Photodegradation of free base and zinc porphyrins in the presence and absence of oxygen.

Comparison of photostability in degassed and aerated toluene solutions is reported for 5,10,15,20-tetraphenylporphyrin, 5,10,15-tri(p-tolyl)porphyrin, and their zinc analogues. After degassing, quantum yields of photodegradation are higher, but the photodecomposition rates decrease. Lower stability in deoxygenated solutions is due to much longer triplet lifetimes: 200-300 microseconds, compared to 200-360 ns in non-degassed toluene. For the zinc porphyrins, the LC-MS results show that the initial photoproduct contains two oxygen atoms. Based on electronic absorption and calculations, it is assigned to dehydrated zinc biladienone structure, relatively stable in toluene, but readily demetallated in dichloromethane. A similar species is formed also in the case of free bases, but it then undergoes hydration due to traces of water present in the solvent. Zinc derivatives were found to form biladienones even in degassed solutions. To explain this observation, we postulate formation of a complex with remaining oxygen or oxygen-containing species which is not removed by freeze-thaw procedure. This hypothesis is confirmed by MS results and by the analysis of photodegradation products obtained when zinc porphyrin is complexed with dimethylsulfoxide (DMSO). Under these circumstances, changes in absorption are the same as in the absence of DMSO when non-degassed toluene is used, but irradiation of deoxygenated solutions leads to a different photoproduct. For both degassed and non-degassed solvents, complexation with DMSO results in the enhancement of photostability.

Spectroscopic investigation of photophysics and tautomerism of amino- and nitroporphycenes.

Parent, unsubstituted porphycene and its two derivatives: 2,7,12,17-tetra-n-propylporphycene and 2,7,12,17-tetra-t-butylporphycene were substituted at the meso position with amino and nitro groups. These two families of porphycenes were characterized in detail with respect to their spectral, photophysical, and tautomeric properties. Two trans tautomers of similar energies coexist in the ground electronic state, but only one form dominates in the lowest excited singlet state. Absorption, magnetic circular dichroism (MCD), and emission anisotropy combined with quantum-chemical calculations led to the assignment of S1 and S2 transitions in both tautomers. Compared with the parent porphycene, the S1-S2 energy gap significantly increases; for one tautomeric form, the effect is twice as large as for the other. Both amino- and nitroporphycenes emit single fluorescence; previously reported dual emission of aminoporphycenes is attributed to a degradation product. Introduction of bulky t-butyl groups leads to a huge decrease in fluorescence intensity; this effect, arising from the interaction of the meso substituent with the adjacent t-butyl moiety, is particularly strong in the nitro derivative.

Towards More Photostable, Brighter, and Less Phototoxic Chromophores: Synthesis and Properties of Porphyrins Functionalized with Cyclooctatetraene.

Free base and zinc porphyrins functionalized with cyclooctatetraene (COT), a molecule known as a good triplet-state quencher, have been obtained and characterized in detail by structural, spectral, and photophysical techniques. Substitution with COT leads to a dramatic decrease of the intrinsic lifetime of the porphyrin triplet. As a result, photostability in oxygen-free solution increases by two to three orders of magnitude. In non-degassed solutions, improvement of photostability is about tenfold for zinc porphyrins, but the free bases become less photostable. Similar quantum yields of photodegradation in free base and zinc porphyrins containing the COT moiety indicate a common mechanism of photochemical decomposition. The new porphyrins are expected to be much less phototoxic, since the quantum yield of singlet oxygen formation strongly decreases because of the shorter triplet lifetime. The reduction of triplet lifetime should also enhance the brightness and reduce blinking in porphyrin chromophores emitting in single-molecule regime, since the duration of dark OFF states will be shorter.

2 + 2 Can Make Nearly a Thousand! Comparison of Di- and Tetra-Meso-Alkyl-Substituted Porphycenes.

