Spectroscopy and photophysics of self-organized zinc porphyrin nanolayers. 3. Fluorescence detected magnetic resonance of triplet states

Article abstract of DOI:10.1021/jp0580569

Fluorescence detected magnetic resonance (FDMR) has been applied to ～25-nm-thick porphyrin films, containing ordered domains of zinc tetra-(p-octylphenyl)-porphyrin (ZnTOPP) spin-coated onto quartz slides. Illuminating the films at 1.4 K with 457.9-nm light from a continuous wave Ar⁺ laser produces at least two different, Jahn-Teller-distorted, ZnTOPP triplet species, labeled i and ii. Microwave-induced magnetic resonance of i and ii in the absence or presence of an externally applied magnetic field affects the fluorescence intensity of ZnTOPP, thus allowing FDMR. For triplet species i, formed in films spin-coated from toluene solution, the zero-field splitting (ZFS) parameters were determined as |D| = (316.9 ± 0.1) × 10^-4 cm^-1 and |E| = (32.0 ± 0.5) × 10 ^-4 cm^-1. By exposure of the spin-coated films to chloroform vapor at room temperature, triplet i is converted into species ii, with |D| = (295 ± 3) × 10^-4 cm^-1 and |E| = (121 ± 3) × 10^-4 cm^-1. For the excited triplet state of ZnTOPP in a toluene glass, ZFS parameters with values of |D| = (295 ±1) × 10^-4 cm^-1 and |E| = (91 ±1) × 10^-4 cm^-1 are found. From a combined study of the FDMR- and microwave-induced fluorescence spectra, i and ii are identified as unligated and ligated ZnTOPP triplet species, respectively. From the asymmetrically shaped zero-field FDMR signals of i, we conclude that the local crystal field perturbations of the stacked molecules are anisotropic. The FDMR results of the ZnTOPP films are compared with those for a film of zinc tetraphenylporphyrin (ZnTPP), which lacks the octyl substituents, and therefore is nonordered. Upon illumination, the ZnTPP films contain only a single, ligated, triplet species with ZFS parameters very similar to those of ligated ZnTOPP. At ～5 K, the lifetime of triplet i is considerably shortened compared to that of ZnTOPP in a glass at the same temperature.

Full text of DOI:10.1021/jp0580569

J. Phys. Chem. B 2005, 109, 17047-17054
17047
Spectroscopy and Photophysics of Self-Organized Zinc Porphyrin Nanolayers. 3.
Fluorescence Detected Magnetic Resonance of Triplet States
,†
†,‡
§
§
|
Tjeerd Schaafsma,* Inbar Dag, Rolf Sitters, Max Glasbeek, and Efrat Lifshitz
Laboratory of Biophysics, Department of Agrotechnology and Food Sciences, Wageningen UniVersity,
Dreijenlaan 3, 6703 HA Wageningen, The Netherlands, Institute for Molecular Chemistry, Department of
Chemistry, Faculty of Mathematics, Computer Science, Physics and Astronomy, UniVersity of Amsterdam,
Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, and Chemistry Department and Solid State
Institute, Israel Institute of Technology, Technion City, Haifa 32000, Israel
ReceiVed: February 16, 2005; In Final Form: May 19, 2005
Fluorescence detected magnetic resonance (FDMR) has been applied to ∼25-nm-thick porphyrin films,
containing ordered domains of zinc tetra-(p-octylphenyl)-porphyrin (ZnTOPP) spin-coated onto quartz slides.
+
Illuminating the films at 1.4 K with 457.9-nm light from a continuous wave Ar laser produces at least two
different, Jahn-Teller-distorted, ZnTOPP triplet species, labeled i and ii. Microwave-induced magnetic
resonance of i and ii in the absence or presence of an externally applied magnetic field affects the fluorescence
intensity of ZnTOPP, thus allowing FDMR. For triplet species i, formed in films spin-coated from toluene
-4
-1
solution, the zero-field splitting (ZFS) parameters were determined as |D| ) (316.9 ( 0.1) × 10 cm and
-
4
-1
|
E| ) (32.0 ( 0.5) × 10 cm . By exposure of the spin-coated films to chloroform vapor at room temperature,
-4 -1 -4 -1
triplet i is converted into species ii, with |D| ) (295 ( 3) × 10 cm and |E| ) (121 ( 3) × 10 cm .
