Oxidation of photochromic spirooxazines by coinage metal cations. Part II. Oxidation by gold(III) compounds and synthesis of gold colloids

Article abstract of DOI:10.1039/b105447n

In our previous paper, we reported that colloidal silver can be prepared by thermal or photo-oxidation processes of spirooxazines (SO). Here we show that SO easily reduce gold(III) ions (1:1 equiv.) to monovalent gold(I) species. A salt composed of oxidized dye/Au^I was isolated and fully characterised by X-ray diffraction, ¹H and ¹³C NMR, UV-vis, fluorescence and FTIR spectroscopies. This salt was further reduced by another SO (2:1 equiv.) leading to gold(0) atoms that coalesce into gold powder. A mechanism for these two reactions is proposed. In the presence of a stabilising agent, such as poly(N-vinylpyrrolidone), the coalescence of Au atoms is limited and nanoparticles with an average size of 8-14 nm and a broad size distribution are formed. The colloidal solution is strongly fluorescent due to the presence of the oxidized SO in the medium. Changes in the absorption spectra from that of the sol and the free oxidized SO are interpreted as a consequence of the adsorption of the cationic dye on the gold surface.

Full text of DOI:10.1039/b105447n

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Oxidation of photochromic spirooxazines by coinage metal cations.
Part II. Oxidation by gold(III) compounds and synthesis of
gold colloidsy
a
b
c
b
b
Pawel Uznanski,* Catherine Amiens, Manuel Bardaji, Bruno Donnadieu, Yannick Coppel,
Bruno Chaudret and Antonio Lagunac
b
a
Centre for Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112,
0-363, Lodz, Poland. E-mail: puznansk@bilbo.cbmm.lodz.pl
9
b
c
Laboratoire de Chimie de Coordination, CNRS, 205 Route de Narbonne, F-31077
Toulouse cedex 4, France. E-mail: chaudret@lcc-toulouse.fr, amiens@lcc-toulouse.fr
Departamento de Quimica Inorganica, Instituto de Ciencia de Materiales de Aragon,
Universidad de Zaragoza-CSIC, E-50009, Zaragoza, Spain
Received (in Montpellier, France) 18th June 2001, Accepted 12th September 2001
First published as an Advance Article on the web
In our previous paper, we reported that colloidal silver can be prepared by thermal or photo-oxidation pro-
cesses of spirooxazines (SO). Here we show that SO easily reduce gold(III) ions (1 : 1 equiv.) to monovalent
I
gold(I) species. A salt composed of oxidized dye=Au was isolated and fully characterised by X-ray di€raction,
1
13
H and C NMR, UV-vis, ¯uorescence and FTIR spectroscopies. This salt was further reduced by another SO
2 : 1 equiv.) leading to gold(0) atoms that coalesce into gold powder. A mechanism for these two reactions is
(
proposed. In the presence of a stabilising agent, such as poly(N-vinylpyrrolidone), the coalescence of Au atoms
is limited and nanoparticles with an average size of 8±14 nm and a broad size distribution are formed. The
colloidal solution is strongly ¯uorescent due to the presence of the oxidized SO in the medium. Changes in the
absorption spectra from that of the sol and the free oxidized SO are interpreted as a consequence of the ad-
sorption of the cationic dye on the gold surface.
Considerable research has focused on the synthesis, physical
and photochemical properties of photochromic spiropyrans
(
degradation of the photochromic dyes involves oxidation
6,7
processes. There have been recent reports concerning the
identi®cation of the degradation product and several
1
SP) and spirooxazines (SO). These compounds show ther-
,2
8
mechanisms of degradation have been proposed. For
mally reversible changes of structure and colour upon light
irradiation, thus attracting interest due to their potential use,
as components of light protective materials, in optical switches
and devices, recording media, etc. Their photochromic prop-
erties are due to the Cspiro±O bond cleavage induced by
ultraviolet irradiation or heating, followed by the formation of
a polar, metastable merocyanine (Mc) entity (Scheme 1).
Unfortunately, the limited stability of the photochromic spiro
compounds when dispersed in various media and after
instance, the Mc form was shown to be reactive towards tri-
methylsilyl cyanide or free radicals and give zwitterionic
9,10
adducts.
In a previous paper we have shown that the oxidative
chemistry of spirooxazines with monovalent silver ions in
alcohols is surprisingly rich, yielding non-photochromic
alkoxy derivatives of the parent spiro compounds and rear-
ranged indolinonaphthoxazole products. These products are
further hydrolysed to a neutral (hydroxyindolino) naphthox-
3
±5
exposure to light, strongly reduces their utility in common
applications. One of the mechanisms recently reported for the
11
azole displaying a characteristic blue emission. We have
proposed a mechanism that clari®es the thermal reaction
channel for the oxidation of SO. Although products involved
1
13
in redox processes were characterised by H and C NMR,
and some of them by X-ray di€raction analysis, we were not
able to isolate single crystals of the major product, namely an
oxazole salt. However, it would be interesting to compare the
crystal structure of such organic=inorganic salts with pre-
viously reported crystal data for a similar but fully organic
system. To simplify the reaction we chose trivalent metal ions
instead of monovalent silver; this should promote a transfer of
two electrons to only one electron acceptor species. In order
to achieve an ecient oxidative reaction that would meet
our expectations, we turned our attention to photochromic
0
5-chloro-1,3-dihydro-1,3,3-trimethylspiro(2H-indole-2,3 -[3H]
naphth[2,1-b]-[1,4]oxazine) (1) and to a tetrachloroaurate(III)
salt. Such compounds crystallise easily and are more resistant
to degradation while the presence of gold atoms may,
accordingly, lead to the formation of a colloid. Furthermore,
Scheme 1
6,11
y For part 1, see ref. 11.
DOI: 10.1039/b105447n
New J. Chem., 2001, 25, 1495±1499
1495
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Scheme 2
III
as the two main oxidation states of gold are Au and Au , the
I
Thermal oxidation of 1 is likely to proceed according to the
�
11
mechanism described before, which includes two com-
plementary reaction paths (Scheme 2). A ®rst electron transfer
use of AuCl4 was supposed to promote a 1 : 1 reaction with
III
I
the SO. Indeed, the reduction of Au to Au turns out to be
fast but the next reduction step is much slower, which allowed
III
occurs from the oxygen atom to the Au ion. A more stable
I
us to isolate the intermediate Au salt as a crystalline com-
0
radical forms by 1,5-hydrogen transfer from C2 . Then two
pound and characterise it by an X-ray di€raction study. The
gold nanoparticles formed at the end of the reaction have been
fully analysed by TEM (transmission electron microscopy) and
UV-vis absorption spectroscopy. Finally, we propose a
mechanism for the redox reactions and compare it with that
paths can be envisaged, whether the second electron transfer
occurs prior to a deprotonation step (route B) or after it
(route A).
III
�
I
�
with elim-
ination of chloro ligands is supposed to proceed through
The conversion of [Au Cl
4
]
ion to [Au Cl
2
]
I
II
2 �
12
already reported for Ag reduction by analogous spirooxazine
compounds.
transient formation of the [Au Cl
4
]
ion. In the presence of
water 2 is hydrolysed to (hydroxyindolino)naphthoxazole
6
(
2a).
�
Salt 2 Á [AuCl ] can be further reduced by 1 (Scheme 3) into
2
Results and discussion
the chloride salt of 2. As the oxidative degradation of spiro-
oxazines involves transfer of two electrons from the open
zwitterionic Mc form to the appropriate electron acceptor
molecule, two gold(I) ions are reduced by one spirooxazine
molecule. As a result of the coalescence of the resulting ele-
mental gold atoms, gold powder precipitates from the reaction
solution.
Thermal oxidation of 1
Addition of sodium tetrachloroaurate to an acetonitrile solu-
tion of 1 leads within a few minutes to a colour change from
colourless to orange. The colour is associated with an elec-
tronic absorption band with a maximum at 465 nm, corre-
sponding to a new compound. This indicates that the p-
electron systems of two reciprocally perpendicular parts of the
parent molecule start interacting strongly due to their copla-
narity. However, the band cannot be attributed to the open Mc
form, the absorbance of which would be expected at ꢀ600 nm.
