Identification of the Excited-State C═C and C═O Modes of trans-β-Apo-8′-carotenal with Transient 2D-IR-EXSY and Femtosecond Stimulated Raman Spectroscopy

Abstract

Assigning the vibrational modes of molecules in the electronic excited state is often a difficult task. Here we show that combining two nonlinear spectroscopic techniques, transient 2D exchange infrared spectroscopy (T2D-IR-EXSY) and femtosecond stimulated Raman spectroscopy (FSRS), the contribution of the C═C and C═O modes in the excited-state vibrational spectra of trans-β-apo-8'-carotenal can be unambiguously identified. The experimental results reported in this work confirm a previously proposed assignment based on quantum-chemical calculations and further strengthen the role of an excited state with charge-transfer character in the relaxation pathway of carbonyl carotenoids. On a more general ground, our results highlight the potentiality of nonlinear spectroscopic methods based on the combined use of visible and infrared pulses to correlate structural and electronic changes in photoexcited molecules.

Femtosecond Stimulated Raman Spectroscopy

“…This compensates for the probe fluctuations on a shot‐to‐shot basis but requires the simultaneous detection of two pulses at a repetition rate of 1 kHz. Therefore, this scheme is most common in connection with photodiode array detectors but is also used with CCD cameras albeit at lower readout rates . Jin et al .…”

“…Therefore, the most common approach is direct supercontinuum generation in transparent dielectrics based on the Kerr effect. Examples include solids like sapphire,, CaF2 and YAG crystals for Yb‐based laser systems, as well as liquids in flow cells, like water and CCl 4 . Liquids, in principle, allow higher fluences, but solids are much easier to use and produce more stable continua.…”

“…Early FSRS experiments used the output of a 100‐fs, 640‐nm colliding‐pulse mode‐locked dye laser to initiate exciton dynamics in polydiacetylene . Similar temporal resolution is achieved by using the frequency‐doubled or tripled output of a Ti:Sa amplifier around 400 and 270 nm, respectively. Without compression, this results in an effective time resolution between 100 and 200 fs, depending on the initial bandwidth of the laser and thickness of the crystal.…”

“…Figure presents an intriguing example where FSRS not only revealed the vibrational modes involved in the internal conversion but also exposed intramolecular vibrational relaxation dynamics on a sub‐ps timescale . The level diagram of trans ‐β‐apo‐8′‐carotenal is sketched in Figure A.…”

Following Coupled Electronic-Nuclear Motion through Conical Intersections in the Ultrafast Relaxation of β-Apo-8′-carotenal

“…The initial ultrafast component may indicate a conical intersection in the vertical Franck−Condon region, directly funneling molecules back to S 0 , or to a lower lying dark state. An alternative explanation is the infrared transition dipole (Table 2), centered roughly at 1590 cm −1 , and from transient 2DIR exchange spectroscopy measurements in cyclohexane, 34 we can assign this to the excited state CO symmetric stretch. The large change in fundamental frequency associated with the C O bond on the excited state (the frequency is reduced by ∼70 cm −1 ), likely reflects a weaker C−O bond on the excited state.…”