Exciton Delocalization Counteracts the Energy Gap: A New Pathway toward NIR-Emissive Dyes.

Alexei Cravcenco,Karl Börjesson,Fredrik Edhborg,Zoltan Takacs,Yizhou Yang,Bo Albinsson,Yi Yu,Jonas F Goebel

doi:10.1021/jacs.1c10654

Abstract

Exciton coupling between the transition dipole moments of ordered dyes in supramolecular assemblies, so-called J/H-aggregates, leads to shifted electronic transitions. This can lower the excited state energy, allowing for emission well into the near-infrared regime. However, as we show here, it is not only the excited state energy modifications that J-aggregates can provide. A bay-alkylated quaterrylene was synthesized, which was found to form J-aggregates in 1,1,2,2-tetrachloroethane. A combination of superradiance and a decreased nonradiative relaxation rate made the J-aggregate four times more emissive than the monomeric counterpart. A reduced nonradiative relaxation rate is a nonintuitive consequence following the 180 nm (3300 cm–1) red-shift of the J-aggregate in comparison to the monomeric absorption. However, the energy gap law, which is commonly invoked to rationalize increased nonradiative relaxation rates with increasing emission wavelength, also contains a reorganization energy term. The reorganization energy is highly suppressed in J-aggregates due to exciton delocalization, and the framework of the energy gap law could therefore reproduce our experimental observations. J-Aggregates can thus circumvent the common belief that lowering the excited state energies results in large nonradiative relaxation rates and are thus a pathway toward highly emissive organic dyes in the NIR regime.

Highlights

Near-infrared (NIR) emissive organic dyes are of importance in applications such as organic light-emitting diodes (OLEDs),[1] bioimaging[2] photodetection,[3] and solar energy harvesting.[4]
A 40% decrease of the nonradiative rate constant was observed, which in combination with the increased radiative rate resulted in a 4-fold increase in the emission quantum yield, this despite a 180 nm red-shift of the absorbance maximum
The key for explaining this unusual observation was found in the delocalized nature of the excited state of the J-aggregate

Summary

■ INTRODUCTION

Near-infrared (NIR) emissive organic dyes are of importance in applications such as organic light-emitting diodes (OLEDs),[1] bioimaging[2] photodetection,[3] and solar energy harvesting.[4]. Englman and Jortner, inspired by the golden rule treatment of nonradiative decay, derived the rate of relaxation from an electronically excited state to an isoenergetic high-energy vibrational mode of a lower electronic state (of any spin multiplicity, but we are only considering singlet states in the analysis).[56,57] When the relative displacement between the two potential energy surfaces is small, the system is in the weak coupling limit (note, the coupling referred to here is between the ground and excited state) This is the regime for both the monomer and J-aggregate, as half the Stokes shift is considerable smaller than the energy of the promoting vibrational mode (365/2 vs 1404 cm−1 for the monomer). The exciton is most likely delocalized over the whole aggregate at optical spectroscopy concentrations

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES