Nonlinear Optical Properties of Ferrocene- and Porphyrin–[60]Fullerene Dyads

Photoinduced Charge Transfer in Single-Molecule p-n Junctions

We measured photoinduced charge separation in isolated individual C₆₀-tethered 2,5-dithienylpyrrole triad (C₆₀ triad) molecules with submolecular resolution using a custom-built laser-assisted scanning tunneling microscope. Laser illumination was introduced evanescently into the tunneling junction through total internal reflection, and the changes in tunneling current and electronic spectra caused by photoexcitation were measured and spatially resolved. Photoinduced charge separation was not detected for all C₆₀ triad molecules, indicating that the conformations of the molecules may affect the excitation probability, lifetime, and/or charge distribution. A photoinduced signal was not observed for dodecanethiol molecules in the surrounding matrix or for control molecules without C₆₀ moieties, as neither absorbs incident photons at this energy. This spectroscopic imaging technique has the potential to elucidate detailed photoinduced carrier dynamics, which are inaccessible via ensemble-scale (i.e., averaging) measurements, which can be used to direct the rational design and optimization of molecular p-n junctions and assemblies for energy harvesting.

Tailoring Colors by O Annulation of Polycyclic Aromatic Hydrocarbons

The synthesis of O-doped polyaromatic hydro- carbons in which two polycyclic aromatic hydrocarbon sub units are bridged through one or two O atoms has been achieved. This includes high-yield ring-closure key steps that, depending on the reaction conditions, result in the formation of furanyl or pyranopyranyl linkages through intramolecular C-O bond formation. Comprehensive photophysical measurements in solution showed that these compounds have exceptionally high emission yields and tunable absorption properties throughout the UV/Vis spectral region. Electrochemical investigations showed that in all cases O annulation increases the electron-donor capabilities by raising the HOMO energy level, whereas the LUMO energy level is less affected. Moreover, third-order nonlinear optical (NLO) measurements on solutions or thin films containing the dyes showed very good values of the second hyperpolarizability. Importantly, poly(methyl methacrylate) films containing the pyranopyranyl derivatives exhibited weak linear absorption and NLO absorption compared to the nonlinearity and NLO refraction, respectively, and thus revealed them to be exceptional organic materials for photonic devices.

Conjugated Porphyrin Dimers: Cooperative Effects and Electronic Communication in Supramolecular Ensembles with C60

Two new conjugated porphyrin-based systems (dimers 3 and 4) endowed with suitable crown ethers have been synthesized as receptors for a fullerene-ammonium salt derivative (1). Association constants in solution have been determined by UV–vis titration experiments in CH₂Cl₂ at room temperature. The designed hosts are able to associate up to two fullerene-based guest molecules and present association constants as high as ∼5 × 10⁸ M^–1. Calculation of the allosteric cooperative factor α for supramolecular complexes [3·1₂] and [4·1₂] showed a negative cooperative effect in both cases. The interactions accounting for the formation of the associates are based, first, on the complementary ammonium-crown ether interaction and, second, on the π–π interactions between the porphyrin rings and the C₆₀ moieties. Theoretical calculations have evidenced a significant decrease of the electron density in the porphyrin dimers 3 and 4 upon complexation of the first C₆₀ molecule, in good agreement with the negative cooperativity found in these systems. This negative effect is partially compensated by the stabilizing C₆₀–C₆₀ interactions that take place in the more stable syn-disposition of [4·1₂].

Effect of alkaline earth metal at the single wall CNT mouth on the electronic structure and second hyperpolarizability

In the present work, electronic structure and second hyperpolarizability of a number of alkaline earth metals (M [Formula: see text] Be, Mg and Ca) complexes with carbon nanotube (CNT) has been studied by using different DFT functional. The complexes have sufficient thermal stability. Significant amount of charge transfer from metal to CNT results in stronger ground state polarization. The second hyperpolarizability obtained at different DFT functional (BHHLYP, CAM-B3LYP, B2PLYP, [Formula: see text]B97XD) showed a consistent trend. The magnitude of second hyperpolarizability of M@CNT[3,0] complexes enhances rather appreciably when a second metal atom is introduced into other mouth position. The longitudinal component of second hyperpolarizability of M@CNT[3,0]@M complexes increases with increasing size of metal atom. The magnitude of second hyperpolarizability of Ca@CNT[3,0]@Ca complex is comparable with Fe([Formula: see text]-C[Formula: see text]B[Formula: see text]. However, widening/lengthening of CNT markedly reduces the cubic responses. The two state model can qualitatively explain the variation of second hyperpolarizability.

