Formation of fluorescence reliefs photocontrolled by collective mass migration

Article abstract of DOI:10.1039/b916454e

Fluorescent and photochromic non-doped thin films have been elaborated from bifunctional molecules exhibiting time-stable glassy properties and combining each a photoisomerizable azo unit covalently linked to a fluorescent moiety through a rigid, saturate

Full text of DOI:10.1039/b916454e

www.rsc.org/materials
Volume 19 | Number 47 | 21 December 2009 | Pages 8889–9076
ISSN 0959-9428
PAPER
Eléna Ishowet al.
FEATURE ARTICLE
Jin-gang Liu and Mitsuru Ueda
Photomigration of azo units connected High refractive index polymers:
to a fluorescent tail conducts to the rise fundamental research and practical
of microscopic emissive patterns
applications

PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Formation of fluorescence reliefs photocontrolled by collective mass migration
a
Aurelie Jacquart, Patrick Tauc, Keitaro Nakatani and Elena Ishow
b
a
a
*
ꢀ
ꢀ
Received 12th August 2009, Accepted 24th September 2009
First published as an Advance Article on the web 19th October 2009
DOI: 10.1039/b916454e
Fluorescent and photochromic non-doped thin films have been elaborated from bifunctional molecules
exhibiting time-stable glassy properties and combining each a photoisomerizable azo unit covalently
linked to a fluorescent moiety through a rigid, saturated and bulky spacer. Photoillumination under
polarized light led to the rise of periodically structured emissive reliefs. The microscopic emission
modulation was addressed by fluorescence confocal microscopy, and the maxima in emission correlated
with the minima in transmission. This clear correspondence features the physical displacement of the
fluorophore units, pulled by the azo moieties during their photoinduced motion in the bulk; this
accumulation of fluorescent materials followed the impinging light pattern, and could be erased and
promoted again under specific illumination conditions.
photochromes and/or generate functional patterns has never
Introduction
really been attempted. Among them, fluorescent probes appear
very attractive due to the high sensitivity of fluorescence detec-
tion even at low concentration, and the recent development of
sophisticated microscopy imaging techniques providing high
spatial resolution through one and two-photon optical address-
ing.12–14 We want to show herein that linking an azo unit to
a bulky fluorescent tail allowed us to obtain fluorescent and
photochromic monomeric thin films where the migration of the
azo units serves as a tool to accumulate the attached fluorophore
and create emissive and fully rewritable photopatterns. Such
a strategy, based on the bulk photoinduced mass transport of
fluorescent entities in non-doped materials, is unexplored to date.
Along to the development of photoadressable functionalized
systems for optical data storage applications, these molecular
systems should help address a stirring fundamental issue related
to the photocontrol of motions at the single molecule level.
Functional molecules undergoing controlled translation and
rotation have recently attracted considerable attention for their
multifold potentialities in the field of nanomechanics and
biology.1 Nature actually displays fascinating examples of
motions through the folding-unfolding processes of proteins, the
rotative flagella responsible for the bacteria motility, the
stretching-contraction of muscle fibers activated by the actine-
myosine complex and eventually the ATP synthase rotor
responsible for the ATP production.2 Various groups have strived
to mimic these natural processes, and elaborated complex inter-
acting supramolecular architectures translating or rotating under
chemical, electrochemical, and photochemical external stimuli.3
Only a few examples of such assembled systems in the solid state
have been studied.4 The need to get molecular systems with
greater structural dynamics conducted to the renewal of photo-
chromes known for their dramatic changes in geometry upon light
irradiation.5 Several seminal works reported directional photo-
control in the bulk leading to the impressive displacement of
deposited micro- and nano-objects such as the rotation of
a micro-bar,6 the slipping of a polystyrene microsphere7 or even
the shooting of a gold nanoparticle.8 The involved collective
motions were even more exemplified with azo-functionalized
materials (polymers, liquid crystal polymers and monomeric thin
films) which cooperatively responded to light polarization, and
caused a dramatic surface deformation of thin films.9,10 More
recently, impressive matter displacement was observed in bulky
azo monomeric thin films subjected to interferential illumination;
the increase in free volume due to the attachment of bulky and
twisted groups to the azo units facilitated migration in the solid
state to form unprecedented micrometer-high periodic patterns.11
Surprisingly, the attachment of functional entities exhibiting
distinguishable signatures to follow the motion of the
Experimental
General methods
1H and 13C NMR spectra were recorded on JEOL 400 MHz
spectrometer and chemical shift were reported in ppm relative to
TMS or referenced to the residual solvent. High resolution mass
spectra were obtained by MALDI-TOF (Voyager DE-STR,
Applied Biosystems). Melting points were measured by using
a Kofler bench. Glass transition temperatures Tg were measured
by using differential scanning calorimetry (Perkin Elmer Pyris
Diamond) in aluminium caps under nitrogen flow at a scan rate
of 20 ꢀC minꢁ1 over the temperature range [30 C–250 C].
ꢀ
ꢀ
Synthesis and materials
All chemical reagents and solvents were purchased from
commercial sources (Aldrich, Acros, SDS) and used as received.
Spectroscopic grade toluene was used for spectroscopic
measurements. All air-sensitive reactions were performed under
argon using a vacuum line. Analytical thin layer chromato-
graphy was performed on Kieselgel F-254 precoated plates. Flash
aENS Cachan, IFR d’Alembert, PRES-UniversSud, PPSM- UMR CNRS
8531, 61 avenue du Preꢀsident Wilson, 94235 Cachan, France. E-mail: elena.
ishow@ens-cachan.fr; Fax: +33-1-4740-7660; Tel: +33-1-4740-7660
bENS Cachan, IFR d’Alembert, PRES-UniversSud, LBPA- UMR CNRS
8113, 94235 Cachan, France
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 8999–9005 | 8999

chromatography was carried out with silica gel from SDS. The
compounds 2,6-dihydroxy-9,10-dihydro-11,12-dicarbomethoxy-
ethenoanthracene 1,15 bis(40-tert-butylbiphenyl-4-yl)amine,16
methyl-5-bromo-2-nitrobenzoate,17 4-[bis(40-tert-butylbiphenyl-
4-yl)amino)]-40-cyanoazobenzene,18 and p-dimethylaminopyr-
idinium-toluenesulfonate19 were synthesized according to
literature procedures. Thin films were obtained by spin-casting
2 wt. % chloroform solution on pre-cleaned glass substrates at
a 500 rpm speed and dried further under vacuum for 24 h.
diisopropylcarbodiimide (8 mg, 0.6 mmol) in dry dichloro-
methane (20 mL) was slowly added to a solution of adduct 1
(44 mg, 0.12 mmol) in dry dichloromethane (40 mL). The reac-
tion mixture was allowed to stir at room temperature for 20 h.
Concentration under vacuum followed by silica gel column
chromatography using petroleum ether: ethyl acetate 7 : 3 as an
eluent, afforded 7 as a glassy orange solid. (38 mg, 65%). Tg 122
ꢀC. 1H NMR (400 MHz, CDCl3, TMS): d ¼ 8.02 (d,
3
3J(H,H) ¼ 9.1 Hz, 1H), 7.62 (d, J(H,H) ¼ 8,7 Hz, 4H), 7.54
3
3
(d, J(H,H) ¼ 8.2 Hz, 4H), 7.48 (d, J(H,H) ¼ 8.2 Hz, 4H),
7.35 (d, 3J(H,H) ¼ 8.2 Hz, 1H),7.30–7.27 (m, 5H), 7.16
(d, 3J(H,H) ¼ 7.8 Hz, 1H), 7.15 (d, 4J(H,H) ¼ 2.7 Hz, 1H), 7.08
(dd, 3J(H,H) ¼ 9.1 Hz, 4J(H,H) ¼ 2.7 Hz, 1H), 6.88 (d, 3J(H,H) ¼
Methyl 5-[bis(40-tert-butylbiphenyl-4-yl)amino]-2-nitrobenzoate
4. Palladium diacetate(II) (9 mg, 0.04 mmol), 1,10-bis(diphenyl-
phosphino)ferrocene (dppf) (66 mg, 0.12 mmol), bis(40-tert-
butylbiphenyl-4-yl)amine (346 mg, 0.80 mmol) and caesium
carbonate (390 mmol, 1.2 mmol) were successively added
to a solution of methyl-5-bromo-2-nitrobenzoate (249 mg,
0.96 mmol) in dry toluene (10 mL) under argon. The reaction
mixture was stirred overnight at 80 ꢀC. After warming up to
room temperature, the reaction mixture was washed with brine,
dried over anhydrous magnesium sulfate, and concentrated
under vacuum. Purification by silica gel column chromatography
with dichloromethane:petroleum ether as an eluent (initial and
final compositions 1 : 1 and 7 : 3 respectively) afforded methyl
4
7.7 Hz, 4J(H,H) ¼ 2.3 Hz, 1H), 6.87 (d, J(H,H) ¼ 2.3 Hz, 1H),
4
6.40 (d, 3J(H,H) ¼ 8.2 Hz, J(H,H) ¼ 1.8 Hz, 1H), 5.38 (s, 1H),
5.37 (s, 1H), 3.77 (s, 6H; –COOCH3), 1.37 (s, 18H; –t-Bu). 13C
NMR (100ꢀMHz, CDCl3, TMS): d ¼ 165.84, 165.74, 165.59,
153.43, 152.77, 150.72, 147.90, 147.45, 146.68, 145.91, 145.47,
143.77, 141.59, 139.15, 137.47, 137.04, 135.48, 131.10, 128.63,
126.73, 126.62, 125.88, 124.67, 124.35, 119.19, 117.84, 117.43,
117.30, 112.21, 111.28, 52.45, 51.91, 51.51, 34.60, 31.35 ppm.
