Tunable aggregation-induced emission of diphenyldibenzofulvenes

Article abstract of DOI:10.1039/b515798f

Nonemissive fulvenes can be induced to luminesce by aggregate formation, with the crystalline aggregates emitting bluer lights more intensely than their amorphous counterparts. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b515798f

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Tunable aggregation-induced emission of diphenyldibenzofulvenes{
Hui Tong,a Yongqiang Dong,ab Matthias Ha¨ußler,a Jacky W. Y. Lam,a Herman H.-Y. Sung,a Ian
D. Williams,a Jingzhi Sunb and Ben Zhong Tang*ab
Received (in Cambridge, UK) 9th November 2005, Accepted 11th January 2006
First published as an Advance Article on the web 25th January 2006
DOI: 10.1039/b515798f
in common organic solvents such as acetonitrile, chloroform and
THF but are insoluble in water.
Nonemissive fulvenes can be induced to luminesce by aggregate
formation, with the crystalline aggregates emitting bluer lights
more intensely than their amorphous counterparts.
Dilute acetonitrile solutions of all the three fulvene derivatives
are practically nonluminescent. However, addition of water, a non-
solvent of the fulvenes, can greatly affect their photoluminescence
(PL) processes (Figs. S1–S3 in the ESI). The isolated species of 1 in
the molecularly dissolved solution displays a PL spectrum that is
almost a flat line parallel to the abscissa (Fig. 1A). Upon addition
of a large amount of water, the mixture becomes visually emissive
(see, for example, the photo shown in the graphical abstract). The
emission and absorption specta of 1 both red-shift with the
addition of water (Table 1), suggestive of the formation of
J-aggregates in the aqueous mixture.8 Clearly, the emission of 1 is
induced by the aggregate formation, or in other words, 1 is AIE-
active. Whereas many dye suspensions in aqueous media gradually
bleach of fade with time, the AIE system of 1 is very stable: little, if
any, change is detected in the PL spectra of its nanoaggregates in
the aqueous mixtures even after the suspensions have been stored
under ambient conditions without protection from light and air for
more than two months.
When chromophoric molecules are dispersed in aqueous media or
fabricated into solid films, they tend to aggregate, due to the strong
electronic and hydrophobic interactions between their aromatic
rings in the polar environment or in the solid state. The aggregate
formation often quenches light emission, which has been a thorny
problem in the development of efficient environmental sensors,
biological probes, and light-emitting diodes.1–3 We have recently
observed an unusual phenomenon of aggregation-induced emis-
sion (AIE): a group of nonemissive silole molecules have been
induced to emit intensely by aggregate formation.4,5 While
crystallization commonly weakens and red-shifts light emission,
the crystalline aggregates of the siloles are found to emit stronger,
bluer lights than their amorphous counterparts.6 The AIE
phenomenon poses a challenge to our current understanding of
light-emitting processes in the solid state, which may spawn new
photophysical theories and spur technological innovations.
Because of its obvious academic value and practical implication,
we have worked on the exploration of more AIE systems.7 In this
paper, we report our results on the development of a new class of
AIE molecules based on a fulvene scaffold structure. Different
from the organometallic silole species, the fulvenes are pure
organic dyes containing no metalloid atoms.
Careful analysis of the PL spectra of 1 in the aquous mixtures
reveals a decrease in its emission intensity and a red-shift in its
emission maximum (lem) when the water content is changed from
95 to 99 vol% (Fig. S1 in the ESI). We speculate that this is due to
the change in the packing order of the aggregates from a crystalline
state to a amorphous one. In a mixture with a ‘‘lower’’ water ratio
(¡95%), the molecules of 1 may slowly assemble in an ordered
fashion to form more emissive, ‘‘bluer’’, crystalline clusters, while
in a mixture with a ‘‘higher’’ water ratio (99%), the molecules of 1
may quickly agglomerate in a random way to form less emissive,
The fulvene derivatives were prepared by the synthetic routes
shown in Scheme S1 (ESI). The lithiation of fluorene followed by
the reaction of the intermediate with 2-aminobenzophenone gave a
diphenyldibenzofulvene derivative with an amino substituent (1).
The treatments of 1 by acetic acid and trifluoroacetic acid yielded
the acylation products carrying acetyl (2) and trifluoroacetyl
groups (3), respectively. All the fulvene derivatives were character-
ized by standard spectroscopic methods, from which satisfactory
analysis data corresponding to their expected molecular structures
were obtained (see ESI for details). The fulvene dyes are all soluble
aDepartment of Chemistry, The Hong Kong University of Science &
Technology, Clear Water Bay, Kowloon, Hong Kong, China.
