Time-Resolved Laser-Induced Ring-Opening and Dissociation Dynamics in Bromothiophene Isomers

Abstract

Photoinduced molecular ring-opening reactions play a critical role in many natural chemical processes; however, there are pending questions regarding the fundamental mechanisms that govern these transformations. Furthermore, the understanding of ring-opening reactions has an important impact on optoelectronic and molecular control applications. These chemical reactions are driven by nonadiabatic coupling between electronic states through conical intersections in their potential energy surfaces, and their excited state dynamics are still a matter of debates. Here, we use a combination of on-the-fly ab initio molecular dynamics simulations and a powerful imaging ultrafast UV pump–IR probe spectroscopy technique to examine in detail the dissociation pathways followed by 2- and 3-bromothiophene. Using time-dependent momentum imaging, we identify clearly three dominant fragmentation channels following UV photon absorption: C–Br bond dissociation and cleavage of either C–S bond, which induce structural changes through molecular ring-opening prior to IR-induced ionization. We also obtain their fragmentation times, which span 600 to 1300 fs. Our findings elucidate both the initial excited states that initiate these dynamics and the nature of the electronic states reached by the nonadiabatic population transfer, a process essential to the observed ring-opening dynamics. Investigations of both 2- and 3-bromothiophene highlight the isomer dependence of UV absorption and reveal differences unknown before. Our results unambiguously support the role of ultrafast internal conversion from the dominant electronic states initiating ring-opening dynamics and identify the conical intersections that determine ring opening. Our work provides findings that advance the understanding of excited-state dynamics in ring molecules.

Introduction

Advances in experimental technologies and theoretical modeling have made it possible to investigate and probe the time evolution of fundamental photoinduced processes in cyclic molecules. − For instance, the biological synthesis of vitamin D₃ (cholecalciferol) in human skin is facilitated by the photoinduced ring-opening of 1,3-cyclohexadiene. ,,− Furthermore, photocyclization reactions have attracted attention for their potential in optoelectronic applications like optical switching and device fabrication, − nanomechanical molecular motors, , molecular logic gates, molecular memory devices, green energy storage, and synthetic organic chemistry.

Most small molecules can absorb ultraviolet (UV) photons (200–400 nm), leading to a wide range of photoinduced dynamics. Upon excitation, these molecules typically undergo efficient rapid relaxation to lower electronic states that determine the resulting reaction outcomes. In cyclic molecules, one such UV-activated pathway involves nuclear motion, like out-of-plane distortion that can drive an opening of the ring structure.

The photodynamics of heterocyclic molecules upon UV absorption have attracted considerable interest in the recent past. Various techniques such as X-ray absorption, time-resolved photoelectron spectroscopy, , ultrafast electron diffraction, , and X-ray scattering, to name just a few, have been used to investigate the role of nonadiabatic molecular dynamics and conical intersections (CIs) in directing excited-state evolution. − It has been established that UV excitation can populate dissociative electronic states through promotion of an electron from a nonbonding (n) or bonding (π) orbital to an antibonding σ* orbital, yielding nσ* and/or πσ* states. In addition, it has been found that in many heterocycles, the initial absorption occurs via a π → π* transition, after which theory predicts ring opening to proceed through πσ*-mediated bond fission, a mechanism previously explored experimentally in the near-UV. Competing pathways have been proposed, including transfer from the ππ* state to a πσ* state followed by relaxation to the S₀ potential energy surface (PES) after asymmetric ring expansion. Alternatively, direct ππ*/S₀ couplings via out-of-plane distortions have also been suggested.

While the previously utilized experimental techniques have their own advantages, they are not sufficiently differential compared to coincidence methods in providing detailed and unambiguous information about UV-induced relaxation pathways and weak processes. Thus, despite extensive prior work, the precise role of nonadiabatic couplings in driving the ring opening remains elusive. Unraveling the reaction mechanisms responsible for the fragmentation and ring-opening dynamics in realistic heterocyclic molecules and elucidating the role of specific excited states and relevant CIs in these processes are open questions that motivated the present study.

In this work, we report a combined experimental and theoretical time-resolved comparative investigation of UV-induced ring-opening and halogen dissociation dynamics in heterocyclic 2- and 3-bromothiophene (C₄H₃BrS) isomers. The measurements employ a 240 nm UV pump–infrared (IR) multiphoton ionization probe scheme, illustrated in Figure a (detailed further in Figure S1.1 of the Supporting Information (SI) document). Initial photoexcitation promotes the electronic population to a dense manifold of dynamically active electronic states, which couple through CIs near the Franck–Condon region, as depicted in Figure b,c. We use the cold target recoil ion momentum spectroscopy (COLTRIMS) technique , to detect molecular fragments resulting from multiphoton ionization by the strong IR pulse. In contrast to previous experimental approaches, the COLTRIMS technique applied here provides complete kinematic information on the time-resolved reaction dynamics through ion–ion coincidence detection. This enables the isolation and imaging of specific fragmentation channels and direct determination of ion fragment momenta and kinetic energy release (KER) that provide unique insight into UV-induced ring opening.

1.

