Synthesis, Physicochemical Properties and Anti-Fungal Activities of New Meso-Arylporphyrins

In this work, we describe the synthesis of three new meso-arylporphyrins, named meso-tetrakis [4-(nicotinoyloxy)phenyl] porphyrin (H2TNPP), meso-tetrakis [4-(picolinoyloxy)phenyl] porphyrin (H2TPPP), and meso-tetrakis [4-(isonicotinoyloxy) phenyl] porphyrin (H2TIPP). These new synthesized meso-arylporphyrins are characterized using spectroscopic analysis: Fourier Transform Infrared Spectroscopy (FTIR) and One-dimensional Nuclear Magnetic Resonance (1D NMR), and mass spectrometry (MS). The photophysical studies (UV-visible absorption, singlet oxygen (1O2) luminescence, and fluorescence emissions) demonstrate their potential uses as photosensitizers (PSs) in photodynamic therapy (PDT) applications. An in vitro investigation of the anti-fungal activity of H2TNPP, H2TPPP, and H2TIPP against Candida (C.) species (C. albicans, C. glabrata, and C. tropicalis) reveals that their minimum inhibitory concentration (MIC) values ranged from 1.25 to 5 mg/mL. In addition, their in vitro anti-fungal susceptibilities against three dermatophyte clinical isolates (Trichophyton rubrum, Microsporum canis, and Trichophyton mentagrophytes) are also evaluated and they demonstrate good anti-fungal activities. A molecular docking study of these meso-arylporphyrins as anti-fungal agents against C. tropicalis extracellular aspartic proteinases, Protein data Bank in Europe (PDBe code: 1J71) and Trichophyton rubrum Sialidases (PDBe code: 7P1D) underlines the possible interactions of H2TNPP, H2TPPP, and H2TIPP with the key amino acid residues of these fungal target proteins.

Luminescent Platinum(II) Complexes with Stimuli-Responsive Flexible Lewis Pair Ligands: Spectroscopic and Computational Studies

A series of new luminescent bimetallic platinum(II) complexes with stimuli-responsive flexible Lewis pair (FlexLP) ligands are described. The FlexLP ligands consist of a dimesitylboron Lewis acid and diphenylphosphine oxide Lewis base which are in equilibrium between the unbound open form and the Lewis adduct, controlled by the hydrogen bond donating strength of the solvent. Spectroscopic techniques and density functional theory (DFT) calculations were used to interpret the photophysics of the platinum(II) complexes. All complexes exhibit tunable absorption in the region of 300-500 nm and green to orange photoluminescence, depending on the ratio of weak (THF) to strong (MeOH) hydrogen bond donating solvent employed. Spectroscopic and computational data shows that phosphine and peripheral acetylide ligands on the platinum(II) centers have limited influence on the emission energy, indicating the emission originates from the FlexLP-dominated fluorescence. Using time-resolved transient absorption spectroscopy it is shown that the complexes undergo intersystem crossing (ISC) to the triplet excited state upon photoexcitation, and the ISC efficiency is affected by the peripheral acetylide ligands. The triplet excited state lifetime can also be manipulated by the state of the FlexLP ligand, with the closed form complexes having longer lifetimes than the open form complexes.

Synthesis and characterization of novel BODIPYs and their antioxidant, antimicrobial, photodynamic antimicrobial, antibiofilm and DNA interaction activities

In the current study, we synthesized and characterized new BODIPY derivatives (1-4) having pyridine or thienyl-pyridine substituents at meso- position and 4-dibenzothienyl or benzo[b]thien-2-yl moieties at 2-,6- positions. We investigated fluorescence properties and the ability to form singlet oxygen. In addition, various biological activities of BODIPYs such as DPPH scavenging, DNA binding/cleavage ability, cell viability inhibition, antimicrobial activity, antimicrobial photodynamic therapy (aPDT) and biofilm inhibition properties were performed. BODIPY derivatives BDPY-3 (3) and BDPY-4 (4) have high fluorescence quantum yields as 0.50 and 0.61 and 1 O2 quantum yields were calculated as 0.83 for BDPY-1 (1), 0.12 for BDPY-2 (2), 0.11 for BDPY-3 and 0.23 for BDPY-4. BODIPY derivatives BDPY-2, BDPY-3 and BDPY-4 displayed 92.54 ± 5.41%, 94.20 ± 5.50%, and 95.03 ± 5.54% antioxidant ability, respectively. BODIPY compounds showed excellent DNA chemical nuclease activity. BDPY-2, BDPY-3 and BDPY-4 also exhibited 100% APDT activity against E. coli at all tested concentrations. In addition to these, they demonstrated a highly effective biofilm inhibition activity against Staphyloccous aureus and Pseudomans aeruginosa. BDPY-4 showed the most effective antioxidant and DNA cleavage activity, while BDPY-3 exhibited the most effective antimicrobial and antibiofilm activity.

