Ultrafast intersystem crossing and spin dynamics of zinc meso-tetraphenylporphyrin covalently bound to stable radicals

Article abstract of DOI:10.1021/jp2021006

Tert-Butylphenylnitroxide (BPNO^?) and α,γ- bisdiphenylene-β-phenylallyl (BDPA^?) stable radicals are each attached to zinc meso-tetraphenylporphyrin (ZnTPP) at a fixed distance using one of the ZnTPP phenyl groups. BPNO^? an

Full text of DOI:10.1021/jp2021006

ARTICLE
pubs.acs.org/JPCA
Ultrafast Intersystem Crossing and Spin Dynamics of Zinc
meso-Tetraphenylporphyrin Covalently Bound to Stable Radicals
Michael T. Colvin, Amanda L. Smeigh, Emilie M. Giacobbe, Sarah M. Mickley Conron, Annie Butler Ricks,
and Michael R. Wasielewski*
Department of Chemistry and ArgonneꢀNorthwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston,
Illinois 60208-3113, United States
S Supporting Information
b
ABSTRACT: tert-Butylphenylnitroxide (BPNO•) and R,γ-bisdipheny-
lene-β-phenylallyl (BDPA•) stable radicals are each attached to zinc
meso-tetraphenylporphyrin (ZnTPP) at a fixed distance using one of the
ZnTPP phenyl groups. BPNO• and BDPA• are oriented para (1 and 3,
respectively) or meta (2 and 4, respectively) relative to the porphyrin
macrocycle. Following photoexcitation of 1ꢀ4, transient optical absorp-
tion spectroscopy is used to observe excited state quenching of 1*ZnTPP
by the radicals and time-resolved electron paramagnetic resonance (TREPR) spectroscopy is used to monitor the spin dynamics of
the paramagnetic product states. The presence of BPNO• or BDPA• accelerates the intersystem crossing rate of 1*ZnTPP about 10-
to 500-fold in 1ꢀ4 depending on the structure compared to that of 1*ZnTPP itself. In addition, the lifetime of 3*ZnTPP in 1 is
shorter than that of 3*ZnTPP itself as a result of enhanced intersystem crossing (EISC) from 3*ZnTPP to the ground state. The
TREPR spectra of the three unpaired spins produced within 1 and 2 show spin-polarized excited doublet (D1) and quartet (Q)
states and subsequent formation of a spin-polarized ground state radical (D0). All three signals are absorptive for 1 and emissive for
2. Polarization inversion of the Q state is observed on a tens of nanoseconds time scale in 2, while no polarization inversion is
observed for 1. The lack of polarization inversion in 1 is attributed to the short lifetime of the doubletꢀquartet manifold as a result of
the very large exchange interaction. The TREPR spectra of 3 and 4 show ground state radical polarization at X-band (9.5 GHz) at
room temperature, but not at 85 K, and similarly no polarization is observed at W-band (94 GHz). No evidence of excited doublet or
quartet states is observed, indicating that the exchange interaction is both weak and temperature dependent. These results show that
although ultrafast EISC produces 3*ZnTPP within 1ꢀ4, the magnitude of the exchange interactions between the three relevant spins
in the resulting 3*ZnTPPꢀBPNO• and 3*ZnTPPꢀBDPA• systems dramatically alters their spin dynamics.
’ INTRODUCTION
spectroscopies,40,41 which allows for a more complete under-
standing of these systems. For example, we observed ultrafast
triplet state formation (τ = 2 ps) in perylene-3,4:9,10-bis-
(dicarboximide) (PDI) covalently attached to tert-butylphe-
nylnitroxide radical (BPNO•) followed by the appearance of
excited doublet and quartet states resulting from spinꢀspin
exchange interactions within this three-spin system.40 We
also examined a similar system in which 2,2,6,6-tetramethyl-1-
piperidinyloxyl (TEMPO) is covalently attached to PDI.41 In this
Developing a fundamental understanding of spin polariza-
tion and the ability to control spin transport through organic
molecules is important to the development of novel molec-
ular materials for electronics and energy production. Photo-
excitation of organic molecules can produce well-defined
initial spin states,1ꢀ6 while modern electron paramagnetic
resonance (EPR) techniques provide an important means of
examining these states with a view toward molecule-based
spintronics.7ꢀ11 The interactions of radicals with excited
states have attracted attention over the past few decades.
