Scramble-Free Synthesis of Unhindered trans-A2B2-Mesoaryl Porphyrins via Bromophenyl Dipyrromethanes.

Trans-disubstituted porphyrins are highly valuable intermediates across diverse fields, but they pose a significant synthesis challenge in some cases due to scrambling and formation of complex mixtures. Conditions that minimize scrambling also lower yields, but steric hindrance around the meso-aryl substituent can effectively suppress scrambling altogether. Here we report a straightforward approach to valuable trans-A2B2 porphyrin intermediates that are otherwise very difficult to obtain, through use of removable blocking bromide substituents.

Protomers of the green and cyan fluorescent protein chromophores investigated using action spectroscopy.

The photophysics of biochromophore ions often depends on the isomeric or protomeric distribution, yet this distribution, and the individual isomer contributions to an action spectrum, can be difficult to quantify. Here, we use two separate photodissociation action spectroscopy instruments to record electronic spectra for protonated forms of the green (pHBDI+) and cyan (Cyan+) fluorescent protein chromophores. One instrument allows for cryogenic (T = 40 ± 10 K) cooling of the ions, while the other offers the ability to perform protomer-selective photodissociation spectroscopy. We show that both chromophores are generated as two protomers when using electrospray ionisation, and that the protomers have partially overlapping absorption profiles associated with the S1 ← S0 transition. The action spectra for both species span the 340-460 nm range, although the spectral onset for the pHBDI+ protomer with the proton residing on the carbonyl oxygen is red-shifted by ≈40 nm relative to the lower-energy imine protomer. Similarly, the imine and carbonyl protomers are the lowest energy forms of Cyan+, with the main band for the carbonyl protomer red-shifted by ≈60 nm relative to the lower-energy imine protomer. The present strategy for investigating protomers can be applied to a wide range of other biochromophore ions.

Solvent Tuning Excited State Structural Dynamics in a Novel Bianthryl.

Symmetry breaking charge separation (SBCS) is central to photochemical energy conversion. The widely studied 9,9-bianthryl (9,9'BA) is the prototype, but the role of bianthryl structure is hardly investigated. Here we investigate excited state structural dynamics in a bianthryl of reduced symmetry, 1,9-bianthryl (1,9'BA), through ultrafast electronic and vibrational spectroscopy. Resonance selective Raman in polar solvents reveals a Franck-Condon state mode that disappears concomitant with the rise of ring breathing modes of radical species. Solvent-dependent dynamics show that CS is driven by solvent orientational motion, as in 9,9'BA. In nonpolar solvents the excited state undergoes multistep structural relaxation, including subpicosecond Franck-Condon state decay and biexponential diffusion-controlled structural evolution to a distorted slightly polar state. These data suggest two possible routes to SBCS; the established solvent driven pathway in rapidly relaxing polar solvents and, in slowly relaxing media, initial intramolecular reorganization to a polar structure which drives solvent orientational relaxation.

Ultrafast Excimer Formation and Solvent Controlled Symmetry Breaking Charge Separation in the Excitonically Coupled Subphthalocyanine Dimer.

Knowledge of the factors controlling excited state dynamics in excitonically coupled dimers and higher aggregates is critical for understanding natural and artificial solar energy conversion. In this work, we report ultrafast solvent polarity dependent excited state dynamics of the structurally well-defined subphthalocyanine dimer, µ-OSubPc2 . Stationary electronic spectra demonstrate strong exciton coupling in µ-OSubPc2 . Femtosecond transient absorption measurements reveal ultrafast excimer formation from the initially excited exciton, mediated by intramolecular structural evolution. In polar solvents the excimer state decays directly through symmetry breaking charge transfer to form a charge separated state. Charge separation occurs under control of solvent orientational relaxation.

Complementary Syntheses Giving Access to a Full Suite of Differentially Substituted Phthalocyanine-Porphyrin Hybrids.

Phthalocyanines and porphyrins are often the scaffolds of choice for use in widespread applications. Synthetic advances allow bespoke derivatives to be made, tailoring their properties. The selective synthesis of unsymmetrical systems, particularly phthalocyanines, has remained a significant unmet challenge. Porphyrin-phthalocyanine hybrids offer the potential to combine the favorable features of both parent structures, but again synthetic strategies are poorly developed. Here we demonstrate strategies that give straightforward, controlled access to differentially substituted meso-aryl-tetrabenzotriazaporphyrins by reaction between an aryl-aminoisoindolene (A) initiator and a complementary phthalonitrile (B). The choice of precursors and reaction conditions allows selective preparation of 1:3 Ar-ABBB and, uniquely, 2:2 Ar-ABBA functionalized hybrids.

