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

Abstract

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 ∼800 to 1000 nm and 1050 nm, respectively, with sheet-like morphologies and high molar absorptivities (ε ≈ 10 5 M −1 cm −1 ). We use Frenkel exciton theory to extend Kasha's model for Jand 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.

Assembly Induced Super-Large Red-Shifted Absorption: The Burgeoning Field of Organic Near-Infrared Materials

“…The effect of in-plane slide on the absorption was also manifested by a cyanine derivative (3,3′-bis(4-sulfobutyl)-5,5′-dichloro-11-diphe-nylamino-10,12-ethylenethiatricarbocyanine, ammonium salt, Cy7-DPA) dye, as investigated by Caram and co-workers. 17 The Cy7-DPA monomers had intrinsic absorption at 807 nm. In comparison, their self-assembled two-dimensional aggregates showed super-large red-shifted absorption lying at ∼1050 nm.…”

Generalized Kasha’s Model: T-Dependent Spectroscopy Reveals Short-Range Structures of 2D Excitonic Systems

“…Second, in order to rapidly characterize the structural parameters of a given sample, we need to establish spectroscopic signatures that provide unambiguous constraints on structural models. While these challenges have been the subject of numerous recent papers, 4,6,14- 23 Kasha's early work on molecular spectroscopy remains the standard for much of the field. 24, 25 Here, we…”

Electron‐Withdrawing Substituted Quinazoline Push‐Pull Chromophores: Synthesis, Electrochemical, Photophysical and Second‐Order Nonlinear Optical Properties

“…[14c] Compound 1a exhibits the highest m parameter and compounds 1 show higher m value than the corresponding compounds 2 indicating higher ICT into cyano derivatives 1 than into trifluoromethyl analogues. For compounds 3 , the fluorosolvatochromism behavior is different as shown on Figure (part B): in solvents with lower polarity ( n ‐heptane, toluene for all compounds as well as 1,4‐dioxane and THF in case of 3c ) the formation of J‐aggregate leading to an emission band in the 550–700 nm range is observed . In higher polarity solvents, compounds 3 exhibit classical positive fluorosolvatochromism.…”