Diffusion-controlled Rate Constants Research Articles

Kinetic Studies of Antioxidant Properties of Ovothiol A.

Ovothiol A (OSH) is one of the strongest natural antioxidants. So far, its presence was found in tissues of marine invertebrates, algae and fish. Due to very low pKa value of the SH group, under physiological conditions, this compound is almost entirely present in chemically active thiolate form and reacts with ROS and radicals significantly faster than other natural thiols. In biological systems, OSH acts in tandem with glutathione GSH, with OSH neutralizing oxidants and GSH maintaining ovothiol in the reduced state. In the present work, we report the rate constants of OSH oxidation by H2O2 and of reduction of oxidized ovothiol OSSO by GSH and we estimate the Arrhenius parameters for these rate constants. The absorption spectra of reaction intermediates, adduct OSSG and sulfenic acid OSOH, were obtained. We also found that OSH effectively quenches the triplet state of kynurenic acid with an almost diffusion-controlled rate constant. This finding indicates that OSH may serve as a good photoprotector to inhibit the deleterious effect of solar UV irradiation; this assumption explains the high concentrations of OSH in the fish lens. The unique antioxidant and photoprotecting properties of OSH open promising perspectives for its use in the treatment of human diseases.

Photoinduced reactions between naphthoquinone and N,N,N′,N′-tetramethyl-p-phenylenediamine in the mixture of ionic liquid [BPy][NTf2] and acetonitrile studied by transient spectroscopy

Effects of the pyridinium ionic liquid N-butylpyridinium bis((trifluoromethyl)sulfonyl)imide ([BPy][NTf2]) on the photolysis behaviors of 1,4-naphthoquinone (NQ) in the acetonitrile solutions have been investigated by using nanosecond transient absorption techniques. When [BPy][NTf2] existed in the solution, the triplet 3NQ⁎ decayed more slowly and its half lifetime became longer. The photoinduced electron transfer rate constants of NQ and N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) in the presence of [BPy][NTf2] were lower than those measured in acetonitrile, but of the same order of magnitude as estimated diffusion controlled rate constant. This finding suggests that [BPy][NTf2] could act as an appropriate solvent for electron transfer reactions. The photoinduced electron transfer reaction rate was determined not only by the viscosity but also by the microstructure of the solvent.

Direct Observation of Solvated Electrons in Deep Eutectic Solvents: Efficient Capture of Presolvated Electrons by DNA Base.

The solvated electron being the simplest form of an extremely reactive intermediate is of great fundamental interest in chemistry, physics, and biology since its discovery. Recently, deep eutectic solvents (DESs) have been in focus as biodegradable and cost-effective alternative to ionic liquids (ILs) for different applications. These include areas where electron transport and transfer processes are involved. Herein, we present the first report on the existence, yield, and properties of solvated electrons in three deep eutectic solvents, reline, ethaline, and glyceline, composed of choline chloride as a hydrogen bond acceptor and urea, ethylene glycol, and glycerol (Gly) as hydrogen bond donors, respectively, at a molar ratio of 1:2. The varied transient absorption spectra of solvated electrons in these DESs have been explained on the basis of polarity, hydrogen-bonding effect, and the moieties responsible for creating the environment for solvation. The yield and average lifetime follow the trends in viscosity as well as the reactivity of electrons with the components. The C37 value, a measure of the efficiency of scavenging presolvated electrons, is the highest in ethaline in the case of nitrate ions, which indicates the slowest solvation process in this DES. The presolvated electron capture by a DNA base, an aspect considered to be important in cancer radiotherapy, could be monitored conveniently in these liquids at a much longer time scale compared to that reported in aqueous solutions. Bimolecular rate constants for the reaction of solvated electrons with nitrate and the DNA base have been calculated and compared in the three DESs. Unlike in ILs, these experimentally obtained values are comparable to the diffusion-controlled rate constants in DESs.

Diffusion of Solvent-Separated Ion Pairs Controls Back Electron Transfer Rate in Graphene Quantum Dots

In the present study, the stability of the photogenerated, solvent-separated charged states of graphene quantum dots (GQDs) in the presence of N,N-diethylaniline (DEA) has been evaluated in a series of organic solvents. The results indicate that the rate constant for back electron transfer (kBET) from GQD radical anion to DEA radical cation is diffusion-controlled. As a result of the diffusion-controlled back electron transfer (BET), kBET exhibits an inverse exponential relation to (a) the viscosity coefficient (η) of the solvent and (b) the average radius of the graphene quantum dots. An analytical expression for the diffusion-controlled back electron transfer rate constant has been formulated. The dependence of kBET on the diffusion of solvent-separated ion pairs has been evaluated for the first time for quantum dot systems and the results provide an efficient method for enhancing the lifetime of the photogenerated charge-separated states from graphene quantum dots. The present findings can potentially improve the performance of GQD-based photovoltaic and optoelectronic devices.