Previous studies of such systems have largely focused on (1)
radical-induced excited state quenching observed optically or
(2) spin polarization that results from three-spin mixing. In
the past, these two topics were studied independently,
with fast quenching primarily studied by fluorescence quan-
tum yield and lifetime measurements,12ꢀ25 and spin polar-
ization studied by time-resolved electron paramagne-
tic resonance (TREPR).26ꢀ39 Recently, we have studied
radicalꢀexcited state interactions from the femtosecond to
microsecond time scales using transient optical and TREPR
3
system we observed ultrafast *PDI formation in toluene, but
electron transfer from TEMPO to PDI in tetrahydrofuran
(THF). The TREPR data on TEMPOꢀPDI in toluene showed
3
distinct *PDI and TEMPO signals as opposed to quartet and
excited doublet states resulting from three exchange-coupled
unpaired spins, which indicates that the exchange interactions
3
between each of the two unpaired electrons comprising *PDI
and the single spin on TEMPO are small. In both of these
Received: March 10, 2011
Revised:
May 27, 2011
Published: June 01, 2011
r
2011 American Chemical Society
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systems 3*PDI formation results entirely from the presence of the
radical and not the usual spinꢀorbit-induced intersystem cross-
ing (SO-ISC) mechanism because the fluorescence quantum
yield of 1*PDI itself is nearly unity.42
ZnTPP is an excellent chromophore to examine the effect of
a stable free radical on excited state dynamics because it has
been well characterized by transient optical experiments.43
Efficient SO-ISC within 1*ZnTPP yields 3*ZnTPP, which has
been examined in detail by TREPR spectroscopy.44,45 In
addition, the spin dynamics of 3*ZnTPP and nitroxide radicals
have been studied extensively in freely diffusing mixtures,28
coordinatively linked27,29,46ꢀ49systems, and in one instance in
a zinc porphyrin with a nitroxide covalently linked to its meso
position.50 The spin dynamics of freely diffusing ZnTPP
and R,γ-bisdiphenylene-β-phenylallyl (BDPA•) mixtures have
also been examined.51
Currently several mechanisms are used to account for spin
polarization that occurs in three-spin systems: the radicalꢀ
triplet pair mechanism (RTPM),26ꢀ29,32,34,35,46,52ꢀ57 electron
spin polarization transfer (ESPT),28,29,51 and the reversed
quartet mechanism (RQM).37,40 The RTPM applies only to
freely diffusing radicalꢀtriplet chromophore mixtures. It is a
spin sorting mechanism, where the triplet and radical are
mixed via the spinꢀspin exchange interaction (J) to produce
excited doublet and quartet states as the radical and triplet
diffuse together.36 Internal conversion (IC) selectively quen-
ches the excited doublet state, with subsequent diffusion resul-
ting in overpopulation of either the R or β state of the radical
depending on the sign of J.
ESPT has usually been observed in freely diffusing systems,
although diffusion is not required for this mechanism to occur.
ESPT requires polarization transfer from a spin-polarized triplet
state to the radical by spin exchange, resulting in quenching of the
triplet state to the ground state singlet.28,29,58 The radical’s
polarization is thus independent of the sign of J and is determined
solely by the polarization of the triplet.
In contrast to the previous two mechanisms, the RQM
applies only to covalent systems. The RQM accounts for the
presence of excited doublets and quartets whose polarization
inverts after several hundred nanoseconds as observed in
some covalently bound systems.37,40 The energy level diagram
for the RQM is shown in Figure 1. Briefly, photoexcitation of
the formally doublet ground state (D0) produces the first
excited state (D2), where the two electrons on the chromo-
phore (1*ZnTPP in our case) are still spin paired. If the
exchange interactions between each of the electrons on
1*ZnTPP and the unpaired electron on the radical are suffi-
ciently large and the experiment is carried out in a static
magnetic field that is much larger than the exchange interac-
tions, excited doublet (D1) and quartet (Q) states are formed,
Figure 1. Energy levels created after doubletꢀtriplet mixing. D2 f D1
internal conversion is more rapid (thick red arrow) than is D2 f Q
intersystem crossing (thin red arrow). The levels shown are for 3J > 0.
initial polarization. D1 is then selectively quenched to D0 by IC,
resulting in the reversed intersystem crossing from Q to D1 and
polarization inversion. There have also been recent reports41,59 of
covalent systems that are in the weak exchange regime, where
radicals and triplets are observed as opposed to doublets and quar-
tets.
In addition to enhanced intersystem crossing (EISC) (A),
electron transfer (B), energy transfer (C), and enhanced internal
conversion (EIC) (D) also need to be considered as possible
mechanisms that can account for radical-induced excited state
quenching. We have discussed these mechanisms in detail
previously.40 Electron transfer has been observed from a radical
3
to a triplet state in the covalently bound TEMPOꢀ *naph-
thalene-1,8:4,5-bis(dicarboximide) system60 and in several dif-
fusing nitroxideꢀC60 systems.61ꢀ63 As mentioned previously, we
have recently reported conclusive evidence of electron transfer
from a radical to an excited singlet state in polar media.41 EIC is
rarely thought to be a dominant quenching mechanism16,24 and
is generally perceived to be a minor contributor to the overall
photophysics.40,41
kEISC
1
2
3
2
ꢁ
ꢁ
ZnTPP þ R f ZnTPP þ R
ðAÞ
kET
1
2
ZnTPP þ R f ZnTPP•ꢀ þ Rþ or ZnTPP•þ þ Rꢀ ðBÞ
ꢁ
kEnT
1
2
2
ꢁ
ꢁ
ZnTPP þ R f ZnTPP þ
R
ðCÞ
kEIC
1
2
2
ꢁ
ZnTPP þ R f ZnTPP þ R
ðDÞ
which are mixtures of the Tþ1, T0, and T
high field
ꢀ1
3
eigenstates of *ZnTPP, and the R and β spin states of the
radical. Transitions then occur from D2 to both Q and D1. The
transition from D2 to Q occurs exclusively by ISC because the
overall spin multiplicity changes and the transition is formally
spin forbidden. The transition from D2 to D1 is partially
allowed since two pathways populate this state: ISC that
F€orster energy transfer is generally thought to be insig-
nificant for nitroxide radicals because their optical transi-
tions in the visible spectrum have low oscillator strengths.
