Communications
The photophysical data for phosphonium- and borate-
shows a faint fluorescence, the distyrylbenzene derivatives 8
and 10 show intense orange emissions with FF values of 0.38
and 0.40, respectively. According to the calculations of the
radiative and nonradiative decay rate constants kr and knr
from FF and the fluorescence lifetime ts, the enhancement in
the fluorescence intensity in 8 and 10 is attributable to both
the increase in kr and the decrease in knr (1a kr = 8.7 106 sÀ1,
knr = 4.3 108 sÀ1; 8 kr = 5.9 107 sÀ1, knr = 9.7 107 sÀ1; 10 kr =
4.0 107 sÀ1, knr = 5.9 107 sÀ1). The increase in kr is mainly
due to the enhancement of the oscillator strength by the
extension of the p conjugation. Meanwhile, the replacement
of the benzene rings of 1a with thiophene also results in a
significant difference in labs and lem. The thiophene derivative
12 shows an intense red emission with a long lem at 623 nm
despite its short p-conjugated length.
bridged compounds 1a, 8, 10, and 12 are summarized in
Table 1, together with those of the valence isomer 2c and the
relevant bridged stilbenes 3, 4, and 5 for comparison. In the
Table 1: Photophysical data for the bridged stilbenes and related
compounds.[a]
Compound
UV/Vis absorption
Fluorescence
FF
labs [nm][b]
loge
lem [nm][c]
ts [ns]
1a
2c
8
10
12
396
336
516
519[f]
483
322
351
395
3.73
4.20
4.25
4.03
4.08
4.45
4.11
3.84
517
452
578
614
623
367
415
480
0.020[d]
0.014[d]
0.38[e]
0.40[e]
0.57[e]
0.92[e]
0.07[e]
0.98[e]
2.3
2.1
6.4
10.1
13.0
1.6
1.4, 9.8
15.7
3[g]
4[h,i]
5[h,i,j]
The electrochemical properties of the series of phospho-
nium- and borate-bridged ladder systems were also studied by
cyclic voltammetry (Table 2). Compound 1a shows irrever-
sible oxidation and reduction waves with peak potentials at
[a] In THF. [b] Only the longest absorption maxima are shown.
[c] Emission maxima upon excitation at the absorption maximum
wavelengths. [d] Relative fluorescence quantum yields determined with
quinine sulfate as a standard. [e] Absolute fluorescence quantum yields
determined by a calibrated integrating sphere system within Æ3%
errors. [f] A shoulder was observed at 550 nm with loge=3.93.
[g] Reference [9a]. [h] Reference [10]. [i] In CH2Cl2. [j] trans isomer.
Table 2: Electrochemical data for the bridged stilbenes and related
compounds.[a]
Compound
Oxidation potential[b]
Reduction potential[c]
Epa [V][d]
Epc1 [V][d]
Epc2 [V][d]
[e]
1a
8
10
+0.57
+0.13
+0.63
À2.55
À2.63
–
UV/Vis absorption spectra, the phosphonium- and borate-
bridged stilbene 1a has an absorption band with the
maximum (labs) at 396 nm and a relatively small molar
absorption coefficient e = 5400. In the fluorescence spectra,
1a exhibits a yellow fluorescence with a maximum (lem) at
517 nm, whereas the quantum yield is very low (FF = 0.020).
Notably, the emission maximum of 1a is the longest among
the series of bridged stilbenes, and about 160 nm longer than
the CMe2-bridged stilbene 3. These facts demonstrate that the
phosphonium and borate zwitterionic substituents substan-
tially alter the nature of the parent stilbene skeleton.
Both labs and lem of 1a are considerably longer than those
of the valence isomer 2c, by 60 and 65 nm, respectively. While
the lowest-energy transition in 2c is the intramolecular charge
transfer from the triarylphosphane moiety to the triarylbor-
ane moiety, that of 1a is from the benzoborole moiety to the
benzophosphole moiety. Thus, the isomerization switches the
nature of the intramolecular charge-transfer transition,
resulting in the significant differences of labs and lem. Such
difference in the nature of electronic transition between 1a
and 2c would affect the solvent dependency of absorption and
emission spectra. In fact, 1a exhibits hypsochromic shifts both
in the absorption and emission spectra (labs = 403 nm in
benzene, 396 nm in THF, 390 nm in DMF; lem = 525 nm in
benzene, 517 nm in THF, 510 nm in DMF), whereas 2c does
not show an obvious solvent effect in the absorption spectra
(labs = 336 nm in benzene, 336 nm in THF, 334 nm in DMF).
These results indicate that the ground state of 1a is more
polar than the excited state.
[e]
–
À2.03
À2.77
(À1.95)[f]
À2.49
(À2.69)[d]
[e]
12
3
+0.21
+0.81
–
[e,g]
À3.27[g]
(À3.17)[f,g]
À1.74
–
(+0.75)[f]
5[g,h]
–
À2.35
[e]
(À1.67)[f]
[a] Determined by cyclic voltammetry under the following condition:
Sample 1 mm; Bu4N+PF6À 0.1m in CH2Cl2 or THF; scan rate 100 mVsÀ1
.
[b] In CH2Cl2. [c] In THF. [d] Peak anodic potential (Epa) and peak
cathodic potential (Epc) against the ferrocene/ferrocenium couple are
given. For reversible processes, the corresponding half redox potentials
(E1/2) are given in parentheses. [e] Not observed. [f] Reversible process.
[g] Reference [10]. [h] trans isomer.
+ 0.57 and À2.50 V, respectively (vs. ferrocene/ferrocenium).
This is in contrast to the fact that the CMe2-bridged stilbene 3
only shows reversible oxidation and reduction waves with
peak potentials at + 0.81 V and À3.27 V, respectively. The
incorporation of the zwitterionic bridges significantly affects
the electronic structure and provides ambipolar character to
the stilbene framework. This observation is consistent with
the previously mentioned results of the molecular orbital
calculations.
Comparison of the redox potentials between 8 and 10 also
reveals noticeable features of the zwitterionic modulation of
the ladder skeleton. These two derivatives are positional
isomers differing only in the arrangement of the bridging
moieties, P+ BÀ BÀ P+ or BÀ P+ P+ BÀ. However, this difference
results in a totally opposite electronic modulation. Thus, the
P+ BÀ BÀ P+-bridged 8 has a very low oxidation potential with
As for the extended analogues, the distyrylbenzene
derivatives 8 and 10 have labs = 516 and 519 nm and lem
=
578 and 614 nm, respectively. These wavelengths are much
longer than those of the stilbene 1a, thus demonstrating the
effective extension of the p conjugation. While the stilbene 1a
E
pa = + 0.13 V, while its reduction peak potential (À2.63 V) is
comparable to that of the stilbene 1a. In contrast, the
5584
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 5582 –5585