Spectroscopy of Free-Base N-Confused Tetraphenylporphyrin Radical Anion and Radical Cation

Photoinduced Electron Transfer in a Porphyrin-Fullerene Dyad at a Liquid Interface

The excited-state properties of an amphiphilic porphyrin-fullerene dyad and of its porphyrin analogue adsorbed at the dodecane/water interface are investigated by using surface second-harmonic generation. Although the porphyrin is formally centrosymmetric, the second-harmonic spectra of both compounds are dominated by the intense Soret band of the porphyrin. Polarization-selective measurements and molecular dynamics simulations suggest an angle of about 45° between the donor-acceptor axis and the interfacial plane, with the porphyrin interacting mostly with the nonpolar phase. Time-resolved measurements reveal a marked concentration dependence of the dynamics of both compounds upon Q-band excitation, indicating the occurrence of intermolecular quenching processes. The significant differences in dynamics and spectra between the dyad and the porphyrin analogue are explained by a self-quenching of the excited dyad via an intermolecular electron transfer.

Creation from Confusion and Fusion in the Porphyrin World─The Last Three Decades of N-Confused Porphyrinoid Chemistry

Confusion is a novel concept of isomerism in porphyrin chemistry, delivering a steady stream of new chemistry since the discovery of N-confused porphyrin, a porphyrin mutant, in 1994. These days, the number of confused porphyrinoids is increasing, and confusion and associated fusion are found in various fields such as supramolecular chemistry, materials chemistry, biological chemistry, and catalysts. In this review, the birth and growth of confused porphyrinoids in the last three decades are described.

UV, FL and IR spectroscopic analysis of N-confused meso-tetra(methoxyphenyl)porphyrins

UV, FL and IR spectra of isomer A (in DCM) and isomer B (in DMAc) of N-confused 5,10,15,20-tetra([Formula: see text]-methoxyphenyl)porphyrin ([Formula: see text]-isomer, 1), N-confused 5,10,15,20-tetra([Formula: see text]-methoxyphenyl)porphyrin ([Formula: see text]-isomer, 2) and N-confused 5,10,15,20-tetra([Formula: see text]-methoxyphenyl)porphyrin ([Formula: see text]-isomer,3) were investigated. Significant red shifts are observed from [Formula: see text]-isomer (1) to [Formula: see text]-isomer (3) in UV-vis absorption and fluorescence spectra. The fluorescence lifetime ([Formula: see text] of isomer A and isomer B of the samples (1, 2 and 3) are ca. 1.39[Formula: see text]1.79 ns and 1.68[Formula: see text]2.09 ns, respectively. In addition, the order of fluorescence quantum yield for isomer A is 1 > 2 > 3, but for isomer B it is 1 < 3 < 2. The isomer B of 2shows the strongest emission in accord with the strongest absorption. The isomer A of 1shows the strongest emission with the fluorescence quantum yield of 0.045, which is nearly double that of 3. The introduction of the methoxy group at the orthoposition increases steric hindrance, the molecular rigidity and the flow of [Formula: see text]-electron, thereby favourable to the fluorescence enhancement. The IR absorption peaks of 1, 2 and 3 were investigated and empirically assigned. The effects of the position of the substituents on the absorption frequency were also discussed. The absorption frequency of the C=C phenyl stretching in plane changed obviously by conjugative effect and steric effect when the methoxy group was at the orthoormetaposition in comparison with paraposition.

Tetracationic porphyrin derivatives against human breast cancer

Photodynamic therapy (PDT) is an approved therapeutic approach and an alternative to conventional chemotherapy for the treatment of several types of cancer with the advantages of reducing the side effects and developing resistance mechanisms. Here, was evaluated the photosensitization capabilities of 5,10,15,20-tetrakis[4-(pyridinium-1-yl-methyl)phenyl]porphyrin (3), its N-confused isomer (4) and of the neutral precursors (1) and (2) and the results were compared with the ones obtained with the cationic 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin (TMPyP). Both regular porphyrin derivatives 1 and 3 showed higher efficiency to generate singlet oxygen than TMPyP. The PDT assays towards MCF-7 cells under red light irradiation (λ > 640 nm, 23.7 mW cm^-2) demonstrated that the cationic porphyrin 3 is an efficient photosensitizer to kill MCF-7 breast cancer cells. The study of the cell death mechanisms induced by the photodynamic process showed that the studied porphyrin 3 and TMPyP caused cell death by autophagic flux and necrosis.

NH Tautomerism of N-Confused Porphyrin: Solvent/Substituent Effects and Isomerization Mechanism

The effects of substituents and solvents on the NH tautomerism of N-confused porphyrin (2) were investigated. The structures, electronic states, and aromaticity of NH tautomers (2-2H and 2-3H) were studied by absorption and nuclear magnetic resonance (¹H, ¹³C, and ¹⁵N) spectroscopies, single-crystal X-ray diffraction analysis, and theoretical calculations. The relative stability of the tautomers is highly affected by solvents, with the 3H-type tautomer being more stable in nonpolar solvents, while the 2H-type tautomer being highly stabilized in polar solvents with high donor numbers such as N,N-dimethylformamide (DMF), pyridine, and acetone. Electron-withdrawing groups on the meso-aryl substituents as well as the methyl group at the ortho position also stabilize the 2H-type tautomer. Kinetically, the tautomerism rate is significantly influenced by solvent and concentration, and a particularly large activation entropy (ΔS^⧧) is obtained in pyridine. The first-order deuterium isotope effect on the reaction rates of NH tautomerism (k_H/k_D) is determined to be 2.4 at 298 K. On the basis of kinetic data, the mechanism of isomerization is identified as an intramolecular process, including the rotation of the confused pyrrole in pyridine/chloroform and DMF/chloroform mixed solvent systems, and as a pyridine-mediated process in pyridine alone.

