Lifetimes of partial charge transfer exciplexes of 9-cyanophenanthrene and 9-cyanoanthracene

The fluorescence decays of several exciplexes with partial charge transfer have been investigated in solvents of various polarity. The measured lifetimes are found to be in reasonable agreement with the activation enthalpy and entropy of exciplex decay obtained earlier from the temperature dependence of the exciplex emission quantum yields. For exciplexes with 9-cyanophenanthrene substantial contribution of the higher local excited state into the exciplex electronic structure is found and borrowed intensity effect enhances the exciplex emission rate constants.     

Effects of magnesium salts on photoinduced electron transfer reaction between ammonia, 2,5-dimethylhexa-2,4-diene, and 9-cyanophenanthrene

Mechanistic studies and product analyses were performed on the single electron transfer (SET) promoted photoamination reaction between 2,5-dimethylhexa-2,4-diene (1) and 9-cyanophenanthrene in order to gain a comprehensive understanding of the effects of additive metal salts. The product selectivity of this process was found to be dramatically increased by the addition of Mg²⁺ salts. An investigation involving the analyses of fluorescence and time-resolved absorption spectroscopic properties reveals that Mg²⁺ enhances the rate of the initial SET step and stabilizes the resulting radical ion pair in this photoreaction.     

Magnetic field effect on an exciplex between N-vinyl carbazole grafted on cellulose acetate film and 1,4-dicyanobenzene

The present work emphasises the investigation of photoinduced electron transfer reaction between an electron-donor fluorophore, N-vinyl carbazole (VCZ), grafted on a polymeric cellulose acetate film and an electron-acceptor, 1,4-dicyanobenzene, in the presence of an external magnetic field that serves as a powerful tool to identify the spin states where the initial electron transfer occurs and to modulate the course of the reaction as desire. Here initial electron transfer occurs in the singlet spin state, and the formation of exciplex increases in the presence of magnetic field. The maximum field effect is obtained in the solvent with medium dielectric constant (ε_max) around 8.0. The ε_max value indicates that the extent of charge transfer in this exciplex is less compared to other similar systems studied so far, owing to the characteristic binding of VCZ to the polymeric backbone through oxygen atom. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3910–3915, 1999     

Radiationless decay processes in benzene; an external deuterium isotope effect

C₆H₆ or C₆D₆ and CHCl₃ form weak complexes in glassy 3-methyl pentane solution at 77°K. Substitution of CDCl₃ for CHCl₃ greatly increases the phosphorescence efficiency and decay time of the benzene and suppresses the photochemical production of hexatrienes. The observations can be understood if there is vibronic coupling of the triplet benzene with the CH motion in the CHCl₃, which accelerates the rate of its radiationless decay.     

Influence of steric factors on exciplex energy and magnetic field effect

With the advent of spin chemistry, magnetic field effect (MFE) on exciplex luminescence has emerged as an important domain of research. MFE is a diffusion controlled phenomenon and hence is solvent dielectric (epsilon) dependent. It maximizes at a particular epsilon (epsilon(max)) for a specific exciplex system. Various attempts have been made to explain the variation of this epsilon(max) from one exciplex to another. In our present work we have succeeded for the first time to enmark the energy of exciplex (E(ex)) as the prime factor in determining the epsilon(max). We have indicated a definite inverse correlation (1:1) between epsilon(max) and E(ex). We have also tried to correlate some parameters that are important in exciplex formation, e.g. Charton's steric constant (nu(c)), repulsive energy (R(e)) and E(ex).     

Exciplex and radical ion intermediates in the photochemical reaction of 9-cyanophenanthrene with 2,3-dimethyl-2-butene

The photochemical reaction of 9-cyanophenanthrene and 2,3-dimethyl-2-butene, first reported by Mizuno, Pac and Sakurai, has been reinvestigated. The formation of a [2+2]-cycloadduct via a singlet exciplex is the exclusive reaction in the nonpolar solvents benzene and ethyl acetate. Photochemical behavior in polar solvents is far more complicated than previously reported. Mechanisms consistent with the effects of solvent polarity, methanol concentration, methanol deuteration, and light intensity upon product yields are proposed. Formation of a 9-cyanophenthrene anion radical and 2,3-dimethyl-2-butene cation radical is the primary photoinitiated process in polar solvent. The cation radical can undergo deprotonation to yield an allyl radical or nucleophilic attack by methanol to yield a methoxyalkyl radical. Covalent bonding of these radicals and the 9-cyanophenanthrene anion radical gives rise to the acyclic adducts obtained in polar solvents. The anion radical can also be protonated, leading ultimately to the formation of 9,10-dihydro-9-cyanophenanthrene.     

