Triplet State Quantum Yields Research Articles

Polarization effect in the electronically excited state of benzene derivatives and related compounds

The energies of intramolecular charge-transfer bands and excited singlet and triplet states, as well as the molar absorption coefficients of mono- and para-disubstituted benzenes and monosubstituted naphthalenes and pyridines depend not only on the inductive and resonance substituent effects, but also on their polarization effect which can be characterized by σα constants. The polarization effect determines the lifetime of the triplet state and phosphorescence quantum yield of monosubstituted naphthalenes.

New photochemically stable riboflavin analogue—3-Methyl-riboflavin tetraacetate

Spectroscopic and photophysical properties of a flavin analogue – 3-methyl-riboflavin tetraacetate – were studied in methanol, ethanol, water and acetonitrile. We compared experimental spectroscopic data with the results of theoretical predictions, obtained using the TD-DFT method. Based on these calculations, we assigned (π,π *) symmetry to both the lowest excited singlet and triplet states. We found the title compound to be a very efficient photosensitizer of singlet oxygen production ( ϕ Δ = 0.61). The triplet state quantum yield of 3-methyl-riboflavin tetraacetate was determined as 0.54 in methanolic solutions. Photodegradation quantum yield measurements demonstrate that the title compound may be used as a much more stable substitute of riboflavin, being two orders of magnitude more photostable ( φ R = 2 × 10 −5). We also present exhaustive crystallographic characteristics of 3-methyl-riboflavin tetraacetate, along with time-resolved fluorescence spectra of its polycrystals.

Triplet-state spectroscopy of dipyrromethene·BF 2 laser dyes

The substituents at positions 2, 6 and 8 in the commercial dye PM567 have been changed and their influence on the triplet state of each dye has been studied in terms of the absorption spectra, lifetimes, quantum yields and triplet–triplet extinction coefficients. The dyes have in position 8 the groups p-acetoxypolymethylene, p-(acetoxypropyl)phenyl or p-(acetoxymethyl)polyphenylene, or in positions 2 and 6 the groups tert-butyl (commercial dye PM597) or 3′-acetoxypropyl. The triplet-state spectroscopy results nearly independent on the molecular structure for the mono- and di-substituted aliphatic derivatives. However, the 8-aryl substitution increases both the triplet absorption over the fluorescence spectral region and the triplet-state quantum yield, but reduces the triplet absorption coefficient.

Anion of hypericin is crucial to understanding the photosensitive features of the pigment

Photosensitive behaviors of hypericin (HYP) have attracted much attention, because of HYP’s great potential in photodynamic therapy. It has been found that HYP differs from homologous pigments, such as hypocrellin A (HA), in photosensitive features. For instance, despite the comparable triplet state quantum yields, HYP holds a much lower singlet oxygen yield than HA. To understand the unique photosensitive behaviors of HYP, time-dependent density functional theory is employed to calculate a series of excited-state properties of HYP and its anion (dominant in polar solvents), which are then compared with excited-state properties of HA. It is revealed that the stronger electron-donating power of HYP anion than that of HA is responsible for the HYP’s photosensitive features.

New photocleavable structures III: Photochemistry and photophysics of pyridinoyl and benzoyl-based photoinitiators

Pyridine ketones have recently been found to be suitable α-cleavage photoinitiators for radical polymerization with excellent water solubility. Nanosecond time-resolved laser flash photolysis and steady-state photolysis were employed to study and compare the triplet photophysics and the photochemistry of 2-hydroxy-2-methyl-1-pyridin-3-yl-propan-1-one ( 1b) and its commercial benzoyl analogue 2-hydroxy-2-methyl-1-phenylpropan-1-one (Darocur 1173 ®, 1a), and of some of their less photoreactive derivatives, in acetonitrile solution. The triplet state quantum yield, measured by energy transfer to naphthalene, was smaller for 1b ( Φ T = 0.10) than for 1a ( Φ T = 0.25), but the photodegradation quantum yields in the presence of the radical trap TEMPO were comparable ( Φ D = 0.6). The discrepancy between the yields of triplet formation and photodegradation points to a possible role of higher excited triplet states in the process of photoinduced α-cleavage. The reactivities of the primary radicals with a series of reaction partners showed that the benzoyl and the nicotinoyl radical have comparable reactivities towards polymerizable double bonds, but that the nicotinoyl radicals reacts 17 times faster with TEMPO. The previously observed reduced oxygen sensitivity in photopolymerization using 1b compared to 1a could be traced to a smaller reactivity towards O 2 of the nicotinoyl radical compared to the benzoyl radical. The photodegradation kinetics of 1a and 1b under steady-state conditions were comparable, but significant differences were observed in a photoproduct study.

