Figure 1 (a) Chemical structures of guest molecules. (b) Schematic diagram of different fluorophores as the guests doped in the matrix of PPh3 to construct the host-guest phosphorescence films.
The synthetic routes to the six luminogens are outlined in Scheme S1. 5-Br-TAT and 6-Br-TAT were synthesized according to our previous work.[38] The bromo substituents of these two intermediates were then replaced by 4-methoxyphenyl, phenyl, and 4-cyanophenyl substituents via Suzuki coupling reactions to afford the target luminogens, respectively. The chemical structures of the resulting luminogens were confirmed by 1H and13C NMR spectra, and their purities were verified by high performance liquid chromatography (HPLC) measurements to avoid the interference of impurities (Figure S1).
The photophysical properties of the individual luminogens are investigated in dichloromethane (DCM) solutions and solid states at ambient conditions. As shown in Figure 2 and Figure S2, 5-TAT-OMe in the solid state emits a blue fluorescence with the emission peak at 418 nm and a lifetime of 4.59 ns. Enhancing the electron-withdrawing ability of the substituents pushes the luminogen fluorescence from blue to green, and the emission peak of 5-TAT-CN redshifts to 476 nm with a shorten lifetime of 2.63 ns. A similar trend is observed from 6-TAT-OMe to 6-TAT-CN in the solid state. In addition, 6-TAT-CN exhibits slightly red-shifted absorption and emission compared with 5-TAT-CN, indicating the position of substituents of the TAT core could effectively regulate the optical bandgap and energy level of luminogens, coinciding with the DFT calculations (Figure S3). The fluorescence quantum yields (ФF) of these luminogens in solid states ranges from 15.73% to 37.58%. The corresponding photophysical data are summarized in Table S1. Note that 6-TAT-CN exhibits the highest intrinsic fluorescence quantum yield of 48.80% among six luminogens, while from dilute solutions to crystalline states, its ФF plummets to 27.10%. To interpret this phenomenon, crystallographic analysis is conducted to understand the molecular stacking of 6-TAT-CN crystals. As shown in Figure S4, the 6-TAT-CN single crystal belongs to the P1 triclinic space group with the unit cell parameters of a = 5.1863 Å, b = 14.0066 Å, c = 18.0705 Å, α = 68.691°, β = 81.788°, and γ = 80.868°. To simplify the packing analyses, dimers with strong intermolecular interactions are selected and illustrated in Figure 3. Dense molecular packing with efficient π-π interaction (3.606 Å) and abundant C-H∙∙∙π (2.528-3.886 Å) and C-H∙∙∙N (3.239-3.898 Å) interactions is observed. Although the C-H∙∙∙π and C-H∙∙∙N interactions could inhibit the non-radiative transitions induced by molecular rotations and vibrations, the strong π-π interaction (3.606 Å) in the aggregated state could also enforce the luminescence quenching,[26,31] thus leading to the reduced ФF of 6-TAT-CN. None of the single component of six luminogens show RTP phenomenon.
Triphenylphosphine (PPh3) is employed as the host material to embed the guest luminogen into its rigid matrix, aiming to break