Subphthalocyanine-polymethine cyanine conjugate: An all organic panchromatic light harvester that reveals charge transfer

Article abstract of DOI:10.1039/c1jm13203b

The synthesis and photophysical characterization of a subphthalocyanine- polymethine cyanine panchromatic light harvester is described. The new conjugate shows the extraordinary absorption features of both constituents and reveals electron transfer that evolves from both constituents.

Full text of DOI:10.1039/c1jm13203b

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PAPER
Subphthalocyanine-polymethine cyanine conjugate: an all organic
panchromatic light harvester that reveals charge transfer†
Carlos Romero Nieto,‡a Julia Guilleme,‡b Carmen Villegas,‡c Juan Luis Delgado,cd
b
David Gonzalez-Rodrıguez, Nazario Martin,
cd
bd
a
*
*
*
and Dirk M. Guldi
ꢀ
ꢀ
ꢀ
Tomas Torres
Received 9th July 2011, Accepted 26th July 2011
DOI: 10.1039/c1jm13203b
The synthesis and photophysical characterization of a subphthalocyanine-polymethine cyanine
panchromatic light harvester is described. The new conjugate shows the extraordinary absorption
features of both constituents and reveals electron transfer that evolves from both constituents.
fullerenes prompted us toexplore subphthalocyanines (SubPcs) as
electron acceptor in a new generation of heptamethine cyanine
conjugates. SubPcs4—non-planar aromatic compounds related to
phthalocyanines5—are known to absorb in the visible region with
high extinction coefficients. This renders them particularly
appealing for the construction of panchromatic molecular
hybrids. To this end, a series of multicomponent systems, in which
SubPcs are linked to either electron acceptors or electron donors,
are known to date.6
Introduction
Solar energy represents an unlimited and renewable energy power
supply. As a matter of fact, in recent years a great deal of research
effort has been invested towards improving artificial systems
capable to collect the photonic energy throughout the entire solar
spectrum.1 In this respect, even if organic sensitizers are known to
commonly absorb only in the UV and visible regions of the solar
spectrum, they have attracted extensive attention because of their
synthetic versatility and electronic tunability.2 In an ideal design,
not only the electrochemical and photophysical properties of
organic sensitizers should be considered, but also their absorption
features. Importantly, the latter determines the upper limit of the
photoconversion performances. Notwithstanding, it is important
to note, that nowadays molecular systems displaying absorption
features in the near-infrared part of the solar radiation, still
remain quite scarce and limited.
Herein, we report the synthesis and photophysical character-
ization of a cyanine derivative, which converge complementarily
the extraordinary near infrared and visible light absorption
features of heptamethine cyanines and dodecafluoro boron-
SubPc, respectively, in a new panchromatic electron donor–
acceptor conjugate (1)—see Fig. 1.
Results and discussion
Motivated by the aforementioned, we were successful in
ascertaining the suitability of cationic and anionic heptamethine
cyanines as near-infrared light harvester in electron donor–
acceptor conjugates.3 Our photophysical studies revealed that
even higher singlet excited states of these cyanines are powerful
promoters for charge separation when they are, for example,
covalently linked to an electron accepting fullerene. Despite this
unique reactivity, the low-to-moderate extinction coefficients of
Synthesis
The synthesis of 1 was carried out by axial substitution on
chloro-SubPc 27 using polymethine cyanine 4 as nucleophile
aDepartment of Chemistry and Pharmacy and Interdisciplinary Center
for Molecular Materials, Friedrich-Alexander-Universita€t, Erlangen-
Nu€rnberg, 91058 Erlangen, Germany. E-mail: guldi@chemie.
uni-erlangen.de
bUniversidad Autoꢀnoma de Madrid, Departamento de Quꢀımica Orgaꢀnica,
Campus de Cantoblanco, Madrid, Spain. E-mail: tomas.torres@uam.es
cDepartamento de Quꢀımica Orgaꢀnica,Facultad de Ciencias Quꢀımicas,
Universidad Complutense de Madrid, Ciudad Universitaria s/n, 28040
Madrid, Spain. E-mail: nazmar@quim.ucm.es
dInstituto Madrilen~o de Estudios Avanzados en Nanociencia (IMDEA-
Nanociencia), Cantoblanco, 28049 Madrid, Spain
† Electronic Supplementary Information (ESI) available: Synthesis and
characterization of 1 and 4, complementary spectroscopic data. See
DOI: 10.1039/c1jm13203b/
Fig. 1 Structure of subphthalocyanine-polymethine cyanine (SubPc-
cyanineꢀ) 1.
