Enantioselective Light Harvesting with Perylenediimide Guests on Self-Assembled Chiral Naphthalenediimide Nanofibers.

Self-assembling molecular systems often display amplified chirality compared to the monomeric state, which makes the molecular recognition more sensitive to chiral analytes. Herein, we report the almost absolute enantioselective recognition of a chiral perylenediimide (PDI) molecule by chiral supramolecular nanofibers of a bichromophoric naphthalenediimide (NDI) derivative. The chiral recognition was evaluated through the Förster resonance energy transfer (FRET) from the NDI-based host nanofibers to the guest PDI molecules. The excitation energy was successfully transferred to the guest molecule through efficient energy migration along the host nanofiber, thus demonstrating the light-harvesting capability of these hybrid systems. Furthermore, circularly polarized luminescence (CPL) was enantioselectively sensitized by the guest molecule as the wavelength band and sign of the CPL signal were switched in response to the chiral guest molecule.

Conjugated Polymer Nanoparticle-Triplet Emitter Hybrids in Aqueous Dispersion: Fabrication and Fluorescence Quenching Behavior.

Conjugated polymer nanoparticles based on poly[9,9-bis(2-ethylhexyl)fluorene] and poly[N-(2,4,6-trimethylphenyl)-N,N-diphenylamine)-4,4'-diyl] are fabricated using anionic surfactant sodium dodecylsulphate in water by miniemulsion technique. Average diameters of polyfluorene and polytriarylamine nanoparticles range from 70 to 100 and 100 to 140 nm, respectively. The surface of the nanoparticles is decorated with triplet emitting dye, tris(2,2'-bipyridyl)ruthenium(II) chloride. Intriguing photophysics of aqueous dispersions of these hybrid nanoparticles is investigated. Nearly 50% quenching of fluorescence is observed in the case of dye-coated polyfluorene nanoparticles; excitation energy transfer is found to be the dominant quenching mechanism. On the other hand, nearly complete quenching of emission is noticed in polytriarylamine nanoparticle-dye hybrids. It is proposed that the excited state electron transfer from the electron-rich polytriarylamine donor polymer to Ru complex leads to the complete quenching of emission of polytriarylamine nanoparticles. The current study offers promising avenues for developing aqueous solution processed-electroluminescent devices involving a conjugated polymer nanoparticle host and Ru or Ir-based triplet emitting dye as the guest.

Giant Amplification of Photoswitching by a Few Photons in Fluorescent Photochromic Organic Nanoparticles.

Controlling or switching the optical signal from a large collection of molecules with the minimum of photons represents an extremely attractive concept. Promising fundamental and practical applications may be derived from such a photon-saving principle. With this aim in mind, we have prepared fluorescent photochromic organic nanoparticles (NPs), showing bright red emission, complete ON-OFF contrast with full reversibility, and excellent fatigue resistance. Most interestingly, upon successive UV and visible light irradiation, the NPs exhibit a complete fluorescence quenching and recovery at very low photochromic conversion levels (<5 %), leading to the fluorescence photoswitching of 420±20 molecules for only one converted photochromic molecule. This "giant amplification of fluorescence photoswitching" originates from efficient intermolecular energy-transfer processes within the NPs.

A multifunctional photoswitch: 6π electrocyclization versus ESIPT and metalation.

A terthiazole-based molecular switch associating 6π electrocyclization, excited state intramolecular proton transfer (ESIPT), and strong metal binding capability was prepared. The photochemical and photophysical properties of this molecule and of the corresponding nickel and copper complexes were thoroughly investigated by steady-state and ultrafast absorption spectroscopy and rationalized by DFT/TDDFT calculations. The switch behaves as a biphotochrome with time-dependent photochemical outcome and displays efficient ESIPT-based fluorescence photoswitching. Both photochemical reactions are suppressed by nickel or copper metalation, and the main factors contributing to the quenching of the electrocyclization are discussed.

Separation of photoactive conformers based on hindered diarylethenes: efficient modulation in photocyclization quantum yields.

Endowing both solvent independency and excellent thermal bistability, the benzobis(thiadiazole)-bridged diarylethene system provides an efficient approach to realize extremely high photocyclization quantum yields (Φo-c , up to 90.6 %) by both separating completely pure anti-parallel conformer and suppressing intramolecular charge transfer (ICT).

Multichromophoric sugar for fluorescence photoswitching.