For the excited triplet state of ZnTOPP in a toluene glass, ZFS parameters with values of |D| ) (295 ( 1)
-
4
-1
-4
-1
×
10 cm and |E| ) (91 ( 1) × 10 cm are found. From a combined study of the FDMR- and
microwave-induced fluorescence spectra, i and ii are identified as unligated and ligated ZnTOPP triplet species,
respectively. From the asymmetrically shaped zero-field FDMR signals of i, we conclude that the local crystal
field perturbations of the stacked molecules are anisotropic. The FDMR results of the ZnTOPP films are
compared with those for a film of zinc tetraphenylporphyrin (ZnTPP), which lacks the octyl substituents, and
therefore is nonordered. Upon illumination, the ZnTPP films contain only a single, ligated, triplet species
with ZFS parameters very similar to those of ligated ZnTOPP. At ∼5 K, the lifetime of triplet i is considerably
shortened compared to that of ZnTOPP in a glass at the same temperature.
Introduction
films favor delocalization by excitons or excitation and energy
transfer. Finally, due to their high surface-to-volume ratio, thin
The two preceding papers report and discuss results of
experiments applying optical spectroscopy to zinc porphyrin
solid films with thicknesses in the nanometer range (“nanolay-
ers”). This paper focuses on the nature and properties of the
photoexcited porphyrin triplet states in these films as studied
by means of optically detected magnetic resonance (ODMR).
In particular, we have applied microwave-induced fluorescence
intensity measurements, i.e., fluorescence detected magnetic
resonance (FDMR), in the absence of a magnetic field, to these
films at 1.4 K. The ODMR technique and its theoretical
background have been treated in several excellent reviews.
Photophysical processes involving triplet states in thin solid
films including inorganic, organic, and polymer films have
frequently been investigated by zero-field ODMR and its high-
films may easily adsorb trace amounts of atmospheric gases,
e.g., moisture or oxygen, resulting in surface traps for excitation
1
2
and/or charge carriers.
The results of this work clearly illustrate that FDMR
spectroscopy, in the absence or presence of an external magnetic
4
,13
field,
offers several strong advantages in the investigation
of molecular triplet species in ultrathin organic solid films. Not
only is the detection limit for triplets in the investigated films
below a nanomole, but in addition, the nature and properties of
the detected triplet species can be determined using its zero-
field splitting (ZFS) values in combination with the kinetics of
the three spin levels. In the present work, the frequencies of
the various FDMR transitions are assigned by combining
the FDMR spectra with the corresponding microwave-
1
2
-8
9
10
1
1
field equivalent electron paramagnetic resonance (EPR). In
such films, excited triplet states have been found for a variety
of different species such as traps, defects, interfaces, and
aggregates. Furthermore, the short intermolecular distances in
1
induced fluorescence (MIF) spectra and comparing the MIF
spectra with the fluorescence spectra reported in the preceding
paper.
FDMR spectroscopy can be applied to localized triplets as
well as to triplet radical ion pairs. Since photogenerated
electron-hole pairs in solid films may interact with fluorescent
species in their immediate environment, FDMR may also be
utilized to report on the fates of these charge carriers when the
latter are formed in the triplet state. Thus, dissociation, trapping,
and/or recombination processes may be followed provided they
*
Author to whom correspondence should be addressed. Fax: +31-31-
7
482725. E-mail: tjeerd_schaafsma@hotmail.com.
†
Wageningen University.
On leave from the Chemistry Department and Solid State Institute, Israel
‡
Institute of Technology.
§
University of Amsterdam.
Israel Institute of Technology.
|
1
0.1021/jp0580569 CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/18/2005

17048 J. Phys. Chem. B, Vol. 109, No. 36, 2005
Schaafsma et al.
contains three nondegenerate spin levels with spin functions |x〉,
2
,3
|
y〉, and |z〉 and zero-field energies
1
3
1
3
2
3
X ) D + E Y ) D - E Z ) -
D
(1a)
In eq 1a
2
2
2
2
3
4
2
2
r - 3z
3
4
2
2
y - x
D ) g â
and E ) g â
(1b)
〈
5
〉
〈
5
〉
r
r
18
π-π* triplet states of oblate-shaped molecules have D > 0,
so the energy of the zero-field |z〉 spin level will be lowest.
Also, the labeling of the molecular x- and y-axes is interchange-
able, and therefore E can always be chosen to have a positive
value.
Figure 1. Molecular structure of ZnTOPP and ZnTPP, where R )
17 and -H, respectively.
8
-C H
affect the fluorescence intensity. High-field FDMR transitions,
In eq 1b, the triplet g-tensor has been assumed to be isotropic
(g ≈ 2), â is the electronic Bohr magneton, and the broken
brackets represent the average over the electronic wave function
of the triplet state. D is a measure for the average distance r
between the electron spins, with components x, y * 0 and z )
0, and E reflects the in-plane asymmetry of the spin distribution
over the porphyrin macrocycle.
