Mimicking the role of the antenna in photosynthetic photoprotection

Article abstract of DOI:10.1021/ja107753f

One mechanism used by plants to protect against damage from excess sunlight is called nonphotochemical quenching (NPQ). Triggered by low pH in the thylakoid lumen, NPQ leads to conversion of excess excitation energy in the antenna system to heat before it

Full text of DOI:10.1021/ja107753f

ARTICLE
pubs.acs.org/JACS
Mimicking the Role of the Antenna in Photosynthetic Photoprotection
Yuichi Terazono,† Gerdenis Kodis,† Kul Bhushan,† Julia Zaks,‡,§ Christopher Madden,† Ana L. Moore,*,†
Thomas A. Moore,*,† Graham R. Fleming,*,§,^ and Devens Gust*,†
†Department of Chemistry and Biochemistry, Center for Bioenergy and Photosynthesis, and Center for Bio-Inspired Solar Fuel
Production, Arizona State University, Tempe, Arizona 85287, United States
‡Applied Science and Technology Graduate Group, University of California, Berkeley, Berkeley, California 94720, United States
§Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
^Department of Chemistry and QB3 Institute, University of California, Berkeley, Berkeley, California 94720, United States
S Supporting Information
b
ABSTRACT: One mechanism used by plants to protect against
damage from excess sunlight is called nonphotochemical quench-
ing (NPQ). Triggered by low pH in the thylakoid lumen, NPQ
leads to conversion of excess excitation energy in the antenna
system to heat before it can initiate production of harmful chemi-
cal species by photosynthetic reaction centers. Here we report a
synthetic hexad molecule that functionally mimics the role of the
antenna in NPQ. When the hexad is dissolved in an organic sol-
vent, five zinc porphyrin antenna moieties absorb light, exchange
excitation energy, and ultimately decay by normal photophysical
processes. Their excited-state lifetimes are long enough to permit
harvesting of the excitation energy for photoinduced charge separation or other work. However, when acid is added, a pH-sensitive dye
moiety is converted to a form that rapidly quenches the first excited singlet states of all five porphyrins, convertingthe excitationenergyto
heat and rendering the porphyrins kinetically incompetent to readily perform useful photochemistry.
by diverting excitation energy into heat before it can be trans-
ferred to the reaction centers and initiate electron transfer. The
process by which this occurs is not fully understood but in some
organisms is known to involve protonation of two residues on a
protein known as PsbS and chemical modification of a carotenoid
polyene. The modified polyene quenches antenna chlorophyll
excited states, possibly by electron and/or energy transfer.3,6-11
Although humans are becoming adept at designing molecules
to perform catalytic or other functions, examples of such molec-
ules that can also regulate their function in response to external
stimuli are rare. Here, we report molecular hexad 1 (Figure 1)
that functionally mimics the role of the antenna in NPQ. The
molecule features five porphyrin antenna moieties and a pH-
sensitive dye, comprising a model multichromophoric photo-
synthetic antenna coupled to a control unit. Under neutral condi-
tions, the porphyrins rapidly exchange excitation energy and have
long-lived excited states capable of doing photochemical work.
When the solution is made acidic, the dye moiety is converted to
a form that quenches the first excited singlet states of all five
porphyrins on the picosecond time scale, rendering them kine-
tically incompetent to carry out additional photochemistry and
converting the excitation energy to heat. In addition to providing
a simple functional model for the antenna in NPQ, the hexad
’ INTRODUCTION
Most of the sunlight used by plants to make fuels through
photosynthesis is harvested by antennas. These chromophore
arrays, excellent examples of natural nanotechnology, transfer
excitation energy to reaction centers where it is converted to
electrochemical potential energy via photoinduced electron
transfer. The electrochemical energy is used in subsequent “dark”
reactions including the synthesis of ATP and of reducing equiv-
alents that are employed to prepare carbohydrate and other fuels.
