Radical-Enhanced Intersystem Crossing in New Bodipy Derivatives and Application for Efficient Triplet-Triplet Annihilation Upconversion

Article abstract of DOI:10.1021/jacs.7b02063

A long-lived triplet excited state of the well-known fluorophore boron dipyrromethene (Bodipy) was observed for the first time via efficient radical-enhanced intersystem crossing (EISC). The triplet state has been obtained in two dyads in which the Bodipy

Full text of DOI:10.1021/jacs.7b02063

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Article
Radical Enhanced Intersystem Crossing (EISC) in New Bodipy Derivatives
and Application for Efficient Triplet-Triplet Annihilation Upconversion
Zhijia Wang, Jianzhang Zhao, Antonio Barbon, Antonio Toffoletti, Yan Liu, Yonglin An,
Liang Xu, Ahmet Karatay, Halime Gul Yaglioglu, Elif Akhuseyin Yildiz, and Mustafa Hayvali
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 19 May 2017
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Radical Enhanced Intersystem Crossing (EISC) in
New Bodipy Derivatives and Application for
Efficient Triplet-Triplet Annihilation Upconversion
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Zhijia Wang,a Jianzhang Zhao,a,* Antonio Barbon,b* Antonio Toffoletti,b Yan Liu,a Yonglin An,a
Liang Xu,a Ahmet Karatay,c Halime Gul Yaglioglu c* Elif Akhüseyin Yildiz,c and Mustafa
Hayvali d
a State Key Laboratory of Fine Chemicals, Dalian University of Technology, E-208 West
Campus, 2 Ling-Gong Road, Dalian 116024, P. R. China E-mail: zhaojzh@dlut.edu.cn
b Dipartimento di Scienze Chimiche, Università degli Studi di Padova, Via Marzolo, 1, 35131
Padova, Italy E-mail: antonio.barbon@unipd.it
c Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100
Beşevler, Ankara, Turkey. E-mail: yoglu@eng.ankara.edu.tr (H.G. Yaglioglu).
d Department of Chemistry, Faculty of Science, Ankara University, 06100 Beşevler, Ankara,
Turkey.
RECEIVED DATE (automatically inserted by publisher)
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Abstract: A long-lived triplet excited state of the well-known fluorophore borondipyrromethene
(Bodipy) was observed for the first time via efficient radical-enhanced intersystem crossing
(EISC). The triplet state has been obtained in two dyads in which Bodipy unit is linked to a
nitroxide radical, 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) with two different length
spacers. The photophysical properties were studied with steady-state and time-resolved transient
optical spectroscopies, and electron spin resonance (cw-ESR and TR-ESR). The fluorescence of
Bodipy units is significantly quenched in the dyads, and the spin polarized TEMPO signal were
observed with TR-ESR, generated by a Radical Triplet Pair Mechanism (RTPM). Efficient EISC
(ΦT = 80%) was observed for the dyad with shorter linker and the triplet state lifetime of Bodipy
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chromophore is exceptionally long (62 μs). The EISC takes 250 ps. Poor ISC was observed for
the dyad with longer linker. The efficient ISC and long-lived triplet excited state in this flexible
system are in stark contrast to the previously studied rigid EISC systems. EISC effect was
employed for the first time to perform triplet-triplet annihilation (TTA) upconversion (quantum
yield ΦUC = 6.7 %).
Keywords: Bodipy; Intersystem Crossing; Radical; Electron Spin Polarization; Upconversion
1. INTRODUCTION
Triplet photosensitizers are versatile compounds which have been widely used in
photocatalysis,1−6 photodynamic therapy,7−12 photovoltaics,13−15 and recently in triplet-triplet-
annihilation (TTA) upconversion.16−20 Triplet photosensitizers are expected to show strong
absorption of visible light, efficient intersystem crossing (ISC) and long-lived triplet excited
state, as well as feasibly derivatizable molecular structures.7,9 However, it is not a trivial task to
tackle all these challenges. For example, it is elusive to enhance the ISC of organic compounds
which are heavy atom-free (contain no transition metal atom or iodine or bromine atom, etc). It
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is still a major challenge to rationally design heavy atom-free organic chromophores to show
predetermined efficient ISC.9,21 Although some methods have been developed recently, such as
using an electron spin converter or singlet fission effect,22−27 much room is left to explore new
approaches to enhance ISC in organic chromophores.
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Quenching of fluorescence by stable radicals, such as 2,2,6,6-tetramethylpiperidinyloxy
(TEMPO), has been known for a long time. In most of the previous studies, the quenching effect
was only used for designing of fluorescent chemosensors (molecular probes) for selective
detection of radicals.28−31 Quenching of the singlet state by stable radical is determined by spin
interactions between the photo-excited chromophores and the radicals. A series of spin states
with different spin multiplicities are generated by this interaction.32−40 Triplet states can be
formed, by enhancing the spontaneous ISC, together with radicals in non-Boltzmann distribution
of population within the different spin states (this is the so-called electron spin polarization,
ESP).41 ESP can be caused by a few different mechanisms, as the most frequent is the radical
triplet pair mechanism (RTPM).32,41 The seminal paper on radical enhanced ISC (EISC) by time-
resolved ESR (TR-ESR) methods is by Kawai and Obi in 1992,42 followed by one of us few
years later.43 Recently the ESP of the radical-labeled perylene,44 phthalocyanines,33,34 and
anthracene chromophores upon photoexcitation were studied.35,41 However, most studies focus
on the ESP mechanisms, and the radical EISC has never been exploited to design a highly
efficient triplet photosensitizers for any applications to drive a photophysical or photochemical
process. On the other hand, paramagnetic compounds (mostly nitroxides) are widely used for in-
vivo applications.45 Thus radical EISC based triplet photosensitizers will be promising
multifunctional photosensitizers. For example, the MRI-guided photodynamic therapy (PDT)
based on a radical triplet photosensitizer will give an accurate delineation of the target tumor
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region, and precise delivery of the treatment light by optical-fiber. As such the tumor detection,
treatment and therapeutic efficiency evaluation can be accomplished with a single compound.
Radical enhanced ISC represents an interesting alternative to the conventional methods to
access the triplet excited states of organic chromophores upon photoexcitation. However, there is
much room remaining for the designing and application of radical EISC-based triplet
photosensitizers. Concerning this aspect, herein we designed an efficient EISC-based triplet
photosensitizer by linking the radical to a chromophore, boron dipyrromethene (Bodipy, Bodipy).
