Quantum Yield Of Triplet Formation Research Articles

Primary processes in excited β-napthol systems: A pulsed laser photolysis study

Using a pulsed N2 laser the rates of formation of solvated electrons, and triplet molecules were measured for β-naphtholate, β-naphthol, and β-naphthyl-methyl-ether in solution. The rate of electron formation is higher than the rate of vibrational relaxation to the lowest excited singlet state, from which intersystem crossing occurs. Quantum yields of triplet formation and rate constants for intersystem crossing were obtained from measurements of fluorescence yields, lifetimes and relative yields of triplets in oxygen free and oxygen containing solutions. For β-naphthol in a high viscosity solvent the sum of the quantum yields of formation of solvated electrons and of population of the lowest excited singlet state is unity whereas the sum is less than unity in low viscosity solvents, indicating that the lowest excited singlet state of β-naphthol is populated by charge recombination in high viscosity solvents but not in low viscosity solvents. A calculation of the energetics yields a maximum charge separation of 5 Å, a value consistent with the occurrence of effective charge recombination.

Tautomerism of uracil and thymine in aqueous solution: spectroscopic evidence.

Excitation spectra for triplet state formation and fluorescence emission from uracil and thymine in neutral aqueous solution at room temperature are anomalous when compared to the absorption spectra of uracil and thymine. The experimental data are critically examined with respect to three molecular models; Model 1, which is characterized by increased intersystem crossing from higher vibrational levels of S(1); Model 2, in which fluorescence is attributed to a (pi,pi(*)) state, whereas intersystem crossing occurs from an (npi(*)) state; and Model 3, in which fluorescence is attributed to a tautomer I, while triplet yields originate from tautomer II. Reasons are presented for discarding Models 1 and 2, and it is demonstrated that the observed excitation spectra complement each other with respect to the absorption spectrum, such that the quantum yields of triplet formation and fluorescence emission become independent of exciting wavelength. It is suggested that the fluorescing tautomer I has the N(3)-C(4) enol structure, while the triplet-forming tautomer II is most likely the predominant diketo form.

The photochemistry of triphenylmethane dyes in films of poly(methylmethacrylate)

AbstractPhotobleaching of the halides of the triphenylmethane dyes crystal violet and malachite green in thin films of poly(methylmethacrylate) is described. The reaction proceeds via the triplet excited state of the dye which undergoes an intramolecular electron transfer reaction, forming the triarylmethyl and halogen free radicals. Quantum yields of triplet formation depend on the concentration of the dye which suggests that the intersystem crossing process involves more than one dye molecule. The rate of reaction depends on the rate at which radicals are formed which are free of the solvent cage and on the reactivity of the radicals.

Heavy atom effect on the luminescence of phenanthrene

The internal heavy atom effect on the radiative and radiationless transitions from the lowest excited states of phenanthrene was investigated. Fluorescence and phosphorescence quantum yields and triplet lifetimes were determined for phenanthrene and its methyl, chlorine and bromine derivatives, and approximate rate parameters to and from the lowest triplet level were calculated. The halogen substitution induces a larger increase of the intersystem crossing S 1-T 1 rate than of the T 1-S 0 one, resulting in a higher overall triplet formation quantum yield and, also, higher radiative decay quantum efficiency from the triplet. An effect of the halogen position on the luminescence properties was also observed.

Quantitative Messung der Singulett- und Triplett-Absorptions-Spektren von Phäophytin a und b / Pheophytin-a and Pheophytin-b; Quantitative Absorption Spectra of the Singuletand Triplet-State

Quantitative singlet absorption spectra dependent on solvent, temperature, and concentration are shown. The bands observed in the visible and UV region are assigned to 3 π → π*-electron transfers. Measurements at different acidities showed that the pheophytin molecule can be present in 3 prototropic forms; as a base, a mono- or a divalent cation. In polar solvents of concentrations >10-4 M dimerization occurred. Larger clusters were formed in non-polar solvents at concentrations <10-4 Μ and at temperatures below ca. 200 °K. The quantitative triplet (T) -absorption spectra for pheophytin a and b are also reported: the longest wavelength T-T-band is expected in the region 5000-8000 cm-1. The relaxation times and quantum yields of triplet formation are given; simple first order kinetics were found to fit the decay curves.

Flash Calorimeter for Measuring Triplet Yields

A fast, sensitive differential calorimeter is described which permits the observation of heating caused by the relaxation of the lowest triplet states of organic molecules formed by flash excitation. From the ratio of slow to total heating after flash excitation, we calculate Φt, the quantum yield of triplet formation. The detection of the small amounts of heat involved (∼10−5 cal) is accomplished by means of a capacitor microphone transducer which measures volume changes caused by the heating. The response time (∼100 μsec) is determined by the lowest cavity resonance of the sample cell. At room temperature we find for anthrance in ethanol Φt=0.66, while for acridine orange in glyerol Φt=0.23.

Quantum yields of triplet formation in solutions of chlorophyll

The method of flash photolysis has been used for direct measurement of the quantum yields of triplet formation in dilute chlorophyll solutions at 23 °C. In ether solution, the triplet yields were found to be 0⋅64 and 0⋅88 for chlorophyll a and b , respectively. In very dry hydro­carbon solution, the yields decrease by at least a factor of 5. It is suggested that the low yields of fluorescence and intersystem crossing in dry solvents are due to dissociation of excited singlet dimer chlorophyll into ground state monomer species.