Exploring organic photosensitizers based on hemicyanine derivatives: a sustainable approach for preparation of amide linkages

Article abstract of DOI:10.1039/C8RA06232C

Hemicyanine derivatives C1-C4 have been synthesized and show strong absorption in the visible region, good water solubility, efficient intersystem crossing, a high singlet oxygen quantum yield and high ability to transport electrons from the donor to acceptor. These hemicyanine derivatives were utilized as photocatalysts in additive/base free oxidative amidation of aromatic aldehydes in mixed aqueous media under visible light irradiation at low catalytic loading. The hemicyanine derivative C4 exhibited recyclability upto four cycles and reusability upto five cycles in oxidative amidation of aromatic aldehydes. Among all the hemicyanine derivatives, C4 shows a high photocatalytic efficiency due to a high singlet oxygen quantum yield. All the mechanism investigations showed involvement of reactive oxygen species generated by the organic triplet photosensitizer based on hemicyanine derivative for carrying out oxidative amidation of aldehyde. Our results will encourage the design of new “metal free” organic photosensitizers and their application in photocatalysis.

Full text of DOI:10.1039/C8RA06232C

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Exploring organic photosensitizers based on
hemicyanine derivatives: a sustainable approach for
Cite this: RSC Adv., 2018, 8, 31237
preparation of amide linkages†
*
Harnimarta Deol, Manoj Kumar and Vandana Bhalla
Hemicyanine derivatives C1–C4 have been synthesized and show strong absorption in the visible region,
good water solubility, efficient intersystem crossing, a high singlet oxygen quantum yield and high ability
to transport electrons from the donor to acceptor. These hemicyanine derivatives were utilized as
photocatalysts in additive/base free oxidative amidation of aromatic aldehydes in mixed aqueous media
under visible light irradiation at low catalytic loading. The hemicyanine derivative C4 exhibited
recyclability upto four cycles and reusability upto five cycles in oxidative amidation of aromatic
aldehydes. Among all the hemicyanine derivatives, C4 shows a high photocatalytic efficiency due to
a high singlet oxygen quantum yield. All the mechanism investigations showed involvement of reactive
oxygen species generated by the organic triplet photosensitizer based on hemicyanine derivative for
carrying out oxidative amidation of aldehyde. Our results will encourage the design of new “metal free”
organic photosensitizers and their application in photocatalysis.
Received 23rd July 2018
Accepted 29th August 2018
DOI: 10.1039/c8ra06232c
rsc.li/rsc-advances
strategy. These limitations have been overcome by utilization of
“photosensitizers” as the photocatalysts for the preparation of
Introduction
the amide bond via oxidative amidation of aldehydes. In this
context, a variety of commercially available ruthenium/iridium
metal based complexes1,5 and dyes have been utilized as pho-
tocatalysts. Very recently, palladium carbodicarbene complex6
has been utilized as photocatalyst for the construction of amide
bond in organic reaction media under visible irradiation. The
main limitations of this approach are the requirement of the
toxic, costly and synthetically difficult organometallic
complexes as the catalytic species. Further, high loading of
these photocatalysts are essential in the reaction mixture to get
the target compound in good yield. The reason behind necessity
of high loading of photocatalyst is weak/moderate absorption of
these materials in visible region. Unfortunately, structural
modication of these photocatalysts to enhance their absorp-
tion in visible region is also difficult. To overcome these limi-
tations, several organic triplet photosensitizers based on
BODIPY,7 phenazinium8 and quinolizinium compounds9 scaf-
fold have been synthesized and tested for the photocatalytic
synthesis of amide bonds (Scheme 1). In comparison to