10
with g ≈ 2, in photoconductive polymers and layered
14
semiconductors have been discussed to involve such electron-
1
5
hole pairs.
In this work, films with thicknesses in the nanometer range
of two different porphyrins on a quartz substrate have been
investigated, one with self-organizing properties, i.e., zinc
tetrakis-(octylphenyl)-porphyrin (ZnTOPP) and, for comparison,
one without these properties, i.e., zinc tetraphenylporphyrin
Microwave transitions between any two spin levels of T1
generally cause a change of their steady-state populations,
resulting in a change of the net decay rate of T1 to the ground-
state S0 and therefore of the S1 f S0 fluorescence intensity.1
Understanding the transport, trapping, and conversion pro-
cesses of excited states in the investigated films and their
structures (cf. previous papers) is important for various applica-
(ZnTPP) (Figure 1).
,4
The level scheme of Figure 2 describes the electronic and
spin levels of a metallo-porphyrin with D4h symmetry, including
the findings reported in the first preceding paper. As a result of
Jahn-Teller distortion,1
6,17
the lowest excited triplet state T1
Figure 2. Energy level scheme for ZnTOPP for various situations A-D. Note the different energy scale used for B-D as compared to A. (A)
Singlet electronic levels S , S , S , and lowest triplet level T , including S exciton components, labeled x and y. The S and T levels consist of
four almost degenerate electronic levels. (B) Zero-field spin levels of the T level of ZnTOPP assuming D4h symmetry, a crystal field ∆, and BB
0. For simplicity, only spin levels of the lowest two electronic levels of the fourfold degenerate manifold are indicated for T . Because of D4h
spin levels of ZnTOPP in the presence of a Jahn-Teller distortion resulting
// z. FDMR transitions in the absence and presence of an external magnetic
0
1
2
1
2
1
1
1
0
)
1
symmetry, the two spin levels |x〉 and |y〉 are twofold degenerate. (C) T
1
from ∆ * 0 and BB
field BB are indicated by dashed arrows
0
0
) 0. (D) Zeeman effect on T
1
spin levels for BB
0

FDMR in Zinc Porphyrin Nanolayers
J. Phys. Chem. B, Vol. 109, No. 36, 2005 17049
tions. Intersystem crossing to triplet exciton states or localized
triplet traps may profoundly affect this excitation transport. Films
of self-organizing (metallo-)porphyrins19 and phthalocyanines20
containing alkyl or alkoxy substituents have been utilized as
energy collecting “antennas” on top of a semiconductor substrate
in dye-sensitized solar cells.2
1-24
These ordered systems show
enhanced excitation transport.25
Ordered ZnTOPP films that have been in contact with
chloroform turn out to contain two different Jahn-Teller-
distorted triplet species,1
6,17
assigned to porphyrins with and
without a ligand, based on their FDMR and MIF spectra. For
nonordered ZnTPP films, these spectra reveal the presence of
ligated triplet species only, irrespective whether the films have
been in contact with chloroform or not.
Experimental Section
Tetraphenylporphyrin free base (H2TPP) and tetra-(p-oct-
ylphenyl)-porphyrin free base (H2TOPP) were synthesized by
condensation of pyrrole with benzaldehyde and 4-(n-octyl)-
Figure 3. FDMR spectrum of 20 µM ZnTOPP in a toluene glass:
+
T ) 1.4 K; excitation, Ar 514-nm laser line; fluorescence detection,
-1
5
90 ( 5 nm; microwave frequency sweep rate, ∼4 MHz s
.
2
6
benzaldehyde, respectively, following standard procedures.
Zinc was inserted into the porphyrins by refluxing a solution
of the free base porphyrin in dimethylformamide (DMF) in the
presence of excess zinc(II) chloride.27 The porphyrins were
purified by chromatography over silica gel (Merck) with
chloroform (Merck, p.a.) as the eluent and estimated to be >99%
pure by thin-layer chromatography, absorption, and fluorescence
spectroscopy.
or Varian VZC-6961/VTC-6160/VA-615M traveling wave tube
amplifiers; the amplified microwave output (∼20 W) was
coupled by coaxial lines to the semirigid cable. The microwaves
were square-wave amplitude-modulated using a Hewlett-Packard
PIN diode switch with a modulation frequency of 280 Hz. The
microwave-induced fluorescence changes were detected using
a lock-in amplifier (Princeton Applied Research, model HR-8)
with the microwave modulation frequency as a reference. The
output of the lock-in amplifier was digitized and further
processed by a PC.