‡ These authors contributed equally to the research.
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(Scheme 1). We employed a recently developed methodology that
makes use of a highly activated triflate intermediate which is
generated in situ by reaction between chloro-SubPc 2 and silver
triflate.8 Such activation was found to be necessary, since the
direct substitution reaction between 4 and 2 ultimately resulted in
SubPc decomposition due to the higher temperatures required.
On the other hand, 4 was prepared by the substitution of the
meso-chlorine atom of 33a by 4-hydroxybenzyl alcohol in DMF. 1
was obtained in 25% yield and characterized by 1H-NMR,
1
MALDI-TOF and HR-MS and UV-vis. Fig. 2 shows the H
NMR spectrum of 1. It is interesting to remark that the signal of
the benzylic protons is affected by the SubPc ring current causing
an upfield shift from 4.39 (4) to 2.49 (1) ppm (see Inset in Fig. 2).
These kind of shifts are very typical in axially substituted SubPcs.
Additional experimental details regarding the characterization of
compounds 1 and 4 are given in the Supporting Information.
Fig. 2 1H-NMR (acetone-d6, 300 MHz) of 1 with proton assignment.
Inset: portion of the 1H-NMR spectra of 4 (a) and 1 (b) showing the
upfield shift experienced by the methylene proton signal.
Photophysics
SubPc-cyanineꢀ conjugate 1 is a black solid, which, importantly,
shows a panchromatic absorption over the entire range between
350 and 1000 nm—see Fig. 3 and S8 (ESI†). Moreover, two
stronger absorption maxima evolve at 571 and 903 nm, which
correspond to the SubPc and the polymethine cyanine constit-
uent, respectively.
800 nm excitation was employed to probe the cyanine fluores-
cence in the near infrared. A fluorescence maximum at 925 nm
and a fluorescence quantum yield of 0.21 are characteristics of
the cyanine reference 4. In the SubPc-cyanineꢀ conjugate 1
quenching reduces the fluorescence quantum yield to 0.07. In
terms of excited state deactivation mechanism, we inspected the
near infrared emission of the cyanine upon SubPc excitation at
560 nm, where the cyanine absorption contributes less than 3% to
the overall absorption in SubPc-cyanineꢀ conjugate 1. Here, the
insignificant cyanine fluorescence suggests an intramolecular
deactivation via electron transfer rather than via energy transfer.
Next, we turned to transient absorption measurements to
confirm the excited state deactivation and to identify the corre-
sponding photoproducts. Like in the fluorescence experiments,
selecting different excitation wavelengths helps to monitor
processes of the SubPc-cyanineꢀ conjugate 1 that exclusively
evolve from SubPc or cyanine. In particular, 560 nm was chosen
to excite SubPc in the reference 2 and in SubPc-cyanineꢀ
conjugate 1. With the conclusion of the excitation of the SubPc
reference the following singlet excited state features evolve—
maxima at 430, 615, 755, 825, 910, 1070 nm and minima at 515,
560, 630 nm—Fig. S9 (ESI†). The latter transforms with 1.6 ꢂ
0.1 ns into the corresponding triplet excited state via intersystem
crossing—Scheme S1 (ESI†). Maxima at 460 and 645 nm as well
as a minimum at 560 nm emerge as fingerprints of the SubPc
Turning to the electronic communication between the SubPc
and polymethine moieties, the first insights into excited state
interactions came from fluorescence experiments by means of
probing either the SuPc in the visible or the cyanine emission in
the near infrared in benzonitrile—see Fig. S8 (ESI†). Firstly, we
focused on the visible fluorescence of SubPc upon excitation at
560 nm. Here, the SubPc reference 2 gives rise to fluorescence
features that include a fluorescence maximum at 583 nm, a fluo-
rescence quantum yield of 0.58, and a fluorescence lifetime of 3.2
ns. In sharp contrast, in the SubPc-cyanineꢀ conjugate 1 the
fluorescence quantum yield is reduced to 8 ꢁ 10ꢀ3 and a lifetime
below the detection range of our apparatus (i.e., 0.1 ns). Not
impacted is, however, the fluorescence maximum. Secondly,
Scheme 1 Synthetic route to 1. Reagents and conditions: (i) silver tri-
fluoromethanesulfonate, chloroform; (ii) 4-hydroxybenzyl alcohol, NaH
(60%), HCl, DMF; (iii) DIPEA, chloroform, 45 ꢃC
Fig. 3 AbsorptionspectrumofSubPc-cyanineꢀ conjugate1in benzonitrile.
