Multi-maleimides bearing electron-donating chromophores: Reversible fluorescence and aggregation behavior

Article abstract of DOI:10.1021/ja0487527

A₍₌₎-D, [A₍₌₎]₂-D and [A ₍₌₎]₃-D multi-maleimides and multi-itaconimides bearing electron-donating chromophores display a strong fluorescence quenching due to an intramolecular charge-transfer interaction. The electron-accepting C=C bond plays a key role in the intramolecular quenching. For the isomerization of these multi-itaconimides and Michael additions of these multi-maleimides, their emission behavior is irreversible. For the Diels-Alder additions of these multi-maleimides, their emission behavior is reversible due to the reversible opening and closing of intramolecular charge-transfer pathway. Tris-maleimide TMPA peripherally modified with furfural alcohol displays not only reversible fluorescence behavior but also reversible aggregation behavior. Copyright

Full text of DOI:10.1021/ja0487527

Published on Web 09/10/2004
Multi-Maleimides Bearing Electron-Donating Chromophores: Reversible
Fluorescence and Aggregation Behavior
Xin Zhang, Zi-Chen Li,* Kai-Bo Li, Fu-Sheng Du, and Fu-Mian Li*
Department of Polymer Science and Engineering, College of Chemistry and Molecular Engineering,
Peking UniVersity, Beijing 100871, China
Received March 4, 2004; E-mail: zcli@pku.edu.cn
Chart 1
Maleimides have been widely used as important monomers,
cross-linking and labeling groups in polymer chemistry, biological
and pharmacological applications because of their high reactivity
toward amine, alcohol, or thiol groups via Michael addition, and
cyclopentadiene or furan via [4 + 2] Diels-Alder addition.1 One
interesting feature of this Diels-Alder reaction is its thermal
reversibility.2 A retro-Diels-Alder addition readily occurs at an
elevated temperature. Several strategies have been developed for
the application of the Diels-Alder addition to envisage various
macromolecular architectures: (1) Diels-Alder polycondensation
between bis-maleimides and difurans can obtain linear reversible
copolymers,3a even optically active polymers;3b (2) thermoreversible
highly cross-linked networks including hydrogels and interpenetrat-
fluorescence spectroscopy are 0.057 and 0.85 ns for TSPA, 0.069
ing polymer networks could be prepared from the Diels-Alder
reaction between the polymers bearing maleimide or furan moi-
eties;4 (3) Diels-Alder reaction has also been used for polymer
chain extension by the reaction between two moieties with
maleimide or diene end groups.5 More recently, McElhanon et al.
reported thermally cleavable/ reassembling dendrons and dendrimers
based on furan-maleimide Diels-Alder reactions.6 However, little
attention was paid to the photophysical and photochemical proper-
ties, as well as aggregation behavior during Diels-Alder addition.
In this study, we synthesize a series of maleimides bearing
triphenylamine chromophore, their model compounds and corre-
sponding itaconimides (Chart 1). These mono-, bis-, and tris-male-
and 1.12 ns for BSPA, 0.051 and 0.82 ns for MSPA, respectively.
However, almost no fluorescence was observed for MMPA, BMPA,
and TMPA. This suggests that the CdC bonds of maleimide group
have a significant fluorescence quenching effect.
Compared with these maleimides, MIPA, BIPA, and TIPA
display a moderate fluorescence quenching at 375 nm. A new broad
structureless emission peak appears at 551 nm with short lifetime:
0.26 ns for MIPA, 0.36 ns for BIPA, 0.39 ns for TIPA. The broad
emission increases gradually from MIPA, BIPA, to TIPA as the
emission at 375 nm decreases (Figure S-11). According to their
absorption and excitation spectra (Figure S-13), the broad emission
is attributed to an ICT interaction between excited donor and A())
.
imides and itaconimides are symbolized as A())-D, [A ())] -D
2
Time-resolved fluorescence spectrum of TIPA indicates that the
emission at 551 nm initially increases as the emission decays at
375 nm (Figure S-17). This result confirms the ICT mechanism
between A()) and D.
and [A ())] 3-D, respectively, where D denotes the electron-donating
chromophore, A ()) denotes the electron-accepting moieties contain-
ing CdC bonds.
