Synthesis and fluorescence studies of porphyrin appended 1,3,4-oxadiazoles

Article abstract of DOI:10.1142/S1088424610002884

A modular synthetic approach for preparing a family of porphyrin appended 1,3,4-oxadiazoles 9 is described. The porphyrin hydrazides are reacted with aryl aldehydes in presence of Yb(OTf)₃ as catalyst to give porphyrin hydrazones 8 which are th

Full text of DOI:10.1142/S1088424610002884

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2010; 14: 1034–1039
DOI: 10.1142/S1088424610002884
Published at http://www.worldscinet.com/jpp/
Synthesis and fluorescence studies of porphyrin appended
1,3,4-oxadiazoles
K.P. Chandra Shekara, Bhupendra Mishraa, Anil Kumara, S. Phukanb, S. Mitrab
and Dalip Kumar*a
a Department of Chemistry, Birla Institute of Technology & Science, Pilani 333 031, India
b Department of Chemistry, North-Eastern Hill University, Permanent Campus, Umshing, Shillong 793 022, India
Received 29 April 2010
Accepted 10 December 2010
ABSTRACT: A modular synthetic approach for preparing a family of porphyrin appended 1,3,4-
oxadiazoles 9 is described. The porphyrin hydrazides are reacted with aryl aldehydes in presence of
Yb(OTf)3 as catalyst to give porphyrin hydrazones 8 which are then cyclized to porphyrin appended
1,3,4-oxadiazoles 9 using iodobenzene diacetate. Photophysical studies in CHCl3 solvent shows that
the electronic structure of the porphyrin chromophore is not greatly perturbed by the incorporation of
the oxadiazole group onto the meso-phenyl ring. Efficient quenching of porphyrin fluorescence was
observed in 9g with a pyridinium moiety.
KEYWORDS: porphyrin, 1,3,4-oxadiazoles, porphyrin hydrazide, porphyrin oxadiazoles, iodobenzene
diacetate.
synthesized novel porphyrins containing 1,3,4-oxadiazole
INTRODUCTION
moiety and studied their physical properties.
Porphyrin and its derivatives have received a significant
Organoiodine(III) reagents are very important for
attention in the last two decades due to their applications in
the oxidation of various functional groups in organic
electrooptical and nonlinear optics [1], two-photon absorp-
chemistry due to their low toxicity, ready availability,
tion [2], field-effect transistors [3], organic synthesis, phos-
ease of manipulation and high stability [10]. Iodoben-
phorescent oxygen sensors [4] and photodynamic therapy
zene diacetate (IBD) mediated oxidation of N-acylhy-
[5]. Porphyrin appended heterocycles have also widely uti-
drazones has been reported to yield 1,3,4-oxadiazoles
lized for their potential applications in medicine, switches,
[11]. Subsequently this protocol has been utilized for
photochemical energy conservation and molecular elec-
the synthesis of naphthyridine analogs by grinding a
tronics [6]. Variation of peripheral and meso-substituents,
mixture of appropriate hydrazones and IBD in solid
and the central metal ion of porphyrins have shown to
state. Recently, we have synthesized a series of biologi-
modulate their physical and biological properties [5]. On
cally interesting indolyl 1,3,4-oxadiazoles from indolyl
the other hand, five-membered 1,3,4-oxadiazoles are an
acylhydrazones [8]. In this paper, we report preparation
important class of heterocyclic compounds with broad
of various porphyrin oxadiazoles using IBD mediated
range of biological activities including antibacterial, anti-
oxidation of porphyrin N-acylhydrazones in very good
mycobacterial, antiinflammatory, anticonvulsant, tyrosi-
yield.
