Novel 5-benzazolyl-10,15,20-triphenylporphyrins and β,meso- benzoxazolyl-bridged porphyrin dyads: Synthesis, characterization and photophysical properties

Article abstract of DOI:10.1016/j.dyepig.2011.07.022

Novel 5-benzazolyl-10,15,20-triphenylporphyrins and β,meso- benzoxazole-linked diporphyrins were synthesized through La(OTf)₃ catalyzed reaction of newly prepared 5-(3,4-diaminophenyl)-10,15,20- triphenylporphyrin or 5-(3-amino-4-hydroxyphenyl)

Full text of DOI:10.1016/j.dyepig.2011.07.022

Dyes and Pigments 92 (2012) 1241e1249
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel 5-benzazolyl-10,15,20-triphenylporphyrins and b,meso-benzoxazolyl-
bridged porphyrin dyads: Synthesis, characterization and photophysical
properties
*
Satyasheel Sharma, Mahendra Nath
Department of Chemistry, University of Delhi, Delhi 110 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel 5-benzazolyl-10,15,20-triphenylporphyrins and
synthesized through La(OTf)3 catalyzed reaction of newly prepared 5-(3,4-diaminophenyl)-10,15,20-
triphenylporphyrin or 5-(3-amino-4-hydroxyphenyl)-10,15,20-triphenylporphyrin with aromatic alde-
b,meso-benzoxazole-linked diporphyrins were
Received 16 February 2011
Received in revised form
12 July 2011
Accepted 29 July 2011
Available online 5 August 2011
hydes in 1,2-dichlorobenzene. On metalation with zinc acetate, freebase
b,meso-benzoxazole-linked
diporphyrin was successfully converted to the ZneZn diporphyrin complex in good yield. The synthe-
sized porphyrin analogues were characterized using electronic absorption, IR and 1H NMR spectroscopy
in addition to mass and elemental analyses. The fluorescence studies of 5-benzazolyl-10,15,20-
triphenylporphyrins showed efficient intramolecular energy transfer from the pyrene and fluorene
Keywords:
Synthesis
Energy transfer
subunits to the porphyrin core. In addition, the fluorescence quenching observed in
b,meso-benzox-
azolyl-bridged porphyrin dyads was attributed to the possible nonplanarity of a component of the
diporphyrins. The freebaseeNi diporphyrin complex underwent strong emission quenching in
comparison to that of freebase diporphyrin and dizinc diporphyrin analogues.
Ó 2011 Elsevier Ltd. All rights reserved.
Fluorescence
meso-Benzazolyl-triphenylporphyrins
b
,meso-Linked diporphyrins
La(OTf)3
1. Introduction
hydrocarbons such as anthraquinone [24], fluorene [25], perylene
[26,27], pyrene [28], fluorescein [29] and anthracene [30] have been
Porphyrins are a unique class of heteroaromatic macrocyclic
ligands that have found wide application in various fields including
catalysis [1,2], molecular sensing [3,4], molecular recognition [5,6]
and as photosensitizers in photodynamic therapy applications
[7e10]. Among various natural pigments, these tetrapyrrole mac-
rocycles have been extensively studied for artificial-light harvesting
systems [11e15] and energy or electron-transfer processes [16] due
to their high absorption coefficients and rapid excited-state energy
transfer characteristics. Thus, these molecules are good candidates
for the development of new molecular materials with enhanced
photochemical and/or electrochemical properties.
covalently attached to the easily accessible meso-tetraarylporphyr-
ins. Besides these, the porphyrin units have also been linked together
in different orientations through meso-meso, b-meso and b-b posi-
tions to modulate the electronic interaction between two porphyrin
units within the porphyrin dyads [31e33]. However, a literature
survey revealed that the porphyrins with benzazole-ring systems
at the meso-positions have not been synthesized and evaluated for
their optical properties. Therefore, the current work is focused
on the synthesis, spectroscopic characterization and photophysical
investigation of novel meso-substituted benzazolyl-10,15,20-
triphenylporphyrins and
systems.
b,meso-benzoxazolyl-bridged diporphyrin
Over the past years, considerable efforts have been made to
synthesize porphyrins with extended
p-systems [17e23] through
peripheral functionalization at the meso- and
b-positions which can
2. Experimental
be useful for many applications in a number of areas ranging from
photosensitizers to molecular devices for energy and electron-
transfer processes. To improve the overall light-harvesting effi-
The reagents and solvents used in this study were purchased
from SigmaeAldrich Chemical Pvt. Ltd., Bangalore, India and Merck
Specialities Pvt. Ltd., Mumbai, India. Spectroscopic grade CHCl3 was
used to measure UVeVisible absorption and emission spectra of the
samples. Thin-layer chromatography (TLC) was conducted on silica
gel 60 F254 (pre-coated aluminum sheets) from Merck. The products
were purified by column chromatography using either activated
ciency, various photon harvesting pigments and highly p-conjugated
* Corresponding author. Tel.: þ91 11 27667794x186; fax: þ91 11 27666605.
E-mail addresses: mnathchemistry@gmail.com, mnath@chemistry.du.ac.in (M. Nath).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.07.022

1242
S. Sharma, M. Nath / Dyes and Pigments 92 (2012) 1241e1249
neutral aluminum oxide (Brokmann grade IeII, Merck) or silica gel
(60e120 mesh). 1H NMR spectra were recorded in CDCl3, using
a Bruker 300 MHz NMR spectrometer and Jeol ECX 400P (400 MHz)
NMR spectrometer. Chemical shifts are expressed in parts per
million (ppm) relative to tetramethylsilane as an internal standard.
Coupling constants (J) are reported in Hertz (Hz). Elemental
analyses for all compounds were performed on Elementar Analy-
sensysteme GmbH VarioEL elemental analyzer. Infrared (IR) spectra
of compounds synthesized were recorded on Perkin Elmer IR
was purified by column chromatography on activated neutral
alumina using chloroform as eluent. Yield: 75% (purple crystalline
solid). 1H NMR (300 MHz, CDCl3)
d
¼ 8.974 (d, J ¼ 4.8 Hz, 2H,
b
-pyrrolic H), 8.817 (d, J ¼ 4.8 Hz, 6H,
b
-pyrrolic H), 8.221e8.196
(m, 6H, meso-ArH), 7.773e7.756 (m, 9H, meso-ArH), 7.581e7.550
(m, 2H, meso-ArH), 7.055 (d, J ¼ 7.8 Hz, 1H, meso-ArH), 3.683 (brs,
4H, 2NH2), ꢀ2.762 (s, 2H, internal NH) ppm. IR (Film)
n
/cmꢀ1: 3421,
3321, 1620, 1595, 1472, 1440, 1350, 1219, 1155, 1070, 969, 800, 751,
701. ESI-MS: m/z ¼ 645 (M þ H)þ. Anal. Calcd. for C44H32N6: C,
81.96; H, 5.00; N, 13.03. Found: C, 82.08; H, 4.84; N, 12.96.
spectrometer and absorption maxima ðymaxÞ are given in cmꢀ1
.