Bodipy is of particular interest due to strong absorption of visible light, high fluorescence
quantum yield (associated to an inhibited non-radiative decay of the electronic excited state) and
feasibly derivatizable molecular structures.8,46−50 Bodipy has been used for preparation of triplet
photosensitizers,7,9 and the ISC of these Bodipy chromophores was achieved with various
mechanisms, such as heavy atom effect,10,11,21 charge recombination,23,24 spin converter,24 and
exciton coupling effect,12,51 etc. Interestingly, production of the triplet excited state of Bodipy
with radical EISC, and the ESP caused by triplet state of Bodipy, were never studied.
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Moreover, application of the high-spin state of Bodipy will open a new avenue for
photochemistry of Bodipy. For example, the TTA upconversion, which exploits the strong
absorption of light by the photosensitizer, can reach an high upconversion quantum yield.16−20
Together with the flexibility in terms of excitation/emission wavelengths, the TTA upconversion
has been used for luminescence bioimaging,52 photovoltaics,53 photocatalysis54 and optical
materials.55−59 Among the other aspects of research on TTA upconversion, designing new triplet
photosensitizers is vital for attaining highly efficient TTA upconversion. However, all the known
photosensitizers for TTA upconversion are based on molecules with efficient triplet state
formation, and most triplet state formation relies on heavy atom effect to enhance the ISC
process. Heavy atom-free photosensitizers for TTA upconversion are rare. Previously we
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developed triplet photosensitizers for TTA upconversion which are based on the use of spin
converter, such as C60, to enhance the ISC.23,24,60 Recently heavy atom-free organic triplet
photosensitizers based on thermally activated delayed fluorescence (TADF) were also
reported.61,62
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Herein we prepared Bodipy-TEMPO dyads to study the EISC effect in Bodipy chromophores
(Scheme 1) and we applied the new systems in TTA upconversion. A dramatic change in the
photophysics of the excited states of Bodipy is induced by the flexibly linked radical unit,
producing a strong visible light absorptive molecule with highly efficient ISC and a long-living
excited triplet state. Linkers between the two units in the dyads with different length were used
to probe the effect of the radical on the singlet excited state of Bodipy. The photophysical
properties were studied with steady-state and time-resolved optical spectroscopies, continuous
wave (cw) and TR-ESR spectroscopies. The fluorescence of the Bodipy unit is significantly
quenched in the dyads. Exceptionally high triplet state quantum yield was observed (ΦT = 80%),
and it was found the ΦT is highly dependent on the length of the linker between the Bodipy and
the TEMPO units. The highly efficient EISC and the long triplet state lifetime (τT = 60 μs) are
unprecedented in the radical-induced EISC studies, and the dyad was used as novel triplet
photosensitizer for TTA upconversion.
2. Results and Discussion
2.1. Design, Synthesis and the Molecular Structure of the Compounds. Two Bodipy-
TEMPO dyads were prepared (BDP-TEMPO-1 and BDP-TEMPO-2, Scheme 1; BDP is the
abbreviation of Bodipy), in which the linkers between BDP and TEMPO units are varied with
different lengths: BDP-TEMPO-1 is with longer linker, and BDP-TEMPO-2 is with shorter
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Scheme 1. Synthesis of the triplet photosensitizers BDP-TEMPO-1 and BDP-TEMPO-2 a
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6
N3
7
8
9
O
O
N
N
O
N
OH
O
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N
O
ii
i
N
N
F
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O
N
O
B
N
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F
B
N
B
BDP-TEMPO-1
2
BDP-1
1
F
F
4
F
F
B
O
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N
iv
+2
iii
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N
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N
N
I
I
B
N
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N
F
B
N
N
O
F
F
B
F
N3
F
F
BDP-2
3
BDP-TEMPO-2
IBDP
a Key: (i) Propargyl bromide, NaH, DMF, rt, 3 h in Ar, 49 %. (ii) Et3N, CuI, 12 h, at rt, in Ar,
65 %. (iii) Phenylacetylene, Et3N, CuI, 12 h, at rt, in Ar, 40 %. (iv) Et3N, CuI, 12 h, at rt, in Ar,
50 %.
linker (Scheme 1). Azide-alkyne Huisgen cycloaddition reaction was used to construct the
linker.63,64 We expected the molecule to adopt both the extended and folded conformations, as
shown from molecular dynamics (MD) simulations (it will be published elsewhere) for which we
give a brief discussion.
MD studies showed that the linkers are flexible, thus these distances and the orientation of the
radical with respect to the chromophore varies substantially with time. The dyads have shown to
be able to display folded conformations which last for only 1 to 2 nanoseconds, then they
transform rapidly to extended conformation. This is expected to play a major role on the
photophysical properties of the BDP chromophore, 34,44,65,66 in particular in enhancing the ISC
process.65
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In the extended conformations the average distance between the chromophore and the
nitroxide (distance between B and O of the nitroxide group) is normally 18 and 13 Å for BDP-
TEMPO-1 and BDP-TEMPO-2, respectively. The first value has been confirmed by single
crystal XRD studies. We failed to obtain the single crystal of BDP-TEMPO-2 due to its poor
crystability.
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The single crystal of BDP-TEMPO-1 was obtained by slow diffusion of n-hexane into
ethanol solution of the compound. The molecular structure of BDP-TEMPO-1 is determined
(Figure 1 and Table S1). BDP-TEMPO-1 crystallized in the triclinic crystal system in the P-1
space group with two molecules in the asymmetric unit.
Figure 1. The ORTEP view of the single crystal structure of BDP-TEMPO-1 with 50% thermal
ellipsoids. Hydrogen atoms are omitted for clarity.
The molecules adopt an extended conformation in the single crystal, with a distance of 18.04
Å between the spin density center (O atom of the nitroxide moiety of the TEMPO unit) and the
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chromophore (B atom of the Bodipy unit), and likely this is the same extended conformation
found in solution by MD.
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MD has also shown that the molecules can also adopt folded conformations, where the
distance between the radical and the chromophore is much shorter. Folded conformations are
obtained because of torsional freedom of the single bonds within the linkers. In folded
conformations the distance between the two can be as short as 8 Å.
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2.2. UV−vis Absorption and Fluorescence Spectra. The UV−vis absorption spectra of the
dyads BDP-TEMPO-1 and BDP-TEMPO-2 do not show any significant difference from that of
reference compounds BDP-1 and BDP-2 (Figure 2a). Thus we propose that electronic
interaction between the Bodipy and TEMPO units at the ground state is small (it must be smaller
than about 10 cm−1, based on the later ESR studies). No enhanced S0→T1 transition was
observed for the dyads, despite of the presence of the radical.67
BDP-TEMPO-1
BDP-1
BDP-2
BDP-TEMPO-2
400
200
0
1.2
0.8
0.4
0.0
a
b
BDP-2
BDP-TEMPO-2+AIBN
BDP-TEMPO-2
300
400
500
600
700
550
600
650
700
Wavelength / nm
Wavelength / nm
Figure 2. (a) UV−vis absorption spectra of BDP-TEMPO-1, BDP-1, BDP-2 and BDP
TEMPO-2, c = 1.0 × 10–5 M in toluene, 20 °C. (b) Fluorescence emission spectra of BDP-2,
BDP-TEMPO-2 and BDP-TEMPO-2 + AIBN (λex = 508 nm) in toluene. Optically matched
solutions were used. c = ca. 1.0 × 10−5 M. 20 °C.
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In stark contrast to the UV−vis absorption spectra, the fluorescence of the dyads are drastically
different from the reference compounds BDP-1 and BDP-2 (Figure 2b). For example, the
fluorescence intensity of BDP-TEMPO-2 is only 25% of that of BDP-2, the latter is known to
have a high fluorescence quantum yield and very weak ISC,46,48,49 i.e. the fluorescence intensity
was quenched by 75% with attachment of the TEMPO unit on the Bodipy chromophore. Similar
trend was observed for BDP-1 and BDP-TEMPO-1 (Supporting Information, Figure S16). We
propose the quenching of the singlet excited state is due to EISC. This postulation was
confirmed by the radical combination reaction with azobisisobutyronitrile (AIBN) under heating
condition to give an ether product (Supporting Information, Scheme S1 and Figure S16 ∼ Figure
S19, Supporting Information). Upon this radical combination reaction and transform of the
TEMPO radical into an ether structure, the fluorescence intensity was fully recovered (Figure
2b).
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Other possible mechanisms for quenching of the fluorescence of Bodipy in the radical-labeled
fluorophore can be ruled out, i.e. intra- or intermolecular Förster resonance energy transfer
(FRET), or Dexter energy transfer to the radical moiety, because of the small oscillator strength
of the n-π* transition of the TEMPO, and the unmatched energy levels of the Bodipy with those
of the radical (which is important for both the Förster and Dexter energy transfer).34,65 Photo-
induced electron transfer (PET) can be also excluded based on both electrochemical
measurements (will be published elsewhere) and TR-ESR measurements. Finally, enhanced
internal conversion (EIC) is also excluded for BDP-TEMPO-2 due to the high yield of the
triplet state (ΦT = 80%, see later section).44 Control experiments proved that the intermolecular
interaction of the Bodipy chromophore with TEMPO is much less efficient (the proof has been
given by studying a solution contained both Bodipy and TEMPO, Figure S14, Supporting
Information). These results indicate that the linker length exerts vital effect on the quenching of
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the fluorescence of Bodipy chromophore, and it is most likely the spin exchange interaction
between the singlet excited chromophore and the radical that quenched the fluorescence.32,68
This electron exchange interaction, is strongly dependent on the molecular orbital overlap,44,65,67
and it is reasonably favoured when the two partners are kept closer, and thus more effective in
BDP-TEMPO-2 than in BDP-TEMPO-1.69
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The photophysical properties were summarized in Table 1. The fluorescence lifetimes of BDP-
TEMPO-1, BDP-1, BDP-TEMPO-2 and BDP-2 were compared (Table 1). The results show
that distinct double-exponential decay were observed for BDP-TEMPO-2, which is tentatively
attributed to the change in the relative geometry between the TEMPO and the Bodipy unit,
provide both large and small interactions between the two units.33 BDP-TEMPO-1 also show
shorter fluorescence lifetime (1.4 ns) compared to BDP-1 (3.4 ns).
Table 1. Photophysical Parameters of the Compounds
b
c
d
g
h
λabs/nm a
503
ε
λem/nm
515
ΦF
τT/μs e
τF /ns f
3.4
ΦΔ
ΦT
0.90i
0.29
0.95
0.99
0.05
j
j
j
BDP-1
8.86
8.78
8.81
8.45
8.33
−
−
−
BDP-TEMPO-1
BDP-TEMPO-1+AIBN
BDP-2
504
515
190
1.4
0.14
0.14
j
j
j
504
515
3.8
−
−
−
j
j
j
522
536
5.6
−
−
−
BDP-TEMPO-2
523
536
62
0.2(96%) 0.56
0.80
5.0 (4%)
j
j
j
BDP-TEMPO-2+AIBN
523
8.40
536
0.98
5.7
−
−
−
a
b
Maximal UV−vis absorption wavelength in toluene (1.0 × 10−5 M). Molar absorption
coefficient at absorption maxima. ε : 104 M−1 cm−1. Maximal emission wavelength in toluene
c
(1.0 × 10−5 M). d The fluorescence quantum yields in toluene, with BDP-1 (ΦL = 0.90, in toluene)
and IBDP (ΦL = 0.036, in toluene) as the standard. e Measured by transient absorption in toluene
(1.0 × 10−5 M). f The fluorescence lifetime, in toluene (1.0 × 10−5 M), λex = 445 nm for BDP-1
g
series, λex = 405 nm for BDP-2 series. Singlet oxygen quantum yields in toluene, IBDP were
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h
used as standard (ΦΔ = 0.85 in toluene). Triplet quantum yields in toluene, Eosin Y were used
as standard (ΦT = 0.56 in methanol). i Literature values.70 j Not applicable.
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2.3. TR-ESR Measurements: EISC Process and Electron Spin Polarization (ESP). We used
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TR-ESR spectroscopy to study the generation of the electron spin polarization (ESP) as a proof
of the activation of the EISC channel; and the involvement of the conformational changes
(folding/unfolding) both on the EISC process and on the lifetime of the triplet. ESP is produced
because the interaction between TEMPO radical and the photoexcited state BDP moieties
activate processes that create a non-Boltzmann population (spin polarization) of the radical
electron spin sublevels that can be detected by TR-ESR: signals in enhanced Absorption (A
polarization, lower spin level, β, is much more populated than upper spin level, α) or in
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Emission (E polarization, upper spin level, α, is more populated than lower spin level, β) can be
observed.33 The analysis of the polarization of the ESR spectra allowed us to obtain information
on the photophysical deactivation path of the dyads.
