High-potential perfluorinated phthalocyanine-fullerene dyads for generation of high-energy charge-separated states: Formation and photoinduced electron-transfer studies

Article abstract of DOI:10.1002/cphc.201402118

High oxidation potential perfluorinated zinc phthalocyanines (ZnF _nPcs) are synthesised and their spectroscopic, redox, and light-induced electron-transfer properties investigated systematically by forming donor-acceptor dyads through metal-lig

Full text of DOI:10.1002/cphc.201402118

CHEMPHYSCHEM
ARTICLES
DOI: 10.1002/cphc.201402118
High-Potential Perfluorinated Phthalocyanine–Fullerene
Dyads for Generation of High-Energy Charge-Separated
States: Formation and Photoinduced Electron-Transfer
Studies
Sushanta K. Das, Andrew Mahler, Angela K. Wilson, and Francis D’Souza*[a]
+
C
Cꢀ
High oxidation potential perfluorinated zinc phthalocyanines
(ZnFnPcs) are synthesised and their spectroscopic, redox, and
light-induced electron-transfer properties investigated system-
atically by forming donor–acceptor dyads through metal–
ligand axial coordination of fullerene (C60) derivatives. Absorp-
tion and fluorescence spectral studies reveal efficient binding
of the pyridine- (Py) and phenylimidazole-functionalised fuller-
ene (C60Im) derivatives to the zinc centre of the FnPcs. The de-
termined binding constants, K, in o-dichlorobenzene for the
1:1 complexes are in the order of 104 to 105 mꢀ1; nearly an
order of magnitude higher than that observed for the dyad
formed from zinc phthalocyanine (ZnPc) lacking fluorine sub-
stituents. The geometry and electronic structure of the dyads
are determined by using the B3LYP/6-31G* method. The
and ZnFnPc –C60Py (n=0, 8 or 16) intra-supramolecular
charge-separated states during electron transfer. Electrochemi-
cal studies on the ZnPc–C60 dyads enable accurate determina-
tion of their oxidation and reduction potentials and the energy
of the charge-separated states. The energy of the charge-sepa-
rated state for dyads composed of ZnFnPc is higher than that
of normal ZnPc–C60 dyads and reveals their significance in har-
vesting higher amounts of light energy. Evidence for charge
separation in the dyads is secured from femtosecond transient
absorption studies in nonpolar toluene. Kinetic evaluation of
the cation and anion radical ion peaks reveals ultrafast charge
separation and charge recombination in dyads composed of
perfluorinated phthalocyanine and fullerene; this implies their
significance in solar-energy harvesting and optoelectronic
device building applications.
HOMO and LUMO levels are located on the Pc and C60 entities,
+
C
Cꢀ
respectively; this suggests the formation of ZnFnPc –C60Im
1. Introduction
Photoinduced electron transfer in donor–acceptor models is of
great interest for researchers designing artificial photosynthetic
systems for light energy harvesting, solar fuel production, and
building opto-electronic devices.[1–13] Porphyrins (P)[14] and
phthalocyanines (Pc)[15] have been studied extensively due to
their roles in mimicking biological systems. Additionally, these
macrocycles offer high chemical and thermal stabilities, ab-
sorption and redox features, and the possibility of tuning
these properties by inserting different metals into the macro-
cycle central cavity or by introducing different substituents at
their ring peripheries. As a result of their remarkable absorp-
tion properties in the visible region, they have been highly
sought out photosensitisers in photovoltaic applications, and
redox-based fluorescence applications.[13]
faces has gained some interest. This is primarily due to the
possibility of generating high-energy charge-separated states
suitable for developing water oxidation/proton reduction cata-
lysts.[16–18] In this context, recently, we reported donor–acceptor
dyads composed of high potential P–fullerene (C60) dyads by
employing P with either fluoro or chloro substituents on the
meso-aryl entities.[18] As expected, the presence of halogens on
the periphery of P made the oxidation processes difficult; how-
ever, it was possible to observe ultrafast photoinduced elec-
tron transfer in these dyads. Encouraged by these findings,
in the present study, we have employed perfluorinated Pcs
as high oxidation potential photosensitisers and built donor–
acceptor dyads by using fulleropyrrolidines (C60Pys) with
either a pyridine or phenylimidazole axial ligating func-
tionality. Two perfluorinated ZnPc derivatives, namely, zinc
2,3,9,10,16,17,23,24,-octafluoro-29H,31H-phthalocyanine
Recently, employing high (oxidation) potential sensitisers to
build donor–acceptor dyads or immobilise semiconductor sur-
(ZnF8Pc) with 8 fluorine substituents on the macrocycle ring
periphery and zinc 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hex-
adecafluoro-29H,31H-phthalocyanine (ZnF16Pc) with 16 fluorine
substituents on the macrocycle ring periphery were synthes-
ised. Donor–acceptor dyads were formed by the metal–ligand
axial coordination approach with either pyridine- (C60Py) or
phenylimidazole-functionalised (C60Im) fulleropyrrolidine (see
Figure 1 for structures). Zinc 2,9,16,23-tetra-tert-butyl-29H,31H-
[a] Dr. S. K. Das, A. Mahler, Prof. A. K. Wilson, Prof. Dr. F. D’Souza
Department of Chemistry, University of North Texas
1155 Union Circle, #305070, Denton, TX 76203-5017 (USA)
Fax: (+1) 940-565-4318
francis
E-mail: .dsouza@unt.edu
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201402118.
