Supramolecular porphyrin as an improved photocatalyst for chloroform decomposition.

In this work, the outlying decoration of the free-base meso-(4-tetra) pyridyl porphyrin (H2TPyP) with the RuCl(dppb)(5,5'-Me-bipy) ruthenium complex (here named Supra-H2TPyP) is observed as an improved molecular photocatalyst for dye-mediated chloroform (CHCl3) decomposition via one-photon absorption operating in the visible spectral range (532 nm and 645 nm). Supra-H2TPyP offers a better option for CHCl3 photodecomposition when compared to the same process mediated by pristine H2TPyP, which requires either excited-state- or UV absorption. The chloroform photodecomposition rates for Supra-H2TPyP as well as its excitation mechanisms are explored as a function of distinct laser irradiation conditions.

Photophysics and visible light photodissociation of supramolecular meso-tetra(4-pyridyl) porphyrin/RuCl2(CO)(PPh3)2 structures.

In the last decades, supramolecular structures have been explored in many technological efforts. One example of such supramolecules is attained when ruthenium complexes are attached in the outer sites of a porphyrin. Ruthenium complexes act as modulators of the photophysical processes of macrocyclic molecules. Besides the investigation of the main changes introduced by the ruthenium complexes in the electronic and vibronic properties, and in the excited state deactivation processes of porphyrins, discussions concerning the photostability of these supramolecules are much needed. Here, we investigate the supramolecular free-base meso-tetra(4-pyridyl) porphyrin decorated with "RuCl2(CO)(PPh3)2" ruthenium species linked at each of its (4-pyridyl) moieties. The modifications in the photophysical processes introduced by the metallic outlying species are discussed and our results suggest an energy transfer process from the porphyrin B-band to the ruthenium complex MLCT-band. The demonstration of visible light photodissociation of the supramolecule, via both pulsed and continuous laser, is also addressed.

Influence of the meso-substituents on the spectral features of free-base porphyrin.

The role of meso-substituents on the spectral features of free-base porphyrins is explored. Meso-tetra(4-pyridyl)porphyrin is compared with meso-tetra(2-thienyl)porphyrin and meso-tetra(pentafluorophenyl)porphyrin. Our results indicate that some of the asymmetric Q-bands in the free-base porphyrin tend to become symmetric relative to the adopted meso-substituent. The results show that the outlying perturbations lead the free-base quasi-degenerated Qx1, Qx2, Qy1, and Qy2 bands to be closer in energy. Combined, absorption, fluorescence and Raman spectroscopies endorse our conclusions showing that both the frequencies and the Huang-Rhys factors associated with every vibronic progression are noticeably affected by the investigated meso-substituents. Our results confirm that the B-band is also multi-featured in agreement with what is found for the Q-bands.

On the excitation dependence of fluorescence spectra of meso-tetrapyridyl zinc (II) porphyrin and its relation with hydrogen bonding and outlying decoration.

Zinc porphyrins are potential candidates for boosting the advancement of various technological applications, including those exploring the molecule's radiative emissions. In this work, the excitation dependence of fluorescence spectra from 5,10,15,20-meso-tetrapyridyl zinc(II) porphyrin dissolved in a binary solvent mixture of CHCl3: MeOH, is reported. Important modifications in the profiles of the fluorescence bands are observed after exciting the molecules in a broad wavelength range from 350 to 565 nm. We attribute such modifications to the existence of two distinct relaxation pathways, related to two quasi-degenerated potential energy surfaces (PES) in the ZnTPyP's first excited state whose population rates changes for different excitation wavelengths. We also observed that by changing the CHCl3:MeOH proportion in the binary mixture, a quenching mechanism mediated by the MeOH hydrogen bondings and ZnTPyP takes place, which allows for tuning the excitation dependence of the aforementioned relaxations pathways. Moreover, our data confirm that the addition of outlying RuCl(dppb)(bipy) ruthenium complex linked to the pyridyl moieties of the ZnTPyP ring is also an excellent strategy to modify the excitation dependence of the fluorescence relaxation pathways.

Evolution of electronic and vibronic transitions in metal(II) meso-tetra(4-pyridyl)porphyrins.

The changing of the electronic and vibronic states due to the insertion of Zn(II), Cu(II), Ni(II) or Co(II) ions in the meso-tetrakis(4-pyridyl)porphyrin ring center is investigated. The combination of absorption, photoluminescence, Raman and infrared spectroscopies with second-derivative-based spectral deconvolution analysis reveals that the structuration of both B- and Q-bands is very sensitive to the decorating ion. Similar to free base porphyrins, metal(II) meso-tetra(4-pyridyl)porphyrins also present their Q-band constituted of multiple electronic transitions, where the central ion plays an important role in the selection of vibration modes that mediate the vibronic transitions. Our novel results will expand and reinterpret current assignments for metal(II) meso-tetra(4-pyridyl)porphyrins vibrational modes available in the literature.

Novel insights on the vibronic transitions in free base meso-tetrapyridyl porphyrin.

We present novel results on the free base 5,10,15,20-meso-tetra(pyridyl)-21H,23H-porphyrin (H2TPyP). This molecule presents complex electronic and vibrational properties and despite the vast literature reporting the transitions observed in its absorption and fluorescence spectra, a more accurate interpretation has been kept elusive. In particular, we show that the molecule's Q-band develops into many electronic and vibronic transitions, whose the well-known "four orbital model" finds it difficult to reconcile. Using distinct spectroscopy techniques, we conclude that both Qx- and Qy-bands comprise, in fact, two quasi-degenerated electronic states together with their respective vibronic progressions each. The analysis of the Huang-Rhys factors and complementary time- and polarization-resolved measurements reinforce the need for the proposed Q-band multi features remodeling.

