Characterization of the excited states of indigo derivatives in their reduced forms

Article abstract of DOI:10.1002/cphc.201000082

A comprehensive characterization of the electronic spectral and photophysical properties of the leuco (reduced) form of several indigo derivatives, including indigo and Tyrian Purple, with di-, tetra-, and hexa-substitution, was obtained in solution. The characterization involves absorption, fluorescence, and triplet-triplet absorption spectra, together with quantitative measurements of quantum yields of fluorescence, Φ_F(0.46- 0.04), intersystem crossing, Φ_T (0.013-0.034), internal conversion, Φ_IC, and the corresponding lifetimes. The position and degree of substitution promote differences in the spectral and photophysical properties displayed by the investigated leuco derivatives. The Φ_F values are about two orders of magnitude higher than those previously obtained for the corresponding keto forms. Also in contrast with the behavior found for the keto forms, the S1~~→T1 intersystem crossing is an efficient route for the excited-state deactivation channel. These findings strengthen the fact that, in contrast to keto indigo where the internal conversion dominates the deactivation of the excitedstate, with leuco indigo (and derivatives), the excited state deactivation involves competition between internal conversion, triplet state formation, and fluorescence. A time-resolved investigation of one of the compounds in glycerol showed the presence of a photoisomerization process.

Full text of DOI:10.1002/cphc.201000082

DOI: 10.1002/cphc.201000082
Characterization of the Excited States of Indigo
Derivatives in their Reduced Forms
Raquel Rond¼o,[a] J. Sꢀrgio Seixas de Melo,*[a] and Gundula Voss[b]
A comprehensive characterization of the electronic spectral
and photophysical properties of the leuco (reduced) form of
several indigo derivatives, including indigo and Tyrian Purple,
with di-, tetra-, and hexa-substitution, was obtained in solution.
The characterization involves absorption, fluorescence, and
triplet–triplet absorption spectra, together with quantitative
measurements of quantum yields of fluorescence, fF (0.46–
0.04), intersystem crossing, fT (0.013–0.034), internal conver-
higher than those previously obtained for the corresponding
keto forms. Also in contrast with the behavior found for the
keto forms, the S1 ~~!T1 intersystem crossing is an efficient
route for the excited-state deactivation channel. These findings
strengthen the fact that, in contrast to keto indigo where the
internal conversion dominates the deactivation of the excited-
state, with leuco indigo (and derivatives), the excited state de-
activation involves competition between internal conversion,
triplet state formation, and fluorescence. A time-resolved inves-
tigation of one of the compounds in glycerol showed the pres-
ence of a photoisomerization process.
sion, f , and the corresponding lifetimes. The position and
IC
degree of substitution promote differences in the spectral and
photophysical properties displayed by the investigated leuco
derivatives. The fF values are about two orders of magnitude
1. Introduction
Indigo is perhaps the oldest dye known as source of blue.[1]
Used for almost four millennia, it is generally regarded as a tex-
tile dye, but its applications are far from being confined to vat-
dyeing processes. Indeed, indigo was used not only for dyeing
textiles, but also with medicinal functions (in particular its
structural isomer indirubin[2]), in cosmetics,[3] tattooing, hair
dye, paintings[4,5] and in sculptures such as those found in the
Maya civilization where this dye extracted from leaves of
plants from the Indigofera family was mixed with a palygorskite
clay.[6,7] Remarkably, indigo has three important forms, which,
besides the neutral form, are the leuco (resulting from reduc-
tion) and dehydroindigo (resulting from oxidation), forms,
shown in Scheme 1. The blue colour of the dye is given by the
Neutral indigo belongs to the group of vat dyes, which are
almost insoluble in most organic solvents and in water, but
can be converted into the water-soluble leuco form which, in
most cases, is pale yellow. It is with the subsequent oxidation
that the textile turns blue. In the antiquity the dyeing of tex-
tiles with indigo demanded fermentation processes that in-
clude urine and bacteria (Clostridium spp.).[11] With the advent
of the modern chemical industry and after the introduction of
chemical methods, at the end of the 19th century, specifically
with the accessibility of the reducing agent sodium dithionite
(Na2S2O4), the dyeing process also become achievable on the
industrial scale. Although there are several modern methods
for reducing indigo to leuco indigo, including electrochemi-
cal[12] and bacterial methods, the more conventional
is still the use of Na2S2O4 in alkaline media.[13,14]
Despite the importance of this form of indigo, the
literature is scarce, particularly with regards to the
spectral and photophysical properties of leuco
indigo.[15,16] Herein, we investigate the electronic
spectral and photophysical properties of eleven dif-
ferent indigo derivatives, including indigo and Tyrian
Scheme 1. The three forms of indigo.