A multichromophoric glucopyranoside 2 bearing three dicyanomethylenepyran (DCM) fluorophores and one diarylethene (DAE) photochrome has been prepared by Cu(I)-catalyzed alkyne-azide cycloaddition reaction. The fluorescence of 2 was switched off upon UV irradiation, in proportion with the open to closed form (OF to CF) conversion extent of the DAE moiety. A nearly 100% Förster-type resonance energy transfer (FRET) from all three DCM moieties to a single DAE (in its CF) moiety was achieved. Upon visible irradiation, the initial fluorescence intensity was recovered. The observed photoswiching is reversible, with excellent photo resistance.

Silica-coated gold nanorod arrays for nanoplasmonics devices.

A facile method for growing silica layer on lithographically designed gold nanorod arrays (GNRAs) using a convenient sol-gel method is presented herein. The silica layer thickness was controlled on GNRAs with the reaction time. The localized surface plasmon resonance (LSPR) spectra of these hybrid metal/dielectric nanoparticles were recorded before and after the coating and the effect of different solvents on the LSPR were also assessed. The change in the fluorescence and SERS intensities of a probe molecule (Rh6G) deposited on GNRAs and silica-coated GNRAs revealed that the as-fabricated silica layer does inhibit the quenching of molecular excited states and enhances photophysical/photochemical processes. This kind of hybrid metal/dielectric nanoparticle arrays hence turn out to be real good candidates to design new "plasmonic-active" devices.

Photoswitchable interactions between photochromic organic diarylethene and surface plasmon resonance of gold nanoparticles in hybrid thin films.

Hybrid materials combining gold nanoparticles (GNP) of variable diameter and an organic thin layer of photochromic diarylethenes were achieved. Solid-state photoswitching based on ring-closure/ring-opening reaction was carried out under alternate UV and visible irradiations. In addition to the spectral changes due to the photochromism itself, the surface plasmon resonance related to the GNP is significantly modified, influenced by a photoinduced change in the refractive index of its environment. These two contributions were sorted out, showing the possibility of probing a photochromic switch by following the plasmon band. The shape change of the plasmon band was consistently compared to calculations based on the Mie theory. Additionally, with one given diarylethene compound, both UV-visible spectroscopy and surface enhanced Raman scattering (SERS) spectroscopy showed an acceleration of the ring-opening photochromic reaction in the presence of GNP.

Aromaticity-controlled thermal stability of photochromic systems based on a six-membered ring as ethene bridges: photochemical and kinetic studies.

Three photochromic compounds--2-butyl-5,6-bis[5-(4-methoxyphenyl)-2-methylthiophen-3-yl]-1 H-benzo[de]isoquinoline-1,3(2 H)-dione (BTE-NA), 4,5-bis[5-(4-methoxyphenyl)-2-methylthiophen-3-yl]benzo[c][1,2,5]thiadiazole (BTA), and BTTA, which contain naphthalimide, benzothiadiazole, and benzobisthiadiazole as six-membered ethene bridges with different aromaticities--were systematically studied in solution, sol-gel, and single-crystal states. They exhibit typical photochromic performance with considerably high cyclization quantum yields. BTE-NA, BTA, and BTTA form a typical donor-π-acceptor (D-π-A) system with significant intramolecular charge transfer (ICT) between HOMO and LUMO upon excitation, thus realizing the fluorescence modulation by both photochromism and solvatochromism. The three ethene bridges with different degrees of aromaticity can provide a systematic comparison of the thermal stability evolution for their corresponding closed forms (c-BTE-NA, c-BTA, and c-BTTA). c-BTE-NA shows first-order decay in various solvents from cyclohexane to acetonitrile. c-BTA only shows first-order decay in polar solvents such as chloroform, whereas it is stable in nonpolar solvents like toluene. In contrast, the less aromatic property of BTTA gives rise to its unprecedented thermal stability in various solvents even at elevated temperatures in toluene (328 K). Moreover, the small energy barrier between the parallel and antiparallel conformers allows the full conversion from BTTA to c-BTTA. In well-ordered crystal states, all three compounds adopt a parallel conformation. Interestingly, BTTA forms a twin crystal of asymmetric nature with interactions between the electron-rich oxygen atom of the methoxy group and the carbon atom of the electron-deficient benzobisthiadiazole moiety. This work contributes to the understanding of aromaticity-controlled thermal stability of photochromic systems based on a six-membered ring as an ethene bridge, and a broadening of the novel building blocks for photochromic bisthienylethene systems.