E-mail: tangbenz@ust.hk; Fax: +852-2358-1594; Tel: +852-2358-7375
bKey Laboratory of Macromolecular Synthesis and Functionalization of
the Ministry of Education, Department of Polymer Science and
Engineering, Zhejiang University, Hangzhou 310027, China
{ Electronic supplementary information (ESI) available: Preparation and
characterization details for fulvenes 1–3, absorption and emission spectra
of 1–3 in solution and aggregation states, emission spectra of 1 and 2 in
amorphous and crystalline states, optimized geometric structures and
molecular orbital amplitude plots of HOMOs and LUMOs of 1–3, and
single crystal data of 1. See DOI: 10.1039/b515798f
Fig. 1 (A) Emission spectra of 1 in acetonitrile/water mixtures and (B)
dependence of quantum yields of fulvenes 1–3 on solvent compositions of
the acetonitrile/water mixtures. Fulvene concentration: 10 mM; excitation
wavelength: 350 nm.
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After studying the optical properties of the fulvenes in the liquid
Table 1 Absorption and emission properties of diphenyldibenzoful-
venes in solution (Soln),a aggregate (Aggr),b crystalline (Cryst), and
amorphous (Amor) states
media (aectonitrile solvent and aqueous mixtures), we investigated
their PL behaviours in the solid state. We prepared two kinds of
solid samples: a crystalline chunk and an amorphous powder. The
crystals of 1 and 2 emit bluer lights more efficiently, in comparison
to their amorphous cousins (Fig. S4 in the ESI). This aggrees well
with our finding described above: the crystalline aggregates of 1
and 2 in the aqueous mixtures emit stronger, bluer lights than their
amorphous ones (Figs. 1B and S5). However, different from other
AIE molecules,4,11 1 and 2 remain nonemissive after they are
l
ab, nmc
l
em, nmd
Soln
Aggr
Soln (WF)
Aggr (WF)
Cryst
Amor
1
2
3
a
316
326
337
335
338
353
439 (0.11)
437 (0.02)
440 (0.01)
545 (9.50)
483 (3.30)
465 (0.03)
b
527
465
550
487
In acetonitrile solutions (10 mM). In acetonitrile/water (1:99 by
c
d
vol) mixtures (10 mM).
maximum [with quantum yield (WF, %) given in the parentheses];
Absorption maximum.
Emission
doped into poly(methyl methacrylate) (PMMA) films (WF
¡
excitation wavelength: 350 nm.
0.5%). This suggests that the molecular interactions play an
important role in the AIE processes of the fulvenes. The PMMA
matrix serves as a solid ‘‘solvent’’, which separates the fulvene
molecules and impedes their aggregate formation. As expected,
almost no PL signals can be captured from 3 in all the cases.
In an effort to understand the photophysical processes of 1–3,
we calculated their geometric structures at the RHF/6-31G* level
(Fig. S6 in the ESI).12 On the basis of the optimized geometries,
their electronic spectra were computed by the semi-empirical
ZINDO/S method in the HyperChem.13 The calculated absorption
maximum (lab,c) of 1 is 317 nm, agreeing well with its experimental
value (lab = 316 nm). Although lab,c’s of 2 (318 nm) and 3 (322 nm)
slightly differ from their lab’s, the trend towards a bathochromic
shift is consistent with the experimental result. The HOMOs and
LUMOs of 1–3 are all dominated by the orbitals originating from
their diphenyldibenzofulvene core, with extremely low electron
densities at the amino and amide pendants (Figs. S7–S9 in the
ESI). This explains why the emissions from all the three fulvene
derivatives in the acetonitrile solutions are almost the same,
independent of their substituent groups.14
‘‘redder’’, amorphous powders. This is proved to be the case by the
electron microscopy and diffraction analyses, which confirm that
the nanoparticles formed in the mixtures with ‘‘lower’’ and
‘‘higher’’ water contents are indeed crystalline and amorphous,
respectively (Fig. S5 in the ESI).