FDMR spectra were recorded by sweeping the microwave
frequency over a 100-1500 MHz range and monitoring the
change in fluorescence intensity, induced by connecting two
spin levels (or three spin levels in a double resonance (FDEL-
DOR) experiment, vide infra) of the lowest porphyrin triplet
state at resonance conditions.
To investigate the connection between the different reso-
nances and their associated MIF spectra, we recorded the
dependence of the amplitude of each resonance on the fluores-
cence wavelength. This was done by amplitude modulation of
the microwaves at a fixed resonance frequency, scanning the
wavelength of the emitted light using a monochromator and
applying phase-sensitive detection (Princeton AppliedResearch,
model HR-8) of the fluorescence intensity at the microwave
modulation frequency.
-3
Solutions (5 × 10 M) of ZnTOPP or ZnTPP in chloroform
(
Merck, p.a.) or toluene (Merck, spectroscopy grade) were spin-
coated at 2500 rpm onto ∼0.5-mm-thick quartz slides (Suprasil
I, P&H Glass Technology), thus resulting in smooth solid films.
Through the use of the optical densities of the films at 550 nm,
their estimated thicknesses were 25 ( 5 nm. The amount of
deposited porphyrin was also determined by dissolving the film
in a known volume of toluene. Through the use of the extinction
coefficient28 ꢀ550 ) 22 000 L M cm , the calculated thickness
was found to be in good agreement with the above-mentioned
value and corresponds to ∼100 porphyrin layers for all
investigated films. After spin-coating, all films were heated to
-1
1
50 °C during 5 min to remove adherent solvent from the film.
For FDMR experiments, small pieces were cut from the same
porphyrin-covered quartz slide to fit into the microwave helix.
All samples were kept in the dark until used in the experiments.
Three different films, in the following denoted as 1-3, were
prepared by spin-coating onto the quartz slide: 1, ZnTOPP from
toluene solution; 2, ZnTOPP from chloroform solution; 3,
ZnTPP from toluene solution.
In some experiments, 1 was exposed to saturated chloroform
vapor at room temperature in a closed vessel during 30 min.
For comparison of ZFS values, ZnTOPP was also studied in a
toluene glass.
Fluorescence detected electron double resonance (FDELDOR)
experiments, again at zero external magnetic field, were
performed using two different microwave sources, coupled to
two concentric slow-wave helixes, surrounding the sample. One
of the oscillators was kept at a constant microwave frequency,
pumping one of the FDMR transitions. The second oscillator
was swept over the frequency range of the other resonances.
High-field FDMR experiments were done at a magnetic field
strength of 1.0 T using a superconducting solenoid coil, placed
in a liquid helium cryostat and pumped at 1.2 K. The 457.9-
Fluorescence and FDMR spectra were recorded with the films
submerged in a bath of pumped liquid helium, at 1.4 K, inside
a homemade cryostat. The films were excited by ∼70-mW
+
4
57.9- or 514-nm radiation from a continuous wave Ar ion
laser (Coherent CR-5). Emission light was detected using an
S20 (EMI) photomultiplier after dispersion through a colored
glass cutoff filter (OG 540) and a monochromator (Jobin-Yvon,
model HR1000). Microwaves were applied using the setup
described previously.8 The sample was mounted inside a slow-
wave helix coupled to a semirigid cable in the cryostat.
Microwave radiation was generated by a sweep oscillator (HP-
+
nm line of an Ar ion laser (Coherent, model Innova 70) was
used for excitation.
Results
,29
Figures 3-7 show the FDMR spectra of ZnTOPP in a toluene
glass and in variously prepared films, respectively, at 1.4 K.
The glass spectrum (Figure 3) contains two transitions corre-
sponding to an increase in the fluorescence intensity, the major
8
4
609B) using appropriate plug-in units (HP-86999B for 100-
000 MHz and 8691B for 1000-2000 MHz) and Hughes 1177H

17050 J. Phys. Chem. B, Vol. 109, No. 36, 2005
Schaafsma et al.
Figure 4. FDMR spectrum of a ZnTOPP film spin-coated from toluene
solution (1): detection, total fluorescence emission; other conditions
as in Figure 3; the dashed line indicates zero FDMR intensity.
Figure 6. FDMR spectrum of a ZnTOPP film (1) spin-coated from
toluene solution and subsequently exposed to chloroform vapor during
3
0 min; other conditions as in Figure 3; the dashed line indicates zero
FDMR intensity.