When the chromophore is in a formal triplet state, the description of the global state of the
molecule is influenced by the possible triplet - radical interaction. The Hamiltonian for the
interaction is given by the exchange term as −2J ST⋅SR, where ST, SR are the spin momentum of
the chromophore triplet state and that of the radical respectively, and J is a measure of the
separation between the doublet and the quartet states (the energy separation is 2J). As detailed in
the specific section in the Supporting Information, the J values are defined as “small” or “large”
when compared to the Zeeman interaction.
The electronic states that are better described in the coupled representation, thus with the total
spin number as quantum number, when the J-value is larger than the Zeeman term.71 When the
exchange interaction is much smaller than the Zeeman term, the uncoupled representation is
preferred, where the two moieties are treated separately; a total spin number can be formally
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provided also in this case. Because of the folding/unfolding motion, both representations have to
be used to describe the molecule in the folded/extended configurations.
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We will use the 2S+1[R−T] notation for the strongly interacting triplet-doublet pairs, forming a
doublet (D, STotal = 1/2 and consequently 2S+1=2) or a quartet (Q, STotal = 3/2 and consequently
2S+1=4) state, whereas the term (2S+1)[R,T] will be used to indicate weakly interacting moieties
in the extended conformations. In this last case the total spin number is less essential for the
description of the state.
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(a)
(b)
2000
3000
4000
5000
Magnetic Field / G
Figure 3. (a) TR-ESR spectrum in frozen toluene solution of BDP-TEMPO-2 (c = 2 × 10−4 M)
at 80 K with excitation at 503 nm. (b) the simulation of the experimental spectrum as sum of two
species: two triplet states (Tr1 and Tr2) spin coupled to the radical. The magnetic parameters of
Tr1 and Tr2 were determined experimentally as reported in the Supporting information. The
polarization pattern of the zero-field states were (0; 0.17; 0; 1.0; 0.67; 1.0) and (1.0; 0.0; 0.0; 0.6;
0.47; 0.33) for 4[R,Tr1] and 4[R−Tr2] respectively, while JTR = 170 and 1.2×104 MHz for the two.
In the simulation the DTR was neglected because its value is within the linewidth value of 100
MHz.
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We begin with the discussion of the results obtained at low temperature in frozen solution (at
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80 K, Figure 3). The most satisfactory simulation was obtained by using two spin-polarized
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species, one with a strong interaction between the radical moiety, and one with a weak
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Exp
interaction (see JTR
values in Table 2). The simulation experimentally confirms the
presence of the two possible conformations (folded and extended) for BDP-TEMPO-2 as
anticipated by MD calculations.
The simulation of this spectrum required, as parameters, the Zero-field splitting (ZFS)
parameters (D, E) of the triplet state, the exchange interaction JTR, the dipolar interaction DTR
between the triplet and the radical, and the polarization of zero-field states. Initially, the fitting
was obtained by using the polarization of the zero-field states as the only free parameters. The
other simulation parameters were taken either from experimental values (the g-values of the
radical and of the triplet, and the zero-field splitting parameters of the triplet), or from
calculation (the radical-triplet exchange interactions). The magnetic parameters of the triplet
were obtained from TR-ESR measurements in a parent molecule having two iodine atoms, after
reduction of the radical (the procedure and the results are reported in the Supporting
Information). We noted the presence of two main conformations also in the triplet state. The
exchange interaction parameter was taken from calculation for the extended conformation,
and initially also for the folded conformation; we finally let also this last parameter to be
varied. The best-fit value for the folded conformations was J = 1.2 ×104 MHz (0.4 cm−1, Table
2). This value is consistent with a radical-triplet distance almost 1 Å larger than the
calculated one (8 Å). The dipolar interaction was neglected because within the linewidth
value (100 MHz).
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Table 2. Estimated distances r between the Bodipy moiety and the nitroxide, ZFS D
parameter and the spin exchange interaction (J) for the molecules in the extended and
folded conformations.
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r (Å)
DTR (MHz)a JTRCalc (MHz) b
JTRExp (MHz) c
BDP-TEMPO-1 18 d
8
0.03
/
8 e
BDP-TEMPO-2 13 d
8 e
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/
2×105
170
170
2×105
1.2×104
a
The calculated values for the dipolar interaction in the point dipole approximation DTR (see
Supporting Information for the calculation).72 b The upper limit of the spin exchange interaction
JTRCalc according to Eq. S4 with respect to the experimental value determined from simulation
c
of the spectrum in Fig. 3b. d Extended conformation. e Folded conformation.
We pass now to present the results obtained for a diluted (10−4 M) fluid toluene solution at 185
K of BDP-TEMPO-2. Representative slices along the field taken at different delays from the
laser pulse (see Figure 4a) show a time evolution of the shape of the spectrum. At early delay
times, a spectrum is observed with complete A polarization and three broad lines centered at g =
2.0077 — relative to a nitroxide with hyperfine interaction of aN = 14.7±0.2 G (dashed spectrum
in Figure 4a); a structureless broad component (ca. 50 G width, see the fitting, i.e. the dotted
curve, in Figure 4a) is also present. Later it evolves to another nitroxide spectrum totally with E
polarization, a narrower linewidth and a nitrogen hyperfine interaction aN = 15.8±0.1 G, and
centered at g = 2.0064. At higher temperatures this is the only species observed.
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b
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*
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time (us)
II
III
0
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Time / μs
Figure 4. (a) TRESR spectra of BDP-TEMPO-2 in toluene (1.0 × 10−4 M at 185 K) upon pulsed
laser excitation (532 nm, 5 ns) at the delay times of 0.3 μs (red, top) and 2 μs (black, medium)
after the laser flash. The bottom spectrum in (a) is the integrated cw-ESR spectrum in the dark
under the same conditions (g = 2.0063). The dashed curve is the simulation of the spectrum at
0.3 μs (g = 2.0077) as described in the text and the dotted curve a broad structureless component.
(b) Time profile curve taken at 3309 G (ꢀ in graph (a)) and the simultaneous simulation of
phase II (the section of the curve representing transition from enhanced absorption to emission)
and III (the section of the curve representing the relaxation of the emission) together by using
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biexponential functions based on RTPM mechanism (red dashed line). Phase I represents the
development of the absorption upon laser excitation. In the inset the only-absorptive signal
obtained at 3304 G (* in graph (a)), relative to the formation and decay of the polarization of the
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species with g = 2.0077. (c) Simplified mechanism explaining the time evolution of the signals
33d
in (a) and (b, phase II and phase III); in grey the species that are observed.
