Fluorescence responsive conjugated poly(tetraphenylethene) and its morphological transition from micelle to vesicle

Article abstract of DOI:10.1039/c5cc00934k

A crown ether-functionalized poly(tetraphenylethene) (AP-TPE) is synthesized and the rotation of the TPE group is successfully restricted via the complexation of crown ether and organic ammonium salts, leading to a stepwise enhanced fluorescence accompani

Full text of DOI:10.1039/c5cc00934k

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Fluorescence responsive conjugated
poly(tetraphenylethene) and its morphological
Cite this:DOI: 10.1039/c5cc00934k
transition from micelle to vesicle†
Received 1st February 2015,
Accepted 17th March 2015
Lipeng He, Xiaoning Liu, Jianjun Liang, Yong Cong, Zhenyu Weng and Weifeng Bu*
DOI: 10.1039/c5cc00934k
www.rsc.org/chemcomm
A crown ether-functionalized poly(tetraphenylethene) (AP-TPE) is can be extended to the AIE and AIEE systems. To date, several non-
synthesized and the rotation of the TPE group is successfully covalent interactions have been used to activate the AIE/AIEE feature
restricted via the complexation of crown ether and organic ammonium successfully.8 However, these systems do not show significant
salts, leading to a stepwise enhanced fluorescence accompanied by fluorescence responsiveness to external stimuli. In this communica-
a morphological transition from micellar to vesicular.
tion, we have synthesized an AIEgen-containing conjugated polymer
(AP-TPE) (Scheme 1). Upon treatment of the solution of AP-TPE
Owing to high quantum yield, excellent photophysical stability successively with acid and base, the fluorescence intensity was
and facile surface modification, conjugated polymers (CPs) have remarkably enhanced and decreased, respectively, together with a
drawn wide attention in the fields of optoelectronics and biology morphological transition from micellar to vesicular aggregates.
in recent years.1 However, the conventional CPs often show weak
AP-TPE was synthesized via the Sonogashira cross-coupling reac-
or quenched fluorescence in their concentrated solutions and tion of dibenzo-24-crown-8 (DB24C8)-substituted 1,4-diiodobenzene
aggregate states because of the aggregation-caused quenching
effect, which has limited their real applications in devices such
as light-emitting diodes (LEDs) and bio-imaging/sensors.2,3
Recently, aggregation-induced emission (AIE) and aggregation-
induced emission enhancement (AIEE) have been demonstrated
as promising methods to tackle the defect.4 In the AIE/AIEE
systems, luminogens are non-emissive or weakly emissive in the
dilute solution but significantly emissive in their concentrated
solutions and/or aggregated states. Therefore, incorporation of
AIE luminogens (AIEgens) into CPs is a significant attempt to
avoid fluorescence quenching.3 In AIEgen-containing CPs, the
steric effect of the rigid chain suppresses intra-molecular motions
of AIEgens and thus their emission is remarkably enhanced.3
On the other hand, supramolecular polymers (SPs) have also
emerged in the optoelectronic and biological fields due to their
multiple responsiveness, superb self-healing and degradability.5
Combining non-covalent interactions with conjugated molecules,
we6 and others7 have developed various fluorescence responsive SPs,
many of which exhibit fluorescence quenching or weakening upon
non-covalent interactions. We therefore ask whether such studies
Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu
Province, State Key Laboratory of Applied Organic Chemistry and College of
Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu, China.
E-mail: buwf@lzu.edu.cn
† Electronic supplementary information (ESI) available: Experimental procedures
Scheme 1 (a) Synthetic approach of AP-TPE; (b) chemical structures of
and full characterizations of AP-TPE, C12-1, C12-1HꢀPF6, C12-2 and C12-2HꢀPF6,
the guest molecules, C12-1HꢀX and C12-2HꢀX, X = Cl and PF6.