ruthenium/iridium heavy metal based complexes, these organic
triplet photosensitizers absorb strongly in visible region and
have long life time of the triplet excited state. Most of these
photocatalytic systems are themselves capable of activating O2
under light irradiation and hence external oxidant is not
needed. However, the economic/environmental advantages of
these approaches are decreased due to their multistep synthetic
routes, tedious purication and also the presence of boron as
heavy atom (in BODIPY based catalytic system). Further, most of
The amide linkage is a structural backbone of proteins and
peptides. These linkages are also predominant in medicinally
important compounds, natural products and materials having
industrial applications. The reason behind the high prevalence
of the amide linkage is its high stability, polarity and confor-
mational diversity.1 Conventional protocols for the preparation
of amide bonds generate a signicant amount of chemical
waste/side products and hence have poor atom economy.2 Thus,
a lot of efforts are being devoted to making the preparation of
the amide linkage more ‘green’. One of the best approache to
achieve the targets of green synthesis is to carry out the reaction
in the presence of some catalytic species. The catalysts are
special as they inuence the kinetics of the reaction positively,
provide an alternative pathway to the reaction, and themselves
remain unchanged. To date, a variety of reaction protocols
centred around various catalytic systems based on transition
metals2,3 and N-heterocyclic carbenes4 have been reported for
amide bond formation. The majority of these catalyzed
synthetic protocols require the presence of an oxidant, strong
thermal conditions and an inert atmosphere which in fact
reduces the economic and environmental advantages of the
Department of Chemistry, UGC Sponsored Centre for Advanced Studies-II, Guru Nanak
Dev University, Amritsar 143005, Punjab, India. E-mail: vanmanan@yahoo.co.in
† Electronic supplementary information (ESI) available: The contents of the ESI
section include 1H, 13C NMR and ESI-MS spectra of compounds; UV-vis and
uorescence studies; time resolved uorescence spectroscopy. See DOI:
10.1039/c8ra06232c
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Scheme 1 Oxidative amidation of aromatic aldehyde.
these systems work only in organic media and in some cases the in this direction, we were then interested to develop a dye based
desired product is furnished in decent yield only in the presence photocatalytic system for amide bond formation via oxidative
of blue LED as source of radiation which again decreases the amidation of aromatic aldehydes in aqueous media under eco-
economic and environmental advantages of the strategy. Thus, friendly conditions. For preparation of such photocatalytic
development of an efficient photocatalyst/organic triplet system, hemicyanine dye is scaffold of our choice due to its
photosensitizer which could work efficiently under visible light strong absorption in visible region, good water compatibility,
irradiation in base/additive free conditions in aqueous media is straightforward synthetic route, ease of modication, long lived
still a challenge.
triplet excited state and its ability to generate reactive oxygen
Our research work involves the development of new photo- species. With these unique photophysical and photochemical
catalytic systems for harvesting solar radiations for pursuing properties hemicyanine dyes have multidisciplinary applica-
various organic transformations.10 In continuation of our efforts tions in various elds such as photodynamic therapy (PDT),
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bioimaging, construction of dye sensitized organic solar cells strip into the solution of derivative C4 for carrying out oxida-
and pharmaceutical industry etc.11
tive amidation of aromatic aldehyde under visible light
Recently, from our lab we reported a hemicyanine deriva- irradiation.
tive C1 which emits in NIR region and showed selective and
sensitive response towards HSA in aqueous media.12 As a test
Results and discussion
Synthesis of the photosensitizers based on hemicyanine
of our hypothesis, we planned to the examine the photo-
catalytic efficiency of NIR active hemicyanine derivative C1 in
additive/base free oxidative amidation of aromatic aldehydes
in mixed aqueous media under visible light irradiation. To our
delight, the reaction worked well and the desired product was
obtained in 72% yield (vide infra). Encouraged by this result,
we plan to synthesized a series of photocatalysts (C1–C4) based
on hemicyanine dye and examine their efficiency in the
oxidative amidation reaction. These photocatalysts based on
hemicyanine dye were prepared by simple condensation of
various aldehydes with 1,2,3,3-tetramethyl-3H-indol-1-ium in
ethanol. The photocatalytic activity of these hemicyanine
derivatives were examined in additive/base free oxidative
amidation of aromatic aldehydes under aerial conditions in
aqueous media using tungsten lament bulb as the irradiation
source. Amazingly, among all the synthesized derivatives, C4
having pyrene as donor unit exhibited high efficiency and
furnished the target compound in good yield. The hemi-
cyanine derivative C4 exhibited recyclability upto four cycles
and reusability upto ve cycles in oxidative amidation of
aromatic aldehyde reaction. The high efficiency of derivative
C4 may be attributed to its strong absorption in visible region,
good water solubility, high singlet oxygen quantum yield (FD ¼
0.85) and efficient intersystem crossing. The work being pre-
sented in this manuscript demonstrates the utilization of
hemicyanine derivatives as efficient photocatalysts for oxida-
tive amidation of aldehydes under solar radiation and under
solvent free conditions. Unprecedented, this is the rst report
regarding utilization of hemicyanine derivatives as organic
photosensitizer in photocatalysis under mild condition
(aqueous media, base/additive free visible light and aerial
derivatives
Derivatives C1, C3 and C4 were synthesized by following the
procedure reported in literature (Fig. S40–S44 in ESI†).12–14
Derivative C2 was synthesized by condensation of 1,2,3,3-tetra-
methyl-3H-indol-1-ium with 2,7-azaindole-3-carboxaldehyde in
ethanol (Scheme 2).15 The structure of C2 was characterized by
various spectroscopic and analytical studies (Fig. S37–S39 in
ESI†).
Photophysical properties
The UV-vis and uorescence studies of derivatives C1–C4 were
examined in DMSO : H2O (1 : 1). The results were summarized
in Table 1 which clearly show that derivatives C1, C3 and C4
exhibit absorption/emission at longer wavelength in compar-
ison to that of derivative C2. The comparatively red shied
emission in case of derivatives C1, C3 and C4 may be attributed
to the presence of relatively stronger donor units and extended
conjugation which strongly facilitates the intramolecular
Table
1 Photophysical parameters of photosensitizers based on
hemicyanine derivativesa
Photosensitizer labs (nm)
3
lem (nm) FF
savg (ns) FD
C1
C2
C3
C4
587
457
540
500
33 000 686
25 000 511
60 000 557
32 000 560/610
0.04 0.78 0.70
0.25 0.85
0.20 0.8
0.03 0.50
a
a
0.85
a
3 ¼ molar extinction coefficient Mꢀ1 cmꢀ1, FF ¼ uorescence
quantum yield, savg ¼ average lifetime of singlet state, FD ¼ singlet
conditions, low catalytic loading). Further, the ‘dip strip’ as oxygen quantum yield, a ¼ could not be detected.
portable catalytic system was prepared by dipping lter paper
Scheme 2 Synthesis of photocatalysts based on hemicyanine derivatives (C1–C4).
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charge transfer in these systems (Fig. S1 and S2 in ESI†).
Further, the singlet state lifetime of the derivatives C1–C4 were
evaluated using the time resolved uorescence spectroscopy
(Fig. S3–S6 in ESI†). In all the derivatives, life time of singlet
state was found to be less than 1 ns, thus, long lifetime of their
respective triplet state is expected. Derivatives C2 and C3
showed decent quantum yields but in case of derivatives C1 and
C4 the low quantum yields were determined (Table 1). The low
quantum yields in case of derivatives C1 and C4 further indicate
efficient intersystem crossing (ISC) form singlet to triplet
excited state in these systems, as a result of which a highly
stable/long lived triplet excited state is expected in these deriv-
atives. Further we calculated the singlet oxygen quantum yields
for derivatives C1 and C4 using 1,3-diphenylisobenzofuran
1
(DPBF) a known O2 quencher (Fig. S7 and S8 in ESI†).16 Both
the derivatives showed good efficiency for generation of singlet
oxygen (Table 1).
Further, we checked the photostability of hemicyanine
derivatives C1–C4 by irradiating their oxygen saturated solu-
tions in mixed aqueous media for 36 h (Fig. S9–S12 in ESI†). The
derivatives C1–C3 showed good photostability even aer
continuous irradiation for 36 h. However, in case of a derivative
C4, 30% decrease in absorbance intensity was observed aer
36 h.
Fig. 1 (A) Absorption spectral changes of derivative C4 (0.02 mM) in
presence of MV2+ (0.2 mM) and TEOA (50 mM) in DMSO under room
light and inert atmosphere. Inset: absorption spectral changes upon
addition of TEOA in solution of derivative C4 and photograph of
solution before and after 6 min. (B) Derivative C4 acts mediator and
transport the electron from sacrificial donor to acceptor under visible
light.
In the next part of our work, we planned to examine the
potential of photosensitizer based on hemicyanine derivative to
transport electron from donor to acceptor unit which is key step
in photocatalysis. For this we chose three component system
consisting of methyl viologen (MV2+) as electron acceptor, trie-
thanolamine (TEOA) as sacricial donor and hemicyanine
derivative as the photosensitizer. We observed that upon addi-
tion of TEOA in solution of derivative C4, the band at 500 nm
due to derivative C4 rapidly decreased along with red shi of
50 nm. These observations indicate a strong interaction
between sacricial donor and derivative C4. Aerwards, we
added methyl viologen (MV2+) and upon exposure to room light
under inert atmosphere within 2 min colour of solution
changed from light pink to green. Further, within next 6 min
colour of solution changed to dark blue. The whole event was
monitored by UV-vis spectroscopy. The UV-vis spectra showed
presence of two new bands at l ¼ 395 and 603 nm which
gradually increased upon exposure to room light (Fig. 1A).
These two bands correspond to the reduced species MVc+ of
methyl viologen. On the basis of these results, we proposed that
derivative C4* reductively quenched by sacricial donor and to
form C4*ꢀ, then C4*ꢀ transports electron to MV2+ and reduced
it (Fig. 1B).17 We also performed the same experiment with other
hemicyanine derivatives (C1–C3), the colour of solution gradu-
ally changed from green to blue (Fig. S13–S15 in ESI†). All above
results showed that hemicyanine derivatives work as photo-
sensitizers which transport the electron from sacricial donor
to acceptor under visible light.
amidation of aromatic aldehyde under visible light irradiation.
The reaction between 4-nitrobenzaldehyde (1a, 1.0 equiv.) and
pyrrolidine (2a, 3.0 equiv.) in DMSO : H2O solvent mixture
under the irradiation of tungsten lament bulb at room
temperature was chosen as the model reaction. Among all the
derivatives, we chose derivative C4 as a photocatalyst for
carrying out model reaction due to its high singlet oxygen
quantum yield and high efficiency to transport the electron
from donor to acceptor. To our delight in presence of 1 mol% of
derivative C4, the reaction was completed in 12 h and the
desired product was obtained in 82% yield (Table 2, entry 1).
Next, we screened different solvents such as THF, MeCN, DMF,
dioxane, DMSO and water (Table 2, entries 2–7) as the reaction
media for the transformation. The desired product was ob-
tained in comparable yields in case of DMSO and DMSO : H2O
mixture and much lower yield of product was obtained in water.
We chose DMSO : H2O (1 : 1) as the reaction media for carrying
out further transformations. Further, we also examined the
inuence of the presence/absence of base in the oxidative
amidation of aromatic aldehyde under visible light irradiation
(Table 2, entry 8). The presence or absence of base did not
inuence the yield of the desired product. Further, in the
absence of a hemicyanine based photosensitizer or in absence
of visible light source, the reaction did not proceed (Table 2,
entries 9 and 10). The desired product was obtained in 10%
yield when model reaction was setup under inert atmosphere
(Table 2, entry 11). This result emphasized that aerial
Photocatalytic amidation of aromatic aldehyde
Aer examining the photophysical properties of derivatives C1–
C4, our initial investigation focused on the direct oxidative
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Table 2 Optimization of reaction conditions
Entry
Photosensitizer
Additive
Solvent
Time
Yield
1
2
3
4
5
6
7
8
C4
C4
C4
C4
C4
C4
C4
C4
—
C4
C4
C4
C4
C4
C1
C2
C3
—
—
—
—
—
—
—
DMSO : H2O (1 : 1)
DMSO
DMF
Dioxane
THF
12 h
12 h
12 h
82%
80%
15%
30%
55%
60%
30%
82%
0%
12 h
12 h
12 h
12 h
12 h
12 h
12 h
12 h
12 h
12 h
20 h
20 h
16 h
MeCN
Water
K2CO3 as base
—
—
—
DMSO : H2O (1 : 1)
DMSO : H2O (1 : 1)
DMSO : H2O (1 : 1)
DMSO : H2O (1 : 1)
DMSO : H2O (1 : 1)
DMSO : H2O (1 : 1)
DMSO : H2O (1 : 1)
DMSO : H2O (1 : 1)
DMSO : H2O (1 : 1)
DMSO : H2O (1 : 1)
9a
10b
11c
12d
13
14
15
16
17
0
10%
60%
88%
85%
72%
50%
75%
—
H2O2
BHT
—
—
—
a
b
d
No photocatalyst. No light. c Inert atmosphere. 4 mol%.
conditions promote the oxidative amidation of aromatic alde- photosensitizers such as Ru(phen)3Cl2, Ir(dtbpy)(ppy)2PF6,
hyde under visible light irradiation in presence of photocatalyst Rhodamine B, Methylene blue and Alizarin red S etc. (Table S1
based on hemicyanine derivative. Further, we examined the in ESI†), the photosensitizers based on hemicyanine derivatives
effect of increase in the catalytic loading on the reaction kinetics worked well in the oxidative amidation of aldehyde under
and efficiency of the reaction. The desired product was obtained visible light irradiation.
in lower yield (60%) (Table 2, entry 12) upon increasing the
In order to get more insight into the efficiency of derivative
catalytic loading from 1 mol% to 4 mol%, which may be C4, we examined the photophysical properties of derivative C4
attributed to the increase in the colour intensity of the solution in presence of pyrrolidine. The solution of derivative C4 in
that hindered the passage of radiations.8 We also examined the DMSO showed emission bands at 560/610 nm. Upon addition of
effect of the presence of external oxidant such as H2O2 on the 0.1 mmol of pyrrolidine to this solution, the intensity of emis-
reaction kinetics. As expected, the reaction was accelerated and sion bands at 560/610 nm were quenched completely (Fig. S16
the desired product was furnished in 88% yield aer 8 h (Table in ESI†). On the other hand, higher equivalents of pyrrolidine
2, entry 13). Further, we also checked the effect of BHT as were required in case of derivatives C1–C3 (Fig. S17–S19 in
additive in the oxidative amidation of aldehyde under visible ESI†). These studies supported the high efficiency of electron
light irradiation. However, no signicant effect on the yield and transfer between pyrrolidine and excited state of derivative C4.7
rate of reaction was observed (Table 2, entry 14).
With the optimized reaction conditions in hand, a variety of
Under these optimized reaction conditions, we also exam- aromatic aldehydes were investigated to illustrate the photo-
ined the photocatalytic activity of other C1, C2 and C3 hemi- catalytic efficiency and scope of the photocatalyst based on
cyanine derivatives in oxidative amidation of aldehyde. The hemicyanine derivative C4 under visible light irradiation and
derivatives C1, C2 furnished the desired product in 72% and aerial conditions. Aromatic aldehydes bearing electron with-
50% yields, respectively aer 20 h. On the other hand upon drawing groups and electron donating groups furnished the
using C3 as the photocatalyst, the desired product was obtained desired products in good yields (Scheme 3).
in 75% yield aer 16 h (Table 2, entries 15–17). Among all the
Furthermore, we examined the substrate scope with respect
derivatives C4 showed high photocatalytic activity due to its to amines having six membered rings such as morpholine and
maximum efficiency for generation of singlet oxygen (FD ¼ 0.85) piperidine. Amazingly, in presence of these amines as the
and high ability to transport the electron as compared to other reaction partner the desired products were obtained in good
hemicyanine derivatives. In comparison to the off shelf yields (Scheme 4).
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Scheme 3 Oxidation amidation of substituted aromatic aldehydes with pyrrolidine using photocatalyst based on hemicyanine derivative C4
under irradiation of visible light at room temperature and aerial conditions.
We also investigated the oxidative amidation of 4-nitro- light accelerate reaction rate upto great extent than tungsten
benzaldehyde (1a, 1.0 equiv.) with pyrrolidine (2a, 3.0 equiv.) bulb using as irradiation source.
under solvent free conditions using of 1.0 mol% of C4 under the
To determine the recyclability of photocatalytic system, we
irradiation of tungsten lament bulb at room temperature chose reaction between of 4-nitrobenzaldehyde (1a, 1.0 equiv.)
(Scheme 5). To our pleasure, the desired product was obtained and pyrrolidine (2a, 3.0 equiv.) in DMSO : H2O (1 : 1) as the
in 70% yield aer 20 h. On the other hand the reaction between model reaction using 1.0 mol% of derivative C4 as photo-
4-methoxybenzaldehyde (1a, 1.0 equiv.) and pyrrolidine (2a, 3.0 catalyst. Aer completion of the reaction, solvent was evapo-
equiv.) under solvent free conditions using 1 mol% of C4 rated under reduced pressure and the resulting residue was
produced the desired product in 60% yield aer 36 h. All these extracted with the ethyl acetate. The organic part was sepa-
studies highlighted the utility of photocatalyst in very mild rated and washed with water, dried over Na2SO4 and concen-
conditions.
trated under reduced pressure. The aqueous layer containing
To further demonstrate the practical application of photo- photosensitizer was concentrated under reduced pressure and
catalyst based on hemicyanine derivative for preparation of reused for next cycle of oxidative amidation of aldehyde
amides, we carried out reaction between 4-nitrobenzaldehyde (Fig. S21 in ESI†). The same procedure was followed for four
(1a, 1.0 equiv.) and pyrrolidine (2a, 3.0 equiv.) in DMSO : H2O cycles. Aer the fourth cycle, the yield of the nal product was
(1 : 1) using of C4 (1 mol%) as photosensitizer one at 0.2 mmol reduced to 62% (Fig. 2).
and another one at gram scale in presence of solar radiations
Further, we examined the reusability of derivative C4 in
(Fig. S20 in ESI†). Remarkably, aer 8 and 36 h, the desired oxidative amidation of aldehyde. A reusability experiment was
product was isolated in 88% and 70% yields respectively carried out using C4 as a catalyst for the oxidative amidation of
(Scheme 6). These results showed that C4 found to be more aldehyde under visible light irradiation. Aer each cycle, reac-
efficient in presence of solar light, which indicate that solar tants were introduced into the solution of original catalytic
Scheme 4 Oxidation amidation of substituted aromatic aldehydes with piperidine/morpholine using photocatalyst based on hemicyanine
derivative (C4) under irradiation of visible light at room temperature and aerial conditions.
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Scheme 5 Oxidation amidation of 4-nitrobenzaldehyde with pyrroli-
dine using photocatalyst based on hemicyanine derivative (C4) under
irradiation of visible light at room temperature, neat and aerial
conditions.
Scheme 8 Proposed reaction mechanism for oxidative amidation of
aldehyde using photosensitizer C4 under visible light irradiation.
We also prepared ‘dip strip’ by dip coating a lter paper
strip in solution of derivative C4 (Fig. S23 in ESI†). The
prepared coated lter paper strip was used as portable cata-
lytic system for carrying out oxidative amidation of aldehyde
under visible light irradiation. To our pleasure the reaction
was completed in 12 h with 80% yield of target product
(Scheme 7).
