Bis(porphyrin)-anthraquinone triads: Synthesis, spectroscopy, and photochemistry

Article abstract of DOI:10.1021/jp312134a

Molecular triads based on bis(porphyrin)-anthraquinone having azomethine bridge at the pyrrole-β position have been designed and synthesized. Both free-base AQ-(H₂)₂ and zinc AQ-(Zn)₂ triads are characterized by elemental analysis, MALDI-MS, ¹H NMR, UV-visible, and fluorescence spectroscopy (steady-state and time-resolved) as well as electrochemical method. The absorption spectra of both Soret and Q-bands of the triads are red-shifted by 12-20 nm with respect to their monomer units. The study supported by theoretical calculations manifests that there exists a negligible electronic communication in the ground state between the donor porphyrin and acceptor anthraquinone of these triads. However, interestingly, both the triads exhibit significant fluorescence emission quenching (51-92%) of the porphyrin emission compared to their monomeric units. The emission quenching is attributed to the excited-state intramolecular photoinduced electron transfer from porphyrins to anthraquinone. The electron-transfer rates (k _ET) of these triads are found in the range 1.0 × 10⁸ to 7.7 × 10⁹ s^-1 and are found to be solvent dependent.

Full text of DOI:10.1021/jp312134a

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Article
Bis-Porphyrin-Anthraquinone Triads:
Synthesis, Spectroscopy and Photochemistry
Lingamallu Giribabu, P.Silviya Reeta, Ravi Kumar Kanaparthi, Malladi Srikanth, and Yarasi Soujanya
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp312134a • Publication Date (Web): 19 Mar 2013
Downloaded from http://pubs.acs.org on March 27, 2013
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The Journal of Physical Chemistry
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BisꢀPorphyrinꢀAnthraquinone Triads: Synthesis, Spectroscopy and
Photochemistry
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L. Giribabu*,a, P. Silviya Reetaa, Ravi Kumar Kanaparthia, Malladi Srikanthb, Y.
Soujanyab
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a
Inorganic & Physical Chemistry Division, CSIRꢀIndian Institute of Chemical Technology,
Habsiguda, Hyderabadꢀ500007, (A.P), India.
b
Molecular Modelling Group, Indian Institute of Chemical Technology, Hyderabadꢀ500067, India.
Abstract
Molecular triads based on bisꢀporphyrinꢀanthraquinone having azomethineꢀbridge at
the pyrroleꢀβ position have been designed and synthesized. Both freeꢀbase AQꢀ(H2)2 and zinc
AQꢀ(Zn)2 triads are characterized by elemental analysis, MALDIꢀMS, 1HꢀNMR, UVꢀVisible
and fluorescence spectroscopy (steadyꢀstate and timeꢀresolved) as well as electrochemical
method. The absorption spectra of both Soret and Qꢀbands of the triads are redꢀshifted by 12ꢀ
20 nm with respect to their monomer units. The study supported by theoretical calculations
manifest that there exists a negligible electronic communication in the ground state between
the donor porphyrin and acceptor anthraquinone of these triads. However, interestingly, both
the triads exhibit significant fluorescence emission quenching (51ꢀ92%) of the porphyrin
emission compared to their monomeric units. The emission quenching is attributed to the
excited state intramolecular photoinduced electron transfer from porphyrins to anthraquinone.
The electron transfer rates (kET) of these triads are found in the range 1.0 x 108 to 7.7 x 109 sꢀ1
and are found to be solvent dependent.
Key Words: Porphyrin, Anthraquinone, Triad, Intramolecular, Electron transfer, Timeꢀ
resolved.
Corresponding author: giribabu@iict.res.in, Phone: +91ꢀ40ꢀ27191724, Fax: +91ꢀ40ꢀ27160921.
