Quantum Yields Of Singlet Oxygen Formation Research Articles

Photophyical and photosensitizing properties of BODIPYs substantially changed by alkyl- and phenyl-amino groups on meso carbon

meso-RNH (R = C3H7, C4H9, PhCH2, H, and Ph) substituted BODIPY compounds have been prepared to examine their photophysical properties and photosensitizing abilities. We have measured the UV–vis absorption, steady state and time resolved fluorescence, excited triplet state formation using laser flash photolysis, singlet oxygen generation ability using chemical trapping method. The results show that the presence of meso-RNH leads to large blue shift of absorption and emission wavelength, remarkable decrease in fluorescence quantum yield and lifetime values, and significant increase in singlet oxygen formation quantum yield. Quantum chemical calculation also reveals the photoinduced charge transfer (PCT) mechanism. We conclude that property changes are due to: 1) S0 and S1 geometry, 2) ground state structural isomerization, and 3) intramolecular PCT. These results and mechanisms are helpful for designing new functional materials.

Halogenated BODIPY photosensitizers: Photophysical processes for generation of excited triplet state, excited singlet state and singlet oxygen

We have systematically examined the formation of singlet oxygen O2(1Δg), the excited triplet state (T1), and excited singlet state (S1) for halogenated BODIPY photosensitizers (halogen = Cl, Br, and I) in eight solvents to understand how halogen atoms and solvent affect these properties. The phosphorescence spectra and lifetimes of singlet oxygen generated by these halogenated BODIPYs have been measured by steady state/time resolved NIR emission, while the formation quantum yield of singlet oxygen (ΦΔ) has been determined by chemical method using diphenylisobenzofuran (DPBF) as the trapping agent. The formation quantum yield ΦΔ of singlet oxygen can be as high as 0.96 for iodinated BODIPY and 0.71 for brominated BODIPY. The triplet state T1 absorption spectra of brominated and iodinated BODIPYs have been recorded by laser flash photolysis method, in which T1 shows high formation efficiency and long lifetime. The formation and decay of excited singlet state S1 of four BODIPYs have been measured by ground state (S0) absorption and steady state/time resolved fluorescence. The results show that larger halogen atoms on BODIPY core lead to smaller fluorescence quantum yield, shorter fluorescence lifetime and higher singlet oxygen formation quantum yield due to heavy atom effect that promotes the formation of triplet state. On the other hand, higher solvent polarity causes lower singlet oxygen formation quantum yield, smaller fluorescence quantum yield, and shorter fluorescence lifetime. This solvent effect is explained by the presence of photoinduced charge transfer (ICT) process from halogen atoms to BODIPY. The ICT efficiency has been estimated and the results are agreed with ICT theory. ICT process in halogenated BODIPYs has never been revealed in literature. HOMO/LUMO obtained from DFT calculation also supports the presence of ICT. The involvement of ICT in the photosensitizing process of halogenated BODIPYs provides new insights for designing BODIPY photosensitizers for photodynamic therapy of tumor.

Deciphering the Structure-Property Relations in Substituted Heptamethine Cyanines.

Heptamethine cyanines (Cy7) are fluorophores essential for modern bioimaging techniques and chemistry. Here, we systematically evaluated the photochemical and photophysical properties of a library of Cy7 derivatives containing diverse substituents in different positions of the heptamethine chain. A single substitution allows modulation of their absorption maxima in the range of 693-805 nm and photophysical properties, such as quantum yields of singlet-oxygen formation, decomposition, and fluorescence or affinity to singlet oxygen, within 2-3 orders of magnitude. The same substituent in different positions of the chain often exhibits distinctly contradictory effects, demonstrating that both the type and position of the substituent are pivotal for the design of Cy7-based applications. The combination of experimental results with quantum-chemical calculations provides insights into the structure-property relationship, the elucidation of which will accelerate the development of cyanines with properties tailored for specific applications, such as fluorescent probes and sensors, photouncaging, photodynamic therapy, or singlet-oxygen detection.

BODIPY parent compound: Fluorescence, singlet oxygen formation and properties revealed by DFT calculations

