Squaraine Appended with Benzodipyrrole for Fluorescent Sensing of Methanol: Exciton Coupling Controls Photophysical Properties

The photophysical properties of dyes composed of two squaraine chromophores fused with a benzodipyrrole central moiety (BS1 and BS2), were investigated using steady‐state absorption, fluorescence, and transient absorption spectroscopy. The dyes exhibit solvent‐independent split electronic absorption due to exciton‐coupling. Interestingly significant solvent‐dependent fluorescence properties were observed. In toluene, they emit from the lowest excited state, while in methanol, they show weak emission in the higher energy region. In the low‐temperature glass matrix, emission from the lowest excited state dominates similarly to that in toluene. The transient absorption spectra exhibit similar ground‐state bleaching in toluene and methanol, revealing the formation of delocalized excited states by exciton coupling independent of solvent. However, the excited state deactivates rapidly in ultrafast time scale in methanol, likely due to solvent interaction, leading to rapid non‐radiative deactivation. The PEG film doped with the exciton‐coupled bis‐squaraine shows a distinct fluorescence response to methanol vapor. The intramolecular exciton‐coupled bis‐squaraine dyes emit low energy in toluene or methanol at 77 K and high energy, deviating from Kasha's rule, in methanol at 273 K. These distinct emissions enable methanol vapor sensing via fluorescence by delocalized and localized excited states.