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1. Introduction
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Photosynthetic reaction centres, which convert solar energy into useful chemical energy,
consist of a protein matrix and donorꢀacceptor redox active pigments. During photosynthesis,
the primary electron transfer (ET) step occurs from a porphyrinꢀbased complex1ꢀ5 and
subsequently, the excited porphyrin–base complex donates an electron to (bacterio)
chlorophylls or their corresponding dimers and then to bacteriopheophytins and quinone
acceptors QA and QB6, resulting in a longꢀlived, chargeꢀseparated state. Apart from this, the
utmost efficiency with which the chlorophyllꢀquinone antenna harvest the solar energy has
also imparted a special interest in designing a wide variety of efficient porphyrin based solar
cells as well.7 Numerous model molecules have been reported in the literature for better
understanding or mimicking the natural photosynthesis.8ꢀ13
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Covalently/nonꢀcovalently linked porphyrin–quinone donorꢀacceptor (DꢀA)
molecules have been studied extensively as model compounds for the lightꢀinduced charge
separation step to get insight into the primary event of the natural photosynthesis.14ꢀ18 Many
current investigations are concerned with porphyrin–quinone molecules in order to
understand the role of distance, orientation, energetic and medium in determining the rate of
intramolecular photoinduced electron transfer (PET).19,20 Most of these studies were mainly
employed either throughꢀspace (πꢀπ interactions) and/or throughꢀbond (superexchange)
mechanism. We are particularly interested in throughꢀbond mechanism because the PET in
many D–A systems is known to be dominated by throughꢀbond interactions.17,21ꢀ23 One must
note that, the spacer between the donor and acceptor play an important role in throughꢀbond
mechanism. As a part of our continuing interest in studying the PET processes,24ꢀ26 hereby,
we report design, synthesis, spectral (UVꢀVisible, MALDIꢀMS and 1H NMR) and
electrochemical characterization of bisꢀporphyrinꢀanthraquinone based molecular triads.
Further, photophysical properties of these newly synthesized azomethineꢀbridged bisꢀ
porphyrinꢀanthraquinone triad AQꢀ(H2)2 and its zinc derivative AQꢀ(Zn)2 have been studied
systematically.
2. Experimental
2.1. Materials.
Commercially available reagents and chemicals were used in the present investigations.
Analytical reagent grade solvents were used for synthesis, and distilled laboratory grade
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solvents were used for column chromatography. Dry chloroform and dichloromethane were
prepared by argonꢀdegassed solvent through activated alumina columns. Nitrogen gas
(oxygenꢀfree) was passed through a KOH drying column to remove moisture. Neutral
Alumina (mesh 60ꢀ325, Brockmann activity1) was used for column purification. All the
reactions were carried out under nitrogen or argon atmosphere using dry degassed solvents
and the apparatus was shielded from ambient light.
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2.2. Synthesis.
5,10,15,20ꢀtetraphenyl porphyrin (H2L1), 5,10,15,20ꢀtetraphenyl porphyrinato zinc(II)
(ZnL1), 2ꢀformylꢀ5,10,15,20ꢀtetraphenyl porphyrin (H2L2) and 2ꢀformylꢀ5,10,15,20ꢀ
tetraphenyl porphyrinato zinc(II) (ZnL2) were synthesized as per the reaction procedures
reported in the literature.27,28
2.2.1. Synthesis of free base triad, AQꢀ(H2)2: This compound was synthesized by following
Schiff’s base reaction. 1,2ꢀdiaminoanthraquinone (71.4 mg, 0.39 mmol) and H2L2 (50 mg,
0.07 mmol) were dissolved in 50 ml of dry toluene. The resultant reaction mixture was
refluxed for 6 h under inert atmosphere. The solvent toluene was removed under reduced
pressure. The obtained solid material was subjected to neutral alumina column and eluted
with CHCl3ꢀHexane (8:2%, v/v). The solvent front running band was the unreacted H2L2 and
the second brown colour band was the desired compound. The solvent was removed under
vacuum and recrystallized from CH2Cl2ꢀhexane to get desired product in 60% isolated yield.
Anal. Calcd. for C104H66N10O2 % (1487.70): C, 83.96; H, 4.47; N, 9.41. Found C, 83.87; H,
4.40; N, 9.35. MALDIꢀMS (TOF) [C104H66N10O2]+: 1486 (10%), [C104H66N10O2
–
C46H32N4]2+ 861 (50%). IR: νmax (KBr)/cmꢀ1 3459, 2918, 1652, 1584, 1429, 1275. H NMR
(CDCl3, δppm): 9.50 (s, 2H); 8.75ꢀ8.88 (m, 12H); 8.16ꢀ8.35 (m, 18H,); 7.65ꢀ7.86 (m, 24H);
7.60 (m, 2H); 7.52 (m, 2H); 6.75 (d, 2H), and ꢀ2.48 (b, 4H).
