  Complex organic molecules are ubiquitous companions of young low-mass
protostars. Recent observations suggest that their emission stems, not only
from the traditional hot corino, but also from offset positions. In this work,
2D physicochemical modelling of an envelope-cavity system is carried out.
Wavelength-dependent radiative transfer calculations are performed and a
comprehensive gas-grain chemical network is used to simulate the physical and
chemical structure. The morphology of the system delineates three distinct
regions: the cavity wall layer with time-dependent and species-variant
enhancements; a torus rich in complex organic ices, but not reflected in
gas-phase abundances; and the remaining outer envelope abundant in simpler
solid and gaseous molecules. Strongly irradiated regions, such as the cavity
wall layer, are subject to frequent photodissociation in the solid phase.
Subsequent recombination of the photoproducts leads to frequent reactive
desorption, causing gas-phase enhancements of several orders of magnitude. This
mechanism remains to be quantified with laboratory experiments. Direct
photodesorption is found to be relatively inefficient. If radicals are not
produced directly in the icy mantle, the formation of complex organics is
impeded. For efficiency, a sufficient number of FUV photons needs to penetrate
the envelope; and elevated cool dust temperatures need to enable grain-surface
radical mobility. As a result, a high stellar luminosity and a sufficiently
wide cavity favor chemical complexity. Furthermore within this paradigm,
complex organics are demonstrated to have unique lifetimes and be grouped into
early (formaldehyde, ketene, methanol, formic acid, methyl formate, acetic
acid, glycolaldehyde) and late (acetaldehyde, dimethyl ether, ethanol) species.