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Figure 2. a) Absorption spectra of ZnPc (i), ZnF8Pc (ii) and ZnF16Pc (iii); and
b) steady-state fluorescence spectra of ZnPc, lex =680 nm (i); ZnF8Pc,
lex =710 nm (ii) and ZnF16Pc, lex =730 nm (iii). All of the compounds were
prepared in degassed DCB.
Figure 1. Structure of the ZnPc, ZnF8Pc, ZnF16Pc, C60Im and C60Py com-
pounds used in this study.
phthalocyanine (ZnPc), with no fluorine substituents, was also
used to form the dyads with C60. Photoinduced electron trans-
fer in these dyads was investigated systematically by using var-
ious spectroscopic, electrochemical, computational and transi-
ent spectral studies to unravel the significance of high poten-
tial Pcs in electron-transfer reactions.
and 680 nm, that is, a gradual redshift of both the Soret and
visible bands of ZnPcs with increasing fluorine substituents
was observed. Interestingly, ZnF8Pc revealed a broad absorp-
tion in the l=475 nm region and a peak at l=860 nm, which
slightly decreased in intensity with time in solution to suggest
appreciable decomposition. However, no such results were
seen for ZnF16Pc, revealing its higher stability. Consequently,
fresh solutions of the studied compounds were prepared prior
to performing spectral and photophysical measurements. A
similar spectral trend was also observed in the fluorescence
spectrum of these compounds. Upon excitation at the wave-
length maxima of the most intense visible band, the emission
maxima were found to be at l=745 nm for ZnF16Pc, l=
727 nm for ZnF8Pc and l=692 nm for ZnPc. In addition, the
fluorescence intensity of the fluorinated Pcs also revealed dras-
tic changes, that is, a 30% reduction in intensity for ZnF8Pc
and 63% reduction intensity for ZnF16Pc compared with the in-
tensity of the ZnPc emission band was observed. Notably, the
redshifted absorbance and emission bands of ZnF8Pc and
ZnF16Pc reveal their significance in harvesting light from the
near-infrared (NIR) portion of the electromagnetic spectrum.
Figure 3 shows the time-resolved fluorescence decay curves
obtained by the TCSPC of various fluorinated Pc derivatives in
degassed toluene. All of the ZnPc derivatives revealed mono-
exponential decay with lifetimes of 3.14, 2.85 and 2.40 ns, re-
spectively, for ZnPc, ZnF8Pc and ZnF16Pc. The lifetimes tracked
the fluorescence intensities shown in Figure 2b.
2. Results and Discussion
2.1. Characterisation of the Perfluorinated ZnPc Derivatives:
Absorbance and Fluorescence Studies
Detailed synthetic details for the perfluorinated Pcs is provided
in the Experimental Section. The one-step synthesis involved
the reaction of either 4,5-difluorophthalonitrile or tetrafluor-
ophthalonitrile in the presence of zinc chloride in dimethyle-
thanolamine (DMAE) at elevated temperature followed by
chromatographic purification on a silica gel column with a mix-
ture of chloroform/methanol as the eluent. The newly synthes-
ised compounds were stored in dark prior spectral and photo-
chemical investigations.
Figure 2a shows the normalised absorption spectrum of the
Pc sensitisers in o-dichlorobenzene (DCB). For ZnF16Pc, the
bands were located at l=380, 656 and 734 nm, whereas for
ZnF8Pc these bands were located at l=352, 477(br), 640 and
712 nm. The position of these bands compared with the con-
trol ZnPc, the bands for which were located at l=350(sh), 614
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involving ZnFnPc and C60Im or ZnFnPc and C60Py can be repre-
sented by Equations (1) and (2), in which the symbol “:” repre-
sents a coordinate bond between zinc–imidazole or zinc–pyri-
dine entities; Fn represents fluorine substituents (n=0, 8 or 16)
on the ZnPc ring.
C60Im þ ZnFnPc $ C60Im : ZnFnPc
C60Py þ ZnFnPc $ C60Py : ZnFnPc
ð1Þ
ð2Þ
Unlike for ZnPc binding to axial coordinating ligands, for
which spectral changes for the visible bands are marginal,
both ZnF8Pc and ZnF16Pc revealed significant changes accom-
panied by one or more isosbestic points during axial coordina-
tion. This was true even for pristine pyridine or phenylimida-
zole binding to these macrocycles. During the course of these
titrations, no evidence for ground-state electron transfer be-
tween ZnPc and C60 (or pristine pyridine or phenylimidazole)
was observed; this suggests that the observed changes in
Figure 4 are indeed due to axial coordination. Further, by using
the absorbance titration data, Benesi–Hildebrand plots[19] were
constructed for all of the dyads (Figure 4b). The formation con-
stant, K, for the C60Py:ZnF16Pc dyad was found to be 3.5ꢁ
105 mꢀ1, whereas for the C60Im:ZnF16Pc dyad it was 5.8ꢁ
105 mꢀ1. The K values for other studied dyads are given in
Table 1 and the spectral data is given in the Supporting Infor-
Figure 3. Time-correlated single photon counting (TCSPC) fluorescence
decay curves of prompt/scatter (i), ZnF16Pc (ii), ZnF8Pc (iii) and ZnPc (iv) re-
corded in degassed toluene. The samples were excited at l=674 nm by
means of a nano-light-emitting diode (nanoLED) with a pulse width of
about 100 ps and the emission was collected at the emission peak maxima
of ZnPcs. The time calibration factor was 0.439 ns per channel.
