Molecular tuning of the crystallization-induced emission enhancement of diphenyl-dibenzofulvene luminogens

Article abstract of DOI:10.1039/d0cc07013k

The absorption and emission properties of various diphenyl-dibenzofulvene (DP-DBF) derivatives were investigated, and their crystallization-induced emission enhancement (CIEE) performances were found to show a clear correlation with the twist angle around

Full text of DOI:10.1039/d0cc07013k

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a
b
b
∗a
Organic luminogens that show aggregation-induced emission
(AIE) and crystallization-induced emission enhancement (CIEE)
properties1–4 have received considerable attention in recent
years, owing to their appealing applications in fluorescent chemi-
cal sensors,5 biological probes,6–8 organic field-effect transistors
(OFETs),9 and organic light emitting devices (OLEDs).10,11 Such
luminogens are generally non-emissive in dilute solution, but
exhibit moderate-to-high degrees of fluorescence in their aggre-
gated states such as amorphous solid or crystalline forms. Many
reported AIE and CIEE luminogens are designed to contain struc-
turally hindered π-conjugated backbones, so that upon aggre-
gation their fluorescence can be enhanced due to restricted in-
tramolecular motion (RIM).12–14 Diphenyl-dibenzofulvene (DP-
DBF) derivatives constitute an intriguing class of AIE and CIEE
lumiogens, featuring relatively simple molecular structures and
easy synthesis. Since the first study by Tang and co-workers
in 2006,15 numerous DP-DBF-based luminogens have been re-
ported in the literature, some of which show very interesting
polymorphism-dependent emission, strong CIEE effects, and ther-
mochromic and mechanochromic fluorescence properties.4,16–20
It is noteworthy that the emission efficiencies of various liter-
ature reported DP-DBFs show a substantial degree of variations
(see the summary in Figure 1). By analyzing these data, one
would reasonably expect to find some correlations between the
Φ
fluorescence properties of DP-DBFs with their molecular param-
eters (e.g., molecular geometry and substitution effects). Pre-
viously, Tang16,17 and Zhang18 attributed the fluorescence en-
hancement and emission wavelength shifts of DP-DBF lumino-
gens 5 and 6 in the aggregated and crystalline states to the twist
angle (θ1) between the phenyl and vinyl units in the structure
of DP-DBF. However, such rationalization was not strongly sup-
ported by theoretical analysis and experimental evidence. Shuai
and co-workers in 2012 performed a molecular dynamics study
to show that both the bond stretching in the DBF unit and the
twisting around the C=C bond play essential roles in the elec-
a
Department of Chemistry, Memorial University of Newfoundland, St. John’s, NL,
Canada A1B 3X7. Fax: 1 (709) 864 3702; Tel: 1 (709) 864 8747; E-mail: yum-
ing@mun.ca
b
Department of Organic Chemistry College of Science, Beijing University of Chemical
Technology Beijing 100029, PR China.
† Electronic Supplementary Information (ESI) available: Synthetic procedures, spec-
troscopic characterization, DFT calculations, and X-ray single crystallographic data
for compounds 5a–c, 8a–c, and 9. See DOI: 00.0000/00000000.

