 2, and 3, respectively, and several derivatives of 1 were synthesized, and their photoisomerization behaviors were examined. The molecular structures of 1 and its derivatives, 2-chloro-5-ferrocenylazobenzene (5) and 3-ferrocenyl-4'-hydroxylazobenzene (11), were determined by X-ray diffraction analysis. 3-Ferrocenyl compound 1 undergoes reversible trans-to-cis isomerization with a single green light source and the Fe(III)/Fe(II) redox change. 4- and 2-Ferrocenyl compounds, 2 and 3, also respond to green light in addition to UV light, exciting the pi-pi* transition, but the cis molar ratio in the photostationary state (PSS) is lower than that of 1. The response to green light in 2 and 3 is caused by the MLCT (from Fe d orbital to azo pi* orbital) band excitation, while the character of the MLCT band, as estimated by time-dependent density functional theory calculations, differs between 1 and 2. The oxidized form of 2 undergoes facile cis-to-trans thermal isomerization. Both 1 and 2 undergo facile protonation and show proton-catalyzed cis-to-trans isomerization. Among the derivatives of 1, 2-chloro-5-ferrocenylazobenzene (5) exhibits the highest cis molar ratio (47%) in the PSS of green light irradiation.     

Redox-conjugated reversible isomerization of ferrocenylazobenzene with a single green light

A new reversible isomerization cycle for meta-ferrocenylazobenzene accomplished by combination of a single green light (546 nm) and redox change between Fe(II) and Fe(III) was discovered. In the Fe(II) state, trans-to-cis isomerization proceeded upon the green light irradiation exciting the metal-to-ligand charge transfer (MLCT) in a high quantum yield (Phit-->c = 0.51) which exceeds that of azobenzene (Phit-->c = 0.12 (313 nm excitation)). The cis molar ratio reached 35% in the photostationary state. The oxidation to the Fe(III) state followed by irradiation with the same green light led to the cis-to-trans back-reaction to recover almost all of the trans-form. The "on-off switching" of the MLCT character played an important role in the redox-dependent response to the green light for the isomerization. The photoisomerization behavior of ferrocenylazobenzenes was strongly dependent on the substitution position of the ferrocenyl moiety on the benzene ring. The MLCT excitation was not effective for the trans-to-cis conversion in para-ferrocenylazobenzene. Time-dependent density functional theory (TD-DFT) calculations for meta- and para-ferrcenylazobenzene showed that the origin of the visible band (MLCT band) is different. The initial orbital for the MLCT in meta-ferrocenylazobenzene is delocalized over Fe and the Cp ring, while that in para-ferrocenylazobenzene is localized on the iron.     

Synthesis of azo-conjugated catecholate complexes and their photo- and proton-responses

A new azo-conjugated catecholate ligand, azocat , and its nickel complexes were synthesized, and their physical and chemical properties were investigated. The complex with one azocat shows no obvious photo-response, whereas the compound with two azocat 's causes a little photoisomerization. Both of these novel azo-conjugated catecholate complexes show remarkable proton responses.     

Probing highly efficient photoisomerization of a bridged azobenzene by a combination of CASPT2//CASSCF calculation with semiclassical dynamics simulation

Mechanism of phototriggered isomerization of azobenzene and its derivatives is of broad interest. In this paper, the S(0) and S(1) potential energy surfaces of the ethylene-bridged azobenzene (1) that was recently reported to have highly efficient photoisomerization were determined by ab initio electronic structure calculations at different levels and further investigated by a semiclassical dynamics simulation. Unlike azobenzene, the cis isomer of 1 was found to be more stable than the trans isomer, consistent with the experimental observation. The thermal isomerization between cis and trans isomers proceeds via an inversion mechanism with a high barrier. Interestingly, only one minimum-energy conical intersection was determined between the S(0) and S(1) states (CI) for both cis → trans and trans → cis photoisomerization processes and confirmed to act as the S(1) → S(0) decay funnel. The S(1) state lifetime is ～30 fs for the trans isomer, while that for the cis isomer is much longer, due to a redistribution of the initial excitation energies. The S(1) relaxation dynamics investigated here provides a good account for the higher efficiency observed experimentally for the trans → cis photoisomerization than the reverse process. Once the system decays to the S(0) state via CI, formation of the trans product occurs as the downhill motion on the S(0) surface, while formation of the cis isomer needs to overcome small barriers on the pathways of the azo-moiety isomerization and rotation of the phenyl ring. These features support the larger experimental quantum yield for the cis → trans photoisomerization than the trans → cis process.     

