  Recently, graphene triangulene chains have been synthesized and their
magnetic response has been analyzed by STM methods by Mishra and coworkers
(Nature 598, 287 (2021)). Motivated by this study, we determine the exchange
bilinear and biquadratic constants of the triangulene chains by calculating
two-spin rotations in the spirit of the magnetic force theorem. We then analyze
open-ended, odd-numbered chains, whose edge states pair up forming a triplet
ground state. We propose three experimental approaches that enable us to
trigger and control a singlet-triplet spin transition. Two of these methods are
based on applying a mechanical distortion to the chain. We finally show that
the transition can be controlled efficiently by the application of an electric
field.
