Ultrafast valence to non-valence excited state dynamics in a common anionic chromophore

James N Bull,Cate S Anstöter,Jan R R Verlet

doi:10.1038/s41467-019-13819-6

Abstract

Non-valence states in neutral molecules (Rydberg states) have well-established roles and importance in photochemistry, however, considerably less is known about the role of non-valence states in photo-induced processes in anions. Here, femtosecond time-resolved photoelectron imaging is used to show that photoexcitation of the S1(ππ*) state of the methyl ester of deprotonated para-coumaric acid – a model chromophore for photoactive yellow protein (PYP) – leads to a bifurcation of the excited state wavepacket. One part remains on the S1(ππ*) state forming a twisted intermediate, whilst a second part leads to the formation of a non-valence (dipole-bound) state. Both populations eventually decay independently by vibrational autodetachment. Valence-to-non-valence internal conversion has hitherto not been observed in the intramolecular photophysics of an isolated anion, raising questions into how common such processes might be, given that many anionic chromophores have bright valence states near the detachment threshold.

Highlights

Non-valence states in neutral molecules (Rydberg states) have well-established roles and importance in photochemistry, considerably less is known about the role of nonvalence states in photo-induced processes in anions
One of the most studied biochromophores is that found in the photoactive yellow protein (PYP)[3], where an anionic thioester derivative (Fig. 1a) of the E-isomer of paracoumaric acid undergoes an ultrafast E → Z isomerisation that triggers a conformational change in the protein and leads to a phototaxis response of the host bacterium[4,5,6,7,8,9]
Photoelectron spectroscopy studies[18,19,20,21], mostly considering deprotonated para-coumaric acid and ester derivatives. These studies utilised light sources with nanosecond pulse duration that are not suitable for characterising the ultrafast excited state dynamics that define the photophysics of the PYP chromophore

Summary

Introduction

Non-valence states in neutral molecules (Rydberg states) have well-established roles and importance in photochemistry, considerably less is known about the role of nonvalence states in photo-induced processes in anions. The consequence of this near degeneracy is that electron loss by autodetachment can compete with excited state nuclear dynamics (isomerisation) Such a competition was evident in the only previous time-resolved study on an isolated deprotonated para-coumaric acid derivative containing the methyl ketone functional group[22]. (ππ*) potential energy surface, including the nature of twisted intermediates and barriers to reach isomerising conical intersection seams[8,9,37,38,39,40,41,42,43,44,45,46] This state of affairs highlights the need for unambiguous measurements of the gas-phase excited state dynamics on model PYP chromophores. Photodetachment action, photoelectron and timeresolved photoelectron spectroscopy are used to probe the deprotonated methyl ester of para-coumaric acid, pCEs−

Methods

Results

Conclusion