The BODIPY (boron-dipyrromethene) parent compound is synthesized to measure its fluorescence properties and singlet oxygen generation ability. The ground state, excited singlet state S1 and triplet state T1 geometry and properties in gas, solid state and organic solvents are calculated by DFT method. This unsubstituted BODIPY (BDP) has good solubility in all solvents ranging from non polar n-hexane to highly polar water. BDP exhibits bright green fluorescence with high fluorescence quantum yield (Φf) and long fluorescence lifetime (τf) values in all solvents, Φf of BDP varies from the lowest 0.79 in toluene to the highest 0.98 in water, while τf varies from the shortest 6.02 ns in toluene to the longest 7.36 ns in water. τf and Φf show good linear correlation with the refractive index (n) of a solvent: τf = 17.13 – 7.36·n and Φf = 2.51–1.16·n (for air-saturated solution). The fluorescence lifetime in each solvent, however, becomes longer in deoxygenated solutions, suggesting that the presence of molecular oxygen leads to the formation of BDP T1 and singlet oxygen: BDP(S1)+O2 → BDP(T1)+1O2. Singlet oxygen formation quantum yield was measured to be up to 0.083. The absorption and emission maximum (λabs and λem), however, are only slightly changed by solvent nature: λabs= 500 ± 4 nm and λem= 510 ± 4 nm. The spectral shape and peak maximum for its absorption and emission are little affected by solvent nature. DFT calculation reveals that the geometry difference between S0 and S1 state of BDP is very small, so that the dipole moment change is also small. The calculated charges of carbon atoms, however, are very different and can well explain their different reactivity to nucleophiles. The calculated absorption spectra (peak maximum λcal) by TD-DFT method shows a good linear relation with experimental value λexp: λexp = 1.19 λcal. These results provide the basis for elucidating the mechanism or photophysical processes involving BODIPY as the functional units.

Photophysical properties of porphyrins with sterically distorted and partially screened macrocycles

We have experimentally studied the spectral-luminescent and photophysical properties, as well as interaction processes with molecular oxygen, of eclipsed β-alkyl-substituted porphyrins para-methoxyphenyl rings of which in opposite meso-positions of the macrocycle are linked with each other in the ortho-position with the (-OCH2)-Ph-(CH2O-) bridge. Using methods of the density functional theory, we have calculated the structures of these porphyrins in the ground and lowest triplet states, electronic transitions and vibrational states, and matrix elements of the direct spin-orbit interaction. It has been found that the lifetime of the T 1 state of the investigated compounds is noticeably shortened due to the enhancement of the internal conversion. This enhancement is caused by the conformational dynamics, which promotes increasing Franck-Condon factors and the spin-orbit interaction between the T 1 and S 0 states. Nevertheless, this does not lead to a considerable decrease in the singlet-oxygen formation quantum yield. We have calculated the structures of a porphin dimer and possible dimeric forms of examined porphyrins. It has been shown that distortion of the porphyrin macrocycle according to type of ruffling weakly affects the binding energy of the porphyrin macrocycles in the dimer, this effect being unable to prevent aggregation. At the same time, the screening of the macrocycle by the phenyl ring lowers the probability of formation of oligomeric structures, which, in view of the determined photophysical properties of investigated compounds, may be used in photochemical applications that require elevated concentrations of porphyrins.

Structural and photophysical properties of [(CO)3(phen)Re(μ-Br)Re(phen)(CO)3]+[(CO)3Re(μ-Br)3Re(CO)3]−: Where does its luminescence come from?

The multimetallic complex [[(CO)3(phen)Re(μ-Br)Re(phen)(CO)3][(CO)3Re(μ-Br)3Re(CO)3]]·CH2Cl2 was prepared by direct reaction of (Re(CO)3(THF)Br)2 (THF: tetrahydrofuran) and 1,10-phenanthroline in a 1:1 ratio, followed by recrystallization in dichloromethane. The compound is an ionic salt where both, cation and anion, are bimetallic complexes. Inside both of them the ReI centers are bridged by one or three bromides respectively. The compound has an absorption band centered at 375nm in CH2Cl2, which has been assigned to a MLCT band. Excitation at 375nm produces luminescent emission at 608nm. Comparison of these results with closely related rhenium complexes, in addition to Time Dependant-DFT analysis, allow us to propose the [(CO)3(phen)Re(μ-Br)Re(phen)(CO)3]+ cation as main responsible for luminescence. Luminescence lifetime and singlet oxygen formation quantum yield suggest also that the emissive excited state has a triplet character.

Concentration Increase of the Singlet-Oxygen Generation Quantum Yield by an Indotricarbocyanine Dye

Regularities in the generation of singlet oxygen sensitized by the indotricarbocyanine dye HITC with I– and ClO4 – anions in highly and slightly polar solvents were studied as functions of dye concentration (Cd). The yield of singletoxygen formation (γΔ) by HITC (I–) increased in slightly polar solvents as its concentration increased. It was shown that HITC with Cd < 10–7 M in CHCl3 and <5·10–6 M in o-dichlorobenzene with any anion was found primarily in the form of free ions. It was established that the concentration increase of singlet-oxygen generation by HITC (I–) was due to the heavy-atom effect of the anion as a result of their existence as contact ion pairs and an increase of the fraction of such pairs in a solution with an increased photosensitizer concentration. The quantum yield of singletoxygen formation for HITC with ClO4 – counterion was stable in various solvents as Cd was varied over a wide range.