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triplet excited state. The triplet is long-lived as we have confirmed
in recent experiments.6a
The SubPc-cyanineꢀ conjugate 1 shows initially the same
SubPc singlet excited state features when exciting at 560 nm,
despite the cyanine presence—see Fig. 4. Maxima at 440, 605,
755, 820, 920, and 1070 nm are complemented by minima at 510,
560 and 630 nm. Importantly however, the cyanine exerts
a strong impact on the excited state dynamics. Instead of the slow
intersystem crossing (i.e., 1.6 ꢂ 0.1 ns) observed for SubPc
reference 2, all of the aforementioned features decay with 5.7 ꢂ
0.5 ps—a finding that agrees with the efficient fluorescence
quenching. With exactly the same time constant new transitions
develop. In fact, two sets of transitions are registered. On one
hand, in the visible range maxima at 460, 505, 520, and 605 go
hand in hand with minima at 495 and 560 nm. On the other hand,
maxima at 622, 660, and 730 nm are accompanied by a strong
minimum in the near infrared at 890 nm.
Previous investigations have shown that electrochemical and
pulse radiolytical reduction of SubPc reveal the same fingerprint
than what is seen in the visible.3a Likewise, oxidation of cyanine
by electrochemistry or pulse radiolysis matches the near infrared
transitions.3a In other words, the SubPc singlet excited state
(2.2 eV) is the inception to a rapid and strongly exothermic
charge transfer process to afford the SubPcꢄꢀꢀcyanine charge
separated state—see Scheme 2. The latter, for which we have
determined an energy of 1.1 eV, is short lived and decays with
205 ꢂ 10 ps. Despite its low energy, charge recombination
populates the triplet excited state of cyanine—vide infra.
Next, we focused on either 387 or 778 nm excitation of the
cyanine in both the SubPc-cyanineꢀ conjugate 1 and the reference
4. 387 nm excitation of the cyanine reference 4 leads to instanta-
neously formed higher singlet excited state features maxima at
490, 600, and 680 nm, minima at 565, 630, and 735, and broad
bleach between 820 and 955 nm – not shown. With a lifetime of
4.4 ꢂ 0.5 ps these features transform rapidly into those of the
lowest singlet excited state with distinct maxima at 520, 585, 625,
and 1240 nm plus distinct minima at 730 and 920 nm.
Next, the slow decay of the singlet excited state follows within
approximately 2.0 ꢂ 0.5 ns to form correspondingly the triplet
excited state—Scheme S1 (ESI†). Population of the higher lying
singlet excited states was successfully bypassed by means of
excitation at 778 nm. This leads to the rapid (>0.5 ps) and direct
formation of the lowest singlet excited state. The lifetime of the
correspondingly formed singlet excited state as determined from
a multi wavelength analysis is 1.9 ꢂ 0.5 ns.
Fig. 4 Upper part – differential absorption spectra (visible) obtained
upon femtosecond flash photolysis (560 nm, 150 nJ) of SubPc-cyanineꢀ
conjugate 1 in benzonitrile with several time delays between 0 and 30 ps at
room temperature – see figure legend of central part for time evolution.
Central part – differential absorption spectra (near-infrared) obtained
upon femtosecond flash photolysis (560 nm, 150 nJ) of SubPc-cyanineꢀ in
benzonitrile with several time delays between 0 and 30 ps at room
temperature – see figure legend for time evolution. Lower part – time-
absorption profile of the spectra at 620 nm, monitoring the charge
separation and charge recombination dynamics.
