Structural Dynamics of Overcrowded Alkene-Based Molecular Motors during Thermal Isomerization

Synthetic light-driven rotary molecular motors show complicated structural dynamics during the rotation process. A combination of DFT calculations and various spectroscopic techniques is employed to study the effect of the bridging group in the lower half of the molecule on the conformational dynamics. It was found that the extent to which the bridging group can accommodate the increased folding in the transition state is the main factor in rationalizing the differences in barrier height and, as a consequence, the rotary speed. These findings will be essential in designing future rotary molecular motors

Chemically Optimizing Operational Efficiency of Molecular Rotary Motors

Unidirectional molecular rotary motors that harness photoinduced cis–trans (E–Z) isomerization are promising tools for the conversion of light energy to mechanical motion in nanoscale molecular machines. Considerable progress has been made in optimizing the frequency of ground-state rotation, but less attention has been focused on excited-state processes. Here the excited-state dynamics of a molecular motor with electron donor and acceptor substituents located to modify the excited-state reaction coordinate, without altering its stereochemistry, are studied. The substituents are shown to modify the photochemical yield of the isomerization without altering the motor frequency. By combining 50 fs resolution time-resolved fluorescence with ultrafast transient absorption spectroscopy the underlying excited-state dynamics are characterized. The Franck–Condon excited state relaxes in a few hundred femtoseconds to populate a lower energy dark state by a pathway that utilizes a volume conserving structural change. This is assigned to pyramidalization at a carbon atom of the isomerizing bridging double bond. The structure and energy of the dark state thus reached are a function of the substituent, with electron-withdrawing groups yielding a lower energy longer lived dark state. The dark state is coupled to the Franck–Condon state and decays on a picosecond time scale via a coordinate that is sensitive to solvent friction, such as rotation about the bridging bond. Neither subpicosecond nor picosecond dynamics are sensitive to solvent polarity, suggesting that intramolecular charge transfer and solvation are not key driving forces for the rate of the reaction. Instead steric factors and medium friction determine the reaction pathway, with the sterically remote substitution primarily influencing the energetics. Thus, these data indicate a chemical method of optimizing the efficiency of operation of these molecular motors without modifying their overall rotational frequency

Cooperative Enhancement of Two-Photon Absorption in Self-Assembled Zinc-Porphyrin Nanostructures

Femtosecond two-photon absorption (2PA) spectra were measured of a series of butadiyne-linked zinc-porphyrin oligomers assembled in single- and double-strand linear and cyclic structures containing 6, 12, or 24 porphyrin units, in the excitation wavelength range 900–1600 nm. We observe strong enhancement of the 2PA cross-section in the Soret region, with maximum values up to σ2PA ∼ 105 GM depending on the geometry of the construct, which we quantitatively explain in terms of a three essential energy level model and cooperative enhancement of the transition dipole moments between the states. The Zn-coordinated template-bound ring of 6 porphyrin units, which resembles some natural light-harvesting systems, results in the highest 2PA cross-section per porphyrin unit (∼104 GM). It is remarkable that this molecule gives such strong σ2PA, when the S0–S1 (0–0) transition is essentially forbidden for one photon transition

Driving Unidirectional Molecular Rotary Motors with Visible Light by Intra- And Intermolecular Energy Transfer from Palladium Porphyrin

Driving molecular rotary motors using visible light (530–550 nm) instead of UV light was achieved using palladium tetraphenylporphyrin as a triplet sensitizer. Visible light driven rotation was confirmed by UV/vis absorption, circular dichroism and ¹H NMR spectroscopy and the rotation was confirmed to be unidirectional and with similar photostationary states, despite proceeding via a triplet instead of a singlet excited state of the molecular motor. Energy transfer proceeds in both inter- and intramolecular fashion from the triplet state of the porphyrin to the motor. Stern Volmer plots show that the rate of intermolecular quenching of the porphyrin excited state by the molecular motor is diffusion-controlled

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