A rigidly bridged squaraine dimer serves as a model compound to study exciton interactions between two chromophores without interfering with conformational or other stereochemical isomers. We describe the synthesis as well as steady state and fs- and ps-time-resolved optical spectroscopic data. The spectra are interpreted using a vibronic coupling model, which considers a single vibrational mode that produces a shallow excited state surface with two minima. These two minima cause symmetry breaking of the excited state, which leads to a partial localization of excitation. The localization of the wave function causes a reduced fluorescence transition moment, although both the absorption and the emission spectra display exchange narrowing typical of excitonically coupled chromophores.

The steady-state and photoinduced dynamical optical properties of two squaraine-bodipy dye conjugates are the focus of this work. While the squared absorption transition moments of the dye conjugates can be traced back in an additive way to the constituents of the conjugates, this is not possible for the squared fluorescence transition moments. We suggest an enhancement of electronic coupling in the relaxed excited state to be responsible for this observation. Transient absorption and fluorescence upconversion experiments with femtosecond-time resolution give insight into the relaxation phenomena of the dye conjugates, in particular concerning the relaxation within the exciton manifold.

The photophysics of a molecular triad consisting of a BODIPY dye and two pyrene chromophores attached in 2-position are investigated by steady state and fs-time resolved transient absorption spectroscopy as well as by field induced surface hopping (FISH) simulations. While the steady state measurements indicate moderate chromophore interactions within the triad, the time resolved measurements show upon pyrene excitation a delocalised excited state which localises onto the BODIPY chromophore with a time constant of 0.12 ps. This could either be interpreted as an internal conversion process within the excitonically coupled chromophores or as an energy transfer from the pyrenes to the BODIPY dye. The analysis of FISH-trajectories reveals an oscillatory behaviour where the excitation hops between the pyrene units and the BODIPY dye several times until finally they become localised on the BODIPY chromophore within 100 fs. This is accompanied by an ultrafast nonradiative relaxation within the excitonic manifold mediated by the nonadiabatic coupling. Averaging over an ensemble of trajectories allowed us to simulate the electronic state population dynamics and determine the time constants for the nonradiative transitions that mediate the ultrafast energy transfer and exciton localisation on BODIPY.