Temperature dependence of local density augmentation around exciplex in supercritical carbon dioxide

To clarify the temperature dependence of local density augmentation around short-lived species, the pressure dependence of the formation and decay processes of exciplex between two neutral species, acetophenone (AP) and N,N,N′,N′-tetramethylbenzidine (TMB), in supercritical carbon dioxide was investigated by a transient absorption technique at 40, 55, and 70 °C. In the high-density (>0.6 g cm⁻³) region, the decay rate constant could be described by the Kirkwood equation. However, in the low-density (0.35–0.6 g cm⁻³) region, the exciplex was more stable than that predicted by Kirkwood analysis, which means that strong influence of local density augmentation around the exciplex occurred. The local density augmentation can be described in terms of an excess density which is defined as a difference between local and bulk density. The excess density was derived from the difference between experimental results and Kirkwood analysis and exhibited a maximum at near-critical density. The excess density decreased with increasing temperature and became negligible at high temperatures and high bulk densities.     

Kinetic model of energy transfer processes between low-coordinated ions on MgO by photoluminescence decay measurements.

Photoluminescence decay studies of emitting species on MgO nanocubes at room temperature provide evidence of three surface species characterized by an excitation and emission wavelength couple {lambda(exc);lambda(em)}. Species A corresponds to {lambda(exc)=240 nm; lambda(em)=380 nm}, whereas the couple {lambda(exc)=280 nm; lambda(em)=470 nm} is assigned to two species: B and B', the former is involved in energy transfer from excited state A* and the latter in direct emission from excited state B'*. A simple model for energy transfer from species A* to B is proposed. The numerical resolution of equations corresponding to this model is in good agreement with experimental data. This method quantifies the kinetics of intrinsic emission and energy transfer processes. Lifetime values indicate that phosphorescence is taking place, and species A, B and B' are identified as edge O(2-) (4 C), corner O(2-) (3 C) and kink O(2-) (3 C) oxide ions respectively.     

Temperature effects on photosensitized processes

The singlet-oxygen-mediated reaction of meso-tetraphenylporphine tetrasulphonate (TPPS4) with different chemical acceptors in buffered aqueous solution was studied as a function of temperature. Imidazole, tryptophan, dimethyl p-nitrosoaniline, (RNO) and furfuryl alcohol served as acceptors. The measurements were performed in real time by spectroscopic or electrochemical monitoring of the consumption of the various reagents, acceptors or dissolved oxygen as a function of the absorbed energy. The results show the following increases in the reaction rate over the temperature range 15-45 degrees C: tryptophan (86%), RNO (90%), furfuryl alcohol (150%) and imidazole (210%). The influence of temperature-correlated changes in the initial oxygen concentration and pH was investigated. Possible implications of the present results for the synergistic influence of hyperthermia and photodynamic therapy are discussed.     

Medium effect on the decay kinetics of benzophenone oxide

The rate constants of benzophenone oxide decay measured at 25°C by flash photolysis (FPh) strongly depend on the nature of the solvent [2k=(2.6±0.3)×10⁷ L mol⁻¹ s⁻¹ in CH₃CN, and (2.0±0.2)×10⁹ L mol⁻¹ s⁻¹ in pentane].     

Solvent effect on conical intersections in excited-state 9H-adenine: radiationless decay mechanism in polar solvent

The ground- and excited-state free energy minima and the conical intersections among these states of 9H-adenine in aqueous and acetonitrile solutions are studied theoretically to elucidate the mechanism of radiationless decay. We employ the recently proposed linear-response free energy (LRFE) to locate the energy minima and conical intersections in solution. The LRFE is calculated by using the reference interaction site model self-consistent field method. The geometry optimizations are carried out at the complete active space self-consistent field level, and the dynamic electron correlation energies are estimated by the multireference M?ller-Plesset method. We find that the conical intersection between the (1)L(a) and (1)L(b) states in aqueous solution occurs at a wide area of the free energy surface, indicating a strong vibronic coupling between them. On the other hand, the (1)npi(*) state is largely blue-shifted at planar geometries in solution, which suggests that the nonadiabatic transition to this state is suppressed. The importance of the (1)pisigma(*) channel is also examined in both the gas phase and solution. Based on the free energy characteristics obtained by the calculations, we intend to explain the experimental observations that the excited state of 9H-adenine decays monoexponentially with shorter lifetimes in polar solvents than that in the gas phase.     

Temperature effects on the intramolecular decay of the lowest triplet state of benzophenone

By monitoring the emission specturm, yield, and lifetime from 77 to 400°K for benzophenone in poly(chlorotrifluoroethylene), we have been able to establish the thermally activated delayed fluorescence process and have also observed a large temperature effect on the rate constant for nonradiative decay from the lowest triplet state to the ground singlet state.     