Schematic of the UV pump–IR probe experiment used to explore the relaxation pathways of 2- or 3-bromothiophene upon UV excitation and the underlying reaction mechanisms. (a) 240 nm UV pulse excites the gas-phase 2- (green) or 3-bromothiophene (orange), and the proceeding dynamics are probed by a 790 nm IR laser pulse. The resulting ions are detected in coincidence using a cold target recoil ion momentum spectrometer (COLTRIMS), yielding their position and time-of-flight information. The coincident ion pairs are identified using photoion-photoion coincidence maps. The 3D momentum vectors of each ion are constructed and used to calculate the kinetic energy released by the formation of the ion pair and mapped as a function of pump–probe delay to identify time-dependent dynamics of each pair. The three primary relaxation pathways proceed through cleavage of one of the three bonds highlighted in red in our molecular diagrams. (b) One-dimensional slice of the spin-free state-averaged adiabatic potential energy surfaces (PESs) for 2-bromothiophene along the C⁽¹⁾–Br coordinate calculated with SA­(9,9)-CASPT2­(12,10)/cc-pVDZ. From the neutral ground state (blue line), the molecule is primarily excited to a dense manifold of singlet states (colored lines), which form conical intersections (CIs) near the Franck–Condon region (highlighted in yellow and expanded in panel (c)). Additional close lying singlet states and triplet states are shown in black and gray, respectively. Dynamics occurring in these neutral states are probed by multiphoton ionization. Cationic fragments, such as those produced by ionization to the example dicationic state in the purple inset in panel (b), are able to be detected experimentally. (c) In the region of the CIs, the dominant electronic configurations of the involved electronic states consist of transitions from occupied π orbitals to π*, σ_{C⁽¹⁾–Br} , and σ_C–S–C orbitals (orbital diabatic character and labeling are described in Figure S5.1 of SI). On either side of the conical intersection, the strong mixing between π* and σ* excited states along adiabatic PESs is observed.

We identify three primary relaxation pathways following UV-activated dynamics of 2- and 3-bromothiophene: (i) C–Br bond fission and ring opening through the lengthening of the C–S bond on (ii) one or (iii) the other side of the thiophene ring. We utilize high-level multireference electronic structure methods and perform coupled electron–nuclear dynamic simulations to access the intermediate stages of the reaction dynamics. We unravel previously unresolved physical and chemical processes associated with heterocyclic molecular dynamics, including the elucidation of ring-opening reaction pathways and CIs along the corresponding PESs. We discuss the essential roles the identified CIs play in the observed ring opening dynamics, both immediately after initial excitation and far from the Franck–Condon region. Finally, we propose additional dynamic processes preceding fragmentation in both bromothiophene isomers.

Results

2-Bromothiophene

We investigated the UV-induced excited-state dynamics of 2-bromothiophene using photoion-photoion coincidence mapping. By employing the COLTRIMS technique, we measured time-of-flight and corresponding mass and charge information on ion pairs (relaxation channels) originating from coincident ionization. This approach enables the determination of the complete momenta vectors of the ions in each channel, which are used to calculate KER as a function of time delay between the UV-pump and IR-probe laser pulses (further explanations in Figure S1.2 of the SI). We found that channels form through one of three predominant relaxation pathways: C⁽¹⁾–Br dissociation, C⁽¹⁾–S ring-opening, and C⁽⁴⁾–S ring-opening (isomer structures in Figure a). KER versus pump–probe delay maps for the most highly populated channels following these pathways that account for the full molecular mass, Br⁺ + C₄H₃S⁺, CHS⁺ + C₃H₂Br⁺, and C₃H₃ ⁺ + SCBr⁺, are shown in Figure a,d,g, respectively. Additional two-ion channels fitting the ring-opening pathways are shown in Figure S2.1. Each KER map has a “static” band at a constant energy and intensity for all pump–probe delays and a “dynamic” band that changes in energy as a function of time delay. Static features are consistent with molecules that do not absorb the UV pump pulse and are instead directly ionized by the IR probe pulse and subsequently fragment without preceding reaction dynamics. Time-dependent features emerge following UV pump excitation as changes in electronic-state populations drive coupled electronic and nuclear motion prior to IR-probe-induced ionization and fragmentation.

2.

Kinetic energy release (KER) versus pump–probe delay for the two-body dissociation pathways in 2-bromothiophene and simulated trajectories with the corresponding electronic state populations. (a) KER as a function of pump–probe delay for the dissociation of bromine into the coincidence channel Br⁺ + C₄H₃S⁺. Each map has two key features: A “static” band that does not significantly vary in intensity over time and a “dynamic” band that decreases in energy for increasing delay. (d) KER map for the formation of CHS⁺ + C₃H₂Br⁺ via C⁽¹⁾–S ring-opening. (g) KER map for the formation of C₃H₃ ⁺ + SCBr⁺ via C⁽⁴⁾–S ring-opening. A model for the ions formed in each channel is shown in the upper right of each plot. (b–h) Overlay of trajectories corresponding to C⁽¹⁾–Br bond dissociation, C⁽¹⁾–S ring-opening, and C⁽⁴⁾–S ring-opening, respectively. In the top inset of each diagram is shown the average geometry under which the trajectories in each ensemble undergo an electronic transition between labeled states. (c), (f), (i) Molecular Coulombic Hamiltonian (MCH) electronic eigenstate populations for respective channels. The PESs and diabatic character of these states near the Franck–Condon region along the C⁽¹⁾–Br coordinate are described in Figure b,c, respectively.

