Design Principles for Two-Dimensional Molecular Aggregates Using Kasha's Model: Tunable Photophysics in Near and Short-Wave Infrared

Technologies which utilize near-infrared (NIR) (700-1000 nm) and short-wave infrared (1000-2000 nm) electromagnetic radiation have applications in deep-tissue imaging, telecommunications, and satellite telemetry due to low scattering and decreased background signal in this spectral region. It is therefore necessary to develop materials that absorb light efficiently beyond 1000 nm. Transition dipole moment coupling (e.g., J-aggregation) allows for red-shifted excitonic states and provides a pathway to highly absorptive electronic states in the infrared. We present aggregates of two cyanine dyes whose absorption peaks red-shift dramatically upon aggregation in water from similar to 800 to 1000 nm and 1050 nm, respectively, with sheet-like morphologies and high molar absorptivities (epsilon approximate to 10(5) M-1 cm(-1)). We use Frenkel exciton theory to extend Kasha's model for J- and H-aggregations and describe the excitonic states of two-dimensional aggregates whose slip is controlled by steric hindrance in the assembled structure. A consequence of the increased dimensionality is the phenomenon of an intermediate I-aggregate, one which red-shifts yet displays spectral signatures of band-edge dark states akin to an H-aggregate. We distinguish between H-, I-, and J-aggregates by showing the relative position of the bright (absorptive) state within the density of states using temperature-dependent spectroscopy. I-aggregates hold potential for applications such as charge injection moieties for semiconductors and donors for energy transfer in NIR and short-wave infrared. Our results can be used to better design chromophores with predictable and tunable aggregation with new photophysical properties.