Electron Spin Resonance of the Lowest Excited Triplet States of .alpha.-Oxo[1.n]paracyclophanes [Cyclophanobenzophenones]. Effect of Molecular Geometry on the Electronic Character of the Triplet State

Lipson, Matthew; McGarry, Peter F.; Koptyug, Igor V.; Staab, Heinz A.; Turro, Nicholas J.; Doetschman, David C.

doi:10.1021/j100082a020

Cited by 21 publications

(22 citation statements)

References 9 publications

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“…This parallels the widely recognized aspect that the kinetics of many intramolecular photocyclizations are subject to strong conformational and stereoelectronic control 42. 43, 45, 51 In nonpolar and polar aprotic solvents, such as CH 3 CN, the flexible construction of the DMBDP molecule renders the needed conformation easily achievable, and this may contribute to the rapid dynamics and high efficiency of the OC addition.…”

Section: Discussionmentioning

confidence: 99%

A Time‐Resolved Spectroscopic Study of the Bichromophoric Phototrigger 3′,5′‐Dimethoxybenzoin Diethyl Phosphate: Interaction Between the Two Chromophores Determines the Reaction Pathway

Kwok

et al. 2010

Chemistry A European J

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3',5'-Dimethoxybenzoin (DMB) is a bichromophoric system that has widespread application as a highly efficient photoremovable protecting group (PRPG) for the release of diverse functional groups. The photodeprotection of DMB phototriggers is remarkably clean, and is accompanied by the formation of a biologically benign cyclization product, 3',5'-dimethoxybenzofuran (DMBF). The underlying mechanism of the DMB deprotection and cyclization has, however, until now remained unclear. Femtosecond transient absorption (fs-TA) spectroscopy and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy were employed to detect the transient species directly, and examine the dynamic transformations involved in the primary photoreactions for DMB diethyl phosphate (DMBDP) in acetonitrile (CH(3)CN). To assess the electronic character and the role played by the individual sub-chromophore, that is, the benzoyl, and the di-meta-methoxybenzylic moieties, for the DMBDP deprotection, comparative fs-TA measurements were also carried out for the reference compounds diethyl phosphate acetophenone (DPAP), and 3',5'-dimethoxybenzylic diethyl phosphate (DMBnDP) in the same solvent. Comparison of the fs-TA spectra reveals that the photoexcited DMBDP exhibits distinctly different spectral character and dynamic evolution from those of the reference compounds. This fact, combined with the related steady-state spectral and density functional theoretical results, strongly suggests the presence in DMBDP of a significant interaction between the two sub-chromophores, and that this interaction plays a governing role in determining the nature of the photoexcitation and the reaction channel of the subsequent photophysical and photochemical transformations. The ns-TR(3) results and their correlation with the fs-TA spectra and dynamics provide evidence for a novel concerted deprotection-cyclization mechanism for DMBDP in CH(3)CN. By monitoring the direct generation of the transient DMBF product, the cyclization time constant was determined unequivocally to be approximately 1 ns. This indicates that there is little relevance for the long-lived intermediates (>10 ns) in giving the DMBF product, and excludes the stepwise mechanism proposed in the literature as the major pathway for the DMB cyclization reaction. This work provides important new insights into the origin of the 3',5'-dimethoxy substitution effect for the DMB photodeprotection. It also helps to clarify the many different views presented in previous mechanistic studies of the DMB PRPGs. In addition to this, our fs-TA results on the reference compound DMBnDP in CH(3)CN provide the first direct observation (to the best of our knowledge) showing the predominance of a prompt (approximately 2 ps) heterolytic bond cleavage after photoexcitation of meta-methoxybenzylic compounds. This provides insight into the long-term controversies about the photoinitiated dissociation mode of related substituted benzylic compounds.

