Triphenylamine substituted dipyrrinato metal complexes: Synthesis, optical and electrochemical studies

Article abstract of DOI:10.1016/j.inoche.2015.07.019

A series of triphenylamine substituted dipyrrinato metal complexes (1-8) have been synthesized. The mononuclear type complexes 1-6 have Ni(II), Co(II), Pd(II), In(III), and Zn(II) metal ions in the core. The binuclear type complexes 7 and 8 have Zn(II) metal ion in the center. All the compounds (1-8) were characterized by HRMS, NMR, IR, UV-vis absorption, cyclic voltammetry and fluorescence techniques. The presence of large electron rich triphenylamine moiety at dipyrrin ligands affected the spectral properties of complexes. Except Co(II) complex, other metal complexes exhibited blue shifted absorption maxima in UV-vis studies. The In(III) and Zn(II) metal complexes 4-6 showed red shifted emission maxima in fluorescence compared to their corresponding phenyl analogues. Complexes 3-8 exhibited good Stokes shifts in the range of 4600 to 7000 cm^{- 1} with reduced quantum yields. Singlet state lifetimes of complexes 3-8 were in the range of 2 to 4 ns; also the decrease in radiative decay constants k_r and the increase in non-radiative decay constants k_nr were in line with the quantum yield data. CV studies of complexes 1-8 showed anodic shifts in the oxidation potentials, suggesting that meso-triphenylamine group has affected the electronic properties of complexes by making them difficult to oxidize.

Full text of DOI:10.1016/j.inoche.2015.07.019

Inorganic Chemistry Communications 60 (2015) 54–60
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Triphenylamine substituted dipyrrinato metal complexes: Synthesis,
optical and electrochemical studies
⁎
Sudipta Das, Iti Gupta
Indian Institute of Technology Gandhinagar, VGEC Campus, Chandkheda, Ahmedabad 382424, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 May 2015
Received in revised form 11 July 2015
Accepted 25 July 2015
Available online 1 August 2015
A series of triphenylamine substituted dipyrrinato metal complexes (1–8) have been synthesized. The mononu-
clear type complexes 1–6 have Ni(II), Co(II), Pd(II), In(III), and Zn(II) metal ions in the core. The binuclear type
complexes 7 and 8 have Zn(II) metal ion in the center. All the compounds (1–8) were characterized by HRMS,
NMR, IR, UV–vis absorption, cyclic voltammetry and fluorescence techniques. The presence of large electron
rich triphenylamine moiety at dipyrrin ligands affected the spectral properties of complexes. Except Co(II) com-
plex, other metal complexes exhibited blue shifted absorption maxima in UV–vis studies. The In(III) and Zn(II)
metal complexes 4–6 showed red shifted emission maxima in fluorescence compared to their corresponding
phenyl analogues. Complexes 3–8 exhibited good Stokes shifts in the range of 4600 to 7000 cm−1 with
reduced quantum yields. Singlet state lifetimes of complexes 3–8 were in the range of 2 to 4 ns; also the
decrease in radiative decay constants kr and the increase in non-radiative decay constants knr were in line with
the quantum yield data. CV studies of complexes 1–8 showed anodic shifts in the oxidation potentials, suggesting
that meso-triphenylamine group has affected the electronic properties of complexes by making them difficult
to oxidize.
Keywords:
Dipyrrins
Triphenylamine dipyrrinato complex
Dinuclear metal complex
© 2015 Elsevier B.V. All rights reserved.
