A New Approach for the Photosynthetic Antenna-Reaction Center Complex with a Model Organized Around ans-Triazine Linker

Design and Synthesis of Porphyrin-Nitrilotriacetic Acid Dyads with Potential Applications in Peptide Labeling through Metallochelate Coupling

The need to detect and monitor biomolecules, especially within cells, has led to the emerging growth of fluorescent probes. One of the most commonly used labeling techniques for this purpose is reversible metallochelate coupling via a nitrilotriacetic acid (NTA) moiety. In this study, we focus on the synthesis and characterization of three new porphyrin-NTA dyads, TPP-Lys-NTA, TPP-CC-Lys-NTA, and Py ₃ P-Lys-NTA composed of a porphyrin derivative covalently connected with a modified nitrilotriacetic acid chelate ligand (NTA), for possible metallochelate coupling with Ni²⁺ ions and histidine sequences. Emission spectroscopy studies revealed that all of the probes are able to coordinate with Ni²⁺ ions and consequently can be applied as fluorophores in protein/peptide labeling applications. Using two different histidine-containing peptides as His₆-tag mimic, we demonstrated that the porphyrin-NTA hybrids are able to coordinate efficiently with the peptides through the metallochelate coupling process. Moving one step forward, we examined the ability of these porphyrin-peptide complexes to penetrate and accumulate in cancer cells, exploring the potential utilization of our system as anticancer agents.

Design, Synthesis, and Utility of Defined Molecular Scaffolds

A growing theme in chemistry is the joining of multiple organic molecular building blocks to create functional molecules. Diverse derivatizable structures—here termed “scaffolds” comprised of “hubs”—provide the foundation for systematic covalent organization of a rich variety of building blocks. This review encompasses 30 tri- or tetra-armed molecular hubs (e.g., triazine, lysine, arenes, dyes) that are used directly or in combination to give linear, cyclic, or branched scaffolds. Each scaffold is categorized by graph theory into one of 31 trees to express the molecular connectivity and overall architecture. Rational chemistry with exacting numbers of derivatizable sites is emphasized. The incorporation of water-solubilization motifs, robust or self-immolative linkers, enzymatically cleavable groups and functional appendages affords immense (and often late-stage) diversification of the scaffolds. Altogether, 107 target molecules are reviewed along with 19 syntheses to illustrate the distinctive chemistries for creating and derivatizing scaffolds. The review covers the history of the field up through 2020, briefly touching on statistically derivatized carriers employed in immunology as counterpoints to the rationally assembled and derivatized scaffolds here, although most citations are from the past two decades. The scaffolds are used widely in fields ranging from pure chemistry to artificial photosynthesis and biomedical sciences.

Synthesis of donor-π-acceptor porphyrins for DSSC: DFT-study, comparison of anchoring mode and effectiveness

In this study two pairs of novel zinc-porphyrin complexes (ZnP1 ZnP2 ZnP3 and ZnP4) were synthesized as sensitizers for DSSC and their photophysical, computational studies and photovoltaic properties were investigated. Structures of proposed dyes are based on a molecular design that relies on donor/[Formula: see text]-bridge/acceptor interactions. Compounds differ by anchoring mode to the titanium dioxide surface: ZnP2 and ZnP4 porphyrins possess carboxyl anchoring groups while ZnP1 and ZnP3 porphyrins have similar structure but without anchors and attached to the surface by isonicotinic acid ligands. All the zinc-porphyrin derivatives bear hexyloxy-chains at the para-positions of their phenyl rings and ZnP3 and ZnP4 contain 1,3,5-triazine fragments as efficient electron transfer bridges. Electron density distribution of the frontier molecular orbitals was calculated based on the density functional theory (DFT). The test DSSC was manufactured and its parameters were measured to compare the effectiveness of the proposed sensitizers. Our results reveal that dyes with an anchoring group directly in their structure demonstrated several times higher efficiency. The use of the triazine fragment proved effective for the introduction of acceptor substituents bearing anchor groups. As a result, the highest efficiency of 4.33% was achieved using the dye ZnP4.