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2.2.2. Synthesis of zinc triad, AQꢀ(Zn)2: This compound was synthesized by following
analogous reaction procedure adopted for AQꢀ(H2)2, in which AQ and ZnL2 were subjected
to a condensation reaction. Anal. Calcd. for C104H62N10O2Zn2 % (1614.45): C, 77.37; H, 3.87;
N, 8.68. Found C, 77.47; H, 3.85; N, 8.655. MALDIꢀMS (TOF) [C104H66N10O2Zn ꢀ
C46H32N4Zn]+: 926 (100%). IR: νmax (KBr)/cmꢀ1 3455, 2929, 1634, 1521, 1278. H NMR
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(CDCl3, δppm): 9.63 (s, 2H); 8.83ꢀ8.99 (m, 12H); 8.16ꢀ8.35(m, 18H,); 7.68ꢀ7.89 (m, 24H);
7.49 (m, 2H); 7.43 (m, 2H); 6.66 (d, 2H).
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2.3. Methods and Instrumentation.
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1HꢀNMR spectra were recorded on a 500MHz INOVA spectrometer. Cyclic and differentialꢀ
pulse voltammetric measurements were performed on a PCꢀcontrolled electrochemical
analyzer (CH instruments model CHI620C). All these experiments were performed with 1
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mM concentration of compounds in dichloromethane at a scan rate of 100 mV s-1 in which
tetrabutyl ammonium perchlorate (TBAP) is used as a supporting electrolyte as documented
in our previous reports.29
2.3.1. Theoretical calculations: Full geometry optimization computations of the AQꢀ(H2)2
and AQꢀ(Zn)2 triads were carried out with the DFTꢀB3LYP method using 6ꢀ31G* basis set
and frequency analysis confirmed that the obtained geometries are to be genuine global
minimum structures. All calculations were performed with the Gaussian G03 (d01) package
on a personal computer.30
2.3.2. Absorption and fluorescence measurements: The optical absorption spectra were
recorded on a Shimadzu (Model UVꢀ3600) spectrophotometer. Concentrations of solutions
are ca. to be 1 x 10ꢀ6 M (porphyrin Soret band) and 5 x 10ꢀ5 M (porphyrin Qꢀbands). Steadyꢀ
state fluorescence spectra were recorded on a Fluorologꢀ3 spectrofluorometer (Spex model,
Jobin Yvon) for solutions with optical density at the wavelength of excitation (λex
Fluorescence quantum yields ( ) were estimated by integrating the fluorescence bands and by
using either [H2L1] ( = 0.13 in CH2Cl2), [ZnL1] (
= 0.036 in CH2Cl2) as reference
) ≈0.05.
φ
φ
φ
compounds.31 Fluorescence lifetime measurements were carried on a picosecond timeꢀ
correlated single photon counting (TCSPC) setup (FluoroLog3ꢀTriple Illuminator, IBH
Horiba Jobin Yvon) employing a picosecond light emitting diode laser (NanoLED, λex=405
nm) as excitation source. The decay curves were recorded by monitoring the fluorescence
emission maxima of the triads (λem=650nm). Photomultiplier tube (R928P, Hamamatsu) was
employed as the detector. The lamp profile was recorded by placing a scatterer (dilute
solution of Ludox in water) in place of the sample. The width of the instrument response
function (IRF) was limited by the full width at half maxima (FWHM) of the excitation
source, ~625 ps at 405 nm. Decay curves were analyzed by nonlinear leastꢀsquares iteration
procedure using IBH DAS6 (version 2.3) decay analysis software. The quality of the fits was
judged by the χ2 values and distribution of the residuals.
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N
N
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N
M
O
O
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N
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H
H
NH2
CHO
O
NH2
N
N
N
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M
dry Toluene, reflux, 6hr
N
N
N
N
O
M
M = 2H, Zn
Figure 1: Synthetic scheme of AQꢀ(H2)2 and AQ(Zn)2 triads.
Both the triads AQꢀ(H2)2 and AQ(Zn)2 were synthesized by using Schiff base
condensation reaction as shown in Figure 1. The desired compounds were obtained with
moderate yields (~ 60%) in both cases after column chromatography purification and
subsequent recrystallization. Preliminary characterization of the both triads was carried out
by MALDIꢀTOF MS and UVꢀVisible spectroscopic methods. The mass spectrum of AQꢀ
(H2)2 showed a peak at m/z = 1486 ([M]+, C104H66N10O2) ascribable to the molecularꢀion
peak. Subsequent peak at m/z = 861 can be ascribed to the detachment of one ([Mꢀ
C104H66N10O2 – C46H32N4]2+ freeꢀbase porphyrin subunit from the triad. A similar type of
fragmentation was also observed in case of AQꢀ(Zn)2. The stretching peak at 1548 & 1521
cmꢀ1 in IR spectra of AQꢀ(H2)2 and AQꢀ(Zn)2, respectively, conforms the presence of C=N
bond in both the triads (S1&S2).