The process for
formation of the A polarization was omitted (phase I). See text for the designation of the rate
constants.
The interpretation of the evolution of the observed spectra is given by the scheme shown in
Figure 4c. The upper (intramolecular) reaction chain accounts for the formation of signals in
absorption, whereas the lower reaction accounts for the generation of signals in emission.
As for the signal in absorption observed at early times, the strong spin interaction between the
radical and the excited chromophore favors the formation of the triplet state of BDP unit through
2
EISC, and consequently generates initially the doublet state [R—T] (D). The A polarization of
the TEMPO moiety may be formed by RTPM with doublet precursor.42 RTPM exploits a partial
mixing between the quartet and the doublet state to transfer some population from the doublet to
the triplet state (Supporting information, Figure S22). According to this mechanism, for the
regular case J < 0,42 one of the sublevels of the doublet (D+1/2) is partially depleted, whereas state
Q−3/2 state is populated. The molecular motion brings the BDP (in triplet state) and the radical far
apart rapidly, through a spin-conserved process (which preserves the total spin number). The
species finally observed by TR-ESR at early times is a nitroxide with weak interaction
(
(2,4)[R,T]); the distribution of population within the six sublevels of these states is the result of a
population transfer from the D/Q states that have the same total MS value; this process leads to
an overall A polarization for the weakly coupled radical species. The magnetic parameters of the
species (g and aN) are slightly different from those of the radical in the ground state D0 because
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the interaction between the radical and the triplet is small but not negligible, and it is responsible
for a quartet state contamination; the interaction, also, favors a line broadening. 44 The presence
of this A polarized species at early times (Figure 4a, top), is the fingerprint of the presence of an
EISC process taking place in the dyad.
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At later delay times, the spectrum in Figure 4a is attributed to that of the dyad in the electronic
ground state D0 (g = 2.0063, aN = 15.75 ± 0.05 G determined from the cw-ESR spectrum in
Figure 4a; note that at room temperature the hyperfine interaction is slightly smaller aN = 15.45 ±
0.05 G, see Supporting Information Figure S21) but spin polarized in emission. The process in
the lower part of Figure 4c accounts for the formation of the ground electronic state D0 with E-
polarization; this polarization is generated by an intermolecular RTPM mechanism with quartet
precursor;33b,65 but also the radical/quartet polarization mechanism (RQPM) is possible.36,40
As regards to the time evolution of the signal of these species, the time trace recorded at 3304
G (see inset in Figure 4b) shows a quick rise (exponential growth with 0.18 μs time constant),
which is time-limited by the response of the instrument (100 − 200 ns), and followed by a rather
fast decay (0.4 μs) likely due to T1 electron spin relaxation; the lifetime of the triplet is ruled out
because much longer (see Table 1) and cannot contribute to the decay.
Figure 4b describes the time profiles at 3309 G, where both (2,4)[R,T] and polarized D0 states
are present. The rapid rise of the signal (phase I) is followed by a rapid decay to an emission (E)
polarization (phase II). The E polarized band is characterized by a very slow decay (phase III of
the time profile curve in Figure 4b). Overall, this time profile is in agreement with the typical
RTPM mechanism, with switching from the doublet precursor to quartet precursor.42
Phase I is determined by the formation/decay of the (2,4)[R,T] and its contribution is negligible
for time larger than ca 1 μs. During phase II and III, a strong polarization of D0 is generated
through intermolecular RTPM as a consequence of an interaction between unpolarized D0 and
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the triplet moiety of (2,4)[R,T] molecules. The time for generation of the maximum of the E
polarization is close to the reported intermolecular ESP generation for TEMPO (1.5 − 2 μs)
through intermolecular processes.36,73
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We used a simple model to quantitatively fit the experimental curve (phase II and III). We
(2,4)
assumed a pseudo-first-order reaction for ESP generation (E polarization) with [D0] »
[R,T]
(see lower reaction in Figure 4c), and included both spin relaxation T1 process for the radical and
the decay of the triplet moiety to ground electronic state (τ is triplet lifetime). The details of the
model are given in the Supporting Information; here we report the analytical solution for the
time dependence of the E polarization, similar to that found for analogous systems,33b,33d
expressed as:
C
[D0 ]EM = −
⋅
[
exp(−kdt) − exp(−k1t)
]
(1)
(kd − k1)
where k1=1/T1, kd = 1/τ and C a scaling factor.
Our fitting of phase II and III signal by using eq. 1 resulted in the determination of τ = 36 ±
10 μs and T1 = 0.33 ± 0.05 μs. We note that lifetime of the triplet state at the same concentration
at room temperature is 17 μs (Supporting information, Figure S27), and that at lower temperature
lifetime usually increases. The T1-value can be compared with that obtained by Pulse ESR
(Inversion-Recovery experiment, run on the narrower line at lower field). After inversion of the
magnetization by a π-pulse, the recovery trace exhibit a biexponential behavior with T1a = 0.2 μs
and T1b = 1.8 μs. The faster component is likely the one to be compared with the observed T1 .
As regards to BDP-TEMPO-1, the low triplet yield of the Bodipy moiety prevented the
observation of a good-quality polarized spectrum, and only very weak emissive lines were
observed (see Figure S24 and discussions in Supporting Information).
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To summarize the results for BDP-TEMPO-2, according to MD simulation, there is a balance
between folded conformation and extended conformation. The results obtained for BDP-
TEMPO-2 at 80 K (in frozen solution, Figure 3) indicated the presence of the two types of
conformers (note that the equilibrium is relative to ground state). During the folding/unfolding
process, for the photoexcited molecule, the interactions between the chromophore and the radical
favor both the EISC and the RTPM mechanisms; the latter interaction generates ultimately
(2,4)[R,T] spin polarized states.