Yet, exceptions have been proposed,64 and when energy-
accepting radicals are freely diffusing in solution, it is also
necessary to consider the distribution of distances and the
presence of multiple radicals in the proximity of one energy
donor.65 However, F€orster energy transfer needs to be
considered for BDPA• because it has an intense absorption
at 485 nm and a much weaker one at 865 nm, which poten-
tially provide the necessary spectral overlap for F€orster
3
populates *ZnTPP, and IC if the overall spin multiplicity
between the initial and final states is unchanged. Since transi-
tions to D1 are more allowed, the D1 sublevels have larger popula-
tions compared to those of Q. The zero-field-splitting interaction
mixes D1 and Q, causing reversible transitions, which results in the
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Scheme 1. Syntheses of 1ꢀ4a
a (a) Pd(PPh3)4, Na2CO3, toluene, 95ꢀ C, 15 h, 35%; (b) TBAF, 0 ꢀC f room temperature, 2 h, PbO2 1 h, 75%; (c) Pd(PPh3)4, Na2CO3, toluene, 80
ꢀC, 14 h, 50%; (d) TBAF, 0 ꢀC f room temperature, 2 h, PbO2 1 h, 76%; (e) Pd(dppf)2Cl2, KOAc, DMF, 85 ꢀC, 4.5 h, 73%; (f) Pd2(dba)3, P(o-tolyl)3,
Ag2O, THF, reflux, 12 h, 52%; (g) DBU, THF, benzoquinone, room temperature, 5 min, quantitative; (h) Pd(dppf)2Cl2, KOAc, DMF, 85 ꢀC, 4.5 h,
73%; (i) Pd2(dba)3, P(o-tolyl)3, Ag2O, THF, reflux, 2 h, 42%; (j) DBU, THF, benzoquinone, room temperature, 5 min, quantitative.
transfer to occur.66 It is also possible that through-bond
Dexter energy transfer may also contribute to quenching of
1*ZnTPP.
interaction, J. Transient optical absorption experiments show
that rapid EISC occurs for 1*ZnTPP in 1ꢀ4 to produce
3*ZnTPP, as well as significantly increasing the rate of
3*ZnTPP decay back to the ground state in 1. For 1 and 2,
TREPR spectroscopy reveals the presence of excited doublet
and quartet states as well as a spin-polarized ground state
doublet consistent with the RQM, indicating that the spinꢀ
In this paper we present data on BPNO• and BDPA• stable
radicals attached to ZnTPP at a fixed distance using one of the
ZnTPP phenyl groups. BPNO• and BDPA• are oriented para
(1 and 3, respectively) or meta (2 and 4, respectively) relative
to the porphyrin macrocycle. The change in orientation is
used to probe how the interaction between 3*ZnTPP and the
radical depends on the magnitude and sign of the exchange
3
spin exchange interaction between *ZnTPP and BPNO• is
large. In contrast, TREPR spectroscopy indicates that spinꢀ
3
spin exchange between *ZnTPP and BDPA• is weaker in 3
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ARTICLE
and 4 at room temperature, and becomes negligible at cryoge-
nic temperatures.
were loaded into quartz tubes (0.84 mm o.d. ꢂ 0.6 mm i.d.) in a
N2-filled glovebox to a height of ∼8 mm, and sealed with a clear
ridged UV doming epoxy (Epoxies, Etc., DC-7160 UV). The
EPR samples were stored in a freezer in the dark, when not
being used.
Steady state CW EPR spectra were measured at X-band using
0.2ꢀ2 mW microwave power and 0.01ꢀ0.05 mT field modula-
tion at 100 kHz. TREPR measurements were made using
continuous wave (CW) microwaves with direct detection, and
field swept electron spin echo (ESE) detection. A 1 kW TWT
amplifier (Applied Systems Engineering 117X) was employed to
generate high power microwave pulses. The resonator was fully
decoupled for all pulsed experiments (Q < 200). ESE was
obtained with a π/2ꢀτꢀπ pulse sequence, with a 8 ns π/2
pulse, a 16 ns π pulse, and a τ of 140 ns. After the transient species
was generated by the laser pulse, the integrated echo intensity
was recorded as a function of magnetic field to yield the spectrum
at a given time after the laser pulse. The temperature was
controlled by an Oxford Instruments CF935 continuous flow
cryostat using liquid N2. Samples 1 and 2 were photoexcited at
532 nm (0.2 mJ/pulse, 7 ns, 10 Hz) using the frequency-doubled
output from a Nd:YAG laser (Quanta-Ray DCR-2), while
samples of 3 and 4 were photoexcited with 6 ns, 550 nm laser
pulses generated using the frequency-tripled output of a Con-
tinuum 8000 Nd:YAG laser to pump a Continuum Panther
OPO, to ensure that BDPA was not photoexcited. The light from
both lasers was coupled to a fiber optic sample holder (Bruker
E-600-1023 L) for W-band experiments. Following photoexcita-
tion, kinetic traces of the transient magnetization were accumu-
lated under CW microwave irradiation (2ꢀ20 mW). The field
modulation was disabled to achieve a Q/πν ≈ 30 ns instrument
response function (IRF), where Q is the quality factor of the
resonator and ν is the resonant frequency, while microwave
signals in emission (e) and/or enhanced absorption (a) were
detected in both the real and the imaginary channels (quadrature
detection). Sweeping the magnetic field gave two-dimensional
spectra versus both time and magnetic field. For each kinetic
trace, the signal acquired prior to the laser pulse was subtracted
from the data. Kinetic traces recorded at magnetic field values off-
resonance were considered background signals, whose average
was subtracted from all kinetic traces. The spectra were subse-
quently phased into a Lorentzian part and a dispersive part, and
the former, also known as the imaginary magnetic susceptibility
χ00, is presented.
’ EXPERIMENTAL SECTION
Synthesis. The syntheses of 1ꢀ4 are summarized in Scheme 1
and described in detail in the Supporting Information.