�
X-Ray crystallographic results for 2 as the [AuCl2] salt
�
Salt 2 Á [AuCl
2
]
has been analysed by a single-crystal X-ray
study, the results of which are presented in Table 1 and
shown in Fig. 1. The solid state structure of 2 is similar to
that of the other oxazole salt characterized by X-ray crys-
tallography. The compound is planar with the indoline part
linked through a C±C bridge to the naphthoxazole ring. The
1
H NMR analysis of the solution also supports the presence of
a new species 2 in the mixture. It shows two singlets at 7.74 and
.74 ppm, corresponding respectively to protons at ±CHˆN±
7
2
and N±CH characteristic for spirooxazine ring opening. The
3
former resonance ®nally disappears from the spectrum
whereas the latter shifts its position to 4.70 ppm, characteristic
of the quaternary nitrogen. Reaction of 1 with Cl AuNa in
unit cell consists of two asymmetric units with two sets of
�
2 Á [AuCl
2
]
cations are stacked and twisted so that their indoline het-
pairs, respectively A and B. In each unit the 2
4
+
re¯uxing acetonitrile under argon for 1.5 h yields a rearranged
product 2 and NaCl crystals. Analytically pure samples of
erocycles overlap with a distance of 7.662(2) A between the
N(2A) and N(2B) atoms. In turn the dyes from the two
�
0
2
and methanol (3 : 1) at 4 C. Product structures were assigned
Á [AuCl
2
]
were crystallised from a mixture of acetonitrile
ꢁ
di€erent units B and B are arranged in a ®shbone-like
pattern. The distances between gold atoms and charged
nitrogen atoms are 5.529(1) and 4.107(2) A, in pairs A and
1 13
on the basis of H and C NMR and X-ray di€raction data.
+
1
496
New J. Chem., 2001, 25, 1495±1499

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Scheme 3
+
B respectively, while gold atoms are separated by 5.630(1) A.
�
The [AuCl2] anion from the B couple is located between
two dyes and the distance Au(1B)±N(2A) equals 3.879(1) A.
+
�
Fig. 1 (a) The structure and atomic numbering of 2 Á [AuCl
2
]
thermal ellipsoids drawn at the 40% probability level. Selected bond
with
Thus, it is shorter than the distance Au(1B)±N(2B), sug-
�
+
ꢁ
lengths (A) and angles ( ) are as follows: Au(1B)±Cl(3B), 2.248(3);
Au(1B)±Cl(2B), 2.256(3); Cl(1B)±C(16B), 1.740(9); O(1B)±C(1B),
.379(1); N(1B)±C(1B), 1.281(13); N(2B)±C(22B), 1.479(1); C(1B)±
C(12B), 1.478(1); C(12B)±C(19B), 1.504(1). Cl(3B)±Au(1B)±Cl(2B),
178.42(1); C(12B)±N(2B)±C(22B), 128.3(9); N(1B)±C(1B)±O(1B),
117.5(10); N(1B)±C(1B)±C(12B), 123.4(1); N(2B)±C(12B)±C(1B),
24.8(9); N(2B)±C(12B)±C(19B), 112.7(9); C(21B)±C(19B)±C(20B),
. Selected intermolecular distances
gesting that the negative charge on [AuCl
�
2
]
anion in A is
is shared
between the two cations. The [AuCl2]
coplanar with 2 and the Au(1A) gold atom is separated from
1
+
the positive charge on N(2B) by as much as 8.952(2) A. The
�
packing of the 2 Á [AuCl
2
]
crystal provides experimental
1
1
(
evidence for its non-conductive properties. The crystal
structure contrasts with that recently reported by Malatesta
et al.: the organic TCNQ2 ion radical salt of an oxazole, in
�
10.8(8). (b) Unit cell of 2 Á [AuCl
2
]
+
A): Au(1A)Á Á ÁN(2A), 5.529(1); Au(1B)Á Á ÁN(2B), 4.107(2); Au(1B)Á Á Á
ꢂ�
N(2A), 3.879(1); Au(1A)Á Á ÁN(2B), 8.952(2); Au(1A)Á Á ÁAu(1B),
7
which anions and cations form segregated stacks. In this
5
.630(1); Au(1B)Á Á ÁAu(1B), 12.572(2); N(2B)Á Á ÁN(2B), 9.198(1);
case, electron delocalisation on helically arranged TCNQ
pairs is in agreement with both a low resistivity for the
microcrystals and paramagnetic character. Selected bond
N(2A)Á Á ÁN(2B), 7.662(2); Au(1A)Á Á ÁC(15A), 3.872(1).
