Quantum Yield Of Triplet Formation Research Articles

Exciplex dynamics in a blend ofπ-conjugated polymers with electron donating and accepting properties: MDMO-PPV and PCNEPV

The photophysical properties of a solution processed blend of two semiconducting polymers with electron donating and electron accepting properties, respectively, as used in polymer photovoltaic devices have been investigated. We show that in the binary mixture of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and poly[oxa-1,4-phenylene-(1-cyano-1,2-vinylene)-(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene)-1,2-(2-cyanovinylene)-1,4-phenylene] (PCNEPV) photoexcitation of either one of the polymers results in formation of a luminescent exciplex at the interface of the two materials. Photoinduced absorption spectroscopy shows that this exciplex can decay to the lowest triplet state $({T}_{1})$ of MDMO-PPV. Application of an electric field results in dissociation of the marginally stable exciplex into charge carriers, which provides the basis for the photovoltaic effect of this combination of materials. Spin allowed recombination of the charge carriers to the MDMO-PPV ${T}_{1}$ state is invoked to explain the field-enhanced quantum yield for triplet formation observed by photoinduced reflection measurements on photovoltaic devices made from the composite films. The field enhanced triplet yield is identified as loss mechanism for the photovoltaic performance of this combination of materials.

Photophysics and photodimerization of 6,5′-dimethylangelicin in different solvents

The photophysical and photochemical behaviour of 6,8-dimethyl-2 H-furo[2,3-h]chromen-2-one (6,5′-dimethylangelicin, 6,5′-DMA) was studied by steady state and pulsed techniques and by semiempirical calculations. The fluorescence characteristics and the triplet state properties were determined in solvents of different polarity/proticity. The obtained values of fluorescence quantum yields and lifetimes together with the triplet formation quantum yield indicate that the energy gap between the two lowest excited states controls the decay pathways of the singlet. In non-polar and non-protic solvents the internal conversion is almost the sole decay process of the singlet, while in protic solvents the intersystem crossing reaches substantial quantum yields. The effect of ground state concentration on the fluorescence lifetime suggested the occurrence of aggregation while the triplet lifetime was quenched and the rate constant of self-quenching was determined. The 6,5′-DMA photodimerization was investigated in different solvents and in different experimental conditions. Two pyrone-pyrone and one furan–pyrone photodimers of 6,5′-DMA were the main photoproducts detected in the irradiated solution. The effects of the 6,5′-DMA concentration and of oxygen on the amounts of the dimers obtained together with the photophysical behaviour gave indications on the formation mechanism of different dimers.

Characterization of the triplet state of tris(8-hydroxyquinoline)aluminium(III) in benzene solution.

The lowest triplet state of tris(8-hydroxyquinoline)aluminium(III) (Alq3) has been prepared by pulse radiolysis/energy transfer from appropriate donors in benzene solutions and has an absorption maximum around 510 nm with a lifetime of about 50 mus. It is quenched by molecular oxygen, leading to singlet oxygen formation. From flash photolysis and singlet oxygen formation measurements, a quantum yield of triplet formation of 0.24 was determined for direct photolysis of the complex. A value of 2.10 +/- 0.10 eV was determined for the energy of the lowest triplet state by energy transfer studies and was confirmed by phosphorescence measurements on Alq3, either in the heavy atom solvent ethyl iodide or photosensitized by benzophenone in benzene. Dexter (exchange) energy transfer was observed from triplet Alq3 to platinum(II) octaethylporphyrin.

Corrigendum to “Quantum yield of triplet formation of riboflavin in aqueous solution and of flavin mononucleotide bound to the LOV1 domain of Phot1 from Chlamydomonas reinhardtii” [Chemical Physics 291 (2003) 97–114

Corrigendum to ‘‘Quantum yield of triplet formation of riboflavin in aqueous solution and of flavin mononucleotide bound to the LOV1 domain of Phot1 from Chlamydomonas reinhardtii’’ [Chemical Physics 291 (2003) 97–114] Shafiqul D.M. Islam, Alfons Penzkofer*, Peter Hegemann a Institut II – Experimentelle und Angewandte Physik, Universit€at Regensburg, Universit€atsstrasse 31, D-93053 Regensburg, Germany b Institut f€ur Biochemie I, Universit€at Regensburg, Universit€atsstrasse 31, D-93053 Regensburg, Germany