Waters LCT Micromass Spectrometer was utilized to obtain the
mass spectra of the compounds. UVeVis absorption and fluores-
cence spectra were recorded using an Analytik Jena Specord 250
UVeVis spectrophotometer and a Varian Cary Eclipse fluorescence
spectrophotometer, respectively.
2.4. General procedure for the synthesis of meso-substituted
benzimidazolyl-triphenylporphyrins (8aee)
To
a solution of 5-(3,4-diaminophenyl)-10,15,20-triphenyl
porphyrin 4 (30 mg, 0.046 mmol) in 1,2-dichlorobenzene (10 mL),
the aryl aldehyde (0.069 mmol) was added followed by the addition
of La(OTf)3 (5 mg, 0.009 mmol). The reaction mixture was stirred at
100 ꢁC for 8 h. The progress of the reaction was monitored by TLC.
After completion of the reaction, the mixture was cooled to room
temperature and loaded onto a silica gel column. The column was
firstly eluted with n-hexane to remove 1,2-dichlorobenzene and
then with chloroform to afford the desired product as a purple
crystalline solid in 78e85% isolated yields.
2.1. Synthesis of 5-(4-acetamido-3-nitrophenyl)-10,15,20-
triphenylporphyrin (2)
To
porphyrin 1 (50 mg, 0.074 mmol) in dichloromethane (15 mL),
conc. HNO3 (50 L) was added. The reaction mixture was stirred at
a solution of 5-(4-acetamidophenyl)-10,15,20-triphenyl
m
25 ꢁC for 2 h. After completion of the reaction, the mixture was
washed with water and the compound was extracted with
chloroform (50 mL). The organic layer was dried over anhydrous
sodium sulphate and then evaporated under reduced pressure to
obtain the crude product. The title compound was purified on
a silica gel column using chloroform/n-hexane (80:20) as an eluent.
The pure product was obtained as a purple crystalline solid in 75%
isolated yield and characterized on the basis of spectral data [34].
2.4.1. 5-[2-(4-Chlorophenyl)-1H-benzimidazol-5-yl]-10,15,20-
triphenylporphyrin (8a)
The compound 8a was prepared from compound 4 (30 mg,
0.046 mmol) and 4-chlorobenzaldehyde (9.8 mg, 0.069 mmol)
according to the general procedure given earlier. Yield: 80%. 1H
NMR (300 MHz, CDCl3)
d
¼ 8.840e8.805 (m, 9H,
b-pyrrolic H &
2.2. Synthesis of 5-(4-amino-3-nitrophenyl)-10,15,20-
triphenylporphyrin (3)
meso-ArH), 8.225e8.173 (m, 7H, meso-ArH), 8.087 (s, 1H, NH), 7.915
(d, J ¼ 7.8 Hz, 2H, ArH), 7.758e7.719 (m, 10H, meso-ArH), 7.406
(d, J ¼ 6.9 Hz, 2H, ArH), ꢀ2.738 (s, 2H, internal NH) ppm. IR (Film)
A solution of 5-(4-acetamido-3-nitrophenyl)-10,15,20-triphenyl
porphyrin 2 (50 mg, 0.069 mmol) in a mixture of TFA (3 mL) and
conc. HCl (4 mL) was stirred at 65 ꢁC for 2 h under N2. After
completion of the reaction, the solution was cooled to room
temperature and then neutralized with 5% aqueous NaOH solution
(50 mL). The desired compound was extracted with chloroform
(50 mL). The organic layer was washed thoroughly with water,
dried over anhydrous sodium sulphate and the solvent evaporated
under reduced pressure. The crude product obtained was purified
on a neutral alumina column using chloroform/n-hexane (80:20) as
n
/cmꢀ1: 3317, 1597, 1473, 1441, 1350, 1219, 1095, 968, 835, 800, 729,
701. ESI-MS: m/z ¼ 764 (M)þ. Anal. Calcd. for C51H33ClN6.3H2O: C,
74.76; H, 4.80; N, 10.26. Found: C, 75.15; H, 4.57; N, 9.85.
2.4.2. 5-[2-(Naphthalen-1-yl)-1H-benzimidazol-5-yl]-10,15,20-
triphenylporphyrin (8b)
The compound 8b was prepared from compound 4 (30 mg,
0.046 mmol) and 2-naphthaldehyde (10.8 mg, 0.069 mmol)
according to the general procedure given earlier. Yield: 82%. 1H
NMR (400 MHz, CDCl3)
d
¼ 8.849e8.826 (m, 6H,
b-pyrrolic H),
b-pyrrolic H), 8.609 (s, 1H, meso-ArH),
eluent. Yield: 95%. 1H NMR (300 MHz, CDCl3)
ArH), 8.870e8.848 (m, 8H, -pyrrolic H), 8.222e8.205 (m, 7H,
d
¼ 8.963 (s,1H, meso-
8.796 (d, J ¼ 4.7 Hz, 2H,
b
8.224e8.137 (m, 9H, meso-ArH & NH), 7.946e7.831 (m, 3H, ArH),
7.757e7.724 (m, 11H, meso-ArH & ArH), 7.507 (t, J ¼ 7.3 Hz, 2H,
meso-ArH), 7.767e7.469 (m, 9H, meso-ArH), 7.115 (d, J ¼ 8.4 Hz, 1H,
meso-ArH), 6.378 (s, 2H, NH2), ꢀ2.781 (s, 2H, internal NH) ppm. IR
ArH), ꢀ2.734 (s, 2H, internal NH) ppm. IR (Film)
n
/cmꢀ1: 3312,1595,
(Film)
n
/cmꢀ1: 3492, 3381, 3320, 1630, 1595, 1559, 1517, 1473, 1345,
1440, 1350, 1261, 1072, 969, 800, 751, 700. ESI-MS: m/z ¼ 780 (M)þ.
Anal. Calcd. for C55H36N6: C, 84.59; H, 4.65; N, 10.76. Found: C,
84.29; H, 4.98; N; 10.56.
1252, 1170, 1074, 968, 800, 749, 701. ESI-MS: m/z ¼ 674 (M)þ. Anal.
Calcd. for C44H30N6O2.2H2O: C, 74.35; H, 4.82; N, 11.82. Found: C,
74.06; H, 5.40; N, 11.95.
2.4.3. 5-[2-(Pyren-1-yl)-1H-benzimidazol-5-yl]-10,15,20-
triphenylporphyrin (8c)
2.3. Synthesis of 5-(3,4-diaminophenyl)-10,15,20-
triphenylporphyrin (4)
The compound 8c was prepared from compound 4 (30 mg,
0.046 mmol) and 1-pyrenecarboxaldehyde (15.9 mg, 0.069 mmol)
according to the general procedure given earlier. Yield: 85%. 1H
To a solution of 5-(4-amino-3-nitrophenyl)-10,15,20-triphenyl
porphyrin
3
(50 mg, 0.074 mmol) in conc. HCl (10 mL),
NMR (400 MHz, CDCl3)
8.879e8.848 (m, 4H,
-pyrrolic H), 8.714 (d, J ¼ 4.5 Hz, 2H,
-pyrrolic H), 8.635 (d, J ¼ 4.5 Hz, 2H, -pyrrolic H), 8.320e8.169
d
¼ 8.927 (d, J ¼ 9.1 Hz, 1H, ArH),
SnCl2$2H2O (100 mg, 0.44 mmol) was added. The reaction mixture
was stirred at 65 ꢁC for 2 h under an inert atmosphere. After
completion of the reaction, the mixture was allowed to cool at room
temperature, neutralized with 10% aqueous NaOH solution and
extracted with chloroform (50 mL). The organic layer was washed
thoroughly with water, dried over anhydrous sodium sulphate and
evaporated to dryness under reduced pressure. The crude product
b
b
b
(m, 8H, meso-ArH & NH), 7.960 (d, J ¼ 7.7 Hz, 1H, meso-ArH), 7.902
(d, J ¼ 9.4 Hz, 1H, ArH), 7.817e7.687 (m, 13H, meso-ArH & ArH),
7.460 (t, J ¼ 7.7 Hz, 1H, ArH), 7.321 (d, J ¼ 7.0 Hz, 1H, ArH),
7.056e6.984 (m, 2H, ArH), ꢀ2.846 (s, 2H, internal NH) ppm. IR
(Film) n
/cmꢀ1: 3318, 1597, 1350, 1218, 969, 846, 801, 753. ESI-MS:

S. Sharma, M. Nath / Dyes and Pigments 92 (2012) 1241e1249
1243
m/z ¼ 854 (M)þ. Anal. Calcd. for C61H38N6: C, 85.69; H, 4.48; N, 9.83.
1070, 969, 871, 799, 727, 700. ESI-MS: m/z ¼ 646 (M þ H)þ. Anal. Calcd.
for C44H31N5O.H2O: C,79.62; H, 5.01; N,10.55. Found: C, 79.54; H, 5.34;
N; 10.36.
Found: C, 85.46; H, 4.76; N; 10.17.
2.4.4. 5-[2-(9H-fluoren-2-yl)-1H-benzimidazol-5-yl]-10,15,20-
triphenylporphyrin (8d)
2.7. General procedure for the synthesis of meso-substituted
The compound 8d was prepared from compound 4 (30 mg,
0.046 mmol) and fluorene-2-carboxaldehyde (13.4 mg, 0.069 mmol)
according to the general procedure given earlier. Yield: 78%.1H NMR
benzoxazolyl-triphenylporphyrins (9aed)
To
a
solution of 5-(3-amino-4-hydroxyphenyl)-10,15,20-
(300 MHz, CDCl3)
d
¼ 8.832e8.765 (m, 8H,
b
-pyrrolic H), 8.375 (s,1H,
triphenylporphyrin 7 (30 mg, 0.046 mmol) in 1,2-dichlorobenzene
(10 mL), the aryl aldehyde (0.069 mmol) was added followed by
the addition of La(OTf)3 (5 mg, 0.009 mmol). The reaction mixture
was heated at 160 ꢁC under stirring for 16 h. After completion of the
reaction, the mixture was allowed to cool at room temperature and
then directly loaded onto a silica gel column. 1,2-Dichlorobenzene
was removed on elution with n-hexane and the desired product
was eluted from the column using chloroform.
NH), 8.300 (s, 1H, meso-ArH), 8.180 (d, J ¼ 6.6 Hz, 6H, meso-ArH),
8.069 (d, J ¼ 7.2 Hz, 1H, meso-ArH), 7.946 (d, J ¼ 7.2 Hz, 1H, meso-
ArH), 7.750e7.706 (m,12H, meso-ArH & ArH), 7.420 (d, J ¼ 6.9 Hz,1H,
ArH), 7.291e7.245 (m, 3H, ArH), 3.809 (s, 2H, CH2), ꢀ2.751 (s, 2H,
internal NH) ppm. IR (Film) n
/cmꢀ1: 3317, 1594, 1441, 1350, 1219,
1072, 969, 939, 801, 734, 701. ESI-MS: m/z ¼ 819 (M þ H)þ. Anal.
Calcd. for C58H38F3N6.0.5H2O: C, 77.31; H, 4.24; N, 10.40. Found: C,
77.22; H, 4.60; N; 10.08.
2.7.1. 5-[2-(4-Chlorophenyl)benzoxazol-5-yl]-10,15,20-
triphenylporphyrin (9a)
The compound 9a was prepared from compound 7 (30 mg,
0.046 mmol) and 4-chlorobenzaldehyde (9.8 mg, 0.069 mmol)
according to the general procedure given earlier. Yield: 68%. 1H
2.4.5. 5-[2-(4-Trifluoromethylphenyl)-1H-benzimidazol-5-yl]-
10,15,20-triphenylporphyrin (8e)
The compound 8e was prepared from compound 4 (30 mg,
0.046 mmol) and 4-trifluoromethylbenzaldehyde (12 mg, 0.069
mmol) according to the general procedure given earlier. Yield: 85%.
NMR (300 MHz, CDCl3)
d
¼ 8.849 (s, 8H,
b-pyrrolic H), 8.618 (s, 1H,
1H NMR (400 MHz, CDCl3)
d
¼ 8.844e8.807 (m, 9H,
b
-pyrrolic H &
meso-ArH), 8.338 (d, J ¼ 7.2 Hz, 2H, ArH), 8.226e8.213 (m, 7H,
meso-ArH), 7.904 (d, J ¼ 8.4 Hz, 1H, meso-ArH), 7.763 (s, 9H, meso-
ArH), 7.576 (d, J ¼ 7.2 Hz, 2H, ArH), ꢀ2.746 (s, 2H, internal NH) ppm.
ArH), 8.226e8.181 (m, 7H, meso-ArH), 8.127e8.109 (m, 3H, ArH &
NH), 7.783e7.725 (m, 12H, meso-ArH & ArH), ꢀ2.7534 (s, 2H,
internal NH) ppm. IR (Film)
n
/cmꢀ1: 3317, 1596, 1474, 1441, 1351,
IR (Film) n
/cmꢀ1: 3317,1595,1560,1465,1440,1352,1260,1188, 969,
1219, 1092, 963, 835, 800, 729, 700. ESI-MS: m/z ¼ 798 (M)þ. Anal.
Calcd. for C52H33F3N6.1.5H2O: C, 75.62; H, 4.39; N, 10.18. Found: C,
75.08; H, 4.53; N, 10.03.
864, 800, 752, 700. ESI-MS: m/z ¼ 765 (M)þ. Anal. Calcd. for
C51H32ClN5O.1.5H2O: C, 77.21; H, 4.45; N, 8.83. Found: C, 76.87; H,
4.57; N, 8.22.
2.5. Synthesis of 5-(4-hydroxy-3-nitrophenyl)-10,15,20-
triphenylporphyrin (6)
2.7.2. 5-[2-(Naphthalen-1-yl)benzoxazol-5-yl]-10,15,20-
triphenylporphyrin (9b)
The compound 9b was prepared from compound 7 (30 mg,
0.046 mmol) and 2-naphthaldehyde (10.8 mg, 0.069 mmol)
according to the general procedure given earlier. Yield: 70%.1H NMR
To a solution of 5-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin
5 (50 mg, 0.079 mmol) in dichloromethane (10 mL), conc. HNO3
(60
m
L) was added slowly and the mixture was stirred at 25 ꢁC for
(400 MHz, CDCl3)
d
¼ 8.939 (s, 1H, meso-ArH), 8.869e8.863
15 min. Then the reaction was quenched with water (20 mL) and
compound was extracted with chloroform (2 ꢂ 20 mL). The organic
layers were combined, washed thoroughly with water and dried over
anhydrous sodium sulphate. On evaporation of the solvent under
reduced pressure, the crude product obtained was purified bycolumn
chromatography on silica gel using chloroform/n-hexane (80:20) as
solvent. The purple crystalline product was obtained in 90% isolated
yield and characterized on the basis of spectral data as reported in the
literature [34].
(d, J ¼ 2.2 Hz, 8H, -pyrrolic H), 8.679 (d, J ¼ 1.2 Hz, 1H, ArH), 8.459
b
(dd, J1 ¼ 8.8 Hz, J2 ¼ 1.6 Hz, 1H, meso-ArH), 8.244e8.224 (m, 7H,
meso-ArH), 8.036 (d, J ¼ 8.0 Hz, 2H, ArH), 7.956 (d, J ¼ 8.0 Hz, 2H,
ArH), 7.769e7.754 (d, 9H, meso-ArH), 7.613 (t, J ¼ 4.0 Hz, 2H,
ArH), ꢀ2.734 (s, 2H, internal NH) ppm. IR (Film)
n
/cmꢀ1: 3318, 1597,
1560, 1468, 1441, 1350, 1260, 1184, 967, 860, 801, 752, 701. ESI-MS:
m/z ¼ 781 (M)þ. Anal. Calcd. for C55H35N5O: C, 85.59; H, 4.36; N,
8.18. Found: C, 85.46; H, 4.71; N, 8.33.
2.7.3. 5-[2-(Pyren-1-yl)benzoxazol-5-yl]-10,15,20-
triphenylporphyrin (9c)
The compound 9c was prepared from compound 7 (30 mg,
0.046 mmol) and 1-pyrenecarboxaldehyde (15.9 mg, 0.069 mmol)
according to the general procedure given earlier. Yield: 75%. 1H
2.6. Synthesis of 5-(3-amino-4-hydroxyphenyl)-10,15,20-
triphenylporphyrin (7)
To a solution of 5-(4-hydroxy-3-nitrophenyl)-10,15,20-triphenyl
porphyrin 6 (50 mg, 0.074 mmol) in conc. HCl (10 mL), SnCl2$2H2O
(100 mg, 0.44 mmol) was added. The reaction mixture was stirred at
65 ꢁC for 2 h under an inert atmosphere. On completion of the reac-
tion, the mixture was allowed to cool at 25 ꢁC and then neutralized
with 5% aqueous sodium hydroxide solution. The title compound
was extracted with chloroform (3 ꢂ 30 mL). The organic layers
were combined, washed thoroughly with water, dried over sodium
sulphate and evaporated to dryness under reduced pressure. The
crude product obtained was purified by column chromatography on
silica gel using 5% methanol in chloroform as eluent. Yield: 85%. 1H
NMR (400 MHz, CDCl3)
d
¼ 9.963 (d, J ¼ 9.4 Hz, 1H, ArH), 9.071
(d, J ¼ 8.0 Hz, 1H, ArH), 8.922e8.861 (m, 8H,
b
-pyrrolic H), 8.802
(d, J ¼ 1.2 Hz,1H, meso-ArH), 8.361e8.314 (m,13H, meso-ArH & ArH),
8.083 (t, J ¼ 7.7 Hz, 1H, ArH), 8.035 (d, J ¼ 8.0 Hz, 1H, ArH),
7.776e7.764 (m, 9H, meso-ArH), ꢀ2.739 (s, 2H, internal NH) ppm. IR
(Film) n
/cmꢀ1: 3317, 1594, 1558, 1473, 1439, 1349, 1256, 1188, 1073,
967, 842, 797, 699. ESI-MS: m/z ¼ 855 (M)þ. Anal. Calcd. for
C61H37N5O.H2O: C, 83.83; H, 4.50; N, 8.01. Found: C, 84.09; H, 5.05; N,
7.53.
NMR (300 MHz, CDCl3)
-pyrrolic H), 8.203 (s, 7H, meso-ArH), 7.749 (s, 10H, meso-ArH), 5.104
(brs,1H, OH), 4.069 (brs, 2H, NH2), ꢀ2.769 (s, 2H, internal NH) ppm. IR
(Film)
/cmꢀ1: 3374, 3312, 1597, 1472, 1439, 1350, 1276, 1217, 1139,
d
¼ 8.936 (s, 1H, meso-ArH), 8.828 (s, 8H,
2.7.4. 5-[2-(9H-fluoren-2-yl)benzoxazol-5-yl]-10,15,20-
triphenylporphyrin (9d)
The compound 9d was prepared from compound 7 (30 mg,
0.046 mmol) and fluorene-2-carboxaldehyde (13.4 mg, 0.069 mmol)
b
n

1244
S. Sharma, M. Nath / Dyes and Pigments 92 (2012) 1241e1249
NH
N
NH
N
N
CH2Cl2
NHCOCH3
NO2
NHCOCH3
HN
Conc. HNO3
N
HN
°
25 C
1
2
TFA-Conc.HCl
°
60 C
NH
N
N
NH
N
N
SnCl2. 2H2O
Conc. HCl
NH2
NO2
NH2
NH2
HN
HN
°
60 C
4
3
Scheme 1. Synthesis of 5-(3,4-diaminophenyl)-10,15,20-triphenylporphyrin.
according to the general procedure given earlier. Yield: 72%. 1H NMR
(400 MHz, CDCl3) -pyrrolic H), 8.631 (s, 1H, meso-
(m, 12H, meso-ArH), 8.136 (dd, J1 ¼ 8.0 Hz, J2 ¼ 1.7 Hz, 1H, meso-ArH),
7.932e7.766 (m, 18H, meso-ArH), 7.611e7.555 (m, 3H, meso-ArH),
7.496 (d, J ¼ 7.7 Hz, 1H, meso-ArH), ꢀ2.519 (s, 2H, internal NH), ꢀ2.694
d
¼ 8.856 (s, 8H,
b
ArH), 8.601 (s, 1H, ArH), 8.460 (d, J ¼ 8.0 Hz, 1H, ArH), 8.235e8.187
(m, 7H, meso-ArH), 7.999 (d, J ¼ 8.0 Hz, 1H, ArH), 7.914 (t, J ¼ 8.0 Hz,
2H, ArH), 7.766e7.751 (m, 9H, meso-ArH), 7.631 (d, J ¼ 6.8 Hz, 1H,
meso-ArH), 7.460e7.403 (m, 2H, ArH), 4.078 (s, 2H, CH2), ꢀ2.739
(s, 2H, internal NH) ppm. IR (Film) n
/cmꢀ1: 3320, 1597, 1469, 1441,
1350, 1260, 1219, 1178, 1072, 1002, 966, 800, 753, 700. ESI-MS:
m/z ¼ 1267 (M)þ. Anal. Calcd. for C89H57N9O.H2O: C, 83.09; H, 4.62;
N, 9.80. Found: C, 82.95; H, 4.97; N, 9.53.
(s, 2H, internal NH) ppm. IR (Film) n
/cmꢀ1: 3317, 1595, 1441, 1350,
1217, 1072, 968, 939, 801, 730, 701. ESI-MS: m/z ¼ 819 (M)þ. Anal.
calcd. for C58H37N5O.H2O: C, 83.13; H, 4.69; N, 8.36. Found: C, 83.09;
H, 4.81; N, 8.01.
2.8.2. Nickel(II) 2-[5-(10,15,20-triphenylporphyrin-5-yl)-
benzoxazol-2-yl]-5,10,15,20-tetraphenylporphyrin (11b)
Yield: 45%. 1H NMR (400 MHz, CDCl3)
d
¼ 9.357 (s, 1H,
-pyrrolic H), 8.864 (s, 6H, -pyrrolic H),
-pyrrolic H), 8.761 (d, J ¼ 4.7 Hz, 1H,
-pyrrolic H), 8.600
b-pyrrolic H), 8.537 (s, 1H, meso-ArH),
b-pyrrolic
H), 8.933 (d, J ¼ 4.0 Hz, 2H,
b
b
2.8. Synthesis of b,meso-benzoxazolyl-bridged porphyrin dyads
(11a and 11b)
8.799 (d, J ¼ 5.0 Hz, 1H,
b
b
-pyrrolic H), 8.724e8.695 (m, 3H,
b
(d, J ¼ 4.6 Hz, 1H,
To
triphenylporphyrin 7 (50 mg, 0.077 mmol) in 1,2-dichlorobenzene
(10 mL), -formyl-5,10,15,20-tetraphenylporphyrin 10a or nick-
el(II) -formyl-5,10,15,20-tetraphenylporphyrin 10b (0.077 mmol)
a
solution of 5-(3-amino-4-hydroxyphenyl)-10,15,20-
8.261e8.210 (m, 6H, meso-ArH), 8.100e8.021 (m, 8H, meso-ArH),
7.786e7.677 (m, 19H, meso-ArH), 7.506e7.411 (m, 4H, meso-
b
ArH), ꢀ2.714 (s, 2H, internal NH) ppm. IR (Film)
n
/cmꢀ1: 3317, 1597,
b
1464, 1441, 1350, 1259, 1219, 1178, 1072, 1002, 966, 800, 754, 700.
was added followed by the addition of La(OTf)3 (9 mg, 0.015 mmol).
The reaction mixture was stirred at 160 ꢁC for 16 h and then allowed
to cool at 25 ꢁC. The reaction mixture was directly loaded onto
a silica-gel column. The column was firstly eluted with hexane to
remove 1,2-dichlorobenzene and then after with chloroform to
afford compound 11a or 11b.
ESI-MS: m/z ¼ 1324 (M þ H)þ.
2.9. Synthesis of di-zinc(II) 2-[5-(10,15,20-triphenylporphyrin-5-
yl)-benzoxazol-2-yl]-5,10,15,20-tetraphenylporphyrin (11c)
To the solution of freebaseefreebase diporphyrin 11a (10 mg,
2.8.1. 2-[5-(10,15,20-triphenylporphyrin-5-yl)-benzoxazol-2-yl]-
0.008 mmol) in dichloromethane (5 mL), a solution of
5,10,15,20-tetraphenylporphyrin (11a)
Zn(OAc)2$2H2O (6 mg, 0.027 mmol) in methanol (0.5 mL) was added
and the reaction mixture was stirred at room temperature for
30 min. After completion of the reaction, the mixture was diluted
with dichloromethane (20 mL) and washed with water (3 ꢂ 25 mL).
Yield: 55%.1H NMR (400 MHz, CDCl3)
d
¼ 9.439 (s,1H,
-pyrrolic H), 8.798 (s, 2H, -pyrrolic H), 8.563
(s, 1H, meso-ArH), 8.350 (d, J ¼ 7.38 Hz, 2H, meso-ArH), 8.316e8.234
b-pyrrolic H),
8.967e8.882 (m, 12H,
b
b

S. Sharma, M. Nath / Dyes and Pigments 92 (2012) 1241e1249
1245
The organic layer was dried over sodium sulphate and evaporated to
dryness under vacuum. The residue obtained was subjected to
column chromatography on silica gel using CHCl3 as eluent. The
pure product was obtained after eluting the column with 5% MeOH
in chloroform in 80% yield. 1H NMR (400 MHz, CDCl3 þ DMSO-d6)
ArCHO, La(OTf)3
NH
N
NH
N
N
XH
NH2
X
HN
N HN
N
1,2-Dichlorobenzene
Ar
d
¼ 9.195 (s, 1H,
b
-pyrrolic H), 9.069 (d, J ¼ 4.7 Hz, 2H,
b
-pyrrolic H),
8.980e8.685 (m, 12H,
b-pyrrolic H), 8.317e8.218 (m, 10H, meso-
ArH), 8.035 (s, 4H, meso-ArH), 7.825e7.752 (m, 18H, meso-ArH),
7.568e7.416 (m, 4H. meso-ArH), 7.272e7.251 (m, 2H, meso-ArH)
8a-e, X = NH
9a-d, X = O
4, X = NH
7, X = O
ppm. IR (Film) n
/cmꢀ1: 1597, 1462, 1265, 1070, 994, 795, 754, 699.
ESI-MS: m/z ¼ 1392 (M þ H)þ. Anal. Calcd. for C89H53N9Zn2O.2H2O:
a
b
c
e
8, 9
d
C, 74.69; H, 4.01; N, 8.81. Found: C, 74.50; H, 4.39; N, 8.64.
Ar
Cl
CF3
3. Results and discussion
3.1. Synthesis and structure characterization
The novel 5-benzazolyl-10,15,20-triphenylporphyrins (8aee and
Scheme 3. Synthesis of meso-substituted benzazolyl-triphenylporphyrins.
cyclization of 5-(3,4-diaminophenyl)-10,15,20-triphenylporphyrin
(4) with 1.5 equivalents of aryl aldehydes in 1,2-dichlorobenzene
using La(OTf)3 as a Lewis acid catalyst at 100 ꢁC temperature for 8 h.
In contrast, meso-substituted benzoxazolyl-triphenylporphyrins
(9aed) were synthesized similarly from 5-(3-amino-4-hydroxy
phenyl)-10,15,20-triphenylporphyrin (7) at 160 ꢁC temperature in
68e75% isolated yields (Scheme 3). In absence of La(OTf)3, the reac-
tion only proceeded at high temperature (>180 ꢁC) and produced
desired products in low yields due to the formation of side products.
The reaction seems to proceed through initial formation of an
iminoporphyrin intermediate, which was possibly activated in the
presence of La(OTf)3, and thus facilitates the intramolecular oxidative
cyclization to afford desired products in good yields.
9aed) and b,meso-benzoxazolyl-bridged diporphyrins (11aec) were
synthesized as outlined in schemes 3 and 4. For the synthesis of these
porphyrin derivatives, two new precursors, 5-(3,4-diaminophenyl)-
10,15,20-triphenylporphyrin (4) and 5-(3-amino-4-hydroxyphenyl)-
10,15,20-triphenylporphyrin (7) were prepared through regioselective
functionalization
of
5-(4-acetamidophenyl)-10,15,20-triphenyl
porphyrin (1) and 5-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin
(5), respectively as shown in schemes 1 and 2. First, 5-(4-acetamido
phenyl)-10,15,20-triphenylporphyrin was synthesized from easily
accessible meso-tetraphenylporphyrins by following the literature
procedure [34]. The regiospecific phenyl nitration of this porphyrin (1)
by using conc. HNO3 in CH2Cl2 under modified reaction conditions at
25 ꢁC afforded 5-(4-acetamido-3-nitrophenyl)-10,15,20-triphenyl
porphyrin (2) in 70% isolated yield. On deprotection of the acet-
amido group using TFA-conc. HCl mixture at 65 ꢁC, porphyrin (2) was
converted to the corresponding 5-(4-amino-3-nitrophenyl)-10,15,20-
triphenylporphyrin (3) which on treatment with SnCl2$2H2O in
conc. HCl at 65 ꢁC for 2 h under nitrogen atmosphere afforded 5-(3,4-
diaminophenyl)-10,15,20-triphenylporphyrin (4). After purification of
the crude product by column chromatography on neutral alumina,
porphyrin (4) was obtained in 75% isolated yield. Similarly, another
novel starting material, 5-(3-amino-4-hydroxyphenyl)-10,15,20-
triphenylporphyrin (7) was synthesized from 5-(4-hydroxyphenyl)-
10,15,20-triphenylporphyrin (5) [35] via nitration by conc. HNO3
followed by reduction of the nitro substituent under standard SnCl2/
HCl conditions as shown in Scheme 2. After purification on silica gel,
porphyrin 7 was obtained in 85% isolated yield. These porphyrins 4
and 7 were characterized spectroscopically and used as precursors
for the preparation of novel meso-substituted benzazolyl-triphenyl
Furthermore, this synthetic protocol was extended to obtain novel
b,meso-linked diporphyrin systems (11aec) with a benzoxazole
spacer as shown in Scheme 4. The freebaseefreebase diporphyrin
(11a) and freebaseenickel(II) diporphyrin (11b) were synthesized in
moderate yields at 160 ꢁC by the reaction of porphyrin 7 with
2-formyl-5,10,15,20-tetraphenyl-porphyrin (10a) and nickel(II)
2-formyl-5,10,15,20-tetraphenylporphyrin (10b), respectively in the
presence of a catalytic amount of La(OTf)3 in 1,2-dichlorobenzene.
The zincezinc diporphyrin analogue (11c) was prepared from free-
baseefreebase diporphyrin (11a) via zinc insertion reaction using
Zn(OAc)2$2H2O in dichloromethaneeMeOH mixture at 25 ꢁC
(Scheme 4).