Fluorescence photoswitching and photoreversible two-way energy transfer in a photochrome-fluorophore dyad.

A thorough photophysical study of a photochrome-fluorophore dyad (3), combining a fluorescent laser dye (DCM-type, , Φ(1) = 0.27) and a photochromic diarylethene (2), obtained by click chemistry, is presented. In addition to photochromism, the open form (OF) of 2 exhibits fluorescence (Φ(-OF) = 0.016), whereas the closed form (CF) does not. Fluorescence is switched upon alternate UV/visible irradiation of 2. The emission band of 2-OF matches the absorption band of 1 (400-550 nm), whereas the emission band of 1 overlaps the absorption band of 2-CF (550-700 nm). Therefore, a photoreversible two-way excitation energy transfer (EET), controlled by the state of the photochromic moiety, is obtained in the dyad 3. Their efficiencies are quantified as Φ(EET)(OF→F) = 85% and as Φ(EET)(F→CF) ~ 100% from the comparison of emission and excitation spectra between 1, 2, and 3. These results are fully compatible with the shortening of fluorescence lifetimes (from τ(-OF) = 70 ps and 170 ps essentially to τ(-OF) < 10 ps) and to the values of Förster radii determined for 3 (R(0)(OF → F) = 29 Å and R(0)(F → CF) = 71 Å), evidencing a Förster-type resonance energy transfer mechanism (FRET). An important outcome of this two-way FRET is the possibility to quench 49% of the fluorescence in 3 at PSS upon UV irradiation, corresponding to the conversion extent of the photochromic reaction, which is different from 2 (α(CF) = 91%). This is a clear example of a situation where the presence of FRET between the photochromic unit and the fluorophore affects noticeably the photochromic properties of the dyad molecule 3.

Topological versatility of oxalate-based bimetallic one-dimensional (1D) compounds associated with ammonium cations.

A new family of oxalate-bridged chains of formula (C(1))[Mn(H(2)O)(3)Cr(ox)(3)]·H(2)O (1), (C(2))(4)[Mn(2)(H(2)O)(3)ClCr(2)(ox)(6)]Cl·H(2)O·2C(2)H(6)O (2a), (C(2))(4)[Co(2)(H(2)O)(3)ClCr(2)(ox)(6)]Cl·2H(2)O·2C(2)H(6)O (2b), [Mn(C(3))(H(2)O)(2)Cr(ox)(3)]·H(2)O (3), and (C(4))(4)[Mn(H(2)O){Cr(ox)(3)}(2)]·H(2)O (4) [C(1)(+) = tetramethylammonium, C(2)(+) = 4-N,N-dimethylaminopyridinium, C(3)(+) = 1-hydroxyethyl-4-N,N-dimethylamino-pyridinium, C(4)(+) = 1-hydroxyethyl-4-(4'-dimethylamino-α-styryl)-pyridinium, ox(2-) = oxalate] have been synthesized by self-assembly of the (C(n))(3)[Cr(ox)(3)] (n = 1-4) mononuclear compound and the chloride salts of the corresponding metal(II) ions. The crystal structures of the five chain compounds have been determined by single-crystal X-ray diffraction. Compounds 1 and 2 crystallize in the Pc and P2(1)/c centrosymmetrical space groups, respectively, whereas 3 and 4 crystallize in the C2cb and P1 noncentrosymmetrical space groups, respectively. Compounds 1, 2, and 3 adopt a zigzag chain structure while 4 exhibits a comb-like chain structure consisting of the repetition of the [Mn(H(2)O){Cr(µ-ox)(ox)(2)}{Cr(µ-ox)(2)(ox)}](4-) entities. Compound 3 displays large second-order optical nonlinearity. The magnetic properties of 1-4 have been investigated in the temperature range 2-300 K. Monte Carlo simulations on 1, 2a, 2b, and 3 provide a quantitative description of the magnetic properties indicating ferromagnetic interactions through the bis(bidentate) oxalate bridges [J = +0.55 cm(-1) (1), J = +1.02 cm(-1) (2a), J = +3.83 cm(-1) (2b), and J = +0.75 cm(-1) (3) using Hamiltonian H = -J(S(i)·S(j))]. On the other side, the fit of the magnetic susceptibility data of 4 by full-matrix diagonalization agrees with a ferromagnetic exchange interaction within the [Mn(H(2)O){Cr(µ-ox)(ox)(2)}{Cr(µ-ox)(2)(ox)}](4-) trinuclear units (J = +2.07 cm(-1)) antiferromagnetically coupled along the chain. Compound 2b exhibits a metamagnetic behavior, the value of the critical field being H(C) = 1000 G, due to the occurrence of weak interchain antiferromagnetic interactions.