2.2. Formation and Characterisation of ZnPc-C60 Dyads Self-
Assembled by Axial Co-ordination
Axial coordination of the electron acceptors C60Im and C60Py to
various perfluorinated ZnPcs was first studied by optical ab-
sorption methods. Figure 4 shows the spectral changes ob-
served during the titration of ZnF16Pc with C60Py in DCB. For
the sake of simplicity, ZnF16Pc and C60Py have been chosen
here to illustrate both the titrations for UV/Vis absorbance and
fluorescence emission titrations. Absorbance and fluorescence
spectral changes for the other ZnPcs during the formation of
the dyads is given in Figures S1–S7 in the Supporting Informa-
tion. Upon increasing the addition of a calculated concentra-
tion of C60Py to ZnF16Pc, peaks at l=430 and 700 nm ap-
peared, whereas the visible band of ZnF16Pc at l=730 nm di-
minished in intensity. The sharp peak at l=430 nm could be
attributed to C60Py, which increased with increasing concentra-
tion of C60Py. An isosbestic point at l=710 nm confirmed the
existence of an equilibrium process. The equilibrium process
Table 1. Formation constant, K, calculated from the Benesi–Hildebrand
method and using the absorbance and fluorescence data, and binding
energies (BEs) obtained from B3LYP/6-31G*-optimised structures.
[a]
[b]
[c]
Donor
ZnPc
Acceptor
K [mꢀ1
]
K [mꢀ1
]
BE [kcalmolꢀ1
]
C60Im
C60Py
C60Im
C60Py
C60Im
C60Py
1.3ꢁ105
8.6ꢁ104
2.2ꢁ105
1.1ꢁ105
6.0ꢁ105
3.7ꢁ105
1.1ꢁ105
7.9ꢁ104
2.1ꢁ105
1.0ꢁ105
5.8ꢁ105
3.5ꢁ105
ꢀ24.95
ꢀ21.60
ꢀ27.68
ꢀ23.95
ꢀ29.71
ꢀ25.63
ZnF8Pc
ZnF16Pc
[a] Formation constant based on fluorescence titration data. [b] Forma-
tion constant based on absorbance data. [c] From B3LYP/6-31G*-opti-
mised structures.
mation. The K values followed the trend ZnF16Pc>ZnF8Pc>
ZnPc for given C60 derivatives, whereas between C60Im and
C60Py higher values were obtained for C60Im binding. These
trends generally agreed with the earlier reported dyads for flu-
orinated ZnP derivatives with C60Im and C60Py.[18] In other
words, electron-deficient ZnPcs form dyads of higher stability.
Better spectral changes seen for axial binding of fluorinated
ZnPcs could be ascribed to higher binding constants.
2.3. Steady-State Fluorescence Measurements
Figure 5 shows the fluorescence spectra during the formation
of the C60Py:ZnF16Pc supramolecular dyad. Upon the addition
of various aliquots of C60Py at the same concentrations as
those used for the absorbance titration, fluorescence quench-
ing of Pc was observed along with a blueshift of 7–8 nm of the
Figure 4. Spectral changes observed during the titration of ZnF16Pc
(0.78ꢁ10ꢀ4 m) upon increasing addition of C60Py (0.113 mm each addition) in
DCB. Inset: Benesi–Hildebrand plot constructed to evaluate the binding con-
stant. A0 and A represent the absorbance of the ZnF16Pc in the absence and
presence, respectively, of added C60Py.
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firms better axial coordination of C60Im with ZnPc than that of
C60Py, as seen from our previous study.[22] With the introduction
of fluorine substituents on the Pc ring, the BE of the supra-
molecular dyads is seen to increase relative to ZnPc-based
dyads. In the case of ZnF8Pc, there was an increase in BE by
2.45 and 2.73 kcalmolꢀ1, respectively, for C60Py and C60Im axial-
ly coordinated moieties. As the number of electron-withdraw-
ing fluorine substituents increased to 16 for ZnF16Pc, a further
increase in BEs amounting to 1.68 and 2.03 kcalmolꢀ1 were cal-
culated for C60Py and C60Im binding, respectively. The ZnꢀN
distance of the axial bond was about 2.1 ꢂ for both structures.