additional DLS plots and TEM images. See DOI: 10.1039/c5cc00934k
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(1) and 1,2-bis(4-ethynylphenyl)-1,2-diphenylethene (2) as shown
in Scheme 1a. 1H NMR (Fig. S4 and S5, ESI†), gel permeation
chromatography methods (Fig. S6, ESI†) and infrared spectra
(Fig. S7, ESI†) confirmed the successful polymerization reaction
and the formation of AP-TPE. UV-vis absorption spectra of 2 and
AP-TPE in their THF solutions (10 mM) are depicted in Fig. S20
(ESI†). In comparison with 2, the absorption band of AP-TPE showed
a large red-shift from 329 to 378 nm, agreeing well with the high
conjugation of AP-TPE.1b,3b Upon excitation at 375 nm, AP-TPE in
pure THF exhibited a weak fluorescence, which was similar with
the reported TPE-based polymer.3 Considering the AIE feature of
AP-TPE, an amount of water was added to the THF solution of
AP-TPE. As shown in Fig. 1, with the increase of water fraction, the
fluorescence intensity of AP-TPE started to increase and reached its
maximum value at 70% water content, which was 4.6 times higher
than that in pure THF solution. However, when the water content
increased to 90%, the fluorescence intensity dropped to some extent.
This was because the precipitation of the large-sized aggregates
decreased the effective AP-TPE concentration in the solution.9
Besides this, amorphous aggregates abruptly formed at 90% water
Fig. 2 Partial 1H NMR spectra (400 MHz, CD2Cl2) of (a) 0.5 mM AP-TPE,
(b) 0.5 mM AP-TPE (monomer concentration) and 1.0 equivalent of C12-2
HꢀPF6 (DBA/DB24C8 1 : 1 molar ratio), (c) obtained by adding 2.0 equiva-
lents of P1-tBu to (b), (d) obtained by adding 2.0 equivalents of CF3COOH
to (c), and (e) guest C12-2HꢀPF6 (CDCl3/CD3CN, 1/1). Here ‘‘u’’ and ‘‘c’’
denote uncomplexed and complexed moieties, respectively.
content could trap the solvent molecules inside. In these loose 2.0 equivalents of CF3COOH were successively added, leading to the
1
appearance of the complexed H NMR signals again, but together
with small amounts of uncomplexed DB24C8 and DBA resonances
(Fig. 2d), which were slightly different from those shown in Fig. 2b.
Similarly, the addition of C12-1HꢀPF6 to AP-TPE also gave rise
to comparable 1H NMR spectral changes as described above
aggregates, the intramolecular motions of TPE groups were not
restricted completely, which also decreased the emission.9 From the
inner photographs in Fig. 1b, we could catch sight of the weak
emission in THF solution but strong emission in the mixed solvent
of 90% water content.
The complexation between AP-TPE and guest groups was initially (Fig. S22, ESI†).
1
investigated by H NMR spectroscopy as shown in Fig. S22 (ESI†)
Next, we examined the sizes of the above-mentioned host–guest
and Fig. 2. Upon treatment of AP-TPE (0.5 mM) with 1.0 equivalent complexes through dynamic light scattering (DLS) and transmission
electron microscopy (TEM) measurements. The results of DLS
of C12-2HꢀPF6, the resonance signals of DB24C8 moieties and
experiments are shown in Fig. S23a (ESI†) and Fig. 3a. In the DLS
plot, AP-TPE exhibited a narrow band centred at a hydrodynamic
diameter (Dh) of 185 nm, which was much larger than the length of
the polymer (ca. 39 nm based on DP = 20), probably due to
dibenzylammonium salts (DBAs) shifted significantly and became
more broadened (Fig. 2a and b). Among these signals, the benzyl
protons H4 of C12-2HꢀPF6 shifted downfield from d = 4.15 to
4.50 ppm, while the others shifted upfield. No uncomplexed signal
was detected, indicating that the crown ether moieties were com- aggregation of AP-TPE under the present solvent condition. After
pletely threaded by the DBA groups and thus the network complex the complexation with C12-2HꢀPF6, the Dh of the network complexes
was formed as shown in Fig. S21 (ESI†). Subsequent in situ addition increased to 294 nm together with a much broader signal. Corre-
of 2.0 equivalents of N-tert-butyl-N0,N0,N00,N00,N0 0 0,N0 0 0-hexamethyl-
phosphorimidic triamide (P1-tBu) caused the deprotonation of
C12-2HꢀPF6 and the complexed resonance consequently dis-
appeared. The resulting 1H NMR spectrum originated from the
combined signals of AP-TPE and neutral C12-2 (Fig. 2c). And then,
spondingly, TEM images revealed that irregular aggregates (Fig. S24,
ESI†) evolved to spherical nanostructures with diameters of
250–400 nm (Fig. 3b and c). Upon addition of 2.0 equivalents of
P1-tBu to the complexes, Dh returned back to the original location.
