Aggregation-induced emission enhancement of polycyclic aromatic alkaloid derivatives and the crucial role of excited-state proton-transfer

Article abstract of DOI:10.1039/c0cc04827e

Aggregation-induced emission enhancement (AIEE) phenomenon is observed in the polycyclic aromatic alkaloid derivatives due to the configuration changes in the excited state, which is attributed to intramolecular proton-transfer and the formation of a new

Full text of DOI:10.1039/c0cc04827e

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 2907–2909
www.rsc.org/chemcomm
COMMUNICATION
Aggregation-induced emission enhancement of polycyclic aromatic
alkaloid derivatives and the crucial role of excited-state proton-transferw
Tao He,a XuTang Tao,*a JiaXiang Yang,b Dan Guo,a HaiBing Xia,a Jiong Jiaac and
MinHua Jianga
Received 6th November 2010, Accepted 21st December 2010
DOI: 10.1039/c0cc04827e
Aggregation-induced emission enhancement (AIEE) phenomenon
is observed in the polycyclic aromatic alkaloid derivatives due to
the configuration changes in the excited state, which is attributed
to intramolecular proton-transfer and the formation of a new
structure of enol form.
has already been rationalized by molecular interactions and
packing. However, the AIEE effect is rarely discussed in the
light of ESPT theory,10 especially from intramolecular inter-
actions. Although a few articles refer to ESPT theory, the
spectrum of the transition state has not been investigated in
depth and crystal structures are not reported.
In the processes of exploring how the ESPT is impacted on
AIEE, two new alkaloid derivatives 4-(2-(3-trifluoroacetyl-
amino)propionylphenylamino)-9-(2-trifluoroacetylamino)ethyl-
acridine-1,2-dione (TBBF) and 4-(2-acetylphenylamino)-9-
methylacridine-1,2-dione (TBBH) were synthesized (Scheme 1).11
They show differences from the general AIEE phenomenon,
because configuration changes have taken place by proton-
transfer when they are excited. In this paper, the optical
properties, crystal structure, pH effect, dynamic conversion
of nanoparticles, NMR and IR spectra were investigated. It is
concluded that TBBF and TBBH are normally more stable as
keto forms in the ground state, whereas, in the excited state,
enol forms are more stable owing to the formation of a new
structure with a conjugated heptacyclic ring. In combination
with the theory of the transition state (Keto–Keto*–Enol*–Enol),
the configuration changes in photoexcitation and an abnormally
large Stokes shift of photoluminescence (PL) spectrum from 470
to 585 nm upon addition of water have been well explained.9,10
Our results will not only enlarge the family of the specific
AIEE-active compounds from polycyclic aromatic alkaloid
derivatives, but also enrich the AIEE mechanism of ESPT theory.
The solutions (such as CH2Cl2, CHCl3, ethanol and DMF)
of TBBH and TBBF were somewhat luminescent, while both
Most organic chromophores are highly emissive in solution but
weakly luminescent in the solid state due to the aggregation-caused
quenching (ACQ) effect. Since the first aggregation-induced
emission (AIE) active material, 1-methyl-1,2,3,4,5-pentaphenyl-
silole, was reported by Tang’s group,1 materials with AIE or AIEE
properties have attracted much attention due to their potential
applications in many fields.2 In the past few years, more and more
compounds with AIE or AIEE properties have been developed,
such as siloles,3 1-cyano-trans-1,2-bis-(40-methylbiphenyl)ethylene
(CN-MBE),4
2,5-diphenyl-1,4-distyrylbenzene
(DPDSB)
derivatives,5 diphenyldibenzofulvene (DPDBF) derivatives,6 con-
jugated polymers,7 and others.8 The previous works have been
focused on the exploration of the mechanism of those AIE- or
AIEE-active molecules. It is concluded that the AIE or AIEE
resulted from the inhibition of the nonradiative channel, that is,
the vibrational/torsional energy relaxation process is blocked by
the stacking of the intermolecular interaction in the aggregate
state. Hence, the AIE or AIEE effect of the molecules has been
found to be usually associated with aromatic groups by rotatable
C–C, C–N or N–N single bonds.3–8 In addition, they are usually
explained on the basis of the theory of restriction of intermolecular
rotation (RIR), intramolecular charge-transfer (ICT), excimer
formation, and excited-state proton-transfer (ESPT).3–9 The
theory of RIR and ICT in explaining the AIEE phenomenon
a State Key Laboratory of Crystal Materials, Shandong University,
Jinan, 250100, P. R. China. E-mail: txt@icm.sdu.edu.cn;
Fax: 0531-88574135
b Department of Chemistry, Anhui University, Hefei, 230039,
P. R. China
c Department of Chemistry, Shandong University, Jinan, 250100,
P. R. China
w Electronic supplementary information (ESI) available: Synthetic
procedures; fluorescence images of solution, aggregates and solid;
time-resolved fluorescence spectra; SEM images; nano-particle size;
HOMO–LUMO energy orbitals; PL spectra in solvents with different
polarity and pH; NMR spectra at various concentrations and IR
spectra; crystallographic data. CCDC 799380. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c0cc04827e
Scheme 1 Synthesis and molecular structure of TBBH and TBBF.