Under intense sunlight, the reactive redox intermediates prod-
uced by reaction centers are generated more rapidly than they
can be used by the dark reactions. The excess reactive species can
cause tissue damage or generate toxins such as singlet oxygen.
One way that plants protect themselves against such harm is non-
photochemical quenching (NPQ), a process whereby excess
excitation in the antenna is degraded to heat, preventing the
generation of surplus redox capacity.1-5
NPQ is triggered by chemistry initiated in the reaction centers.
Electron transfer in reaction centers releases protons into the
thylakoid lumen, leading to a pH and charge gradient across the
photosynthetic membrane. Chemical potential stored in this gra-
dient powers many of the subsequent reactions of energy conver-
sion. At high light levels the lumen pH drops, signaling that
electron transfer is outpacing the fuel-producing reactions that
consume the pH gradient. Low pH activates NPQ, wherein
changes in the antenna system regulate reaction center function
Received: August 27, 2010
Published: February 11, 2011
r
2011 American Chemical Society
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Figure 1. Structures of the hexads. Hexad 1o is generated from closed
form 1c upon treatment with an acid, H-X, where X represents the
conjugate base.
demonstrates the kind of self-regulatory behavior that will be
needed if the promise of nanotechnology is to be fully realized.12
’ RESULTS
Synthesis. The five zinc porphyrin moieties that comprise
the antenna portion of hexad 1 are covalently linked to a
hexaphenylbenzene core at porphyrin meso positions. The
hexaphenylbenzene also bears a rhodamine-type pH-sensitive
dye, and serves as a relatively rigid framework that controls elec-
tronic interactions among the attached chromophores. Under
neutral conditions, the hexad exists with the dye in the spirocyclic
form 1c, but when the medium becomes sufficiently acidic, the
molecule is protonated and converted to 1o, in which the dye
is in the open, positively charged form. The synthesis and char-
acterization of hexad 1 and model compounds 2-4 (Figure 2)
are detailed in the Supporting Information. The general outline
of the synthetic route to 1 is as follows. The basic hexaphenylben-
zene skeleton was prepared by a variation of the cyclotrimeriza-
tion method of Takase, et al.13 Two acetylene moieties, each
bearing two nickel porphyrins, and a third acetylene bearing a
nickel porphyrin and a substituted aniline were cyclized using a
dicobalt octacarbonyl catalyst. After removal of the nickel with
sulfuric acid and insertion of zinc into the porphyrins, the result-
ing hexaphenylbenzene precursor bearing five zinc porphyrins
was coupled to the dye precursor via a palladium-catalyzed ami-
nation. The pH-sensitive dye precursor was synthesized on the
basis of a method reported by Woodroofe, et al.14 for related
compounds. Models 2, 3, and 4 were synthesized using similar
methods. New compounds were characterized by mass spectro-
metry, 1H NMR, and UV-vis spectroscopy.
Figure 2. Structures of model compounds. In model dyad 3 and model
dye 4 the dye moiety is shown only in the closed, spirocyclic form. The
open form is analogous to that shown in Figure 1.
shown in Figure 4, the Soret band of 1c is broader (see shoulder
at ∼408 nm) and slightly shifted to longer wavelengths relative to
that of model compound 2 comprising a single zinc porphyrin
(Figure 2), indicating excitonic interactions among adjacent por-
phyrin moieties. Excitonic interactions of this type have been observed
in similar compounds,15-17 including a closely related porphyrin
array, and the nature of the interactions discussed in some detail.18
The Q-band shapes and wavelengths are essentially identical to those
in 2. As acetic acid is added, portion-wise, to the solution of 1, a new
absorbance with a maximum at 656 nm grows in (Figure 3a). The
new band is characteristic of the open form of the dye, signaling
conversion of 1c to 1o by protonation. The zinc porphyrin spectral
features are unchanged upon addition of acetic acid, indicating that
zinc is not removed from the porphyrins by the acid.
The conversion of 1c to 1o is accompanied by profound changes
in the porphyrin fluorescence emission (Figure 3b). Hexad 1c
shows typical zinc porphyrin emission at 602 and 650 nm. As acid is
added to the solution, the shape of the emission is unchanged,
but the emission intensity decreases substantially. The decrease
indicates that in the open form of the dye, which predominates in
the solution, emission from all five porphyrin moieties is strongly
quenched. Treatment of acidified solutions of 1 with sodium
carbonate to neutralize the acid results in recovery of the original
absorption and emission spectra (Figure 3). Thus, the proton-
ation is reversible, and the ratio of 1o to 1c in the solution is a
function of the amount of acid added.
Absorption and Fluorescence Spectra. The absorption
spectrum of 1c in dichloromethane is shown in Figure 3a. It fea-
tures the porphyrin Soret (B) at 423 nm and Q-bands at 514, 551,
and 590 nm. The dye portion of the molecule does not absorb in
the visible spectral region but has bands at 309 and 336 nm. As
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Figure 5. Emission decays for 1 at 650 nm following excitation at
550 nm with a 130 fs laser pulse. Decays were obtained from 1c in
dichloromethane (squares) and from an acidified solution highly
enriched in 1o (circles). The solid lines are best fits to exponential
functions with time constants as described in the text.
550 nm, where the light is absorbed by the porphyrin Q-bands, the
decay of the fluorescence emission (monitored at 650 nm) could
be fitted with a single exponential having a time constant of 2.14 ns
(Figure 5, χ2 = 1.16). Such a lifetime is typical of zinc porphyrins of
this general type (vide infra).
The fluorescence decay of the solution of 1 was measured again
(Figure 5) after addition of acetic acid, which converted most of the
sample to 1o. The decay was fitted (χ2 = 1.21) with exponential
components having time constants of 10.3 ps (49%), 39.4 ps
(31%), and 2.10 ns (20%). The 2.1 ns contribution is due to resi-
dual 1c, and the picosecond components represent drastic reduc-
tion of the porphyrin first excited singlet state lifetimes; for 1o, the
rate constants of the quenching processes are 9.7 Â 1010 s-1 and
2.5 Â 1010 s-1. The quantum yield of fluorescence quenching is
essentially unity, as calculated on the basis of either of the two
picosecond time constants and the lifetime of 1c.
Figure 3. Absorption and emission spectra of the hexads. (a) Absorp-
tion spectra of 1c in dichloromethane (solid) and after addition of acetic
acid, which converts some of the 1c to 1o: acetic acid at 123 mM (dash),
244 mM (dot), 482 mM (dash-dot), and 826 mM (dash-dot-dot).
(b) Fluorescence emission spectra (λex = 396 nm) of the solutions des-
cribed in (a). The spectral amplitudes in (a) and (b) have been corrected
for dilution by acid. After completion of these measurements, the acetic
acid was removed by treating the solution with excess granular sodium
carbonate and filtering. The absorption and emission spectra of the
resulting solution are shown (circles). This treatment led to the rever-
sion of 1o to 1c. It also caused the appearance of a very small absorption
at 670 nm, and a small emission band (not shown) at ∼720 nm. These
features are ascribed to a small amount of impurity or decomposition
appearing after the neutralization process.
In order to better understand the nature of the quenching pro-
cess in 1o, we prepared and studied a model dyad 3 (Figure 2),
which consists of a pH-sensitive dye identical to that in 1 linked
to a hexaphenylbenzene core that features a single zinc porphyrin
at a position ortho to the dye. The dyad with the dye in the closed,
spirocyclic form, dissolved in dichloromethane, showed Soret and
Q-band absorption maxima and fluorescence maxima at wave-
lengths similar tothoseobservedfor2. Time-resolved fluorescence
experiments using the single-photon timing method with excita-
tion at 425 nm and emission at 600 nm yielded a lifetime for the
porphyrin first excited singlet state of 2.09 ns (Figure 6). The
fluorescence of model porphyrin 2, measured using the same
apparatus, decayed as a single exponential with a lifetime of 2.13 ns
(not shown, χ2 = 1.02).