UV-Vis, NMR, and time-resolved spectroscopy analysis of photoisomerization behavior of three- and six-azobenzene-bound tris(bipyridine)cobalt complexes

The photoisomerization properties of tris(bipyridine)cobalt complexes containing six or three azobenzene moieties, namely, [Co(II)(dmAB)3](BF4)2 [dmAB = 4,4'-bis[3'-(4'-tolylazo)phenyl]-2,2'-bipyridine], [Co(III)(dmAB)3](BF4)3, [Co(II)(mAB)3](BF4)2 [mAB = 4-[3' '-(4' '-tolylazo)phenyl]-2,2'-bipyridine], and [Co(III)(dmAB)3](BF4)3, derived from the effect of gathering azobenzenes in one molecule and the effect of the cobalt(II) or cobalt(III) ion were investigated using UV-vis absorption spectroscopy, femtosecond transient spectroscopy, and 1H NMR spectroscopy. In the photostationary state of these four complexes, nearly 50% of the trans-azobenzene moieties of the Co(II) complexes were converted to the cis isomer, and nearly 10% of the trans-azobenzene moieties of the Co(III) complexes isomerized to the cis isomer, implying that the cis isomer ratio in the photostationary state upon irradiation at 365 nm is controlled not by the number of azobenzene moieties in one molecule but rather by the oxidation state of the cobalt ions. The femtosecond transient absorption spectra of the ligands and the complexes suggested that the photoexcited states of the azobenzene moieties in the Co(III) complexes were strongly deactivated by electron transfer from the azobenzene moiety to the cobalt center to form an azobenzene radical cation and a Co(II) center. The cooperation among the photochemical structural changes of six azobenzene moieties in [Co(II)(dmAB)3](BF4)2 was investigated with 1H NMR spectroscopy. The time-course change in the 1H NMR signals of the methyl protons indicated that each azobenzene moiety in [Co(II)(dmAB)3](BF4)2 isomerized to a cis isomer with a random probability of 50% and without interactions among the azobenzene moieties.     

Photo-induced structural changes of azobenzene Langmuir-Blodgett films

Structural changes of the Langmuir-Blodgett (LB) films of azobenzene accompanied by photoisomerization are described. First, photoisomerization is explained in terms of 'free volume'. In the polyion complex monolayers of amphiphiles having two azobenzene units at the air-water interface, the area per molecule depends on the polycation species. The fraction of cis-azobenzene in the LB films at the photostationary state under the illumination with UV light increased with increasing area per molecule, which is consistent with the concept of free volume. Second, a counter example of the concept of free volume is presented. Three-dimensional cone-shaped structures developed with trans-to-cis photoisomerization in the polyion complex LB film of a water-soluble amphiphilic azobenzene. These structures appeared and disappeared reversibly by alternate illumination with UV and visible light. The results indicate that the two-dimensional LB film structure exerts significant modification by photoisomerization. This is against the concept of free volume because this concept does not consider the possibility that the two-dimensional LB film structures may change into three-dimensional ones. Finally, photo-induced J-aggregate formation of non-photochromic and photochromic dyes is described. Two cyanine dyes were each mixed with an amphiphilic azobenzene in the LB films. These cyanine dyes are known to form J-aggregates in single-component LB films. In the mixed LB films, the J-aggregate formation was suppressed to some extent. The alternate illumination of the films with UV and visible light caused the photoisomerization of azobenzene in the mixed LB films, which triggered the J-aggregate formation of the cyanine dyes. The J-aggregate formation was accompanied by the development of three-dimensional cone-shaped structures from the film surface. When an amphiphilic merocyanine was mixed with the azobenzene in the LB films, J-aggregate formation was also induced by the alternate illumination with UV and visible light. This J-aggregate formation was also accompanied by a large morphological change: circular domains changed into fractal-like ones. The J-aggregate formation of the dyes and the concomitant morphological change were irreversible. In these cases, the photoisomerization of azobenzene served as a trigger to induce self-organization of the dye molecules.     

