Philip G. Smith

Structural measurements of hydrogen‐bonded van der Waals dimers and trimers by rotational coherence spectroscopy: 2,5‐diphenyloxadiazole with argon, methane, water, and alcohols

Picosecond time‐resolved polarized fluorescence experiments involving time‐correlated single‐photon counting have studied rotational coherence phenomena of hydrogen‐bonded and other molecular aggregates in their lowest excited singlet states. The experiments are supported by detailed simulations. Using the molecule 2,5‐diphenyl‐1,3,4‐oxadiazole (PPD) as a host species, experiments have compared van der Waals aggregates with Ar1,2, (CH4)1,2, (H2O)1,2, (CH3OH)1,2, (C2H5OH)1, and (1‐C3H7OH)1. Bare PPD, and the argon and methane aggregates all exhibit prominent J‐type recurrences. The resulting sums of rotational constants (B+C) are consistent with center‐of‐mass‐bound, three‐dimensional structures, having out‐of‐plane distances for the attached species of 3.3–3.5 A. The 1:2 aggregates involving argon and methane exhibit additive spectral shifts and nearly additive rotational recurrence times. This shows that the sites for addition of consecutive species are equivalent.Calculations of rotational constants con...

Structures of perylene complexes with long‐chain alkanes and alkyl halides examined by rotational coherence spectroscopy

Fluorescence excitation spectra of jet‐cooled van der Waals complexes of the planar aromatic hydrocarbon perylene with the n‐alkanes pentane, hexane, octane, and decane show in each case a single structural form. Rotational coherence transients observed for these species, spaced by 2.5–4.4 ns, are consistent with the n‐alkane chain oriented parallel to the long axis of perylene and placed 3.6 A above its surface. In contrast, the 1‐chloropentane and 1‐fluoropentane complexes of perylene both exhibit three conformational isomers (α,β,γ) in the electronic ground state. Rotational coherence experiments have measured the structures of these different species in the S1 state, via 000 excitation, proving the existence of three distinct isomers in each case. Dispersed emission spectra following vibronic excitation at 355 cm−1 (A10) indicate in each case that the γ structure relaxes to the α form. Knowledge of the structures of the different forms provides a basis to identify the photoisomerization trajectories. ...

Structural measurements of hydrogen-bonded van der Waals dimers and trimers by rotational coherence spectroscopy. 2,5-diphenyloxadiazole (PPD) with water

Abstract Rotational coherence spectroscopy involving polarized, time-correlated single-photon counting techniques has been used to measure the excited-state structures of hydrogen-bonded complexes between PPD and water at the 1:1 and 1:2 levels of aggregation. Structure determinations involved comparison of measured and simulated RCS traces using the measured B and C rotational constants. PPD/(H2O)1 reveals a singly hydrogen-bonded, in-plane structure with an N…HO distance of 2.95±0.1 A. The best structural estimate for the 1:2 complex, PPD/(H2O)2, consists of a water dimer, hydrogen bonded to one nitrogen atom of PPD.

Electronic coupling and conformational barrier crossing of 9,9’‐bifluorenyl studied in a supersonic jet

Fluorescence excitation, dispersed fluorescence, and picosecond time‐resolved fluorescence spectroscopies have been used to study the dimeric molecule 9,9’‐bifluorenyl, isolated under supersonic jet conditions. The excitation spectrum reveals a splitting in several principal resonances of the S0→S1 excitation spectrum, which can be attributed to electronic coupling between the two fluorenyl moieties. The splitting, which for different vibronic transitions correlates with Franck–Condon factors, is consistent with an exciton model that includes higher electronic states. The splitting is reasonably well reproduced by a multipole interaction potential. Calculations have verified that the electron exchange coupling is small. Furthermore, the relative intensities of the resonances allow an estimate of the equilibrium geometry, suggesting that the molecular long axes have an angular displacement of 60°–70°, consistent with the geometry found by x‐ray diffraction. A most interesting feature of this species is tha...

Photoisomerization of molecular van der Waals aggregates. Structural assignments of isomeric forms of 1-chlorobutane/perylene via rotational coherence spectroscopy

Abstract Van der Waals complexes of perylene with 1-chlorobutane, prepared under supersonic jet conditions, exist in three distinct isomeric forms distinguishable by their spectral red-shifts of 265,238 and 184 cm −1 . Following vibronic excitation at 355 cm −1 , two of these isomers yield similarly shaped and broadened fluorescence bands of ≈ 180 cm −1 width, suggesting a rapid isomerization. A third isomer is not coupled, showing a distinct spectrum even for a vibrational energy of 700 cm −1 . Structure determinations of all three isomers are reported using rotational coherence spectroscopy. The existence is demonstrated of three remarkably different binding sites for 1-chlorobutane on the perylene surface.

Vibrational relaxation of isomerizing alkyl halide complexes of perylene measured by stimulated emission spectroscopy

Abstract Experiments involving picosecond stimulated-emission spectroscopy have monitored the resonance emission following vibronic excitation of the 1:1 complexes ofperylene with n -hexane, n -octane and the three isomers each with 1-chlorobutane and 1-chloropentane. The experiments confirm that photoisomerization is occurring in the alkyl halide complexes, by labelling the transient resonant emission. The rates of vibrational relaxation (IVR) at 355 cm −1 excess energy differ by up to a factor of three for different isomers, and are independent ofisomeric stability.