Xian-Sheng Ke

Porphodilactones as Synthetic Chlorophylls: Relative Orientation of β-Substituents on a Pyrrolic Ring Tunes NIR Absorption

Porphodilactones represent the porphyrin analogues, in which the peripheral bonds of two pyrrole rings are replaced by lactone moieties. They provide an opportunity to investigate how β-substituent orientation of porphyrinoids modulates the electronic structures and optical properties, in a manner similar to what is observed with naturally occurring chlorophylls. In this work, a comprehensive description of the synthesis, characterization, and optical properties of meso-tetrakispentafluorophenylporphodilactone isomers is first reported. The β-dilactone moieties are found to lie at opposite pyrrole positions (trans- and cis-configurations are defined by the relative orientations of the carbonyl group when one lactone moiety is fixed), in accordance with earlier computational predictions (Gouterman, M. J. Am. Chem. Soc. 1989, 111, 3702). The relative orientation of the β-dilactone moieties has a significant influence on the electronic structures and photophysical properties. For example, the Qy band of trans-porphodilactone is red-shifted by 19 nm relative to that of the cis-isomer, and there is a 2-fold increase in the absorption intensity, which resembles the similar trends that have been reported for natural chlorophyll f and d. An in depth analysis of magnetic circular dichroism spectral data and TD-DFT calculations at the B3LYP/6-31G(d) level of theory demonstrates that the trans- and cis-orientations of the dilactone moieties have a significant effect on the relative energies of the frontier π-molecular orbitals. Importantly, the biological behaviors of the isomers reveal their different photocytotoxicity in NIR region (>650 nm). The influence of the relative orientation of the β-substituents on the optical properties in this context provides new insights into the electronic structures of porphyrinoids which could prove useful during the development of near-infrared absorbing photosensitizers.

Ytterbium(III) Porpholactones: β-Lactonization of Porphyrin Ligands Enhances Sensitization Efficiency of Lanthanide Near-Infrared Luminescence

The near-infrared (NIR) luminescence efficiency of lanthanide complexes is largely dependent on the electronic and photophysical properties of antenna ligands. Although porphyrin ligands are efficient sensitizers of lanthanide NIR luminescence, non-pyrrolic porphyrin analogues, which have unusual symmetry and electronic states, have been much less studied. In this work, we used porpholactones, a class of β-pyrrolic-modified porphyrins, as ligands and investigated the photophysical properties of lanthanide porpholactones Yb-1 a-5 a. Compared with Yb porphyrin complexes, the porpholactone complexes displayed remarkable enhancement of NIR emission (50-120 %). Estimating the triplet-state levels of porphyrin and porpholactone in Gd complexes revealed that β-lactonization of porphyrinic ligands lowers the ligand T1 state and results in a narrow energy gap between this state and the lowest excited state of Yb(3+) . Transient absorption spectra showed that Yb(III) porpholactone has a longer transient decay lifetime at the Soret band than the porphyrin analogue (30.8 versus 17.0 μs). Thus, the narrower energy gap and longer lifetime arising from β-lactonization are assumed to enhance NIR emission of Yb porpholactones. To demonstrate the potential applications of Yb porpholactone, a water-soluble Yb bioprobe was constructed by conjugating glucose to Yb-1 a. Interestingly, the NIR emission of this Yb porpholactone could be specifically switched on in the presence of glucose oxidase and then switched off by addition of glucose. This is the first demonstration that non-pyrrolic porphyrin ligands enhance the sensitization efficiency of lanthanide luminescence and also display switchable NIR emission in the region of biological analytes (800-1400 nm).

