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