Tracking the Relaxation of 2,5-Dimethylpyrrole by Femtosecond Time-Resolved Photoelectron and Photoion Detection

The relaxation of 2,5-dimethylpyrrole after excitation in the 290–239 nm range, which covers the weak absorption of the S₁ ¹A₂ πσ* state, dissociative along the N–H bond, and the stronger band mostly attributed to the ¹B₂ ππ* state, has been investigated by time-resolved ion and photoelectron techniques. The measurements yield an invariant lifetime of ∼55 fs for the ¹πσ* state, after preparation in its Franck–Condon region with increasing vibrational content. This ultrafast rate indicates that, contrary to the observations made in pyrrole (Roberts et al.<i> Faraday Discuss.</i> <b>2013</b>, <i>163</i>, 95–116), the molecule reaches the dissociative part of the potential without any barrier effect, although calculations predict the latter to be higher than in the pyrrole case. The results are rationalized in terms of a barrier free multidimensional pathway that very likely involves out-of-plane vibrations. Additionally, a lifetime of ∼100 fs is found after excitation along the higher ¹B₂ ππ* ← S₀ transition. The relaxation of this state by coupling to a very short living S₁ ¹πσ* state, or by alternative routes, is discussed in the light of the collected photoelectron measurements

Triplet Mediated C–N Dissociation versus Internal Conversion in Electronically Excited <i>N</i>‑Methylpyrrole

The photochemical and photophysical pathways operative in <i>N</i>-methylpyrrole, after excitation in the near part of its ultraviolet absorption spectrum, have been investigated by the combination of time-resolved total ion yield and photoelectron spectroscopies with high-level ab initio calculations. The results collected are remarkably different from the observations made for pyrrole and other aromatic systems, whose dynamics is dictated by the presence of πσ* excitations on X–H (X: N, O, S, ...) bonds. The presence of a barrier along the C–N dissociation coordinate that can not be tunneled triggers two alternative decay mechanisms for the S₁ A″ πσ* state. While at low vibrational content the C–N dissociation occurs on the surface of a lower ³ππ* state reached after efficient intersystem crossing, at higher excitation energies, the A″ πσ* directly internally converts to the ground state through a ring-twisted S₁/S₀ conical intersection. The findings explain previous observations on the molecule and may be relevant for more complex systems containing similar C–N bonds, such as the DNA nucleotides

Ultrafast Nonradiative Relaxation Channels of Tryptophan

The nonradiative relaxation channels of gas-phase tryptophan excited along the S₁–S₄ excited states (287–217 nm) have been tracked by femtosecond time-resolved ionization. In the low-energy region, λ ≥ 240 nm, the measured transient signals reflect nonadiabatic interactions between the two bright L_a and L_b states of ππ* character and the dark dissociative πσ* state of the indole NH. The observed dynamical behavior is interpreted in terms of the ultrafast conversion of the prepared L_a state, which simultaneously populates the fluorescent L_b> and the dissociative πσ* states. At higher energies, after excitation of the S₄ state, the tryptophan dynamics diverges from that observed in indole, pointing to the opening of a relaxation channel that could involve states of the amino acid part. The work provides a detailed picture of the processes and electronic states involved in the relaxation of the molecule, after photoexcitation in the near part of its UV absorption spectrum

Ultrafast Evolution of Imidazole after Electronic Excitation

The ultrafast dynamics of the imidazole chromophore has been tracked after electronic excitation in the 250–217 nm energy region, by time delayed ionization with 800 nm laser pulses. The time-dependent signals collected at the imidazole⁺ mass channel show the signature of femtosecond dynamics, originating on the πσ*- and ππ*-type states located in the explored energy region. The fitting of the transients, which due to the appearance of nonresonant coherent adiabatic excitation requires a quantum treatment based in the Bloch equations, yields two lifetimes of 18 ± 4 and 19 ± 4 fs. The first is associated with the πσ* ← ππ* internal conversion, while the second reflects the loss of ionization cross-section as the system evolves along the dissociative πσ* surface. This study provides a comprehensive picture of the photophysics of the molecule that agrees with previous experimental and theoretical findings