Coherent ultrafast torsional motion and isomerization of a biomimetic dipolar photoswitch

Abstract

(<250 words). Femtosecond fluorescence up-conversion, UV-Vis and IRtransient absorption spectroscopy are used to study the photo-isomerization dynamics of a new type of zwitterionic photoswitch based on a N-alkylated idanylidene pyrroline Schiff base framework (ZW-NAIP). The system is biomimetic, as it mimics the photophysics of retinal, in coupling excited state charge translocation and isomerisation. While the fluorescence lifetime is 140 fs, excited state absorption persists over 230 fs in form of a vibrational wavepacket according to twisting of the isomerising double bond. After a short "dark" time window in the UV-visible spectra, which we associate to the passage through a conical intersection (CI), the wavepacket appears on the ground state potential energy surface, as evidenced by the transient mid-IR data. This allows for a precise timing of the photoreaction all the way from the initial Franck-Condon region, through the CI and into both ground state isomers, until incoherent vibrational relaxation dominates the dynamics. The photo-reaction dynamics remarkably follow those observed for retinal in rhodopsin, with the additional benefit that in ZW-NAIP the conformational change reverses the zwitterion dipole moment direction. Last, the pronounced low-frequency coherences make these molecules ideal systems for 3 investigating wavepacket dynamics in the vicinity of a CI and for coherent control experiments.4

Mode specific excited state dynamics study of bis(phenylethynyl)benzene from ultrafast Raman loss spectroscopy

“…We have assigned the 8.6 ps and 12.1 ps components as vibrational relaxation (cooling), which could be coupled with torsional degrees of freedom by transferring energy to torsional degrees of freedom and may contribute to the rate of planarization. 48 As discussed above for the URLS intensities, the faster relaxation of the CC triple bonds is probably due to a steeper potential energy surface as compared to the aromatic CC double bonds. In acetonitrile and butyronitrile the frequency upshift is slightly slower as compared to methanol.…”

Influence of the Chemical Design on the Coherent Photoisomerization of Biomimetic Molecular Switches

“…For the previously reported IPs the fluorescence is shorter-lived than the Excited State Absorption (ESA) observed in UV transient absorption experiments [2][3][4]. A more accurate measurement of the residence time in S 1 resides in the crossover between SE and PA.…”

“…We perform transient absorption spectroscopy on two new, cationic members of the family, the NAIP and dMe-MeO-NAIP, which are respectively missing the MeO group on C 6' or the Me group on C 5 . We compare the dynamics to that of ZW-MeO-NAIP [3], itself very similar to MeO-NAIP [4]. In the VIS range, the successive signatures of Stimulated Emission (SE) and Photoproduct Absorption (PA) may be observed, allowing us to pinpoint accurately in time the passage of the system through the CI, as was also observed in Rho [5].…”

“…To this end, the indanylidene-pyrroline (IP) chemical skeleton has been designed to reproduce the excited state (S 1 ) potential energy surface of retinal in rhodopsin (Rho) [1,2]. As a result, these biomimetic molecules undergo an ultrafast photoisomerization involving coherent dynamics through a conical intersection (CI) [3]. This is a remarkable biomimetic process in which a significant amount of mechanical energy is delivered into a few specific vibrational modes.…”

Linear dichroism amplification: Adapting a long-known technique for ultrasensitive femtosecond IR spectroscopy

“…Isomerization is ultrafast (return to the ground state in less than 1 ps), efficient (≥20%), and the switches are robust under illumination. 16 For this N-alkylated Schiff base switch, the E isomers generated by the pump pulse relax within a few seconds back to Z at room temperature. The ground state barrier is 22.4 (±2 ) kcal mol −1 .…”