Tunable double photochromism of a family of bis-DTE bipyridine ligands and their dipolar Zn complexes.

The photoinduced ring-closure/ring-opening reactions of a series of bis-dithienylethene derivatives, as free ligands and Zn(II)-complexes, are investigated by resorting to theoretical (time-dependent density functional theory) and kinetic analyses in solution. The originality of the system stems from the tunability of the photoreaction quantum yields and conversion yields as a function of the electronic structure. The latter could be varied by modifying the electron-donating character of the DTE-end substituents L(a-d) (o,o) (a, D = H; b, D = OMe; c, D = NMe(2); d, D = NBu(2)) and/or the Lewis character of the metal ion center L(a-d)ZnX(2) (o,o) (L(a-c), X = OAc; L(d), X = Cl). The orbital description of the doubly-open form (o,o) and half-closed form (o,c) predicts that double closure to the form (c,c) would occur using UV irradiation. Photokinetic studies on the complete series demonstrate that photocyclization proceeds following a sequential ring closure mechanism. They clearly point out distinct quantum yields for the first and second ring closures, the latter being characterized by a significantly lower value. Dramatic decrease in both the quantum yields of the ring-closure and ring-opening processes is demonstrated for the complex L(d)ZnCl(2) exhibiting the strongest charge-transfer character in the series investigated. These studies show that this series of DTE derivatives provides an efficient strategy to tune the photochromic properties through the combination of the electron-donor and electron-acceptor (D-A) groups.

Photochromic and reductive electrochemical switching of a dithiazolylethene with large redox modulation.

A new dipyridylthiazolylethene (1a) and its dicationic analogue (2a), with two N-methylated pyridyl rings, have been synthesized and structurally characterized. Due to the N-methylation of the pyridyl rings, 2 a displays not only very different photochromic properties, but also undergoes a reductive ring-closing reaction to generate its closed-ring isomer 2b. Careful electrochemical studies coupled with EPR spectroscopy show that this reductive ring-closing reaction takes place when 2a is two-electron reduced. DFT calculations suggest that such a ground-state electrocyclization is driven by a very large stabilization of the reduced closed-ring isomer 2b relative to the reduced open-ring isomer 2a. In addition, 2b exhibits two successive and reversible one-electron reductions at half-wave potentials of 0.04 and -0.14 V versus SCE and a redox modulation as large as 1 V is achieved when passing from 2a to 2b.

Probing photochromic properties by correlation of UV-visible and infra-red absorption spectroscopy: a case study with cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene.

Quantification of the relative composition of the isomers in a photochromic system at any irradiation time interval is a critical issue in determining absolute quantum yields. For this purpose, we have developed a simple and convenient protocol involving combination of UV-visible and infra-red absorption spectroscopy. Photochromic cyclization reaction of cis-l,2-dicyano-l,2-bis(2,4,5-trimethyl-3-thieny1)ethene (CMTE) is analyzed to demonstrate the efficiency of the proposed methodology. This approach is based on the fact that the two isomers show distinctive infra-red bands. Detailed investigations of the UV-visible and infra-red spectra of the mixture obtained at different irradiation times in CCl(4) supported by quantum chemical computations lead to the unambiguous estimation of molar absorption coefficients of the closed isomer (epsilon(CF) = 4650 L mol(-1) cm(-1) at 512 nm). It facilitates the first determination of absolute quantum yields of this reversible photochromic reaction in CCl(4) by fitting the UV-visible spectral data (Phi(OF-->CF) = 0.41 +/- 0.05 and Phi(CF-->OF) = 0.12 +/- 0.02 at 405 nm and 546 nm, respectively).

Second-order nonlinear optical properties of polyoxometalate salts of a chiral stilbazolium derivative.

