Making Azobenzene Fluorescent: Substituent Effects on Fluorescence Enhancementof Azobenzene Derivatives
Mina Han,1,2 Yuuki Hirayama3 and Masahiko Hara2
1

Department of Chemistry and 2Department of Electronic ChemistryTokyo Institute of Technology
3
New Technology Laboratory, Merck Ltd.

Azobenzene is a well known chromophore, which exhibits photoinduced and reversible
trans/cis isomerization. The photoisomerization of azobenzene and its derivatives influences
versatile physical properties such as wettability, viscosity, and aggregation behaviors becauseof changes in molecular structure and dipole moment (from 0 of trans form to ca. 3 Debye ofcis form) between the rod-shaped trans form and the bent-shaped cis form. Owing to theirability to undergo trans/cis isomerization, generally azobenzene molecule does not fluorescewith appreciable quantum yield (about 10–7~10–5). However, a few exceptional
self-assembled bilayer aggregates of azobenzene-containing amphiphiles and of
azobenzene-functionalized dendrimers exhibited fluorescence emission at ~600 nm. Thisemission is attributed to the densely packed arrangement of azobenzene chromophores in thebilayer structure. It has also been reported that orthometallated azobenzene complexes wereweakly fluorescent. By cyclometallation, the photoisomerization of trans-blocked azobenzene
is suppressed and the efficiency of fluorescence increases significantly.
(a)

XN
N

O

NMe2: X = NMe2
1 : X = MeO
2 : X = EtO
3 : X = Hexyl

4: X = H
5: X = F
6: X = CF3
7: X = CN

(b)

(c)
0.005

300

2250

1

Fluorescence quantum yield (ΦΦ f)

350
Relative intensity (Arb. Unit)

Unlike the attempts reported so far,here we describe that the first stage oftrans-to-cis photoisomerization by UVlight at 365 nm was slowly followed by asignificant
enhancementof
thefluorescence from azobenzene molecule,which seems to be related to thespontaneous formation of sphericalorganic aggregates. In addition, to obtainan insight into significant substituent
effect on the fluorescence enhancement ofazobenzenes, we synthesized azobenzene
derivatives
para-substitutedwithelectron-donating(EDGs)
orelectron-withdrawing groups (EWGs).
The fluorescence enhancement is closely
related to cis-azobenzene showing bothsufficient lifetime and a larger dipole
moment. The fluorescence quantum yield
increased linearly with increasing
electron-donating
abilities
ofthesubstituents.

200
3150
100
50
0400

456, 7500

Φyf ==–0.0083σ-0.0083xp ++ 0.0011
0.00112RR2==0.934
0.934

0.004
2=EtO

0.003

0.002

0.001

0.000
600

Wavelength (nm )

700

1=MeO

3=Hexyl

4=H
5=F
6=CF3 7=CN-0.4 -0.2 0.0 0.2 0.4 0.6 0.8

Hammett constant (σp)(a) Molecular structure. (b) Fluorescence spectra of
1–7 after UV light irradiation (365 nm excitation). (c)Correlation of Hammett substituent constant (σp) andfluorescence quantum yield (Φf) of UV-exposed
azobenzene molecules (1–7).

[1] Shimomura, M.; Kunitake, T. J. Am. Chem. Soc. 1987, 109, 5175.
[2] Ghedini, M.; Pucci, D.; Calogero, G.; Barigelletti, F. Chem. Phys. Lett. 1997, 267, 341[3] Han, M.; Hara, M. J. Am. Chem. Soc. 2005, 127, 10951-10955[4] Han, M.; Hara, M. New J. Chem. 2006, 30, 223-227.
[5] Han, M.; Hirayama, Y.; Hara, M. Chem. Mater. 2006, 18, 2784-2786.