Soret Band Research Articles

Enhanced Femtosecond Nonlinearities and Multiphoton Absorptions in Discrete Bands of Porphyrins.

Optical nonlinearities of discrete absorption energy levels of one of the typical heterocyclic aromatic molecules, free-base porphyrins, have been probed over a broad spectral region (400-1600 nm) utilizing intense femtosecond pulses. A wide range of strong one- and multiphoton-induced nonlinear absorptions of both the blue-end Soret (B) band (au → b1g) and red-end orbital mixing split quasi-allowed Q-bands (Qx(0,0; 0,1), Qy(0,0; 0,1), au → eg) are critically probed and reported. During the resonant excitation within B- (400 nm) and Q-bands (600-750 nm), the nonlinear absorption has become predominant by the saturation of absorption (SA) of the one-photon absorption (1PA) process due to ground-state bleaching. At nonresonant wavelengths, it is dominated by the reverse saturation of absorption (RSA), involving various nonlinear processes of two-, three-, and four-photon (2PA, 3PA, and 4PA) absorptions, either to B- or Q-bands (1100-1600 nm). The laser intensity-dependent nonresonant (2PA, 800 nm) excitations for the prominent B-band show a distinct cross-over from SA to RSA, contributed by the excited-state absorption (ESA) utilizing a three-photon induced (3PA) process, whereas resonant (1PA, 400 nm) excitation reveals a systematic strong SA process. Both wavelength- and intensity-dependent nonlinear refractive index studies exhibit positive electronic Kerr-based self-focusing effects, with prominent contributions of nonlinear absorption and higher-order effects. The spectrally discrete, highly intense laser probing of individual energy bands and the consequent variety of nonlinearities can be broadly generalized for many free-base porphyrins and metalloporphyrins. The present studies provide a strong foundation and new insight into the broad categories of macrocycles, such as porphyrins and phthalocyanines, for myriad applications in nonlinear optics and bio/optophotonics.

NanomolarFluorescentDetectionofGuanine Using Tin Porphyrin.

5,10,15,20-tetramethoxyphenylporphyrinatotin (IV) (SnTMPP) was synthesised. SnTMPP exhibited Soret band at 432nm and emission peaks at 629 and 682nm. The fluorescence intensity of SnTMPP was quenched in the presence of guanine linearly in the range 4 × 10-9M to 7.2 × 10-8M and the quenching response was found to be stable even in the presence of other nucleosides such as adenine, cytosine, uracil, thymine, alanine, aspartic acid and ascorbic acid. The detection limit was found to be 0.17nM and the mechanism behind the decrease in the fluorescence intensity of SnTMPP in the presence of guanine is due to dynamic quenching, which was confirmed by cyclic voltammetric studies and life time studies. The CV studies illustrates the possibilty for an electron transfer between the guanine and the electron deficient metal core of SnTMPP.

Photophysical properties of 5-(4-acetamidophenyl)-10,15,20-tris(N-methylpyridinium-3-yl)porphyrin trichloride and its Zn(II) complex

The spectral and photophysical properties of two porphyrins, 5-(4-acetamidophenyl)-10,15,20-tris(N-methylpyridinium-3-yl)porphyrin trichloride (1) and its Zn(II) complex 2 were investigated in different solvents by absorption, steady-state and time-resolved fluorescence spectroscopy, and laser flash photolysis. The findings were corroborated by computations at the TD-B3LYP/6-31++G(d,p) level of theory. The absorption spectra of 1 and 2 are the typical for porphyrins and their Zn complexes with the Soret bands at ≈ 420 nm and 430 nm, respectively, and the Q-bands at 515–650 nm, and 560–605 nm. Smaller differences in the absorption spectra compared to the symmetric porphyrins were assigned to more pronounced HOMO and HOMO-1 energy levels splitting in the asymmetric molecules. The temperature dependence of the absorption spectra with hypsochromic shifts suggests the formation of H-aggregates. The emission spectra are situated at 600–700 nm, and the vibronic resolution of the spectra for 1 depends on the solvent and the excitation wavelength, which was assigned due to the tautomerism by H-transfer within the porphyrin core. Quantum yields of fluorescence for 1 and 2 in methanolic solutions are Φf = 0.072 and 0.051, respectively. Although, the steady-state fluorescence spectra do not indicate that 1 and 2 form aggregates in methanolic and aqueous solutions (at <10-4 M), singlet decays measured by time-correlated single photon counting for both compounds were fit to multiexponential functions, which was explained by formation of aggregates in more concentrated solutions. Furthermore, at very high concentrations of 1 in CH3OH (>10-4 M), the dynamic formation of aggregates was evidenced by new bathochromically shifted band in the emission spectrum and the growth kinetics in the time-resolved fluorescence measurement. Triplet excited states of 1 and 2 were probed by laser flash photolysis. The triplets are populated with quantum yields of intersystem crossing, ΦISC = 0.34 and 0.41, respectively and lifetimes of the triplet excited states are in millisecond time-scale. Both triplets are quenched by O2 with the rate constants approaching diffusion limits. Relative quantum yields for the singlet oxygen formation are 0.86 and 0.92 respectively, rendering porphyrins 1 and 2 applicable for photodynamic therapy.

Electronic properties and redox chemistry ofN-confused metalloporphyrins

Here we study the effect of metals on the characteristic Soret band of N-confused porphyrins. We used DFT calculations to study how this low-lying region of the spectrum of the NCP-2H isomer is affected by the introduction of transition metals with various ([Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] d-electron configurations. The spin ground state of these complexes is mostly dependent on the number of unpaired electrons, both with and without the presence of an axial ligand. The analysis of the electronic distribution and spin density showed that these unpaired electrons are often harbored by the N-confused porphyrin ring instead of on the metal. Time-dependent DFT results indicated that the aromatic system of porphyrin is disrupted in the N-confused isomer: instead of the typical large Soret band, this now gives rise to two peaks of much lower intensity. Most metallo-porphyrins exhibited similar optical properties, with the HOMO/LUMO orbitals showing a mixed metal/porphyrin character. The only exception was the Rh metalloporphyrin that exhibited a ligand-to-metal charge transfer band with increasing intensity as function of the ligand field. This suggests Rh is the only metal whose orbitals are higher in energy than the ligand’s, indicating that it is the only system where the redox processes occur on the metal.

Photoconductive metal-organic frameworks based on 10,20-meso-substituted Zn-porphyrin and fullerene C60

Metal-organic frameworks (MOFs) with electrical conductivity have potential applications as electrochemical sensors, electrocatalysis, and energy storage. We present the design of a new class of conductive MOFs based on porphyrin, substituted in the 10,20 meso-positions (1 amine, 2 hydroxy, 3 methoxy, 4 nitro, and 5 dimethylamine). Furthermore, these MOFs are doped with fullerene molecules (C60) within their pores, resulting in 1@C60, 2@C60, 3@C60, 4@C60, and 5@C60. To study the electronic and optical properties of these MOFs before and after the incorporation of C60, theoretical calculations based on density functional theory (DFT) (PBE+D3/DZP) and time-dependent DFT (TPSSh/6-31G) were performed. Incorporating C60 causes a decrease in the bandgap of MOFs due to its unoccupied crystalline p orbitals (LUCOs). It also promotes the Q and Soret (B) bands of porphyrin in the MOF to absorb at longer wavelengths. Charge transport properties were calculated using the Keldysh nonequilibrium Green's function (NEGF) formalism and molecular junctions with Au(111)-based electrodes for 1 and 1@C60. When a bias voltage of 0.2 V is applied, an increase in current is obtained for 1@C60 concerning 1. Our results suggest that MOF@C60 is a promising alternative for designing new photoconductive materials because of the donor-acceptor interactions generated.

Combining Pr3+-Doped Nanoradiosensitizers and Endogenous Protoporphyrin IX for X-ray-Mediated Photodynamic Therapy of Glioblastoma Cells.

