UV-Induced Isomerization Dynamics of N-Methyl-2-pyridone in Solution

Abstract

The photoisomerization dynamics of N-methyl-2-pyridone (NMP) dissolved in CH3CNhave been interrogated by time-resolved electronic and vibrational absorption spectroscopy. Irradiation at 330 or 267 nm prepares NMP(S1) molecules with very different levels of vibrational excitation, which rapidly relax to low vibrational levels of the S1 state. Internal conversion with an associated time constant of 110(4) ps, leading to reformation of NMP(S0) molecules, is identified as the dominant (>90%) decay pathway.Much of the remaining fraction undergoes a photo-initiated rearrangement to yield two ketenes (revealed by their characteristic antisymmetric C=C=O stretching modes at 2110 and 2120 cm -1 ), which are in equilibrium. The rate of this isomerization is found to be pump-wavelength dependent, consistent with ab initio electronic structure calculations which predict a barrier on the S1 potential energy surface en route to a prefulvenic conical intersection, by which isomerization is deduced to occur. Two kinetic modelsdifferentiated by whether product branching is deduced to occur in the S1 or S0 electronic 2 states -are presented and used with nearly equal success in the analysis of the experimental data, highlighting the difficulties associated with deducing mechanistic information from kinetic data alone.

Contrasting ring-opening propensities in UV-excited α-pyrone and coumarin

“…In contrast, the 901 out-of-plane MECI is accessed by remaining on the S 1 (pp*) potential, without the need for any curve crossing. Such pp* mediated ring-opening was not implicated in any of our prior work on heterocyclic photoisomerisation reactions, 16,40 however those investigations did not include such an exhaustive search of the potential energy landscape and attendant MECIs.…”

Shedding light on the effective fluorophore structure of high fluorescence quantum yield carbon nanodots

“…Similar molecular structures have been studied by Murdock and co-workers, who focused on the photo-isomerization dynamics of aqueous N-methyl-2-pyridone. 41 They observed that, during the excited state decay, a considerable fraction of CDs undergo photoinitiated rearrangement and carbonization. Furthermore, they demonstrated that the photochemically formed radicals contribute to destroying the pyridone and, in turn, the effective uorescent structure.…”

Exploring the Dynamics of the Photoinduced Ring-Opening of Heterocyclic Molecules

“…TRIR data for the 1500–1800 and 1950–2250 cm –1 regions following excitation of 2-thiophenone at 267 nm (panels a and f), 2-furanone at 225 nm (b and g), N -methyl-2-pyridone at 330 nm (c and h), α-pyrone at 310 nm (d and i) and coumarin at 330 nm (e), all in solution in acetonitrile, along with the corresponding equilibrium structures. These data are adapted from refs , , and , and are also displayed with the time delays specified in the Supporting Information.…”