The photophysical characteristics of 2- butylamino-2-demethoxy-hypocrellin B

2- Butylamino-2-demethoxy-hypocrellin B (BAHB), is an efficient photodynamic sensitizer, excited singlet-state energies are determined from the overlap of the normalized from absorption and emission spectra to be 176.4 kJ mol −1 ( E S CT ) and 202.8 kJ mol −1 ( E S 1 ), respectively. The quantum yields of BAHB fluorescence are 0.0098 ( Φ f1) and 0.0076 ( Φ f2), respectively. The energies and quantum yields of excited triplet states of BAHB are examined from the external effect of heavy atom on its florescence emission in the mixture of n-propyl iodide and benzene. The energies of excited triplet states of BAHB are 134.4±10.5 kJ mol −1 ( E T CT ) and 158.6±10.5 kJ mol −1 ( E T 1 ), respectively. The quantum yields of excited triplet states of BAHB are 0.73±0.04 ( Φ T CT ) and 0.56±0.04 ( Φ T 1 ), respectively. Meanwhile, the kinetic constants of fluorescence emission, intersystem crossing and internal conversion are also calculated from relative equations.

Time resolve study on isopropyl-ether porphyrindiol

A novel fat-soluble porphyrin derivative——isopropyl-ether porphyrindiol (IPEP) has been investigated using the nanosecond laser photolysis and pulse radiolysis. In acetonitrile, IPEP-excited triplet state shows two relative absorption areas peaking at 320 and 430 nm, molar extinction coefficients by energy transfer method (in benzene solution) are proved to be e (315) = 1.07×10 5 , e (450) = 3.3×10 4 L 3 ·mol -1 ·cm -1 and the excited triplet state quantum yield by comparative method is 0.18. Laser photolysis of IPEP in dGMP-containing solution shows that the triplet state cannot excite/oxidize dGMP via the energy/electron transfer process.

Photophysical Efficiency Factors of Singlet Oxygen Generation from Core-modified Trithiasapphyrin Derivatives

The photophysical properties and the singlet oxygen generation efficiencies of meso-tetraphenyl-trithiasapphyrin (S 3 TPS) and meso-tetrakis(p-methoxyphenyl)-trithiasapphyrin ((p-MeO)-S 3 TPS) have been investigated, utilizing steady-state and time-resolved spectroscopic methods to elucidate the possibility of their use as photosensitizers for photodynamic therapy (PDT). The observed photophysical properties were compared with those of other porphyrin-like photosensitizers in geometrical and electronic structural aspects, such as extended π conjugation, structural distortion, and internal heavy atoms. The steady-state electronic absorption and fluorescence spectra were both red-shifted due to the extended π-conjugation. The fluorescence quantum yields were measured as very small. Even though intersystem crossing rates were expected to increase due to the increment of spin orbital coupling, the triplet quantum yields were measured as less than 0.15. Such characteristics can be ascribed to the more enhanced internal conversion rates compared with the intersystem crossing rates. Futhermore, the triplet state lifetimes were shortened to ∼1.0 μs as expected. Therefore, the singlet oxygen quantum yields were estimated to be near zero due to the fast triplet state decay rates and the inefficient energy transfer to the oxygen molecule as well as the low triplet quantum yields. The low efficiencies of energy transfer to the oxygen molecule can be attributed to the lower oxidation potential and/or the energetically low lying triplet state. Such photophysical factors should be carefully evaluated as potential photosensitizers that have extended π-conjugation and heavy core atoms synthesized for red-shifted absorption and high triplet state quantum yields.