Potentiation of the cage effect in binary solvents detected by magnetic modulation of exciplex fluorescence

It was shown that selective solvation potentiates the cage effect in recombination of radical ion pairs formed in electron phototransport in binary solvents with components with strongly differing dielectric properties. This is manifested by a more pronounced (in comparison to single solvents) increase in the exciplex fluorescence in a magnetic field in solutions of pyrene with an excess of N,N-dimethylaniline. The magnitude of the magnetic effect, which is a function of the composition of the solvent, is equal to a maximum of 18% (B= 30 mT) for the benzene/dimethyl sulfoxide binary solvent with a 0.26 volume fraction of the latter. A simple model which explains the results obtained is proposed.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2456–2463, November, 1991.     

Temperature effect on the fluorescence anisotropy decay dynamics of Coumarin-153 dye in Triton-X-100 and Brij-35 micellar solutions

The fluorescence anisotropy decay dynamics of the fluorescent probe Coumarin-153 (C153) have been investigated in two neutral micelles, Triton-X-100 (TX-100) and Brij-35 (BJ-35), at different temperatures and analyzed on the basis of the well-known two-step model. Because steady-state fluorescence spectra of the above probe do not show any noticeable changes with respect to temperature, for either of the studied micelles, suggests a similar polarity in the microenvironment around the probe at all the temperatures studied. The anisotropy results indicated that, for both the micelles, the fluidity inside the Palisade layer increases with temperature. However, the temperature effect on the anisotropy decay is relatively more pronounced in TX-100 than in BJ-35. It is inferred that the temperature effect on the anisotropy decay in the BJ-35 micelle is mainly due to the thermal effect on the microviscosity in the micellar phase. In the case of TX-100, the results indicate that, along with the above thermal effect, an additional effect is observed due to the increased size and hydration of the micelle with temperature, with the result being that the fluorescence anisotropy decay in TX-100 is more sensitive to temperature than in BJ-35. In the TX-100 micelle, our studies show that with an increase in temperature, even though the micellar size increases substantially, the distance of the probe from the micellar core does not increase that significantly. Thus, with increasing temperature, the probe undergoes a relative migration toward the micellar core to avoid the increased hydration in the micellar Palisade layer.     

Temperature dependence of the nonradiative decay of indoles in solution

The nonradiative decay rate k_nr. of some indole derivatives has been measured as a function of temperature in three solvents : n-heptane, methanol and water. A temperature-independent component (attributed to S₁ → T₁ intersystem crossing) and a temperature-dependent component (attributed to S₁ → S₀ internal conversion) are present in all case. In aqueous solutions, our results indicate the existence of a second temperature-dependent process which can be identified with photoionization.     

Characteristics of the kinetics of exciplex formation between pyrene and N,N-dimethylaniline on a porous silica surface

The bimolecular photochemical quenching of the fluorescence of pyrene by N,N-dimethylaniline on the surface of various porous silicas with low coverage of the surface (less than a monolayer) has been studied. The kinetics of this process on porous and nonporous silica have been compared. Institute of Surface Chemistry, National Academy of Sciences of Ukraine, 31 Prosp. Nauki, 252039, Kiev. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 33, No. 3, pp. 188–191, May–June, 1997.     

Temperature effect on the adsorption and desorption processes in retinoic acid crystals: An electrical conductivity study

Using electrical conductivity as a probe, the effect of crystal temperature on the process of adsorption and desorption of some chemical vapours on retinoic acid semiconductor has been investigated. It has been observed that a three-stage adsorption process in a sample at low temperature changes over to a two-stage process at higher temperature. Measures of activation energies of adsorption and desorption at different stages have been estimated from kinetic data analysis by using a modified Roginsky-Zeldovich equation.     

Magnetic field effect on the exciplex between all-s-trans- 1,4-diphenylbuta- 1,3-diene and 1,4-dicyanobenzene: a comparative study with other alpha,omega-diphenyl polyenes

The exciplex between all-s-trans-1,4-diphenylbuta-1,3-diene and 1,4-dicyanobenzene has been studied by steady state fluorescence along with the magnetic field effect (MFE) and compared with the other alpha,omega-diphenyl polyenes. The exciplex formation and magnetic field effect are dictated by the chain length of the polyene rather than the electronic requirement of these phenomena. The wavelength dependence of the MFE confirms the presence of two different charge-transfer complexes.     

Water effect on the excited-state decay paths of singlet excited cytosine

Two low-energy deactivation paths for singlet excited cytosine, one through a S₁/S₀ conical intersection of the ethylene type, and one through a conical intersection that involves the (n_N, π^*) state, are calculated in the presence of water. Water is included explicitly for several cytosine monohydrates, and as a bulk solvent, and the calculations are carried out at the complete active space self-consistent field (CASSCF) and complete active space second order perturbation (CASPT2) levels of theory. The effect of water on the lowest-energy path through the ethylenic conical intersection is a lowering of the energy barrier. This is explained by stabilization of the excited state, which has zwitterionic character in the vicinity of the conical intersection due to its similarity with the conical intersection of ethylene. In contrast to this, the path that involves the (n_N, π^*) state is destabilized by hydrogen bonding, although the bulk solvent effect reduces the destabilization. Overall, this path should remain energetically accessible.