Chem. Commun., 2011, 47, 2907–2909 2907
c
This journal is The Royal Society of Chemistry 2011

View Online
Fig. 1 Absorption and PL spectra of TBBF in ethanol–water
mixtures with different fractions of water; c = 5 mM.
Fig. 3 SEM images of TBBF (volume percent of water: (a) 0, (b) 30,
(c) 50, (d) 70, (e) 90, (f) 100%).
of the solids exhibit strong orange fluorescence under UV-lamp
illumination (365 nm) (Fig. S1, ESIw). The AIEE feature is
also quantitatively characterized by the measurement of their
PL spectra and UV-visible absorption spectra in ethanol
and in water–ethanol mixtures (concentration kept at 5 mM)
(Fig. 1 and S2, ESIw). The main absorption band peaks are
located at around 295 and 460 nm, respectively.
the enhancement of planarity and the increase of conjugation
lead to weak and broad emission, and also promote the
aggregation of nanoparticles to some extent owing to the
increase of intermolecular interactions. Because water is a
poor solvent of TBBF, some molecules begin to pack in the
aggregate state upon addition of water. Therefore the restric-
tion imposed on the intramolecular torsional motions is
strengthened, which aids proton transfer by the formation of
O–H and N–H hydrogen bonds. In 0–30% water–ethanol
mixtures, one-dimensional nanowires were obtained after solvent
evaporation (Fig. 3a and b), while when the water fraction is
above B40%, a large number of nanoparticles are formed
(Fig. 3c and d). Despite this, all the solutions prepared are
homogeneous without visible deposits observed after mixing.
The large portion of the TBBF molecules remaining in the
solvent mixture gradually deposit in a way similar to recrystalli-
zation. So the rotation has been partially limited and fluores-
cence intensity is enhanced in the ‘‘half free’’ state. Upon using
a higher fraction of water in the system, for example, up to
90%, the TBBF molecules quickly agglomerate to form
reddish amorphous ramiform aggregates (Fig. 3e and f). The
aggregate particles predominate in the mixture and their size is
large enough to be easily observed. One can see that the
diameter of particles is hundreds of nanometres (Fig. S4b, ESIw).
With increasing water content molecules gradually change
from the ‘‘half free’’ state to the ‘‘locked’’ state (Fig. 2). Thus
the fluorescence intensity at 481 nm decreases and then
disappears gradually while a peak of 570 nm becomes evident.
Similar results are obtained to TBBH (Fig. S3 and S4a, ESIw),
which exhibit weaker fluorescence in the aggregate state,
owing to the shorter substituent group.
For TBBF, the emissions from pure ethanol solution is weak
and the fluorescence quantum yield (FF) value is small (6.5%)
(Fig. 1 and Table S1, ESIw). With increasing the fraction of
water, the PL of TBBF is switched on and red-shifted from 469
to 481 nm, accompanied by a gradual increase in the emission
intensity which reaches a maximum for 70% water–ethanol
solutions. Then the fluorescence at 481 nm decreases gradually,
and a new peak at 570 nm develops and becomes stronger with
further increasing the water fraction. When the water fraction
is increased to 90%, the FF value rises to 71.9% with a much
longer lifetime (3.795 ns) shown, which is about 50-fold higher
than that of (0.072 ns) in pure ethanol (Table S1, ESIw). The
PL of TBBF in the film state is red-shifted to 585 nm. The
Stokes shift for TBBF is 116 nm in the film compared with that
of in the pure ethanol solution, indicating that there is a large
configurational conversion from the twisted ground state to a
more planar excited state (Fig. 2). For the sake of better
understanding the role of ESPT, we only consider the single-
molecular rotation group. In dilute ethanol solution without
water added, molecules have two points that can twist without
any restriction imposed on the intramolecular torsional
motions. This is regarded as the hypothetical ‘‘free’’ state
and TBBF should have no emission. However, in the actual
excited state, intramolecular proton transfer occurs via N–H
and O–H bonds, which hinders molecular rotation. In this
‘‘half free’’ state, due to restriction of intramolecular rotation,
In order to verify whether proton-transfer exists and how it
impacts on the excited state of TBBF and TBBH, we measured
the PL spectra in solvents with different polarity and pH. At
the same concentration, it is noticed that the emission peaks
increase with the increase of the polarity of the solution
(Fig. S5, ESIw). This indicates that a significant intramolecular
electronic push–pull phenomenon exists in the bonds of
N–H–O. Proton-transfer leads to occupied orbital energy transfer
from the aniline group in the ground state to the whole
molecule in the excited state (Fig. S6, ESIw). In addition,
TBBF is weakly luminescent in acidic solution when the pH
value is lower than 4.31. At a pH of 12.64, the PL intensity is
15-fold higher than that at pH of 3.09 (Fig. 4a). The peak
intensity at 475 nm is decreased and then increased compared
with the peak at 580 nm when the pH is reduced from 12.64 to
3.09 (Fig. 4b), which demonstrates that N–H–O bond is
broken in strong acid or alkali solutions, though it is easier
Fig. 2 Configuration changes from keto to enol forms are influenced
by proton-transfer in the excited-state.