Figure 5. FDELDOR spectrum, 1046-MHz pumped, of a ZnTOPP
film spin-coated from toluene solution (1), other conditions as in Figure
4
.
Figure 7. FDMR spectrum of the ZnTOPP film of Figure 6 after
prolonged exposure to chloroform vapor; other conditions as in Figure
one at 613 ( 1 MHz and a weaker resonance at 1160 ( 6 MHz,
whereas the spectrum of 1, using the entire fluorescence
emission for detection, has two major, broad transitions (also
reflecting fluorescence intensity increases) with maxima at 854
6
.
corresponding to a fluorescence decrease. For a different part
of the same film, only resonance III was reproducibly found
(
5 and 1046 ( 5 MHz, denoted I and II (Figure 4). Note the
(cf. Figure 7), indicating that the composition of various parts
different frequencies and intensity distributions of the glass and
film resonances, both with an full width at half-maximum
of the films is not uniform. Although the films obviously are
not homogeneous, measurements were quite reproducible,
provided that the sample in the microwave helix was not moved.
Also, the same transitions, but not necessarily with the same
relative intensities, were found for several films, when treated
under identical conditions. Prolonged exposure of the film to
chloroform vapor always produced spectra with a single, major
resonance III, indistinguishable from spectra of 2. A general
finding is that when the resonances I, II, and IV are absent only
resonance III remains with a large amplitude, apart from some
spurious weak satellites (Figure 7).
(
fwhm) of ∼50-100 MHz. The average of each pair of
resonances (887 and 950 MHz, respectively) are representative
of the D-value of ZnTOPP in toluene and 1. These D-values
are typical for a porphyrin triplet.1
,4,16
The notably asymmetric
line shape of the film spectrum was unaffected by varying the
instrumental settings, in particular when the microwave sweep
rate was lowered or the sweep direction was reversed. No
resonance was observed for 1 at the difference frequency of
∼
190 MHz. Since the samples may contain more than one triplet
species (see below), it is not a priori obvious whether the
resonances I and II (Figure 4) belong to one and the same triplet
species. Pumping resonance II of 1 in a fluorescence detected
electron double resonance (FDELDOR) experiment resulted in
a relatively narrow (∼20 MHz fwhm) resonance (denoted as
I-II), appearing at the difference frequency of 192 MHz of
resonances I and II, showing that both resonances belong to
one and the same triplet.
The FDMR spectrum of 3 (not shown) was detected using
the full fluorescence emission. Invariably a single, strong
resonance with a positive sign at 1262 MHz is found with a
minor satellite at 1300 MHz. Contrary to 1 and 2, the FDMR
spectra of 3 did not show any dependence on the sample
treatment, e.g., exposure to chloroform.
The MIF spectrum associated with resonance I has main peaks
at 584 and 642 nm (Figure 8A), whereas the MIF spectrum
associated with resonance III has peaks at 594 and 652 nm
(Figure 8B). Both spectra closely agree with the fluorescence
spectra of Figures 10A and 10C, respectively, in the first paper.
Exposure of 1 to chloroform vapor initially results in
considerable narrowing of resonances I and II (Figure 6) and
the appearance of two additional resonances: III, corresponding
to a fluorescence increase, at 1248 MHz, and IV at ∼948 MHz,

FDMR in Zinc Porphyrin Nanolayers
J. Phys. Chem. B, Vol. 109, No. 36, 2005 17051
Figure 9. High-field FDMR spectrum at 1.0 T of a ZnTOPP film
spin-coated from chloroform solution. The horizontal axis has been
converted from a frequency scale to that of X-band electron spin
resonance (ESR). The dashed line indicates the baseline.
by the environment of the triplet species, giving rise to quite
different splittings between the resonance pairs as shown, for
example, by comparing Figures 3 and 4. The large differences
between the glass and film FDMR spectra strikingly illustrate
the quite different triplet-state properties in both media. ZnTOPP
in toluene glass has an E/D ratio of 0.31, close to its maximum
1
possible value of /3 for a molecule with an oblate shape. By
contrast, the same molecule in the film is much less distorted,
as shown by the E/D ratio of 0.10. The shortened lifetime of
the triplet state in film 1 as compared to that in the glass
indicates the presence of additional triplet relaxation and/or
decay channels in the film.
Figure 8. Microwave-induced fluorescence spectra, (A) associated with
resonance II and (B) associated with resonance III.
The results of the FDELDOR experiment of 1 (Figure 5),
when pumping at 1046 or 854 MHz, show that the resonances
I and II in Figure 4 belong to one and the same triplet and can
be assigned as its zero-field D + E and D - E transitions.
