Photophysics of Threaded sp-Carbon Chains: The Polyyne is a Sink for Singlet and Triplet Excitation

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 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

Insights into Magneto-Optics of Helical Conjugated Polymers

Materials with magneto-optic (MO) properties have enabled critical fiber-optic applications and highly sensitive magnetic field sensors. While traditional MO materials are inorganic in nature, new generations of MO materials based on organic semiconducting polymers could allow increased versatility for device architectures, manufacturing options, and flexible mechanics. However, the origin of MO activity in semiconducting polymers is far from understood. In this paper, we report high MO activity observed in a chiral helical poly-3-(alkylsulfone)­thiophene (<b>P3AST</b>), which confirms a new design for the creation of a giant Faraday effect with Verdet constants up to (7.63 ± 0.78) × 10⁴ deg T^–1 m^–1 at 532 nm. We have determined that the sign of the Verdet constant and its magnitude are related to the helicity of the polymer at the measured wavelength. The Faraday rotation and the helical conformation of <b>P3AST</b> are modulated by thermal annealing, which is further supported by DFT calculations and MD simulations. Our results demonstrate that helical polymers exhibit enhanced Verdet constants and expand the previous design space for polythiophene MO materials that was thought to be limited to highly regular lamellar structures. The structure–property studies herein provide insights for the design of next-generation MO materials based upon semiconducting organic polymers

Electronic Delocalization in the Radical Cations of Porphyrin Oligomer Molecular Wires

The radical cations of a family of π-conjugated porphyrin arrays have been investigated: linear chains of <i>N</i> = 1–6 porphyrins, a 6-porphyrin nanoring and a 12-porphyrin nanotube. The radical cations were generated in solution by chemical and electrochemical oxidation, and probed by vis–NIR–IR and EPR spectroscopies. The cations exhibit strong NIR bands at ∼1000 nm and 2000–5000 nm, which shift to longer wavelength with increasing oligomer length. Analysis of the NIR and IR spectra indicates that the polaron is delocalized over 2–3 porphyrin units in the linear oligomers. Some of the IR vibrational bands are strongly intensified on oxidation, and Fano-type antiresonances are observed when activated vibrations overlap with electronic transitions. The solution-phase EPR spectra of the radical cations have Gaussian lineshapes with linewidths proportional to <i>N</i>^–0.5, demonstrating that at room temperature the spin hops rapidly over the whole chain on the time scale of the hyperfine coupling (ca. 100 ns). Direct measurement of the hyperfine couplings through electron–nuclear double resonance (ENDOR) in frozen solution (80 K) indicates distribution of the spin over 2–3 porphyrin units for all the oligomers, except the 12-porphyrin nanotube, in which the spin is spread over about 4–6 porphyrins. These experimental studies of linear and cyclic cations give a consistent picture, which is supported by DFT calculations and multiparabolic modeling with a reorganization energy of 1400–2000 cm^–1 and coupling of 2000 cm^–1 for charge transfer between neighboring sites, placing the system in the Robin–Day class III

Electronic Delocalization in the Radical Cations of Porphyrin Oligomer Molecular Wires

The radical cations of a family of π-conjugated porphyrin arrays have been investigated: linear chains of <i>N</i> = 1–6 porphyrins, a 6-porphyrin nanoring and a 12-porphyrin nanotube. The radical cations were generated in solution by chemical and electrochemical oxidation, and probed by vis–NIR–IR and EPR spectroscopies. The cations exhibit strong NIR bands at ∼1000 nm and 2000–5000 nm, which shift to longer wavelength with increasing oligomer length. Analysis of the NIR and IR spectra indicates that the polaron is delocalized over 2–3 porphyrin units in the linear oligomers. Some of the IR vibrational bands are strongly intensified on oxidation, and Fano-type antiresonances are observed when activated vibrations overlap with electronic transitions. The solution-phase EPR spectra of the radical cations have Gaussian lineshapes with linewidths proportional to <i>N</i>^–0.5, demonstrating that at room temperature the spin hops rapidly over the whole chain on the time scale of the hyperfine coupling (ca. 100 ns). Direct measurement of the hyperfine couplings through electron–nuclear double resonance (ENDOR) in frozen solution (80 K) indicates distribution of the spin over 2–3 porphyrin units for all the oligomers, except the 12-porphyrin nanotube, in which the spin is spread over about 4–6 porphyrins. These experimental studies of linear and cyclic cations give a consistent picture, which is supported by DFT calculations and multiparabolic modeling with a reorganization energy of 1400–2000 cm^–1 and coupling of 2000 cm^–1 for charge transfer between neighboring sites, placing the system in the Robin–Day class III