Optical Spectroscopy. Ground state absorption measure-
ments were made on a Shimadzu (UV-1601) spectrophot-
ometer. The optical density of all samples was maintained
between 0.1 and 0.6 at 532 nm, for both femtosecond and
nanosecond transient absorption spectroscopies. Femtosecond
transient absorption measurements were made using the 590 nm,
110 fs output from a optical parametric amplifier using techni-
ques described earlier.67 Samples were placed in a 2 mm path
length glass cuvette and bubbled with nitrogen to prevent sample
degradation. The samples were irradiated with 1.0 μJ/pulse fo-
cused to a 200 μm spot. The total instrument response function
(IRF) for the pumpꢀprobe experiments was 180 fs. Samples for
nanosecond transient absorption spectroscopy were placed in a
10 mm path length quartz cuvette equipped with a vacuum
adapter and subjected to five freezeꢀpumpꢀthaw degassing
cycles. The samples were excited with 6 ns, 1 mJ, 416 and 550 nm
laser pulses generated using the frequency-tripled output of a
Continuum 8000 Nd:YAG laser to pump a Continuum Panther
OPO. The excitation pulse was focused to a 5-mm-diameter spot
and matched to the diameter of the probe pulse generated using a
xenon flashlamp (EG&G Electro-Optics FX-200). The signal
was detected using a photomultiplier tube with high voltage
applied to only 4 dynodes (Hamamatsu R928). The total instru-
ment response time is 7 ns and is determined primarily by the
laser pulse duration. Transient absorption kinetics were fit to a
sum of exponentials convoluted with a Gaussian instrument
function using LevenbergꢀMarquardt least-squares fitting.
EPR Spectroscopy. EPR measurements at both X-band (9.5
GHz) and W-band (94 GHz) were made using a Bruker Elexsys
E680-X/W EPR spectrometer outfitted with a variable Q di-
electric resonator (ER-4118X-MD5-W1) at X-band and a cy-
lindrical resonator (EN-680-1021H) at W-band. For EPR
measurements at X-band, toluene solutions of 1ꢀ4 (∼10ꢀ4 M)
were loaded into quartz tubes (4 mm o.d. ꢂ 2 mm i.d.),
subjected to five freezeꢀpumpꢀthaw degassing cycles on a
vacuum line (10ꢀ4 mbar), and sealed using a hydrogen torch.
For EPR measurements at W-band, samples of 1ꢀ4 (∼10ꢀ4 M)
’ RESULTS
Synthesis and Steady State Characterization. The synth-
eses of 1ꢀ4 are described in detail in the Supporting Informa-
tion. Briefly, 1 and 2 were synthesized by first making the corres-
ponding monobrominated zinc tetraphenylporphyrin68 and then
using a Suzuki cross-coupling reaction to attach the silyl-pro-
tected BPNO• radical precursor,69,70 as shown in Scheme 1.
BPNO• was prepared by deprotection of the silyl ether using
TBAF to give the hydroxylamine, which was oxidized with PbO2
to give BPNO•. The residual hydroxylamine precursor was
separated from the radical by chromotography. Molecules 3
and 4 were synthesized by generating the boronic ester from
brominated zinc 5-(4-bromophenyl)-10,15,20-triphenylpor-
phyrin and performing a Suzuki cross-coupling reaction with
iodo-BPDA(H)71,72 in THF. The BDPA radical was then
prepared by treatment of the BDPA(H) porphyrin with
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1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and benzoquinone
in THF at room temperature.
photophysics of ZnTPP. Selective photoexcitation of ZnTPP in
toluene to its lowest excited singlet state (1*ZnTPP) at 590 nm
produces a transient absorption spectrum having an absorption
band at 465 nm, which then red shifts to about 480 nm with τ =
2.0 ns as 3*ZnTPP is formed (Figure S1, Supporting In-
formation). Selective photoexcitation of the lowest energy
absorption of ZnTPP in 1ꢀ4 with 590 nm, 110 fs laser pulses
initially forms 1*ZnTPP followed by ultrafast formation of
3*ZnTPP in both toluene (Figure 3) and THF (Figure S2,
Supporting Information) with time constants that are all much
The UVꢀvis spectra of 1 and 2 are almost identical, with both
showing the well-known Soret band of ZnTPP at 420 nm and
Q-band at 550 nm (Figure 2).43 There is little contribution to the
UVꢀvis spectrum from BPNO•, indicating that there is weak
electronic coupling between radical and chromophore. The
absorption spectra of 3 and 4 display an additional absorption
band at 500 nm resulting from BDPA•, as well as a broad,
featureless absorption centered at approximately 330 nm, which
continues to increase into the UV. The BPDA• extinction
coefficient at 485 nm is 29 000 Mꢀ1 cmꢀ1, and that of ZnTPP
is 24 000 Mꢀ1 cmꢀ1 at 548 nm.66,73
1
shorter than the intrinsic SO-ISC time constant for *ZnTPP
(Table 1). The triplet yields were determined by comparing the
absorption changes at 465 nm and 0.5 ps following the laser flash
with those at 480 nm and 4.7 ns. The extinction coefficients for
Transient Absorption Spectroscopy. Transient absorption
studies on 1ꢀ4 in toluene and THF were conducted to deter-
mine how the presence and connectivity of the radicals affects the
3
1*ZnTPP and *ZnTPP at 465 and 480 nm are 5.6 ꢂ 104 and
7.4 ꢂ 104 Mꢀ1 sꢀ1, respectively.43 The low signal-to-noise ratio
from 3*ZnTPP in 3 and 4 is due to the constraint of photoexciting
at 590 nm to avoid BDPA excitation. Since the extinction
Table 1. EISC Kinetics and Triplet Yields for 1ꢀ4
compound
τ (1*ZnTPP f 3*ZnTPP) (ps)
ϕT
1 (TOL)
1 (THF)
2 (TOL)
2 (THF)
3 (TOL)
3 (THF)
4 (TOL)
4 (THF)
4.0 ( 0.1
4.6 ( 0.3
164 ( 5
144 ( 5
19.4 ( 0.7
19.7 ( 0.7
34 ( 1
0.46 ( 0.02
0.49 ( 0.02
0.60 ( 0.02
0.63 ( 0.02
0.10 ( 0.02
0.10 ( 0.02
0.20 ( 0.02
0.16 ( 0.02
Figure 2. Normalized UV/vis of 1ꢀ4 in toluene.
