Synthesis and Characterization of Fluorescent Pyrrolyldipyrrinato Sn(IV) Complexes

Preparation of seven-coordinated hypervalent tin(IV)-fused azobenzene and applications for stimuli-responsive π-conjugated polymer films

Heavy atoms can form highly coordinated states, and their optical properties have attracted much attention. Recently, we have demonstrated that a reversible coordination-number shift of hypervalent tin(IV) from five to six can provide predictable hypsochromic shifts in light absorption and emission properties in small molecules and a π-conjugated polymer film. Herein, we show the preparation of seven-coordinated tin and reveal that the binding constant of the seven coordination with ethylenediamine (EDA, K = 2900 M-1) is 200 times higher than that of six coordination with propylamine (PA, K = 14 M-1) owing to the chelate effect. Moreover, reversible vapochromism of the π-conjugated polymer film was observed upon exposure (λabs = 598 nm and λPL = 697 nm) and desorption (λabs = 641 nm and λPL = 702 nm) of EDA vapor. Furthermore, as a unique demonstration, the thermochromic film was prepared by fixing the seven coordination as the initial state using 1,10-phenanthroline. These optical variations are predictable by quantum chemical calculations. Our findings are valuable for the development of designable and controllable stimuli-responsive materials focusing on the inherent properties of the elements.

Synthesis and optical properties of antimony(V) complexes of a trianionic N2O2-type tetradentate dipyrrin ligand

LSbCl2, an air- and moisture-stable antimony(V)-dipyrrin complex at room temperature, was obtained by treating an excess amount (20 eq.) of SbCl3 with a trianionic N2O2-type tetradentate dipyrrin ligand (L) under visible-light irradiation and O2 atmosphere. The Cl ligands in LSbCl2 were replaced by OH ligands via hydrolysis, yielding LSb(OH)2. Further, the molecular structures and optical properties of the Sb(V)-dippyrin complexes were investigated. While LSbCl2 was non-fluorescent, LSb(OH)2 exhibited an intense red fluorescence with a photoluminescence quantum yield of 68%.

Synthesis, Structure, Spectral, Electrochemical, and Theoretical Studies of Ru(II) Complexes of 3-Pyrrolyl BODIPY-Based Ligands

3-Pyrrolyl BODIPY having an appended pyrrolyl group at the 3-position of BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) and its functionalized derivatives have been used as ligands to prepare one example of Ru(II) complex of pyrrolyl dipyrrin and three unique examples of bichromophoric BODIPY-Ru(II) complexes in good yields. The complexes were characterized by HR-MS, 1D and 2D NMR, X-ray analysis for two complexes, absorption, cyclic voltammetry, and DFT/TD-DFT techniques. The X-ray structure of the Ru(II) complex of pyrrolyl dipyrrin revealed that the geometry around the Ru(II) ion was pseudo-octahedral with an arene unit occupying three coordination sites in η6-fashion and two nitrogen atoms of the dipyrrin ring and one chloro group adopting the "three legs piano-stool" structure. The X-ray structure of the bichromophoric BODIPY-Ru(II) complex revealed that the BODIPY core was planar and the central B(III) was coordinated with two pyrrole nitrogens of the dipyrrin unit and two axial fluoride ions in a tetrahedral geometry, and Ru(II) was bonded to appended pyrrole "N" and "N" of benzimidazole substituent present at the α-position of appended pyrrole, one chloro group, and one arene ring in a pseudo-octahedral geometry. The spectral studies revealed that the electronic properties of the BODIPY unit in BODIPY-Ru(II) complexes were significantly altered, and electrochemical studies indicated that the BODIPY-Ru(II) complexes exhibit one oxidation corresponding to Ru(II) to Ru(III) and one reduction corresponding to the BODIPY unit. The DFT/TD-DFT studies corroborated the experimental observations.

Recent advancements of fluorescent tin(IV) complexes in biomedical molecular imaging

In the last few years, tin(IV) complexes have emerged as very attractive candidates in the field of molecular imaging due to their unique photophysical properties. Despite the few reviews published to date covering the chemistry of organotin and tin complexes and their cytotoxic potential, there are no reviews devoted to their live cell imaging properties. Therefore, this feature article summarizes the discussion of the fundamental photophysical properties of fluorescent tin metal complexes focusing on their recent advances in "biomedical molecular imaging". A debate on the design of tin complexes as cellular imaging agents relating to their chemical, electronic and photophysical properties is enclosed. This paper also discusses the imaging applications of tin complexes in cells, tissues, and organisms via confocal and multiphoton imaging for sensing mechanisms in cellular media, bioimaging, and therapeutic labeling. In addition, it explores and explains the current challenges and prospects associated with these tin complexes as emerging luminescent cellular agents for potential clinical use.

