Pseudo‐Bimolecular [2+2] Cycloaddition Studied by Time‐Resolved Photoelectron Spectroscopy

Abstract

The first study of pseudo‐bimolecular cycloaddition reaction dynamics in the gas phase is presented. We used femtosecond time‐resolved photoelectron spectroscopy (TRPES) to study the [2+2] photocycloaddition in the model system pseudo‐gem‐divinyl[2.2]paracyclophane. From X‐ray crystal diffraction measurements we found that the ground‐state molecule can exist in two conformers; a reactive one in which the vinyl groups are immediately situated for [2+2] cycloaddition and a nonreactive conformer in which they point in opposite directions. From the measured S1 lifetimes we assigned a clear relation between the conformation and the excited‐state reactivity; the reactive conformer has a lifetime of 13 ps, populating the ground state through a conical intersection leading to [2+2] cycloaddition, whereas the nonreactive conformer has a lifetime of 400 ps. Ab initio calculations were performed to locate the relevant conical intersection (CI) and calculate an excited‐state [2+2] cycloaddition reaction path. The interpretation of the results is supported by experimental results on the similar but nonreactive pseudo‐para‐divinyl[2.2]paracyclophane, which has a lifetime of more than 500 ps in the S1 state.

Through-Bond Interactions and the Localization of Excited-State Dynamics

“…This notion is common within aromatic systems, for example, when functional groups are attached to a benzene ring where the whole molecule acts as a chromophore but the dynamics mainly take place at the functional groups (e.g., stilbene 53À55 or substituted paracyclophanes 56 ). We emphasize that the realization that even small polyenes tend to localize their dynamics is relatively new.…”

Hexamethylcyclopentadiene: time-resolved photoelectron spectroscopy and ab initio multiple spawning simulations

“…Similar but weaker photoelectron band shifts were indeed observed earlier in cyclic polyenes like cyclohexa-1,4-diene (100 fs), cyclohexene (90 fs) and cycloheptatriene (30 fs) 12,17 and also in considerably larger polyenic systems. 11 They were associated to large amplitude motions of the nuclear wavepacket and localization of the dynamics on the way to the CoIn with the ground state. However, a quantitative assignment to particular degrees of freedom was not possible due to the lack of detailed dynamics simulations of these molecules.…”

Predistortion amplified in the excited state

“…This includes -electrocyclic ring opening of cyclohexadiene and derivatives (even large molecules -although with relatively localized excitation -such as 7-dehydrocholesterol [40,[51][52][53]), as described above; ring closure of dienes to form cyclobutenes [54,55] and the corresponding ring opening [56], ring opening of cyclooctatriene to octatetraene [57], -formation of bicyclo [110]alkanes from dienes [48,55,58]. Also formation of bicyclo[3.1.0]alkenes from trienes must be ultrafast, as it can compete with cis-trans isomerization [28,41,42,48,49,59], -[2 + 2]-cycloaddition to cyclobutanes [60][61][62], including also the dimerization of cyclohexadiene, which at high concentration can compete with the ultrafast ring opening [2] ([2 + 2]-cycloaddition in a divinylcyclophane takes longer -13.5 ps [63] -perhaps due to unfavourable geometry and/or too much delocalization of the excitation), -sigmatropic hydrogen 1,3-and 1,7-migration [55,64-68] and a suggested antarafacial H 1,5-migration (Scheme 4 and context in [69], proposed geometry confirmed in Fig. 13 of [70]), sigmatropic alkyl 1,3-migration [60].…”