*
b
*
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ꢀ2.5
ꢀ3.0
10.0 9.5
9.0
8.5
8.0
ppm (δ)
7.5
7.0
6.5
6.0
Figure 2: 1HꢀNMR spectra of a) AQꢀ(H2)2 and b) AQꢀ(Zn)2 in CDCl3. The peak labeled with
asterisk (*) is due to solvent.
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1HꢀNMR spectral data of both the triads have been summarized in the experimental
section and the spectra of AQꢀ(H2)2 and AQꢀ(Zn)2 are shown in Figure 2. Comparison of
these spectra with those of individual constituents (H2L1, ZnL1and AQ) reveals that there is
no appreciable change in resonance positions of various protons present on porphyrin
macrocycle and that of anthraquinone part of the triad AQꢀ(H2)2, whereas in case of AQꢀ
(Zn)2, subtle shift of protons probably due to the interaction of anthraquinone part of the triad
with zinc porphyrin is observed.17 This is reasonable if one considers that the two aromatic
rings are in ‘trans’ configuration with respect to the C=N spacer, avoiding steric interaction.
This is confirmed by energy minimization study of the triads which is discussed in section
3.3.
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AQ
H2L1
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ZnL
X 10
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AQꢀ(Zn)2
X 10
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0
300
400
500
600
700
300
400
600
700
Wavelen5g0t0h (nm)
Wavelength(nm)
Figure 3: Absorption behaviour of AQ, H2L1, AQꢀ(H2)2, ZnL1 and AQꢀ(Zn)2 in CH2Cl2.
Table 1: UVꢀvisible and electrochemical data of AQ, H2L1, AQꢀ(H2)2 and AQꢀ(Zn)2.
Absorption, λmax, nm (log ε, Mꢀ1 cmꢀ1)a
Porphyrin Bands AQ bands
Potential V vs. SCEb
Compound
Reduction
Oxidation
416 515 549
(6.06) (4.27) (3.90) (3.77) (3.69)
592 646
ꢀ
ꢀ
H2L1
ꢀ1.22, ꢀ1.56
1.01, 1.25
419 547 586
(6.17) (4.34) (3.60)
ZnL1
ꢀ1.45,ꢀ1.71
ꢀ0.96, ꢀ1.38
0.72, 1.06
ꢀ
ꢀ
256
(7.40)
AQ
ꢀ0.86, ꢀ1.10,
ꢀ1.50
1.00, 1.27
0.89, 1.21
433 527
(5.40) (4.69) (4.39) (4.34) (3.77)
580 608
668
257
(4.91)
AQꢀ(H2)2
AQꢀ(Zn)2
ꢀ0.83, ꢀ1.28
ꢀ1.54
434 526 560 613
(5.6) (4.72) (4.71) (4.72)
257
(4.90)
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aSolvent CH2Cl2, Error limits: λmax, ± 1 nm, log ε, ± 10%. CH2Cl2, 0.1 M TBAP; Glassy carbon working
electrode, Standar calomel electrode is reference electrode, Pt electrode is auxillary electrode. Error limits, E1/2
±0.03 V.
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3.1. Ground state properties.
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Electronic absorption profile of AQꢀ(H2)2 and AQꢀ(Zn)2 have been measured in
CH2Cl2 and the spectral behaviour is compared with individual monomer units, H2L1 and
ZnL1 (Figure3). Spectroscopic data which include wavelength of maximum absorbance
(λmax), molar extinction coefficients (log ε) of both triads and their constituent individual
components are summarized in Table 1. As shown in figure 3, the absorption spectrum is a
combination of constituent porphyrin and anthraquinone. Both triads exhibit characteristic
intense Soret band (~ 430 nm), which is an a1u(π)/eg(π*) electronic transition, assigned to the
second excited state (S2) and Qꢀbands (500ꢀ700nm) originated from a2u(π)/eg(π*) electronic
transition, attributed to the first excited state (S1). These characteristic bands are similar to
many other dimers or trimers.32,33 The band centered at ~260 nm is largely due the
anthraquinone moiety as the porphyrin have very limited/poor absorption at that region (230ꢀ
350 nm). It is also observed that the bands are redꢀshifted and slightly broadened compared to
simple H2L1 and ZnL1 porphyrin units. Similar such redꢀshifts and broadening were observed
in other pyrroleꢀβ substituted porphyrins as well.28,34,35 This spectral redꢀshift (20ꢀ30nm) can
be explained based on the extended πꢀ electron conjugation of porphyrin unit to the
anthraquinone, from there to the other porphyrin unit through the two imine (ꢀC=Nꢀ) links.
