Zinc porphyrin-anthraquinonylimidazole supramolecular dyads

Article abstract of DOI:10.1142/S1088424619501943

In this work, the synthesis and spectroscopic characterization of new zinc porphyrin-anthraquinone dyads is proposed. In particular, electron donor units based on zinc meso-tetraphenylporphyrin (ZnTPP) and zinc octaethylporphyrin (ZnOEP) have been coupled

Full text of DOI:10.1142/S1088424619501943

This is a JAM - Just Accepted Manuscript
Journal of Porphyrins and Phthalocyanines
Article Title:
Zinc Porphyrins-Anthraquinonylimidazoles supramolecular dyads
Author(s):
DOI:
Federica Sabuzi, Alessia Coletti, Valeria Conte, Barbara Floris, Pierluca
Galloni
10.1142/S1088424619501943
To be cited as:
Federica Sabuzi, Alessia Coletti, Valeria Conte, Barbara Floris, Pierluca Galloni, Zinc
Porphyrins-Anthraquinonylimidazoles supramolecular dyads, Journal of Porphyrins and
Phthalocyanines, doi: 10.1142/S1088424619501943
Received: 6 November, 2019
Accepted: 5 December, 2019
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Zinc
Porphyrins-Anthraquinonylimidazoles
supramolecular
dyads
#
#
Federica Sabuzi , Alessia Coletti , Valeria Conte, Barbara Floris and Pierluca Galloni*
Department of Chemical Sciences and Technologies, University of Rome Tor Vergata, Via della Ricerca Scientifica snc,
0
0133 Rome, Italy
Dedicated to Professor Roberto Paolesse on the occasion of his 60th birthday
Received date (to be automatically inserted after your manuscript is submitted)
Accepted date (to be automatically inserted after your manuscript is accepted)
ABSTRACT: In this work, the synthesis and spectroscopic characterization of new zinc porphyrins –
anthraquinones dyads is proposed. In particular, electron donor units based on zinc meso-
tetraphenylporphyrin (ZnTPP) and zinc octaethylporphyrin (ZnOEP) have been coupled with differently
substituted anthraquinones as acceptors. The quinone moiety was properly functionalized with imidazole, thus
ensuring porphyrin complexation through zinc ion coordination. Accordingly, absorption and emission
measurements demonstrated that the coordination occurred, and calculated binding constants were in the
3
4
-1
range 6.6∙10 - 3.9∙10 M . Transient absorption spectroscopy for ZnTPP and ZnOEP dyads demonstrated
that the electron transfer occurred, with the formation of the corresponding charge separated state, ZnTPP+-
-
AQ . Moreover, in ZnOEP complexes, a strong correlation between the chain length and flexibility with the
charge separated state lifetime was observed.
KEYWORDS: zinc tetraphenylporphyrin, zinc octaethylporphyrin, anthraquinones, dyads, electron transfer.
*
Correspondence to: Pierluca Galloni, tel +390672594380, fax +390672594328 email galloni@scienze.uniroma2.it
these two authors contributed equally
#

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INTRODUCTION
To mimic the natural photosynthesis has been a goal pursued by scientists for many years. Nowadays, it is common
knowledge that the simplest chemical system has to be formed by at least two moieties, an electron donor and an electron
acceptor unity, connected in a variety of ways. A 2019 review discussed more than 150 papers relative to organic electron
Donor-Acceptor (D-A) dyads and their features (charge separation and recombination, long-lived charge transfer states,
intersystem crossing) [1]. Computational models were developed as a tool for a rational design of covalently linked dyads
[
2]. However, the way to D-A dyads as efficient as natural systems is still long. Porphyrins and zinc porphyrins have been
the donors of choice in most cases, being the molecules closest to the natural system, whereas a wide range of acceptors
were investigated. Apart from fullerenes, for which a plethora of papers appeared (to cite only a few in the last three years
see [3-11]), a wide range of acceptor structures were investigated, from naphthalenediimide [12, 13] to coumarin [14] and
carbazole [15-17] or corrole [18]. Moreover, a number of papers appeared dealing with porphyrin-quinone D-A dyads. As
an example, a free base tetra-meso-substituted porphyrin was coupled to anthraquinone via azo functionality [19]. Later,
1
,4-benzoquinone was directly linked at the meso position of tri-meso-substituted zinc porphyrins [20] or to a cyclohexane
ring as a spacer [21]. More complex were the triads published in 2014, with zinc 1,10-diaryl-porphyrins, bearing a N-
ferrocenyl-4-benzenamide at position 5 and a 4-(2-anthraquinonecarboxylic anilide) at position 15 [22]. Finally, zinc
porphyrin-anthraquinone dyads were prepared with donor and acceptor covalently linked through different spacers [23].
All these systems suffer of the problem of low yields and difficult preparation methodologies. Therefore, considering our
experience in quinoidal systems [24-28] and taking advantage of the coordinating ability of imidazole for the metal of Zn-
porphyrins [29,30], we prepared a series of anthraquinone functionalized with different spacers bearing imidazole, in order
to obtain porphyrin anthraquinone dyads (P-AQ). The donor and acceptor moieties prepared are summarized in Chart 1. In
particular, anthraquinone, which is one the most common quinonic systems, has been chosen as the acceptor, whereas zinc
meso-tetraphenylporphyrin (ZnTPP) and zinc octaethylporphyrin (ZnOEP) were chosen as the donors, so that electronic
and steric effect of substituents could be appreciated. The interaction between the porphyrin and the quinone has been
ensured functionalizing the quinone with imidazole, thus allowing the formation of a metal-ligand coordination bond. Three
different spacers were used to link the imidazole and the anthraquinone scaffold, in order to change the distance between the
donor and the acceptor moieties and the flexibility of the chain. Herein, the study of the photoinduced electron transfer in
such new P-AQ dyads is presented.