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Different from other systems having rigid linker between the radical and the chromophores,
we attribute the high triplet quantum yield and long triplet lifetime of the dyads in this paper to
the efficient EISC from S1→T1 but inefficient EISC of T1→S0 on the base of the following
points: first of all the decay to the ground state is a first order perturbation and its efficiency is
reciprocal to the energy gap of the states.34,44 As second point, we note that the folding/unfolding
might disfavor EISC to the ground state by limiting the interactions between the radical and the
chromophore; from our results we must conclude that there is a stiffening of the molecule in the
(2,4)[R,T] states which prevents the folding of the dyad and therefore the EISC to the singlet (in
this case S0). It should be pointed out that the available examples of the radical enhanced ISC
systems are not sufficient to draw an conclusion on the quantitative relationship between the
magnitude of the exchange (J values) and the ISC efficiency (i.e. the triplet state quantum yield,
ΦT) because in most cases not both the J and the ΦT values were reported.34,44,65
2.4. Nanosecond Transient Absorption Spectroscopy: Triplet Electronic Excited States.
In order to study the triplet excited state of both dyads upon photoexcitation, optical methods
were applied, and in particular nanosecond transient absorption spectroscopy (Figure 5). With
BDP-TEMPO-1, intense ground state bleaching (GSB) band at 500 nm was observed upon
pulsed laser excitation (Figure 5a). Weak excited state absorption (ESA) bands were observed in
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the regions of 410 nm – 460 nm and 550 nm – 700 nm. These are the ESA band of the T1 state of
Bodipy.74−76 Using triplet-triplet energy transfer (TTET) method (Eosin Y was used as reference
compound, optically matched solutions were used and equal proper amount of acceptor was
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0.01
0.00
0.00
-0.01
746.5 μs
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......
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0 μs
τ = 190.0 μs
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Time / μs
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......
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-0.09
τ = 62 μs
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c
d
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Wavelength / nm
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Time / μs
Figure 5. Nanosecond time resolved transient absorption spectra of BDP-TEMPO-1 and BDP-
TEMPO-2. (a) BDP-TEMPO-1 upon ns pulsed laser excitation (λex = 500 nm) and (b) decay
trace of BDP-TEMPO-1 at 510 nm. The spike is due to scattered laser. (c) BDP-TEMPO-2
upon pulsed laser excitation (λex = 516 nm) and (d) decay trace of BDP-TEMPO-2 at 490 nm. c
= 1.0 × 10−5 M in deaerated toluene, 20 °C.
added), according to equation S5, (Supporting Information), the triplet state quantum yield of
BDP-TEMPO-1 (ΦT) was determined as 14% (Supporting Information, Figure S28, Table S3).
This ISC efficiency is significant as compared with unsubstituted Bodipy (less than 1%). The
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triplet excited state lifetime of BDP-TEMPO-1 was determined as 190 μs (Figure 5b). It should
be pointed out that the fluorescence quenching of the Bodipy moiety in BDP-TEMPO-1 by the
TEMPO unit is much more significant (75%) than the triplet state quantum yield of BDP-
TEMPO-1 (14%), thus non-radiative decay of the S1 state such as EIC is probable for BDP-
TEMPO-1 upon photoexcitation.44
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With BDP-TEMPO-2, the transient signal is more intense (Figure 5c). For example, the
ΔO.D. (optical density) value at the bleaching band increased to 0.09, which is much larger than
that of the BDP-TEMPO-1 (ΔO.D. = 0.025). Δ O.D. values are proportional to the concentration
of the excited molecules.77,78 The triplet state lifetime of BDP-TEMPO-2 was determined as 62
μs (Figure 5d), and the triplet state quantum yield (ΦT) was determined as 80% (Supporting
Information, Figure S29 and Table S3). By reaction with AIBN (actually 2-cyanoprop-2-yl
radical) under heating conditions to produce ether, we found that no triplet state can be detected
for the resulted dyads upon photoexcitation (Figure S25, Supporting Information). This evidence
is a further proof that the radical moiety is vital to the formation of triplet state with the dyads.
To the best of our knowledge, this is the first report of the detail study of the triplet state
property of Bodipy, accessed by the radical enhanced ISC (EISC). Previously Bodipy and
verdazyl radical moieties were connected by using 9,10-anthryl linker and PET effect was
observed for the dyad, but the triplet state generated by photoexcitation is localized on
anthracene moiety, not on Bodipy chromophore.36,79
We also studied the effect of the TEMPO on production of triplet state of Bodipy
chromophore by intermolecular interaction (studied with mixture of TEMPO and BDP, Figure
S26, Supporting Information), the results show that the production of triplet state is much less
efficient than the intramolecular interaction (however the triplet state lifetime is longer, which is
due to the diminished self-quenching effect). These results unambiguously confirmed that the
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formation of the triplet excited state with BDP-TEMPO-1 and BDP-TEMPO-2 is via the
intramolecular interaction between the singlet excited state of Bodipy and the radical, and the
efficiency of this radical enhanced ISC is highly dependent on the length of the linker between
the Bodipy chromophore and the TEMPO unit.34,65,66
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The high triplet state quantum yield of our BDP-TEMPO-2 dyad is unprecedented. In
previous studies on radical labeled chromophores, normally the triplet state quantum yield for
EISC is much lower,65 or not reported. On the other hand, previously the triplet state lifetime of
the chromophore obtained with EISC is much shorter than the intrinsic triplet state of the organic
chromophore.34,44,65 For example, perylenebisimide (PBI) chromophore was rigidly connected
with TEMPO moiety with a specific distance and orientation, and radical-enhanced ISC was
observed.34,44 However, the triplet state lifetimes are shorter than 1 μs, although the intrinsic T1
state lifetime of PBI is over 100 μs (accessed by methods other than EISC).65 With attachment of
TEMPO moiety to the phthalocyaninatosilicone, the triplet state lifetime was greatly reduced
from 500 μs to ca. 15 μs.33b In our case with BDP-TEMPO-2, the triplet state lifetime is
exceptionally long (62 μs). Moreover, the triplet excited state quantum yield of BDP-TEMPO-2
is high (ΦT = 80%). We attribute the efficient ISC and the long-lived triplet state of BDP-
TEMPO-2 to the weak and transient interaction between the radical moiety and the singlet
excited state of Bodipy, which is sufficient for S1→T1 ISC, but nonsufficient for the T1→S0 ISC.
