Issue 7, 2020

Effects of ring-strain on the ultrafast photochemistry of cyclic ketones

Abstract

Ring-strain in cyclic organic molecules is well-known to influence their chemical reactivity. Here, we examine the consequence of ring-strain for competing photochemical pathways that occur on picosecond timescales. The significance of Norrish Type-I photochemistry is explored for three cyclic ketones in cyclohexane solutions at ultraviolet (UV) excitation wavelengths from 255–312 nm, corresponding to an π* ← n excitation to the lowest excited singlet state (S1). Ultrafast transient absorption spectroscopy with broadband UV/visible probe laser pulses reveals processes common to cyclobutanone, cyclopentanone and cyclohexanone, occurring on timescales of ≤1 ps, 7–9 ps and >500 ps. These kinetic components are respectively assigned to prompt cleavage of an α C–C bond in the internally excited S1-state molecules prepared by UV absorption, vibrational cooling of these hot-S1 molecules to energies below the barrier to C–C bond cleavage on the S1 state potential energy surface (with commensurate reductions in the energy-dependent α-cleavage rate), and slower loss of thermalized S1-state population. The thermalized S1-state molecules may competitively decay by activated reaction over the barrier to α C–C bond fission on the S1-state potential energy surface, internal conversion to the ground (S0) electronic state, or intersystem crossing to the lowest lying triplet state (T1) and subsequent C–C bond breaking. The α C–C bond fission barrier height in the S1 state is significantly reduced by the ring-strain in cyclobutanone, affecting the relative contributions of the three decay time components which depend systematically on the excitation energy above the S1-state energy barrier. Transient infra-red absorption spectra obtained after UV excitation identify ring-opened ketene photoproducts of cyclobutanone and their timescales for formation.

Graphical abstract: Effects of ring-strain on the ultrafast photochemistry of cyclic ketones

Supplementary files

Article information

Article type
Edge Article
Submitted
15 Oct 2019
Accepted
13 Jan 2020
First published
13 Jan 2020
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2020,11, 1991-2000

Effects of ring-strain on the ultrafast photochemistry of cyclic ketones

M. Kao, R. K. Venkatraman, M. N. R. Ashfold and A. J. Orr-Ewing, Chem. Sci., 2020, 11, 1991 DOI: 10.1039/C9SC05208A

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