Addition of acetic acid to the solution of 3 converted most of
the dyad to the open, protonated form, which had a porphyrin
first excited singlet state lifetime, measured using single-photon
timing, of 23 ps (Figure 6). This time constant represents quench-
ing of the first excited singlet state of the porphyrin by the open
form of the adjacent dye.
Turning again to the hexad, we associate the 10-ps decay ob-
served for 1o mainly with quenching of the two porphyrins ortho
to the dye by the dye moiety. The time constant is comparable to
that measured for the similar process in 3 (23 ps), given the fact
that the two molecules are not identical. In the hexad, the por-
phyrins next to the dye moiety experience strong steric interac-
tions with it and the adjacent porphyrins (not present in dyad 3),
and these result in conformational changes that undoubtedly
Figure 4. Absorption spectra in the Soret region of dichloromethane
solutions of model porphyrin 2 (solid), porphyrin-dye dyad 3 with the dye
in the closed, spiro form (dash), and hexad 1c with the dye in the closed,
spiro form (dot). Note the broadening and slight shift of the hexad Soret
band, which indicates excitonic interactions among the porphyrin moieties.
Transient Spectroscopy. Information about the quenching
process was obtained from transient spectroscopy. Fluorescence
emission data were obtained from two different spectrometers: a
streak camera setup and a conventional single-photon timing appa-
ratus. In each case, the decays were analyzed by least-squares fitting
as a sum of exponentials, with reconvolution to remove effects from
the instrument response as described in the Experimental Section.
Both apparatus produced reliable time constants down to ∼5 ps,
with the goodness of fit (χ2) values reported below.
Fluorescenceemissionstudieswith the streakcamera(Figure5)
showed that when a dichloromethane solution of 1c was excited at
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Figure 6. Fluorescence emission decays at 600 nm following excitation
at 425 nm of a solution of dyad 3 in 2.5 mL of dichloromethane. The
squares indicate the data for the dyad in the closed, spirocyclic form, and
the accompanying solid line is a best exponential fit to the data that yields
lifetimes of 2.09 ns (99.3%) and 8.2 ns (0.7%), with χ2 = 1.16. The small
amount of 8.2 ns decay is ascribed to the free base form of the porphyrin.
The circles indicate data for the solution after addition of 250 μL of
acetic acid, which converts most of the dyad to the open, protonated
form. Exponential fitting gives time constants of 23 ps (70%) and 2.12 ns
(30%), with χ2 = 1.13. The shorter time constant is associated with
the protonated form of the dyad, and the longer time constant with the
closed, spirocyclic form. The apparent rise at early times is due to the
convolution of the decay kinetics with the instrument response function.
alter the energy transfer time constants. The 39 ps decay must
then result from some combination of direct quenching of the
porphyrins meta and para to the dye by the dye moiety and sin-
glet energy transfer among the porphyrins. In a detailed study of a
porphyrin array with very similar, but not identical, porphyrin
structures, a time constant for exchange of energy between adja-
cent porphyrins of ∼180 ps was estimated from fluorescence
anisotropy results.18 If we assume that a similar time constant is
appropriate for 1, then it is clear that the rapid (∼39 ps) transfer
of excitation to the open dye from the porphyrin moieties meta
and para to it must involve energy transfer directly from these
porphyrins to the dye moiety. In principle, the kinetics for direct
transfer will be convoluted to some extent with excitation hopp-
ing between porphyrins. However, kinetic simulations reveal that
if we assume that meta and para transfer both occur with time
constants of 39 ps and ortho transfer occurs with a time constant
of 10 ps, inclusion of the time constant for exchange of energy
between porphyrins has a negligible effect on the porphyrin first
excited singlet state decay profile. The time constants for energy
transfer involving the porphyrins in the meta and para locations
are undoubtedly different from one another, but extraction of
additional time constants was not justified by data analysis. It is
clearly evident that the dye in its open, protonated form rapidly
quenches the excited singlet states of all five porphyrins.