Fluorescence from an azobenzene-containing diblock copolymer micelle in solution

We report the observation of unusual fluorescence emission from an azobenzene-containing polymer micellar solution. An amphiphilic diblock copolymer composed of the hydrophilic quaternized poly(4-vinyl pyridine) (QP4VP) and a hydrophobic liquid crystalline polymethacrylate bearing azobenzene side groups (PAzoMA) is nonfluorescent in molecularly dissolved state in N,N-dimethyl formamide (DMF) but becomes fluorescent as a result of the micellization upon addition of water, which confines azobenzene groups into the core region of micellar aggregates. Experimental results suggest that the micellization-enhanced fluorescence was caused by a slowdown, due to the confinement effect, in the rate of the trans-to-cis photoisomerization that is the main nonradiative relaxation process for excited azobenzene groups in the trans form. Furthermore, it was found that the fluorescence intensity of aqueous micellar solution is sensitive to changes in pH (reversible fluorescence variation) and to illumination (irreversible fluorescence variation). The results indicate that a subtle change in the state of polymer micellar association may alter the confining state of azobenzene groups responsible for the fluorescence emission.     

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.     

Synthesis and Surface-Active Properties of New Photosensitive Surfactants Containing the Azobenzene Group

Several water-soluble cationic surfactants, 4-alkylazobenzene-4'-(oxy-2-hydroxypropyl)trimethylammonium methylsulfate (AZMS) (AZMS-0, AZMS-1, AZMS-2, AZMS-4, and AZMS-8), containing alkylglycidylether and azoarene have been synthesized with high yields of 63-78% and their surface-active properties have been investigated upon irradiation with UV/vis light. All of the trans-AZMS surfactants are isomerized to cis-trans mixtures containing 92.5% cis isomer by UV light irradiation at 350 nm. The cis isomers in the mixtures are reverted to trans isomers by visible light irradiation (lambda>445 nm). Such photoisomerization induces changes in the surface activity of each surfactant. The critical micelle concentration (cmc) of the trans form of AZMS-8 surfactant is about 1.28x10(-4) mol/l. At the photostationary state, 92.5% of the trans form is changed to the cis form which exhibits a slightly higher cmc (3.41x10(-4) mol/l). The new cmc of AZMS surfactants upon photoisomerization is similar to that of the ideal mixed micellar system. In particular, the ratio of cmc(cis) to cmc(trans) of AZMS derivatives is about 1.87-2.85 which increases proportionally with the chain length of alkyl group. The minimum average area per molecule (A(min)(a/w)) for the trans and cis isomers of AZMS-8 is 0.60 and 0.74 nm(2), respectively. The difference in the A(min)(a/w) may originate from the structural differences in the two isomers. These values are quite different as compared to those of the conventional azobenzene surfactants. Copyright 2000 Academic Press.     

The kinetics of helix unfolding of an azobenzene cross-linked peptide probed by nanosecond time-resolved optical rotatory dispersion

The unfolding dynamics of a 16 amino acid peptide (Ac-EACAREAAAREAACRQ-NH(2), FK-11-X) was followed using nanosecond time-resolved optical rotatory dispersion (ORD). The peptide was coupled to an azobenzene linker that undergoes subnanosecond photoisomerization and reisomerizes on a time scale of minutes. When the linker is in the trans form, the peptide favors a more helical structure (66% helix/34% disordered) and when in the cis configuration the helical content is reduced. Unfolding of FK-11-X was rapidly triggered by a 7-ns laser pulse at 355 nm, forming cis azobenzene-linked peptides that maintained the secondary structure (helical or disordered) of their trans azobenzene counterparts. The incompatibility of the instantaneous cis photoproduct with helical secondary structure drives the subsequent peptide unfolding to a new conformational equilibrium between cis helix and cis disordered structures. The kinetic results show a approximately 40% decrease in the time-dependent ORD signal at 230 nm that is best fit to a single-exponential decay with a time constant of 55 +/- 6 ns. Folding and unfolding rates for cis FK-11-X are estimated to be approximately 3.0 x 10(6) s(-)(1) (1/330 ns) and approximately 1.5 x 10(7) s(-)(1) (1/66 ns), respectively.     