Phosphorescence quenching of neutral and cationic iridium(III) complexes by molecular oxygen and aromatic electron acceptors

Phosphorescence quenching of neutral and cationic Ir(III) complexes by molecular oxygen and aromatic electron acceptors (AEAs) have been systematically investigated in acetonitrile. The phosphorescence quenching rate constant (kq) by AEAs increased with decreasing Gibbs energy change (ΔGel) of electron transfer and gave a diffusion-controlled rate constant (kd) of 2.0 × 1010 M−1s−1. The occurrence of electron transfer was substantiated by transient absorption measurements, although the separated ion yields were as low as 0.10–0.17. The oxygen quenching of the Ir(III) complexes originated mainly from two competing pathways through singlet and triplet encounter complexes: a noncharge-transfer (nCT) channel, which is regarded as internal conversion to produce singlet oxygen (1O2), and a charge-transfer (CT) channel, whose rate is governed by ΔGel. The kd value for the Ir(III) complex/O2 systems was estimated to be 4.1 × 1010 M−1s−1. For the systems with ΔGel ≲ −0.8 eV, the limiting kq value was 2.5 × 1010 M−1s−1 and the efficiencies (fΔ) of 1O2 production from triplet states quenched by oxygen were 0.40. The limiting kq value being greater than (4/9)kd and the fΔ value being larger than 0.25 suggested the involvement of 1O2 formation through intersystem crossing between singlet, triplet, and quintet CT complexes.

A Molecular Dynamics simulation study on nitrobenzene and [rad]OH radical in supercritical water

With Molecular Dynamics simulation method, the diffusion of nitrobenzene and OH radical in supercritical water was studied at the temperature of 643.2K–843.2K and under the pressure of 23MPa–29MPa. The diffusion-controlled reaction rate constant was calculated. Thus, guidance for reaction kinetics study on the nitrobenzene oxidation in supercritical water process can be provided. Additionally, the microstructure of supercritical water-nitrobenzene system under different conditions was investigated. The deviation of the calculation results of infinite dilute diffusion coefficients obtained from the data processing method in this work is within 9%, compared with experimental values in the literatures. With MD simulation method, at the temperature of 643.2K–823.2K and under the pressure of 23MPa–29MPa, the infinite dilute diffusion coefficients of nitrobenzene and OH radical in supercritical water were obtained. Based on the infinite dilute diffusion coefficients of nitrobenzene and OH radical in supercritical water, the rate constant of the diffusion-controlled reaction was calculated.

Investigation of the fluorescence quenching of 1-aminoanthracene by dissolved oxygen in cyclohexane.

This study provides a detailed investigation of the fluorescence quenching mechanisms of the fluorophore, 1-aminoanthracene, by dissolved oxygen in cyclohexane. Dynamic/collisional quenching dominates in the system studied, but there is also a small component of static quenching. Stern-Volmer plots revealed that the dynamic quenching constant is 0.445 ± 0.014 mM(-1) and represents ∼95% of total quenching in the system. The static quenching rate constant is 0.024 ± 0.001 mM(-1), and mechanisms by complex formation and "sphere of action" static quenching were examined. Compensation of steady-state fluorescence data for solvent loss during the gradual deoxygenation period of the experiment was found to be important in order to conduct a thorough evaluation of the different quenching mechanisms of the system. The enhancement factors, (F(o)/F) and (τ(o)/τ), for 1-aminoanthracene were determined to be 2.20 ± 0.01 and 2.08 ± 0.01, respectively, and the diffusion-controlled bimolecular rate constant was found to be 2.1 × 10(10) ± 0.2 × 10(10) M(-1) s(-1). The work involved the development of a novel instrumental setup that simultaneously measures several important spectroscopic parameters (steady-state fluorescence intensity, absorbance, fluorescence lifetime, and dissolved oxygen concentration) for the careful study of oxygen quenching mechanisms of 1-aminoanthracene in a cyclohexane solution.

