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
89.35o. 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.