The dependence of the fluorescence quantum yield (WF) of 1 on
the solvent composition of the acetonitrile/water mixture is shown
in Fig. 1B. In the mixtures with water contents below y70%, the
WF values of 1 are very small because its molecules are dissolved in
the mixtures. Its WF starts to increase at a water content of y70%,
suggesting that its molecules begin to aggregate in the mixture with
this solvent composition. The WF of 1 continuously increases with
an increase in the water content, reaching a maximum of 9.8% at a
water content of 95%, which is y90-fold higher than that of its
dilute acetonitrile solution. Further increasing the water content to
99% results in a decrease in WF (9.5%) due to the morphological
change discussed above. It should be pointed out that the absolute
WF values of the aggregates of 1 could be much higher than its
relative values given in Fig. 1B and Table 1, as the apparent high
absorbance caused by the light-scattering or Mie effect of the
nanoparticle suspensions9 results in underestimations of the
relative WF values.4,10
To identify the origin of the AIE phenomenon of the fulvenes,
their molecular geometries and electronic structures in the solid
state need to be investigated. Delightfully, the crystal structure of 1
can be determined by the X-ray diffraction (XRD) measurement.{
As can be seen from Fig. 2A, there exist two fulvene molecules in
one asymmetric unit that mainly differ in the torsion angles of the
phenyl groups with respect to the dibenzofulvene ring, presumably
Similar to 1, its derivative carrying an acetyl group (2) also
shows typical AIE behaviours (Fig. S2 in the ESI). Whereas its
acetonitrile solution is virtually nonluminescent with a near-zero
PL efficiency (WF = 0.016%), its aggregates are emissive. Its WF
displays a similar trend to that of 1 (Fig. 1B). At a water content of
90%, its WF is increased to 6.6%, which is two orders of magnitude
(or 416-fold) higher than that of its acetonitrile solution. Again, at
very high water contents, its WF drops. Close inspection reveals
that the lem’s of the aggregates of 2 are blue-shifted from those of
1. The steric hindrance of acetylamino group of 2 may have
weakened the intermolecular electronic interaction and the
hydrogen bonding between the amide groups may have changed
the packing structure: both of these two effects may have
contributed to the blue-shift in the lem’s of its aggregates.
In sharp contrast to 1 and 2, their congener bearing a trifluoro-
acetylamino group (3) is totally AIE inactive. Its dilute solution is
nonemissive (WF = 0.009%), and adding water exerts little effect
on its emission efficiency. The photoinduced electron-transfer
caused by the strong electron-withdrawing trifluoroacetyl group
may have effectively quenched its PL in the solution and solid
states. It now becomes clear that the AIE process of fulvene can be
turned on and off by a simple structural modification of its
substituent.
…
…
as a result of the C–H p and N–H p intermolecular interactions
(Fig. 2B).3c
On the basis of these geometric structures, the electronic spectra
of 1 in different packing states are computed using the ZINDO/S
method.15 For a single molecule of 1, three possible nonradiative
transitions (with zero oscillator strength) at 437, 524 and 539 nm
are found, which may have served as the nonradiative decay paths
that effectively quenched the PL of 1 in the dilute solution. In the
crystalline state (Fig. 2B), a set of three other possible nonradiative
transitions at 439, 442 and 447 nm are identified, which obviously
cannot quench the strong emission of the crystal of 1 at 527 nm.
From the solution state to the crystalline state, the nonradiative
transition of 1 at y527 nm disappears. Clearly the closure of this
nonradiative channel has endowed 1 with the AIE property. This
channel closure is perhaps caused by the restriction of molecular
motions due to the strong intermolecular interactions in the
fulvene aggregates. On the other hand, the nonradiative transition
at y439 nm always exists, which makes this emission invisible
even in the solid state.
The structure shown in Fig. 2A may serve as a model for the
amorphous state, where the intermolecular interactions are limited
1134 | Chem. Commun., 2006, 1133–1135
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to the group of Prof. Y. Luo at the Royal Institute of Technology
in Sweden for its technical assistance in the theoretical calculations.
B.Z.T. thanks the support from the Cao Guangbiao Foundation
of the Zhejiang University.
Notes and references
{ Crystal data for 1: C26H19N, M = 345.42, monoclinic, P2(1), a =
˚
10.6320(18), b = 9.2506(15), c = 18.993(3) A, b = 105.409(3)u, V =
1800.8(5) A , Z = 4, Dc = 1.274 Mg m23, m = 0.073 mm21, F(000) = 728,
3
˚
T = 100(2) K, 2hmax = 25.00u, 10729 measured reflections, 4273
independent reflections (Rint = 0.0543), R1 = 0.0768, wR2 = 0.1450 (all
data), De 0.334 and 20.219 e A23. CCDC 289588. For crystallographic
˚
data in CIF or other electronic format see DOI: 10.1039/b515798f
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In summary, we have investigated the photophysical processes
of three novel fulvene derivatives. We have provned that 1 and 2
are AIE-active and demonstrated that the fulvene emission can be
tuned by changing the substituent structure and by varying the
aggregate morphology. We have further shown that the blockage
of the nonradiative channel of 1 at y527 nm is the cause of its
AIE behaviours. The fulvene-based AIE molecules may find an
array of high-tech applications as luminescent and sensory
materials in optical display, enviromental protection, and biome-
dical imagining.
This project was partially supported by the Hong Kong
Research Grants Council, the National Science Foundation and
the Ministry of Science and Technology of China. We are grateful
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This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 1133–1135 | 1135