Azophenine as Central Core for Efficient Light Harvesting Devices

The notoriously non-luminescent uncycled azophenine (Q) was harnessed with Bodipy and zinc(II)porphyrin antennas to probe its fluorescence properties, its ability to act as a singlet excited state energy acceptor and to mediate the transfer. Two near-IR emissions are depicted from time-resolved fluorescence spectroscopy, which are most likely due to the presence of tautomers of very similar calculated total energies (350 cm^-1 ; DFT; B3LYP). The rates for energy transfer, k_ET (S₁ ), for ¹ Bodipy*→Q are in the order of 10¹⁰ -10¹¹  s^-1 and are surprisingly fast when considering the low absorptivity properties of the lowest energy charge transfer excited state of azophenine. The rational is provided by the calculated frontier molecular orbitals (MOs) which show atomic contributions in the C₆ H₄ C≡CC₆ H₄ arms, thus favoring the double electron exchange mechanism. In the mixed-antenna Bodipy-porphyrin star molecule, the rate for ¹ Bodipy*→porphyrin has also been evaluated (≈16×10¹⁰  s^-1 ) and is among the fastest rates reported for Bodipy-zinc(II)porphyrin pairs. This astonishing result is again explained from the atomic contributions of the C₆ H₄ C≡CC₆ H₄ and C≡CC₆ H₄ arms thus favouring the Dexter process. Here, for the first time, this process is found to be sensitively temperature-dependent. The azophenine turns out to be excellent for electronic communication.

An artificial photosynthetic model based on a molecular triad of boron dipyrromethene and phthalocyanine

A boron dipyrromethene (BDP) unit and its monostyryl derivative (MSBDP) were introduced at the axial positions of a silicon(iv) phthalocyanine (SiPc) core. The absorption spectrum of this compound virtually covered the entire visible region (300-700 nm) and could be interpreted as a superposition of the spectra of individual components. The intramolecular photoinduced energy and charge transfer processes of this triad were studied using steady-state and time-resolved spectroscopic methods in polar and nonpolar solvents. Upon BDP-part excitation, a fast and highly efficient excitation energy transfer (EET) occurred resulting in strong quenching of its fluorescence and the formation of the first excited singlet state of SiPc or MSBDP. It was found that both EET and charge transfer (CT) processes competed with each other in the depopulation of the first excited singlet state of the MSBDP moiety. The former strongly superseded CT in nonpolar toluene, whereas the latter was dominant in a polar environment. Direct or indirect (via EET) excitation of the SiPc-part of the triad was followed by CT yielding the charge-separated (CS) species BDP-SiPc(˙-)-MSBDP(˙+). The energy gap between the CS state and the S1-state of the SiPc moiety was found to be only 0.06 eV in toluene, which facilitated the back CT process and resulted in the appearance of thermally activated delayed fluorescence. With increasing solvent polarity, the energy of the CS state reduced resulting in the disappearance of the delayed fluorescence in CHCl3, tetrahydrofuran or N,N-dimethylformamide. The charge recombination rate, k(CR), was very fast in polar DMF (3.3 × 10(10) s(-1)), whereas this process was two-orders of magnitude slower in nonpolar toluene (k(CR) = 4.0 × 10(8) s(-1)).

Conjugated Porphyrin Dimers: Cooperative Effects and Electronic Communication in Supramolecular Ensembles with C60

Two new conjugated porphyrin-based systems (dimers 3 and 4) endowed with suitable crown ethers have been synthesized as receptors for a fullerene-ammonium salt derivative (1). Association constants in solution have been determined by UV–vis titration experiments in CH₂Cl₂ at room temperature. The designed hosts are able to associate up to two fullerene-based guest molecules and present association constants as high as ∼5 × 10⁸ M^–1. Calculation of the allosteric cooperative factor α for supramolecular complexes [3·1₂] and [4·1₂] showed a negative cooperative effect in both cases. The interactions accounting for the formation of the associates are based, first, on the complementary ammonium-crown ether interaction and, second, on the π–π interactions between the porphyrin rings and the C₆₀ moieties. Theoretical calculations have evidenced a significant decrease of the electron density in the porphyrin dimers 3 and 4 upon complexation of the first C₆₀ molecule, in good agreement with the negative cooperativity found in these systems. This negative effect is partially compensated by the stabilizing C₆₀–C₆₀ interactions that take place in the more stable syn-disposition of [4·1₂].

Photocurrent in Multilayered Assemblies of Porphyrin-Fullerene Covalent Dyads: Evidence for Channels for Charge Transport

Specially designed porphyrin-fullerene dyads have been synthesized to verify literature predictions based on quantum chemistry calculations that certain porphyrin-fullerene dyads are able to self-arrange into specific structures providing channels for charge transport in a bulk mass of organic compound. According to AFM and SEM data, the newly synthesized compounds were indeed prone to some kind of self-arrangement, although to a lesser degree than was expected. A dispersion corrected DFT study of the molecular non-covalent interactions performed at the DFT-D3 (B3LYP, 6-31G*) level of theory showed that the least energy corresponded to head-to-head dimers, with close contacts of porphyrin-porphyrin and fullerene-fullerene fragments, thus providing a unit building block of the channel for charge transport. Experimental proof for the existence of channels for charge transport was obtained by observing a photocurrent in a simple photovoltaic cell.