Brownian aggregation rate of colloid particles with several active sites.

We theoretically analyze the aggregation kinetics of colloid particles with several active sites. Such particles (so-called "patchy particles") are well known as chemically anisotropic reactants, but the corresponding rate constant of their aggregation has not yet been established in a convenient analytical form. Using kinematic approximation for the diffusion problem, we derived an analytical formula for the diffusion-controlled reaction rate constant between two colloid particles (or clusters) with several small active sites under the following assumptions: the relative translational motion is Brownian diffusion, and the isotropic stochastic reorientation of each particle is Markovian and arbitrarily correlated. This formula was shown to produce accurate results in comparison with more sophisticated approaches. Also, to account for the case of a low number of active sites per particle we used Monte Carlo stochastic algorithm based on Gillespie method. Simulations showed that such discrete model is required when this number is less than 10. Finally, we applied the developed approach to the simulation of immunoagglutination, assuming that the formed clusters have fractal structure.

Characterization of the Triplet State of Tanshinone IIA and its Reactivity by Laser Flash Photolysis

Tanshinone IIA (Tan IIA) has the properties of cardiovascular protection, anti-inflammation, antioxidation and anticancer. Its light-induced instability has drawn our interests in its photochemistry. Therefore, laser flash photolysis herein was used to investigate the transient photochemistry of Tan IIA. Our results show that direct photoexcitation by 355 nm laser pulses or photosensitization by energy transfer can lead to the formation of the triplet state of Tan IIA ((3)Tan IIA*). The triplet absorption spectrum and molar absorption coefficient, and ISC quantum yield were determined. Self-quenching of (3)Tan IIA* by its ground state was identified as an autooxidation reaction. (3)Tan IIA* was proved to react quickly with N, N-dimethylaniline, tert-butylhydroquinone and propyl gallate via electron transfer with the diffusion-controlled rate constants. One of the products with maximum absorption around 390 nm was assigned to the radical anion of Tan IIA. Our results indicate that (3)Tan IIA* is a reactive transient species and can be generated by photosensitization or direct photoexcitation. According to our results, the possible role of Tan IIA as a photosensitizer to induce potential phototoxicity via Type-II pathway in the presence of O2 can be predicted.

Electron transfer in N-butylpyridinium tetrafluoroborate ionic liquid by pulse radiolysis

The radiolysis behavior of neat pyridinium ionic liquids (ILs) and their aqueous solutions was investigated using nanosecond pulse radiolysis techniques. Radiolysis of the ionic liquids, such as N-butylpyridinium tetrafluoroborate (BuPyBF4), resulted in the formation of solvated electrons and organic radicals. Solvated electrons reacted with the pyridinium moiety to produce a pyridinyl radical, which can transfer electrons to various acceptors. The electron-transfer rate constants of the solvent-derived butylpyridinyl radicals in BuPyBF4 and in several compounds (for example, duroquinone, 4,4′-pyridine, benzophenone, and 1,1′-dimethyl-4,4′-bypyridinium dichloride) (k of the order 108 L/(mol s) were lower than those measured in water and in i-PrOH but were significantly higher than the diffusion-controlled rate constants estimated based on viscosity. The electron-transfer rate constants in neat BuPyBF4 were one order of magnitude faster than the diffusion-controlled values. This finding suggests that Bu-PyBF4 acts not only as solvent but also as active solute, such as in solvent-mediated reactions. These reactions result in electrons reaching their final destinations via intervening pyridinium groups without requiring the diffusion of a specific radical.

Mechanism of radiation-induced reactions in aqueous solution of coumarin-3-carboxylic acid: Effects of concentration, gas and additive on fluorescent product yield