Expanded Rosarin: A Versatile Fullerene (C60) Receptor

An expanded rosarian (P3P6) with a bowl-like conformation has been prepared and characterized in a one-pot procedure that involves condensing a bispyrrole pyridine precursor (P1P2) with benzaldehyde, followed by oxidation. Single crystal X-ray diffraction analysis reveals a bowl-like conformation in the solid state with an upper rim diameter defined by the meso-phenyl substituents of ca. 13.5 Å and a depth of roughly 6.3 Å. P3P6 forms both 1:1 and 2:1 complexes with C60 in the solid state. DFT reveals similar energies for the two binding modes. A 1:1 binding stoichiometry dominates in 1,2-dichlorobenzene-d4 at the millimolar concentrations dictated by solubility consideration. The solution phase interactions between rosarian and C60 were studied using 1H NMR, UV-vis, and femtosecond transient absorption spectroscopies in 1,2-dichlorobenzene-d4 or toluene. To our knowledge, this is the first report of an unfunctionalized porphyrinoid that forms a well-defined complex with C60 in solution as well as in solid state.

β-conjugation of gadolinium(III) DOTA complexes to zinc(II) porpholactol as potential multimodal imaging contrast agents

Synthesis of two bimodal imaging agents consist of a hydrophobic zinc(II) tetrapentafluorophenylporpholactol core and a β-substituted hydrophilic Gd(III) DO3A (ZnLGd539) or DOTA (ZnLGd595) like moiety has been described (DO3A = 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid, DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). Through β-conjugation approach, the absorption at deep red region increases compared to traditional conjugation methods at meso-position. More importantly, these new complexes possess largely improved ionic relaxitivites relative to the sole Gd(III) magnetic resonance imaging (MRI) agents such as Gd(III) DO3A and Gd(III) DOTA like complexes. Combining the optical and magnetic resonance measurements in aqueous media, the largely enhanced r1 relaxivities was attributed to the aggregation of ZnLGd539 and ZnLGd595 in aqueous media. Furthermore, fluorescence and magnetic resonance imaging experiment showed that both ZnLGd539 and ZnLGd595 can be applied as potential bimodal imaging contrast agents. Finally, both compounds showed no dark cytotoxicity and good photocytotoxicity (IC50 = 1.73 ± 0.13 and 1.52 ± 0.10 μM for ZnLGd539 and ZnLGd595 respectively) on Hela cells.

Encoding, Reading, and Transforming Information Using Multifluorescent Supramolecular Polymeric Hydrogels

Traditional (1D, 2D, and 3D) codes are widely used to provide convenient readouts of encoded information. However, manipulating and transforming the encoded information is typically difficult to achieve. Here, the preparation of three fluorescent (blue, green, and red) hydrogels containing both tetracationic receptor-anion recognition motifs and gel-specific fluorophores is reported, which may be used as building blocks to construct through physical adhesion fluorescent color 3D codes (Code A, Code B, and Code C) that may be read out by a smartphone. As a result, parts of the individual gel components that make up Code B can be replaced with other gel building blocks to form Code A via a cut and adhesion approach. A fluorophore responsive to ammonia is further incorporated into one of the hydrogels. This allows the gel block-derived pattern that makes up Code C to be converted to Code A by chemical means. Therefore, the encoded information produced by patterns of the present hydrogels may be transformed through either physical action or by exposure to a chemical stimulus. Due to the nature of the soft materials involved, the codes can be used as wearable materials.

Flattened Calixarene-like Cyclic BODIPY Array: A New Photosynthetic Antenna Model

A cyclic BODIPY array, characterized by a rigid flattened calixarene-like conformation, acts as a photosynthetic antenna mimic. The system in question, triBODIPY, is a better antenna than the corresponding BF2-free ligand. On the basis of absorption spectral studies and supporting calculations, it is concluded that exciton coupling between the BODIPY subunits occurs readily. TriBODIPY supports a complex with Li+@C60 that bring an antenna-like light absorber into close proximity to an electronic acceptor without the need for linking spacers. As inferred from X-ray diffraction analyses of complexes between triBODIPY and C60, it is inferred that the fullerene complexes are stabilized via convex-concave donor-acceptor interactions. Steady state absorption/fluorescence, time-correlated single photon counting, and transient absorption measurements have allowed a complete characterization of the complexes in both the ground and excited states, including the host-guest recognition features, fluorescence quenching effects, and charge separation/recombination dynamics.

