Photoswitchable fluorescence on/off behavior between cis- and trans-rich azobenzenes

Article abstract of DOI:10.1039/c2jm34776h

We report rapid photoswitchable fluorescence on/off behavior between cis- and trans-rich 4-butyl-4′-methoxyazobenzene and its analogue 4-butyl-2,6-dimethyl-4′-methoxyazobenzene.

Full text of DOI:10.1039/c2jm34776h

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COMMUNICATION
Photoswitchable fluorescence on/off behavior between cis- and trans-rich
azobenzenes†
a
a
a
b
Bo-Kai Tsai, Chien-Hong Chen, Cheng-Hsiang Hung, Vincent K. S. Hsiao and Chih-Chien Chu
ac
*
Received 20th July 2012, Accepted 28th August 2012
DOI: 10.1039/c2jm34776h
the bent-shaped cis-isomers with a larger dipole moment favored the
formation of head-to-tail intermolecular J-aggregates.7,8
We report rapid photoswitchable fluorescence on/off behavior
between cis- and trans-rich 4-butyl-40-methoxyazobenzene and its
analogue 4-butyl-2,6-dimethyl-40-methoxyazobenzene.
According to the previous finding, it is noticed that fluorescent Az-
based aggregates continued to show a trans 4 cis photo-
isomerization; however, their fluorescence expression was unaffected
upon switching between UV and visible light irradiation. In other
words, once the light-driven AIE was saturated, a reversible photo-
isomerization process did not alter the emission intensity. In contrast,
this communication provides unprecedented results of photo-
switchable fluorescence behavior of 4-butyl-40-methoxyazobenzene
(BMAB),9 a widely used LC azo guest, and its structural analogue
4-butyl-2,6-dimethyl-40-methoxyazobenzene (BDMAB, Scheme 1).
The newly designed BDMAB bearing two methyl groups ortho to
the azo moiety could reduce the coplanarity of the Az-conjugated
system.10 Therefore, compared with BMAB, BDMAB has slightly
blue-shifted p–p* (from 350 to 338 nm) and red-shifted n–p* (from
434 to 446 nm) maximum absorption wavelengths. Moreover, ortho-
dimethylation significantly reduces the rate of thermal cis-to-trans
relaxation, efficiently retaining the longer lifetime of cis BDMAB.11
On the basis of Han et al.’s assumption, the long-term stability of the
cis-Az isomer has a potentially crucial function in producing aggre-
gation-induced fluorescence.12
Azobenzene (Az) groups are typically used in a broad range of
photochromic molecules that undergo rapid trans/cis reversible
isomerization by switching the irradiation beam between UV and
visible light.1 Therefore, the incorporation of Az moieties into various
functional materials was explored intensively during the previous
decade to develop intelligent photoresponsive molecular systems for
sensing, optical switching, and image-storing applications.2 The
molecular conformation of trans-Az isomers is rod-like, and they can
be used as a photoresponsive calamitic liquid crystal (LC) mesogen or
as the photochromic guest for host LC molecules.3 Moreover,
introducing a flexible alkyl chain and a polar group onto the 4,40-
positions of the two benzene rings also imparts liquid crystallinity for
the Az scaffold, favoring molecular assembly through p–p stacking
between the aromatic rings and van der Waals interactions between
the alkyl chains, and increasing the compatibility with host LCs.
The characteristic p–p* (UV region) and n–p* (visible region)
transitions corresponding to the trans- and cis-rich states, respec-
tively, allow rapid scanning of photoswitchable behavior using
UV-visible (UV-Vis) spectroscopy. However, it is known that they
are incapable of emitting fluorescence with appreciable quantum
yield upon UV light irradiation, despite the intense absorption of Az
derivatives in the UV region.4 Recently, Han and Hara reported
intense fluorescence from light-driven self-assembled spherical
aggregates of nonionic Az derivatives;5 they have further demon-
strated color-tunable fluorescence of a series of Az compounds con-
taining electron-donating and -withdrawing pendant groups.6 The
authors proposed that this unusual fluorescence phenomenon was
attributed to the aggregation-induced emission (AIE) and that only
Initially, both trans- and cis-BMAB solution were non-fluorescent
upon excitation at UV 365 nm. Following the previous procedure for
generating observable fluorescence of cis-rich Az derivatives, trans-
BMAB solution (5 ꢀ 10ꢁ4 M in THF) was then continuously
exposed to UV light (365 nm, 100 W) for 30 min.5 The UV-Vis
absorption spectra shown in Fig. 1 exhibit a significant decrease of
p–p* transition at 350 nm and an increase of n–p* transition at
aSchool of Applied Chemistry, Chung Shan Medical University, Taichung
City 40201, Taiwan. E-mail: jrchu@csmu.edu.tw; Fax: +886-4-23248189
bDepartment of Applied Materials and Optoelectronic Engineering,
National Chi Nan University, Puli, Nantou County 54561, Taiwan
cDepartment of Medical Education, Chung Shan Medical University
Hospital, Taichung City 40201, Taiwan
† Electronic supplementary information (ESI) available: Experimental
details including the synthesis and preparation of fluorescent azo
compounds. Additional TEM images, UV-Vis absorption and
fluorescence spectra. See DOI: 10.1039/c2jm34776h
Scheme 1 Structures of the mesogen-like azobenzene derivatives.