UV-vis (toluene), lmax (3max (molꢁ1Lcmꢁ1)): 416 (1.30 ꢂ 104), 318
(1.94 ꢂ 104) nm. HRMS (MALDI-TOF), m/z (M+ + Na, 100%):
for C37H43NO2Na calculated 955.3571; found 955.3565.
5-(bis(40-tert-butylbiphenyl-4-yl)amino)-2-nitrobenzoate
4
as
a fluffy orange solid (500 mg, 95%). Tg 101 ꢀC. 1H NMR
(400 MHz, CDCl3, TMS): d ¼ 7.96 (d, 3J(H,H) ¼ 9.5 Hz, 1H), 7.6
4-{40-[bis(40-tertbutylbiphenyl-4-yl)amino]phenylazo}benzoic
acid 2. To a solution of 4-[bis(40-tert-butylbiphenyl-4-yl)amino)]-
40-cyanoazobenzene (400 mg, 0.62 mmol) in 30 wt. % aqueous
sodium (20 mL) hydroxyde heated at reflux was added ethoxy-
ethanol (20 mL) during 2 h. The reaction mixture was cooled
with an ice bath. Hydrogen chloride 6M was added under
vigorous stirring to neutralise and precipitate the azo-acid 2 as
a pure orange product which was filtered off, washꢀed with water
3
3
(d, J(H,H) ¼ 8.7 Hz, 4H), 7.54(d, J(H,H) ¼ 8.2 Hz, 4H), 7.48
3
3
(d, J(H,H) ¼ 8.2 Hz, 3H), 7.26(d, J(H,H) ¼ 8.7 Hz, 4H), 7.05
(m, 2H), 3.87 (s, 3H; –COOCH3), 1.36 ppm (s, 18H;t-Bu). 13C
NMR (100ꢀMHz, CDCl3, TMS): d ¼ 167.4, 152.6, 150,7, 143.9,
139.0, 137.8, 137.1, 131.6, 129.1, 128.6, 128.3, 126.7, 126.6, 125.9,
119.1, 117.5, 53.4, 34.6, 31.4 ppm. UV-vis (toluene), lmax (3max
(molꢁ1Lcmꢁ1)): 411 (1.68 ꢂ 104), 318 (2.44 ꢂ 104) nm. HRMS
(MALDI-TOF), m/z (M+, 100%): for C40H40N2O4 calculated
612.2988, found 612.2982.
1
and dried under vacuum (391 mg, 95%). Tg 115 C. H NMR
3
(400 MHz, CDCl3, TMS): d ¼ 8.23 (d, J(H,H) ¼ 8.7 Hz, 2H),
7.92 (d, 3J(H,H) ¼ 8,3 Hz, 2H), 7.86 (d, 3J(H,H) ¼ 9.1 Hz, 2H),
7.56–7.52 (m, 8H), 7.46 (d, 3J(H,H) ¼ 8.7 Hz, 4H), 7.27
(d, 3J(H,H) ¼ 8.7 Hz, 4H), 7.20 (d, 3J(H,H) ¼ 9.1 Hz, 2H), 1.37
(s, 18H; –t-Bu). HRMS (MALDI-TOF), m/z (M+, 100%): for
C45H43N3O2 calculated 657.3355, found 657.3349.
5-[bis(40-tert-butylbiphenyl-4-yl)amino]-2-nitrobenzoic acid 5. A
30 wt. % sodium hydroxide aqueous solution (5 mL) was added to
a solution of 4 (100 mg, 0.16 mmol) in ethoxyethanol (5 mL) heated
at reflux. After heating for 30 min., the reaction mixture was cooled
with an ice bath. A 6 mol Lꢁ1 hydrogen chloridesolution was added
under vigorous stirring until pH equal to 1 to precipitate 5-(bis
(40-tert-butylbiphenyl-4-yl)amino)-2-nitrobenzoic acid 5 as a pure
orange product. The compound was filtered off, washed with water
to neutrality and dried under vacuum (95 mg, 95%). Tg 110 ꢀC. 1H
NMR (400 MHz, CDCl3, TMS): d ¼ 7.96 (d, 3J(H,H) ¼ 9.2 Hz,
1H), 7.60 (d, 3J(H,H) ¼ 8,5 Hz, 4H), 7.54(d, 3J(H,H) ¼ 8.5 Hz, 4H),
4-[bis(40-tert-butylbiphenyl-4-yl)amino]-40-{[(6-hydroxy-910-
dihydro-11,12-dicarbomethoxyetheno)anthracene-2-yl]oxy
carbonyl}azobenzene 3. To a solution of adduct 1 (200 mg,
0.56 mmol) in dry dichloromethane (200 mL) was slowly added
a solution of 2 (184 mg, 0.28 mmol), dimethylaminopyridinium
p-toluenesulfonate (26 mg, 0.3 mmol) and diisopropylcarbodii-
mide (DIPC) (53 mg, 0.42 mmol) in dry dichloromethane
(100 mL). The reaction mixture was allowed to stir at reflux
overnight. Concentration under vacuum followed by silica gel
column chromatography using petroleum ether/ethyl acetate 7/3
as eluant, afforded 3 as a deep red solid (180 mg, 65%). Tg 123 ꢀC.