Synthesis of patched or stacked graphene and hBN flakes: a route to hybrid structure discovery.

Two-dimensional (2D) materials such as graphene and hexagonal boron nitride (hBN) have attracted significant attention due to their remarkable properties. Numerous interesting graphene/hBN hybrid structures have been proposed but their implementation has been very limited. In this work, the synthesis of patched structures through consecutive chemical vapor deposition (CVD) on the same substrate was investigated. Both in-plane junctions and stacked layers were obtained. For stacked layers, depending on the synthesis sequence, in one case turbostratic stacking with random rotations were obtained. In another, "AA-like", slightly twisted stacking between graphene and hBN was observed with lattice orientation misalignment consistently to be <1°. Raman characterizations not only confirmed that hBN is a superior substrate but also revealed for the first time that a graphene edge with hBN passivation displays reduced D band intensity compared to an open edge. These studies pave the way for the proposed well-ordered graphene/hBN structures and outline exciting future directions for hybrid 2D materials.

Phonon self-energy corrections to nonzero wave-vector phonon modes in single-layer graphene.

Phonon self-energy corrections have mostly been studied theoretically and experimentally for phonon modes with zone-center (q=0) wave vectors. Here, gate-modulated Raman scattering is used to study phonons of a single layer of graphene originating from a double-resonant Raman process with q≠0. The observed phonon renormalization effects are different from what is observed for the zone-center q=0 case. To explain our experimental findings, we explored the phonon self-energy for the phonons with nonzero wave vectors (q≠0) in single-layer graphene in which the frequencies and decay widths are expected to behave oppositely to the behavior observed in the corresponding zone-center q=0 processes. Within this framework, we resolve the identification of the phonon modes contributing to the G(⋆) Raman feature at 2450 cm(-1) to include the iTO+LA combination modes with q≠0 and also the 2iTO overtone modes with q=0, showing both to be associated with wave vectors near the high symmetry point K in the Brillouin zone.

In situ atomic force microscopy tip-induced deformations and Raman spectroscopy characterization of single-wall carbon nanotubes.

In this work, an atomic force microscope (AFM) is combined with a confocal Raman spectroscopy setup to follow in situ the evolution of the G-band feature of isolated single-wall carbon nanotubes (SWNTs) under transverse deformation. The SWNTs are pressed by a gold AFM tip against the substrate where they are sitting. From eight deformed SWNTs, five exhibit an overall decrease in the Raman signal intensity, while three exhibit vibrational changes related to the circumferential symmetry breaking. Our results reveal chirality dependent effects, which are averaged out in SWNT bundle measurements, including a previously elusive mode symmetry breaking that is here explored using molecular dynamics calculations.

Using the G' Raman cross-section to understand the phonon dynamics in bilayer graphene systems.

The G' (or 2D) Raman band of AB stacked bilayer graphene comes from a double resonance Raman (DRR) process and is composed of four peaks (P(11), P(12), P(21), and P(22)). In this work, the integrated areas (IA) of these four peaks are analyzed as a function of the laser power for different laser lines. We show that the dependence of the IA of each peak on temperature is different for each distinct laser excitation energy. This special dependence is explained in terms of the electron-phonon coupling and the relaxation of the photon-excited electron. In this DRR process, the electron is scattered by an iTO phonon from a K to an inequivalent K' point of the Brillouin zone. Here, we show that this electron relaxes while in the conduction band before being scattered by an iTO phonon due to the short relaxation time of the excited electron, and the carrier relaxation occurs predominantly by emitting a low-energy acoustic phonon. The different combinations of relaxation processes determine the relative intensities of the four peaks that give rise to the G' band. Some peaks show an increase of their IA at the expense of others, thereby making the IA of the peaks both different from each other and dependent on laser excitation energy and on power level. Also, we report that the IA of the G' mode excited at 532 nm, shows a resonance regime involving ZO' phonons (related to the interlayer breathing mode in bilayer graphene systems) in which a saturation of what we call the P(12) process occurs. This effect gives important information about the electron and phonon dynamics and needs to be taken into account for certain applications of bilayer graphene in the field of nanotechnology.

Diameter dependence of the dielectric constant for the excitonic transition energy of single-wall carbon nanotubes.

The measured optical transition energies Eii of single-wall carbon nanotubes are compared with bright exciton energy calculations. The Eii differences between experiment and theory are minimized by considering a diameter-dependent dielectric constant kappa, which comprises the screening from the tube and from the environment. Different kappa dependencies are obtained for (E11(S), E22(S), E11(M)) relative to (E33(S), E44(S)). A changing environment changes the kappa diameter dependence for (E11(S), E22(S), E11(M)), but for (E33(S), E44(S)) the environmental effects are minimal. The resulting calculated exciton energies reproduce experimental E_{ii} values within +/-70 meV for a diameter range (0.7

Deformation induced semiconductor-metal transition in single wall carbon nanotubes probed by electric force microscopy.

We report the direct experimental observation of the semiconductor-metal transition in single-wall carbon nanotubes (SWNTs) induced by compression with the tip of an atomic force microscope. This transition is probed via electric force microscopy by monitoring SWNT charge storage. Experimental data show that such charge storage is different for metallic and semiconducting SWNTs, with the latter presenting a strong dependence on the tip-SWNT force during injection. Ab initio calculations corroborate experimental observations and their interpretation.