In self-assembled molecular aggregates, transition dipole moments (TDM) of individual molecules couple over long distances (upto micron scale) forming delocalized excitons. Depending on the arrangement of the molecules, the excitonic transitions maybe red or blue shifted from the monomer (J-/ H-aggregates). Non-linearly enhanced oscillator strengths and radiative rates are facilitated by long-range TDM coupling. Therefore, molecular aggregation can be utilized as a new avenue for tuning excitonic transitions and energy transport. We prepare the aggregates of thiacarbocyanine dyes in methanol-water mixtures where higher volume fractions of water yield aggregates with red-shifted transitions in shortwave infrared (SWIR, 1-2 microns) and high molar absorptivities (~105 M-1cm-1). Strong absorption in SWIR is particularly desirable for deep-tissue imaging and telecommunications. Cryo-electron microscopy reveals the morphology of SWIR aggregates as 2-dimensional (2D) sheets. We construct a model based on brick arrangement of TDMs showing direct relationship between the nanoscale packing and observed photophysics and thus, develop general design principles for 2D aggregates analogous to Kasha’s model for dimers and linear chains. A unique consequence of the 2D morphology is that the bright state is not necessarily restricted to the band-edge, giving rise to a new aggregate type, intermediate or I-aggregate, which has a red-shifted absorption (similar to J-aggregates) but has dark states lying below the bright state (similar to H-aggregates). Using temperature dependent absorption, we experimentally confirm that the sheets are indeed I-aggregates. These aggregates pave the way for novel photophysical applications such as SWIR energy transfer via near-field coupling and charge-injection into semiconductor moieties.

Design principles for near-infrared and shortwave infrared molecular aggregates of cyanine dyes

2019 Fall National Meeting

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