Two porphycenes, substituted at the meso positions with two and four methyl groups, respectively, reveal similar absorption spectra, but their photophysical properties are completely different. 9,20-dimethylporphycene emits fluorescence with about 20% quantum yield, independent of the solvent. In contrast, fluorescence of 9,10,19,20-tetramethylporphycene is extremely weak in nonviscous solvents, but it can be recovered by placing the chromophore in a rigid environment. We propose a model that explains these differences, based on calculations and structural analogies with other extremely weakly emitting derivatives, dibenzo[cde,mno]porphycenes. The efficient S1 deactivation involves delocalization of two inner cavity protons coupled with proton translocation toward a high-energy cis tautomer. The latter process leads to distortion from planarity. The probability of deactivation increases with the strength of the intramolecular NH···N hydrogen bonds. The model also explains the observation of biexponential fluorescence decay in weakly emitting porphycenes. It can be extended to other derivatives, in particular, the asymmetrically substituted ones. We also point to the possibility of using specific porphycenes as viscosity sensors, in particular, when working in single molecule regime.

Goodbye to Quinine in Sulfuric Acid Solutions as a Fluorescence Quantum Yield Standard.

Quinine is by far the most popular fluorescence quantum yield standard used nowadays. In this work, we exploit a modified version of a recently developed SAFE (simultaneous absorption and emission measurement) method for investigating the temperature dependence of fluorescence quantum yields. The accuracy of the method was verified using rhodamine 6G and rhodamine B solutions, which have well characterized quantum yield temperature dependences. Subsequently, we investigated the quantum yield temperature dependence of quinine solutions in 0.05 M sulfuric acid and 0.1 M perchloric acid. The results show a large temperature dependence of fluorescence quantum yield for quinine in 0.05 M sulfuric acid. This temperature dependence is particularly pronounced near room temperature (from 20 to 25 °C), where the quantum yield changes by -0.45% per Celsius degree. In contrast, the fluorescence quantum yield of quinine in 0.1 M perchloric acid shows no temperature dependence between 20 and 45 °C, where, in the entire range, the luminescence quantum yield value remains constant and equal to 0.60 ± 0.007. These results clearly indicate that it should be recommended to use quinine in 0.1 M perchloric acid rather than quinine in 0.05 M sulfuric acid as a standard solution for the determination of luminescence quantum yields.

Photoinduced oxidation of an indole derivative: 2-(1'H-indol-2'-yl)-[1,5]naphthyridine.

The UV-induced oxidation of 2-(1'H-indol-2'-yl)-[1,5]naphthyridine acetonitrile solution in the presence of air leads to the formation of 2-(1,5-naphthyridin-2-yl)-4H-3,1-benzoxazin-4-one as a major product and N-(2-formylphenyl)-1,5-naphthyridine-2-carboxamide as a minor one. The probable reaction mechanisms are different for the two photoproducts and may involve both the reaction with singlet oxygen generated by the excited substrate or the reaction of the excited substrate with the ground state oxygen molecule. Electronic absorption and IR spectra indicate that both photoproducts are formed as mixtures of syn and anti-rotameric forms. The obtained results indicate an efficient and easy method for the synthesis of molecules with a benzoxazinone structure.

Antiaromatic or Nonaromatic? 21 H,61 H-2,6(2,5)-Dipyrrola-1,5(2,6)-dipyridinacyclooctaphane-3,7-diene: a Porphycene Derivative with 4 N π Electrons.

A porphycene-derived compound with a 20 π-electron skeleton has been obtained by replacing two pyrrolene units of porphycene by pyridine rings. NMR, electronic absorption and MCD spectra, and the lack of fluorescence are typical for 4 N cyclic π electron systems. The electronic structure and the differences with respect to porphycene can be rationalized by treating these compounds as perturbed, doubly positively charged [22]annulene and [20]annulene perimeters, respectively. Even though the spectroscopic and photophysical criteria proposed for antiaromatic systems are fulfilled, the molecule is very stable. We argue that the compound should be characterized as nonaromatic rather than antiaromatic. The perimeter model is recommended as a powerful tool for predicting the electronic structure and spectra and as a useful addition to other methods that probe the aromaticity.

Versatile Approach for Reliable Determination of Both High and Low Values of Luminescence Quantum Yields.