[a] R. Rond¼o, Dr. J. S. Seixas de Melo
Department of Chemistry
University of Coimbra
neutral keto form; dehydroindigo[8]—the oxidized form of
indigo—is known to be present in Maya Blue samples.[9,10] Re-
duced leuco indigo is the watersoluble form of indigo from
which the dyeing process emanate (Scheme 1).
3004-535 Coimbra (Portugal)
Fax:+351 239 827703
E-mail: sseixas@ci.uc.pt
Indigo, although known and used for millennia still has, with
its three forms, mysteries to unveil. In particular, the reduction
of indigo to leuco indigo has a remarkable history, almost
linked to that of the dye itself.
[b] Dr. G. Voss
Department of Bioorganic Chemistry
University of Bayreuth
95440 Bayreuth (Germany)
ChemPhysChem 2010, 11, 1903 – 1908
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1903

J. S. Seixas de Melo et al.
Purple, in their reduced (leuco) forms. Our aim is to understand
the differences of these in relation to those displayed by the
neutral (keto) form.
which partially precipitates when added to the solution.
Indeed, the presence of this tail in the absorption does not
change the pale yellow color of the solution.
As previously seen for indigo and some of its derivatives, a
significant blue-shift of the absorption spectra of the leuco
forms when compared with the keto is observed.[15,17]
In Table 1, it can be seen that the Stokes shift (SS, DSS) dis-
play values of ~90 nm, which contrasts with the smaller values
found for the keto forms, ~40 nm.[17] This suggests that, in the
leuco form, the ground- and excited-state structures of the in-
vestigated indigo derivatives are significantly different and that
this is presumably related to a photoisomerization process oc-
curring with the leuco forms. Moreover, with the leuco form,
the hydrogen bonds that keep indigo (and indigo-like struc-
tures) in a trans planar configuration, and is responsible for the
fast and dominant internal conversion deactivation channel
(through intra or intermolecular proton transfer),[18] are no
longer present. Indeed, it is not likely that efficient proton
transfer in the excited state occurs for the leuco form. Several
arguments can be given to ad-
2. Results and Discussion
The structures and acronyms for the compounds investigated
were divided in three families depending on the degree of
substitution (di-, tetra- or hexa-substitution), and are depicted,
in their leuco forms, in Scheme 2 where the numbering of the
positions is given as well.
Figure 1 shows the absorption, fluorescence emission and
excitation spectra for selected compounds of the three fami-
lies: compounds 4, 5, and 9 in Scheme 1. Data for leuco indigo
and leuco Tyrian purple were previously obtained in dimethyl-
formamide (DMF),[15] and for the correspondent keto forms of
the compounds in Scheme 2 in dioxane.[17] In Figure 1 the tail
observed at longer wavelengths (in the absorption spectra) is
due to scattering promoted by particles of the reduced agent,
dress this point. Although it is
likely that the excited-state
proton transfer (ESPT) in the
keto form is linked to the effi-
cient radiationless deactivation
process, the nature of this ESPT
(intra or intermolecular) is still
under debate. Recent works by
Iwakura et al.[18,19] with indigo-
carmine (a water-soluble indigo)
have provided additional evi-
dence relative to the occurrence
of an excited proton transfer
with this compound. Neverthe-
less, these experiments were
done in methanol (or deuterat-
ed methanol) and therefore the
ESPT mechanism may or may
not be assisted by the sol-
vent.[20] In addition, there are
several other reasons that
strongly decrease the possibility
that the same mechanism (ESPT)
is operative with the leuco form.
