Photophysical properties of benzil in solution: triplet state deactivation pathways

The fluorescence and phosphorescence from benzil in dilute benzene and cyclohexane solutions (2 × 10⁻⁴ M) were studied by both conventional luminescence and time-correlated single-photon techniques in the temperature range 8 – 69 °C. The fluorescence (λ = 502 nm) did not show a substantial temperature dependence and was free from thermal and triplet-triplet annihilation delayed contributions at the low concentration used. The phosphorescence (λ = 562 nm) was temperature dependent and its decay was controlled by an activation energy (E_a = 7.4 ± 0.5 kcal mol⁻¹) which was slightly larger than the spectroscopic single-triplet splitting (6.1 kcal mol⁻¹). The photophysical parameters derived from the lifetimes of the two emissions was not consistent with the model of thermal equilibration between S₁ and T₁.     

Association equlibria in acetonitrile solution of tetracyanoquinodimethanides

Trimetric ion (TCNQ)^2?₃ has been discovered in mixed solutions of TCNQ and LiTCNQ in acetonitrile. The estimated association constant isK_T = 5.6 × 10¹⁰ øl² mol^?2.     

Prediction of acidity in acetonitrile solution with COSMO-RS

The COSMO-RS method, a combination of the quantum chemical dielectric continuum solvation model COSMO with a statistical thermodynamics treatment for realistic solvation simulations, has been used for the prediction of pK(a) values in acetonitrile. For a variety of 93 organic acids, the directly calculated values of the free energies of dissociation in acetonitrile showed a very good correlation with the pK(a) values (r(2) = 0.97) in acetonitrile, corresponding to a standard deviation of 1.38 pK(a) units. Thus, we have a prediction method for acetonitrile pK(a) with the intercept and the slope as the only adjusted parameters. Furthermore, the pK(a) values of CH acids yielding large anions with delocalized charge can be predicted with a rmse of 1.12 pK(a) units using the theoretical values of slope and intercept resulting in truly ab initio pK(a) prediction. In contrast to our previous findings on aqueous acidity predictions the slope of the experimental pK(a) versus theoretical DeltaG(diss) was found to match the theoretical value 1/RT ln (10) very well. The predictivity of the presented method is general and is not restricted to certain compound classes. However, a systematic correction of -7.5 kcal mol(-1) is required for compounds that do not allow electron-delocalization in the dissociated anion. The prediction model was tested on a diverse test set of 129 complex multifunctional compounds from various sources, reaching a root mean square deviation of 2.10 pK(a) units.     

NH-acid properties of porphyrins in acetonitrile

The thermodynamics of acid dissociation of synthetic porphyrins (H₂P) in alkaline acetonitrile solution is studied. The low stability of the anionic species is attributed to the previously formulated electronic, steric, and solvation factors. The solvent plays an important role in the stabilization of anionic porphyrin species in acetonitrile. The electronic absorption spectra of anionic species of H₂P are analyzed.     

Quasi-isoenergetic triplet energy migration in homogeneous solution

Intermolecular triplet energy migration in alkyl aryl ketones is shown to be a very fast process. In quasi-isoenergetic examples the rate constants are in the neighborhood of 10¹⁰ M^? s^? in n-heptane at room temperature. The process is rather insensitive to alkyl α-substitution in the donor triplet state and is substantially faster than the analogous transfer in aliphatic ketones.     

Raman study of vibrational relaxation of acetonitrile in solution

Line widths of isotropic Raman spectra of the ν₁ (a₁) CH or CD stretching bands of acetonitrile CH₃CN and CD₃CN were measured in a number of solvents. The vibrational dephasing theory, modified for use in binary mixtures, predicts quantitatively the solvent dependence of the Raman line widths.     

Photo-oxidation of triplet aromatic dianions in rigid solution

An ESR study of the photo-oxidation of the triplet dianions of triphenylbenzene and triphenylene in rigid MTHF provided information about the ion pair structures of both mono- and dianions.     

Photosensitizing properties of the porphycene immobilized in sol-gel derived silica coating films

Porphycene was covalently immobilized in a sol-gel silica film deposited on a glass plate, and the immobilized porphycene showed a photosensitizing property with recycling for the photo-oxidation of 1,5-dihydroxynaphthalene.     

Adiabatic triplet state tautomerization of p-hydroxyacetophenone in aqueous solution

The primary photophysical processes of p-hydroxyacetophenone (HA) and the ensuing proton transfer reactions in aqueous solution were investigated by picosecond pump-probe spectroscopy and nanosecond laser flash photolysis. Previous studies have led to mutually inconsistent conclusions. The combined data allow us to rationalize the excited-state proton transfer processes of HA in terms of a comprehensive, well-established reaction scheme. Following fast and quantitative ISC to the triplet state, (3)HA*, adiabatic proton transfer through solvent water simultaneously forms both the anion, (3)A(-)*, and the quinoid triplet enol tautomer, (3)Q*. The latter subsequently equilibrates with its anion (3)A(-)*. Ionization and tautomerization are likely to compete with the desired release reactions of p-hydroxyphenacyl photoremovable protecting groups.     

