Coupled Oscillators for Tuning Fluorescence Properties of Squaraine Dyes

Combining a squaraine (S) and a BODIPY (B) chromophore in a heterodimer (SB) and two heterotrimers (BSB and SBS) by alkyne bridges leads to the formation of coupled oscillators whose fluorescence properties are superior compared to the parent squaraine chromophore. The lowest energy absorption and emission properties of these superchromophores are mainly governed by the squaraine part and are shifted by more than 1000 cm^–1 to the red by excitonic interaction between the squaraine and the BODIPY dye. Employing polarization-dependent transient absorption and fluorescence upconversion measurements, we could prove that the lowest energy absorption in SB and BSB is caused by a single excitonic state but by two for SBS. Despite the spectral red-shift of their lowest absorption band, the fluorescence quantum yields increase for SB and BSB compared to the parent squaraine chromophore SQA. This is caused by intensity borrowing from the BODIPY states, which increases the squared transition moments of the lowest energy band dramatically by 29% for SB and 63% for BSB compared to SQA. Thereby, exciton coupling leads to a substantial enhancement of fluorescence quantum yield by 26% for SB and by 46% for BSB and shifts the emission from the red into the near-infrared. In this way, the BODIPY-squaraine conjugates combine the best properties of each class of dye. Thus, exciton coupling in heterodimers and -trimers is a valuable alternative to tuning fluorescence properties by, e.g., attaching substituents to chromophores

Exciton Dynamics from Strong to Weak Coupling Limit Illustrated on a Series of Squaraine Dimers

We present a joint theoretical and experimental study on the light-induced exciton relaxation dynamics in a series of three squaraine dimers spanning the range from weak to intermediate to strong excitonic coupling strength regime. As revealed by transient-absorption spectroscopy and mixed quantum-classical dynamics simulations that explicitly take into account excitation by the laser pulse, three different types of exciton dynamics could be observed, although the investigated systems exhibit very similar spectral features. While in the strongly coupled system (Frenkel limit), the exciton remains delocalized over both dye monomers, in the system with intermediate coupling, transient localization–delocalization on a femtosecond time scale can be observed. Finally, in the weakly coupled heterodimer (Förster limit), efficient exciton transfer, mediated by transient delocalization that correlates with a strong nonadiabatic coupling, takes place. By delivering the first systematic microscopic study on different regimes of exciton transfer, our findings shed new light on the possible mechanisms of energy transport in organic molecular excitonic materials