The determination of luminescence quantum yields by means of relative methods requires setting identical experimental conditions for both sample and reference compounds. This requirement has a critical impact on the applicability of these protocols, as it does not allow for the precise determination of low quantum-yield values using well-characterized high-quantum-yield standards. We show that using the simultaneous absorption and fluorescence-emission measurement (SAFE) approach [ Nawara and Waluk. Anal. Chem. 2017 , 89 , 8650 ], the sample and reference compounds can be effectively measured with different excitation-slit spectral bandpass or integration times, separately optimized for each chromophore. This unique feature simplifies the determination of luminescence quantum yields, allowing measurements of low quantum-yield values using well-characterized high-quantum-yield standards.

Combined effect of hydrogen bonding interactions and freezing of rotameric equilibrium on the enhancement of photostability.

The photophysics and photostability of 12,13-dihydro-5H-indolo[3,2-c]acridine (IA), a rigid bifunctional indole derivative with proton donor/acceptor functionalities, can be drastically changed by the environment. The formation of hydrogen bonds with alcohols leads to a significant decrease of the triplet formation efficiency and an increase of photostability. The photodegradation yield was found to be about two hundred times lower in methanol and 1-propanol than in n-hexane or acetonitrile. A similar effect has been reported for two indole-naphthyridines, molecules that can exist in syn and anti rotameric forms. We demonstrate that IA, which can exist only in the syn form, is more photostable in alcohols than similar, but non-rigid molecules. This additional photostability enhancement is due to the elimination of a slower channel of excited state deactivation in alcohol complexes, S0 ← S1 internal conversion. The dominant, faster channel of S1 depopulation is the excited state double proton transfer, manifested by the presence of low energy tautomeric fluorescence.

Improved Method of Fluorescence Quantum Yield Determination.

In the most widely used procedure for luminescence quantum yield determination, absorption and emission spectra are measured on two different instruments. This leads to errors caused by wavelength misalignment and different monochromator characteristics of the spectrophotometer and the spectrofluorometer. These errors can be avoided using a method for fluorescence quantum yield determination that relies on simultaneous absorption and fluorescence emission (SAFE) measurement using a single commercial spectrofluorometer. The method's performance is compared with the standard routinely used procedure for the relative quantum yield determination. The advantages of SAFE measurement are discussed. The proposed novel approach eliminates a number of potential errors in quantum yield determination protocol and provides higher accuracy.

Parent, Unsubstituted Hemiporphycene: Synthesis and Properties.

Among seven possible nitrogen-in constitutional isomers of porphyrin only one, porphycene, has been obtained so far in the free, unsubstituted form. Herein, the synthesis of another isomer, parent hemiporphycene (HPc), and its thorough structural, spectral, photophysical, electrochemical, and theoretical characterization are reported. Most of the properties of HPc are intermediate between those of porphyrin and porphycene, as evidenced by the values of inner-cavity dimensions, orbital-energy splittings, absorption coefficients, magnetic circular dichroism parameters, NH-stretching frequencies, fluorescence quantum yields, tautomerization rates, and redox potentials. The largest differences arise with respect to tautomerism, due to the low symmetry of HPc and inequivalence of the four nitrogen atoms that define the inner cavity. Two trans tautomers are observed, separated in energy by about 1 kcal mol-1 . Tautomerization from the higher- to the lower-energy form is detected in the lowest-excited singlet state and occurs at a rate that is about four orders of magnitude lower than that observed for porphycene. Hemiporphycene is a very good model for the investigation of inequivalent intramolecular H-bonds present in one molecule; two such bonds in HPc reveal unusual characteristics, and the bond strength results from the interplay between the N⋅⋅⋅N distance and the N-H-N angle.

Photoinduced reactivity of doxorubicin: catalysis and degradation.

Doxorubicin exhibits unusual photoreactivity in aqueous solutions. Our data show that there are two distinct photoreactive pathways for doxorubicin. One is a two-step process that leads to the formation of 3-methoxysalicylic acid, a stable degradation product. The other pathway is a photoreduction of doxorubicin to form the corresponding dihydroquinone, which undergoes spontaneous oxidation mediated by dissolved oxygen to recover doxorubicin with the formation of hydrogen peroxide. Our data account for the known nonlinear dependence of doxorubicin fluorescence intensity on concentration.

A step towards gadolinium-free bioresponsive MRI contrast agent.