The chemistry of dipyrranes [1] has grown tremendously in last two
decades because they have been used as key synthetic precursors for
variety of boron based dyes and porphyrin derivatives [2]. The oxidized
form of dipyrrane is known as dipyrrin, it was first reported by Hans
Fischer in 1937 [3]. Due to unsaturation the electron density gets
delocalized over two pyrrolic rings attached via sp2 carbon atom in
dipyrrins. The presence of two coordinating nitrogen atoms in dipyrrin
makes it a good bidentate ligand. The initial efforts to isolate free
dipyrrins were not so successful; however in the last decade the chem-
istry of dipyrrins has been evolving [4,5]. Dipyrrins have been used as
ligands to form stable and neutral metal complexes, called metal
dipyrrinato complexes. Such dipyrrinato metal complexes are more
stable than free dipyrrin ligands. Two kinds of metal–dipyrrinato com-
plexes have been reported, first: homoleptic type (contains two similar
dipyrrin ligands) and second: heteroleptic type (contains two different
dipyrrin ligands) [6]. A variety of metal complexes has been reported
with 3rd row transition metals [7] as well as with 4th and 5th row [8]
transition metals. The borondifluoride complexes of dipyrrins have
found applications in materials and biology due to good photostability
and high quantum yields [2]. As compared to dipyrrin–borondifluoride
complexes their metal counterparts have been ignored due to lack of
fluorescence properties (Ni(II), Cu(II), Co(II) dipyrrinato complexes)
or low emission efficiencies (Ga(III), In(III), Pd(II) dipyrrinato com-
plexes). However, coordination chemistry of dipyrrinato ligands con-
tinues to attract researchers due to their potential applications in
photochemistry. Cohen and co-workers [9] have synthesized supramo-
lecular systems such as metal–organic frameworks (MOFs) based on
pyridyl and cyanophenyl dipyrrinato complexes. Recently, alkali metal
(Li+, Na+, K+) based dipyrrinato complexes have also been reported
[10] and their applications in salt elimination reactions have been test-
ed. Also, the heteroleptic dipyrrinato complexes with Rh(II) have been
used as sensitizers for DSSCs (Dye Sensitized Solar Cells) [11]. Most of
the metal complexes exhibited interesting luminescent and optical
properties. Homoleptic Zn(II) and heteroleptic Re(II) dipyrrinato com-
plexes have been found to be highly fluorescent with multi-nanosecond
lifetime [12,13]. The bulky mesityl group restricted the rotation around
the meso-carbon single bond thereby enhancing the excited state life-
time. Lindsey and co-workers have reported the synthesis of zinc bis-
dipyrrinato bridged porphyrin dyads and energy transfer studies of
such systems [14]. Extended conjugation across the two pyrrole rings
renders promisingly useful optical properties to the dipyrrin deriva-
tives. Nishihara and co-workers have used this feature and reported
luminescent In(III) and Zn(II) containing heteroleptic complexes
with high quantum yields [15,16]. This paper details the synthesis,
photophysical and electrochemical properties of six novel triphenylamine
⁎
Corresponding author.
E-mail address: iti@iitgn.ac.in (I. Gupta).
http://dx.doi.org/10.1016/j.inoche.2015.07.019
1387-7003/© 2015 Elsevier B.V. All rights reserved.

S. Das, I. Gupta / Inorganic Chemistry Communications 60 (2015) 54–60
55
Scheme 1. Synthesis of triphenylamine substituted dipyrrinato metal complexes 1–6.
dipyrrinato metal complexes having Zn(II), In(III), Pd(II), Ni(II), Co(II)
metal ions in the core. Two binuclear zinc complexes based on
tripheynylamine dipyrrinato ligand have also been synthesized and
studied. It is anticipated that replacement of meso-phenyl groups at
dipyrrin ligand by bulky and electron rich triphenylamine moiety af-
fects the electronic, photophysical and electrochemical properties of
dipyrrinato metal complexes.
The triphenylamine aldehyde and corresponding dipyrrane
were synthesized as per the reported procedure [17]. The
triphenylaminedipyrrane was oxidized using 2,3-dichloro-5,6-
dicyanobenzoquinone (DDQ) dissolved in benzene and the resulting
dipyrrin was used immediately to make metal complexes without any
purification. Metal salts dissolved in methanol were added to the
dipyrrin solution and stirred at room temperature for several hours
(Scheme 1). Progress of the reaction was monitored by TLC (thin layer
chromatography) and reaction was stopped when new spot developed
on TLC plate corresponding to metal complex. Crude metal complexes
1–6 were subjected to silica gel column chromatography and were
eluted with 30–50% ethyl acetate/hexane mixture. The pure metal
complexes were obtained as reddish maroon color solid in the range
of 11–13% yields. Binuclear type zinc dipyrrinato complexes 7 and 8
were prepared as per the synthetic procedure shown in Scheme 2.
meta-benzi and para-benzi-bisdipyrranes were prepared as per the
literature reported procedure [18]. Tripheynylaminedipyrrane and
Scheme 2. Synthesis of binuclear triphenylamine substituted dipyrrinato complexes 7 and 8.

56
S. Das, I. Gupta / Inorganic Chemistry Communications 60 (2015) 54–60
Fig. 1. 1H NMR spectrum of Pd(II) complex 3 recorded in CDCl3.
meta-benzi- and para-benzi-bisdipyrranes were oxidized separately
in chloroform using DDQ. Then the oxidized dipyrrins were mixed to-
gether and zinc acetate dissolved in methanol was added to the mix-
ture. Reaction mixture was allowed to stir at room temperature for
several hours. New orange color spot developed on TLC indicated the
formation of dinuclear zinc complex. Complexes 7 and 8 were purified
by silica gel column chromatography in 20–40% ethyl acetate/pet
ether mixture in 10–12% yields.
to complex 1 (ESI). In the case of complex 5 three dipyrrin units were co-
ordinated to indium metal thus six α-pyrrole protons showed up as sin-
glet at 7.87 ppm and twelve β-pyrrole protons appeared as two singlets
at 6.99 and 6.59 ppm. The remaining forty two aryl protons showed up
as three multiplets at 7.39, 7.23 and 6.99 ppm (the aryl and β-pyrrole
proton signals were merged here). In case of heteroleptic complex 6
showed several signals for eighteen aryl protons between 7.41 to
6.98 ppm. The eight β-pyrrole protons showed up as four set of signals
between 6.87 and 6.41 ppm, whereas four α-pyrrole protons appeared
as two singlets at 7.64 and 7.52 ppm. Three protons of methoxy group
appeared as singlet at 3.89 ppm confirming the heteroleptic nature of
the complex. The dinuclear zinc complexes 7 and 8 showed doublet
for aryl protons of central benzene unit and three set of multiplets for
aryl and α-pyrrole protons between 7.31 and 7.11 ppm. The remaining
aryl and β-pyrrole protons appeared in between 7.06 and 6.82 ppm in
the NMR.
The absorption studies of complexes 1–8 were carried out by UV–vis
spectroscopy and solvent dependent molar extinction coefficients in
five different solvents were also calculated for all the compounds
(Table 1). The comparison of absorption spectra of all the complexes
1–8is shown in Figs. 2 and 3. The absorption spectra of metal dipyrrinato
complexes primarily exhibit the π–π* transition of the dipyrrin ligand
[20]. All the complexes 1–8 exhibited intense absorption band in the re-
gion of 430–480 nm in toluene, which corresponds to π–π* transitions
originated from the dipyrrin ligand. The nickel complex 1 showed single
absorption band at 460 nm in dichloromethane (DCM) which was
10 nm blue shifted as compared to the reported diphenyldipyrrinato
nickel complex [21]. The cobalt complex 2 showed one major absorp-
tion band at 473 nm in DCM (Table 1) which was 10 nm red shifted
compared to the corresponding dipyrrin metal complex [22]. Palladium
complex 3 showed only one band in DCM at 435 nm (Table 1), whereas
the reported dipyrrinato palladium complex exhibited two bands at 400
and 496 nm in the same solvent [23]. The homoleptic zinc complex 4
showed blue shifted absorption band at 426 nm in toluene as compared
to the reported diphenyl-dipyrrinato zinc complex [24]. This blue shift
All compounds were characterized by Mass, IR, 1H and 13C NMR
spectroscopy (ESI). Compounds 1–8 were confirmed by the molecular
ion peak in mass spectrometry. Typical HRMS mass analysis showed
[M+1]+ peak at 831.3316 for complex 1. In IR spectra of complexes
1–8 different vibration bands were observed in the range of 654 to
3308 cm−1 corresponding to dipyrrin and triphenylamine groups.
Typically, C–H stretching vibrations were observed around 2920 to
3271 cm−1 for the dipyrrin and triphenylamine moieties [19]. The
ring skeleton C–C stretching and C–H bending vibrations were observed
around 1410 to 1709 cm−1. The characteristic C–N stretching vibrations
for triphenylamine group were observed between 1328 and 1397 cm−1
.