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DOI: 10.1039/D0CC07013K
12a–c, which subsequently underwent Williamson etherification
with benzyl bromide to give OBn-substituted DP-DBFs 8a–c. Ad-
ditionally, the etherification of 12a with diiodomethane in the
presence of Cs2CO3 produced DP-DBF 9, the structure of which
contains a methylenedioxy (OCH2O) group to bridge the two
phenyl groups at their ortho-positions. This compound serves as
a special DP-DBF model where the rotations of the phenyl groups
are completely restricted.
With these substituted DP-DBFs in hand as models, we then
carried out comparative analyses focusing on their structural,
electronic, and fluorescence properties. Figure 4 shows the elec-
tronic absorption spectra of DP-DBFs 5a–c, 8a–c, and 9 measured
in the solution phase. Ortho-OMe substituted 5a gives three dis-
tinct π → π* absorption peaks at 316, 302, and 282 nm. In addi-
tion, there is broad absorption shoulder ranging from ca. 330 to
420 nm. Comparatively, the spectrum of ortho-tethered 9 shows
π → π* absorption peaks at similar wavelengths; however, its
long-wavelength shoulder band at 329 nm appears to be much
steeper and narrower. The different spectral patterns can be at-
tributed to the different rotational modes between 5a and 9; the
phenyl groups of 5a can rotate to form various conformers in the
solution phase, but the tethered phenyl rings of 9 are inhibited
from such rotational modes. The spectrum of meta-OMe substi-
tuted 5b shows two absorption peaks at 320 and 287 nm, along
with a very broad absorption shoulder in the region of 330 to 440
nm. The edge of the long-wavelength absorption shoulder of 5b is
significantly redshifted relative to that of 5a, suggesting a higher
degree of π-conjugation in 5b. For the spectrum of para-OMe sub-
stituted 5c, a broad absorption band at 369 nm can be seen and
its absorption edge is the most redshifted among the DP-DBFs.
The spectra of OBn-substituted DP-DBFs show similar features to
those of their OMe-substituted analogues, indicating little contri-
bution of the benzyl groups to the electronic absorption features.
To determine the S0 → S1 vertical transition energies for the DP-
DBFs, the half-height wavelength of the lowest-energy maximum
absorption band (λ1a/b2s) in each absorption spectrum was consid-
ered. The experimental values show good agreement with time-
dependent density functional theory (TD-DFT) calculated S0 →
tronic non-adiabatic transition of DP-DBF.21 Recently, it has been
reported that restriction of the twisting around the C=C bond
plays a crucial role in the AIE of various π-conjugated systems.22
In 2013, Li and Blancafort developed a conical intersection (CI)
model to explain the AIE properties of DP-DBF.23 According to
their model, the first excited state (S1) of DP-DBF can be deacti-
vated to the ground state (S0) through an S1/S0 CI seam. As illus-
trated in Figure 2, DP-DBF in the solution phase can readily reach
an energetically accessible region of the CI seam, where molecu-
lar geometries show a high twist angle (θ2 > 50◦) about the C=C
bond of DP-DBF. This non-radiative decay pathway quenches the
fluorescence of DP-DBF in the solution phase. In the solid state,
however, the twist around the C=C bond is restricted and there
is only a high-energy region of CI seam existing on the poten-
tial energy surface (PES), which in turn results in enhanced solid-
state fluorescence. This modelling study thus motivated us to pro-
pose that the twist angle about the C=C bond of DP-DBF (θ2) in
the minimum-energy structure of the ground state is a key factor
controlling its fluorescence behavior, especially the fluorescence
quantum yield in the aggregated state. Upon aggregation, re-
stricted intramolecular motion should significantly influence the
internal conversion of the first excited state (S1) to the CI seam
on the PES. If θ2 is relatively large, the amount of energy required
for the S1 state to reach the CI seam should be small and the flu-
orescence is thus well quenched. If θ2 is small, the non-radiative
decay pathway through the CI seam becomes difficult to access.
As a result, the radiative decay acts as the dominant deactivation
pathway (i.e., fluorescence enhancement).
Validation of the above hypothesis requires systematic exam-
ination of DP-DBFs with varied θ2 values. To do so, we chose
the approach of attaching functional groups to different positions
of the two phenyl rings in DP-DBF to finely tune the twist angle
θ2 by steric effects. A series of DP-DBFs derivatives functional-
ized with methoxy (OMe) or benzyloxy (OBn) substituents was
thus designed and prepared in this work. Figure 3 outlines our
synthetic methods, in which 9-(dibromomethylene)-9H-fluorene
(10) was first subjected to Suzuki-Miyaura cross coupling with
methoxyphenylboronic acids 11a–c to yield OMe-substituted DP-
DBFs 5a–c in good yields. Note that compounds 5a–c are re-
gioisomers with OMe groups substituted at the ortho, meta, and
para-positions of the phenyl rings, respectively. Compounds 5a–
c were then treated with BBr3 to afford demethylated products
S1 energies (see Table S-2, ESI). For both groups of OMe and
abs
1/2
OBn-substituted DP-DBFs, a redshift trend in λ
can be clearly
observed as: ortho < meta < para. It is worth mentioning that
the solid-state UV-Vis absorption spectra of the DP-DBFs were also
measured, showing very small variations in comparison with the
solution-phase spectra (see Figure S-23, ESI).
The single crystals of DP-DBFs 5a–c, 8a–c, and 9 were grown
and their molecular structures were elucidated by single crystal X-
ray diffraction (XRD) analysis (Figure 5). In the crystal structure
of ortho-OMe substituted 5a, the two OMe groups take two differ-
ent orientations (cis and trans) with respect to the fluorene plane.
These two conformations exist in a ratio of ca. 1:1, and they show
similarly large twist angles around the C=C bond. In contrast to
5a, the crystal structure of ortho-OBn-substituted 8a only shows
the trans conformation with a small twist angle around the C=C
bond. Both meta-substituted 5b and 8b adopt a trans confor-
mation in terms of the relative orientation of the OMe or OBn
substituents, while the benzyl groups in the structure of 8b show