c
2908 Chem. Commun., 2011, 47, 2907–2909
This journal is The Royal Society of Chemistry 2011

View Online
that the AIEE effect is attributed to ESPT, as established by
observation of optical spectra, solution polarity effects,
pH effect, NMR spectra, IR data and crystallography.
ESPT plays a critical role not only on explaining the AIEE
effect, but also on the synthesis of further AIEE-active
compounds. In addition, we forecast that they could be used
as metal ion probes by replacing the water molecules in the
crystal structure.
We thank the financial support from the National Natural
Science Foundation of China (Grant No. 50721002, 50990061,
50802054 and 51002086), 973 program of PR China (Grant No.
2010CB630702).
Fig. 4 PL spectra of TBBF (20 mM) at different pH values (left) and
in normalized form (right) in ethanol–water mixture (1 : 9 v/v).
to form the enol molecular structure. At low pH, molecules in
the ‘‘locked’’ state are destroyed and converted into an
ammonium salt form, which dissolves in water, so the solution
is weakly emissive; at high pH, the ammonium salt form of
TBBF returns to its free amine form, which is insoluble and
aggregates in water. Thus, the stronger fluorescence coming
from the aggregate state is turned on. A similar phenomenon is
observed for TBBH (Fig. S7, ESIw). In fact, the configuration
changes caused by aggregation can be further confirmed by
NMR and IR. The ‘‘half-free’’ state of the larger conjugated
structure gives rise to an increase of the chemical shift upon
increase in concentration (Fig. S8, ESIw). From the IR spectra,
the rocking vibration peak of N–H (1661 cmꢀ1), the stretching
vibration peak of CQO (1737 cmꢀ1) and the enhanced symmetric
stretching vibration of H bonds in CH3 (2852 cmꢀ1) are
observed in solution (Fig. S9a, ESIw), however, they can not
be seen in the solid state (Fig. S9b, ESIw). All of these results
indicate the formation of new rings of the enol form due to the
configuration changes.
Notes and references
1 J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu,
H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang,
Chem. Commun., 2001, 1740.
2 (a) R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes,
R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. D. Santos,
J. L. Bredas, M. Logdlund and W. R. Salaneck, Nature, 1999, 397,
121; (b) J. Chen, C. C. W. Law, J. W. Y. Lam, Y. Dong, S. M. F. Lo,
I. D. Williams, D. Zhu and B. Z. Tang, Chem. Mater., 2003, 15,
1535; (c) G. Qian, B. Dai; M. Luo, D. Yu, J. Zhan, Z. Zhang, D. Ma
and Z. Y. Wang, Chem. Mater., 2008, 20, 6208; (d) Y. N. Hong,
J. W. Y. Lam and B. Z. Tang, Chem. Commun., 2009, 4332;
(e) Y. Liu, X. T. Tao, F. Z. Wang, X. N. Dang, D. C. Zou,
Y. Ren and M. H. Jiang, J. Phys. Chem. C, 2008, 112, 3975.
3 (a) B. Mi, Y. Dong, Z. Li, J. W. Y. Lam, M. Haussler, H. H.
Y. Sung, H. S. Kwok, Y. Dong; I. D. Williams, Y. Liu, Y. Luo,
Z. Shuai, D. B. Zhu and B. Z. Tang, Chem. Commun., 2005, 3583;
(b) G. Yu, S. Yin, Y. Liu, J. Chen, X. Xu, X. Sun, D. Ma, X. Zhan,
Q. Peng, Z. Shuai, B. Z. Tang, D. B. Zhu, W. Fang and Y. Luo,
J. Am. Chem. Soc., 2005, 127, 6335.
4 (a) S.-J. Li, B.-K. An, S.-D. Jung, M.-A. Chung and S. Y.