The synthetic routes and characterization of these compounds
are provided in Supporting Information. TMPA, TSPA, TIPA,
MIPA, MSPA are new compounds. The single-crystal X-ray analy-
ses indicate that TMPA and TSPA have a C3 centro-symmetrical
propeller-like geometry arrangement with the center N-C bonds
lying in one plane. These maleimides tend toward a more planar
structure than their model compounds due to the enhanced electron
delocalization interaction.7
It should be noted that the ICT interaction is distinct from the
intermolecular interaction. There is only a slight intermolecular
fluorescence quenching of triphenylamine by maleimide even in
1:30 molar ratio, where no new broad emission or absorption band
was observed (Figure S-15).
It is interesting to see that, after the N,N-dimethyl acetamide
(DMAC) solution (5 × 10-5 M) of TIPA was heated at 120 °C for
1
2 h, its fluorescence disappeared (Figure 1a). H NMR spectra
The absorption at 310.1 nm (π-π* transition) of these male-
imides and their model compounds in 1,2-dichloroethane (DCE)
was found to decrease gradually in the order of TSPA > TMPA >
TIPA, BSPA > BMPA > BIPA, MSPA > MMPA > MIPA (Fig-
ure S-8). A weak broad absorption band at 439.1 nm increased
gradually in an opposite order. The concentration independence of
the broad absorption of TMPA in DCE and its disappearance in
methanol (Supporting Information Figures S-9, S-14) reveal that
the broad absorption results from an intramolecular charge transfer
(ICT).
indicate that TIPA was isomerized into the corresponding tris-
citraconimide TCPA. The same phenomenon was observed for
MIPA and BIPA. The citraconimide group is an R-methyl male-
imide analogue. Electrochemical data obtained from cyclic volta-
mmetry indicate that the citraconimide (EA ) 0.571 eV) and
maleimide (0.565 eV) groups possess a stronger electron-accepting
ability than itaconimide acceptor (1.16 eV) according to their
electron affinity (EA).8a The changes in the free energy (∆G) of
charge transfer determined from the Rehm-Weller equation8b
indicate that the extent of ICT increases in the order of BIPA (∆G
) -1.72 eV), BCPA (∆G ) -1.86 eV), BMPA (∆G ) -1.88
eV); and TIPA (∆G ) -1.82 eV), TCPA (∆G ) -1.95 eV),
TMPA (∆G ) -1.98 eV). Therefore, the isomerization process
Model compounds MSPA, BSPA, and TSPA display a strong
fluorescence at 375 nm in DCE. Their fluorescence quantum yields
and lifetimes determined from steady-state and time-resolved
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J. AM. CHEM. SOC. 2004, 126, 12200-12201
10.1021/ja0487527 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. AFM image (1.0 mg/mL, water (a)) and TEM images (10 mg/
mL, water (b); 1.0 mg/mL, ethanol (c)) of the exo,exo,exo-TMPA-FA
aggregates.
data confirm that the red precipitate is TMPA, suggesting that a
retro-Diels-Alder reaction occurred. After the above solution was
stirred at room temperature for 6 h, the red precipitate disappeared.
These aggregates were observed again (Figure S-20), and the
fluorescence was restored. The same aggregation behavior was also
observed for a mixed exo,exo,exo-/endo,endo,endo-TMPA-FA
(1.0/0.59). To the best of our knowledge, this is the first report on
the reversible aggregation companied with reversible fluorescence
behavior based on the Diels-Alder addition of maleimides.
Figure 1. Irreversible fluorescence spectra and schematic illustration for
the isomerization of TIPA in DMAC (a), Michael addition of TMPA with
piperidine (b), and reversible fluorescence spectra for Diels-Alder addition
of TMPA with furan in chloroform (c). λex ) 310.0 nm.
Acknowledgment. Financial support from the National Natural
Science Foundation of China (No. 50003001) is gratefully ac-
knowledged.
causes a stronger ICT interaction between A()) and D, which leads
to a significant fluorescence quenching. During the isomerization,
the fluorescence is switched from “ON” to “OFF” (Figure 1a).
However, this fluorescence behavior is irreversible.
