  One-dimensional (1D) linear nanostructures comprising sp-hybridized carbon
atoms, as derivatives of the prototypical allotrope known as carbyne, are
predicted to possess outstanding mechanical, thermal, and electronic
properties. Despite recent advances in the synthesis, their chemical and
physical properties are still poorly understood. Here, we investigate the
photophysics of a prototypical polyyne (i.e., 1D chain with alternating single
and triple carbon bonds), as the simplest model of finite carbon wire and as a
prototype of sp-carbon-based chains. We perform transient absorption
experiments with high temporal resolution (<30 fs) on monodispersed
hydrogen-capped hexayne H$-$(C$\equiv$C)$_6-$H synthesized by laser ablation in
liquid. With the support of detailed computational studies based on ground
state density functional theory (DFT) and excited state time-dependent (TD)-DFT
calculations, we provide a comprehensive description of the excited state
relaxation processes at early times following photoexcitation. We show that the
internal conversion from a bright high-energy singlet excited state to a
low-lying singlet dark state is ultrafast and takes place with a 200-fs time
constant, followed by thermalization on the picosecond timescale and decay of
the low-energy singlet state with hundreds of picoseconds time constant. We
also show that the timescale of these processes does not depend on the end
groups capping the sp-carbon chain. The understanding of the primary
photo-induced events in polyynes is of key importance both for fundamental
knowledge and for potential optoelectronic and light-harvesting applications of
low dimensional nanostructured carbon-based materials.
