Design and Synthesis of Highly Efficient Nonlinear Optical Chromophores
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ple MOPAC modeling. The calculation reveals that the α-perfluoroaryl or the αphenyl dicyanovinyl group severely twists out of the plane consisting of amino, π-bridge, and malanonitrylene. These conformational spacers create significant steric hindrance to prevent the chromophore molecules from electrostatic aggregation. The synthetic routes for these two series of new chromophores are quite different. Perfluoroaryl dicyanovinyl chromophores 4-9 are directly derived from corresponding tricyanovinyl chromophores 1-3. As shown in the scheme, tricyanovinyl chromophore was added in one portion into a perfluoroaryl lithium solution of THF at -70 oC. The A: Synthesis of Perfluoroaryl dicyanovinyl Chromophores
NC
R N
CN
R S Ar Li
R = C 4H 9 , P h , ( c 4 H 9 )C 6 H 4
CN o C
THF, -78
F
F
F
F
F
F
NC
R N
CN
Ar =
N
or
F
R S
F
Ar
F
B: Synthesis of Ph-TCBD Chromophores R N R
Ph n TCNE
S R = C 2H 5 , C 4H 9 , P h , ( c 4 H 9 )C 6 H 4
C H C l3 , R T .
n = 0, 1
NC
R N
CN
R
CN n
S Ph
CN
substitution reaction mixture was stirred at -70 oC for 1 h then allowed to warm to room temperature. The key in the multistep synthesis of Ph-TCBD chromophores is a [2+2] cycloaddition between tetracyanoethylene (TCNE) and an electron-rich bis-aryl acetylene in a chloroform solution at room temperature [10]. The following quick ring-opening reaction of cyclobutene resulted in Ph-TCBD containing chromophores 10-15. Although we had some difficulty in growing single crystals for all the chromophores, the interesting 3D structure of chromophore 10 predicted by MOPAC calculation was well matched with the results from the X-ray crystallography. The single crystal structure of 10 reveals that a malanonitrylene moiety is linked in co-plane with the donor-substituted π-electron connective segment and forms an efficient push-pull system while the α-phenyl dicyanovinyl substituent is twisted out of the plane. The dihedral angle between two malanonitrylene groups is 78.3 degrees [9]. Both perfluoroaryl dicyanovinyl chromophores and Ph-TCBD chromophores showed much improved chemical stability compared to their tricyanovinyl analogs. The chemical stability of each chromophore was tested in a dilute chloroform solution that was treated with large excess of diethyl amine (strong nucleophile). The chromophore's tolerance to the nucleophile was elucidated from the UV-Vis spectra. Tricyanovinyl chromophores readily discolored almost instantaneously while none of the new chromophores showed color change even after having been exposed to amine for more than 24 h. Eοred 3 NEB
1
R
EοOx 3
λmax
2
n
(V)
(V)
(nm)
1
-
R1
1
-0.80
0.56
640
2
-
R2
1
N/A
N/A
601
3
-
R3
1
N/A
N/A
625
R1
1
-1.02
0.52
595
5
a
R2
1
-0.98
0.73
554
6
a
R3
1
-1.01
0.61
577
7
b
R1
1
-1.04
0.56
591
1
-1.03
0.72
553
0.61
573
4
8
a
b
R2
9
b
R3
1
-1.04
10
c
R4
0
-0.71
0.53
562
0.71
542
11
c
R2
0
-0.70
12
c
R3
0
-0.71
0.52
561
13
c
R4
1
-0.71
0.53
604
14
c
R2
1
-0.67
0.7
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