Each dissociation pathway is characterized by distinct KER distributions: C⁽⁴⁾–S ring-opening has the largest energy release centered at 4 eV, C⁽¹⁾–S ring-opening near 3 eV, and C⁽¹⁾–Br bond dissociation has the lowest at 2 eV. Interestingly, static and dynamic features are well-separated for both ring-opening pathways but overlap for C⁽¹⁾–Br bond dissociation. These behaviors reflect the distinct regions of the PESs accessed within each reaction mechanism. The total KER of each pathway combines kinetic energy accumulated during evolution on the manifold of the relevant PESs with additional energy gained after excitation to a certain dicationic state. In C⁽¹⁾–Br bond dissociation, for example, a sharp rise in kinetic energy for short pump–probe delays, which corresponds to small changes in C⁽¹⁾–Br separation, indicates that the pump-induced C⁽¹⁾–Br cleavage accesses a dissociative neutral PES with a steeper gradient than that associated with simple Coulombic repulsion. Variations in the center-of-mass of the molecular fragments upon ionization further influence the energy of static features relative to the dynamic ones. The fragmentation time scales for each pathway can be extracted from the dynamic band of each KER map in Figure a,d,g. An edge-fitting algorithm, described in Figures S3.1 and S3.2 of the SI, was applied to identify the lower energy boundary of each time-dependent feature. This boundary is then fit to a decaying exponential of the form Ae ^–Δt/τ + y ₀ to extract the characteristic time constant of each fragmentation pathway as summarized in Table . We found that the fragmentation occurs most rapidly for the C⁽⁴⁾–S ring-opening C₃H₃ ⁺ + SCBr⁺ channel over approximately 600 fs. The C⁽¹⁾–S ring-opening channel, CHS⁺ + C₃H₂Br⁺, follows on a time scale of about 800 fs, and C⁽¹⁾–Br bond dissociation proceeds more slowly near 1300 fs.

1. Exponential Fragmentation Times for the Three Dissociation Processes Present in the Two-body Fragmentation Channels of 2-Bromothiophene .

ion fragments	channel description	fragmentation time τ (fs)
Br+ + C4H3S+	C(1)–Br bond dissociation	1350 ± 205
CHS+ + C3H2Br+	C(1)–S ring-opening	834 ± 73
C3H3 + + CSBr+	C(4)–S ring-opening	634 ± 83

^aThe bottom edge of each time-varying KER band is isolated and fit to a decaying exponential to identify the fragmentation time for each coincident ion pair.

While fragmentation time scales can be extracted from the KER spectra, the femtosecond dynamics and the underlying reaction mechanisms preceding ionization are not directly accessible experimentally. Hence, to elucidate the ultrafast processes that contribute to the rich molecular dynamics within bromothiophene, we performed an extensive theoretical analysis following the procedure described in the Methods section. The obtained molecular dynamics trajectories were grouped based on the final geometry and compared with the experimentally observed fragments. Statistics and parameters defining this pathway assignment are discussed in Table S5.1. In addition, for each fragmentation channel trajectories were further classified based on transient ring deformations and near-transition geometries (see Figure S5.2 for details).

A two-dimensional projection of the trajectories, shown in Figure b,e,h, reveals that while bromine dissociates fully from the molecule, the ring-opening channels remain bound even after complete C–S bond cleavage. This distinction is further supported by the evolution of relevant interatomic distances across the full ensemble, as shown in Figure . Within each channel, we see bond lengthening beginning within 100 fs. While for C⁽¹⁾–Br bond dissociation (Figure a) the bond lengthening continues, for the C–S ring opening channels (Figure b,c) an equilibrium internuclear distance of around 4 Å is reached following the initial bond lengthening. A majority of ring-opened trajectories oscillate about this equilibrium, with some evolving back to a ring-closed structure (see Figure S5.3). Analysis of ring-bending parameters shows pronounced out-of-plane motion in the ring-opening trajectories, in contrast to the C⁽¹⁾–Br dissociation channel (see Figure S5.2). The separation of time scales between C–S bond elongation and the onset of large-amplitude out-of-plane motion suggests that ring opening is initiated by bond cleavage, followed by out-of-plane rotation of the fracturing ring and subsequent oscillatory motion about an extended geometry.

3.

Dynamic bond lengths of interest for the whole ensemble for 2-bromothiophene: (a) C⁽¹⁾–Br, (b) C⁽¹⁾–S, and (c) C⁽⁴⁾–S. In blue are those trajectories which fully complete the defined reaction pathways (i.e., reach a bond length of >4 Å for C⁽¹⁾–Br or >2.5 Å for C–S). The corresponding adiabatic populations for these trajectories are described in Figure c,f,i, respectively. In black are trajectories that do not reach these specified bond lengths. In red are those that reach the bond lengths at some point but finish the simulation below these thresholds.

During the observed molecular dynamics, all three channels exhibit ultrafast (<50 fs) nonadiabatic population transfer between the singlet excited states, as depicted in Figure c,f,i. The involved electronic states are predominantly S₁, S₂, and S₃, shown by the colored lines in Figure b,c. Furthermore, on longer time scales (>200 fs), we observed significant population transfer to the S₀ ground state. The identified average nuclear geometries at which the most important population transfers occur are depicted in the top insets of Figure b,e,h. In particular, it is seen that the transition to the ground electronic state S₀ happens at molecular geometries located far from the Franck–Condon region.

To gain insight into the electronic character associated with each pathway, we analyzed overlaps between the initial excited-state wave functions and those at the equilibrium geometry (see Figure S5.4 and discussion in the Methods section for more details). Across the ensemble, the dominant populated configurations at t = 0 were π₂σ_C–Br , π₂σ_C–S–C , π₁σ_C–S–C , and π₂π*. We found that the C⁽¹⁾–Br dissociation channel is correlated with early time population of a π₂σ_C–Br configuration, while in contrast, the ring-opening channels display more heterogeneous electronic character, with early time contributions from π₂σ_C–Br , π₂σ_C–S–C , π₁σ_C–S–C , and π₂π* configurations. Within the ring-opening ensemble, trajectories with stronger π₂π* character are statistically biased toward C⁽¹⁾–S cleavage, whereas enhanced π₂σ_C–S–C character is more frequently associated with C⁽⁴⁾–S opening. This behavior is consistent with the bonding character of π₂, which is weakly π bonding along the C⁽¹⁾–S bond but antibonding along C⁽⁴⁾–S, such that the redistribution of population involving this orbital preferentially weakens the C⁽¹⁾–S bond. Consistent with this picture, C⁽¹⁾–S cleavage occurs nearly twice as often as C⁽⁴⁾–S cleavage within the simulated ensemble.