The radiation-induced reactions of a water-soluble coumarin derivative, coumarin-3-carboxyl acid (C3CA), have been investigated in aqueous solutions by pulse radiolysis with a 35 MeV electron beam, final product analysis following 60Co γ-irradiations and deterministic model simulations. Pulse radiolysis revealed that C3CA reacted with both hydroxyl radicals (•OH) and hydrated electrons (e− aq) with near diffusion-controlled rate constants of 6.8 × 109 and 2.1 × 1010 M−1 s−1, respectively. The reactivity of C3CA towards O2• − was not confirmed by pulse radiolysis. Production of the fluorescent molecule, 7-hydroxy-coumarin-3-carboxylic acid (7OH-C3CA), was confirmed by final product analysis with a fluorescence spectrometer coupled to a high performance liquid chromatography (HPLC) system. Production yields of 7OH-C3CA following 60Co γ-irradiations depended on the irradiation conditions and ranged from 0.025 to 0.18 (100 eV) −1. Yield varied with saturating gas, additive and C3CA concentration, implying the presence of at least two pathways capable of providing 7OH-C3CA as a stable product following the scavenging reaction of C3CA with •OH, including a peroxidation/elimination sequence and a disproportionation pathway. A reaction mechanism for the two pathways was proposed and incorporated into a deterministic simulation, showing that the mechanism can explain experimentally measured 7OH-C3CA yields with a constant conversion factor of 4.7% from •OH scavenging to 7OH-C3CA production, unless t-BuOH was added.

Study of diffusion-controlled and activation–diffusion-controlled electron-transfer processes from quenching measurements

The diffusion-controlled rate constants, k d , of various quenching reactions, [Ru(L) 3] 2+∗ (L = bpy, phen and 4,7-(CH 3) 2phen) + [Fe(CN) 6] 3−, were measured through fluorescence measurements. From them, the effective values of viscosity coefficients for several methanol + water mixtures were calculated. These coefficients were checked through calculations of the rate constants of the reaction [IrCl 6] 2− + [Ru(bpy) 3] 2+∗, which were also obtained by fluorescence quenching measurements. The agreement between the two sets of data (experimental and predicted) is excellent. Besides, the trends of association, k d , and dissociation, k − d , rate constants for 2+/3−, 2+/2− and 2+/2+ reactions in methanol–water mixtures are discussed. The use of effective diffusion coefficients for estimating k d and k − d allowed us to obtain the intrinsic electron transfer rate constant, k et , for the activation-diffusion-controlled process between [Ru(bpy) 3] 2+∗ and [Co(NH 3) 5Cl] 2+ complexes from the observed (quenching) rate constant. The influence of methanol–water mixtures on k et was rationalized by using the Marcus electron-transfer treatment.

Spectral Characterization of Guanine C4-OH Adduct: A Radiation and Quantum Chemical Study

The reaction of hydroxyl radical ((•)OH) with guanine was investigated under restricted pH condition (pH 4.6) using pulse radiolysis technique. The time-resolved optical transient absorption spectra showed two peaks centered at 300 and 330 nm at 4 μs after the pulse which exhibited different reactivity toward molecular oxygen (O(2)). The peak at 300 nm was found to be relatively more stable than the peak at 330 nm. The peak corresponding to 330 nm decayed within 20 μs having a first order rate constant 4-7 × 10(4) s(-1) and was pH dependent. On longer time scale, the species decayed by a bimolecular process. The presence of O(2) did not affect its decay rate constant. The (•)OH reacts with guanine at pH 4.6 with a diffusion-controlled second order rate constant of ≥1 × 10(10) mol(-1) dm(3) s(-1). The reaction of Br(2)(•-), O(2)(•-), and 2-hydroxy-2-propyl radical with guanine was also investigated to differentiate among the one-electron oxidized, one-electron reduced species of guanine and the guanine-OH adducts formed in the reaction of (•)OH at pH 4.6. On the basis of the spectral characteristics and reactivity toward O(2), two guanine-OH adduct species were identified (i) the C4-OH adduct species absorbing at 330 nm which has not been reported so far and (ii) the C8-OH adduct species absorbing at 300 nm in agreement with the known literature absorption features. Quantum chemical calculations using BHandHLYP with 6-31+G(d,p) basis set and excited state calculations using TDDFT for all possible transients complement the assignment of the observed spectral peak at 330 nm to the C4-OH adduct of guanine. Furthermore, steady state radiolysis revealed the formation of 8-hydroxy-guanine whose precursor is known to be the C8-OH adduct species.