However, almost no change in Dhs was detected during the self-
assembly of AP-TPE and C12-1HꢀPF6 (Fig. S23a, ESI†).
Interestingly, we therefore asked whether the aggregation could
activate the AIEE nature of AP-TPE. Upon addition of C12-2HꢀPF6,
the fluorescence band of AP-TPE (0.5 mM) centred at 525 nm
showed a clear increase in intensity (Fig. 3d), which agreed well
with the increase in Dh as described above. Instead of treatment
with extra host and/or guest molecules as demonstrated in previous
pillar arene-based AIEE systems,8 the facile addition of P1-tBu
completely restored the enhanced emission to the original level.
The difference was that only less fluorescence enhancement was
observed in the case of C12-1HꢀPF6 and AP-TPE, consistent with the
slight increase in Dh (Fig. S23, ESI†). Corresponding to previous
experimental studies,4,6d the host–guest recognition restricts the
intramolecular rotation of TPE moieties or other AIEgens at the
Fig. 1 (a) Fluorescence spectra excited at 375 nm and (b) the plot of
fluorescence intensity of AP-TPE (10 mm) in THF–water mixture solvents
with different water fractions. The inner photographs show AP-TPE in the
mixed THF–water solvent at 0% and 90% water content under UV irradiation.
The concentration referred to the monomer concentration.
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fact that there was a 2.2 times increase in the fluorescence intensity
(Fig. 4a and b). Subsequent addition of 2.0 equivalents of NaOH led
to a transparent solution accompanied by a decrease in fluores-
cence. Afterwards, the solution was treated with the acid–base
cycles twice and the fluorescence became stronger step by step
(Fig. 4a and b). The fluorescence intensity increased nearly 4.3 times
after the third acidification. Furthermore, we carried out several
control experiments to explain the phenomenon (Fig. S27, ESI†)
and found that the interaction between DB24C8 and DBA and the
salting-out effect played leading roles in such a stepwise enhanced
fluorescence. According to the previous studies,11 when the TPE
moiety took a tighter packing in crystals or highly ordered struc-
tures, the intra-molecular motion was restricted more completely,
thus leading to stronger and blue-shifted emissions. In our system,
the blue-shift of the emission peak from 523 to 514 nm had
certified a more and more compact packing of AP-TPE molecules.
No significant change was detected in the fluorescence intensity
when the THF solution of AP-TPE and C12-1 was treated with the
same acid–base cycles (Fig. S25, ESI†).
Fig. 3 (a) DLS plot of AP-TPE, (b and c) TEM images of aggregates of
AP-TPE and (d) fluorescence responsiveness of AP-TPE upon successive
treatment with C12-2HꢀPF6 and P1-tBu (CH2Cl2, 0.5 mM).
molecular level and the non-radiative pathway is blocked, lead-
To understand the observed phenomena, DLS and TEM mea-
ing to a visible fluorescence enhancement. Comparatively, in the surements were employed to check the Dhs and aggregate struc-
case of the network complex formed by C12-2HꢀPF6 and AP-TPE, tures of the complexes. In the initial solution of AP-TPE and C12-2,
both the intra- and intermolecular rotations were restricted due the DLS plot exhibited a narrow band centred at 338 nm (Fig. 5a).
to the cross-linked topological structure and thus the fluores- Upon acid treatment with HCl, the complexation of DB24C8 and
cence was significantly enhanced. When the base was added, DBA moieties occurred, leading to the formation of supramolecular
the intra- and intermolecular motions were not suppressed any assemblies. Consequently, the value of Dh increased to 492 nm and
more and the fluorescence was thus recovered. These facts spherical micelles were observed in TEM images (Fig. 5b). The
highlighted the fact that the aggregation benefiting from the ordered micellar aggregates resulted in a much stronger fluores-
host–guest interaction of DB24C8 and DBA groups can activate cence as shown in Fig. 4. When the solution was subsequently
the AIEE feature to a great extent in the present case.
treated with NaOH, the fluorescence intensity dropped remarkably.