The centre-to-centre distances were 12.2 and 10.1 ꢂ for
C60Im:ZnPc and C60Py:ZnPc dyads, respectively, that is, the
close proximity of C60 to the Pc ring in the case of C60Py:ZnPc,
compared with that of C60Im:ZnPc, was evident from these cal-
culations. The frontier orbitals, namely, the HOMO and LUMO,
were generated along with the supramolecular electrostatic
potential maps. As shown in Figure 6a and b, the HOMO was
Figure 5. Fluorescence spectral changes observed during the titration of
ZnF16Pc (0.78ꢁ10ꢀ4 m) upon increasing addition of C60Py (0.113 mm each ad-
dition) in DCB, excited at l=730 nm. Inset: Benesi–Hildebrand plot con-
structed to evaluate the binding constant. I0 and I represent the fluores-
cence intensity of ZnF16Pc in the absence and presence, respectively, of
added C60Py.
emission band. To verify that the shift was due to binding of
C60Py with ZnF16Pc, fluorescence emission was recorded by ex-
citing ZnF16Pc at l=692 nm, corresponding to the shoulder
band of ZnF16Pc. A blueshift under these conditions was also
observed. A similar spectral trend was observed during the for-
mation of C60Im:ZnF16Pc, as shown in Figure S7 in the Support-
ing Information. The formation constant for the supramolec-
ular dyads, calculated by constructing Benesi–Hildebrand
plots[19] of fluorescence quenching data, is listed in Table 1. For
the given dyad, the value of K was comparable to the corre-
sponding value obtained by absorption studies. The trend in K
values with respect to the nature of the Pc macrocycle and C60
ligating functionality discussed earlier also were observed
herein.
2.4. Computational Study
Figure 6. a) Frontier HOMO and b) LUMO of the B3LYP/6-31G*-optimised
supramolecular C60Py:ZnF16Pc dyad. Electrostatic potential map for
c) C60Im:ZnF16Pc and d) C60Py:ZnF16Pc. For a coloured image, please see Fig-
ure S15 in the Supporting Information.
To understand the geometry and electronic structures of the
supramolecular dyads, computational studies were performed
with density functional theory. The Becke three-parameter,
Lee–Yang–Parr hybrid functional (B3LYP) was chosen for these
calculations due to the well-documented success of this func-
tion in geometry optimisations and predicted electronic prop-
erties, including Pcs,[21a] although linear Hartree–Fock calcula-
fully localised on the ZnPc macrocycle, irrespective of the large
number of fluorine substituents, whereas the LUMO was on
the C60 entity (see Figure S11 in the Supporting Information for
the HOMOs and LUMOs of ZnF8Pc-derived dyads). The electro-
static potential map was also indicative of the electron-rich
nature of ZnPc relative to the electron-deficient nature of C60
(Figure 6c and d).
tions to predict properties of Pcs are also well known.[21b,c]
A
Pople-style 3-21G* basis set was used for geometry optimisa-
tions and 6-31G* was used to calculate BEs and the HOMO/
LUMO energies. All calculations were performed by using the
Gaussian 09 computational software package.[20] The optimised
structures were not much different from that reported earlier
for dyads formed from ZnPs or zinc naphthalocyanine
(ZnNc).[21a] The BEs, calculated from the difference between the
energy of the optimised structure minus the sum of individual
energies of ZnPc and C60, are listed in Table 1. Interestingly, the
trend in these values tracked that of the binding constants,
that is, ZnPc upon axial ligation of C60Im and C60Py revealed
BEs of ꢀ24.95 and ꢀ21.60 kcalmolꢀ1, respectively. This con-
2.5. Redox Potentials and Energy Level Diagram
Electrochemical redox potentials of the various ZnPc and C60Py
derivatives obtained by using cyclic voltammetry techniques
were evaluated in DCB containing 0.1m tetrabutylammonium
perchlorate as a supporting electrolyte. Under the solution
conditions, the perfluorinated Pcs were sparingly soluble, just
enough to measure the redox potentials. As predicted, the per-
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Table 2. Electrochemical redox potentials (versus the ferrocene/ferroceni-
um couple, Fc/Fc+),[a] free energy change for charge separation (ꢀDGCS
and charge recombination (ꢀDGCR) for the investigated dyads in DCB.
)
[b]
[c]
[d]
[e]
Donor
ZnPc
Acceptor
Eox
Ered
DGCS [eV]
DGCR [eV]
C60Im
C60Py
C60Im
C60Py
C60Im
C60Py
0.08
0.03
0.39
0.34
0.58
0.57
ꢀ1.07
ꢀ1.08
ꢀ1.03
ꢀ1.04
ꢀ0.95
ꢀ0.96
ꢀ0.68
ꢀ0.72
ꢀ0.30
ꢀ0.34
ꢀ0.15
ꢀ0.16
ꢀ1.15
ꢀ1.11
ꢀ1.42
ꢀ1.38
ꢀ1.53
ꢀ1.52
ZnF8Pc
ZnF16Pc
[a] From differential pulse voltammetry in DCB, 0.1m tetrabutylammoni-
um perchlorate [(TBA)ClO4]. [b] ZnPc-centred oxidation. [c] C60-centred re-
duction. [d] DGCS =EoxꢀEredꢀE(0,0). [e] DGCR =EoxꢀEred
, in which E(0,0) for
ZnPc=1.83 eV, E(0,0) for ZnF8Pc=1.72 eV and E(0,0) for ZnF16Pc=1.68 eV
based on absorbance and emission data in DCB; Eox and Ered are the first
oxidation potential of the donor, ZnPc and the first reduction potential of
the acceptor, C60 recorded in DCB containing 0.1m (TBA)ClO4 obtained by
differential pulse voltammetry. The solvation energy was neglected in the
free energy calculations due to the rigid nature of the donor and accept-
or entities (Ref. [25]).