Mechanism studies
We believe that oxidative amidation of aromatic aldehyde was
mediated by reactive oxygen species generated by photosensi-
tizer based on hemicyanine derivative in excited state. For
generation of reactive oxygen species, two pathways are sug-
gested. First, energy transfer pathway in which energy of triplet
Scheme 6 Oxidation amidation of 4-nitrobenzaldehyde with pyrroli-
dine using photocatalyst based on hemicyanine derivative (C4) under
solar radiation at room and aerial conditions.
Fig. 2 Recyclability of photocatalyst based on hemicyanine derivative
(C4) for carrying oxidative amidation of aromatic aldehyde under
visible light irradiation.
system. The derivative C4 was successfully reused upto ve
cycles and no signicant change in the yield of target product
was observed (Fig. S22 in ESI†).
Scheme 9 Oxidation amidation of 4-nitrobenzaldehyde with pyrroli-
Scheme 7 Oxidation amidation of 4-nitrobenzaldehyde with pyrroli- dine in the presence of ROS quenchers using photocatalyst based on
dine using dip strip of derivative C4 under visible irradiation at room hemicyanine derivative (C4) under irradiation of visible light at room
temperature and aerial conditions.
temperature and aerial conditions.
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excited state C* transferred to dioxygen and singlet oxygen
species was generated, which further reacts with amine to
produce H2O2. The second proposed pathway is activation of
dioxygen through a single electron transfer (SET) route to form
the superoxide radical O2cꢀ from the radical anion [C]cꢀ, which
is generated by SET from the amine to the excited state3 [C]*.
Then, the active species O2cꢀ produced H2O2. The in situ
generated H2O2 further oxidized a-hydroxy amine to afford the
amide (Scheme 8).
To check in situ generation of H2O2, we added KI/CH3COOH
to the reaction mixture and brown coloration was observed
which conrms the presence of H2O2 in the reaction mixture
(Fig. S24 in ESI†).7,16c
To conrm our assumption regarding the involvement of
singlet oxygen/superoxide in reaction, we carried out the
model reaction in the presence of singlet oxygen scavenger
such as DABCO (1,4-diazabicyclo[2.2.2]-octane), sodium azide
and in presence of superoxide scavenger such as 1,4-benzo-
quinone. We found that the photocatalytic reaction was
affected signicantly and the desired product was obtained in
20%, 18% and 22% yields respectively (Scheme 9). These
observations support that both 1O2 and O2cꢀ are involved in
the photocatalytic reaction.
Determination of singlet oxygen quantum yield. Singlet
oxygen quantum yields of derivatives C1 and C4 were obtained
by the measurement of quenching of DPBF in the presence of
C1 and C4 as photosensitizers taking the reference of methylene
blue under same conditions (FD ¼ 0.52) in DMSO as solvent.
The irradiation was done in a uorescence spectrophotometer
equipped with a Xe lamp at the excitation wavelength of C1 and
C2 for 50 min. UV-visible measurements were performed using
UV-vis spectrophotometer. All the experiments were carried out
ꢁ
at 25 C.
Synthesis of derivative C2. To a the mixture of 1,2,3,3-tetra-
methyl-3H-indol-1-ium (205 mg, 0.68 mmol) and 2, 7-azaindole-
3-carboxaldehyde (100 mg, 0.68 mmol) in anhydrous ethanol
(20.0 mL) were added two drops of piperidine. Then the reaction
mixture was reuxed for 48 h under inert atmosphere. Aer
completion of reaction, red coloured solid was ltered and
washed with ethanol (yield ¼ 80%). 1H NMR (300 MHz, DMSO-
d6) d ¼ 13.29 (s, 1H, NH), 8.87–8.83 (m, 2H), 8.74 (d, J ¼ 15 Hz,
2H), 8.54 (d, J ¼ 6 Hz, 2H), 7.90 (t, J ¼ 7.5 Hz, 2H), 7.69–7.59 (m,
2H), 7.52–7.48 (m, 1H), 7.31 (d, J ¼ 15 Hz, 2H), 4.13 (s, 3H), 1.89
(s, 6H). 13C NMR (125 MHz, DMSO-d6) d ¼ 181.15, 151.00,
148.90, 145.71, 143.14, 140.44, 130.52, 129.45, 123.16, 119.11,
114.41, 106.93, 56.50; mass, m/z ¼ 302.16 [M + 1]+.
