Femtosecond evolution of the pyrrole molecule excited in the near part of its UV spectrum

The evolution of the isolated pyrrole molecule has been followed after excitation in the 265–217 nm range by using femtosecond time delayed ionization. The transients collected in the whole excitation range show the vanishing of the ionization signal in the femtosecond time scale, caused by the relaxation along a πσ* type state (3s a1←π 1a2), which is the lowest excited electronic state of the molecule. This surface is dissociative along the NH bond, yielding a 15 ± 3 fs lifetime that reflects the loss of the ionization cross-section induced by the ultrafast wavepacket motion. Although a weak πσ* absorption is detected, the state is mainly reached through internal conversion of the higher bright ππ* transitions, which occurs with a 19 ± 3 fs lifetime. In addition to its resonant excitation, the intense ππ* absorption extending in the 220–190 nm interval is also out-of-resonance populated at energies far to the red from its absorption onset. This coherent adiabatic excitation of the ππ* transition should follow the excitation pulse (coherent population return effect), but instead the system relaxes toward the lower πσ* surface through a conical intersection during the interaction time, leading to the population of πσ* state at wavelengths as long as 265 nm. According to the observed behavior, the time evolution of the system in the full excitation range studied is modeled by a coherent treatment that provides key insights on the photophysical properties of the molecule.This study was funded by Spanish MICINN (Grant No. CTQ2010-17749) and Consolider Program “Science and Applications of Ultrafast Ultraintense Lasers” (Grant No. CSD2007-00013), and by the Basque Government through the “Ayudas para apoyar las actividades de grupos de investigación del sistema universitario vasco” program. The experiments and theoretical calculations were carried out at the SGIker laser facility, and IZO-SGI of the UPV/EHU, respectively

FEMTOSECOND TIMESCALE EVOLUTION OF PYRROLE ELECTRONIC EXCITATION

Author Institution: Departamento de Quimica Fisica, Facultad de Ciencia y Tecnologia, Universidad del Pais Vasco (UPV-EHU), Ap. 644, E-48080 Bilbao, SpainPyrrole is a simple aromatic molecule with relevantchromophoric properties in biology. Although its apparent simplicity, it shows a complicated dynamics after excitation in the near part of the UV absorption spectrum, which results from the interplay between the bright $\pi\pi^*$ and the dark dissociative $\pi\sigma^*$ electronic transitions., nderline{\textbf{312}}, 1637-1640, 2006.} Herein, we present a time resolved study with ultrafast resolution on the relaxation dynamics of isolated pyrrole, after excitation in the 265-217 nm range. Two lifetimes of 19 and 15 fs, which are associated with the internal conversion from the bright $^{1}$B$_{2}$ $\pi\pi^*$ state and the propagation of the wavepacket on the $\pi\sigma^*$ state, respectively, are found in the studied energy interval. The work also explores the consequences of non resonant adiabatic excitation of the system when broadband femtosecond pulses are employed to prepare the molecule in the targeted electronic states, revealing the key implication of this type of coherent phenomena. The collected data reveal that the bright $^{1}$B$_{2}$ $\pi\pi^*$ state is adiabatically populated at excitation wavelengths far away from resonance, providing an efficient way to reach the $\pi\sigma^*$ state. The recorded transients are fit employing a coherent model that provides a comprehensive view of the dynamical processes pyrrole undergoes after excitation by ultrashort light pulses

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