In the SubPc-cyanineꢀ conjugate 1, when exciting at 387 nm,
the higher singlet excited state internally convert with the same
dynamics that the cyanine reference 4 shows, that is, 4.4 ꢂ 0.5
ps, into the lowest singlet excited state (1.35 eV)—see Fig. S10
(ESI†). Commencing with the completion of this process starts
the decay of the 490, 600, 680 nm maxima as well as the 565,
630, 735, 905 nm minima. The dynamics of these new transients
(i.e., 215 ꢂ 30 ps) are, however, notably faster than the inter-
system crossing (i.e., 2.0 ꢂ 0.5 ns) observed for the cyanine
reference 4 and, interestingly, match those of the charge
recombination seen upon excitation of the SubPc moiety at
560 nm—vide supra. Nevertheless, despite these fast dynamics
the spectral features of the cyanine triplet excited state emerge
simultaneously. A likely rationale must infer a slow charge
separation (i.e., 550 ꢂ 50 ps), which has been derived from the
fluorescence experiments, followed by a faster charge recom-
bination (i.e., 215 ꢂ 30 ps) to populate the lowest triplet excited
state, that is, the cyanine centered one—Scheme 2. In other
words, the charge separated state originated from the lowest
excited state of the cyanine decays faster than being formed,
which renders its detection impossible.
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Femtosecond transient absorption studies were performed with
775 and 387 nm laser pulses in benzonitrile (1 kHz, 150 fs pulse
width, 200 nJ) from an amplified Ti/sapphire laser system (Model
CPA 2101, Clark-MXR Inc.—output 775 nm).
Chemicals were purchased from commercial suppliers and
used without further purification. Solid, hygroscopic reagents
were dried in a vacuum oven before use. Reaction solvents were
thoroughly dried before use by means of standard methods. The
synthesis and characterization of the starting anionic poly-
methine cyanine3a 3 and dodecafluorosubphthalocyanine7 2 have
been previously reported.
Cyanine 4
4-Hydroxybenzyl alcohol (33.6 mg, 0.27 mmol, 1.2 equiv.) was
dissolved in dry DMF (8 ml) under argon atmosphere, then NaH
60% (10.8 mg, 0.27 mmol, 1.2 equiv.) was added. After 30 min of
stirring at room temperature, 3 (175 mg, 0.23 mmol, 1 equiv.)
dissolved in DMF (8 ml), was added dropwise to this solution.
The solution was stirred for 6 h at RT under argon atmosphere,
and then poured into ice and hydrochloric acid (0.4 ml). The
resulting green solid was filtered off, redissolved in dicholoro-
methane, and washed with water and Na2CO3. The organic layer
was dried with MgSO4, and precipitated in pentane, to afford 4
Scheme 2 Energy diagram of the SubPc-cyanineꢀ conjugate 1. The
dynamics when exciting at 387 and 560 nm are represented.
Conclusions
We have described the synthesis of a boron subphthalocyanine/
polymethine cyanine conjugate. In the new conjugate, the
complementary characteristics of the heptamethine cyanines, that
is, near-infrared absorption and electron donation, with those of
dodecafluoro SubPc, that is, visible absorption and electron
acceptance, are combined. Although in the ground state no
appreciable interactions are seen between the different constitu-
ents, in the excited state an intramolecular electron transfer
prevails. Importantly, the different singlet excited state energies of
2.2 eV (SubPc) and 1.35 eV (cyanine) relative to the charge
separated state energy of 1.1 eV evoke very different charge
separation dynamics—5.7 ꢂ 0.5 ps versus 550 ꢂ 50 ps, while the
charge recombination is not effected at all with 215 ꢂ 30 ps.
1
as a green solid (160 mg, 84%). H-RMN dH(DMSO-d6; 300
MHz) 7.67 (2H, d, J ¼ 14 Hz), 7.22 (2H, d, J ¼ 8 Hz), 6.91 (2H, d,
J ¼ 8 Hz), 5.96 (2H, d, J ¼ 14 Hz), 4.39 (2H, s), 3.18–3.13 (8H,
m), 2.59–2.56 (4H, m), 1.88–1.83 (2H, m), 1.59–1.54 (8H, m),
1.38 (12H, s), 1.34–1.24 (8H, m), 0.93 (12H, t, J ¼ 7 Hz). 13C-
NMR dC(DMSO-d6; 125 MHz) 176.1, 166.5, 160.9, 158.0, 137.0,
136.0, 128.0, 123.2, 115.4, 114.5, 114.3, 113.9, 105.9, 95.0, 82.5,
62.3, 57.5, 57.5, 57.5, 44.5, 26.1, 23.8, 23.0, 20.7, 19.1, 13.4. MS
(ESI -): m/z calculated for C37H27N6O4: 621.22558 [M]: found
621.22291.