Photochemistry of thioxanthen-9-one-10,10-dioxide: A remarkably reactive triplet excited state

The steady state photolysis of thioxanthen-9-one-10,10-dioxide ( 1) in dichloromethane does not result in any product formation. In hydrogen donor solvents, such as 2-propanol, toluene or cyclohexane, the formation of the corresponding pinacol was readily observed. The phosphorescence spectrum of 1 in EPA, at 77 K, reveals a triplet energy of 66.3 kcal mol −1. Laser flash photolysis irradiation of a degassed acetonitrile solution of 1 led to the formation of its triplet excited state ( λ max = 375 and 520 nm, τ = 11 μs) that was quenched by oxygen, β-carotene and 1,3-cycloexadiene at a diffusion controlled rate constant. The quenching rate constants for the triplet of 1 with 1,4-cyclohexadiene, methanol, ethanol, 2-propanol, cyclohexane, substituted phenols and amines were determined, ranging from 7.1 × 10 6 (for methanol) to 3.1 × 10 10 L mol −1 s −1 (for triethylamine). From the quenching rate constants observed for the triplet excited state and from the steady state photolysis, it was concluded that the remarkable reactivity of 1 is due to the n,π* character of the carbonyl chromophore.

Insights into the effect of ketylimine, aldimine, and vinylene group attachment and regiosubstitution on the fluorescence deactivation of fluorene

The spectroscopic and electrochemical properties of a 9-substituted fluorene ketylimine (3) were investigated and compared with those of its vinylene analogue (4) to determine the origins of the quenched fluorescence of these compounds. The predominate mode of singlet excited state deactivation of the heteroatomic fluorene was found to be internal conversion involving bond rotation. Meanwhile, its carbon counterpart was found to undergo deactivation preferentially by intersystem crossing to form its triplet, which was confirmed by laser flash photolysis. Both 3 and 4 quenched the fluorescence of fluorene with diffusion-controlled rate constants, implying that the singlet excited states of 3 and 4 are also quenched by intramolecular photoinduced electron transfer (PET). This deactivation mode was found to be exergonically favorable (–90 kJ/mol for 3 and –81 kJ/mol for 4) according to the Rehm–Weller equation. The position of the heteroatomic bond on the fluorene moiety was further found to influence the singlet excited state deactivation pathway. The 2-substituted regioisomer decayed predominately by intramolecular PET and its fluorescence can be restored by acid protonation. Conversely, the PET mechanism is a minor deactivation mode for the 9-substituted fluorene derivative and its fluorescence can be enhanced by suppressing bond rotational modes, possible at low temperature and potentially in thin films.

Photoinduced Electron-Transfer Reactions: A Study of the Diffusion-Controlled and Activation-Diffusion-Controlled Processes

The diffusion-controlled electron transfer rate constants (k(d)) of several quenching reactions of ruthenium complexes [Ru(L)(3)](2+*) (L = bpy, phen, and 4,7-(CH(3))(2)phen) with [Fe(CN)(6)](3-) were experimentally determined at different concentrations of NaNO(3). From these rate constants, the effective values of viscosity coefficients for NaNO(3) solutions were calculated using EMSA (exponential mean spherical approximation) and EF (Eigen-Fuoss) approaches in order to take into account the mean force potential between reactants. The reliability of the effective parameters were checked through calculations of the rate constants of the reaction [IrCl(6)](2-)+ [Ru(bpy)(3)](2+)* in these NaNO(3) solutions. The rate constants of this reaction were also obtained by fluorescence quenching measurements. The agreement between the two sets of data (experimental and predicted) is excellent. The trends of association (k(d)) and dissociation (k(-d)) rate constants for 2+/3-, 2+/2-, and 2+/2+ reactions in NaNO(3) solutions are discussed. The use of effective diffusion coefficients for estimating k(d) and k(-d) allowed us to obtain the intrinsic electron transfer rate constant (k(et)) for the activation-diffusion-controlled process between [Ru(bpy)(3)](2+)* and [Co(NH(3))(5)Cl](2+) complexes from the observed (quenching) rate constant. The trend of electron-transfer rate constant in NaNO(3) for this reaction was rationalized by using the Marcus electron-transfer treatment.

A comprehensive Monte Carlo simulation of styrene atom transfer radical polymerization

An optimized and high-performance Monte Carlo simulation is developed to take thorough account of four different cases of termination in styrene ATRP. According to the simulation results, the bimolecular termination rate constant sharply drops throughout the polymerization when either chain-length dependency of termination rate constant, gel effect, or both together is applied to the simulation. In addition, as expected, the initiator is quickly decomposed at the early stages of the polymerization. The concentration of the catalyst in lower oxidation state decreases at first and then plateaus at higher conversion; furthermore, the steady concentration of M t Y/L in the polymerization is the highest when the chain-lengthdependent diffusion-controlled termination rate constant is employed in the simulation. The rates of deactivation and chain end degradation reactions are also smaller in this case. Therefore, the fraction of dormant chains is higher throughout the reaction and consequently the portion of dead polymers decreases. Besides, molecular weight increases linearly with conversion; however, when neither gel effect nor chain-length dependency of termination rate constant is considered, the molecular weight deviates from linearity at the end of the reaction. The peak of chain length distribution shifts toward higher molecular weight too during the reaction. Finally, the molecular weight distribution broadens at higher conversion; however, the chain length distribution of polymers produced under conditions of applying chain-length-dependent diffusion-controlled termination rate constant is narrower.