However, the present fluorescence enhancement was rather less However, the DLS plot extraordinarily displayed that Dh of the
than that of TPE-based CPs in the mixed THF–water solvents mixture did not revert back to its original value but increased to
(Fig. 1).3 In contrast with the threading structure of DB24C8 with 631 nm with a broad distribution (Fig. 5a). We therefore speculated
DBAꢀPF6,10a–d DB24C8 can bind DBA with the chloride counter that AP-TPE might form some large but unordered aggregates in
anion via the N–HꢀꢀꢀO hydrogen bond in a face-to-face manner, the solution after the first acid–base cycle. As expected, irregular
and form a more compact and ordered matrix.10e, f Upon successive and unordered aggregates were clearly observed in TEM images
treatment of HCl and NaOH, we therefore monitored fluorescence (Fig. S30, ESI†), substantiating our speculation. In the latter two
changes of the THF solution containing AP-TPE and C12-1/C12-2. cycles, the DLS plots exhibited a similar trend and the only
The experimental data are depicted in Fig. S25 (ESI†) and Fig. 4. difference from the first cycle was that the sizes of aggregates grew
When 2.0 equivalents of HCl were added to the neutral solution of stepwise to 586 and 622 nm upon acidification, respectively.
AP-TPE/C12-2, the solution became turbid immediately (Fig. S26,
ESI†). Meanwhile, the fluorescence spectra revealed an amazing
Fig. 4 (a) Fluorescence spectra and (b) the plot of fluorescence intensity
of AP-TPE/C12-2 in THF treated with HCl and NaOH, repeatedly. The
concentrations of AP-TPE and C12-2 were 0.5 mM. The concentrations of
HCl and NaOH were 1.0 M in H2O. Here, N, A and B denote the neutral
solution and its acid and base treatments, respectively. 1, 2 and 3 denote
the sequence of the acid–base cycle.
Fig. 5 (a) DLS plot of AP-TPE and C12-2 in THF treated with HCl and
NaOH repeatedly. (b–d) TEM images of AP-TPE and C12-2 after the first,
second and third acidification. (e and f) SEM images of AP-TPE and C12-2
after the third acidification.
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C. K. W. Jim, J. W. Y. Lam, M. M. F. Yuen, G. Ramos-Ortiz and
B. Z. Tang, J. Mater. Chem., 2012, 22, 232.
As shown in Fig. 5c, the self-assembly of AP-TPE with C12-2 formed
spherical aggregates with diameters of 400–600 nm after the second
acidification. The spherical aggregates showed a clear contrast
between the exterior and interior, which was consistent with the
typical feature of vesicles. After the third acidification, many more
legible vesicles were observed with diameters ranging from ca.
500 to 700 nm (Fig. 5d). To confirm the vesicular structure,
scanning electron microscopy (SEM) measurement was further
performed (Fig. 5e and f). Several collapsed and ruptured vesicles
were captured. Furthermore, the inside and outside of ruptured
vesicles were clearly observed. Usually, vesicular aggregates origi-
nated from the solvophobic self-assembly of molecules in a specific
solvent.12 In our case, DB24C8 bound with DBA to form polar
groups after the first acidification. As a result of the solvophobic
effect, the complex formed the micelle and the polar groups were
located on the inner side of the micelle. The subsequent acid–base
reaction led to the generation of NaCl, which further promoted the
aggregation due to the salting-out effect. Until the next acidifica-
tion, both the interaction between DB24C8 and DBA and the
salting-out effect modified the solvophilic–solvophobic effect and
the system reached a new balance, and thus a highly ordered
vesicle was formed. These changes were consistent with the step-
wise enhanced fluorescence and blue-shifted emission peaks.11 To
the best of our knowledge, such a morphological transition-
induced step-by-step enhancement has seldom been brought out
in the previous AIE/AIEE systems.3,4,8,11
4 (a) 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;
(b) B.-K. An, S.-K. Kwon, S.-D. Jung and S. Y. Park, J. Am. Chem. Soc.,
2002, 124, 14410; (c) L. Liu, G. Zhang, J. Xiang, D. Zhang and D. Zhu,
Org. Lett., 2008, 10, 4581; (d) J. He, B. Xu, F. Chen, H. Xia, K. Li, L. Ye
and W. Tian, J. Phys. Chem. C, 2009, 113, 9892; (e) Q. Chen, N. Bian,
C. Cao, X.-L. Qiu, A.-D. Qi and B.-H. Han, Chem. Commun., 2010,
46, 4067; ( f ) H. Lu, B. Xu, Y. Dong, F. Chen, Y. Li, Z. Li, J. He, H. Li
and W. Tian, Langmuir, 2010, 26, 6838; (g) X. Wang, J. Hu, T. Liu,
G. Zhang and S. Liu, J. Mater. Chem., 2012, 22, 8622; (h) Z. Wang,
S. Chen, J. W. Y. Lam, W. Qin, R. T. K. Kwok, N. Xie, Q. Hu and
B. Z. Tang, J. Am. Chem. Soc., 2013, 135, 8238; (i) Z. Wang, B. Xu,
L. Zhang, J. Zhang, T. Ma, J. Zhang, X. Fu and W. Tian, Nanoscale,
2013, 5, 2065; ( j) R. Yoshii, A. Hirose, K. Tanaka and Y. Chujo, J. Am.