3
3
7.48 (d, J(H,H) ¼ 8.5 Hz, 4H), 7.27 (d, J(H,H) ¼ 8.5 Hz, 4H),
4
7.15 (d, 4J(H,H) ¼ 2.5 Hz,1H), 7.07 (dd, J(H,H) ¼ 2.5 Hz,
3J(H,H) ¼ 9.2 Hz, 1H), 1.37 ppm (s, 18H;t-Bu). 13C NMR
(100ꢀMHz, CDCl3, TMS): d ¼ 152.49, 150.48, 144.10, 138.71,
137.77, 128.53, 126.66, 126.61, 126.46, 125.87, 119.19, 117.88,
34.63, 31.42. HRMS (MALDI-TOF), m/z (M+, 100%): for
C39H38N2O4 calculated 598.2832, found 598.2826.
3
1H NMR (400 MHz, CDCl3, TMS): d ¼ 8.27 (d, J(H,H) ¼ 8.7
3
3
Hz, 2H), 7.94 (d, J(H,H) ¼ 8,7 Hz, 2H), 7.88(d, J(H,H) ¼ 9.1
3
Hz, 2H), 7.58–7.53 (m, 8H), 7.47 (d, J(H,H) ¼ 8.7 Hz, 4H),
[(6-hydroxy-9,10-dihydro-11,12-dicarbomethoxyetheno)anthra-
cene-2-y]-5-[bis(40-tert-butylbiphenyl-4-yl)amino]-2-nitrobenzoate
7. A solution of compound 5 (38 mg, 0.06 mmol), diaminopyr-
idinium p-toluenesulfonate (6 mg, 0.02 mmol), and
7.40 (d, 3J(H,H) ¼ 8.2 Hz, 1H), 7.28–7.19 (m, 8H), 6.92 (d,
3
4
4J(H,H) ¼ 2.3 Hz, 1H) 6.88 (dd, J(H,H) ¼ 7.7 Hz, J(H,H) ¼
2.3 Hz, 1H), 6.45 (d, 3J(H,H) ¼ 7.7 Hz, 4J(H,H) ¼ 2.3 Hz, 1H),
5.41 (s, 1H), 5.40 (s, 1H), 3.80 (s, 3H; –COOCH3), 3.79
9000 | J. Mater. Chem., 2009, 19, 8999–9005
This journal is ª The Royal Society of Chemistry 2009

(s, 3H; –COOCH3), 1.37 (s, 18H; –t-Bu). 13C NMR (100ꢀMHz,
CDCl3, TMS): d ¼ 166.06, 165.99, 164.97, 156.03, 153.75, 151.19,
150.21, 148.27, 147.52, 147.10, 146.75, 145.95, 145.46, 145.24,
141.28, 137.40, 137.14, 134.92, 131.19, 129.98, 128.03, 126.45,
125.84, 125.77, 124.88, 124.55, 125.35, 122.48, 121.36, 117.98,
117.59, 112.18, 111.30, 52.54, 51.85, 51.56, 34.53, 31.35ppm. UV-
vis (toluene), lmax (3max (molꢁ1Lcmꢁ1)): 474 (2.28 ꢂ 104), 328
(2.96 ꢂ 104) nm. HRMS (MALDI-TOF), m/z (M+ + H, 100%):
for C65H57N3O7H calculated 992.4275; found 992.4269.
laser beam working at 488 nm into two coherent beams of
equal intensity (160 mW cmꢁ2 each).11 Interference gratings of
a 1.38 mm spatial period were obtained in accord with the inci-
dent angle 8ꢀ between both interfering beams. Fluorescence
imaging was obtained with a Leica SP2 fluorescence confocal
microscope equipped with a Ar+ laser (488 nm) working at
20 mW. Topographic imaging of recorded SRGs and super-
structures was done by using a Veeco Explorer atomic force
microscope working in a tapping mode.
