Construction of D-π-A Emitters with High Molar Extinction Coefficient via Two-state Resonance

Abstract

The molar extinction coefficient (ε) is a fundamental physical parameter for optoelectronic molecules, critically determining the energy conversion efficiencies of corresponding devices. Benefiting from strong intramolecular resonance enabled by donor-acceptor (D-A) structural modulation, merocyanine dyes are capable of delivering large ε at relatively low molecular weights. Such resonance originates from the equilibrium between neutral and zwitterionic resonance forms, whose magnitude can be quantitatively evaluated using the resonance coefficient c². Specifically, a c² of 0.5 corresponds to ideal strong resonance and superior photophysical performance. Herein, two D-π-A emitters, namely N-O-2CN and N-S-S, were rationally developed. Both molecules possess prominent two-state resonance characteristics, with calculated c² values of 0.44 and 0.45, respectively. Consistently, the two compounds display outstanding molar absorptivity: N-O-2CN affords a maximum ε of 6.43×10⁴ L·mol⁻¹·cm⁻¹ at 468 nm. More impressively, N-S-S reaches an exceptionally high ε of 1.23×10⁵ L·mol⁻¹·cm⁻¹ at 509 nm while bearing a low molecular weight of merely 377, which facilitates favorable processing capability. In addition, N-O-2CN and N-S-S exhibit narrowband blue and green luminescence, with full-width-at-half-maximum of 0.26 eV at 491 nm and 0.20 eV at 532 nm, respectively.

Keywords

Narrow spectra / Donor-acceptor modulation / Organic fluorescence material

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