�
lengths and angles for the 2 Á [AuCl
2
]
salt are given in
Fig. 1. The crystals were not stable and turned to powder
within a few weeks, probably due to their sensitivity to
water.
�
2
Au]
Table 1 Crystallographic data for 2 Á [Cl
Formula
FW
Crystal system
22 3
C H18NOCl Au
629.70
Triclinic
Electronic and luminescence spectra
Space group
P
ꢀ1
+
a=A
+
12.5891(2)
13.6746(2)
14.2577(2)
114.178(1)
97.965(2)
98.965(1)
2155.3(5)
4
180
15984
5839
0.0720
In agreement with our previous studies on oxidation products
11
of the photochromic spirooxazines, the salt prepared in this
study has similar absorption spectra with a broad and intense
transition between 400±500 nm (the chlorine substituent has
little e€ect on the photophysical properties). The excited state
b=A
+
c=A
ꢁ
a=
ꢁ
b=
ꢁ
g=
U=A
+
3
f
is strongly ¯uorescent (F > 0.85 in acetonitrile) with an
Z
T=K
emission maximum at 535 nm (Fig. 2). This is not surprising,
as the ¯uorescence of oxazoles is known to be strongly
13
Re¯ections collected
Unique re¯ections
enhanced when attached to an aromatic hydrocarbon. The
hypsochromic shift of the absorption band and blue ¯uores-
cence of 2a is connected with its non-planar geometry. For
both products a large Stokes shift and a high ¯uorescence
R
R
int
0.0364
0.0650
wR
11
quantum yield with fast emission decay are observed.
New J. Chem., 2001, 25, 1495±1499
1497

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Fig. 2 UV-vis absorption (A), ¯uorescence excitation (B) and emis-
]
�
sion (C) spectra. (a) 2 Á [AuCl
2
in acetonitrile. (b) 2a in EtOH.
Gold nanoparticles
I
As was mentioned earlier, further reduction of the Au anion
led to the formation of gold powder. Unfortunately, in the
acetonitrile±PVP solution 1 itself undergoes dark degrada-
tion processes to products with characteristic strong green-
ish-orange ¯uorescence, which are competitive with gold(I)
reduction bonds and prevented any clear observation of the
gold sol. However, as this e€ect was not observed for an
EtOH±PVP mixture, the reaction was repeated in this
medium.
Fig. 3 (a) Extinction spectrum of gold colloid prepared from re-
duction of Cl AuNa by 1 in the presence of PVP in ethanol (A). For
comparison the normalised spectrum of 2 in EtOH is also shown (B).
Inset. Plasmon spectrum of gold obtained by subtraction of spectrum
B from spectrum A. (b) Electron micrograph showing the size of gold
particles. The alignment of these particles is due to drying effects on
the copper grid. (c) Size distribution of the sample shown in (b).
4
A typical absorption spectrum of the solution, after the
�
reaction of the gold salt 2 Á [AuCl
2
]
with 1 (2 : 1 equiv.) in
the presence of PVP in EtOH is shown in Fig. 3. The orange
opalescent colour results from a mixture of two components:
a strong absorption by the indolinooxazole cation 2 and the
extinction band of colloidal gold. 2 absorbs with a maximum
at 462 nm, while the less intense part of the spectrum
extending into the red is a signature of gold particles. The
absorption due to the nanoparticles was calculated as the
di€erence between the two spectra presented in Fig. 3(a) and
its broad maximum falls around 510 nm (inset). This is in
good agreement with the characteristic value of 520 nm
corresponding to the collective oscillation of free conduction
Conclusions
We have shown that gold(III) salts can be reduced to gold(0)
atoms with a stable transient dye=gold(I) salt in a two-step
reaction by photochromic spirooxazines. Dispersions of small
gold particles were obtained using a stabilising PVP solution.