Glioblastoma multiforme is an aggressive type of brain cancer with high recurrence rates due to the presence of radioresistant cells remaining after tumor resection. Here, we report the development of an X-ray-mediated photodynamic therapy (X-PDT) system using NaLuF4:25% Pr3+ radioluminescent nanoparticles in conjunction with protoporphyrin IX (PPIX), an endogenous photosensitizer that accumulates selectively in cancer cells. Conveniently, 5-aminolevulinic acid (5-ALA), the prodrug that is administered for PDT, is the only drug approved for fluorescence-guided resection of glioblastoma, enabling dual detection and treatment of malignant cells. NaLuF4:Pr3+ nanoparticles were synthesized and spectroscopically evaluated at a range of Pr3+ concentrations. This generated radioluminescent nanoparticles with strong emissions from the 1S0 excited state of Pr3+, which overlaps with the Soret band of PPIX to perform photodynamic therapy. The spectral overlap between the nanoparticles and PPIX improved treatment outcomes for U251 cells, which were used as a model for the thin tumor margin. In addition to sensitizing PPIX to induce X-PDT, our nanoparticles exhibit strong radiosensitizing properties through a radiation dose-enhancement effect. We evaluate the effects of the nanoparticles alone and in combination with PPIX on viability, death, stress, senescence, and proliferation. Collectively, our results demonstrate this as a strong proof of concept for nanomedicine.

Monitoring the solid-state VIS profiles of degrading bloodstains

Determining the time since deposition (TSD) of bloodstained evidence can be an important process in forensic investigations. Hemoglobin is often examined as a biomolecule of interest for these purposes due to the known ex vivo oxidative changes to its structure. These time-dependent oxidative processes have previously been probed using UV–VIS spectroscopy following the resuspension of bloodstains. Our study investigated the solid-state VIS spectra of degrading bloodstains without sample pre-treatment, effectively bypassing the need for resuspension. A total of 128 bloodstains from eight biological replicates were created and stored on glass slides in four temperature conditions: −20 °C, 4 °C, 22 °C, and 45 °C (see graphical abstract, created with BioRender.com). Spectra were acquired from 380−800 nm at five time points spanning 96 h. The peak area of the methemoglobin (metHb) band displayed the largest time and temperature differences, an interesting contrast to previous literature using the Soret band for TSD. Principal Component Analysis (PCA) demonstrated that storage temperature delineated the data, with the metHb band showing the greatest contributions to PC1. Linear mixed models from the PCA data with time showed clear TSD relationships with temperature, and with minimal inter-donor variability. Overall, this work complements the UV–VIS analysis of bloodstains for TSD estimation, with the importance of noting clear differences between phases and sample preparation methods.

Rhenium Biscorrole Sandwich Compounds: XAS Evidence for a New Coordination Motif.

The interaction of three free-base meso-tris(p-X-phenyl)corroles H3[TpXPC] (X = H, CH3, OCH3) with Re2(CO)10 at 235 °C in the presence of K2CO3 in o-dichlorobenzene has led to putative rhenium biscorrole sandwich compounds with the formula ReH[TpXPC]2. Density functional theory calculations and Re L3-edge extended X-ray absorption fine structure measurements suggest a seven-coordinate metal center, with the "extra" hydrogen located on one of the corrole nitrogens. The complexes can be deprotonated by a base such as 1,8-diazabicyclo[5.4.0]undec-7-ene, resulting in a substantial sharpening of the UV-vis spectra and split Soret bands, consistent with the generation of C2-symmetric anions. Both the seven-coordinate neutral and eight-coordinate anionic forms of the complexes represent a new coordination motif in the field of rhenium-porphyrinoid interactions.

CuTPP films grown on ceramic and ITO-coated glasses

α, β, γ, δ—Copper(II) tetraphenylporphyrinate (CuTPP) has been investigated as an active material in composites for optoelectronic devices, such as organic light-emitting diodes (OLEDs). This study focused on the spectroscopic, structural, and morphological analysis of CuTPP thin films obtained using layer-by-layer deposition. The infrared spectra of the films, deposited both on ceramic glass and indium tin oxide-coated glass, revealed that the substrate interacts with CuTPP, inducing the formation of aliphatic CH2 or CH3 and a general rearrangement of the bands related to the bending of phenyl groups. UV–vis spectroscopy indicated the presence of two bands at 350 and 472 nm in the films, instead of the intense band in the region between 380 and 500 nm corresponding to the Soret band. The structure of the complex was elucidated by single-crystal X-ray diffraction. X-ray powder diffraction analysis showed that films exhibited two peaks associated with the 101 and 103 planes, which noticeably increased after heating at 200 °C. Scanning electron microscopy images confirmed that the annealing favored the growth of two-dimensional square-shaped aggregates on the surface of the substrate.

