green to blue region tend to show larger Stokes shifts than those
in the yellow to red region. It is intriguing to ask what is the
other factor, in addition to energy gap law, in controlling the
fluorescence intensity and Stokes shift of these maleimides,
which are key properties in the application of fluorophores for
OLEDs. Spectroscopic data of 1-NPHMLMe and
2-NPHMLMe may provide
a
clue to the answer.
2-NPHMLMe has a much higher ff and extinction coefficient
(e) than 1-NPHMLMe. Here the electronic factor is unlikely to
be decisive because both maleimides are relatively similar in
absorption and emission energy (Table 1). Steric hindrance
appears to be a possible reason for the affect on the magnitude
of ff in 1-NPHMLMe and 2-NPHMLMe. The sterically
hindered 1-NPHMLMe shows one of the lowest fluorescence
intensities on the list (Table 1). It is also interesting to note that
em
Fig. 1 Fluorescence image of 3,4-diaryl-substituted maleimide derivatives
in dichloromethane showing varied colours. From left to right, the first row
EtAMLMe, NPAMLMe, PhAMLMe, and TPAMLMe; the second row
1-NPHMLH, MLMe, 1-NPHMLMe, 2-NPHMLH, and 2-NPHMLMe;
the third row 3-PYDMLH, 4-MeOMLMe, 2-THPMLH, 2-THPMLMe,
and MLH; the fourth row 3-CF3MLH, 3-CF3MLMe, 4-CF3MLH,
4-CF3MLMe, and 4-MeOMLH.
the sterically less hindered 2-NPHMLMe has a shorter lmax
wavelength than that of 1-NPHMLMe, although the former has
ab
a longer lmax wavelength. Insightful structure–property
relationships are currently being pursued through theoretical
calculations and we are preparing more maleimide derivatives
for spectroscopic examination.
In summary, we have developed a one-step synthesis of
3,4-diaryl-substituted maleimides directly from easily-acces-
sible acetonitrile derivatives. Our preliminary examination
indicates that steric factors in addition to electronic effects play
an important role in determining the fluorescence energy and
intensity of these colourful fluorophores.
Table 1 Spectroscopic data of 3,4-diaryl-substituted maleimides
lmaxabs, e
em
(nm/nm)a,
lmax
D
ff
(%)
(M21cm21 b
)
(nm/nm)a
(cm21/cm21 a
)
Financial support from the National Science Council and
Academia Sinica is gratefully acknowledged.
MLH
MLMe
350/357, 6030
361363, 4010
373/384, 4690
377/383, 4410
391/392, 10510
395/398, 9250
339/344, 5710
352/359, 3630
339/340, 2180
350/353, 3220
387/404, 7560
399/408, 7140
340/346, 5640
414/421, 7650
423/428, 6400
474/485, 13900
476/485, 14830
480/485, 13690
481/512, 11050
471/493
490/503
497/544
507/547
488/523
499/526
460/471
477/490
459/470
475/487
506/548
518/551
464/482
514/545
530/552
572/642
575/650
581/653
581/659
7300/7700
7300/7700
6700/7700
6800/7800
5100/6400
5300/6100
7800/7800
7400/7400
7700/8100
7500/7800
6100/6500
5800/6400
7900/8200
4700/5400
4800/5300
3600/5000
3600/5200
3600/5300
3600/4400
21c
31c
2c
1-NPHMLH
1-NPHMLMe
2-NPHMLH
2-NPHMLMe
3-CF3MLH
3-CF3MLMe
4-CF3MLH
4-CF3MLMe
4-MeOMLH
4-MeOMLMe
3-PYDMLH
2-THPMLH
2-THPMLMe
NPAMLMe
TPAMLMe
PhAMLMe
EtAMLMe
Notes and references
† Full synthesis details and characterization data will be given elsewhere.
‡ ff of each maleimide was determined by the literature procedure of J. N.
Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991.
3c
12c
22c
30d
53d
8d
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26d
25c
26c
6d
7c
5c
13e
11e
10e
5e
a In hexane and dichloromethane solutions, respectively. b In dichloro-
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in 1,4-dioxane (68%).13
shifts in more polar solvents (dichloromethane vs. hexane in
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to our surprise, considering the unavoidable nonplanar con-
formation of the adjacent 3,4-disubstituted phenyl rings that
partially block the interaction of substituents to the five-member
maleimide ring. Nevertheless, there are some general trends in
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em
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methylation, maleimides have a longer wavelength lmax
(such as 470 nm for purple blue 4-CF3MLH vs. 487 nm for sky
blue 4-CF3MLMe), a higher solution fluorescence quantum
yield‡ (ff) (such as 30% for 3-CF3MLH vs. 53% for
3-CF3MLMe), and a smaller Stokes shift (such as 6534 cm21
for 4-MeOMLH vs. 6361 cm21 for 4-MeOMLMe).
The simple energy gap law does not work well for 3,4-diaryl-
substituted maleimides in term of ff. However, the magnitude
of Stokes shift seems to obey, i.e., those emitting light in the
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