Analysis of the electronic structure along trajectories leading to ring opening reveals (see Figure S5.5) a region of strongly coupled electronic–nuclear dynamics in which the overlap between sulfur- and carbon-centered electronic wave functions (at C⁽¹⁾ or C⁽⁴⁾) plays a decisive role in determining whether ring opening proceeds or the thiophene ring subsequently recloses. This overlap is governed by the interplay between nonadiabatic coupling among in-plane and out-of-plane sulfur and carbon p orbitals, the evolving C–S internuclear distance, and torsional motion about the C–S bond. Together, these factors define the electronic character at the surface crossings and control the branching between irreversible ring opening and reannulation.

In addition to the discussed dominant pathways, a variety of less frequent intermediates is observed, including cyclopropene-like structures, thiol species formed via hydrogen migration, and rare instances of bromine atom migration (see Figures S5.6 and S5.7). The presence of these motifs highlights the complexity of the nonadiabatically coupled multidimensional PESs explored during the dynamics. The observed behavior is consistent with the measured experimental fragmentation channels such as CH₃ ⁺ + C₃SBr⁺ and C₃HS⁺ + CH₂Br⁺ depicted in Figure S2.1c,d, respectively, and require intramolecular rearrangement prior to fragmentation.

3-Bromothiophene

The UV-induced dynamics of 3-bromothiophene were examined with the same approach as its 2-bromothiophene isomer. Similar channels were identified, albeit with changes in the required dissociation pathway and different branching ratios. The KER versus pump–probe delay maps for C⁽²⁾–Br bond dissociation (Br⁺ + C₄H₃S⁺), C⁽¹⁾–S ring-opening, and C⁽⁴⁾–S ring-opening are shown in Figure a,d,g, respectively. As in 2-bromothiophene, C⁽²⁾–Br bond dissociation in 3-bromothiophene exhibits overlapping static and dynamic KER features centered near 2 eV, lower than either ring-opening pathways. However, the dominant ionic products following strong field ionization of the neutral ring-opened structures differ between the bromothiophene isomers. In 3-bromothiophene, C⁽¹⁾–S ring-opening primarily yields C₂H₂S⁺ + C₂HBr⁺, whereas in 2-bromothiophene it more commonly produces CHS⁺ + C₃H₂Br⁺. While experimental data alone cannot uniquely identify the C–S bond cleavage yielding CHS⁺ + C₃H₂Br⁺ in 3-bromothiophene, calculations indicate a C⁽⁴⁾–S ring opening origin. Since CHS⁺ + C₃H₂Br⁺ is the highest-population channel in both isomers, C⁽⁴⁾–S ring opening is the dominant dissociation pathway in 3-bromothiophene compared to C⁽¹⁾–S in 2-bromothiophene. The dominant C⁽⁴⁾–S ring opening channel in 2-bromothiophene, C₃H₃ ⁺ + CSBr⁺, is also observed in 3-bromothiophene (Figure S2.2b) but requires additional postring-opening dynamics due to the changed bromine position and shows a significant reduction in channel population (Section 4 of the SI).

4.

Kinetic energy release (KER) versus pump–probe delay for the two-body dissociation pathways in 3-bromothiophene and simulated trajectories with corresponding electronic state populations. (a) KER as a function of pump–probe delay for the dissociation of bromine into the coincidence channel Br⁺ + C₄H₃S⁺. Like in 2-bromothiophene, the static and dynamic bands overlap one another. (d) KER map for the formation of C₂H₂S⁺ + C₂HBr⁺ via C⁽¹⁾–S ring-opening. (g) KER map for the formation of CHS⁺ + C₃H₂Br⁺ via C⁽⁴⁾–S ring-opening. C⁽¹⁾–S ring-opening would also yield the ions for this channel in 3-bromothiophene, but simulations reveal that C⁽⁴⁾–S bond-lengthening is the dominant process that leads to this pair. A model for the ions formed in each channel is shown in the upper right of each plot. (b–h) Overlay of trajectories corresponding to Br dissociation, C⁽¹⁾–S ring-opening, and C⁽⁴⁾–S ring-opening, respectively. In the top inset of each diagram is shown the average geometry under which the trajectories in each ensemble undergo an electronic transition between labeled states. (c), (f), (i) Molecular Coulombic Hamiltonian (MCH) electronic eigenstate populations for respective channels.

The simulated molecular dynamics trajectories, shown in Figure b,e,h, as well as the observed population transfer, depicted in Figure c,f,i, indicate that while the S₂ and S₃ singlet states are initially less populated than in the case of 2-bromothiophene, similar dissociative channels nevertheless occur. As in the case of 2-bromothiophene, in 3-bromothiophene we see C⁽²⁾–Br bond dissociation, which leaves behind a planar thiophene ring, as well as two ring-opening channels which are characterized by out-of-plane motion of the thiophene ring and an intact ring-opened structure (Table S5.2 and Figures S5.2 and S5.8). Like for 2-bromothiophene, the dominant electronic configuration that populates the C⁽¹⁾–Br dissociation channel is π₂σ_C–Br . For the ring opening channels, π₁σ_C–S–C and π₂π* transitions are more dominant, with the π₂π* configuration slightly favoring C⁽⁴⁾–S ring opening S5.4. An investigation of the internal conversion within these channels reveals that, like for 2-bromothiophene, both ring-opening channels are mediated by population transfer between in-plane and out-of-plane sulfur and carbon p orbitals as well as their relative orientation governed by nuclear motion (see Figure S5.9).