Without pumping, the 2E resonance is not observed, due to
almost equal steady-state populations of the |x〉 and |y〉 spin
levels (Figure 2), as commonly observed for porphyrins.1
Pumping at the maximum of one of the resonances I or II (Figure
From the response of the major ∆mS ) (1 X-band EPR
transition to a 14-ns, 532-nm laser flash at T ≈ 5 K, it can be
concluded that the triplet lifetime of ZnTOPP in films of type
1
1
has an upper limit of 1 ms,30 shortened by at least a factor of
0 as compared to that of a zinc porphyrin in an organic glass
,4,16
31
in the same temperature range.
4) causes a population redistribution over the triplet spin levels,
thus allowing the 2E resonance to be observed in FDELDOR
Discussion
(Figure 5).
Both the FDMR spectra and the MIF spectra of 1-3 present
The values of the zero-field splitting parameters as concluded
from the FDMR and FDELDOR experiments for 1 are included
in Table 1.
The transitions I and II in the FDMR spectrum of 1 (Figure
4) exhibit some interesting features. (i) The line shapes are quite
asymmetric and roughly each other’s mirror image around the
average frequency of 950 ( 7 MHz; we note that a similar
mirror asymmetry has been reported for the D + E and D - E
1
,4,16
clear evidence for the presence of porphyrin triplet states
The nonzero E-value for each of the observed porphyrin triplet
species reflects a nonaxial symmetry of ZnTOPP and ZnTPP
in their lowest triplet states; the scheme of Figure 2 includes
this result that is thought to arise from a static Jahn-Teller
1
6,17
distortion of the porphyrin molecule with D4h symmetry.
The E-value strongly depends on the crystal field ∆, imposed
TABLE 1: ZFS Parameters D and E (units 10-4 cm-1) of Zinc Porphyrin Triplet States in Films and Solid Solutions
compound
ZnTOPP
medium
Db
Eb
methodc
reference
T (K)
film (1)
film (2)
toluene glass
316.9 ( 0.1
295 ( 3
295 ( 1
32.0 ( 0.5
A, B
A
A
this work
this work
this work
1.4
1.4
1.4
d
121 ( 3
91 ( 1
ZnTPP
film (3)
benzene/1% pyridine
toluene glass
315 ( 5
308 ( 5
315 ( 5
105 ( 5d
96 ( 5
e
A
A
C
this work
ref 38
ref 35
1.4
4.2
5
a
b
D and E are both assumed to be greater than 0. Errors have been estimated considering the uncertainties in the position of the resonance peaks
and the frequency markers. A, FDMR; B, FDELDOR; C, ESR. d Calculated using the data from the high-field spectrum (Figure 9). e Not determined.
c

17052 J. Phys. Chem. B, Vol. 109, No. 36, 2005
Schaafsma et al.
resonances observed for a dilute solution of scandium(III)
octaethylporphyrin monomers and dimers in a low-temperature
3
the spread of E-values albeit that the asymmetric line shape
and their mirror symmetry are maintained. From the broadening
of the zero-field transitions depicted in Figure 4, one might
conclude that the ZnTOPP triplets in the stacks have a certain
degree of in-plane disorder that appears to decrease upon
exposure of the film to chloroform (Figure 6).
-methyl pentane glass.32 (ii) Both resonances are considerably
broadened (fwhm ≈ 50-100 MHz) as compared to those of
the same films exposed to chloroform vapor (fwhm ≈ 30 MHz)
(cf. Figures 4 and 6). (iii) Apart from the splittings, also the
relative amplitudes of both transitions of ZnTOPP in a film are
quite different from those in a glass (cf. Figures 3 and 4).
Exposure of 1 to chloroform vapor also gives rise to a weak
resonance III at 1248 MHz (Figure 6) of a second triplet species
and a weak resonance IV of unknown origin (vide infra). It is
noted that resonance III is the only one surviving a prolonged
exposure to chloroform vapor (Figure 7). Also, its resonance
frequency (1248 MHz) is not far from that of the D + E
transition for the triplet state of ZnTOPP in a glass. In line with
the discussion given above, we ascribe resonance III to strongly
perturbed ZnTOPP molecules, probably ligated ZnTOPP, the
ligand possibly being a small molecule such as water.
Since the asymmetric line shape of both resonances does not
change upon varying the microwave frequency sweep rate and
the resonance line shapes were found to be independent of the
sweep direction, slow spin level decay as the cause of the tails
can be excluded. Thus, the line shape asymmetry must originate
from the intrinsic properties of the porphyrin triplet state itself.