Absorption spectra of the ®nal sol suggest the adsorption of
the oxidised spirooxazine on the gold surface. The results may
14
electrons at the surface of spherical gold nanoparticles. The
presence of nanoparticles was demonstrated by TEM. In the
micrograph of Fig. 3(b) the particles are non-uniform and
can be described rather by a bimodal distribution. The sizes
of the majority of them are around 14 nm. The second
component of the size distribution has an average size of 8
nm [Fig. 3(c)]. Apparently, the particles are not aggregated.
We ascribe their alignment on the micrograph to drying
e€ects on the copper grid. The colloidal solution was stable,
demonstrating its colour and ¯uorescence properties for
months.
15,17
be relevant to various surface enhancement phenomena
and to the understanding of fundamental processes occurring
in photochromic compounds.
The properties of spirooxazines and the strong ¯uorescence
of the oxidative product make it suitable for metal ion sensing,
for example. The ability to employ a reversible acid±base
reaction to control access to its excited state (Scheme 2) may
prove useful in applications using ¯uorescence methods.
Finally we have also found that complex gold precursors, such
as ClAuTHT (THT ˆ tetrahydrothiophene), can be reduced by
spirooxazines to molecular gold species. The reaction is
induced either thermally or by UV irradiation, leading with
PVP to formation of gold nanoparticles.
Nucleation and growth of gold clusters is expected to be
�
speci®c as they are generated by the reduction of [AuCl2]
ions by an electron transfer reaction from the Mc entity,
which is oxidised to a positively charged species. Indeed, the
absorption by 2 is slightly larger than in neat EtOH (458 nm)
and the plasmon band is blue shifted, which may suggest
that the dye is chemisorbed on the gold particle surfaces.
This point, however, needs elucidation using surface
Experimental
1
5
enhancement spectroscopic techniques. Some recent results
11,16
Materials
on the adsorption of cationic dyes on gold or silver
show that dye±metal systems on the nanometric scale are
possible.
0
5-Chloro-1,3-dihydro-1,3,3-trimethylspiro(2H-indole-2,3 -[3H ]-
naphth[2,1-b]-[1,4]oxazine) (1) and poly(N-vinylpyrrolidone)
1
498
New J. Chem., 2001, 25, 1495±1499

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4
0
(
chased from Aldrich Chemical Co. and used as received.
PVP) with an average molecular weight of 4 Â 10 were pur-
138.78 (C11 ). IR (thin ®lm on SiO support): 1634, 1599, 1571,
1555, 1523, 1489, 1455, 1443, 1426, 1412, 1367, 1342, 1281,
1243, 1225, 1192, 1153, 1111, 1092, 1061, 1012, 925, 822, 808,
2
Tetrachloroaurate(III) sodium salt hydrate (NaCl
analytical reagent) was obtained from Montplet & Esteban
Barcelona, Spain). Acetonitrile was dried over appropriate
4
Au Á 2H
2
O,
�
1
18 2 3
759, 730, 696 cm . Anal. calc. for C22H N OCl Au: C,
41.92; H, 2.86; N, 4.44; found: C, 42.5; H, 2.9; N, 4.3%.
(
drying agents and distilled under argon immediately prior to
use. All reactions were performed under an atmosphere of
argon using standard Schlenk line techniques.
Preparation and characterization of colloidal gold
�
5
1
ethanol (10 mL) and the solution was de-aerated under
(5.4 mg, 1.5 Â 10 mol) and PVP (22 mg) were dissolved in
Spectroscopic measurements
�
5
vacuum. NaAuCl
4
(4 mg, 10
mol) was added under argon.