nase inhibitory and anticancer activities [7, 8]. Some of the
1,3,4-oxadiazoles have also been investigated as electron-
transportingmaterialswithinmultilayereddevicesandcolor
tunable luminescence [9]. Due to interesting physical and
EXPERIMENTAL
biological properties of porphyrin heterocycles, we have
General
The synthesized porphyrin derivatives (1–7) and
the free base (8) were characterized by UV-vis and
*Correspondence to: Dalip Kumar, email: dalipk@bits-pilani.
ac.in, tel: +91 1596-245073-279, fax: +91 (1596)-244183
Copyright © 2010 World Scientific Publishing Company

SyntheSIS anD fluOreSCenCe StuDIeS Of POrPhyrIn aPPenDeD 1,3,4-OxaDIazOleS 1035
fluorescence spectroscopy. Steady-state absorption spec-
triflate (18 mg, 0.029 mmol) and the reaction mixture
was stirred for 4 h at room temperature. After comple-
tion of the reaction, the solvent was distilled off and the
residual mass was taken into water (5 mL) and basified to
pH ~ 8 with sodium carbonate. The aqueous solution was
extracted with dichloromethane (3 × 20 mL), and com-
bined organic phase was dried over sodium sulphate and
distilled off the excess solvent to afford porphyrin hydra-
zone 8 in 69–83% yields. 5-(N-4-methylbenzylidene-4-
phenylhydrazino)-10,15,20-triphenylporphyrin (8a).
Yield 78%. 1H NMR (500 MHz; CDCl3; Me4Si): H, ppm
9.79 (s, 1H), 8.99–8.71 (m, 8H), 8.52 (s, 1H), 8.33–8.35
(m, 4H), 8.21–8.19 (m, 6H), 7.89 (d, 2H, J = 8.2 Hz),
7.80–7.71 (m, 9H), 7.69 (d, 2H, J = 8.4 Hz), 2.41 (s, 3H).
MS: m/z 774.5, calcd. for [M + H]+ 773.9. 5-(N-4-pyri-
dylidene-4-phenylhydrazino)-10,15,20-triphenylpor-
tra were recorded on a Perkin-Elmer model Lambda25
absorption spectrophotometer. Fluorescence spectra
were taken in a Hitachi model FL4500 spectrofluorim-
eter and all the spectra were corrected for the instrument
response function. Quartz cuvettes of 10 mm optical
path length received from PerkinElmer, USA (part no.
B0831009) and Hellma, Germany (type 111-QS) were
used for measuring absorption and fluorescence spec-
tra, respectively. For fluorescence emission, the sample
was excited at 515 nm unless otherwise mentioned,
whereas, excitation spectra were obtained by monitor-
ing at the respective emission maximum. In both cases,
5 nm bandpass was used in the excitation and emission
side. Fluorescence quantum yields (φf) were calculated by
using compound 8 as reference. The relative experimen-
tal error of the measured quantum yield was estimated
1
phyrin (8b). Yield 70%. H NMR (500 MHz; CDCl3;
1
within 5%. H NMR spectra were recorded on Brucker
Me4Si): H, ppm 10.48 (s, 1H), 8.99–8.71 (m, 8H),
8.58 (s, 1H), 8.52 (d, 2H, J = 7.9 Hz), 8.36–8.33 (m,
4H), 8.21–8.19 (m, 6H), 7.84–7.65 (m, 9H), 7.57 (d, 2H,
J = 8.3 Hz). MS: m/z 761.8, calcd. for [M + H]+ 761.9.
5-(N-4-methoxy-benzylidene-4-phenylhydrazino)-10,
Heaven 11400 (400 MHz) and mass spectra were recorded
on QSTAR Elite LX/MS/MS from applied biosystems.
The 5-(4′-methylcarboxylate)-10,15,20-triphenylporphyrin
4 and 5-[(4′-methylcarboxylate)-10,15,20-triphenylpor-
phyrinato]zinc(II) 5 were prepared according to literature
procedure [12].
1
15,20-triphenyl-porphyrin (8c). Yield 74%. H NMR
(400 MHz; CDCl3; Me4Si): H, ppm 10.05 (s, 1H), 8.99–
8.74 (m, 8H), 8.45 (s, 1H), 8.31–8.29 (m, 4H), 8.23-8.20
(m, 6H), 7.82–7.79 (m, 9H), 7.65 (d, 2H, J = 8.2 Hz),
7.35 (d, 2H, J = 8.3 Hz), 3.85 (s, 3H). MS: m/z 790.8,
calcd. for [M + H]+ 790.9. 5-(N-4-chlorobenzylidene-4-
phenylhydrazino)-10,15,20-triphenylporphyrin (8d).
Synthesis
Preparation of (5-(4′-phenylhydrazide)-10,15,20-
triphenylporphyrinato)zinc(II) 6.
A
solution of
1
compound 5 (0.6 g, 0.81 mmol) in 15 mL of dimeth-
ylformamide was treated with 98% hydrazine hydrate
(20 mL) and heated to 80 °C for 4 h. After the completion
of the reaction, the reaction mixture was cooled 10–15 °C
Yield 76%. H NMR (500 MHz; CDCl3; Me4Si): H,
ppm 9.66 (s, 1H), 8.99–8.65 (m, 8H), 8.38 (d, 2H, J = 7.5
Hz), 8.31–8.29 (m, 4H), 8.26–8.17 (m, 6H), 8.03 (d, 2H,
J = 7.8 Hz), 7.84–7.66 (m, 9H), -2.78 (s, 2H). MS: m/z
795.3, calcd. for [M + H]+ 795.3. 5-(N-benzylidene-4-
phenylhydrazino)-10,15,20-triphenylporphyrin (8e).
Yield 83%. 1H NMR (500 MHz; CDCl3; Me4Si): H, ppm
9.93 (s, 1H), 8.97–8.76 (m, 8H), 8.62 (d, J = 8.4 Hz, 2H),
8.34 (d, 2H, J = 7.8 Hz), 8.23–8.20 (m, 6H), 7.88 (d, 3H,
J = 8.2 Hz), 7.78–7.73 (m, 9H), 7.45 (d, 2H, J = 8.4 Hz),
-2.79 (s, 2H). MS: m/z 760.7, calcd. for [M + H]+ 760.9.
5-(N-4-nitrobenzylidene-4-phenylhydrazino)-10,15,20-
triphenylporphyrin (8f). Yield 69%. 1H NMR (400 MHz;
CDCl3; Me4Si): H, ppm 9.66 (s, 1H), 8.99–8.65 (m, 8H),
8.38 (d, 2H, J = 7.5 Hz), 8.31–8.28 (m, 4H), 8.26–8.17
(m, 6H), 8.03 (d, 2H, J = 7.8 Hz), 7.84–7.66 (m, 9H), -2.78
(s, 2H). MS: m/z 805.9, calcd. for [M + H]+ 805.9.
1
and filtered to afford 6 in 96% (0.58 g) yield. H NMR
(500 MHz; CDCl3; Me4Si): H, ppm 8.97–8.74 (m, 8H),
8.29–8.03 (m, 6H), 7.92 (s, 1H), 7.81–7.63 (m, 9H), 7.34
(d, 2H, J = 8.2 Hz), 7.06 (d, 2H, J = 8.2 Hz). MS: m/z
735.3, calcd. for [M + H]+ 735.5.
Preparation of 5-(4′-phenylhydrazide)-10,15,20-
triphenylporphyrin 7. To a solution of porphyrin
hydrazide 6 (0.73 g, 0.99 mmol) in dichloromethane was
added 10 mL of dilute HCl (6N) and the reaction mix-
ture was stirred for half an hour at room temperature. The
reaction contents were basified to pH ~ 8 with sodium
carbonate and the organic layer was separated. Remain-
ing aqueous layer was extracted with dichloromethane
(3 × 15 mL). The combined organic phase was dried over
sodium sulphate and distilled off at reduced pressure to
obtain pure hydrazide 7 in 91% (0.51 g) yield. 1H NMR
(500 MHz; CDCl3; Me4Si): H, ppm 8.98–8.67 (m, 8H),
8.29–8.07 (m, 6H), 7.82–7.60 (m, 9H), 7.33 (d, 2H, J =
8.2 Hz), 7.06 (d, 2H, J = 8.3 Hz). MS: m/z 672.5, calcd.
for [M+H]+ 672.3.
