Probing ultrafast dynamics in photoexcited pyrrole: timescales for 1πσ* mediated H-atom elimination

Abstract

The heteroaromatic ultraviolet chromophore pyrrole is found as a subunit in a number of important biomolecules: it is present in heme, the non-protein component of hemoglobin, and in the amino acid tryptophan. To date there have been several experimental studies, in both the time- and frequency-domains, which have interrogated the excited state dynamics of pyrrole. In this work, we specifically aim to unravel any differences in the H-atom elimination dynamics from pyrrole across an excitation wavelength range of 250–200 nm, which encompasses: (i) direct excitation to the (formally electric dipole forbidden) 1¹πσ* (¹A₂) state; and (ii) initial photoexcitation to the higher energy ¹ππ* (¹B₂) state. This is achieved by using a combination of ultrafast time-resolved ion yield and time-resolved velocity map ion imaging techniques in the gas phase. Following direct excitation to 1¹πσ* (¹A₂) at 250 nm, we observe a single time-constant of 126 ± 28 fs for N–H bond fission. We assign this to tunnelling out of the quasi-bound 3s Rydberg component of the 1¹πσ* (¹A₂) surface in the vertical Franck–Condon region, followed by non-adiabatic coupling through a 1¹πσ*/S₀ conical intersection to yield pyrrolyl radicals in their electronic ground state (C₄H₄N()) together with H-atoms. At 238 nm, direct excitation to, and N–H dissociation along, the 1¹πσ* (¹A₂) surface is observed to occur with a time-constant of 46 ± 22 fs. Upon initial population of the ¹ππ* (¹B₂) state at 200 nm, a rapid ¹ππ* (¹B₂) → 1¹πσ* (¹A₂) → N–H fission process takes place within 52 ± 12 fs. In addition to ultrafast N–H bond cleavage at 200 nm, we also observe the onset of statistical unimolecular H-atom elimination from vibrationally hot S₀ ground state species, formed after the relaxation of excited electronic states, with a time-constant of 1.0 ± 0.4 ns. Analogous measurements on pyrrole-d₁ reveal that these statistical H-atoms are released only through C–H bond cleavage.