Experimental section
General Procedures
Subphthalocyanine-polymethine cyanine 1
LSI-MS and HRMS spectra were determined on a VG AutoSpec
apparatus, ESI-MS spectra were obtained from an Applied
Biosystems QSTAR equipment, and MALDI-TOF-MS spectra
were obtained from a BRUKER ULTRAFLEX III instrument
equipped with a nitrogen laser operating at 337 nm. NMR
spectra were recorded with a BRUKER AC-300 (300 MHz)
instrument. The temperature was actively controlled at 298 K.
Chemical shifts are measured in ppm relative to tetramethyl-
silane (TMS). Carbon chemical shifts are measured downfield
from TMS using the resonance of the deuterated solvent as the
internal standard. Column chromatography was carried out on
In a 25 mL round-bottomed flask equipped with a magnetic
stirrer,
2 (70 mg, 0.11 mmol, 1 equiv.) and silver tri-
fluoromethanesulfonate (36 mg, 0.14 mmol, 1.3 equiv.) were
placed. Freshly distilled chloroform (2 mL) was added and the
mixture was stirred for 24 h under argon atmosphere until the
starting subphthalocyanine was consumed. After that time,
compound 4 (20 mg, 0.023 mmol, 0.2 equiv.) and N,N-diiso-
propylethylamine (0.23 mmol, 0.2 equiv.) were added. The
mixture was stirred at 45 ꢃC until the reaction was completed (the
reaction was monitored by TLC). The solvent was removed by
evaporation under reduced pressure and the product was purified
by chromatography on silica gel using DCM/ethyl acetate (3 : 1)
as eluent. Recrystallization from DCM/hexane afforded
compound 1 as a black solid. Yield: 25%. 1H-NMR dH(300 MHz;
(CD3)2CO) 7.37 (2H, d, J ¼ 14.7 Hz), 6.53 (2H, d, J ¼ 8.7 Hz),
6.27 (2H, d, J ¼ 14.7 Hz), 5.79 (2H, d, J ¼ 14.7 Hz), 3.32 (8H, m),
2.49 (2H, s), 2.45–2.35 (4H, m), 2.16–2.10 (2H, m), 1.80–1.61
(8H, m), 1.39–1.23 (8H, m), 1.19 (12H, s), 0.85 (12H, t, J ¼ 7.2).
MS (MALDI, DCTB): m/z ¼ 1473 (Mꢀ, 5%), 1231.2 ([M-
NBu4+]ꢀ, 100%), 621.0 ([Cy]ꢀ). HRLSI-MS (C77H64BF12N13O4)
[M]ꢀ Calculated: 1473.5106; Found: 1473.5082. UV-vis:
lmax(CHCl3)/nm 892, 789sh, 569, 554sh, 524sh, 430, 313, 282.
ꢁ
silica gel Merck-60 (230–400 mesh, 60 A), and TLC on aluminum
sheets precoated with silica gel 60 F254 (E. Merck).
UV/Vis spectra were recorded with a Hewlett-Packard 8453
and Varian Cary 5000 UV-VIS-NIR instruments. The fluores-
cence experiments were carried out using a Horiba Jobin Yvon
Fluoromax 3P and Horiba Jobin Yvon Fluorolog-3 spectrometer.
The quantum yields were determined using cresyl violet in meth-
anol (QY ¼ 0.54) and IR-140 in ethanol (QY ¼ 0.167)9 as stan-
dards for the SubPc and cyanine respectively. The fluorescence-
lifetime measurements were performed by time correlated single
photon counting (TCSPC) by using a FluoroLog-3 spectrometer.
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Acknowledgements
Financial support by the MICINN, Spain (CTQ2008-00418/
BQU, CONSOLIDER-INGENIO 2010 CDS 2007-00010,
PLE2009-0070, CTQ2008-00795/BQU, PIB2010 JP-00196), and
CAM (MADRISOLAR-2, S2009/PPQ/1533), as well as by SFB
583 (DFG, Germany) and EU (FUNMOLS,FP7-212942-1), is
gratefully acknowledged.
ꢀ
ꢀ
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ꢀ
ꢀ
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15918 | J. Mater. Chem., 2011, 21, 15914–15918
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