Protonation of tetrasulfonated zinc phthalocyanine in aqueous acetonitrile solution.

The phenomenon of protonation of phthalocyanines (Pc) and its effect upon their photophysical properties has seen considerable neglect in the literature. The work reported here clearly shows that tetrasulfonated zinc Pc, a known photodynamic therapy (PDT) agent, is strongly susceptible to protonation at the azomethine bridges. Absorption and fluorescence spectra demonstrate the absolute dependence of the redshifted peak on the pH of the solution. The fluorescence spectra and lifetimes of the protonated Pc are reported, and the potential application of this phenomenon to the development of a PDT agent with increased selectivity is discussed.     

Substituent effect on triplet lifetimes of arylketones in benzene solution

The triplet yields and lifetimes of a series of substituted benzophenones have been detemined by their efficiency in sensitizing biacetyl phosphorescence, in benzene solution.The triplet decay constants of nine para-substituted derivatives show a satisfactory Hammett-type correlation. The results are discussed on the basis of a reversible interaction of the excited ketones with the solvent. Possible factors which may be responsible for the inefficiency of sensitization are also discussed.     

Low temperature properties of acetonitrile confined in MCM-41

The effect of confinement on the phase changes and dynamics of acetonitrile in mesoporous MCM-41 was studied by use of adsorption, FT-IR, DSC, and quasi-elastic neutron scattering (QENS) measurements. Acetonitrile molecules in a monolayer interact strongly with surface hydroxyls to be registered and perturb the triple bond in the C[triple bond]N group. Adsorbed molecules above the monolayer through to the central part of the cylindrical pores are capillary condensed molecules (cc-acetonitrile), but they do not show the hysteresis loop in adsorption-desorption isotherms, i.e., second order capillary condensation. FT-IR measurements indicated that the condensed phase is very similar to the bulk liquid. The cc-acetonitrile freezes at temperatures that depend on the pore size of the MCM-41 down to 29.1 A (C14), below which it is not frozen. In addition, phase changes between alpha-type and beta-type acetonitriles were observed below the melting points. Application of the Gibbs-Thomson equation, assuming the unfrozen layer thickness to be 0.7 nm, gave the interface free energy differences between the interfaces, i.e., Deltagamma(l/alpha) = 22.4 mJ m(-2) for the liquid/pore surface (ps) and alpha-type/ps, and Deltagamma(alpha/beta) = 3.17 mJ m(-2) for alpha-type/ps and beta-type/ps, respectively. QENS experiments substantiate the differing behaviors of monolayer acetonitrile and cc-acetonitrile. The monolayer acetonitrile molecules are anchored so as not to translate. The two Lorentzian analysis of QENS spectra for cc-acetonitriles showed translational motion but markedly slowed. However, the activation energy for cc-acetonitrile in MCM-41 (C18) is 7.0 kJ mol(-1) compared to the bulk value of 12.7 kJ mol(-1). The relaxation times for tumbling rotational diffusion of cc-acetonitrile are similar to bulk values.     

Kinetic and fluorescent properties of tetraphenylporphine derivatives in acetonitrile

The kinetics of formation of zinc complexes with 5,10,15,20-tetra(4-Cl-phenyl)porphine and 5,10,15,20-tetra(4-NH₂-phenyl)porphine in acetonitrile in the range of 298–318 K are studied. It is found that peripheral substituents in tetraphenylporphine can slow down or accelerate the reaction of formation of zinc complexes in acetonitrile as observed by changes in kinetic rate constants. The fluorescence quantum yields of tetraphenylporphine derivatives and their zinc complexes also depend on the presence of electrondonating or electron-drawing substituents in the ligand structure.     

Photolysis of alpha-azidoacetophenones: trapping of triplet alkyl nitrenes in solution

[reaction: see text] Selective excitation of the ketone chromophore in alpha-azidoacetophenones, 1, leads to intramolecular triplet energy transfer to the azido group, which forms the corresponding triplet alkyl nitrene, 2. Azides 1 also undergo alpha-cleavage to form benzoyl and methyl azido radicals in competition with nitrene formation. Thus the major photoproduct, 2-benzoylamino-1-phenylethanone, 3, comes from trapping of 2 with a benzoyl radical. This appears to be the first observation of bimolecular trapping of triplet alkyl nitrenes in solution.