Polarized photolysis

Work by Koga et aJ [62] demonstrates how the polarization iiiechanism can change upon alteration of the chemical enviromnent. Upon laser flash photolysis, excited xanthone abstracts a proton from an alcohol... 

Figure Bl.16.19. (a) CIDEP spectrum observed in die photolysis of xanthone (1.0 x 10 M) in cyclohexanol at room temperature. The stick spectra of the ketyl and cyclohexanol radicals with RPM polarization are presented, (b) CIDEP spectrum after the addition of hydrochloric acid (4.1 vol% HCl 0.50 M) to the solution above. The stick spectra of the ketyl and cyclohexanol radicals with absorptive TM polarization are presented. The bold lines of the stick spectra of the cyclohexanol radical show the broadened lines due to ring motion of the radical. Reprinted from [62].

Closs G L and Miller R J 1979 Laser flash photolysis with NMR detection. Microsecond time-resolved CIDNP separation of geminate and random-phase polarization J. Am. Chem. Soc. 101 1639—41... 

Morozova O B, Tsentalovich Y P, Yurkovskaya A V and Sagdeev R Z 1998 Consecutive biradicals during the photolysis of 2,12-dihydroxy-2,12-dimethylcyclododecanone low- and high-field chemically induced dynamic nuclear polarizations (CIDNP) study J. Rhys. Chem. A 102 3492-7... 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Nanosecond flash photolysis studies of Ct-nitronaphthalene in polar and nonpolar solvents led to the detection of the trip let-triplet absorption spectrum of this nitrocompd (Ref 26). 

Nanosecond flash photolysis of 1,4-dinitro-naphthalene in aerated and deaerated solvents showed a transient species with absorption maximum at 545nm. The maximum of the transient absorption was independent of solvent polarity and its lifetime seemed to be a function of the hydrogen donor efficiency of the solvent. The transient absorption was attributed to the lowest excited triplet state of 1,4-dinitronaphthalene. Based on spectroscopic and kinetic evidence, the triplet state of 1,4-dinitronaphthalene behaved as an n - Tt state in nonpolar solvents,... 

Photolysis at about 40°C of the /ra is-isomer of the unsymmetrical azo-compound 4 in benzene solution yields dicumyl and biphenyl as the major products, together with a little a-methylstjTene, cumene, and 2,2-diphenylpropane. All these products show polarization of their... 

A partially oriented (using polarized, visible photolysis) sample of Fe(CO)s in solid CO at 20 K has been prepared (193). It was subsequently found that no change in the polarization properties of the system occurred during several hours of spectroscopic observation. It was concluded that the fluxionality of Fe(CO)5 had been quenched under these conditions, as, were this not the case, maintenance of polarization for more than a fraction of a second would be impossible. 

Much has been learned in recent years about the 00 dimer , O2O2, produced in reaction 17. It is actually dichlorine peroxide, OOOCl its geometry is now well established from submillimeter wave spectroscopy (15). Photolysis of OOOO around 310 nm the atmospherically important wavelengths -- yields chlorine atoms and ClOO radicals (16), as given in reaction 18, rather than two OO radicals, even though QO-OQ is the weakest bond (it has a strength of about 17 Kcal/mol (17)). Thermal decomposition of QOOQ (the reverse of reaction 17) occurs very fast at room temperature, but more slowly at polar stratospheric temperatures. Hence, photolysis is the predominant destruction path for CIOOQ in the polar stratosphere and two Q atoms are produced for each ultraviolet photon absorbed. 

Carbocations are intermediates in several kinds of reactions. The more stable ones have been prepared in solution and in some cases even as solid salts, and X-ray crystallographic structures have been obtained in some cases. An isolable dioxa-stabilized pentadienylium ion was isolated and its structure was determined by h, C NMR, mass spectrometry (MS), and IR. A P-fluoro substituted 4-methoxy-phenethyl cation has been observed directly by laser flash photolysis. In solution, the carbocation may be free (this is more likely in polar solvents, in which it is solvated) or it may exist as an ion pair, which means that it is closely associated with a negative ion, called a counterion or gegenion. Ion pairs are more likely in nonpolar solvents. 

The a- and [3-isomers of endosulfan undergo photolysis in laboratory tests after irradiation in polar solvents and upon exposure to sunlight on plant leaves. The a-isomer also undergoes isomerization to the P-isomer, which is relatively more stable (Dureja and Mukerjee 1982). A photolytic half-life of about 7 days was reported for endosulfan by EPA (1982c). The primary photolysis product is endosulfan diol, which is subsequently photodegraded to endosulfan a-hydroxyether. Endosulfan sulfate is stable to direct photolysis at light wavelengths of >300 nm however, the compound reacts with hydroxy radicals, with an estimated atmospheric half-life of 1.23 hours (HSDB 1999). 

The difference in structure between 16 and 17 is readily understood in terms of the addition of strongly electron-donating substituents, but the contrast between 16 and 20 is less easily rationalized. Photolysis of 19 was carried out in HFIP (dielectric constant (e) = 16.75), while TRIR experiments with diphenyl diazomethane (22) were carried out in dichloromethane (e = 9.08), suggesting that a-lactone structure may be dependent on solvent polarity. 

To substitute the strongly bound axial CO ligand of the ruthenium or osmium center, it is necessary to employ more drastic conditions than simple stirring at room temperature. Imamura (11,20) used photolysis to synthesize porphyrin trimers on the basis of simultaneous coordination of two 4-pyridyl porphyrins to the same ruthenium porphyrin (12, Fig. 3). Some interesting photophysical behavior was observed for these systems. The trimers have an extra UV-Vis absorption band at about 450 nm which is ascribed to metal-ligand charge transfer (MLCT), a d7r(Ru(II))-7r (OEP) transition. This band shows a batho-chromic shift in more polar solvents, and decreased in intensity when... 

Fig. 7. Raw TOF spectra for the photolysis of C2H2, obtained with two polarization configurations. The laser frequency was set at the center of the Doppler profile, i.e. u)q.

Fig. 9. The translational energy distributions of H2O photodissociation at 121 nm obtained with photolysis laser polarization parallel to the detection direction, (a) The upper trace was acquired experimentally, (b) The lower trace is the simulated distribution.

CM) frame of 0cm relative to the photolysis laser polarization can be represented by the following formula ... 

---