Influence of Collective Excitonic Effects on the Photophysics of Flexibly- and Rigidly-Bridged Squaraine Dimers Targeted for DNA-Based Applications

Squaraines are conjugated organic molecules that exhibit advantageous optical properties in the visible and near-infrared spectral regions; as such, they are featured in a variety of applications. Specifically, squaraines offer improved performance in solar cells, nonlinear optics, bioimaging, organic photodiodes, and emerging applications involving deoxyribonucleic acid (DNA) self-assembly. To improve their optical properties (and ultimately performance) further via collective excitonic effects, we synthesized three squaraine dimers featuring either a rigid or a flexible bridge. We characterized their optical properties and excited-state dynamics in chloroform and methanol via steady-state and time-resolved optical spectroscopies. Compared to the corresponding monomers, we found that both rigidly- and flexibly-bridged squaraine dimers exhibit signatures of collective excitonic effects─that is, red-shifted absorption, band narrowing, small Stokes shift, and superradiance─in both polar and nonpolar solvents. We also found that while rigidly-bridged dimers featured all of these properties, the flexibly-bridged dimers did not feature significant band narrowing and a small Stokes shift. Interestingly, the fluorescence quantum yields and singlet excited-state lifetimes of both monomers and dimers decreased in the polar solvent. As a first step toward the assembly of a functional dimer aggregate network, we covalently tethered the flexibly-bridged dimer and its corresponding monomer to single- and double-stranded DNA and characterized their photophysical properties in an aqueous buffer solution. We observed that the optical properties of the monomer served as a sensitive probe of the local polarity of the environment. In the case of the dimer, we observed a large distribution of conformations via absorption spectroscopy and sampled a subset of the distribution via fluorescence spectroscopy. Our work is an important first step toward implementing molecular aggregates preprogrammed to exhibit desired collective excitonic effects in DNA-assembled networks.