Knowledge of the factors controlling excited state dynamics in excitonically coupled dimers and higher aggregates is critical for understanding natural and artificial solar energy conversion. In this work, we report ultrafast solvent polarity dependent excited state dynamics of the structurally well‐defined subphthalocyanine dimer, μ‐OSubPc2. Stationary electronic spectra demonstrate strong exciton coupling in μ‐OSubPc2. Femtosecond transient absorption measurements reveal ultrafast excimer formation from the initially excited exciton, mediated by intramolecular structural evolution. In polar solvents the excimer state decays directly through symmetry breaking charge transfer to form a charge separated state. Charge separation occurs under control of solvent orientational relaxation.

Efficient photomolecular motors will be critical elements in the design and development of molecular machines. Optimisation of the quantum yield for photoisomerisation requires a detailed understanding of molecular dynamics in the excited electronic state. Here we probe the primary photophysical processes in the archetypal first generation photomolecular motor, with sub‐50 fs time resolved fluorescence spectroscopy. A bimodal relaxation is observed with a 100 fs relaxation of the Franck‐Condon state to populate a red‐shifted state with a reduced transition moment, which then undergoes multi‐exponential decay on a picosecond timescale. Oscillations due to the excitation of vibrational coherences in the S1 state are seen to survive the ultrafast structural relaxation. The picosecond relaxation reveals a strong solvent friction effect which is thus ascribed to torsion about the C−C axle. This behaviour is contrasted with second generation photomolecular motors; the principal differences are explained by the existence of a barrier on the excited state surface in the case of the first‐generation motors which is absent in the second generation. These results will help to provide a basis for designing more efficient molecular motors in the future.

Molecular structure designs inspired by naturally occurring light harvesting systems have been intensely pursued over the last couple of decades. Interesting new structures include the π-conjugated porphyrin nanorings, which show promising features such as ultrafast excitedstate delocalization, leading to suppressed radiative rates, superradiance with increasing temperature and energy transfer times comparable to their natural counterparts. An important question to be addressed in such systems is the role and time scale of structural motions and how they affect excited-state delocalization. Here it is shown that porphyrin nanorings which are not rigidified by a template are structurally heterogeneous in the ground state and evolve dynamically on a tens of picoseconds timescale. In the excited state, there is a reduction of the torsion angle between neighboring porphyrin units, on a timescale of hundreds of femtoseconds. Furthermore ultrafast excitation delocalization is observed, by anisotropy measurements, being insensitive to structural motions of the nanorings.