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Section: Discussionmentioning

confidence: 99%

A Time‐Resolved Spectroscopic Study of the Bichromophoric Phototrigger 3′,5′‐Dimethoxybenzoin Diethyl Phosphate: Interaction Between the Two Chromophores Determines the Reaction Pathway

Kwok

et al. 2010

Chemistry A European J

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“…Further details are described elsewhere. [13][14][15] Solutions of 2 (0.5 mM in label, 50 mM in PMA according to monomer concentration) in 80% aqueous buffer (acetate for pH below 5.7, phosphate for pH above ([K + ] ) 0.2 M)) and 20% methanol were used for the TR-ESR experiments. The integration window used in these studies was 400-600 ns after the laser pulse.…”

Section: Methodsmentioning

confidence: 99%

Chemically Induced Dynamic Electron Polarization Studies of a pH-Dependent Free Radical Cage Formed in a Photoinitiator Labeled Poly(methacrylic acid)

Maliakal¹,

Weber²,

Turro³

et al. 2002

Macromolecules

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Time-resolved electron spin resonance (TR-ESR) has been used to investigate the effect of pH-dependent poly(methacrylic acid) clustering on the radical pair generated from photolysis of a poly(methacrylic acid) acid sample randomly labeled with the common R-hydroxy ketone photoinitiator 1. The TR-ESR spectra for the ketyl radical show a dramatic change in electron spin polarization mechanism, from a predominantly triplet mechanism (TM) at low pH (below 6) to a predominantly radical pair mechanism (RPM) at high pH (above 7). The ratio of RPM/TM polarization exhibits a sigmoidal behavior as a function of pH which passes through an inflection point at pH 6.9 ( 0.1. The inflection point agrees well with previous estimates for pH-dependent cluster opening in poly(methacrylic acid). These results indicate that the pH-dependent cluster is capable of forming a cage around the geminate radical pair which restricts its motion at low pH. At higher pH, the cluster opens, allowing the ketyl radical to escape the pair. Studies of the benzoyl partner in the radical pair, which is attached covalently to the polymer backbone, indicate differing environments within the cluster even at pH below cluster opening. This study represents the first application of TR-ESR and electron spin polarization (ESP) to the problem of pH-dependent clustering and reveals new features of the pH-dependent poly(methacrylic acid) cluster.

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“…Low-temperature TR-ESR experiments employed the pulses from a Lambda Physik LPX 100 excimer laser (308 nm, ca. 80 mJ/pulse, 20 ns), a Bruker ESP-300 X-band ESR spectrometer equipped with a dielectric ring cavity (ESP380-1052 DLQ-H) in a high-Q mode and an Oxford cryostat (GFS 300), and the boxcar averager and signal processor as previously described . The signals were averaged over a 5 μs period following the laser flash.…”

Section: Methodsmentioning

confidence: 99%

A Steady-State and Picosecond Pump-Probe Investigation of the Photophysics of an Acyl and a Bis(acyl)phosphine Oxide

et al. 1997

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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.

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Electron Spin Resonance of the Lowest Excited Triplet States of .alpha.-Oxo[1.n]paracyclophanes [Cyclophanobenzophenones]. Effect of Molecular Geometry on the Electronic Character of the Triplet State

Cited by 21 publications

References 9 publications

A Time‐Resolved Spectroscopic Study of the Bichromophoric Phototrigger 3′,5′‐Dimethoxybenzoin Diethyl Phosphate: Interaction Between the Two Chromophores Determines the Reaction Pathway

A Time‐Resolved Spectroscopic Study of the Bichromophoric Phototrigger 3′,5′‐Dimethoxybenzoin Diethyl Phosphate: Interaction Between the Two Chromophores Determines the Reaction Pathway

Chemically Induced Dynamic Electron Polarization Studies of a pH-Dependent Free Radical Cage Formed in a Photoinitiator Labeled Poly(methacrylic acid)

A Steady-State and Picosecond Pump-Probe Investigation of the Photophysics of an Acyl and a Bis(acyl)phosphine Oxide

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