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DOI: 10.1039/D0CC07013K
θa
θb
a certain degree of rotational disorders. It is also worth mention-
ing that there was another crystalline form of 5b obtained during
the crystallization process. Such crystals were formed in a rel-
atively small quantity, and the molecular structure adopts a cis
conformation (see Fig. S25A, ESI). Two distinct crystal structures
of para-OMe-substituted 5c were previously reported by Zhang
and co-workers.18 One is colorless with blue emission, and the
other is yellow in color with green emission. In our experiment,
we obtained the single crystal of 5c that is similar to the first one.
The molecular structure of para-OBn-substituted 8c shows a sim-
ilar DP-DBF conformation to that of 5c, but the twist angle about
the C=C bond appears to be smaller. In the crystal structure of
9, the two oxygen groups are in a cis conformation and the C=C
bond shows a moderate degree of twisting. Overall, the twist
angles around the C=C bonds in the crystal structures of 5a–c,
8a–c, and 9 show a wide range of variation as expected. To quan-
titatively assess the degree of twisting, the sum of two dihedral
angles (θa and θb, see the inset of Figure 3 for definition) about
the C=C bond were measured and compiled in Table 1.
Meta-substituted DP-DBFs 5b and 8b afford moderate-to-weak lu-
minescence in yellowish green color, which is very different from
the other DP-DBFs. Solid-state fluorescence spectra of the DP-
DBF crystals were measured (Figure 6), and detailed emission
properties are provided in Table 1. All the DP-DBF crystals give
a relatively broad emission band without distinctive vibronic fea-
tures. Both the OMe and OBn-substituted series show a trend
of increasing maximum emission wavelength (λem) as follows:
para < ortho < meta. The trend is different from the shift of
λabs observed in the UV-Vis analysis. The much longer emission
wavelengths of meta-substituted 5b and 8b relative to the other
DP-DBFs are consistent with their yellowish green-colored lumi-
nescence in the solid state. Stokes shifts (∆ν) were calculated ac-
cording to the difference between λabs and λem in the solid state
(see Table 1). It is noteworthy that the Stokes shifts of para-
substituted 5c and 8c are considerably smaller than those of ortho
and meta-substituted DP-DBFs, suggesting that para-substituted
DP-DBFs undergo a much smaller degree of structural variations
during the S0 → S1 excitation in the crystallin state. The strong
resonance effects arising from para-substitution are therefore be-
All the DP-DBF derivatives 5a–c, 8a–c, and 9 show insignificant
fluorescence in the solution phase; however, their crystals exhibit
luminescence with various intensities and colors under UV-light
irradiation (see Figure 5). Para-substituted DP-DBFs 5c and 8c
give strong bright blue emission, while ortho-substituted DP-DBFs
5a and 8a exhibit moderate blue emission in the crystalline state.