The C–H bending (ring) vibrations were observed between 1042 and
1276 cm−1 and out of plane bands were observed around 752 to
976 cm−1. Vibrational bands around 654 to 696 cm−1 were ascribed
to ring bending originated from triphenylamine groups [19]. The repre-
sentative 1H NMR spectrum of homoleptic palladium complex 3 is
shown in Fig. 1. In 1H NMR of complex 1, eight β-pyrrole protons
showed up as two singlets at 6.42 and 6.76 ppm and four α-pyrrole pro-
tons showed up as singlet at 7.63 ppm. Twenty eight aromatic protons of
triphenylamine appeared as four sets of multiplets between 7.07 and
7.37 ppm. In 1H NMR of complex 2, eight β-pyrrole signals were split
into three sets and showed up as multiplet at 6.80 and two singlets at
6.48 and 6.19 respectively. Four α-pyrrole proton signals were down-
field shifted as compared to complex 1 and appeared as singlet at
8.09 ppm. Twenty eight aromatic protons of triphenylamine appeared
as three sets of multiplets between 7.07 and 7.33 ppm. In 1H NMR of
complexes 3 and 4 the signal pattern chemical shift values were similar
Table 1
Absorption data of complexes 1–8 in various solvents. Concentration used was 2 × 10−5 M.
Compound
Solvent
Dichloromethane
Chloroform
Ethyl Acetate
Acetonitrile
Toluene
1
2
3
4
5
6
7
λ
λ
λ
λ
λ
λ
λ
abs (ε × 104)
abs (ε × 104)
abs (ε × 104)
abs (ε × 104)
abs (ε × 104)
abs (ε × 104)
abs (ε × 104)
460 (7.83)
473 (7.83)
435 (10.82)
461 (12.13)
459 (11.64)
481 (12.76)
462 (8.58)
462 (8.10)
475 (7.83)
460 (11.40)
462 (12.17)
462 (11.07)
482 (12.83)
458 (8.39)
480 (7.74)
472 (7.83)
480 (10.07)
425 (10.16)
428 (10.20)
480 (10.87)
427 (6.99)
428 (7.43)
470 (7.83)
432 (8.98)
460 (10.51)
458 (9.27)
479 (10.69)
426 (7.88)
432 (7.73)
476 (7.83)
434 (9.86)
426 (9.93)
435 (8.91)
486 (11.12)
365 (17.86)
421 (9.21)
370 (8.71)
428 (9.06)
8
λ
abs (ε × 104)
436 (8.61)
462 (8.52)
425 (7.56)
421 (8.11)

S. Das, I. Gupta / Inorganic Chemistry Communications 60 (2015) 54–60
57
acetate (Table 1). On the other hand complexes 1and 3–8 showed
red shifted absorption band upon changing the solvent from non-
polar to polar one (Table 1).
The fluorescence properties of compounds 3–8 in ethyl acetate were
investigated both by steady state and time resolved fluorescence tech-
niques. Complexes 1 and 2 containing nickel and cobalt ions respective-
ly were non-fluorescent in nature. Time dependent fluorescence studies
were performed in ethyl acetate and single excited state lifetimes were
measured. Quantum yields, Stokes shifts and singlet state lifetimes of
complexes 3–8 are presented in Table 2. Also, the photographs of di-
chloromethane (DCM) solutions of 3–8 are shown in Fig. 4. The solu-
tions of complexes 3, 7 and 8 looked brightly luminescent under UV
light. The comparison of the normalized emission spectra of 3–8 is
shown in Fig. 5 and lifetime decay profiles of 4–6 are shown in Fig. 6.
The typical mirror–image relationship was observed for the lowest en-
ergy absorption band and emission spectra of all the complexes 3–8.