Figure 7. Energy level diagram showing photochemical events occurring in
the C60Py:ZnFnPc and C60Im:ZnFnPc (n=0, 8, 16) dyads, leading to charge
separation and charge recombination.
fluorinated ZnPcs revealed facile reduction, but more difficult
oxidations as a result of electron-withdrawing fluorine substitu-
ents on the macrocycle periphery.[23] Upon coordinating C60Py
or C60Im, small changes in the potentials of both ZnPc and C60
were observed. Table 2 lists the first oxidation potential of the
ZnPc and the first reduction potential of C60 needed to esti-
mate free energy changes for charge separation (DGCS) and
charge recombination (DGCR); the voltammograms are shown
in Figures S8–S10 in the Supporting Information.
2.6. Femtosecond Transient Absorption Studies
To establish the mechanistic details of the formation of charge-
separated states, and to evaluate kinetic information on
charge separation and recombination processes in the supra-
molecular dyads, transient absorption studies were performed
by using the femtosecond pump–probe technique with l=
400 nm laser excitation. Toluene, instead of DCB, was used as
the solvent due to better solubility of the investigated com-
pounds. C60 in toluene upon excitation showed the instantane-
The DGCS and DGCR values were evaluated by using the
Rehm–Weller approach[24] and are also given in Table 2. In
these calculations, the solvation energies were omitted due to
the rigid nature of both C60 and Pc entities, for which highly
delocalised p-cation and p-anion radicals were expected to
form.[25] From the values listed in Table 2, it is clear that substi-
tution of fluorine atoms on the Pc ring lowers DGCS in the
order of ZnPc>ZnF8Pc>ZnF16Pc. Concurrently, the calculated
DGCR values increase with an increasing number of fluorine
substituents on the Pc macrocycle.
1
ous formation of an intense C60* broad peak centred at l=
950 and 500 nm that decayed at a rate of 7.9ꢁ1010 ꢀ1, as
s
shown in Figure S12 in the Supporting Information. The decay
1
of the singlet C60* at l=950 nm was accompanied by an in-
3
1
crease of the C60* state at l=760 nm. The lifetime of the C60*
state was calculated to be 2.6 ns by using a mono-exponential
decay fit.
Figure 8a shows the femtosecond transient absorption spec-
tra recorded for ZnPc in toluene at different time intervals.
Upon excitation, instantaneous singlet–singlet spectral features
were observed with characteristic excited singlet peaks at l=
438, 478, 594, 634 and 840 nm accompanied by transient
bleaching at l=610 and 684 nm; this is opposite to the ab-
sorbance spectral features of ZnPc.[26] The singlet features de-
cayed with a concomitant increase in the triplet features, both
in the visible and NIR regions. The peak at l=478 nm could
Figure 7 shows the energy level diagram related to photoin-
duced electron transfer from singlet excited perfluorinated
ZnPc to coordinated C60. Owing to the redshift in the absorp-
tion and emission of fluorinated Pcs (Figure 2), there was a de-
crease in the singlet–singlet energy from 1.83 eV for ZnPc to
1.72 eV for ZnF8Pc to 1.68 eV to ZnF16Pc. Even then, the calcu-
lated DGCS values suggest that electron transfer is indeed pos-
sible from their singlet excited states. Interestingly, owing to
the more difficult oxidation of perfluorinated Pc, the energy of
the charge-separated states gradually becomes higher. The
generation of high-energy radical ion pairs implies maximum
utilisation of light energy. The radical ion pairs thus formed
3
be attributed to the ZnPc* state. Similar spectral features were
also observed for perfluorinated ZnPcs. As shown in Figure 8b,
upon excitation, ZnF8Pc revealed transient features with
bleaching at l=630 and 710 nm, corresponding to the ab-
sorbance bands and positive peaks at l=500, 550, 578, 745
and 1130 nm. The singlet peaks decayed to populate the trip-
let state in the l=500–600 nm range with a lifetime of 2.8 ns.
ZnF16Pc also revealed such singlet–singlet features that de-
cayed with a lifetime of 2.4 ns.
3
could populate low-lying 3ZnPc* (ET ꢁ1.1 eV) and not C60*
(ET =1.56 eV)[25] due to the high energy of the latter relative
to that of the radical ion pairs prior returning to the ground
state.
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from these data were 4.0ꢁ108 and 2.1ꢁ108 sꢀ1 for the corre-
sponding radical ion pairs; this indicated a slower charge re-
+
Cꢀ
C
combination process in the case of the C60 Im:ZnPc radical
ion pair due to increased donor–acceptor separation than that
in the former dyad.
Figure 10 shows the femtosecond transient absorption spec-
tra of C60Py:ZnF8Pc and C60Im:ZnF8Pc dyads at different delay
times. Upon photoexcitation of the dyads, slightly different
spectral features were observed compared with dyads com-
posed of ZnPc. This could be attributed to fluorine substitu-
ents on the Pc ring. The instantaneous formation of singlet
spectral features was observed with a characteristic excited sin-
glet peak corresponding to 1ZnF8Pc* in the l=480–500 nm
region. However, the decay of these peaks, including recovery
of the absorption band in the l=700 nm region, was much
faster due to the occurrence of an intramolecular process ac-
companied by characteristic peaks at l=893 and 1020 nm.
