Up-converted emission in a series of phenylazomethine dendrimers with a porphyrin core

Metallopolymers for advanced sustainable applications

Since the development of metallopolymers, there has been tremendous interest in the applications of this type of materials. The interest in these materials stems from their potential use in industry as catalysts, biomedical agents in healthcare, energy storage and production as well as climate change mitigation. The past two decades have clearly shown exponential growth in the development of many new classes of metallopolymers that address these issues. Today, metallopolymers are considered to be at the forefront for discovering new and sustainable heterogeneous catalysts, therapeutics for drug-resistant diseases, energy storage and photovoltaics, molecular barometers and thermometers, as well as carbon dioxide sequesters. The focus of this review is to highlight the advances in design of metallopolymers with specific sustainable applications.

Assembly, growth and nonlinear thermo-optical properties of nitropeptides

The molecular self-assembly, growth and nonlinear thermo-optical properties of three synthetic aromatic–aliphatic hybrid nitropeptides have been investigated. The X-ray crystallography of nitropeptide 2 containing a glutamic acid moiety shows that the peptide adopts a dimeric structure using intermolecular hydrogen bonding as well as face to face π–π stacking interactions. Moreover, nitropeptide 2 exhibits nonlocal nonlinear optical properties. When a Gaussian laser beam passes through nitropeptide 2, the peptide shows several concentric rings due to spatial self-phase modulation (SSPM). However, the homologous peptide 1 containing an aspartic acid moiety and peptide 3 containing an achiral α-aminoisobutyric acid (Aib) moiety adopt sheet-like structures and have no self-phase modulation effect. The report describes the thermo-optical properties consistent with assumption and calculation and is promising for their applications in nonlinear optical modulation devices.

Theoretical investigations on one- and two-photon absorptions for a series of covalently functionalized hybrid materials based on graphene

Graphene-based (Gn) hybrids with porphyrin possess improved solubility and good nonlinear optical properties, especially excellent two-photon absorption (TPA) features.

Dendritic upconverting nanoparticles enable in vivo multiphoton microscopy with low-power continuous wave sources

We report a group of optical imaging probes, comprising upconverting lanthanide nanoparticles (UCNPs) and polyanionic dendrimers. Dendrimers with rigid cores and multiple carboxylate groups at the periphery are able to tightly bind to surfaces of UCNPs pretreated with NOBF(4), yielding stable, water-soluble, biocompatible nanomaterials. Unlike conventional linear polymers, dendrimers adhere to UCNPs by donating only a fraction of their peripheral groups to the UCNP-surface interactions. The remaining termini make up an interface between the nanoparticle and the aqueous phase, enhancing solubility and offering multiple possibilities for subsequent modification. Using optical probes as dendrimer cores makes it possible to couple the UCNPs signal to analyte-sensitive detection via UCNP-to-chromophore excitation energy transfer (EET). As an example, we demonstrate that UCNPs modified with porphyrin-dendrimers can operate as upconverting ratiometric pH nanosensors. Dendritic UCNPs possess excellent photostability, solubility, and biocompatibility, which make them directly suitable for in vivo imaging. Polyglutamic dendritic UCNPs injected in the blood of a mouse allowed mapping of the cortical vasculature down to 400 μm under the tissue surface, thus demonstrating feasibility of in vivo high-resolution two-photon microscopy with continuous wave (CW) excitation sources. Dendrimerization as a method of solubilization of UCNPs opens up numerous possibilities for use of these unique agents in biological imaging and sensing.

Self-assembled monolayers of metal-assembling dendron thiolate formed from dendrimers with a disulfide core

Novel phenylazomethine dendrimers with a disulfide core (SS-DPA G1-4) were synthesized in nearly quantitative yields. Although the disulfide core is shielded by the rigid dendron shell, direct formation of the self-assembled monolayers of metal-assembling dendron thiolate was observed by XPS and electrochemical reduction of the self-assembled monolayer substrates. The dendrimers showed a similar metal-assembling manner with other derivatives. The metal assembly to the self-assembled monolayers of metal-assembling dendron thiolate was also confirmed.

Photochemical conversion of solar energy

Energy is the most important issue of the 21st century. About 85% of our energy comes from fossil fuels, a finite resource unevenly distributed beneath the Earth's surface. Reserves of fossil fuels are progressively decreasing, and their continued use produces harmful effects such as pollution that threatens human health and greenhouse gases associated with global warming. Prompt global action to solve the energy crisis is therefore needed. To pursue such an action, we are urged to save energy and to use energy in more efficient ways, but we are also forced to find alternative energy sources, the most convenient of which is solar energy for several reasons. The sun continuously provides the Earth with a huge amount of energy, fairly distributed all over the world. Its enormous potential as a clean, abundant, and economical energy source, however, cannot be exploited unless it is converted into useful forms of energy. This Review starts with a brief description of the mechanism at the basis of the natural photosynthesis and, then, reports the results obtained so far in the field of photochemical conversion of solar energy. The "grand challenge" for chemists is to find a convenient means for artificial conversion of solar energy into fuels. If chemists succeed to create an artificial photosynthetic process, "... life and civilization will continue as long as the sun shines!", as the Italian scientist Giacomo Ciamician forecast almost one hundred years ago.

