Trans-cis molecular photoswitching in interstellar Space

TL;DR

This study reports the first detection of cis-HCOOH in interstellar space, demonstrating UV-induced photoswitching between trans and cis conformers, supported by quantum calculations, revealing a new mechanism affecting molecular structures in space.

Contribution

It introduces the first observational evidence of UV-driven conformational change in interstellar molecules, expanding understanding of molecular dynamics under space conditions.

Findings

Detection of cis-HCOOH in the Orion Bar region

Trans-to-cis abundance ratio of 2.8+-1.0

UV irradiation induces conformational photoswitching

Abstract

As many organic molecules, formic acid (HCOOH) has two conformers (trans and cis). The energy barrier to internal conversion from trans to cis is much higher than the thermal energy available in molecular clouds. Thus, only the most stable conformer (trans) is expected to exist in detectable amounts. We report the first interstellar detection of cis-HCOOH. Its presence in ultraviolet (UV) irradiated gas exclusively (the Orion Bar photodissociation region), with a low trans-to-cis abundance ratio of 2.8+-1.0, supports a photoswitching mechanism: a given conformer absorbs a stellar photon that radiatively excites the molecule to electronic states above the interconversion barrier. Subsequent fluorescent decay leaves the molecule in a different conformer form. This mechanism, which we specifically study with ab initio quantum calculations, was not considered in Space before but likely induces structural changes of a variety of interstellar molecules submitted to UV radiation.