Effect of aggregation on bacteriochlorin a triplet-state formation: a laser flash photolysis study.

Bacteriochlorin a (BCA) is a potential photosensitizer for photodynamic therapy of cancer. It has been shown previously that the photoefficiency of the dye is mainly dependent on singlet oxygen (1O2) generation. Nanosecond laser flash photolysis was used to produce and to investigate the excited triplet state of the dye in methanol, phosphate buffer and dimiristoyl-L-alpha-phosphatidylcholine (DMPC) liposomes. The transients were characterized in terms of their absorption spectra, decay kinetics, molar absorption coefficients and formation quantum yield of singlet-triplet intercrossing. The lifetime of the BCA triplet state was measured at room temperature. The triplet-state quantum yield is quite high in methanol (0.7) and in DMPC (0.4) but only 0.095 in phosphate buffer. In the last case, BCA is in a monomer-dimer equilibrium, and the low value of the quantum yield observed was ascribed to the fact the triplet state is only formed by the monomers.

Effect of core atom modification on photophysical properties and singlet oxygen generation efficiencies: tetraphenylporphyrin analogues core-modified by oxygen and/or sulfur

Photophysical properties and singlet oxygen generation efficiencies of oxa- and/or thia-porphyrins have been investigated by time-resolved and steady-state spectroscopic methods. Introduction of sulfur atom into the porphyrin core induces a slight increment in the internal conversion rate and it also largely increases the intersystem crossing rate due to spin–orbit coupling enhancement, resulting the enhanced triplet state and singlet oxygen quantum yields. However, the quantum yields of mono oxygen-substituted porphyrin are reduced due to relatively more enhanced internal conversion. These suggest that sulfur modification in the core improves the photophysical efficiency of photosensitizers in the application respect of photodynamic therapy (PDT).

Optical limiting performance of dicyanodihydrofullerene

The properties of the excited triplet state of dicyanodihydrofullerene [C 60(CN) 2] were determined with nanosecond laser photolysis and a picosecond/nanosecond optical limiting (OL) system. It was found that C 60(CN) 2 possessed a lower quantum yield and higher extinction coefficient of the excited triplet state, which in turn resulted in its similar OL performance for nanosecond laser pulse compared with pristine C 60. However, C 60(CN) 2 shows a better OL performance for picosecond laser pulse, which can be understood according to a three-level model.

Photophysical and electron transfer studies of a stable carbocation

Photophysical and electron transfer properties of the stable trioxatriangulenium carbocation ( 1) are reported. Photophysical studies include absorption, fluorescence and phosphorescence spectra, singlet and triplet state quantum yields and lifetimes. Both the singlet and triplet excited states of 1 can accept an electron from donor molecules leading to the formation of the donor radical cation and the radical of 1. In aqueous solution, 1 can photo-oxidize DNA nucleosides such as guanosine and adenosine indicating that 1 may have potential use as a DNA cleaving agent.

Maleimides as electron-transfer photoinitiators: quantum yields of triplet states and radical-ion formation

The knowledge of quantum yields of triplet states and, moreover, of initiating radicals is a crucial prerequisite to evaluate any novel photoinitiator system. This work provides triplet quantum yields of N-methylmaleimide (0.03±0.01), N-ethylmaleimide (0.07±0.01) and N-propylmaleimide (0.05±0.02) determined by relative actinometry using laser flash photolysis and acetone sensitisation. Unsubstituted maleimide on the other hand shows rapid triplet state tautomerisation, not allowing the application of relative actinometry with triplet sensitisation. The triplet quantum yield was then determined by comparing the transient conductivity of the enolate formed with conductivity actinometry; it is unity.The yield of initiating radicals is only a fraction of the triplet yield, as the electron transfer reaction includes an efficient back donation. The radical ion yields measured span from 2% (thiocyanate) to 28% (allylthiourea) of the triplet quantum yield.

