Photophysics
of Threaded sp-Carbon Chains: The Polyyne
is a Sink for Singlet and Triplet Excitation
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Abstract
We
have used single-crystal X-ray diffraction and time-resolved
UV–NIR–IR absorption spectroscopy to gain insights into
the structures and excited-state dynamics of a rotaxane consisting
of a hexayne chain threaded through a phenanthroline macrocycle and
a family of related compounds, including the rhenium(I) chlorocarbonyl
complex of this rotaxane. The hexayne unit in the rhenium-rotaxane
is severely nonlinear; it is bent into an arc with an angle of 155.6(1)°
between the terminal C1 and C12 atoms and the centroid of the central
C–C bond, with the most acute distortion at the point where
the polyyne chain pushes against the Re(CO)<sub>3</sub>Cl unit. There
are strong through-space excited-state interactions between the components
of the rotaxanes. In the metal-free rotaxane, there is rapid singlet
excitation energy transfer (EET) from the macrocycle to the hexayne
(τ = 3.0 ps), whereas in the rhenium-rotaxane there is triplet
EET, from the macrocycle complex <sup>3</sup>MLCT state to the hexayne
(τ = 1.5 ns). This study revealed detailed information on the
short-lived higher excited state of the hexayne (lifetime ∼1
ps) and on structural reorganization and cooling of hot polyyne chains,
following internal conversion (over ∼5 ps). Comparison of the
observed IR bands of the excited states of the hexayne with results
from time-dependent density functional calculations (TD DFT) shows
that these excited states have high cumulenic character (low bond
length alternation) around the central region of the chain. These
findings shed light on the complex interactions between the components
of this supramolecular rotaxane and are important for the development
of materials for the emerging molecular and nanoscale electronics
