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Suppression of Kasha's rule as a mechanism for fluorescent molecular rotors and aggregation-induced emission

Nature Chemistry volume 9, pages 83–87 (2017)Cite this article

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Abstract

Although there are some proposed explanations for aggregation-induced emission, a phenomenon with applications that range from biosensors to organic light-emitting diodes, current understanding of the quantum-mechanical origin of this photophysical behaviour is limited. To address this issue, we assessed the emission properties of a series of BF2–hydrazone-based dyes as a function of solvent viscosity. These molecules turned out to be highly efficient fluorescent molecular rotors. This property, in addition to them being aggregation-induced emission luminogens, enabled us to probe deeper into their emission mechanism. Time-dependent density functional theory calculations and experimental results showed that the emission is not from the S1 state, as predicted from Kasha's rule, but from a higher energy (>S1) state. Furthermore, we found that suppression of internal conversion to the dark S1 state by restricting the rotor rotation enhances fluorescence, which leads to the proposal that suppression of Kasha's rule is the photophysical mechanism responsible for emission in both viscous solution and the solid state.

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Figure 1: The absorption (dashed line) and fluorescence (solid lines) spectra of compound 4 in mixtures of Gly and EG of increased solvent viscosity.
Figure 2: Ground (blue diamonds) and excited (shaded red diamonds) PBE TDDFT state energies of 1 as a function of rotor angle.
Figure 3: Excitation-dependent corrected emission spectra of compound 1 in dichloromethane.

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Acknowledgements

I.A. and M.D.L. acknowledge the support of the National Science Foundation (DMR-1506170 and DMR-1506248, respectively). A.W. acknowledges the support of the ACS Project SEED program.

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Authors and Affiliations

  1. Department of Chemistry, Dartmouth College, Hanover, 03755, New Hampshire, USA

    Hai Qian, Audrey Wakefield & Ivan Aprahamian

  2. Department of Chemistry, University of Vermont, Burlington, 05405, Vermont, USA

    Morgan E. Cousins, Erik H. Horak & Matthew D. Liptak

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Contributions

All the experiments were conducted by H.Q., M.E.C., E.H.H. and A.W. with input from I.A. and M.D.L. All the computational work was conducted by M.E.C. and E.H.H. with the supervision of M.D.L. The manuscript was written jointly by H.Q., I.A., M.E.C. and M.D.L.

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Correspondence to Matthew D. Liptak or Ivan Aprahamian.

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Crystallographic data for compound 2. (CIF 242 kb)

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Structure factors file for compound 2. (FCF 132 kb)

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Qian, H., Cousins, M., Horak, E. et al. Suppression of Kasha's rule as a mechanism for fluorescent molecular rotors and aggregation-induced emission. Nature Chem 9, 83–87 (2017). https://doi.org/10.1038/nchem.2612

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