A facile strategy for realizing room temperature phosphorescence and single molecule white light emission

Abstract

Research on materials with pure organic room temperature phosphorescence (RTP) and their application as organic single-molecule white light emitters is a hot area and relies on the design of highly efficient pure organic RTP luminogens. Herein, a facile strategy of heavy-atom-participated anion-π⁺ interactions is proposed to construct RTP-active organic salt compounds (1,2,3,4-tetraphenyloxazoliums with different counterions). Those compounds with heavy-atom counterions (bromide and iodide ions) exhibit outstanding RTP due to the external heavy atom effect via anion-π⁺ interactions, evidently supported by the single-crystal X-ray diffraction analysis and theoretical calculation. Their single-molecule white light emission is realized by tuning the degree of crystallization. Such white light emission also performs well in polymer matrices and their use in 3D printing is demonstrated by white light lampshades.

Fig. 1

Molecule design and photophysical properties. a Chemical structures of TPO-I, TPO-Br, TPO-Cl, TPO-F, and TPO-P. b Luminescent photographs and c PL spectra of TPO-I, TPO-Br, TPO-Cl, TPO-F, and TPO-P in the solid state. Luminescent photographs were taken under 365 nm UV irradiation. Time-resolved PL decay of d TPO-I (@ 559 nm) and TPO-Br (@ 549 nm) and e TPO-Cl (@435 nm), TPO-F (@420 nm), and TPO-P (@ 422 nm) in the solid state at room temperature under air

Fig. 2

Single-crystal X-ray diffraction analysis. Torsion angles and anion–π⁺ interactions with distances for a, c TPO-I and b, d TPO-Br, respectively

Fig. 3

Theoretical calculation for fluorescence and RTP behaviors of organic salts. The calculated energy level diagram, spin–orbit couplings (ξ) between singlet and triplet states, and the oscillator strengths (f) of the S₁ state of a TPO-P and b TPO-Br in crystal based on the optimized ground-state geometries using ONIOM method. The natural transition orbitals (NTOs) (hole ones at the bottom and electron ones on the top) and the corresponding proportions for c TPO-P and d TPO-Br, as well as the bromine components in NTOs for TPO-Br

Fig. 4

OSMWLEs constructed by organic salts. a TPO-Br only, c TPO-I/TPO-Cl and e TPO-I/TPO-P with different counterions molar ratios. Commission Internationale de L’Eclairage (CIE) 1931 coordinates of prompt emission of films: b TPO-Br only, d TPO-I/TPO-Cl and f TPO-I/TPO-P with different molar ratios. Insets show luminescent photographs of these thin films taken under 365 nm UV irradiation

Fig. 5

White light emitters in polystyrene (PS) films based on organic salts. PL spectra and CIE 1931 coordinates of prompt emission of a, b TPO-Br, c, d TPO-I/TPO-Cl (molar ratio = 1:2), and e, f TPO-I/TPO-P (molar ratio = 1:3) in polystyrene (PS) (2%, m/m). Insets show luminescent photographs of these PS films taken under 365 nm UV irradiation. g Luminescent photographs of UV-LED lamps (Emission wavelength: 360–370 nm) coated with PS films without (left) and with (right) TPO-Br. h Luminescent pattern of a Chinese character “Tang” based on the PS film (3 cm × 3 cm) containing TPO-Br (2%, m/m) taken under 365 nm UV irradiation

Fig. 6

White light emitters in 3D printing. a PL spectra and b CIE 1931 coordinates of prompt emission of the PEG film containing TPO-Br (2%, m/m). Photographs of 3D printed lampshades without (left) and with (right) TPO-Br taken under c daylight, d 365 nm UV light irradiation, and e UV-LED lamps. The size of lampshades is 8.2 × 8.2 × 9.2 mm