Model dye 4 (Figure 2) in its protonated form did not show
measurable fluorescence when studied by single-photon timing
methods. Therefore, the excited singlet state lifetime was mea-
sured in ethanol with excitation at 640 nm using femtosecond
transient absorption spectroscopy. Figure 8 shows excited-state
relaxation to the ground state (decay of ground-state bleaching at
640 nm) and an exponential least-squares fit with a time constant
of 5.0 ps.
Figure 7. Cyclic voltammograms of (a) model dye 4 dissolved in
dichloromethane containing 0.1 M tetra-n-butylammonium hexafluor-
ophosphate and (b) the same solution after addition of sufficient acetic
acid to convert most of the dye to the open, protonated form. The
potentials were measured with ferrocene as an internal reference and are
reported relative to SCE. The waves around 0.5 V are due to ferrocene.
transfer yielding a charge-separated state. In order to investigate
the thermodynamic requirements for quenching by photoinduced
electron transfer, we measured the electrochemical properties of
model dye 4 (Figure 2). The results of cyclic voltammetric experi-
ments in dichloromethane containing 0.1 M tetra-n-butylammo-
nium hexafluorophosphate are shown in Figure 7. The working
electrode was glassy carbon, the counter electrode was platinum
foil, and the reference electrode was a silver wire in 10 mM silver
nitrate in acetonitrile containing the same electrolyte. The poten-
tials were converted to the SCE scale using ferrocene as an internal
reference compound.
Representative results for 4 in the closed, spirocyclic form are
shown in Figure 7a. The first oxidation was observed at 0.94 V vs
SCE, and the first reduction at -1.11 V vs SCE. Both waves were
essentially reversible. The waves around 0.5 V are due to ferro-
cene. A representative voltammogram after addition of 80 μL of
acetic acid in order to convert most of the dye to the open,
protonated form is shown in Figure 7b. Although the voltammo-
gram is not reversible, it is clear that reductive processes occur in
the -0.5 to -0.6 V region.
On the basis of the oxidation potential of zinc tetramesityl-
porphyrin (0.68 V vs SCE19) and the reduction behavior of
model dye 4, quenching via photoinduced electron transfer from
the excited porphyrin to the protonated dye is thermodynami-
cally possible. However, electron transfer requires overlap of
donor and acceptor molecular orbitals, which is expected to be
rather weak in 1o, given the large number of bonds between the
porphyrins and dye moiety ortho, meta, and para to them and
the fact that steric hindrance at the center of the hexaphenylbenzene
Quenching Mechanism. Two possible mechanisms for
quenching of the porphyrin excited states by the protonated
form of the dye are singlet-singlet energy transfer to the dye,
yielding an excited state of the dye, and photoinduced electron
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Figure 8. Transient absorption at 640 nm (arbitrary units) of an ethanol
solution of model dye 4 excited with a 54-fs laser pulse at 640 nm. The
solution contained sufficient formic acid to convert the molecule to its
open, protonated form. The data show the decay of the dye ground-state
bleach as the first excited singlet state returns to the ground state. The
smooth curve is an exponential fit to the decay with a time constant of
5.0 ps. The χ2 value (unreduced) is 0.16. The initial part of the decay is
convoluted with coherence artifacts arising from cross-phase modula-
tion, and were omitted from the data fitting.
Figure 9. Hexad 1 fluorescence quenching upon addition of acid. A
solution of 1 in dichloromethane was prepared, and the zinc porphyrin
fluorescence intensity at 650 nm (circles) was monitored as the acetic
acid concentration (squares) was increased. The fluorescence data have
been corrected for dilution of the solution by the added acid.
of dichloromethane (9), and electron transfer rates are strongly
dependent on solvent polarity, which affects both reorganization
energies and driving force. Energy transfer is generally not a
strong function of solvent polarity. Direct observation of energy
transfer by transient spectroscopy in 1o would be difficult be-
cause the lifetime of the first excited singlet state of model dye 4
in its protonated form, measured in ethanol by transient absorp-
tion spectroscopy, is only 5 ps (Figure 8), which is shorter than
the time constants of the processes that populate it.