+
C
These peaks could be assigned to the ZnF8Pc cation and
Cꢀ
C60 anion radicals, respectively, resulting in the formation of
+
ꢀ.
+
Cꢀ
C
C
C60 Py:ZnF8Pc and C60 Im:ZnF8Pc radical ion pairs. The
+
C
identity of the ZnF8Pc cation radical at l=893 nm was con-
firmed by performing chemical oxidation of ZnF8Pc by using
nitrosonium hexafluoroborate as an oxidising agent (see Fig-
ure S13 in the Supporting Information for the spectrum of the
Figure 8. Femtosecond transient spectra of a) ZnPc and b) ZnF8Pc at an exci-
tation wavelength of l=400 nm by using a 100 fs pulsed laser in nitrogen-
saturated toluene at the indicated time intervals.
+
+
C
C
ZnF8Pc cation radical). The nearly 50 nm redshift of ZnF8Pc
+
C
relative to ZnPc is consistent with the previously discussed
absorption spectra of neutral Pc derivatives (Figure 2). The evo-
lution of the transient bands was much faster than that ob-
The transient absorption spectra of C60Py:ZnPc and
C60Im:ZnPc dyads at different delay times in toluene are shown
in Figure 9a and b, respectively. The instantaneously formed
singlet decayed with the generation of new transient bands at
l=848 and 1020 nm. The former transient band is attributed
served for the ZnPc sensitiser. The time constants were 20
+
Cꢀ
C
and 52 ps for the formation of C60 Py:ZnF8Pc
and
ꢀ.
C60 Im:ZnF8Pc +, respectively, which resulted in kCS values of
C
5.0ꢁ1010 and 1.9ꢁ1010 ꢀ1, respectively, for radical ion pair for-
s
mation. These values are four to five times larger than those
to the formation of ZnPc +, whereas the latter is attributed to
observed for ZnPc-derived dyads. The decay of the radical ion
C
Cꢀ [27,28]
Cꢀ
C
C
+
the formation of C60
;
thus providing evidence for the for-
pairs was also faster: whereas the decay of C60 Py:ZnF8Pc
+
+
+
Cꢀ
C
Cꢀ
C
Cꢀ
mation of C60 Py:ZnPc and C60 Im:ZnPc radical ion pairs.
was over by 3 ns, the decay of C60 Im:ZnF8Pc persisted
for little over 3 ns. The measured kCR values were 7.9ꢁ108
1
These results demonstrate that ZnPc* formed undergoes pho-
+
Cꢀ
C
toinduced electron transfer instead of intersystem crossing to
populate the triplet excited state of either ZnP or C60. The ki-
netics of charge separation, kCS, and charge recombination, kCR,
were monitored by following the time profiles of the radical
cation and radical anion bands, as shown in Figure 9c and d,
and 6.4ꢁ109 sꢀ1, respectively, for the C60 Py:ZnF8Pc
and
+
Cꢀ
C
C60 Im:ZnF8Pc radical ion pairs.
Figure 11a and
b
shows the transient spectra of
C60Py:ZnF16Pc and C60Im:ZnF16Pc dyads at different delay times
in toluene. Due to limited solubility of the dyads, it was diffi-
cult to obtain the transient spectra with a better signal/noise
ratio. However, in the NIR region, the transient bands at l=
+
+
Cꢀ
C
Cꢀ
C
respectively, for the C60 Py:ZnPc and C60 Im:ZnPc radical
ion pairs. The rise time was found to be smaller for C60Py:ZnPc
(75 ps) than that of C60Im:ZnPc (200 ps) due to an increased
distance between the donor and acceptor entities in the latter
dyad. The rise time obtained here compared well with the rise
time reported earlier for a C60Im:ZnNc dyad from picosecond
907 and 1020 nm were clearly observed. These peaks could be
+
C
Cꢀ
assigned to the ZnF16Pc radical cation and C60 radical anion,
+
Cꢀ
C
resulting in the formation of C60 Py:ZnF16Pc
and
+
Cꢀ
C
C60 Im:ZnF16Pc radical ion pairs. Similar to previously dis-
cussed ZnF8Pc-based dyads, the rise time of charge separation
was smaller: 17 and 21 ps for C60Py- and C60Im-based dyads,
transient studies.[28] The kCS values calculated from the rise time
were 13.3ꢁ109 sꢀ1 for C60 Py:ZnPc
109 sꢀ1 for C60 Im:ZnPc formation. The decay of both the
radical cation and radical anion bands followed monoexponen-
tial decay and persisted for a little over 3 ns; the time window
of our instrumental setup. The time constants estimated from
+
Cꢀ
C
formation and 5.0ꢁ
respectively. The determined kCS values were 5.9ꢁ1010
ꢀ.