Complex 3 exhibited one emission band at 625 nm in ethyl acetate,
which matched with the corresponding reported complex [24]. The
mononuclear zinc complexes 4 and 6 showed about 100 nm red
shifted emission bands at 605 and 611 nm respectively (Table 2). The
emission maximum for complex 5 with indium center was observed
at 560 nm, which was 38 nm red shifted compared to the corresponding
compound [27]. Emission bands of binuclear zinc complexes 7 (533 nm)
and 8 (503 nm) were 45–75 nm blue shifted compared to the similar
binuclear zinc complex [16]. The quantum yields for all compounds 3–
8 were not quite high which was ascribed to the non-radiative
decay processes involved. Stokes shifts for complexes 3–8 were calcu-
lated in ethyl acetate and found to be in the range of 3648 to
7000 cm−1 (Table 2). Fluorescence lifetime experiments indicated
that complexes 3–6 had first order decay with lifetimes (τ1) in the
nano-second range. The binuclear zinc complexes 7 and 8 showed bi-
exponential decay (Table 2) [16]. The lifetime data for complexes 4–8
was similar to the reported values of their corresponding complexes,
except for 3 where the singlet state lifetime was significantly higher
than the reported palladium dipyrrinato complex [13]. The non-
radiative relaxation from dipyrrin centered excited states primarily
takes place depending upon the rotation of the meso-aryl substituents.
If rotation of meso-aryl group is hindered then emission gets enhanced.
The meso-aryl substituents though, affect the quantum yield and the ex-
cited state (S1) lifetime (τ); it does not influence much on the absorp-
tion and emission patterns. It is clear from the emission data (Table 2)
that the rate of the non-radiative decay process was higher than the ra-
diative process for all the complexes.
Fig. 2. The comparison of absorption spectra of complexes 1, 2, 3 and 5 in toluene.
could be due to the presence of two large electron rich tri-phenylamine
groups in the complex. On the other hand, a heteroleptic zinc complex 6
showed maximum intensity absorption band at 486 nm in toluene
(Table 1). This is very close to the reported value (485 nm) of
heteroleptic dipyrrinato zinc complex [25]. The binuclear zinc com-
plexes 7 and 8 exhibited two absorption bands in toluene, one band of
higher intensity around 380 nm and another band around 427 nm.
The absorption pattern of 7 and 8 matched with the reported
binuclear zinc dipyrrinato complexes [16,26]. The high energy band
was due to vibronic coupling and the lower energy band was
assigned to the π–π* transitions of dipyrrin ligands [16]. The area
under the absorption curve is proportional to the number of ligands
attached to the metal in the complex. Therefore, higher area under
the curve represents higher number of ligands in homoleptic class
of complexes. This is indeed the case with the indium complex 5,
which exhibited higher area under the absorption curve. The indium
ion having +3 oxidation state binds with three dipyrrin ligands;
whereas, the other metals in their +2 oxidation state bind with
only two dipyrrin ligands. Complex 5 exhibited blue shifted band at
435 nm in toluene as compared to the reported dipyrrinato indium
complex, which showed two bands at 444 and 496 nm in non-polar
solvent [15,27]. Complexes 2 and 6 did not show much shift in ab-
sorption band upon changing the solvents from toluene to ethyl
Cyclic voltammetry studies of all complexes 1–8 were conducted
using BASi epsilon electrochemical workstation. The cyclic voltammetry
measurements were carried out in dry DCM at the scan rate of 50 mV/s
using 0.1 M tetra-butylammoniumperchlorate (TBAP) as supporting
electrolyte. The redox potential data and a comparison of oxidation
Table 2
Emission data and singlet excited state lifetimes of 3–8 in ethyl acetate.
a
b
Compound λex
λem
φf
Stokes λex
shift
(nm) (ns) (ns) (109s−1
(cm−1
τ1
τ2
kr
knr
c
d
(nm) (nm)
)
(109s−1
)
)
3
4
5
6
7
8
480
458
418
480
432
415
625
605
560
611
533
503
0.015 4833
0.013 7000
0.014 5507
0.012 4466
0.022 4657
0.021 3648
445
445
405
445
445
405
3.48
3.90
1.99
4.01
–
–
–
–
0.004
0.003
0.007
0.003
0.283
0.253
0.495
0.246
0.361
0.374
2.71 0.50 0.008
2.62 0.08 0.008
a
b
c
Excitation wavelength.
Laser source excitation wavelength.
Data collected at their respective emission maxima.
d
Fig. 3. The comparison of absorption spectra of Zn(II) complexes 4, 6, 7 and 8 in toluene.
These data fitted to biexponential curve.