The broadening is more pronounced at the longer wavelength region (>600 nm) which might
be due to charge or electron transfer because of donorꢀacceptorꢀdonor type of molecular
arrangement. However, the spectral broadening does not affect further with the solvent
polarity indicates the lack of ground state charge or electron transfer (figure S9). So, the
broadening which we observe perhaps is due to conformers originated from different
orientations of porphyrin units around the two –CꢀNꢀ bonds.36
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Figure 4: Differential pulse voltammogrames of AQ, H2L1, AQꢀ(H2)2 and AQꢀ(Zn)2. in CH2Cl2 with
0.1 M TBAP.
Figure 4 illustrates the differential pulse voltammogrammes of AQ, AQꢀ(H2)2 and
AQꢀ(Zn)2 and Table 1 summarises the redox potential data (CH2Cl2 and 0.1 M TBAP) of the
D2ꢀA systems investigated in this study along with that of the corresponding individual
constituents. Figure 4 and data in Table 1 indicates that each new compound investigated
shows three reduction peaks and two oxidation peaks under the experimental conditions
employed in this study. Wave analysis suggested that, in general, while the first two
reduction steps and first two oxidation steps are reversible (ipc/ipa = 0.9ꢀ1.0) and diffusionꢀ
controlled (ipc/ν1/2 = constant in the scan rate (
Ep = 60ꢀ70 mV; Ep = 65 ± 3 mV for ferrocenium/ferrocene couple) reactions, the
subsequent steps are, in general, either quasiꢀreversible (Epa – Epc = 90ꢀ200 mV and ipc/ipa =
0.5ꢀ0.8 in the scan rate (
) range 100ꢀ500 mV sꢀ1) or totally irreversible. The peaks occurring
ν) range 50ꢀ500 mV/s) oneꢀelectron transfer
(ꢀ
ꢀ
ν
at anodic potentials are ascribed to successive oneꢀelectron oxidations of the porphyrin parts
of AQꢀ(H2)2 and AQꢀ(Zn)2. As seen in Figure 4, the differential pulse voltammogrames of
AQꢀ(H2)2 and AQꢀ(Zn)2 in CH2Cl2 contains three reduction peaks corresponding to the
reduction of porphyrins (H2/Zn)/anthraquinone moiety. Comparing the reduction potentials of
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AQꢀ(H2)2 and AQꢀ(Zn)2 with its reference compounds AQ, H2L1 and ZnL1, it was found
that the reduction potentials for the first peak purely belongs to the reduction of
anthraquinone part of both the triads. The second reduction belongs to the first reduction of
porphyrin moiety and third reduction peak belongs to the second reduction of porphyrin and
anthraquinone of the D2ꢀA systems.
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The spectroscopic and electrochemical features described above suggested that in the
ground state electronic communication between porphyrin and anthraquinone chromphores is
quite negligible in these new DꢀA systems. More importantly, one can exploit the excited
state properties by selective excitation of the individual chromophore units.
1
H L
(a)
2
(b)
4
3
2
1
0
AQꢀ(H )
2 2
1
Zn L
2
AQꢀZn
2
550 600 650 700 750
600
700
Wavelength (nm)
Figure 5: Fluorescence spectra (λex= 420 nm) of equiabsorbing solutions of H2L1
and AQꢀ(Zn)2 (OD λex = 0.05) in cyclohexane (a) and CH2Cl2 (b).
