Accelerating charge transfer in a triphenylamine-subphthalocyanine donor-acceptor system

Article abstract of DOI:10.1039/b719226f

We have designed, synthesized and probed a dodecafluoro-subphthalocyaninato boron(iii) unit, which bears a triphenylamine moiety in its axial position, as a novel electron donor-acceptor system. The Royal Society of Chemistry.

Full text of DOI:10.1039/b719226f

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Accelerating charge transfer in a triphenylamine–subphthalocyanine
donor–acceptor systemw
Anaı
¨
s Medina,a Christian G. Claessens,a G. M. Aminur Rahman,b Al Mokhtar Lamsabhi,c
c
c
b
a
´ ´ ´
˜
Otilia Mo, Manuel Yanez,* Dirk M. Guldi* and Tomas Torres*
Received (in Cambridge, UK) 13th December 2007, Accepted 18th January 2008
First published as an Advance Article on the web 11th February 2008
DOI: 10.1039/b719226f
We have designed, synthesized and probed a dodecafluoro-
subphthalocyaninato boron(III) unit, which bears a triphenyl-
amine moiety in its axial position, as a novel electron donor–
acceptor system.
The synthetic pathway to SubPc–TPA dyad 1z (Scheme 1)
involves the cyclotrimerizarion of the corresponding tetra-
fluorophthalonitrile in the presence of boron trichloride to
obtain SubPc 4a followed by axial substitution of the axial
chlorine atom with the corresponding phenol derivative 3.6
Previously, pinacol borate 27 was obtained in a one-pot
procedure from 4-bromotriphenylamine in 67% yield.8
Palladium-catalyzed Suzuki cross-coupling reaction between
pinacol boronate 2 and 4-iodophenol in the presence of CsF
and catalytic amounts of Pd(PPh3)4 in THF gave rise to
phenol 3 in 54% yield.9 Substitution of the chlorine atom of
chlorododecafluorosubphthalocyanine 4a with phenol 3 in
toluene in the presence of stoichiometric amounts of NEt3
gave SubPc–TPA dyad 1 in 45% yield.
Designing, synthesizing and studying multicomponent elec-
tron donor–acceptor systems that efficiently separate photo-
generated charges over long distances is a multidisciplinary
field of research. It still requires substantial efforts—both
theoretically and experimentally—to achieve performances
that are typically seen in nature.1 Among the broad range of
possible categories of photoactive ensembles, the donor–
acceptor approach is still the most widespread one. Subphtha-
locyanines (SubPcs),2 non-planar 14 p-electron aromatic chro-
mophores, have recently emerged as a platform for novel
electron acceptor units. Importantly, they show great potential
in artificial photosynthetic systems3—a consequence that is
largely based on their outstanding light harvesting features
(550–650 nm), their excitation energies above 2.0 V, and their
low reorganization energies.4 On the other hand, triphenyl-
amine (TPA), a large conjugated tertiary amine system, not
only acts as a strong electron donor but may also stabilize the
charge-transferred state.5
According to TD-DFT calculations at the B3LYP/6-31G(d)
level on ground-state geometries optimized at the same level of
theory, three absorption bands are predicted at 882, 870 and
512 nm in SubPc–TPA 1 (Scheme 1). The former corresponds
essentially to the HOMO–LUMO transition and does not lead
to a charge-separation state and has negligible oscillator
strength. The same applies to the second band which corre-
sponds basically to the HOMO–LUMO+1 transition. The
third band, however, corresponding to the SubPc Q band, has
as dominant contribution the HOMOꢀ1/LUMO+1 excita-
tion leading to a clear charge-separation state (see Fig. 1) and
presents a rather large oscillator strength (0.219), indicating
In the current work we have pursued an approach in which
we have started with a theoretical survey to determine the
frontier orbitals that are involved in the charge separation
process in covalently linked subphthalocyanine–triphenyl-
amine SubPc–TPA-systems. This was followed by the subse-
quent design, synthesis and finally characterization of dyad 1
(Scheme 1), especially in terms of charge transfer features.