Excited state energy landscape of phosphorescent group 14 complexes

Great progress has been achieved on phosphorescent or photoactive complexes of the Earth-abundant transition metals, while examples for phosphorescent heavy main group element complexes are rare, in particular for group 14 complexes in the oxidation state +II. The known compounds often show only weak phosphorescence with fast non-radiative deactivation. The underlying photophysical processes and the nature of the phosphorescent electronic states have remained essentially unexplored. The present combined photophysical and theoretical study on tin(ii) and lead(ii) complexes E(bpep) with the dianionic tridentate ligand bpep^2- (E = Sn, Pb; H₂bpep = 2-[1,1-bis(1H-pyrrol-2-yl)ethyl]pyridine) provides unprecedented insight in the excited state energy landscape of tetrel(ii) complexes. The tin complex shows green intraligand charge transfer (ILCT) phosphorescence both in solution and in the solid state. In spite of its larger heavy-atom effect, the lead complex only shows very weak red phosphorescence from a strongly distorted ligand-to-metal charge transfer (LMCT) state at low temperatures in the solid state. Detailed (TD-)DFT calculations explain these observations and delineate the major path of non-radiative deactivation via distorted LMCT states. These novel insights provide rational design principles for tetrel(ii) complexes with long-lived phosphorescence.

Luminescence from open-shell, first-row transition metal dipyrrin complexes

Several first-row transition metal complexes of the 1,9-bis(2',4',6'-triphenylphenyl)-5-mesityl dipyrrinato ligand and its tetrahalogenated analogues have been synthesized and their luminescence spectra obtained. The protonated ligands, as well as the Li(i), Mn(ii), Cu(i), Cu(ii), and Zn(ii) chelates show appreciable luminescence, despite the paramagnetism of the Mn(ii) and Cu(ii) ions. Fluorescence quantum yields (ΦF) as high as 0.67 were observed for the zinc complex. Luminescence was partially quenched by the introduction of heavy halogens to the backbone of the ligand, as well as by the introduction of paramagnetic metal ions. Room-temperature, solution state phosphorescence was observed from the halogenated dipyrrinato lithium salts, as well as from the non-halogenated Mn(ii) complex.

Highly Luminescent Heavier Main Group Analogues of Boron-Dipyrromethene

The preparation and photophysical properties of two heavier main group element analogues of boron-dipyrromethene (BODIPY) chromophores are described. Specifically, we have prepared dipyrrin complexes of dichlorogallate (GADIPY) or phenylphosphenium (PHODIPY) units. Whereas cationic PHODIPY is labile, decomposing to a phosphine over time, GADIPY is readily prepared in good yield as a crystalline solid having moderate air- and water-stability. Crystallographically characterized GADIPY displays intense green photoluminescence (λ_em = 505 nm, Φ_em = 0.91 in toluene). These inaugural heavier main group element analogues of BODIPY offer a glimpse into the potential for elaboration to a panoply of chromophores with diverse photophysical properties.

Prodigiosenes conjugated to tamoxifen and estradiol

We report the synthesis of the first click-appended prodigiosene conjugates. Four prodigiosene conjugates of estradiol functionalised at the 7α-position were prepared, as were three prodigiosene conjugates of tamoxifen. The coupling between a prodigiosene and an 11-hydroxy estradiol derivative via an ether linkage was investigated, as was the 11- and 7-functionalisation of the estradiol core. The robustness of estradiol protecting groups was severely challenged by reactions typically used to equip such frameworks for 11- and 7-functionalisation. Specifically, and important to synthesis involving estradiol, TBS, TMS and THP are not useful protecting groups for the functionalisation of this core. When the chemical features of the therapeutic agent limit the choice of protecting group (in this case, prodigiosenes bearing aryl, NH, alkenyl and ester groups), click chemistry becomes an attractive synthetic strategy. The anti-cancer activity of the seven click prodigiosene conjugates was evaluated.

Synthesis and characterization of pyrrolyldipyrrin F-BODIPYs

A series of synthetic analogs of the tripyrrolic natural product prodigiosin were complexed with boron trifluoride to generate the corresponding F-BODIPYs.

Effects of five membered aromatic heterocycles at the meso-position on the electronic properties of 3-pyrrolyl BODIPY

The synthesis, structures and properties of 3-pyrrolyl BODIPYs containing five membered aromatic heterocycles such as pyrrole, thiophene and furan at the meso-position are described.

New application of fluorescent organotin compounds derived from Schiff bases: synthesis, X-ray structures, photophysical properties, cytotoxicity and fluorescent bioimaging

A novel application of fluorescent organotin compounds derived from Schiff bases was developed for imaging cells.