39 ( 3
Figure 3. Transient absorption spectra of 1ꢀ4 at times indicated in toluene and following a 110 fs, 590 nm laser pulse.
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Figure 4. Transient absorption spectra of 1 at the indicated times in toluene and THF following a 7 ns, 416 nm laser pulse.
coefficient of ZnTPP is small at 590 nm, we cannot increase the
concentration because it diminishes probe beam transmission
through the sample. The formation of 3*ZnTPP within 1ꢀ4 was
also observed using nanosecond transient absorption spectros-
copy (Figure 4 and Figure S3 in the Supporting Information).
longer times (700 ns). However, the polarization inversion
(a f e) depends on the microwave power incident on the
sample, indicating that the polarization inversion is due to
transient nutations.11 The line width of the transient polarized
signal matches that of BDPA• measured by CW EPR spectros-
copy. The TREPR spectrum of 4 shows an emissive signal that
also oscillates with the incident microwave power (Figure 6c).
No transient signal is observed at W-band.
By examining the spin dynamics in frozen solution, one can
determine the ISC mechanism.45 Thus we acquired TREPR
spectra of 1 and 2 at 85 K (Figure 7) to determine the influence of
the radical on the ISC mechanism. The TREPR spectrum of 1
shows two overlapping features, each with absorption at low field
and emission at high field. The wide feature (75 mT baseline to
baseline) matches the TREPR spectrum of 3*ZnTPP, while the
narrower feature (45 mT baseline to baseline) is attributed to the
quartet state resulting from exchange coupling the two electrons
on 3*ZnTPP with that on BPNO•. The absorptive feature in the
center of the spectrum is assigned to the ground state radical.
Similar features are observed in 2, where a narrow feature (45 mT
baseline to baseline) is observed with absorption at low field and
emission at high field. In contrast, the TREPR spectra of 3 and 4
at 85 K (Figure 8) show a broad feature that is absorptive at low
field and emissive at high field, matching the D and E values of
3*ZnTPP.45 No polarization is observed in the center of the
spectra of 3 and 4 at 85 K.
3
The decay of *ZnTPP in 1 is significantly faster than that of
3*ZnTPP43 itself with τ = 77 ( 1 ns in both toluene and THF,
while the corresponding decay time constants of 2ꢀ4 are all
>50 μs and limited by bimolecular collisions in solution. The
shorter lifetime in 1 most likely results from the quenching of
3*ZnTPP by EISC back to ground state in the presence of BPNO•.
EPR Spectroscopy. The steady state CW EPR spectra of 1 and
2 at 295 K (Figure 5a) consist of a dominant hyperfine triplet
splitting due to nitrogen (I = 1) that is split further by the protons
responsible for the smaller hyperfine splittings. The hyperfine
coupling constants are reported in Table 2 and were obtained by
simulations using WINSIM.74 The TREPR spectrum of 1
(Figure 5b) at X-band initially consists of a broad (∼20 mT
baseline to baseline) feature that decays with τ = 161 ( 4 ns, and
three absorptively polarized signals with hyperfine splittings of
1.2 mT appearing at later times that decay with τ = 1206 ( 77 ns.
No signal is observed at W-band. The TREPR spectrum of 2 at
W-band (Figure 5c) shows three strongly emissive signals that
appear within the instrument response time. The corresponding
X-band spectrum is shown in Figure S6 in the Supporting
Information. The low field signal (D0) appears at 3362.1 mT
(g = 2.0057) with a hyperfine splitting of 1.2 mT, characteristic of
the nitrogen in the BPNO•. The second signal at higher field (Q)
appears at 3367.0 mT (g = 2.0027), while the third signal (D1) is
at 3372.1 mT (g = 1.9998). The emissive Q, D1, and D0 signals
appear within our resonator response time, and the Q and D1
states evolve to absorptive polarization over time with rise times
of 38 ( 2 and 25 ( 45 ns, respectively, followed by decay to zero
with τ = 1036 ( 56 ns for Q and 934 ( 340 ns for D1, while the
polarization of the D0 signal does not invert and decays to zero
with τ = 425 ( 14 ns. In contrast to our previously reported
results, we do not observe hyperfine splitting of the quartet signal
(Q). This is likely a result of uncertainty broadening due to the
fast initial decay.
’ DISCUSSION
Ultrafast Excited State Dynamics. We have previously dis-
cussed in detail the most common radical-induced excited state
quenching mechanisms.40 To briefly summarize, radical-induced
quenching of excited states usually proceeds by one or more of
the following mechanisms: electron transfer, F€orster and/or
Dexter energy transfer, electron-exchange-induced enhanced
intersystem crossing (EISC), and enhanced internal conversion
(EIC), i.e., rapid vibrational relaxation.12ꢀ16,18,19,21ꢀ24,60,61,75,76
The predicted rates for F€orster energy transfer were calculated
using PhotochemCAD software.77 The rate constant for F€orster
energy transfer is given by65
Figure 6a shows the steady state EPR of 3 and 4 at X-band.
BDPA• is a carbon-centered radical with most of its spin density
localized on the allyl fragment and partial delocalization into the
fluorenyl rings as evidenced by the hyperfine coupling constants
given in Table 2. The TREPR spectra of 3 (Figure 6b) show an
absorptive signal at early times (200 ns) and an emissive signal at
ꢀ
ꢁ
Z
¥
ϕDk2 9000ðln 10Þ
kFEnTðrÞ ¼
FDðλÞ εAðλÞλ4 dλ ð1Þ
τDr6 128π5Nη4
0
where ϕD is the quantum yield of fluorescence of the donor, k is a
factor describing the orientation of the transition dipoles with
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energy transfer an unlikely quenching mechanism. Given that
Dexter energy transfer is an electron exchange process, it is
possible that rapid through-bond energy transfer from 1*ZnTPP
to BDPA• occurs in 3 and 4.
To examine this possibility, femtosecond transient absorption
spectra of 2*BDPA• in toluene were obtained by direct excitation
of BDPA•. These transient spectra display a distinct absorption
band at 527 nm in addition to a ground state bleach at 485 nm
(Figure S4, Supporting Information). These transient features
decay completely with τ = 14.9 ( 0.3 ps. The 527 nm transient
absorption has an extinction coefficient of about 104 Mꢀ1 sꢀ1
based on the bleach of the 485 nm ground state absorption (ε =
2.9 ꢂ 104 Mꢀ1 sꢀ1). Given these extinction coefficients, it should
2
be possible to observe *BDPA• produced by energy transfer
from 1*ZnTPP; however, the transient absorption data for 3 and
4 show no evidence of 2*BDPA• formation. The time constants
for 1*ZnTPP decay given in Table 1 are only slightly longer than
the 2*BDPA• lifetime, so it is unlikely that the inability to observe
the formation of 2*BDPA• is a consequence of inverted kinetics.
The free energies of reaction for electron transfer from
1*ZnTPP to BPNO• (ΔG = 0.9 eV in toluene and ΔG =
0.3 eV in THF) or from BPNO• to 1*ZnTPP (ΔG = 1.0 eV in
toluene and ΔG = 0.4 eV in THF) are all positive, so electron
transfer is ruled out as a 1*ZnTPP quenching mechanism for 1
and 2 (see the Supporting Information). Similarly, electron
transfer from BDPA• to ZnTPP in both solvents is not energe-
tically feasible (ΔGCS = 1.0 eV in toluene and ΔG = 0.4 eV in
THF). In contrast, electron transfer from ZnTPP to BDPA• is
energetically favorable in both 3 and 4 (ΔGCS = ꢀ0.1 eV in
toluene and ΔG = ꢀ0.6 eV in THF), but the absence of the
strong BDPAꢀ (absorption at 600 nm (ε = 4.5 ꢂ 104 Mꢀ1 sꢀ1 80
)
in the transient absorption spectra of 3 and 4 in both toluene
(Figure 3) and THF (Figure S2, Supporting Information) in-
dicates that electron transfer is not occurring in either solvent. It
is highly unlikely that the electron transfer kinetics would be
inverted in both low- and high-polarity solvents. This leaves
enhanced internal conversion (EIC) as the principal quenching
mechanism competitive with EISC. In the case of 1 and 2, the
measured triplet yields indicate that EISC and EIC each account
for about half the total quenching of 1*ZnTPP by BPNO•, while
for 3 and 4, EIC is the dominant mechanism.
The lifetime of 3*ZnTPP in 1 is shorter than that of 3*ZnTPP
Figure 5. CW EPR (a) and TREPR spectra (b and c) of 1 and 2 at times
indicated following a 7 ns, 532 nm laser pulse at 295 K. 1 is shown at
X-band (9.5 GHz), while 2 is shown at W-band (94 GHz).
3
itself, while the lifetimes of *ZnTPP in 2ꢀ4 are limited by
bimolecular collisions in solution, and are too long be accurately
measured by our instrument (>50 μs). The short lifetime of 1
compared to 2ꢀ4 is most likely a result of an increase in the ISC
rate of 3*ZnTPP to its ground state, which is a consequence of the
very large exchange interaction between 3*ZnTPP and BPNO• in
1, and is consistent with the TREPR data discussed in the next
section. In contrast, the diffusion-limited lifetimes of 3*ZnTPP in
2ꢀ4 suggest that the radical’s presence does not play a sub-
stantial role in the quenching of 3*ZnTPP.
