The UV-visible action-absorption spectrum of all-trans and 11-cis protonated Schiff base retinal in the gas phase

Abstract

The UV-visible absorption of retinal in its protonated Schiff-base form is studied in the gas phase. In particular, transitions to highly-excited electronic states, S, in the all-trans and 11-cis forms are considered, and several new states are discovered. Their positions and strengths are compared to state of the art quantum calculations. The location of these states are particularly important when new fs pump-probe experiments are designed to investigate the fast excited-state dynamics of retinal chromophores.

Intrinsic photoisomerization dynamics of protonated Schiff-base retinal

“…Years of studies have been concerned with the related subject of spectral tuning (opsin shift) in proteins, where the band maximum traditionally has been compared to that obtained in MeOH solution (440 nm) 8 . This value is far from the intrinsic transition wavelength of about 610 nm obtained in the gas phase for both all- trans and 11- cis RPSB chromophores 31–33 , as well as those found in retinal-containing proteins. Interactions inside proteins, as well as in solutions, hence significantly change the energy gap between the ground and excited electronic states in the Franck–Condon region, and may also change the excited-state potential-energy surface around conical intersections.…”

“…The present new approach of two-dimensional time-resolved action spectroscopy works as follows. A pump pulse first excites the RPSB chromophore in the broad S 0 → S 1 absorption band, which in the gas phase is between 550 and 650 nm 31–33 . To follow the evolution in the excited state , a probe wavelength of either 800 or 900 nm was used.…”

“…To follow the evolution in the excited state , a probe wavelength of either 800 or 900 nm was used. At these wavelengths, subsequent S 1 → S n excitation may take place 33 causing fast statistical fragmentation of the chromophore ions after sequential absorption of two photons (pump and probe) and internal conversion to the S 0 ground state. The ground state is not re-excited at these probe wavelengths, and hence the fast two-photon fragmentation signal disappears as soon as the system has left the S 1 excited state and returned to S 0 (Fig.…”

“…A fast decay is observed when the 580 nm pump comes prior to the 800 nm probe (positive delay). Here the RPSB chromophore is first excited to S 1 by the pump pulse, and further excited to S 3 by the 800 nm probe pulse 33 . No fast, two-photon, action is observed when the pulses arrive in the reversed order (negative delay) due to the lack of ground-state excitation with 800 nm pulses.…”

“…This gives a signal at asymptotic negative as well as positive delay times. The S 0 → S 2 transition has a low oscillator strength, and the 400 nm probe pulse is absorbed from the S 1 state more efficiently than from S 0 after internal conversion 33 , resulting in the overall decay of the signal at positive delay times. Notably, when the RPSB is excited to S 2 by 400 nm excitation, it returns to the ground state in ca.…”