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BDMAB solution containing 7% of cis-isomers initially showed
greater trans-to-cis photoisomerization efficiency than BMAB solu-
tion, indicating relatively greater stability of the ortho-disubstituted
Az compounds in the cis state.
Visible light irradiation causes rapid cis-to-trans isomerization of
Az compounds, and thus, the trans/cis states for pristine BMAB and
BDMAB could be reversibly switched by alternating UV and visible
light irradiation. A reversible photoisomerization reaction of the
fluorescent BMAB solution was also conducted using a handheld UV
lamp (365 nm, 4 W) and white light bulb irradiation within 3 min to
achieve cis- and trans-rich PSS, respectively. The change in maximum
absorbance for n–p* transition confirms the trans 4 cis photo-
isomerization by alternating the irradiation light source (Fig. S3†).
Furthermore, the photo-induced cis-to-trans back reaction regen-
erated approximately 83% of trans-BMAB, and 43% of trans-
BDMAB, indicating that BDMAB favors the relatively stable cis
state even following light irradiation.
Because Az compound fluorescence correlates highly with cis-
isomer content, it is anticipated that the fluorescence intensity can
also be reversibly switched by alternating irradiation light sources to
adjust the trans- and cis-contents in the system. However, Han and
co-workers only observed a steady fluorescence expression when the
trans and cis states of Az derivatives were switched. In sharp contrast,
the emission intensity of fluorescent BMAB and BDMAB decreased
significantly when exposed to white light irradiation, and then
increased to the original magnitude when irradiated with UV light
(Fig. S4†). Fig. 2 shows the visualized images of this unprecedented
photoswitchable fluorescence ‘‘on/off’’ behavior of BMAB and the
repeating cycles of switchable fluorescence accompanied by the
trans 4 cis photoisomerization monitored by the absorbance change
of the n–p* transition. Because the two switched cycles correlate well,
the cis-contents in the PSS dominates the change of the observed
fluorescence intensity. Although the detailed mechanism for this
Fig. 1 UV-Vis spectra of UV-exposed BMAB solution. The change in
absorbance of (a) p–p* and (b) n–p* transition. (c) Excitation (red line)
and emission spectra (blue line) of cis-rich BMAB solution (5 ꢀ 10ꢁ4 M
in THF); inset: time-dependent fluorescence intensity increase. TEM
images of BMAB before (d) and after (e) UV light irradiation.
445 nm, which suggests a cis-rich photostationary state (PSS).
Although the BMAB solution was dominated by the cis-isomers
when exposed to UV light, its fluorescence intensity remained negli-
gible. An unexpected blue fluorescence gradually increased as the
solution was agitated gently over the UV light irradiation, and the
intensity was rapidly saturated within 10 min (see ESI†). Fig. 1c
shows the excitation, emission, and time-dependent intensity increase
profiles for BMAB, and the obtained quantum yield referring to the
standard was 0.036.13 Additionally, an identical preparation method
also produced the rapid fluorescence enhancement of the cis-rich
BDMAB solution, suggesting that these two azo compounds
possessed similar trans-to-cis photoisomerization kinetics and
molecular assembly behavior in their cis-rich PSS states, regardless of
the dimethyl groups at the ortho position.
Because the formation of ordered aggregated structures is critical
to this fluorescence behavior, appropriate shaking during UV light
exposure appears to have assisted the ordered molecular assembly
pattern of cis-isomers. The UV light-driven fluorescence for cis-rich
BMAB and BDMAB could be readily performed in various organic
solvent systems, including THF, chloroform, toluene, and hexane,
although the emission intensity increase in ethanol and DMF was
minor. The observed solvent effect implies that a less-polar envi-
ronment favors the assembly of azo molecules, and the consequent
fluorescence intensity could increase significantly. Moreover, Fig. 1d
and e, and Fig. S1† (higher magnification) show the TEM micro-
graphs of BMAB before and after UV exposure, confirming the light-
driven assembly of BMAB particles in the submicron dimension.
