Angewandte
Communications
Chemie
The thermodynamically stable,
showed, in CHCl3 solution, the typical absorption band of
AB dyes with maximum at 360 nm and e = 2.7 ꢀ
A
trans isomer, tA
units, as well as between tA and the gold core, were weak in
the ground state.
a
To investigate the influence of the binding to the GNP on
the PI t!c process, tA-GNP were irradiated at 360 nm in the
same conditions used for the reference compound tA. As
shown in Figure 1, the peaks at 314 nm and 448 nm, typical of
cA, were observed at the PSS. Moreover, the same two
isosbestic points at 319 nm and 430 nm, observed during the
PI of the free ligand tA, were maintained during the
irradiation of tA-GNP. In particular, at the PSS, the absorp-
tion spectrum of the NPs (cA-GNP) perfectly matched the
one expected in the case of > 95% t!c conversion. The large
extent of photoswitching was the result of specific engineering
of the ligand, according to studies previously reported for
analogous 4,4’-dialcoxiazobenzene derivatives.[19] Interest-
ingly, the PI quantum yield measured for tA-GNP was
Ft-c = 0.15, a value that matched the one observed for the
reference compound tA.
As far as luminescence is concerned, a broad emission
band in the NIR region, with maximum at about 930 nm, was
observed upon excitation of either tA-GNP or cA-GNP in
CHCl3. The emission spectral profile was, for both of the
samples, independent of the excitation wavelength, in the
300–600 nm range, and consistent with data reported for
similar gold NPs.[15d] To investigate the effect of ligand PI on
the NIR luminescence, we compared the emission spectra of
the same sample of NPs recorded first at the thermodynami-
cally stable state (tA-GNP) and then at the PSS (cA-GNP).
The emission spectra acquired upon direct excitation of the
gold core (lexc,2 = 550 nm, where absorption by the ligands is
negligible) were identical within the experimental error (see
the Supporting Information). On the contrary, upon excita-
tion at 360 nm, a decrease of about 60% of the intensity of the
emission band was observed going from tA-GNP to cA-GNP
as an effect of the t!c PI (Figure 2). These observations
allowed us to conclude that: i) the emission quantum yield of
the gold NP did not change because of the PI, and ii) part of
the excitation energy adsorbed by tA was transferred to the
gold leading to sensitized emission. Sensitization was con-
104 mꢀ1 cmꢀ1.[12a] A gradual decrease of the intensity of this
band was observed, as reported for similar molecules, upon
irradiation at 360 nm, because of t!c PI. At the photo-
stationary state (PSS), the absorbance at 360 nm was
decreased to about 5% of the initial one. This observation
allowed us to conclude that: i) almost complete t!c con-
version occurred at the PSS, and ii) the molar absorption
coefficient of cA at 360 nm was negligible with respect to the
one of tA.
Going more into detail, the absorption spectra recorded at
different irradiation times (see the Supporting Information)
showed two isosbestic points at 320 nm and 429 nm. This
behavior demonstrated that only the two species tA and cA
were present in the solution, while no side photo-products
were formed upon irradiation. Moreover, the absorption
spectrum at the PSS strongly resembles cA. This spectrum
showed two peaks at 314 nm and 448 nm (e = 1.0 ꢀ
104 mꢀ1 cmꢀ1 and e = 3.0 ꢀ 103 mꢀ1 cmꢀ1 respectively). The
measured PI quantum yield was Ft-c = 0.15, as reported for
analogous AB derivatives.[12a] No fluorescence was observed
either for tA (lexc = 360 nm) or for cA (lexc = 480 nm).
As far as the NPs are concerned, the absorption spectrum
of the tA-coated nanoclusters tA-GNP (average diameter of
the gold core 1.7 nm, estimated formula Au144tA60) presented,
in CHCl3, both the band at 360 nm of the tA chromophore
and the weak surface plasmon resonance band of the gold
core,[18] with the latter one dominant in the region above
550 nm (Figure 1). More precisely, the absorption spectrum of
tA-GNP matches the one calculated for Au144tA60 as the
linear combination of the spectra of tA and of a reference
sample of CH3(CH2)11SH-stabilized GNP (Figure 1). This
spectral matching, and thus the lack of spectral perturbation,
showed that electronic interactions between adjacent tA
Figure 1. Continuous lines: absorption spectra of the thermodynami-
cally stable tA-GNP in CHCl3 (red) of the photo-isomerized NPs cA-
GNP (green) and of reference CH3(CH2)11SH stabilized GNP (black)
Dashed lines: linear combinations of the absorption spectrum of the
reference GNP with those of the ligands tA (red) and cA (green).
Inset: Absorption spectra of a CHCl3 solution of tA-GNP during
irradiation at 360 nm.
Figure 2. Continuous lines: excitation spectra of the thermodynami-
cally stable tA-GNP in CHCl3 (black) and of the photo-isomerized NPs
cA-GNP (red). Dashed lines: luminescence spectra (lexc =360 nm) of
the thermodynamically stable tA-GNP in CHCl3 (black) and of the
photo-isomerized NPs cA-GNP (red).
2
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
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