All the newly synthesized compounds 3, 4, 7, 8aee, 9aed and
11aec were purified by column chromatography and characterized
on the basis of 1H NMR, IR, UVeVis and mass spectral data. The 1H
NMR of porphyrins 3, 4, 7, 8aee and 9aed showed a characteristic
singlet at about
core. The characteristic
d
¼ ꢀ2.7 ppm for the NH protons of the porphyrin
porphyrins (8aee and 9aed) and
porphyrin dyads (11a,b), respectively.
b,meso-benzoxazolyl-bridged
b-pyrrolic protons appeared in the
downfield region between
substituted benzimidazolyl porphyrins (8aee), the NH proton of
d
¼ 8.6e8.9 ppm. In the case of meso-
The
meso-substituted
benzimidazolyl-triphenylporphyrins
(8aee) were synthesized in 75e85% isolated yields by condensation
SnCl2.2H2O
NH
N
N
NH
N
N
CH2Cl2
NH
N
N
OH
NO2
OH
OH
NH2
Conc.HCl
60 °C
Conc. HNO3
25 °C
HN
HN
HN
5
7
6
Scheme 2. Synthesis of 5-(3-amino-4-hydroxyphenyl)-10,15,20-triphenylporphyrin.

1246
S. Sharma, M. Nath / Dyes and Pigments 92 (2012) 1241e1249
NH
N
NH
N
N
M
N
OHC
O
OH
NH2
La(OTf)3
N
N
N HN
N
N HN
N
1,2-Dichlorobenzene
N
N
160 °C
N
10a, M = 2H
10b, M= Ni
7
11a, M = 2H
11b, M = Ni
N
N
N
Zn(OAc)2 , MeOH
CHCl3 , 25
Zn
11a
O
N
°
C
N
N
N
N
N
11c
Scheme 4. Synthesis of
b,meso-benzoxazolyl-bridged porphyrin dyads.
the benzimidazole ring was present as
a
singlet between
The electronic absorption spectra of b,meso-benzoxazolyl-
d
¼ 8.0e8.3 ppm. In addition, porphyrins 8d and 9d showed
a characteristic singlet for the CH2 protons of the fluorenyl moiety
at
¼ 3.8 and 4.0 ppm, respectively. In contrast, the proton NMR of
freebaseefreebase diporphyrin (11a) showed a characteristic peak
for the -proton adjacent to the benzoxazole moiety as a singlet at
¼ 9.43 ppm. The remaining 14 -pyrrolic protons appeared as
a multiplet at
¼ 8.82e8.96 ppm for 12 protons and a singlet at
bridged porphyrin dyads (11aec) are shown in Fig. 1b. The visible
absorption spectra of porphyrin dyads displayed an intense Soret-
band with slight splitting at 421, 428 nm for freebaseefreebase
d
b
Table 1
d
b
Electronic absorption and emission data for porphyrins 3, 4, 7, 8aee, 9aed and
11aec.
d
d
¼ 8.79 ppm for 2 protons, respectively. A singlet at
d
¼ 8.56 ppm
Compd no.
Absorptiona lmax
,
Fluorescencea,b
lem/nm)
for one proton was assigned to the ortho-position of the meso-
phenyl ring. The remaining 37 meso-phenyl protons appeared
either as doublets or multiplets between
nm (
3
ꢂ 10ꢀ4, Mꢀ1 cmꢀ1
)
(
3
422(47.33), 518(2.01), 551(1.04),
592(0.58), 648(0.26)
422(48.90), 518(2.57), 555(1.26),
594(0.68), 650(0.59)
421(46.33), 518(2.35), 552(1.02),
591(0.70), 648(0.51)
421(49.28), 518(2.04), 553(0.99),
592(0.50), 648 (0.35)
422(49.93), 517(1.97), 552 (0.92),
593(0.55), 648(0.38)
422(53.60), 518(2.51), 553(1.29),
593(0.66), 649(0.45)
422(51.26), 518(2.28), 553(1.19),
593(0.64), 649 (0.49)
421(46.64), 517(1.93), 553(0.92),
593(0.39), 648(0.17)
421(49.63), 517(2.09), 551(0.95),
593(0.53), 647(0.33)
421(50.25), 517(1.94), 551(0.82),
591(0.54), 646(0.38)
422(54.11), 517(2.45), 552(1.18),
593(0.69), 647(0.44)
421(52.13), 517(2.33), 552(1.15),
591(0.61), 647(0.36)
422(56.44), 429(61.94), 519(4.78),
552(2.01), 595(1.23), 651(0.92)
420(48.26), 517(3.39), 543(2.61),
585(1.26), 647(0.45)
425(41.05), 432(47.54), 557(3.18),
597(1.19)
652, 717
655, 719
654, 717
652, 717
652, 718
652, 718
652, 719
651, 716
651, 715
651, 717
651, 717
651, 717
670, 722
651, 717
621, 654
d
¼ 7.49e8.35 ppm.
The two singlets at
d
¼ ꢀ2.51 and ꢀ2.69 ppm for two protons each
4
were assigned to the internal NH for the two porphyrin rings. The IR
spectrum of diporphyrin (11a) showed a peak at 3320 cmꢀ1 due to
the NH bond stretching. The structure of dyad (11a) was further
supported by the mass, which showed molecular ion peak (M)þ at
m/z 1267. Similarly, porphyrin dyads 11bec were characterized and
their spectral data are presented in the experimental section.
7
8a
8b
8c
8d
8e
9a
9b
9c
9d
11a
11b
11c
3.2. Photophysical properties
The electronic absorption and emission data of the newly
synthesized compounds are presented in Table 1. The meso-
benzazolyl-triphenylporphyrins (8aee and 9aed) exhibited typical
intense Soret- or B-bands at w421 nm and four weaker Q bands
at w517, 552, 592 and 647 nm. The UVeVis spectra of porphyrins 4,
7, 8c, 8d, 9c and 9d are shown in Fig. 1a. Besides the Soret and
Q bands in porphyrins (8ced and 9ced), an additional absorption
peak originates from the presence of pyrenyl and fluorenyl units at
280 nm and 320 nm, respectively. Thus, the absorption spectra of
these compounds showed the features of both porphyrin and
pyrene or fluorene subunits and suggest that there is no substantial
interaction between the attached moiety and the porphyrin ring
in the ground state. This observation is in agreement with a nearly
perpendicular orientation of the aryl group relative to the
porphyrin plane in the freebase meso-tetraarylporphyrins [36].
a
Absorptionand emission data weretaken for CHCl3 solutions ofporphyrinsat 298 K.