Three-Body Coincidences

Determining the physical and chemical processes related to the dynamic evolution of heterocyclic molecules upon UV photoexcitation primarily relies on the analysis of two-body processes discussed above. However, we also observe three-body fragmentation channels, indicating the presence of further reaction dynamics taking place in 2- and 3-bromothiophene. Unlike two-body relaxation pathways that demonstrate a single dynamic KER band, triple coincidence channels contain two dynamic features that diverge at longer delays. This phenomenon indicates multiple dissociation pathways that give the same ionic products, as in the formation of C₂H₂ ⁺ + C₂SH⁺ + Br⁺ and C₃H₂ ⁺ + CHS⁺ + Br⁺ fragments depicted in Figure a,b, respectively. To distinguish the pathways, Newton diagram analysis can be applied to each band isolated using a KER range at long delays, where the separation between features is maximal. Newton diagrams are a method of visualizing the momentum correlations among multibody ion fragments. ,− Such maps provide direct structural information about fragmentation processes, making it possible to distinguish between instantaneous and sequential dissociation, thereby identifying the order of ion formation. In general, direct fragmentation appears as a localized distribution in a Newton diagram, reflecting that a breakup of chemical bonds occurred on a short time scale and the molecular geometry at the moment of fragmentation is largely preserved. Sequential processes yield a semicircular distribution due to rotation of the intermediate fragments relative to the initial ion dissociation. ,, By plotting the momentum of one of the ions on the horizontal axis and analyzing the distributions of sequential fragments in both the top and bottom parts of the Newton plots, it is possible to identify which ion dissociates first by determining which orientation of the Newton diagrams produces the characteristic semicircular features.

5.

Kinetic energy release (KER) versus pump–probe delay plot for two three-body coincidence channels observed in 2-bromothiophene. Each KER has three distinct bands at different energy levels: one static band that does not vary significantly across delay and two dynamic bands that separate for longer delays. The events associated with each band are isolated by an energy range and plotted in Newton diagrams to determine the mechanisms associated with the different dissociation bands. (a) KER vs pump–probe delay map for C₂H₂ ⁺ + C₂SH⁺ + Br⁺ fragmentation and its associated Newton diagrams for the regions 9.5–15 eV (a.1), 4.5–7.75 eV (a.2), and 1.0–4.5 eV (a.3). Br⁺ is plotted on the horizontal axis with C₂H₂ ⁺ and C₂HS⁺ on the top and bottom parts, respectively, for both (a.1) and (a.2). The semicircular distributions indicate that Br⁺ dissociates first in both regions. The static band can also form via the direct fragmentation of the molecule into the three ions, resulting in an asymmetrical intensity distribution. (a.3) has C₂H₂ ⁺ on the horizontal axis with C₂HS⁺ and Br⁺ on the top and bottom parts, indicating that the intense low energy band forms following C₂H₂ ⁺ dissociation. (b) KER vs delay map for the C₃H₂ ⁺ + CHS⁺ + Br⁺ channel and Newton diagrams for the regions 8.5–14 eV (b.1), 3.75–6.5 eV (b.2), and 1.0–3.0 eV (b.3). (b.1) has Br⁺ on the horizontal axis and C₃H₂ ⁺ and CHS⁺ on the top and bottom parts, but it could form through dissociation of each of the three ions (see Figure S2.4). (b.2) uses the same orientation but exclusively forms via sequential C⁽¹⁾–Br bond dissociation. The lowest energy region (b.3) has CHS⁺ on the horizontal axis with C₃H₂ ⁺ and Br⁺ on the top and bottom parts, but due to low statistics, the mechanisms leading to its formation are difficult to discern experimentally. A model for the ions formed in each channel is shown in the bottom left of each plot.

The Newton diagrams for the static features in KER maps of C₂H₂ ⁺ + C₂SH⁺ + Br⁺ and C₃H₂ ⁺ + CHS⁺ + Br⁺ channels are shown in Figure a.1,b.1, respectively. Both are shown with Br⁺ on the horizontal axis. Since in the case of static KER the pump pulse does not contribute to the measured spectra, the observed distributions originate from probe-only induced instantaneous and sequential processes. It is seen from Figure a.1 that the sequential fragmentation to the C₂H₂ ⁺ + C₂SH⁺ + Br⁺ channel proceeds via Br⁺ dissociation followed by some later fragmentation of the C₄H₃S⁺ intermediate. Plotting other orientations of the Newton diagrams (see Figure S2.3) for this channel reveals very minor contributions of alternative fragmentation pathways to the sequential dynamics. The observed asymmetry in the intensity distributions is attributed to the overlap of the sequential and direct fragmentation pathways. The Newton diagram in Figure b.1, showing the fragmentation to C₂H₂ ⁺ + C₂SH⁺ + Br⁺ channel, has similar structure but unlike Figure a.1 where Br⁺ always dissociates first in sequential processes, alternative pathways formed by initial fragmentation of Br⁺, C₃H₂ ⁺, and CHS⁺ ions are possible (see Figure S2.4).

Figure a.2,a.3 show the Newton diagrams for the dynamic KER bands of the C₂H₂ ⁺ + C₂SH⁺ + Br⁺ channel in the 4.5–7.75 and 1.0–4.5 eV energy ranges, respectively. The higher energy band forms when the Br⁺ ion, plotted on the horizontal axis with C₂H₂ ⁺ and C₂HS⁺ on the top and bottom parts, respectively, dissociates first. The lower energy band forms in the process when the C₂H₂ ⁺ ion dissociates first. Similarly, the highest energy (3.75–6.5 eV) dynamic pathway producing C₃H₂ ⁺ + CHS⁺ + Br⁺ forms through C⁽¹⁾–Br bond dissociation followed by the breakup of the intermediate to C₃H₂ ⁺ and CHS⁺ fragments, as shown in Figure b.2. The lower energy (1.0–3.0 eV) band of the C₃H₂ ⁺ + CHS⁺ + Br⁺ channel, shown in Figure b.3, has extremely low statistics and thus the mechanisms leading to its formation are difficult to discern, but this feature is likely to originate from CHS⁺ dissociation.