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Chart 1. Formulae of porphyrins (donors) and imidazolyl substituted anthraquinones (acceptors) used in P-AQ dyads.
EXPERIMENTAL
General
1H-NMR spectra were recorded on a Bruker Avance 300 MHz spectrometer. 13C-NMR spectra were recorded on Bruker
4
00 MHz spectrometer. LC-MS analysis have been performed with Shimadzu LCMS-2010 EV. UV-visible spectra were
recorded on a SHIMADZU 2450 spectrometer equipped with the UV Probe 2.34 program. Fluorescence spectra were
recorded on a spectrofluorimeter SPEX 1681 Fluorolog, equipped with two double monochromators (excitation and
emission).
Cyclic voltammetries were performed with a Palm Sens instrument using PS Trace software.
Femtosecond pump and probe transient absorption measurements were performed using a Spectra-Physics Hurricane
titanium/sapphire regenerative amplified laser system. The full spectrum pumping tunability was based on an optical
parametric amplifier (Spectra-Physics OPA 800). A residual fundamental light was used for generation of a white light
continuum from a sapphire window (450-1500 nm) as the probe, which was detected with two separated diode array based
spectrometers (OceanOptics and Control Development). The laser output (800 nm) was typically 0.8 íJ pulse-1 (130 fs

Accepted manuscript to appear in JPP
fwhm) with a repetition rate of 1 kHz. The OPA was used to generate excitation pulses at different wavelengths (1.5 íJ
pulse-1). To avoid local heating by the laser beam, the sample cuvette (1 mm) was placed on a moving kinematics stage.
Synthesis
Tetraphenylporphyrin and octaethylporphyrin were used as purchased and treated with zinc acetate to form the
corresponding zinc complexes, ZnTPP and ZnOEP.
Preparation of zinc-5,10,15,20-meso-Tetraphenylporphyrin (ZnTPP). The synthesis of ZnTPP was obtained upon
complexation of the commercially available free base H
31]. A solution of 0.6 g (3.3 mmol) of Zn(OAc) in 3 mL of MeOH was added to a solution of 204 mg (0.33 mmol) H
in 10 mL of CHCl and the mixture was refluxed under stirring for 3 hours. After disappearance of H TPP (TLC analysis),
the reaction mixture was washed three times with water, dried with Na SO and the solvent was reduced to the minimum
volume. After addition of hexane to the concentrated solution, the product crystallized. Yield, 84% (189 mg, 0.28 mmol).
1H-NMR in CDCl
(ppm): 8.96 (s, 8H, pyrrole rings); 8.23 (m, 8H, orto positions of the phenyl rings); 7.78 (m, 12H, para
2
TPP with Zn(OAc)
2
using a slightly modified literature procedure
[
2
2
TPP
3
2
2
4
3
and meta positions of the phenyl rings).
Preparation of zinc-2,3,7,8,12,13,17,18-Octaethylporphyrin (ZnOEP). ZnOEP was synthesized from 77 mg (0.14
2 2
mmol) of the commercially available free base porphyrin H OEP and 0.6 mg of Zn(OAc) (3.3 mmol) following the
procedure described for ZnTPP. To isolate the product, the solvent was evaporated from the reaction mixture and the solid
1
residue was crystallized once from THF-pentane and twice from THF-diethyl ether. Yield, 90% (77 mg, 0.13 mmol). H-
NMR in CDCl
3
(ppm): 10.19 (s, 4H, meso positions of the porphyrin core); 4.14 (q, 16H, J=7.8Hz, methylene groups);
1
.96 (t, 24H, J=7.5Hz, methyl groups).
Preparation of 2-imidazoylmethyl-9,10-anthraquinone (AQ1). 300 mg (1 mmol) 2-bromomethyl-9,10-anthraquinone
and 137 mg (2 mmol) imidazole were dissolved in 20 mL CH Cl previously dried over CaCl , in anhydrous atmosphere.
The solution was kept under stirring overnight and then refluxed for 5 hours. A yellow precipitate was observed that was
recovered by filtration, dissolved in CHCl , and extracted with an aqueous alkaline solution. The organic phase was dried
over Na SO and filtered and the solvent was distilled under reduced pressure. To purify the product, the solid obtained was
dissolved in the minimum amount of CHCl
2
2
2
3
2
4
3
; after addition of hexane, a precipitate formed that was recovered by filtration
1
and further purified by repeating the same procedure. Yield, 40% (110 mg) H-NMR in CDCl
3
(ppm): 8.34 (m, 3H); 8.17
1 2
=2Hz, J
=7Hz); 7.17 (s, 1H); 6.97 (s, 1H); 5.32 (s, 2H). 13C-
(
d, 1H, J=1.5Hz); 7.84 (m, 2H); 7.64 (s, 1H); 7.53 (dd, 1H, J
NMR in CD Cl (ppm): 182.98 and 182.73 (C=O); 140.35, 136.34, 133.62, 133.46, 132.88, 132.43, 132.13, 130.45,
29.15, 128.85, 126.36, 126.14, 125.86, 123.71 and 119.78 (sp carbons); 53.45 (sp carbon). MS (ESI in CH
M+H+).
3
3
2
3
+
1
3
CN): 289
(
Preparation of (2-anthraquinonyl)methyl 6-bromohexanoate. In a round bottomed flask, kept under nitrogen, 250 mg
1 mmol) 2-hydroxymethyl-9,10-anthraquinone, 100 µL (1.1 mmol) pyridine dried over K CO and then 160 µL (1 mmol)
-bromohexanoyl chloride in 50 mL of CH Cl dried over CaCl were added. The reaction mixture became completely
homogeneous only after the addition of the carbonyl chloride. The solution stirred under anhydrous atmosphere overnight.
TLC analysis (SiO , CH Cl as the eluent) showed uncomplete consumption of the substrate, even after additional 1 hour at
(
2
3
6
2
2
2
2
2
2

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reflux. The reaction mixture was washed four times with water, dried with Na
product was first crystallized with toluene and then purified by column chromatography, using SiO
and CH Cl as the eluent. The product obtained was crystallized from dichloromethane-hexane. Yield, 57.7 % (240 mg, 57.7
mmol). H-NMR in CDCl (ppm): 8.35 (m, 3H); 8.30 (d, 1H, J=1.5 Hz); 7.84 (m, 2H); 7.78 (dd, 2H, J =2.0 Hz, J =8.0
Hz); 5.30 (s, 2H); 3.43 (t, 2H, J=6.5 Hz); 2.48 (t, 2H, J=7.5 Hz); 1.9 (m, 2H); 1.74 (m, 2H); 1.53 (m, 2H).
2
SO
4
and the solvent was evaporated. The
2
as the stationary phase
2
2
1
3
1
2
Preparation of (2-anthraquinonyl)methyl 6-imidazolylhexanoate (AQ6). The reaction was carried out in dried
glassware and nitrogen atmosphere. 0.5 mmol 2-(6-bromohexanoyloxymethyl)-9,10-anthraquinone were dissolved in 5 mL
2 3
of anhydrous acetonitrile containing 117 mg (1.1 mmol) of anhydrous Na CO ; 34 mg (0.5 mmol) imidazole was
subsequently added. The reaction was refluxed for 24 hours. The formed solid precipitate was recovered by filtration and
the solution was distilled and combined with the precipitate. The powder obtained was dissolved in dichloromethane,
extracted with water and the organic phase was dried over anhydrous Na
2
SO
4
and distilled. The compound was purified by
OH in CH Cl as the eluent. The
column chromatography, using SiO as the stationary phase and a mixture of 3% v/v CH
2
3
2
2
first fraction eluted was the compound of interest, that was concentrated and precipited with hexane. Yield, 7.2% (14.5 mg,
1
0
4
1
1
2
.036 mmol). H-NMR in (CD
3
)
2
CO (ppm): 8.29 (m, 4H); 7.95 (m, 3H); 7.56 (s, 1H); 7.1 (s, 1H); 6.9 (s, 1H); 5.36 (s, 2H);
CO (ppm):
.06 (t, 2H, J=7.2 Hz); 2.49 (t, 2H, J=7.2 Hz); 1.84 (m, 2H); 1.72 (m, 2H); 1.38 (m, 2H). 13C-NMR in (CD
)
3 2
83.13 and 182.70 (aromatic C=O); 172.80 (aliphatic C=O); 142.76, 135.17, 134.41, 134.35, 133.73, 133.49, 133.43,
2
33.13, 132.96, 127.85, 127.36, 127.34, 125.76, 120.57 and 120.37 (sp carbons); 65.12, 49.34, 33.67, 30.22, 25.71 and
3
+
3 3
4.08 (sp carbons). MS (ESI in CH CN): 466 (M+CH
CN+Na+).
Preparation of (2-anthraquinonyl)methyl p-bromomethylbenzoate [32]. 646.2 mg (3 mmol) p-
bromomethylbenzencarboxylic acid was dissolved in about 5 mL dichloromethane dried over CaCl in dried glassware,
2
under gentle stream of nitrogen. 742.8 mg (3.6 mmol) dicyclohexylcarbodiimide (DCC) and 20.3 mg (0.16 mmol) N,N-
dimethylaminopyridine (DMAP) were added with consequent formation of a white precipitate. 715 mg (3 mmol) 2-
hydroxymethylanthraquinone was added and the reaction mixture was kept under stirring for 24 hours in anhydrous
atmosphere. The reaction mixture was diluted with 100 mL of dichloromethane; the solid dicyclohexylurea formed was
removed by filtration and the solvent was distilled. The solid obtained was dissolved with boiling toluene and further
filtered, then kept at -18°C overnight. The yellow solid formed was recovered by decantation and further purified by
crystallisation with toluene. The compound was pure at TLC analysis (eluent: dichloromethane). Yield, 64% (840 mg, 1.93
1
mmol). H-NMR in CDCl
3
(ppm): 8.35 (m, 4H); 8.09 (d, 2H, J=8.0 Hz); 7.85 (m, 3H); 7.53 (d, 2H, J=8.0 Hz); 5.54 (s,
2
H); 4.53 (s, 2H).
Preparation of (2-anthraquinonyl)methyl p-imidazolylmethylbenzoate (AQPh). 70 mg (1.03 mmol) imidazole was
dissolved in 5 mL of CH Cl (dried over CaCl ) in dried glassware and, after addition of 235 mg (0.54 mmol) (2-
2
2
2
anthraquinonyl)methyl p-bromomethylbenzoate, the solution was diluted to 10 mL and kept stirring under anhydrous
atmosphere for 3 days. The precipitate and the solution obtained were dissolved in 75 mL of dichloromethane and extracted
2 4
with NaCl aqueous solution. The organic phase was dried over anhydrous Na SO , filtered and concentrated until reaching
the minimum volume. The compound precipitated upon addition of hexane and recovered by filtration. Yield, 23% (52.1
1
mg, 0.12 mmol). H-NMR in (CD
3
)
2
SO (ppm): 8.35 (m, 4H); 8.09 (d, 2H, J=8.0 Hz); 7.85 (m, 3H); 7.78 (s, 1H); 7.53 (d,

