Ming-Fea Chow

CONVENIENT AND SIMPLE METHODS FOR THE OBSERVATION OF PHOSPHORESCENCE IN FLUID SOLUTIONS. INTERNAL AND EXTERNAL HEAVY ATOM AND MICELLAR EFFECTS

Abstract— Phosphorescence of organic molecules in fluid solutions may be conveniently and readily observed under certain conditions. If kp (radiative phosphorescence rate constant) is 10s‐1, then (in the absence of photoreaction) phosphorescence is observable upon N2 purging. For example, nitrogen purged, acetonitrile solutions of bromo and dibromonaphthalene display readily observable phosphorescence as a result of internal heavy atom enhancement of πS, and kp. External heavy atom enhancement of k, (CH2BrCH2Br solvent) of aromatic hydrocarbons even allows observation of phosphorescence from these compounds in N2 purged fluid solutions. Although bromonaphthalenes are not significantly phosphorescent in N2 purged aqueous solution, phosphorescence is readily observed in N2 purged detergent (HDTBr, HDTCl, and SDS) solutions above the critical micelle concentration. The general factors which determine whether phosphorescence is “readily” obervable in fluid solution are briefly discussed and the results are interpreted in light of these factors.

Time-resolved laser flash spectroscopic study of benzyl radical pairs in micelle cages

Photolysis of benzyl ketones provides a convenient means of producing triplet, spin-correlated geminate radical pairs.’ In nonviscous, homogeneous solution these radical pairs rapidly (5 s) become random free radicals.* Spin correlation is destroyed when the radical fragments have separated sufficiently to allow exchange forces to become negligible relative to weak, random, magnetic forces experienced by the individual radical centers, Le., the spin-correlated triplet radical pair develops singlet character. Micelles have been shown to provide a unique type of “solvent cage” for a radical pair.j The hydrophobic portion of the micelle provides a restricted volume of space of dimensions which allow the exchange interaction between the radical center to rapidly decrease to very small values, but a t the same time the micelle preserves the compositional correlation of geminate radical pairs for relatively long periods of time (5 s ) . ~ We report here a time-resolved laser flash spectroscopic investigation of triplet benzyl radical pairs produced in micelles. A study of transient absorption of benzyl radicals as a function of (1) isotopic composition and structure of the parent ketone, (2) magnetic field, and (3) the presence or absence of an aqueous phase radical trap provides support for the occurrence of two types of geminate benzyl radical pairs: a fast decaying pair and a slow decaying pair. The structures of the ketones6 employed in this study are shown in Chart I. Typically, a given ketone in aqueous solutions of hexadecyltrimethylammonium chloride (HDTCl) was excited by a 15-11s pulse of 249-nm light produced by an excimer laser.7 Under these conditions a transient, whose spectral characteristics (A, 320 nm) are in good agreement with those of benzyl radical in homogeneous solution, is produced.8a The decay of benzyl radical absorption in methanol and in isooctane cleanly follows second-order kinetics in the time range of 50-50 000 ns. Thus, as expected,8b the disappearance of benzyl radicals in homogeneous solution, in the time range of interest, occurs predominantly via combination of random free radicals. In the same time domain, the decay of benzyl radicals (monitored by absorbance or optical density, Figure 1) in micellar solution is complex, but can be analyzed in terms of a “fast” first-order decay (rate constant, k f ) and a “slow” first-order decay (rate constant, k,), Le., in terms of eq 1, where I(?) is the measured optical density a t time t, Zf

SOLVENT SENSITIVITY OF TYPE II PHOTOREACTIONS OF KETONES AS A DEVICE TO PROBE SOLUTE LOCATION IN MICELLES

Abstract. The solvent sensitivity of the Type II reaction is employed as a device to investigate the properties of detergent solutions. The quantum yield for Type II reaction and the ratio of cleavage to cyclization are used as specific monitors of local solvent properties of micelles. Our results indicate that the average position of phenyl alkyl ketones is either on the micelle surface or in the Stern layer. The absence of significant residence time of the excited ketone in the aqueous phase is confirmed by the lack of quenching of Type II reaction by Eu3+ in micellar solutions.

Magnetic isotope effect on the thermolysis of 9,10-diphenylanthracene endoperoxide as a means of separation of /sup 17/O from /sup 16/O and /sup 18/O

The results appear to be consistent with the postulate that the thermolysis of 9,10-diphenylanthracene endoperoxide (DPA-O/sub 2/) produces a mixture of singlet oxygen and triplet oxygen as primary products via an initial singlet diradicaloid intermediate which can either fragment to yield singlet oxygen or intersystem cross to yield a triplet diradicaloid which can fragment to yield triplet oxygen. When the singlet diradicaloid possesses an /sup 17/O atom which can interact, via hyperfine coupling, with an electron spin, the rate of intersystem crossing is accelerated relative to the rate for singlet diradicaloids that possess only /sup 16/O or /sup 18/O atoms. The experimental manifestations of this magnetic isotope effect are a lower quantum yield for singlet oxygen formation in /sup 17/O-enriched DPA-O/sub 2/ and a selective isotopic enrichment of /sup 17/O in the untrappable molecular oxygen generated in the thermolysis of DPA-O/sub 2/. These results appear to represent the first of selective separation of a middle isotope by a chemical process. 1 table, 3 figures.

Investigation of triplet—triplet energy transfer in films of poly-n-vinylcarbazole employing chemiluminescence techniques

Abstract Chemiluminescent techniques are employed to investigate energy transfer and exciton in polyvinylcarbazole films at 34°C. Tetramethyl-1,2-dioxetane is used as a source of chemiexcited acetone triplets and 1,4-dibromonaphthalene and 1,10-phenanthroline are used as triplet acceptors which phosphoresce under the conditions of measurement. A study of the relationship of acceptor chemiluminescence intensity to acceptor concentration leads to the conclusion that extensive energy transfer by triplet excitons can occur in polyvinylcarbazole films.

The dynamics of the photodecarbonylation of dibenzylketone in a micellar detergent solution: effect of temperature on the absolute quantum yields and on 13C enrichment

Abstract An investigation is reported of the influence of temperature on the quantum efficiencies and 13 C enrichment efficiency of photolysis of dibenzylketone in micellar solution.

Reactivity of singlet oxygen toward strained substrates and a novel non-photochemical method for determination of quenching constants of singlet oxygen.

Abstract Rate constants for quenching of 1 O 2 by a number of strained molecules have been determined by the competitive rubrene photooxidation method; the rate constant for quenching by Q may be evaluated by adaption of a kinetic analysis already in the literature for the rubrene photooxidation method.

Photophysical properties of poly(n-vinylcarbazole) solid films at elevated temperatures. Evidence for a third phosphorescent species

Abstract The λ max for phosphorescence emission of poly(N-vinylcarbazole) (PVCz) shifts to lower values as the temperature is raised from 278 to 363 K. A corresponding long-lived phosphorescent emission is found to have a temperature-independent lifetime from 220 to 298 K. Neither of two identified excimer-Iike phosphorescent components of PVCz have properties consistent with this emission; its properties suggest it is excitonic in origin.