Quantum yield of triplet formation of riboflavin in aqueous solution and of flavin mononucleotide bound to the LOV1 domain of Phot1 from Chlamydomonas reinhardtii

Abstract The quantum yields of triplet formation, φ T , of riboflavin, and of flavin mononucleotide (FMN) bound to wild-type LOV1 domain (LOV1-WT) or bound to a mutated LOV1 domain (LOV1-C57S, cysteine 57 replaced by serine) of the photo-receptor Phot1 from the green alga Chlamydomonas reinhardtii are determined by picosecond laser double-pulse excitation and time-resolved fluorescence detection. The determined quantum yields of triplet formation are φ T =0.375±0.05 for riboflavinin in aqueous solution at pH=7, φ T =0.225±0.04 for FMN in LOV1-C57S in aqueous solution at pH=8, and φ T =0.255±0.04 for FMN in LOV1-WT in aqueous solution at pH = 8.

Photophysical and theoretical studies of naphthalene-substituted oligothiophenes

A comprehensive photophysical characterization of a new class of naphthalene derivatives of the important α-oligothiophenes (αn’s) has been undertaken in solution at room (293 K) and low (77 K) temperature. This includes absorption and fluorescence spectra, fluorescence quantum yields (φF), and lifetimes (τF), as a function of temperature and solvent. Triplet–triplet absorption spectra and triplet formation quantum yields (φT) were also determined. From the above, all the rate constants for the radiative (kF) and radiationless (kIC and kISC) have been calculated. It is shown that the lowest singlet excited state is an allowed π,π* state in all solvents. The results show that although the behavior of the oligomers is similar to their parent compounds (αn’s), significant differences are observed. By comparison of the extinction coefficients of the naph(αn’s) and the (αn’s), a good correlation was found between naph(αn) and α(n+1). On the basis of this proposed pairing, a consistent blue-shift was observed in the absorption maxima between the compounds here considered and the reference α-oligothiophenes. This indicates that there is some significant twist between the naphthalene and the αn chromophores. The most favorable inter-ring angle between the two chromophores, naphthalene and αn, was predicted on the basis of comparison with theory.

On the Phosphorescence of 1H-Phenalen-1-one

Combining the effects of heavy atom and low temperature, the phosphorescence spectrum from 1H-phenalen-1-one has been unveiled. The 0-0 band is located at 649 nm in methylcyclohexane and shifted to 646 nm in ethanol, which sets the triplet-energy level to 185 and 186 kJ⋅mol−1, respectively. The emission is unambiguously identified as phosphorescence originating from 1H-phenalen-1-one through complementary transient absorption and emission studies. The quantum yield for triplet formation is confirmed to be unity.

Phthalocyanine photophysics and photosensitizer efficiency on human embryonic lung fibroblasts

This study correlates the photophysical properties of aluminum phthalocyanine tetrasulfonate (AlPcTs), zinc phthalocyanine tetrasulfonate (ZnPcTs), and phthalocyanine tetrasulfonate (PcTs) with their ability to kill human embryonic lung (HEL) cells. Photofrin, a proven anti-cancer drug, was used for comparison. The photophysical properties include fluorescence emission spectra, fluorescence quantum yields, singlet state and radiative lifetimes, quantum yield for triplet formation, and fluorescence rate constants. Results indicate that photodynamic efficacy correlates inversely with fluorescence quantum yield and fluorescence lifetime and directly with quantum yield for triplet formation.

Triplet state properties of N-mTEG[60]fulleropyrrolidine mono and bisadduct derivatives

Laser flash photolysis and pulse radiolysis techniques were used to determine triplet state properties of N-mTEG[60]fulleropyrrolidine mono and bisadduct derivatives (FP) (mTEG=CH 2CH 2OCH 2CH 2OCH 2CH 2OCH 3) in benzene. These properties include the triplet absorption spectra between 550 and 900 nm, quantum yields of triplet photosensitized singlet oxygen production ( Φ Δ ) and triplet molar absorption coefficients ( ε T) with the assumption that the quantum yield of triplet formation ( Φ T) is equal to Φ Δ . Our results demonstrate that the triplet properties depend on the number of addends and the addition pattern.