Laterally Bound Co Porphyrin on CdTe QD: A Long-Lived Charge-Separated Nanocomposite

Cobalt porphyrin (CoP) derivatives are potential compounds for photocatalytic CO2 reduction which must be activated by photoinduced electron transfer from a suitable electron donor. Herein, we have prepared and studied the photophysics of CdTe quantum dots (CQD) coupled with CoP derivatives where CQDs act as the light antenna and the electron donor and CoP acts as the electron acceptor. To facilitate the nanocomposite formation of CoP with CQD, CoP has been equipped with a −COOH anchoring group which leads to strong complexation between CQD and CoP as observed in the absorption spectra by a gradual shift in the Soret absorption band. This is attributed to the lateral binding geometry of CoP through the −COOH anchoring group and Co-center coordination to CQD, which helps to bring CoP close to the CQD. Our DFT calculations have identified that this lateral geometry is more favorable than the upright orientation on the CdTe (110) surface. The redox levels have been determined from cyclic voltammetry which shows that the electron transfer (ET) from CQD to CoP is feasible. The strong luminescence quenching of CQD in the presence of CoP has also suggested quantitative CQD/CoP nanocomposite formation and pointed to the ET from QDs to CoP. The charge carrier dynamics have been monitored using femtosecond transient absorption (TA) spectroscopy. The TA spectral analysis has shown efficient ET in CQD/CoP which proves that our 4 nm CQD acts as an efficient electron donor for the CoP counterpart. The CQD excited state lifetime is shortened along with delayed Soret band bleaching of CoP in this nanocomposite. From the global fitting of TA data, the estimated average ET time constant from CQD to a CoP molecule is approximately 70 ps, and the charge recombination time is ≫5 ns. Also, differences in the TA spectra after ET have been observed which can be associated with the changes in the binding geometry of CoP on the CQD surface, which is lateral in the case of the ground-state complex to the upright orientation after the ET process. Hence, the studied CQD/CoP nanocomposites are promising materials to initiate CO2 reduction through photoexcitation of the CQD that activates the CoP molecular catalyst through the ET.

Dioxygen and glucose force motion of the electron-transfer switch in the iron(III) flavohemoglobin-type nitric oxide dioxygenase

Kinetic and structural investigations of the flavohemoglobin-type NO dioxygenase have suggested critical roles for transient Fe(III)O2 complex formation and O2-forced movements affecting hydride transfer to the FAD cofactor and electron-transfer to the Fe(III)O2 complex. Stark-effect theory together with structural models and dipole and internal electrostatic field determinations provided a semi-quantitative spectroscopic method for investigating the proposed Fe(III)O2 complex and O2-forced movements. Deoxygenation of the enzyme causes Stark effects on the ferric heme Soret and charge-transfer bands revealing the Fe(III)O2 complex. Deoxygenation also elicits Stark effects on the FAD that expose forces and motions that create a more restricted NADH access to FAD for hydride transfer and switch electron-transfer off. Glucose also forces the enzyme toward an off state. Amino acid substitutions at the B10, E7, E11, G8, D5, and F7 positions influence the Stark effects of O2 on resting heme spin states and FAD consistent with the proposed roles of the side chains in the enzyme mechanism. Deoxygenation of ferric myoglobin and hemoglobin A also induces Stark effects on the hemes suggesting a common ‘oxy-met’ state. The ferric myoglobin and hemoglobin heme spectra are also glucose-responsive. A conserved glucose or glucose-6-phosphate binding site is found bridging the BC-corner and G-helix in flavohemoglobin and myoglobin suggesting novel allosteric effector roles for glucose or glucose-6-phosphate in the NO dioxygenase and O2 storage functions. The results support the proposed roles of a ferric O2 intermediate and protein motions in regulating electron-transfer during NO dioxygenase turnover.

Photoinitiated Energy Transfer in Porous-Cage-Stabilised Silver Nanoparticles.

We report a new composite material consisting of silver nanoparticles decorated with three-dimensional molecular organic cages based on light-absorbing porphyrins. The porphyrin cages serve to both stabilize the particles and allow diffusion and trapping of small molecules close to the metallic surface. Combining these two photoactive components results in a Fano-resonant interaction between the porphyrin Soret band and the nanoparticle-localised surface-plasmon resonance. Time-resolved spectroscopy revealed the silver nanoparticles transfer up to 37 % of their excited-state energy to the stabilising layer of porphyrin cages. These unusual photophysics cause a 2-fold current increase in photoelectrochemical water-splitting measurements. The composite structure provides a compelling proof of concept for advanced photosensitiser systems with intrinsic porosity for photocatalytic and sensing applications.