respect to each other, η is the refractive index of the solvent, τD is
the fluorescence lifetime of the donor, FD(λ) is the normalized
fluorescence intensity of the donor and εA(λ) is the extinction
coefficient of the acceptor. The calculated F€orster energy transfer
rates are summarized in Table 3. For 1 and 2, this mechanism is
unlikely because the BPNO• absorbance is weak (ε = 10.5
M
ꢀ1 cmꢀ1 at 476 nm), so the spectral overlap with the 1*ZnTPP
emission is poor. Moreover, the 1*ZnTPP energy (∼2.06 eV)78 is
well below the lowest excited doublet state energy of 2*BPNO•
(∼2.6 eV),40 making both F€orster and Dexter energy transfer
unlikely mechanisms for *ZnTPP quenching. In contrast, energy
RadicalꢀTriplet Spin Interactions. The spin Hamiltonian
for radicalꢀtriplet interactions is given as
^
^
^
^
^
H ¼ HZ þ Hhf þ HDip þ HEx
1
transfer is possible for 3 and 4 because the *ZnTPP energy is
T
Z
R
Z
R
2.06 eV,78 which is well above that of the lowest energy electronic
transition of BDPA• at 1.4 eV (865 nm).66 However, the
calculated F€orster energy transfer rates for 3 and 4 (Table 3)
are about an order of magnitude slower than the experimental
time constants observed for 3*ZnTPP formation, making F€orster
^
^
^
S
¼ gTβ BS þ gRβ BS þ
A Ii
∑
e
e
3
3
i
T
T
T
R
T
R
^
^
^
^
^
^
S
þ S DT S þ S DTR S þ JTRS
ð2Þ
3
3
3
3
3
where βe is the Bohr magneton, gT and gR are the g values for the
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Table 2. EPR Hyperfine Couplings for 1ꢀ4 (mT)
1
aDN ¼ ꢀ aRN
3
triplet and ground state radicals, respectively, B is the magnetic
field, A is the hyperfine tensor and becomes a, the isotropic
hyperfine coupling constant, in fluid solution, S and I are the
ð5Þ
^
electron and nuclear spin operators, D is the dipolar interaction,
and J is the exchange interaction.When the exchange interaction
between the electron on the radical and the electrons on the
triplet, |JTR|, is larger than the other magnetic interactions
(strong exchange regime), the exchange interaction removes
the spin degeneracy resulting in the formation of excited doublet
and quartet states.81 In the strong exchange regime the g values of
the excited doublet (gD) and quartet (gQ) should be linear
combinations of the ground state doublet (gR) and excited triplet
(gT), according to eqs 3 and 4,81 while the hyperfine coupling
constants aN should follow eqs 5 and 6. The zero-field-splitting
parameters for a quartet state are different from those for a triplet
state and obey eq 7, where DQ is the zero field splitting of the
quartet state, DT is the zero field splitting of the triplet state, and
DTR is the dipoleꢀdipole interaction between the electrons on
the triplet and the electron on the radical. In the weak exchange
limit the EPR spectrum should show separate resonances for the
triplet and ground state radicals with the corresponding hyper-
fine splitting and g values.
1
aNQ
¼
aNR
ð6Þ
ð7Þ
3
1
DQ
¼
ðDT þ DTRÞ
3
TREPR Spectroscopy. We employed TREPR spectroscopy at
both X- and W-bands to probe the time evolution of the spin
polarization of the three-spin systems, where two electrons reside
on *ZnTPP and one electron is on BPNO•. The TREPR
3
spectrum of 1 at 295 K and X-band consists of a broad absorptive
feature due to the Q and D1 signals overlapping at early times that
evolves into an absorptively polarized signal having a nitrogen
hyperfine splitting matching that of the ground state radical. The
overlapping Q and D1 signals are slightly broader than those
observed in 2, which results from lifetime broadening. The fast
decay observed by TREPR is consistent with the transient optical
results. The absorptive polarization is consistent with the sign
of J.40 No signal was observed at W-band due to the short lifetime
of the three-spin system in 1.
The TREPR spectrum of 2 at room temperature at both X-
and W-bands differs from that of 1. The TREPR of 2 at W-band
shows an emissively polarized signal at g = 2.0057, with a 1.2 mT
hyperfine splitting, which matches the g value and hyperfine split-
ting of the BPNO• ground state, and is thus assigned to that
1
4
gD ¼ ꢀ gR þ gT
ð3Þ
ð4Þ
3
3
1
3
2
gQ
¼
gR þ gT
3
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ARTICLE
compared to that of 2. Since the spin polarization pattern of
the quartet state in 1 and 2 is the same as that of ZnTPP, it
appears that the quartet state is still formed by SO-ISC and is
merely accelerated by the presence of the radical.
We consider two spin mechanisms to explain the spin polar-
ization observed in 1 and 2: ESPT and the RQM. The RTPM is
not considered as it requires diffusive motion that cannot occur
in our system. The results of 2 are consistent with the RQM, as
we observe excited doublet and quartet states as well as polariza-
tion inversion. ESPT is ruled out for 2 since it results in
quenching of the triplet state, which is not observed optically
or by TREPR spectroscopy. Assigning the spin polarization
mechanism for 1 is more difficult, as the polarization inversion
of the quartets and doublets observed in 2 are not observed.
However, the hyperfine lines are polarized in absorption. On the
basis of previous results,40 we know that changing the position of
the radical from para to meta results in a change in the sign of J
and therefore we would expect to observe absorptive features
in 1. The observation of a quartet at 85 K indicates that D1ꢀQ
mixing is occurring. The lack of polarization inversion results
from fast EIC from D1 to D0, which is a result of a large exchange
coupling between 3*ZnTPP and BPNO•. This shows that despite
the observed presence of doublets and quartets at early times
indicative of the RQM, fast EIC from D1 to D0 kinetically out-
competes the equilibrium between D1 and Q that results in the
usual observation of polarization inversion at later times for
the RQM.
The TREPR spectra of 3 and 4 at X-band and 295 K show
polarized signals matching the width and g value (2.0026) of
ground state BDPA• and are assigned to the polarized ground
state. Although polarization inversion occurs in 3, it depends on
microwave power (Figure S7, Supporting Information) and is
thus due to transient nutations, not polarization inversion as a
result of the RQM. Transient nutations are also observed in 4
(Figure S7, Supporting Information), but they do not result in
polarization inversion. No evidence of quartet and excited
doublet states is observed, indicating that the RQM is not
occurring in 3 and 4. It is likely that the polarization results from
a weak, but nonnegligible exchange interaction between
3*ZnTPP and BDPA•. This is not without precedent as we have
recently reported a system in which a covalently bound TEMPO
radicalꢀPDI triplet state is weakly exchange coupled.41 Whether
a three-spin system is in the strong or weak exchange regime as
regards TREPR observation depends on the relative contribu-
tions from the exchange and Zeeman interactions, respectively.