Preparation of 5-(4-phenyl-(2-aryl-1,3,4-oxadia-
zol-5-yl)-10,15,20-triphenylporphyrin 9a–f. Iodobenzene
diacetate (0.039 g, 0.114 mmol) was added to a solu-
tion of porphyrin hydrazones 8 (0.118 mmol) in dichlo-
romethane (5 mL) and the reaction mixture was stirred at
room temperature for 8 h. After completion of reaction
as indicated by TLC, the contents were poured into water
(5 mL). The product was extracted with dichloromethane
(3 × 15 mL). Combined organic phase was dried over
sodium sulphate and distilled off the excess solvent to
afford 9 which was purified by column chromatography
Preparation of 5-(4′-benzoylhydrazono-phenyl)-10,
15,20-triphenylporphyrin 8. To a solution of porphy-
rin hydrazide 7 (0.1 g, 0.148 mmol) and aldehyde (15.8
mmol) in tetrahydrofuran (5 mL) was added ytterbium
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 1035–1039

1036 K.P.C. SheKar et al.
using chloroform-hexane (3:2, v/v) as a solvent system.
5-(4-phenyl-(2-(4-nitrophenyl)-1,3,4-oxadiazol-5-yl)-
10,15,20-triphenylporphyrin (9f). Yield 80%. 1H NMR
(400 MHz; CDCl3; Me4Si): δH, ppm 8.89 (d, 2H, J = 4
Hz), 8.85 (s, 8H), 8.55 (d, 2H, J = 8 Hz), 8.42 (d, 2H, J =
8 Hz), 8.23 (d, 6H, J = 8 Hz), 7.79 (d, 9H, J = 8 Hz), 7.61
(d, 2H, J = 8 Hz), -2.75 (s, 2H). MS: m/z 803.1592, calcd.
for [M + H]+ 803.2654. 5-(4-phenyl-(4-methylpyridin-
ium-1,3,4-oxadiazol-5-yl)-10,15,20-triphenylporphy-
5-(4-phenyl-(2-tolyl-1,3,4-oxadiazol-5-yl)-10,15,20-
1
triphenylporphyrin (9a). Yield 71%. H NMR (400
MHz; CDCl3; Me4Si): δH, ppm 8.89 (d, 2H, J = 4 Hz),
8.85 (s, 8H), 8.56 (d, 2H, J = 12 Hz), 8.41 (d, 2H, J =
8 Hz), 8.23 (d, 6H, J = 8 Hz), 8.17 (d, 2H, J = 8 Hz),
7.77 (d, 9H, J = 8 Hz), 3.71 (s, 3H), -2.75 (s, 2H). MS:
773.1698, calcd. for [M + H]+ 773.3029. 5-(4-phenyl-(4-
pyridyl-1,3,4-oxadiazol-5-yl)-10,15,20-triphenylpor-
1
rin (9g). Yield 95%. H NMR (400 MHz, CDCl3) H,
1
phyrin (9b). Yield 75%. H NMR (400 MHz; CDCl3;
ppm 9.08 (d, J = 7.6 Hz, 2H), 8.97–8.77 (m, 8H), 8.59 (d,
2H, J = 7.9 Hz), 8.49 (d, 4H, J = 7.9 Hz), 8.30–8.11 (m,
6H), 7.88–7.66 (m, 9H), 4.53 (s, 3H), -2.85 (s, 1H). MS:
m/z 774.5, calcd. for [M + H]+ 774.7.