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S. Das, I. Gupta / Inorganic Chemistry Communications 60 (2015) 54–60
Fig. 4. The photographs of 3–8 dissolved in DCM; (a) under ambient light, (b) under UV light. Absorbance was fixed at similar values for all samples.
waves are summarized in Table 3 and Fig. 7 respectively. All the com-
plexes 1–8 showed only reversible or quasi reversible one electron oxi-
dation wave. Also, none of the complex showed any redox wave in
negative potential window during electrochemical analysis. All the
metal complexes 1–8 have shown ligand centric electrochemistry
which is consistent with the literature reports [28,29]. Complex 1
exhibited one reversible oxidation peak at 1.282 V, which was
higher than the reported oxidation potentials of bis-dipyrrinato nickel
complex (0.80 and 1.09 V) [30]. Complex 2 showed reversible oxidation
peak at 1.145 V, whereas the reported ferrocynyldipyrrinato-cobalt
complex exhibited oxidation waves at 0.364 V corresponding to
Fc/Fc+ based oxidations [31]. Same group reported oxidation
potential of bis-ferrocenyldipyrrinato palladium complex at 0.349 V,
which corresponded to meso-ferrocenyl unit in the complex [31]. The
palladium complex 3 exhibited quasi-reversible oxidation waves at
1.361 V (Table 3). In the series of all homoleptic type complexes
highest oxidation potential was found in the case of palladium
complex. On the other hand, lowest value of oxidation potential
was found in the case of indium complex 5 at 1.08 V (Table 3). For
mononuclear zinc complexes 4 and 6 and binuclear complexes 7
and 8, only one reversible oxidation wave was observed (Table 3).
The corresponding bis-dipyrrinato zinc complex showed two
oxidation waves at 0.81 and 0.94 V and no reduction waves were ob-
served for the corresponding oxidation processes [24]. On the other
hand the complexes 1–8 described in this report did not show any
reduction peaks in the negative potential window. All the complexes
1–8 exhibited higher oxidation potentials than their reported corre-
sponding metal–dipyrrinato complexes, thus making them difficult
to oxidize.
In summary, a series of triphenylamine substituted dipyrrinato
metal complexes have been presented. Both homoleptic and
heteroleptic type complexes as well as binuclear metal complexes
have been synthesized and characterized. The presence of bulky
electron rich triphenylamine moiety on the dipyrrinato ligands affected
the photophysical properties of the complexes. The absorption maxima
were blue shifted for Ni(II), Zn(II), Pd(II), and In(III) dipyrrinato com-
plexes. Except for complex 8, all the complexes (3–7) exhibited red
shifted emission maxima and reduced quantum yields with respect to
their corresponding phenyl substituted metal complexes. Descent
Stokes shifts (from 3648 to 7000 cm−1) were observed for complexes
3–8. The luminescent complexes of zinc and indium have potentials to
be used in organic photovoltaics as sensitizers with appropriate
functionality.
Fig. 6. Fluorescent lifetime decay profiles of complexes 4, 6 (λex = 445 nm) and 5
(λex = 405 nm) in ethyl acetate.
Table 3
Electrochemical redox data (V) of complexes 1–8 in DCM,
containing 0.1 M TBAP as supporting electrolyte recorded
at 50 mV/s scan speed (V vs SCE).
Compound
Eox (V vs SCE)
1
2
3
4
5
6
7
8
1.282
1.145
1.361
1.286
1.088
1.17
1.268
1.209
Fig. 5. The comparison of normalized emission spectra of 3–6 and 7–8 (inset) in ethyl
acetate.

S. Das, I. Gupta / Inorganic Chemistry Communications 60 (2015) 54–60
59
Fig. 7. Cyclic voltammograms of complexes 1–8 in DCM, containing 0.1 M TBAP as supporting electrolyte recorded at 50 mV/s scan speed (V vs SCE).
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Funding for this research was provided by CSIR Govt. of India, Grant
No: 02(0055)/12/EMR II. IG acknowledges IIT Gandhinagar for providing
infrastructure and IIT Bombay, Department of Chemistry for CV studies.
SD thanks IIT Gandhinagar for the fellowship.
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The experimental section and selected characterization data like ESI-
MS, 1HNMR spectra, fluorescence decay profiles and absorption spectra
of reported compounds are available as electronic supplementary mate-
rial. Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2015.07.019.
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