First of all, the proton transfer in
the excited state is, with the
keto form, associated with the
internal conversion radiationless
deactivation channel, where it
represents more than 99% of
the quanta lost, whereas with
the leuco the two other deacti-
vation channels (fluorescence
and singlet–triplet intersystem
crossing) are clearly operative.
Secondly, the efficiency of the
Scheme 2. Structures and acronyms for the substituted indigo derivatives (leuco structures) studied.
intramolecular ESPT is, with
1904
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ChemPhysChem 2010, 11, 1903 – 1908

Excited States of Indigo Derivatives
carbon–carbon bond has single-bond character, thus allowing
different non-planar conformations, both in the ground and
excited state. Indeed, in the case of dehydroindigo (the oxi-
dized and third form of indigo), in which the central carbonꢀ
carbon bond has single-bond character, it was shown that dif-
ferent geometries were obtained in both the ground and sin-
glet excited states.[8] Thirdly, intermolecular proton transfer in
the excited state is also strongly improbable in the view of the
alkaline media. Furthermore it is worth noting that the meas-
urements of the decay profiles on a picosecond time scale
(time resolution of ~2 ps) are bi-exponentials (Figures 2, 3),
and do not seem compatible with ESPT since the contribution
of the two components/species is dependent on the irradia-
tion time (Figure 3).
The coexistence of two conformers (close to, but not neces-
sarily trans and cis geometries), with different relative propor-
tions, should therefore be equated and is justified by the biex-
ponential nature of the decays (Figures 2, 3). In experiments in
the nanosecond time-range (time resolution of ~200 ps), the
decays were found to be strictly single exponential.
In order to further explore the possibility of the presence of
two conformers, we have conducted a time-resolved experi-
ment with compound 8 in highly viscous glycerol (h=
1.412 Pas). The decays were obtained with what we may con-
sider as a time zero—that is, the solution is prepared and im-
mediately measured (within 5 min)—and then after 60 min of
irradiation with a wavelength of 392 nm (laser wavelength of
the ps-TCSPC equipment, see the Experimental Section for fur-
ther details). The fluorescence decays, obtained under these
two conditions, depicted in Figure 3, clearly show two compo-
nents whose pre-exponential factors (concentrations at time
zero) change with the time of irradition. The shorter compo-
nent (tF ꢁ140–160 ps) increases, after one hour of irradiation,
with the concomitant decrease of the longer component
(tFꢁ1.32–1.53 ns). This clearly indicates that a light-induced
process (nonexistent in the keto structures) is present, and
that it is compatible with a photoisomerization process.
Figure 1. Absorption, fluorescence emission and excitation spectra (normal-
ized) for representative families 1 (4), 2 (5) and 3 (9) compounds in dioxane
at T=293 K in their leuco forms.
indigo, strongly associated with the planar geometry (experi-
mentally observed in the crystal), which is responsible for (and
the consequence of) the intramolecular hydrogen bonds be-
tween the NꢀH and C=O groups. To this planar conformation
two structural features are fundamental: the close proximity
between the NꢀH and C=O groups and the double-bond
nature of the central carbon–carbon bond connecting the two
indole-like moieties. In the case of the leuco form, the central
Table 1. Spectroscopic [absorption and fluorescence wavelength maxima together with singlet (eSS) molar extinction coefficients] and photophysical data
including fluorescence quantum yields (fF), lifetimes (tF) and rate constants (kF, kNR) for the leuco forms of the studied compounds in dioxane solution, T=