When the asymmetry of the FDMR transitions of the porphyrin
triplet state in films is discussed, several possibilities may be
considered.
The triplet species associated with resonance III is also found
in film 2, which was spin-coated from chloroform solution. It
can be clearly distinguished from the species with resonances
I and II (cf. Figures 6 and 7). The MIF spectrum associated
with resonance III (Figure 8B) identifies it as a ligated ZnTOPP
A first possibility to explain the asymmetry would be to
ascribe the resonance line shapes to triplet excitons. In ordered
domains of ZnTOPP films, the intermolecular plane-to-plane
distance is short enough to allow for some π-π orbital overlap
and thus, in principle, allowing for triplet exciton motions to
3
4
monomer. Evidently, part of the nonligated traps become
increasingly ligated by exposure to chloroform or its vapor;
hence resonances I and II will disappear, and resonance III is
the only resonance observed (Figure 7). Figures 6 and 7
demonstrate that the degree of ordering is not uniformly
distributed over the film; i.e., some parts exhibit only the single
resonance III, very similar to that in nonordered films of ZnTPP
(see below).
3
3
occur. Brenner has shown for (quasi-)one-dimensional mo-
lecular crystals that D + E and D - E ODMR transitions of
triplet excitons may indeed exhibit asymmetrical line shapes
due to k-dispersion. Simulated spectra using Brenner’s equations
indeed yield asymmetric line shapes for both the D + E and
the D - E transitions. However, the simulations also show that
these line shapes are not related by mirror symmetry. Moreover,
mobile triplet excitons are unlikely to occur in the films since
The MIF spectrum of signal III (Figure 8B) is red-shifted
compared to that of signal II (Figure 8A). This agrees with the
observed red shift of the fluorescence spectrum upon exposure
to chloroform (cf. Figures 10A and 10C of the first paper),
indicating that signal II (and as can be concluded from the
FDELDOR experiment, also signal I) is due to the unligated
monomer.
the porphyrin molecules in the lowest excited triplet state are
Jahn-Teller systems1
6,17
and local crystal field perturbations
resulting from Jahn-Teller active vibrations may readily
outweigh the weak excitonic interactions (a few cm ), causing
the excitation to remain largely localized.
-1
Another possible cause for the asymmetric broadening of the
zero-field triplet-state transitions of ZnTOPP in films may be
inhomogeneous broadening of the spin levels of the lowest triplet
state.
The smaller E-value for the lowest triplet state of ZnTOPP
in the film and the larger width of the D + E and D - E
transitions compared to that for the porphyrin in the glass implies
that the crystal field perturbation exerted by the environment
that leads to a nonaxially symmetric spin distribution is weaker
and has a wider distribution for the porphyrin in the film as
compared to that in the glass (cf. Figures 3 and 4).
The above-mentioned crystal field perturbations are induced
by imperfections in the environment of the stacked molecules
and give rise to an inhomogeneous distribution of E-values. The
influence of inhomogeneously distributed local fields is one-
sided or asymmetric in the sense that only larger E-splittings
Also, the Q(0,0) and Q(1,0) fluorescence wavelengths of 584
and 642 nm, found for the MIF spectrum of signal II in Figure
8A, indicate that ZnTOPP is nonligated, as reported values for
the Q(0,0) wavelength change from 594 to 611 nm and for the
Q(1,0) wavelength from 639 to 664 nm, respectively, upon
3
4
ligation to pyridine.
MIF spectra (not shown) obtained by modulated pumping of
resonance IV are identical to those of resonances I and II.
Therefore, this species is most likely also a nonligated monomer
but with different ZFS parameters and an opposite sign of the
fluorescence response to microwave resonance as compared to
the first triplet species.
The observation of only one zero-field resonance, III, for
chloroform-treated ZnTOPP and ZnTPP films is remarkable.
This situation is different from that for ZnTOPP in a toluene
glass (Figure 3) and unligated ZnTOPP in a film (Figure 4) in
which cases two resonances are observed. Generally, microwave-
induced transitions between the three spin levels as in Figure 2
are expected to result in 0, 2, or 3 FDMR resonances, unless E
≈ 0. We can exclude the latter possibility, considering the Jahn-
(with no smaller E-splitting values as counterparts) are obtained.
Referring to Figure 2C, we thus propose that inhomogeneous
broadening leads to a spreading of the spin level X to higher
energies and of spin level Y to lower energies. The distribution
function, characterizing this spreading, is assumed to be
asymmetric, attaining its maximum for the smallest E-values.