Photoinitiated Energy Transfer in Porous‐Cage‐Stabilised Silver Nanoparticles

AbstractWe report a new composite material consisting of silver nanoparticles decorated with three‐dimensional molecular organic cages based on light‐absorbing porphyrins. The porphyrin cages serve to both stabilize the particles and allow diffusion and trapping of small molecules close to the metallic surface. Combining these two photoactive components results in a Fano‐resonant interaction between the porphyrin Soret band and the nanoparticle‐localised surface‐plasmon resonance. Time‐resolved spectroscopy revealed the silver nanoparticles transfer up to 37 % of their excited‐state energy to the stabilising layer of porphyrin cages. These unusual photophysics cause a 2‐fold current increase in photoelectrochemical water‐splitting measurements. The composite structure provides a compelling proof of concept for advanced photosensitiser systems with intrinsic porosity for photocatalytic and sensing applications.

Near-infrared absorption of fused core-modified expanded porphyrins for dye-sensitized solar cells.

Photophysical, photovoltaic, and charge transport properties of fused core-modified expanded porphyrins containing two pyrroles, one dithienothiophene (DTT) unit, and 1-4 thiophenes (1-4) were inspected by using density functional theory (DFT) and time-dependent DFT. Compounds 1-3 have been investigated experimentally before, but 4 is a theoretical proposal whose photophysical features match those extrapolated from 1 to 3. They exhibit absorption in the range of 700-970nm for their Q bands and 500-645nm for their Soret bands. The rise of thiophene rings placed in front of the DTT unit in the expanded porphyrin ring causes a bathochromic shift of the longest absorption wavelength, leading to near-infrared absorptions, which represent 49% of the solar energy. All the systems show a thermodynamically favorable process for the electron injection from the dye to TiO2 and adsorption on a finite TiO2 model. The electron regeneration of the dye is only thermodynamically feasible for the smallest expanded porphyrins 1 and 2 when I-/I3- electrolyte is used. The charge transport study shows that for voltages lower than 0.4V, junctions featuring pentaphyrin 1 and octaphyrin 4 are more conductive than those containing hexaphyrin 2 or heptaphyrin 3. The results showed that the four fused core-modified expanded porphyrins investigated are potential dyes for applications in dye-sensitized solar cells, mainly pentaphyrin 1 and hexaphyrin 2. Moreover, increasing the number of thiophene rings in the macrocycle proved fruitful in favoring absorption in the near-infrared region, which is highly desired for dye-sensitized solar cells.

A porphyrin derivative with highly red shifted and intensified absorption bands immobilized into the mesopores of nanostructured amberlite: Synthesis, characterization and photocatalytic activity

The BF3 complex of meso-tetra(4-sulfonatophenyl)porphyrin (H2TPPS4), immobilized into the pores of a new nano-structured mesoporous anion exchange polymer, nanoAmbN(Me)3Cl (Amb = Amberlite IRA-410(Cl)) was used as the photosensitizer for aerobic oxidation of organic compounds. NanoAmbN(Me)3Cl, nanoAmbN(Me)3@H2TPPS4 and nanoAmbN(Me)3@H2TPPS4(BF3)2, were characterized by IR and UV–vis diffuse reflectance spectroscopy (UV–vis DRS) methods, TGA, FESEM, elemental mapping (SEM-EDX) and BET. The nanocomposite was used as photosensitizer in the aerobic oxidation of 1,5-dihydroxynaphthalene (DHN) and cyclohexene in acetonitrile. The results were compared with those obtained in the presence of the BF3 complex of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (H2TMPyP) immobilized on nanostructured amberlyst 15. The observed red shifts in the UV–vis DRS UV–vis spectrum upon the formation of nanoAmbN(Me)3@H2TPPS4(BF3)2 were much larger than those of nanoAmbSO3@H2TMPyP(BF3)2. The singlet quatum yield of nanoAmbN(Me)3@H2TPPS4 and nanoAmbN(Me)3@H2TPPS4(BF3)2 was determined by using 1,3-diphenylisobenzofuran (DPBF) and DHN as the quenchers of singlet oxygen, for the excitation of the photosensitizers in the Q and Soret band regions, respectively.

