Paper and Pencil Design of Color-Pure Organic Emitters: The Curious Case of Xanthene Dyes

The quest for color-pure emitters for multicolor bioimaging as well as for ultrahigh definition organic light-emitting diodes demands facile design concepts to avoid tedious synthetic or computational trial-and-error procedures. We have recently presented a simple recipe to construct color-pure blue emitters, which combines basic resonance structure and frontier molecular orbital treatments; this recipe applies to multiresonant type emitters and allows to enlarge the chemical space toward novel structural motifs. In the current work, we show that such fundamental considerations further apply to the structurally entirely different family of xanthene dyes. Opposite to the related polymethine dye family with small bond length alternation (BLA) in the ground and in the excited state (S0, S1), however, xanthene dyes display large BLA in S0 and in S1, so that the overall change in BLA, ΔBL, is small. This gives equally rise to color-pure emission; the underlying reasons for this curious behavior are carved out in the current study. This generalization of the recipe in fact constitutes the desired "paper and pencil" design strategy, spanning now the whole visible range.