Conformation-Specific Spectroscopy and Excited State Photophysics of 5-Phenyl-1-pentene

Single-conformation spectroscopy of 5-phenyl-1-pentene has been studied in a supersonic expansion by using a combination of methods, including resonant two-photon ionization (R2PI), ultraviolet hole-burning (UVHB), and rotational band contour analysis. Five conformational isomers (labeled A−E) have been identified in the spectrum, with S0−S1 origins at 37518, 37512, 37526, 37577, and 37580 cm−1, respectively. Rotational band contours of these origin transitions recorded at 0.08 cm−1 resolution reflect the sensitivity of the direction of the transition dipole moment (TDM) to the conformation of the pentene side chain. On the basis of a comparison of the observed rotational band contours with that predicted by M05-2X/6-31+G* and CIS/6-31G calculations, firm assignments have been obtained for four of the five conformers, while the fifth is constrained to one of two possibilities. On the basis of values of three torsional angles along the pentene chain [about the C(α)−C(β) (τ1), C(β)−C(γ) (τ2), and C(γ)−C(δ) (τ3) bonds], the conformers can be uniquely labeled. By using this scheme, the assigned conformations are ggHγ′ for A, gaHγ′ for B, gaHγ for C, agHγ for D, and either aaHγ or agHγ′ for E. Single vibronic level fluorescence lifetimes have been recorded for a series of vibronic levels in the range 0−1500 cm−1 for all five conformers. A sharp drop in lifetime of all five conformers at ∼1000 cm−1 is proposed to accompany overcoming a rate-limiting barrier to exciplex formation.