Accepted manuscript to appear in JPP
2
H, J=8.0 Hz); 7.21 (s, 1H); 6.93 (s, 1H); 5.54 (s, 2H); 5.43 (s, 2H). 13C-NMR in CDCl
(ppm): 182.53 and 182.10
3
(
aromatic C=O); 169.04 (aliphatic C=O); 145.12, 142.13, 134.98, 134.61, 134.05, 133.65, 133.33, 133.12, 132.83, 132.77,
2
1
30.45, 130.12, 129.95, 129,67, 128.34, 127.98, 127.55, 127.11 126.86, 122.54 and 119.78 (sp carbons); 69.43 and 55.88
sp carbons). MS (ESI in CH
3
CN): 445 (M+Na+).
3
+
(
RESULTS AND DISCUSSION
In this work, different zinc porphyrin-quinone dyads have been synthesized. In particular, ZnTPP and ZnOEP have
been coupled with differently substituted anthraquinones to study the interaction between the D-A units upon excitation.
Noteworthy, the quinone moiety was properly functionalized with imidazole, that can efficiently coordinate the zinc ion in
the porphyrin macrocycle, thus allowing the formation of the dyad. Three different spacers were used to link the imidazole
and the anthraquinone scaffold, in order to tune donor-acceptor distance and spacer flexibility. More in detail, a methylene
unit was used to prepare the acceptor with the shortest distance, namely 2-imidazolylmethylanthraquinone (AQ1), obtained
from the nucleophilic bimolecular substitution of 2-bromomethylanthraquinone with two equivalents of imidazole. An ester
function was used to obtain (2-anthraquinonyl)methyl-6-imidazolylhexanoate (AQ6) from 2-hydroxymethylanthraquinone
N
and 6-bromohexanoyl chloride. Bromine atom was then substituted with imidazole by a S 2 reaction. A similar approach
was used to synthesise (2-anthraquinonyl)methyl-p-imidazolylmethylbenzoate (AQPh), reacting 2-hydroxynaphthoquinone
with p-bromomethylbenzoic acid, to form the ester function, with a reaction mediated by DCC and DMAP. Imidazole was
then introduced by nucleophilic substitution of the bromine atom. All the dyads have been obtained in satisfactory yields
and properly characterized.
0
Calculation of ΔG for the photoinduced electron transfer. To check if the photoinduced electron transfer between the
two electroactive units could occur in solution, the Gibbs energy for the mentioned process was calculated in
dichloromethane. In particular, ∆G0ET value was estimated for all the dyads according to Rehm-Weller equation [24]:
+∙
-
2
ΔGET = E (D /D) – E (A /A) - ΔG00 - e /(4πε
0
εR
C
)
(eq. 1)
To solve such equation, the zero-zero transition energies ΔG0,0 for ZnTPP and ZnOEP were estimated considering the
emission wavelength in toluene (593 nm for ZnTPP, 573 nm for ZnOEP), while the redox potentials of the donor and
0
+∙
0
-∙
acceptor moieties (E (D /D) and E (A/A ), respectively) were determined by cyclic voltammetry. The mean distance
between the D-A units (R ) was estimated from the optimized geometries, calculated using DFT models (see Supp. Info).
values for each acceptor were supposed to be the same for both porphyrins. The obtained data are listed in Table 1 and
ΔG0ET values are collected in Table 2.
C
R
C

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Table 1. Summary of the structural features for calculation of G0ET
Donor
ZnTPP
ZnOEP
E0 (V)
0.75
Acceptor
AQ1
E0 (V)
-0.92
-0.91
-0.92
R
C
(Å)
9.6
0.62
AQ6
16.3
11.9
AQPh
Table 2. G0ET values for the investigated porphyrin-anthraquinone complexes.
ΔG0ET
ΔG0ET
(eV)
ZnTPP-AQ
ZnOEP-AQ
(eV)
ZnTPP-AQ1
ZnTPP-AQ6
ZnTPP-AQPh
-0.59
-0.53
-0.55
ZnOEP-AQ1
ZnOEP-AQ6
ZnOEP-AQPh
-0.78
-0.73
-0.76
Obtained ΔG0ET values suggest that the photoinduced electron transfer can occur in all the synthesized dyads, being more
favourable for the closest D-A units, as in the case of AQ1 acceptor. As a matter of fact, for both porphyrins, the longer the
distance between the units, the more disadvantaged the electron transfer. Conversely, a rigid acceptor (as AQPh) led to a
more energetically favoured processes with respect to AQ6 acceptor. Moreover, using the same acceptor unit, the
photoinduced electron transfer process was favoured with ZnOEP donor with respect to ZnTPP, likely because of the
electronic effect of the alkyl substituents on the porphyrin macrocycle.
UV and Fluorescence Titration. The interaction between ZnTPP and ZnOEP with the imidazolyl functionalized
anthraquinones was studied by UV-vis spectroscopy. Specifically, small amounts of a concentrated solution of the
anthraquinone derivatives were added to a solution of each porphyrin, and the absorption spectra were registered (see Supp.
Info). As an example, UV-vis titration of ZnTPP with AQ1 is shown in Figure 1. It can be observed that in the presence of
the acceptor unit, the intensity of the Soret band at 420 nm gradually decreased, while the band at 428 nm increased of
intensity. Moreover, an intense red shift of the Q-bands was detected. The presence of two isosbestic points in each titration
indicates the existence of only two species in solution, i.e. the free porphyrin and the dyad.

Accepted manuscript to appear in JPP
-6
-7
-5
Fig. 1. UV-vis titration of ZnTPP (2.3 ∙ 10 M) with AQ1 (from 6.0 ∙ 10 M to 4.0 ∙ 10 M).
Using the Scatchard plot equation (eq. 2) [33], it was possible to calculate the binding constants for all the complexes, in
order to estimate the concentration where the porphyrin is almost completely coordinated to the imidazole of the acceptor
unit:
(
eq. 2)
In eq. 2, A indicates the absorbance of the porphyrin in the presence of the anthraquinone moiety, A
0
the absorbance in its
absence, [AQ] is the anthraquinone concentration and K is the association constant. Soret bands were chosen to analyze the
a
absorption data, since anthraquinones do not absorb in that region.
As a comparison, in the case of ZnTPP and ZnOEP, which are fluorescent compounds, the binding constants was
calculated also by fluorescence measurements. Therefore, the emission spectra were registered for the same solutions: the
addition of increasing amounts of quinones to the porphyrin solutions resulted in fluorescence quenching (see Supp. Info).
To prove that the quenching was not exclusively due to the complexation with imidazole, but it was also due to the
occurring electron transfer between the D-A units, fluorescence titrations with 1-buthylimidazole were performed. The
results show that in the absence of the quinone moiety, the quenching effect was significantly lower (see Supp. Info).
Therefore, electron transfer between the porphyrins and anthraquinone units was effectively occurring.
Obtained data were fit by the Stern-Volmer equation (equation 3) [33]:
(
eq. 3)
0
where I is the fluorescence in the presence of the quencher (i.e. the anthraquinone derivative), I the fluorescence in its
absence, [AQ] is the quencher concentration and KSV represents the binding constant. Fluorescence titration of ZnTPP with
AQ1 and the corresponding Stern-Volmer plot are reported in Figure 2.

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Fig. 2. Fluorescence titration of ZnTPP (2.3 ∙ 10-6 M) with AQ1 (from 6.0 ∙ 10-7 M to 4.0 ∙ 10-5 M) in toluene, normalised for the
absorbance value. exc= 550 nm. Inset: Stern-Volmer plot calculated at 593 nm.
a
Binding constants obtained from Uv-vis (K ) and fluorescence (KSV) measurements are summarised in Table 3.
a
Table 3. Binding constants for P-AQ complexes, obtained from Scatchard plot (K ) and Stern-Volmer plot (KSV), in toluene, at 25°C.
AQ
ZnTPP
ZnOEP
-
1
-1
K
a
(M )
K
SV
K
a
(M )
K
SV
AQ1
AQ6
2.6 x 104
3.8 x 104
2.2 x 104
3.9 x 104
2.2 x 104
6.6 x 103
1.4 x 104
1.2 x 104
7.6 x 103
1.3 x 104
1.1 x 104
AQPh 2.4 x 104
From data reported in Table 3 it is possible to note that similar binding constant values were achieved with absorption and
emission spectroscopies. Conversely to what expected from Gibbs free energy values reported in Table 2, for both
porphyrins the highest binding constant was obtained with the AQ6 acceptor. Moreover, with the same acceptor, higher
constants have been achieved with ZnTPP donor species. To verify that the imidazole unit in the acceptor was the Zn-
a a
coordinating species, K for 1-buthylimidazole was measured. The obtained K was similar to those obtained with the
3
-1
imidazolyl-anthraquinone derivatives (9.7 × 10 M for ZnTPP), thus confirming that the complexation occurred through
the imidazole in the dyad.
As for ferrocene-naphthoquinone dyads [24], the fluorescence quenching indicates that an electron transfer from the
donor moiety, i.e. the porphyrin, to the acceptor quinone, likely occurs. The linear correlation in the Stern-Volmer plot
indicates that only one type of fluorophore is involved. However, the mechanism of the quenching can be either collisional
or static [33]. To better understand the observed phenomena, femtosecond time resolved transient absorption spectroscopy
has been performed, aiming at obtaining a clear evidence of the formation of radical cation or radical anion of the charge
+
-
separated state P -AQ .

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Transient absorption spectroscopy. Transient absorption spectra were recorded upon excitation of porphyrin,
porphyrin-methylimidazole and P-AQ solutions. The complexes were obtained by addition of a toluene solution of the
quinone (or 1-methylimidazole) to a solution of the porphyrin; the absorbance was checked before and after the laser
excitation, to test the stability of the complexes. Concentrations of porphyrin and ligands were chosen in order to have an
optical density value between 0.3 and 0.9 a.u. and to have less than 15% of free porphyrin, according to the measured K
values.
a
The porphyrin Q bands correspond to S
and porphyrin complexes) were excited to the second excited state, the first process that occurred was the relaxation to the
first excited state, S . Therefore, the observed decay was the one from the first excited state, even if the system was excited
0
→S
1
transitions, whereas the Soret band to the S
0
2
→S ones. When porphyrins
(
1
in correspondence of the Soret band. Hence, transient absorption spectra were recorded exciting both, at the Q band
wavelength and at the Soret band, without noting any significant difference. Figure 3 shows the absorption spectra recorded
at different times after the pump pulse at 430 nm, for ZnTPP, ZnTPP-1-methylimidazole (MeIm) and ZnTPP-AQ1.
B) ZnTPP+MeIm
C) ZnTPP+AQ1
A) ZnTPP
0
0
0
0
0
0
.025
.020
.015
.010
.005
.000
0.050
0.04
5ps
5
ps
30ps
ns
3ns
5ps
30ps
1ns
3ns
3
0ps
1ns
0.025
0.000
0.025
0.050
0.075
1
0
0
.02
.00
3
ns
-
-
-
-
-
-
-
-
0.005
0.010
0.015
0.020
0.025
-0.02
-0.04
400
450
500
550
(nm)
600
650
700
750
400
450
500
550
600
650
700
750
400
450
500
550
(nm)
600
650
700
750