40-{[(6-{5-[bis(40-tert-butylbiphenyl-4-yl)amino]-2-nitrobenzoy
loxy}-9,10-dihydro-11,12-dicarbomethoxyetheno)anthracene-2-
yl]oxycarbonyl}-4-[bis(40-tert-butylbiphenyl-4-yl)amino]azo-
benzene 6. To a solution of compound 3 (130 mg, 0.13 mmol) in
dry dichloromethane 20 mL) was added slowly a solution of 5
(77 mg, 0.13 mmol), DPTS (12 mg, 0.04 mmol), diisopro-
pylcarbodiimide (DIPC) (25 mg, 0.20 mmol) in dry dichloro-
methane (10 mL). The reaction mixture was allowed to stir at
reflux overnight. Concentration under vacuum followed by two
successive silica gel column chromatographies using first
dichloromethane and second petroleum ether/ethyl acetate 7/3 as
eluant, afforded 6 as a glassy red solid (120 mg, 60%). Tg 174 ꢀC.
Results and discussion
Synthetic strategy
The elaboration of fluorescent azo compounds obtained by
attaching a fluorophore to an azo unit is quite challenging given
the competitive ultra-fast photochromic reaction of the azo unit.
The E to Z photoisomerization process occurs in a few picosec-
onds, and deactivates upon energy transfer any excited state
when too close.20,21 The choice of red-emitting species, far from
the azo absorption spectrum to avoid emission energy transfer, is
thus mandatory. However, most of the red emitters present
extended p-conjugated structures which lead to high intermo-
lecular p–p stackings and fluorescence quenching when working
in the solid state.
1H NMR (400 MHz, CDCl3, TMS): d ¼ 8.28 (d, J(H,H) ¼ 8.2
3
Hz, 2H), 8.04 (d, 3J(H,H) ¼ 9.1 Hz, 1H), 7.95 (d, 3J(H,H) ¼ 8.2
Hz, 2H;), 7.89 (d, 3J(H,H) ¼ 8.7 Hz, 2H), 7.63 (d, 3J(H,H) ¼ 8.7
Hz, 4H), 7.58–7.54 (m, 12H), 7.50–7.47 (m, 8H), 7.42
(d, 3J(H,H) ¼ 8.7 Hz, 1H), 7.40 (d, 3J(H,H) ¼ 8.7 Hz, 1H), 7.35
(d, 4J(H,H) ¼ 2.3 Hz, 1H), 7.31–7.25 (m, 10H), 7.22 (d,
3J(H,H) ¼ 9.2 Hz, 2H), 7.16 (d, 4J(H,H) ¼ 2.3 Hz, 1H), 7.10 (dd,
3J(H,H) ¼ 9.6 Hz, 4J(H,H) ¼ 2.7 Hz, 1H), 6.92 (m, 2H), 5.51 (s,
1H), 5.49 (s, 1H), 3.81 (s, 3H;–COOCH3), 3.80 (s, 3H;–
COOCH3), 1.38 (s, 18H; –t-Bu), 1.37 (s, 18H;t-Bu). 13C NMR
(100ꢀMHz, CDCl3, TMS): d ¼ 165.62, 164.82, 156.13, 152.85,
151.29, 150.79, 150.32, 148.51, 148.05, 147.25, 147.10, 146.94,
145.60, 145.40, 145.28, 143.84, 141.56, 140.98, 139.22, 137.53,
137.49, 137.25, 137.11, 131.28, 131.17, 130.16, 129.11, 128.71,
128.30, 128.13, 126.82, 126.71, 126.56, 125.96, 125.86, 125.36,
124.94, 124.74, 124.68, 122.55, 121.49, 119.25, 118.38, 118.23,
118.01, 117.73, 117.49, 52.60, 51.93, 51.88, 34.67, 34.63, 31.44
ppm. HRMS (MALDI-TOF), m/z (M+ + H): for C104H93N5O10H
calculated 1572.7001; found 1572.6995. UV-vis (toluene), lmax
(3max (molꢁ1Lcmꢁ1)): 439 (2.95 ꢂ 104), 320 (5.41 ꢂ 104) nm.
The azo fluorescent compound 6 could be obtained from
coupling the azo carboxylic acid 3 to the fluorophore 5 whose
derivatives are known to emit far from the azo absorption range
(labs ¼ 420–510 nm) as non-doped amorphous thin films
(lem > 520 nm).22 The introduction of the spacer 1 between the
azo and the emitting units allowed us to further decrease the risks
of fluorescence quenching upon energy transfer (Scheme 1).