N-Confused Porphyrin - A Unique "Turn-On" Chemosensor for CN- and F- ions and "Turn-Off" Sensor for ClO4 - ions

N-Confused meso-tetrakis(4-carbomethoxyphenyl)porphyrin (1) and its Ni(II) complex (1 a) have been synthesized and utilized for anion sensing studies, and the results are compared with N-confused meso-tetraphenylporphyrin (NCTPP). Anion susceptibilities of 1 and 1 a were investigated using spectroscopic, electrochemical, and DFT studies. Porphyrins 1 and 1 a were able to detect CN^- , F^- , and ClO₄ ^- ions selectively over the tested set of anions even at ppm level. Interestingly, the addition of ClO₄ ^- ions resulted in fluorescence quenching (turn off) whereas the addition of F^- or CN^- resulted in fluorescence enhancement (turn on). Notably, the TFA addition resulted in fluorescence quenching, whereas the fluorescence enhancement was observed while adding TBAOH. The higher association constant (K_a ) values with anions, lower detection limit, and shifts in redox potentials are due to the electron-withdrawing effect of the -COOCH₃ group at the para-position of the meso-phenyl ring. This electron-withdrawing nature is crucial for the higher affinity towards anions. The anion sensing description in this article may not only unveil the built-in nature of N-confused porphyrins, but may also provide a general proposal for the development of novel anion sensors based on porphyrinoids. The electron-deficient porphyrin framework, large polarisable π-system, and anion binding through the outer NH or a combination of the above factors serve as a foundation for N-confused porphyrin to act as an anion sensor.

N-confusedmeso-tetraaryl-substituted free-base porphyrins: determination of protonation and deprotonation constants in nonaqueous media

The protonation and deprotonation reactions for a series N-confused meso-tetraaryl-substituted free-base porphyrins containing electron-donating or electron-withdrawing substituents was monitored in CHCl3and DMF by UV-visible spectroscopy during titrations with trifluoroacetic acid or tetra-n-butylammonium hydroxide. The spectroscopic data was also used to calculate equilibrium constants for these reactions. The examined compounds are represented as (XPh)4NCPH2, where "NCP" represents the N-confused porphyrin π-conjugated macrocycle and X is a CH3O , CH3, H or Cl para-substituent on the four meso-phenyl rings (Ph) of the compound. The porphyrins can exist in two tautomeric forms depending upon the solvent and each tautomer undergoes two stepwise protonation reactions leading to formation of the mono- and bis-protonated porphyrins, [(XPh)4NCPH3]+and [(XPh)4NCPH4]2+. A single step deprotonation is observed for the same compounds in DMF and the product is assigned as [(XPh)4NCPH]-. Comparisons are made between UV-visible spectra of the protonated, neutral and deprotonated forms of the porphyrin and the effect of the porphyrin ring substituents and tautomeric form of the neutral porphyrin on the UV-visible spectra and protonation constants is discussed along with data from DFT calculations.

Electronic state dependence of heterogeneous electron transfer: injection from the S1 and S2 state of phlorin into TiO2

Ultrafast time-resolved measurements were performed on a novel pentafluorophenyl substituted 5,5-dimethyl phlorin derivative in solution and when attached to TiO2 colloidal films. The complex excited state dynamics of this porphyrinoid after S1 and S2 excitation was compared at different wavelengths and can be assigned to several subsequent relaxation mechanisms. The difference between excited state dynamics in the free molecule and when attached to an electron accepting electrode was measured. For both cases the dynamics was compared after excitation to the S1 and the S2 state. For the free molecule in solution an intermediate relaxation step was identified and assigned to a buckling motion of the tetrapyrrole ring. On the electrode, heterogeneous electron transfer (HET) times from both states were very similar and around 50 fs. Surprisingly, the large difference in the density of acceptor states that are resonant with the respective donor level of the molecule does not significantly influence HET dynamics. This result indicates that HET proceeds into intermediate transition states that are different from steady state surface states obtained from experiments or computations. The density of states (DOS) of these transient acceptor states appears not to be directly related to the corresponding surface or bulk DOS.

New donor–acceptor conjugates based on a trifluorenylporphyrin linked to a redox–switchable ruthenium unit

Spectroelectrochemical studies show four accessible redox states on dyad 3 with the first oxidation process taking place at the Ru(ii) centre and the second oxidation and first reduction processes at the porphyrin centre.

Extraction of photogenerated electrons and holes from a covalent organic framework integrated heterojunction

Covalent organic frameworks (COFs) offer a strategy to position molecular semiconductors within a rigid network in a highly controlled and predictable manner. The π-stacked columns of layered two-dimensional COFs enable electronic interactions between the COF sheets, thereby providing a path for exciton and charge carrier migration. Frameworks comprising two electronically separated subunits can form highly defined interdigitated donor-acceptor heterojunctions, which can drive the photogeneration of free charge carriers. Here we report the first example of a photovoltaic device that utilizes exclusively a crystalline organic framework with an inherent type II heterojunction as the active layer. The newly developed triphenylene-porphyrin COF was grown as an oriented thin film with the donor and acceptor units forming one-dimensional stacks that extend along the substrate normal, thus providing an optimal geometry for charge carrier transport. As a result of the degree of morphological precision that can be achieved with COFs and the enormous diversity of functional molecular building blocks that can be used to construct the frameworks, these materials show great potential as model systems for organic heterojunctions and might ultimately provide an alternative to the current disordered bulk heterojunctions.