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DOI: 10.1039/D0CC07013K
entry [θa +θb] (◦)
λabs (nm)
319
λem (nm)
473
Φf (%)
1.44
∆ν (cm−1
10206
10306
4904
)
5a
5b
5c
8a
8b
8c
9
40.2
32.6
12.4
3.2
31.6
5.4
16.2
333
497
4.22
376
468
89.81
27.89
2.95
86.01
17.63
319
454
10117
10665
4113
332
515
379
449
319
457
9561
lation. Our structure-property studies not only provide in-depth
understanding of the physical origin of the AIE and CIEE perfor-
mances of DP-DBF-based luminogens, but disclose an effective
molecular approach to rationally design and finely tune the fluo-
rescence properties of various related AIE and CIEE luminogens.
The authors thank the Natural Sciences and Engineering Re-
search Council of Canada (NSERC), Canada Foundation for In-
novation (CFI), and Memorial University of Newfoundland for
funding support. Dr. Jian-Bin Lin of Memorial University of New-
foundland and Dr. Michael Ferguson of University of Alberta are
acknowledged for solving the single crystal structures. Compute
Canada is acknowledged for assistance in our DFT calculations.
θa +θb
Φf
lieved to result in significantly rigid π-conjugated framework of
DP-DBF.
Figure 6C illustrates the correlations of the fluorescence quan-
tum yields (Φf ) of DP-DBF crystals with the degree of twist-
ing about the C=C bond ([θa + θb]). In line with our hypoth-
esis, the experimental results show clear correlations between
the twisting of C=C bond and the fluorescence efficiency in the
crystalline state. The Φf values of DP-DBF crystals show an in-
creasing trend with decreasing twist angle about the C=C bond.
In particular, meta and ortho-substituted DP-DBFs give an excel-
lent linear relationship between Φf and [θa + θb]. This result
proves that restricted torsional vibration about the C=C bond
is the origin of the AIE and CIEE properties of DP-DBF. Given
that ortho-tethered 9 also clearly follows the same linear corre-
lation, rotational modes of the phenyl groups can be ruled out
as a significant factor to the fluorescence efficiency in the solid
state. It is also remarkable to note that the literature reported DP-
DBF 7, the structure of which contains an ethylene bridge link-
ing the ortho positions of the two phenyl rings, also follows the
same linear trend.20 In contrast to meta and ortho-substituted DP-
DBFs, para-substituted 5c and 8c do not follow the linear trend
of fluorescence quantum yields. Actually, both compounds exhibit
very high Φf values in the crystalline state, and these values are
comparable with those of the para-substituted DP-DBFs reported
in the literature.17,18 Their high CIEE performances can be at-
tributed to enhanced π-delocalization arising from para-alkoxy
substitution. Finally, it is worth noting that 5a and 9 are not CIEE
luminogens, since their powders give greater Φf values than their
crystals (see Table S-1, ESI).
There are no conflicts to declare.
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In conclusion, with the access to a range of substituted DP-
DBF single crystals, we have unambiguously confirmed that the
twist angle around the C=C bond is a key molecular factor con-
trolling their solid-state fluorescence properties. For non-para-
substituted DP-DBFs, their fluorescence quantum yields in the
crystalline state can be predicted by a linear correlation with the
twist angle. Para-substituted DP-DBFs, on the other hand, show
very strong CIEE performances but do not follow the linear corre-

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DOI: 10.1039/D0CC07013K
Table of Content Entry
The CIEE performances of diphenyl-dibenzofulvene luminogens are controlled by the twist
angle of the C=C double bond.