The three-body fragmentation channels of 3-bromothiophene were also investigated, revealing the same primary channels as 2-bromothiophene, C₂H₂ ⁺ + C₂SH⁺ + Br⁺ and C₃H₂ ⁺ + CHS⁺ + Br⁺ (see Figure S2.5a,b, respectively). As in 2-bromothiophene, these channels have two dynamic features that differ in their dissociation processes. Limited channel populations for 3-bromothiophene make conclusive interpretation of the ion formation order challenging; however, meaningful comparisons are still possible. Unlike in 2-bromothiophene where these channels are roughly equal in population, C₃H₂ ⁺ + CHS⁺ + Br⁺ is significantly more probable to form in 3-bromothiophene (see Table S4.4). Furthermore, while both ring-opening pathways are present in 2-bromothiophene, CHS⁺ + C₃H₂Br⁺ forming via C⁽⁴⁾–S rather than C⁽¹⁾–S ring opening in 3-bromothiophene suggests that C⁽¹⁾–S is not present in the three-body fragmentation pathways of this isomer.

In our complementary simulations of 2-bromothiophene, we see a small number of trajectories that undergo both C⁽¹⁾–Br bond dissociation and ring opening (see Figure S5.10), consistent with the measured three-body fragmentation channels. Although C⁽¹⁾–S ring opening occurs more frequently than C⁽⁴⁾–S ring opening overall, in the subset of trajectories that undergo both ring opening and Br dissociation, the pathway involving C⁽⁴⁾–S and C⁽¹⁾–Br is significantly more likely. Specifically, this combined channel occurs nearly 7 times more frequently than the C⁽¹⁾–S and C⁽¹⁾–Br channel. For 3-bromothiophene, the opposite trend occurs: while the C⁽⁴⁾ – S ring opening channel is dominant, the combined C⁽¹⁾–S and C⁽¹⁾–Br channel is more frequent than the C⁽⁴⁾–S and C⁽¹⁾–Br channel with a ratio of 16:0. While the channels are rather weak, we observe that the C⁽¹⁾–S and C⁽¹⁾–Br pathway originates primarily from populating (π₂σ_C–Br ) configuration, while the C⁽⁴⁾–S and C⁽¹⁾–Br channel is dominantly populated by (π₂σ_C–S–C ) and (π₂π*) configurations, in a notable inversion of the trend for ring opening alone.

Discussion

We have identified the importance of the initially populated πσ_C–Br , πσ_C–S–C , and ππ* configurations for each reaction channel of 2- and 3-bromothiophene, and clarified that thiophene ring opening proceeds through a S₁ to S₀ CI, from which the molecules can either annulate or evolve into twisted ring-opened conformers. This CI appears to be located at regions where in-plane (σ-bonding along the ring-opened coordinate) and out-of-plane (nonbonding along the ring-opened coordinate) sulfur orbitals are nearly degenerate. The nonadiabatic population transfer between the electronic states corresponding to these molecular orbitals determines the final conformer reached by a molecule undergoing ring-opening dynamics.

Our results indicate that nonadiabatic population transfer is essential, not incidental, to ring-opening reactions. Experimentally, coincident ion detection has shown that C–Br bond dissociation and ring opening are indeed the dominant 240 nm UV-induced dynamic processes in these molecules, corroborating the role πσ* and ππ* electronic configurations play in light-induced reactions. The experiment also suggests that further dynamics take place on longer time scales, as evidenced by time-dependent KER features extending into picosecond time scales. In particular, in 2-bromothiophene, we observe C⁽¹⁾–Br bond dissociation, C⁽¹⁾–S, and C⁽⁴⁾–S ring opening proceeding on a time scales of about 1300, 830, and 630 fs, respectively. The behavior of C–Br bond dissociation is expected from theory, as simulations suggest that the bromine atom fully fragments from the molecule leading to a continuous decrease in KER over a long time scale (Figure S5.11).

In the case of the ring-opening channels, our simulations suggest that the molecules reach stable ring-opened conformers on subpicosecond time scales. We note that the experimental findings include post-UV-photoexcitation dynamics of the molecular fragments following ionization by the probe pulse. As such, the reported experimental time scales reflect both the fragmentation process and postionization evolution of the corresponding fragments, leading to longer time scales than our simulations predict. It may be suggested that the C–C bond breakages leading to the experimentally observed coincident fragment detection of the C⁽¹⁾–S and C⁽⁴⁾–S ring opening channels may occur outside of the simulated time domain, leading to the long time KER dependence. Moreover, experimental observation of fragmentation channels not accessible through direct bond cleavage, e.g., CH₃ ⁺ + C₃SBr⁺ in 2-bromothiophene and C₄H₃ ⁺ + SBr⁺ in 3-bromothiophene (Figures S2.1c and S2.2d, respectively), provides evidence for additional isomerization processes occurring after the initial ring-opening step. These channels suggest subsequent rearrangements not explained by the ring-opening picture alone and remain to be understood. Interestingly, in simulations, we see occasional C⁽³⁾–C⁽⁴⁾ and C⁽¹⁾–C⁽²⁾ bond lengthening to ∼2 Å in each bromothiophene isomer, particularly after cyclopropene formation (Figures S5.6 and S5.7). However, full dissociation of these bonds is not theoretically observed, given the employed computational methods. Attempts to use different electronic structure methods to potentially include more active electrons that may contribute to the experimentally observed bond breaking were not successful. Indeed, because of the computational cost of multiconfigurational electronic structure methods and the difficulty of simulating the multiphoton ionization process and the resulting dissociative dynamics of the dication, these long-time dynamics remain an area of uncertainty and interest.