Chem. Soc., 2014, 136, 18131.
5 (a) P. Cordier, F. Tournilhac, C. Soulie-Ziakovic and L. Leibler,
Nature, 2008, 451, 977; (b) M. Burnworth, L. Tang, J. R. Kumpfer,
A. J. Duncan, F. L. Beyer, G. L. Fiore, S. J. Rowan and C. Weder,
Nature, 2011, 472, 334; (c) S. Seiffert and J. Sprakel, Chem. Soc. Rev.,
2012, 41, 909; (d) S.-L. Li, T. Xiao, C. Lin and L. Wang, Chem. Soc.
Rev., 2012, 41, 5950; (e) T. Aida, E. W. Meijer and S. I. Stupp, Science,
2012, 335, 813; ( f ) B. C. Tee, C. Wang, R. Allen and Z. Bao, Nat.
Nanotechnol., 2012, 7, 825; (g) Z. Huang, L. Yang, Y. Liu, Z. Wang,
O. A. Scherman and X. Zhang, Angew. Chem., Int. Ed., 2014, 53, 5351;
(h) P. Wei, X. Yan and F. Huang, Chem. Soc. Rev., 2015, 44, 815.
6 (a) B. Yu, B. Wang, S. Guo, Q. Zhang, X. Zheng, H. Lei, W. Liu, W. Bu,
Y. Zhang and X. Chen, Chem. – Eur. J., 2013, 19, 4922; (b) B. Yu,
S. Guo, L. He and W. Bu, Chem. Commun., 2013, 49, 3333; (c) S. Guo,
J. Zhang, B. Wang, Y. Cong, X. Chen and W. Bu, RSC Adv., 2014,
4, 51754; (d) L. He, J. Liang, Y. Cong, X. Chen and W. Bu, Chem.
Commun., 2014, 50, 10841.
7 X. Ji, Y. Yao, J. Li, X. Yan and F. Huang, J. Am. Chem. Soc., 2013,
135, 74.
In summary, we have confirmed in our attempt that the
complexation of DB24C8 and DBA groups can suppress the motion
of the TPE moiety and thus activate the AIEE feature of AP-TPE.
Particularly, the reduplicative complexation of AP-TPE and C12-2Hꢀ
Cl not only enhances the fluorescence remarkably, but also induces
a morphological transformation from micellar to vesicular. More-
over, it is emphasized that such acid–base responsive vesicular
assembly/disassembly has a potential for encapsulation and release
of small molecules such as drugs and dyes.
This work was supported by the NSFC (51173073 and
J1103307), the Fundamental Research Funds for the Central
Universities (lzujbky-2014-74) and the Open Project of State Key
Laboratory of Supramolecular Structure and Materials of Jilin
University (sklssm201502).
8 (a) P. Wang, X. Yan and F. Huang, Chem. Commun., 2014, 50, 5017;
(b) J. Wu, S. Sun, X. Feng, J. Shi, X.-Y. Hu and L. Wang, Chem.