The synthesis of nonlinear optical (NLO) active salts with stilbazolium derivatives and polyoxometalate (POM) counterions has been investigated. With known nonchiral stilbazolium derivatives, such as MOMS(+), compounds with centrosymmetric structures have been isolated, like for instance the centrosymmetric salt (MOMS)(4)[Mo(8)O(26)] (1), synthesized under hydrothermal conditions. A new chiral derivative of the known DAMS(+) molecules, named here CHIDAMS(+), has therefore been synthesized in order to force the crystallization of the hybrid ionic salts in noncentrosymmetric space groups. The CHIDAMS(+) cation has been crystallized under two polymorphic PF(6)(-) salts, (CHIDAMS)PF(6) (2a and 2b), and its reactivity with various POMs has been investigated. The ionic salt (CHIDAMS)(2)[Mo(5)O(13)(OEt)(4)(NO){Na(H(2)O)(0.5)(DMF)(0.5)}] (4) crystallizes in the noncentrosymmetric P2(1) group, but the push-pull axis of the CHIDAMS(+) cations adopts a quasi-antiparallel alignment. The ionic salt (CHIDAMS)(3)[PW(12)O(40)].2DMF (5) associating three CHIDAMS(+) cations and a PW(12)O(40)(3-) Keggin anion crystallizes also in the P2(1) space group, but the disposition of the cations in the solid state is far more favorable. Diffuse reflectance experiments have evidenced a charge transfer between the organic and inorganic components in 5, and Kurtz-Perry experiments show that this salt exhibits a second harmonic generation efficiency more than 10 times higher than those of the PF(6)(-) salts 2a and 2b, the hybrid salt 4, and all of the other NLO active POM molecular materials reported in the literature.

Light-Controlled Release and Uptake of Zinc Ions in Solution by a Photochromic Terthiazole-Based Ligand.

We have synthesized and fully characterized a photochromic zinc complex with a terphenylthiazole-based ligand with a salen-like cavity. The solution stability of the complex was found to be greatly dependent on the state of the photochromic ligand and an interesting photo-triggered release and uptake of zinc ions was found as well as monitored by its fluorescence. The contrasting stability difference of the zinc complex between its two isomeric states was rationalized by DFT calculations.

One- and two-photon absorption of a spiropyran-merocyanine system: experimental and theoretical studies.

We report on the nonlinear optical properties measurements and quantum-chemical calculations of a well-known photochromic system consisting of spiropyran and the merocyanine photoproduct. The study of nonlinear absorption and refraction properties of the molecules dissolved in chloroform were performed with the Z-scan technique, using femtosecond pulses in a wide range of wavelengths. Maxima in the two-photon absorption spectrum at 700 and 1050 nm were found for the merocyanine form, and the corresponding two-photon absorption cross section is 80 GM and 20 GM, respectively. The latter feature does not vanish completely in the nonlinear spectrum of the spiropyran form, possibly because of the existence of some photoconversion caused by the laser beam during the measurements. A nonlinear absorption peak at 900 nm is found in the spiropyran form with an effective cross section of about 20 GM; it is likely due to three-photon absorption or to absorption by some intermediate species. The experimental data are supported by calculations performed with the use of a hybrid quantum mechanics-molecular mechanics approach.

Syntheses, Crystal Structures, and NLO Properties of New Chiral Inorganic Chromophores for Second-Harmonic Generation.

A new chiral ligand (H(2)L) based on the condensation of diaminocyclohexane and 4-(diethylamino)salicylaldehyde is reported along with its nickel(II) and manganese(III) complexes. These compounds all crystallize in noncentrosymmetric space groups. Crystal data are as follows. For the ligand (C(28)H(40)N(4)O(2)): monoclinic, I2, a = 15.487(2) Å, b = 8.1988(6) Å, c = 20.958(3) Å, beta = 95.92(1) degrees, Z = 4. For the Ni(II)L complex: monoclinic, P2(1), a = 8.613(2) Å, b = 21.714(4) Å, c = 15.870(4) Å, beta = 91.02(2) degrees, Z = 4. For the Mn(III)LCl complex: orthorhombic, P2(1)2(1)2(1), a = 7.233(2) Å, b = 15.159(4) Å, c = 25.591(7) Å, Z = 4. The manganese derivative exhibits an efficiency 8 times that of urea in second-harmonic generation at 1.9 &mgr;m. INDO/SCI-SOS quantum-chemical calculations predict a sizable molecular nonlinear response and an enhancement of the nonlinearity after metal complexation.