 (nm)

Fig. 3. Comparison between the transient absorption spectra in toluene of ZnTPP (A), ZnTPP-MeIm complex (B) and ZnTPP-AQ1
complex (C) at different times, after excitation at 430 nm.
The transient absorption spectrum of ZnTPP shows a strong absorption band at 460 nm, due to the singlet excited state,
1*ZnTPP [34]. With prolonged times (3 ns), such band was gradually red-shifted at 475 nm, because of the formation of the
ZnTPP triplet excited state, 3*ZnTPP [35]. Complexation of ZnTPP with imidazole does not significantly change the
spectra: in fact, both, the singlet and the triplet excited states of the porphyrin were detected, whereas no other absorption
bands were observed. In the case of ZnTPP-AQ1 dyad, the formation of 3*ZnTPP was less pronounced. Moreover, a sharp
absorption band appeared around 410 nm (see Figure 3c, red line). This latter can be attributed to the formation of the
+
∙
porphyrin radical cation, ZnTPP . As a matter of fact, a similar band (cantered at 407 nm) was observed in
spectroelectrochemical measurements of the porphyrin in dichloromethane (see Supp. Info). As a confirmation, transient
absorption spectra of ZnTPP in the presence of 9,10-anthraquinone as acceptor have been recorded, since such molecule
cannot interact with the zinc porphyrin. As a result, similar spectra to those obtained for the ZnTPP alone have been
recorded, proving that no electron transfer occurred by an intermolecular process. Moreover, data suggested that, in the
presence of the anthraquinone moiety, after the excitation at 420 nm, the formation of the singlet excited state evolved into a
+
-
charge separated state, ZnTPP AQ , that competes with the intersystem crossing (Scheme 1). The lifetime of the charge
separated state, calculated by a triexponential decay fit of the kinetic trace at 410 nm (Figure 4), was 350 ps for ZnTPP-
AQ1, whereas it increased up to 1.1 ns for ZnTPP-AQ6 (Table 4). Therefore, the long distance between the porphyrin and

Accepted manuscript to appear in JPP
the anthraquinone core stabilised the charge separated state in ZnTPP-AQ6, likely reducing the charge recombination
process. The best fitting for ZnTPP-AQPh data at 410 nm was biexponential and no similar decays could be detected. A
reasonable explanation could be that in the presence of a rigid spacer, the approach of the quinone to the porphyrin is
hampered. Notably, the transient spectrum of the ZnTPP-AQ1 dyad, where the quinone is quite close to the porphyrin, is
characterized by the most intense signal at 410 nm. In other words, the proximity of the donor and acceptor moieties could
promote the electron transfer process, whereas a longer distance between them could prevent the charge recombination, thus
extending the charge separated state lifetime.
1*ZnTPP-AQ
S1
ISC
CS
3*ZnTPP-AQ
+
-
ZnTPP -AQ
CR
S0
ZnTPP-AQ
Scheme 1. Proposed ET transfer processes for ZnTPP-AQ systems.
Table 4. Kinetic parameters obtained for the investigated ZnTPP-AQ complexes, at 410 nm, at 25°C.
Compound
A
1
(%)
τ
1
(ps)
A
2
(%)
τ
2
(ps)
A
3
(%)
τ
3
(ps)
χ2
ZnTPP-AQ1
ZnTPP-AQ6
ZnTPP-AQPh
-33
-53
-85
83
-18.4
-13
7
48
34
-
356
0.002
0.001
0.0015
210
229
27
26
1130
-
-15
Fig. 4. Kinetic trace (blue dots) and fitting (red line) at 410 nm for ZnTPP-AQ1 complex. Excitation wavelength: 430 nm.
A further confirmation of the occurring electron transfer can be achieved by studying the kinetic decay at 550 nm,
•
-
corresponding to the signal of AQ in accordance with spectroelectrochemistry measurements of AQ1 in dichloromethane