Saturated backbones were preferred over insaturated ones to
bring sufficient electronic insulation and consequently restrict the
probability of fluorescence quenching via a through-bond elec-
tron exchange mechanism. Long alkyl chains should however be
ruled out to limit folding of the molecule giving birth to a non-
radiative species through intramolecular p-stacking between the
azo and the fluorescent moieties. To this aim, rigid saturated
structures like adamantyl,23 carboranyl,24 and triptycenyl, an
anthryl Diels–Alder adduct,25 have successfully been proposed.
Additionally they bring enough free volume which is compulsory
for improved molecular migration in thin films. Yet, the distance
separating the azo and fluorescent moieties must at least be of the
Photochromic experiments
ꢁ
€
same range of order as the Forster radius R0, calculated at 16 A
from considering the fluorophore and the azo units as the energy
donor and acceptor respectively. This last criterium prompted us
to select the anthryl Diels–Alder adduct 115 as a spacer since its
DFT-minimized geometry between positions 2- and 6- was
Photoisomerization kinetics were recorded on magnetically stir-
red solutions using a continuous wave argon ion laser working at
488 nm (30 mWcmꢁ2) as a pump source while simultaneous
probing was performed with a continuous Xe lamp (450 W) and
a CCD camera coupled with a spectrometer (Princeton Instru-
ments). After reaching the photostationary state, the back
thermal reaction was modelled in fluid solution and thin films by
a monoexponential fit a ꢂ exp(ꢁkt) and a biexponential fit
a1 ꢂ exp(ꢁk1t) + a2 ꢂ exp(ꢁk2t) respectively.
ꢁ
ꢁ
evaluated to be 14 A compared to the 5 A-large scaffold of the
carboranyl and adamantyl units.
The bis-phenol compound 1 was thus reacted with the azo
unit 2 to provide the azo precursor 3 following an esterification
reaction (Scheme 1). The ester linkage ensured a dipolar char-
acter for the azo moiety, strong enough to produce efficient
migration in the solid state and the use of a single irradiation
wavelength to perform successive E-Z photoisomerization cycles
(vide infra). Final coupling of 3 with the hydrolyzed fluorophore
5 yielded the bifunctional compound 6. The synthetic strategy
Holographic setup and fluorescence relief imaging
Holographic inscriptions were carried out on spin-coated thin
films by means of a two-arm interferometer setup splitting an Ar+
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 8999–9005 | 9001

Scheme 1 Synthesis of the bifunctional compound 6. i- Diisopropylcarbodiimide, dimethylaminopyridinium p-tosylate, CH2Cl2, 24h, RT toluene
solution. ii- NaOH 30%, methoxymethylether, 90 ꢀC, 3h.
adopted here allowed us to easily separate the final ester product
from the reagents by column chromatography due to their
marked difference in polarity, and avoid misinterpretation
caused by the presence of fluorescent contaminants.
Exclusive participation of the fluorophore moiety to the
emission of 6 was established from recording an excitation
spectrum which corresponds to the typical absorption spectrum
of the fluorophore 4. Two bands were displayed at 318 nm and
411 nm. The low-energy band, ascribed to a charge transfer (CT)
transition from the triarylamino to the nitro group corresponds
to the final radiative state, while biphenylamino-centered p–p
transitions, coupled to the CT excited state, were involved in the
UV range.22
Fluorescence in solution and thin films
The bifunctional compound 6 exhibited a broad absorption band
in the visible resulting from the superimposed contributions of
the fluorophore and azo units, centered at 411 nm and 474 nm
respectively in toluene solution (Fig. 1).
The fluorescence quantum yield Ff of compound 6 in dilute
toluene solution was measured to be 0.006, compared to 0.025
for the fluorophore 4. When the azo and the fluorophore units
were directly connected without any spacer, a 30-fold decrease in
Ff was observed with respect to the fluorophore alone, which
validated the efficient insulating role played by the Diels–Alder
adduct.16
The spectral match of the sum of the absorption spectra of 3
and 4 in a ratio 1 : 1 with the absorption spectrum of 6 proves the
lack of significant interactions in the ground state between the
azo and fluorescent chromophore in agreement with the satu-
rated character of the bridge. The emission spectrum of 6,
peaking at 618 nm, resembled that of compound 4, slightly blue-
shifted (lem ¼ 613 nm) (Fig. 1, Table 1).
Working in toluene solution or in glassy monolithic thin films
4 and 6 (obtained from spin-coating 2 wt. % chloroform solu-
tions) gave similar absorption and emission spectra with a slight
shift of the maxima in the solid state as usually encountered. Thin
films of azo 3 were found little emissive, with no influence on the
molecular migration or photochromism ability by contrast to
previous studies on self-assembled azo aggregates.26 All the thin
films showed time-stable amorphous properties with glass tran-
ꢀ
sition temperatures Tg above 100 C (Table 1). The high trans-
parency of the thin films was not affected after months of storage
at room temperature according to the lack of recrystallization
transition evidenced by differential scanning calorimetry (DSC)
analyses over the temperature range [30–250 ꢀC]
Fig. 1 Absorption and emission spectra (isoabsorbing at lexc ¼ 400 nm)
of compounds 3, 4 and 6 in toluene solution.