, , AQꢀ(H2)2
ZnL1
3.2. Singlet state properties.
Figure 5 illustrates the steady state fluorescence spectra of AQꢀ(H2)2 and AQꢀ(Zn)2
triads along with their individual constituents H2L1 and ZnL1 measured in cyclohexane and
dichloromethane (CH2Cl2). Corresponding emission maxima and quantum yields have been
collected in Table 2. The fluorescence emission maxima of free base AQꢀ(H2)2 and AQꢀ
(Zn)2 are redꢀshifted ~25 nm and ~4 nm, respectively with respect to isolated free base
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porphyrin and zinc porphyrin units. A similar redꢀshift in emission maxima was also
observed in other pyrroleꢀβ substituted porphyrins.34,35 The redꢀshift of these triads is due to
the more polar nature of the emitting state originated from the donorꢀacceptorꢀdonor (DꢀAꢀD)
type molecular arrangement in which the anthraquinone is acceptor and porphyrin acts as
donor. Interestingly, we observe remarkable decrease in porphyrin emission intensity when
equiabsorbing solutions of triads were excited at 420 nm (λmax, where prophyrin absorbs
predominantly). The posiblity of selfꢀaggregation of porphyrins which leads to quenching of
fluorescence intensity (as a result of non radiative path in the excited state) can be ruled out
as all experiments were carried out with very dilute solutions (10ꢀ6 M). Moreover, we could
not observe substantial changes in the absorption spectra as it happens in few porphyrin
systems.37
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Table 2: Fluorescence data.a
λem, nm (φ, %Q)b
Compound
Cyclohexane
Toluene
CH2Cl2
CH3CN
λex = 420 nm
653, 719
(0.122)
653, 719
(0.137)
652,716
(0.130)
651,716
(0.130)
H2L1
595, 642
598, 647
646, 596
657, 603
ZnL1
(0.037)
(0.046)
(0.036)
(0.048)
671, 742
674 , 740
676,738
675, 737
AQꢀ(H2)2
(0.044, 64)
670,737
(0.042, 69)
674, 737
(0.035, 75)
651
(0.015, 89)
675, 739
AQꢀ(Zn)2
(0.022, 52)
(0.020,51)
(0.005, 86)
(0.004, 92)
b
aError limits: λex, ± 2 nm, φ ± 10%. Q is defined in equation 2 (see text).
The excited state energy transfer (EET) or photoinduced electron transfer (PET) could
also be responsible for the decrease in fluorescence intensity. For EET, the emission spectra
of donor has to be well matched with absorption of the acceptor. It is evident from the
Figures 3 & 5 that the EET is not possible between anthraquinone and porphyrin due to lack
of proper spectral overlap. Alternative pathway for emission quenching is the intramolecular
PET from the singlet state of porphyrin donor to the ground state of anthraquinone acceptor.
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The E0ꢀ0 (0ꢀ0 spectroscopic transition energy) values of porphyrin part of triads, 1.86±0.05
eV, 1.96±0.05 eV for AQꢀ(H2)2 and AQꢀ(Zn)2 respectively as estimated from overlap of
their absorption and emission spectra were found to be in the same range as the E0ꢀ0 values of
free H2L1 ZnL1 38
/ . The change in free energy for PET from the porphyrin to the
anthraquinone can be calculated using the equation,
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ꢀG(1Por→AQ) = ECT(Por+AQꢀ) – E0ꢀ0(Por)
(1)
ꢀG was found to be ꢀ0.096 & ꢀ0.24 eV for AQꢀ(H2)2
&
AQꢀ(Zn)2 respectively, when excited
at 420 nm. This ꢀG clearly indicates that there could be a possibility of PET from free
base/metal porphyrin to anthraquinone.
Fluorescence quantum yields of the triads and individual constituents have been
estimated (Table 2) by comparing the emission curves of reference compounds (i.e., H2L1 or
ZnL1, φ = 0.13±0.01 and 0.036±0.001, respectively in CH2Cl2 solvent) with that of triads. It
was found that the fluorescence quantum yield (φ) of AQꢀ(H2)2 and AQꢀ(Zn)2 are 0.042
and 0.022 respectively in CH2Cl2 solvent. The quenching efficiency values (Q) can be
estimated from,
1
1
φ
(H2 L )or(ZnL ) −φ[AQ ꢀ (H2 )2 ]or[AQ ꢀ (Zn)2 ]
Q =
(2)
1
1
φ
(H2 L )or(ZnL )
Where φ(H2L1)/(ZnL1) and φ( AQꢀ(H2)2
of H2L1
/
AQꢀ(Zn)2) refer to the fluorescence quantum yields
/
ZnL1 and the triads respectively; (λex = 420 nm). As the static dielectric constant of
the solvent is increased, the fluorescence quantum yield decreased gradually (Table 2),
indicate the excited state electron transfer mechanism. For example, quenching efficiencies in
cyclohexane and toluene are less compared to the CH2Cl2 and CH3CN. In general, the PET
process is accelarated by polar solvents than the nonꢀpolar solvents, so the present results are
in consistant with the literature.39 Efficient quenching of the AQꢀ(Zn)2 triad relative to that
of the AQꢀ(H2)2 can be directly attributed to the more exothermic value of ꢀGPET of AQꢀ
(Zn)2
.
