Mapping the Regioisomeric Space and Visible Color Range of Purely Organic Dual Emitters with Ultralong Phosphorescence Components: From Violet to Red Towards Pure White Light

Abstract

We mapped the entire visible range of the electromagnetic spectrum and achieved white light emission (CIE: 0.31, 0.34) by combining the intrinsic ns-fluorescence with ultralong ms-phosphorescence from purely organic dual emitters. We realized small molecular materials showing high photoluminescence quantum yields (Φ_L ) in the solid state at room temperature, achieved by active exploration of the regioisomeric substitution space. Chromophore stacking-supported stabilization of triplet excitons with assistance from enhanced intersystem crossing channels in the crystalline state played the primary role for the ultra-long phosphorescence. This strategy covers the entire visible spectrum, based on organic phosphorescent emitters with versatile regioisomeric substitution patterns, and provides a single molecular source of white light with long lifetime (up to 163.5 ms) for the phosphorescent component, and high overall photoluminescence quantum yields (up to Φ_L =20 %).

Keywords: dibenzofuran; luminophores; phosphorescence; ultralong lifetimes; white light emission.

Scheme 1

Chemical formulae of the investigated regioisomers.

Figure 1

Persistent phosphorescence (after UV excitation is turned off) and prompt emission (under 365 nm UV light, during irradiation with a hand‐lamp) of all regioisomers as crystalline solids at room temperature (qualitative experiment, the actual lifetimes can be found in Table 1). The phosphorescence afterglow can be seen up to 2 s by naked eye upon interruption of the photoexcitation. Note: the delay times depicted should be regarded as ±40 ms, due to the experimental uncertainty, see Supporting Information for details. Parts of the characterization of 1‐PhDBF and 3‐PhDBF has been reported beforehand..

Figure 2

A) Photographs of selected compounds under 365 nm‐light excitation and immediately after switching off the UV‐light source (0–40 ms due to the experimental setup, see Supporting Information for details). In each case, the wavelength of the emission maximum is detailed in parentheses; the corresponding wavelengths of the afterglow are given after 50 ms delay (λ _ex=376,7 nm). B) Photographs of 2‐PhBrDBF and 3‐PhBrDBF showing white light emission (λ _ex=365 nm). The wavelengths correspond to the maxima of the two emission shoulders (λ _ex=350 nm). C) Emission spectra of 2‐PhBrDBF and 3‐PhBrDBF at 350 nm excitation. D) CIE diagram of 2‐PhBrDBF and 3‐PhBrDBF at 350 nm excitation.

Figure 3

Demonstration of regioisomerism‐controlled chromophore stacking in the crystal of the bromo‐ and chloro‐ substituted isomers. The isomers that show strong perpendicular DBF‐DBF stacking also show longer lifetimes (e.g. 2‐PhClDBF and 2‐PhBrDBF). All other interactions have been omitted for clarity.

Figure 4

Calculated ISC channels for monomer and the dimer of 2‐PhClDBF, 2‐PhBrDBF and 2‐PhIDBF isomers in a crystalline environment obtained from two‐layer ONIOM calculations at the quantum mechanics/molecular mechanics (QM/MM) level. Solid green lines indicate effective ISC channels within energy level between S₁ and T₁ that is, |ΔE _ST|≤0.3 eV. Calculated SOC values (ξ, in cm⁻¹) were provided in each case.