Room‐Temperature Near‐Infrared Metastable Species Measured by Phase‐Sensitive Lock‐in and Fourier Transform Techniques

AbstractAn ultrasensitive detecting system coupled with either a lock‐in or Fourier transform technique has been used to detect near‐infrared (1000‐2500 nm) metastable species in room temperature solutions. These species include O2(1Δg), O2(1σ+g), the solvent induced satellite peak of molecular singlet oxygen (1Δg) emission, and the triplet state of bis(triisobutylsiloxy)silicon‐2,‐3‐naphthalocyanine (SilNC). Using the O2(1Δg) emission in benzene as a standard, the quantum yield and radiative decay rate of SiINC triplet state have been determined. Depending on types of spectral acquisition, special techniques such as phase‐distinguishing and step scan capabilities were utilized. Their advantages and disadvantages are discussed.

Photophysical studies on the interaction of two water-soluble porphyrins with bovine serum albumin. Effects upon the porphyrin triplet state characteristics

The interaction of two metal-free water soluble porphyrins (PPh): meso-tetrakis ( p-sulfo-natophenyl)porphyrin (TPPS 4) and meso-tetrakis(4- N-methyl-pyridiniumyl)porphyrin (TMPyP), with bovine serum albumin (BSA) was investigated in the pH range from 4.0 to 8.5 using the flash-photolysis technique in comparison with results obtained by optical absorption, fluorescence and resonance light scattering. It was found that in the presence of BSA, TPPS 4 can exist in aqueous solutions as free monomers, aggregates and/or monomers bound to a single BSA molecule and monomers inside the BSA aggregates, while TMPyP does not form aggregates at binding. Binding to BSA transforms the profile of the triplet decay curve from monoexponential to biexponential form with the component lifetimes and relative amplitudes depending on binding characteristics. The triplet lifetime of a bound porphyrin monomer is longer than that of a free one. The aggregation of TPPS 4 observed at [TPPS 4]/[BSA] > I reduces the T-T absorption since the lifetimes of the aggregate excited states are very short and/or quantum yield of the aggregate triplet state is very low. The porphyrin binding to BSA reduces the quenching constants of the porphyrin triplet states by molecular oxygen due to obstacles produced by binding. This effect is especially pronounced for the porphyrin molecules located inside BSA aggregates formed around the porphyrin molecules in excess BSA.

Efficiency of singlet oxygen generation from the triplet states of nitrophenyl ethers

The bimolecular rate constants for oxygen quenching, k O 2 T and the quantum yields of singlet oxygen production, Δ, for 4-nitroveratrole, 4-nitroanisole and 3-nitroanisole triplet states in air-equilibrated deuterated water and acetonitrile solutions were determined where possible from nanosecond laser flash photolysis and time-resolved O 2 ( 1Δ g) phosphorescence measurements. The results show that the efficiency of singlet oxygen production, ƒ ΔT, for 4-nitroveratrole and 3-nitroanisole is moderately high and is related with the nature of the low-lying triplet state and that 4-nitroanisole is not able to sensitize singlet oxygen under these conditions.

Efficiency of singlet oxygen generation by alloxazines and isoalloxazines

Some spectroscopic properties of alloxazines and the lifetimes of their singlet and triplet states in acetonitrile are reported. In addition, the efficiencies of singlet oxygen production by energy transfer from the excited states of a range of substituted alloxazines and lumiflavine have been determined in aerated acetonitrile solution. Triplet state quantum yields have also been measured and it has been shown that the efficiency of singlet oxygen production from the triplet state is equal to unity within experimental error in all cases.