In this way, one anticipates asymmetrically shaped zero-field
transitions such that the broadening for the D - E transition is
toward its low-frequency side, whereas for the D + E transition
the broadening is toward its high-frequency side. Thus, the
model can qualitatively account for the experimental results.
1
6,17,32
Teller distortion of the porphyrin triplet state.
Moreover,
the calculated value for D of 416.3 × 10 cm-1 assuming E
≈ 0 is far outside the range typical for Zn porphyrin triplets.
We may also exclude a low or vanishing microwave power in
the frequency range around the |D - E| transition, in view of
the results for ZnTOPP in a toluene glass (Figure 3).
-4
The narrowing effect on the spectra by exposure of the films
to chloroform vapor (Figure 6) is attributed to a reduction of
The high-field FDMR results (Figure 9) suggest the reason
for the absence of the D - E and 2E resonances.. The outer Z
+

FDMR in Zinc Porphyrin Nanolayers
J. Phys. Chem. B, Vol. 109, No. 36, 2005 17053
-
and Z features in the high-field FDMR spectrum at 1.0 T are
perturbed by the crystal field exerted by the surrounding ground-
state porphyrins. The asymmetric resonance line shapes of the
|D + E| and |D - E| FDMR transitions can be explained by
anisotropic, inhomogeneous broadening of the Jahn-Teller
distortions.
• The stacked ZnTOPP molecules in films exhibit localized
triplet states, implying that the combined Jahn-Teller and
crystal field interactions dominate the triplet exciton interaction.
• In contrast to ZnTPP, the presence of hydrocarbon chains
in ZnTOPP films prevents the access of small polar ligands, in
particular, water molecules, to the zinc center of ZnTOPP.
• Finally, the present work shows that FDMR and its
complementary counterpart, microwave-induced fluorescence
spectroscopy, may be successfully applied to sensitively and
selectively study triplet states in nanolayers.
quite pronounced, but resonances in the XY regions are severely
broadened and hardly detectable. The absence of the D - E
and 2E resonances in Figure 7 and similarly for ZnTPP films
can then be explained by assuming that their zero-field |y〉 spin
level (Figure 2, column C) is severely broadened, so that only
the D + E resonance is observed. The selective broadening of
the |y〉 spin level is interesting, since it does not occur for
ZnTOPP in toluene glass (Figure 3), suggesting that this
selective broadening is related to the anisotropic environment
of the porphyrin molecule in the film.
3
3
The ZFS values D and E for ZnTOPP and ZnTPP from
FDMR and high-field FDMR and EPR spectra, both from this
and published work, are summarized in Table 1.
As for chloroform-treated ZnTOPP films, a single resonance
(
not shown) at about the same frequency is also observed for
nonordered ZnTPP films 3 (Table 1). The single resonance at
254 MHz found for 3 is very close to that found for ZnTOPP
Acknowledgment. This work has been supported as part of
the research program “Organic Solar Cells; A Feasibility Study”
by The Netherlands Organization for Energy and Environment
Contract No. 146.100-024.4. T.J.S. gratefully acknowledges
financial support from the Nata H. Sherman Lectureship Fund
for research during a sabbatical stay at the Department of
Chemistry, Technion, Haifa. The authors are indebted to Dr.
T. J. Savenije for providing electron spin resonance data. Drs.
H. Donker and W. van Schaik contributed to this work by useful
comments. T.J.S. thanks Dr. J. Schmidt for illuminating
discussions.
1
after prolonged chloroform treatment (Figure 7), suggesting that
the triplet species in both films are very similar. The presence
of a different type of porphyrin, e.g., the free base, can be
excluded as the cause of resonances III in 2 and 3 in view of
their absorption, fluorescence, and MIF spectra. The 1254 MHz
resonance of ZnTPP can be identified as the D + E transition
of the ligated porphyrin monomer; this assignment is compatible
with published ZFS parameters for the compound in a low-
3
5
temperature glass. Since the ZnTPP film was prepared using
toluene as the solvent in the spin-coating procedure, the ligand
could possibly involve a trace of water present in the toluene
solvent. However, even when using sodium-dried toluene,
resonance III survived for the ZnTPP film. Evidently, the ZnTPP
molecules in the film appear as almost completely ligated, even
when spin-coated from dry toluene. We tentatively ascribe the
strikingly different ligation properties of ZnTPP and ZnTOPP
films, both spin-coated from toluene, to the influence of the
substitutional group -R, where -R is octyl for ZnTOPP and
H for ZnTPP. In ZnTOPP films, the zinc center is protected
from becoming ligated by small hydrophilic oxygen-containing
References and Notes
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6
37
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