+
C
+
Cꢀ
C
and 4.7ꢁ1010 sꢀ1
,
respectively, for C60 Py:ZnF16Pc
and
+
Cꢀ
C
C60 Im:ZnF16Pc radical ion pair formation. The decay of
the radical ion pairs was also fast relative to the previously
discussed dyads. The measured kCR values were 12.1ꢁ108
C
respectively, for C60 Py:ZnF16Pc
C60 Im:ZnF16Pc radical ion pairs.
the decay curve at Ao/e of the monoexponential decay curve
+
+
Cꢀ
C
Cꢀ
were 2500 and 4800 ps, respectively, for the C60 Py:ZnPc
and 10.2ꢁ109 sꢀ1
,
and
ꢀ.
+
+
C
Cꢀ
C
and C60 Im:ZnPc radical ion pairs. The kCR value calculated
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Figure 9. Femtosecond transient spectra of a) C60Py:ZnPc and b) C60Im:ZnPc dyads at the excitation wavelength of l=400 nm using 100 fs pulsed laser in ni-
+
Cꢀ
C
trogen saturated toluene at the indicated time intervals. The time profile of the C60 band at l=1020 nm and ZnPc band at l=840 nm of the
+
+
Cꢀ
C
Cꢀ
C
C60 Py:ZnPc (c) and C60 Im:ZnPc (d) radical ion pairs are also shown.
Table 3 lists kinetic data for the various dyads discussed
herein. Both kCS and kCR increased with an increasing number
of fluorine substituents on the Pc ring periphery, and the rates
were generally higher for C60Py-derived dyads than those of
C60Im-derived ones. This trend agrees well with recently report-
ed dyads based on halogenated P and C60 assembled using
the metal–ligand axial coordination approach.[18] Additionally,
for a given dyad, the kCS value was one to two orders of mag-
nitude higher than that of kCR; a result that can be attributed
to a low reorganisation energy demand of the donor and ac-
ceptor employed herein.[29] Interestingly, the kinetic results did
not follow the expected trend in the free energy change rela-
tionship (Table 2), according to Marcus theory.[30] The geometry
of the dyads and populating the phthalocyanine triplet state
during charge recombination (Figure 7) are attributed to this
cause. Furthermore, the extent of charge stabilisation was eval-
uated by taking the ratio of kCS/kCR (Table 3). These values for
the C60Py-derived dyads were slightly higher than those of the
C60Im-derived dyads. Higher values of kCS/kCR for perfluorinated
Pc derived dyads were obtained; this implies better charge sta-
bilisation.
Table 3. Rise time, rate of charge separation (kCS), decay time constant
and rate of charge recombination (kCR) calculated for various supramolec-
ular dyads of perfluorinated ZnPc–C60Py dyads in nitrogen-saturated tolu-
ene.
[a]
[b]
Donor
ZnPc
Acceptor tCS [ps] kCS [sꢀ1
]
tCR [ps] kCR [sꢀ1
]
kCS/kCR
C60Py
C60Im
C60Py
C60Im
75
200
20
13.3ꢁ109 2507
5.0ꢁ109 4800
50ꢁ109
19ꢁ109
58ꢁ109
47ꢁ109
4.0ꢁ108
33
24
63
30
2.1ꢁ108
7.9ꢁ108
6.4ꢁ108
ZnF8Pc
1262
1568
822
52
3. Conclusions
ZnF16Pc C60Py
C60Im
17
21
12.1ꢁ108 50
10.2ꢁ108 46
980
High oxidation potential perfluorinated ZnPcs were employed
as electron donors in donor–acceptor dyads with C60 as an
electron acceptor. A self-assembly method using metal–ligand
axial coordination was employed to form these dyads. The
[a] kcs =1/tCS. [b] kCR =1/tCR; kCR calculated based on A0/e calculations with
mono-exponential decay fit. The error in the measured kinetic values is
ꢂ5.
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Figure 10. Femtosecond transient spectra of a) C60Py:ZnF8Pc and b) C60Im:ZnF8Pc dyads at an excitation wavelength of l=400 nm by using a 100 fs pulsed
+
Cꢀ
C
laser in nitrogen-saturated toluene at the indicated time intervals. The time profile of the C60 band at l=1020 nm and the ZnF8Pc band at l=893 nm of
+
+
Cꢀ
C
Cꢀ
C
the C60 Py:ZnF8Pc (c) and C60 Im:ZnF8Pc (d) radical ion pairs are also shown.
whereas bulk solvents utilised in the syntheses were from Fischer
Chemicals (Plano, TX). Fulleropyrollidine derivatives, C60Im and
C60Py, were synthesised according to literature procedures.[26]
spectroscopically determined binding constants were higher
than those observed for dyads formed by ZnPc lacking fluorine
substituents. By using the B3LYP/6-31G* method, the geome-
tries and electronic structures of the dyads were determined,
and the frontier HOMO and LUMO levels and electrostatic po-
tential maps were derived. The energy of the charge-separated
states was estimated from electrochemical and spectral stud-
ies. The energy of the charge-separated state for the dyads
composed of perfluorinated ZnPc was higher than that of the
dyad assembled with ZnPc. By using femtosecond transient
absorption studies in nonpolar toluene, evidence for charge
separation and kinetics of charge separation and recombina-
tion was determined. The occurrence of ultrafast charge sepa-
ration and charge recombination, resulting in the formation of
high-energy charge-separated states in dyads composed of
perfluorinated Pc and C60, was observed.