Bicyclic Baird-type aromaticity

Classic formulations of aromaticity have long been associated with topologically planar conjugated macrocyclic systems. The theoretical possibility of so-called bicycloaromaticity was noted early on. However, it has yet to be demonstrated by experiment in a simple synthetic organic molecule. Conjugated organic systems are attractive for studying the effect of structure on electronic features. This is because, in principle, they can be modified readily through dedicated synthesis. As such, they can provide useful frameworks for testing by experiment with fundamental insights provided by theory. Here we detail the synthesis and characterization of two purely organic non-planar dithienothiophene-bridged [34]octaphyrins that permit access to two different aromatic forms as a function of the oxidation state. In their neutral forms, these congeneric systems contain competing 26 and 34 π-electronic circuits. When subject to two-electron oxidation, electronically mixed [4n+1]/[4n+1] triplet biradical species in the ground state are obtained that display global aromaticity in accord with Bairds rule.

Hetero Cu(III)-Pd(II) Complex of a Dibenzo[g,p]chrysene-Fused Bis-dicarbacorrole with Stable Organic Radical Character.

Bis-dicarbacorrole (bis-H3) with two adj-CCNN subunits was synthesized by incorporating a dibenzo[g,p]chrysene moiety into the macrocyclic structure. The two trianionic cores in bis-H3 can stabilize two Cu(III) ions (bis-Cu) or concurrently a Cu(III) cation and a Pd(II) ion in the form of a hetero bis-metal complex (mix-Cu/Pd). As prepared, mix-Cu/Pd displays organic π radical character, as confirmed by various techniques, including electron paramagnetic resonance spectroscopy, cyclic voltammetry, femtosecond transient absorption measurements, and DFT calculations. Radical formation is ascribed to one-electron transfer from the dicarbacorrole backbone to the Pd center allowing the d8 Pd(II) center to be accommodated in a square planner coordination geometry. Nucleus-independent chemical shift and anisotropy of the induced current density calculations provide support for the conclusion that bis-H3 and bis-Cu both display antiaromatic character and contain two formally 16 π-electron dicarbacorrole subunits. On this basis, we suggest that mix-Cu/Pd is best considered as containing a fused 15 π-electron nonaromatic radical subunit and a 16 π-electron antiaromatic subunit. The spectroscopic observations are consistent with these assignments.

Metal-Stabilized Quinoidal Dibenzo[g, p]chrysene-Fused Bis-dicarbacorrole System

We report here a metal complexation-based strategy that permits access to a highly stable expanded porphyrin-type quinoidal polycyclic aromatic hydrocarbons (PAH). Specifically, double insertion of Pd(II) ions into a dibenzo[ g, p]chrysene-fused bis-dicarbacorrole (bis-H3) gives rise to a bis-metalated species (bis-Pd) that undergoes a facile benzenoid-quinonoid transformation. In contrast to what is true for the corresponding mono-Pd(II) complex, which has organic radical character, well resolved 1H NMR and 19F NMR spectra are seen for bis-Pd. This complex is also electron paramagnetic resonance (EPR) silent over a range of temperatures. On the basis of crystallographic analyses, Raman spectroscopic studies, harmonic oscillator model of aromaticity (HOMA), and nucleus-independent chemical shift (NICS) calculations, we suggest that the dibenzo[ g, p]chrysene bridge in bis-Pd has quinoidal character and that the system as a whole is a closed shell species. As expected for a quinoidal system, bis-Pd is characterized by a lowest energy absorption band that is shifted into the NIR (λmax = ca. 1420 nm (ε > 1.5 × 105 M-1 cm-1) for bis-Pd vs 780 nm (ε < 5.0 × 103 M-1 cm-1) for bis-H3). On the other hand, bis-Pd displays solvent dependent ground state and transient absorption spectral features. Such findings provide support for a zwitterionic resonance contribution to what is a predominantly a quinonoid-type ground state. The use of specific metalation to fine-tune the electronic features of polytopic ligands, as reported here, opens the door to what might be a potentially generalizable approach to the design of quinoidal PAH structures with long wavelength solvatochromic absorption features.