9002 | J. Mater. Chem., 2009, 19, 8999–9005
This journal is ª The Royal Society of Chemistry 2009

Table 1 Spectroscopic and photochromic properties of compounds 3, 4, 6 and 7 in toluene solution and thin films
b e
,
Tg/ꢀCa
lmaxabs/nmb
lmaxem/nmbc
Ffd
k/10ꢁ5 sꢁ1
rmin [%]b f
,
Compound
3
4
6
7
123
101
174
122
474, 328 (475, 329)
411, 318 (418, 315)
439, 320 (450, 324)
416, 318 (422, 322)
ꢁ(665)
—
9.8 (37; 2.4)
—
10.6 (53; 2.5)
—
47 (15.5)
—
39 (13.5)
—
613 (604)
618 (645)
621 (615)
0.025
0.006
0.017
a
DSC analyses with a 20 ꢀC minꢁ1 thermal gradient. b 5 ꢂ 10ꢁ5 mol Lꢁ1 toluene solution (thin film). c lexc at the absorption maximum. d Measured in
e
toluene solution using coumarine 540 A in ethanol (Ff ¼ 0.38) as a fluorescence standard. After excitation at 488 nm (Ar+ laser). Mono- and
f
biexponential decays in solution and thin films resp. Assuming no absorption of the Z form at the E absorption maximum.
Fig. 2 Left: AFM imaging of surface reliefs for non-doped thin film of 6
(film thickness 260 nm) and Z profile. Right: 1st order diffraction
intensity h ¼ I+1/I0 as a function of time during the migration process at
lexc ¼ 488 nm.
Photoinduced fluorophore migration and fluorescence patterning
Mass transfer in thin film 6 was obtained by interferential illu-
mination at 488 nm using two isoenergetic polarized (+45ꢀ/ꢁ45ꢀ)
argon laser beams (160 mW cmꢁ2 each). The dynamics of the
surface deformation was followed by recording the change in the
first-order diffraction efficiency h ¼ I+1/I0 during the irradiation
step, where I+1 and I0 feature the first diffraction order and the
initial incident intensity respectively of a low-power He–Ne laser
beam working at 633 nm, far from the azo absorption band.
Even though h evolved more slowly for 6 compared to the azo
model compound 3, the plateau reached at the end was signifi-
Fig. 3 Absorption spectra of compounds 3 and 6 before (bold line) and
cantly larger, signing a higher relief amplitude. This was
after irradiation at 488 nm (dashed line) at the photostationary state. a)
confirmed by atomic force microscopy (AFM) yielding a peak-
Toluene solution. Inset: fit of the back thermal recovery of the E-isomer
to-trough amplitude of 190 nm for 3 against 240 nm for 6 (Fig. 2)
at labs ¼ 473 nm. b) Monolithic thin films (200 nm- thick). Inset: fit of the
for thin films of identical thickness (260 nm).
back thermal recovery of the E-isomer at labs ¼ 477 nm.
This discrepancy in migration dynamics cannot be related to
the photochromic properties of the azo units in 3 and 6 which
were found to be very similar from complementary photo-
physical investigations in free-constraint toluene solution and
also in thin films (Fig. 3.). By following the time-evolution of the
absorbance maxima at 473 nm of thin films 3 and 6 subjected to
an homogeneous irradiation at 488 nm, the minimum photo-
conversion yields rmin were estimated to be 15.5% and 13.5%
respectively. Once the photostationary state was reached, the
back thermal recovery of the E form was successfully modeled by
a biexponential law [E] ¼ [E]0 + {[E]PSS ꢁ [E]0} ꢂ [a1exp(ꢁk1t) +
a2exp(ꢁk2t)] where [E]0 and [E]PSS represented the concentra-
tions of the E form before irradiation and at the photostationary
state. The minor rate constants k1 (fractional amplitude f1 of 3%
with f1 ¼ a1/(a1 + a2)) were found unusually larger than those in
toluene solution. This component actually features the fraction
of molecules which occupy very constraint surroundings and
cannot adapt their new Z geometries after photoisomerizing.11,27
Given the steric crowding brought by the spacer, a small fraction
of azo derivatives reverts back to the initial E state in thin films
even quicker than in solution. The major rate constants k2
(f2 ¼ 97%), describing stabilized Z geometries, were found very
close at 2.4 ꢂ 10ꢁ5 sꢁ1 and 2.5 ꢂ 10ꢁ5 sꢁ1 for 3 and 6 respectively,
proving that both kinds of molecules photoisomerize in the same
way as non-doped materials.
