Photophysicsof Threaded sp-Carbon Chains: The Polyyneis a Sink for Singlet and Triplet Excitation

摘要

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)3Cl 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 ³MLCT state to the hexayne (τ = 1.5 ns). This study revealed detailed information on theshort-lived higher excited state of the hexayne (lifetime ∼1ps) and on structural reorganization and cooling of hot polyyne chains,following internal conversion (over ∼5 ps). Comparison of theobserved IR bands of the excited states of the hexayne with resultsfrom time-dependent density functional calculations (TD DFT) showsthat these excited states have high cumulenic character (low bondlength alternation) around the central region of the chain. Thesefindings shed light on the complex interactions between the componentsof this supramolecular rotaxane and are important for the developmentof materials for the emerging molecular and nanoscale electronics.