PH-Dependent Association of Proteins. The Test Case of Monoclonal Antibody HyHEL-5 and Its Antigen Hen Egg White Lysozyme

We describe a method for determining diffusion-controlled rate constants for protein-protein association that explicitly includes the solution pH. The method combines the transient-complex theory for computing electrostatically enhanced association rates with an approach based on a rigorous thermodynamic cycle and partition functions for energy levels characterizing protonation states of associating proteins and their complexes. To test our method, we determine the pH-dependent kinetics of association of the HyHEL-5 antibody with its antigen hen egg white lysozyme. It was shown experimentally that their association rate constant depends on pH, increasing linearly in the pH range 6-8 and saturating or even exhibiting a flat maximum in the pH range 8-10. The presented methodology leads to a qualitative agreement with the experimental data. Our approach allows one to study diffusion-controlled protein-protein association under different pH conditions by taking into account the ensembles of protonation states rather than just the most probable protonation state of each protein.

Bimolecular-reaction effect on the rate constant of electron transfer at the oil/water interface as studied by scanning electrochemical microscopy

Two interfacial electron transfer (ET) systems, i.e., the decamethylferrocene ( DcMFc ) – Fe ( CN ) 6 3 - system and the MTPP + – Fe ( CN ) 6 4 - system (M = Zn or Cd; TPP = 5,10,15,20-tetraphenylporphyrin) at oil/water interfaces were studied by means of scanning electrochemical microscopy. The second-order rate constants ( k) for the ET systems could be accurately determined at different “standard” Gibbs energies (Δ G°) by changing the nature and concentration ratio of a common ion added to both phases. The driving force dependence, i.e., the log k vs. Δ G° plot did not show a simple upward parabola, with the k values being limited in a certain range of Δ G°. This clearly suggested that there should be a bimolecular-reaction effect, as predicted previously [T. Osakai, H. Hotta, T. Sugihara, K. Nakatani, J. Electroanal. Chem. 571 (2004) 201–206]. However, the observed diffusion-controlled rate constants are typically one-order smaller than the theoretical ones, and have shown only small dependence on the viscosity of organic solvents for the DcMFc – Fe ( CN ) 6 3 - system. These results were unexpected from the previous model based on “microscopic diffusion” of a redox species in the vicinity of the interface, and then suggested an alternative model, in which the rate-determining step is “interfacial diffusion” of a redox species across the oil/water interface.

Synthesis, photophysical and photochemical studies on long chain zinc phthalocyanine derivatives

The synthesis and characterization of 2,9,16,23-chloro-3,10,17,24-triethyleneoxysulphanylphthalocyaninato zinc(II) (CTESZnPc) is described. The photophysics and photochemistry of CTESZnPc and those of tetrakis(triethyleneoxysulphanyl)zinc phthalocyanine (TTESZnPc) and octakis(triethyleneoxysulphanyl)zinc phthalocyanine (OTESZnPc), are presented and compared with those of unsubstituted zinc phthalocyanine (ZnPc). The presence of triethyleneoxysulphanyl substituents on the ZnPc ring gave rise to higher values of singlet oxygen ( Φ Δ) and photodegradation ( Φ Pd) quantum yields in DMF. However, TTESZnPc, OTESZnPc and CTESZnPc are less fluorescent than ZnPc, judging from their fluorescence quantum yield ( Φ F) values. Fluorescence was lower in toluene than in DMF due to aggregation in the former solvent. Triplet quantum yield ( Φ T) values were found to increase with the presence of substituents on the ZnPc ring, while triplet lifetimes ( τ T) were found to vary linearly with the logarithms of solvent viscosities. The fluorescences of the substituted ZnPc complexes were effectively quenched by benzoquinone (BQ), and the quenching data analyzed by the Stern–Volmer equation. The Stern–Volmer constants and the diffusion-controlled bimolecular rate constants were calculated.