Me4Si): δH, ppm 8.94 (d, 2H, J = 8 Hz), 8.84 (s, 8H), 8.63
(d, 2H, J = 8 Hz), 8.52 (d, 2H, J = 8 Hz), 8.24 (d, 6H,
J = 8 Hz), 8.21 (d, 2H, J = 8 Hz), 7.85 (m, 9H ), -2.86 (s,
2H). MS: m/z 760.1547, calcd. for [M + H]+ 760.2825.
5-(4-phenyl-(2-(4-methoxyphenyl)-1,3,4-oxadiazol-5
1
-yl)-10,15,20-triphenylporphyrin (9c). Yield 64%. H
RESULTS AND DISCUSSION
NMR (400 MHz; CDCl3; Me4Si): δH, ppm 8.89 (d, 2H,
J = 4 Hz), 8.85 (s, 8H), 8.54 (d, 2H, J = 8 Hz), 8.41 (d,
2H, J = 8 Hz), 8.23 (d, 6H, J = 8 Hz), 7.78 (d, 9H, J = 8
Hz), 7.12 (d, 2H, J = 8 Hz), 3.93 (s, 3H), -2.75 (s, 2H).
MS: m/z 789.1601, calcd. for [M + H]+ 789.2978. 5-(4-
phenyl-(2-(4-chlorophenyl)-1,3,4-oxadiazol-5-yl)-10,
Synthesis and characterization
The synthesis of porphyrin 4 was carried out from the
reaction of 4-formyl methylbenzoate 1 (1 equiv.), benz-
aldehyde 2 (3 equiv.) and pyrrole 3 (4 equiv.) in refluxing
propionic acid (Scheme 1) followed by oxidation with
DDQ in dry chloroform [13]. The desired porphyrin 4
was separated from the crude mixture by column chro-
matography in 7% yield. The metallation of the porphy-
rin 4 was carried out with zinc acetate in a mixture of
chloroform-methanol (1:1, v/v) to give porphyrin 5 in
95% yield. The reaction of porphyrin 5 with hydrazine
hydrate in N,N′-dimethylformamide resulted in the for-
mation of hydrazide 6 in 96% yield.
1
15,20-triphenyl-porphyrin (9d). Yield 74%. H NMR
(400 MHz; CDCl3; Me4Si): δH, ppm 8.89 (d, 2H, J = 4
Hz), 8.85 (s, 8H), 8.55 (d, 2H, J = 8 Hz), 8.42 (d, 2H,
J = 8 Hz), 8.23 (d, 6H, J = 8 Hz), 7.79 (d, 9H, J = 8 Hz),
7.61 (d, 2H, J = 8 Hz), -2.75 (s, 2H). MS: m/z 793.1147,
calcd. for [M + H]+ 793.2483. 5-(4-phenyl-(2-phenyl-
1,3,4-oxadiazol-5-yl)-10,15,20-triphenylporphyrin
1
(9e). Yield 66%. H NMR (400 MHz; CDCl3; Me4Si):
δH, ppm 8.89 (d, 2H, J = 4 Hz), 8.86 (s, 8H), 8.57 (d, 2H,
J = 12 Hz), 8.42 (d, 2H, J = 8 Hz), 8.23 (d, 6H, J = 8Hz),
7.77 (d, 9H, J = 8 Hz), 7.61 (t, 3H, J = 8 Hz), -2.75 (s,
2H). MS: m/z 759.1574, calcd. for [M + H]+ 759.2872.