Indeed, a rapid charge transfer, evolving between the photo-
excited SubPc and TPA fragments, has been confirmed in
femtosecond transient absorption measurements.
that
1 should exhibit photoinduced charge separation
behavior.
A first set of electrochemical and photophysical experiments
was conducted to establish the optical markers of the radical
a
´
Departamento de Quımica Organica, C-1, Universidad Autonoma de
Madrid, Cantoblanco, 28049 Madrid, Spain. E-mail:
´
´
tomas.torres@uam.es; Fax: +34 91 4973966; Tel: +34 91 4973966
b Universitat Erlangen, Department of Chemistry and Pharmacy
¨
Interdisciplinary Center for Molecular Materials (ICMM),
Egerlandstr. 3, 91058 Erlangen, Germany. E-mail:
dirk.guldi@chemie.uni-erlangen.de; Fax: +49 9131 8527341; Tel:
+49 9131 8528307
Departamento de Quımica, C-9, Universidad Autonoma de Madrid,
c
´
Cantoblanco, E-28049 Madrid, Spain. E-mail:
´
manuel.yanez@uam.es; Fax: +34 4974953; Tel: +34 91 4975238
w Electronic supplementary information (ESI) available: Experimental
procedures and complete structural characterization of all new com-
pounds. See DOI: 10.1039/b719226f
Scheme 1 Synthesis of subphthalocyanine–TPA dyad 1.
Chem. Commun., 2008, 1759–1761 | 1759
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Fig. 1 Electron density plots associated with the HOMOꢀ1 and
LUMO+1 orbitals of SubPc–TPA dyad 1.
Fig. 2 Room-temperature fluorescence spectra of SubPc 4b (black)
and SubPc–TPA dyad 1 (grey) in toluene recorded with solutions that
exhibit optical absorption of 0.5 at the 530 nm excitation wavelength.
ions involved in photoinduced charge separation processes
(Fig. S18 in ESIw). In particular, we tested the one-electron
reduction of the SubPc reference 4b10 (in a deoxygenated
toluene–acetone–2-propanol solvent mixture) and the one-
electron oxidation of the TPA reference 3 (in oxygenated
dichloromethane). In the oxidation experiments a major peak
is noted, with a maximum at 460 nm, and on onset of broad
absorption, which seems to extend out into the near-infrared.
Notably, the latter band is extremely broad and is likely to
reflect the delocalization of the positive charge. Radiolytic
reduction of reference SubPc 4b, on the other hand, leads to
the following features: a transient maximum at 490 nm that is
followed by a sharp bleaching of the ground state between 510
and 610 nm. Moreover, a series of sharp transient peaks in the
range between 800 and 900 nm complete the spectral features.
It should be noted that this feature is unique for perfluorinated
SubPc derivatives such as 4b.11
resolution of approximately 0.1 ns. In fact, considering the
fluorescence quantum yields of 4b and 1 we can extrapolate the
fluorescence lifetime for 4b as B3 ps. The presence of the
electron donor TPA thus causes a rapid intramolecular deac-
tivation of the SubPc fluorescence (i.e., c1010 ꢀ1). A possible
s
rationale implies an exothermic transformation of the SubPc
singlet excited state (2.16 eV) into an intramolecular radical
ion pair state (1.61 eV). Interestingly, such a fast charge
transfer contrasts the behavior seen in a comparable SubPc-
ferrocene (Fc) conjugate, where charge separation occꢀurs with
+
B1010 sꢀ1 to generate a similarly energetic SubPcꢂ ꢀFcꢂ
(B1.4 eV).3
Conclusive information about the photoproducts came
from transient absorption spectroscopy. In particular, with
the help of femtosecond laser pulses (i.e., 580 nm) the fate of
the SubPc singlet excited state was probed in the SubPc
reference 4b and in SubPc–TPA 1. Following, for instance,
the time evolution of the characteristic singlet excited state
features of SubPc is a convenient approach toward identifying
spectral features of the resulting photoproducts (i.e., triplet
excited state in SubPc and radical ion pair state in SubPc–TPA
dyad 1) and determining absolute rate constants for these
intramolecular decays.