The last 30 years of gadolinium-based "static" MRI contrast agents motivated to investigate bioresponsive agents with endogenous paramagnets. Iron(III) chelated by N,O-aminophenol skeleton of high versatility, and tuning potential was studied. The two-step convenient route of the ligand is characterized by high selectivity and allows for building a tunable chelate system. Functionalization with galactose endows a bioresponsive character sensitive to the enzyme activity. Direct relaxometric measurements of the resulting complexes revealed extremely high relaxivity of 5.62 mmol/dm3·s-1 comparable to classic gadolinium complexes. Enzymatic hydrolysis leads to relaxivity change by over 80%. Phantom MRI studies prove the bioresponsive character by contras percentage change within the range 40-275%. Cytotoxicity studies showed 70-90% viability of HeLa cells of the iron complexes. Proposed iron-based chelates with galactosidase-sensitive fragment express unequivocal relaxivity and MRI contras change and good biocompatibility. Therefore, these complexes are a promising step towards modern, bioresponsive MRI contrast agents with a "human-friendly" metal.

Spectrofluorometer: the excitation beam intensity calibration using a single standard solution.

The accurate, periodically updated excitation beam intensity correction is essential for conducting fluorescence spectroscopy research. This article describes a simple and inexpensive approach to reevaluate the excitation calibration curve of a spectrofluorometer using a single dye solution. The method shows excellent agreement with the data obtained using a certified calibration detector for the broad spectral range from 290 nm to 700 nm.

Triarylisocyanurate-Based Fluorescent Two-Photon Absorbers.

The synthesis and characterization of six triarylisocyanurates, featuring 2,7-fluorenyl or 9,10-anthracenyl groups incorporated in their peripheral arms are reported. Photophysical studies reveal that these new octupolar derivatives are more fluorescent (ΦF ≥0.60 for all new compounds except for 1,3,5-tri(9H-fluoren-2-yl)-1,3,5-triazinane-2,4,6-trione 3) and present a red-shifted lowest absorption and emission compared to their known phenyl analogues of comparable size. Depending on the nature of the terminal substituent, fast intramolecular energy transfer among the three arms or localization of the excitation on a single branch occurs after population of their first singlet excited state. The latter effect was only observed in the presence of strongly electron-releasing substituents in polar media. These new chromophores are also better two-photon absorbers than the 1,4-phenylene-based isocyanurates reported so far, with cross sections σ2 ≥500 GM at 770 nm for 4-NPh2 the fluorenyl group containing (13) and the anthracenyl group containing (14) chromophores. All these spectroscopic features, analyzed with the help of quantum chemical calculations, are crucial for the design of new biphotonic fluorescent dyes.

The effect of locking π-conjugation in organoboron moieties in the structures of luminescent tetracoordinate boron complexes.

A series of 8 luminescent borafluorene complexes were extensively studied both experimentally and theoretically in order to elucidate the effect of organoboron moiety rigidification on the physicochemical properties of these compounds. Due to the spiro geometry of the boron atom, borafluorene and ligand units are perpendicularly aligned, which considerably affects the flexibility of the molecule as well as its solid-state structure. Through comparative analysis with close diphenyl analogues, we show how these structural features influence the thermal, photoluminescent and charge mobility behaviour of the studied compounds. Crystal structural analysis revealed that the molecules are connected mostly through C-HO and C-Hπ interactions formed between perpendicularly aligned borafluorene and ligand moieties from neighboured molecules, serving as a complementary donor and acceptor of electron density, respectively. This also efficiently prevents molecules from engaging in unfavoured π-stacking contact. Furthermore, structural analysis suggests that borafluorene complexes possess a considerable degree of flexibility due to OBN heterocycle distortions and mutual borafluorene-ligand plane movements. The magnitude of these effects strictly depends on the ligand structure and may lead either to enhancing or lowering the quantum yield value with respect to BPh2 analogues, while the absorption and emission wavelength are slightly affected. The measured photophysical parameters for solid-state samples showed that the studied complexes are much better emitters in their crystalline states that in amorphous films. The TD-DFT and NTO calculations revealed a significant change in frontier molecular distribution, with the HOMO localized on the borafluorene moiety. However, as the HOMO-LUMO transition is geometrically not favoured, excitation occurred from HOMO-1 localized on the ligand. Finally, aggregation effects were discussed based on supramolecular arrangements in crystal structures and charge transfer rates obtained from theoretical calculations in the framework of the Marcus-Hush approximation. They suggest that borafluorene complexes are much better electron carriers with respect to non-annulated BPh2 complexes.

