Nonlinear optical signatures of ultraviolet light-induced ring opening inα-terpinene

Abstract

Photoinduced electrocyclic ring opening reactions in conjugated cylcoalkenes are among the most elementary processes in organic chemistry. One prototypical ring opening reaction transforms cyclohexadiene into hexatriene. It is known that a sequence of sub-100 fs internal conversion transitions precedes bond breaking in cyclohexadiene and some of its derivatives. However, these excited state dynamics have never been directly monitored in solution because of insufficient time resolution. Here we aim to uncover the extraordinary photophysics behind related ultrafast internal conversion processes in a derivative of cyclohexadiene, α-terpinene (α-TP), solvated in cyclohexane. Transient absorption anisotropy experiments conducted with 20 fs laser pulses at 267 nm expose non-exponential depopulation kinetics for the ππ * electronic state of α-TP. Our data show that population transfer rapidly accelerates within the first 100 fs after photoexcitation. In addition, recurrences in twodimensional photon echo (2DPE) line shapes reveal strong vibronic coupling in a normal mode near 523 cm −1 , which involves torsions of the C C bonds and hydrogen out-of-plane (HOOP) wagging on a vinyl group. With the support of several experiments, we hypothesize that the excited state wavepacket in α-TP undergoes several recurrences in the C C stretching coordinate before

Coherent multidimensional optical spectra measured using incoherent light

“…Nevertheless, the overall rectangular shape-consistent with numerous femtosecond 2D ES measurements of similar carbocyanine molecules where vibronic splittings do not exceed absorption linewidths 22,25,28,41,46,47 -of the IR144 2D spectra indicate that both diagonal and cross peaks are present.…”

“…The spectroscopic technique-analogous to multidimensional NMR spectroscopy 14 has been performed in the infrared, visible and ultraviolet spectral regions [15][16][17][18][19][20][21][22][23][24][25][26][27][28] . In the visible region, the technique is often called two-dimensional electronic spectroscopy (2D ES), and it has been used to study important topics such as photosynthetic energy transfer [29][30][31][32] .…”

“…Indeed, to date, all coherent multidimensional optical spectra have been measured using coherent laser excitation [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] . The vast majority of measurements used femtosecond pulses and even small deviations from a perfectly linear spectral phase profile introduce artefacts 33,34 .…”

Uncovering molecular relaxation processes with nonlinear spectroscopies in the deep UV

“…Such setups are often operated in a configuration where each beam propagates in the lowest order EH 11 mode. In our experiments, the UV bandwidth is maximized by operating the setup near 850 Torr in a cross modal phase matching condition because relatively narrowband fundamental pulses at 800 nm are available [56]. Selfphase modulation of the incoming beams at 850 Torr results in thirdharmonic pulses with bandwidths as large as 1500 cm À1 .…”

“…Another innovative new approach utilizes a birefringent delay line [53]. Specialized second harmonic generation methods in BBO crystals [49,54,55], self-phase modulation in CaF 2 [52], and nonlinearities in noble gases [56,57] have all been leveraged to enhance the laser bandwidths employed in 2DUV experiments. Because of these technical advances, biologically interesting applications to DNA and proteins are now on the horizon [44,46,47,49,57].…”