core leads to near-orthogonality of the central and peripheral
benzene rings.20-22
Singlet-singlet energy transfer, on the other hand, usually
occurs by the Coulombic (F€orster) mechanism,23 which does
not require orbital overlap and depends in part on interchromo-
phore separation and orientation. In 1o, the spatial proximity of
the dye moiety and the zinc porphyrins is favorable for singlet-
singlet energy transfer, as is the energy overlap of porphyrin
emission and dye absorption. Rate constants for singlet-singlet
energy transfer from porphyrin moieties of 1o to the dye moiety
in the open, protonated form were calculated using the F€orster
theory. In these calculations, the transition dipole moment for
the protonated dye molecule was assumed to lie in the same
position as it does in the related rhodamine 6G.24 The fluores-
cence quantum yield for the zinc porphyrins in the absence of
energy transfer was taken as 0.04.25 The extinction coefficient for
the dye in the open form at 656 nm was measured as 5.7 Â 104
M-1 cm-1 for model dye 4. Modeling using molecular me-
chanics (MM2) showed that several conformations for the dye
relative to the porphyrin moieties were likely. For the porphyrin
para to the dye, distances between the transition dipoles ranged
from 20.9 Å to 21.5 Å. Assuming an orientation factor κ2 equal to
2/3 (random orientation), these distances gave energy transfer
time constants of 40.6 and 48.1 ps, respectively. Energetically
reasonable conformations for the molecule featured distances be-
tween the dipoles of the dye and a porphyrin ortho to it, ranging
between 12.5 Å and 15.3 Å. A κ2 of 2/3 yielded time constants of
1.9 and 6.25 ps for these separations. Transfer rates between the
open dye and a porphyrin meta to it were not calculated, but
would lie between those for the ortho and para porphyrins.
The calculated time constants for energy transfer from a por-
phyrin to a dye moiety ortho and para to it are relatively close to
the measured lifetimes of 10 and 39 ps. Thus, the quenching in 1o
is consistent with energy transfer from porphyrin to the proto-
nated dye. Assignment of the quenching to energy transfer is
strongly supported by the observation that the time constants for
decay of the zinc porphyrin excited states of 1o in ethanol (9 and
39 ps) are nearly identical to those in dichloromethane. The
dielectric constant of ethanol (24) is significantly larger than that
’ DISCUSSION
It is clear from the above results that absorption of visible light
by hexad 1c results in the formation of zinc porphyrin excited
singlet states that exchange excitation energy by singlet-singlet
energy transfer. The ensemble of excited singlet states decays
exponentially with a time constant of 2.1 ns, which is essentially
the same lifetime as observed for related monomeric zinc por-
phyrin 2. As has been shown by numerous studies,26-28 such
long-lived singlet states are ideal for driving photoinduced elec-
tron transfer which converts excitation energy to chemical poten-
tial. When a solution of 1 is acidified, protonation of the dye con-
verts 1c to 1o, in which the porphyrin first excited singlet states
are quenched to <40 ps, rendering them kinetically incompetent
to readily donate electrons to useful acceptor moieties and ulti-
mately converting the excitation energy to heat. The effect is
shown graphically in Figure 9, where the porphyrin fluorescence
at 650 nm is seen to decrease as the acid concentration increases.
Each porphyrin moiety in the array is quenched by the proto-
nated dye via a process ascribed to singlet-singlet energy trans-
fer. The transfer rates are consistent with those calculated using
simple F€orster theory. The resulting excited dye moiety rapidly
(5 ps) decays to the ground state. Thus, as the acidity of a solu-
tion of 1 is increased, the fraction of hexad molecules able to
perform useful photochemistry decreases accordingly: the sys-
tem down-regulates its photochemical efficiency as acidity in-
creases. The hexad photochemistry thus mimics the antenna sys-
tem function in the NPQ photoprotective process in green plants,
where antenna chlorophyll excited-state quenching is triggered by
decreasing pH in the thylakoid lumen. In addition to providing a
model framework in which to investigate the principles underlying
naturalNPQ, thehexad shows that incorporation of self-regulation
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of function in response to external stimuli is also possible in
human-made nanosystems for solar energy conversion.