293 K.
Abs
max
Fluo
max
[b]
[c]
[c]
Compound
l
[nm] eSS [104 mꢀ1 cmꢀ1
]
l
[nm] fF
0.462
tF [ns] kF [nsꢀ1
]
kNR [nsꢀ1
]
DSS [cmꢀ1
]
Indigo
415
430
21160
13050[a]
500
505
3.33
0.225[a] 3.77[a]
0.139
0.06[a]
0.161
0.206[a]
4096
3454
Tyrian Purple (6,6’-dibromo-indigo)
Family 1
4,4’-dibromo-indigo (1)
4,4’-dichloro-indigo (2)
6,6’-difluoro-indigo(3)
6,6’-di(MeO)-indigo (4)
Family 2
430
430
412
413
14780
18300
11400
17540
505
503
518
515
0.367
0.223
0.0393 0.55
0.0468 0.55
2.89
3.20
0.128
0.218
0.238
1.719
1.730
3454
3375
4967
4796
0.0684
0.0703
0.0849
4,4’-(diOC9H19)-7,7’-di(MeO)-indigo (5)
5,5’,6,6’-tetra(MeO)-indigo (6)
Family 3
397
410
24170
15430
475
510
0.221
0.0702 0.52
2.20
0.0995
0.134
0.351
1.388
4136
4782
4,4’-di(OC6H13)-5,5’-diamino-7,7’-di(MeO)-indigo (7) 408
13470
8280
24580
490
480
490
0.160
0.234
0.213
2.30
2.50
2.28
0.0678
0.0928
0.0859
0.356
0.304
0.317
4102
3858
4406
4,4’-di(OC4H9),5,5’-diamino-7,7’-di(MeO)-indigo (8)
4,4’-di(OC4H9)-5,5’-dinitro-7,7’-di(MeO)-indigo (9)
405
403
ꢀF
[a] Data in DMF taken from ref. [15]. [b] Major component of a biexponential decay. [c] kF ¼ ; kNR
1ꢀꢀF
tF
¼
.
tF
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1905

J. S. Seixas de Melo et al.
Figure 3. Fluorescence decays and pulse instrumental response obtained for
the leuco form of 8 in glycerol at T=293 K, with an irradiation time=0 min
(upper curve) and after 60 min of irradiation (lower curve). The experimental
conditions and decay times obtained are shown as insets. Also shown are
the weighted residuals, autocorrelation functions (A.C.) and the c2 values for
a better judgment of the quality of the fits. The excitation wavelength is
392 nm and the emission wavelength 480 nm.
Figure 2. Fluorescence decays and pulse instrumental response obtained for
the leuco form of compounds 2 and 6 in dioxane at T=293 K. The experi-
mental conditions and obtained decays times are shown as insets in the
figure. Also shown are the weighted residuals, autocorrelation functions
(A.C.) and the c2 values for a better judgment of the quality of the fits. The
excitation wavelength is 392 nm.
However, more than the degree of substitution, the factor
that seems to be affecting the spectral behaviour of these
compounds the most is the substitution position. Indeed, if
one observes family 1 compounds 1 and 2, which have bro-
mine and chlorine atoms (different halogen atoms) in the
4,4’-positions, these display similar spectral (absorption and
emission wavelength maxima) and the same happens with
compounds 3 and 4 which are substituted in the 6,6’-positions
by different groups. However, when substitution takes place in
the 6,6’-positions, a much more significant Stokes shift is seen
when compared with the derivatives with substitution in the
4,4’-positions. The same is valid for the fluorescence quantum
yield values, which are one order of magnitude lower when
substitution is at the 6,6’-position and the lifetimes (~5 times
lower with 6,6’-compounds with regards to 4,4’-substitution),
see Table 1.
Table 1 summarizes the photophysical parameters for indigo
and the investigated derivatives in their leuco forms. With the
keto forms, the fF values are known to be low
(~10ꢀ3–10ꢀ4),[15,17] whereas with the leuco forms (Table 1) they
increase by about two orders of magnitude. Moreover, the
same applies to the fluorescence lifetimes for the keto and
leuco forms whose major component fall, respectively, in the
picosecond and nanosecond time domains.[15,17] This shows
that, in addition to internal conversion (dominant with the
keto forms),[15,17] with the leuco forms fluorescence (and sin-
glet–triplet intersystem crossing) now becomes an efficient de-
activation pathway for the excited state of the indigo deriva-
tives.
