The Journal of Physical Chemistry A
Article
light. This clearly demonstrates a photoinduced chemical
transformation.
To our best knowledge, it is the first time that a
photoisomerization process is reported for the leuco form of
indigo.
The quantum yield of reaction for leuco indigo (at λexc = 416
nm) and its derivative DBMNI (λ = 405 nm) was calculated
exc
21
from eq 2:
ΔAV
ε1000I Δt
ϕR
=
(2)
0
EXPERIMENTAL SECTION
■
where I
is the light absorbed by the solution at the irradiation
0
wavelength (calculated from eq 1), V is the volume of irradiated
solution (in mL), ΔA is the change in absorbance (at the
monitoring wavelength) over the irradiation time period, Δt,
and ε is the molar absorption coefficient of the compound at
the monitoring wavelength.
Considering that in the current experiments the changes
observed were registered in terms of fluorescence emission,
instead of absorbance, the factor (ΔA/ε) in eq 2 has been
replaced by the change in normalized concentration C(t) along
the irradiation time, according to eq 3:
Indigo and 1,10-phenanthroline were purchased from Aldrich,
potassium ferrioxalate was from Alfa Aesar, and the indigo
derivatives (4,4′-dibutoxy-7,7′-dimethoxy-5,5′-dinitroindigo
(
DBMNI) and methylated preciba (MpreCiba)) were synthe-
8
,24
sized as elsewhere reported.
The solvents used were all of spectroscopic or equivalent
grade.
The leuco species was prepared by adding 2−3 drops of
concentrated sodium dithionite/NaOH solution (0.15 g of
Na S O in 10 mL of NaOH (1 M)) to the dye in dioxane,
2
2
4
submitted to constant and gentle bubbling with Ar. The
solution was left bubbling for further 20 min and then sealed in
a proper device described elsewhere.
fluor. int.(t) × final concentration
C(t) =
25
final fluor. int.
(3)
Absorption and fluorescence spectra were recorded on
Shimadzu UV-2100 and Horiba-Jobin-Yvon Spex Fluorolog 3-
where “fluor. int. (t)” stands for the intensity of the
fluorescence emission at time t, “final concentration” stands
for the concentration of the leuco form (when the total
isomerization has been achieved), and “final fluor. int.” stands
for the intensity of fluorescence emission upon total isomer-
ization, that is, at the end of the experiment. As a consequence
of this, eq 2 now transforms into eq 4:
2
.2 spectrophotometers, respectively. Fluorescence spectra
were corrected for the wavelength response of the system.
The fluorescence quantum yield of the compounds was
26
determined using quinquethiophene (ϕ = 0.36 in dioxane )
F
as standard.
Photoreaction figures and yields for the leuco forms were
performed with a Horiba-Jobin-Yvon Spex Fluorolog 3-2.2
spectrophotometer, with irradiation at 370 nm for indigo, 408
nm for Mpreciba, and 405 nm for 4,4′-dibutoxy-7,7′-dimethoxy-
V ΔC
sol
ϕR = 1
000I Δt
(4)
0
where the ratio ΔC/Δt is obtained from the slope of the plot of
C(t) versus time.
Fluorescence decays were measured using a home-built
TCSPC apparatus elsewhere described and were analyzed
using the modulating functions method implemented by
Striker. The experimental excitation pulse (fwhm = 21 ps)
was measured using a LUDOX scattering solution in water.
After deconvolution of the experimental signal, the time
5
,5′-dinitroindigo (3 mm bandwidth). To obtain the I from the
0
irradiation source (450 W Xe lamp), the actinometer potassium
ferrioxalate was used [ϕ = 1.14 (405 nm) and ϕ = 1.12 (416
29
R
R
nm) in aqueous 0.05 M H SO ], using the “micro-version”
2
4
2
7
procedure, and by collecting the change in the optical density
at 510 nm, after several periods of time of irradiation (λexc
05 nm, 4 mm bandwidth for DBMNI and λexc = 416 nm, 2
30
=
4
mm bandwidth for indigo) in the fluorimeter. At each
irradiation wavelength (405 and 416 nm), a solution of 3 mL
29
resolution of the apparatus is ca. 2 ps.
(
0.006 M) of the actinometer was irradiated, keeping constant
RESULTS AND DISCUSSION
■
stirring, for 10 min. At the end of this, the cell was immediately
taken out of the fluorimeter, and 0.5 mL of buffered
phenanthroline was added. The absorbance at 510 nm was
measured immediately and after 10, 20, and 30 min.
Meanwhile, the solution was kept in the dark. The same was
done with a reference solution that has been kept in the dark.
The experiment was repeated, irradiating for 20 and 30 min.
Assuming that the entire incident light is absorbed by the
solution, the intensity of light is given by eq 1:
Indigo. The observation that, upon UV irradiation of the
leuco form of indigo, an increase was observed in both the
emission wavelength maxima and the fluorescence quantum
yield (Figure 1) led us to go into a deeper and detailed study of
this and related molecule(s). Considering that in the reduced
(
leuco) form of indigo the C−C bond connecting the two
indole moieties allows considerable rotation (particularly in the
lowest excited state), this behavior was interpreted to be due to
a (trans−cis) isomerization induced by light (photoisomeriza-
tion). The experimental support will be given below. In
addition, and supporting this, is the fact that we note a similar
small red shift in the absorption spectrum of butadiene on
ΔAV
I0 =
ε510dϕRt
(1)
31
going from the trans to the cis isomer.
As can be seen from Figure 1, the fluorescence quantum yield
where ΔA is the absorbance of the irradiated solution at 510
nm, corrected by the absorbance of the reference solution, d is
the optical path of the absorption cell measured in centimeters,
ε510 = 1.11 × 10 M cm , ϕ is the quantum yield of
production of Fe at the wavelength used in the photolysis, t is
−
2
values are low for t = 0 (ϕ ≈ 10 ), increasing to ϕ ≈ 0.2−0.3
F
F
when a steady-state equilibrium has been accomplished, and
4
−1
−1
R
these latter values are in agreement with those previously
2+
10,23
published
for the leuco form of indigo (and related
3
the irradiation time (minutes), and V is the volume (dm ) of
the solution used in the determination of the absorbance.
molecules) where full conversion was achieved by leaving the
sample under daylight exposure for approximately 20 min.
28
2
827
dx.doi.org/10.1021/jp211996f | J. Phys. Chem. A 2012, 116, 2826−2832