Energetics of photocyclization of polyphenylamines and assignment of the intermediate: A time-resolved photoacoustic calorimetric study

The photocyclization reactions of diphenylamine (DPA), N-methyldiphenylamine (MDPA) and triphenylamine (TPA) have been studied in methanol solvent through nanosecond/microsecond flash photolysis and time-resolved photoacoustic calorimetry (TRPAC). It is confirmed for all the amines that the long-lived dihydrocarbazole intermediate ( 1 XDHC 0 ) is formed through a cascade of processes. However, DPA behaves differently from the other two members of the series so far as the rates of the individual processes are concerned. The quantum yield of triplet formation and the quantum yield of the photocyclization of the amine triplet have been estimated for the DPA system. In conjugation with the experiments, theoretical calculations, employing AM1-SCI method, have been performed to propose a structure for the intermediate. For all the amines, the intermediate has been proposed to be of zwitterionic nature in the ground state. The enthalpies of formation of the intermediate ( 1 XDHC 0 ) from MDPA and TPA have been determined experimentally to be around 51 kcal mol -1 and the values agree well with the calculated ones.

Study of the photophysics and energy transfer of 9,10-diphenylanthracene in solution

Photophysics of 9,10-diphenylanthracene (DPA) in benzene solution was studied with time-resolved thermal lensing (TRTL) technique. The TRTL signal was composed of the fast and slow-rising components. The heat conversion efficiency, which is defined as the ratio of energy released as heat against energy absorbed, was determined to be 0.31±0.01, revealing that the fluorescence quantum yield is fairly large (0.88±0.03), while the quantum yield for triplet formation is quite small (0.04±0.02) by analyzing the slow component of the TRTL signal. The triplet–triplet (T–T) absorption of DPA in benzene appeared for the wavelength region of 420–470nm, obtained by energy transfer with triplet benzophenone used as a photosensitizer. The quenching rate constant of DPA by triplet benzophenone was determined to be (3.6±0.3)×109M−1s−1 with Stern–Volmer plots. The intrinsic energy transfer rate was also estimated by analyzing the time profile of the TRTL signal.

Fast Internal Conversion in Nonfluorescent Carbonyl Anthracenes

Abstract The quantum yield of triplet formation (φT) for two nonfluorescent molecules of 9-acetylanthracene and 9-benzoylanthracene was determined with a time-resolved thermal lensing method (TRTL). The value of φT for 9-benzoylanthracene was 0.19 in cyclohexane, indicating that a fast internal conversion process with a rate constant of 4.0 × 1010 s−1 exists in the deactivation from the first excited singlet state (S1).

A Laser Flash Photolysis and Pulse Radiolysis Study of Primary Photochemical Processes of Flumequine¶

The 355 nm laser flash photolysis of argon-saturated pH 8 phosphate buffer solutions of the fluoroquinolone antibiotic flumequine produces a transient triplet state with a maximum absorbance at 575 nm where the molar absorptivity is 14,000 M(-1) cm(-1). The quantum yield of triplet formation is 0.9. The transient triplet state is quenched by various Type-1 photodynamic substrates such as tryptophan (TrpH), tyrosine, N-acetylcysteine and 2-deoxyguanosine leading to the formation of the semireduced flumequine species. This semireduced form has been readily identified by pulse radiolysis of argon-saturated pH 8 buffered aqueous solutions by reaction of the hydrated electrons and the CO2*- radicals with flumequine. The absorption maximum of the transient semireduced species is found at 570 nm with a molar absorptivity of 2,500 M(-1) cm(-1). In argon-saturated buffered solutions, the semireduced flumequine species formed by the reaction of the flumequine triplet with TrpH stoichiometrically reduces ferricytochrome C (Cyt Fe3+) under steady state irradiation with ultraviolet-A light. In the presence of oxygen, O2*- is formed but the photoreduction of Cyt Fe3+ by O2*- competes with an oxidizing pathway which involves photo-oxidation products of TrpH.