General procedure for oxidative amidation of aromatic aldehyde
using hemicyanine derivative under visible light irradiation
Conclusion
Hemicyanine derivatives C1–C4 have been synthesized and The mixture of aromatic aldehydes (0.2 mmol, 1.0 equiv.) and
utilized as photocatalysts in oxidative amidation of aromatic amine (0.6 mmol, 3.0 equiv.) in DMSO : H2O (1 : 1) in presence
aldehydes under mild conditions. Among all the synthesized of C4 (1 mol%) as a photocatalyst was stirred at room temper-
derivatives, C4 exhibited high efficiency which is attributed to ature for 12 h under aerial condition and visible light irradia-
its strong absorption in visible region, good water solubility, tion (for products 5a, 6a, 8a, 7e, 5e), 20 h (for 5c, 5d, 7c, 7d) and
high singlet oxygen quantum yield (FD ¼ 0.85), efficient inter- 30 h (for substrate 5b, 7b, 8b). Aer the completion of the
system crossing and high ability to transport electron from reaction (monitored by TLC), organic part was extracted with
donor to acceptor. All the mechanism investigations show EtOAc. The organic layer dried over anhydrous sodium sulphate
participation of reactive oxygen species generated by triplet and concentrate under reduced pressure. The crude was puri-
organic photosensitizer in oxidative amidation of aldehyde. The ed by column chromatography using EtOAc : hexane (1 : 1) to
mild reaction conditions such as aerial conditions, aqueous furnished the desired products. All the products were identied
media, low catalytic loading and visible light irradiation with by 1H NMR spectroscopy (Fig. S25–S36 in the ESI†).
broad substrate scope make this approach practically useful
and eco-friendly for the synthesis of amide bond. Findings of
this work will inspire the designing of new of “metal free”
photosensitizers as photocatalyst in organic transformations.
Preparation of photocatalyst coated dip strip
The lter paper strip was dipped in DMSO solution of C4 pho-
tocatalyst (10.0 mM) for 1 h. Aerwards, removed the lter paper
and air dried. The dried lter paper strip has been used for
carrying out the oxidative amidation of the aromatic aldehydes
under visible light.
Experimental section
General experimental methods and materials18
The general experimental methods, quantum yield calculations,
and materials used are the same as those reported earlier by us.
UV-vis and uorescence experiment.18 The stock solutions
(10ꢀ3 M) of hemicyanine derivatives were prepared by dissolv-
ing 3.33 mg, 3.02 mg, 2.78 mg and 3.86 mg of compound C1, C2,
C3 and C4 respectively in 10.0 mL of DMSO. 15.0 mL of this stock
Conflicts of interest
There are no conicts to declare.
Acknowledgements
solution further diluted with add 1485 mL of DMSO and 1500 mL V. B. is thankful to Science and Engineering Research Board
of water (pH ¼ 7.05) prepare 3.0 mL solution of derivative C1–C4 (SERB), New Delhi (ref. no. EMR/2014/000149) for nancial
(5.0 mM) and this solution was used for each UV-vis and uo- support. M. K. is thankful to Science and Engineering Research
rescence experiments.
Board (SERB) (ref no. EMR/2016/003473). H. D. is thankful to
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UGC-BSR for Senior Research Fellowship (SRF). We are also
thankful to UGC (New Delhi) for ‘‘University with Potential for
Excellence’’ (UPE) project.
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