UV-visible and infrared absorption spectra were obtained with
an HP diode array 8452A spectrophotometer and with an ITI
Mattson In®nity FT system. Room-temperature excitation and
emission spectra were acquired with a Perkin±Elmer LS50
The solution was stirred under re¯ux for 1 h. Electron
micrographs were taken with a JEOL transmission electron
microscope model 200CX. TEM samples were obtained by
slow evaporation of one drop of the crude solution on a copper
grid coated with amorphous carbon. The particle size dis-
tribution was obtained from the sample placed on a copper
grid coated with amorphous carbon.
1
13
luminescence spectrometer. The H and C NMR spectra
were recorded at 305 K on a Bruker AMX400 spectrometer
1
13
operating at 400.13 MHz for H and at 100.61 MHz for C.
1
All chemical shifts for H and C were related to TMS using
13
1 13
H (residual) or C chemical shifts of the solvent as a sec-
ondary standard. NMR studies were carried out either by one-
1
or two-dimensional NMR methods. The assignments of the H
Acknowledgements
resonances were made by 1D or 2D nuclear Overhauser
exchange spectroscopy (NOESY) (t
This work was completed with the support of the European
Associated Laboratory ®nanced by CNRS (France), the Polish
Committee for Scienti®c Research (KBN 7 T08E 018 17 pro-
ject) and Programme de Recherche Franco-Espagnol 2000
PICASSO (no. 00688XH and HF 1999-0122). We thank
V. Colliere for technical assistance with the TEM experiments.
m
ˆ 600 ms) and 1D total
correlation spectroscopy (TOCSY) (t ˆ 60 ms), whereas 2D
m
heteronuclear correlation techniques (HMQC, HMBC) were
1
used for the complete C chemical shift assignments.
3
Crystallographic structural determination
The single-crystal X-ray di€raction data were collected at
1
80 K on a Stoe imaging plate di€raction system (IPDS) dif-
fractometer equipped with an Oxford Cryosystems Cryo-
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1
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�
Preparation of 2 Á [AuCl2]
�
5
A mixture of 7.2 mg (2 Â 10
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�
5
1
0
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4
2
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0
ꢁ
ꢁ
4
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1
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�
1
1
3
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3
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4
5
5
0
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3
HH
HH
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4
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0
HH HH HH
14
3
3
5
(
ddd, J ˆ 8.1 Hz, J ˆ 7.0 Hz, J ˆ 1.3 Hz; 1H, C8 -
3
4
H), 7.80 (dd, JHH ˆ 8.8 Hz, JHH ˆ 2.0 Hz; 1H, C6-H), 4.70 (s;
15 J. Aubard, in Organic Photochromic and Thermochromic Com-
pounds, ed. J. C. Crano and R. J. Guglielmetti, Kluwer Academic=
Plenum Publishers, New York, 1999, vol. 2, ch. 8.
‡
13
1
C{ H} NMR
3
H, N ±CH ), 2.04 (s; 6H, gem-CH ).
3
3
(
(
3
CD CN): d 170.14 (C2), 55.55 (C3), 145.54 (C4a), 124.36
3
16
(a) N. Chandrasekharan, P. V. Kamat, J. Hu and G. Jones II,
J. Phys. Chem. B, 2000, 104, 11 103.(b) L. Jeunieau, V. Alin and
J. B. Nagy, Langmuir, 2000, 16, 597.
C4), 137.64 (C5), 130.52 (C6), 118.31 (C7), 141.70 (C7a),
0
8.73 (C8), 24.28 (C9), 24.28 (C10), 150.90 (C2 ), 150.72
0
0
0
0
0
(
1
C4 a), 111.80 (C5 ), 134.86 (C6 ), 132.47 (C6 a), 130.11 (C7 ),
0
17 N. Felidj, L. J. Pantigny and J. Aubard, New J. Chem., 1998, 22,
725.
0
0
0
28.40 (C8 ), 129.83 (C9 ), 122.70 (C10 ), 126.62 (C10 a),
New J. Chem., 2001, 25, 1495±1499
1499