Brightening triplet excitons enable high-performance white-light emission in organic small molecules via integrating n-π*/π-π* transitions

Abstract

Luminescent materials that simultaneously embody bright singlet and triplet excitons hold great potential in optoelectronics, signage, and information encryption. However, achieving high-performance white-light emission is severely hampered by their inherent unbalanced contribution of fluorescence and phosphorescence. Herein, we address this challenge by pressure treatment engineering via the hydrogen bonding cooperativity effect to realize the mixture of n-π*/π-π* transitions, where the triplet state emission was boosted from 7% to 40% in isophthalic acid (IPA). A superior white-light emission based on hybrid fluorescence and phosphorescence was harvested in pressure-treated IPA, and the photoluminescence quantum yield was increased to 75% from the initial 19% (blue-light emission). In-situ high-pressure IR spectra, X-ray diffraction, and neutron diffraction reveal continuous strengthening of the hydrogen bonds with the increase of pressure. Furthermore, this enhanced hydrogen bond is retained down to the ambient conditions after pressure treatment, awarding the targeted IPA efficient intersystem crossing for balanced singlet/triplet excitons population and resulting in efficient white-light emission. This work not only proposes a route for brightening triplet states in organic small molecules, but also regulates the ratio of singlet and triplet excitons to construct high-performance white-light emission.

Fig. 1. The molecular structure and photoluminescence (PL) properties of IPA.

a The crystal structure of IPA. b Photographs of the pristine and pressure-treated IPA taken at different time intervals before and after turning off the laser excitation (355 nm) at ambient conditions. The photographs (UV off) were taken immediately after turning the UV light off. A mass of quenched products (decompression from 20.0 GPa) was harvested by high-pressure experiments in a Walker-type large-volume press. c PL spectra of IPA upon compression to 9.0 GPa using symmetric diamond anvil cell (DAC) devices under the 355 nm laser excitation. d PL spectra of IPA (left) at ambient conditions and after pressure was released from 18.0 GPa. The corresponding PL photographs (right) were taken in DAC devices. e Changes of chromaticity coordinates at ambient conditions (0.22, 0.22) and after releasing pressure from 18.0 GPa (0.28, 0.36). f PL (blue solid line), fitting fluorescence (green dashed line), and fitting phosphorescence (orange dashed line) spectra of the targeted IPA treated by the Walker-type large-volume press.

Fig. 2. Crystal structure evolution upon compression and decompression.

a IR spectra of IPA in the region of C=O stretching vibrational mode ν (C=O) at ambient conditions and after pressure was released from 20.0 GPa. b Molecular packing along the b-axis (left). The schematic diagram of the parallel misalignment angle (σ) of IPA (right). c The compression rate of three lattice constants (a, b, c) at different pressures. d Time-of-flight (TOF) neutron diffraction patterns of IPA-d₆. The TOF neutron diffraction patterns of the pristine (blue line) and recovered (magenta line) samples were collected at BL11 PLANET in the MLF at J-PARC. The asterisks indicate the peaks of the Pb marker. The neutron diffraction patterns of the pristine (green line) and recovered (orange line) samples were collected at the High-pressure neutron diffractometer (Fenghuang) at the CMRR neutron science platform. e Pressure-dependent hydrogen-bond distances D₁₇···O₉ (d₁) and D₂₁···O₅ (d₂) evolution of IPA. f Pressure-dependent σ evolution of IPA. The d₁, d₂, and σ of IPA-d₆ were determined by Rietveld refinement of neutron diffraction patterns. The error bars in (e) and (f) represent the standard error in the Rietveld refinement.

Fig. 3. The SOC and NTOs of the pristine and pressure-treated IPA.

a The calculated ξ (ξ(S₁ − T₆), ξ(S₁ − T₅), ξ(S₁ − T₄), ξ(S₁ − T₃), ξ(S₁ − T₂), and ξ(S₁ − T₁)) for the pristine and pressure-treated IPA. b The NTOs of T₅ and T₆ in the pristine and treated IPA. At 1 atm, the T₅ and T₆ states feature ³(n, π^*) transition. After pressure treatment (Released), the T₅ and T₆ states feature a mixture of ³(π, π^*) and ³(n, π^*) transitions. c Schematic representation of hydrogen bonds acting on the ISC process. The strengthened hydrogen bonds promote the mixture of ³(π, π^*) and ³(n, π^*) transitions, thus accelerating the ISC process.

Fig. 4. The effects of HFC and singlet-triplet energy gap (ΔE_ST) on the ISC process of IPA before and after pressure treatment.

a Magnetic-field effects on the PL intensity of IPA. The data at 1 atm and released represent the mean value from five and three experiments, respectively. The error bars are ±s.e.m. b The energies of singlet and triplet states of IPA before and after pressure treatment. c The calculated ΔE_ST (ΔE(S₁ − T₆), ΔE(S₁ − T₅), ΔE(S₁ − T₄), ΔE(S₁ − T₃), ΔE(S₁ − T₂), and ΔE(S₁ − T₁)) for the pristine and targeted IPA. d Proposed energy transfer processes for fluorescence and phosphorescence in IPA before and after pressure treatment. The narrowed ΔE_ST effectively accelerated the ISC process after pressure treatment.