Structure–Kinetics Correlations in Isostructural Crystals of α-(ortho-Tolyl)-acetophenones: Pinning Down Electronic Effects Using Laser-Flash Photolysis in the Solid State

Abstract

Aqueous suspensions of nanocrystals in the 200-500 nm size range of isostructural α-(ortho-tolyl)-acetophenone (1a) and α-(ortho-tolyl)-para-methylacetophenone (1b) displayed good absorption characteristics for flash photolysis experiments in a flow system, with transient spectra and decay kinetics with a quality that is similar to that recorded in solution. In contrast to solution measurements, reactions in the solid state were characterized by a rate limiting hydrogen transfer reaction from the triplet excited state and a very short-lived biradical intermediate, which does not accumulate. Notably, the rate for δ-hydrogen atom transfer of 1a (2.7 × 10(7) s(-1)) in the crystalline phase is 18-fold larger than that of 1b (1.5 × 10(6) s(-1)). With nearly identical molecular and crystal structures, this decrease in the rate of δ-hydrogen abstraction can be assigned unambiguously to an electronic effect by the para-methyl group in 1b, which increases the contribution of the (3)π,π* configuration relative to the reactive (3)n,π* configuration in the lowest triplet excited state. These results highlight the potential of relating single crystal X-ray structural data with absolute kinetics from laser flash photolysis.

Photochemistry and Transmission Pump–Probe Spectroscopy of 2-Azidobiphenyls in Aqueous Nanocrystalline Suspensions: Simplified Kinetics in Crystalline Solids

“…However, it is known that the spectroscopic analyses of crystals and bulk powders is complicated by their high optical densities, strong light scattering, and the potential interference by the accumulation of photoproducts. To address these challenges, our group has developed a flow method that uses nanocrystals suspended in a nondissolving fluid such as water. − We have shown that low-loading suspensions of crystals in the ca. 50–500 nm size range are suitable for transmission spectroscopy because of their homogeneous illumination and reduced light scattering.…”

Mechanistic Studies of Adamantylacetophenones with Competing Reaction Pathways in Solution and in the Crystalline Solid State

“…), stable suspensions of crystalline particles with an average size in the submicrometer regime can be prepared. We term such mixtures nanocrystalline suspensions, which have been shown to be amenable to spectroscopic analysis using standard solution-phase methods. ,− For each derivative, nanocrystalline suspensions were prepared using a 0.04 mM CTAB surfactant solution and stock solutions in acetonitrile. For ketone 4 , 0.2 mL of a 2.5 mg/mL acetonitrile solution was added dropwise to ∼20 mL of a vortexing surfactant solution.…”

Capturing Conformation‐Dependent Photoreactivity of Crystalline 3‐Azido‐1,3‐diphenylisobutyrophenone

“…Laser flash photolysis experiments were performed with an excimer laser (308 nm, 17 ns pulse width). [9] An anocrystalline suspension of azide 1 was prepared by following the reprecipitation technique described by Simoncelli et al [5] As olution of azide 1 (0.7 mm,2 00 mL) in acetone was slowly injected into 10 mL of Millipore water.T he resulting suspension was sonicated three times at room temperature for 6min, allowing 3min rest between each run. The absorbance of the resulting nanocrystalline suspensions was between 0.3 and 0.5 at 308 nm.…”

“…[3a, 4] Because solid-statep hotoreactions cannot always be sufficiently explained by crystal structure/reactivity correlations, Garcia-Garibay and co-workers developed at echnique using laser flash photolysis of nanocrystalline suspensions to detect excited states and reactive intermediates within crystal lattices. [5] This approach makes it possible to elucidate the mechanisms of solid-state photoreactions based on spectroscopy.…”