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

Abstract

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 S(0)-S(1) 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(alpha)-C(beta) (tau(1)), C(beta)-C(gamma) (tau(2)), and C(gamma)-C(delta) (tau(3)) bonds], the conformers can be uniquely labeled. By using this scheme, the assigned conformations are ggH(gamma)' for A, gaH(gamma)' for B, gaH(gamma) for C, agH(gamma) for D, and either aaH(gamma) or agH(gamma)' 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 approximately 1000 cm(-1) is proposed to accompany overcoming a rate-limiting barrier to exciplex formation.

Conformational Isomerization of 5-Phenyl-1-pentene Probed by SEP-Population Transfer Spectroscopy

“…The results of ground state optimizations performed at the DFT Becke3LYP 17,18 /6-31+G*, MP2 21-26 /6-311++G**, and DFT M05-2X 19 /6-31+G* levels of theory were reported in the preceding paper. 1 These results will be useful when predicting the barrier heights of specific XfY isomerizations. QST3 transition state calculations were performed by using DFT B3LYP/6-31+G* and M05-2X/6-31+G* levels of theory.…”

“…In the preceding paper, 1 the conformation-specific spectroscopy of the alkylbenzene, 5-phenyl-1-pentene (5PPene, Figure 1), was discussed. Five conformational isomers, labeled A-E, were observed in the free jet environment with S 0 -S 1 origin transitions at 37 518, 37 512, 37 526, 37 577, and 37 580 cm -1 , respectively.…”

“…Knowledge of the energy barriers to isomerization has significant consequences in a range of contexts. First, one of the motivations for the study of the spectroscopy of 5-phenyl-1-pentene 1 was that the photophysics and photochemistry of the molecule involved exciplex formation between the phenyl ring and vinyl groups, which should have different energy thresholds in the excited state depending on starting conformation. We argued in that work 1 for the transferability of ground state barriers in the pentene chain (determined in this paper) to the S 1 state, providing a rationalization for the precipitous drop in S 1 lifetimes at a threshold of ∼1000 cm -1 .…”

“…First, one of the motivations for the study of the spectroscopy of 5-phenyl-1-pentene 1 was that the photophysics and photochemistry of the molecule involved exciplex formation between the phenyl ring and vinyl groups, which should have different energy thresholds in the excited state depending on starting conformation. We argued in that work 1 for the transferability of ground state barriers in the pentene chain (determined in this paper) to the S 1 state, providing a rationalization for the precipitous drop in S 1 lifetimes at a threshold of ∼1000 cm -1 . Second, those seeking to understand the conformational dynamics of liquid alkanes, 2,3 self-assembled monolayers, 4 membranes, 5,6 and other environ-ments in which alkyl or alkenyl chains are involved require as fundamental input isomerization barrier height to understand the types and time scales governing their motions.…”

Photochemistry of Benzylallene: Ring-Closing Reactions to Form Naphthalene

“…The proximity of the π-systems in the gauche structure may play a role in ring-closure mechanisms. Thus, as a precursor to photochemical studies, the conformation specific spectroscopy of 4-phenyl-1-butyne and several related C 10 and C 11 molecules − has been carried out under jet-cooled conditions. In the case of 4-phenylbutyne, it was found that both the anti and gauche conformers were present in the expansion.…”