Scholl Reaction of ortho-Phenylene-Bridged Cyclic Pyrrole-Thiophene Hybrid Hexamer

The Scholl reaction of ortho-phenylene-bridged cyclic pyrrole-thiophene hybrid hexamer gave cyclophane-type [5]heterohelicene exclusively in 45% yield. The structure was unambiguously revealed by X-ray diffraction analysis. This helicenophane-type compound showed sharp absorption and fluorescence spectra, reflecting its rigid structure. The reaction path was analyzed on the basis of DFT calculations, and it was found that the formation of the [5]helicene-dimer is thermodynamically favored and further oxidation is prohibited due to the increased strain energy.

Charge Transfer, Intersystem Crossing, and Electron Spin Dynamics in a Compact Perylenemonoimide-Phenoxazine Electron Donor-Acceptor Dyad

With phenoxazine (PXZ) as the electron donor and perylene-3,4-dicarboximide (PMI) as the electron acceptor, we prepared a compact, orthogonal electron donor-acceptor dyad (PMI-PXZ) to study the spin-orbit charge transfer-induced intersystem crossing (SOCT-ISC). A weak charge transfer (CT) absorption band, due to S₀ → ¹CT transition, was observed (ε = 2840 M^-1 cm^-1 at 554 nm, FWHM: 2850 cm^-1), which is different from that of the previously reported analogue dyad with phenothiazine as the electron donor (PMI-PTZ), for which no CT absorption band was observed. A long-lived triplet state was observed (lifetime τ_T = 182 μs) with nanosecond transient absorption spectroscopy, and the singlet oxygen quantum yield (Φ_Δ = 76%) is higher than that of the previously reported analogue dyad PMI-PTZ (Φ_Δ = 57%). Ultrafast charge separation (ca. 0.14 ps) and slow charge recombination (1.4 ns) were observed with femtosecond transient absorption spectroscopy. With time-resolved electron paramagnetic resonance spectroscopy (TREPR), we confirmed the SOCT-ISC mechanism, and the electron spin polarization phase pattern of the triplet-state TREPR spectrum is (e, e, a, e, a, a), which is dramatically different from that of PMI-PTZ (a, e, a, e, a, e), indicating that the triplet-state TREPR spectrum of a specific chromophore in the electron donor-acceptor dyads is not only dependent on the geometry of the dyads but also dependent on the structure of the electron donor (or acceptor). Even one-atom variation in the donor structure may cause significant influence on the electron spin selectivity of the ISC of the electron donor-acceptor dyads.

Prompt and Long-Lived Anti-Kasha Emission from Organic Dyes

Anti-Kasha behavior has been the subject of intense debate in the last few years, as demonstrated by the high number of papers appearing in the literature on this topic, dealing with both mechanistic and applicative aspects of this phenomenon. Examples of anomalous emitters reported in the last 10 years are collected in the present review, which is focused on strictly anti-Kasha organic molecules displaying radiative deactivation from S_n and/or T_n, with n greater than 1.

Photocytotoxicity of Thiophene- and Bithiophene-Dipicolinato Luminescent Lanthanide Complexes

New thiophene-dipicolinato-based compounds, K₂nTdpa (n = 1, 2), were isolated. Their anions are sensitizers of lanthanide ion (Ln^III) luminescence and singlet oxygen generation (¹O₂). Emission in the visible and near-infrared regions was observed for the Ln^III complexes with efficiencies (ϕ^Ln) ϕ^Eu = 33% and ϕ^Yb = 0.31% for 1Tdpa^2- and ϕ^Yb = 0.07% for 2Tdpa^2-. The latter does not sensitize Eu^III emission. Fluorescence imaging of HeLa live cells incubated with K₃[Eu(1Tdpa)₃] indicates that the complex permeates the cell membrane and localizes in the mitochondria. All complexes generate ¹O₂ in solution with efficiencies (ϕO12) as high as 13 and 23% for the Gd^III complexes of 1Tdpa^2- and 2Tdpa^2-, respectively. [Ln(nTdpa)₃]^3- (n = 1, 2) are phototoxic to HeLa cells when irradiated with UV light with IC₅₀ values as low as 4.2 μM for [Gd(2Tdpa)₃]^3- and 91.8 μM for [Eu(1Tdpa)₃]^3-. Flow cytometric analyses indicate both apoptotic and necrotic cell death pathways.

Intersystem Crossing and Electron Spin Selectivity in Anthracene-Naphthalimide Compact Electron Donor-Acceptor Dyads Showing Different Geometry and Electronic Coupling Magnitudes

