SHG-active molecular nanorods with intermediate photochromic properties compared to solution and bulk solid states

Article abstract of DOI:10.1039/c0cc00985g

A convenient protocol has been developed to fabricate well-defined nanorods of a photochromic molecule N-(3,5-di-tert-butylsalicylidene)-4-aminopyridine (DBSAP) by a laser ablation method. Reversible and fatigue-resistant optical switching and SHG activity were demonstrated. Interestingly, it was observed that DBSAP nanorods exhibit intermediate photochromic properties compared to solution and solid.

Full text of DOI:10.1039/c0cc00985g

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SHG-active molecular nanorods with intermediate photochromic
properties compared to solution and bulk solid statesw
´ ´
Abhijit Patra, Remi Metivier, Jonathan Piard and Keitaro Nakatani*
Received 16th April 2010, Accepted 5th July 2010
DOI: 10.1039/c0cc00985g
A convenient protocol has been developed to fabricate well-
defined nanorods of a photochromic molecule N-(3,5-di-tert-
butylsalicylidene)-4-aminopyridine (DBSAP) by a laser ablation
method. Reversible and fatigue-resistant optical switching and
SHG activity were demonstrated. Interestingly, it was observed that
DBSAP nanorods exhibit intermediate photochromic properties
compared to solution and solid.
on non-amphiphilic photochromic molecules.6 This communi-
cation focuses on fabrication and photochromic and SHG
properties of DBSAP nanorods. The slow kinetics of thermal
back reaction in nanoparticles enabled us to evaluate the
absorption spectrum of the pure keto form and further
determine the absolute quantum yields for enol-keto and
keto-enol reactions which, to the best of our knowledge,
have not been reported so far for anils.
DBSAP in its more stable enol form was synthesized
following a reported procedure.8 Needle-like crystals of
DBSAP were introduced into a quartz cuvette containing an
aqueous solution of surfactant (Triton X-100 reduced). The
mixture was stirred vigorously for 1 h and then exposed to
the third harmonics of a nanosecond Nd:YAG pulsed laser
(355 nm). The intense laser pulses induce a gradual photo-
fragmentation of DBSAP bulk crystals and yield a light
orange colloidal suspension, which was found to be stable
for several days with no detectable precipitation.w AFM
images of the DBSAP colloids deposited on an alumina
membrane reveal well-defined nanoparticles with rod like
morphology (Fig. 1a,b). Nanorods fabricated through laser
ablation for 10 min with 30 mJ cmÀ2 per pulse energy have
average length ranging from 0.6–1.5 mm (Fig. 1a); length
further decreased to 0.3–0.8 mm when laser ablation was
continued for 16 min (Fig. 1b). Width and height of the
nanorods do not vary much with the laser ablation time and
typically range from 70–200 nm. FESEM investigations also
indicate similar size-distributions.w Fig. 1c depicts the gradual
decrease of particle size by each step of the fabrication process
discussed above: size distribution histograms (based on length)
of bulk crystals, and those obtained after 1 h stirring and after
10 and 16 min of laser ablation are shown, effectively reflecting
the impact of the laser ablation process.
Photochromic materials have been investigated extensively
during the past few decades because of their unique photo-
chemically reversible structural changes leading to interesting
optical responses and several nonlinear optical, electronic and
magnetic properties as well.1–3 The domain of solid state
photochromism has gained considerable attention due to their
varied applicability from ophthalmic lenses to optical memories,
sensors and switches.1,3 In recent years photochromic materials
based on molecular nanoparticles have emerged as an active
field of research due to their dispersibility in colloidal aqueous
suspensions and ease of integration in thin layer matrices,
avoiding the problem of shallow light penetration in bulk
materials and yielding optically suitable media.4–6
Among various classes of photochromes, salicylidene-
anilines, salicylidene-aminopyridines and related Schiff bases
(generally called anils) have received significant interest since
their photochromism involves an excited state proton transfer
leading to enol(-imine) to keto(-amine) transformation
(Scheme 1).3,7 Its fast kinetics is particularly suitable for rapid
switching applications. Earlier studies in our laboratory have
shown interesting photochromic properties of N-(3,5-di-tert-
butylsalicylidene)-4-aminopyridine (DBSAP) in solution, thin
films and crystalline phase.8,9 DBSAP bulk crystals in the enol
form belong to noncentrosymmetric space group P32 and
second order nonlinear optical studies were carried out in
solution and the solid state.8 Due to its promising properties
and its intriguing difference of behavior between solution
and bulk solid states (keto isomers’ stability usually in the
millisecond range in the former and up to several months in
the latter), it has become interesting to investigate the properties
of DBSAP in the nanoscopic state. However, a conventional
reprecipitation method10 failed to produce stable and well
dispersed nanoparticles. Recently we have employed the laser
ablation technique5,11 to fabricate nanorods of DBSAP. Even
though fabrication of molecular 1D nanomaterials has been
well documented,12 only a very few examples are found based
The formation of nanoparticles by the laser ablation process
was followed by UV-visible spectroscopy. When exposed to
the laser beam, the absorbance of the suspension increased
dramatically (Fig. 2a). The 355 nm beam is absorbed by enol
and induces enol to keto transformation, which is indicated by
the gradual evolution of the absorption band of the keto form
at 470 nm (Fig. 2a, inset). The general increase of absorption
PPSM, ENS Cachan, CNRS, UniverSud, 61 av President Wilson,
94230 Cachan, France. E-mail: nakatani@ppsm.ens-cachan.fr
w Electronic supplementary information (ESI) available: Details of
laser ablation method, microscopic and spectroscopic investigations,
and SHG measurements. See DOI: 10.1039/c0cc00985g
Scheme 1 Light induced intramolecular proton transfer leading to
enol to keto transformation in DBSAP.
c
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Chem. Commun., 2010, 46, 6385–6387 6385

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Fig. 2 (a) UV-visible spectral evolution during the laser ablation
process. Inset: gradual emergence of the band at 470 nm indicating
simultaneous enol to keto transformation during laser ablation. Curve
(A) corresponds to the extrapolated spectrum of the keto form
determined by Fischer’s method (see text for details). (b) Time profile
of ratio of absorbance at 470 and 390 nm demonstrating the progress
of the photochromic reaction during the laser ablation process.
Fig. 1 AFM images of DBSAP nanoparticles fabricated by laser
ablation at different time of laser exposure: (a) 10 min and (b) 16 min.
(c) Size distribution histogram based on length: (A) crystals, (B) after
1 h stirring in surfactant solution, (C) after 10 min of laser ablation
and (D) after 16 min of laser ablation.
and the decrease of scattering at 800 nm (where neither the
enol nor the keto form absorbs) during the ablation process
indicate a concomitant increase in the number of particles and
decrease of their average size to the nanometric range.w
Enol-keto transformation and fragmentation processes occur
simultaneously during laser ablation. Fig. 2b shows the plot of
the ratio of absorbance at 470 and 390 nm (corresponding to
the maximum of the band of the keto form and the isosbestic
point of the enol–keto mixture, respectively, see Fig. S12)w
with the time of laser ablation. This ratio increases as a
function of the laser ablation time, indicating that the photo-
chromic reaction occurs efficiently at the very beginning of the
laser ablation leading to the photostationary state in around
8 min (a = 31%) whereas microscopy investigations mentioned
earlier show that the laser ablation process was still effective
even after 16 min.
molecules at the surface and bulk of the nanocrystals may lead
to the biexponential decay.
Determination of photochromic quantum yields in presence
of thermal back reaction is not simple. Due to the meta-
stability of the keto form, its absorption coefficients are usually
difficult to estimate for anils. In the present case, the relatively
slow kinetic behavior of thermal back reaction in DBSAP
nanorods allowed us to apply Fischer’s method15 and obtain
the absorption spectrum of the pure keto form. The absorp-
tion spectrum of colloids containing only enol form was
obtained by irradiating the laser ablated samples at 488 nm,
where only keto form absorbs. Subsequently, under irradia-
tion at 405 and 365 nm, the colloidal suspension reached the
respective photostationary states. Applying Fischer’s method
the conversion yields obtained at the two photostationary
states are a(405 nm) = 10 Æ 2% and a(365 nm) = 19 Æ 2%.
The extrapolated absorption spectrum of the keto form using
these values is depicted in Fig. 2a, curve A.
The rate of thermal back reaction of nanoparticle disper-
sions was found to follow a double exponential kinetics
(Table 1). The rates of thermal back reaction of DBSAP in
solution and bulk solid state are also indicated in Table 1.8
This reaction occurs within a few milliseconds in solution, and
takes more than one year in the bulk solid. Interestingly,
nanoparticles exhibit kinetic behaviour intermediate between
that of solution and solid where the colored form reverts to the
enol form within 4–4.5 h.w The origin of the biexponential
decay for nanoparticles is not straightforward. The existence
of two different photocoloured species in the crystals, namely
cis-keto and trans-keto, was evidenced in some anils.13 The
fast component of the decay is believed to be due to the
cis-keto form whereas the slow component can be attributed
to the trans-keto form. On the other hand, owing to the larger
ratio of surface-to-bulk molecules, the lattice of nanoparticles
is likely to be less rigid than that in the bulk crystal.14 So it can
also be argued that the different relaxation behavior of the
The quantum yields of enol-keto (FE-K) and keto-enol
(FK-E) reactions at room temperature were determined
by irradiating the nanoparticle dispersions with a very low
power of a Hg/Xe lamp through appropriate band-pass filters.
Table 1 Kinetic constants of keto-enol thermal back reaction of
DBSAP in different physical states
Kinetic constants
k1/sÀ1 (a1)a
k2/sÀ1 (a2)a
DBSAP
Ethanol solution
Nanoparticlesb
Bulk solid
320 (1)
2.6 Â 10À3 (0.25)
—
3.2 Â 10À4 (0.75)
3.6 Â 10À5 (0.01)
1.8 Â 10À8 (0.99)
a
b
a1, a2 are the respective fractional contributions. Nanoparticles
obtained after 16 min of laser ablation. Rate constants of larger
particles (10 min of ablation) are found to be similar.
c
6386 Chem. Commun., 2010, 46, 6385–6387
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Dr G. Truan for providing lyophilized samples. PRES
UniverSud is acknowledged for providing a post-doctoral fellow-
ship to A. Patra, and ANR PNANO for supporting this research
work (Nanophotoswitch project, ANR-07-NANO-025).
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Fig. 3 (a) Time profile of absorbance at 470 nm of DBSAP nano-
particles upon UV (366 nm, 0.70 mW) and visible (488 nm, 0.26 mW)
irradiation. The continuous red line is the numerical fitting to the
experimental traces. (b) Photoswitching of absorbance at 470 nm
under alternate irradiation at 366 nm (66 mW for 20 s) and 488 nm
(4.6 mW for 40 s).
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and corresponding numerical fittings are provided in Fig. 3a.
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F
K-E is found to be nearly unity and FE-K is estimated to be
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back reaction (Fig. 3a).w This FE-K determination is consis-
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fitting from the experimental data indicates a possible distri-
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scopic heterogeneity in the nanoparticles. When irradiated
with alternate UV and visible light, DBSAP nanoparticles
exhibit fast switching of color which is demonstrated to be
well-repeatable through several cycles (Fig. 3b). Solid containing
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its fatigue resistant switching properties and SHG-activity will
be another interesting issue to pursue from the point of view of
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Chem. Commun., 2010, 46, 6385–6387 6387