Straightforward and Controlled Synthesis of Porphyrin-Phthalocyanine-Porphyrin Heteroleptic Triple-Decker Assemblies.

A versatile and straightforward protocol is disclosed for controlled synthesis of complex lanthanide-bridged heteroleptic porphyrin-phthalocyanine triple-decker assemblies. Two porphyrins, linked by a flexible spacer chain of intermediate length, sequentially capture lanthanide ions and a phthalocyanine to efficiently form the triple-decker complex. The bridge directs assembly, but also controls the mobility of the central macrocycle and further imparts a fully eclipsed arrangement of all three rings.

Evolutionary Computation for Parameter Extraction of Organic Thin-Film Transistors Using Newly Synthesized Liquid Crystalline Nickel Phthalocyanine.

In this work, the topic of the detrimental contact effects in organic thin-film transistors (OTFTs) is revisited. In this case, contact effects are considered as a tool to enhance the characterization procedures of OTFTs, achieving more accurate values for the fundamental parameters of the transistor threshold voltage, carrier mobility and on-off current ratio. The contact region is also seen as a fundamental part of the device which is sensitive to physical, chemical and fabrication variables. A compact model for OTFTs, which includes the effects of the contacts, and a recent proposal of an associated evolutionary parameter extraction procedure are reviewed. Both the model and the procedure are used to assess the effect of the annealing temperature on a nickel-1,4,8,11,15,18,22,25-octakis(hexyl)phthalocyanine (NiPc6)-based OTFT. A review of the importance of phthalocyanines in organic electronics is also provided. The characterization of the contact region in NiPc6 OTFTs complements the results extracted from other physical-chemical techniques such as differential scanning calorimetry or atomic force microscopy, in which the transition from crystal to columnar mesophase imposes a limit for the optimum performance of the annealed OTFTs.

Electronic Energy Transfer in a Subphthalocyanine-Zn Porphyrin Dimer Studied by Linear and Nonlinear Ultrafast Spectroscopy.

The efficient harvesting and transport of visible light by electronic energy transfer (EET) are critical to solar energy conversion in both nature and molecular electronics. In this work, we study EET in a synthetic dyad comprising a visible absorbing subphthalocyanine (SubPc) donor and a Zn tetraphenyl porphyrin (ZnTPP) acceptor. Energy transfer is probed by steady-state spectroscopy, ultrafast transient absorption, and two-dimensional electronic spectroscopy. Steady-state and time-resolved experiments point to only weak electronic coupling between the components of the dimer. The weak coupling supports energy transfer from the SubPc to the zinc porphyrin in 7 ps, which itself subsequently undergoes intersystem crossing to populate the triplet state. The rate of the forward energy transfer is discussed in terms of the structure of the dimer, which is calculated by density functional theory. There is evidence of back energy transfer from the ZnTPP on the hundreds of picoseconds time scale. Sub-picosecond spectral diffusion was also observed and characterized, but it does not influence the picosecond energy transfer.

Synthesis of Porphyrin-CdSe Quantum Dot Assemblies: Controlling Ligand Binding by Substituent Effects.

Cadmium selenide quantum dots of 2.2-2.3 nm diameter were prepared by phosphorus-free methods using oleic acid as stabilizing surface ligand. Ligand exchange monitored quantitatively by (1)H NMR spectroscopy gave an estimate of 30-38 monodentate ligands per nanocrystal, with a ligand density of 1.8-2.3 nm(-2). The extent of ligand exchange with macrocycles carrying one or more functional groups was investigated, with the aim of producing nanocrystal-macrocycle conjugates with a limited number of coligands. Metal-free porphyrins are able to sequester the Cd(2+) ions from the Cd(oleate)2 outer layer of the nanocrystals. Zinc porphyrin complexes carrying one carboxylate function displace oleate efficiently to give porphyrin/CdSe composites with porphyrins stacked upright on the crystal surface. Porphyrins with four potential ligating sites are able to bind to the crystal surface only if the donors are at the end of sufficiently long and flexible tethers. High-dilution methods allowed the synthesis and isolation of well-defined composites of composition [CdSe{porphyrin}2], where porphyrin = 5,10,15,20-tetrakis{3-(carboxy-n-alkyloxy)phenyl}porphyrinato zinc (n = 5 or 10) and 5,10,15,20-tetrakis{3-(11-undecenyloxythiol)phenyl}porphyrinato zinc. Comparison of the composition data obtained by (1)H NMR spectroscopy with luminescence quenching behavior suggests a dependence of quenching efficiency on the tether length. Luminescence quenching was also observed for porphyrins that, according to (1)H NMR results, do not undergo surface ligand exchange.