Synthesis of ZnF8Pc
4,5-Difluorophthalonitrile (700 mg, 4.27 mmol) and ZnCl2 (2.32 g,
0.017 mol) were kept in a 100 mL round-bottomed flask under N2
for 30 min. Then 2-(dimethylamino)ethanol (DMAE; 8 mL) was
added and the whole mixture was heated at 1508C for 18 h. After
cooling the mixture to room temperature, the solution was diluted
with methanol and water (15:5 mL each) and centrifuged for 1.5 h.
The obtained green-coloured residue was dissolved in a minimum
of chloroform and purified by column chromatography on silica
gel. The desired compound (120 mg, 25%) was obtained as the
second fraction eluted by chloroform/MeOH (85:15 v/v). MS (ESI):
m/z: 722.67 [M+H]+.
Synthesis of ZnF16Pc
Tetrafluorophthalonitrile (500 mg, 2.5 mmol) and ZnCl2 (1362 mg,
10 mmol) were kept in a 100 mL round-bottomed flask under N2
for 30 min. Then, DMAE (4 mL) was added and the whole mixture
was heated at 1508C for 18 h. After cooling the mixture to room
temperature, the solution was diluted with methanol and water
(15:5 mL each) and centrifuged for 1.5 h. The obtained green-col-
Experimental Section
Chemicals
Reagents, ZnPc and C60 were purchased from Aldrich Chemicals
(Milwaukee, WI), TCI America and SES Research (Houston, TX),
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Figure 11. Femtosecond transient spectra of a) C60Py:ZnF16Pc and b) C60Im:ZnF16Pc dyads at an excitation wavelength of l=400 nm by using a 100 fs pulsed
+
Cꢀ
C
laser in nitrogen-saturated toluene at the indicated time intervals. The time profile of the C60 band at l=1020 nm and ZnF16Pc band at l=907 nm of the
+
+
Cꢀ
C
Cꢀ
C
C60 Py:ZnF16Pc (c) and C60 Im:ZnF16Pc (d) radical ion pairs are also shown.
oured residue was dissolved in a minimum of chloroform and puri-
fied by column chromatography on silica gel. The desired com-
pound (140 mg, 28%) was obtained as the second fraction eluted
by chloroform/MeOH (75:25 v/v). MS (ESI): m/z: calcd. 863.93,
found 863.9 (see Figure S14 in the Supporting Information).
used as the reference electrode. The Fc/Fc+ redox couple was
used as an internal standard. All solutions were purged with nitro-
gen gas prior to electrochemical and spectral measurements.
Femtosecond transient absorption spectroscopy experiments were
performed by using an Ultrafast Femtosecond Laser Source (Libra)
by Coherent incorporating a diode-pumped, mode locked Ti:sap-
phire laser (Vitesse) and diode-pumped intracavity doubled Nd:YLF
laser (Evolution) to generate a compressed laser output of 1.45 W.
For optical detection, a Helios transient absorption spectrometer
coupled with a femtosecond harmonics generator both provided
by Ultrafast Systems LLC was used. The source for the pump and
probe pulses were derived from the fundamental output of Libra
(compressed output 1.45 W, pulse width 91 fs) at a repetition rate
of 1 kHz; 95% of the fundamental output of the laser was intro-
duced into a harmonic generator, which produced second and
third harmonics of l=400 and 267 nm in addition to the funda-
mental l=800 nm for excitation, whereas the rest of the output
was used to generate a white-light continuum. Herein, the second
harmonic l=400 nm excitation pump was used in all experiments.
Kinetic traces at appropriate wavelengths were assembled from
the time-resolved spectral data. Data analysis was performed by
using Surface Xplorer software supplied by Ultrafast Systems. All
measurements were conducted at 298 K as solutions in degassed
N2.
Instrumentation
The UV/Vis/NIR spectral measurements were performed with a Shi-
madzu 2550 UV/Vis spectrophotometer or a Jasco V-670 spectro-
photometer. The steady-state fluorescence emission was moni-
tored by using a Varian (Cary Eclipse) fluorescence spectrophotom-
eter or a Horiba–Jobin–Yvon Nanolog UV/Vis/NIR spectrofluorome-
ter equipped with PMT (for UV/Vis) and InGaAs (for NIR) detectors.
Lifetimes were measured with the TCSPC lifetime option with
a nanoLED excitation source (l=674 nm) on the Nanolog. The
time correlation factor for the system was 0.439 ns per channel. All
solutions were purged prior to spectral measurements with nitro-
gen gas.
Cyclic and differential pulse voltammograms were recorded on
a Princeton Applied Research potentiostat/galvanostat Model 263A
by using a three-electrode system. A platinum button electrode
was used as the working electrode, whereas a platinum wire
served as the counter electrode and an Ag/AgCl electrode was
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Acknowledgements
This work was supported by National Science Foundation (grant
no. 1110942 to F.D.). We also thank the National Science Founda-
tion for equipment support via NSF CRIF (CHE-0741936). Addi-
tional computing resources were provided by the Academic Com-
puting Services at the University of North Texas.
Keywords: donor–acceptor systems
fullerenes · phthalocyanines · zinc
· electron transfer ·
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Received: March 11, 2014
Published online on May 21, 2014
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