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Further evidence of the intramolecular PET process has been evaluated by measuring
the excited state decay curves. Figure 6 illustrates excited state decay curves of triads,
collected in CH2Cl2 solvent upon excitation at 405 nm and the decay parameters collected in
four different solvents are shown in Table 3. From the data, it is clear that the fluorescence
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10000
(a)
1000
100
Prompt
TPP
10
10000
AQꢀ(H
)
2 2
(b)
ZnTPP
AQꢀZn
1000
100
10
2
10
20
30
40
time (ns)
Figure 6: Fluorescence decay (λex= 405 nm, λem= 650 nm) in cyclohexane (a) and CH2Cl2 (b).
Table 3: Fluorescence decay parameters† and electron transfer rate constants (kET, Sꢀ1). ‡
(τ, ns (A%), kET, sꢀ1)
Compound
H2L1
Cyclohexane
Toluene
CH2Cl2
CH3CN
9.47
9.71
8.81
9.40
1.93(93)
2.07(7)
2.07(95)
1.11(5)
1.76(95)
1.17(5)
1.72(89)
2.86(11)
ZnL1
1.07(44)
9.12(35)
5.14(21)
8.3×108
3.08(57)
9.26(23)
4.60 (20)
2.2×108
0.13(66)§
6.36(12.0)
1.57(22)
7.6×109
0.69 (57)
7.60 (32)
2.28 (11)
1.3×109
AQꢀ(H2)2
1.60(85)
1.16(6)
1.48(68)
2.04(11)
0.33(65)§
2.46(13)
0.60(51)
1.82(26)
AQꢀZn2
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0.57(9)
1.0×108
0.29(21)§
1.9×108
1.22(22)
2.4×109
1.00(23)
1.1×109
6
7
8
9
†
‡
All lifetimes are in nanoseconds (ns). Error limits of τ and kET ~ 10%. Values in parenthesis are relative
amplitude of corresponding decay component. § These short lifetime components are limited by the instrument
response function (IRF).
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lifetimes,τ (λex = 405 nm and λem = 650 nm) of both AQꢀ(H2)2 and AQꢀ(Zn)2 are decreased
/
in all solvents when compared to H2L1 ZnL1. In all solvents, AQꢀ(H2)2 decay curves are
fitted with a triꢀexponential expression and major component is having shorter lifetime which
is attributed to the deactivation (quenching) of the excited porphyrin by anthraquinone.17 The
other two lifetime components are assigned either to an unquenched decay or decay of the
porphyrinꢀhydroquinone generated by porphyrin sensitized photoreduction40 and different
conformers of the molecule with flexible linker in solutionꢀstate41,42. On the other hand, AQꢀ
(Zn)2 decay curves are fitting into a threeꢀexponential expression composed of one major (see
table 3, shorter lifetime) component and two minor components (shorter and longer lifetime).
From this data, the shorter lifetime with higher amplitude and longer lifetime components
have the same origin as in AQꢀ(H2)2, but the additional shorter lifetime component may be
due to the nonꢀcovalent interaction of oxygen lone pair of electrons (anthraquinone) with zinc
metal present in other porphyrin of AQꢀ(Zn)2. We believe that this anthraquinoneꢀligated
state is responsible for the additional shortꢀlived component obtained from the TCSPC
experiments.17 In all four investigated solvents, it is observed that the shorter lifetime of AQꢀ
(H2)2 have much larger amplitudes relative to the longer lifetime components. Similar
features are also seen in AQꢀ(Zn)2 triad.
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Figure 7: Photoinduced electronꢀtransfer scheme of bisꢀporphyrinꢀanthraquinone triads.
Further, rate constant (kET) for the charge transfer state Por+.AQꢀ. have been calculated
using eq (3) and summarized in Table 3.
kET = (1/τf) ꢀk
(3)
Where k is the reciprocal of the lifetime of the H2L1
/
ZnL1, τf is the lifetime of AQꢀ
(H2)2/
AQꢀ(Zn)2. The solventꢀdependent kET values are in the range of 1.0 x 108 to 7.7 x 109 sꢀ
1. The observed general increase of the kET values with increasing polarity of the solvent is
consistent with the participation of a charge transfer state in the excited state deactivation of
the porphyrin components of these D2ꢀA systems. Figure 7 shows the energy levels of the
porphyrin singlet excited state and the charge transfer state that participates in the PET
processes in bisporphyrin–anthraquinone triads. In order to establish the intramolecular PET
process of these two triads, we have carried out computational calculations to generate
frontier molecular orbitals of both triads.
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Figure 8: Frontier molecular orbitals, HOMO and LUMO of a) AQꢀ(H2)2 and b) AQꢀ(Zn)2
optimized at the B3LYP/6ꢀ31G(d) level.