Michael and Diels-Alder additions are two typical reactions
related to the electron-accepting CdC bonds of maleimides.1
Michael addition of TMPA with piperidine was carried out in
acetone at room temperature. As the Michael addition proceeds,
the electron-accepting CdC bonds are consumed. The deactivated
ICT pathway is closed, which leads to the fluorescence switched
from “OFF” to “ON” (Figure 1b). The fluorescence switch is also
irreversible. A retro-Michael addition is difficult, which can only
occur at 180 °C.9
Supporting Information Available: Synthetic routes, characteriza-
tion, fluorescence and UV-vis spectra, electrochemical data, cyclic
voltammograms of MMPA, MSPA, MIPA, BMPA, BIPA, BSPA,
TMPA, TIPA, TSPA and their energy diagrams of photoinduced charge
transfer, 2D 1H-13C HSQC NMR spectra of exo,exo,exo-TMPA-F and
exo,exo,exo-TMPA-FA; MALDI-TOF mass spectra of TMPA-F,
TMPA-FA, TMPA-P, fluorescence decay curves of MSPA, BSPA, and
TSPA, time-resolved fluorescence spectrum of TIPA, and crystal-
lographic data (CIF) of BMPA, BSPA, TMPA and TSPA. This material
is available free of charge via the Internet at http://pubs.acs.org.
Diels-Alder adduct of TMPA with furan (exo,exo,exo-TMPA-
F)10 also displays a strong fluorescence at 375 nm. Although a new
CdC bond was formed in the adduct TMPA-F, it does not show
any fluorescence quenching. This result supports that the fluores-
cence quenching is only due to the electron-accepting CdC bond,
which can interact with donor in excited state. At 60 °C, a retro-
Diels-Alder addition readily occurs in TMPA-F to give out furan,
where the electron-accepting CdC bond is formed again. Therefore,
for TMPA, the deactivated ICT pathway is open between A()) and
D, and its fluorescence is switched in an “OFF” state. For TMPA-
F, the ICT pathway is closed, and its fluorescence is switched in
an “ON” state. The two states are reversible due to a reversible
Diels-Alder addition leading to a reversible opening and closing
of the ICT pathway. To the best of our knowledge, this is the first
report on the reversible fluorescence behavior based on the Diels-
Alder reaction.
References
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(7) The single-crystal X-ray analyses indicate that TMPA and BMPA display
a smaller torsion angle (53.7° and 62.1°) between its phenyl ring and
adjacent maleimide ring, and higher crystal density (1.450 and 1.376
Mg/m3) than corresponding saturated compounds TSPA (59.2°, 1.432
Mg/m3) and BSPA(67.3°, 1.341 Mg/m3), respectively.
The Diels-Alder adduct of TMPA with furfural alcohol
(exo,exo,exo-TMPA-FA)10 was found to self-organize into spheri-
cal aggregates (1.0 mg/mL) and dendritic morphology (10 mg/mL)
in deionized water (Figures 2a,b, S-20, and S-21). In ethanol,
interestingly, helical morphology was observed (Figures 2c and
S-21). The helical formation may be due to the inducing effect of
the propeller-like structure of the TSPA core. The detailed study
on the aggregation behavior is underway. These aggregates display
a strong fluorescence at 378 nm. More interestingly, these ag-
gregates disappeared at the elevated temperature of 75-80 °C. A
red precipitate appeared, and no fluorescence was observed. Spectral
(8) (a) Electron affinity was calculated from reduction potential Ered(A-/A)
of the acceptor using the relationship of Chen (Chen, E. C. M.; Wentworth,
W. E. J. Chem. Phys. 1975, 63, 3183). (b) ∆G ) Eox(D/D+) - Ered(A-/
A) - E00(D*) - C, where Eox is the oxidation potential of the donor.
E00(D*) is the excitation energy, and C is the electrostatic interaction term.
(9) Mison, P.; Sillion. B. AdV. Polym. Sci. 1999, 140, 137.
(10) TMPA-F and TMPA-FA obtained initially are a mixed exo,exo,exo-/endo,-
endo,endo-isomers. The pure exo, exo, exo-isomer can be obtained by
heating the mixed isomers in chloroform with an addition of furan or
furfural alcohol at 40 °C for 2 days, where the endo,endo,endo-isomer
was converted completely into the (exo,exo,exo)-isomer. The exo,exo,-
exo-isomer is relative stable, which does not reconvert into endo,endo,-
endo-isomer before retro-Diels-Alder occurring.
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