A π-Extended Donor-Acceptor-Donor Triphenylene Twin Linked via a Pyrazine Bridge.

ß-Amino triphenylenes can be accessed via palladium catalyzed amination of the corresponding triflate using benzophenone imine. Transformation of amine 6 to benzoyl amide 18 is also straightforward, and its wide mesophase range demonstrates that the new linkage supports columnar liquid crystal formation. Amine 6 also undergoes clean aerobic oxidation to give a new twinned structure linked through an electron-poor pyrazine ring. The new discotic liquid crystal motif contains donor and acceptor fragments and is more oval in shape rather than disk-like. It forms a wide range columnar mesophase. Absorption spectra are strong and broad; emission is also broad and occurs with a Stokes shift of ca. 0.7 eV, indicative of charge-transfer character.

Synthesis of meso-substituted subphthalocyanine-subporphyrin hybrids: boron subtribenzodiazaporphyrins.

The first syntheses of hybrid structures that lie between subphthalocyanines and subporphyrins are reported. The versatile single-step synthetic method uses a preformed aminoisoindolene to provide the bridging methine unit and its substituent while trialkoxyborates simultaneously act as Lewis acid, template, and provider of the apical substituent. The selection of each component therefore allows for the controlled formation of diverse, differentially functionalized systems. The new hybrids are isolated as robust, pure materials that display intense absorption and emission in the mid-visible region. The new compounds are further characterized in solution and solid state by variable-temperature NMR spectroscopy and X-ray crystallography, respectively.

Tetrabenzoporphyrin and -mono-, -cis-di- and Tetrabenzotriazaporphyrin Derivatives: Electrochemical and Spectroscopic Implications of meso CH Group Replacement with Nitrogen.

Nonperipherally hexyl-substituted metal-free tetrabenzoporphyrin (2H-TBP, 1a) tetrabenzomonoazaporphyrin (2H-TBMAP, 2a), tetrabenzo-cis-diazaporphyrin (2H-TBDAP, 3a), tetrabenzotriazaporphyrin (2H-TBTAP, 4a), and phthalocyanine (2H-Pc, 5a), as well as their copper complexes (1b-5b), were synthesized. As the number of meso nitrogen atoms increases from zero to four, λmax of the Q-band absorption peak becomes red-shifted by almost 100 nm, and extinction coefficients increased at least threefold. Simultaneously the blue-shifted Soret (UV) band substantially decreased in intensity. These changes were related to the relative electron-density of each macrocycle expressed as the group electronegativity sum of all meso N and CH atom groups, ∑χR. X-ray photoelectron spectroscopy differentiated between the three different types of macrocyclic nitrogen atoms (the Ninner, (NH)inner, and Nmeso) in the metal-free complexes. Binding energies of the Nmeso and Ninner,Cu atoms in copper chelates could not be resolved. Copper insertion lowered especially the cathodic redox potentials, while all four observed redox processes occurred at larger potentials as the number of meso nitrogens increased. Computational chemical methods using density functional theory confirmed 1b to exhibit a Cu(II) reduction prior to ring-based reductions, while for 2b, Cu(II) reduction is the first reductive step only if the nonperipheral substituents are hydrogen. When they are methyl groups, it is the second reduction process; when they are ethyl, propyl, or hexyl, it becomes the third reductive process. Spectro-electrochemical measurements showed redox processes were associated with a substantial change in intensity of at least two main absorbances (the Q and Soret bands) in the UV spectra of these compounds.

Synthesis of a class of core-modified aza-BODIPY derivatives.

The convenient synthesis of a new class of conjugated aza-BODIPY derivatives from readily available precursors has been achieved. The new materials bear close structural similarity to BODIPYs but differ significantly in electronic configuration from known derivatives, leading to markedly different absorption and emission properties.

Expanded porphyrin-like structures based on twinned triphenylenes.

Triphenylene twins are intriguing structures, and those bridged through their 3,6-positions by dipyrromethene units give a new class of macrocycles that can be viewed as rigid, expanded porphyrin derivatives in which coplanarity is enforced in a formally antiaromatic π system. Somewhat surprisingly, however, macrocyclization leads to significant overall stabilization of the dipyrromethene chromophores.

