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

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) (1)pi sigma* ((1)A(2)) state; and (ii) initial photoexcitation to the higher energy (1)pi pi* (B-1(2)) 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 (1)pi sigma* ((1)A(2)) 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 (1)pi sigma* ((1)A(2)) surface in the vertical Franck-Condon region, followed by non-adiabatic coupling through a (1)pi sigma*/S-0 conical intersection to yield pyrrolyl radicals in their electronic ground state (C4H4N((X) over tilde)) together with H-atoms. At 238 nm, direct excitation to, and N-H dissociation along, the (1)pi sigma* ((1)A(2)) surface is observed to occur with a time-constant of 46 +/- 22 fs. Upon initial population of the (1)pi pi* (B-1(2)) state at 200 nm, a rapid (1)pi pi* (B-1(2)) --> (1)pi sigma* ((1)A(2)) --> 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 S-0 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-d(1) reveal that these statistical H-atoms are released only through C-H bond cleavage.