Accepted manuscript to appear in JPP
•
-
(
see Supp. Info). Although the signal of the singlet excited state of the porphyrin hides the signal of the AQ , it is possible
to observe that such decay remains unchanged when the zinc-porphyrin is coordinated with methylimidazole, whereas it is
faster in the presence of AQ1. Kinetic decays at =550 nm are reported in Figure 5 and the corresponding fitting data are
collected in Table 5.
Fig. 5. Kinetic decay for ZnTPP-MeIm and ZnTPP-AQ1 at 550 nm. exc=430 nm.
Table 5. Kinetic parameters obtained at =550 nm for some of the ZnTPP-AQ complexes, at 25°C.
A
1
Compound
τ
1
(ps)
A
2
(%)
τ
2
(ps) χ2
0.0001
(%)
ZnTPP
100
100
98
1272
1796
410
-
-
-
ZnTPP-MeIm
ZnTPP-AQ1
-
0.0002
0.0003
2
39
Femtosecond transient absorption spectra for ZnOEP-AQ dyads were registered exciting in the Q band region, at 530
nm (see Supp. Info). In the case of ZnOEP dyads, no other absorption bands were observed with respect to the porphyrin
alone. However, it is possible to observe changes in the kinetic decays of the spectra at 380 nm, as shown in Figure 6, where
the kinetic curves of the different dyads are compared. At that wavelength, the radical cation of the porphyrin should
absorb, as already observed by spectroelectrochemical measurements.

Accepted manuscript to appear in JPP
Fig. 6. Kinetic decays for ZnOEP complexes at =380 nm (left) and fitting of the absorption data at =380 nm for ZnOEP-AQ1 (right).
As in the case of ZnTPP, the evidence of weak bands might be due to the formation of the charge separated state,
+
-
ZnOEP AQ . The kinetic decays at 380 nm were triexponentially fitted in order to have an idea of the lifetime of all the
investigated dyads (Table 6). The lifetime of the radical cation was 140 ps for the short quinone AQ1 and, similarly to
ZnTPP complexes, it increased with increasing distances between the donor and the acceptor. Conversely, the rigid spacer
in AQPh determined a shorter, but still remarkable, lifetime of 1.3 ns. The lower reduction potential of ZnOEP with respect
to ZnTPP, due to the presence of the electron donating group in the aromatic core, together with the lower steric hinderance
of ethyl substituents, could make the difference in terms of energetically favoured electron transfer. In fact, also the way
porphyrins approach the acceptor unit can be influenced by the different steric hinderance of substituents.
Table 6. Kinetic parameters obtained for the fitting of ZnOEP-AQ complexes at 380 nm.
Sample
A1 (%)
τ1 (ps)
A2 (%)
τ2 (ps)
A3 (%)
τ3 (ps)
ZnOEP+AQ1
ZnOEP+AQ6
ZnOEP+AQPh
47
43
50
140
911
-29
-14
-28
37
156
199
-23
-42
-21
2
2.5
3
1285
CONCLUSION
In this work, new P-AQ dyads have been prepared. In particular, ZnTPP and ZnOEP were used as electron donor units,
coupled with different imidazolyl functionalised anthraquinones as acceptors. Analysis of Gibbs free energy demonstrated
that the photoinduced electron transfer was energetically favoured for all the synthesised dyads. UV-vis and fluorescence
studies allowed to directly follow the association between the D-A units and to measure the binding constants. Association
3
4
-1
(
Ka) and Stern-Volmer (KSV) constants were in the range 6.6∙10 - 3.9∙10 M for all the dyads.
Femtosecond absorption spectroscopy of the dyads was performed. In the case of ZnTPP, the formation of the charge
separated (CS) state was indicated by the formation of a new absorption band due to the radical cation of the porphyrin.
This band was particularly evident for ZnTPP-AQ1 complex and the measured decay lifetime was about 300 ps. For longer
D-A distances, as in ZnTPP-AQ6 dyad, a similar but slower decay was detected at 410 nm with a longer lifetime.

Accepted manuscript to appear in JPP
Considering the ZnOEP complexes, the kinetic decays and the detection of a new absorption band (corresponding to the
radical cation) also suggested the formation of the CS state. Obtained results indicated a strong correlation between the
spacer length and flexibility with the CS state lifetime.
Furthermore, the observed differences for the dyad composed by ZnTPP or ZnOEP donor with AQPh acceptor have
been attributed both, to electronic and steric effect of substituents. In fact, in the case of ZnOEP, alkyl groups lower the
oxidation potential of the porphyrin, and they can also influence the way porphyrin approaches the acceptor unit.
To conclude, in this work we found that, although at the ground state ZnOEP exhibited a lower oxidation potential than
ZnTPP, at the excited state the latter resulted the most efficient electron donor unit in the studied P-AQ dyads. Therefore,
modulation of porphyrin substituents, as well as D-A spacer nature (length, flexibility), allows to tune the electron transfer
properties of the dyads.
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