Time-resolved CIDEP

Time-resolved CIDEP and optical emission studies provide further definitive characterization of the triplet and excited singlet states followed by their primary photochemical reactions producing transient radicals in individual mechanistic steps in the photolysis of a-guaiacoxylacetoveratrone. Both fluorescence and phosphorescence are observed and CIDEP measurements confirm the mainly n,n character of the lowest triplet state. The results indicate a photo triplet mechanism involving the formation of the ketyl radical prior to the P-ether cleavage to form phenacyl radicals and phenols. Indirect evidence of excited singlet photo decomposition mechanism is observed in the photolysis at 77 K. 

Time-Resolved Triplet CIDEP Spectra at 77 K. The time-resolved CIDEP spectra of the triplet state of GAV in a solid solution at 77 K are given in Figure 2. 

Figure 2. Time-resolved CIDEP spectrum of the lowest triplet of GAV at 77 K. a, Am = 2 transitions at half field, b, Ani = 1 transition at the canonical orientations. The strong emissive signal at g = 2 is due to phenoxy radicals.

Until recently, the mechanism of the inhibition of light-induced yellowing was subject to speculation. However, solid state ESR and CIDEP have provided insight into the mode of inhibition by thiols (Wan, J.K.S., et al, J. Wood Chem. Technol., in press). Near-uv irradiation of unbleached and peroxide bleached thermomechanical pulp impregnated with thiols caused a rapid increase of the thiol radicals. The time resolved CIDEP spectrum, however, shows a symmetric broad band characteristic of the polarized phenoxy radical. This result suggests that thiols quench triplet generated phenoxy radicals in a secondary thermal process. 

T has to be obtained from independent experiments the obvious choice would be to take the value from the time-resolved CIDEP study. However, if T values are not available, we do not need to know the absolute value of V in those experiments where one is measuring relatives kq s of a particular triplet molecule reacting with a number of different substrates (49) as long as the same radical, derived from the same parent triplet state, is being monitored. 

A typical CIDEP initial polarization system is summarized in Fig. 11. For the quenching by a hydrogen donor (SH) of triplet quinone molecules, a plot of V-- - versus [SH] 1 will yield a straight line (see eq. 38), the ordinate intercept of which will give V"1 and from the slope an estimate of Tjt (or Tf) kq can be made. Because Vo can be obtained from the intercept and eliminated from the slope to obtain kq, it can be seen that only relative values of V are needed (i.e., the absolute are not required). (Absolute can be directly measured in a time-resolved CIDEP experiment, if needed.) Obviously, if the value of 3Tl is known, kq (absolute) can be calculated. Unfortunately, Tj values are usually not known. However, by studying a series of hydrogen donors, the relative quenching efficiencies can be estimated from the ratios of the slopes for plots of eq. 38. 

A time resolved CIDEP study of radical pair systems derived by electron-transfer photooxidation of carbazole derivatives using maleic anhydride has revealed that the singlet state of the radical ion-pair has an energy which is greater than that of the triplet state. From this evidence the conclusion is drawn that the sign of the apparent exchange interaction is positive. 

Wan et al. [18] have established the major role played by the radical anion SO2 originating from many different parent ions and chemical environments. Their time-resolved CIDEP observations showed that the radical species can be generated either by photolysis, or by the thermal dissociation as shown in Equation 13.2 ... 

Time resolved CIDEP study. 38) CIDNP study. ... 

Goudsmit G H and Paul H 1993 Time-resolved EPR investigation of triplet state Cgg. Triplet-triplet annihilation, CIDEP, and quenching by nitroxide radicals Chem. Phys. Lett. 208 73-8... 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

Time-resolved laser flash ESR spectroscopy generates radicals with nonequilibrium spin populations and causes spectra with unusual signal directions and intensities. The signals may show absorption, emission, or both and be enhanced as much as 100-fold. Deviations from Boltzmann intensities, first noted in 1963, are known as chemically induced dynamic electron polarization (CIDEP). Because the splitting pattern of the intermediate remains unaffected, the CIDEP enhancement facilitates the detection of short-lived radicals. A related technique, fluorescence detected magnetic resonance (FDMR) offers improved time resolution and its sensitivity exceeds that of ESR. The FDMR experiment probes short-lived radical ion pairs, which form reaction products in electronically excited states that decay radiatively. ... 