Temperature-Dependent Lifetimes and Quantum Yield of the Singlet and Triplet States of the B820 Subunit of LHI Antenna Complex of Purple Bacterium Rhodospirillum rubrum

The quantum yield and lifetimes of fluorescence and triplet state of the B820 subunit of core antenna of photosynthetic purple bacterium Rhodospirillum rubrum are reported at temperature range from 4.2 to 300K. The fluorescence quantum yield of B820 is strongly dependent on temperature. From 4.2 to about 200 K the yield remains nearly constant. A nonradiative decay process becomes activated around 170−200 K. Between 200 K and room temperature the quantum yield drops by a factor of 4. The fluorescence lifetimes and the B820 triplet yield follow the same temperature dependence as the fluorescence quantum yield. It is concluded that a change in the rate of internal conversion from the excited singlet state to the ground state produces the observed change. The change is modeled by using the energy gap law in the strong coupling limit. An activation energy of 65 kJ/mol and average frequency of 450 cm-1 for the promoting vibrational mode of the nonradiative decay are found by fitting the data by energy gap law ...

Mechanistic quantitative structure–activity relationship model for the photoinduced toxicity of polycyclic aromatic hydrocarbons: II. An empirical model for the toxicity of 16 polycyclic aromatic hydrocarbons to the duckweed Lemna gibba L. G‐3

AbstractPhotoinduced toxicity of polycyclic aromatic hydrocarbons (PAHs) occurs via photosensitization reactions (e.g., generation of singlet‐state oxygen) and by photomodification (photooxidation and/or photolysis) of the chemicals to more toxic species. The quantitative structure–activity relationship (QSAR) described in the companion paper predicted, in theory, that photosensitization and photomodification additively contribute to toxicity. To substantiate this QSAR modeling exercise it was necessary to show that toxicity can be described by empirically derived parameters. The toxicity of 16 PAHs to the duckweed Lemna gibba was measured as inhibition of leaf production in simulated solar radiation (a light source with a spectrum similar to that of sunlight). A predictive model for toxicity was generated based on the theoretical model developed in the companion paper. The photophysical descriptors required of each PAH for modeling were efficiency of photon absorbance, relative uptake, quantum yield for triplet‐state formation, and the rate of photomodification. The photomodification rates of the PAHs showed a moderate correlation to toxicity, whereas a derived photosensitization factor (PSF; based on absorbance, triplet‐state quantum yield, and uptake) for each PAH showed only a weak, complex correlation to toxicity. However, summing the rate of photomodification and the PSF resulted in a strong correlation to toxicity that had predictive value. When the PSF and a derived photomodification factor (PMF; based on the photomodification rate and toxicity of the photomodified PAHs) were summed, an excellent explanatory model of toxicity was produced, substantiating the additive contributions of the two factors.

Photophysics of cationic 5,10,15,20-tetrakis-(4- N-methylpyridyl) porphyrin bound to DNA, [poly(dA-dT)] 2 and [poly(dG-dC)] 2: on a possible charge transfer process between guanine and porphyrin in its excited singlet state

The photophysical parameters (fluorescence quantum yield and lifetime, triplet state formation and lifetime) of a cationic prophyrin, i.e. 5,10,15,20-tetrakis(4- N-methylpyridyl) porphyrin (H 2(TMpy-P4)), were determined directly in phosphate buffer solution in the native state and in complexes with calf thymus DNA, [poly(dA-dT)] 2 and [poly(dG-dC)] 2. In [poly(dG-dC)] 2-porphyrin complexes (mostly intercalative), interaction induced an efficient fluorescence quenching and decrease in the triplet state quantum yield. No such effects were observed in [poly(dA-dT)] 2-pophyrin complexes (mostly non-intercalative). The case of DNA was intermediate between the other two. On the basis of the redox properties of the various nucleic bases, it was shown that these observations can be interpreted in terms of fast reversible intermolecular electron transfer from the guanine moiety to the porphyrin in the excited state, when the porphyrin is intercalated with an appropriate geometry within the DNA duplex. These observations and interpretations support a type I (electron transfer) mechanism as the primary event leading to the photodynamic activity of cationic free base porphyrins used as photosensitizers in the process of photoinduced DNA strand breaks.