Symmetry-Breaking Charge Transfer of Visible Light Absorbing Systems: Zinc Dipyrrins

Zinc dipyrrin complexes with two identical dipyrrin ligands absorb strongly at 450-550 nm and exhibit high fluorescence quantum yields in nonpolar solvents (e.g., 0.16-0.66 in cyclohexane) and weak to nonexistent emission in polar solvents (i.e., <10^-3, in acetonitrile). The low quantum efficiencies in polar solvents are attributed to the formation of a nonemissive symmetry-breaking charge transfer (SBCT) state, which is not formed in nonpolar solvents. Analysis using ultrafast spectroscopy shows that in polar solvents the singlet excited state relaxes to the SBCT state in 1.0-5.5 ps and then decays via recombination to the triplet or ground states in 0.9-3.3 ns. In the weakly polar solvent toluene, the equilibrium between a localized excited state and the charge transfer state is established in 11-22 ps.

Prodigiosin analogue designed for metal coordination: stable zinc and copper pyrrolyldipyrrins

The pyrrolyldipyrrin motif is found in several naturally occurring prodigiosin pigments. The potential roles of the interactions of prodigiosins with transition metals and the properties of metal-bound pyrrolyldipyrrins, however, have been difficult to assess because of the very limited number of well-characterized stable complexes. Here, we show that the introduction of a meso-aryl substituent and an ethyl ester group during the sequential assembly of the three heterocycles affords a pyrrolyldipyrrin of enhanced coordinating abilities when compared to that of natural prodigiosins. UV-visible absorption studies indicate that this ligand promptly binds Zn(II) ions with 2:1 ligand-to-metal stoichiometry and Cu(II) ions with 1:1 stoichiometry. Notably, no addition of base is required for the formation of the resulting stable complexes. The crystal structures reveal that whereas the tetrahedral zinc center engages two nitrogen donors on each ligand, the pseudosquare planar copper complex features coordination of all three pyrrolic nitrogen atoms and employs the ester group as a neutral ligand. This first example of coordination of a redox-active transition metal within a fully conjugated pyrrolyldipyrrin framework was investigated spectroscopically by electron paramagnetic resonance to show that the 1:1 metal-to-ligand ratio found in the crystal structure is also maintained in solution.

Imino–phenolic–azodye appended rhodamine as a primary fluorescence “off–on” chemosensor for tin (Sn4+) in solution and in RAW cells and the recognition of sulphide by [AR–Sn]

A new azo-rhodamine based, AR was developed for fluorescent ‘turn-on’ sensing of Sn⁴⁺ ion.

α-Monoacylated and α,α'- and α,β'-diacylated dipyrrins as highly sensitive fluorescence "turn-on" Zn2+ probes

With the purpose of developing readily synthesized CHEF (chelation-enhanced fluorescence) type Zn(2+) probes with relatively simple molecular structures and excellent sensing behavior, p-anisoyl chloride was used for the acylation of 5-(pentafluorophenyl)dipyrromethane. Interestingly, the α,β'-diacylated product PS2 with a unique substitution mode was obtained in high yield in addition to the normal α-substituted mono- and diacylated products PS1 and PS3. Further oxidation of PS1-PS3 afforded dipyrrins S1-S3. Crystal structure and (1)H NMR measurements of S2 demonstrate the existence of a pure tautomer, which is consistent with DFT calculations. S1-S3 show highly Zn(2+) selective "turn-on" fluorescence based on a CHEF mechanism by the formation of 2:1 (probe:metal) Zn(2+) complexes. The emission colors can be easily tuned from green to red by changing the dipyrrin substitution modes. Furthermore, these probes demonstrate fast responses and wide applicable pH ranges. Among them, S2 shows the highest Zn(2+) sensitivity, with a detection limit of 4.4 × 10(-8) M.

Coordination programming of photofunctional molecules

Our recent achievements relating to photofunctional molecules are addressed. Section 1 discloses a new concept of photoisomerization. Pyridylpyrimidine-copper complexes undergo a ring inversion that can be modulated by the redox state of the copper center. In combination with an intermolecular photoelectron transfer (PET) initiated by the metal-to-ligand charge transfer (MLCT) transition of the Cu(I) state, we realize photonic regulation of the ring inversion. Section 2 reports on the first examples of heteroleptic bis(dipyrrinato)zinc(II) complexes. Conventional homoleptic bis(dipyrrinato)zinc(II) complexes suffered from low fluorescence quantum yields, whereas the heteroleptic ones feature bright fluorescence even in polar solvents. Section 3 describes our new findings on Pechmann dye, which was first synthesized in 1882. New synthetic procedures for Pechmann dye using dimethyl bis(arylethynyl)fumarate as a starting material gives rise to its new structural isomer. We also demonstrate potentiality of a donor-acceptor-donor type of Pechmann dye in organic electronics.