[60]Fullerene–porphyrin [n]pseudorotaxanes: self-assembly, photophysics and third-order NLO response

Herein we report a series of porphyrin and methano[60]fullerene that undergo self-assembly.

Ultrafast third order nonlinearities of organic solvents

The third order nonlinear optical parameters of a variety of organic solvents are determined under picosecond and femtosecond laser irradiation. Z-scan measurements have been carried out at 35 ps, 532 nm and at 40 fs, 800 nm, while Optical Kerr Effect studies were performed at 35 ps, 532 nm. The target of the present work is to provide a reference of the nonlinear optical response of various widely used organic solvents under the same experimental conditions and render feasible the direct comparison between their nonlinearities.

Static polarization of the supramolecular dyads of fullerene C60 with porphyrin derivatives

The nonlinear optical properties of four supramolecular dyads consisting of fullerene C 60 non-covalently bonded to porphyrin, porphyrazine, tetrabenzoporphyrin and phthalocyanine were investigated by calculating their electronic polarizability and first- and second-order hyperpolarizabilities using the finite field method and the density functional theory with the Grimme dispersion correction. Large first- and second-order hyperpolarizabilities result in nonlinear dependence of the polarization of dyads on the strength of external electric field. The increase in the size of the π-conjugated electron system of the porphyrin analogs leads to the increase of the polarizability and first- and second-order hyperpolarizabilities of the dyads. The absence of the covalent bonds between the components of the dyads prevents the field-induced electron transfer from porphyrin analogs to fullerene. The main reason for the nonlinear behavior of the polarization of dyads is the mutual polarization of fullerene and porphyrin analogs amplified by the external electric field.

Controlling the growth of porphyrin based nanostructures for tuning third-order NLO properties

A series of porphyrin nanospindles with controlled long axis length distributions of 330, 550, 800 nm, and 4 μm have been successfully fabricated via hierarchical self-assembly of cationic porphyrin (H₆TPyP)(4+) with the help of anionic surfactant sodium dodecyl sulfonate (SDS) due to the effective electrostatic interaction. These newly fabricated nanostructures are characterized by TEM and SEM techniques, powder X-ray diffraction analysis, electronic absorption spectroscopy, and confocal laser scanning microscopy (CLSM). The Z-scan technique with a laser duration of 5 ns at the wavelength of 532 nm reveals unreported size-dependent third-order NLO switching properties: the nonlinear absorption changes from saturation absorption to reversed saturation absorption and the nonlinear refraction from self-defocus to self-focus due to the change of the dominant scattering effect, from Rayleigh scattering for nanostructures with a smaller size than the wavelength of laser light, to Mie scattering for nanostructures with a larger size than the laser wavelength. This result is useful for the development of organic nanostructures with desired NLO properties, in particular the optical limiting properties.

Large excited state two photon absorptions in the near infrared region of surprisingly stable radical cations of (ferrocenyl)indenes

Multiphoton absorptions are important non-linear optical processes which allow us to explore excited states with low energy photons giving rise to new possibilities for photoinduced processes. Among these processes, multiphoton absorptions from excited states are particularly interesting because of the large susceptibilities characteristic of excited states. Here we explore the nonlinear transmission measurements recorded with 9 ns laser pulses at 1064 nm of the radical cations of (2-ferrocenyl)indene and of (2-ferrocenyl)-hexamethylindene, two interesting very stable molecules. The non-linear transmission data can be interpreted with a multiphoton sequence of three photon absorptions, the first being a one photon absorption related to the intramolecular charge transfer and the second a two photon absorption from the excited state created with the first process. The two photon absorption cross section is found to be several orders of magnitude larger than those usually found for two photon absorbing systems excited from the ground state.

