Comparative Electrochemical and Photophysical Studies of Tetrathiafulvalene‐Annulated Porphyrins and Their ZnII Complexes: The Effect of Metalation and Structural Variation

A series of tetrathiafulvalene (TTF)‐annulated porphyrins, and their corresponding Zn^II complexes, have been synthesized. Detailed electrochemical, photophysical, and theoretical studies reveal the effects of intramolecular charge‐transfer transitions that originate from the TTF fragments to the macrocyclic core. The incremental synthetic addition of TTF moieties to the porphyrin core makes the species more susceptible to these charge‐transfer (CT) effects as evidenced by spectroscopic studies. On the other hand, regular positive shifts in the reduction signals are seen in the square‐wave voltammograms as the number of TTF subunits increases. Structural studies that involve the tetrakis‐substituted TTF–porphyrin (both free‐base and Zn^II complex) reveal only modest deviations from planarity. The effect of TTF substitution is thus ascribed to electronic overlap between annulated TTF subunits rather than steric effects. The directly linked thiafulvalene subunits function as both π acceptors as well as σ donors. Whereas σ donation accounts for the substituent‐dependent charge‐transfer transitions, it is the π‐acceptor nature of the appended tetrathiafulvalene groups that dominates the redox chemistry. Interactions between the subunits are also reflected in the square‐wave voltammograms. In the case of the free‐base derivatives that bear multiple TTF subunits, the neighboring TTF units, as well as the TTF ^{? +} generated through one‐electron oxidation, can interact with each other; this gives rise to multiple signals in the square‐wave voltammograms. On the other hand, after metalation, the electronic communication between the separate TTF moieties becomes restricted and they act as separate redox centers under conditions of oxidation. Thus only two signals, which correspond to TTF ^{. +} and TTF²⁺, are observed. The reduction potentials are also seen to shift towards more negative values after metalation, a finding that is considered to reflect an increased HOMO–LUMO gap. To probe the excited‐state dynamics and internal CT character, transient absorption spectral studies were performed. These analyses revealed that all the TTF–porphyrins of this study display relatively short excited‐state lifetimes, which range from 1 to 20 ps. This reflects a very fast decay to the ground state and is consistent with the proposed intramolecular charge‐transfer effects inferred from the ground‐state studies. Complementary DFT calculations provide a mechanistic rationale for the electron flow within the TTF–porphyrins and support the proposed intramolecular charge‐transfer interactions and π‐acceptor effects.     

A Second-generation photocage for Zn2+ inspired by TPEN: characterization and insight into the uncaging quantum yields of ZinCleav chelators

Photocages have been used to elucidate the biological functions of various small molecules and Ca²⁺; however, there are very few photocages available for other metal ions. ZinCleav‐2 (1‐(4,5‐dimethoxy‐2‐nitrophenyl)‐N,N,N′,N′‐tetrakis‐pyridin‐2‐ylmethyl‐ethane‐1,2‐diamine) is a second‐generation photocage for Zn²⁺ that releases the metal ion after a light‐induced bifurcation of the chelating ligand. The structure of ZinCleav‐2 was inspired by TPEN (N,N,N′,N′‐tetrakis(2‐pyridylmethyl)ethylenediamine), which is routinely used to sequester metal ions in cells owing to its high binding affinity. Inclusion of a 2‐nitrobenzyl chromophore leads to the formation of two more weakly binding di‐(2‐picolyl)amine (DPA) fragments upon photolysis of the TPEN backbone. The desired ligand was prepared using a modified procedure used to access ZinCleav‐1 (1‐(4,5‐dimethoxy‐2‐nitrophenyl)‐N,N′‐dimethyl‐N,N′‐bis‐pyridin‐2‐ylmethyl‐ethane‐1,2‐diamine). ZinCleav‐2 has a conditional dissociation constant (K_d) of ～0.9 fM as measured by competitive titration with a quinoline‐based fluorescent sensor for Zn²⁺. The K_d of the Zn²⁺ complex of the DPA photoproducts is ～158 nM ; therefore, the ΔK_d for ZinCleav‐2 photocage is ～10⁸. A large ΔK_d is required to significantly perturb free metal ion concentrations in biological assays. The quantum yield of photolysis of apo ZinCleav‐2 and the [Zn(ZinCleav‐2)]²⁺ complex are 4.7 and 2.3 %, respectively, as determined by HPLC analysis. Proof of concept Zn²⁺ release upon photolysis of [Zn(ZinCleav‐2)]²⁺ was demonstrated using the fluorescent sensor Zinpyr‐1, and the speciation of Zn²⁺ complexes was simulated using computational methods. The influence of benzylic substituents on the quantum yield of uncaging is also analyzed with the aim of tuning the photochemical properties caged complexes for in vivo experiments.     