This is reasonable since ISC is driven by first-order perturbation which is dependent on the
energy gap of the states,34 which leads to a relatively slow T1→S0 ISC. To some extent, the weak
and transient interaction in our flexible system contribute to the high triplet quantum yield and
long triplet lifetime, which have never been realized in rigid system. It should be pointed out that
the quantitative relationship between the magnitude of the exchange (J values) and the ISC
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efficiency (i.e. the triplet state quantum yield, ΦT) is unclear because in most cases not both the J
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and the ΦT values were reported.34,44,65
Considering the wide application of Bodipy as triplet photosensitizers,21 we believe our results
are useful for preparation of heavy atom-free organic triplet photosensitizers and for study of the
fundamental photochemistry of spin-labeled chromophores. One reason for designing of heavy
atom-free triplet photosensitizers is that with bulky chromophore, the ISC may become less
efficient even with the heavy atom attached.67 Heavy atom-free triplet photosensitizers, which
are based on ISC via the mechanisms other than heavy atom effect, such as those with radical
EISC, discussed herein, may overcome this bottleneck.
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In order to study the kinetics of the EISC in the dyads, the femtosecond transient absorption
spectroscopy (fs TA) was used for characterization of the photophysical processes involved in
BDP-TEMPO-2 (the detail will be published elsewhere). The results show that the ISC of BDP-
TEMPO-2 takes 250 ps. Previously a compact ring-fused perylene-nitroxide was prepared, due
to the smaller distance between the radical and the perylene chromophore, a sub-ps ISC was
observed.44 In another TEMPO-labeled PBI compound with longer linker,34 EISC is slower (τ =
45 ps). The electron exchange interaction between the radical and the chromophore serves as the
first-order perturbation that drives EISC, the magnitude of this perturbation is highly dependent
on the orbital overlaps of the two moieties. Thus shorter distance between the radical and the
chromophore may result in faster ISC. However, in that case the triplet state lifetime of the
perylene-nitroxide dyad is only 100 ns − 500 ns,44,65 which is not satisfactory to be used as triplet
photosensitizer for intermolecular electron transfer or energy transfer processes (such as
photocatalysis). It should be pointed out that the photophysics of the radical-labeled organic
chromophore is elusive and further investigation is needed. For example, previously pentacene
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was connected with TEMPO through semi-rigid, short linker of a C=N bond, however, the ISC
of the pentacene was not affected by the nitroxide radical.80
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2.5. Application of the EISC in Triplet-Triplet-Annihilation (TTA) Upconversion. To date
most of the photosensitizers for TTA upconversion are with triplet state accessed with heavy
atom effect.16,18,19,59 Herein we demonstrated for the first time that photosensitizers with the
radical group attached to visible light-harvesting chromophores, having EISC, are also efficient
for TTA upconversion (BDP-TEMPO-2). These results are useful in terms of both fundamental
photochemistry and the development of TTA upconversion.
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Based on the T1 state energy level of the Bodipy, we selected perylene as the triplet
acceptor/emitter for the TTA upconversion.18,59,76 The overall process involves the excitation of
the photosensitizer which populates the triplet excited state of the BDP unit through an EISC
process. This state is responsible for photosensitization of the perylene to its triplet state, which
then drives the TTA upconversion process.
First we demonstrated that excitation of BDP-TEMPO-2 or perylene alone at 520 nm did not
produce any emission in the range of 430 nm − 500 nm, where perylene is expected to give
fluorescence emission (Figure 6). As second step, we determined that a solution containing the
mixture of the two components (BDP-TEMPO-2 and the perylene) produced upconverted
emission in the range 430 nm – 500 nm upon excitation with 520 nm laser. The upconversion
quantum yield was determined to be 6.7% (Figure 6). To the best of our knowledge, this is the
first time that a photosensitizer with radical induced EISC process is exploited to produce TTA
upconversion.
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IBDP+perylene
IBDP
BDP-TEMPO-2+perylene
BDP-TEMPO-2
1500
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1000
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0
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Wavelength / nm
Figure 6. Upconversions with sensitizers BDP-TEMPO-2 and IBDP. Excited with 520 nm laser
(210 mW cm−2). Optically matched solutions were used (the solutions show the same absorbance
at the excitation wavelength, A = 0.2); [perylene] = 1.1 × 10−4 M, in deaerated toluene, 20 °C.
TTA upconversion with BDP-TEMPO-1 as the triplet photosensitizer was also studied, but
much lower upconversion quantum yield (ΦUC = 0.2 %) was observed. This is within expectation
because the triplet state quantum yield of BDP-TEMPO-1 (ΦT = 14 %) is much lower than
BDP-TEMPO-2 (ΦT = 80 %).
Scheme 2. Proposed Mechanism for the EISC promoted TTA Upconversion
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We propose to use the following scheme (Scheme 2) to describe the production of the perylene
triplet (3pery) and the TTA upconversion with the formation of perylene S1 excited state (1pery*).
Based on all the above discussions, the photophysical processes of BDP-TEMPO-1 and BDP-
TEMPO-2 upon photoexcitation can be summarized in Scheme 3. Irradiation of the two dyads
at around 500 nm excite the chromophore of the dyads to an excited singlet state. The triplet
state of the chromophore (state [R,T] of the molecule) is located at 1.65 eV for both molecules
above the ground state. Exchange interactions between the radical and triplet moieties slightly
vary the energy of the states by some GHz (expanded in the diagram) according to their total
spin number (D or Q states).
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Scheme 3. Jablonski Diagram Illustrating the Photophysical Processes in BDP-TEMPO-1
and BDP-TEMPO-2
BDP-TEMPO-1
BDP-TEMPO-2
CTS
2[R*,S0]
CTS
2.34
2[R*,S0]
2.7 eV
3.21
2[R,S1]
2.46
2[R,S1]
2.31
Q
[R,T]
1.65
Q
[R,T]
1.65
D
D
2[R,S0]
2[R,S0]
The radical-chromophore pair is designated as [R,S] or [R,T] according to the spin state of the
chromophore: singlet (S) or triplet (T). The superscript on left indicate the overall spin
multiplicity of the pair state. For strongly interacting [R,T] pairs, exchange interaction separates
doublet states (total spin S = 1/2, D in the scheme) from quartet states (S = 3/2, Q in the scheme).
CTS stands for charge transfer state. The excited state of the nitroxide group is designated with
red asterisks. Photoexcitation of the nitroxide moiety is obtained by irradiation at 459 nm.
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Our future plan is to design radical labeled visible light-harvesting organic chromophore as
novel photosensitizers, to study the photo-induced spin alignments and the high spin states,
preferably to be with high yield of the long-lived quartet state, as well as the application of these
molecules in optical or magnetic materials, spintronic materials, and TTA upconversion.