Discovery of novel sulfonamide chromone-oxime derivatives as potent indoleamine 2,3-dioxygenase 1 inhibitors

A series of chromone-oxime derivatives containing piperazine sulfonamide moieties were designed, synthesized and evaluated for their inhibitory activities against IDO1. These compounds displayed moderate to good inhibitory activity against IDO1 with IC50 values in low micromolar range. Among them, compound 10m bound effectively to IDO1 with good inhibitory activities (hIDO1 IC50 = 0.64 μM, HeLa IDO1 IC50 = 1.04 μM) and were selected for further investigation. Surface plasmon resonance analysis confirmed the direct interaction between compound 10m and IDO1 protein. Molecular docking study of the most active compound 10m revealed key interactions between 10m and IDO1 in which the chromone-oxime moiety coordinated to the heme iron and formed several hydrogen bonds with the porphyrin ring of heme and ALA264, consistent with the observation by UV–visible spectra that 10m induced a Soret peak shift from 403 to 421 nm. Moreover, compound 10m exhibited no cytotoxicity at its effective concentration in MTT assay. Consistently, in vivo assays results demonstrated that 10m displayed potent antitumor activity with low toxicity in CT26 tumor–bearing Balb/c mice, in comparison with 1-methyl-l-tryptophan (1-MT) and 4-amino-N-(3-chloro-4-fluorophenyl)-N′-hydroxy-1,2,5-oxadiazole-3-carboximidamide (IDO5L). In brief, the results suggested that chromone-oxime derivatives containing sulfonamide moieties might serve as IDO1 inhibitors for the development of new antitumor agents.

Multifunctional heterogeneous catalysts: Tetrakis(pentafluorophenyl)porphinato]iron(III) immobilized on amine–functionalized Diatomaceous Earth for catalytic and adsorption applications

The use of Diatomaceous Earth (DE) as a promising support of a synthetic metalloporphyrin is reported, trying to heterogenize metalloporphyrin catalysts to mimicking enzyme site isolation and improving reaction selectivity. New multifunctional hybrid materials consisting of DE amino–functionalized with aminopropyltriethoxysilane (DE–APTES), followed by grafting with [meso–tetrakis(pentafluorophenyl)porphinato]iron(III) (DE–APTES–FeTFPP), were prepared and fully characterized. FeTFPP was grafted into DE–APTES via the amine groups (band at 1570 cm–1). The brown color of the materials indicated that FeTFPP was immobilized in the matrix; a Soret band characteristic of ironporphyrin located in a confined space, was found at 416 nm. Leaching studies confirmed that the ironporphyrin was entrapped and not just adsorbed on the silica surface. DE was composed of typical quartz and cristobalite crystalline phases and amorphous silica. The intensity of its characteristic reflection at 22° (2θ) decreased in the presence of FeTFPP, evidencing that the ironporphyrin influenced the organization of the material. Catalytic tests using DE–APTES–FeTFPP in cyclooctene epoxidation to cyclooctene oxide (56 % yield, with complete selectivity for the epoxide) and cyclohexane oxidation (4 % yield of oxidized products, with ketone/alcohol selectivity ∼ 3:1), evidenced the versatility of the catalyst and the multifunctionality of the resulting hybrid materials and the ability of DE as a promising natural support for ironporphyrin catalysts. Finally, the capacity of the materials as CO2 adsorbents was evaluated in the temperature range 100–200 °C. DE–APTES showed a maximum adsorption capacity of 1.26 mmol/g at 100 °C, 18 times higher than the value found under the same conditions for the non–functionalized support.

Highly Coupled Heterobicycle-Fused Porphyrin Dimers: Excitonic Coupling and Charge Separation with Coordinated Fullerene, C60.