Energy and electron transfer in enhanced two-photon-absorbing systems with triplet cores

Enhanced two-photon-absorbing (2PA) systems with triplet cores are currently under scrutiny for several biomedical applications, including photodynamic therapy (PDT) and two-photon microscopy of oxygen. The performance of so far developed molecules, however, is substantially below expected. In this study we take a detailed look at the processes occurring in these systems and propose ways to improve their performance. We focus on the interchromophore distance tuning as a means for optimization of two-photon sensors for oxygen. In these constructs, energy transfer from several 2PA chromophores is used to enhance the effective 2PA cross section of phosphorescent metalloporphyrins. Previous studies have indicated that intramolecular electron transfer (ET) can act as an effective quencher of phosphorescence, decreasing the overall sensor efficiency. We studied the interplay between 2PA, energy transfer, electron transfer, and phosphorescence emission using Rhodamine B-Pt tetrabenzoporphyrin (RhB-PtTBP) adducts as model compounds. 2PA cross sections (sigma2) of tetrabenzoporphyrins (TBPs) are in the range of several tens of GM units (near 800 nm), making TBPs superior 2PA chromophores compared to regular porphyrins (sigma2 values typically 1-2 GM). Relatively large 2PA cross sections of rhodamines (about 200 GM in 800-850 nm range) and their high photostabilities make them good candidates as 2PA antennae. Fluorescence of Rhodamine B (lambda(fl) = 590 nm, phi(fl) = 0.5 in EtOH) overlaps with the Q-band of phosphorescent PtTBP (lambda(abs) = 615 nm, epsilon = 98 000 M(-1) cm(-1), phi(p) approximately 0.1), suggesting that a significant amplification of the 2PA-induced phosphorescence via fluorescence resonance energy transfer (FRET) might occur. However, most of the excitation energy in RhB-PtTBP assemblies is consumed in several intramolecular ET processes. By installing rigid nonconducting decaproline spacers (Pro10) between RhB and PtTBP, the intramolecular ETs were suppressed, while the chromophores were kept within the Förster r0 distance in order to maintain high FRET efficiency. The resulting assemblies exhibit linear amplification of their 2PA-induced phosphorescence upon increase in the number of 2PA antenna chromophores and show high oxygen sensitivity. We also have found that PtTBPs possess unexpectedly strong forbidden S0 --> T1 bands (lambda(max) = 762 nm, epsilon = 120 M-1 cm-1). The latter may overlap with the laser spectrum and lead to unwanted linear excitation.

Entangled photon absorption in an organic porphyrin dendrimer

Two-photon absorption spectroscopy is an intensity dependent nonlinear effect related to the excitation of virtual intermediate states. The classical two-photon absorption has an extremely low efficiency which is quantified by its cross-section (delta approximately 10(-48) cm4 s at 800 nm). To overcome this limitation, we demonstrate a novel effect of the two-photon absorption method utilizing the high degree of quantum optical correlation between photon pairs created by the process of spontaneous parametric downconversion. A large entangled two-photon absorption cross-section (delta(e) approximately 10(-17) cm2 at 800 nm) was measured in an organic porphyrin dendrimer. We also discuss the nonmonotonic behavior of variation of the entangled two-photon absorption cross-section by controlling the entanglement time. This novel effect may open new avenues for ultrasensitive detection in chemical and biological systems. TPA spectroscopy has been considered as a powerful tool in physics, chemistry, and biology. The inherent nonlinear process of the classical TPA is distinguishable from the single photon absorption (SPA) linear process. Although the benefits of greater penetration depth and better control and reduction of scattering, the TPA spectroscopy has been restricted by the necessity of a high power optical source due to the low efficiency of the TPA effect. The use of entangled photons from a correlated source for the purpose of the two-photon effect is promising in this regard as one may obtain two-photon effects with very small numbers of photons.

Strong Two-Photon Absorption in New Asymmetrically Substituted Porphyrins: Interference between Charge-Transfer and Intermediate-Resonance Pathways

We study two-photon absorption (2PA) in two series of new free-base porphyrins with 4-(diphenylamino)stilbene or 4,4'-bis-(diphenylamino)stilbene (BDPAS) attached via pi-conjugating linkers at the porphyrin meso-position. We show that this new substitution modality increases the 2PA cross section in the Soret band region (excitation wavelength 750-900 nm) of the core porphyrin by nearly 2 orders of magnitude, from sigma(2) approximately 10 GM for the meso-phenyl-substituted analogue to sigma(2) approximately 10(3) GM for the ethynyl-linked BDPAS-porphyrin dyad. The 2PA properties are quantitatively described by considering two different and interfering 2PA quantum transition pathways. The first path involves virtual transition via intermediate one-photon resonance. The second path bypasses the intermediate resonance and occurs due to a large permanent dipole moment difference between the ground and the final electronic states. To our best knowledge, this is the first experimental observation of the combined effect of these two pathways on one particular two-photon transition, resulting in quantum-interference-modulated 2PA strength.