We also experimentally resolved several competing three-body processes in 2-bromothiophene. In particular, two distinct three-ion fragmentation coincident events are observed stemming from ring-opening on either C–S bond. Each channel exhibits multiple dynamic dissociation pathways leading to the same final ion products: a lower-energy pathway in which a portion of the thiophene ring dissociates first and a higher-energy pathway where bromine is the first ion formed. We observe that three-body channels involving C⁽⁴⁾–S ring-opening (Figure a) form more readily through the dissociation of a C₂H₂ ⁺ ion while C⁽¹⁾–S ring-opening channels (Figure b) are more likely following the higher-energy C⁽¹⁾–Br bond dissociation pathway. We note that for C⁽¹⁾–Br bond dissociation in both channels, while bromine is the first ion detected, simulations indicate that both pump-induced C⁽¹⁾–Br or C–S bond cleavage can precede ionization. Theoretically, while three-body fragmentation is not explicitly observed, we note that these processes may result from an interaction of the pump and probe laser on the system. While it is thermodynamically possible that the 240 nm pump alone could induce three-body fragmentation (for example, unrestricted Hartree–Fock calculations indicate that the ground state energy of the Br^•+C₂HS^•+C₂H₂ radical is only 4.3 eV above the ground state of 2-bromothiophene), we suggest that these three body channels could be accessed via a pump-induced two-body fragmentation, followed by probe-induced further fragmentation and ionization. Indeed, such a process would explain the difference in the KER for these different channels. We propose that when fragmentation is initiated by the pump pulse alone, the resulting KER is lower, as the radical fragments would have greater spatial separation at the time of probe ionization and subsequent Coulomb explosion. Conversely, when the probe is involved in fragmentation, the KER is larger, as the charged fragments remain in closer proximity prior to the Coulomb explosion. Hence, we suggest that the pathways described in Figure a.2,b.2 likely result from a combined pump–probe bond fragmentation, while those in a.3,b.3 are solely pump-induced (see Figure S5.12).

Conclusions

This work has uncovered remarkable reaction mechanisms responsible for laser-induced ring-opening and -dissociation dynamics in heterocyclic molecules. We investigated the UV-induced processes in the bromothiophenes using state-of-the-art ion coincidence spectroscopy combined with high-level theoretical simulations. Our results elucidate the nuclear and electronic dynamics leading to C–Br bond dissociation and ring-opening reactions on time scales of several hundred femtoseconds.

Our experimental and theoretical effort supports the role of ultrafast internal conversion from the dominant ππ* to πσ* electronic states in initiating ring-opening dynamics and identifies the conical intersections that determine whether ring opening proceeds to completion or reannulation. Although the dominant ring-opening products differ between the bromothiophene isomers, the observation of analogous mechanisms in both 2- and 3-bromothiophene indicates that these dynamics are not specific to a single substitution pattern, suggesting broader applicability across heterocyclic systems. Coincidence-resolved measurements using the COLTRIMS microscope corroborate theoretical findings and establish a powerful, versatile experimental platform for investigating ultrafast nonadiabatic molecular dynamics.

Methods

Experiment

The experimental setup used in this study has been described elsewhere previously. − A schematic of the experimental scheme is shown in Figure . We used a 5 kHz Ti:sapphire laser producing 35 fs pulses with a central wavelength of 790 nm. The beam was split by using a 70:30 beam splitter into two independent arms that were time-delayed relative to one another. Each arm had an optical grating compressor to generate transform-limited pulses and had variable intensity controlled by a combination of λ/2 waveplate and polarizer. Both beams were linearly polarized in the direction of the time-of-flight axis of the spectrometer. 240 nm generation was accomplished using a commercial optical parametric amplification (Light Conversion, TOPAS Twins) scheme − to first generate 480 nm light, which was frequency doubled with a second harmonic β-barium borate (BBO) crystal to output 240 nm laser pulses. A schematic of the optical arrangement is shown in Figure S1.1. For this experiment, the pump arm intensity was in the range of 1 × 10¹³ W/cm² for both isomers and was tuned to minimize ionization induced by the UV pulses. IR probe intensity was in the range of 10¹⁴ W/cm² for both isomers and was selected to optimize the population of the two-ion coincidence channels with the C₄H₃BrS target. Both beams were directed into the interaction region, where they were overlapped and back-focused to spot sizes of about 6 and 3 μm for the UV and IR arms, respectively. Our temporal resolution was approximately 125 fs. A cold molecular jet of bromothiophene from a room-temperature bubbler was produced by expansion through a 30 μm nozzle seeded with 1 bar of helium gas. This jet was propagated into the spectrometer interaction region perpendicular to the two laser pulses. The bromothiophene was ionized and the resulting ions were detected using our COLTRIMS. ,, A weak, homogeneous electric field within the COLTRIMS directed ions to a position-sensitive detector that collected the three-dimensional momentum distributions of the charged fragments. For Coulomb explosion imaging, we primarily focus on measuring the fragmented ions, and for the current experimental conditions, we have an ion momentum resolution of 0.1 au. By applying the coincidence technique, we can isolate specific relaxation channels of bromothiophene and gain the most relevant kinematically complete information about the fragmentation dynamics.

Theory

To describe theoretically the excited state dynamics of 2- and 3-bromothiophene, we performed a series of fully ab initio simulations using multireference methods based on the state-averaged complete active space self-consistent field (SA-CASSCF) and the multistate multiconfigurational second-order perturbation theory (MS-CASPT2) approaches. All electronic structure simulations were performed in OpenMolcas version 25.06 using the cc-pVDZ basis set. Dynamical simulations were performed using the trajectory surface hopping (TSH) method in the SHARC version 4.0 software package. ,

We began by performing a geometry optimization of the neutral system in the ground electronic state using the second-order Møller–Plesset perturbation (MP2) level of theory. The Hartree–Fock (HF) molecular orbitals (MOs) for this optimized geometry, shown in Figure S5.1, were analyzed in order to determine the relevant electronic active space for the follow-up simulations.

