Despite the above efforts, a complementary and alternative methodology is to create an environment to insulate the active groups from the oxygen molecules. According to this idea, encapsulation technology has been developed in numerous applications to improve the stability of materials and devices spanning from macro- to nano-levels, as concluded in Figure 1a.^[12-17] For instance, device encapsulation is an indispensable procedure in the fabrication of OLED, which incorporates glass or metal cover sealed with ultraviolet (UV)-cured epoxy resin, multilayer thin films of polymers, and metal complexes by deposition technologies.^[18] The strategy of encapsulation is so powerful that have been applied from the hermetic packaging of macroscopic devices to the microscopic molecules or nanoparticles in many areas such as catalysts and drug delivery.^[19-20] The related numerous coating materials involve host macrocycles,^[21]cross-linked copolymers,^[22-25] and carbon materials.^[26] Besides these coatings, groups of molecules can also act as the barriers and exhibit a self-encapsulation effect. Conjugated polymer polydiarylfluorene (PHDPF-Cz) shows an excellent single-chain featured emission, which is attributed to the defense of polymer backbone by the π-π stacking encapsulation of carbazole side group,^[17] inspired a novel encapsulation method in term of molecular design.

Herein, we demonstrate an encapsulation paradigm to dramatically increase the stability of oxygen-sensitive blue fluorescent emitters via a “steric armor” strategy as shown in Figure 1b. The model molecule14H-spiro[dibenzo[c,h]a-cridine-7,9′-uorene] (SFDBA) in amorphous state tends to be oxidized within 2 h under sunlight irradiation, while SFDBA crystals sheltered by the closely packed fluorenyl defense are able to remain the same under sunlight irradiation for 8 h. The self-encapsulation packing is due to the well-designed bi-planar cruciform-shaped molecular structure. The effective stability strategy is due to two interrelated reasons: one is the insulation of active naphthylamine groups away from oxygen by inert fluorenyl groups as sterically hindered walls; the other is the low-dimensional morphology further maximization the exposed plane paved by the fluorenyl walls. Furthermore, the photo-

Figure 1 The comparison between previous encapsulation methods and the crystallization strategy. a) Encapsulation examples with different scales and package modes including macroscopic device encapsulation, microscopic core/shell heterostructure, ligand coating and self-sealing of single molecule. b) Schematic representation exhibits the distinct photooxidation stabilities of different aggregation states, which is closely related to the SFDBA molecular arrangements.

luminescence quantum yields (PLQYs) of microcrystal films increase by 48.18 % and 327.37 % compared with solutions and amorphous films. This work provides an ingenious example for the rational noncovalent interaction design of molecule which leads the favorable molecular arrangement to enhance the oxidative stability and luminescence.

Results and Discussion

SFDBA is a typical spirocyclic molecule which can be deemed as a combination of a heterocyclic aromatic plane and a fluorene plane as shown in Figure 1b. Naphthylamine group in the heterocyclic aromatic plane is oxygen-sensitive under sunlight irradiation, thus SFDBA should be freshly synthesized through the one-pot synthesis and characterized by ¹H NMR in Figure S1 (Supporting Information, SI), then keep away from light and oxygen. To explore the influence of crystallization on the photooxidation process, single crystals and microcrystals of SFDBA were cultivated. Single crystals of SFDBA were prepared via solvent diffusion and the growth condition is listed in Table S1. The single crystal data in Table S2 indicate that the SFDBA crystal is assigned to monoclinic space group of P 2₁/c, with cell parameters of a= 12.293 Å, b = 23.528 Å, c = 7.576 Å, α = 90°,β = 104.476°, γ = 90°. The crystal molecular structure and the corresponding schematic diagram are illustrated in of Figure 2a, revealing a bi-planar cross-shaped conformation with a dihedral angle of 89.35^o. For SFDBA, all the sites of supramolecular interactions are found in the heterocyclic plane with larger conjugate area. As demonstrated in our previous work,^[27] the concentrated distribution of supramolecular weak interaction sites in the heterocyclic plane would make it attractive and tend to link with other heterocyclic planes, while the chemically stable fluorene planes line up on both sides. A dimer in Figure 2b expresses this interdigital lipid bilayer-like (ILB) packing mode, which signify layer-by-layer structures as revealed in Figure 2c. Viewed from b -axis, the ILB units arrange alongc -axis to form an ILB chain, while the lack of adequate supramolecular forces between the upper and lower chains alonga -axis (Figure S2) would contribute to the growth suppression of this direction and lead to the exposure of (100) plane. Figure 2d shows the perspective of the ILB chains in (100) plane as a single layer, revealing that the non-covalent interactions intrachain are π-π interactions (distances of 3.356 and 3.372 Å) and interchain are relatively weak C-H…π interactions (distances of 2.843 and 2.787 Å). The corresponding interaction energies (IE) (Table S3) within these molecules further indicate that the binding interaction of intrachain molecules along c -axis is the strongest, followed by the intrachain molecules along b -axis, and the interlayer alonga -axis is the weakest. As a result, the prepared microcrystals are verified to exhibit a one-dimensional (1D) belt-like morphology using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) characterizations. Uniform SFDBA microcrystals with long strip shapes can be easily formed by reprecipitation without any additives as the SEM image disclosed in Figure 2e. Figure 2f show the TEM image of an individual SFDBA microcrystal with evident selected-area electron diffraction (SAED) dots (inset in Figure 2f, upper left) identified as (002) and (040). The crystal orientations of [001] and [010] are also indicated in the microcrystal. The cartoon model of SFDBA microcrystal (inset in Figure 2f, bottom left) exhibits two major crystal faces of (100) and (010). The height profile of a SFDBA microcrystal in Figure 2g indicates a thickness of about 130 nm. The average length: width proportion is about 33:1 (16.5 um in length, 0.5 um in width) for SFDBA microcrystals from the size statistics in Figure S3. Moreover, the attachment energies of SFDBA crystal faces calculated by Materials Studio further confirm that the most thermodynamically stable crystal faces are {100} due to the lowest attachment energy (Table S4).^[28] X-ray powder diffraction (XRD) patterns of SFDBA microcrystals in Figure 2h are completely in conformity with the SFDBA single crystals, and the strong {100} peaks parallel to the substrate represent the layer-by-layer structure of SFDBA microcrystals.