Additional experiments by means of cyclic voltammetry
further underline these data. (1) these studies confirmed the
ease of reduction and oxidation of SubPc and TPA references,
4b and 3, respectively. The following redox potentials were
established in these experiments: E1/2 (4b/4bꢂꢀ) = ꢀ1.06 V vs.
Fc/Fc+; E1/2 (3/3ꢂ+) = 0.55 V vs. Fc/Fc+. (2) When adding
the potentials of the first SubPc 4b reduction and the first TPA
3 oxidation an energy of the radical ion pair of 1.61 eV was
estimated.y
Next, we tested the SubPc reference 4b and SubPc–TPA 1 in
steady-state and time-resolved fluorescence experiments. As
Fig. 2 illustrates, common to both compounds is a distinct
fluorescence pattern, which includes a strong *0–0 maximum
around 580 nm (Esinglet = 2.16 eV) followed by a set of *0–1,
*0–2, etc. transitions in the form of poorer resolved shoulders
at 600, 620 and 660 nm. Such features are essentially a mirror
image to the ground-state absorption (i.e., 0–*0 at 573 nm).
Different are, however, the fluorescence quantum yields, which
are 0.31 for the SubPc reference 4b and 1.2 ꢃ 10ꢀ3 for
SubPc–TPA 1 in toluene. This corresponds to an overall
quenching of at least 250. Importantly the SubPc fluorescence
quantum yield depends primarily on the solvent polarity:
toluene 4 anisole 4 THF. In time-resolved fluorescence
measurements, that is, probing the SubPc fluorescence deacti-
vation in the SubPc reference 4b and in SubPc–TPA 1 at
580 nm, reveal for 4b a strictly mono-exponential decay from
which we derived a fluorescence lifetime of 1.6 ns. Not
surprising is the observation that no appreciable SubPc fluor-
escence was registered for 1, that lies outside of a time
The differential spectrum recorded immediately after the
laser pulse for the SubPc reference 4b gives rise to the
instantaneous formation (i.e., 41 ꢃ 1012
s
ꢀ1) of a transient
species, which exhibits the following characteristics: maxima
at 415 and 770 nm and minima at 525 and B575 nm (Fig. S19
in ESIw). The minima are a good reflection of the ground-state
absorption maxima. These spectral attributes are indicative of
the SubPc singlet excited state—Esinglet = 2.16 eV. This decays
slowly (9.1 ꢃ 108 sꢀ1) to the energetically lower-lying triplet
excited state via intersystem crossing—ETriplet = 1.45 eV.
Kinetically, an important aspect is that the intersystem cross-
ing goes well in hand with the lifetimes derived in the fluor-
escence experiments. Spectroscopically, the newly formed
features include maxima at 455 and 611 nm, which are further
accompanied by a strong transient bleach in the region of the
ground state absorption (i.e., 510–590 nm). Complementary
nanosecond experiments helped to corroborate these transient
changes and their oxygen sensitive nature through generation
of singlet oxygen (not shown).
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1760 | Chem. Commun., 2008, 1759–1761

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triphenylamine moiety through a rigid phenylene spacer,
undergo fast photoinduced electron transfer.
Financial support from the Ministerio de Ciencia y Tecno-
logıa (CTQ2005-08933/BQU and Consolider-Ingenio 2010
´
CSD2006-0015), Comunidad de Madrid (MADRISOLAR,
S-0505/PPQ/0225) EU (MRTN-CT-2006-035533, Solar-N-
type) the Deutsche Forschungsgemeinschaft through
SFB583, DFG (GU 517/4-1), FCI, and the Office of Basic
Energy Sciences of the US Department of Energy is gratefully
acknowledged.
