The role of surface plasmon in second harmonic generation from arrays of gold nanorod particles excited by femtosecond laser pulses is investigated as a function of incident light polarization and irradiation wavelength. In addition to photoluminescence, a peak of second harmonic is observed and is found to depend on the polarization and wavelength of the fundamental frequency laser beam. In particular, the authors found similarities between extinction spectra of the nanoparticles and spectra of emmitted second harmonic. This behavior can be explained by resonant excitation of localized surface plasmon resonances.

The rod-like molecule bis((4-(4-pyridyl)ethynyl)bicyclo[2.2.2]oct-1-yl)buta-1,3-diyne, 1, contains two 1,4-bis(ethynyl)bicyclo[2.2.2]octane (BCO) chiral rotators linked by a diyne fragment and self-assembles in a one-dimensional, monoclinic C2/c centrosymmetric structure where two equilibrium positions with large occupancy imbalance (88% versus 12%) are identified on a single rotor site. Combining variable-temperature (70-300 K) proton spin-lattice relaxation, (1)H T1(-1), at two different (1)H Larmor frequencies (55 and 210 MHz) and DFT calculations of rotational barriers, we were able to assign two types of Brownian rotators with different activation energies, 1.85 and 6.1 kcal mol(-1), to the two (1)H spin-lattice relaxation processes on the single rotor site. On the basis of DFT calculations, the low-energy process has been assigned to adjacent rotors in a well-correlated synchronous motion, whereas the high-energy process is the manifestation of an abrupt change in their kinematics once two blades of adjacent rotors are seen to rub together. Although crystals of 1 should be second harmonic inactive, a large second-order optical response is recorded when the electric field oscillates in a direction parallel to the unique rotor axle director. We conclude that conformational mutations by torsional interconversion of the three blades of the BCO units break space-inversion symmetry in sequences of mutamers in dynamic equilibrium in the crystal in domains at a mesoscopic scale comparable with the wavelength of light used. A control experiment was performed with a crystalline film of a similar tetrayne molecule, 1,4-bis(3-((trimethylsilyl)ethynyl)bicyclo[1.1.1]pent-1-yl)buta-1,3-diyne, whose bicyclopentane units can rotate but are achiral and produce no second-order optical response.

A functionalized polymer film allowing for a complete and straightforward second-harmonic generation (SHG)-assisted high-contrast writing-reading-erasing-writing sequence is proposed. The whole process is supported by the reversible photoinduced dimerization of a coumarin chromophore and enables efficient optical data storage that can be detected only by SHG imaging.

A small push-pull molecule involving a diphenylamine substituted by an oligo-oxyethylene chain is described. The compound exhibits aggregation-induced emission with solvent-dependent emission wavelength. Spin-cast deep-red amorphous films rapidly self-reorganize into colorless crystalline films which exhibit mechanofluorochromism and aggregation-induced second-harmonic generation.

We propose to encode optical information through the localized depoling of polar chromophores in thin films of grafted polymeric materials with a femtosecond near IR laser source. This disorientation is promoted through the photoisomerization of the azo-dye component induced by a twophoton absorption process. We show that the resulting localized loss in second harmonic generation efficiency can be exploited in data storage applications. The low irradiation powers used allow for a recycling by reheating and repoling the films leading to a rewritable system.

We report on a crystalline rotor that undergoes a reversible phase transition at 145 K. Variable-temperature X-ray and 1 H spin−lattice relaxation experiments, and calculations of rotational barriers, provide a description (i) of the way in which the rotators' dynamics changes back and forth at the onset of the phase transition and (ii) of the mechanism responsible for the abrupt switching of the crystalline rotors from a very low-energy 4-fold degenerate equilibrium state, in which the rotation is ultrafast (9.6 GHz at 145 K), to a single higher-energy state associated with a slower motion (2.3 GHz at 145 K). Our results provide evidence that the reversible change observed in the rotational barriers at the transition is due to a cooperative modulation of the C−H rotator •••I stator hydrogen bond cloud across a C−I stator •••I stator −C halogen bond-mediated phase transition. In addition, we report evidence for second-harmonic generation from this material, thereby confirming with a second example the benefit of using polarized light to probe the torsional degree of freedom of chiral helix blades, as well as symmetry and dimensionality of large collections of chiral rotors in the solid state.

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Preparation of the Self-Assembled T1 Nanofibril: T1 was synthesized by a solid-phase synthetic method based on Fmoc chemistry. The first amino acid Phe was loaded to Wang resin (substitution, 0.8 mmol g ±1 ) by the preformed symmetrical anhydride method using N,N-dimethylaminopyridine as a catalyst. 2-(1H-Benzotriazol-1-yl)-1,1,3,3-tetramethyl uranium hexafluorophosphate/1-hydroxy-benzotrizole (HBTU/HOBT) was employed as the coupling reagent. The peptide product was cleaved from the resin using K reagent (82.5 % TFA, 5 % thioanisole, 2.5 % ethanedithiol, 5 % phenol, and 5 % H 2 O), purified by RP-HPLC (Gilson Inc., France, zorbax C 18 column, 9.4 250 mm), and confirmed by MALDI-TOF mass spectrometry ([M+H]: observed 1562.0/ calc. 1558.7).Incubation of T1 in phosphate buffered saline (PBS) at 37 C for 2 weeks resulted in T1 fibril as confirmed by TEM. The TEM sample was prepared by transferring a colloidal solution of T1 fibril onto a TEM grid coated with a carbon film. After drying, the sample was stained with uranyl acetate.Preparation of Au Colloids: A Au colloidal solution was prepared according to the procedure described by Brown et al. [19]. 0.5 mL of a HAuCl 4 solution (2 %) and 2 mL of a Na 3 citrate solution (38.8 mM) were added to 90 mL of H 2 O. After about 1 min stirring, 1 mL of freshly prepared NaBH 4 /Na 3 citrate aqueous solution (13 mg NaBH 4 in 5 mL of a Na 3 citrate solution (38.8 mM)) was added. A transparent ruby-red colloidal solution formed immediately, which was stirred for a further 30 min and stored at 4 C. The pH value of the prepared Au colloidal solution was about 6. The UV-vis absorption spectrum of the Au sol exhibited a surface plasmon band at 522 nm.Assembly of Au Nanoparticles on T1 Fibril: The preformed Au nanoparticles were attached to the T1 fibril template by a simple procedure. A typical experiment involved adding 5 lL of T1 fibril colloidal solution into 0.5 mL of the Au colloidal solution. After keeping the mixture at 4 C for 24 h, a red precipitate appeared, indicating that Au nanoparticles had been attached onto the T1 fibrils. The precipitate was isolated by centrifuging at 4000 rpm for 30 min and then dispersed in H 2 O by ultrasonic treatment for several seconds. The obtained suspension was used in the preparation of TEM samples. To investigate the effect of pH on the structure of the Au nanoparticle assembly, the pH value of the obtained mixed colloidal solution was adjusted to pH 3.5, 4.5, 5.4, 6.5, and 9 by adding different amounts of a H 3 citrate solution (0.1 M) or a NaOH solution (0.1 M).Assembly of Pd Nanoparticles onto T1 Fibril: To assemble Pd nanoparticles, a different strategy was adopted. A saturated aqueous solution of PdCl 2 was prepared by adding excessive amount of PdCl 2 in ultra-pure water and then treating the mixture in an ultrasonic bath for 10 min. The mixture was centrifuged for 10 min at 4000 rpm to separate undissolved PdCl 2 . The obtained PdCl 2 solution was mixed with the colloidal solution of T1 fibril in a ratio of 1:1 and the...