With family 3, since the position of substitution is the same
for the three compounds (7, 8 and 9), the spectral and photo-
physical parameters are very similar, although compound 8
emits at shorter wavelengths. By now comparing the values
between families 1 and 2—for example compounds 4 and 6—
it can be seen that substitution in the 6,6’-positions is responsi-
ble for the low fF and tF values, thus showing that this substi-
tution lowers the contribution of the radiative decay channel.
From Table 1 it can be seen that the wavelength absorption
maxima can generally be found in the ~400–430 nm range.
This indicates that substitution does not significantly affect the
absorption characteristics of the leuco forms, in contrast to
what was found with the corresponding keto forms.[17] The ex-
tinction coefficients are also generally high, characteristic of al-
lowed p,p* transitions.[22]
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ChemPhysChem 2010, 11, 1903 – 1908

Excited States of Indigo Derivatives
Leuco, contrary to keto forms, presents strong transient trip-
let–triplet signals. The transient triplet–triplet absorption spec-
trum for one representative compound of each family is de-
picted in Figure 4, and data (extinction coefficients, wave-
length maxima and intersystem crossing yields) in Table 2.
Compared with indigo and indigo di-substituted derivatives
(family 1), the more substituted compounds (tetra- and hexa-),
families 2 and 3, present a more pronounced depletion in the
singlet absorption region, followed by intense signals in the
450–500 nm region. This is evidenced by the increase on the
fT value, Table 2, going from indigo to 4,4’-di(OC4H9)-5,5’-dia-
mino-7,7’-di(MeO)-indigo (8). This growth shows that the
degree of substitution is decisive for the efficiency of the
S1 ~~!T1 intersystem crossing process.
An estimation of the triplet location (and consequently of
the singlet–triplet difference) can also be made by using the
phosphorescence spectrum onset value of indigocarmine in
methanol, with a value of ~475 nm (2.61 eV).[15] If this value is
used for the other compounds and for the energy of the S1
state we consider it to be found in the interval between
403 nm (3.07 eV) and 430 nm (2.88 eV), and we obtain, for the
S1–T1 energy splitting, values in the 0.46–0.27 eV range. More-
over, the rate of intersystem crossing (kISC) falls in the range of
~107–108 sꢀ1 which, with the singlet–triplet splitting obtained,
are values close to those of typical aromatic hydrocarbons.[23,24]
This provides convincing evidence that the triplet state of the
leuco forms of indigo and its derivatives is, similarly to keto
indigo,[23] of p,p* origin.
From Table 2 it can be seen that although the dominant
decay channel is the internal conversion (~80 to 89 percent of
the quanta is lost through this channel), this contribution is de-
creased relative to keto indigo where 99.99% of the quanta is
lost through this channel.[15–17,23] Moreover the solvent seems
to play a role, although not decisively, since in DMF the inter-
system crossing values were higher.[15]
Finally it is worth commenting on the degradation of the
indigo skeleton through hydrolytic cleavage in basic conditions
of the central C=C bonds. This is reported in the literature for
N,N’-diacetylindigo and indigo, originating, respectively, from
N-acetylanthranilic acid and anthranilic acid.[25–27] However, in
these works the yields of degradation are not reported and
keto indigo is known to be a stable molecule that only suffers
some degradation in extreme conditions, such as the presence
of nitric acid or under light excitation, and even at these condi-
tions with very low quantum yields of reaction (10ꢀ4 or
lower).[28] The significantly different photophysical behaviour
shown by the leuco forms strongly suggest that this form is
more prone to reactions and subsequently to degradation.
From Tables 1, 2 the rate constants of fluorescence and in-
ternal conversion are of the same order of magnitude for all
compounds, whereas the intersystem crossing values are one
to two orders of magnitude lower (Table 2).