The effect of conformational isomerism on the optical properties of bis(8-oxyquinolato) diboron complexes with a 2,2'-biphenyl backbone.

A fluorescent bis(8-oxyquinolato) diborinic complex with a central 2,2'-biphenyl backbone 1 and its octafluoro analogue 2 were synthesized to study the optical-structural relationship of sterically encumbered molecules featuring close intramolecular π-stacking interactions involving chromophore units. The crystal structure of 1 revealed a unique π-stacked arrangement of two pendant phenyl groups and two 8-oxyquinolato ligands (Q) located in the inner part of the complex. Unlike 1, the closely related complex 2 features conformational isomerism, and two major forms, namely 2-syn and 2-anti, are observed in solution to a varying extent depending on the solvent polarity. Form 2-syn, a geometrical analogue of 1, is preferable in polar solutions, whereas its rotational isomer 2-anti featuring π-stacking interactions between the terminal phenyl group and Q ligand dominates in benzene and chloromethane solutions. The observed conformational equilibria strongly affect the optical properties of the system, specifically leading to a significant increase of the quantum yield of emission (from 22% in MeCN to 38% in benzene) accompanied by a bathochromic shift (Δλ = 10 nm) of absorption and hypsochromic shifts (Δλ = -8 nm) of emission spectra with decreasing solvent polarity. This effect was ascribed to the variation in frontier orbital distributions.

Structure-dependent complexation of Fe3+ by anthracyclines. 1. The importance of pendent hydroxyl functionality.

We have investigated the stability of doxorubicin and daunorubicin complexes with Fe(3+) in aqueous solution. Doxorubicin and daunorubicin are anthracycline chemotherapeutic agents that differ structurally by the presence of a hydroxymethylketone functionality in doxorubicin versus a methyl ketone moiety in daunorubicin. We demonstrate that the presence of the hydroxyl group in doxorubicin enhances its 1:1 complexation with Fe(3+) relative to that seen for daunorubicin. We utilize UV-visible absorbance, circular dichroism, fluorescence and EPR spectroscopies, molecular dynamics, and semiempirical calculations to understand how the presence of an additional hydroxyl group affects the interactions of anthracyclines with Fe(3+). Our data indicate that the binding mode of iron in the complex is different for doxorubicin and daunorubicin.

Structure-dependent complexation of Fe3+ by anthracyclines. 2. The roles of methoxy and daunosamine functionalities.

We have investigated the effects of the methoxy and daunosamine sugar moieties on the stability of anthracycline complexes with Fe(3+) in aqueous solution. Idarubicin and daunorubicin are structurally very similar, differing only by the presence of the methoxy substituent at the 4-position. We demonstrate that the methoxy group interacts sterically with the proximal quinone oxygen atom and this interaction affects the stability of the anthracycline-iron(III) complex. A similar steric effect is seen for the pendent daunosamine moiety. Free daunosamine in solution does not show any significant interactions with iron(III), whereas the pendent daunosamine functionality destabilizes anthracycline-iron(III) complex formation. This body of information illustrates the important role of methoxy and daunosamine moieties in anthracycline-iron(III) complex formation and stability.

Solid-core and hollow magnetic nanostructures: Synthesis, surface modifications and biological applications.

In the need of development of versatile and flexible platforms for sensing, nanostructured particles are one of the systems of choice. Additionally, the state-of-the-art, controlled surface modifications of these structures offer broad possibilities of using such systems for diagnostics and therapy, often referred to as thera(g)nostics. In this brief review we will focus on the synthesis and surface modifications of solid-core magnetic nanostructures and polymeric capsules containing nanoferrites modified with anti-cancer drug--doxorubicin, designed for magnetic field-driven drug delivery for cancer therapy. We will also outline some problems related to the usage of such structures. The encapsulation and distribution of magnetic iron oxide nanoparticles modified with doxorubicin will be demonstrated in the polypyrrole spherical microvessels.