Primary photophysical properties of ofloxacin.

Steady-state fluorescence has been used to study the excited singlet state of ofloxacin (OFLX) in aqueous solutions. Fluorescence emission was found to be pH dependent, with a maximum quantum yield of 0.17 at pH 7. Two pKa*s of around 2 and 8.5 were obtained for the excited singlet state. Laser flash photolysis and pulse radiolysis have been used to study the excited states and free radicals of OFLX in aqueous solutions. OFLX undergoes monophotonic photoionization from the excited singlet state with a quantum yield of 0.2. The cation radical so produced absorbs maximally at 770 nm with an extinction coefficient of 5000 +/- 500 dm3 mol-1 cm-1. This is confirmed by one-electron oxidation in the pulse radiolysis experiments. The hydrated electron produced in the photoionization process reacts with ground state OFLX with a rate constant of 2.0 +/- 0.2 x 10(10) dm3 mol-1 s-1, and the anion thus produced has two absorption bands at 410 nm (extinction coefficient = 3000 +/- 300 dm3 mol-1 cm-1) and at 530 nm. Triplet-triplet absorption has a maximum at 610 nm with an extinction coefficient of 11,000 +/- 1500 dm3 mol-1 cm-1. The quantum yield of triplet formation has been determined to be 0.33 +/- 0.05. In the presence of oxygen, the triplet reacts to form both excited singlet oxygen and superoxide anion with quantum yields of 0.13 and < or = 0.2, respectively. Moreover, superoxide anion is also formed by the reaction of oxygen with the hydrated electron from photoionization. Hence the photosensitivity due to OFLX could be initiated by the oxygen radicals and/or by OFLX radicals acting as haptens.

Photo-isomerisation, triplet formation, and photo-degradation dynamics of indocyanine green solutions

Indocyanine green sodium iodide (ICG-NaI) in dimethylsulfoxide (DMSO), methanol, water, and human plasma is excited with a chopped powerful diode laser at 812.5nm (time resolution 20μs). Small transmission changes observed are analysed in terms of triplet formation, photo-isomerisation, photo-degradation, and thermal heating. Quantum yields of P-isomer formation and T1-triplet formation as well as P-isomer and T1-triplet lifetimes are determined. For nitrogen-bubbled ICG-NaI in DMSO a quantum yield of triplet formation of φT≈5×10−5 and a triplet-state lifetime of τT≈700μs are determined. For ICG-NaI in methanol a quantum yield of P-isomer formation of φP≈2.4×10−4 and a P-isomer lifetime of tP≈430μs are obtained.

Photophysical and electron-transfer properties of a few 2,6-dimethyl-4-arylpyrylium derivatives

Photophysical and electron transfer properties of pyrylium salts 1–4 were investigated. The photophysical properties include absorption and fluorescence spectra, quantum yields, fluorescence lifetimes and phosphorescence spectra. Laser flash photolysis of 1–4 in degassed dichloromethane led to the formation of transients assigned to the triplets of these molecules. The quantum yields of triplet formation and the extinction coefficients of the triplet–triplet absorption were determined. Electron transfers to the excited states of 1–4 from biphenyl were studied by fluorescence quenching and laser flash photolysis. The intermediate radical ions formed in the electron transfer reactions were characterized. In the case of 1–3, only the singlet excited states were involved in electron transfer reactions, whereas, in the case of 4, the singlet as well as the triplet excited states act as electron acceptors. Involvement of the triplet in the electron transfer reactions, led to higher Φ ion and lower k bet values in the case of 4. Thus, suitable modification of the structure of pyrylium salts can lead to molecules with very good photoelectron transfer properties.