synchronized with a mechanical chopper modulating the pump path at
499 Hz. The observed kinetics were independent of both pump and
probe intensity. The measured transient absorption signal was fit with a
single exponential decay using least-squares analysis, and goodness of fit
was established from residuals. The fitting was restricted to the part of
the curve where the effects of the excitation pulse and coherence artifacts
were negligible.
’ EXPERIMENTAL SECTION
The details for the synthesis and characterization of all new com-
pounds reported are given in the Supporting Information. Spectroscopic
studies were carried out on dilute (∼10-5 M) solutions in dichloro-
methane, and acetic acid was added to convert the dye to the protonated
form, unless otherwise specified. Random errors associated with the
reported lifetimes were typically e5%.
Steady-State Spectroscopy. Absorption spectra were measured
on a Shimadzu UV2100U UV-vis and/or UV-3101PC UV-vis-NIR
spectrometer. Steady-state fluorescence spectra were measured using a
Photon Technology International MP-1 spectrometer and corrected for
detection system response. Excitation was provided by a 75 W xenon-arc
lamp and single grating monochromator. Fluorescence was detected 90ꢀ
to the excitation beam via a single grating monochromator and an R928
photomultiplier tube having S-20 spectral response and operating in the
single-photon counting mode.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details of the
b
synthesis and characterization of the molecules described. This
material is available free of charge via the Internet at http://pubs.
acs.org.
’ AUTHOR INFORMATION
Corresponding Author
gust@asu.edu; GRFleming@lbl.gov
Time-Resolved Fluorescence. Fluorescence decay measure-
ments were performed employing two different systems: a conventional
single-photon timing apparatus and a streak camera setup. The excita-
tion source for the single-photon timing system was a mode-locked Ti:
Sapphire laser (Spectra Physics, Millennia-pumped Tsunami) with a
130-fs pulse duration operating at 80 MHz. The laser output was sent
through a frequency doubler and pulse selector (Spectra Physics model
3980) to obtain 370-450 nm pulses at 4 MHz. Fluorescence emission
was detected at the magic angle using a double grating monochromator
(Jobin Yvon Gemini-180) and a microchannel plate photomultiplier
tube (Hamamatsu R3809U-50). The instrument response function was
35-55 ps. The spectrometer was controlled by software based on the
LabView programming language, and data acquisition was done using a
single-photon counting card (Becker-Hickl, SPC-830).
Time vs wavelength fluorescence intensity surfaces were recorded on
the streak camera system, with excitation provided by an ultrafast laser.
Laser pulses of ∼130 fs at 800 nm were generated from an amplified,
mode-locked titanium-sapphire 250 kHz laser system (Verdi/Mira/
RegA, Coherent). The excitation light wavelength was adjusted using an
optical parametric amplifier (Coherent). Fluorescence with polarization
set to the magic angle was collected 90ꢀ to the excitation beam and focused
on the entrance slit of a Chromex 250IS spectrograph, which was coupled
to a Hamamatsu C5680 streak camera with a M5675 synchroscan sweep
unit. The streak images were recorded on a Hamamatsu C4742 CCD
camera and curvature corrected (corrections for shading and system
detection sensitivity at different wavelengths were not performed). The
instrument response function was ∼5-20 ps.
’ ACKNOWLEDGMENT
This work was funded by the Helios Solar Energy Research
Center, which is supported by the Director, Office of Science,
Office of Basic Energy Sciences of the U.S. Department of Energy
under Contract No. DE-AC02-05CH11231. G.K. was supported
as part of the Center for Bio-Inspired Solar Fuel Production, an
Energy Frontier Research Center funded by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences
under Award Number DE-SC0001016. C.M. thanks the Science
Foundation Arizona for financial support.
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