Muoniated spin probes in the discotic liquid crystal HHTT: rapid electron spin relaxation in the hexagonal columnar and isotropic phases.

Avoided level crossing muon spin resonance (ALC-µSR) spectroscopy was used to study radicals produced by the addition of the light hydrogen isotope muonium (Mu) to the discotic liquid crystal (DLC) 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT). Mu adds to the secondary carbon atoms of HHTT to produce a substituted cyclohexadienyl radical, whose identity was confirmed by comparing the measured hyperfine coupling constants with values obtained from DFT calculations. ALC-µSR spectra were obtained in the isotropic (I), hexagonal columnar (Col(h)), helical (H), and crystalline (Cr) phases. In the I and Col(h) phases the radicals, which are incorporated within the stacks of HHTT molecules as isolated paramagnetic defects, undergo extremely rapid electron spin relaxation, on the order of a hundredfold faster than in the H or Cr phases. The electron spin relaxation rate increases significantly with increasing temperature and appears to be caused by the liquidlike motion within the columns, which modulates the overlap between the π system of the radical and the π systems of the neighboring HHTT molecules, and hence, the hyperfine coupling constants. Rapid electron spin relaxation should occur for any π radical incorporated within the columns of a DLC, which may limit the utility of DLCs for future spin-based technologies.

Synthesis, characterization, MCD spectroscopy, and TD-DFT calculations of copper-metalated nonperipherally substituted octaoctyl derivatives of tetrabenzotriazaporphyrin, cis- and trans-tetrabenzodiazaporphyrin, tetrabenzomonoazaporphyrin, and tetrabenzoporphyrin.

Synthesis of the title compounds has been achieved through refinement of a recently reported synthetic protocol whereby varying equivalents of MeMgBr are reacted with 1,4-dioctylphthalonitrile to produce mixtures favoring specific hybrid structures. The initially formed magnesium-metalated compounds are obtained as pure materials and include, for the first time, both isomers (cis and trans) of tetrabenzodiazaporphyrin. The compounds were demetalated to the metal-free analogues, which were then converted into the copper-metalated derivatives. The X-ray structure of the copper tetrabenzotriazaporphyrin derivative is reported. The metal-free and copper-metalated macrocycles exhibit columnar mesophase behavior, and it is found that the mesophase stability is unexpectedly reduced in the diazaporphyrin derivatives compared to the rest of the series. The results of time-dependent density functional theory calculations for the copper complexes are compared to the observed optical properties. Michl's perimeter model was used as a conceptual framework for analyzing the magnetic circular dichroism spectral data, which predicted and accounted for trends in the observed experimental spectra.

Discotic triphenylene twins linked through thiophene bridges: controlling nematic behavior in an intriguing class of functional organic materials.

Substituted triphenylenes and similar discotic molecules have a strong tendency toward columnar organization. Nematic mesophases are much less commonly observed in discotic systems. We have demonstrated a general strategy whereby discotic triphenylenes can be twinned to form stable, boardlike materials that display only nematic mesophases. The dominant structural feature that leads to nematic behavior is an enforced void region in the center of the macrocycle that results from bridging through the triphenylene 3,6-positions. This precludes simple columnar assembly because it would lead to free space through the center of each stack. Selection of appropriate bridging units allows materials to be designed which combine molecular features, such as the optoelectronic properties of electron-rich triphenylenes and conjugated thiophene units, with the processability, self-healing, and alignment features inherent in nematic mesophases. In addition, communication across twinned structures can lead to additional enhancement of optoelectronic behavior. This is particularly apparent in fully conjugated, planar twin 12 which is formally expected to have some antiaromatic character. This character is manifested in its spectral properties, and particularly noteworthy is its strong, Stokes shifted emission at around 500 nm.

Unexpected phenol production from arylboronic acids under palladium-free conditions; organocatalyzed air oxidation.

An intriguing class of quinones that efficiently catalyze the air oxidation (overall hydroxylation) of arylboronic acids to the corresponding phenol is reported. Autocatalysis in the parent system is particularly efficient and leads to rapid, quantitative synthesis of quinones such as 4 from boronic acid 1 at room temperature using air as stoichiometric oxidant. The efficiency results from a balance between two-stage conjugate addition and migration with each step driven by aromatization of a naphthalene fragment.