Novel Luminescent Electrospun Fibers Prepared From Conjugated Rod–Coil Block Copolymer of Poly[2,7‐(9,9‐dihexylfluorene)]‐block‐Poly(methyl methacrylate)

Novel luminescent electrospun (ES) fibers have been successfully prepared from a conjugated rod–coil block copolymer, poly[2,7‐(9,9‐dihexylfluorene)]‐block‐poly(methyl methacrylate) (PF‐b‐PMMA) using a single‐capillary spinneret. Experiment results indicate that PF‐b‐PMMA ES fibers prepared from THF, THF/DMF (50/50), and DMF contain PF block aggregated structures of dot‐like (5–10 nm), line‐like (10–20 nm), and ellipse‐like structure (25–50 nm), respectively. Such variation of aggregation size leads to a red‐shift of the absorption or luminescence spectra. In addition, the fiber diameters decrease upon enhancing the DMF content. The present study demonstrates that blue light‐emitting ES fibers are successfully prepared from conjugated rod–coil diblock copolymers and their aggregate morphology and photophysical properties could be tuned through use of selective solvent.

     

荧光化学传感器研究中的光化学与光物理问题

在对分子识别用的荧光化学传感器化合物进行设计合成时,必须对传感器分子在工作中所涉及的发光光化学及光物理问题有一基本的了解.本文从几个方面分别对传感器的报告和中继部分中的光化学、光物理问题作了较充分的讨论,也对传感器分子中接受体部分和光化学相联系的内容作了介绍.     

Near-infrared Piezochromic Materials at High Pressure

The study of piezochromic materials (PCMs) has become an attractive field and numerous scholars have reported various material structures and phenomena. PCMs incorporating near-infrared (NIR) emission have led to a broader range of applications due to the strong penetration and interference resistance of longer wavelength light sources. However, NIR PCMs are still rare due to difficulties in tuning molecular configuration, conformation and stacking structure. In this review, organic compounds are classified according to their types and structures, and recent advances in NIR PCMs are comprehensively summarized and described. The various factors affecting the piezochromic properties from the perspective of the compound structure are shown. The effects of pressure on the photophysical changes of different compounds are discussed. It is expected to provide ideas for subsequent NIR PCMs, from structural design to predicting their photophysical properties under pressure.     

Coordination/organometallic polymers based on diphosphine and diisocyanide ligands

This paper reviews various coordination/ organometallic polymers in which the metal atoms are incorporated in the backbone using diphosphine and diisocyanide ligands. Such ligands includes diphosphines of the type bis(diphenylphosphino)alkane where alkane is (CH₂)_m with m = 1, 3-6, bis(diphenylphosphino)acetylene (dpa), and bis(dimethylphosphino)methane (dmpm), and diisocyanides such as 1,8-diiso-cyano-p-menthane (dmb) and p-diisocyanotetra-methylbenzene (ditmb). The metal fragments are monocations such as Cu⁺, Ag⁺, and Au⁺, dinuclear species such as Pd₂(dmb)₂²⁺, Pd₂(dppm)₂²⁺, M₂(dmpm)₃²⁺ (M = Cu, Ag), and clusters such as M₄(dmb)₄²⁺ (M = Pd, Pt).     

Exciton dissociation in conjugated polymers

In conjugated polymers, a majority of photogenerated charges form metastable geminate pairs (GPs), of which only some fraction can dissociate completely. Both the yield of GP photogeneration and the probability of further dissociation of GPs into free charges depend upon an external electric field. In the present article we discuss several experimental methods to detect the existence of geminate pairs such as delayed field collection of charges, field quenching of fluorescence, and field-assisted photoinduced optical absorption. It is shown that the field dependences of the exciton dissociation into GPs and of the free carrier photogeneration yield are rather similar. This is in contrast with the traditional Onsager theory, which assumes field-independent yield of primary photoionization and disregards the field dependence of the initial separations between carriers in GPs.     

Photophysics of Iridium(III) (2-phenylpyridine)(2,2’,6,2”-terpyridine) Chloride Encapsulated within the Zinc(II) Polyhedral MOF,USF-2

Photoactive metal-organic frameworks (MOFs) provide an important class of functional porous materials for a wide range of applications including light harvesting, photocatalysis and photodynamic therapy. Two strategies have been employed in the development of photoactive MOFs, one in which the photoactive element is incorporated as an element of the framework itself and the other in which the photoactive element serves as a guest within the MOF cavities. Transition metal polyimines have now been non-covalently incorporated within the cavities of a large number of MOFs with the Ru^II-polyimine being the most widely examined guest complex. Previous studies have demonstrated that the nature of the cavity modulates the Ru-polyimine photophysics. Here, an Ir^III(terpyridine)(phenylpyridine)Cl complex has been encapsulated within the Zn-polyhedral MOF, USF-2. Unlike the Ru-polyimines, the excited state photophysics associated with the encapsulated Ir polyimine shows very little change in either the steady state emission and emission lifetime. The slight decrease in emission lifetime is attributed to energy transfer between encapsulated Ir complexes. These results indicate that transition metal polyimines that exhibit excited state structural changes demonstrate the largest perturbations upon confinement.     

Engineered Green Fluorescence Proteins for Proteomics and Biomolecular Electronic Applications

The discovery of the photochromic characteristics of engineered green fluorescent proteins (GFPs) allows new proteomics and biomolecular electronic applications. In particular, photoreversibility among two distinct optical states can lead to the realization of a bio-optical high density storage memory. Here we review our recent work on an optically bistable GFP and we report the recent developments of self-assembly methods for spatial immobilization of proteins into well-definite 2D patterns.