Figure 4. a, CIDEP spectrum observed during the photolysis of a flowing GAV solution (flow rate at 5 mlVmin). b, The time-resolved polarization profiles monitored at selected magnetic fields 1, ketyl radicals and 2, phenacyl radicals. 

Appropriate modifications of the ESR spectrometer and generation of free radicals by flash photolysis allow time-resolved (TR) ESR spectroscopy [71]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They may be enhanced as much as one hundredfold and may appear in absorption, emission, or in a combination of both modes. These spectra indicate the intermediacy of radicals with substantial deviations from equilibrium populations. Significantly, the splitting pattern characteristic for the spin density distribution of the intermediate remains unaffected thus, the CIDEP (chemically induced dynamic electron polarization) enhancement facilitates the detection of short-lived radicals at low concentrations. 

Fig. 1.14. Comparison between integrated continuous light-induced (upper trace) and time-resolved pulsed laser-induced (lower trace) EPR spectra from 45A Ti02 (0.3M) modified with dopamine (0.08 M).The lower trace was obtained with a 550 nm laser (laser intensity 10 mJ per pulse, 10 ns pulse duration, 20 scans), 1 ps after the laser pulse. Both spectra were recorded at 8 K. Right section shows how triplet radical pair mechanism of CIDEP in addition to fast exchange can contribute to the observed polarized spectrum.

Direct recording of the kinetics on an absolute time scale can be achieved by the time-resolved e.s.r. method. However, special attention has to be paid to CIDEP effects which may distort the observation of chemical kinetics. When the rate of radical decay is... 

In a time-resolved experiment, the intrinsic polarization of the system is measured with very little correction except for power saturation (98,99). To measure the initial polarization, the signal heights S and Sq (hence the enhancement factor V and the polarization factor P = /So) can be obtained directly from the time profile. For a photo-CIDEP system induced only by the triplet mechanism and the triplet reaction involves a substrate SH, a kinetic analysis yields the following relationship ... 

Many new physical methods were developed in response to needs of spin chemists. In particular, the time-resolved EPR (TREPR) ° and time-resolved NMR (CIDNP) techniques were found to be of unparalleled utility in terms of mechanistic understanding of radical chemistry. Theoretical work to explain CIDNP and CIDEP phenomena was able to link, for the first time, the spin physics of radical pairs to their diffusion, molecular tumbling, confinement (solvent cages versus supramo-lecular environments ), and the effects of externally applied magnetic fields. ... 

Analysis of a CIDEP pattern with time-resolved ESR (TR ESR) spectra provides a solid conclusion to be made on the spin multiplicity of molecular precursors of polarized free radicals (a singlet or a triplet excited molecule) and the tracking of fast reactions of polarized radicals leading to secondary radicals. Thus, TR ESR is a convenient method in mechanistic photochemistry and free radical chemistry. Continuous wave TR ESR (CW TR ESR) devices are widely used for detection of photogenerated radicals. They usually consist of a pulsed ns laser with detection of transients by their ESR spectra with a X band ESR spectrometer in the direct detection mode (no hied modulation).Time-resolved Eourier transform ESR (FT ESR) has some advantages and drawbacks with respect to CW TR ESR. Rather sophisticated FT ESR devices have become available, and FT ESR studies become more common. 

A review of flash photolysis spin resonance describes CIDEP techniques which can be used to identify free radicals. Time resolved ESP and ENDOR has been used during the photolyses of M,W-dimethoxy-W-phenylacetophenone, an efficient UV curing initiator. ... 

The overall diagram of evolution of the excited states and reactive intermediates of a photoinitiating system working through its triplet state can be depicted in Scheme 10.2 [249]. Various time resolved laser techniques (absorption spectroscopy in the nanosecond and picosecond timescales), photothermal methods (thermal lens spectrometry and laser-induced photocalorimetry), photoconductivity, laser-induced step scan FTIR vibrational spectroscopy, CIDEP-ESR and CIDNP-NMR) as well as quantum mechanical calculations (performed at high level of theory) provide unique kinetic and thermodynamical data on the processes that govern the overall efficiency of PIS. 

---