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2.6. Conclusion. In summary, it is the first time that the triplet excited state of Bodipy
chromophore is accessed with the radical enhanced intersystem crossing (EISC), and the electron
spin polarization (ESP) along with the EISC was studied. Our strategy is to label the Bodipy
chromophore with a stable radical, 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) via different
linkers, and the EISC was studied with steady state and transient optical and ESR spectroscopies
(cw ESR and TR-ESR). Significant fluorescence quenching and triplet electronic excited state
production upon visible light photoexcitation was observed, with triplet state quantum yield of
ΦT = 80%. The EISC process takes 250 ps. The triplet state of the Bodipy chromophore is
exceptionally long (62 μs). These efficient EISC and the long-lived triplet excited state are
unprecedented for the radical-chromophore EISC systems. Previously the triplet state of
chromophore accessed with EISC effect is short-lived (less than 1 μs). The dyad with shorter
linker between the Bodipy and the TEMPO unit shows much stronger quenching of the Bodipy
fluorescence and more efficient triplet state production. The interaction of the singlet excited
state of the Bodipy moiety and the TEMPO (radical moiety) was studied with TR-ESR. The
analysis of the spectra confirmed the presence of EISC route to the formation of the triplet state,
followed by a radical triplet pair mechanism (RTPM) leading to ESP. This heavy atom free
photosensitizer with BDP triplet state accessed via EISC was used for triplet-triplet annihilation
upconversion (quantum yield is 6.7%). Our future plan is to control the electron exchange
interactions between the radical and the excited state of the organic chromophores and to access
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high spin state of organic chromophores. These studies are useful for investigation of the
interaction between triplet state of organic chromophore and radical, photo-induced electron spin
polarization in organic magnetic materials and triplet-triplet-annihilation upconversion, as well
as fundamental photochemistry.
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3. Experimental Section
3.1. General Methods. UV−vis absorption spectra were taken on a HP8453 UV−vis
spectrophotometer. Fluorescence spectra were recorded on Shimadzu RF-5301PC
spectrofluorometer. Luminescence lifetimes were measured on an OB920 fluorescence/
phosphorescence lifetime instrument (Edinburgh, U.K.).
3.2. Compound BDP-TEMPO-1. Under N2 atmosphere, compound 4 (40.9 mg, 0.1 mmol) and
2 (31.0 mg, 0.15 mmol) were dissolved in mixed solvent CHCl3/EtOH/H2O (12 mL, 10:0.8:0.8,
v/v), then the mixture was stirred at room temperature (RT). One drop of Et3N was added and
the mixture was stirred for 5 min. Then CuI (20 mg) were added. The mixture was stirred at RT
for 12 h. Then water was added and the organic layer was collected and dried over anhydrous
MgSO4. The solvent was evaporated under reduced pressure. The crude product was purified
using column chromatography (silica gel, DCM: MeOH = 30:1, v/v) to give a red solid. Yield:
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40 mg (65%). Mp: 180.5 − 181.5 °C. The H NMR spectra was recorded after treatment of the
product with phenylhydrazine, to reduce the nitroxide to hydroxyl structure (no satisfactory
NMR spectra can be recorded for paramagnetic compound). 1H NMR (400 MHz, CDCl3): δ =
7.76 (s, 1H), 7.20 (d, 2H , J = 8.0 Hz), 6.99 (d, 2H, J = 8.0 Hz), 5.97 (s, 2H), 4.80 (t, 2H, J = 4.0
Hz), 4.68 (s, 2H), 4.42 (t, 2H, J = 4.0 Hz), 3.85 (s, 1H), 2.55 (s, 6H), 2.09 (d, 2H, J = 8.0 Hz),
1.82 (d, 2H, J = 8.0 Hz), 1.40−1.28 (m, 18H). 13C NMR (125 MHz, CDCl3) δ = 158.75, 156.78,
145.80, 145.24, 140.96, 131.61, 128.33, 123.57, 115.36, 113.19, 85.69, 74.19, 70.61, 66.39,
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61.84, 55.32, 49.65, 44.11, 31.39, 20.94, 17.24, 16.02. ESI-HRMS(C33H42BF2N6O3 + Na+): calcd.
m/z = 642.3277; found m/z = 642.3262.
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3.3. Compound BDP-TEMPO-2. Similar procedure with compound BDP-TEMPO-1. Yield:
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26 mg (50%). Mp: 161.5 − 162.5 °C. The H NMR spectra was recorded after treatment of the
product with phenylhydrazine (no satisfactory NMR spectra can be recorded for paramagnetic
1
compound). H NMR (400 MHz, CDCl3): δ = 7.48 (s, 1H), 6.13 (d, 2H , J = 8.0 Hz), 5.84 (d,
2H, J = 8.0 Hz), 4.60 (d, 2H, J = 8.0 Hz), 3.84−3.78 (m, 1H), 2.57 (s, 6H), 2.24 (d, 6H, J = 4.0
Hz), 2.13−2.06 (m, 2H), 1.98−1.96 (m, 2H), 1.49−1.13 (m, 12H). 13C NMR (125 MHz, CDCl3):
δ = 157.98, 157.91, 141.52, 141.45, 132.32, 129.95, 128.48, 123.39, 123.27, 121.46, 121.38,
61.88, 45.82, 32.04, 31.71, 29.81, 22.82, 22.78, 16.23, 15.99, 15.09, 14.98, 14.29. ESI-
HRMS(C26H36BF2N6O2 + Na+): calcd. m/z = 536.2859; found m/z = 536.2850.
3.4. Steady State and Time-Resolved (TR) ESR Spectroscopy. Both continuous wave and
TR-EPR spectra were recorded with two different generations of Bruker spectrometers, both
with an extended detection bandwidth (6 MHz): a ER200D spectrometer equipped with a
cylindrical cavity with optical access and cooled by a nitrogen flow for temperature control, and
a ELEXSYS E580 spectrometer equipped with a dielectric cavity inside an Oxford CF900
cryostat cooled by liquid nitrogen.94 The time resolutions for the two systems in the time-
dependent experiments were different, and of the order of 150 ns for the first, and 500 ns for the
second, this last having a slightly better sensitivity. Direct-detected signals (no modulation of the
magnetic field was used) were recorded with LeCroy digital oscilloscopes with GHz bandwidth.
Photoexcitation was obtained from Nd: YAG pulsed lasers (Quantel Brilliant, λ = 532 nm, pulse
length = 5 ns; E/pulse = ca. 5 mJ) equipped with superior harmonic modules and optical
parametric oscillators (OPOTECH) for tunable irradiation in the visible.
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