Anthracene-naphthalimide (An-NI) compact electron donor-acceptor dyads were prepared, in which the orientation and distance between the two subunits were varied by direct connection or with intervening phenyl linker. Efficient intersystem crossing (ISC) and long triplet state lifetime (Φ_Δ =92 %, τ_T =438 μs) were observed for the directly connected dyads showing a perpendicular geometry (81°). This efficient spin-orbit charge transfer ISC (SOCT-ISC) takes 376 fs, inhibits the direct charge recombination (CR) to ground state (¹ CT→S₀ , takes 3.04 ns). Interestingly, efficient SOCT-ISC for dyads with intervening phenyl linker (Φ_Δ =40 % in DCM) was also observed, although the electron donor and acceptor adopt almost coplanar geometry (dihedral angle: 15°). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy shows that the electron spin polarization of the triplet state, i. e. the electron spin selectivity of ISC, is highly dependent on the dihedral angle and the linker. For the dyads showing weaker coupling between the donor and acceptors, the charge separation and the intramolecular triplet energy transfer are inhibited at 80 K (frozen solution), because both the ³ An and ³ NI states were observed and the ESP are same as compared to the native anthracene and naphthalimide, which unravel their origin. The dyads were used as triplet photosensitizers for triplet-triplet annihilation upconversion (TTA UC). High UC quantum yield (Φ_UC =12.9 %) as well as a large anti-Stokes shift (0.72 eV) was attained by excitation into the CT absorption band.

Anthryl-Appended Platinum(II) Schiff Base Complexes: Exceptionally Small Stokes Shift, Triplet Excited States Equilibrium, and Application in Triplet-Triplet-Annihilation Upconversion

Two anthryl platinum(II) N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-benzenediamine Schiff base complexes were synthesized, with the anthryl attached via its 9 position (Pt-9An) or 2 position (Pt-2An) to the platinum (Pt) Schiff base backbone. The complexes show unusually small Stokes shifts (0.23 eV), representing a very small energy loss for the photoexcitation/intersystem crossing process, which is beneficial for applications as triplet photosensitizers. Phosphorescence of the Pt(II) coordination framework (Φ_P = 11.0%) is quenched in the anthryl-containing complexes (Φ_P = 4.0%) and shows a biexponential decay (τ_P = 3.4 μs/87% and 18.2 μs/13%) compared to the single-exponential decay of the native Pt(II) Schiff base complex (τ_P = 3.7 μs). Femtosecond/nanosecond transient absorption spectroscopy suggests an equilibrium between triplet anthracene (³An) and triplet metal-to-ligand charge-transfer (³MLCT) states, with the dark ³An state slightly lower in energy (1.96 eV for Pt-9An and 1.90 eV for Pt-2An) than the emissive ³MLCT state (1.97 eV for Pt-9An and 1.91 eV for Pt-2An). Intramolecular triplet-triplet energy transfer (TTET) and reverse TTET take 4.8 ps/444 ps for Pt-9An and 55 ps/1.7 ns for Pt-2An, respectively. The triplet-state equilibrium extends the triplet-state lifetime of the complexes to 103 μs (Pt-2An) or 163 μs (Pt-9An), in comparison to the native Pt(II) complex, which shows a lifetime of 4.0 μs. The complexes were used for triplet-triplet-annihilation upconversion with perylene as the triplet acceptor. The upconversion quantum yield is up to 15%, and a large anti-Stokes shift (0.75 eV) is achieved by excitation into the singlet metal-to-ligand charge-transfer absorption band (589 nm) of the complexes (anti-Stokes shift is 0.92 eV with 9,10-diphenylanthracene as the acceptor).

A novel BODIPY-based photosensitizer with pH-active singlet oxygen generation for photodynamic therapy in lysosomes

A novel photosensitizer BDPI-lyso has been developed for photodynamic therapy (PDT). The photosensitizer BDPI-lyso showed a high singlet oxygen quantum yield (ΦΔ = 0.95) and low fluorescence quantum yield (ΦF = 0.05) in EtOH. Different singlet oxygen quantum yields were found in acidic solution (pH = 5, ΦΔ = 0.51) and in neutral solution (pH = 7, ΦΔ = 0.38). DFT and TD-DFT calculations of BDPI-lyso and its protonated product BDPI-lysoH+ indicated that the S1/T3 transition was responsible for the intersystem crossing (ISC) enhancement which would promote the production of singlet oxygen. The negligible dark cytotoxicity toward the hepatoma cell line Bel-7402 was confirmed by MTT assay, AO/EB dual staining, and cell images. Upon exposure to a low dose of light illumination, the disruption of the cell plasma membrane and the calculated half-maximal inhibitory concentration (IC50) of 0.4 μM showed a high phototoxicity of the photosensitizer BDPI-lyso. The light-induced intracellular ROS generation was verified as the PDT mechanism of BDPI-lyso. Colocalization experiments of LysoTracker Green and BDPI-lyso in the dark indicated the good lysosome-targeting ability of BDPI-lyso. The images of cells costained with LysoTracker Green and BDPI-lyso, and the appearance of intracellular and extracellular blebs with green fluorescence after light illumination revealed the light-induced dysfunction of lysosomes and cell apoptosis.

Towards better understanding of the photophysics of platinum(ii) coordination compounds with anthracene- and pyrene-substituted 2,6-bis(thiazol-2-yl)pyridines

The photophysical properties of platinum(ii) compounds with 4-(9-anthryl)-2,6-bis(thiazol-2-yl)pyridine (1) and 4-(1-pyrenyl)-2,6-bis(thiazol-2-yl)pyridine (2) were widely investigated.