However, direct attempt to prepare porphyrin hydraz-
ide failed to yield desired porphyrin hydrazide. Demeta-
lation of 6 was effected with dilute hydrochloric acid
CO2CH3
NH
N
N
N
N
N
N
CHO
Zn(OAc)2
propionic acid
reflux
CO2CH3
CO2CH3
Zn
2
CHCl3,MeOH
NH
CHO
1
N
H
3
4
5
N
N
N
NH
N
N
NH2NH2.H2O
DMF
dil.HCl
CONHNH2
Zn
N
CONHNH2
NH
6
7
Scheme 1. Synthesis of porphyrin hydrazide 7
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 1036–1039

SyntheSIS anD fluOreSCenCe StuDIeS Of POrPhyrIn aPPenDeD 1,3,4-OxaDIazOleS 1037
Table 1. Synthesis of porphyrin oxadiazoles (9a–g)a
presence of hypervalent iodine reagent, iodobenzene
diacetate (1 equiv.) in dichloromethane to give desired
porphyrin 9 in good yields (Scheme 2).
Entry
R
Product
Yield, %b
1
2
3
4
5
6
7
4-CH3C6H4
4-pyridyl
9a
9b
9c
9d
9e
9f
71
75
64
74
66
80
94c
Photophysical properties
4-OCH3C6H4
4-ClC6H4
The most acceptable interpretation of electronic
absorption spectra of porphyrin derivatives is based on
theoretical analysis of the experimental data and quan-
tum chemical calculations. According to this model, the
porphyrins do not exhibit n→π* transition due to the
symmetry of the n orbitals and asymmetry of the π orbit-
als. All bands are of π→π* type. The Soret band is due to
C6H5
4-NO2C6H4
4-methylpyridinium
9g
Reaction conditions: a 8 PhI(OAc)2, DCM; b isolated yield; c 9b
CHCl3, CH3I, reflux, 36 h.
1
an allowed A1g→1E′u transition, and consequently, the
intensity of this band is very high [14]. The characteristic
peak positions for absorption spectra are given in Table 2
along with the molar absorption coefficient of each of
them. In general, all the compounds exhibited character-
istically one intense Soret band and four weak Q-bands.
The peak position of the Soret band is around 420 nm;
in dichloromethane to obtain pure porphyrin hydrazide
7. The porphyrin hydrazone 8 was synthesized from
the reaction of 7 with aldehydes in presence of ytter-
bium triflate (0.2 equiv.) in tetrahydrofuran at room
temperature (Scheme 2). This reaction was found to be
sluggish in ethanol and catalytic amount of conc. HCl
due to insolubility of porphyrin 7 even at higher tem-
perature. The formation of hydrazone with compound
6 was also attempted but it took 48 h for the comple-
tion of the reaction whereas with metallated porphyrin
7, the reaction completed within 3–5 h. Further, the
oxidative cyclization of compound 8 was carried out in
whereas theY-polarized bands IV [QY(0,1)] and III [QY(0,0)
]
around 515 and 551 nm, respectively, and the X-polar-
ized bands II [QX(0,1)] and I [QX(0,0)] around 590 and 648
nm, respectively. Compared to the H2TPP (418 nm), the
Soret band of all the prophyrin derivatives appear slightly
red-shifted at 420 nm except in case porphyrin 9g, where
it appears at ca. 416 nm. Similar observation is made for
NH
N
N
NH
N
N
N
N
HN
O
N
RCHO
IBD
R
7
O
R
Yb(OTf)3,THF
HN
HN
8a-f
9a-f
a) R = 4-CH3C6H4; b) 4-pyridyl; c) 4-OCH3C6H4; d) 4-ClC6H4; e) C6H5; f) 4-NO2C6H4
Scheme 2. Synthesis of porphyrin oxadiazoles 9
Table 2. Absorption and emission data of porphyrin 1,3,4-oxadiazoles 9a–g in chloroform
Compound
(Absorption) λmax nm (ε/104 M-1.cm-1)
(Emission) λmax nm Quantum
yield, Φf
(Soret)
[QY(0,1)]
[QY(0,0)
]
[QX(0,1)
]
[QX(0,0)]
9a
9b
420 (14.00)
420 (7.10)
420 (36.80)
420 (21.30)
420 (34.60)
420 (11.80)
416 (4.20)
418 (42.00)
516 (2.30)
515 (1.30)
516 (4.50)
515 (3.00)
516 (4.20)
517 (2.60)
540 (0.79)
516 (4.60)
552 (2.00)
551 (1.10)
550 (3.70)
551 (2.60)
551 (3.70)
550 (2.30)
614 (0.65)
552 (3.70)
592 (1.90)
591 (1.09)
590 (3.50)
590 (2.40)
590 (3.30)
590 (2.20)
653 (0.64)
592 (3.40)
647 (1.80)
649 (1.08)
646 (3.30)
647 (2.30)
646 (3.10)
648 (2.10)
682 (0.61)
647 (3.20)
647
648
648
647
647
648
707
707
708
707
708
708
0.063
0.102
0.160
0.073
0.085
0.062
9c
9d
9e
9f
9g
no characteristic emission
647 707 0.110
H2TPP
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 1037–1039