Synthesis of chiral azobenzene-based compounds for use in the photochemical tuning of the helical structure of liquid crystals

We have synthesized azobenzene-based molecules containing either one or two chiral groups. A cholesteric phase can be induced by adding the chiral azobenzene compounds to a host nematic liquid crystal. We investigated the effects of the trans - cis photoisomerization of the chiral azobenzene compounds on the properties of the cholesteric phase, such as the helical pitch length. This can be increased or decreased by the photoisomerization of the chiral azobenzene compounds. We discuss the photochemically driven change in the helical pitch of the cholesteric phase with respect to structural effects involving the chiral groups.     

Photoswitching behavior of a novel single molecular tip for noncontact atomic force microscopy designed for chemical identification

A tripod molecule with an azobenzene arm was designed as a single molecular tip for noncontact atomic force microscopy (NC-AFM). The azobenzene moiety showed photoisomerization that enabled measurements of the same position of the sample by different tip apexes with different interactions. Photoswitching behavior of the molecule synthesized and adsorbed on Au surfaces was examined and reversible switching between the trans- and cis forms was successfully confirmed by NC-AFM measurements.     

Local free volume and structural relaxation studied with photoisomerization of azobenzene and persistent spectral hole burning in poly(alkyl methacrylate)s at low temperatures

The final extent of trans‐to‐cis photoisomerization of an azobenzene probe in various amorphous polymers has been used in previous studies for estimating local free volume and its fluctuation in polymer solids. However, there have been few studies on what kinds of molecular motion cause the fluctuation of local free volume at low temperatures. The onset of local structural relaxation (molecular motion) can be observed with temperature cycling experiments in persistent spectral hole burning (PSHB). Thus, in the present article, the fluctuation of local free volume observed in trans‐to‐cis photoisomerization of azobenzene is related to the local structural relaxation observed in PSHB for poly(alkyl methacrylate)s with various ester groups, i.e., methyl (PMMA), ethyl (PEMA), n‐propyl (PnPMA), isopropyl (PiPMA), and isobutyl (PiBMA) groups. In the final cis fraction, rapid decrease, from 20 to 4 K in PEMA, PnPMA, and PiPMA, and from 86 to 20 K in PiBMA, is observed. These temperature regions of the rapid decrease in final cis fraction in these polymers agree well with those where the hole width in PSHB temperature cycling experiments begins to increase for the same polymers. For example, PEMA begins its ester ethyl group rotation at 17 K, which was primarily observed with PSHB, causing the drastic decrease in final cis fraction of azobenzene from 20 to 4 K. The final cis fractions at 4 K for these poly(alkyl methacrylate)s reflect the intrinsic sizes of the local free volume, except in the case of PMMA, and are compared with the reported results of positron annihilation lifetime measurements. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3098–3105, 2000     

侧基含偶氮苯生色团的短梗霉多糖的制备及光致变色性

利用4-(4-硝基苯偶氮)-1-萘酚的丙烯酸酯与短梗霉多糖进行接枝共聚,制备了侧基含偶氮苯生色团的短梗霉多糖聚合物,并用红外光谱、扫描电镜、X射线衍射、热重分析等手段对聚合物进行了表征.用紫外光谱研究了聚合物的光致变色性,结果表明聚合物具有良好的热稳定性和光致变色性.     

Photoinduced alignment of azobenzene moieties in the side chains of polyglutamate films

The ‘trans↔cis’ reversible photoisomerization process tends to align azobenzene derivatives perpendicular to the polarization direction of the pumping beam. It is shown that in the trans→cis optical transition the cis state is aligned perpendicular to the pumping light polarization. This is shown for spin-coated films of ‘hairy-rod’ polyglutamate with azobenzene moieties in the side chains. Photoselection in both trans and cis states is demonstrated using the attenuated total reflection method.