Coupled Oscillators for Tuning Fluorescence Properties of Squaraine Dyes

Abstract

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.

Novel pyran based dyes for application in dye sensitized solar cells

“…furthermore two main transitions occur in the same spectral region, this is the consequence of the heterotrimer structure of the dyes that allow for exciton coupling, as previously reported [50]. …”

Exciton Transport in Molecular Aggregates – From Natural Antennas to Synthetic Chromophore Systems

“…Likewise, oligomers also incorporating other dyes such as BODIPYs help for a better understanding of exciton coupling effects in heterosystems. [236] Besides that, squaraine oligomers with cyclic or branched structures were synthesized and their optical properties were studied ( Figure 17). The cyclic trimer c[SQB] 3 derived from bent SQB and an analogous tetramer display unusual absorption and fluorescence spectra that may only be understood if structural distortions of the excited states (symmetry breaking) and vibronic coupling is taken into account.…”

Energy Transfer Between Squaraine Polymer Sections: From Helix to Zigzag and All the Way Back

“…[72][73][74][75][76][77][78][79][80] Unlike conjugated polymers that are based on very small monomers such as styrene (e.g., MEH-PPV) or thiophene (e.g., P3HT) and whose polymer properties are totally different from those of the monomers, 81 squaraine homo-and copolymers are based on squaraine dyes which already show a strong absorption in the red region of the visible spectrum. [32][33][34][35][36]82 The optical properties of the squaraine polymer, although distinct from those of the monomeric dye, can be explained by exciton coupling of localized squaraine chromophore transition moments which in general leads to broadened and red-shifted spectra reflecting the excitonic manifold of states.…”