The 1H NMR analyses (Fig. S2†) show that no structural
decomposition or additional photoreaction of the Az unit (6.5–
8.0 ppm) and the alkyl chain (0.5–2.8 ppm) occurs during UV light
irradiation, and the ratio of cis- and trans-isomers in the UV light PSS
can be determined by calculating the integral areas of the methoxy
protons in each state. The NMR data estimate approximately 75%
and 94% of cis-BMAB and BDMAB isomers, respectively, in the
UV-exposed solutions, confirming that cis-Az was the primary
component for generating light-driven fluorescence. Moreover,
Fig. 2 The visualized images of photoswitchable fluorescence ‘‘on/off’’
behavior of BMAB and the repeating cycles of switchable fluorescence
intensity (blue line) and trans 4 cis photoisomerization (red line),
monitored by fluorescence and UV-Vis absorption analyses, respectively.
The fluorescence intensity was integrated from 400 to 700 nm.
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fluorescence behavior is not well established, we proposed that the
formation of intermolecular aggregation of cis-isomers is a dynamic
process, in which the trans-isomers that are back generated by visible
light irradiation trigger the disassembly of the aggregated structures.
Therefore, the AIE phenomenon of azo compounds diminishes
temporarily until the next UV irradiation cycle. Additionally,
BDMAB solution that showed similar photoswitchable fluorescence
maintained moderate emission levels following white light irradiation,
whereas BMAB was almost non-fluorescent. The difference in the
PSS of visible light between these two azo compounds is ascribed to
the relatively abundant cis-content of BDMAB, allowing the partial
retention of the assembled structure and the AIE.
In summary, we successfully prepared blue-fluorescent cis-rich
BMAB and BDMAB solutions upon UV light irradiation and
demonstrated an unprecedented result of photoswitchable fluores-
cence ‘‘on/off’’ behavior for these two azo compounds. The change in
emission intensity could be controlled by alternating UV and white
light exposure within several minutes, which is also accompanied by a
reversible trans 4 cis photoisomerization. Although the reasons for
the fluorescence enhancement and rapid photoswitchable behavior
are not yet understood, it is proposed that the dynamic molecular
assembly and disassembly processes during trans 4 cis isomerization
through external stimuli (e.g., light and heat) was crucial to this
unusual phenomenon. Moreover, compared with BMAB, BDMAB
composed of two methyl groups on the ortho position showed
extremely slow thermal cis-to-trans isomerization and a stable fluo-
rescence expression in the dark for a longer period. This steady
emission allows cis-BDMAB to be maintained in the fluorescent ‘‘on’’
state at ambient temperature toward the fields of molecular sensing
and imaging with spatiotemporal concerns until we switch off the
fluorescence by visible light irradiation.
Alternatively, cis-to-trans isomerization can also be carried out
using thermal treatment following two possible mechanisms: simple
rotation around the N–N bond or inversion through an sp-hybrid-
ized transition state.14 On the basis of the decrease in the absorbance
of the n–p* transition at ambient temperature (Fig. 3a), fitting the
experimental data to the first-order kinetics produced a thermal
relaxation rate and the half-life of cis-BMAB of 5.76 ꢀ 10ꢁ2 hꢁ1 and
approximately 12 h, respectively.15 Conversely, cis-BDMAB was
extremely stable in the dark, and the thermal back isomerization was
significantly slower at room temperature. Presumably, the introduc-
tion of methyl groups on the two ortho positions restricts the rotation
or the inversion process because of the proximity of the benzene ring
to the methyl groups, which suppresses the cis-to-trans thermal
isomerization.16
The authors would like to thank the National Science Council of
Taiwan (NSC100-2113-M-040-007-MY2) and AOARD (FA2386-
12-1-4023), for financially supporting this research. NMR analyses
were performed in the Instrument Center of Chung Shan Medical
University, which is supported by the National Science Council,
Ministry of Education, and Chung Shan Medical University.
The change in fluorescence intensity also reflects the distinct
thermal relaxation processes of the two azo compounds. Spectral
tracing in Fig. 3b shows that BDMAB had a much slower fluores-
cence decrease in the dark than BMAB. This is consistent with the
concept that ortho-dimethylation retards the cis-to-trans thermal
isomerization rate, thereby retaining the fluorescence intensity for a
longer period. Several reports have shown that cis-Az derivatives
coupled with fluorophores that are thermally isomerized to the trans
form are accompanied by a fluorescence-fading behavior. This is
because the trans-isomers favor photoinduced electron transfer from
the lone pair electrons on the azo group to the HOMO of excited
fluorophores through effective p-conjugation (reductive electron
transfer).17 However, it is believed that the thermal-induced fluores-
cence quenching for BMAB and BDMAB that lack fluorophore
coupling is primarily attributed to the dynamic molecular disas-
sembly that occurs during cis-to-trans isomerization.
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Fig. 3 The change in (a) absorbance and (b) fluorescence intensity for
cis-to-trans thermal isomerization. BMAB (-), BDMAB ( ).
15 The first-order rate constant (k) was determined by fitting the
experimental data to the equation: ln[(At ꢁ AN)/(A0 ꢁ AN)] ¼ ꢁkt,
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where At, A0, and AN are the absorbance at 445 nm at time t, time
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