Park, Angew. Chem., Int. Ed., 2004, 43, 6346; (b) B.-K. An,
S.-K. Kwon, S.-D. Jung and S. Y. Park, J. Am. Chem. Soc.,
2002, 124, 14410.
5 (a) Z. Xie, B. Yang, G. Cheng, L. Liu, F. He, F. Shen, Y. Ma and
S. Liu, Chem. Mater., 2005, 17, 1287; (b) Y. Li, F. Li, H. Zhang,
Z. Xie, W. Xie, H. Xu, B. Li, F. Shen, L. Ye, M. Hanif, D. Ma and
Y. Ma, Chem. Commun., 2007, 231.
6 (a) H. Tong, Y. Dong, M. Haussler, J. W. Y. Lam, H. H.-Y. Sung,
I. D. Williams, J. Sun and B. Z. Tang, Chem. Commun., 2006, 1133;
(b) H. Tong, Y. Dong, Y. Hong, M. Haussler, J. W. Y. Lam,
H. H.-Y. Sung, X. Sun, J. Yu, I. D. Williams, H. S. Kwok and
B. Z. Tang, J. Phys. Chem. C, 2007, 111, 2287.
7 (a) R. Deans, J. Kim, M. R. Machacek and T. M. Swager, J. Am.
Chem. Soc., 2000, 122, 8565; (b) W. Holzer, A. Penzkofer,
R. Stockmann, H. Meysel, H. Liebegott and H. H. Horhold,
Polymer, 2001, 42, 3183.
8 (a) S. Kim, Q. Zheng, G. S. He, D. J. Bharali, H. E. Pudavar,
A. Baev and P. N. Prasad, Adv. Funct. Mater., 2006, 16, 2317;
(b) S. Kim, T. Y. Ohulchanskyy, H. E. Pudavar, R. K. Pandey and
P. N. Prasad, J. Am. Chem. Soc., 2007, 129, 2669; (c) J. Xu, X. Liu,
J. Lv, M. Zhu, C. Huang, W. Zhou, X. Yin, H. Liu, Y. Li and J. Ye,
Langmuir, 2008, 24, 4231; (d) A. Qin, J. W. Y. Lam, F. Mahtab,
C. K. W. Jim, L. Tang, J. Sun, H. H. Y. Sung, I. D. Williams and
B. Z. Tang, Appl. Phys. Lett., 2009, 94, 253308; (e) C. X. Yuan,
X. T. Tao, Y. Ren, Y. Li, J. X. Yang, W. T. Yu, L. Wang and
M. H. Jiang, J. Phys. Chem. C, 2007, 111, 12811.
The proposal that ESPT is operative is also supported by
crystallography (Fig. 5). The torsional angle between the back-
bone and adjacent aniline group is 29.111 and rotational
motions of the aniline group leads to weak fluorescence in
ethanol solution. In the single molecule, there are strong intra-
molecular interactions, which make TBBH easily form a new
structure containing a conjugated heptacyclic ring by O–H
and N–H bonds (Fig. S10a, ESIw) and is the reason why
TBBH has only one emission peak at 580 nm in the film state.
Besides the intramolecular interactions, there are also strong
intermolecular interactions (Table S2, ESIw). Each molecule is
fixed with eight molecules up and down by C–H–p interactions
and H–O bonds (including one water molecule) (Fig. S10b and
S10c, ESIw). These interactions lead to the large Stokes shift of
the emission peak and a gradual increase in the emission
intensity upon the addition of water.
In summary, in this work, we have successfully developed
two novel alkaloid derivatives with an AIEE effect. We propose
9 (a) O. Vendrell, M. Moreno, J. M. Luch and S. Hammes-Schiffer,
J. Phys. Chem. B, 2004, 108, 6616; (b) Chattopadhyay Nitin, Int. J.
Mol. Sci., 2003, 4, 460; (c) T. K. Mukherjee, P. Ahuja, A. L. Koner
and A. Datta, J. Phys. Chem. B, 2005, 109, 12567; (d) E. Abd
E1-Hakam Abou E1-Nasr, A. Fujii, T. Ebata and N. Mikami,
Chem. Phys. Lett., 2003, 376, 788.
10 Y. Qian, S. Li, G. Zhang, Q. Wang, S. Wang, H. Xu, C. Li, Y. Li
and G. Yang, J. Phys. Chem. B, 2007, 111, 5861.
11 (a) G. Gellerman, M. Babad and Y. Kashman, Tetrahedron Lett.,
1993, 34, 1827; (b) G. Gellerman, A. Rudi and Y. Kashman,
Tetrahedron Lett., 1993, 34, 1823.
Fig. 5 Molecular structure of TBBH (left) and packing arrangement
of TBBH in a unit cell (right).
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2907–2909 2909