3. Conclusions
We have investigated the electronic spectral and photophysical
properties of eleven indigo derivatives, including indigo and
Tyrian purple, in their leuco forms. These were found to
depend on the location and number of substitution. With the
di-substituted derivatives, the substitution in the diagonal 6,6’-
positions leads to fF values that are one order of magnitude
lower when compared to the disubstituted in the 4,4’-posi-
tions. With the tetra-substituted compounds (family 2) the sub-
stitution in the 6,6’-position also leads to a three-fold decrease
when compared to other substitution(s).
The internal conversion is the dominant deactivation chan-
nel for the leuco compounds in dioxane, although fluores-
cence and intersystem crossings are also important deactiva-
tion pathways for the excited state. This is in sharp contrast to
what was found for the neutral keto form of indigo and its de-
rivatives where more than 99.99% of the quanta is lost
through the internal conversion
Figure 4. Transient triplet–triplet absorption spectra obtained for the leuco
forms of some selected indigo and indigo derivatives (1, 5, 8) in dioxane, ob-
tained with different delay times after laser flash [181.0 ns for indigo,
437.7 ns (1), 190.5 ns (5) and 200.0 ns (8)], T=293 K.
channel. This may explain the
Table 2. Transient triplet–triplet parameters (triplet–triplet extinction coefficients, absorption wavelength
maxima, intersystem crossing yields), together with radiationless rate constants (kIC and kISC) and the S1 ~~!S0
internal conversion yield for the leuco forms of indigo and three representative derivatives of families 1, 2 and
3 in dioxane, T=293 K.
loss of stability of the leuco
when compared to the keto
forms. This possibility has not
been generally considered but
may be explained by a more
populated triplet excited state
in the leuco form which would
then prime the molecule to be
more reactive.
Compound
eTT [104 mꢀ1 cmꢀ1
]
lmT ax
fT
f
IC
1!Tn
Indigo
4,4’-dibromo-indigo (1)
4,4’-di(OC9H19)-7,7’-di(MeO)Indigo (5)
4,4’-di(OC4H9)-5,5’-diamino-7,7’-di(MeO)-indigo (8)
31030
20500
28710
20170
480
480
470
470
0.0127
0.0130
0.0278
0.0342
0.525
0.620
0.751
0.732
ChemPhysChem 2010, 11, 1903 – 1908
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1907

J. S. Seixas de Melo et al.
A time-resolved study showed that the decay profiles
change with the time length of irradiation, which is compatible
with a light-induced process (photoisomerization) and the
presence of two conformers.
Acknowledgements
Financial support from FEDER, FCT and POCI (PTDC/QUI-QUI/
099388/2008) are acknowledged. R. Rond¼o acknowledges FCT
for a PhD grant (SFRH/BD/38882/2007).
Experimental Section
Keywords: fluorescence
·
dyes/pigments
·
indigo
·
The indigo derivatives were synthesized and characterized as re-
ported elsewhere.[17,29] Indigo was purchased from Aldrich and
used as received. The solvents used were of spectroscopic or
equivalent grade.
photophysics · redox chemistry
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[3] J. Balfour-Paul, XXth International Congress of History of Science (Liꢀge,
Belgium) 1997, pp. 29–39.
[4] M. van E. Hommes, Discoloration in Renaissance and Baroque Oil Paint-
ings, Archetype Publications, London, 2004.
[5] M. J. Melo in Handbook of Natural Colorants (Eds.: T. Bechtold, R.
Mussak), John Wiley and Sons, Hoboken, 2009, p. 20.
[6] F. Museum, Centuries-Old Maya Blue Mystery Finally Solved, Science Daily,
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[7] H. Berke, Chem. Soc. Rev. 2007, 36, 15.
[8] R. Rond¼o, J. S. Seixas de Melo, V. D. B. Bonifꢂcio, M. J. Melo, J. Phys.
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[9] A. Domꢀnech, M. T. Domꢀnech-Carbꢃ, M. L. V. de A. Pascual, J. Phys.