We found that in both isomers, the first 33 MOs are those forming the localized core states as well as those responsible for forming C–C and C–H σ bonds. MOs 34 and 35 are σ-bonding orbitals responsible for the formation of C⁽¹⁾–Br and C–S–C bonds in the system. The corresponding σ-antibonding MOs are found to be orbitals 41, 42, and 47. Thiophene ring π-bonding and antibonding interactions are found in MOs 38, 39, 40, and 43. Interestingly, MOs 36 and 37 of the C⁽¹⁾–Br bond are found to be highly localized on the bromine atom, with dominant 4p _y and 4p _x character, which indicates that they are not relevant for the formation of the corresponding C⁽¹⁾–Br bond. We note that while it is hard to exclude these occupied orbitals from the consideration due to their close energy spacing with other MOs, we do not take any corresponding antibonding pairs into consideration. Accordingly, for 2-bromothiophene, a computationally affordable active space for our simulations consists of the following 10 MOs: all σ-bonding and antibonding orbitals of the C–S–C bonds and C⁽¹⁾–Br bond (34, 35, 41, 42, 47), as well as the energetically intermediate MOs (36, 37, 38, 39, 40). Interestingly, for 3-bromothiophene, orbital 43 was necessary to consider as well, due to the importance of the dihedral ring bending vibrational modes for geometry optimization and normal-mode analysis.

The chosen set of HF MOs described in the previous paragraph were further optimized using the SA(9)-CASSCF­(12,10) and SA(9)-CASSCF­(12,11) approaches for 2- and 3-bromothiophene, respectively. The numbers in brackets indicate that the optimization has been performed targeting nine, in this case, singlet, electronically excited states of the system, distributing 12 active electrons over ten or 11 MOs, respectively. The generated set of optimized MOs has been used in follow-up SA(9)-CASSCF­(12,10)/SA(9)-CASSCF­(12,11) geometry reoptimization of the neutral systems in their ground electronic state.

The found ground state minimum energy structures were subjected to calculations of normal vibrational frequencies by using the same electronic structure method. The obtained normal coordinates and frequencies were used to generate 1000 initial conditions for each isomer employing the Wigner distribution. At each generated geometry, excitation energies and oscillator strengths were calculated by using MS(9)-CASPT2­(12,10)/MS(9)-CASPT2­(12,11). The photoabsorption cross sections, plotted in Figure S5.13, were constructed using the nuclear ensemble approach and convoluted by Gaussian functions with a phenomenological broadening parameter of 0.2 eV. From the 1000 sampled geometries, we generated an ensemble of 262 initial conditions (2-bromothiophene) and 223 initial conditions (3-bromothiophene), determining the initially populated electronic state for each of the geometries based on an excitation energy window of 5.05–5.35 eV, as well as the calculated oscillator strengths. Of the 262 initial conditions for 2-bromothiophene, 145 were in the first excited state, 102 in the second, and 15 in the third. Of the 223 initial conditions for 3-bromothiophene, 166 were in the first excited state, 54 in the second, and 3 in the third.

For the follow-up dynamical simulations, we mapped the accurate MS-CASPT2 excitations to the more computationally affordable CASSCF scheme. Although the initial excitation prepared using MS-CASPT2 is dominated by ππ* electronic character, mapping onto the CASSCF states used in dynamics lead to population of states with mixed electronic character, including significant πσ* contributions. To clarify the impact of this initial population distribution, we note that the subset of trajectories initially assigned to bright ππ* states undergo rapid nonadiabatic relaxation into πσ* configurations within the first 50 fs (see Figure S5.14).

For both isomers, the ensemble of trajectories was propagated using the TSH approach on the manifold of 9 singlet electronic states computed at the SA(9)-CASSCF­(12,10) level of theory. We note that while SA(9)-CASSCF­(12,11) was used for the geometry optimization and excited state calculation for 3-bromothiophene, orbital 43 was removed from the active space for the following dynamic simulations due to the extreme computational complexity of the scheme. The dynamical simulations were executed for 650 fs (or until convergence failed for a limited number of trajectories in the ensemble) at a 0.5 fs time step using a local diabatization scheme. Statistics of the survival time for the ensemble are shown in Figure S5.15.

In addition, to check the consistency of our calculations, we performed the dynamical simulations using the time-dependent density-functional theory (TD-DFT) approach (see Figure S5.16) and the restricted active space self-consistent field (RASSCF) scheme with a larger active space but limited orders of excitations (see Figure S5.17). In both cases, we observed breaking of same bonds and comparable ratio between reaction channels as in the case of the CASSCF scheme discussed above. However, due to the single-reference nature of the TD-DFT method, bond dissociation of the main reaction channels was not well represented. As for the RASSCF simulations, while we observed similar dynamics, we ultimately discarded this alternative and clearly very powerful approach in favor of the more general CASSCF scheme, which provides a more balanced and systematic description of the system. We also analyzed the effect of including spin–orbit coupling (SOC) on the nuclear and electronic evolution (see Figure S5.18). We found that while SOC allows access to triplet states, the nuclear dynamic pathways, dissociation time scales, and approximate branching ratios are not significantly affected between methods.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.6c05063.

• Additional details on the performed experiment. Additional analysis of measured spectra. The results of supplementary theoretical simulations supporting the conclusions presented in the main text (PDF)

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N.F. and M.T. contributed equally to this work.

The authors declare no competing financial interest.