1038 K.P.C. SheKar et al.
Fig. 1. (a) Fluorescence excitation spectra of different porphyrin derivatives in chloroform obtained by monitoring the fluorescence
emission at 648 nm. (b) Fluorescence emission spectra of porphyrin derivatives in chloroform (λexc = 515 nm)
the Q-bands also; where all the porphyrin derivatives are
observed to behave similarly except 9g.
“donor-space-acceptor” intramolecular PET transfer sys-
tem. The fluorescence of porphyrin is quenched by way
of transfer of the excited state electron from porphyrin to
pyridinium ring through the oxadiazole spacer. The fluo-
rescence quantum yields of 9b and 9c was higher than that
of H2TPP, indicating the substantial electronic interaction
between porphyrin ring and aryl ring through oxadiazole
spacer whereas fluorescence quantum yields decreased
in other porphyrin derivatives (9a, 9d–e and 9f). Also,
it has been observed that fluorescence excitation spectra
(Fig. 2) for all the porphyrins are in very good agreement
with the absorption data, given in Table 2.
The small shift in both the Soret and Q-bands from
the model compound indicates that introduction of new
groups in meso-phenyl of the porphyrin moiety did not
affect the energy levels substantially. The electronic
structure of the porphyrin chromophore is not greatly per-
turbed by the incorporation of the oxadiazole group onto
the meso-phenyl ring. Some of the representative fluores-
cence spectra are shown in Fig. 1. The fluorescence peak
position of all the compounds in chloroform are given
in Table 2 along with the relative yield of fluorescence.
The fluorescence spectra of all the compounds except 9g
are characterized by two emission bands at around 647
and 707 nm, when excited at 515 nm. Excitation at Soret
band also gives similar emission spectra (not shown).
Interestingly, the fluorescence emission peak position
is remarkably insensitive to the nature of substitution.
However, porphyrin 9g that contains a positively charged
pyridinium moiety does not show any fluorescence emis-
sion behavior in the whole spectral range. This is also in
accordance with the observation that compound 9f con-
taining a neutral electron withdrawing nitro group show
comparable fluorescence property (although having rela-
tively lesser magnitude of fluorescence quantum yield)
when compared with other fluorescing porphyrin deriva-
tives however, the fluorescence quantum yield is signifi-
cantly quenched. Complete quenching of fluorescence in
case of 9g may be due to some specific interaction of
the porphyrin ring with pyridinium moiety. Table 2 also
shows substantial difference in Q-band positions for 9g
with other synthesized porphyrin derivatives as well as
the model compound H2TPP. This further indicates that
the energy levels are perturbed in 9g and the nature of
interaction is different in this case from the rest of the
group. In porphyrin 9g, pyridyl ring is connected with
porphyrin through a linking bridge; it would make up a
Acknowledgements
The authors thank the Department of Science andTech-
nology, New Delhi for the financial support and SAIF,
Panjab University for providing analytical support.
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