3.3. Theoretical calculations.
In general, intramolecular electron transfer operates either throughꢀspace or throughꢀ
bond mechanism. In the former mechanism, (through space) electron transfer occurs from
HOMO (the highest occupied molecular orbital) of the donor moiety to LUMO (the lowest
unoccupied molecular orbital) of acceptor moiety when they are in close proximity and
(depends strongly) on the orientation of donor and acceptor moieties.43ꢀ44 The later
mechanism (through bond) is rather less sensitive to geometrical position/orientation of
donor and acceptor moieties but strongly depends on the number of atoms connecting them.
In this case, electronic states of spacer (moieties between the donor and the acceptor) will
mix with those of the donor and acceptor, thus the throughꢀbond mechanism is synonymous
with superexchange.45,46 In order to examine the nature and position of frontier molecular
orbitals, density functional theory (DFT) calculations have been carried. Complete
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optimization without any constraint was done using B3LYP and 6ꢀ31g(d) basis set to see the
structural parameters of the two systems, AQꢀ(H2)2 and zinc AQꢀ(Zn)2
.
6
7
8
9
Figure 8 displays the HOMO and LUMO distribution and geometrical optimized
structures of the triads AQꢀ(H2)2 and AQꢀ(Zn)2, obtained by DFT calculations. From Figure
8, it is clear that the HOMO is mainly distributed at porphyrin moiety while LUMO mainly
distributed at anthraquinone moiety. The energies of HOMO and LUMO for triads AQꢀ(H2)2
and AQꢀ(Zn)2 were found to be ꢀ4.958 & ꢀ5.054 eV and ꢀ2.578 & ꢀ2.511 eV, respectively.
From the geometrical optimized structures (Fig.8), the two porphyrin rings are away from
each other as the connecting bonds between porphyrin ring and anthraquinone moieties are
not long enough to have πꢀπ interaction. Similarly, the porphyrin and anthraquinone rings
are also far apart which clearly suggest that there is very less chance to have facial interaction
between donor porphyrin and acceptor anthraquinone. Moreover, phenyl rings at ꢀmeso
positions are found to be perpendicular to the plane of porphyrin ring, which prevent the
stacking interaction. (All these observations corroborate the prevalence of throughꢀbond
intramolecular electron transfer mechanism in the current systems). As discussed in earlier
sections, the fluorescence quantum yields, lifetimes, and electron transfer rates of these triads
are found to be strongly dependent on solvent polarity. One cannot expect this solvent
dependency if electron transfer occurs throughꢀspace mechanism. Therefore the results are
best compatible with “throughꢀbond” mechanism for electron transfer pathway.
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4. Conclusions
In conclusion, bisporphyrinꢀanthraquinone triads having azomethineꢀbridge at
pyrroleꢀβ position of porphyrin macrocycle were successfully shown to be a model
photosynthetic reaction center to simulate the electron transfer from chlorophylls to the
electron acceptors. The ground state properties indicate that there are no interactions between
porphyrin macrocycles and anthraquinone moieties in both the triads. Steadyꢀstate and timeꢀ
resolved spectroscopy showed that AQꢀ(H2)2 and AQꢀ(Zn)2 undergo rapid intramolecular
electron transfer from singlet state of porphyrin to anthraquinone. The electron transfer rates
are found in the range 1.0 × 108 to 7.7 × 109 sꢀ1 and are solvent dependent.
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Acknowledgement
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6
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We are grateful to Department of Science and Technology (DST, SR/S1/IC21/2008)
for financial support of this work. One of the authors PS is thankful to CSIR for the
fellowship. YS thank BRNS (DAE) for fiancial support.
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Supporting Information
1
Characterization data of the compounds such as IR spectra (Figure S1
NMR (Figures S3 S4 S5 S6) and MALDIꢀMS spectra (Figure S7
spectra of AQꢀ(H2)2 in different solvents (Figure S9), fluorescence behaviour of the
compounds in toluene and acetonitrile (Figure S10 S11), fluorescence decay curves
(Figure S12 S13) and optimized geometries of molecules (Figure S14) are available free of
&
S2), H
,
,
&
& S8), absorption
&
&
charge via the Internet at http://pubs.acs.org.
References and notes
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BisꢀPorphyrinꢀAnthraquinone Triads: Synthesis, Spectroscopy and
Photochemistry
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L. Giribabu*,a, P. Silviya Reetaa, Ravi Kumar Kanaparthia, Malladi Srikanthb, Y.
Soujanyab
Table of Contents
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