To confirm that molecules containing the spacer 1 require
larger surroundings for moving, we measured the maximum
absorbance of thin films of azo 3 and fluorophore 7. We
compared them with thin films of similar thicknesses made of the
glass-forming cyano analog of the azo acid 211 and fluorophore 4
respectively. We thus systematically found a smaller absorbance
for thin films 3 and 7 as compared to thin films of the cyano azo
analog and 4, devoid of the anthryl adduct 1. These results as well
as the very fast minor components k1 tend to prove that 1
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 8999–9005 | 9003

induced larger free volume in the azo’s microenvironment,
rendering the material less dense and favoring migration in the
solid state.28 The slower motion, noticed for 6, may thus rely on
the additional mass brought by the attached fluorescent tail
which slowed down the azo drag. Such effects had already been
noticed in azo-functionalized polymers which form surface reliefs
ten times slower than azo monomeric thin films exempt of chain
entanglement.11,29 Such a subtle migration difference between
two compounds, which only differ from the size of the tail
attached to the azo units and not from their photochromic
properties, confirms the results of previous studies which have
established the crucial need of free volume for the molecules to
cooperatively move in the absence of polymer chain or Van der
Waals interactions as in liquid crystalline polymers.11
Fig. 5 Time evolution of the diffraction efficiency h ¼ Iꢃ1/I0 of a He–Ne
beam by a 410 nm-thick film of compound 6. The height of the relief was
measured to be 395 nm from AFM. Two writing-erasure cycles were
performed corresponding to the orange and blue curves respectively.
Anisotropic migration was performed by using two polarized Ar+ beams
The periodic migration of emissive azo derivatives 6 was
eventually investigated by laser confocal scanning microscopy
(LCSM) at lexc ¼ 488 nm, corresponding to the red edge of the
fluorophore absorption band (Fig. 4). No destruction of the
reliefs was observed under the focusing illumination conditions.
A periodically structured emission was imaged over 570–700 nm
in the area exposed to interferential illumination. By contrast,
only homogenous emission was detected for non-irradiated thin
films or thin films exposed to uniform irradiation. Photo-
degradation was also ruled out since thin films of 4 or 7 were found
to be stable under the same experimental conditions, and could
not exhibit periodic emission if subjected to light fringes as for 6.
Remarkably, the maxima in emission (red lines) matched the
minima in transmission (black lines) corresponding to the accu-
mulation of absorbing material after photomigration. The inter-
fringe periodic spacing was found to be 1.38 mm in perfect
agreement with the value determined from the sinusoidal profile
by AFM. Reversibility of the migration process could be per-
formed by erasing first the modulated surface with one beam at
a higher power (320 mW cmꢁ2) and shining again light interfer-
ences at a lower power (Fig. 5). The slightly slower dynamics
noticed was assumed to be due to mechanical effect history.
From all these observations, we can conclude that the fluo-
rescent moiety acts as a real molecular tail attached to the pho-
toactivable azo group which could be used as a tag to create
functional reliefs addressable by various optical and near-field
spectroscopies. Investigations of the microscopic displacement in
very diluted media would open the horizon toward the under-
standing of the molecular trajectories and the involved mecha-
nisms as well as the further manipulation and orientation of
single photoresponsive molecules.
(+ 45ꢀ/ꢁ45ꢀ configuration, each working at 160 mWcmꢁ2
) while
randomization and surface flattening involved one polarized Ar+ beam
(+45ꢀ: 320 mWcmꢁ2).
Conclusions
In summary, we have devised a new concept to get emissive
micrometric reliefs based on the photomigration of fluorophores
attached to azo moieties. This strategy implies the respect of
several simple spectral and structural guidelines to avoid inter-
molecular p-stacking in monomeric thin films and quenching of
the fluorescence and photomigration ability. The fluorophore
emission needs to be red-shifted regarding the absorption of the
azo unit which is able to deactivate any excited state, when too
close, by photoisomerizing in the picosecond time range. As for
the fluorophore attachment to the azo unit, a saturated, rigid and
bulky spacer is highly favorable to keep both chromophores
quite isolated and get efficient migration in the solid state thanks
to the free volume additionally involved. Fluorescence and
photochromic properties of the bifunctional and model
compounds have been investigated in solution and thin films.
When subjected to light interferences, the azo molecules migrate,
pulling the attached fluorescent tail during their motion. Fluo-
rescence confocal microscopy of illuminated films allowed us to
evidenced fluorescence reliefs where the intensity maxima exactly
matched the accumulation areas of the azo units along with their
fluorescent tags. To our knowledge, this work illustrates for the
first time the microscopic motion of two connected independent
units and should help better control photoinduced motions at the
single molecule level for the possible development of photo-
oadressable (bio-)active vectors.
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