At X-band we observe only ground state polarization, indicating
that the exchange interaction is weak, while no transient signal is
observed at W-band. This indicates that the exchange interaction
in the three-spin system at W-band is negligible relative to the
Zeeman interaction, which increases 10-fold while the exchange
interaction remains unchanged. The lack of polarization ob-
served at 85 K coupled with the presence of a triplet signal and
absence of a quartet signal at 85 K indicates that the exchange
interaction is also weak at low temperatures. A decrease in J as the
temperature is lowered has been observed previously in radical
pairs within fixed distance donorꢀbridgeꢀacceptor mole-
cules.82ꢀ84 The exchange interaction depends on overlap of
the relevant orbitals of ZnTPP with those of BDPA• or BPNO•.
The dihedral angles between the phenyl rings joining ZnTPP to
BDPA• or BPNO• undergo restricted rotation in fluid solution,
allowing access to conformations with greater πꢀπ orbital
overlap, which increases the coupling between them, while in
Figure 6. CW EPR(a) and TREPRspectra(b and c) of 3 and4 at 295 K,
following a 6 ns, 550 nm laser pulse at times indicated.
radical. Using the g value of 3*ZnTPP (2.001)30 and eqs 3 and 4,
the predicted g values of the Q and D1 states (gQ = 2.0026, gD
=
1.999) correspond well to the measured g values gQ = 2.0027 a1nd
gD = 1.998, respectively. The hyperfine splittings for Q and D1
1
predicted by eqs 5 and 6 are not observed, which likely results
from lifetime broadening.
The TREPR spectra observed at X-band for 1 and 2 at 85 K
look similar, and display a broad feature that matches the
3*ZnTPP spectrum, while the narrow feature at center field is
assigned to the 1/2 T ꢀ1/2 transitions of the overlapping Q and
D1 states resulting from the interaction of 3*ZnTPP with BPNO•.
The quartet state spectrum observed in Figure 7 has been
observed in other ZnTPP systems when a radical is coordina-
tively attached to the Zn atom.29,31,32 The wide feature is
3
assigned to *ZnTPP, which results from a small amount of
sample degradation resulting in loss of the radical due to laser
damage. Similar to the transient optical and TREPR data at room
temperature, the TREPR spectrum of 1 decays much faster
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Figure 7. TREPR spectra of 1 and 2 in toluene at 85 K following a 7 ns, 550 nm laser pulse at X-band at the indicated times.
Figure 8. TREPR spectra of 3 and 4 at 85 K at X-band, following a 6 ns, 550 nm laser pulse 150 ns after the laser pulse.
Table 3. Calculated Fo€rster Energy Transfer Rates for 1ꢀ4 in
that other differences between BDPA• and BPNO• may account
for the difference in quenching mechanisms. IC should be related
to the number of vibrations coupled to the excited state. The
exchange interaction allows the electrons on 1*ZnTPP to spend
some time on the radical. The unpaired electron in BDPA• is
delocalized into the two fluorene rings which have more bonds
compared to BPNO• and therefore more vibrations to accelerate IC.
Toluene
molecule
k2
ra (Å)
τEnT (ns)
1
2
3
4
0.75
1
10.8
10.4
13.7
11.8
82
61
0.75
1
6.6
2.7
a DꢀA distances modeled using Hyperchem MMþ.79
’ CONCLUSION
Transient optical spectroscopy reveals that BPNO• quenches
1*ZnTPP competitively by EISC and EIC mechanisms, while
BDPA• does so by a dominant EIC mechanism in covalent
ZnTPPꢀradical systems at fixed distances. Both quenching
mechanisms are essentially independent of solvent polarity.
TREPR spectroscopy shows that, when J is sufficiently large,
EISC results in fast quenching of 3*ZnTPP as well, and as a result,
polarization inversion is not observed in the RQM. TREPR
spectroscopy also shows that there is a temperature-dependent
frozen solution the phenyl rings are restricted to dihedral angles
which diminish the coupling. Although tripletꢀradical systems
have received significant attention, relatively few systems are
weakly coupled and none have negligible interaction at any
temperature. It is difficult to measure J with certainty in these
systems; however, we can deduce upper or lower limits based on
whether 1ꢀ4 are in the strong or weak exchange regime at a
given applied magnetic field. Compounds 1 and 2 are in the
strong exchange regime even at W-band and thus 3J should be
larger than 3500 mT. However, 3 and 4 are in the weak exchange
regime at X-band, implying that 3J is smaller than 350 mT at
room temperature.
Finally, we address a possible reason as to why EIC is the
dominant quenching mechanism in 3 and 4, whereas EISC is the
dominant quenching mechanism in 1 and 2. It is possible that the
smaller exchange interaction between 3*ZnTPP and BDPA• in 3
and 4 results in slower EISC, which allows EIC to dominate
kinetically; however, both EIC and EISC may depend on J. In
addition to the small exchange interaction in 3 and 4, we believe
exchange interaction between *ZnTPP and BDPA• which is
3
weak at room temperature and negligible at cryogenic tempera-
tures because no quartet signal is observed and BDPA• polariza-
tion is absent. This is the first time that a negligible exchange
interaction is shown at any temperature in a covalently bound
radicalꢀtriplet system. This work shows that by decreasing the
exchange interaction it is possible to slow down EISC to permit
EIC to dominate. Moreover, it demonstrates that the magnitude
of the exchange interaction is not only important in determining
the rate of excited state quenching, but it can also drastically alter
the excited state quenching mechanism.
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