Volume Change Associated with Large Photoinduced Dipole Formation in a Rigid Donor−Acceptor Compound: New Approach to Optoacoustic Volume Determination

Time-resolved laser-induced optoacoustic spectroscopy was used to study the charge separation process taking place in the excited state of a donor−bridge−acceptor (D-br-A) compound in alkane solvents. This molecule contains a D−A pair separated by a rigid saturated hydrocarbon bridge 6 sigma bonds long. Excitation at 308 nm results in very efficient long-range charge separation, leading to formation of a charge-transfer (CT) state with a dipole moment of 38 D. By monitoring the pressure waves generated during the decay of the excited species, we could discern three consecutive relaxation processes. To separate the contributions of enthalpy and structural volume changes (ΔVstr), experiments were carried out with a series of n-alkanes having different photothermal properties. However, the usual separation method clearly failed for this system, and a new approach was developed to ensure that ΔVstr remained constant across the alkane series. This consisted of selecting conditions where the solvent compressibility could be considered constant over the experimental range. In this way, ΔVstr values between −110 and −200 mL/mol were obtained, depending on alkane length and temperature. These are remarkably large contractions, which can be attributed only to electrostriction of the alkane solvent around the dipolar CT species. Interestingly, the contractions are as large as predicted by classical electrostatic theory. The quantum yield of triplet formation, ΦT, determined from the optoacoustic data, revealed a strong dependence on the oxygen content of the solution. A value of ΦT = 0.04 was obtained for the intrinsic quantum yield in the absence of oxygen.

Intrazeolite Photochemistry. 23. Transparent PDMS Films of Zeolites Incorporating Organic Guests: Quantitative Determination of Photophysical Parameters by Transmission Techniques

Zeolites containing embedded organics have been made into transparent films. These new materials have interesting properties that allow the measurement and quantification of optical and photophysical properties of the guest molecules that are either unattainable using current diffuse reflectance techniques or significantly less quantitative. These include the determination of the ground-state extinction coefficients and the quantum yield of triplet formation. It was determined that within experimental error, the values of the photophysical parameters of the guest molecules included within the zeolite are very similar to those measured in solution. This indicates that in the cases under consideration, incorporation of an organic molecule or cation into the zeolite pores does not significantly alter its molecular properties with respect to those in nonrigid media. These transparent films open the possibility of determining the extinction coefficient of cations that are not persistent in solution but can be ...

Higher excited-state photoisomerization and singlet to triplet intersystem-crossing in DODCI

An intensity dependent N-isomer fluorescence reduction was observed by picosecond laser pump and probe experiments for DODCI (3,3′-diethyloxadicarbocyanine iodide) in ethylene glycol and methanol. The S 1–S 0 fluorescence signal caused by time-delayed probe pulse excitation is reduced due to pump pulse induced enhanced N → P photoisomerisation, higher excited-state photodisintegration, and S m –T m′ intersystem-crossing. The transmission of a time-delayed picosecond probe light continuum is lowered by enlarged P-isomer absorption and triplet–triplet absorption. The excited-state absorption dynamics, photoisomerisation, photodisintegration and intersystem-crossing dynamics are simulated numerically. The quantum yields of P-isomer formation caused by higher excited-state singlet absorption are φ NP′=0.06±0.02 and 0.1±0.02 for DODCI in ethylene glycol and methanol, respectively. S m –T m′ quantum yields of triplet formation of φ S m T m′ =0.05±0.02 for DODCI in ethylene glycol and φ S m T m′ =0.04±0.02 for DODCI in methanol are determined. Triplet–triplet absorption cross-section spectra are deduced from the picosecond light continuum transmission measurements.

Photophysical properties of C 76

The ground-state absorption spectrum of C 76 was measured in toluene solution over the interval 300–1100 nm. Pulse radiolysis and laser flash photolysis have been used to determine properties of the lowest triplet-state of C 76 in benzene, including the triplet difference spectrum between 310 and 1100 nm, the difference and absolute molar absorption coefficients at 840 nm (ε=4460 and 5730 M −1 cm −1, respectively), the quantum yield of triplet formation ( Φ T =0.03±0.01) and the quantum yield for C 76-sensitized formation of singlet oxygen in aerated benzene ( Φ Δ =0.04±0.01). Our results suggest that the S 1–T 1 interval is smaller than in C 60. The strongly lipophilic nature of C 76, coupled with its low triplet and sensitized singlet oxygen yield, suggests its use as a carrier for delivery of highly polar drugs to hydrophobic sites and tissues without phototoxic sensitization.