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

preprint
revised on 17.05.2019 and posted on 20.05.2019 by Arundhati Deshmukh, Danielle Koppel, Chern Chuang, Danielle Cadena, Jianshu Cao, Justin Caram
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 ~ 105 M-1cm-1). 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.

Funding

Clare Booth Luce Fellowship

UCLA faculty research grant

History

Email Address of Submitting Author

arundhatipd@ucla.edu

Institution

University of California, Los Angeles

Country

United States

ORCID For Submitting Author

0000-0002-7901-8814

Declaration of Conflict of Interest

Authors declare no conflict of interest.

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