Notes and references
z SubPc–TPA dyad 1 was isolated as a purple solid. Mp 4250 1C. 1H
NMR (500 MHz, CDCl3): d 7.15 (t, 4H), 7.14 (d, 2H), 6.99 (d, 2H),
6.98 (d, 2H), 6.95 (d, 2H), 6.93 (d, 2H), 5.32 (d, 2H) ppm. 13C NMR
(126 MHz, CDCl3): d 154.9, 147.8, 146.7, 138.4, 135.0, 133.4, 129.9,
129.3, 127.9, 127.4, 124.3, 124.2, 122.8, 117.9, 115.7 ppm. 19F NMR
(470.5 MHz, CDCl3): d ꢀ147.38 (m, 6F), ꢀ136.65 (m, 6F) ppm. 11B
NMR (160.5 MHz, CDCl3): d ꢀ15.1 (s) ppm. MS (FAB, m-NBA): m/z
Fig. 3 Differential absorption spectrum (visible and near-infrared)
obtained upon femtosecond flash photolysis (580 nm, 150 nJ) of
SubPc–TPA 1 in nitrogen-saturated toluene with time delays of 0 ps
(black spectrum), 1 ps (red spectrum) and 5 ps (orange spectrum) at
room temperature. Inset: time-absorption profiles of the spectra at
500 nm, monitoring the formation and decay of the radical ion pair
state.
=
947.2 [M]+
,
611.1 [M
ꢀ
axial group]+
. HRMS calc. for
C48H18BF12N7O: 947,1474; found, 947.1454. UV-Vis (CHCl3): lmax
/
nm (log e) = 571 (4.6), 552 (sh), 531 (sh), 310 (4,4). FT-IR (KBr):
n/cmꢀ1 3031, 2925, 1534, 1485, 1318, 1032, 881, 680. Anal. Calc.
for C48H18BF12N7O: C, 60.85; H, 1.91; N, 10.35. Found: C, 60.71;
H, 2.00; N, 10.26%.
y The corresponding experiments with SubPc–TPA 1 yield an energy
of 1.68 eV.
Regarding the femtosecond transient absorption measure-
ments of dyad 1, immediately, after the laser excitation, the
strong singlet–singlet absorption of the SubPc grows in in-
s
stantaneously (i.e. 1012 ꢀ1). This confirms that, despite the
presence of TPA, the SubPc singlet excited state is successfully
formed. Instead of seeing, however, the slow intersystem
crossing dynamics, as we have discussed in the context of
the SubPc reference 4b, the singlet–singlet absorptions decay
1 Electron Transfer in Chemistry, ed. V. Balzani, Wiley-VCH, Wein-
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(1 ꢃ 1012
s
ꢀ1) in the presence of the electron donating TPA via
3 (a) D. Gonza
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´ ´
an intramolecular charge transfer reaction to yield
ꢀ
+
SubPcꢂ –TPAꢂ
.
´
-
Spectroscopic proof for the radical ion pair formation was
obtained from the features developing in parallel with the
disappearance of the SubPc singlet–singlet absorption in the
visible and the near-infrared, which are exemplified in Fig. 3.
In the visible region, two sharp absorptions correspond to the
one-electron oxiꢀdized TPAꢂ+ (460 nm) and to the one-electron
reduced SubPcꢂ (495 nm). We noted, on the other hand, in
the near-infrared region fine-structured peaks in the 700–
800 nm range and a 1100 nm maximum, whicꢀh resemble the
signatures of the one-electron reduced SubPcꢂ and the one-
electron oxidized TPAꢂ+, respectively. In other words, the
transient features all the spectroscopic markers that were
found in the electrochemical and radiolytical assays.
´
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Both radical ion pair attributes are only short lived and start
to decay on the picosecond time scale. To examine the charge-
recombination dynamics all of the spectral fingerprints are
useful probes. Important is the fact that their decays resemble
each other and give rise to kinetics that obey a clean unim-
olecular rate law. The lifetimes range from 17 ps (toluene) to
9 ps (THF) and 7 ps (benzonitrile).
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´ ´
In view of these studies it can be concluded that dodeca-
fluorosubphthalocyanine is an excellent electron-acceptor unit
which, when covalently linked in its axial position to a
Herranz and L. Echegoyen, J. Am. Chem. Soc., 2004, 126,
6301–6313.
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Chem. Commun., 2008, 1759–1761 | 1761