The photochemistry of two phosphine oxides and the rate constants of reaction of their daughter radicals with several alkenes, halocarbons, and oxygen have been determined. Photolysis of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (1) and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (4) in each case affords a phosphinoyl and a benzoyl radical. The phosphinoyl radicals are readily detected by laser flash photolysis and exhibit absorption maxima at 325 and 450 nm for the diphenylphosphinoyl (3) and 2,6-dimethoxybenzoyl-2,4,4-trimethylpentylphosphinoyl (6) radicals, respectively. The rate constants for reaction of the phosphinoyl radicals with alkyl halides, alkenes, and oxygen range from 104 to 109 M-1 s-1. Radical 3 is 2−6 times more reactive than radical 6. For example, 3 adds to methyl methacrylate with a rate constant of (11 ± 2) × 107 M-1 s-1 whereas 6 has an addition rate constant for the same reaction of (2.3 ± 0.3) × 107 M-1 s-1. The rate constants for reaction with alkyl halides decrease with increasing C−X bond strength, while the rate constants for quenching by acrylates decrease with increasing methyl substitution on the β-carbon. The 2,6-dimethoxybenzoyl (5) and phosphinoyl (6) radicals derived from 4 are readily detected by time-resolved ESR (TR ESR); benzoyl radical 5 appears as a singlet and phosphinoyl radical 6 appears as a doublet of triplets (A(P) = 285 G, A(H) = 4.8 G). The CIDEP patterns of 5 and 6 indicate that the radicals are formed from α-cleavage of the triplet excited state of 4. TR ESR has also proved useful in the direct detection of the polarized benzyl radicals formed from addition of phosphinoyl radicals 3 and 6 to styrene and 2,4,6-trimethoxystyrene. The lower reactivity of 6 compared to 3 is attributed to its more planar structure and lower degree of spin localization in a s-orbital on phosphorus.

The photophysics (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (1) and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (4) have been investigated by fluorescence, phosphorescence, and low temperature time resolved electron spin resonance. Both 1 and 4 undergo α-cleavage to produce benzoyl and phosphorous centered radicals. The photochemistry of 1 and 4 has been investigated by nanosecond laser flash photolysis, picosecond pump-probe spectroscopy, and steady-state photolysis. The singlet states of 1 and 4 and the phosphorous centered radicals produced by α-cleavage were characterized directly by time resolved absorption spectroscopy. The triplet states of 1 and 4 were characterized indirectly by quenching with 1-phenylnaphthalene as a selective triplet quencher. The use of 1-phenylnaphthalene indicates that α-cleavage occurs mainly from the triplet states of 1 and 4. However, the observed rate of formation of phosphorous centered radicals derived from picosecond investigations is experimentally indistinguishable from the rate of disappearance of the singlet states of 1 and 4. The results are compatible with mechanisms for which the rate of intersystem crossing of the S1 states of 1 and 4 limits the observed rate of α-cleavage, because the rate of α-cleavage is of the same order or faster than the rate of intersystem crossing. This relatively uncommon situation appears to have an analogy in the well investigated photochemistry of dibenzyl ketone.

2-(2′-Hydroxy-5′-methylphenyl)benzotriazole, 1, Tinuvin P, and o-hydroxybenzophenone, 2, are thought to achieve exceptional photostability through highly reversible deactivation associated with their intramolecular hydrogen bonds. Excimer laser excitation (10 mJ, 20 ns at 308 nm) of these molecules in argon-bubbled hexane solution at room temperature affords no discernible transient signals (absorption or emission) between 320 and 800 nm. In DMSO solution under the same conditions, however, strong transient absorptions in the visible (λ max 410 and 425 nm for 1 and 2, respectively) are observed. The transients responsible for the absorptions are quenched by oxygen and acid, and the spectra match well with the ground state difference spectra generated from the corresponding phenolate ions. When a bulky group is incorporated ortho to the hydroxyl function such as in 2-(2′-hydroxy-3′-cumyl-5′-(1,1,3,3-tetramethylbutyl)phenyl)benzotriazole, 3, Tinuvin 928, no appreciable transient absorption is observed even in DMSO solution. These results are consistent with the disruption of the intramolecular hydrogen bond of 1 and 2 (but not 3) as the result of intermolecular hydrogen bond formation with DMSO. The "dramatic" ortho effect implies that the bulky group adjacent to the hydroxyl function sterically shields the intramolecular hydrogen bond from disruption by polar basic environments.

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The photochromic valence isomerization of the system norbornadiene (N)-quadricyclane (Q) has attracted much attention in the field of photochemical energy storage1 and, more recently, has been considered as the basis for an optical memory system.2 The energy-releasing conversion of Q to N can be achieved with high efficiency via a free-radical-cation chain reaction, initiated by chemical? electrochemical$ or photosensitized one-electron oxidations (eql).s The existence of two distinct radical cations,