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[11] A. N. Padden, V. M. Dillon, P. John, J. Edmonds, M. D. Collins, N. Alvarez,
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The solution of leuco indigo were prepared by adding 2–3 drops
of concentrated sodium dithionite/NaOH solution [0.15 g Na2S2O4
in 10 mL NaOH (1m)] to the dye in dioxane (or glycerol), submitted
to constant and gentle bubbling with Ar. The solution was left to
bubble for 20 min and sealed in a device (which keeps the solution
in inert atmosphere) as described elsewhere.[30]
Absorption and fluorescence spectra were recorded on Shimadzu
UV-2100 and Horiba-Jovin-Yvon Spex Fluorog 3–2.2. spectropho-
tometers, respectively. Fluorescence spectra were corrected for the
wavelength response of the system. The fluorescence quantum
yield of the compounds were determined using indigo (leuco form
in DMF, fF =0.35[15,17]) and quinquethiophene (fF =0.36 in diox-
ane[31]).
Fluorescence decays were obtained using the time-correlated
single-photon-timing technique with two different home-built ap-
[12] A. Roessler, D. Crettenand, Dyes Pigm. 2004, 63, 29.
[13] G. Voss, Col. Techn. 2006, 122, 317.
[14] G. Voss, J. Soc. Dyers Colour. 2000, 116, 87.
[15] J. Seixas de Melo, A. P. Moura, M. J. Melo, J. Phys. Chem. A 2004, 108,
6975.
[32,33]
paratus (with ns and ps time resolution) described elsewhere
and were analyzed using the method of modulating functions im-
plemented by Striker.[34]
[16] J. Seixas de Melo, R. Rond¼o, H. D. Burrows, M. J. Melo, S. Navaratnam,
R. Edge, G. Voss, J. Phys. Chem. A 2006, 110, 13653.
[17] J. S. Seixas de Melo, R. Rond¼o, H. D. Burrows, M. J. Melo, S. Navaratnam,
R. Edge, G. Voss, ChemPhysChem 2006, 7, 2303.
[18] I. Iwakura, A. Yabushita, T. Kobayashi, Chem. Lett. 2009, 38, 1020.
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The experimental setup used to obtain triplet-state absorption
spectra and quantum yields involves an Applied Photophysics laser
flash photolysis apparatus pumped by a Nd:YAG laser (Spectra
Physics), as described in detail elsewhere.[35] Transient spectra were
obtained by monitoring the optical density change at intervals of
10 nm over the 250–800 nm range and averaging at least
10 decays at each wavelength. Second-order kinetics were ob-
served for the decay of the lowest triplet state. Excitation was at
355 nm with an unfocused beam. Special care was taken in deter-
mining triplet yields to have optically matched dilute solutions
(abs. ꢁ0.2 in a 10 mm square cell) and low laser energy (2 mJ) to
avoid multiphoton and triplet–triplet annihilation effects. The trip-
let molar absorption coefficients for the leuco forms were deter-
mined by the singlet depletion technique, according to Equa-
tion (1):[36]
ðeSÞðDODTÞ
ð1Þ
eT ¼
DODS
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where both DODS and DODT were obtained from the triplet transi-
ent absorption spectra, and triplet formation quantum yields were
derived from these and actinometry with benzophenone. The in-
tersystem-crossing yields (fT) for the compounds were obtained by
comparing DOD at 530 nm of benzene solutions of benzophenone
(standard) optically matched (at the laser excitation wavelength)
and of the compound using Equation (2):
Benzophenone
TT
Indigo derivative
TT
Indigo derivative
DOD
max
e
ꢀTIndigoderivative
¼
ꢀBTenzophenone
ð2Þ
Benzophenone
e
DOD
Received: February 2, 2010
Revised: March 5, 2010
Published online on April 16, 2010
max
with eBTTenzophenone =7200 mꢀ1 cmꢀ1 and ꢀTBenzophenone ¼ 1.[36]
1908
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