Photochemical properties of squarylium cyanine dyes

This study presents several new squarylium dyes derived from benzothiazole and benzoselenazole with several structural variations, namely the nature of the heteroaromatic ring and the length of the N,N'-dialkyl groups. Before being investigated in connection with their effect on living cells and/or tissues, these novel compounds were characterized, namely with respect to the determination of their main photophysical parameters. Therefore, a study of the ground state absorption, fluorescence emission (quantum yields and lifetimes) and singlet oxygen generation quantum yields was performed for all the compounds synthesized in order to evaluate their efficiency as photosensitizers. An increase of the alkyl chain length from ethyl to hexyl did not produce a clear change in the fluorescence quantum yields, showing no influence on the photoisomerization process. Heavy atom inclusion (Se instead of S) enhanced the singlet oxygen generation efficiency and decreased the intensity of the fluorescence emission. The external heavy atom effect (I(-) as a counterion instead of CF3SO3(-)) produced a significant increase in the singlet oxygen formation quantum yield (about 20%). Transient absorption studies in aerated and oxygen free samples revealed that the photoisomerization process, which could compete with the triplet state formation for all dyes in solution, is a negligible pathway for the excited state deactivation, in accordance with the rigidity introduced by the squaric ring into the polymethine chain of the dye, both in chloroform and ethanol. However, in the case of the chloroform solution a new transient was detected in air equilibrated solutions, resulting from a reaction of the excited squarylium dye in the singlet state with CHCl3˙, and assigned to the radical cation (SQ(+)˙) of the dye.

Effect of pH on Methylene Blue Transient States and Kinetics and Bacteria Photoinactivation

We have measured directly by time-resolved spectroscopy the transient spectra and kinetics of the methylene blue (MB) excited singlet and triplet state as a function of pH from a few picoseconds to several microseconds. The data show that the acidic triplet state (3)MBH(2+) is the protonated analogue of the basic (3)MB(+). It is also shown that the singlet oxygen formation quantum yield is much higher in basic than in acidic media. The transient spectra and their kinetics suggest that because pH exerts a large influence in singlet oxygen and radical formation, it may also be important in bacteria inactivation. Therefore, we performed experiments, which showed that the rate of gram-positive and gram-negative bacteria inactivation at pH 9 is 3-25 times higher than the rate at pH 5.

In Vitro Photodynamic Activity of 5,15‐bis(3‐Hydroxyphenyl)porphyrin and Its Halogenated Derivatives Against Cancer Cells

5,15-Diarylporphyrins (1-5) with hydroxyl groups and halogens as substituents were prepared by condensation between unsubstituted dipyrromethane and halogenated m-hydroxybenzaldehydes. Photophysical properties show that the nonhalogenated porphyrin 1 has higher fluorescence yield but lower singlet oxygen formation quantum yield than the halogenated derivatives due to the heavy atom effect. The in vitro activity of these derivatives was tested against WiDr colorectal adenocarcinoma and A375 melanoma cancer cells. All porphyrins present a much higher phototoxicity than Photofrin with IC(50) values lower than the 50 nM level for WiDr cells and 25 nM level for A375 cancer cells. The most photoactive compound is the nonhalogenated porphyrin 1 which also presents the highest uptake. Halogenated derivatives present much lower uptakes than 1. However, their photoactivity is similar to compound 1 showing that their intrinsic photoactivity (ISP) is very high. Iodinated compound 4 presents the highest ISP. The greater ability of these porphyrins to destroy cancer cells could be related to their photophysical and photochemical properties.

The influence of the support on the singlet oxygen quantum yields of porphyrin supported photosensitizers

A halogenated porphyrin was covalently supported on aminoalkylated silica and aminoalkylated Merrifield polymer through a chlorosulfonation reaction. The supported porphyrin was tested for the photoxidation of α-terpinene. The kinetics of photocatalysis were evaluated using 9,10-dimethylanthracene as target and singlet oxygen formation quantum yield was calculated. The influence of the support, silica, Merrifield and other polymers, on the quenching of singlet oxygen were also evaluated.

Singlet oxygen quantum yields of potential porphyrin-based photosensitisers for photodynamic therapy

The singlet oxygen formation quantum yield (Phi(Delta)) for solutions of the di-cation, free-base and metallated forms of hematoporphyrin derivative (HpD), hematoporphyrin IX (Hp9) and a boronated protoporphyrin (BOPP) are reported using the method of direct detection of the characteristic phosphorescence following polychromatic excitation. Values of Phi(Delta) for the free-base form of all the porphyrins and the di-cation forms of Hp9 and HpD are in the range of 0.44 to 0.85 in the solvents investigated. Incorporation of zinc ions into the macrocycle reduces Phi(Delta) for all three porphyrins. BOPP facilitates the coordination of certain transition metals (Mn, Co and Cu) compared to Hp9 and HpD and results in a dramatic decrease in Phi(Delta). The experimental data suggest the introduction of low energy charge transfer states associated with the disruption of the planarity of the macrocyclic ring provides alternative non-radiative deactivation pathways. In BOPP, this non-planarity is augmented by the large closo-carborane peripheral substituent groups.