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

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 (S 1 ). 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 S 1 -state molecules prepared by UV absorption, vibrational cooling of these hot-S 1 molecules to energies below the barrier to C-C bond cleavage on the S 1 state potential energy surface (with commensurate reductions in the energy-dependent α-cleavage rate), and slower loss of thermalized S 1 -state population. The thermalized S 1 -state molecules may competitively decay by activated reaction over the barrier to α C-C bond fission on the S 1 -state potential energy surface, internal conversion to the ground (S 0 ) electronic state, or intersystem crossing to the lowest lying triplet state (T 1 ) and subsequent C-C bond breaking. The α C-C bond fission barrier height in the S 1 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 S 1 -state energy barrier. Transient infra-red absorption spectra obtained after UV excitation identify ring-opened ketene photoproducts of cyclobutanone and their timescales for formation. Ultrafast spectroscopy of ring-opening in three cyclic ketones reveals how ring-strain affects Norrish Type-I α-cleavage mechanisms.