Commun., 2014, 50, 9122; (c) N. Song, D.-X. Chen, M.-C. Xia,
X.-L. Qiu, K. Ma, B. Xu, W. Tian and Y.-W. Yang, Chem. Commun.,
2015, 51, 5526.
9 (a) Y. Hong, S. Chen, C. W. T. Leung, J. W. Y. Lam, J. Liu, N.-W.
Tseng, R. T. K. Kwok, Y. Yu, Z. Wang and B. Z. Tang, ACS Appl. Mater.
Interfaces, 2011, 3, 3411; (b) E. Wang, J. W. Y. Lam, R. Hu, C. Zhang,
Y. S. Zhao and B. Z. Tang, J. Mater. Chem. C, 2014, 2, 1801;
(c) J. Huang, Y. Jiang, J. Yang, R. Tang, N. Xie, Q. Li, H. S. Kwok,
B. Z. Tang and Z. Li, J. Mater. Chem. C, 2014, 2, 2028.
10 (a) P. R. Ashton, P. J. Campbell, E. J. T. Chrystal, P. T. Glink,
S. Menzer, D. Philp, N. Spencer, J. F. Stoddart, P. A. Tasker and
D. J. Williams, Angew. Chem., Int. Ed. Engl., 1995, 34, 1865;
(b) A. G. Kolchinski, D. H. Busch and N. W. Alcock, J. Chem. Soc.,
Chem. Commun., 1995, 1289; (c) M. Han, H.-Y. Zhang, L.-X. Yang,
Q. Jiang and Y. Liu, Org. Lett., 2008, 10, 5557; (d) H.-B. Cheng,
H.-Y. Zhang and Y. Liu, J. Am. Chem. Soc., 2013, 135, 10190;
(e) S. Dong, B. Zhang, D. Xu, X. Yan, M. Zhang and F. Huang, Adv.
Mater., 2012, 24, 3191; ( f ) L. Gao, D. Xu and B. Zheng, Chem.
Commun., 2014, 50, 12142.
Notes and references
11 (a) X. Wang, J. Hu, T. Liu, G. Zhang and S. Liu, J. Mater. Chem., 2012,
22, 8622; (b) Z. Zhao, J. W. Y. Lam and B. Z. Tang, J. Mater. Chem.,
2012, 22, 23726; (c) J. Ma, T. Lin, X. Pan and W. Wang, Chem. Mater.,
2014, 26, 4221; (d) W. Bai, Z. Wang, J. Tong, J. Mei, A. Qin, J. Z. Sun
and B. Z. Tang, Chem. Commun., 2015, 51, 1089.
12 (a) W. Bu, S. Uchida and N. Mizuno, Angew. Chem., Int. Ed., 2009,
48, 8281; (b) L. He, S. Bi, H. Wang, B. Ma, W. Liu and W. Bu,
Langmuir, 2012, 28, 14164; (c) Q. Zhang, L. He, H. Wang, C. Zhang,
W. Liu and W. Bu, Chem. Commun., 2012, 48, 7067; (d) P. Yin, T. Li,
R. S. Forgan, C. Lydon, X. Zuo, Z. N. Zheng, B. Lee, D. Long,
L. Cronin and T. Liu, J. Am. Chem. Soc., 2013, 135, 13425.
1 (a) S. W. Thomas, G. D. Joly and T. M. Swager, Chem. Rev., 2007,
107, 1339; (b) T. L. Andrew and T. M. Swager, J. Polym. Sci., Part B:
Polym. Phys., 2011, 49, 476; (c) B. Liu and G. C. Bazan, Conjugated
Polyelectrolytes: Fundamentals and Applications, Wiley, 2013;
(d) K. Mu¨llen, J. R. Reynolds and T. Masuda, Conjugated Polymers:
A Practical Guide to Synthesis, RSC, 2014.
2 (a) S. A. Jenekhe and J. A. Osaheni, Science, 1994, 265, 765; (b) B. Valeur,
M. N. Berberan-Santos, Molecular Fluorescence: Principles and Applications,
Wiley-VCH Verlag & Co., Weinheim, Germany, 2nd edn, 2013.
3 (a) R. Hu, N. L. C. Leung and B. Z. Tang, Chem. Soc. Rev., 2011,
40, 5361; (b) R. Hu, J. L. Maldonado, M. Rodriguez, C. Deng,
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2015