The excitation wavelength for emission data is 420 nm.
b

S. Sharma, M. Nath / Dyes and Pigments 92 (2012) 1241e1249
1247
Fig. 1. (a) Electronic absorption spectra of porphyrins 4, 7, 8c, 8d, 9c and 9d in CHCl3 at 298 K. (b) Electronic absorption spectra of
(11aec) in CHCl3 at 298 K. Inset in both (a) and (b) shows the Q bands.
b,meso-benzoxazolyl-bridged porphyrin dyads
porphyrin dyad (11a) and 425, 432 nm for the zincezinc porphyrin
dyad (11c), whereas the freebaseenickel complex (11b) exhibited
only a Soret peak at 420 nm along with four broadened Q bands at
517, 543, 585 and 647 nm due to the overlapping of absorptions of
freebase and nickel(II) porphyrin units. The splitting of Soret bands
in the case of diporphyrins (11a and 11c) possibly arises due to
the non-planar deformation of the porphyrin core of
substituted component of the two compounds [37].
b,meso-
The fluorescence spectra of compounds 4, 7, 8c, 8d, 9c and 9d in
CHCl3 solution at excitation wavelength 420 nm are shown in Fig. 2.
All these porphyrins showed typical emission bands at w650 nm
and a shoulder at w717 nm with almost equal intensities. The
fluorescence bands of the benzazolylporphyrins (8ced and 9ced)
are found to be blue-shifted by 3e4 nm in comparison to their
corresponding precursors 4 or 7, which suggests that there is no
increment in the
p-conjugation in these molecules. The energy
transfer behavior of pyreneeporphyrin dyads (8c and 9c) was
studied by analyzing their absorption and emission spectra. Fig. 3a
shows the emission spectra of pyrene-1-carboxaldehyde (P-1) and
porphyrins (4, 7, 8c and 9c) following 280 nm excitation. It was
observed that the fluorescence of P-1 at 420 nm due to the pyrene
moiety is almost completely quenched on excitation of porphyrins
8c and 9c at 280 nm and there is an increase in the intensity of
fluorescence band of these porphyrin dyads at w652 and w718 nm
with respect to the starting porphyrin compounds 4 and 7. Thus, it
Fig. 2. Fluorescence spectra of porphyrins 4, 7, 8c, 8d, 9c and 9d in CHCl3
(2 ꢂ 10ꢀ6 mol Lꢀ1) at 298 K, lex ¼ 420 nm.
Fig. 3. (a) Emission spectra of P-1 and porphyrins 4, 7, 8c and 9c in CHCl3 (2 ꢂ 10ꢀ6 mol Lꢀ1) at 298 K, lex ¼ 280 nm (b) UVeVis spectra of 8c and 9c and fluorescence spectrum of
P-1 (lex ¼ 280 nm) in CHCl3.

1248
S. Sharma, M. Nath / Dyes and Pigments 92 (2012) 1241e1249
Fig. 4. (a) Fluorescence spectra of F-1 and porphyrins 4, 7, 8d and 9d in CHCl3 (2 ꢂ 10ꢀ6 mol Lꢀ1) at 298 K, lex ¼ 280 nm (b) UVeVis spectra of 8d and 9d and fluorescence spectrum
of F-1 (lex ¼ 280 nm) in CHCl3.
shows that there is an efficient intramolecular energy transfer from
the pyrene unit to the porphyrin ring in the dyads 8c and 9c.
Further, the significant overlapping of fluorescence spectrum of P-1
with the absorption spectra of 8c and 9c (Fig. 3b) also supports the
above statement as it fulfills the first condition of energy transfer
[38]. Similarly, the intramolecular energy transfer from the fluorene
moiety to the porphyrin core was also observed in porphyrine
fluorene dyads 8d and 9d (Fig. 4a,b).
porphyrins are practically known to exhibit poor emission due to
enhanced rates of internal conversion [39e42].
4. Conclusions
In summary, we have successfully synthesized and characterized
two new porphyrin precursors, 5-(3,4-diaminophenyl)-10,15,20-
triphenylporphyrin (4) and 5-(3-amino-4-hydroxyphenyl)-10,15,20-
triphenylporphyrin (7), which can be used as starting materials for
the synthesis of a variety of porphyrinic molecules and their metal
complexes. To this end, a series of meso-substituted benzazolyl-
The fluorescence spectra of 5,10,15,20-tetraphenylporphyrin
(TPP), zinc(II) 5,10,15,20-tetraphenylporphyrin (Zn-TPP) and
b,meso-benzoxazolyl-bridged porphyrin dyads (11aec) in chloro-
form solution at excitation wavelength 420 nm are presented in
Fig. 5. The fluorescence spectra of diporphyrins (11aec) showed
two fluorescence bands at 670, 722 nm for freebaseefreebase
diporphyrin (11a), at 651, 717 nm for freebase-nickel(II) dipor-
phyrin (11b) and at 621, 654 nm for zincezinc diporphyrin (11c).
On excitation at 420 nm, the fluorescence bands of both dipor-
phyrin 11a and 11c were significantly quenched and red shifted
by w20 nm with respect to TPP and Zn-TPP, respectively. In
contrast, the strong emission quenching was observed in the case of
diporphyrin (11b) at 651 and 717 nm with respect to TPP. These
observations may be explained by the possible non-planarity of
a component of the diporphyrins (11aec) as the non-planar
triphenylporphyrins and
b,meso-benzoxazolyl-bridged porphyrin
dyads have been efficiently synthesized in good yields through
La(OTf)3-catalyzed condensation cyclization reaction of porphyrin 4
or 7 with variousaromatic aldehydes. Onphotophysical investigation,
a significant intramolecular energy transfer was observed from the
pyrene and fluorene subunits to the porphyrin core in the case of
porphyrins (8ced and 9ced). Furthermore, the significant emission
quenching observed in
b,meso-benzoxazolyl-bridged porphyrin
dyads (11aec) was attributed to the possible non-planarity of
synthesized diporphyrin macrocycles. These photophysical results
are significantly encouraging and henceforth may be useful
for designing new porphyrin molecules for various applications
including light harvesting and photodynamic agents for photody-
namic therapy.
Acknowledgments
We thank the University of Delhi, India for providing necessary
financial support for this work. Satyasheel is grateful to Council
of Scientific and Industrial Research, New Delhi, India for Senior
Research Fellowship. The Jeol ECX 400P (400 MHz) NMR facility at
USIC is acknowledged for NMR spectra.
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298 K, lex ¼ 420 nm.

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