ELECTRONIC STRUCTURES, EXCITED SPECTRA, AND NONLINEAR OPTICAL PROPERTIES OF N-METHYL–FULLEROPYRROLIDINE–OLIGOANILINE DYADS NMPC60–ANn (n = 1–5): QUANTUM CHEMISTRY STUDY

The geometrical and electronic structures of a series of dyads NMPC60–AN n (n = 1–5) were investigated at B3LYP/6-31G* level. Then, the excited states were calculated at TDB3LYP/3-21G* level to investigate the photophysical properties. The third-order polarizabilities of degenerate four-wave mixing (DFWM), electric-field-induced second-harmonic generation (EFISHG), and third-harmonic generation (THG) were calculated in combination with the sum-over-states (SOS) method. The obtained absorption spectra show a remarkable red shift. The average value of third-order polarizabilities 〈γ〉 away from the resonant region increases with n for NMPC60–AN n. The charge transfers from the oligoanilines to C60 cage make the main contribution to the large third-order polarizabilities of this series of compounds. It is found that 〈γ〉 can be expressed as a function of the chain length of linked oligoaniline and 11.74 × 10-34 esu is the upper limit after n = 7 for NMPC60–AN n in the static state.

Synthesis, strong two-photon absorption, and optical limiting properties of novel C(70)/C(60) derivatives containing various carbazole units

An approach was demonstrated toward the design and synthesis of a series of novel C(70) and C(60) derivatives for large two-photon absorption (TPA). The molecular structures of fullerene derivatives were confirmed by MALDI-MS, (1)H NMR, and FT-IR. With femtosecond open-aperture Z-scans and frequency-degenerate pump-probe measurements at 780 nm, the TPA cross sections of up to 3.47 x 10(-46), 1.64 x 10(-46), 1.1 x 10(-46), and 7.82 x 10(-47) cm(4) s photon(-1) molecule(-1) were determined for C(70)-TCTA, C(60)-TCTA, C(70)-BCzMB, and C(70)-MQEtCz in toluene with concentrations of 10(-4) M, respectively. The normalized light transmittances of solutions of these molecules were attenuated to the range between 33% and 50% for C(70)-TCTA, C(60)-TCTA, C(60)-BCzMB, and C(70)-MQEtCz as the input irradiance was increased to about 150 GW/cm(2), showing that they are effective optical limiters. Both intensity-dependent Z-scans and pump-probe experiments confirmed that the reduction in the light transmittance results mainly from the TPA process. In addition, the molecule concentration dependence of the TPA cross sections was also investigated. It was found that the TPA cross sections are extremely sensitive to the concentration with the greatest TPA cross-section of 1.0 x 10(-45) cm(4) s photon(-1) molecule(-1) for C(70)-TCTA measured in the low concentration regime ( approximately 10(-5) M).

Direct electron-beam writing of highly conductive wires in functionalized fullerene films

This work demonstrates the patterning of thin films (approximately 25 nm) of a newly synthesized fullerene derivative by direct-write electron-beam lithography to produce highly conducting carbon microstructures. Scanning electron microscopy and atomic force microscopy are used to characterize the resulting microstructure morphology, whilst the resistivities of the structures are probed using four-point probe electrodes deposited on the microstructures by lift-off. The microstructures have a resistivity of approximately 9.5 x 10(-3) Omega cm after exposure to an electron dose of 0.1 C cm(-2). The method may have applications in the generation and electrical contacting of organic electronics, organic photovoltaics, and lab-on-a-chip devices.

A carbon nano-onion-ferrocene donor-acceptor system: synthesis, characterization and properties

We describe the synthesis and characterization of a novel ferrocene-carbon onion derivative, where ferrocene acts as an electron-donating moiety, while the carbon nano-onion (CNO) serves as the electron acceptor. CNOs were functionalized by 1,3-dipolar cycloaddition and the resulting products were characterized by transmission electron microscopy, thermogravimetric analysis, Raman and energy dispersive spectroscopies. The electronic properties of the Fc-CNO derivative were investigated by electrochemical and photophysical techniques, complemented by quantum chemical calculations. On average, the CNOs have a spherical appearance with six shells. Functionalization saturates one carbon atom in 36 carbon atoms on the outer cage of the CNO. Through-space interactions between the Fc moiety and the CNO core were detected electrochemically. Fluorescence was observed at low and high energies with an intrinsic decay that is faster at lower energies. Based on theory and experiment, we conclude that, after absorption of a photon at low energy, there is emission from CNOs characterized by larger external shells and a lower degree of functionalization. At high energy, emission comes from CNOs with smaller external shells and a higher degree of functionalization.