Porphyrin dimers have been widely explored and studied owing to their importance in photosynthetic systems. A vast variety of dimers linked by different groups and at different angles have been synthesized and studied; however, the means by which to synthesize rigidly fused porphyrins with direct conjugation of the chromophores remains limited. Such a class of porphyrins may possess interesting properties that unconjugated or stacked dimers may not exhibit. In this study, bisbenzimidazole-fused porphyrin dimers and their mono- and bis-zinc derivatives are synthesized and characterized. As a consequence of excitonic coupling, these dimers exhibit a split Soret band irrespective of the metal ion in the porphyrin cavity. Steady-state fluorescence and excitation spectra followed by femtosecond transient absorption spectral studies of the heterometallated dimer, (free-base and zinc porphyrin) reveals the occurrence of efficient singlet-singlet energy transfer (>95 % efficiency and rate constant >1012 s-1 ) within the dyad. Further, donor-acceptor conjugates were formed by metal-ligand axial coordination of phenyl imidazole functionalized C60 and were characterized by a variety of physicochemical techniques. Excited state charge separation from both singlet and triplet excited states of ZnP in the conjugates has been established. The lifetime of the final charge-separated state was in the 30-40 μs range revealing charge stabilization. Interestingly, no charge separation in the conjugate derived from the heterometallated dimer was observed wherein excitation transfer dominated the process. The present study brings out the importance of the rigid π-spacer connecting porphyrin dimers in governing the energy and electron transfer events when coupled with an electron acceptor.

FORMATION FEATURES AND ELECTROCHROMIC PROPERTIES OF POLYPORPHYRIN FILMS BASED ON Zn-TETRAKIS(3-(HYDROXYPHENYL)PORPHYRIN

This paper presents the results of electrochemical deposition and the redox behavior of polyporphyrin films based on Zn(II) tetrakis(3–(hydroxyphenyl)porphyrin (ZnT(3–OHPh)P). It has been shown that electrochemical deposition initiated by superoxide anion–radical can be used to form films based on ZnT(3–OHPh)P. The potential ranges at which the electrochemical deposition of poly–ZnT(3–OHPh)P occurs on ITO electrodes were determined. The thickness of the obtained films based on ZnT(3–OHPh)P was determined by atomic force microscopy (AFM) and is about 90 nm. The presented results of studying the electronic absorption spectra of films based on ZnT(3–OHPh)P showed that the tetrapyrrole macrocycle is not destroyed during electrochemical deposition. The paper presents the results of Raman spectroscopy, which confirm that the main reason for the binding of hydroxyphenylporphyrin molecules in a polyporphyrin film is the formation of C–O–C bridges between side substituents. The redox processes in poly–ZnT(3–OHPh)P were studied by the spectroelectrochemical method. Electrochromic responses of films based on ZnT(3–OHPh)P have been determined in a wide range of potentials. Process of electroreduction leads to a decrease and a shift to the short–wavelength region of the Soret band. Electrooxidation of films based on ZnT(3–OHPh)P leads to a decrease in the intensity of the Soret band and the disappearance of the Q bands. It is shown that the observed spectral changes are in good agreement with the known literature data and indicate the localization of excess negative or positive charges on the macrocycle. Due to the change in their optical properties depending on the applied voltage, films based on ZnT(3–OHPh)P can be recommended as a promising electrochromic coating.

The binding of nitrogen-donor ligands to the ferric and ferrous forms of cytochrome P450 enzymes

The cytochrome P450 superfamily of heme-thiolate monooxygenase enzymes can catalyse various oxidation reactions. The addition of a substrate or an inhibitor ligand induces changes in the absorption spectrum of these enzymes and UV–visible (UV–vis) absorbance spectroscopy is the most common and readily available technique used to interrogate their heme and active site environment. Nitrogen-containing ligands can inhibit the catalytic cycle of heme enzymes by interacting with the heme. Here we evaluate the binding of imidazole and pyridine-based ligands to the ferric and ferrous forms of a selection of bacterial cytochrome P450 enzymes using UV–visible absorbance spectroscopy. The majority of these ligands interact with the heme as one would expect for type II nitrogen directly coordinated to a ferric heme-thiolate species. However, the spectroscopic changes observed in the ligand-bound ferrous forms indicated differences in the heme environment across these P450 enzyme/ligand combinations. Multiple species were observed in the UV–vis spectra of the ferrous ligand-bound P450s. None of the enzymes gave rise to the isolation of a single species with a Soret band at ∼442–447 nm, indicative of a 6-coordinate ferrous thiolate species with a nitrogen-donor ligand. A ferrous species with Soret band at ∼427 nm coupled with an α-band of increased intensity was observed with the imidazole ligands. With some enzyme-ligand combinations reduction resulted in breaking of the iron‑nitrogen bond yielding a 5-coordinate high-spin ferrous species. In other instances, the ferrous form was readily oxidised back to the ferric form on addition of the ligand.