Design Principles for 2-Dimensional Molecular Aggregates using Kasha’s Model: Tunable Photophysics in Near and Shortwave Infrared

Abstract

Technologies which utilize near-infrared (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 redshifted excitonic states and provides a pathway to highly absorptive electronic states in the infrared. We present aggregates of two cyanine dyes whose absorption peaks redshift dramatically upon aggregation in water from ~800 nm to 1000 nm and 1050 nm respectively with sheet-like morphologies and high molar absorptivities (e ~ 10<sup>5 </sup>M<sup>-1</sup>cm<sup>-1</sup>). We use Frenkel exciton theory to extend Kasha’s model for J and H aggregation and describe the excitonic states of 2-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 redshifts 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 as charge injection moieties for semiconductors and donors for energy transfer in NIR and SWIR. Our results can be used to better design chromophores with predictable and tunable aggregation with new photophysical properties.

Designing Highly Luminescent Molecular Aggregates via Bottom-Up Nanoscale Engineering

“…2D molecular aggregates can host H-, I-, and J-type excitonic coupling between their constituent molecules. 20,40,41,46 In both H-and I-type excitonic coupling, fluorescence is almost completely quenched, whereas J-type excitonic coupling leads to a radiative rate enhancement and an increased QY. 27 Therefore, it is necessary to tune the structures of the monomers to favor the formation of highly emissive J-aggregates over I-or H-aggregates.…”

“…This type of absorption and emission behavior suggests J-type excitonic coupling, which results in temperature dependent linewidths. 20,46 There are homogenous and inhomogenous contributions to the linewidth, where the former contribution arises due to the exciton's interaction with the environment at timescales faster than the lifetime of the exciton.…”

“…17 Many J-Aggregates emitting in the visible and NIR have QYs below 10%. [18][19][20] The low QY of J-Aggregates has been attributed to large homogenous and inhomogenous disorder, 12,21 charge-transfer states, 12 vibronic coupling and self-trapped excitons. [22][23][24] The type of nonradiative pathways dominant in a J-aggregate is different for different classes of chromophores.…”

Designing Highly Luminescent Molecular Aggregates via Bottom-Up Nanoscale Engineering

“…(2) spontaneous emission of phonons and relaxation to the band edge. 20 The stimulated absorption/emission of phonons depends on the thermal occupation of phonons, and therefore, it is temperature dependent. In contrast, the spontaneous emission of phonons and relaxation to the band edge depends on the available density of states below the band edge, and is temperature independent.…”

“…17 Many J-Aggregates emitting in the visible and NIR have QYs below 10%. [18][19][20] The low QY of J-Aggregates has been attributed to large homogenous and inhomogenous disorder, 12,21 charge-transfer states, 12 vibronic coupling and self-trapped excitons. [22][23][24] The type of nonradiative pathways dominant in a J-aggregate is different for different classes of chromophores.…”